Biostratigraphy, paleoecology and paleogeography of ...

----------------------------------------------------------------------------------------------------------------------N. Jb. Geol. Paläeont. Abh. 207 (3) 311-394 Stuttgart, März 1998 -----------------------------------------------------------------------------------------------------------------------

Biostratigraphy, paleoecology and paleogeography of the middle and late Tertiary deposits of the northern Western Desert, Egypt Khaled Ouda, Department of Geology, Faculty of Science Assiut University, Assiut, 71516, Egypt

Abstract: A new and improved Late Eocene-Pliocene biostratigraphic sequence is achieved in the northern part of the Western Desert. The sequence concentrates on global planktic foraminiferal paleoevents and involves a number of larger benthic foraminiferal datum planes that are considered to be correlative and isochronous over the Mediterranean region. The paleontologic events are correlated to the recently revised Cenozoic time scale, and the biostratigraphic sequence is compared with the generalized zonal

scheme

of

the

tropical/subtropical

areas.

The

stage

boundaries

are

comprehensively evaluated in the light of the new definition of stages by the GSSPs of their lower boundaries rather than by their historical type-localities. The study reveals stratigraphic results; Unconformities in the stratigraphic record are

evidently

major

and

lithologically

distinct

at

the

Middle/Late

Eocene,

Oligocene/Miocene and Miocene/ Pliocene boundaries. The Priabonian/Rupelian, Aquitanian/Burdigalian, Burdigalian / Langhian and Zanclean/ Piacenzian boundaries are paleontologically conformable and have no visible lithologic or facies Change. A wide depositional disturbance marks the transition from the Rupelian to the chattian. A regional drop in the sea level occurred during the Late Langhian-Serravallian, and the sea withdrew completely during the full entirety of the Late Miocene. Paleogeographic maps of the northern Western Desert during the different stages of the Mid-Late Tertiary period are constructed.

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INTRODUCTION Recently, the most potentially exciting news of Oil and Gas discoveries are coming from the northern part of the Western Desert where several outstanding discoveries have been achieved since the early eighties. Since then, a large amount of data have become available, and several publications concerning the subsurface structural pattern of this part of the desert as well as the reservoirs characteristics and the isopachous variation of the subsurface sedimentary units, have been presented in the 7th to 11th. Exploration Seminars of the Egyptian General Petroleum Corporation, 1982-1992). However, not much attention has been paid to the regional biostratigraphy of the MidLate Tertiary sequence of the northern Western Desert, although open marine facies of this sequence is almost confined to the subsurface of this area. The classical lithostratigraphic work of Shata (1955, 1957), Said (1962), Abdallah (1966), Norton (1967),

Marzouk (1970) and

Omara & Ouda (1972) is still forming the base of recent rock-stratigraphy of the Mid-Late Tertiary deposits of the northern Western Desert. Earlier attempts to divide the Oligocene, Miocene or Pliocene section (s) of the northern Western Desert into a number of biostratigraphic zones include those of Beckmann et al. (1969), Fahmy et al. (1969); Mansour et al. (1969), Omara & Ouda (1969); Ouda (1971), Bassiouni et al. (1976), Sadek et al. (1977), Masoud (1983), Salloum et al. (1985) and El Shiekh & Faris (1985). Preliminary paleogeographic maps during the different stages of the Oligocene-Miocene time interval have been constructed by Marzouk (1970), Ouda (1971) and Salem (1976). The present study is concerned with the proposal of a sequence of biostratigraphical zones based essentially on planktonic foraminifera covering the time interval from the Late Eocene to Pliocene in the northern part of the Western Desert of Egypt. Larger foraminifera such as nummulitids, miogypsinids, heterosteginids, and alveolinids are used instead of planktonic foraminifera in order to establish a corresponding biostratigraphical sequence in equivalent shallow water sediments, and to fill gabs in the planktonic foraminiferal sequence formed due to the regional drop in sea level. For this purpose, samples from 24 wells (Fig. 1) drilled along the entire northern stretch of the Western Desert have been investigated micropaleontologically. The wells include Burg El Arab-1, Mersa Matruh-1, Dabaa-1, Mamura-1, Sallum Hole-1, Wadi Natrun-1,Shaltut-1, Ziebida-1, Alamein IX, Abu Gharadig-1, Betty-1, Ghazalat-1, North Ghazalat-1, Naqb Khalda-1, Khalda SD-1, Khalda-1, Zahra-1X; Umbarka1&2, Qaret Shushan-1, .Bir El Nuss WW-1, Gib Afia-1, Faghur-1 and Faghur WW-1. 133

Fig.1. Location Map The paleontological data brought by Bassiouni et al. (1976), from Dabaa-1, by Masoud (1983) from Umbarka-2 and by Salloum et al. (1985) from Abu Tunis IX, East Faghur-1 and Khalda-1 are incorporated (with emendations) in this study. Also, the stratigraphical or paleoenvironmental informations given by Shata (1955), Said (1962), Said & Issawi (1963), Abdallah (1966), Bown & Vondra (1974) Vondra (1974), Hammad et al. (1976), Abdel Kireem et. al. (1985), Strougo & Hottinger (1987) and Haggag (1990) from the surface exposures have served the construction of revised paleogeographic maps for the northern part of the Western Desert during the different stages of the Mid-Late Tertiary Period.

THE LATE EOCENE (PRIABONIAN) The Priabonian stage has generally been accepted by the International Subcommission on Paleogene Stratigraphy at the 28th International Geological Congress in Washington, July 1989 and in Tubingen, December, 1990 as constituting the late part of the Eocene Epoch. It lies immediately above the Bartonian and is the only stage of the Upper Eocene. Its type area is in northeastern

Italy (Barrin, 1988) and its upper boundary in the Massignano Section near

131

Ancona marks is considered to be a GSSP for the Eocene/Oligocene boundary (Jenkins & Luterbacher, 1992). Radiometric age determinations by Odin et al. (1991), Odin (1992) and Odin & Luterbacher (1992) suggested the placement of the base of the Priabonian in its type area close to 37 Ma (upper Anomaly 17 N), whereas the top of the Priabonian stage as defined in the Massignano Section has been placed within the interval 33.2 to 34.2 Ma, with an average age of 33.7+0.5. This age concept for the Priabonian has recently been confirmed by Berggren et al. (1995) who correlated the Bartonian/Priabonian boundary with the younger part of Chron C17n, with an estimated age of 36.9 Ma, and the Priabonian/Rupelian boundary with the Chron C13r, with an estimated age of 33.7 Ma. Correlation of the magnetostratigraphy and the planktic foraminiferal zonation (e.g. Berggren et al., 1985b, 1995; Napoleone, 1988) reveals that the Priabonian is equivalent to the later part of Zone P15, Zone P16 and Zone P17, corresponding to the nanoplankton Zones NP18, NP19, NP20 and early part of Zone NP21 (Aubry, 1986). The lower limit of the stage is marked by the last occurrence of associated

Acarinina and Truncorotaloides whose disappearance is

with mid Chron C17n, a short stratigraphic interval above the datum of first

appearance of Gk. semiinvoluta (e.g. Poore et al., 1982, 1983; Lowrie et al., 1982; Napoleone et al., 1983; Berggren et al., 1985b, 1995). A-Lithostratigraphy and Facies:( Figs.2, 3) Open marine, clastic sediments of transgressive nature belonging to the Priabonian age are widely distributed in the subsurface of the northern Western Desert. They are recovered from the majority of wells drilled in the area to the north of Lat. 29o30’ and east of Lat. 26o, except of some major subsurface highs (e.g. Katanyia Horst, Mamura-Zayed-Matruh Trend). The sediments are composed predominantly of deep water shales which are partly calcareous and enclosing, in places very thin streaks of limestone. They constitute the lower part of the Dabaa Formation (Norton, 1967) and are usually unconformably underlain by a Middle Eocene carbonate sequence which belongs to the Apollonia Formation. The latter formation is a deep neritic, chalky limestone with subordinate shale ranging in age from the Paleocene to the Middle Eocene, overlying unconformably the Cretaceous rocks and changing southward into shallow water, thick-bedded nummulitic limestone. Its type locality has been defined south of the Village of Apollonia (Susah), along the northern coastal escarpment of Jabal Al-Akadar, Libya (e.g.

133

Roehlich, 1974). A sharp lithologic and paleontologic break usually marks the boundary between the Apollonia and the overlying Dabaa Formation in the northern Western Desert.

Fig. 2. Composite subsurface columnar section of the marine Late Eocene-Pliocene sequence of the northern Western Desert (compiled from the studied wells).

133

Fig.3. The Late Eocene stratigraphic sequence and its lateral variation from the West (left side) to the East (right side) of the northern Western Desert, correlated with the planktonic foraminiferal zonation of Berggren et al. (1995). Ages are given after Cande and Kent (1992/1995) and Berggren et al. (1995). Legend is given in Figure 2.

The shaly facies of the Priabonian yields a rich plarktonic-benthonic foraminiferal association of the middle-to upper -slope type. In the northern Qattara Depression and in the area lying between this depression and the Mediterranean, the Priabonian shales vary in thickness between 150 and 300 feet (with a normal thickness of 200-220 feet) and pass conformably with gradual shoaling into the Lower Oligocene (Rupleian). However, a marked thickening of the Priabonian section accompanied by an increase in the sandstone content occurs toward the east and southeast. In the area to the east of Qattara Depression, between Abu Gharadig and Wadi Natrun, the Priabonian section assumes 510-660 feet and encloses several thin horizons of brackish water sandstone (with agglutinated foraminifera) in the youngest part. The sediments pass upward into sandstone-dominated fluviatile sequence of Oligocene to Lower Miocene age.

133

A gradual enrichment in the sandstone content accompanied by a vertical increasing of the deltaic influences occurs southeastward toward the Fayoum province. In the north of Fayoum, the Upper Eocene sediments crop out and cover an extensive area including the stretch separating the Fayoum from the Nile Valley. They assume a thickness of about 770 feet and are composed of a basal, normal marine unit made up of fossiliferous shale and calcareous sandstone (Lower Birket Qarun Formation, Beadnell, 1905) and an upper thick fluvio-marine unit composed of alternations of gypseous clays, carbonaceous marls, sandstones and sands containing shallow water littoral to sublittral molluscs, together with fresh water ostracads and land vertebrate remains (Upper Birket Qarun and Qasr El Sagha Formation, Beadnell, op. cit.). On the other hand, a marked thinning of the Upper Eocene sediments accompanied by a progressive enrichment in the carbonate content occurs toward the west and southwest. In the area to the west of the Qattara Depression, theUpper

Eocene section is predominantly

composed of shallow water, reefal limestone and dolomite, attaining a thickness of about 150 feet

and yielding Nummulites spp., Orbitolites sp., Rotalia sp., together with miliolids,

pelecypods and echinoids (e.g. Faghur-1, Gib Afria-1 , Bir El Nuss WW-1). Southeastward toward the Baharyia Oasis, the sediments become very thin (100 feet) and are made up of richly fossiliferous arenaceous bioclastic carbonates which seem to have been deposited in lacustrines along the Eocene shoreline (Said & Issawi, 1963; Said, 1990). B-Planktic Foraminiferal Zonal Stratigraphy (Figs. 3, 8, 13, 14) Globigerinatheka semiinvoluta Zone Category:

Partial-range Zone

Age:

Late Eocene (Priabonian).

Author:

Bolli (1957), modified by Proto Decima and Bolli (1970).

Definition: Interval with zonal marker from the last occurrence of spinose Globorotaliidae including, Truncorotaloides, Acarinina and Morozovella to the last occurrence of Globigerinatheka semiinvoluta. Remarks: This zone is equivalent only to the later part of Zone P15 as emended by Blow (1979) and Berggren & Miller (1988). According to these authers, the last occurrence of Tr. rohri occurs within the life range of Gr. semiinvoluta, and their Zone P15 seems therefore to span the Middle/Late Eocene boundary. The lower limit of this zone is marked in the northern Western Desert by the sudden disappearance of all diagnostic Middle Eocene spinose forms together with earlier members of the T. cerroazulensis lineage (i.e. T.c. frontosa and T.c. possagnoensis), as well as Gk. 133

subconglobata s.s., Gk. s. curryi, Gk. mexicana kugleri and Hk. mexicana. On the other hand, highly developed members of the T. cerroazulensis lineage (i.e. T.c. cerroazulensis and T.c. cocoaensis) together with the zonal marker and large-sized grobigerinids including Gg. yeguaensis, Gg. tripartita, Gg. praeturritilina and Gg. angiporoides begin to appear suddenly at the base of this zone. Occurrence: In the northern Western Desert, Egypt, this zone has a sporadic distribution, being only represented in the area to the east of Qattara Depression (e.g. Wadi Natrun-1, Zibeida-1, Abu Gharadig-1) and in Sallum Hole-1 at the northwestern corner of the Western Desert (Fig. 16).

Fig.4. The Oligocene stratigraphic sequence and its characteristic datum levels in the northern Western Desert, correlated with the planktonic foraminiferal zonation of Berggren et al. (1995). Ages are given after Cande and Kent (1992/1995) and Berggren et al. (1995). Legend is given in Figure 2.

Haggag (1990) recorded Gk. semiinvoluta above the datum of extinction of spinose forms, within the uppermost part of Gehanum Formation (later revised as basal part of Birket 133

Qarun in Haggag & Bolli, 1995) in Fayoum area. According to Haggag (op.cit.), the extinction datum of Truncorotaloides and Morozovella is separated from the datum of first appearance of Gk. semiinvoluta by a thin stratigraphic interval containing mixed Middle-Late Eocene forms together with minute spinose globigerinids and acarininids (Globigerina pseudoampliapertura Zone). Since, the datum of last occurrence of Truncorotaloides and Acarinina is proved to be younger and only subsequent (Chron 17, Poore et al., 1982; 1983) to the datum of first appearance of Gk. semiinvoluta (Chron 18, Lowrie et al., 1982 and Napoleone et al., 1983), it would be advisable to equate the transitional interval of Haggag (op. cit.) with the Middle Eocene rather than Late Eocene (see discussion on the Middle/Late Eocene boundary). Turborotalia cerroazulensis s.l. Zone Category:

Partial-range Zone

Age:

Late Eocene.

Author:

Bolli (1957) renamed by Bolli (1966 and 1972).

Definition: Interval with zonal marker from the last occurrence of Globigerinatheka semiinvoluta to the last occurrence of all representatives of T. cerroazulensis s.l. Remarks: The above definition of the zone is only possible in areas where Gk. semiinvoluta Zone occurs in the bottom part of the Dabaa Formation. However, in the Qattara Depression and in most of the area lying between this depression and the Mediterranean, the Gk. semiinvoluta Zone is missing together with the underlying Middle Eocene Zones P13 and P14 (Fig. 3). In such a case, the zone occupies the vertical interval from the last occurrence of Truncorotaloides and other spinose forms to the last occurrence of T. cerroazulensis s.l. This interval should not be considered synchronous to the T.c. cerroazulensis/ T.c. cocoaensis and T.c. cunialensis zones of Toumarkine and Bolli (1970, emended by Toumarkine (in Toumarkine and Luterbacher, 1985). The presence of a well marked stratigraphic hiatus at the base of this interval makes it only correlatable to Bolli’s Zone T. cerroazulensis s.l. or a part of it (1966). The zone is characterized in all localities by the presence of highly evolved members of the T. cerroazulensis lineage including T.c. cerroazulensis, T.c. cerroazulensis-T.c. cocoaensis, T.c. cocoaensis together with frequent large-size globigerinids (Fig. 8). It is also marked by the appearance of P. naguewichiensis, Gg. gortanii s.s., Gg. ouachitaensis ouachitaensis, Gg.o. ganucki, Gg. prasaepis, Gg. praebulloides leroyi, C. martini martini, C.m. scandretti, C. howi, C. pera and Gr. obesa.

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Occurrence:

The T. cerroazulensis s.l. Zone is widely distributed in the northern Western

Desert where it occupies the basal part of the Dabaa Formation. It is recorded from the majority of wells drilled in the northern area lying to the north of Lat. 29o 30, and bounded from the west by East Faghur-Gib Afia line, from the northwest by Mamura-Zayed-Matruh high, from the east by Maryout-Khatatba line and from the southeast by Kattanyia-SW Mubarak line (Figs. 1, 16). The faunal association of this zone is replaced by a shallow water, neritic foraminiferal assemblage in Faghur-1,Gib Afia-1 and Bir El-Nuss WW-1.

Fig.5. The Miocene stratigraphic sequence and its characteristic datum levels in the northern Western Desert, correlated with the planktonic foraminiferal zonation of Berggren et al. (1995). Ages are given after Cande and Kent (1992/1995) and Berggren et al. (1995). Legend is given in Figure 2.

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C- The Priabonian Larger Foraminifera: (Figs. 11, 13)

Fig.6. The Miogypsina biostratigraphic sequence; rock units, ages, planktonic foraminiferal events in the northern Western Desert, correlated with planktonic foraminiferal zonation of Berggren et al. (1995) In Sallum Hol-1, the T. cerroazulensis s.l. Zone encloses in the higher levels very thin horizons of limestone containing Nummulites variolarius gandinus, N. orbignyi and N. fabianii. The same forms are recorded in abundance together with Orbitolites, Rotalia, milioliods, pelecypods and echinoids from the shallow water carbonate facies of the Late Eocene which prevailed along the Faghur-Gib Afia line and extends southeastward toward Baharyia Oasis. In this area, the Upper Eocene section is represented by dolomitic limestone which is unconformably underlain by Lutetian limestone containing N. gizehensis, and unconformably overlain by either Lower Miocene carbonates with Miogypsina s.s. (e.g. Gib Afia-1, Bir El Nus WW-1) or Middle Miocene carbonates with Borelis melo s.s. (e.g. Faghur-1, Faghur WW-1).

The shallow water, carbonate facies of the Upper Eocene extends south of the Qattara Depression, between Siwa and Baharyia Oasis where Cuvillier (1930) described fossiliferous limestone containing mixed Middle-Upper Eocene Nummulites (N. fabianii, N. variolarius, N. 133

contortus, N. chavannesi), together with molluscs and echinoids. The limestone is underlain by unfossiliferrous marl which indicates according to cuvillier (op.cit.) a probable rising above sea level of this region before deposition of the Upper Eocene in a small gulf east of Siwa. In the northern plateau of Baharyia Oasis, Strougo & Hottinger, (1987) recorded N. fabianii from limestone beds lying at the base of the clastic series of Qasr El Sagha Formation. The latter formation is well developed at the north of Fayoum where it reflects a prodelta to delta front facies. No Nummulites species were encountered in this series in the vicinity of Fayoum (Strougo, 1992), but the underlying marine to prodeltaic unit, Birket Qarun Formation yields in its lower part planktonic foraminiferal fauna referable to the latter part of Zone P15 (Haggag & Bolli, 1995). This would suggest the equation of Qasr El Sagha Formation with the youngest part of the Eocene, corresponding to the age of the T. cerrozulensis s.l. Zone.

Fig.7. The Pliocene stratigraphic sequence and its characteristic datum levels of the northern Western Desert. Ages are given after Spovieri (1993), Cande and Kent (1995) and Shipboard Scientific Party (1996 a & b). Legend is given in Fig.2.

D-The Bartonian-Priabonian (Middle/Late Eocene) Boundary: In Egypt, the Middle/Late Eocene boundary was traditionally correlated by many workers with the P14/P15 boundary which (according to the original definition of Blow, 1969) is marked by the extinction of Truncorotaloides rohri. However, it is important to note that Blow (1979) has 133

emended the definition of his Zones P14 and P15. The extinction datum (LAD) of the Tr. rohri group becomes placed within the range of Zone P15, and the appearance datum (FAD) of Gk. seminovluta is consequently used to define the P14/P15 zonal boundary. This concept has been reemphasized by magnetobiostratigraphic studies on the deep sea cores, and surface sections in the type area of the Priabonian (see Berggren et al. 1985b), and which indicate that the LAD of Tr. rohri is not contiguous with the FAD of Gk. semiinvoluta. Both taxa overlap in deep sea sequences and the Truncorotaloides rohri/”Globigerapsis” semiinvoluta zonal boundary of Bolli (1966 = P14/ P15 boundary of Blow, 1969) would fall within Chron C17 N, close to the boundary between Zones NP17 and NP18 (Poore et al., 1983). On these grounds the Middle/Late Eocene (=Bartonian/Priabonian) boundary is placed at a level within the later part of Zone P15, (e.g. Berggren et al., 1985b, 1995; Napoleone, 1988; Odin & Luterbacher, 1992). In the northern Western Desert of Egypt, a prominent stratigraphic hiatus exists between the Middle and Late Eocene. The hiatus coincides with the sharp lithologic boundary which occurs everywhere between the Apollonia and the overlying Dabaa Formation. It can be traced in the majority of wells drilled in the central and eastern parts of the northern Western Desert as well as along the Mediterranean coastal plain. In these wells, the Middle/Late Eocene boundary is marked by an abrupt change in fauna, from those referable to the Morozovella lehneri Zone (Zone P12) just immediately below the boundary, to fauna belonging to either the Globigerinatheka semiinvoluta Zone (later part of Zone P15) or most commonly the Turborotalia cerroazulensis s.l. Zone (Zones P16- P17) just immediately above the boundary. The foraminiferal Zones P13 and P14 are entirely missing in all studied subsurface sections of the northern Western Desert, whereas Zone P15 occurs sporadically and only partially. When occurring, the latter zone is only represented by its later part (above the datum of extinction of spinose Globorotaliidae), covering a thin stratigraphic interval in the low-lying areas to the east and southeast of the Qattara Depression where the Late Eocene section attains its greatest development. In the northern Qattara Depression and the area between this depression and the Mediterranean coast, the Zone P12 is generally reduced in thickness (40-110 feet thick), being almost conformably underlain by Zone P11 and unconformably overlain by the Late Eocene Zones P16 - P17. (e.g. Ghazalat-1, North Ghazalat-1, Betty-1, East Faghur-1, Um Barka-2, Abu Tunis-1, Sallum-1, Dabaa-1, Burg El Arab-1, Alam El Buieb-1). In these localities, the Zone P12 occupies the upper part of the Middle Eocene section (topmost of Apollonia Formation and yields the following planktonic foraminiferal assemblage:Truncorotaloides rohri, Tr. topilensis, 131

Morozovella lehneri, M. spinulosa, Acarinina spinuloinflata, Globigerinatheka mexicana kugleri, Gk.m. barri , Gk. index s.s., Gk. subconglobata s.s., Gk. s. curryi, Turborotalia cerroazulensis pomeroli (=Gr. centralis), T.c. frontosa (=Gr. boweri), T.c. possagnoensis, Hantkenina mexicana and Globigerina senni.

Fig.8. Distribution of Middle (Late Lutetian-Early Bartonian) –Late (Priabonian) Eocene planktonic foraminifera in the northern Western Desert This faunal assemblage covers the stratigraphic interval from the datum of last occurrence of Morozovella aragonensis (a bioevent indicative of P11 /P12 zonal boundary) to the datum of last occurrence of all diagnostic Middle Eocene spinose Globorotaliidae. The lower limit of this interval forms a well recognizable datum level in the northern Western Desert that can be considered to be correlatable and synchronous. It is marked by the extinction of several diagnostic Eocene forms such as M. aragonensis, Pseudohastigerina wilcoxensis, Hastigerina bolivariana and Acarinina broedermanni. The upper limit of this interval coincides with the lithologic boundary between the Apollonia and the overlying Dabaa Formation. It is marked by the sudden disappearance of all representatives of the genera Truncorotaloides, Morozovella and Acarinina together with the members of the Gk. subconglobata and Gk. mexicana groups as well as T.c. frontosa, T.c. possagnoensis, H. maxicana s.s and Gg. senni.

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No specimens referable to Orbulinoides beckmanni are encountered any where within this assemblage, and the fauna appear consequently to be fully determinative of the Morozovella lehneri Zone of Bolli (1957). This zonal concept is substantiated by the concurrent association of T.c. pomeroli and T.c. frontosa, an association which is indicative of Pre Zone P13 (Toumarkine & Luterbacher, 1985). On the other hand, T.c. cerroazulensis, T.c. cocoaensis, T. increbescens, Gr. opima nana, Gg. ampliapertura, Gg. pseudoampliapertura, Gg. tripartite, Gg. angiporoides, Gg. praeturritilina begin to appear only at the base of the Dabaa Formation , just immediately above the datum of disappearance of the spinose Gk. No specimens of Gk. semiinvoluta are recorded in this area, but only specimens referable to Gk. index tropicalis are found in common numbers above the Middle/Late Eocene boundary. T. c. pomeroli is the most conspicuous Middle Eocene form which extends above this boundary where it forms frequent transitional forms toward Gg. ampliapertura. Thus, the whole assemblage which directly overlies the datum of extinction of spinose forms in almost localities seems to be entirely indicative of the T. cerroazulensis s.l. Zone of Bolli (1966).

Fig.9. Distribution of the Oligocene (Rupelian) planktonic foraminifera in the northern Western Desert. The Chattian is predominantly represented by shallow water deposits. However, in the area to the east and southeast of the Qattara Depression, the Zone P12 attains its greatest development (210-340 feet thick) and is marked in its upper part (upper 150 feet in Wadi Natrun-1) by the general impoverishment of T.c. frontosa, Gk. mexicana kugleri,

133

Gk. subconglobata curryi and Hk. mexicana (e.g. Wadi Naturn-1, Zibeida-1, Abu Gharadig-1). On the other hand, the spinose Globorotaliidae in particular Tr. rohri, M. spinulosa and Ac. spinuloinflata predominate upward together with Gk. mexicana barri, T.c.

pomeroli, T.c.

possagnoensis and Gg. senni. The fauna still beyond zone P14 and is unconformably overlain by the assemblage of the Gk. semiinvoluta Zone. The latter occupies the basal part of the Dabaa Formation (reaches up 210 feet thick) and is characterized by the appearance of the zonal marker together with T. cerrazulensis s.s., T.c. cocoaensis, Gg. pseudoampliaperture, Gg. yeguaensis, Gg. praeturritilina, Gg. angiporoides, Gg. tripartita, Hk. alabamensis and Gk. index tropicalis. No spinose forms are encountered within this assemblage and the zone seems therefore to be synchronous with that of Proto Decima & Bolli (1970) , corresponding to the later part of Zone P15 as emended by Blow (1979).

Fig.10. Distribution of the Miocene (Late Aquitanian-Langhian) planktonic foraminifera in the northern Western Desert. Most of the Aquitanian and the entire Late Miocene are missing, while the Serravallian is predominantly represented by shallow water deposits. The assemblage of the Gk. semiinvoluta Zone gives rise upward to that of the T. cerrozulensis s.l. (up to 300 feet thick) Zone which covers stratigraphically that part of the range of T.c. premoli, T.c. cerroazulensis s.s. and T.c. cocoaensis above the datum of extinction of Gk. semiinvoluta. Frequent specimens of Gg. ampliapertura, Gg. tripartita, Gg. gortanii s.s., Gg. praeturritilina, Gg. ouachitaensis are usually found in the latter zone (Fig. 8).

133

Fig.11. Distribution of selected Late Eocene- Middle Miocene larger foraminiferai n the northern Western Desert. The stratigraphic hiatus between the Middle and Late Eocene in the northern Western Desert was previously detected by Sadek et al. (1977) in Burg El Arab-1 , using both planktonic and nanoplankton fauna. According to them, the Middle Eocene carbonate bed which lies just immediately below the Upper Eocene clastic series of the Dabaa Formation, (depth from 5100 to 5020 feet) belongs to Zone NP15 (Nanotiterina fulgens Zone) . This zone is now equated with the Middle-Late part of the Lutetian, being equivalent to the later part of Zone P10 , Zone P11 and early part of Zone P12 in the standered planktonic foraminiferal scheme (e.g. Berggren et al., 1985b, Aubry; 1986; Odin & Luterbacher 1992). Thus, a stratigraphic hiatus not less than 3.5 Ma in age is now documented between the Middle and Late Eocene in the northern Western Desert, at least from the age of appearance of 133

O. beckmanni, dated 40.5 Ma to the age of last occurrence of spinose forms (Truncorotaloides and Acarinina), dated 36.9 Ma (Berggren et al., 1995). The hiatus is essentially attributed to the epierogenic uplift which seems to have affected most of North Africa during the Late LutetianEarly Bartonian time (Pyrenean Tectonic Phase). The active uplifting caused the abrupt withdrawal of the Middle Eocene sea toward the north, and a period of wide subaerial erosion prevailed before the advent of the Late Eocene transgression. Deep-sea drilling in the Levantine Basin has proved the existence of a major stratigraphic hiatus at the top of zones P11, P12, involving the Late Eocene, Oligocene and the greater part of the Miocene in Holes 966F, 967A and 967E (Shipboard Scientific Party, 1996a).

Fig.12. Distribution of the Pliocene (Zanclian-Piacenazian) planktonic foraminifera in the northern Western Desert. Evidences of uplif of previously Middle Eocene submerged areas along the entire northern stretch of the Western Desert, and tilting toward the northeast during the close of the Middle Eocene are reflected by isopach maps and seismic sections given by Norton, (1971), Moussa (1986), and Hantar (1990). According to Moussa (1986), the tilting ended the carbonate deposition, and the associated slight rejuvenation of the stable shelf area to the south allowed subaerial erosion to take place with a resulting supply of fine terrigenous clastics. Hantar (1990) showed that the northern part of the Western Desert was shaped for the first time, into a number

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of lows and highs during the Late Eocene, in contrast to forming a platform during the EarlyMiddle Eocene. The tectonic influence of the epeirogenic movement belonging to the Pyrenean Tectonic phase was first noted in Egypt by Omara (1954) who recognized the occurrence of two significant periods of tectonic disturbances affecting structurally the Eocene rocks of the Pyramids Plateau near Cairo, during the Late Lutetian and the Early Bartonian. The uplifting seems to have accompanied by important fracturing volcanicity and alkaline magnetism, in the southern Western Desert. The K/Ar age determinations on some volcanic rocks and alkaline granites in the south Western Desert indicate the presence of a period of alkaline magmatism between 45 Ma and 41 Ma (Marholz, 1968 and Klerkx & Rundle, 1976), and volcanic activities at 40 Ma (Greenwood, 1969 and Meneisy & Kreuzer, 1974) and 45 Ma (Meneisy, 1990). This period corresponds to the Late Lutetian-Early Bartonian Pyrenean movement. E-Paleogeography: (Fig. 17) The Late Eocene-Oligocene succession of the northern Western Desert represents one sedimentary cycle starting with a marine transgression at the beginning of Priabonian and ending with a complete retreation of sea and prevalence of continental circumstances at the close of the Chattian. The cycle began after a period of epeirogenic uplifting movement which has affected the entire northern part of Egypt during the latest Lutetian-early Bartonian. The Late Eocene sea invaded most of the northern stretch of the Western Desert and transgressed the eroded surfaces of the Middle Eocene strata in the area to the north of Lat. 29o 30’, except of some major structural highs. It was relatively reduced than the Lutetian sea, being bounded from the west and southwest by the emergent area lying to the west of Faghur-Gib Afia line, and extending southeastward to the north of Baharyia, then northeastward to the north of Fayoum. The sea was moderately deep to deep in the central and eastern parts of the northern Western Desert where open marine shales rich in planktonic foraminifera laid down on the expenses of the Middle Eocene carbonates. A progressive shoaling of the sea occurred west-and southward, where shallow water carbonates of reefal nature predominate. The nature of the Priabonian transgression resulted in a sudden change in lithology, from predominantly carbonates of the Middle Eocene (Apollonia Formation) to predominantly montomorellonite shales (Dabaa Formation) of the Late Eocene and grading conformably upward into the Oligocene. No transitional beds (e.g. conglomerates sandstone) suggestive of a normal transgression exist between both rock units, and the Late Eocene transgression seems, therefore, to be the function of sudden drawing of the subsided surface by sea water. A 133

sharp paleontologic break usually marks the base of the Priabonian section, involving the absence of a stratigraphic interval corresponding in age to the greater part of the Bartonian stage. The basal part of the Priabonian is also missing in several localities in the central part and along the Mediterranean coastal plain, thus suggesting that the surface-bottom topography of the Late Eocene sea was irregular (lows and highs). Toward the west and the south, the shaly facies of the Upper Eocene alternates with calcareous facies consisting mainly of biomicrite. The latter facies predominated along FaghurGib Afia line and in the area extending south of the Qattara Depression, between Siwa and north of Bahayria Oasis. The regression phase of this cycle began earlier in the south than in the north. In the area to the north of Fayoum, the marine Priabonian strata (lower shaly part of Birket Qarun Formation) belonging to the later part of Zone P15 (Haggag & Bolli, 1995) are rapidly followed by a coarsening-upward sequence of reduced salinity containing a mixture of shallow water fauna and land vertebrate remains (Upper Birket Qarun and Qasr El Sagha Series, Beadnell, 1905). The latter sequence seems to have deposited under coupled conditions of retreating shallow sea and prograding delta with distributary channels. The retreat of the sea seems to have taken place episodically as deduced from the presence of lacustrine limestones which are generally impure and containing vertebrate and fish remains interrupting the coarsening-upward trend of the Upper Eocene north of Fayoum and east of the Siwa Oasis (e.g. Beadnell, op. cit.; Cuvillier, 1930; Said, 1962).

THE OLIGOCENE The two-fold chronostratigraphic subdivision of the Oligocene into Rupelian (lower) and Chattian (upper) stages, which was recommended by the International Subcommission on Paleogene Stratigraphy at the 28th International Congress in Washington 1989, is justified by recent magnetobiostratigraphic correlations (e.g. Odin & Luterbacher, 1992, Odin, 1992, Berggren et al., 1992, 1995). The Lattorfian of Germany which was previously considered as the lowest stage of the Oligocene in Europe, includes beds equivalent in age to the Priabonian. Evidences of the Mammals, Mollusca, Nummulites and planktonic organisms occurring both in the beds at Latdorf (=Lattorf) and in those of the Priabonian of northern Italy suggest the placement of both stages in the Late Eocene (e.g. Blondeau, 1969; Cavelier, 1972, 1979; Pomerol, 1982; Berggren et al., 1985b).

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Fig.13. Generalized planktic and larger foraminiferal zonal schemes and zonal markers of the Mid-Late Tertiary sequence of the northern Western Desert. 113

I. THE EOCENE/OLIGOCENE BOUNDARY: According to Nocchi et al. (1988), the extinction of Hantkeninidae is the most reliable bioevent for the location of the Eocene/Oligocene boundary in the Massignano Section. The latter section has been accepted as constituting a Global Stratotype Section and Point (GSSP) for the base of Rupelian (and consequently the Eocene/Oligocene) boundary. This boundary which can be traced at Meter 19 of the Massignano Section corresponds to the limit between planktonic foraminiferal zones P17 and P18 (Coccioni et al., 1988). It is generally considered to be correlative and synchronous with the datum of the last occurrence of all representatives of the T. cerroazulensis group (Berggren et al., 1992, 1995). The same boundary is placed in shallow water deposits at the datum between the Nummulites fabianii and N. intermedius zones(Pomerol, 1982). Chronologically, the Eocene/Oligocene boundary is dated 33.7+0.5 Ma by Odin & Luterbacher (1992),based on data with direct magnetostratigraphic control from the Upper Eocene and Lower Oligocene of the Central Apenines and the Southern Alps. The same age estimate has been proposed by Cande & Kent (1992) and Berggren et al. (1992, 1995) who placed the boundary at the level marked by the LAD of Hantkenina spp. in the youngest part of Chron C13r. In the northern Western Desert, the Eocene/Oligocene boundary is considered to be the most reliable conformable epoch-boundary in the entire Tertiary. It is located in the lower onethird part of the Dabaa Formation with no visible lithologic or paleontologic break. No facies changes across the boundary is observed in the area lying between the Qattara Depression and the Mediterranean.

However, eastward of the Qattara Depression, the Oligocene section

begins with a marked facies change leading to the widespread accumulation of shallow water to prodelta deposits which give rise southeastward into delta plain ones (see later). Paleontologically, all representatives of the T. cerroazulensis lineage including the highly evolved angled forms, with compressed tests have their last occurrence in the open marine facies just immediately below the Eocene/Oligocene boundary. On the other hand, Globigerina tapuriensis evolves from its immediate ancestor Gg. tripartita above the boundary. Many largesized Late Eocene globigerinids such as Gg. gortanii s.s., Gg. angiporoides, Gg. yeguaensis, Gg. pseudovenezuelana and Gg. tripartita, together with Gg. ampliapertura and Gg. pseudoampliapertura cross the Eocene/Oligocene boundary.

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Fig. 14. Correlation of the Late Eocene-Miocene foraminiferal zones of the northern Western Desert with those of the tropical/subtropical (N and M) and temperate (Mt) areas of the Atlantic-and Indo-Pacific provinces. The chronostratigraphic units and foraminiferal events are calibrated to the geomagnetic polarity time scale of Cande and Kent(1992,1995) and Berggren et al. (1995). 111

Fig. 15. Correlation of the Pliocene planktonic foraminiferal zones of the northern Western Desert with those of the Mediterranean region and calibration of the foraminiferal events and stage boundaries to the time scale of Sprovieri (1993) and Cande and Kent (1995). II. THE EARLY OLIGOCENE (RUPELIAN) A. Lithostratigraphy and Facies:(Figs. 2, 4) The Priabonian basinal shaly facies of the central part of the northern Western Desert and along the Mediterranean coastal plain grades conformably upward into the Rupelian. Openmarine sediments belonging to the latter stage are recorded from the subsurface of the northern Qattara Depression and the area lying between this depression and the Mediterrean coast, except of few subsurface structural highs (e.g. Mersa Matruh). The sediments are predominantly composed of shales (main part of Dabaa Formation) which are lithologically indistinguishable from those of the underlying Priabonian ones (basal Dabaa Formation). They exhibit an upper slope to outer shelf environment and are susually overlain by shallow water sediments of various litho- and biofacies belonging to the Chattian. The Rupelian sediments are considerably variable in thickness in the area lying to the west of Longitude 27o30’ where the thickness ranges from 300-330 feet (e.g. North Ghazalat-1, Um Barka-1 and 2, Khalda-1) to 640-740 feet (e.g. Ghazalat-1, Lotus IX, Brady-1, Zahra IX, Meleiha SE-1). Northward, along the coastal plain, the Rupelian is represented only by its upper part (100-330 feet thick) and 113

unconformably underlain either by Upper Cretaceous (e.g. Mamura-1) or Middle Eocene rocks (e.g. Abu Tunis IX, Meleiha IX), whereas in Mersa Matruh-1, it is entirely missing and Chattian rocks follow directly on the top of Upper Cretaceous carbonates. A marked thinning of the Rupelian sediments occurs also westward until they disappear along the Faghur-Gib Afia line. In Sallum Hole-1, East Faghur-1 and Bir El Nuss WW-1, the Rupelian section is represented by its lower part (100 feet thick or less), exhibiting predominantly shallow water facies and it is unconformably overlain by Lower Miocene (Aquitanian) carbonates. On the other hand, a marked thickening of the marine Rupelian shales is observed eastward along the Mediterranean coast. In the area to the northeast of the Qattara Depression, the open marine Rupelian section attains 740-850 feet thick and passes conformably upward into nearshore sandy shale and sandstone belonging to the Chattian (e.g. Razak-1, Yidma IX, Alamein IX, Dabaa-1, Burg El Arab-1 and Alam El Buieb-1). Southeastward, the Rupelian sediments exhibit a prodelta facies east of the Qattara Depression. In North Abu Gharadig-1, Zibeida-1 and Wadi Natrun-1, the Rupelian is represented by a thick clastic succession (up to 1200 feet) made up of a coarsening-upward sequence of clays, silts and sandstone. The sequence follows conformably the Upper Eocene strata and contains in the lower part several marine benthonic foraminifera together with few globigerinids, whereas the upper part is generally poorly fossiliferous and yields predominantly agglutinated foraminifera. Further in the southeast, to the north of Fayoum, the Ruplian is represented by an alluvial complex which seems to have been deposited in a low-forested deltaic plain by a sinuous to meandring stream (Bown and Vondra, 1974). These sediments which are known in literatures as Qatrani Formation (Beadnell, 1905) cover conformably the prodelta to delta front succession of the Upper Eocene Birket Qarun -Qasr El Sagha Formation and are generally thinning out eastward toward Cairo. Neumerous mammalian fauna were encountered in certain beds of this formation together with silicefied wood, plant remains and freshwater ostracods (Beadnell, 1905 and Cuvillier, 1930). No marine Oligocene sediments were encountered south and southwest of the Qattara Depression. In the area to the north and northwest of the Baharyia Oasis, the Oligocene is represented by unfossiliferous continental sandstone which is dark-coloured, cross-bedded and forming a continuous unit between the fossiliferous Upper Eocene and the Lower Miocene (Shata, 1955).

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Fig. 16. Distribution of the Mid-Late Tertiary planktonic and larger foraminiferal zones in selected subsurface sections in the northern Western Desert, compared to those of the Nile Delta area. The zonal and stage boundaries are calibrated to the time scale of Cande & Kent (1992/1995) and Berggren et al. (1995). * After Taylor & Jones (1982), ** After Zaghloul et al. (1979), *** After Ouda & Obaidallah (1995)

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B. Planktic Foraminiferal Zonal Stratigraphy: (Figs. 4, 9) Three planktonic foraminiferal zones could be distinguished in the basinal facies of the Rupelian of the northern Western Desert. The zones are corresponding to zones P18, P19, P 20 and the early part of Zone P21 of the tropical/subtropical areas (Blow, 1969 & 1979; Berggren & Miller, 1988 and Berggren et al., 1995). The zones, their characteristic datum planes corresponding rock units and equivalent zones in Berggren & Miller’s zonal scheme (1988) are given in Fig. 4 Pseudohastigerina spp. Zone (= Zone P 18 of Berggren & Miller, 1988). Category:

Interval zone

Age:

Early Rupelian, from 33.8 Ma to 32 Ma according to Berggren et al. (1995).

Author:

Berggren et al. (1995), here shortened.

Definition: Biostratigraphic interval from the last occurrence of T. cerroazulensis s.l. to the last occurrence of Pseudohastigerina spp. Remarks:

This zone represents the earliest Oligocene paelobiologic unit in the northern

Western Desert, where it covers conformably the T. cerroazulensis s.l. Zone. It is generally characterized by the frequent occurrence of P. naguewichiensis, Globigerina tapuriensis, Gg. tripartita, Gg. angiporoides, Gg. prasaepis, Gg. pseudoampliapertura, Gg. ampliapertura, Gg. gortanii s.s. and Gg. yeguaensis (additional forms are listed in Fig. 9 ). The zone is corresponding, but not closely equivalent to zones P18 and P19 of Blow (1969, 1979). It is entirely equivalent to the Cassigerinella chipolensis/Pseudohastigerina micra Zone as defined by Bolli (1966) and Bolli & Saunders (1985). Occurrence: This zone is recorded in the northern Qattara Depression and in the area between this depression and the Mediterranean coast except of the Mamura-Zayed-Mersa Matruh Trend (Fig. 16). The planktonic association of this zone is entirely replaced by shallow water neritic assemblage in Sallum Hole-1, East Faghur-1, Gib Afia-1 and Bir El Nurs WW-1. In the area to the east of the Qattara Depression, the planktonic foraminifera become very rare and occurring only sporadically whereas benthic faunas are predominantly represented by agglutinated species(e.g. Betty-1, Abu Gharadig-1, Zibeida-1, Wadi Natrun-1).

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Globigerina ampliapertura Zone (=Zone P19 of Berggren & Miller, 1988) Category: Age:

Interval zone.

Middle Rupelian, from 32 Ma to 30.3 Ma according to Berggren et al. (1995).

Author: Originally proposed by Bolli (1957, 1966), later emended by Blow (1969) and redefined by Berggren & Miller (1988). The definition of the latter authors is here applied. Definition: Biostratigraphic interval from the last occurrence of Pseudohastigerina spp. to the last occurrence of Globigerina ampliapertura. Remarks: This zone is marked in the northern Western Desert by the concurrent occurrence of Gr. opima opima and Gg. ampliapertura, together with frequent Gr. increbescens as well as several transitional forms between Gr. opima nana and Gl. opima opima. The lower limit of the zone is also characterized by the extinction of Gg. gortanii s.s. and Gg. tripartita. On the other hand, Gg. angiporoides, Gg. tapuriensis, Gg. pseudoampliapertura, Gg. senilis, C. howi and C. martini have their last occurrence at or slightly before the upper limit of this zone (Fig. 9). The concurrent occurrence of Gr. opima opima and Gg. ampliapertura is in accordance with Blow’s viewpoint (1969) that in many parts of the world the latter form is associated in the higher part of its range with Gr. opima opima (e.g. southern Trinidad, Barbados, Libya). A similar association was encountered by Reiss & Gvirtzmann (1966b) in the upper part of the Gg. ampliapertura Zone of Israel. Miller et al.

(1985) also recorded Gr. opima opima

immediately after the last appearance of Pseudohastigerina in the western North

Atlantic.

These occurrences indicate that Gg. ampliapertura has its last occurrence at a horizon which is distinctly younger than and directly subsequent to the datum of first appearance of Gr. opima opima. Globigerina ciperoensis angulisuturalis makes its first appearance at or near the top of this zone, and the zone seems consequently to be entirely equivalent to Zone P20 of Blow (1969), but is not completely corresponding to the Gg. ampliapertura Zone of Bolli (1966) (Fig.14). Occurrence: This zone and the overlying Gr. opima opima Zone have been recorded in the majority of wells drilled in the northern Qattara Depression and the area lying between this depression and the Mediterranean coast, except of Mersa Matruh (Fig. 16). The zones are missing westward along the Sallum-East Faghur-Gib Afia line, whereas eastward of the Qattara Depression, they are entirely replaced by poorly fossiliferous, sandstone-dominated sequence of prodelta to delta front facies(e.g. Abu Gharadig-1, Zibedia-1, Wadi Natrun-1).

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Globorotalia opima opima Zone (=Zones P 20-P 21 of Berggren & Miller, 1988) Category:

Interval zone.

Age: al. (1995).

Late Rupelian-Early Chattian, from 30.3 Ma to 27.1 Ma according to Berggren et

Author: Originally defined by Bolli (1957) as a total range zone, later emended by Blow (1969), redefined and subdivided by Berggren & Miller (1988). Definition: Interval zone with zonal marker from the last occurrence of Globigerina ampliapertura to the last occurrence of Globorotalia opima opima. Remarks:

The lower limit of this zone is also marked by the first appearance of Gg.

ciperoensis angulisuturalis, Gg. anguliofficinalis, Gg. angustiumbilicata, and Gg. praebulloides occlusa. Its upper limit cannot be defined in most localities because of the large drop in sea level which occurred in the northern Western Desert at the beginning of the Chattian, and led to the withdrawal of planktonic foraminifera from the area. Berggren et al. (1985b and 1995) and Berggren & Miller (1988) showed that the datum of last occurrence of Chiloguemelina cubensis, which is the most reliable marker for the Rupelian/Chattian boundary, lies within the Gr. opima opima Zone. In the northern Western Desert, no representatives belonging to this form have been encountered anywhere within the Oligocene section. Its absence is not only restricted to the northern Western Desert, but also to the Nile Delta area (Ouda & Obaidallah, 1995). However, the common occurrence of Nummulites species of Rupelian age (see later) at different levels (particularly the youngest part) of the Gr. opima s.s. Zone indicates that this zone is represented in the northern Western Desert only by its lower (Rupelian) part, corresponds to Zone P20 and to the early part of Zone P21 (Subzone P 21a) of Berggren & Miller (1988). The zone is equivalent only to the early part of Zone P21 of Blow (1969, 1979) (Fig. 14). Nevertheless, in a few localities (Fig.6) northward of the Qattara Depression the top of the Gr. opima opima Zone could be tentatively identified; it corresponds to the prevailance of sallow water, Chattian carbonates (e.g. Mamura-1, Khalda SD-1). In these localities, Gr. opima opima become extinct together with Gg. yeguaensis, Gg. sellii, Gg. pseudovenezuelana, T. increbescens and C. unicava s.s. Occurrence: See occurrence of the Gg. ampliapertura Zone (Fig. 16).

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Fig.17. Paleogeographic map of the northern Western Desert during the Late Eocene (Priabonian), Estimated age: 35.3-33.9 Ma. The offshore Mediterranean partremains undifferentiated until additional data become available from the offshore. C. The Rupelian Larger Foraminifera: (Figs. 11,13). In Sallum Hole-1 and Bir El-Nuss WW-1 (Fig. 1), the Oligocene section exhibits a shallow water neritic facies and occupies a thin stratigraphic interval between the underlyin Upper Eocene shales (or carbonates in Bir El-Nuss) and the overlying Lower Miocene carbonates. The sediments yield common Nummulites incrassatus, N. intermedius and N. ramondiformis (A & B forms). The co-occurrence of these forms points to an Early Rupelian age. being younger in age than the underlying Priabonian Nummulites (N. fabiani, N. orbigny and N. variolarius gandinus) which occur (in Sallum Hole-1) among the T. cerroazulensis s.l. Zone.

They are unconformably followed (in both localities) by the assemblage of the

Miogypsina tani Zone which marks the base of the overlying Lower Miocene sediments. In the Aquitanine Basin, southwestern France, N. incrassatus ranges from the Ludian (=Priabonian) to the Stampian (=Rupelian) where it becomes associated with N. intermedius (see Eames, 1971). Middle to Upper Rupelian Nummulites are commonly recorded at different horizons within the Gg. ampliapertura and Gr. opima opima zones in Ghazalat-1, North Ghazalat-1 and 133

Qaret Shushan Hole-1. They are mainly represented by N. vascus and N. boucheri; both forms have their last occurrence at or slightly below the top of the Gr. opima opima Zone, before the appearance of Miogypsinoides. Salloum et al.

(1985) recorded N. intermedius Zone from the Oligocene limestone

section of East Faghur-1. They equated this zone with the Upper Oligocene (Chattian) and correlated it with the Miogypsinoides complanata Zone of Omara & Ouda (1972). However, no Nummulites species have been encountered by the present author in the assemblage of Miogypsinoides complanata Zone which caps the Gr. opima opima Zone. intermedius Zone of Salloum et al.

Thus, the N.

(op. cit.) appears to be of Rupelian age, being

corresponding in age to the Gg. ampliapertura and Gr. opima opima zone. Nummulites vascus and N. boucheri represent the final representatives of the genus Nummulites in the Paleogene section of the northern Western Desert. Their disappearance datum is more or less coincident with the Rupelian/Chattian boundary, thus being in accordance with the datum of last occurrence of Nummulites in most countries of the Mediterranean region and the Far East. According to Eames et al. (1962), Clarke & Blow (1969), Eames (1971) and Adams (1971,1976,1981,1984) all representatives of Nummulites have their last occurrence before the beginning of Chattian and, thus, before the appearance of Miogypsinoides in the Western Tethys and Europe, Mediterranean region, India, Indonesia, Malaysia, Melansia and other localities in the Western Pacific. In East Indeis, the Nummulites have their last occurrence , with N. fichteli and N. vascus as the youngest forms, at the boundary between the stages Td and Te (Adams, op.cit.). This datum of extinction of Nummulites has been correlated by Clarke & Blow (1969) and Jenkins et. al. (1985) with the zonal P19/P20 boundary. However, in the Mediterranean area, N. fichteli and N. vascus extend more younger in the Upper Rupelian strata (corresponding to Zone P21), until become extinct at or near the base of Zone P22 (Cahuzac & Poignant, 1997). In a few restricted occurrences in France, N. bouilli which is usually found in strata of Upper Eocene and Lower Oligocene has been recorded together with Miogypsinoides in sediments referable to Zone P22 (e.g.Drooger,1979; Cahuzac & Poignant, 1997). The geographic restriction of this high-level occurrence of Nummulites hinders its validity as a marker species for the Chattian.

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Fig.18. Paleogeographic map of the northern Western Desert during the Early Oligocene (Rupelian), Estimated age: 33.9-28.4 Ma III. THE LATE OLIGOCENE (CHATTIAN) A- The Rupelian/Chattian Boundary This boundary is still somewhat vague owing to the uncertain position of the Chattian as represented by the Kassel Sands in northern Germany. A number of K-Ar (glauconitic) dates from NW Germany gave an age ranging from 25 Ma to 26.2+0.5 Ma for the EoChattian succession at Deberg (Kreuzer et al., 1980, and Gramann et al. 1980). This age estimate does not conform with that given by Berggren et al. (1985b) and Aubry et al. (1988) who showed that the Rupelian/Chattian boundary is closely linked with Chron C10 N, equivalent to the LAD of Chiloguembelina and/or the NP23/NP24 boundary, with an estimated age of 29.5-30 Ma. Berggren et al’s biochronocogical concept (op. cit.) is based on paleontological data brought from the upper part of the Rupelian stage (Rupel 4) and the succeeding Eochattian (Beds 1-25 of Doberg section) in northwest Germany.

133

However, radiometric ages obtained from the pelagic series of the Central Apennines (Odin & Luterbacher 1992) gave a date of 28.1+ 0.4 Ma for the base of the magnetic episode of 9 N which is correlated with the basal Chattian (Montanari et al., 1985 and Odin et al., 1991). This has led Odin & Luterbacher (op. cit) to place the Rupelian/Chattian boundary in its present usage just below and close to 28 Ma, although they noted that the lower boundary of the Chattian could also be placed at about 29 Ma. Recently, Berggren et al. (1995) gave a revised age of 28.5 Ma instead of 30 Ma for the Rupelian/Chattian boundary, corresponding to the last occurrence of “common” Chiloguembelina in the mid Chron C10 n (within the upper part of the range of Gr. opima opima). In the northern Western Desert, the Rupelian/Chattian boundary cannot be defined on the basis of planktonic foraminifera. This is attributed to the regional drop in the sea level which occurred during the close of Rupelian, and led to hindrance of open-marine circumstances and widespread accumulation of shallow marine to brackish water deposits during the Chattian. Thus, a marked faunal as well as lithological change characterizes the Rupelian-Chattian transition in the whole area. This facies change is linked to the paleogeographic events which affected the circum Mediterranean and led to the reconstitution of seas and land-masses during the Late Oligocene-Early Miocene (e. g. Steininger & Rِgl,1985). B- Lithostratigraphy and Facies: (Figs. 2, 4) In the area to the northeast of the Qattara Depression and extending eastward along the Mediterranean coast, the Chattian section is made up of sandy shale and sandstone which are poorly fossiliferous and exhibiting a littoral facies, which is interrupted by sandstone-dominated fluviatile horizons in the uppermost part. The sediments attain a thickness of 250 to 300 feet in Razzak-1, Yidma-IX and Alamein IX, increasing north-and north eastward to 430-480 feet in Dabaa-1, Burg El Arab-1 and Alam El Buieb-1. In all these localities the Chattian sediments overly directly the open-marine Rupelian strata belonging to the Gr. opima opima Zone, and pass insensibly upward into a deltaic series of clays and sandstone belonging to the Lower Miocene (Moghra Formation, Said, 1962). South-and southeastward, the prodeltaic facies of the Rupelian gives rise upward to predominantly fluviatile facies made up of sands, conglomerates and sandstones which are completely devoid of fossils and ranging in age from the Chattian to the Early Miocene (e.g. North Abu Gharadiq-1, Zibeida-1, Wadi Natrun-1). Basaltic sheets are commonly found in the basal part of the fluviatile Chattian section in the northeastern part of the Western Desert (e.g. Wadi Natrun-1, Shaltut-1, SW Mubarak-1, Gebel Qatrani, West Samalut). The basalts are 131

comparable to those of Widan El Faras which cap the Qatrani Formation north of Fayoum (Bowen & Vondra, 1974). Isotopic age determination gave dates of 27.4+2.7-31.7+3.2 Ma for the basalt of SW-Mubarak-1 (Greenwood, 1970) 27.0+31+1 Ma for the basalt of Gebel Qatrani (e.g. El-Shazly et al. , 1975; Fleagle et al., 1986), and 28.4+1.8 Ma for the basalt of Samalut (Meneisy & Kreuzer, 1974). In the area to the west of Long. 27o30’, a marine carbonate series of considerably shallower depths prevailed during the Chattian. The series caps the open marine Rupelian strata in the northern Qattara depression and in the area lying to the northwest of this depression (e.g. Ghazalat-1, North Ghazalat-1, Naqb Khalda-1, Khalda-1, Qarat Shushan-1, Mamura-1, Umbarka wells). It is composed of calcareous shale and sandy limestone at the basal part, changing rapidly upward into a massive unit of white reefal limestone (Shushan Formation, Omara & Ouda, 1972). The limestone is variable in thickness (35-480 feet thick), attaining its greatest thickness in Umbarka-2 and Qaret Shushan-1, and thinning out considerably toward the north and east. In Mersa Matruh-1, the Shushan limestone overlies unconformably the Upper Cretaceous carbonates, whereas in Abu Tunis-IX and Sallum-1, it is entirely missing and the Rupelian sediments are unconformably overlain by Lower Miocene sediments. In most localities, the Chattian carbonates include Miogypsinoides Lepidocyclina, Spiroclypeus and Cycloclypeus, together with several species of larger foraminifera which live preferably in warm, shallow wter environments in the proximity of reefs, e.g. Heterostegina, Operculina, Amphistegina, and large- sized Rotalia spp.

Northwestward of the Qattara

Depression, the Chattian deposits (Shushan Formation) are unconformably overlain by a series of alternations of sandstone, shale and limestone of transgressive nature, belonging to the Lower Miocene. C- The Chattian Larger Foraminifera: (Figs. 11, 13) The Chattian section of the western part of the northern Western Desert yields a rich benthic foraminiferal assemblage comprising several larger rotaliid species which can be grouped in one biozone, the Miogypsina (Miogypsinoides) complanata Zone. This zone was originally proposed by Ouda (1971) and has generally been considered as a well recognizable and correlatable biostratigraphic unit in the area lying between longitudes 26o and 27o 30’ (e.g. Omara & Ouda, 1972, Masoud, 1983, Salloum et al., 1985). Its stratigraphic relation to the Oligocene and Miocene biostratigraphic units and characteristic datum planes are given in Figs. 4 & 6. 133

Miogypsina (Miogypsinoides) complanata Zone Category:

Total-range Zone

Age: Late Oligocene (Chattian), corresponding to the age of the uppermost part of Zone P21 and Zone P22 as defined by Berggren & Miller (1988), not as given by Blow (1969, 1979). Author:

Ouda (1971).

Definition:

Range of the zonal marker.

Remarks:

The faunal assemblage of this zone is marked by the first appearance of the

primitive miogypsinid form (with solid lateral walls) M. (Ms.) complanata, which possesses a large number of nepionic chambers (X=15-23). This form gives rise upward to M. (Ms.) bantamensis which is marked by its lower rate of nepionic acceleration (X=11). In some places, the latter form shows a tendency toward M.(Ms.) formosensis (X=13-14). The Miogypsinidae are intimately associated with Lepidocyclina (Nephrolepidina) morgani which possesses a smaller number of auxiliary chambers than those of the Lower Miocene Lepidocyclina. Other distinctive larger foraminiferal species which are commonly recorded in this zone include representatives of the genera Cycloclypeus and Spiroclypeus together with Operculina complanata, Amphistegina lessonii, Operculina complanata japonica, O. carpenteri, Heterostigina cf. praecursor, H. borneensis and H. costata s.l., arranged in order of abundance. M. (Ms.) complanata Zone is restricted to the carbonate rock unit (Shushan Formation) which caps the open marine, Rupelian shales belonging to the Gr. opima opima Zone in the western part of the northern Western Desert. The lower limit of this zone is characterized by the last occurrence of the Rupelian Nummulites (Viz. N. intermedius, N. vascus, and N. boucheri). The upper limit of the zone is marked in all localities by the last occurrence of primitive Miogypsinoides with a higher number of nepionic chambers (viz. M. (Ms.) complanata and M.(Ms.)

ex.

interc.

formosensis-bantamensis),

Heterostegina borneensis, H.

cf.

together

with

Lepidocyclina

morgani,

praecursor, Operculina complanata japonica and

Spiroclypeus spp. On the other hand, Miogypsina s.s. begins to appear just above the top of this zone, starting with M. (M.) tani as the oldest member of the Miogypsina s.s. lineage in the northern Western Desert. Northward, along the coastal plain, the upper limit of the M.(Ms.) complanata Zone coincides with the level of first appearance of Neogene planktonic foraminifera in the northern

133

Western Desert. However, eastward of Long. 27o 30’, this zone is entirely missing and the Chattian section is more thicker, sandstone- dominated and very poorly fossiliferous. Occurrence: M.(Ms.) complanata Zone is recorded in Mamura-1, Mersa Matruh-1, Qaret Shushan Hole-1, Umbarka-2, Ghazalat-1, North Ghazalat-1, Naqb Khalda-1,Zahra-1 and Khalda-1(Fig. 16).

D- Age Concept and Correlation: (Figs. 13, 14) Miogypsina (Miogypsinoides) complanata is a useful cosmopolitan

taxon for

distinguishing post Rupelian-Pre-Aquitanian strata, corresponding to the Chattian. It has been recorded from the Upper Oligocene section of Northern Italy (Drooger, 1954a), Spain (Drooger, 1956a), Algeria (Drooger & Magné, 1959), Venezuela and Mexico (Clarke & Blow, 1969), India (Raju, 1974), East Indies (Adams, 1981,1984), Sicily (Wildenborg, 1991) and Trinidad (Caudri, 1996). In the Aquitaine Basin, SW France, this primitive miogypsinid form is restricted to the pre-Aquitanian rocks of Escornebéou (Drooger et al., 1955). In the Mediterranean region, M. (Ms.) complanata is intimately associated with Lepidocyclina morgani and Cycloclypeus eidae. The co-occurrence of the three forms has been considered as being stratigraphically valuable for the recognition of the Chattian (e. g. Drooger, 1963; Wildenborg, 1991).

Recent work by Cahuzac & Poignant (1997) showed that the

assemblage of M. (Ms.) complanata (Zone SB 23) in the neritic area of Southern European basins is entirely equivalent to the planktic foraminiferal Zone P22. However, in Venezuela, Mexico (Clarke & Blow, 1969) Trinidad (Caudri, 1996) and the Far East (Adams, 1984; Jenkins et. al., 1985), M.(Ms.) complanata occurs in beds which were considered to be referable to the interval covered by Zones P21 and P22. In the northern Western Desert, the thickness of the M.(Ms.) complanata Zone is variable and the lower limit of the zone seems to be not isochronous everywhere. In some localities (e.g. Ghazalat-1, North Ghazalat-1), the carbonate sediments of this zone (Shushan Formation) overly directly with a sharp boundary the open marine shales belonging to Zone P21, whereas in other localities northward (seaward), a thin transitional interval containing only smaller benthonic foraminifera exists between both zones (e.g. Umbarka-2, Qaret Shushan Hole-1). In Mamura-1, along the Mediterranean coastal plain, this transitional interval yields few planktonic forams which are probably belonging to the basal part of Zone P22 of Berggren & Miller (1988).

133

On the other hand, N. vascus and N. boucheri disappeard together with the plnktonic foraminifera just below the base of the M.(Ms.) complanata Zone in the south (e.g. Ghazalat-1), whereas northward (e.g. Shushan Hole-1),the planktonic foraminifera of Zone P21 extend a short stratigraphic interval above the datum of last occurrence of Nummulites,before the advent of shoaling conditions and appearance of, M.(Ms.) complanata. Thus, it appears that the lower limit of the M.(Ms.) complanata Zone ranges from the uppermost part of Zone P21 ( Subzone P21b of berggren & Miller (op. cit.) to the lowermost part of Zone P22 (as defined by Berggren & Miller, 1988, not as given by Blow, 1969,1979). The upper limit of the M.(Ms.) complanata Zone is marked in all localities to the northwest of Qattara Depression, by the disappearance of all primitive representatives of Miogypsinoides (viz. M.(Ms.) complanata and M.(Ms.) ex. interc. formosensis-bantamensis) and appearance of Miogypsina s.s., thus being coincident with the top of Zone P22 of Berggren & Miller (op. cit.),and consequently the top of the Chattian stage. However, younger miogypsinoid forms with a lower rate of nepionic acceleration (viz. M. (Ms.) bantamensis ) are normally found to extend above the last occurrence of M. (Ms.) complanata into the Lower Miocene where they become associated with different members of Miogypsina s.s. For the discussed reasons, the M.(Ms.) complanata Zone is considered indicative of the Chattian interval corresponding in time to the upper part of Zone P21 (Subzone P21 b of Berggren & Miller, 1988) and/or Zone P22 (as redefined by Berggren & Miller , op. cit.). This biostratigraphic interval range in age from 28.5 Ma to 23.8 Ma, according to Berggren et. al.(1995). IV. PALEOGEOGRAPHY: (Figs. 18, 19) During the Rupelian, the sea retreated north and north-westward, and this was contemporaneously accompanied in the eastern part of the northern Western Desert by the progradation of the delta shoreline toward the Qattara Depression and the Mediterranean, in front of the advancing distributaries of the old Fayoum Delta. This resulted in the deposition of thick terrigenous prodeltaic deposits overlying the normal marine Priabonian strata in the basinal area lying to the east of Qattara Depression (Abu Gharabiq-Wadi Natrun basin). Meanwhile, open marine sedimentation prevailed in the northern Qattara Depression and in the area between this depression and the Mediterranean coast. The

lithologic

characters, faunal content and geographic distribution of the Lower Rupelian sediments, corresponding in age to Zone P18 ( Berggren & Miller, 1988 = Zones P18-P19 of Blow, 1969,

133

1979), indicate that these sediments were deposited during an interval of progressive shoaling of the sea. However, a distinct rise in the sea level, coupled with the subsidence of several old highs in the northwestern portion of the Western Desert occurred at the time of disappearance of Pseudohastigerina species ( 32 Ma according to the time scale of Cande & Kent ,1995). This had led to the widespread accumulation of Middle-Upper Rupelian sediments (corresponding in age to zones P19-P21 of Berggren & Miller, op.cit.), overlying the pre-existing Cretaceous (e.g. Mamura-1),Eocene (e.g. Abu Tunis-1) or the lowermost Oligocene (most localities) rocks.

Fig. 19. Paleogeographic map of the northern Western Desert during the Late Oligocene (Chattian), Estimated age: 28.4-23.03 Ma.

A similar stratigraphic situation has been encountered by Taylor & Jones (1982) in the western side of the Nile Delta and by Ouda & Obaidalla, (1995) in the onshore and offshore area lying to the northeast of the Nile Delta. In the latter area which extends from Monaga-1 in the south to El Temsah-2 in the north, the marine strata referable to the Globigerina ampliapertura

and

Globorotalia

opima

opima

133

zones

overly unconformably a

thick,

unfossiliferous,continental sandy succession which covers, in turn, the Upper Cretaceous carbonates. Also, Andrawis et. al. (1986) recorded in Misri-1, northeastern Sinai, the assemblage of zones P19-P21 (= zones P20-P21 of Blow,1969,1979) overlying unconformably those of the Middle Eocene. This would imply that most of northern Egypt was a deep sea during the Middle-Late Rupelian, corresponding to an age from 32 Ma to 28.5 Ma, according to Berggren et. al. (1995). However, slightly prior to the age of extinction of Gr. opima opima ( 27.1 Ma)

the

northern Western Desert underwent a regional facies change; the open-marine conditions ceased completely and either a shallow marine, brackish or fluviatile sedimentation prevailed.. This facies change reflects a regional drop in the sea level at the beginning of Chattian, followed by major biological events involving the prevailance of warming conditions and widespread appearance of larger foraminifera in the wetern part of the area. On the other hand, a large delta prograded northwestward toward the Mediterranean and the Qattara Depression, and a thick coarsening-upward, clastic succession laid down in the area to the northeast of this Depression and along the eastern coastal plain.

THE MIOCENE I. THE OLIGOCENE/MIOCENE BOUNDARY Recently, it has been recommended and ratified by the IUGS that the Global Stratotype Section and Point (GSSP) for the base of the Neogene be placed at the 35 m level of the Rigorosa Formation in the Carrosio-Lemme section of NW Italy (Steininger, 1994, leader of the Paleogene/Neogene Working Group of the IUGS Neogene Subcommission). This level corresponds to the base of Chron 6 Cn. 2n and the first appearance datum of Globorotalia kugleri, with an age estimate of 23.8 Ma (Cande & Kent, 1992, 1995; Berggren et al., 1995). This recommendation necessitates the abandonment of the usage of either the Globigerinoides primordius datum (e.g. Cati et al., 1968, Blow, 1969, Bizon & Bizon, 1972, Cita, 1976, Borsetti et al., 1979, Bolli & Saunders, 1985) or the Globoquadrina dehiscens s.s. datum (e.g. Srinivasan & Kennet, 1982, Iaccarino, 1985) as reliable datum levels for the recognition of the Oligocene/Miocene boundary. It supports Berggren et al.’s concept (1985a) concerning the relocation of this boundary at a level which is stratigraphically older than the base of Zone N4 (sensu Blow, 1969), but not older than the first appearance of primitive Globigerinoides (Gs. primordius). The latter form has been recorded from pre-Aquitanian levels 133

in the Aquitaine Basin of SW France (Butt, 1966; Scott, 1972) and the Rhone Valley (Anglada, 1971). Its appearance predates the FAD of Gr. kugleri s.s. in the DSDP site 516 F south Atlantic (Berggren in Berggren et al.

1985a), in the Ashmore Reef No. 1 well, northwest

Australia (Chaproniere in Shafik & Chaproniere, 1978; Chaproniere, 1981) and in the Piedmont Basin, NW Italy (Baldi-Beke et al., 1978). This would eliminate the utility of Zone N4 of Blow (1969). However, a different point of view concerning the stratigraphic relationship between the Globigerinoides datum and the FAD of Gr. kugleri has been presented by Bolli & Saunders (1985). Whatever the stratigraphic relationship between the FADs of Gr. kugleri and Gs. primordius, the Oligocene/Miocene boundary has to be placed within Zone P22 (sensu Blow, 1969, 1979), at a level which is equivalent to the boundary between the Gg. ciperoensis s.s. Zone and Gr. kugleri Zone in Bolli’s zonal scheme (1957, 1966). It is note worthy that Banner & Blow (1965) placed the base of the Aquitanian stage at the datum of first appearance of Gr. kugleri which marks the base of their Zone N4 (not same usage as in Blow, 1969). However, a precise correlation between the first appearance of Gr. kugleri and the base of the stratotype Aquitanian near Saucats, Aquitaine Basin, has not been demonstrated. Berggren et al. (1985a) believed that the stratotype section which is characterized at its base by an unconformity, corresponds to the FAD of Gr. kugleri.

This concept does not precisely

conform with paleontological data reported by different authors from the type section and nearby areas. Studies of the planktonic foraminiferal fauna by Jenkins (1966), Clarke & Blow (1969), Scott (1972), and Poignant & Püjol (1976) revealed that the type Aquitanian section lies within the limits of Zone N4 (corresponding to the later part of the range of Gr. kugleri as given in the usage of Blow, 1969) and the early part of Zone N5 (older than the first appearance of Gs. altiaperturus). The lowest samples of the section contain Gq. dehiscens s.s., whose appearance is known to occur younger than the appearance of Gr. kugleri, both in land surface sections (Srinivasan & Kennett, 1982) and in deep-sea sequences (e.g. Sites 77B, 149, 24, 237, 357, 362, 363, 522; for references see Berggren et al., 1985a). In addition , Miogypsina (M.) gunteri gives rise to M. (M.) tani via M. (M.) ex. interc gunteri-tani in the basal part of the stratotype Aquitanian, whereas the remaining part of the section corresponds to the range of M.(M.) tani (Drooger et al., 1955; Clarke & Blow, 1969, Blow, 1969). The datum of first evolution of M. (M.) tani from M.(M.) gunteri has generally been considered as marking the later part of the Aquitarian stage in the Mediterranean ( e.g. Drooger 133

, 1956b, 1963; Cahuzac & Poignant, 1997 ), the Caribbean (Clarke & Blow, 1969) and the Indian region (Raju, 1974). In the Caribbean region, M.(M.) tani begins to occur at a level which is more or less coincident with the last occurrence of Gr. kugleri (N4/N5 zonal boundary of Blow,1969, 1979), e.g. Mexico, Venezuela, Carriacou and Jamaica (Clarke & Blow, 1969). M. (M.) gunteri on the other hand, has been considered in the western Mediterranean, Gulf Coast area and Indo Pacific as a very good marker for the beginning of the Aquitanian (Drooger 1956b, 1963; Drooger et al., 1955; Akers & Drooger, 1957; Drooger & Magné, 1959; Raju, 1974; Adams, 1970,1981,1984; Wildenborg, 1991; Cahuzac & Poignant, 1997). In all these regions M.(M.) gunteri begins just immediately above the last occurrence of M. (Ms.) complanata. The latter form which possesses a higher number of nepionic chambers more than 13, has never been recorded in these regions in sediments younger than the Chattian. In the Caribbean region, M. (Ms.) complanata has its last occurrence at a level within Zone P22, immediately below the appearance of Gr. kugleri, while M. (M.) gunteri extends in the younger beds until becomes replaced by M.(M.) tani at or near the top of Zone N4 (e.g. Mexico, Venezuela, Clarke & Blow, 1969). In Sicily, M. (M.) gunteri begins together with Gr. kugleri at the base of the Aquitanian, just immediately above the last occurrence of M. (Ms.) complanata (Wildenborg, 1991) It is, thus, apparent that the stratigraphic hiatus which exists at the lower boundary of the stratotype section of the Aquitanian is much longer than supposed by Berggren et al. (1995). The hiatus involves most of the Aquitanian stage, equivalent to the greater part of the Gr. kugleri Zone (as originally defined by Bolli, 1957, not as emended by Bolli & Saunders, 1985), corresponding to Zone M1 of Berggren et al. (1983, 1995). It also corresponds to the greater part of the range of M. (M.) gunteri. The Aquitanian section in its type area seems to represent only the later part of the stage; that part which straddles the Gr. kugleri/C. dissimilis Zonal boundary. The same conclusion was reached by Scott (1968) on bases of a quantitative comparative study of the genus Globigerinoides. It was also expected by Clarke and Blow (1969) who maintained that “the lowest samples from the stratotype Aquitanian yielded faunas referable to horizons which were significantly above the base of Zone N4”, (note that Zone N4 of Blow, 1969, corresponds only to the later part of the range of Gr. kugleri). A similar stratigraphic hiatus at the base of the Aquitanian has been recorded in Morocco (Drooger, 1954b), Algeria (Drooger & Magné, 1959) and Tunisia (see Pomerol, 1982). In these countries, the Aquitanian exhibits a shallow water facies and begins at base with M. 133

(M.) tani or M.(M.) ex. interc. gunteri-tani, overlying unconformably theUpper Oligocene (Chattian) carbonates with M. (Ms.) complanata. A comparable stratigraphic hiatus can be distinguished at the base of the Neogene section of the northern Western Desert. The Aquitanian section begins in most localities with M. (M.) tani as the oldest member of the Miogypsina s.s. lineage. The Miogypsinidae

are

accompanied by a planktonic foraminiferal association in the coastal area extending from Mamura and Mersa Matruh. The planktonic fauna are characterized by the appearance of Gq. dehiscens s.s., Gs. primordius, Gs. sacculiferus, Gs. trilobus immaturus and Gg. woodi connecta. No specimens belonging to either Gr. kugleri or M. (M.) gunteri have been encountered anywhere, and the association seems consequently to be fully indicative of the early part of Zone N5. The association occupies the stratigraphic interval from the last occurrence of M.(Ms.) complanata which marks the top of the Chattian to the first appearance of Gs. altiaperturus. The latter datum marks the Aquitanian/Burdigalian boundary in the Mediterranean region and is usually characterized by the evolution of M.(M.) ex. interc. taniglobulina which in turn gives rice to M.(M.) globulina directly above the base of the Burdigalian. Thus, it appears that the base of the Miocene (base of Mamura Formation) is separated from the top of the Oligocene (top of Shushan Formation) by a stratigraphic gab, corresponding in age to the entire Gr. kugleri Zone (=Zone M1 of Berggren et al., 1995). This can also be indicated by the entire absence of M.(M.) gunteri either below or above the base of the Aquitanian, although conditions of sedimentation allowed the flourishing of Miogypsinidae across the Oligocene/Miocene boundary in the northwestern part of the Western Desert. The absence of Gr. kugleri is also difficult to be interpreted as being due to facies or climatic reasons. This diagnostic Miocene globorotaliid is quite common in the Aquitanian sediments of the Sirte Basin, Libya (Berggren, 1969), the Nile Delta Basin (Ouda & Obaidallah, 1995) and the Gulf of Suez Basin (e.g., Ouda & Masoud, 1993). In these regions, the basinal Aquitanian facies begins at the base with Gr. kugleri which is either concurrently associated with or slightly preceding the appearance of Gs. primordius. Along the Mediterranean coastal plain (Mamura-Mersa Matruh line), the open marine Aquitanian section corresponds to the concurrent range of Gq. dehiscens s.s., Gs. primordius, Gs. sacculiferus, Gs. trilobus immaturus and M. (M.) tani. Thus, it would be reasonable to consider this section as being equivalent only to the later part of the Aquitanian stage, i.e. that

133

part which is stratigraphically younger than the age of the last occurrence of Gr. kugleri and M.(M.) gunteri, and older than the age of the appearance of Gs. altiaperturus and M.(M.) globulina. This stratigraphic interval is equal to about 1 Ma, from Chron C6 Ar, dated 21.5 Ma to the top of Chron C6 An, dated 20.52 according to the GPTS of Cande & Kent (1992, 1995) and Berggren et al. (1995). II. THE EARLY MIOCENE (AQUITANIAN-BURDIGALIAN) A- Lithostratigraphy and Facies: (Figs. 2, 5) a) The Aquitanian Marine sediments belonging to the Aquitanian are known only in the area lying to the northwest of the Qattara Depression. They exhibit a transgressive nature, being composed of alternations of calcareous sandstone and shale with thin stringers of limestone (lower part of Mamura Formation of Marzouk, 1970). The sediments vary in thickness between 90 feet and 285 feet and cover with a marked lithologic break the Chattian carbonates (Shushan Formation). They exhibit a shelf-type facies of comparatively shallower depths in the south, but with a marked increase in the shale content, corresponding to a relative increase in the planktonic foraminiferal content northward, toward the coastal plain (e.g. Mamura-1, Mersa Matruh-1). On the other hand, the Aquitanian sediments show a progressive enrichment in the carbonate content toward the west and southwest until the sediments become predominantly composed of coralline limestone and are unconformably underlain by the Upper Eocene or Lower Oligocene carbonates (e.g. East Faghur-1, Bir El Nuss WW-1, Gib Afia-1, Sallum Hole1). In Faghur-1 and Faghur WW-1, along the Egyptian-Libyan border, the Lower Miocene together with the underlying Upper Eocene-Oligocene succession are missing, and the Middle Miocene rests directly over the Middle Eocene nummulitic limestone. Eastward of Long. 27o30’, no marine sediments belonging to the Lower Miocene are recorded. In the area to the northeast of the Qattara Depression, the Aquitanian and the greater part of the overlying Burdigalian section are made up of a thick series of sandstone, silts and clays of brackish water nature (Moghra Formation of Said, 1962). They are almost devoid of fossils, being strongly influenced by deltaic processes and grading downward into nearshore sandy shale and sandstone of Chattian age. (e.g. Dabaa-1, Burg El Arab-1, Aram El Buieb-1, 131

Razzak-1, Yidma-IX, Alamein IX. Southeastward, the Early Miocene sediments are composed predominantly of unfossiluferous loose sands, sandstone and conglomerate of fluviatile origin (Shata, 1955). b) The Burdigalian The marine facies of the Aquitanian of the northwestern part of the Western Desert passes conformably upward into the Burdigalian. The latter is represented by alternations of grey calcareous shales and white to greenish, argillaceous limestone, while sandstone is inconspicuous. The sediments constitute the upper part of Mamura Formation of Marzouk (1970) and generally exhibit a progressive deepening toward the north (seaward). They vary in thickness between 240 and 1000 feet and are conformably overlain by the Middle Miocene (Langhian) in the area between the Qattara-Siwa depressions and the Mediterranean coast. Along the coastal plain (Mamura-Mersa Matruh stretch) the Burdigalian section attains 500 ft. thick and yields a mixed planktonic and benthonic (both larger and smaller) foraminiferal association. However, southward (landward) the basal part of the Burdigalian section contains a warm shallow-water marginal benthic foraminiferal association including Miogypsina s.s., whereas the remaining part of the section contains few planktonic species and exhibits a gradual shoaling upward particularly in the higher levels (e.g. Khalda SD-1, Qaret Shushan Hole-1, Um Barka-2, Khalda-1). A marked increase in the carbonate content accompanied by a progressive thinning of the Burdigalian section exists westward of the Qattara Depression (e.g. Bir El Nuss WW-1, Gib Afia-1) until the section entirely disappears together with the underlying Aquitanian in Faghur-1 and Faghur WW-1. Further northwestward, in the Sallum area (both on surface and in the subsurface) the Burdigalian sediments are exclusively composed of coralline dolomitic limestone which is difficult to be separated from the underlying Aquitanian carbonates unless the Miogypsinidae are used (Omara & Ouda, 1972). Eastward, along the northeastern escarpment of the Qattara Depression, the Lower Miocene sediments become very sandy, poorly fossiliferous and strongly influenced by deltaic facies. To the west of the Moghra Oasis, the Lower Miocene interval is represented on the surface by about 785 feet thick delta plain deposits, composed by sandstones and clays (Moghra Formation of Said, 1962). These deposits thin out eastward (toward Wadi El Faregh, SW Wadi Natrun) and yield, in places, land vertebrates, fossil wood, shark teeth and marine molluscs (Blanckenhorn, 1901, Fourteau, 1918, Said, 1990). They grade southeastward (toward

133

Fayoum) into coarse, red sands of exclusively fluviatile origin (Gebel Khashab Formation, Said, 1962). Northeastward in Burg El Arab-1 along the coastal plain where the Early Miocene section becomes much more thicker and reflecting in its greater part a delta front facies, the upper part of the section yields a fairly rich benthic framiniferal association together with few planktonic forams of Late Burdigalian age. B- Planktic Foraminiferal Zonal Stratigraphy: (Figs. 5, 10, 13, 14) Three planktic foraminiferal zones are distinguished in the open marine, Early Miocene succession of the northern Western Desert. The zones, their characteristic datum planes, ages and corresponding rock units are given in Fig. 5. Globoquadrina dehiscens s.s. Zone Category:

Interval Zone

Age:

Late Aquitanian, from 21.5 Ma to 20.5 Ma according to Berggren et al. (1995).

Authors:

Iaccarino & Salvatorini (1982) as Gq. dehiscens s.s. Subzone, here emended.

Definition:

Biostratigraphic interval with zonal marker between the last occurrence of Gr.

kugleri and the first appearance of Gs. altiaperturus. Remarks:

This zone represents the earliest Neogene planktonic foraminiferal zone in the

northern Western Desert. It lies unconformably over the Chattian M.(Ms.) complanata Zone along the coastal area extending from Mamura to Mersa Matruh. Its base is marked by the common appearance of the genus Globigerinoides including Gs. primordius, Gs. sacculiferus, Gs. trilobus immaturus, together with Gq. dehiscens s.s. and Gg. woodi connecta. Additional forms recorded from this zone are listed in Fig.10 The upper limit of this zone is also characterized by the last occurrence of Gg. anguliofficinalis,

Gg.

ouachitaensis

gnaucki,

Gg.

praebulloides

leroyi

and

Gg.

pseudociperoensis. No specimens referable to Gr. kugleri have been recorded in this zone, and , consequently, the zone seems to be younger than the top of Zone N4 of Blow (1969, 1979), and Zone M1 of Berggren et al. (1983), being corresponding to the lowermost part of Zone N5 of Blow (op.cit.) or Zone M2 of Berggren et al. (op.cit.)

133

Fig. 20. Paleogeographic map of the northern Western Desert during the Early Miocene (late Aquitanian-early Burdigalian), Estimated age: 21.5-18.8 Ma.

The Gq. dehiscens s.s. Subzone of Iaccarino & Salvatorini (1982) lies within the greater part of the range of Gr. kugleri (see Iaccarino, 1985) and appears therefore to be considerably older than that defined in the northern Western Desert (Fig. 14). Occurrence: This zone has been recorded in Mamura-1 and Mersa Matruh-1 (Fig. 16). Globigerinoides altiaperturus Zone Category:

Interval Zone

Age:

Early Burdigalian, from 20.5 Ma to 17.3 Ma according to Berggren et al. (1995).

Authors:

Bizon & Bizon (1972).

Definition: Biostratigraphic interval with zonal marker between the first occurrence of Gr. altiaperturus to the last occurrence of C. dissimilis. Remarks:

This zone is comparable to the yonger part of the C. dissimilis Zone and the C.

stainforthi Zone of Bolli (1957), corresponding to the younger part of Zone N5 and Zone N6 of 133

Blow (1969, 1979). The same stratigraphic interval is designated by Berggren et al. (1995) as Zone M2 (upper part) and Zone M3. Several Globigerinoides and Globoquadrina species begin to occur successively within this zone; Gs. quadrilobatus, D. altispira globosa and Gq. laramui appear at or near the base of the zone, then followed upward by Gs. subquadratus and finally by Gs. trilobus s.s. which predominates in the younger part of the zone. Occurrence: This zone is recorded in Mamura-1, Mersa Matruh-1. Globigerinoides trilobus trilobus Zone Category:

Interval Zone

Age:

Late Burdigalian, from 17.3 Ma to 16.4 Ma according to Berggren et al. (1995).

Authors:

Bizon & Bizon (1972).

Definition:

Biostratigraphic interval with zonal marker from the last occurrence of C.

dissimilis to the first appearance of Pr. sicana (not Gs. bisphericus). Remarks:

This zone is entirely equivalent in time to Zone M4 of Berggren et al. (1995). It

also corresponds closely to Zone N7 of Blow (1969, 1979). The lower limit of the zone is marked by the first appearance of several forms such as D. altispira s.s., Gq. dehiscens advena, Gg. concinna, Gg. foliata, N. continuosa and Ge. praesiphonifera. D. altispira s.s. is a useful taxon for the recognition of this zone, not only in the northern Western Desert, but also in other parts of the Mediterranean region (e.g. Iaccarino, 1985). The upper limit of this zone coincides with the Early/Middle Miocene boundary (see later). It is placed at the first evolution of the genus Praeorbulina (as manifested by Pr. sicana) from Globigerinoides bisphericus. For distinction between both species see Bolli & Saunders (1985). Occurrence: This zone is widely distributed along the Mediterranean coastal plain of the Western Desert. It is recorded in Mamura-1, Mersa Matruh-1, Dabaa-1 and Burg El Arab-1. It is also represented in Khalda SD-1 and Qarat Shushan Hole-1, although it contains fewer planktonic foraminifera as compared to other localities (Fig. 16).

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C-The Early Miocene Larger Foraminifera: (Figs. 6,11) A Miogypsina s.s. lineage in which members evolve uniformly according to the principle of nepionic acceleration (Drooger, 1952, 1963) has been recognized by Ouda (1971) within the Early Miocene succession of the northern Western Desert. The lineage is here redefined and stratigraphically reevaluated on the basis of additional data from other localities as well as recent informations concerning the chronology and chronostratigraphy of the different Late Oligocene-Early Miocene stages. The lineage comprises five members arranged from older to younger as follows: M.(M.) tani Drooger M.(M.) globulina (Michelotti) M.(M.) intermedia Drooger M.(M.) cushmani Vaughan M.(M.) antillea (Cushman) The total range of each member of this lineage constitutes in itself a distinctive paleobiologic unit (i.e. biozone) that can be considered to be stratigraphically correlatable and isochronous allover the western part of the northern Western Desert. These zones, their datum planes, ages and corresponding planktic foramaniferal zones are given in Figs 6 and 13.

Miogypsina (Miogypsina) tani Zone Category:

Total-range zone

Age:

Late Aquitanian.

Author:

Ouda (1971).

Definition:

Range of zonal marker.

Remarks:

M.(M.) tani is the oldest member of the Miogypsina s.s. lineage in the northern

Western Desert. It begins at the base of the Miocene section, just immediately above the last occurrence of M.(Ms.) complanata. Specimens belonging to this species start with individuals having Mx value (average number of nepionic chambers in the main spiral) varying between 7.4 and 7.9, and 200 / value (Drooger, 1952) equal to zero, at the base of the Gq. dehiscens s.s. Zone. The relatively younger specimens of M.(M.) tani show a higher M 200 / value ranging between 2.8 and 3.3, at a horizon which is immediately preceding the first appearance of Gs. altiaperturus.

133

Fig. 21. Paleogeographic map of the northern Western Desert during the Early-Middle Miocene (late Burdigalian-Early Langhian). Estimated age: 17.3- 15.1 Ma. A few specimens of M. (Ms.) bantamensis, with Mx values ranging between 8 and 12, are recorded in association with M.(M.) tani at different levels within the Gq. dehiscens s.s. Zone in Mamura-1. This miogypsinoid form developed from M.(Ms.) complanata via M. (Ms.) formosensis, in the Late Oligocene and continued in the Lower Miocene where it became associated with Miogypsina s.s. Occurrence : M.(M.) tani and M.(M.) globulina have been recorded from the majority of wells drilled in the area lying between the Siwa-Qattara depressions and the Mediterranean, except of Faghur region (Fig. 16). Miogypsina (Miogypsina) globulina Zone Category:

Total-range Zone

Age:

Early Burdigalian.

Author:

Ouda (1971).

Definition:

Range of zonal marker.

133

Remarks:

M.(M.) globulina evolves from M.(M.) tani at a level which is nearly coincident

with the base of the Gs. altiaperturus Zone (e.g. Mamura-1, Mersa Matruh-1). This evolution is marked in all localities by the development of transitional samples having M 200 / values ranging in different sections between 13.1 and 19.5, and Mx values ranging between 6.6 and 7.1. Typical specimens of M.(M.) globulina having M 200 / values ranging between 33.5 and 36.3 are recorded from the early part of the Gs. altiaperturus Zone, below the datum of first appearance of Gs. trilobus s.s. Occurrence: See M.(M.) tani Miogypsina (Miogypsina) intermedia Zone Category:

Total-range Zone

Age:

Early Burdigalian.

Author:

Ouda (1971).

Definition:

Range of zonal marker.

Remarks:

M.(M.) intermedia evolves from M. globulina within the younger part of the Gs.

altiaperturus Zone in Mamura-1, and continues upward until the upper limit of this zone where it is replaced by M.(M.) cushmani. It starts with individuals having a M 200 / value of 60 in the older horizons and ends with those which are near the transition of M.(M.) ex. interc. intermediacushmani (M 200 /=75.4) in the youger levels,

just immediately below the level of last

occurrence of C. dissimilis. The stratigraphic interval occupied by the total range of M.(M.) intermedia is also marked by the frequent occurrence of Lepidocyclina tournoueri which possesses a higher number of primary auxiliary chambers than the Late Oligocene representatives of Lepidocyclina.

M.

(Miogypsinoides) bantamensis is also recorded with rare specimens scattered sporadically within the youger part of the range of M.(M.) intermedia. In the Sallum surface section M.(M.) intermedia appears in the higher levels of the Lower Miocene carbonate section, but the limit between this form and its ancestral form M.(M.) globulina is difficult to fix because of the unfavorable facies conditions which badly affected the preservation of miogypsinids (Ouda, 1971). It is worth of mention that in the Sallum Hole-1, to the west of the Sallum surface section, no Miogypsina specimens younger than M.(M.) globulina

133

are recorded, and the Middle Miocene limestone with Borelis melo s.s. covers unconformably the lower part of the Burdigalian carbonates which contains M.(M.) globulina. Occurrence: This zone is recorded in Mamura-1, Qaret Shushan Hole-1 and Sallum surface section. Miogypsina (Miogypsina) cushmani Zone Category:

Total-range Zone.

Age:

Late Burdigalian.

Author:

Ouda (1971).

Definition:

Range of zonal marker.

Remarks:

M. (M.) cushmani replaces M.(M.) intermedia in Mamura-1, at a level which is

coincident with the last occurrence of C. dissimilis. It starts with individuals having M 200 / values ranging between 81.5 and 83.5 in the lower levels of the Gs. trilobus s.s. Zone, whereas upward the representatives of this species show a tendency toward the transition between M.(M.) cushmani and M. (M.) antillea, with M 200 / values varying from 85 and 88. The evolution of M.(M.) cushmani is characterized by a marked change in the foraminiferal content leading to the appearance of several planktonic forms (see above) and the last occurrence of Lepidocyclina spp. Miogypsina (Ms.) bantamensis has its last occurrence within the later part of the life range of M.(M.) cushmani. Occurrence: This zone is recorded in Manura-1.

Miogypsina (Miogypsina) antillea Zone Category:

Total-range Zone.

Age:

Latest Burdigalian.

Author:

Ouda (1971).

Definition:

Range of zonal marker.

Remarks:

M.(M.) antillea has a short stratigraphic range as compared to the older members

of the Miogypsina s.s. lineage. It is restricted in Mamura-1 to the uppermost part of the Burdigalian succession, where it replaces M. (M.) cushmani via M.(M.) ex interc. cushmaniantillea. Representatives belonging to this youngest member possess M 200 / values of 90.4 and  value of 20o. The upper limit of the range of this form is immediately below the first

133

appearance of Praeorbulina. No Miogypsina specimens are recorded above this datum in the northern Western Desert. Occurrence: This zone is recorded in Mamura-1.

D- Chronostratigraphy of the Miogypsinidae: (Figs. 13, 14) The primitive populations of Miogypsinoides with a higher number of nepionic chambers (viz. M.(Ms.) complanata) are restricted to the Late Oligocene (Chattian) interval, corresponding to the later part of Zone P21 and early part of Zone P22 of Blow (1969, 1979). The same interval is designated by Berggren et al. (1995) as P 21 B and P 22 , with estimated age of 27.1-23.8 Ma. On the other hand, younger Miogypsinoides forms with a slower rate of nepionic acceleration (i.e. M.(Ms.) bantamensis) extend from the higher levels of the Chattian (Zone P22 sensu Berggren et al. op.cit.) to the Late Burdigalian, corresponding to Zone M 4a of Berggren et al. (op.cit.) . The co-occurrence of Miogypsina s.s.and younger Miogypsinoides (viz, M.(Ms.) bantamensis) in the Burdigalian deposits has been reported in Morocco (Drooger, 1954b), Egypt (Souaya, 1961) and Sicily (Wildenborg, 1991). The Miogypsina s.s. lineage begins with M.(M.) tani as the oldest member at the base of the Miocene section which seems to commence at a level corresponding to the base of Zone N5 of Blow (1969, 1979) with an estimated age of 21.5 Ma (Berggren et al., 1995). Both M.(M.) globulina and M.(M.) intermedia have their total ranges during the time interval between the first appearance of Gs. altiaperturus and the last occurrence of C. dissimilis. This interval is correlated with Late Zone N5 and Zone N6 of Blow (op. cit.), corresponding to the younger part of Zone M2 and the entire Zone M3 of Berggren et al. (1995), with an estimated age from 21.5 Ma to 17.3 Ma. On the other hand, M.(M.) cushmani and its descendant form M.(M.) antillea have their total ranges within the time interval between the last occurrence of C. dissimilis and the first appearance of of Pr. sicana. This interval is entirely equivalent to Zone N7 of Blow (op. cit.), corresponding to Zone M4 of Berggren et al. (op.cit.) with an estimated age from 17.3 to 16.4 Ma, according to Berggren et al. (1995). E- The Aquitanian/Burdigalian Boundary Micropaleontological investigations of the larger and planktonic foraminiferal content of the stratotype Burdigalian or equivalent sections in the Aquitaine Basin or the Rhône Valley, southwest France, revealed that the Aquitanian/Burdigalian boundary be best placed within the older part of Zone N5 (e. g. Drooger et al., 1955; Jenkins, 1966; Clarke & Blow, 1969; Poignant 133

& Püjol, 1978). The boundary has been accepted as being coincident with the level of first appearance of M. (M.) globulina (=irregularis) and/or Gs. altiaperturus (e. g. Drooger, 1963, 1979; Iaccarino,1985; Wildenborg, 1991; Montanari et al., 1991; Cahuzac & Poignant, 1997). In the Contessa Highway Section, Gubbio, Italy, Gs. altiaperturus begins to appear in the older part of Chron C6r, with an estimated age of 20.5 Ma, while Gr. kugleri disappears at the base of Chron C6 An, with an estimated age of 21.32 Ma (Montanari et al., 1991). A similar age for the first appearance of Gs. altiaperturus (20.52 Ma, at the top of Chron C6 An) has been recorded in the DSDP Site 516 F (Berggren et al., 1995). It is now apparent that the appearance of Gs. altiaperturus and/or M.(M.) globulina which marks the base of Burdigalian is separated from Zone N5 of Blow (1969) or Zone M2 of Berggren et al. (1995) by an interval equal in time to about 1 Ma. This interval should be treated as an independent stratigraphic unit in order to facilitate inter-regional correlation of the Aquitanian/ Burdigalian boundary. However, a different viewpoint has been presented by the Shipboard Scientific Party (1996) who recorded both events (FAD of Gs. altiaperturus and LAD of Gr. kugleri) at nearly the same stratigraphic level in the deep-sea sequences of the Mediterranean, thus suggesting the placement of the Aquitanian/ Burdigalian boundary at the contact between Blow`s zones N4/N5. In the northwestern part of the Western Desert, the Aquitanian/Burdigalian boundary can be defined on the basis of both planktonic foraminifera and Miogypsinidae in those wells which are drilled along the coastal plain from Mamura to Mersa Matruh. However, toward the west and the south, planktonic fauna disappear and the Miogypsimidae become the main tool for the definition of this boundary. In most localities, the base of Burdigalian is marked by the last occurrence of typical specimens of M.(M.) tani, and the first evolution of M.(M.) ex. interc. tani-globulina. This evolution took place at a level which is nearly coincident with the first appearance of Gs. altiaperturus in the Mamura and Mersa Matruh wells. However, typical specimens of M.(M.) globulina begin to occur slightly above the first evolution of Gs. altiaperturus. which is usually accompanied by the appearance of D. altispira globosa and Ge. siphonifera praesiphonifera (Fig. 4.3), together with numerous benthonic species among which Bulimina elongata subalata is the most common one.

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III- THE MIDDLE MIOCENE (LANGHIAN-SERRAVALLIAN) A- Lithostratigraphy and Facies: (Figs. 2, 5) The Middle Miocene sediments cover most of the northern stretch of the Western Desert extending from the Libyan border to Siwa eastward along the northern escarpment of the Qattara Depression. They are composed of two distinct rock units, a lower thin shaly unit made up of alternations of shale, marl and grey limestone (=topmost part of Mamura Formation of Marzouk, 1970), and an upper thick calcareous unit composed of white limestone which is partly chalky downward and strongly dolomitized westward (=Al Jaghbub Formation of Desio, 1928). The lower shaly unit overlies conformably the Burdigalian sequence in the subsurface of the area lying to the north and northwest of the Qattara depression. It is generally thin, ranging in thickness from 60 feet to 150 feet and yielding in most localities a shallow-water benthic foraminiferal assemblage of the inner-shelf type. The faunas include representatives of the genera Eponides, Cibicides, Elphidium, Quinqueloculina, Triloculina, Ammonia, Guttulina, Textularia, Operculina, Heterostegina and Amphistegina. Northward, along the coastal plain, the sediments of this unit exhibit a comparatively deeper water facies, being composed predominantly of shales and yielding, in places (e.g. Mamura-1), several planktonic species together with many benthonic forms such as Bulimina ovata, Uvigerina hispidocostata, Cancris auriculus, Lagena striata, Orthomorphina proxima and Heterostegina costata. The planktonic foraminifera of this unit are indicative of Zone M5 of Bergren et al. (1995) which marks the early part of the Langhian age. The upper calcareous unit (referred to as Marmarica limestone by Said 1962) is coeval and wholly equivalent to the Al-Jaghbub Formation which was introduced by Desio (1928) to represent the Middle Miocene calcareous outcrop bordering along the escarpment of the AlJaghbub Oasis (Lat. 29o 48’ N, Long. 24o 30’ E). It extends eastward into the northern Western Desert where its base becomes well defined along the northern escarpments of the Siwa and Qattara depressions, and in the subsurface area between these depressions and the Mediterranean. The sediments of this unit attain a thickness of 400-600 feet in the central part of the northern Western Desert and along the Mediterranean coastal plain. In these areas, the limestone unit follows conformably the Langhian shaly unit (topmost Mamura Formation) without distinct stratigraphic gaps (e.g. Mamura-1, Mersa Matruh-1, Qaret Shushan Hole-1, Khalda SD133

1, Khalda-1, North Ghazalat-1, Alamein-1, Burg El Arab-1). However, westward of Long. 26o the Middle Miocene limestone becomes relatively thicker (660-730 feet) and covers unconformably either the Lower Burdigalian limestone with M.(M.) globulina (e.g. Sallum Hole-1), the Aquitanian limestone with M.(M.) tani (e.g. Gib Afia-1, Bir El Nuss WW-2), or the Middle Eocene limestone with Nummulites gizehensis (e.g. Faghur-1, Faghur WW-1). In this area, attention must be paid to the fact that the entire Lower Miocene section is composed of massive limestone which is lithologically indistinguishable from the overlying Middle Miocene one. The presence of a distinct stratigraphic hiatus between the Lower and Middle Miocene limestone makes it difficult to place the entire carbonate section within a single rock unit. A gradual thinning of the Middle Miocene limestone unit occurs southward. In the area to the north of the Siwa Oasis, Said (1962) measured 260 feet of this unit, the lower 100 feet of which include some marl intercalations. On the other hand, a strong reduction in thickness and increase in the sand content of this unit occurs eastward along the northeastern wall of the Qattara Depression. In the Moghra Oasis and the area lying between Qattara and Alamein, this unit attains 20-60 feet, being composed mainly of sandy limestone intercalated with clay covering conformably the Early Miocene Moghra Formation (Hammad et al., 1976). The Middle Miocene calcareous unit (Al Jaghbub Formation) reflects a predominantly warm, shallow water, carbonate environment of reasonable confidence and precision. The limestone is generally rich in calcareous algae, corals, milleporides, amphisteginids, alveolinids together with other forms which live preferably in the proximity of reefs (Bellini, 1969, Omara & Ouda, 1972). Borelis melo s.s. which is a diagnostic Late Langhian-Serravallian form, occurs frequently in these sediments. In the Caribbean, Borelis specimens occupy the clean, washed sands and oolites of the carbonate banks and inter-reef channels (Brasier, 1975). According to Bignot & Guernet (1976), the presence of Borelis melo curdica in the Miocene of the Isle of Kos, Greece, is thought to indicate very shallow, clear water of the order of 1 m in depth, temperatures from 25o to 30oC, strong currents and elevated salinities (35-50 ppm). Bellini (1969) concluded also that the Al-Jaghbub Formation has been deposited in a very shallow sea on a broad, open shelf extending from Libya eastward along almost the entire northern stretch of the Western Desert. This shelf was dominated by detrital limestone, with minor terrigenous deposits, and fringed to the west and south by reef shoals.

133

Fig. 22. Paleogeographic map of the northern Western Desert during the Middle Miocene (Late Langhian- Serravallian). Estimated age: 15.1-11.6 Ma.

B- Planktic Foraminiferal Zonal Stratigraphy: (Figs. 5, 10, 13, 14) Two planktonic foraminiferal zones are distinguished in the open-marine, Langhian facies of the northwestern part of the Western Desert, a lower Pr. sicana Zone, and an upper Pr. glomerosa s.s. Zone. Both zones are synchronous with Zone M5 of Berggren et al. (1995), which corresponds closely to Zone N8 of Blow (1969, 1979). The zones are well represented along the coastal area extending between Mamura and Mersa Matruh. Southward, the planktonic foraminifera become very rare or even missing, and the same stratigraphic interval could only be recognized on basis of larger benthic foraminifera (i.e., the Hetelostegina costata s.l. Zone). The Late Langhian-Serravallian interval (corresponding to Zones M6-M11 of Berggren et al. op. cit., or Zones N9-N14 of Blow, op. cit.) is exclusively represented by shallow water carbonate deposits belonging to the Borelis melo s.s. Zone. The stratigraphic and geographic relation between these zones, their datum planes and corresponding rock units are given in Fig. 5.

133

Praeorbulina sicana Zone (= Subzone M5 a of Berggren et al., 1995) Category:

Interval Zone

Age: Middle Miocene (Early Langhian), from 16.4 Ma to 16.1 Ma according to Berggren et al.,(1995). Author: Berggren et al. (1995), name is shortened here. Definition: Stratigraphic interval with zonal marker (not Gs. bisphericus), from its first appearance to the first appearance of Pr. glomerosa s.s. Remarks:

This zone overlies conformably the Gs. trilobus s.s. Zone without a marked

change in facies or lithology. It stratigraphically corresponds to the early part of Zone N8 of Blow (1969, 1979). Several long-ranging Oligocene-Early Miocene forms have their last occurrence at the base of this zone (e.g. Gg. angustiumbilicata, Gg. eamesi, Gg. officinalis), together with Gs. altiaperturus. On the other hand, Gg. bulloides, Gg. juvenilis, Gs. bolli, and Pr. transitoria begin to appear at the base of this zone. Occurrence: This zone has been recorded from Mamura-1 and Mersa Matruh-1. Praeorbulina glomerosa s.s. Zone (= Subzone M5 b of Berggren et al., 1995) Category:

Interval Zone

Age: Middle Miocene (Middle Langhian), from 16.1 Ma to 15.1 Ma according to Berggren et al.,(1995). Author:

Berggren et al. (1995), name is shortened here.

Definition: This zone is originally defined as the stratigraphic interval with the zonal marker from its first appearance to the first appearance of O. suturalis. However, in the northern Western Desert the upper limit of this zone is marked by a distinct faunal change involving the last occurrence of Miocene planktonic foraminifera and their displacement by a shallow water foraminiferal fauna containing Borelis melo s.s. The appearance of the latter form occurs elsewhere at the base of Zone N9 (see below) and can therefore be considered as being coincident and isochronous with the datum of appearance of O. suturalis. Remarks:

This zone represents the youngest planktic foraminiferal zone in the Miocene

sequence of the northern Western Desert. It is comparable in time to the youngest part of Zone N8 of Blow (1969, 1979), but occupies a relatively smaller stratigraphic interval than the Pr. glomerosa curva Zone of Jenkins (1966), and the Pr. glomerosa Zone of Bolli (1966), Srinivasan & Kennett (1981) and Iaccarino (1985). Occurrence: This zone has been recorded from Mamura-1 and Mersa Matruh-1 (Fig.16).

133

D- The Middle Miocene Larger Foraminifera: (Figs. 6, 11) Heterostegina costata s.l. Zone Category:

Interval Zone

Age: Middle Miocene (Langhian), equivalent in age to the Pr. sicana and Pr. glomerosa s.s. Zones. Author:

Ouda (1971), here emended.

Definition: Biostratigraphic interval with zonal marker from the last occurrence of the youngest representatives of Miogypsinidae (i.e. M.(M.) antillea) to the first appearance of Borelis melo melo. Remarks:

Representatives belonging to the Heterostegina s.l. group are recorded from

older strata ranging in age from the Late Oligocene to the Early Miocene. They include H. costata s.s. d’Orbigny, H. costata politatesta Papp & Kupper, and H. granulatatesta s.s. Papp & Küpper. In the Langhian section these forms are commonly associated with Amphistegina lessonii and rarely with H. complanata spiralis and Operculina frizzelli. No members of the H. costata s.l. group are recorded in the overlying Borelis melo s.s. Zone, although conditions of sedimentations of the latter zone allowed the flourishing of larger rotalids. This would suggest the use of the partial range of H. costata s.l. above the last occurrence of Miogypsina s.s. and below the first appearance of Borelis melo s.s., to recognize the lower part of the Langhian section, corresponding to Zone N8 of Blow (1969, 1979)( = Zone M5 of Berggren et al., 1995). Occurrence: This zone has been recorded in most localities situated north of the Qattara Depression and east of Longitude 26° (Fig. 16).

Borelis melo melo Zone Category:

Interval Zone

Age:

Middle Miocene (Late Langhian-Serravallian).

Author:

Bellini (1969).

Definition: Biostratigraphic interval with zonal marker, from its first appearance to the last appearance of marine Miocene fauna in the northern Western Desert. Remarks:

This zone is widely distributed in the northern Western Desert where it

represents the youngest marine biostratigraphic unit of the Miocene sequence. It overlies directly the Pr. glomerosa s.s. Zone or its coeval H. costata s.l. Zone and begins at the base with a marked faunal and lithological change. The change involves the entire disappearance of Miocene planktonic foraminifera together with several diagnostic larger rotaliids such as H. costata s.l., H. complanata spiralis and Operculina frizzelli. 133

The lower limit of this zone coincides with the first appearance of Orbulina suturalis in open marine successions. In Africa and the Indo-Pacific region, B. melo has its first occurrence in beds containing Orbulina (Eames et al., 1962). In Isril (Reiss & Gvirtzmann, 1966a), the Orbulina datum is coincident with the initial appearance of B. melo, whereas in Sicily (Blow, 1957) this species occurs with planktonic foraminifera of the Gr. fohsi Zone (corresponding to Zones N8-N12 of Blow, 1969). Clarke & Blow (1969) and Blow (1969) placed the world-wide date of appearance of B. melo s.s. at the contact between Zone N8 and Zone N9, closely coincident with the Orbulina datum (15.1 Ma, according to Berggren et al., 1995). Borelis melo s.s. is associated in most localities by Elphidium antoninum, E. advenum, E. macellum, Ammonia beccarii, Asterigerina planorbis, Discorbis orbicularis, D. globularis, Cibicides boueanus, Quinqueloculina bradyana and Q. candeiana, Q. laevigata, Q. vulgaris, Textularia agglutinans, T. gramen and Guttulina spp. In the western portion of the northern Western Desert, this zone yields a poor foraminiferal association comprising Amphistegina lessonii and Quinqueloculina spp. The B. melo s.s. Zone is unconformably overlain along the Mediterranean coastal plain by Pliocene sediments. No marine faunas younger, than the Borelis melo melo Zone and older than the Pliocene are recorded in the northern Western Desert (see also Bassiouni et al.,1975 and Hammad et al., 1976). Occurrence: This zone has been recorded both on surface and in the subsurface of the area lying between the Siwa-Qattara depressions and the Mediterranean coast (Fig. 16). C- The Early/Middle Miocene (Burdigalian/Langhian) Boundary According to Cita & Premoli Silva (1968) and Cita & Blow (1969), the base of the Langhian stage (either as designated by the type section of Bricco della Croce of Cita & Premoli Silva ,1960 or the Cessole Formation of Vervloet, 1966) is marked by the first evolutionary appearance of Pr. glomerosa. The latter form gives rise to Orbulina suturalis within the upper part of the section and becomes extinct at its top, thus supporting its usage as a useful marker for the recognition of the time duration of the Langhian age. As a consequence, the Early/Middle Miocene boundary has been traditionally placed in the Mediterranean region at the contact between the Gs. sicanus (= Gs. bisphericus) Zone and the Pr. glomerosa Zone( e.g. Cati et al.,1968; Cita, 1976; Bizon, 1979; Borsetti et al., 1979; Iaccarino, 1985).

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However, another point of view is recently presented by Berggren et al. (1995) who suggested the readjustment of the Burdigalian/Langhian boundary at the first appearance of Pr. sicana (= Gs. sicanus non Gs. bisphericus). The latter form represents an intermediate stage in the lineage between Gs. bisphericus and Pr. glomerosa curva (for distinction between Pr. sicana and Gs. bisphericus see Jenkins et al., 1981 and Bolli & Saunders (1985). The author is inclined to accept the concept of Berggren et al. (op. cit.) for the following considerations: 1- The lowest sample of the type Langhian section, collected by D.D. Bayliss (in Cita & Blow, 1969) from the northern part of the Village of Cessole (sample By 249) contains Gs. sicanus but without specimens referable to Pr. glomerosa s.l. This would indicate that the base of the stage was developed at a level which is nearly coincident with the base of Blow’s Zone N8 and not slightly above the base of this zone as suggested by Cita & Blow (1969). These authors based their concept on the assumption that sample. By 249 is probably younger than level 7 of Cita & Premoli Silva (1960) and that the lack of Pr. glomerosa s.l. in this sample is probably due to facies control. It is worth to mention that Blow (1969, P. 229) proposed a paratype locality for his Zone N8 at the locality and level of sample By 249. 2- The first appearance of representatives of the genus Praeorbulina belonging to Pr. sicana occurs in the northern Western Desert at a level which is immediately subsequent to the last occurrence of M.(M.) antillea, the youngest member of the Miogypsina s.s. lineage in Egypt. This bioevent provides a good indicator for recognizing the Burdigalian/Langhian boundary in shallow-water successions where planktonic formainifera are either absent or very subordinate. In the Mediterranean region, all representatives of the Miogypsina lineage have their last occurrence at or near the top of the Burdigalian, a short stratigraphic interval before the first evolution of Orbulina (e.g. Drooger, 1963; Adams, 1981, Wildenborg, 1991, Cahuzac & Poignant, 1997).

Planktonic foraminiferal evidences from Sicily (Wildenborg, 1991), the

southern European basins (Cahuzac & Poignant, 1997) and the northern Western Desert of Egypt (present study) indicate that the extinction of the Miogypsinidae is coincident with the N7/N8 zonal boundary of Blow (1969,1979). A comparable stratigraphic position was previously suggested by Akers & Drooger (1957) in the Gulf of Coast area where M.(M.) antillea is the final member of the Miogypsina s.s. lineage. Clarke & Blow (1969) also recorded the disappearance of the youngest members of Miogypsina s.s. (viz M. cushmani and M. mexicana) in Mexico at

133

the top of Zone N8. Raju (1974) also recognized the extinction of M.(M.) antillea and thus the last occurrence of Miogypsina s.s. in India in the uppermost Burdigalian, just below the base of the Middle Miocene. However, in the Far East, Miogypsina s.s. species have been found to occur abundantly above the Orbulina datum (e.g. Eames et al., 1962; Clarke & Blow, 1969; Adams, 1970, 1981, 1984). They extend to a level high in the “f” stage, which is within the range of Zone N12 and the early part of Zone N13 of Blow (1969). Also, in Venezuela, M.(M.) cushmani gives rise to M.(M.) antillea within Zone N10, and the latter becomes extinct at the N10/N11 zonal boundary (Clarke & Blow, 1969). This would suggest that Miogypsina species persisted into younger strata in tropical areas than in temperate regions. 3- The evolutionary appearance of the genus Praeorbulina as manifested by Pr. sicana is easily recognizable because of its distinctive morphology (with 4 apertures around the base of the last chamber; Jenkins et al. , 1981) with respect to the genus Globigerinoides. However, it is not always easy to place a sharp boundary between Pr. sicana and its descendant form Pr. glomerosa curva; the latter differs only in having more than 4 apertures at the base of the last chamber (Blow, 1969).Therefore, the writer follows Berggren et al’s (1995) concerning the usage of the datum of appearance of Pr. glomerosa s.s. to identify the upper limit of the Pr. sicana Zone. Thus, it appears that the span of time between the last occurrence of the Mediterranean Miogypsinidae and the appearance of Orbulina suturalis (or Borelis melo s.s. in equivalent shallow water sequences) is occupied by the Early Langhian Pr. sicana and Pr. glomerosa s.s. Zones, corresponding to the entire Zone N8 of Blow (1969, 1979). The same time interval has been defined by Berggren et al. (1995) as Zone M5, with an estimated duration of 1.3 Ma ( from 16.4 Ma to 115.1 Ma). The conformable relation between the Langhian and Burdigalian sediments is not only found in the northern Western Desert, but also recognized in other parts of Egypt, e.g. the Gulf of Suez region (e.g. Ouda & Masoud 1993), the Nile Delta area (e.g. Taylor & Jones, 1982, Ouda & Obaidallah, 1995). The boundary between both stages cuts through the uppermost part of the Mamura Formation in the western part of the northern Western Desert, the uppermost part of the Qantara Formation (or within the Sidi Salim Formation) in the Nile Delta and the uppermost part of the Rudeis Formation in the Gulf of Suez.

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IV. PALEOGEOGRAPHY: (Figs. 20-22) The presence of a regional biostratigraphic hiatus at the base of the Miocene section of the northwestern portion of the Western Desert, corresponding to the Gr. kugleri Zone (Zone M1 of Berggren et al., 1995) indicates that the entire northern stretch of the Western Desert was a high that stood well above sea level during most of the Aquitanian. Marine sediments belonging to the youngest part of the Aquitanian were deposited unconformably over preexisting rocks in a relatively narrow, open shelf extending in N-S direction to the west of Longitude 27o30’. The narrow shelf was bounded in the west by reefs and corals, whereas eastward it was strongly influenced by appreciable amount of terrigenous clastics. Generally there is a distinct decrease in the amount of carbonates as the depth of the shelf increases northward (seaward). Normal marine sedimentation straddled the Aquitanian-Burdigalian transition with a noticeable rise in sea level at the beginning of the Burdigalian. The shelf-type conditions prevailed in the same area during most of the Burdigalian age, with a progressive shoaling and increasing of the lime content toward the south and west. However, a tectonic pulse seems to have occurred during the time interval of the Gs. altiaperturus Zone, a short stratigraphic interval after the appearance of M.(M.) globulina. This is manifested by the discontinuity of the sedimentary succession and the Miogypsina lineage in some localities situated southward of the coastal plain. On the other hand, the sea abandoned the northeastern part of the Western Desert during most of the Early Miocene, except in the area just to the south of the coastal plain between Dabaa and Burg El Arab. In this area, the Lower Miocene is represented by a thick, sandstone-dominated sequence of brackish facies. The coarsening-upward structure exhibited by this sequence suggests a continuous progradation of the delta shoreline toward the Qattara Depression and the Mediterranean coast during the Early Miocene. Paleontological evidences by previous authors from the Early Miocene clastic sediments which crop out 30 Km west of the Moghra Oasis substantiate the predominance of delta front environments along the eastern coastal plain of the Western Desert during the Aquitanian and most of the Burdigalian. However, the uppermost part of the Burdigalian section in Burg El Arab-1 contains several benthonic foraminiferal fauna of the inner shelf type, together with few, sporadic planktonic species. This would suggest that a shallow sea with normal marine conditions enchroached the eastern part of the coastal plain before the close of the Burdigalian. A gradual lowering of the sea level occurred during the Early Langhian (age of the Pr. sicana and Pr. glomerosa s.s. zones), until open marine conditions ceased completely and warm, shallow-water, carbonate environment prevailed during the Late Langhian-Serravallian in 133

the entire area between the Siwa-Qattara depressions and the Mediterranean. The regional shoaling conditions of the Middle Miocene are essentially attributed to the incipient isolation of the Levantine Basin from the Mesopotamian trough (Steininger & Rogl,1985) which occurred around the time of the Orbulina datum (15.1 Ma.). During this time, the marine connection from the Indo-Pacific to the Eastern Paratethys were interrupted and narrowed by uplift of the Caucasus mountain ranges, turning the Eastern Paratethys into a low salinity realm (Steininger & Rogl, op. cit ). The Late Miocene (Tortonian-Messinian) was a period of regional emergence and active subaerial erosion in the entire northern Western Desert. Paleontological criteria from the Miocene sediments of the Gulf of Suez Basin (Ouda & Masoud, 1993) and the Nile Delta area (Ouda & Obaidallah, 1995) indicate that a vigorous drop in sea level occurred in northern Egypt at the beginning of Tortonian (probably as early as the latest Serravallian), leading to the complete withdrawal of the Mediterranean.

THE PLIOCENE A- Lithostratigraphy and Facies: (Figs. 2, 7) a. The Early Pliocene (Zanclean). Open marine deposits of Zanclean age are restricted to the Sallum Gulf in the northwestern corner of the Western Desert. They are recorded both on the surface (SE Sallum, Mansour et al., 1969) and in the subsurface (Sallum Hole-1, at Lat. 31o28’ 36.6” N and Long. 25o 29’ 32.4” E). The sediments cover unconformably the shallow-water, Middle Miocene carbonates and pass conformably upward into the Piacenzian.

They attain a maximum

thickness of 200 feet and exhibit exclusively a deep water facies. The lower part of the Zanclean section is composed predominantly of planktonic foraminiferal tests (more than 2/3 of the residues), thus constituting a true calcareous ooze which marks the deeper parts of the modern oceans and seas at depths more than 1000 m. The pelagic oozes are overlain in Sallum Hole-1 by hemipelagic sediments made up of soft sticky, clays of light grey colour, intercalated with soft sandy biogenic limestone. The clays are rich in foraminifera but contain more terrigenous quartz than in the underlying calcareous ooze. They seem to have deposited on continental slopes under very low current velocities. The transition from the pelagic oozes to the hemipelagic clays is gradational and marked by a gradual change

131

in the carbonate content, corresponding to a visible change in the planktonic / benthonic foraminiferal content. The Zanclean/Piacenzian boundary lies in Sallum Hole-1 at depth 220 feet, about 200 feet above the base of the Zanclean section. A comparable thickness for the Zanclean has been recognized by Mansour et al. (1969) in the surface section to the southeast of

Sallum.

However, in the latter section, no planktonic faunas younger than the Zanclean were encountered, and the sediments belonging to this age are unconformably overlain by tanish to brownish, shallow-water limestone of latest Pliocene to Pleistocene age. b- The Late Pliocene (Piacenzian) The Piacenzian deposits are much widely distributed along the coastal plain of the Western Desert than the Zanclean ones. In Sallum Hole-1, these deposits follow conformably the Zanclean, whereas in Mersa Matruh-1 and Burg El Arab-1, they rest unconformably on the Middle Miocene limestone belonging to the Al-Jaghbub Formation. The sediments vary in thickness between 30 feet and 180 feet, and are mainly composed of soft, sticky clays which are very richly fossiliferous, with thin layers of soft, grey limestone and grading southward (landward) from an outer shelf to an inner shelf environment. In all localities, the Piacenzian clays are directly overlain by a thick limestone cap rock of shallow-water origin, ranging in age from the latest Piacenzian to the Pleistocene. The limestone is white to pink in colour, soft to moderately hard mostly cryptocrystalline, partly oolitic, non porous and iron-stained. It is comparable to the Surur Formation (Hammad et al., 1976), attaining a thickness of 80-100 feet and comprising a shallow-water, benthic foraminiferal assemblage including Elphidium, Ammonia, Asterigerina, Discorbis as well as various miliolids. The basal part of the limestone is pink to brown in colour, hard, dense and contains abundant pelecypods and echinoid spines. According to Hammad et al. (op. cit.), this type of limestone is well developed in the eastern part of the coastal plain, while scattered in the form of isolated hills in the western part. It also appears on the northeastern part of the plateau and in the subsurface. On the surface, it takes the form of consolidated ridges arranged parallel to the present coast. It is essentially composed of carbonate granules, shell fragments, quartz sand, echinoid spines and is white in colour particularly the younger parts. Boukhary et al. (1976) studied the microfacies of this formation and concluded that the environment was sublittoral, highly agitated with marine clear 133

water. Bassiouni et al. (1975) gave a Pleistocene age to this formation and the underlying Cardium limestone. The present investigation indicates that this limestone is younger than the Gs. obliquus extremus Zone. Comparable Piacenzian deposits of rather shallow water facies are known to occur in the Wadi Natrun district (both surface and subsurface) and extending between Wadi Natrun and Gebel Mansuriya, and northwestward for about 350 kms until the reaches of Mersa Matruh. According to Shata (1955) these deposits are divisible into two series, a lower series made up of black shale alternating with argillaceous sandstone of unknown thickness, and an upper series composed of porcellaneous limestone with flint, alternating with quartz sands and having a thickness of about 35 m. At Qaret Mashukra, the limestone is characterized by the development of gypsum beds which attain a maximum thickness of 10 m and extend for at least 50 km (Shata, op. cit.). Abdallah (1966) proposed the formational name of Alam El Milh for these sediments in the area between Alamein and the Qattara Depression. The formation includes few miliolids together with Ammonia beccarii as well as pelecypods and few gastropods, an association which would suggest a shallow water littoral environment. Bassiouni et al. (1975) gave these sediments a Pliocene age. The stratigraphic position of this formation below the Surur Formation and above the “Marmarica” Formation indicates that this rock unit represents the nearshore facies of the Upper Piacenzian, corresponding in age to the Gs. obliquus extremus Zone which is now known to represent the main open marine facies of the Pliocene in both Burg El Arab and Mersa Matruh wells. B- Planktic Foraminiferal Zonal Stratigraphy: (Figs. 7, 12, 13) Many of the taxa used in the definition of the Pliocene planktonic foraminiferal zones in the tropical/subtropical and temperate areas of the Atlantic and Indo. Pacific provinces have been found to be absent or occurring very rarely and only sporadically in sediments of the same age in the Mediterranean region e.g. Globorotalia tumida, Gr. miozea, Gr. cibaoensis, Gr. miocenica, Gr. exilis, Pulleniatina obliquiloculata, Sphaeroidinella dehiscens. This has been interpreted by Cita (1976) as being due either to climatic reasons or to some kind of ecologic control that occurred at the reimmigration of the marine fauna after the sterilization of the Mediterranean induced by the Salinity Crisis. On the other hand, the entry of the cosmopolitan forms Gr. margaritae, Gr. puncticulata and Gr. crassaformis in the Mediterranean was delayed by the re-establishment of normal marine conditions following the “terminal Miocene desiccation phase. Thus, it is difficult to correlate the Pliocene of northern Egypt with the Pliocene planktic 133

zonal schemes of the tropical/subtropical provinces (e.g. Blow, 1969, 1979, Berggren, 1973, Bolli & Premoli Silva, 1973, Srinivasan & Kennett, 1981, Berggren et al. 1995).

Fig. 23. Paleogeographic map of the northern Western Desert during the Early Pliocene (Zanclean). Estimated age: 5.3-3.6 Ma. In the Mediterranean region, several zonal schemes have been proposed for the Pliocene interval including those of Cati et al. (1968), Cita (1973, 1975, 1976), Bizon (1979), Borsetti et al. (1979), Iaccarino & Salvatorini (1982) and Iaccarino (1985). A comparison of the zonal boundaries and the stratigraphic ranges of faunas shows that good agreement exists between these schemes, although the taxa used for the definition of zones are apparently different. The biostratigraphic subdivision worked out for the Mediterranean deep-sea Pliocene sediments (i.e. Cita, 1973, 1975, 1976), and the planktic foraminiferal zonation established in sections outcropping in Sicily (including the stratotype Zanclean, Cita & Gartner, 1973) can be applied without difficulties to the Pliocene section of the northern Western Desert. The zones, their datum planes and ages are summarized as below; reference is made to Cita (1973, 1975),

133

Cita & Gartner (1973), Thunell (1979) and Sprovieri (1992, 1993) for the distribution and correlation of these zones in the Mediterranean region. Sphaeroidinellopsis spp. Zone Category:

Interval zone.

Age: Earliest Pliocene (Zanclean), from 5.33 Ma to 5.1 Ma, according to Sprovieri (1993) and Cande & Kent (1995). Author: Bizon et al. (in Cati et al., 1968), emended by Borsetti et al. (1979) and redefined by Iaccarino & Salvatorini as Ss. seminulina s.l. Zone (1982). Definition: According to the definition by Iaccarino (1985) this zone covers the stratigraphic interval which postdates the Messinian salinity crisis and predates the first appearance of Gr. margaritae in the Mediterranean region. In Sallum Hole-1, this zone is manifested by the appearance of a flood of Sphaeroidinellopsis spp. within a pelagic ooze devoid of Gr. margaritae and immediately following the shallow water, littoral to sublittoral limestone of the Middle Miocene. Remarks:

The Sphaeroidinellopsis specimens of this zone constitute more than 50% of the

whole foraminiferal association and include forms referable to Ss. seminulina s.l., Ss. hancocki and Ss. sphaeroides. Additional forms which are recorded frequently in this zone include : Gg. nepenthes, Gg. decoraperta, Gg. falconensis, Gg. porabulloides, Gg. bulloides, Gs. conglobatus, Gs. irregularis, Gs. obliquus, Gs. obliquus extremus, Gs. trilobus s.s., Gs. trilobus immaturus, Gs. bolli, Gs. sacculiferus, Gs. subsacculiferus, N. acostaensis, N. humerosa, B. bilobata, O. universa, O. suturalis and H. pelagica (Fig.

). The upper limit of this zone is characterized by the first

appearance of Gr. margaritae s.s. It is also marked by the last occurrence of Ss. hancocki (=Ss. kochi Caudri) whereas Ss. seminulina extends upward into the overlying zones until it becomes extinct at the top of the Ss. seminulina s.l. Zone. The Sphaeroidinellopsis spp. Zone of Sallum Hole-1 corresponds closely but not precisely to the Sphaeroidinellopsis Acme Zone of Cita (1973, 1975, 1976). The latter zone is by definition an ecozone which according to its author extends slightly younger than the age of first appearance datum of Gr. margaritae (see comments by Iaccarino, 1985). Occurrence: The Sphaeroidinellopsis spp. Zone is recorded only in the Sallum area where it covers the interval from 420 ft to 390 ft. in Sallum Hole-1. It has also been recorded by Mansour et al. (1969) in the Pliocene surface section SW of Sallum.

133

Fig. 24. Paleogeographic map of the northern Western Desert during the Late Pliocene (Piacenazian). Estimated age: 3.2-2.6 Ma.

Globorotalia margaritae Zone Category:

Total-range Zone.

Age: Early Pliocene (Zanclean), from 5.1 Ma to 3.94 Ma, according to Sprovieri (1993) and Cande & Kent (1995). Authors:

Bolli & Bermudez (1965).

Definition:

Range of zonal marker..

Remarks:

The base of the zone is also marked by the first appearance of Gs. elongatus.

The latter part of the zone is characterized by the evolution of Gr. margaritae evoluta which according to Cita (1973) differs from Gr. margaritae s.s. by its greater size, less elongated equatorial and more symmetrical axial profile. Several diagnositic planktonic foraminiferal forms make their last occurrence within and around the top of the Gr. margaritae Zone, such as Gg. nepenthes, N. humerosa, N. 133

acostaensis, D. altispira s.s. and Ge. praesiphonifera. On the other hand Globorotalia puncticulata begins to appear within the younger part of this zone, corresponding to the Gr. margaritae evoluta Subzone of Cita (1973). Occurrence: This zone is only recorded in Sallum Hole-1 where it occupies the interval from 390 feet to 220 feet. It has also been recorded by Mansour et al. (1969) in the Pliocene surface section SW of Sallum. Sphaeroidinellopsis seminulina s.l. Zone Category:

Interval Zone.

Age: Late Pliocene (Piacenzian), from 3.94 Ma to 3.22 Ma, according to Sprovieri (1993) and Cande & Kent (1995). Author: Cita (1973); the change in the taxonomy of Sphaeroidinellopsis follows recent studies of that group according to Bolli & Saunders (1985). Definition: Interval with zonal marker between the last occurrence of all members of the Gr. margaritae group and the last occurrence of Ss. seminulina s.l. including specimens referred to by Blow (1969) as “Ss. subdehiscens s.s.”. Remarks:

This zone is fully synchronous with Zone P13 of Berggren et al. (1995). Its lower

part is marked by the conspicuous occurrence of Gr. puncticulata whereas Gr. crassaformis s.l. appears in rare specimens at or near the top of this zone. Gs. elongatus is usually recorded in frequent numbers throughout this zone. In case of absence or rarity of Sphaeroidinellopsis species, the part of the range of Gr. puncticulata above the extinction of Gr. margaritae and below the appearance of Gr. crassaformis s.l. can be used as an indicative marker for the definition of the greater part of this zone. It is thus considered to be stratigraphically equivalent to the Gr. puncticulata Zone and older part of the Gr. aemiliana Zone of. Iaccarino & Salvatorini (1982) and Iaccarino (1985) Occurrence: This zone is recorded only in Sallum Hole-1 from 220 feet to 150 feet. Globigerinoides obliquus extremus Zone Category:

Interval zone.

Age: Late Pliocene (Piacenzian), from 3.22 Ma to 2.14, according to Sprovieri (1993) and Cande & Kent (1995). Author: Cita (1973), not Gs. obliquus extremus Zone of Iaccarino & Salvatorini (1982) and Iaccarino (1985).

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Definition: Interval with zonal marker from the last occurrence of Ss. seminulina s.l. to the first appearance of Gr. inflata. Remarks:

Gr. crassaformis s.l. is the most distinctive diagnostic globorotaliid form in this

zone. Specimens belonging to this form are usually variable in abundance and showing a wide morphological variability between those of Gr. crassaformis s.s. and Gr. aemiliana. However, there is no a general agreement on the age of the first appearance and the range of Gr. crassaformis and its related forms in the Mediterranean. Therefore, the writer follows Cita’s concept (1973, 1975, 1976) to use the younger part of the range of Gs. obliquus extremus above the last occurrence of Ss. seminulina s.l. to define the yongest open marine part of the Pliocene interval in the northern Western Desert. Neither Gr. puncticulata nor Gr. inflata has been encountered throughout this interval, and the zone seems consequently to correspond fully to the yonger part of the Gr. aemiliana Zone (Gr. crassaformis s.s. Subzone) of Iaccarino & Salvatorini (1982). Globorotalia specimens which were previously referred to as “Gr. puncticulata” by Omara & Ouda (1969) from the Pliocene section of Burg El Arab-1 are actually belonging to the Gr. crassaformis s.l. group and the section at this locality seems consequently to belong to the Gs. obliquus extremus Zone. This zone is the most widespread Pliocene biostratigraphic unit in the northern Western Desert. It is generally variable in thickness and covering either conformably the Sphaeroidinellopsis seminulina s.l. Zone (e.g. Sallum Hole-1) or unconformably the Middle Miocene, shallow water limestone with Borelis melo (e.g. Mersa Matruh-1, Burg El Arab-1). The upper limit of this zone is marked in all localities by the disappearance of the Neogene planktonic foraminiferal and their displacement with a shallow-water benthic foraminiferal assemblage of the inner shelf-type. The latter assemblage is confined to the white tanish, oolitic limestone (Surur Formation) which caps the open marine Pliocene clays and ranges in age from the latest Pliocene (age of the Gr. inflata Zone) to the Pleistocene. Occurrence: The Gs. obliquus extremus Zone covers the interval from 420 feet to 240 feet in Burg El Arab-1, from 130 feet to 100 feet in Mersa Matruh-1, and from 160 to 120 feet in Sallum Hole-1. C- Chronostratigraphy and Stage Boundaries: (Fig. 15). Paleontological evidences as well as correlation with the stratotype section of the Zanclean at Capo Rossello in Sicily and the Pliocene sections penetrated in the Tyrrhenian 133

Basin indicate that no important strata at the base of the Zanclean are missing in Sallum region. The same planktonic foraminiferal zonation proposed for deep-sea Mediterranean sediments (Ryan,1973, Hsü et al., 1973, Cita, 1975, Shipboard Scientific Party, 1996 a,b), and established in sections outcropping in Sicily (Cita & Gartner, 1973) could be easily applied to the pelagic sediments of Sallum Hole-1 The consistency of biostratigraphic correlation, based on the marker levels of first appearance of the Gr. margaritae s.s., last occurrence of Gr. margaritae s.l., and last occurrence of Ss. seminulina s.l., strongly supports the assumption that these bioevents are virtually isochronous in the Mediterranean region. It also allows to estimate the sedimentation rate for the Zanclean succession. Comparing their thickness in the Mediterranean, at Capo Rossello and in Sallum Hole-1, we find the following values from top to bottom:

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The ratio of sediments thickness measured in Sallum Hole-1 versus Site 132 is given in column 8. This stratigraphic interval, on the whole, can be equated to the time elapsed from the base of the Pliocene, corresponding to the re-establishment of marine conditions in the Mediterranean

after

the

Late

Miocene

Salinity

Crisis,

to

the

last

occurrence

of

Sphaeroidinellopsis seminulina s.l. The age determination of the different datum planes which mark the zonal boundaries depends essentially on correlation of the Pliocene planktic foraminiferal events in the Mediterranean Sea (Sprovieri, 1993, Shiphoard Scientific Party, 1996 a&b) with the geomagnetic polarity time scale of Cande & Kent (1995). The base of the Sphaeroidinellopsis Acme Zone of Cita (1973, 1975) which marks the base of the Zanclean, was encountered in the lowermost part of the Gilbert Epoch, few meters below Chron C3n,.4n, with an estimated age of 5.3 Ma. The value is in good agreement with that given previously by Ryan (1973) and Cita (1975), and with the age resulting from the study of deep-sea cores outside the Mediterranean (e.g. Berggren, 1973, Berggren et al., 1985a; Saito et al., 1975). The lower boundary of the Gr. margaritae Zone which coincides with the first common appearance of Gr. margaritae s.s. (according to Cita & Gartner, 1973 and Cita, 1975, 1976), has been recorded within the later part of Chron C3n.4n, with an estimated age of 5.1 Ma. However, Langereis & Hilgen (1991) recorded the initial appearance of Gr. margaritae in the Mediterranean just immediately above the base of the Zanclean in the mid-Thvera Subchron at 4.93 Ma. Berggren et al. (1995) showed that this taxon has an erratic datum of appearance in the open ocean (non-Mediterranean), spanning over 1 My from Chron C3 An to basal Chron C3r, whereas in the Mediterranean, the datum of the oldest common occurrence of this species is associated with the Thvera (Cr 3n.4n) Subchron, with an estimated age of 5.2 Ma. The first appearance of Gr. puncticulata which is nearly coincident with the Gr. margaritae s.s.- Gr. margaritae evoluta evolutionary transition has been placed in the Mediterranean Sea at or near the top of Chron C3n.2n, with an estimated age of 4.52 Ma according to the time scale of Cande & Kent (1995). This datum is followed by the extinction of Gr. margaritas s.l. which has been identified within the topmost part of Gilbert Epoch, midway between Chron C3n.1n. and Chron C2An.3n (Gauss Epoch), with an estimated age of 3.94 Ma. Berggren et al. (1995), however, gave a younger age of 3.58 Ma for the same datum, at the Gauss/Gilbert boundary, a short stratigraphic interval younger than the datum of extinction of Gg. nepenthes in subtropical areas. In the northern Western Desert Gg. nepenthes makes its 133

last occurrence at a datum which is nearly coincident with the extinction of all representatives of the Gr. margaritae lineage, a case which has been recognized by Bolli & Saunders (1985) in the tropical/subtropical areas. The extinction of Sphaeroidinellopsis was previously recognized by Ryan (1973) and Cita (1975) in Site 132 near the top of the Kaena event of the Gauss Epoch, dated 2.9 Ma. A similar age (3.0 Ma) was given by Berggren et al. (1983), Leonard et al. (1983) and Püjol (1983) in deep-sea cores from outside the Mediterranean. However, recent evalution of this bioevent (Shipboard Scientifc Party, 1996a & b) on the basis of the time scale of Cande & Kent (1995) suggests an age of 3.22 Ma to the bottom of Chron C2An.2n. The sedimentation rate during the stratigraphic interval from the base of the Sphaeroidinellopsis spp. Zone to the top of the Ss. seminulina s.l. Zone (equivalent to the last occurrence of Sphaeroidinellopsis spp. in the Mediterranean region) is calculated to be about 37 m/my in both Site 132 and Sallum Hole-1. Although the Miocene/Pliocene boundary in Sallum Hole-1 is marked by a major stratigraphic hiatus, no significant strata of Early Pliocene age are missing. The lowermost Pliocene biozone (viz. the Sphaeroidinellopsis spp. Zone) in Sallum Hole-1 attains a thickness of 9 m which is similar to those in the subbottom of the Tyrrhenian Sea (Cita & Gartner, 1973, Thunell, 1979) and in the outcrops from central and southwest Sicily ( in Cita, 1975). In the Mediterranean this zone is generally less than 11.5 m. thick in pelagic Pliocene sediments (Cita, 1976, Shipboard Scientific Party,1996 a & b.). The interval occupied by the Gr. margaritae Zone assumes nearly the same thickness (51-55 m) in Sallum Hole-1 and Sites 132, 134 and 977. However, the Zanclean stratotype in Capo Rossello exhibits a notable increase in the sedimentation rate which is interpreted by Cita & Gartner (1973) as the result of the geodynamic evolution of the Argrigento Basin involving a decrease of the water depth. This is supported by the relative abundance and diversity of benthic foraminifera in the upper part of the Trubi Formation, which is increasing in the overlying Monte Nabrone Formation. A similar trend has been recorded in the Western Mediterranean basins (Sites 975 and 977) during the interval from the datum of extinction of Gr. margaritae to the datum of extinction of Sphaeroidinellopsis spp., as compared to the Tyrrhenian Basin (Shipboard Scientific Party, op. cit.). The boundary between the Early and Late Pliocene is placed in Sallum Hole-1 at the last occurrence of Gr. margaritae s.l. which is recorded 200 feet above the base of the Zanclean.. According to the original definition of the Zanclean stratotype and later references (Cita & 131

Gartner, 1973, Cita, 1975; Cita & Decima, 1975), the Zanclean stage extends upwards above the extinction datum of Gr. margaritae until the extinction of Sphaeroidinellopsis, and thus included the time interval represented by the stratotypes of the Tabianian (Iaccarino, 1967) and a part of the Piacenzian (Barbieri, 1967). Later, Mazzei et al. (1978) redefined the top of the Zanclean by the base of the overlying Piacenzian, following the rules of the International Stratigraphic Guide (Hedberg, 1976). Accordingly, the Zanclean/Piacenzian boundary coincides with the last occurrence of Gr. margaritae s.l. which marks the base of the stratotype Piacenzian near Piacenzia in northern Italy (e.g. Barbieri, 1967; Mazzei et al., 1978, Iaccarino, 1985, Berggren et al., 1995). Thus, the Pliocene interval which postdates the extinction of Gr. margaritae s.l. in Sallum Hole-1 (viz. the Ss. seminulina s.l. Zone and the overlying Gs. obiquus extremus Zone) should be equated with the Late Pliocene (Piacenzian). No planktonic foraminifera referable to the Gr. inflata Zone have been encountered anywhere in the onshore of the Mediterranean coastal plain, and the sediments younger than the Gs. obliquus extremus Zone are predominantly shallow water in facies, suggestive of a regional drop in the sea level, a very short time interval before the beginning of the Pleistocene. D- Paleogeography: (Figs.23-24). After a long period of emergence and subaerial erosion ertending during the entire Late Miocene, the Sallum basin was flooded by an abrupt transgression at the beginning of the Pliocene. This transgression is partly attributed to the sudden drowning of the Mediterranean Basin by Atlantic water masses due to the collapse of the dam formely existing at Gibralter (Cita, 1976). However, the absence of transitional sediments at the base of the Zanclean section of Sallum area, but a sharp sedimentary break marked by pelagic oozes directly overlying the eroded surfaces of the Middle Miocene carbonates, indicates that this abrupt transgression was the function of the combined effect of rapid subsidence of the Sallum basin and a global rise of sea level. Meanwhile, most of the coastal area stretching between Sallum in the west and Burg El Arab in the east, remained above sea level, acting as a structurally positive land that was partially and intermittantly submerged by shallow water. A gradual retreat of the Pliocene sea toward the north (seaward) occurred during the Late Zanclean-Early Piacenzian, leading to the deposition of hemipelagic clays belonging to the youger part of the Gr. margaritae Zone and the Ss. seminulina s.l. zones in the Sallum basin. 133

However, at a time which is nearly coincident with the age of last occurrence of Ss. seminulina s.l. (3.22 Ma), tectonic subsidence took place along almost of the coastal area extending from Mersa-Matruh to Burg El Arab, and the sea invaded the area just to the south of the coast and progressed southeastward with gradual shallowing until the vicinity of Wadi Natrun. This led to the deposition of richly fossiliferous clays of outer shelf-type (with high benthic/planktic foraminiferal ratio), belonging to the Late Piacenzian Gs. obliquus extremus Zone, on the expense of the Middle Miocene limestone along the coastal area between Mersa Matruh and Burg El Arab. The wide variation in thickness of these sediments (30-180 feet thick) indicates that the surface topography of the coastal plain was irregular during the progress of the Late Piacenzian sea. The latter extended as a shallow embayment southeastward of Mersa Matruh until the Gebel Mansuriya-Wadi Natrun line, where poorly fossiliferous black shales alternating with sandstone and changing upward into thin, restricted-marine carbonate layers laid down. A regional drop in the sea level occurred in the northern Western Desert during the latest Piacenzian, at a time which is more or less corresponding to the age of appearance of Gr. inflata in the Mediterranean basin. In all localities along the coastal plain, the open marine clays belonging to the Gs. obliquus extremus Zone change upward into cryptocrystalline, pinkish limestones which are, partly oolitic, iron-stained and containing a shallow water littoral benthic foraminiferal fauna (Surur Formation).

ACKNOWLEDGEMENT The author is greatful to Prof. Dr. H. P. Luterbacher, Institute of Paleontology, University of Tubingen, Tubingen, Germany, for detailed critical comments of an earlier draft and to Prof. Dr. A. I. Kenawy, Geology Department, University of Assiut, Assiut, Egypt, for identification of the Nummulites species included in the text. This research was not possible without the support of the management of the Egyptian General Petroleum Coroporation who put under the author’s disposal the well samples and logs.

APPENDIX Abbreviations used in the text are as follow: Ac: Acarinina; B: Biorbulina ; C: Catapsydrax; D: Dentoglobigerina;Gd : Globorotaloides ; Ge : Globigerinella; Gg : Globigerina; Gk : Globigerinatheka; Gq : Globoquadrina ; Gr : Globorotalia; Gs: Globigerinoides; H : Hastigerina; Hk : Hantkenina; M : Morozovella; N : Neogloboquadrina; O: Orbulina; P : Pseudohastigerina; Pr : Praeorbulina. Ss : Sphaeroidinellopsis; T: Turborotalia, Tr: Truncorotaloides.

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