Sedimentary facies, mineralogy, and geochemistry of ...

Sedimentary Geology 174 (2005) 63 – 96 www.elsevier.com/locate/sedgeo

Research paper

Sedimentary facies, mineralogy, and geochemistry of the sulphate-bearing Miocene Dam Formation in Qatar H.G. Dilla,*, R. Botzb, Z. Bernerc, D. Stqbenc, S. Nasird, H. Al-Saadd a

Federal Institute for Geosciences and Natural Resources, P.O. Box 510163, D-30631 Hannover, Germany University Kiel, Geological-Palaeontological Department, Olshausenstraße 40-60, D-24118 Kiel, Germany c Technical University Karlsruhe, Institute for Mineralogy and Geochemistry, Fritz- Haber-Weg 2, D-76131 Karlsruhe, Germany d Qatar University, Geological Dept. P.O. Box 2713, Doha, Qatar b

Received 6 January 2004; received in revised form 30 August 2004; accepted 22 November 2004

Abstract The Miocene deposits of the Dam Formation were deposited in a narrow seaway stretching along the western edge of the Qatar Arch. During the initial stages of basin evolution the rising Zagros Mts. delivered debris in this fore deep basin. The paleocurrent and paleogeographic zonation are reflected by the heavy mineral assemblage, by the spatial distribution of phyllosilicates and the various types of sulphate. From NW towards the SE, the contents of smectite and palygorskite increase, whereas the illite and kaolinite contents decrease. Mega crystals of gypsum are found in the NW and massive fine-grained gypsum in the SE of the basin. During the waning stages of basin subsidence, the Arabian Shield became more and more important as a source for the Miocene sediments. In this study, the Dam Formation was subdivided into 7 members/lithofacies associations (lower, middle, upper Salwa, and Al Nakhsh Members, Abu Samrah Member). The Salwa Members at the base of the Dam Formation consists of heterolithic siliciclasticcalcareous sediments which were laid down under meso- to microtidal conditions. The Al Nakhsh Members formed under macrotidal conditions with sub- to supratidal depositional environments passing into continental ones. Celestite, gypsum, and microbial mats (stromatolites) are very widespread in these sabkha sediments. Crystals of gypsum and the thickness of stromatolites tremendously increase towards younger sediments indicating thereby a close genetic link between growth of microbial domes and gypsum precipitation. Throughout the Abu Samrah Member marine calcareous sediments were deposited in a microtidal wave-dominated environment. Dissolution of Eocene evaporites at depth governed the lithofacies differentiation in the Miocene Dam Formation. D 2004 Elsevier B.V. All rights reserved. Keywords: Evaporite; Siliciclastic-calcareous; Sabkha; Deposition; Miocene; Qatar

1. Introduction * Corresponding author. Fax: +49 511 6432304. E-mail address: [email protected] (H.G. Dill). 0037-0738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2004.11.004

The southern Dukhan Anticline, SW Qatar, provides an excellent target site to study a complete

64

H.G. Dill et al. / Sedimentary Geology 174 (2005) 63–96

Fig. 1. (a) Sketch map to show the Miocene rocks and the study area at Al Nakhsh (framed area). The Dukhan Anticline extends in NNW–SSE direction along the western coast of Qatar. Locations referred to in the text are labeled in the map. The dotted bold lines delimit the occurrence of sulphate in the underlying Eocene Rus Formation. See for the occurrence of the sulphate zone in the subsurface also the cross-section below (b). The surface expression of the cross-section is marked in the map by the transect A–B. (b) W–E cross-section through the southern part of the Central Qatar Arch. For position of cross-section see (a).


pp. 65 – 70

Fig. 2. Litholog of the Miocene Dam Formation and their depositional environments. All depth-related data are given in meters, all dimensions in the litholog are given in centimeters. The wedge denotes which way a texture or structure fades out. LLH—laterally linked hemispheroids, SH—vertically-stacked hemispheroids.

H.G. Dill et al. / Sedimentary Geology 174 (2005) 63–96 71

Fig. 3. Geological map of the Al Nakhsh study area covering the southern sector of the Dukhan anticline. For position see Fig. 1a. Azimuth of the paleocurrent at each site is expressed by arrows. A–B denotes the bsulphate lineQ also shown in Fig. 1a.

72


sabkha sequence in Miocene sediments from off-shore to continental deposits (Figs. 1–3; Table 1). A joint research project was launched, involving sedimentary petrography, mineralogy, and chemistry. Supplementary paleontological data were obtained by the study of body and ichnofossils (Fig. 2; Table 2). The scope of the present inquiry into these evaporitic deposits is to shed some light on the evolution of the depositional environments and the paleoecology of the Late Tertiary sedimentary series during an early stage of the evolution of the Arabian Gulf (Fig. 2; Tables 1 and 2). Biodata may be useful to refine the paleogeographic interpretation of the ancient depositional environment, that was interpreted based on mineralogical, sedimentological, and chemical data. Vice versa, the living habitat of some species (e.g. foraminifera) whose palaeoecology is still uncertain may be constrained by the help of inorganic data (see above). Giant crystals of gypsum, attaining a size hitherto unknown from evaporite sequences elsewhere, are encountered in the sediments at Al Nakhsh (Fig. 4A). The abundance in gypsum and celestite and a peculiar clay assemblage render the sedimentary series of the Dam Formation in SW Qatar especially attractive for economic geologists who look for nonmetallic deposits or construction material.

prominent structure in western Qatar (Fig. 1a). It hosts the major on-shore oil field on the peninsula and stretches NNW–SSE across western Qatar (Sugden, 1962; Dill et al., 2003).

3. Methodology The entire Miocene sequence in the Al Nakhsh region, about 50 km SW of Doha, was mapped and cross-sectioned with about 250 samples taken for laboratory-based mineralogical investigations, such as examinations of thin and particulate sections and Xray diffraction analysis (XRD) of the insoluble residue of calcareous rocks (Figs. 2 and 3). Major and minor elements were analyzed using routine X-ray fluorescence (XRF). Isotope analyses were carried out in the common way described by McCrea (1950). Analytical details on the method are given in Berner et al. (2002). During the microfacies analysis, calcareous rocks are generally named according to the proposals published by Folk (1959), Dunham (1962) with amendments by Wright (1992). The limestones under consideration, however, contain considerable amounts of impurities as silt, sand, and clay as well as several evaporites in discrete seams. Therefore, the classification scheme of Fookes (1978) was applied in the reference crosssection of Fig. 2.

2. Geological setting The Qatar Peninsula is the surface expression of the Qatar Arch, a projection of the Arabian platform towards the N into the Arabian Gulf (Cavelier, 1970). The majority of sediments exposed at the surface are Quaternary in age. These sediments rest on Miocene and Eocene rocks which crop out mainly in SW Qatar (Fig. 1). In mainland Qatar, series older than the Paleogene were only hit by oil wells. Lithological investigation of the Qatar geology is scarce. It was mainly stratigraphic studies on the calcareous Tertiary sediments and numerous unpublished reports commissioned by oil companies which contribute to the present knowledge of Qatar geology (Powers, 1968; Powers et al., 1966; Boukhary, 1985; El Beialy and Al-Hitmi, 1994; Alsharhan and Nairn, 1995; Al-Hinai et al., 1997; Al-Saad and Ibrahim, 2002). The Tertiary sedimentary rocks under study are exposed at best in the Dukhan Anticline, a

4. Results 4.1. Stratigraphic subdivision The Miocene Dam Formation was subdivided by Cavelier (1970) into a lower and upper subformation. During the field work in 2003, the succession of sedimentary rocks subjacent to the Hofuf Formation was subdivided into seven lithofacies associations (Fig. 2). Following the recommendations by Hedberg and George (1976), these lithofacies associations may stratigraphically be grouped from top to bottom into three members called Abu Samrah, Al Nakhsh, and Salwa Members (Fig. 2; Tables 1 and 2). Al Nakhsh and Salwa Members both allowed for a refinement of the stratigraphy, each comprising an upper, middle, and lower unit (lithofacies associations). Within each member different units, reflecting different facies,

Table 1 Stratigraphy, lithology, sedimentology, and mineralogical composition of the Miocene sedimentary rocks with their depositional environment inferred from the inorganic data listed herein and the biodata listed in Table 2 Lithology Rock color

Classification of sedimentary rocks and grain size FS: Fookes (1988), FL: Folk (1959); DU: Dunham (1962)

Grain shape and sorting

Type-grading/ cyclicity

Hofuf Formation

Brown to gray mottled

FS: - - FS: calcareous conglomerate FL: - - DH: - - -

Rounded to well-rounded pebbles and cobbles, moderately well to well sorted

Planar crossbedding, horizontal planar, massive

Dam Formation Abu Samrah Member Lithofacies Association VII

Bright gray to white with reddish interbeds

FS: pure limestone to clayey marlstone FS: - - FL: intraclastbearing oobiosparite, sparite, biosparite, peloomicrite, intrasparite DH: grainstone, packstone, sparstone, rudstone, floatstone, bindstone, bafflestone

Well sorted

FS: marlstone FS: silt to mediumgrained calcareous sandstone gypsum bed

Al Nakhsh Upper Al Member Nakhsh Member Lithofacies Association VI

Middle Al Nakhsh Member Lithofacies Association V

Red to brown with gray interbeds

Bright gray to white with red interbeds

Stratification

FS: pure limestone to marly limestone FS: claystone

Sedimentary structures and Paleocurrent textures vectors Biogenic (azimuth/dip) Inorganic

Micro- and macrofossils (for genus and species of invertebrates see Table 2)

Mineralogical composition LP: limestone particles AP: authigenic particles TP: terrigenous particles

n.d.

---

---

LP: calcite fluvial AP: - - TP: lithoclasts of metamorphic, igneous, sedimentary origin (no detailed investigations

Thinly laminated CUNFU fine-grained interbeds, trough, planar and heringbone cross-bedding, wave ripple crosslamination

228/188 calcarenite 418/198 calcarenite

Irregular burrows giving the sedimentary rocks a mottled outward appearance rip-up clasts

Shark teeths forams mollusks bryozoans ostracodes

LP: calcite, dolomite

Beachrock

AP: gypsum, celestite

Tidal channel, delta, mudflats Coastal/ beach ridges

Moderately well to well sorted grain size distribution with grains ranked subrounded to subangular

Planar and trough CU (?), (in parts largeFU (?) scale) crossbedding,

1938/278large-sacle cross-bedding in siliciclastic arenaceous rocks

Poorly sorted to well sorted in calcarenites and calcrudites

A few bedsets showing planar cross-bedding, wavy parallel to algal (sensu Cole and Picard 1975), massive

n.d.

TP: quartz, plagioclase, orthoclase, paligorskite, kaolinite

LP: dolomite goethite, hematite AP: gypsum, F celestite Ferricretes and concretions, collapse breccia, hardpan

CU 368/168 (in places calcarenite capped) 2948/198 calcarenite

Laterally linked (LLH) and vertically stacked hemispheroids (SH) in \stromatolites (giant stromatolitethrombolite association), tepee structures

TP: muscovite, illite, kaolinite, F palygorskite, plagioclase, orthoclase, quartz Microbial mats forams mollusks ostracodes

LP: dolomite AP: gypsum, celestite, bassanite, anhydrite, halite, goethite

Environment of deposition

Aeolian deposits barchanoid dunes Calcretes (groundwaterrelated) Supratidal (intertidal) salinasabkha cycles


Stratigraphy (formation/member/lithofacies unit)

Intertidal flats and subtidal channels with seabed lithification and reworking convert into continental red beds

(continued on next page)

73

74

Stratigraphy (formation/member/lithofacies unit)

Dam Formation Al Nakhsh Lithofacies Member Association V

Lower Al Nakhsh Member

Lithology Rock color

Stratification Classification of sedimentary rocks and grain size FS: Fookes (1988), FL: Folk (1959); DU: Dunham (1962) FL: biopelmicrite, bio oomicrite, pelbiomicrite, microsparite, biolitite, micrite DH: mudstone, wackestone, packstone to floatstone, rudstone dolomitic, bindstone, bafflestone gypsum beds FS: pure limestone to marly claystone

Bright gray to white with layers of pinkish tint (sulfate-free lithofacies only) Lithofacies FS: - - Association IV FL: ooid-bearing bio pelmicrite, dolomicrite, pelmicrite, biomicrite dolomitic, biolitite DH: packstone to floatstone, rudstone dolomitic, bindstone, bafflestone gypsum lenses and concretions

Grain shape and sorting

Sedimentary structures and Paleocurrent textures vectors Biogenic (azimuth/dip) Inorganic

Type-grading/ cyclicity

Micro- and macrofossils (for genus and species of invertebrates see Table 2)

mud cracks, and seabed-lithification, solution casts

Poorly sorted A few bedsets to well sorted showing planar in calcarenites cross-bedding and calcirudites

CU

168/138 calcarenite

Mud cracks, laterally linked (LLH) and vertically stacked hemispheroids (SH) in stromatolites, tepee structures Seabed-lithification, solution casts

Microbial mats forams mollusks echinoids

Mineralogical composition LP: limestone particles AP: authigenic particles TP: terrigenous particles

Environment of deposition

TP: muscovite, illite, kaolinite, palygorskite, plagioclase, orthoclase, quartz

Supratidal deposits prevail over intertidal flats and subtidal channels, tempestites, mangrove swamps, with seabed lithification and reworking

LP: dolomite AP: gypsum, celestite, anhydrite, halite, goethite TP: muscovite, illite, kaolinite, palygorskite, quartz

Intertidal flats prevailing over subtidal channels with seabed lithification and reworking and supratidal levees and little marshes. In places, emersion with subaerial dissolution


Table 1 (continued)

Salwa Member

Bright gray to white with reddish and greenish interbeds

Middle Salwa Member Lithofacies Association II

Bright gray to white with reddish and greenish interbeds

Lower Salwa Member Lithofacies Association I

Bright gray to white

FS: pure limestone to marly limestone FS: calcareous siltstones and fine-grained sandstone FL: ooid-bearing biopelmicrite altered into dolomicrite, oosparite, dolomicrosparite, biomicrite DH: rudstones to packstones, mudstone FS: clayey marlstones to marly claystones FS: calcareous siltstones and claystones FL: bio- and oosparite, -micrite DH: rudstones to packstones, mudstone

FS: pure dolomitic limestone to clayey marlstone, FS: fine-grained calcareous sandstone FL: bio- and oosparite, -micrite DH: rudstones to packstones, mudstone

Poorly sorted to well sorted, siliciclastics subangular to subrounded clasts

A few bedsets showing planar cross-bedding and horizontal cross-bedding

Poorly sorted to moderately well sorted

Little ripple cross-lamination and heringbone cross-bedding, horizontal bedding

FU NCU

Poorly sorted to moderately well sorted

Horizontal bedding to massive

FU

CU

1588/298 calcarenite 1318/168 calcarenite 708/178 calcarenite

Burrows (vertical zhorizontal) (Callianassa, Thalassinoides, Alpheus (?)) Rip-up clasts,

Forams mollusks echinoids

LP: dolomite AP: goethite TP: muscovite, illite, kaolinite, smectite, (palygorskite), quartz, plagioclase, orthoclase, zircon, rutile, epidote-clinozoisite s.s.s

Subtidal to lower intertidal with mangrove and open ponds with seabed lithification and reworking

---

Shark teeths, ostracodes forams mollusks echinoids

LP: dolomite, calcite

Shark teeths, ostracodes forams mollusks

LP: dolomite, calcite

Restricted platform/ lagoonal dysaerobic with surf zones and intertidal/beach deposits and intercalation of distal tidal deltas with seabed lithification and reworking Restricted platform-open marine well oxygenatedintertidal/ beach

Rip-up clasts, ferricretes and concretions

1408/78 calcarenite

Burrows mainly horizontal (Planolites, Thalassinoides) Ferricretes concretions, solution casts

AP: gypsum, goethite, (iron disulfide) TP: muscovite, illite, kaolinite, quartz, plagioclase, orthoclase, zircon, rutile, smectite, Fpalygorskite, epidote-clinozoisite s.s.s.

AP: goethite TP: muscovite, illite, kaolinite, quartz, plagioclase, orthoclase, smectite, F palygorskite, zircon, rutile, epidoteclinozoisite s.s.s


n.d.—not determined.

Upper Salwa Member Lithofacies Association III

75

76


Table 2 The fauna of the Miocene Dam Formation in the Al Nakhsh study area, SW Qatar (see also Cavelier, 1970; Al-Saad and Ibrahim, 2002) Fossil

Foraminifers

Bivalves

Members (Lithofacies)

Depositional environment

Agglutinella spp. Ammonia beccarii Amphisorus sp. Borelis melo melo Cancris auricula Cibicides sp. Cibicidoides unbonatus Cibroelphidium spp. Clavulina cf. mexicana Clavulinoides sp. Coscinospira spp Cribroelphidium spp. Dendritina spp. Elphidum crispum Haplophragmoides sp. Lenticulina cf. rotulata Miliolinella sp. Peneroplis carinata Peneroplis cristata Proemassilina rugosa Pyrgo levis Quinqueloculina bicarinata Quinqueloculina lamarchiana Quinqueloculina spp. Sigmoilina sp. Sigmoilina tenuis Spirolina arietina Spiroloculina excavata Triloculina linneiana Triloculina subgranula Triloculina trigonula Triloculinella sp. Clausinella persica Clementia papyracea Pectinidae Corbula sp. Cardita sp. Cardium sp. Modiola sp.

Lower Salwa

Middle Salwa

Upper Salwa

Lower Al Nakhsh

Middle Al Nakhsh

Upper Al Nakhsh

Abu Samra

Subtidal (little intertidal/ beach) well oxygenatedrestricted platform

Subtidal (little intertidal/beach) -limited oxygenation intercalation of distal tidal deltas +++

Subtidal to lower intertidal

IntertidalN subtidalN channels supratidal marshes

SupratidalN intertidal flatsN subtidal channels

Supratidal to continental

Beach ridges, tidal flats and channels

+++ +

+

+

+ + ++ +

+

+++ +

+ + + + + ++

++ +

+ + +

+ ++

+

++

++ ++

++

++ ++ +

+ +

+ + ++ +

+ +

+ + +

++ +

++ + +

+ ++

+

++ + + +

+ +

+ +


77

Table 2 (continued) Fossil

Members (Lithofacies) Lower Salwa

Bivalves Gastropods

E O B

Ostrea sp. Lima sp. Natica sp. Hydrobia sp. Cerithidae Xenophora sp. Conus sp. Turitella sp. Echinocyamus sp. Ostracods Bryozoa

Middle Salwa

Upper Salwa

Lower Al Nakhsh

+

+ +

Middle Al Nakhsh

Upper Al Nakhsh

Abu Samra

+ + + + + + ++

+ ++

+ +

+ +

E, echinoids; O, ostracodes; B, bryozoans; +++, abundant; ++, common; +, rare.

were coded with Roman numerals (e.g. Salwa 1 to 7). The key elements for the distinction of the seven lithofacies associations are (1) lithology, including rock color, grain size, shape and sorting; (2) stratification; (3) inorganic structures; (4) biogenic structures; (5) paleontological composition; (6) mineralogical composition; (7) chemical composition (Fig. 2; Tables 1 and 2). 4.2. Lithology of the Salwa Member The Salwa Member is a calcareous-siliciclastic series. A few brown and red interbeds were encountered in this overall pale gray lithofacies associations. These red beds particularly widespread in the Upper Salwa Member, locally, alternate with dark gray and green beds (Fig. 2; Table 1). A characteristic feature of some of these marly limestones is ripple cross-lamination. The internal structure of the ripples is described as unidirectional cross-lamination. Horizontal stratification with even bedding planes and bedsets measuring up to 1 m is widespread particularly in the siltstones and finegrained sandstones of the Lower and Upper Salwa Members. Some fine-grained siliciclastics of the Salwa lithofacies associations also developed planar cross stratification. Measurements of the paleocurrent show a trend towards the SE in the Salwa Member. Bioturbation has disrupted the primary sedimentary structures in many calcareous rocks of the Lower and Upper Salwa Members. In the Lower Salwa Member the ichnofossil assemblage is characterized

by straight and branched burrows in sediment composed of Cardita debris. 4.3. Lithology of the Al-Nakhsh Member The rock of the Al Nakhsh Member is predominantly grey and white in colour. The Upper Al Nakhsh Member is brown and red in colour. This upper series also contains argillaceous and arenaceous rocks resting on top of a thick gypsum seam (Fig. 2). The brown sandstones are very well sorted and display a wide range of bedding types. Set heights may exceed more than 0.5 m in the red medium-grained sandstones of the Upper Al Nakhsh Member. They are overlain by bedsets displaying large-scale planar cross stratification with an angular or tangential basal contact. Their mean vector suggests a strong paleocurrent towards the S at a rather steep angle of repose of 278. Red argillaceous beds with planar stratification were found intercalated among the calcareous rocks and evaporites of the Middle Al Nakhsh Member (Fig. 2). Claystones are more widespread and increase in thickness in stratigraphically equivalent beds of the Middle Al Nakhsh Member that contain no evaporites. In this non-evaporite facies, the thickness of the massive clayand siltstones, rich in nodules, may attain more than one meter. Only the Al Nakhsh Member in the Dam Formation contains stromatolites of varying size and shape (Logan et al., 1964, 1974). A wide range from laterally-linked hemispheroids (LLH) to verticallystacked hemispheroids (SH) are recorded from the

78


study area. Some of these stromatolites may attain a considerable size of as much as 2 m in diameter and 0.5 m in height. The climax of stromatolites growth is reached in the Middle Al Nakhsh Member. They evolve from centimeter-thick LLH stromatolites in the lower part to giant SH stromatolites in the upper part

of the Al Nakhsh Member. At 48 m a.m.s.l. LLH stromatolites appear in the Lower Al Nakhsh Member with tepee structures and centimeter-thick laminae (Figs. 2 and 4B). At 50 m a.m.s.l. patches of columnar microbial structures (SH), covering several hundreds of square meters developed on top of LLH stromato-


lites (Fig. 2). The individual columns form a sort of a bstromatolite pavementQ (Fig. 4C). In the Middle Al Nakhsh Member transitional types between SH and LLH stromatolites occur. Their internal structure closely resembles that of oncoids exfoliating in an onion-shell style (Fig. 4D). The maximum size of stromatolites (SH) is achieved in beds immediately underneath the boundary between the Middle and Upper Al Nakhsh Member (Fig. 4E), where domal structures, measuring 2 m across cover a wide platform. Internally, these domal structure or thrombolite buildups–microbial mounds sensu James and Bourque (1992), Leinfelder and Schmid, 2000–consist of a dense irregularly-shaped network. There is one type of stromatolite which does not fit well in this pattern of LLH and SH microbial/stromatolitic structures (Fig. 4F). This stromatolite extends horizontally across several square meters and is akin to the LLH stromatolites. Some concentric ring structures are randomly distributed among an irregularly-shaped network. In places, these rings have a central knob or a column erected like a broken tree trunk. 4.4. Lithology of the Abu Samrah member The Abu Samrah is similar to the lower Salwa Member with respect to its lithology, but it ranks much higher than the older series as to the sorting value. Trough cross stratification with set heights from less than 0.1 m to 0.3 m occur in the dolomitic pure limestones/fossiliferous grain- to rudstones. Towards younger series of the Abu Samrah Member, type and dip angle considerably change from planar crossbedding with very low angle into horizontal bedding (Fig. 2). The most conspicuous type of cross stratification observed in these Miocene beds is herringbone cross stratification. Upward herringbone

79

cross-bedding alters into trough-shaped cross-bedding and eventually fades out into wave ripple crosslamination. In the Abu Samrah Member, irregular burrows occur and a shell bed marks the boundary between Abu Samrah and Al Nakhsh Members. This fossiliferous layer is contained in a thinly bedded sequence of calcareous and siliciclastic rocks. There are no vertebrate remains (Table 2). 4.5. Paleontology of the Dam Formation A list of body fossils found in the Dam Formation at Al Nakhsh is given in Table 2. The various species and genus are listed as a function of the stratigraphy and the depositional environment (for more details see the chapter on Discussion). Although bathymetry, salinity, or the state of oxygenation may sometimes well be determined by means of microfossils, there is as much uncertainty about some taxa as to their paleoecological value (Nagy, 1992). A correlation of biodata and depositional environment (Table 2) may help to constrain the habitat for some of the fauna as paleoecological markers, including fish bones and shark teeth (Fig. 2). 4.6. Mineralogical composition In all members under study, excluding the Lower Salwa Member dolomite predominates over calcite (Fig. 2; Table 1). Both carbonate minerals occur in particles, form part of the matrix or of the cement (Table 1). Second in abundance are gypsum and celestite which were concentrated in the Middle and Upper Al Nakhsh Members. Bassanite also occurs. The sulphate minerals form several seams, the thickest of which is the seam No 8 (Fig. 2). Gypsum crystals may attain a length of more than

Fig. 4. (A) Giant gypsum rosettes (secondary gypsum II) from the Middle Al Nakhsh Member resting on top of domal SH stromatolites (see, for stratigraphic position, Fig. 2). See 15-cm yardstick for scale in the center of the gypsum aggregate. (B) LLH stromatolite with tepee structures in clayey marlstones of the Lower Al Nakhsh Member. Edge of field notebook is 20 cm. (C) LLH stromatolites (a) underneath columnar SH stromatolites (b) in the Lower Al Nakhsh Member. This columnar type has a vast aerial extension. Columnar stromatolites may laterally grade into more domal structures. This columnar stromatolites occur in protected basins with rather high tidal range under arid climatic conditions. (D) Oncoid-like stromatolites with typical onion-shell exfoliation structure in the Lower Al Nakhsh Member (Al Nakhsh 2). Edge of field notebook is 20 cm. (E) Internal structure of the giant domal stromatolites in the Upper Al Nakhsh Member. The massive to crudely bedded broofQ (a) of the stromatolite dome was cut by erosion and reveals the internal irregular network of a giant microbial dome (b). The overall size of the stromatolite may be inferred from the size of the car in comparison with the size of the stromatolite (inset top left). (F) Concentrically structured stromatolites from the Lower Al Nakhsh Member (Al Nakhsh 1). These biogenic structures are interpreted as microbial mat blisters caused by gas which raises the thin microbial mats. After collapse blister mash down and form ring-shaped structures. Part of them has had also harder cores which survived this decay.

80


1 m. The single crystals are arranged in aggregates bcoiledQ like a giant ammonite (Fig. 4A). Downward and upward from the Middle Al Nakhsh Member, the size of sulphate crystals diminishes. Gypsum no longer forms discrete seams but comes up only sporadically in some thin lenses or disseminated in rosette-like concretions, e.g. gypsum mineralization 1a to 1c (Fig. 2). These types of sulphate mineralization are diagenetic in origin. Fibrous aggregates of gypsum infill a few joints in the

Salwa Member, where the enrichment of gypsum is always coupled with an enrichment of goethite. Halite is as scarce as anhydrite and only visible under the petrographic microscope. Quartz tops the list of silica minerals. It is present mainly in the Salwa and the Upper Al Nakhsh Members. Plagioclase and K feldspar are also present in these members, but trailing behind quartz by a wide margin (Fig. 5A). In the siliciclastics of the Salwa Members, the rock-forming silicates mentioned

Fig. 5. (A) Micrograph giving an overview of a typical fine- to medium-grained sandstone of the Middle Salwa Member (Salwa 6). The field of view shows angular clasts of quartz (a) and plagioclase (b) with some of their grains supporting each others while others bfloatingQ in an microsparitic cement of dolomite (c). In the center of the micrograph dolomitized pellets and ooids (d) are recognizable. Crossed polars. (B) Micrograph showing light minerals such as muscovite (a), K feldspar (b) side-by-side with heavy minerals zoisite (c) and rutile (d). The cement consists of dolomite (e). In some parts of the thin section nests of vermiform kaolinite (f) may be identified. The fine-grained sandstone forms part of the Middle Salwa Member (Salwa 6). Crossed polars. (C) Micrograph to show gypsum (II) growing interstitially in sediments of the Al Nakhsh Members. Gypsum II (a) infills mouldic pores, solution casts and heals out vugs. Rosettes of celestite (b) incompletely rim dolomicritized peloids/ooids (?) (c). Crossed polars. (D) Micrograph illustrating the cementation and dissolution in beachrocks (Abu Samrah Member). Moulds (a) and interparticle pores are partially filled with different types of calcite cement. Particles are mainly pellets (b). Somewhat irregular fringe cement (c) was originally aragonite or high-Mg calcite (?). It was transformed into calcite I. Calcite I is replaced by a blocky calcite II (d). Moulds are filled with an acicular calcite III (e). Plan polarized light.

Table 3 Correlation coefficients of major and trace elements of sedimentary rocks of the Miocene Dam Formation in the Al Nakhsh study area SiO2 TiO2 Al2O3 Fe2O3 Na2O MgO CaO 1.00 0.83 0.80 0.49 0.48 0.49 0.85 0.49 0.28 0.86 0.88 0.66 0.30 0.00 0.31 0.14 0.31 0.19 0.46 0.21 0.13 0.08 0.15 0.22 0.92

1.00 0.99 0.83 0.66 0.39 0.87 0.39 0.32 0.77 0.96 0.86 0.30 0.00 0.35 0.37 0.57 0.16 0.79 0.01 0.11 0.11 0.04 0.04 0.81

1.00 0.87 0.69 0.39 0.85 0.39 0.31 0.76 0.96 0.86 0.29 0.01 0.36 0.39 0.62 0.15 0.82 0.05 0.12 0.10 0.05 0.08 0.75

1.00 0.62 0.17 0.65 0.17 0.31 0.47 0.75 0.84 0.30 0.01 0.36 0.55 0.73 0.09 0.89 0.02 0.10 0.07 0.01 0.06 0.44

1.00 0.23 0.61 0.23 0.24 0.49 0.68 0.49 0.21 0.63 0.27 0.34 0.42 0.01 0.61 0.01 0.01 0.14 0.00 0.03 0.44

1.00 0.18 1.00 0.63 0.77 0.45 0.17 0.52 0.10 0.10 0.12 0.18 0.17 0.21 0.17 0.06 0.09 0.13 0.17 0.41

1.00 0.18 0.64 0.75 0.87 0.71 0.31 0.11 0.42 0.23 0.41 0.07 0.61 0.06 0.04 0.16 0.21 0.08 0.81

dolo

1.00 0.63 0.77 0.45 0.17 0.52 0.10 0.10 0.12 0.18 0.17 0.21 0.17 0.06 0.09 0.13 0.17 0.41

calc

1.00 0.01 0.29 0.37 0.57 0.14 0.40 0.04 0.13 0.10 0.25 0.08 0.03 0.07 0.12 0.07 0.30

rest

1.00 0.82 0.52 0.21 0.02 0.20 0.19 0.33 0.14 0.47 0.16 0.05 0.17 0.26 0.17 0.77

K2O

1.00 0.80 0.28 0.06 0.32 0.29 0.56 0.15 0.71 0.07 0.10 0.12 0.09 0.09 0.81

P2O5 SO3

1.00 0.44 0.04 0.39 0.43 0.62 0.13 0.72 0.02 0.15 0.04 0.03 0.02 0.66

1.00 0.12 0.27 0.09 0.16 0.02 0.24 0.05 0.09 0.08 0.24 0.06 0.32

Cl

1.00 0.09 0.02 0.04 0.23 0.01 0.05 0.12 0.10 0.03 0.04 0.04

F

Ce

1.00 0.16 0.26 0.01 0.26 0.08 0.07 0.02 0.06 0.10 0.27

1.00 0.44 0.33 0.46 0.06 0.38 0.46 0.04 0.05 0.12

Co

1.00 0.01 0.66 0.01 0.06 0.08 0.01 0.02 0.27

Cu

1.00 0.06 0.02 0.93 0.86 0.18 0.01 0.25

Ni

1.00 0.03 0.06 0.10 0.14 0.07 0.41

Pb

1.00 0.03 0.00 0.09 1.00 0.10

Sr

1.00 0.94 0.15 0.02 0.19

Th

U

Zn Zr


SiO2 TiO2 Al2O3 Fe2O3 Na2O MgO CaO dolo calc rest K2O P2O5 SO3 Cl F Ce Co Cu Ni Pb Sr Th U Zn Zr

1.00 0.20 0.01 0.02

1.00 0.09 1.00 0.06 0.10 1.00

dolo—dolomite; calc—calcite; rest—insoluble residue after dissolution with HCl.

81

82 H.G. Dill et al. / Sedimentary Geology 174 (2005) 63–96 Fig. 6. Chemologs of major and trace elements as well as isotope ratios relevant for the analyses of the depositional environment. Dolomite and calcite contents were calculated based on the whole rock chemical composition. Depth is given in meter. For lithological reference see Fig. 2: (a) SiO2–Zr–P2O5–U–K2O–Na2O, (b) Sr–SO3–Cl–dolomite–calcite–F, (c) oxygen–carbon–sulfur isotopes.

83

Fig. 6 (continued).


84


Fig. 6 (continued).


above are accompanied, in places, by some heavy minerals such as zircon, zoisite, epidote-zoisite s.s.s., and rutile (Fig. 5B). Illite, muscovite, biotite, kaolinite, smectite, chlorite, and palygorskite occur mainly in sediments of the Salwa Members (Table 1). Palygorskite was also identified in Paleogene sedimentary rocks which constitute the roof in the northern sector of the Dukhan Anticline (Dill et al., 2003). The amount of palygorskite increases significantly in the uppermost part of the Upper Salwa Member (Salwa 7) and climaxes in the evaporitic parts of the Al Nakhsh Member (Al Nakhsh 4, 11, 12). An antithetic trend exists between the kaolinite and palygorskite distributions. Palygorskite gradually disappears from the phyllosilicate mineral assemblage towards the NNW, whereas the amount of kaolinite increases. The same holds true for the smectite/palygorskite ratio. 4.7. Chemical composition Based upon the correlation coefficients listed in Table 3 and the paragenetic relations which exist among some major elements, trace elements, and the contents of rock-forming calcareous minerals a set of chemologs was compiled in Fig. 6: (a) Si–Zr–P–K–Na, (b) Sr–S–Cl–dolomite–calcite–F. The oxygen and carbon isotope compositions of the bulk carbonates are illustrated in the diagrams of Fig. 6c. Carbon and oxygen isotopes have a spread of 2.1x to +4.6 x (y 13 C) and 9.9x to 1.4x (y18O), respectively, reflecting the complex processes which affected the carbonate fraction of the rock samples in these marine environments of formation. The y34S values of bulk samples from the evaporite layers of the Middle Al Nakhsh Member are clustered in a relative narrow range between 21.6x and 23.5x (Fig. 6c). These values are similar to those reported for the sulphur isotope composition of Miocene seawater sulphate and evaporitic deposits of the same age from the Mediterranean area (Claypool et al., 1980; Pierre and Rouchy, 1990; Peebles et al., 1997; Playa et al., 2000; Lu et al., 2001). However, the sulphur isotope ratios are significantly lower in the Salwa and Abu Samrah Members situated below (16.3x) and above (17.8x), respectively. There is also a clear vertical trend in the distribution of the values, which is expressed by a continuous increase upsection (with a maximum of 23.5x in the central

85

part of the Middle Al Nakhsh Member) followed by a gradual decrease afterwards.

5. Discussion 5.1. Provenance and paleocurrent Epidote–zoisite s.s.s. and orthozoisite point to basic igneous rocks which underwent low-grade regional metamorphism or were subject to autohydrothermal alteration (Fig. 5B). Metabasic igneous rocks were exposed to erosion in the provenance area as the sediments of the Middle and Upper Salwa Members were deposited in the Al Nakhsh Region. Zircon and rutile two ultrastable heavy minerals cannot exactly be attributed to a definite provenance (Dill, 1998). The plot of SiO2 against stratigraphy shows several remarkable highs in the Middle and Upper Salwa Members (Fig. 6a). Some of these SiO2 peaks in the Middle Salwa Member well correlate with maxima in the Zr, K2O, and Na2O plots (Fig. 6a). The graphic correspondence is substantiated by the correlation coefficients r Si–Zr=0.92, and r Si–K=0.88 (Table 3). Quartz, closely associated with mica-type phyllosilicates, and abundant zircon are responsible for these strongly positive correlations. The contents of Si, Zr, and K mirror the terrigenous input into the basin, mainly by wind and to a lesser extent through rivers (see grain sorting). The sheet silicate mineral assemblage in the study area shows a marked zonality along strike of the Dukhan Anticline. Kaolinite is known to be the prime alteration product during continental weathering of silicates, mainly feldspar. Not surprisingly, it is deposited proximal to the source area. It has been delivered from a source area lying roughly north to northwest of the study area. Illite contents also slightly increase this way towards the north. Mineralogical and geological studies on palygorskite in mainly modern marine environments, indicate that the lateral and vertical changes of this chainstructured phyllosilicate may be a valuable tool to describe the climatic and bathymetric conditions in the environment of deposition (Weaver, 1989; Pletsch et al., 2000). The phyllosilicate is a common constituent of sediments in perimarine environments where it formed under semi-arid climatic conditions.

86


In the area under study, palygorskite is assumed to have been formed under schizohaline conditions on a shallow dolomitic carbonate shelf. As the hydrological conditions get closer to normal-marine, as observed in the Salwa Members, this phyllosilicate is no longer stable and smectite forms instead (Table 1). Towards the NW, more landward, palygorskite is substituted for by terrigenous phyllosilicates such as kaolinite and illite. The structural elements, e.g. cross-bedding, used for the determination of the paleocurrent in sediments of the Salwa Member show that the detritus was supplied mainly from the NW to NNW (Fig. 3). For the Lower and Middle Al Nakhsh Member, the measurements delivered readings with a general trend mainly towards the NE. The red beds of the Upper Al Nakhsh Member came into existence under an aeolian regime, with winds gusting landward from N to S. When the heterolithic sediments of the Salwa Member developed, the source area was situated in the N to NW. The Zagros Mts. were rapidly emerging throughout that time and delivered their debris in the fore deep basin (Murris, 1980a,b). During the final stage of the Dam Formation the Arabian Platform with the Arabian Shield as a backbone has been the source area for the Miocene sediments (Fig. 2). 5.2. Stromatolite evolution a function of sediment accumulation and sea level fluctuation Thin microbial mats are persistent over square kilometers and generated various types of stromatolites (Figs. 2 and 4). Based upon the amount of noncarbonate impurities trapped in these microbial mats, the stromatolites are denominated in the Al Nakhsh Area as marly limestone through pure limestones. Most modern stromatolites and microbial mats are reported from intertidal and shallow marine settings, generally characterized by fluctuating water supply and salinity (Browne et al., 2000). Some are said to form down into the upper subtidal environments (Aitken, 1967; Burne and Moore, 1987). The microbial mats grew in a wide range of tidal environments, from the lower intertidal to the lowermost supratidal which is characterized by extensive precipitation of sulphate. Broken crusts and tepee structures are cogent evidence for emersion and supratidal conditions (Fig. 4B). Being situated at the edge of tidal channels,

some microbial mats may have also extended into the upper subtidal environment-Al Nakhsh 10 and 11 (Fig. 2). The morphological changes from LLH towards SH stromatolites and vice versa are illustrated in Figs. 2 and 4C. This morphological change is an immediate response to environmental changes, an adaptation to a higher energy regime in course of the overall basin subsidence and the episodic lowering of the sea level in each unit of the Al Nakhsh Members. In general, LLH stromatolites found favorable conditions to develop in intertidal sediments of a moderate energy regime, whereas their domal counterparts which are more resistant to stronger wave and tidal actions could also survive in shallower bathymetric conditions. Judging by the variation of size of these SH stromatolites the energy regime increased toward younger series and the basin subsidence slowed down from the Lower Al Nakhsh towards the top of the Middle Al Nakhsh Member (Figs. 2 and 4). The gradual increase of thickness of gypsum seams, their emplacement immediately on top of the giant stromatolites as well as red beds associated in time and space with them, would discard any hypothesis of a deep water marine setting for the sites under study (Figs. 2 and 4E). By and large, the microbial bindstones of the Al Nakhsh Members are transitional between normal marine and hypersaline evaporitic conditions. The stromatolites from the lower Al Nakhsh Member which are shown in Fig. 4F were interpreted in terms of microbial mat blisters and gas escape structures. The ring structures in an overall network are produced by degassing processes. The microbial mats are expanded to form mat blisters. After the leathery skin of the mat is being destroyed by the gas overpressure the structure collapses and the circular remains get preserved. These biogenic decay structures are held to be indicative of a rather low rate of sedimentation. Otherwise such delicate structures would have not been preserved. 5.3. Carbon and sulphur sources Marine carbonates which formed from atmospheric CO2 (y13C–CO2 atm= 7x; Keeling, 1958) have y13C values near 3x (with a 10x fractionation at 20 8C between solid carbonate and gaseous CO2; Emrich et al., 1970). Hence, the y13C values of marine carbonates (near 0x) investigated in our study most likely are


attributed to the primary (atmospheric) carbon source (Fig. 6c). Some samples from the Lower and Middle Salwa Members and the topmost part of the Abu Samrah Member stand out from the overall oxygen isotope composition by their extremely light isotope values falling in the range 8.8x to 9.9x. Another set of samples from the upper part of the Middle Salwa Member has isotope values somewhat isotopically heavier than the aforementioned group of isotope ratios ( 3.0x and 3.7x). The extremely light isotope values suggest that freshwater impacted on the carbonate precipitation during the lower and middle Salwa Members. This is also invoked for the sample from the Abu Samrah Member. The samples under consideration are enriched in calcite following precipitation of dolomite. The intermediate oxygen isotope values of 3.0x and 3.7x reflect brackish conditions, whereas the bulk of samples with isotopically heavier oxygen isotope values formed under marine conditions. The temperature of formation of carbonates lies in the range 10 to 40 8C. The isotope fractionation associated with the precipitation of sulphates from solution is very low (1.65x for CaSO4; Thode and Monster, 1965). The y34S values of evaporitic minerals reflect closely the isotope composition of fluids from which they precipitate and consequently were used to reconstruct the isotope composition of seawater sulphate during geologic times (Claypool et al., 1980; Strauss, 1997; Paytan et al., 1998). The measured isotope ratios are in agreement with the precipitation of gypsum from Miocene seawater. However, the relatively low y34S values in the lowest and upper part of the section and the trend in their vertical distribution are sufficiently significant to claim for an adequate explanation. Concerning the vertical trends, several possibilities may be considered. Although the isotope fractionation coupled with the sulphate precipitation is low, a breservoir effectQ may occur in a closed system with limited sulphate supply (Lu et al., 2001). According to this, due to the preferential precipitation of 32SO42 , the residual brine, i.e. later precipitated sulphate is progressively enriched in the 32S isotope. However, such a model could explain only the decreasing (upper) segment of the isotope profile. If it had been effective, it is very unlikely to have affected only the upper part of the section. A further possibility to be considered for the interpretation of the isotopic

87

features is the gradual mixing of the marine brines with non-marine fluids (fresh water) of continental origin (e.g., run-off, ground water) enriched in 32S. Due to the re-oxidation of reduced sulphur species depleted in 34S such sulphate of terrestrial origin is mostly in a range between 5x and +5x (e.g., Nriagu et al., 1991). Our assumptions seem to be supported by a general covariance of the S isotope data with the y18O values of carbonate, the lowest S-isotope values in the Salwa and Abu Samrah Members corresponding to y18O values as low as 10x PDB, suggesting the admixture of fresh water depleted in 18O to the system (Fig. 6c). Although this model may apply for the lowest and uppermost part of the section it cannot adequately explain the systematic changes of y34S in the Al Nakhsh Member, in which the y18O values of carbonate are almost constantly at about 0x PDB. The strongest fractionation of the sulphur isotopes in nature is associated with bacterial sulphate reduction (Canfield, 2001). This process is coupled with changes in the redox state of the depositional environment and also reported in hypersaline brines of marine salt pans (Pierre, 1985). In an environment with restricted sulphate supply, due to the preferential consumption of the energetically more advantageous 32SO42 ions, sulphide strongly depleted in 34 S precipitates, whereas sulphate in the residual brine is getting progressively enriched in 34S. The reduction could occur within the free brine column or the porous sediments below (Lu et al., 2001), but also in the capillary space of a supratidal flats. The re-oxidation of reduced sulphide species takes place without a notable isotope fractionation (e.g., Mizutani and Rafter, 1969), so that such a process will considerably lower the isotope composition of the bulk sulphate. The oxidation of reduced S species may have been promoted by interaction with well-oxygenated fresh water (Lu et al., 2001). This redox model is in agreement not only with the abovementioned covariance between y34S and y18Ocarbonate, but it is supported also by a significant negative correlation between the y34S values and the amount of detrital components [r(y34S–SiO2)= 0.73; r(y34S–TiO2)= 0.67; r(y34S–Al2O3)= 0.64; r(y34S–Fe2O3)= 0.63; r(y34S– MnO)= 0.84; r(y34S–K2O)= 0.76], which indicates a higher detrital input during periods of interaction

88


with oxygenated fresh water. Because the measurements were carried out on bulk samples, the possibility must be considered that the low y34S values represent a mixed sulphate–sulphide signal. 5.4. Dolomite and sulphate formations Dolomite occurs as cement and in calcareous particles. Oval and rod-shaped ooids and peloids are entirely dolomitized (Fig. 5A). Micritization has partially destroyed the tangential lamellar texture of the ooids. It is normal ooids sensu Flu¨gel (1978); only a few of them with one or a two laminae may be classified as superficial. Based upon the overall very fine grain size, the intergrowth and the good preservation of sedimentary textures in the calcareous particles as well as the lamination in stromatolites, the dolomite is held to be syn(dia)genetic (Fig. 5). The Ca content of dolomite is around 50%. According to Fu¨chtbauer and Goldschmidt (1965), syn(dia)genetic dolomite has almost stoichiometric composition. Dolomite formed by the reaction of brines with the host carbonate sediments. Per ascensum and per descensum modes of formation may be discussed for the dolomite. Under arid climatic conditions, capillary ascent of fluids is triggered by strong evaporation. It is accompanied by brine reflux from the sea—evaporative pumping model of Hsu¨ and Siegenthaler (1969). Periodic flooding of the sabkha by marine floods causes an increase of concentration and provokes a downward percolation of the brine under the force of gravityflood recharge model of McKenzie et al. (1980). The isotopic composition of the dolomite does not totally rule out the first model but is a strong argument in favor of the second mode of formation (Fig. 6c). Moulds, vugs, and interparticle pores of the uppermost Abu Samrah Member are partially filled with different types of calcite cement (Fig. 5D). Relic calcite I is replaced by a blocky calcite II infilling the voids. These morphological changes from an evenrim-calcite cement I to a calcite-II mineralization constituting a mosaic of equant crystals mirrors the transition from marine phreatic into freshwater vadose conditions—see also isotope data (Fig. 6c). Replacement of dolomite by calcite occurred in a beachrock environment. Microbial lumps found in samples from the Abu Samrah Member suggest

biological processes might have mediated this mineral transformation, a process that was experimentally proved by Neumeier (1999). Fine-grained, massive gypsum I currently mined near Sawda Nathil possess structures and texture very much distinctive from the giant gypsum II in the Al Nakhsh study area (Fig. 1a). Gypsum I precipitated subaqueously in a shallow marine environment at a very early stage. These paleogeographic and chronological assumptions are based upon its formation more basinward than giant gypsum II and on its fine-grained texture—see Tucker (2001). At Al Nakhsh, gypsum II grows interstitially in early-formed sediments of the Al Nakhsh Member (Fig. 5C). It replaces anhydrite, bassanite, and rosettes of celestite which rims dolomicritized peloids/ooids (?) (Fig. 5C). Transformation of anhydrite into gypsum is impeded when the mean annual temperature exceeds 20 8C, anhydrite is kept off the reaches of the groundwater and not flooded any more during marine transgressions (Kendall, 1992). These conditions did not occur all the time round since the sulphate has been precipitated in the Al Nakhsh Sabkha. Gypsum II forming the giant crystals is of secondary/diagenetic origin and younger than dolomite. Crystallization of these giant crystal starts of from a nucleus in the center of coiled snail-like structures (Fig. 4A). These gypsum aggregates are juxtaposed to large domal SH stromatolites (Fig. 4E). The porosity of these microbial build-ups are supposed to have favored the growth of gypsum and allowed it to gain such an exceptional size. Experimental studies involving micro-analytical techniques and isotope studies were performed by Garcia-Guinea et al. (2002) to tackle the problem of growth of gigantic gypsum crystals. In view of their fluid inclusion studies a direct sedimentary (evaporitic or vadose) origin during the Mediterranean Messinian salinity crisis has to be disregarded for huge gypsum crystals. The y34S values of the gypsum crystals spread the range from +18.6x to +19.8x. The authors conclude from their data that there existed a genetic link with marine sulphates via freshwater dissolution–recrystallization of earlier marine evaporites. At their study site near Pulpi, Spain, the host rock is dolomitic. Prior to the precipitation of huge gypsum crystals acicular crystals of celestite formed at the rim of the karst cavities. Conclusively, the gigantic gypsum crystals may be accounted for as follows.


Stromatolitic structures were crucial as to the permeability and porosity for the percolating fluids. A tremendous increase in the size of SH stromatolite may be recognized along with an increase in thickness and crystal size of gypsum. The giant gypsum crystals resulted from an early diagenetic reaction at shallow depth. Freshwater and seawater fluids involved in this process lead to a dissolution and recrystallization of earlier marine evaporites. Celestite is typical of mineralizations of secondary gypsum II. No discrete Sr mineral phase is identified within the primary anhydrite nor in the fine-grained gypsum I. 5.5. Synthesis: evolution of the depositional environments 5.5.1. The Lower Salwa Member—lithofacies association I In the Lower Salwa Member, Salwa 1a exhibit a deepening-upward and Salwa 1b a shallowing-upward trend. Fine-grained siliciclastics were deposited at the base and stand for a deeper subtidal environment. Calcitic clayey marlstone forms the topstratum and stands for a intertidal to beach environment. Large quantities of dolomite leave no doubt that these marine sediments were in the reaches of a Mg-enriched brine reflux under arid conditions and located not far away from the coast line. Based upon the isotope signature, freshwater was a key factor during precipitation of calcite which makes up a near-surface carbonate mineralization as it was also found higher up in the stratigraphic record, in the topmost part of the Abu Samrah Member (detailed discussion of cementation and beachrock formation see the chapter on lithofacies VII). Rock colors with bright gray and brownish tints indicate well-oxygenated conditions. The assemblage of foraminifers corroborate these constraints on the chemo-physical conditions. Observations from Yeu Island and the Bay of Bourgneuf on the Atlantic coast of France reveal that Ammonia beccarii is epiphytic, living on the calcareous algae and prefers sediments of brackish environments (Debenay et al., 1998; Table 2). The state of oxygenation reported for the habitat of modern species of Ammonia sp. is similar to that of its Neogene predecessors. The foraminifer burrows very shallow and is limited to the oxygenated sediments (Saffert and Thomas, 1998). Haplophragmoides sp. is

89

recorded from salt marshes in recent environments. In equivalent marshy environments of the Miocene it is absent but typical of the normal marine Salwa Members. Benthic foraminiferal distribution and microhabitat occupation are often regulated by the interplay of organic flux, oxygen, and competition. Each of these factors is acting in a different way and leads to the complex pattern as found in living associations (Van der Zwaan et al., 1999). Other genus of forams such as Pyrgo, Quinqueloculina and Spiroloculina are found at water depth of as much as 3000 m (Ohkushi and Natori, 2001), a bathymetric situation that has no meaning for the near-shore marine Miocene depocenter under consideration. Trace fossils are ubiquitous in lithofacies association I. The organisms burrowed their shaft and tunnels in a low to moderate energy regime. The creators are widely used to indicate patterns of erosion and sedimentation and assist in the analysis of the depositional environment (Zonneveld et al., 2001). The density of bioturbation in a stratum varies inversely with the rate of sedimentation. In the Lower Salwa Members, a decreasing-upward trend shown by the bioturbation index correlates inversely with an increase in the rate of sedimentation. The very complex surface tracks and trails in the Lower Salwa Member belong to the Cruziana Facies of Seilacher (1967). The ichnofossils are Planolites sp. and Thalassinoides sp., both of which are observed on bedding planes of sedimentary rocks which formed in a subtidal environment between 10 and 100 m water depth. They were recorded also from lower intertidal sediments (Zonneveld et al., 2001). Scoffin (1987) described these crawling and grazing feeding trails from less turbulent waters (restricted) carbonate platforms at water depths between 5 and 25 m, a bathymetric situation most likely also for the lower parts of the Lower Salwa Member. 5.5.2. The Middle Salwa Member—lithofacies association II The Middle Salwa Member (lithofacies association II) has all the hallmarks of a transitional period during the evolution of the Miocene basin. The ripplelaminated beset suggest a rather low water level and well-oxygenated bathymetric conditions. A subtidal environment with less intensive tidal currents than in the intertidal environments is invoked. The ripples in

90


the bedsets are akin to herringbone cross-lamination and common to tidal environments. Lithological trends, in context with the herringbone cross-bedding and ripple lamination are supposed to have been created in the distal part of a tidal delta complex (Sha, 1990; Fig. 2). Several of such couplets evolved in the topmost part of the middle Salwa Member and are indicative of a strongly varying flow strength. The sea level was strongly fluctuating with gradual changes in the water chemistry from freshwater to brackish. Although the built-up of the Middle Salwa Member is more complex than the Lower Salwa Member, both lithofacies types have much in common. Deepening-, e.g., Salwa 2, and shallowing-upward cycles e.g., Salwa 4 are characteristic of a restricted platform sedimentation (Fig. 2). Bonebeds full of shark teeth, locally, containing also invertebrate fossil hash came into being at the base of the cyclothems. Reworking is indicated at the base of each cycle by rip-up clasts and bioclasts. These basal beds were interpreted as having formed in a surf zone. The topstrata of Salwa 3 and 5 cyclothems were interpreted as a beachrock (intertidal) environment very much like the lithologies in Salwa 1 (Fig. 2). The transitional character of this member applies also to the rock color. Red and green rock colors observed in the Middle Salwa Member are indicative of varying oxidizing and reducing conditions, respectively. Ubiquitous goethite and hematite are responsible for the red color. The green color is due to bivalent iron in some phyllosilicates. Benthic foraminifera such as Aggluntinella spp. were recorded by Samir and El-Din (2001) from modern depocenters in Egypt. According to the authors, the presence of Aggluntinella in modern environments reflect environmental perturbation and strong organic coastal pollution. During the Miocene, these foraminifers found the most favorable conditions in lagoonal environments and bloomed in the Middle Salwa Member, that was susceptible to little environmental stress. They are absent from the intertidal and wave-induced subenvironments (Table 2). Cancris subconicus population increases towards the higher energetic sections of the Middle Salwa Member, an environment of deposition quite similar to the tide-dominated delta recorded from temperate climates in north Germany by Dill et al. (1996). Borelis sp. is also found in shallow water (Mandic

et al., 2002). The bathymetric conditions concluded from investigations of the inorganic and sedimentological data accord with those derived from biodata of the Salwa Members. The complex lithofacies changes may be summarized as follows. The basin begun deepening during the passage into the Middle Salwa Member. The state of oxygenation deteriorated (dysaerobic reducing conditions), so that part of the environment is most conveniently described as lagoonal. The water depth in the basin under study reached a maximum in the Salwa 2 and Salwa 3 (approximately 20 m; Fig. 2). A significant grain size increase among the bio- and lithoclasts from Salwa 3 into Salwa 4, Si and Zr peaks in the chemologs, ripple bedding, and herringbone cross-bedding point to an increase in the energy regime, placer-like deposition of accessory minerals and an overall shallowing of the basin. The environment during Salwa 4 unit may be described as a distal tidal delta (Dill et al., 1996). In the upper subtidal to lower intertidal environment of the Salwa 4 winnowed grainstone layers became cemented and the resultant very much resistant crust was broken and ripped up. During Salwa 5 a relapse to deeper lagoonal conditions is recognizable in the sedimentary record. Finally, in the shallow-marine basin-and-swell topography of the Middle Salwa Member, there is a shift from a microtidal to a mesotidal regime, both terms used in the sense of Barwis and Hayes (1979), is to be seen (Fig. 2). During the Middle Salwa Member, the basin saw a strong terrigenous input from the NW to N. 5.5.3. The Upper Salwa Member—lithofacies association III The Upper Salwa Member consists of two coarsening- or shallowing-upward sequences, very much similar to the sequence in the Lower Salwa Member but without any calcite precipitation during the waning stages of the cycles. Trace fossils in the Salwa 6 unit, attesting to more hospitable conditions than in the underlying Middle Salwa Member. The burrow morphology attributes the ichnofossil assemblages to the Callianassa Facies. The creators live in subtidal to lower intertidal environments (Scha¨fer et al., 1996). The predominance of vertical burrows over horizontal burrows clearly attest to a shallowing upward in the Upper Salwa Member. The ramifying


burrows in the Salwa 7 unit look the same as burrows produced by Alpheid shrimps (?) which occur in mangrove and open ponds. Sediment reworking and placer-like concentrations of heavy minerals were as intensive in the Salwa 7 and 6 units as in the Salwa 4 unit. Ostrea, known to be widespread in estuaries and tidal flats (Wilson, 2002), bpaves the wayQ from the subtidal environment of the Salwa Members into the inter- to supratidal subenvironments of the Al Nakhsh Members (Table 2). 5.5.4. The Lower Al Nakhsh Member—lithofacies association IV The Lower Al Nakhsh Member encompasses three fully-developed coarsening- or shallowing-upward sequences, named Al Nakhsh 1 through Al Nakhsh 3, each starting off with bioclastic calcareous rocks and ending up with stromatolites. Locally, the calcareous sediments are intercalated with some gypsum lenses or peppered with gypsum concretions. Similar cycles were termed as peritidal cycles by Pratt (2002). At the base of Al Nakhsh 1 a hardground developed (Fig. 2). The siliciclastic has almost ceased, whereas the evaporation has increased. Tidal channels up to 5 m deep are indicated by the bioclastic limestones in the lower part of each cycle (subtidal). Fossil hash and intraclasts represent lag deposits at the base of the channels. A complex system of tidal channels dissected the tidal flats, which occur in the middle part of the each cycle. The drained flats are made up of bioclastic limy marlstones through clayey marlstone and stromatolite of different types. Absence of lamination in the intertidal zone is due to burrowing. Microbial-laminated sediments formed in the upper intertidal zone. Small-scale representatives of the supratidal environments occur as levees on the outer bend of tidal channels within the intertidal zone. There is an overall decrease in the rate of basin subsidence. A good ecological agreement between modern and ancient representatives is found among the Peneroplidae. These forams live attached to fine microbial mats which cover vast areas in the Lower and Middle Al Nakhsh Member. These microbial mats provide shelter to them against high turbulence (Renema and Troelstra, 2001). In the Lower and Middle Al Nakhsh Members, the diversity of species of the macrofossil assemblages is rather

91

low, but the number of individuals is considerably high, reflecting a strong environmental stress on the fauna (Fig. 2). 5.5.5. The Middle Al Nakhsh Member—lithofacies association V Many of the units from Al Nakhsh 4 through Al Nakhsh 16 resemble those treated in the previous chapter. This is especially true for the non-evaporite facies of the Middle Al Nakhsh Member (Fig. 3). Some very distinctive features between lithofacies IV and V refer to the size and type of evaporites and the bounding surfaces between each unit. Most cycles encountered in the evaporite-bearing facies of the Middle Al Nakhsh Member are topped by a seam of gypsum, e.g., Al Nakhsh 4 (Figs. 2 and 4A). A few cycles were deprived of such seams by the succeeding flood event, e.g., Al Nakhsh 11 (Fig. 2). Towards the NE evaporites do no longer occur in the cyclothems of the Middle Al Nakhsh Member (Fig. 3 bsulphate lineQ). Vertically upward, the sedimentary rocks in the non-evaporitic Middle Al Nakhsh Member takes on an outward appearance which closely resembles that of the Upper Al Nakhsh Member. Fully developed cycles may be denominated as brining-upward cycles reflecting a shallowing-upward trend in a supratidal-dominated regime—see also Warren (1999). Such salina cycles are known from numerous epochs in the geological past, e.g., the Permian Zechstein Basin, where several cycles exist. Although varying in thickness and in completeness, they may be used for basinwide (sequential) stratigraphic correlation (Leyer et al., 1999). The thickness of gypsum seams, the supratidal increments, increases vertically upward in the lithofacies V and centimeterthick LLH stromatolites tend to evolve into giant SH stromatolites in the Middle Al Nakhsh Member. Both changes manifest a gradually shallowing of the basin throughout the Middle Al Nakhsh Member in the evaporite-bearing facies. This shallowing was not a steady process. Some rhizoliths underneath the gypsum seams of the Al Nakhsh 6 unit were interpreted as mangrove swamps caused by aquatic pedologic process. They mark phases during which the basin subsidence was retarded (Fig. 2). In the equivalent section of the Middle Al Nakhsh Member, where gypsum seams are absent, the intertidal to subtidal facies abruptly converts into a continental red bed

92


facies due to an increase in the rate of uplift. Gypsum seams located west of the sulphate line are correlative with collapse breccia and cellular silcretes east of the sulphate line. This relative increase in relief on a rather small scale may have resulted from differential salt dissolution at depth and halokinetic processes along the Dukhan Anticline which gave rise to a rugged topography in the basin with swells topped by red beds and evaporites precipitating in some local sinks. A fairly high degree of physical and chemical stress and strongly saline conditions are achieved in the Al Nakhsh Members. Corbula, Cardita, and the Cerithidae seem(ed) to be very tolerant as to such very inhospitable conditions. Present-day representatives of gastropods and bivalves and their Neogene predecessor which live in intertidal to supratidal environments indicate that there was no significant change as their habitats and these invertebrates got well adapted to the physico-chemical conditions (Mandic et al., 2002). The substrate and the stenohyline conditions contributed to the small number of species in the Al Nakhsh Members. Bivalves were preserved disarticulated and articulated. Reworking and transport of fossils may be neglected in this case. The fossil assemblage reflect the community of invertebrates which lived in that area. In some shell lags disarticulated valves are aligned in stable position with their convex side up and the breakage of shells only at a moderate level. They reflect energy conditions as they might be expected from tempestites. The sequences with different types of stromatolites were interrupted by successive key-beds ravining into the underlying limestones and interpreted as calcareous tempestite storm deposit as shown from evaporite sequences of Tunisia (Kamoun et al., 2001). The thicker sequences of shell hash bound to oolites are caused by short-lasting ingressions along (sub)tidal channels or wave-induced currents which swept into the intertidal environment (Fig. 2). 5.5.6. The Upper Al Nakhsh Member—lithofacies association VI The red bed facies of lithofacies association VI resembles in many respects that of the non-evaporite facies equivalent of the Middle Al Nakhsh Member (lithofacies association V). The lack of depositional sedimentary structures and the homogenization in the multicolored argillaceous beds may have been brought

about by the calcretisation (Al Nakhsh 16 and 17). Based on the criteria compiled by Mack and James (1992) and Pimentel et al. (1996) the mottled massive rocks devoid of any rhizocretions are likely to have formed under the influence of a fluctuating water table rather than by pure pedogenic processes under aquatic conditions. In contrast with the ichnofabric mentioned in the previous chapters, there are no burrows or textures which might indicate that fauna was responsible for the bioturbation. Lithofacies association VI is characterized by a lateral facies change much stronger than anywhere else in the study area and erosional/ bounding surfaces play a more important role than in lithofacies V. In the reference cross-section of Fig. 2 a salina-sabkha cycle is shown. This gypsum-bearing coarsening-upward cycle means that the supratidal regime known from the Middle Al Nakhsh Member has taken to the extreme in the Upper Al Nakhsh Member. It is the most landward (inland sabkha) equivalent of the Al Nakhsh Member. It passes into mottled argillaceous calcretes which evolved on top of shoals in the sabkha or may grade into arenaceous aeolian deposits (Fig. 3). Mega cross-bedding in the Upper Al Nakhsh Member with foresets dipping at an angle of 278 suggests that these clastic sediments are of aeolian origin (Clarke and Rendell, 2003). The azimuth of the direction of transport did not change very much. The arrow in Fig. 3 showing the azimuth of the mean vector points to 1978. The wind was blowing towards the present-day coast line where it brought about barchanlike ridge dunes. 5.5.7. Abu Samrah Member-lithofacies VII Fining-upward cycles alternating with coarseningupward cycles, strong and abrupt changes in the lithology as well as a great variety of cross-bedding types attest to fluctuating energy regimes (Fig. 2). A conspicuous change in shape and dip angle of cross-bedding towards younger series is provoked by an increase in flow strength. The calcarenites in the Abu Samrah Member were deposited in a highenergy near-shore marine environment with its flow strength increasing towards younger series. Herringbone cross-bedding is undoubtedly the result of strong tidal currents whose mean flow velocity gradually decreased towards younger series (Fig. 2). The lowest level in flow strength is achieved in the bedsets displaying wave ripple cross-lamination.


These hydrodynamic conditions provoked development of well sorted calcareous sediment. The onset of the Abu Samrah Member is marked by a hardground. It is equivalent to a new transgressive phase. The mesoto macrotidal regime known from the Al Nakhsh Member is replaced by a microtidal regime (Fig. 2). Tidal flats deduced from the litholog of Abu Samrah 3, 4, and 5 may rather be termed mudflats (Warren, 1999; Fig. 2). In the supratidal to continental Upper Al Nakhsh Member the tidal forces have already come close to nil. Supratidal sediments deposited above normal or mean high tide are only flooded by spring tides or episodic storms. In the Abu Samrah Member the marine setting has almost completely turned from a tide-dominated into wave-dominated beach environment. The CU sequences of Abu Samrah 1 and 2 are interpreted as coastal/beach ridge which were basinward attached at different distances off-shore to the abandoned sabkha of the Al Nakhsh Members. Vertical upward in the stratigraphic column two FU sequences– Abu Samrah 3 and 4–mark a breakthrough in the beach ridge in form of tidal channels (re-entrant) which basinward faded out into a tidal delta and landward into some mudflats (Abu Samrah 5). This is the only environment in the Abu Samrah Member where subordinate amounts of gypsum concretions were produced by supratidal evaporation and thin LLHstromatolite may still be recognized. The calcareous beds in Abu Samrah 6 immediately beneath the unconformity were named beachrocks that underwent calcitic cementation (Fig. 5D). It is overlain by gravelly sediments of the Hofuf Formation that is alluvial to fluvial by origin. Modern mollusks and their ancient equivalents show good accordance with respect to their living habitat. Hydrobia is a snail that needs a wet habitat to be active either covered by seawater or by moving in fluid layers for low-tide conditions (Armonies and Hartke, 1995). Not surprisingly, these gastropods appear in a great number in the Abu Samrah Member as the supratidal/continental environment of the Upper Al Nakhsh Member became re-inundated.

6. Summary and conclusions During the Early Eocene dolomitic and evaporitic limestones of the Rus Formation were laid down on a

93

shallow shelf (Dill et al., 2003). In the map of Fig. 1a the aerial extension of sulphate in the subsurface is indicated by the stippled line and in Fig. 1b the lateral facies changes of the Rus Formation are displayed. During the Oligocene the Qatar dome was uplifted (Cavelier, 1970). Salt was dissolved in the underground during that uplift provoking basins and swells and a conspicuous facial differentiation in the succeeding Dam Formation (Fig. 1b). Miocene deposits are found in a narrow seaway on the western edge of Qatar (Fig. 1a). The sulphate line delineating the zone barren with respect to sulphate in the underlying Rus Sabkha coincides with the boundary between the evaporitic and non-evaporitic facies of the Miocene Middle Al Nakhsh Member (Figs. 1a and 3). West of this boundary gypsum developed and was well preserved in the Al Nakhsh Area. Towards the east the seams are replaced by dolomitic limestones, cellular dolomites, red beds, and halite (only dolines indicate the former presence of salt). Provenance studies and paleocurrent measurements carried out in sedimentary rocks of the Salwa Member suggest a transport of debris from a source area, what is called today Mesopotamia, being located NW to NNW of present-day Qatar. In the Lower and Middle Al Nakhsh Member the continental run-off in the basin under study came almost to a halt. A NE-trending paleocurrent was superimposed on the aforementioned NNW paleocurrent trend. In the Upper Al Nakhsh Member coastward winds were blowing from the NE. During deposition of the Al Nakhsh Member, mineral zonation in the basin still follows the NNW–SSE trend that is marked by the axis of the Dukhan Anticline. Primary gypsum I developed in the south while secondary gypsum II formed in the north. Palygorskite and smectite developed off-shore in the south, kaolinite and illite settled near-shore in the north. The widespread occurrence of mega crystals of secondary gypsum II in the Middle Al Nakhsh Member is due to rehydration. Alternating burial at shallow depth and exhumation, favorable porosity and permeability conditions created by the microbial domes and mats as well as a high concentration of sulphate in the pore water were crucial for the precipitation of gypsum II. From the sequence stratigraphic point of view, the maximum flooding surface is likely to lie within the Middle Salwa Member (Fig. 2). The succeeding evaporite-bearing sediments of the Middle Al Nakhsh through Abu Samrah Members

94


could be highstand deposits. Envisaging a Middle to Upper Miocene age for the Dam Formation would well agree with the chronological assumptions of Cavelier (1970).

Acknowledgment The senior author is grateful for the support provided by the College of Science, University of Qatar. Gratitude is especially expressed to the Scientific and Applied Research Centre (SARC) which was catering for the transport in the field. We are indebted to the staff members of the Qatar Centre of GIS who did not spare efforts in their assistance and provision with topographic data. Chemical analyses have been performed in the chemical labs of the Federal Institute for Geosciences and Natural Resources, Hannover, under the conductance of D. Rammlmair. The paper was reviewed by B. Sellwood, A.S.Alsharhan, and another anonymous reviewer. Their comments to the manuscript are acknowledged with thanks.

References Aitken JD. Classification and environmental significance of cryptmicrobial limestones and dolomite with illustrations from the Cambrian and Ordovician of southwestern Alberta. J Sediment Petrol 1967;37:1163 – 78. Al-Hinai KG, Dabbagh AE, Gardner WC, Khan MA, Saner S. Shuttle imaging radar views of some geological features in the Arabian Peninsula. GeoArabia 1997;2:165 – 78. Al-Saad H, Ibrahim MI. Stratigraphy, micropaleontology, and paleoecology of the Miocene Dam Formation, Qatar. GeoArabia 2002;7:9 – 28. Alsharhan AS, Nairn AEM. Tertiary of the Arabian Gulf: sedimentology and hydrocarbon potential. Palaeogeogr Palaeoclimatol Palaeoecol 1995;114:369 – 84. Armonies W, Hartke D. Floating of mud snails Hydrobia ulvae in tidal waters of the Wadden Sea, and its implications in distribution patterns. Helgol Meeresunters 1995;49:529 – 38. Barwis JH, Hayes MO. Regional patterns of modern barrier island and tidal inlet deposits as applied to paleoenvironmentals studies. In: Ferm JC, Horne JC, editors. Carboniferous depositional environments in the Appalachian region Carolina Coal Group. University of South Carolina; 1979. p. 472 – 98. Berner Z, Stqben D, Leosson M, Beushausen M, Klinge H. Sand O-isotopy of dissolved sulphate in the cover rock aquifer of a Permian salt dome. Appl Geochem 2002;17: 1515 – 28.

Boukhary M. Paleontological studies on the Eocene succession in western Qatar Arabian Gulf. Rev Paleóbiol 1985;4:183 – 202. Browne KM, Golubic S, Seong-Joo L. Shallow marine microbial carbonate deposits. In: Riding RE, Awramik SM, editors. Microbial sediments. Springer-Verlag; 2000. p. 233 – 49. Burne RV, Moore LS. Microbialites: organosedimentary deposits of benthic microbial communities. Palaios 1987;2:241 – 54. Canfield DE. Isotope fractionation by natural populations of sulphate-reducing bacteria. Geochim Cosmochim Acta 2001; 65:1117 – 24. Cavelier C. Geologic description of the Qatar Peninsula (Arabian Gulf). Publ Government of Qatar, Dept of Petroleum Affairs; 1970. [39 pp]. Clarke ML, Rendell HM. Late Holocene dune accretion and episodes of persistent drought in the Great Plains of Northeastern Colorado. Quat Sci Rev 2003;22:1051 – 8. Claypool GE, Holser WT, Kaplan IR, Sakai H, Zak I. The age curves of sulfur and oxygen isotopes in marine sulphate and their mutual interpretation. Chem Geol 1980;28:199 – 260. Debenay J-P, Beńe´teau E, Stouff V, Geslin E, Redois F, FernandezGonzalez M. Ammonia beccarii and Ammonia tepida (Foraminifera): morphofunctional arguments for their distinction. Mar Micropaleontol 1998;34:235 – 44. Dill HG. A review of heavy minerals in clastic sediments with case studies from the alluvial-fan through the nearshore-marine environments. Earth-Sci Rev 1998;45:103 – 32. Dill HG, Kfthe A, Gramann F, Botz R. A palaeoenvironmental and paleoecological analysis of fine-grained Paleogene estuarine deposits of North Germany. Palaeogeogr Palaeoclimatol Palaeoecol 1996;124:273 – 326. Dill HG, Nasir S, Al-Saad H. Lithological and structural evolution of the northern sector of the Dukhan Anticline, Qatar, during the early Tertiary: with special reference to bounding surfaces of sequence stratigraphical relevance. GeoArabia 2003;8:201 – 26. Dunham RJ. Classification of carbonate rocks according to depositional texture. In: Ham WE, editor. Classification of Carbonate Rocks. American Association of Petroleum Geology Memoir, vol. 1. 1962. p. 108 – 21. El Beialy SY, Al-Hitmi HH. Micropalaeontology and palynology of the lower and middle cretaceous thamama and wasia groups, DK-C well, Dukhan oil field, western Qatar. Sci Geól, Bull 1994;47:67 – 95. Emrich K, Ehhalt DH, Vogel JC. Carbon isotope fractionation during the precipitation of calcium carbonate. Earth Planet Sci Lett 1970;8:363 – 71. Flqgel E. Mikrofazielle Untersuchungsmethoden von Kalken. Berlin7 Springer-Verlag; 1978. [633 pp]. Folk RL. Practical petrographic classification of limestones. Am Assoc Petrol Geol 1959;19:730 – 81. Fookes PG. The geology of carbonate soils and rocks and their engineering characterisation and description. In: Jewell RJ, Korshid MS, editors. Engineering for Calcareous Sediments. Rotterdam7 Balkema; 1978. p. 787 – 806. Fqchtbauer H, Goldschmidt H. Beziehungen zwischen Calciumgehalt und Bildungsbedinungen der Dolomite. Geol Rundsch 1965;55:29 – 40.

H.G. Dill et al. / Sedimentary Geology 174 (2005) 63–96 Garcia-Guinea J, Morales S, Delgado A, Recio C, Calaforra JM. Formation of gigantic gypsum crystals. J Geol Soc (Lond) 2002;159:347 – 50. Hedberg HD, George N. International stratigraphic guide—a guide to stratigraphic classification, terminology and procedure. New York7 Wiley; 1976. [200 pp]. Hsq KJ, Siegenthaler C. Preliminary experiments on hydrodynamic movement induced by evaporation and their bearing on the dolomite problem. Sedimentology 1969;12:11 – 25. James NP, Bourque P-A. Reefs and mounds. In: Walker RG, James NP, editors. Facies Models: Response To Sea Level Change. Geol Assoc Can; 1992. p. 323 – 47. Kamoun F, Peyberne`s B, Ciszak R, Calzada S. Triassic palaeogeography of Tunisia. Palaeogeogr Palaeoclimatol Palaeoecol 2001;172:223 – 42. Keeling CD. The concentration and isotopic abundance of carbon dioxide in rural areas. Geochim Cosmochim Acta 1958; 13:322 – 34. Kendall AC. Evaporites. In: Walker RG, James NP, editors. Facies models-response to sea level change. Geol Assoc Canada; 1992. p. 375 – 409. Leinfelder RR, Schmid DU. Mesozoic reefal thrombolites and other microbolites. In: Riding RE, Awramik SM, editors. Microbial sediments. Berlin7 Springer; 2000. p. 289 – 94. Leyer K, Strohmenger C, Rockenbauch K, Bechst7dt T. Highresolution forward stratigraphic modeling of Ca2-carbonate platforms and off-platform highs (Upper Permian, Northern Germany). In: Harff J, Lemke W, Stattegger K, editors. Computerized Modeling of Sedimentary Systems. Berlin7 Springer; 1999. p. 307 – 39. Logan BW, Rezak R, Ginsburg RN. Classification and environmental significance of algal stromatolites. J Geol 1964; 72:68 – 83. Logan BW, Hoffman P, Gebellin CD. Algal mats, cryptalgal fabrics and structures, Hamelin pool, western Australia. AAPG Mem 1974;22:141 – 94. Lu FH, Meyers WJ, Schoonen MA. S and O(SO4) isotopes, simultaneous modeling, and environmental significance of the Nijar Messinian gypsum, Spain. Geochim Cosmochim Acta 2001;65:3081 – 92. Mack GH, James WC. Calcic paleosols of the Plio-Pleistocene Camp Rice and Palomas Formations, southern Rio Grande rift, USA. Sediment Geol 1992;77:89 – 109. Mandic O, Harzhauser M, Spezzaferri S, Zuschin M. The paleoenvironment of an early Middle Miocene Paratethys sequence in NE Austria with special emphasis on paleoecology of mollusks and foraminifera. Geobios 2002;35:193 – 206. McCrea JM. The isotope chemistry of carbonates and a paleotemperature scale. J Chem Phys 1950;18:849 – 957. McKenzie JA, Hsq KJ, Schneider JF. Movement of subsurface waters under sabkha, Abu Dhabi, UAE, and its relation to evaporative dolomite genesis. In: Zenger DH, Dunham JD, Ethington RC, editors. Concepts and Models of Dolomitization. Spec Publ Soc Econ Palaeont Mineral. vol. 28. Tulsa7 American Association of Petroleum Geologists; 1980. p. 11 – 30. Mizutani Y, Rafter TA. Oxygen isotopic composition of sulphates: Part 4 Bacterial fractionation of oxygen isotopes in the reduction of sulphate and in the oxidation of sulphur. NZ J Sci 1969;12:60 – 8.

95

Murris RJ. The Middle East: stratigraphic evolution and oil habitat. Am Assoc Pet Geol Bull 1980a;64:597 – 618. Murris RJ. Hydrocarbon habitat of the middle East. In: Miall AD, editor. Facts and Principles of World Petroleum Occurrence. vol. 6. Canad Soc of Petrol Geol Mem; 1980b. p. 765 – 800. Nagy J. Environmental significance of foraminiferal morphogroups in Jurassic North Sea deltas. Palaeogeogr Palaeoclimatol Palaeoecol 1992;95:111 – 34. Neumeier U. Experimental modeling of beachrock cementation under microbial influence. Sediment Geol 1999;126: 35 – 46. Nriagu JO, Rees CE, Mekhtiyeva A, Lein Yu, Fritz P, Drimmie RJ, Pankina RG, Robinson RW, Krouse HR. Hydrosphere. In: Krouse HR, Grinenko VA, editors. Stable Isotopes: Natural and Anthropogenic Sulphur in the Environment, SCOPE vol. 43. Wiley and Sons; 1991. p. 1 – 26. Ohkushi K, Natori H. Living benthic foraminifera of the Hess Rise and Suiko Seamount, central North Pacific. Deep-Sea Res, Part 1, Oceanogr Res Pap 2001;48:1309 – 24. Paytan A, Kastner M, Campbell D, Thiemens MH. Sulfur isotopic composition of Cenozoic seawater sulphate. Science 1998;282: 1459 – 62. Peebles RG, McBride J, Shaner M. Chemostratigraphy of carbonate-evaporite successions from the southern Arabian Gulf 1997 AAPG international conference and exhibition. AAPG Bull 1997;81:1403. Pierre C. Isotopic evidence for the dynamic redox cycle of dissolved sulphur compounds between free and interstitial solutions in marine salt pans. Chem Geol 1985;53:191 – 6. Pierre C, Rouchy JM. Sedimentary and diagenetic evolution of Messinian evaporites in the Tyrrhenian Sea (ODP Leg 107, sites 652, 653, and 654); petrographic, mineralogical, and stable isotope records. In: Stewart N, editor. Proceedings of the Ocean Drilling Program, vol. 107. Scientific Results; 1990. p. 187 – 210. Pimentel NL, Wright VP, Azevedo TM. Distinguishing early groundwater alteration effects from pedogenesis in ancient alluvial basins: examples from the Paleogene of southern Portugal. Sediment Geol 1996;105:1 – 10. Playa E, Orti F, Roselli L. Marine to non-marine sedimentation in the upper Miocene evaporites of the Eastern Betics, SE Spain; sedimentological and geochemical evidence. Sediment Geol 2000;133:135 – 66. Pletsch T, Barrera E, Botz R, Hisada KI, Kajiwara Y. Marine palygorskite clay and early Paleogene oceanography. Mitt Ges ¨ sterr 2000;43:106 – 7. Geol Bergbaustud O Powers, RW. Lexique stratigraphique international Asie, vol IIISaudi Arabia, CNRS, Paris, 1968. 177 pp. Powers RW, Ramirez LF, Redmont CD, Elberg EL. Geology of the arabian peninsula Sedimentary geology of Saudi Arabia. US Geol Surv Prof Paper. vol. 560 D. Washington7 US Government Printing Office; 1966. [147 pp]. Pratt BR. Tepees in peritidal carbonates: origin via earthquakeinduced deformation, with example from the Middle Cambrian of western Canada. Sediment Geol 2002;153:57 – 64. Renema W, Troelstra SR. Larger foraminifera distribution on a mesotrophic carbonate shelf in SW Sulawesi (Indonesia). Palaeogeogr Palaeoclimatol Palaeoecol 2001;175:125 – 46.

96


Saffert H, Thomas E. Living foraminifera and total populations in salt marsh peat cores: Kelsey Marsh (Clinton, CT) and the Great Marshes (Barnstable, MA). Mar Micropaleontol 1998;33:175 – 202. Samir AM, El-Din AB. Benthic foraminiferal assemblages and morphological abnormalities as pollution proxies in two Egyptian bays. Mar Micropaleontol 2001;41:193 – 227. Sch7fer A, Hilger D, Gross G, von der Hocht F. Cyclic sedimentation in tertiary lower-rhine basin (Germany)—the Liegendrqcken of the brown-coal open-cast Fortuna mine. Sediment Geol 1996;103:229 – 47. Scoffin TP. An introduction to carbonate sediments and rocks. Glasgow7 Blackie; 1987. [274 pp]. Seilacher A. Bathymetry of trace fossils. Mar Geol 1967;5:413 – 28. Sha LP. Sedimentological studies of the ebb-tidal deltas along the West Frisian Islands, the Netherlands. Geol Ultraiectina 1990:(64). [160 pp]. Strauss H. The isotopic composition of sedimentary sulfur through time. Palaeogeogr Palaeoclimatol Palaeoecol 1997;132:97 – 118. Sugden W. Structural analysis, and geometrical prediction for change of form with depth, of some Arabian plains-type folds. Bull Am Assoc Pet Geol 1962;48:2213 – 28.

Thode HG, Monster J. Sulfur-isotope geochemistry of petroleum, evaporites, and ancient seas. Fluids in subsurface environments—a symposium. Mem-Am Assoc Pet Geol 1965; 367 – 77. Tucker M. Sedimentary petrology. London7 Blackwell; 2001. [262 pp]. Van der Zwaan GJ, Duijnstee IAP, den Dulk M, Ernst SR, Jannink NT, Kouwenhoven TJ. Benthic foraminifers: proxies or problems? A review of paleoecological concepts. Earth-Sci Rev 1999;46:213 – 36. Warren JK. Evaporites—their evolution and economics. Oxford7 Blackwell; 1999. [438 pp]. Weaver CE. Clays, muds, and shales. Amsterdam7 Elsevier; 1989. [819 pp]. Wilson JG. Productivity, fisheries and aquaculture in temperate estuaries. Estuar Coast Shelf Sci 2002;55:953 – 67. Wright VP. A revised classification of limestones. Sediment Geol 1992;76:177 – 85. Zonneveld J-P, Gingras MK, Pemberton SG. Trace fossil assemblages in a Middle Triassic mixed siliciclastic-carbonate marginal marine depositional system, British Columbia. Palaeogeogr Palaeoclimatol Palaeoecol 2001;166:249 – 76.