Sedimentology and depositional environments of the ...

Arab J Geosci (2017) 10:178 DOI 10.1007/s12517-017-2977-1

ORIGINAL PAPER

Sedimentology and depositional environments of the Ordovician Umm Sahm Sandstone Formation in southern Jordan Issa Makhlouf 1 & Abdalla Abu Hamad 2 & Basem Moh’d 3

Received: 12 January 2016 / Accepted: 29 March 2017 # Saudi Society for Geosciences 2017

Abstract The Ordovician Umm Sahm Sandstone Formation of Jordan comprises approximately 200-m-thick succession of fluvial quartzarenites with subordinate claystone and siltstone lithologies of shallow marine conditions. The Umm Sahm Formation is characterized by its dark brown color, frequent jointing, and steep scarps. The Umm Sahm Formation is bounded by the marine claystones of Hiswah Formation at the top and the fluvial sandstones of the Disi Formation at the bottom. The Umm Sahm Formation is composed of two main facies: fluvial facies and tidal facies. The fluvial facies constitutes about 93% of the total thickness. The lower few meters of the succession passes upward from the Disi Sandstone Formation into similar massive white sandstone facies exhibiting similar white color, fine- to coarse-grained sandstone, with round-shaped pebbles. Trough and planar cross-bedding show a northwest unidirectional palaeocurrent trend. Light brown colored quartzarenites similar to those of the Cambrian Umm Ishrin Sandstone Formation are most common in the upper part of the succession. The tidal facies occupies three intervals in the middle part of the succession. It is composed of laminated and thin-bedded sandstones, siltstones, and claystones. They are rippled and varicolored with abundant trace fossils (Cruziana, Harlania, ruzophycus). The presence of hummocky cross stratification indicates the

* Issa Makhlouf [email protected]

1

Department of Earth and Environmental Sciences, Hashemite University, Zarqa, Jordan

2

Department of Applied and Environmental Geology, University of Jordan, Amman, Jordan

3

Department of Civil Engineering, Applied Sciences Private University ASU, Amman, Jordan

earliest short-lived tempestite conditions during the Paleozoic erathem of Jordan. The first appearance of Graptolites in the Ordovician rocks of Jordan was recorded during this study in the tidal facies of the Umm Sahm Formation. The vertical arrangement of both fluvial and tidal facies indicates three successive short-lived transgressions and regressions. These marine incursions indicate the successive shoreline advances of the Tethys Ocean, which was located northward, and inundated the southern braid plain. The three short-lived transgressive events took place, and the Tethys marine margin was displaced southward, giving rise to deposition of tidal facies in an open coast tidal flat. Following the transgressive events, renewed progradation and strandline retreat took place, fed by large amounts of siliciclastics derived from the continent and transported by braided streams across the intertidal zone. Keywords Umm Sahm Formation . Jordan . Ordovician . Fluvial . Tidal

Introduction and objectives The Palaeozoic succession in Jordan comprises sediments of Cambrian to Silurian age, formed by an up to 1800-m-thick succession of fluvial, glaciofluvial, and marine siliciclastic sediments with rare carbonates. The succession was subdivided into two groups: Ram and Khreim. The Ram Group includes the Cambrian Salib and Umm Ishrin formations, the Cambro-Ordovician Desi Formation and the Early Ordovician Umm Sahm Formation. The Khreim Group encompasses the Silurian Dubaydib, Mudawwara, Ammar, and Khushsha formations. The arrangement of the facies suggests their deposition along the margins of a continental seaway in several environmental settings from continental to marine

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environments (Makhlouf 1992, 1995, 1998; Selley 1972; Powell 1989; Amireh et al. 2001; Abed et al. 1993; Turner et al. 2005; Armstrong et al. 2005, 2009). Sedimentation of the Ordovician succession occurred on the margins of the Palaeotethys ocean at the northern margins of the Gondwana land (Quennell 1951; Burdon 1959; Lloyd 1968; Bender 1968). The Early Ordoviacian Umm Sahm Formation studied here is relevant to understand the sedimentary conditions in these marginal areas, as this unit records both continental and marine facies in response to relative sea-level fluctuations, that preceeded the complete submergence of the marginal area. The underlying CambroOrdovician Desi Formation was deposited in a braided fluvial environment. Nevertheless, shortly before the end of the Disi times, a short-lived incursion of the sea was responsible for the deposition of a Cruziana-rich siltstone horizon. This marine incursion was followed by a rapid retreat of the sea shoreline, which allowed the reestablishment of the fluvial conditions. These were the depositional conditions before the deposition of the Umm Sahm Formation. This unit was developed in the context of a gradual rise of sea level that was followed by a further southward transgression and deposition of tidal and deep shelf Hiswa and Dubaydib formations (Makhlouf 1992, 1995, 1998; Selley 1972; Powell 1989; Amireh et al. 2001; Abed et al. 1993; Turner et al. 2005; Armstrong et al. 2005, 2009). This paper aims to reconstruct the main depositional environments of the Lower Ordovician Umm Sahm succession, after looking in detail at the stratigraphy and the sedimentary facies, including fossil content of the sediments, geometry, Fig. 1 Location map showing the outcrops of the Umm Sahm Sandstone Formation and the studied stratigraphic log in the study area at Wadi Al Hiswah, South Jordan (after Qannassiya map and report, Khalil 1989a, b, 1994)

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sedimentary structures, and facies architecture in an stratigraphic reference section of the unit located near Wadi Al Hiswa, in southern Jordan. Although there are some similarities with the sedimentary facies and depositional environments described in previous studies of the Umm Sahm strata (Selley 1970, 1972; Powell 1989; Amireh et al. 2001), the facies associations and sedimentary structures and stacking patterns display notable differences.

Geological and geographical context of the studied stratigraphic section The Umm Sahm Formation is a sandstone-dominated succession, ∼200 m thick, well exposed in the southern desert of Jordan (Fig. 1). The name of this unit was first used by Quennell (1951), Burdon (1959), and Lloyd (1968) and is derived from Jabal Umm Sahm in the southern desert, which is located to the southeast of Wadi Ram. This formation was named by Bender (1974) as the Bedded Brownish Weathered Sandstone. The studied stratigraphic section locates near Wadi Al Hiswa, in the greographical coordinates 29° 36′ 36.94″ N, 35° 41′ 14.23″ E (Fig. 1). The synthethic log including the vertical distribution of main sedimentary facies of the Umm Sahm Formation is offered in Fig. 2, and general and detailed images of the outcrop are included in Figs. 3 and 4. The unit is characterized by laterally persisting tabular bedding with steep scarps, highly oxidized rock surfaces with dark black tarnish films and extensive joins (Figs. 3 and 4).


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Fig. 2 Synthetic stratigraphic log of the Umm Sahm Sandstone Formation

The dark color of sandstone weathered surfaces is a few microns thick superficial film, which is a result of desert tarnish (oxidation). This gives the Umm Sahm outcrops a diagnostic coloring feature among the Ram Group units (Fig. 4a). The extensive joints that characterize the Umm Sahm outcrops occasionally obscure the bedding planes and other sedimentary features (Fig. 4b). Two sets of joints were observed: a predominant set trending roughly NNW-SSE (150°–300°), and a subsidary set trending NNE-SSW (20°, 200°) (Fig. 3a, b). Intersecting joint sets are encountered (Fig. 3c). Some of these joints are up to 25 cm wide and are infilled by clayey material with rock fragments up to 5 cm in diameter (Fig. 3d). The color and extensive joints of the Umm Sahm Formation allow its distinction form the underlying and overlying units

(Fig. 2). The lower boundary of the sandstones of the Umm Sahm Formation is transitional with the fluvial sandstones of the Disi Sandstone Formation (∼330 m thick). Nevertheles, both units are clearly distinguishable by their different color and hardeness, from white friable sandstones of the Disi Sandstone Formation to dark hard sandstones of the Umm Sahm Formation (Fig. 4c). The Umm Sahm Formation is overlaid by the marine claystones of the Hiswah Formation (∼60 m thick; Fig. 4d) (Moumani 2003). The Hiswah Formation is rich in the graptolite Didymograptus bifidus, which indicates a Lower Llanvirn age. This unit is the Graptolite Sandstone Formation of Bender (1963, 1968, 1974), the base of Khreim Group of Lloyd (1968), and correlates with the Hanadir Shale member of the Tabuk Formation of Saudi Arabia (Helal 1965).

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Fig. 3 Extensive joins in the Umm Sahm Formation. a Joint set with narrow spacing. b Joint set with clay and clasts infill. c Two intersecting sets of joints. d Close-up of jointed rocks with wide spacing infilled with claystone and rock fragments (hammer is 28 cm long)

Fig. 4 a General view of Umm Sahm Sandstone Formation in the studied area. b Extensive jointing of the Umm Sahm Formation. c Lower contact with the Disi Sandstone Formation. d Upper contact with the Hiswah Formation



Although the Early Ordovician sedimentation experienced a braided fluvial system during which most of the Disi Sandstone Formation, thin marine intervals, ∼20 m thick, with some ichnofossils (Cruziana, Harlania, ruzophycus) deposited during the late Disi time (Powell 1989; Amireh et al. 2001).

Facies analysis Two main facies, fluvial and tidal facies, characterize the Umm Sahm Formatiom at the Wadi Al Hiswah stratigraphic section (Fig. 2).

Fig. 5 Stratigraphic log shows the stratigraphic positions of the fluvial facies and tidal facies of Umm Sahm Formation

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Fluvial facies The fluvial facies comprises the bulk of the Umm Sahm Formation (about 93%) (Fig. 5) and is mainly composed of brownish weathered quartzarenitic pebbly sandstones, rich in quartz granules and pebbles (rosy, milky and smoky colored up to 2 cm in diameter), and flat intraclasts (up to 3 cm long and 5 mm thick). This facies has trough cross-bedding, with foresets commonly graded (small pebbles and granules to coarse-grained sandstone) (Fig. 6a–c), and occasional planar cross-bedding (Fig. 6d). Sets of trough cross-bedding are 25 to 55 cm thick and 150 to 370 cm wide. Palaeocurrents are mainly orientated

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towards the northwest direction and occasionally to the north and northeast (Table 1 and Fig. 7). The presence of quartz pebbles (up to 3 cm long, Fig. 6e), groove casts (at the channel floor, Fig. 6f), and parting lineation are indicators of high energy and upper flow regime conditions. Some beds within this facies are characterized by overturned (trough or planar) cross-bedding which are orientated almost in the same direction of the original cross-beds. Deformation of sediments occurs either contemporaneous with or shortly after deposition, while sediments remain water saturated, as a result of liquefaction, slumping, or seismic shocks (Allen and Banks 1972; Makhlouf and Abed 1991). The deformation ranges from simple synsedimentary recumbent folds to complex convoluted forms. These result from the oversteepening of cross-laminae to form a single recumbent fold whose axial plane is parallel or nearly parallel to the base

Fig. 6 Field images of sedimentary structures in fluvial facies. a Trough cross-bedding. b, c Graded trough cross-bedding foresets. d Planar cross-bedding. e Pebbly sandstone (up to 3 cm long). f Groove casts at the channel floor (hammer is 28 cm long and pen is 15 cm long)

Table 1 Palaeocurrent measurements in fluvial facies of the Umm Sahm Formation

Trough cross-bedding 320°

240°

345°

335°

352°

360°

240°

10°

350°

340°

280°

300°

and top of the deformed stratum. The fold nose (hinge zone) in a cross-bedded unit may be at almost any level, although it commonly occurs in the central portion (Allen and Banks 1972; Makhlouf and Abed 1991). The overturned cross-bedding mostly shows palaeocurrent orientation towards the northwest direction. Deformed crossbed sets are contorted and overturned at their basal (Fig. 8a),


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Rose Diagram Title N (330.6)

W

E

middle (Fig. 8b), and top parts (Fig. 8c, d), whereas others are S-shaped due to increase of pressure exerted on the top of the cross-bed set (Fig. 8e, f). Superimposed shallow channels with variable dimensions ranging from 1 to 2 m deep, and 2 to 6 m wide are also present. Common channel floor strewn pebbles and granules are indicative of upper flow regime conditions (Fig. 9). The predominance of cross-bedded sandstone, the presence of channel fills, the high proportion coarse sediments (pebbly sandstones), and the relative to the low proportion of fine sediments (claystones and siltstones) are indicative of braided fluvial facies (Fig. 9). Tidal facies

S

Fig. 7 Rose diagram of the palaeocurrent measurements in the fluvial facies of the Umm Sahm Formation (number of measurements = 12; see Table 1) Fig. 8 Overturned cross-bed sets in fluvial facies occurring at a basal part, b middle part, c, d upper part (note truncated foresets and tangential bottomsets), and e, f overturned cross-bed showing S-shaped deformation (hammer is 28 cm)

The tidal facies is located in the middle part of the succession and occurs in three stratigraphic levels (2, 0.5, and 7 m, respectively) sandwiched between the fluvial facies (Figs. 5 and 10). This facies is composed of siltstones and claystones, with

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Fig. 9 a Adjacent superimposed channels. b Close-up to show the lensoidal sand bodies of channel infill. c Quartz pebbles strewn channel floor (coin is 3 cm in diameter). c Dark stained channel floor (hammer is 28 cm long)

subordinate thin-bedded and laminated fine-grained sandstones. They are easily identified by their fine-grained nature and gray greenish color, with subordinate chocolate brown and violet colors (Fig. 10). Bedding planes show abundant trace fossils (Fig. 11). The most frequent traces are intersecting Cruziana goldfusi traces (Fig. 11a, b) and Harlania and Cruziana rusophycus traces (Fig. 11b, c). Graptolites have also been observed in this tidal facies (siltstones) for the first time in Umm Sahm Formation (Fig. 11d). Siltstones, claystones, and thin-bedded and laminated finegrained sandstones form rhythmites (tidalites) with flaser bedding and oscillatory ripples (Fig. 12a, b). Hummocky cross stratification (HCS) and swaley cross stratification have also been observed for the first time in the Umm Sahm Formation, indicating short-lived storms events during deposition (Harms et al. 1975; Dott and Bourgeois 1982) (Fig. 13a, b). The wavelength of the (HCS) is about 1 m which is relatively short as compared with those deposited on the shelf (>2 m) as reported by Yang et al. (2005). This may suggest their deposition in an open coast tidal flats (Yang et al. 2005; Basilici et al. 2012).

bedding, with subordinate planar cross-bed sets, indicate deposition in the proximal setting of a braided fluvial system. The dominant palaeocurrents towards the north, i.e., towards the Palaeotethys strandline, indicates that the provenance terrain was located to the south of the basin during the Early Ordovician times (Torsvik and Cocks 2009) (Fig. 14). This is in accordance with previous authors (e.g., Bender 1974; Amireh et al. 2001). The dominance of trough cross-bedding is more common in the braided rivers where the hydrodynamic energy is high, and the upper flow regime conditions prevail.

Interpretation The Umm Sahm Formation is widespread throughout the southern desert of Jordan and northern Saudi Arabia. The non-fossilliferous, coarse-grained channels infill, the sedimentary structures including abundant unidirectional trough cross-

Fig. 10 Contact between the greenish gray, laminated, fine-grained sandstone tidal facies (third level) and the quartzarenite fluvial facies (see Figs. 2 and 5) (hammer is 28 cm long)

Arab J Geosci (2017) 10:178 Fig. 11 Tidal facies shows bedding planes of marooncolored, fine-grained sandstones displaying a abundant intersecting Cruziana (goldfusi) traces, b, c abundant intersecting Cruziana (goldfusi), Harlania and Cruziana rusphycus traces, and d Graptolite (coin is 3 cm in diameter)

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a

b

c

d

Despite the fluvial character of the Umm Sahm Formation, three marine incursions took place during its deposition (Powell 1989; Amireh et al. 2001). The presence of siltstones and claystones and thin-bedded and laminated fine-grained sandstones with flaser bedding, ripple marks, and trace fossils points to a tidal environment. Interruption by storm conditions occurred, as evidences the presence of hummocky cross-stratification (Harms et al. 1975; Dott and Bourgeois 1982; Makhlouf 1992, 1995, 1998). The occurrence of hummocky cross-stratification and graptolite has been reported from the Umm Sahm Formation for the first time during the present work. The presence of the short wavelength of the HCS in the tidal facies is an indication of the possible development of an open coast tidal flat characterized by the episodic storm action (similar to that described by Yang et al. 2005, in the Yellow Sea coast of Korea, and those described by Basilici et al. 2012, in the Cambrian-Ordovician Lagarto and Palmares formations in north-eastern Brazil), instead of a Fig. 12 Upper level of marine facies showing thin-bedded tidalites displaying flaser bedding with oscillatory ripples. a Tidal facies is truncated by fluvial facies above. b Close-up view of oscillatory ripples (hammer is 28 cm long)

sheltered of protected tidal flat. This finding is relevant for further detailed palaeogeographic reconstructions of the studied area during the Early Ordovician. Deposition of shallow marine sediments at three intervals between the dominant braided fluvial facies indicates three sea transgressions and subsequent regressions took place, in the northern borders of the Gondwanaland (Torsvik and Cocks 2009) (Fig. 14). The topmost fluvial facies of Umm Sahm was truncated by a major transgression of the Palaeotethys Ocean, that gave rise to the prevailance of the marine conditions, characterized by the presence of abundant graptolite, Skolithos and Cruziana rich mudstones of the Hiswa Formation. Marine sedimentation continued during the deposition of the Dubaydib and Mudawwara formations until the end of the Ordovician when the shelf was affected by the Late Ordovician glaciation. At that time, Jordan was situated at ∼65° latitude at the northeastern margin of Gondwana and

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Fig. 13 Fine-grained sandstone marine facies displaying a hummocky cross-stratification (HCS), and b swaley cross-stratification (SCS) (hammer is 28 cm long)

deposition was characterized by the striated, faceted and erratic boulders of the glacio-fluvial Ammar Formation (Abed et al. 1993; Turner et al. 2005; Armstrong et al. 2005, 2009; Torsvik and Cocks 2009). Although there are some similarities with the sedimentary facies and depositional environments described in previous studies of the Umm Sahm strata in the southern desert of Jordan (Selley 1970, 1972; Powell 1989; Amireh et al. 2001), the facies associations and sedimentary structures and stacking Fig. 14 Palaeogeographic map showing the location of Jordan during the Early Ordovician (Arenig) when the Umm Sahm Formation was deposited. The map of Torsvik and Cocks (2009) is slightly modified by highlighting the Gondwanan palaeoshoreline and adding the location of Jordan at that time. The dot indicates the approximate position of the studied stratigraphic section

patterns display three notable differences: (1) the presence of hummocky cross-stratification in the tidal facies have been described for the first time in Umm Sahm succession (Ram Group), and this finding is relevant as it indicates storm action in an open coast tidal flat; (2) the presence of graptolites have been also decribed for the first time in the tidal facies, which is relevant for biostratigraphic zonation and precise age determination; and (3) the reconstruction of a palaeogeographic map to show the location of Jordan during the Early Ordovician


(Arenig Epoch) when the Umm Sahm Formation was laid down (Fig. 14). HCS indicates an open coast tidal flat this is relevant for further detailed palaeogeographic reconstructions of the studied area during the Early Ordovician.

Conclusions Fluvial conditions prevailed throughout the Early Ordovician, as evidenced by the very thick sandstone succession of the Umm Sahm Formation. However, three short-lived sea incursions were responsible for the sedimentation of shallow water marine deposits. The tidal processes were responsible for sedimentation. The associated fluvial/tidal facies (Fig. 5) during Umm Sahm times may be related to fluctuations of the Tethyan strandline due to fluctuations in the relative sea level and the development of transgressive regressive episodes (Makhlouf 2003). Such variations are attributable to minor shifts of the strandline position due to local subsidence-uplift rates. The overall sequence is interpreted in terms of fluctuating fluvial-dominated strandline model as a result of local tectonism and uplift. The three short-lived transgressive events took place, and the Tethys marine margin was displaced southward, giving rise to deposition of tidal facies in an open coast tidal flat (Yang et al. 2005; Torsvik and Cocks 2009; Basilici et al. 2012) (Fig. 14). Following the transgressive events, renewed progradation and strandline retreat took place, fed by large amounts of siliciclastics derived from the continent and transported by braided streams across the intertidal zone. The recognition of these transgressive pulses is important to understand the sedimentary evolution during the Ordovician at the northern margins of the Gondwana land, as they preceded the deposition of the deep water sediments of the Hiswa graptolitic mudstones, which set sharply at the top of Umm Sahm sediments. Acknowledgements The authors wish to thank the Deanship of Scientific Research of the Hashemite University for financial support and facilitating field work. Miss. Reem Al-Shuaibi is thanked for drafting diagrams and Tariq Al Bashiti for lab work. Two anonymous reviewers are thanked for their critical and helpful comments. Prof. Beatriz Badenas (Associate Editor) is thanked for constructive remarks and suggestions.

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