The stratigraphy, depositional environments and periods of deformation of the Wajid outcrop belt, southwestern Saudi Arabia

Jwmal of Africanlhrrk Sciences. Vol. 21. No. 3. pp. 421.441, 1595 Copyright 0 1995 ElscvicrScienceLtd F’rinled ia GreatBrilaio.All rights reserved am-s3a95 $9.50 + 0.00

Pergamon

The stratigraphy, depositional environments and periods of deforination of the Wajid outcrop belt, southwestern Saudi Arabia T. E. STUMP and J. G. VAN DER EEM

Aramco, PO Box 11123, Dhahran 31311, Saudi Arabia (Received 7 December 1994: revised version received 11 July 1995) Abstract - The Wajid outcrop belt in southwestern Saudi Arabia is an area of over 22,000 square kilometres where strata of Middle(?) Cambrian through to Jurassic crop out. The Early Palaeozoic sediments in southwestern Arabia have been placed in the Wajid sandstone. This paper elevates this unit to group status and recogmzes its former members as formations. The remaining Palaeozoic successions are placed in the Qalibah and Khuff ~formations. The Middle(?) Cambrian through to Early(?) Ordovician Dibsiyah Formation, Wajid Group, correlates with the Saq sandstone of central and northern Saudi Arabia. The ‘lower’ Dibsiyah buries a topography develo ed on the Arabian shield during the Late Precambrian Pan-African and/or the late Early Cambrian periods of up &t . The late Early Cambrian period of deformation is documented in southwestern Saudi Arabia and adjacent Oman by an angular unconformity which separates the Dibsiyah Formation correlative from the underlying Late PrecartnbrianEarly Cambrian Lower Haima Group. The ‘lower’ Dibsiyah was deposited in a northward prograding fluvial apron. The ‘upper’ Dibsiyah was deposited in intertidal to shallow subtidal marine settings during a period of coastal onlap. The Late(?) Ordovician Sanamah Formation overlies the Dibsiyah Formation in angular and/or erosional unconformity. This unconformity was developed during a period of regional uplift which began near the close of the Middle Ordovician. The Sanamah Formation was deposited in broad valley complexes excavated in fluvial settings. The Early Sihuian Qusaiba Member of the Qalibah Formation unconformably overlies the Sanamah. The Qalibah Formation was deposited in shallow marine to mid-shelf settings during a major period of coastal onlap. The angular unconformity to disconformity, which separates the Qalibah Formation from underlying units, resulted from diastrophtsm of Late Ordovician to earliest Sihrrian age. The Early Devonian to Middle(?) Carboniferous Khusayyayn Formation unconformably overlies the Qalibah Formation. The Khusayyayn ‘Formation was deposited m shallow marine, lagoonal, eolian and fluvio-deltaic settings. The unconformity separating the Khusayyayn Formation from the Qalibah Formation documents a period of regional diastrophism of late Early Silurian to late Early Devonian age. Unconformably overlying the Khusayyayn is the Middle Carboriiferous through Early Permian Juwayl Formation. This unit was deposited in broad valley complexes excavated in fluvial settings. The base Juwayl unconformity was produced by the Hercynian orogeny. The Permian Khuff Formation unconformably overlies the Juwayl and the former was deposited in shallow marine settings. The base Khuff Formation unconformity resulted from a period of major coastal onlap. The Early Triassic Sudair shale succeeds the Khuff and the former was deposited during a period of coastal offlap in lagoonal to coastal plain settings. Near the end of the Early Triassic the Arabian Peninsula underwent another period of tectonism. In central and southwest Saudi Arabia this event is reflected by the erosion of the Sudair shale and older units from the higher portions of reactivated fault blocks and the syndepositional thickening of the Jilh Formation on the down thrown sides of these fault blocks. R&umC - La chaine de Wajid dans le sud-ouest de YArabie saoudite couvre une surface de 22000 km2 a l’affleurement. Des sediments du Cambrien ?moyen au Jurassique y sont presents. Les sediments du Paleozoique inf&ieur y sont classes comme Gres de Wajid que nous avons eleves au rang de Groupe. Ses anciens membaes sont done devenus des formations. Le reste de la succession pal6ozoIque est subdivise en Formations de Qalibah et de Khuff. Au sein du Groupe de Wajid, la Formation de Dibsiyah, s’etageant du Cambrien ?moyen a YOrdovicien ?inf&ieur, est co&lee avec les Gres de Saq de l’Arabie Saoudite centro-septentrionale. Le Dibsiyah ‘inferieur’ recouvre une surface topographique heri& de la remon& du bouclier arabe lors de la p&ode pan-africatne finipr6cambrienne et/au a la fin du Cambrien inf6rieur. La p&ode de deformation de la fin du Cambrien inf&ieur se traduit darts le sud-ouest de YArabie saoudite et darts l’Oman voisin par une discordance angulaire. Cette derniere Separe l’equivalent de la Formation de Dibsiyah de la partie inf&ieure du Groupe de Haima, sous-jacent et d’age Precambrien superieur a Cambrien inf&ieur. Le Dibsiyah ‘inf&ieur’ s’est depose dans une plaine alluviale progradant vers le nord. Le Dibsiyah ‘superieur’ s’est par contre depose dans tm environnement intertidal a subtidal peu profond lors dune periode de transgression. La Formation de Sanamah d’lge Ordovicien ?i&rieur surmonte la Formation de Dibsiyah en discordance angulaire et/au de ravinement. Cette discordance dest developpee lors dun uplift regional a la fin de YOrdovicien moyen. La Formation de Sanamah s’est depo$5e dans un complexe de larges vallees fluviatiles. Le Membre de Qusaiba, Sihuien inferieur, de la Formation de Qahbah, recouvre en discordance cette demiere. Qusaiba s’est depose darts un environnement marin de faible profondeur ou de shelf mterm&liaire lors dune periode majeure de transgression. La discordance angulaire ou la lacune qui s6parent la Formation de Qalibah des unites sous-jacentes r6sulte du diastrophisme d’lge Ordovicien su+ieur a Sihuien le plus inf&ieur. La Formation de Khusayyayn, d’age D6vonien inf&ieur a Carbonifere ?moyen, surmonte en discordance la Formation de Qalibah. Cette formation s’est depos6e darts un environnement marin peu profond, de lagon, eolien et fluvio-delta’ique. La discordance la s6parant de la Formation de Qalibah corresponkl ?I une

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T. E. STUMP and J. G. VAN DER EEM

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p&ode de diastrophisme regional d’lge Silurien inferieur a Devonien inferieur. A son tour, la Formation Carbonifere moyen a Permien inferieur de Juwayl surmonte en discordance la Formation de Khusayyayn. Cette unite s’est deposee dans un complexe de larges vallees fluviatiles. La discordance situee a la base de cette formation est lice a l’orogenese hercynienne. La Formation permienne de Khuff recouvre en discordance la Formation de Juwayl et s’est deposee dans un milieu marin peu profond. Cette discordance rest&e dune periode majeure de transgression. Les Shales de Sudair, du Trias inferieur, succedent a la Formation de Khuff lors dune regression darts un milieu de lagune ou de plaine c&i&e. A la fin du Trias inferieur, la p&insule arabique a subi une nouvelle phase tectonique. En Arabie saoudite cent&e et sud-occidentale, cet evenement s’est traduit par l’erosion des Shales de Sudair et des formations plus anciennes sur les blocs bascules les plus eleves et par l’epaississement syn-depot de la Formation de Jilh sur les compartiments affaisses de ces blocs bascules.

INTRODUCTION

AND REGIONAL

SETTING

Pre-Permian outcrops are found in three main areas of Saudi Arabia: in the Tayma/Tabuk regions, the Qasim area and the Wajid plateau (Fig. 1). This paper briefly discusses the Wajid plateau and places it in a regional context with central and northern Arabia. The periods of tectonism and the depositional settings of the Palaeozoic through to Early Triassic units of the Wajid plateau have been shown to extend into central and northern Arabia (Stump ef al., 1993, in press). In this context, the geohistory of the Wajid plateau illustrates sequences of events which occurred across Saudi Arabia. The Late(?) Precambrian through to Palaeozoic deposits of Saudi Arabia occur in six major depositional packages, informally termed ‘Super Groups’ (Stump et al., 1993, in press; Fig. 2). Their geohistories resulted from ‘sedimentation along fluvial to midshelf facies tracts, eustatic oscillations and periodic uplifts. The boundaries of each Super Group are defined by major periods of diastrophism and the resultant interregional unconformities, plus associated fundamental changes in sedimentation. The Wajid outcrop belt, due to the well-exposed nature of the area, allows the direct study of the sedimentological and tectonic events which are inferred in the subsurface of central and northern Arabia. New biostratigraphic control has also clarified the timing of uplift periods and some of the lateral facies relationships of units in southwestern Arabia. Several workers since the 1940’s have proposed subdivisions for strata which crop out on the Wajid plateau. The term ‘Wajid Sandstone’ was first used in an unpublished Aramco report for these strata. Powers et al. (1966) and Powers (1968) formally proposed the term and described a type section. Pallister (1982), mapping in the Wajid plateau, proposed two members of the ‘Wajid Sandstone.’ The upper ‘Wajid Sandstone’ was termed the Ilman Member, while the lower was called the Shum (see Table 1). The criteria used to distinguish the members was largely based on the degree of tectonism. The Shum Member in Pallister’s study area was intensely folded, while the Ilman Member was not. These, terms, while recognizing an integral feature of the geological history of Saudi Arabia, did not gain wide

acceptance and are considered abandoned terms. Kellogg et al. (1985) proposed the terminology followed by most subsequent workers. They named four members of their ‘Wajid Sandstone’ based upon lithostratigraphic criteria and field relationships. At the time, no age diagnostic fossils were known from the ‘Wajid Sandstone’ and Kellogg et al. (1985) thought their members were Cambrian through to Ordovician in age. Evans et al. (1991) proposed to drop the term Wajid sandstone and replace it with recognized divisions of the Saq and Qasim formations. They also recognized in outcrop the Qusaiba Member of the Qalibah Formation. The authors retain the nomenclature of Kellogg ef al. (1985) recognizing the regional characteristics of the stratigraphical succession in southwestern Saudi Arabia. Subsequent to these author’s work, additional biostratigraphically significant taxa have been found in southwestern Arabia and field studies have recognized several periods of uplift which produced an intraregional hiatus between units of the Wajid sandstone. These new additions to the authors understanding has highlighted the necessity for a revision of the stratigraphic nomenclature of Kellogg et aI. (1985). The authors elevate the ‘Wajid Sandstone’ of Kellogg et al. (1985) to the Wajid Group and its former members to the rank of formations. The authors also recognize the Qusaiba Member, Qalibah Formation (Mahmoud et al., 1992), as a separate unit within the Wajid Group. The Wajid sandstone is elevated to group status for three reasons. First, each of its ‘members’ are separated from one another by hiatuses of varying but significant durations. Second, each unit is part of a sequence whose bounding unconformities resulted from periods of regional deformation and erosion. Third, the depositional history of each formation reflects a unique series of events which is distinct from the units preceding and succeeding it. SUPER GROUP Yatib/Siq

I

Formations

Regional setting and physical stratigraphy. Super Group I of Late(?) Precambrian through to Early Cambrian age includes the Yatib/Siq Formations of northern Saudi Arabia (Stump et al., in press). Strata of

The stratigraphy,

depositional environments

and periods of deformation of the Wajid outcrop belt

Wajid outcrop

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Belt

AL SULAYYIL

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INDEX

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4

BORE HOLES

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Figure 1.Index map to major localities or wells mentioned in the text (map adopted from Kellogg et al., 1985 as amended by Evans et al., 1991).

Super Group I are unknown in the Wajid plateau, but were probably deposited there and eroded near the close of the Early Cambrian. These conclusions are inferred from the regional distributions and unconformable relationships of strata of this age in northern Saudi Arabia and interior Oman. The Yatib/Siq Formations were deposited by braided streams that eroded and buried a topography which was developed on the crystalline rocks of the Arabian shield during a late stage of the PanAfrican orogeny (Fig. 3). The Siq/Yatib Formations lie in angular unconformity in northwestern Saudi Arabia below the Middle Cambrian to Early Ordo-

vician Saq sandstone (Clark, 1986; Grainger and Hanif, 1989). In Oman, the Middle Cambrian Amin Formation (Hughes-Clarke, 1988) has a similar unconformable relationship with its Yatib/Siq equivalents, the Lower Haima Group. These angular unconformities, on either end of the Arabian Peninsula, document regional uplift of about late Early Cambrian age. Erosion at this time probably stripped the Wajid plateau of its Siq/Yatib and Lower Haima equivalent strata. This period of erosion limited the sediments of Super Group I to northern Saudi Arabia/ Jordan AndyOman, respectively, and they may have served as la source terrain for the Saq sandstone and its corollaries.

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T. E. STUMP and J. G. VAN DER EEM

WAJID OUTCROP BELT

GENERALIZED SAUDI ARABlA

SUPER cRoups

(Stump et al 1995)

JUWAYL FM. WESTTPHALIAN ‘/

6 0

M /;

N

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Figure 2. The Late Precambrian their equivalents in southwestern

to Early Triassic Super Groups of Saudi Arabia and Saudi Arabia.

SUPER GROUP II Dibsiyah Formation

Regional setting and physical stratig-raphy. Super Group II (Stump et al., in press), Middle Cambrian to Late Ordovician, includes the Burj and Saq Formations and the lakers probable Wajid temporal equivalent, the Dibsiyah Formation. The Hanadir, Kahfah and Ra’an Members of the Qasim Formation of east central through northern Saudi Arabia are also included in Super Group II. The Dibsiyah Formation is thought to be the lateral equivalent of the Saq sandstone based upon ik stratigraphical position and lithological similarity to the Saq sandstone. The Dibsiyah Formation rests nonconformably on the Ara-

bian shield and is unconformably overlain by the Sanamah Formation or younger units. In the Wajid plateau the graptolitic shales of the Hanadir, Kahfah and Ra’an Members of the Qasim Formation are missing. Consequently, it may be that the Dibsiyah Formation is the proximal, coarse-&&c equivalent of the Saq through to the Qasim Formations (Evans et al., 1991). The lithological similarities of the Dibsiyah Formation with the Saq sandstone and the locally intense deformation (Stoeser and Greenwood, 1984) and erosion of the Dibsiyah Formation, however, suggests that a large hiatus exists between the Dibsiyah Formation and the overlying Sanamah Formation. It is probable, therefore, that the Hanadir, Kahfah and Ra’an Members of the Qasim Formation were deposited in southwestern Arabia,

The stratigraphy,

depositional

environments

and periods of deformation

WAJID OUTCROP BELT

PERloDoF DEFORMATlON E. KlMMERlAN

-

of the Wajid outcrop belt

425

MAjQR EVENTSIN SWARABlA

i-

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Figure 3. Major events in the Palaeozoic

Uplift. erosion of Shuawra h okkr units Uplift+erosion of Sanamah & older units

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geohistory

but they were removed by the erosional event which separates the Dibsiyah Formation from the overlying Sanamah Formation. The Dibsiyah/Saq Formations thicken from about 152 m in the Wajid plateau to over 1524 m in the subsurface of southern Jordan. Palaeocurrent data from the Qasim (Vaslet et aZ., 1987) and Wajid areas suggests a regional palaeoslope during Dibsiyah/Saq formation deposition from the south to the northnortheast or north-northwest. The Saq sandstone is divided in outcrop into two members, the Lower F&ha and the Upper Sajir (Vaslet ef al., 1985). Likewise, the Dibsiyah occurs in two major lithofacies, herein informally termed the ‘lower’ and ‘upper’ Dibsiyah Formation. Both the ‘lower’ Dibsiyah and the

of southwestern

Arabia.

Risha Member of the Saq sandstone bury a palaeotopographic surface, which is locally several metres high developed on the crystalline rocks of the Arabian shield. Compositionally, the ‘lower’ Dibsiyah Formation and the Risha Member of the Saq sandstone are similar. Their basal deposits usually consists of a palaeosol which is succeeded by a well-rounded, quart&e pebble to cobble-bearing litharenite/sub-litharenite of highly variable thickness. The remainder of the Risha Member of the Saq sandstone and the ‘lower’ Dibsiyah Formation are composed of texturally and compositionally mature quartzarenite. The sandstones are usually intensely cross-stratified and, their clasts are imbricated within broad, relatively wide and moder-

T. E. STUMP and 1. G. VAN DER EEM

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ately deep channels. The ‘upper’ Dibsiyah Formation consists of cross bedded, variably sorted, locally and intensely Scolifhos-bioturbated quartzarenite. Siltstones and shales are rare in both units. ln some outcrops of the Dibsiyah Formation, bioturbation extends almost to the base of the unit. This suggests either an interfingering of the nonmarine ‘lower’ Dibsiyah Formation with the marine ‘upper’ Dibsiyah Formation or that erosion produced by the ‘upper’ Dibsiyah Formation transgression partially removed the ‘lower’ Dibsiyah Formation.

Depositional environments and age. The ‘lower’ Dibsiyah Formation and the Risha Member of the Saq sandstone were deposited by braided streams which prograded in a elastic apron from southern Arabia northward to Jordan. The marine facies of the ‘upper’ Dibsiyah Formation and the Sajir Member, Saq sandstone, were deposited in settings ranging from shoreface to tidal flat/tidal channel. No agediagnostic fossils have been discovered in the Dibsiyah Formation. The trace fossils Scolifhos and Cncziana, while important facies indicators, have no biostratigraphical value in southwestern proven Arabia. The age dating of the Dibsiyah Formation is inferred by its correlation with the Saq sandstone of east central and northern Saudi Arabia. The Saq sandstone here is Middle Cambrian to Early Ordovician in age (Stump et al., in press). SUPER Sanamah

GROUP

III

Formation

Regional setting and physical stratigraphy. Super Group III (Stump et al., in press), of Late Ordovician through to earliest Silurian age, includes the Zarqa/Sarah Formations, the Quwarah Member of the, Qasim Formation and its corollaries in central Arabia and the Sanamah Formation of the Wajid plateau. All these units were deposited in facies tracts ranging from fluvio-deltaic to mid-shelf marine. The Sanamah Formation lies in angular unconformity to disconformity on the Dibsiyah Formation and deeply erodes it. The Sanamah Formation is overlain in angular unconformity to disconformity by either the Qusaiba Member of the Qalibah Formation or directly by the Khusayyayn Formation. The Sanamah Formation probably correlates with the Zarqa/Sarah formations of central through northem Arabia. This correlation is suggested by their similar lithological makeup, common depositional settings and their position in the stratigraphic colUI.IlIl. The Sanamah Formation consists of a series of stacked, cross-cutting channel sands which infill palaeovalleys. The formation consequently varies

widely in thickness, sorting and textural maturity. The Sanamah Formation has a maximum known thickness of about 152 m (Evans et al., 1991). In its type locality it is 55 m thick (Kellogg et al., 1985). The basal Sanamah Formation consists here of a quartzite clast-bearing conglomerate, which is about 23 m thick. The clasts range up to about twenty centimeters in length and are mostly multicycle quartzite and chert. The remainder of the type section is typical of the formation as a whole. It consists of cross-cutting channel sands which are red to white in color, poorly to well-sorted, fine- to coarse-grained and are subarkose to lithic arkose. Most of the Sanamah Formation in outcrop appears to have been deposited in nonmarine settings, but bioturbation is present in the uppermost part of the type section in Jebel Sanamah. In other areas, such as Jebel Qahr (lat. 20” 06’ 28” N, long. 44” 22’ 10” E), debris flows, slumps and grain flows are present, implying rapid sedimentation and liquefaction during peak periods of discharge.

Depositional environments, age and periods of &formation. The Sanamah and Juwayl Formations share many common features which have suggested to workers similar palaeoenvironmental interpretations. The palaeoenvironmental interpretation of the Sanamah and/or the Juwayl Formations has ascribed them to glacial (McClure ef al., 1988; Vaslet, 1987, 1989,1990), glaciofluvial (Dabbagh and Rogers, 1983), glacial/periglacial (Evans ef al., 1991), or to purely fluvial processes (Hadley and Schmidt, 1975; Alsharhan et al., 1991). The Sanamah and Juwayl Formations are interpreted by th authors as being a product of mostly fluvial deposition. This interpretation is supported by the geometry of the valleys excavated during the deposition of the Sanamah Formation, the formation’s contained bed forms and by the composition of the Sanamah Formation valley fill. The supposed ‘tillites’ of the Sanamah or Juwayl formations are extremely rare and poorly developed when compared with their temporal equivalents in north Africa (Beuf et al., 1971) or Oman (Hughes-Clark, 1988). Typical tillites composed of basement blocks floating in an unweathered matrix have not been observed in the Wajid plateau. Likewise the Sanamah and Juwayl valleys are not filled with poorly sorted ground or terminal moraines, which would be expected if the valleys were excavated and occupied by glaciers. The clasts in both units instead characteristically show extensive fluvial transport and abrasion. These particles are usually of cobble size or smaller and they tend to be subrounded to rounded, percussion marked and are typically imbricated along channel bases. The Sanamah Formation valleys appear to have a geometry similar to those of the better known Zarqa Formation. The Zarqa Formation valleys in the sub

The stratigraphy,

Finure 4. Antia unconformities Nylong. 44’ 68’ 20” E).

depositional environments

and periods of deformation of the Wajid outcrop belt

between the Qusaiba/Sanamah

surface of central and northern Arabia have low gradient walls with wide and deep floors. They also meander in plan view and on seismic reflection data many are tens of kilometres wide, scores of kilometres long and some are over 600 mebes deep (Saudi Aramco geophysical staff, pers. comm.). In the outcrop belts of northwestern and southwestern Arabia, the Zarqa and Sanamah Formation valleys are filled mostly with nonmarine sediments. Eastward into the subsurface, the nonmarine Zarqa Formation grades into sediments deposited in shallow marine conditions. In east central Arabia the Zarqa valleys are backfilled almost entirely with marine sediments, which were deposited in inner to mid shelf water depths. To date, few wells have penetrated the Sanamah Formation, so its lateral facies relationships remain speculative. But on regional seismic lines from northern through southern Arabia, the Zarqa and Sanamah Formation valley systems are pervasive features of the Late Ordovician landscape. ‘Striated pavements’ have been observed in the Sanamah and the Juwayl Formations, which have been attributed to glacial abrasion. These features, as far as is known, occur exclusively interbedded within the Sanamah and Juwayl Formation sandstones, which at the time of their deposition would

and the Khusayyayn/Qusaiba

427

in the Al Madarah &ea (lat. 19” 36’ 38”

have been unconsolidated or semiconsolidated. No ‘striated’ surface has been observed ‘by the authors, or reported by others on the Arabian shield or on other penecontemporaneously lithifibd substrates. It is difficult to see how, from a mechanical point of view, these striations were preserved within the Sanamah Formation, if they originated as tool marks formed at the base of valley glaciers. The Sanamah/ Juwayl Formation valleys are characteristically filled by sediments deposited in high flow regime conditions. The erosion of these striated sandstones would be expected during peak periods of glacial runoff. The striations most frequently occur at or near the base of subaqueously deposited density or debris flows. They may therefore have formed as tool marks produced by clasts at the bases of these density flows as the flows moved across their substrate. The preservation of the striated surfaces would be assured by the overlying debris flow deposits until exposed by recent erosion. Rare striated boulders and equaBy rare ‘faceted’ pebbles are also present in the Sanamah Formation. Even if we accept these particles as being stiated or faceted by glacial processes, their presence only suggests that glaciers were present somewhere along the

T. E. STUMP and J. G. VAN DER EEM

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Sanamah Formation’s facies tract. Their presence does not prove that glaciers once resided in Arabia. The drainage area for the fluvial systems of the Zarqa and Sanamah Formations extended well into central Africa. Here, glacially striated boulders of Precambrian age, as well as Late Ordovician age, are common (see Beuf et al., 1971). Their transport by fluvial processes into Arabia is to be expected. To date, no biostratigraphically significant taxa have been recovered from the Sanamah Formation. the Sanamah Formation’s The Zarqa Formation, probable corollary, has been shown to be Late Ordovician through to Early Silurian in age (Stump et aI., in press). It is assumed that the Sanamah Formation has a similar age range. The oldest formation which overlies the Sanamah Formation is the Qusaiba Member of the Qalibah Formation (Mahmoud et al., 1992). This unit has been dated in southwestern Arabia as Early Silurian (Llandoverian) in age (see discussion later). In north Africa, fluvioglacial deposits range in age from Middle Caradocian (Klitzsch, 1981) to Middle Llandoverian (Legrand, 1985a, b; Destombes et al., 1985). In the Late Ordovician to Early Silurian of Arabia, two periods of uplift can be demonstrated which coincide with the ‘Taconic’ uplift (Fig. 3). The first period occurred during the Early to Middle Caradocian. In the Wajid plateau, this period of tectonism is documented by the uplift and deformation of the Dibsiyah Formation and the penecontemporaneous erosion of valley systems filled by the Sanamah Formation. During the Late Ordovician/Early Silurian, a second phase of the Taconic uplift affected most of Saudi Arabia (Stump et al., 1993, in press). In the Wajid plateau this event is indicated by an angular unconformity, which separates the Sanamah Formation from the Qusaiba Member of the Qalibah Formation (Fig. 4). This period of uplift is also reflected throughout Arabia by the renewed erosion of the Zarqa and Sanamah Formation valleys and their further eastward erosion onto the Late Ordovician continental shelves. Rising sea levels during the latest Ordovician to earliest Silurian backfilled the valleys of the Zarqa and Sanamah Formations with marine

sediments. SUPER GROUP IV Qalibah Formation, Qusaiba Member Regional setting and physical stratig-raphy. Strata of Super Group IV (Stump et al., in press) range in age from Early Silurian to Middle Carboniferous. These units rest on the Upper Ordovician/Lower Silurian unconformity and below the Middle Carboniferous Hercynian unconformity. The Qalibah, Tawil, Jauf,

Jubah, Berwath and Khusayyayn Formations compose this Super Group. The Khusayyayn Formation is present in the Wajid outcrop belt and is the lithological equivalent of the Tawil through to Berwath Formations of central Arabia. The Qalibah Formation consists of the Qusaiba and the Sharawra Members and the formation is found throughout much of Saudi Arabia. The Qusaiba Member of the Qalibah Formation is composed mostly of shale (mudstone) and thin interbeds of sandstone. The Sharawra Member of the Qalibah Formation is composed mostly of micaceous sandstone and subordinate siltstone and shale. The Qusaiba Member has only recently been discovered in the Wajid plateau (Evans et al., 1991). Previously, Kellogg et al. (1985) placed in their ‘Sanamah Member’ of the Wajid sandstone an upper, flaggy, silty sandstone and siltstone succession and a lower ‘channel sandstone’ series. The flaggy, silty sandstone series rests upon the ‘channel sandstone’ series in disconformity (Kellogg et al., 1985) to angular unconformity. Subsequent work by Evans et al. (1991) and herein have demonstrated that the ‘upper’ Sanamah Member of Kellogg et al. (1985) is lithologically and temporally equivalent to the Qusaiba Member of the Qalibah Formation. The Qusaiba Member of the Qalibah Formation in the Wajid plateau has a maximum known thickness of about 61 m and it rests unconformably on a variety of substrates (Fig. 5). The Sharawra Member is not recognized in outcrop but is present in the adjacent subsurface in Well V-608 (see Fig. 1) The Khusayyayn or Juwayl Formations rest unconformably on the Qusaiba Member. The Qalibah Formation has been drilled and cored in many wells in Saudi Arabia and across Arabia the Qusaiba Member has been divided into ‘upper’ and ‘lower’ parts (Stump et al., 1993). The lower contact of the ‘lower’ Qusaiba Member is placed at the base of a transgressive sandstone. The lower contact of the ‘upper’ Qusaiba Member is placed at the top of an upward coarsening series of sandstones, which are interbedded within the mudstones of the Qusaiba Member (Fig. 5). On a regional basis, the ‘lower’ Qusaiba Member is characteristically an upward coarsening series while the ‘upper’ part of the Qusaiba Member is upward fining. In the Wajid plateau, the ‘lower’ Qusaiba Member has a maximum known thickness of about 50 m. In most subsurface sections, the basal part of the ‘lower’ Qusaiba Member is composed of a relatively thin sandstone. In the Wajid outcrop belt the basal sandstone is about 1.5 m thick and consists of a poorly to moderately sorted, fine-grained to conglomeratic subarkose to quartzarenite. Lying above the basal sand is a variable thickness of sometimes highly fossiliferous, pyritic and richly

The stratigraphy,

depositional environments

429

and periods of deformation of the Wajid outcrop belt

A

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Figure 5. The Qusaiba Member in the Wajid Plateau. (a) Jebel Al Qu’ad (lat. 17” 57’ 27” N, long. 44” 27’ 02” E); (b) Jebel Karmah (lat. 18” 37’ 30” N, long. 44” 12’ 45” E); (c ) south of Jebel Ilman (lat. 18” 43’ 43” N, long. 44” 17’ 30” E); (d) Al Madarah area (lat. 19” 36’ 38” N, long. 44” 02’ 20” E); (e) north Al Madarah area (lat. 19” 37’ 36” N, long. 44” 02’ 20” E); ‘Biostratigraphic control points (see Fig. 1 for line of section).

organic series of claystones and shales. This unit is the ‘hot shale’ (sensu Mahmoud et al., 1!392), which is the main hydrocarbon source rock for the oil found within the Palaeozoic successions of Saudi Arabia. The ‘hot shale’ is much less bioturbated than the shales which succeeded it, but it collectively contains an abundant and diverse megafauna of graptolites, molluscs, brachiopods, trilobites, fragmental eurypterids and hyolithids. In the subsurface, the ‘hot shale’ is gradational upwards with the black to varicolored shales of the middle and upper parts of the ‘lower’ Qusaiba Member. The ‘hot shale’ is best developed in the subsurface of central and eastern Saudi Arabia and is difficult to identify in outcrop in southwestern Arabia. In the Wajid outcrop belt the ‘hot shale’ is in a shallow marine facies and typically consists of a gray to varicolored, bioturbated, sandy, micaceous claystone. The sandstones interbedded within the ‘lower’ Qusaiba Member (Fig. 5) are usually moderate- to well-sorted, ripple-laminated to cross-bedded and contain common soft sediment deformation and dewatering structures. The top of this upward coarsening sequence is a parasequence bedset which defines the ‘lower’ from the ‘upper’ parts of the Qusaiba Member. In the Wajid outcrop belt, this sand series occurs at the top, or in the upper two-thirds, of all the known Qusaiba Member exposures. This sandstone

series has a maximum known thickness of about 7.6 m and is composed of red/brown/white, finecross-stratified quartzarenite grained, well-sorted, beds, which usually have erosional bases and gradational tops. In the subsurface of Arabia, open shelf taxa are the most diverse and individuals more abundant in the lower parts of the ‘lower’ Qusaiba yember. Up section, these species, while still present, are much less numerous and fewer individuals are found. This feature, plus the upward coarsening nature of the ‘lower’ Qusaiba Member and the sedimentary structures present in the interbedded sands, suggest that the ‘lower’ part of the Qusaiba Member was rapidly deposited in an upward shoaling setting. The ‘upper’ Qusaiba Member in the Wajid plateau has a maximum known thickness of about 10.6 m and most or all of the unit has been removed by preKhusayyayn Formation erosion. The ‘upper’ Qusaiba Member here consists of varicoloured siltstone/claystones, which are finely laminated and bioturbated. Interbedded within the ‘upper’ Qusaiba Member are thin sandstones similar to those found in the ‘lower’ Qusaiba Member. The Sharawra Member of the Qalibah Formation in southwestern Arabia is known with certainty only from well V-608 (Fig. 1). In well V-698, the Sharawra Member is 67 m thick and is composed mostly of mi-

430

T. E. STUMP and J. G. VAN DER EEM

~oyefj& cabottii (Cramer) MilCer ;EnEames 1982 (X 1400)

~~u~os~or~ ~ufus~e~sa (Strother & Traverse) Burgess 6t Richardson 1991 (X 1400)

~o~os~ofu ~uf~~~~e~sj~ Strother & Traverse 1979 (X 1400) Figure 6. Characteristic palynomorphs

of the Early Silurian (Early/Middle

caceous fine- to medium-grained, variably sorted sandstone with subordinate interbeds of siltstone and shale. The northwestwards coarsening trend of the ‘lower’ Qusaiba Member (see Fig. 5) implies that the depositional edge of the Qusaiba lay west of the present outcrop limit.

Llandovery)

of southwestern Saudi Arabia

Depositional environments, age and periods of upLift. Strata of Super Group IV were deposited in a fluvio-deltaic-marine, river dominated system of vast proportions. The Qusaiba and Sharawra Members of the Qalibah Formation are interpreted to be, respectively, the offshore clays and pro-deltaic sands of this deltaic system. The Tawil through to Jubah Forma-

The stratigraphy,

depositional environments

Figure 7. The Qusaiba Member resting in angular unconformity 36’06.5” N, long. 44” 03’ 2.8” E). -

and periods of deformation of the Wajid outcrop belt

431

below the basal Khusayyayn Formation in the Al Madarah area (lat. 19”

tions and the Khusayyayn Formation are the fluvial to delta front deposits. The Berwath Formation was deposited mostly on the delta plain (Stump et al., 1993). The transgression which deposited the graptolitic muds of the Qusaiba Member of the Qalibah Formation resulted from the ablation of the continental glaciers in central Africa. The high organic C content of the basal Qusaiba Member, its diminished infaunal bioturbation and diverse benthose suggests that the unit may have accumulated as a consequence of high primary productivity. These qualities also suggest that the muds were anaerobic but the water column above the sea floor was normally oxygenated. The deglaciation of Central Africa appears to have occurred as a series of advances and retreats and not as a single episode. The major shallow marine sandstones, which occur at the top of the ‘lower’ Qusaiba Member, probably resulted from a period of coastal offlap produced by renewed glacial advance. The graptolitic shales of the ‘upper’ Qusaiba Member, which overlie these sands, record a period of renewed coastal onlap produced by continued glacial retreat. The innumerable parasequence bed sets within the Qalibah Formation document either the short term

seasonality of the continental ice cap, or storms passing over the Silurian continentaP shelves of Arabia. To date, no megafossils have been recovered from the Qusaiba Member in the Wajid plateau. Microfossils of Early to Middle Llandoverian age have been recovered from cores in the basal’ lower Qusaiba submember of well V-608, the middle and uppermost parts of the unit in the Al Midarah, Jebel Karmah sections and in the basal part of thesJebe1 Ilman section (see Fig. 5; Aramco proprietary reports). In the wells and outcrops available for study, the Qusaiba Member is characterized by abundant cryptospores with several species of Nodospora, Tafrahedralefes, Dyadospora and Pseudodydospora present (Fig. 6). Rare to common sphaeromorphic acritarclis compose the Qusaiba flora, as do rare forms such as Ambifisporifes, Archaeozonofrilefes, Moyeria caboffii (Cramer) Miller and Eames 1982 Torfofubus profuber@s Johnson 1985 and Eviffia denticulafa. (Cramer) LeI-Ikkisd 1989. Very rare individuals of Belonechifina spp.; Conochifina spp. and fragmental chitinozoans com#ete the known flora of the Qusaiba Member in southwestern Arabia. The basal Khusayyayn Formation lies in angular unconformity (Fig. 7) or disconformity on a deeply eroded Qalibah Formation. In southwestern Arabia

T. E. STUMP and J. G. VAN DER EEM

432

D

(4 North

? “upper” Khusayyayn

\\ I’ (0

“lower” Khusayyayn Formation

(9

??

Datum Base Khusayyayn Fm.

Figure 8. The Khusayyayn Formation in the Wajid Plateau. (f) south Qara al Midarah roadcut (lat. 17” 35’ 52” N, long. 44” 06’ 26” E);(g) Jebel Ar Ra Lat. (lat. 17” 33’ 12.6” N, long. 43” 52’ 13.4” E); (e ) north Al Midarah area (lat. 19” 37’ 36” N, long. 44” 02’ I) south Jebel Sanamah (lat. 20” 12’ 47.7” N, long. 44” 14’ 54” E); (h) Bani Khurb area (lat. 19” 43’ 26.1” N, long. 44” 39’ 46.5” E); (‘ 44” E). *Biostratigraphic control points (see Fig. 1 for line of section).

which separates the Qalibah Formation from the Khusayyayn/Tawil Formations is of Llandoverian (Lower Silurian) to Siegenian/Emsian (Lower Devonian) age. The hiatus narrows slightly into central Arabia where it ranges from Middle or Late Llandoverian to Pridolian (Upper Silurian) in age. In northern Arabia the Sharawra Member is conformable below the Upper Silurian part of the Tawil sandstone (Aramco proprietary reports). In central Arabia the Tawil Formation has a similar unconformable relationship with the Qalibah Formation and older units. This period of uplift indicated by the Lower Silurian to Lower Devonian hiatus is temporally equivalent to the Acadian orogeny. Angular unconformities of similar ages have been described from Libya, Egypt (Klitzsch, 1981) and Morocco (Destombes et al., 1985). Periods of renewed folding and metamorphism also date from about this time in Mauritania and Senegal (Lecorche et al., 1983; Deynoux, 1985). During the Acadian deformation phase, large parts of the Arabian platform were drained by the Sharawra sea. Marine conditions continued to prevail in north and northeastern Arabia and probably in the Arabian Gulf (Stump et al., 1993). In westernmost Arabia, post Devonian erosion has removed all of the sedimentary section, so the amount of uplift here is conjectural. In the Wajid plateau a minimum of bethe hiatus

tween 300 m to as much as 600 m of uplift can be documented by the progressive erosion, from the east to the west, of the Sharawra and Qusaiba Members. Khusayyayn Formation Regional setting and physical stratigraphy. The Khusayyayn Formation is the lithological equivalent of the Tawil through to Berwath Formations of central and north-central Arabia. The Khusayyayn Formation rests unconformably on a variety of substrates from the Qusaiba Member down to the Arabian shield (Fig. 7). The Juwayl Formation in outcrop lies unconformably on the Khusayyayn Formation. In broad terms, the Khusayyayn Formation is an upward coarsening sequence and is 200 m thick at its type section (Kellogg et al., 1985). The true thickness of the unit is unknown because of the erosional nature of its upper contact. In the type section, the base of the Khusayyayn Formation is composed of a white/brown, mediumto coarse-grained, quartzite-bearing, pebbly and conglomeratic sandstone. Cross-beds consisting of remarkably long foresets, composed mostly of wellto moderately-sorted quartzarenite, are characteristic features of the formation. In outcrops beyond the type area, the lower part of the Khusayyayn Formation shows considerable variation in its physical

The stratigraphy,

Figure 9. Characteristic

depositional environments

palynomorphs

and periods of deformation of the Wajid outcrop belt

of the Early Devonian (Emsian) of southwestern Saudi Arabia.

stratigraphy. The outcrops at the South Qara al Midarah area (section f of Fig. 8) consists of about 21 m of interbedded white/brown mudstone and brown/red sandstone. In hand specimen the basal mudstones here resemble bentonite. In other outcrops (sections g, e and h of Fig. B), finely bedded, silicified units resembling porcellanites were found. Petrographic examination of these bentonites and porcellanites

proved all of them to be of indeterminate origin. The bentonites of section f (Fig. 8) rest on weathered basement and are mudcracked and contain fish and plant remains. The sandstones in this section have a wide range of sorting and textural maturity. The lowest sandstone bed is subarkosic and planar bedded, while the upper sands are moderate- to wellsorted, cross-bedded quartzarenites. The top of this

434

T. E. STUMP and J. G. VAN DER EEM

section consists of red/brown, well- to moderatelysorted, parallel to festoon-bedded quartzarenite. The section at Jebel Ar Ra (section g of Fig. 8) is one of the most complete outcrops of the Khusayyayn Formation. The formation here is estimated to be about 213 m thick and it rests on a pediment eroded onto the Arabian shield. This section is an upward coarsening sequence of which the lower 61 m consists of red/ brown, well-sorted, large-scale cross-bedded and festoon-bedded sandstone. Interbedded within this section are thin quartzite, pebble-bearing litharenites and parallel to cross-bedded sandy mudstones/siltstones, plus the bentonites mentioned above. The thin red/brown sandy, micaceous siltstone interbeds in this section are usually bioturbated. The remaining upper 152 m consists primarily of thick bedsets of strongly cross-bedded fine- to medium-grained, moderate- to well-sorted quartz- are&es. In a series of outcrops in the Bani Khurb area (section h of Fig. 8) the Khusayyayn Formation is divisible into ‘upper’ and ‘lower’ parts. Dip measurements of about 150 cross-bed sets were made here in the ‘lower’ Khusayyayn Formation, which indicated a palaeocurrent direction that was predominantly to the north or north-northeast. The ‘upper’ Khusayyayn Formation at Bani Khurb is composed of laterally discontinuous channelized sandstones, which are unconformably overlain by the Juwayl Formation. These channels cut into equally variably sorted sandstone and thick beds of bentonite and porcellanite. Approximately forty dip measurements of clasts and cross-bedding surfaces were made in this part of the Bani Khurb section. Judging from this limited sample, the sediments of the ‘upper’ Khusayyayn Formation have a slightly more easterly palaeocurrent direction than do the sediments of the underlying ‘lower’ Khusayyayn Formation. Field relationships in the Bani Khurb area between the ‘upper’ and ‘lower’ parts of the Khusayyayn Formation suggests that the contact may be an angular unconformity. Depositional environments, age and periods of deformation. The lower part of the Khusayyayn Formation consists of two distinct, interfingering lithofacies. The sections composed mostly of sandstone were probably deposited in marginal marine to upper composed mostly of shoreface settings. Sections claystone, which contain fish remains and terrestrially derived spores, were probably laid down in shallow lagoons. The succeeding parts of the Khusayyayn Formation were deposited in an immense, high flow regime, fluvial, lacustrine(?) and eolian system, which prograded north and northeastwards away from southern Arabia. The age of the Khusayyayn Formation is Early through to at least Middle Devonian (Saudi Aramco

proprietary reports) and it may be as young as Middle Carboniferous (Saudi Aramco proprietary reports). Fish fossils consisting of Acanthodian spines and plates resembling Uruniacunfhus and Gyracanthus of late Early Devonian age (Forey et al., 1992) have been recovered at several widely scattered locations in the basal part of the Khusayyayn Formation (Saudi Aramco proprietary reports). These forms are known from the basal mudstones of the South Qara al Midarah, north Al Midarah area and Jebel Sanamah areas (see Fig. 8). Additional specimens of Early Devonian fish have come from Jebel ad Dirar (lat., 19” 21’ 25. 3” N, long. 43” 56’ 06.3” E) and Jebel al Qahr (lat., 19” 36’ 30.8” N, long., 44” 02’ 27.3” E). The Early Devonian (Emsian) spores Refusofrilefes maculufus McGregor and Camfield 1976, Dibolisporifes eifeliensis (Lanninger) McGregor 1973, Apiculirefusisporu cf. brundfii Streel 1964, Dicfyofrilefes emsiensis (Allen) McGregor 1973, Archueozonofrilefes chulus Richardson and Lister sexfanfii McGregor and Cam1969, Cumerozonofrilefes field 1976, Clivosporu verrucufu var. convolufu McGregor and Camfield 1976 and Emphunisporifes spp. have also been recovered from wells or outcrops in southwestern Arabia (Fig. 9). A less diverse Early through Middle Devonian microflora, consisting of Refusofrilefes, Puncfutisporifes, Hymenozonofrilefes, Apiculirefusisporu and Apiculufisporifes, has also been recovered from southwestern Arabia. Megascopic plant remains have been found in the basal Khusayyayn Formation at Jebel Gillman (lat. 18” 46’ 34.8” N, long. 44” 15’ 07.8” E) where they occur in thin coal and carbonaceous mudstone beds. These forms proved unidentifiable due to advanced coalification. Identifiable Middle Devonian megascopic plant remains consisting of Oresfoviu-like forms (Saudi Aramco proprietary reports, 1992) have been recovered from red, micaceous siltstones and shales in the Jebel Fard al Ban area (lat. 20” 06’ 18” N, long. 44” 31’ 36” E) and at Jebel al Qahr (lat. 19” 58’ 09” N, long. 44” 46’ 08” E) The latter locality correlates with the upper part of the ‘lower’ Khusayyayn of Fig. 8. No fossils of any type have been recovered from the ‘upper’ Khusayyayn. If the unconformable relationship between the ‘upper’ and ‘lower’ Khusayyayn Formation of the Bani Khurb area proves to be an angular unconformity, this would document a late stage movement of the Acadian orogeny. In central Arabia, unconformities are known between Middle Devonian through to Middle Carboniferous Formations (Stump et al., 1993, in press). SUPER GROUP

V

Juwayl Formation Regional setting and physical stratigraphy. Super of the Group V (Stump et al., in press) consists

The stratigraphy,

depositional environments

and periods of deformation of the Wajid outcrop belt

Unayzah Formation (sensu Ferguson and Chambers, 1991) and the Juwayl Formation (Kellogg et al., 1985). These units were deposited on the erosional surface produced by the Hercynian orogeny and below the basal Khuff Formation flooding surface. The Unayzah Formation is known from central to northeast Saudi Arabia while the Juwayl is present in the Wajid plateau. The Juwayl Formation is probably the more proximal facies while the Unayzah is the more distal part of the depositional system that was established during and following the Hercynian orogeny. In the Wajid, the Juwayl Formation rests unconformably on units ranging from the Khusayyayn Formation to the Arabian shield and it is unconformably overlain by the Khuff Formation. The Juway1 Formation is a elastic valley fill succession, which varies widely in thickness and composition. The type area for the formation is in the Wadi Tathlith Quadrangle (Kellogg et al., 1985) where at least two major, discrete Juwayl Formation outcrop belts are found. The westerly belt is a northwest-southeast trending valley system, while the easterly outcrop trends north-south. The westerly Juwayl Formation valley system has an estimated depth of at least 152 m and is up to eight kilometres wide with a preserved length of about 50 kilometres. The Juwayl Formation .valley walls are exceptionally steep here with an estimated gradient of about 40 degrees. They are lined with a very poorly-sorted, caliche-cemented, boulder conglomerate and breccia. The clasts are mostly composed of the valley wall host rock in the form of clasts and large slump blocks. Less abundant basement-derived clasts are present and consist of metasedimentary and igneous/plutonic clasts, which were probably derived from the Arabian shield. This valley is mostly exhumed so the sediments which filled the center of the valley are poorly exposed. Juwayl Formation valleys are well exposed at Jebe1 al Ahmar (lat. 20” 00’ 53.3” N, long. 44” 35’ 05.8” E) and Bani Khurb (lat. 19” 50’ 34.5” N, long. 44” 39’ 39” E). At both localities the Juwayl Formation consists of a series of stacked channel sandstones and debris/grain flows. The sands are fine- to coarsegrained, moderate- to very poorly-sorted and locally conglomeratic. The bulk lithology ranges between sublitharenite/subarkose to quartzarenite. Intraformational slumps and rafted blocks of Juwayl Formation sandstone up to several metres long are incorporated within the channel fill. An exceptionally well exposed section of the Juway1 Formation is found at Bani Khurb. Here the Juway1 Formation is about 39.6 m thick and rests unconformably on the ‘upper’ Khusayyayn Formation. The basal Juwayl Formation consists of about 6.1 m of poorly-sorted conglomerate and pebbly sandstone. The pebbles and cobbles are mostly subrounded to

435

rounded quartzite and rare chert. Ripups of the underlying ‘upper’ Khusayyayn Formation form a conspicuous part of the basal Juwayl Formation. Both populations of clasts are imbricated along channel bases. The remaining 33.5 m of the section is composed mostly of sandstone and occasional thin conglomerate beds of widely variable gram size, sorting and textural maturity. At the Bani Khurb exposures dip measurements on about 45 clasts and crossbedding surfaces were made. These measurements suggests a more northerly transport direction for the Juwayl Formation than for the underlying ‘upper’ Khusayyayn Formation. Depositional environments, age and periods of deformation. The Juwayl Formation has been interpreted to be of fluvial to glacial origin. In several widely scattered Juwayl Formation outcrops, ‘striated’ sandstones are found interbedded within the Juwayl Formation valley sediments. These striated surfaces, and/or the Juwayl sediments, have been attributed by some workers to the processes of glacial sedimentation (Helal, 1968; McClure et al., 1988; Vaslet, 1989). The Juwayl Formation ‘striations’ are similar to those observed in the Sanamah Formation and both probably had similar origins ~(see discussion of the Sanamah Formation later). The glacial depositional model for the Juwayl Formation has been in dispute almost since its first proposal (see Hadley and Schmidt, 1975). A reevaluation of the formation suggests that, taken as a whole, it was probably deposited in fluvial through to shallow marine settings. This conclusion is based on field studies of the reported ‘tillites’ in the Juwayl Formation, the gross composition of the valley sediment fill, the sedimentary structures present within these sediments and the geometry of the Juwayl Formation valleys present in the outcrop and in the adjacent subsurface. It is probable that the lower part of the Juwayl Formation was deposited in fluvial settings while the sections similar to those of the Bani Khurb area may have been deposited in subaqueous settings. The Juwayl Formation in outcrop has produced no biostratigraphically significant taxa, but microfossils found in nearby wells have dated the unit as Stephanian: Late Carboniferous to early Late Permian age. In the lower parts of the JuwayE Formation, the Early through to Middle Stephanian forms Vullatispori tes, Cirrutrirudites, Cris tutisporites and Krueuselisporites urnutus (News) Owens, Mishell and Marshall 1976 are rare to common (Fig. 10). Abundant monosaccate pollen, rare bisaccate pollen and miospores including Densosptites, Rudiizonuhts and Cycdldopitessp. cf. C. cymbutus (Balme and Hennelly) Bharadwaj and Salujha 1964 also compose the known flora of the lower Juwayl. The Lower Permian (Artinskian to Asselian)

T. E. STUMP and J. G. VAN DER EEM

436

ii

.” .-

(X 800) ’ (x

Figure 10. Characteristic

palynomorphs

of the Late Carboniferous

(Early/Middle

--

600)

Stephanian) of southwestern Saudi Arabia.

The stratigraphy,

depositional environments

Figure11.Characteristic palynomorphs

and periods of deformation of the Wajid outcrop belt

of the Farly Permian (Sakmarian/Artinskian)

of southwestern

Saudi Axlabia.

437

T. E. STUMP and 1. G. VAN DER EEM

438

~o#osuccute pollen (X 500)

Corisaccites alutcu Venkatachala & Kar 1966

-4

fuec~i~~~rite~

vir~~iue

Vittuti~u SD. (X 400)’

Potonie 6 Klaus 1954 (X 400) Figure 12. Characteristic

palynomorphs

of the Late Permian (Kazanian) of southwestern

portion of the Juwayl Formation is characterized by abundant large bisaccate pollen, common to abundant monosaccate pollen and striate, bisaccate pollen dominated by Protohuploxypinus. Common Vittufina and the forms CosfupoZZenifes ellipticus Tschudy and Kosanke 1966, Cuheniusuccites, Buruka-rites rofutus (Bahne and Hennelly) Bharadwaj and Tiwari 1964, IZZinites units Kosanke 1950 and Kingiucolpifes subcircuZuris Tiwari and Moiz 1971 (Fig. 11) are also present in this part of the unit. In the Upper Permian (Kazanian) portion of the Juwayl Formation, abundant small to large bisaccate and common monosaccate pollen and the forms Lueckisporites virkkiue Potonie and Klaus 1954, Vittutinu spp. and Corisuccites ulufus Venkatachala and Kar 1966 are present. In Saudi Arabia, the Hercynian orogeny occurred during the Middle Carboniferous (Stump et al., 1993). The Juwayl Formation valleys present in outcrop and on the available seismic data were probably formed as a response to this uplift.

Saudi Arabia.

SUPER GROUP VI Khuff and Sudair Formations Sediments of Super Group VI (Stump et al., in press) of Early Permian through to Early Triassic age unconformably overlie the Juwayl/Unayzah Formations and are composed of the Khuff and Sudair Formations. The Khuff Formation was deposited during a major period of coastal onlap which, for the first time in the Phanerozoic, brought widespread carbonate/evaporite deposition to the Arabian platform. The Sudair shale is composed of mudstones/siltstones and minor sands. This formation was deposited in settings ranging from tidal flat and lagoon to coastal plain and was deposited during a period of coastal offlap. Regionally, sediments of Super Group VI rest unconformably below the Jilh Formation of latest Early through early Late Triassic age. The Khuff Formation in southwestern Arabia is

The stratigraphy,

depositional environments

Figure 13. Characteristic

palynomorphs

and periods of deformation of the Wajid outcrop belt

of the Late Permian (Tartarian) of southwestern Saudi Arabia.

Late Permian (Tatarian) in age. In the formation, abundant small to large bisaccate and common to rare monosaccate pollen are seen. Striate, bisaccate pollen dominated by Stiutoahieitesand common Lueckisputites vi&k& Potonie and Klaus 1954 are characteristic floral elements (Fig. 12). Rare forms, such as Gnisuccifessp. A and the fungal spore ChurdecysfiachalustuFoster 1979, are also found (Fig. 13). Near the end of the Early Triassic, the Arabian Peninsula underwent another period of tectonism. At this time, the Qatar-South Fars arch was reactivated

(Alsharhan and Kendall, 1986) and parts of Arabia were uplifted and exposed to erosion: This period of tectonism corresponds to the Early Kimmerian orogenie phase. In central and southwestern Saudi Arabia this period of deformation is reflected by the thickening of the Jilh Formation on the down-thrown sides of basement seated faults (see McGillivray and Husseini, 1992) and the local erosion of the Sudair/Khuff formations from the higher fault blocks. These movements were the most pronounced during the deposition of the Lower to Middle Jilh Formation

T. E. STUMP and J. G. VAN DER EEM

440

(cu Upper Scythian to Ladinian) and had mostly ceased by the time the Upper Jilh Formation was deposited (ca Early to Middle Carnian). Acknowledgments Many useful and valuable discussions were conducted with several Saudi Aramco personnel during the completion of this work. Many stimulating discussions, mostly held in the field, concerning regional relationships and environments of deposition occurred between the authors, D. S. Evans and T. C. Connally. Highly productive sedimentological debates also occurred between the authors and W. A. King, G. H. Al-Aida, J. A. Al-Hajhog, C. J. Heine and W. M. Petersen. The authors also appreciate the skillful editing of H. Al-Tayyar and J. Filatoff and also express their appreciation to the Ministry of Petroleum and Minerals and Saudi Aramco for permission to share previously unpublished data. The authors and their colleagues agree that much has yet to be learned from the vast, well exposed geological laboratory that is southwestern Saudi Arabia. REFERENCES Alsharhan, A. S. and Kendall, C. 1986. Precambrian to Jurassic rocks of Arabian Gulf: Their facies, depositional setting, and hydrocarbon habitat. Bulletin American Association Petroleum Geologis fs 70, 9771002. Alsharhan, A. S., Nairn, A. E. M. and Shegewi, 0. 1991. The Paleozoic sandstones of the Rub Al Khali Basin, Arabia, a review. Palaeogeography, Palaeoclimatology, Palaeoecology 85161-168. Beuf, S., Biju-Duval, B., de Charpal, O., Rognon, D., Gariel, 0. and Bennacef, A. 1971. Les gres du Paleozoique inferieur du Sahara. Ed. Technip, Insfitut Francais Petrole, Science Technique Petrole, 18,464~. Clark, M. D. 1986. Explanatory notes to the geologic map of the Al Bad quadrangle, Sheet 28A, Kingdom of Saudi Arabia. Saudi Arabian Deputy Ministry for Mineral Resources. Dabbagh, M. E. and Rogers, J. J. W. 1983. Depositional environments and tectonic significance of the Wajid Sandstone of southern Saudi Arabia. Journal African Earth Sciences 1,47-57. Destombes, J., Holland, H. and Willefert, S. 1985. Lower Paleozoic rocks of Morocco. In: Lower Paleozoic of northwestern and west-central Africa, 4, Lower Paleozoic rocks of the world (Edited by Holland, C. H.) 512~. John Wiley. Deynoux, M., Sougy, J. and Trompette, R. 1985. Lower Paleozoic rocks of west Africa and the western part of central Africa. In: Lower Paleozoic of northwestern and west-central Africa, 4, Lower Paleozoic rocks of the world (Edited by Holland, C. H.)

512~. John Wiley. Evans, D. S., Lathon, R. B., Senalp, M. and Connally, T. C. 1991. Stratigraphy of the Wajid sandstone of southwestern Saudi Arabia. Society Petroleum Engineering. SPE publication SPE 21449,947-960. Ferguson, G. S. and Chambers, T. M. 1991. Subsurface stratigraphy, depositional history and reservoir development of the Early to Late Permian Unayzah Formation in central Saudi Arabia. Society Petroleum Engineering. March 2-5, Manama, Bahrain. Forey, I’. L., Young, V. T. and McClure, H. A. 1992. Lower Devonian fishes from Saudi Arabia. Bulletin British Museum (Natural Histo y) GeologtJ 48‘25-43. Grainger, D. J. and Hanif, M. R. 1989. Explanatory notes to the geologic map of the Shaghab quadrangle, Sheet 27l3, Kingdom of Saudi Arabia. Ministry Petroleum Mineral Resources. Hadley, D. G. and Schmidt, D. L. 1975. Non-glacial origin for conglomerate beds in the Wajid sandstone of Saudi Arabia. In: Gondwana Geology (Edited by Campbell, K. S. W.) ~~357-371. Nat. Univ. Press, Canberra Australia. Helal, A. H. 1968. Stratigraphy of outcropping Paleozoic rocks around the northern edge of the Arabian Shield (within Saudi Arabia). Zeitschrift Deutschengeologischen Gesellschaft 117,506-543. Hughes-Clark, M. W. 1988. Stratigraphy and rock unit nomenclature in the oil-producing area of interior Oman. ]ournaZ Petroleum Geology 11,5-60. Kellogg, K. S., Janjou, D., Minoux, L. and Fourniguet, J. 1985. Explanatory notes to the geological map of the Wadi Tathlith quadrangle, Sheet 20G, Kingdom of Saudi Arabia. Ministry Petroleum Mineral Resources, 27~. Klitzsch, E. 1981. Lower Paleozoic rocks of Libya, Egypt, and Sudan. In: Lower Paleozoic of the Middle Easf, Eastern and Southern Africa and Antarcfica, 3, Lower Paleozoic rocks of the world (Edited by Holland, C. H.) 331~. John Wiley. Lecorche, J. I’., Roussel, J., Sougy, J. and Guetat, Z. 1983. An interpretation of the geology of the Mauritanides erogenic belt (west Africa) in the light of geophysical data. In: Contributions fo the tectonic of mountain chains (Edited by Hatcher, R. D. Jr and Zietz, I.) Geological Sociefy America Memoir 158,131-149. Legrand, I’. 1985a. Lower Paleozoic rocks of Tunisia. In: Lower Paleozoic of northwestern and west-central Africa, 4, Lower Paleozoic rocks of the world (Edited by Holland, C. H.) 512~. John Wiley. Legrand, I’. 1985b. Lower Paleozoic rocks of Algeria. In: Lower Paleozoic of northwestern and west-central Africa, 4, Lower Paleozoic rocks of the world (Edited by Holland, C. H.) 512~. John Wiley. Mahmoud, M. D., Vaslet, D. and Husseini, M. I. 1992. The lower Silurian Qalibah Formation of Saudi Arabia: an important hydrocarbon source rock. Bulletin American Association Petroleum Geologisfs 76,1491-1506.

The stratigraphy, depositional environments and periods of deformation of the Wajid outcrop belt

McClure, H. A., Husset, E. and Kaill, P. 1988. Permian-Carboniferous glacial deposits in southern Saudi Arabia. Geologisches Jahrbuch 68,3-31. McGill&ray, J. G. and Husseini, M. I. 1992. The Paleozoic petroleum geology of Central Arabia. Bullefin American Association Petroleum Geologists 76, 1473-1490. Pallister, J. S. 1982. Reconnaissance geological of the Jabal al Ilman quadrangle, Sheet 18/44A, Kingdom of Saudi Arabia. United States Geological Suwey, Open-File Report USGS-OF-02~90. Powers, R. W. 1968. Lexique stratigraphique international. Vol III, Asie, East. lO.Bl, Saudi Arabia. Centre National Recherche Scienfifique, Paris 177~. Powers, R. W., Ramirez, L. F., Redmond, C. D. and Elberg, E. L. Jr. 1966. Sedimentary geology of Saudi Arabia. In: Geology of the Arabian Peninsula United States Geological Survey Professional. Paper 56OD, ppl-147. Stoeser, D. 8. and Greenwood, W. R. 1984. The Hijinah uplift and regional gravity sliding in the Wajid sandstone, Kingdom of Saudi Arabia. Ministry Petroleum Mineral Resources, Technical Record USGSTR-O4-18,23p. Stump, T. E., Connally, T. C. and van der Eem, J. G. L. A. 1993. The ?Late Precambrian to Early Triassic stratigraphy of the Kingdom of Saudi Arabia. Saudi Aramco Miscellaneous Geological Report No. 1014.

441

Stump, T. E., Al-Hajri S., van der Eem, J. G. L. A. (In press). The geology and palyno-stratigraphy of the Late l&Cambrian through Paleozoic sediments of of Saudi Arabia. In: The Paleozoic Palyno-sfrafigraphy the Kingdom of Saudi Arabia (Edited by Owens, B., Al-Tayyar, H. A., van der Eem, J. G. L. A. and AlHajri, S.). Reviau Paleobofany Palynolom. Vaslet, D. 1987. The Paleozoic (Pre-Late Permian) of Central Arabia and correlations with neighboring regions. Minisfry Petroleum Mineral Resources Technical Record BRGM-TR-O7-2,161~. Vaslet, D. 1989. Late Ordovician glacial deposits in Saudi Arabia: a lithostratigraphic revision of the early Paleozoic succession. Deputy Ministry Mineral Resources, Project paper 3,44p. Vaslet, D. 1990. Upper Ordovician glacial deposits in Saudi Arabia. Episodes 13,147-161. Vaslet, D., Beurrier, M., Villey, M., Manivit, J., le Strat, P., le Nindre, Y., Berthiaux, A., Brosse, Jr and Fourniguet, J. 1985. Exploratory notes to the geologic map of the Al Faydah quadrangle, Sheet 25G, Kingdom of Saudi Arabia. Ministry Petroleum Mineral Resources 28~. Vaslet, D., Kellogg, K. S., Berthiaux, A., le S&at, I’. and Vincent, I’. L. 1987. Explanatory notes to the geologic map of the Baq’a quadrangle, Sheet 27F, Kingdom of Saudi Arabia. Ministry Petroleum Mineral Resources 45~.