The Middle to Upper Ordovician Qasim Formation is well exposed in the Qasim region of central Saudi Arabia and is recognized from many wells to the north and east. It consists of the Hanadir, Kahfah, Ra’an, and Quwarah members that are arranged in two coarsening-upward progradational sequences that have a total thickness of about 200 m in outcrops. Two sections were measured in each sequence.
The lower sequence is a storm-dominated, shallow-marine depositional system composed of the late Llanvirn to middle Caradoc Hanadir and Kahfah. At least five progradational beach parasequences were identified. The Hanadir overlies deltaic deposits of the upper Saq Formation and marks a major marine transgression onto Gondwana. It is composed of laminated fissile shale formed in an offshore marine environment together with siltstone laminae that indicate periodic influxes of low-density turbidity currents. The overlying Kahfah is composed of shale and sandstone.
The upper progradational sequence consists of the middle Caradoc to middle Ashgill Ra’an and Quwarah members. The Ra’an is a succession of homogeneous fissile shale and minor siltstone laminae formed in an offshore marine environment following a major transgression onto the Kahfah. The overlying Quwarah consists of sandstone and minor amounts of siltstone and shale in its lower parts, and a thickly bedded sandstone facies above. The diagnostic sedimentary structures in the Quwarah are large-scale lens-shaped tidal sand waves composed of sigmoidal bundles deposited in a mesotidal setting of barrier bars, tidal channels, ebb-dominated deltas and lagoons.
In the subsurface, the Formation thickens northeastward (basinward) to over 4,000 ft (1,220 m). In the same direction, the sand-dominated Kahfah and Quwarah gradually become shale-dominated, small-scale bed forms are more abundant, and identification of members based on lithofacies is more difficult.
After deposition of the Quwarah, present-day Saudi Arabia was affected by a Late Ordovician (late Ashgill) glaciation. The Qasim and older formations are deeply incised by glacial channels at the base of the Zarqa and Sarah formations. Basinward, the glacial unconformities (or their submarine erosion surface equivalents) become less significant and a thick succession of the Formation is preserved.
Outcrops of lower Paleozoic siliciclastic rocks in central Saudi Arabia (Figure 1) provide excellent opportunities for the analysis of sedimentary facies and the development of a sequence stratigraphic framework. Shallow-marine and shelf-clastic sequences of the Middle to Upper Ordovician Hanadir, Kahfah, Ra’an, and Quwarah members of the Qasim Formation are well exposed and display a variety of depositional facies (Figure 2). The exceptional exposure allows stratigraphic correlations to be made in the field by tracing the various sedimentary sequences between measured sections. Used with subsurface data from wells to the north and east of the outcrop belts, they allow the identification of the proximal and distal developments of the Qasim shallow-marine sequences.
Part I of this paper is an overview of the regional setting of the Qasim outcrops on the margin of the Arabian Shield and the distal development of the Formation in the subsurface to the north and east. We then describe the Qasim and its members in a lexicon style that includes lithology, thickness, fossil content and age. Each stratigraphic unit is defined from literature sources and the extensive fieldwork and drill-core investigations made by the authors. Much of the work presented here has been documented by M. Senalp in unpublished internal Saudi Aramco reports on core descriptions (Sedimentological Memoranda 71, 73, 74, 79 and 80; 1992) and field trips (for example, Northern Area Field Trip, 1992).
Part II consists of the description of four measured sections, principally in the arenaceous Kahfah and Quwarah members, and an interpretation of their depositional environments. The deposition of the constituent sand bodies is discussed in terms of storm- and wave-dominated processes and tidal effects, and the relationships between the sandbody architecture and relative sea-level fluctuations are described.
The Hanadir and Khafah are placed in a sequence stratigraphic context. The terms ‘marine flooding surface’ (mfs) and ‘parasequence’ are used in the manner of Van Wagoner et al. (1990) and ‘Maximum Flooding Surface’ (MFS) is after Sharland et al. (2001). The marine flooding surface is a surface separating younger from older strata across which there is evidence of an abrupt increase in water depth. The parasequence is a relatively conformable succession of genetically related beds or bedsets bounded by marine flooding surfaces or their correlative surfaces. A succession of genetically related parasequences was defined as a parasequence set by Van Wagoner et al. (1990). Stacking patterns of parasequences within a parasequence sets are progradational, retrogradational or aggradational, depending on the ratio of depositional rates to accommodation rates. A Maximum Flooding Surface (MFS) caps a transgressive system tract and it is commonly characterized by extensive condensation and the widest landward extent of the marine facies. The MFS is believed to be isochronous and primarily controlled by eustatic fluctuation. The use of MFS allows the existing disparate lithostratigraphic schemes across the Arabian Plate to be placed for the first time within a unifying, sequence stratigraphic framework (Sharland et al., 2001).
The subsurface geology of the Qasim Formation is based on information from 176 wells drilled either for exploration by Saudi Aramco or for water by the Ministry of Agriculture. Of the 24 key wells shown in Figure 3, 22 have complete sections through the Formation and 2 are cut by the Sarah glacial channels. Other wells mentioned in the text and shown on Figure 3 do not have complete sections through the Qasim. Figure 4 illustrates the subsurface stratigraphy and log characteristics of the Qasim Formation in a representative exploratory well in central Saudi Arabia. Biostratigraphic data were obtained from palynological reports prepared by contractors for Saudi Aramco.
PART I REGIONAL GEOLOGICAL SETTING
The Qasim Formation was deposited during Middle and Late Ordovician times on a gently dipping, wide and stable marine shelf bordering the Paleo-Tethys ocean. The shelf was tilted toward the present-day northeast to provide accommodation space for the deposition of a siliciclastic wedge more than 4,000 ft (1,220 m) thick on the edge of Gondwana.
In Saudi Arabia, the Qasim Formation and the overlying glacial/periglacial Zarqa and Sarah formations are exposed in two outcrop belts that have an aggregate length of approximately 900 km along the northern and northeastern margins of the Arabian Shield (Figures 1 and 3).
Within the Arabian sequence stratigraphic framework of Sharland et al. (2001), the Qasim is located in the upper part of tectonostratigraphic megasequence AP2. Maximum flooding surfaces (MFS) designated as Ordovician 30 and Ordovician 40 (MFS O30 and MFS O40), occur near the base of the Hanadir and near the base of the Ra’an, respectively. Both MFS are major regional events that are recognizable across the Arabian Plate. A major erosional surface dated at 445 Ma marks the top of the Quwarah Member and corresponds to glacial channeling at the base of the Zarqa Formation.
According to Sharland et al. (2001), the O30 maximum flooding event caused deposition of ‘outer shelf’ shales far into western areas of the Arabian Plate along practically the whole length of the plate.
Earlier sequences of mainly continental sediments were rapidly transgressed by outer shelf shales in Jordan, Saudi Arabia, and Oman. Black graptolitic shales of late Llanvirn age represent this O30 MFS, dated at 465 Ma. Sharland et al. (2001) interpret the marked environmental change as most likely due to increased rates of subsidence resulting from a phase of rifting. They consider that the subsidence may have been enhanced by a ‘lower order’ rise in eustatic sea level and that a rapid transgressive systems tract (TST) was followed by a long-lived period of deposition during highstand conditions.
In central Saudi Arabia, the shallow-marine sedimentation in the Saq Formation ended with possible development of a minor unconformity at around the close of the Arenig (Vaslet, 1989). This locally eroded part of the underlying upper Saq, and the poorly sorted basal transgressive sandstones of the predominantly shaly Hanadir Member were deposited in the O30 TST.
According to Sharland et al. (2001), the lateral equivalents of the members of the Qasim Formation have Arabian Plate-wide distribution (Tables 1 and 2). The tables show plate-wide correlations at the levels of MFS O30 and MFS O40.
The reference section for MFS O30 is in deep-water shales that have an abundant pelagic fauna within the lower part of the Hanadir Member (El-Khayal and Romano, 1988). Age-diagnostic fossils are graptolites of the Didymograptus murchisoni Zone (El-Khayal and Romano, 1988), trilobites, chitinozoa (McClure, 1988), and acritarchs. The age of MFS O30 is early Middle Ordovician (early late Llanvirn) (465 Ma).
The reference section for MFS O40 is in deep-water shales near the base of the Ra’an Member of the Qasim Formation (Vaslet et al. 1987). The shales have an abundant pelagic fauna and age-diagnostic fossils are graptolites of the Dicranograptus clingani Zone. MFS O40 is dated at 453 Ma (Late Ordovician; late Caradoc).
LITHOSTRATIGRAPHY AND DATING
Definition and Type Section
Figure 5 is a comparison chart that shows the evolution of nomenclature relating to the Qasim Formation. The Formation was first defined by Williams et al. (1986) in the Jabal Habashi quadrangle in the Qasim region. The Qasim corresponds to the lower part of the Tabuk Formation1 as defined by Powers et al. (1966) and amended by Powers (1968). It was also described by Manivit et al. (1986) in the Buraydah quadrangle, and by Vaslet et al. (1987) in the Baq’a quadrangle of the Ha’il region (Figure 1). No specific type section for the Formation has been designated, although Williams et al. (1986) report that the Qasim reference section corresponds to the reference sections of its four members, the Hanadir, Kahfah, Ra’an and Quwarah (Figure 2).
The Qasim crops out in two major belts around the northern and northeastern margin of the Arabian Shield over a distance of about 900 km. It was the area between Baq’a and Buraydah (Figure 1) in which Vaslet et al. (1987) reported outcrops as much as 37 km wide that was the subject of this present study.
Lithology and Thickness
In outcrop in Saudi Arabia, the Qasim Formation consists of two siliciclastic coarsening-upward sequences, each with a basal shale/siltstone member and an overlying sandstone member. The siliciclastic sequence of shales, siltstones and sandstones has a thickness of 194 m in the central part of its outcrop belt in the Baq’a quadrangle. In the subsurface, the Qasim thickens towards the north and northeast from less than 39 m near the Arabian Shield to more than 1,220 m in Iraq and Syria.
The basal argillaceous member of the Qasim (the Hanadir) conformably overlies the arenaceous Saq Formation. The contact is lithologically defined: the sandy and low gamma ray Saq is abruptly overlain by high-gamma ray fissile shales of the Hanadir. In the subsurface, the lower contact is intersected in the Qasim–801 water well. In outcrop, the contact is seen near Jal Al-Aswad at the northern end of the Al Hanadir cuesta (Figure 6a).
The upper boundary is marked by a pronounced unconformity. Along the Qasim outcrop belts in northern and central Saudi Arabia, significant parts of the formation have been removed by the effects of the Ashgillian (Late Ordovician) glacial activity. At the northern end of Khashm Al Madba’ah (26°23’08.2”N; 43°45’19”E), and in many places along the Khashm Ar-Ra’an, Jal As-Saqiyah, and Jal Az-Zarqa cuestas between Buraydah and Baq’a, the uppermost part of the Qasim is deeply incised and eroded and directly overlain by the glacial Zarqa (Williams et al., 1986), and Sarah formations (Vaslet et al., 1987). The Zarqa and Sarah formations can be distinguished in outcrop from the underlying Qasim by glacial features such as basement-derived boulders occurring as dropstones and in tillites, and the occurence of glacial pavements and striations (Senalp and Al-Laboun, 2000).
In the subsurface, the upper contact of the Qasim changes basinward as the magnitude of erosion at the base of the Zarqa and Sarah diminishes toward the north and northeast. In some areas, the ‘hot shales’ of the Qusaiba Formation directly overlie the Qasim and elsewhere the Qasim subcrops the pre-Unayzah unconformity.
Age and Fossils
The Ordovician Qasim is dated as late Llanvirnian to late Caradocian (possibly Ashgillian). Dating at outcrops is based mainly on the identification of graptolites, trilobites and chitinozoa that are common in the shales of the Hanadir and Ra’an members. Details of age diagnostic fossils are discussed in the sections dealing with individual members.
Definition and Type Section
The Hanadir Member is the lowermost unit of the Qasim Formation (Figure 2). The term was informally introduced by Holm et al. (1948; unpublished report in Al-Laboun, 1993) after the Al-Hanadir cuesta between Buraydah and Qusayba (Figure 1). The name was later applied as either the Hanadir Member (Powers et al., 1966; Powers, 1968; Clark-Lowes, 1980; Williams et al., 1986), or the Hanadir Shale (McClure, 1978; Al-Laboun, 1982). Helal (1965) used the term ‘ Didymograptus Shaly Member’ for the same unit (Figure 4). El-Khayal and Romano (1985, 1988) re-defined the Hanadir Shale as the Hanadir Formation and raised the Tabuk Formation (now discarded) to group status.
Lithology and Thickness
The units of the Hanadir are well exposed between Al-Hanadir and Jal At-Tiraq (Figure 1) where they form a steep SW-facing cuesta. In this area, the Member is between 22 and 25 m thick. The lower 5 m (Figure 6a), is composed of reddish-brown to greenish-gray mottled shale rich in graptolites and trilobites. The upper 20 m in the Jal Al-Aswad cuesta (Figure 6b), is a grayish, laminated, fissile and bioturbated shale with some graded-bedded siltstone laminae. The occurrence of siltstone laminae gradually increases upward.
In subsurface, the thickest section of the Hanadir (1,962 ft; 598 m) in Saudi Arabia is present in the Jalamid-1 well (Figure 3) and the log data show that it is represented by a homogeneous shale facies. The position of the boundary is confirmed palynologically. The anomalous thickness of the Hanadir in Jalamid-1 may be caused by a shale diapir.
The Hanadir was cored in the Kahfah-1 and Ain Dar-196 wells, and partly cored in Uthmaniyah-630 and Qasim-801. In Kahfah-1 and Qasim-801, core was recovered across the Saq/Hanadir boundary. The same facies is present in all the wells. The lowermost 6.5 ft (2 m) of the Hanadir consists of interbedded and interlaminated micaceous and strongly bioturbated shale and siltstone. The upper 71 ft (21.6 m) of the cored interval is gray, graptolite-rich, homogeneous shale with less than 2 percent of 1- to 3-mm-thick siltstone laminae.
In the Kahfah-1 well, 265 ft (81 m) of core was cut continuously from the Hanadir and the underlying Saq. The Hanadir is represented by a dominantly shale facies 198 ft (60 m) thick. The lower part consists of dark-gray to black, homogeneous, fissile, pyritic, micaceous shale, whereas in the upper part the homogeneous shale is interbedded with laminated shale. Light-gray siltstone and locally very fine-grained sandstone layers, together form less than 10 percent of the cored interval. The siltstone/sandstone layers are sharp-based but have diffused upper contacts. Graded bedding, well-developed current ripples, and bioturbation are the most common sedimentary features in the siltstone/sandstone units. In the basal part of the Hanadir, the black shales show a ‘hot’ gamma-ray log signature.
In outcrop, the Hanadir conformably overlies strongly burrowed and bioturbated sandstone of the Saq Formation (Figure 6a). The sharp lithologic break provides an easily recognizable landmark. The lower boundary of the member is intersected in several wells. In the water well Qasim-801 (M. Senalp, Saudi Aramco internal communication, 2000), the lower part of the Hanadir (77.5 ft; 23.6 m) and the upper part of the Saq were cored continuously and the boundary between them is conformable. The argillaceous Hanadir is lithologically well defined in outcrop but in Qasim-801 the underlying Saq Formation is interbedded with shales.
The member is conformably/gradationally overlain by the interbedded sandstone, siltstone, and shale sequence of the Kahfah Member (Figure 6b). The upper Hanadir/lower Kahfah boundary is placed where the interbedded sandstone and siltstone units dominate.
Age and Fossils
Graptolites of the Didymograptus murchisoni Zone were collected from the lower half of the Hanadir at the At Tiniyat cuesta in the Baq’a quadrangle (Vaslet et al., 1987). They indicate a late Llanvirnian age. Al-Hajri (1995) reported Llanvirn chitinozoans, such as Siphonochitina formosa and Laufeldochitina robusta. The upper part of the Hanadir has been assigned to the early Llandeilo by the presence of chitinozoans such as Linochitina pissotensis, Pognochitina spnifera and Hercochitina spp. (Al-Hajri, 1995). Shales in the Kahfah-1 well contain a diverse fauna of brachiopods, molluscs, ostracods, trilobites, and graptolites. El-Khayal and Romano (1988) described and identified the trace fossils that are abundant at the Saq/Hanadir boundary in the Qasim region.
Definition and Type Section
The Kahfah Member crops out in a NW-trending belt from 10 to 15 km wide between major cuestas of the Hanadir and Ra’an members. The term Kahfah was first introduced by Williams et al. (1986) and formally defined by Vaslet et al. (1987) from a locality near Al-Kahfah (Figure 1). It corresponds to the informal unit 2 (‘Scolithus sandstone’) of Powers (1968); the ‘Lower Tabuk Sandy Member’ of Helal (1964, 1965); the ‘Lower Sandstone Member’ of Clark-Lowes (1980); and the unnamed ‘Ordovician Sandstone’ of Al-Laboun (1982). All these units belonged to the now-discarded Tabuk Formation (Figure 5).
Lithology and Thickness
Figure 7 is an outcrop view of the Kahfah Member that shows two incomplete beach parasequences separated by a marine flooding surface. The thickness of the Member at Wadi Al-Makrim is reported to be 110 m by Williams et al. (1986). The Kahfah shows both lateral and vertical facies variations in the subsurface. The thickest section is in the Zalma-1 well. The sedimentology of the Member is summarized in limited studies of core from the Ain Dar-196, Farwan-1, Uqab-1, and Zalma-1 wells (M. Senalp, Saudi Aramco Sedimentology Memoranda, 1992).
In Farwan-1, the Kahfah is represented by a single coarsening-upward parasequence that consists predominantly of shale with lesser amounts of siltstone and sandstone. The entire section is dark-gray, micaceous, pyritic and fissile, and mottled due to strong bioturbation. There are many pelecypod molds in the shales.
In contrast, in the Ain Dar-196, Shedgum-212, and Hamdan-1 wells the Kahfah is composed of two well-developed coarsening-upward sequences. In Ain Dar-196, the upper parasequence is 207 ft (63 m) thick and the lower one is 245 ft (74.7 m) thick. The lower half of the each parasequence consists of interbedded shale and sandstone, and the upper half consists predominantly of sandstone. Except for thickness variations, the lithofacies are very similar to those in the measured sections. The Kahfah is represented by 27 ft (8.2 m) of fine to medium-grained, moderate- to well-sorted, slightly shaly and intensely tigillite-burrowed sandstone. Sedimentary structures have been destroyed due to the bioturbation.
In Fadhili-15, the Kahfah is about 500 ft (150 m) thick and its top is eroded by the pre-Khuff Unconformity and its lower boundary with the Ra’an is gradational. It is a good example of a coarsening-upward progradational sequence. The 114 ft (34.8 m) of core was cut from the sand-dominated upper part of the sequence. The Kahfah consists of 10- to 15-ft-thick (3–4.6 m) parasequence sets. It is composed of sandstone, shaly sandstone with thin beds of dark-gray shale between the parasequences. Clean sandstone facies at the top of the parasequences are white to gray, fine-grained, gently cross-bedded, partly micaceous and firmly cemented by silica. The shaly sandstone facies is very fine-grained, moderately sorted and silica cemented. The entire core is strongly burrowed by tigillites.
The lower and upper boundaries of the Kahfah are conformable. The lower contact of the Kahfah Member with the underlying Hanadir Member is gradational (Figure 6b) and the boundary is placed where the interbedded siltstone and sandstone facies dominate the section. In Ain Dar-196, the lowermost part of the Ra’an and the upper part of the Kahfah was cored continuously. The contact between these two members is sharp but conformable. The upper boundary of the Kahfah with the Ra’an is at the top of the upward-coarsening parasequence. The contact is sharp, which indicates either a rapid rise of sea level or basin subsidence (Senalp and Al-Duaiji, 1996). In many places along the Al-Hanadir cuesta, the Kahfah is cut into deeply by glacially formed paleochannels at the base of the Zarqa Formation. The contact between the red, tigillite-burrowed sandstones of the Kahfah Member and the thick disordered tillite of the Zarqa Formation can be seen in a road cut about 2 km from Uyun Al-Jiwa on the highway to Al-Quwarah.
Age and Fossils
The Kahfah Member was assigned by Vaslet et al. (1987) to the Llandeilian, based on its stratigraphic position between the Llanvirnian Hanadir, and the Caradocian Ra’an, and the presence of trilobites (Plaesiacomia vacuvertis Thomas; and Kerfornella sp.). Recently, Al-Hajri (1995) reported chitinozoans from water wells and exploratory petroleum wells in northwestern Saudi Arabia. He reported the presence of Lagenochitina ponceti that is a good marker of late Llandeilo age, and Calpichitina lenticularis that indicates an age not older than middle Caradoc. This association of chitinozoans provides a late Llandeilo to middle Caradoc age for the Kahfah.
Definition and Type Section
The Ra’an Member is named after Khashm Ar-Ra’an (Figure 1) and was re-defined by Williams et al. (1986). It corresponds to the informal unit 3 (or ‘Ra’an Shale member’) of Powers (1968); to the ‘Diplograptus shaly member’ of Helal (1964, 1965); to the ‘Ra’an shale’ of McClure (1978); to the Ra’an Member of Clark-Lowes (1980); and to the ‘Ra’an shale member’ of Al-Laboun (1982). In all these early studies, the Ra’an was included within the now discarded Tabuk Formation (Figure 5).
Lithology and Thickness
The measured thickness of the Ra’an Member at its type locality is almost 40 m. It consists of a basal 3.5-m-thick unit of gray to dark-red, finely laminated fissile shale and dark-brown laterally continuous graptolitic, micaceous siltstone layers. These beds are overlain by a 36.3-m-thick unit of cliff-forming gray to dark-gray, fissile shales, silty shales, and laterally discontinuous, partially burrowed sandy siltstones. Vaslet et al. (1987), Manivit et al. (1986) and Ekren et al. (1986) described similar successions of the Ra’an Member between the Buraydah and Ha’il.
The Ra’an was cored in the Ain Dar-196, Ain Dar-277 and Haradh-1 wells. In Ain Dar-196, the 60-ft-thick (18.3 m) cored section consists of the upper part of the Kahfah and the lower part of the Ra’an. The Ra’an core is 33 ft (10 m) of laminated and interbedded shale and sandstone. Dark-gray to black shale forms about 80 percent of the cored interval and the remainder is light-gray sandstone laminae and sandstone beds. The shale is organic rich, particularly in its lower parts. It is bioturbated, but the tigillite burrows that are abundant in the Kahfah are missing from the Ra’an due to the significant increase in water depth. Muscovite flakes are concentrated in some sandstone laminae but are not abundant. The sandstone intervals range in thickness from less than one millimeter to about 450 mm. They are fine- to very fine-grained, parallel laminated, wave rippled and bioturbated. Thick sandstone beds have undulatory, erosional bases that are in places covered with rip-up shale pebbles (M. Senalp, Saudi Aramco internal report, 2000).
An anomalously thick Ra’an succession is present in the Jalamid-1 well. The Ra’an and the Quwarah together are 2,556 ft (779 m) thick and appear to form one single progradational sequence. The lower boundary with the Kahfah is difficult to define but the upper boundary of the sequence is sharply cut by the glacial paleovalleys at the base of the Sarah Formation. The entire section consists of regularly interbedded shale, siltstone and sandstone facies.
In the type locality along the Khashm Ar-Ra’an cuesta, the lower contact of the Ra’an Member is covered by recent sabkha deposits (Figure 8a). However, the contact can be seen clearly along the Tiraq road where dark-gray fissile shales overlie the coastal eolian sandstones of the Kahfah Member. Manivit et al. (1986) and Ekren et al. (1986) described a thin bioclastic conglomerate of fragmental lingulids, conodonts and fish debris at the base of the Ra’an Member. The sharp contact of the Ra’an on the Kahfah indicates a sudden deepening of the sea floor during a major marine transgression. The lower boundary of the member is well defined in the subsurface where dark-gray shales overlie the Kahfah sandstones.
The contact of the Ra’an with the overlying Quwarah Member is gradational and both members together form the upper coarsening-upward progradational sequence of the Qasim Formation (Figure 2). The Ra’an and Quwarah members crop out at the southern end of the Khashm Ar-Ra’an cuesta (at the Quwarah type section), and in the Khashm Al-Madba’ah cuesta near Al Qar’a (Figure 8b). The gradational contact is placed where the interbedded sandstone and minor shale sequence of the Quwarah Member starts. In most wells, the upper boundary of the Ra’an is gradational with siltstone and sandstone of the overlying Quwarah. The contact between these genetically related members is placed at the base of the predominantly sandy Quwarah. However, in some places, as at Jal As-Saqiyah (Figure 1), the upper Ra’an is deeply incised by glacial paleochannels at the base of the Zarqa (Figure 8c)
Age and Fossils
The late middle to late Caradocian age of the Ra’an Member is well established. The range of the graptolite Orthograptus amplexicaulis that occurs in the lower part of the Member, is from the clingani Zone (middle Caradoc) to the anceps Zone (middle Ashgill) as reported by McClure (1988). Al-Hajri (1995) documented typical middle Caradoc chitinozoans, such as Jenkinochitina tanvillensis, and restricted the age of the Ra’an to the late middle to late Caradocian.
Definition and Type Section
The Quwarah Member was first defined by Williams et al. (1986) near Al-Quwarah (Figure 1), as the uppermost member of the Qasim Formation (Vaslet et al., 1987). The unit is a synonym and/or entirely or partially equivalent to the ‘Upper Tabuk Sandy Member’ of the now obsolete Tabuk Formation (Helal, 1964, 1965); the ‘Middle Sandstone Member’ of the Tabuk Formation (Clark-Lowes, 1980); and the lower part the unnamed Ordovician-Silurian sandstone member of the Tabuk Formation of Al-Laboun (1982; Figure 3). The Quwarah Member was also described by Manivit et al. (1986), Ekren et al. (1986), and Vaslet et al. (1987). The type section of the Member is located to the southeast of the Khashm Ar-Ra’an cuesta, about 7 km west of Al-Quwarah.
Lithology and Thickness
Williams et al. (1986) measured an 87.5-m-thick section of interbeded silty clay, ferruginous sandstone, micaceous sandstone, and bioclastic layers. The thickest section of the Quwarah (1,039 ft; 317 m) is in the Farwan-1 well.
Cores were cut from various parts of the Quwarah in the Kahfah-1, Zalma-1, Uqab-1, Farwan-1, Saqqar-2, and Ain Dar-196 wells. The Quwarah in Saqqar-2 is sandstone-dominated and the sand/shale ratio, bedding thickness, and sedimentary structures indicate that deposition took place in shallower water closer to the coastline than in the same section in other wells. The sandstone facies in Saqqar-2 is thickly bedded fine-grained micaceous, horizontal planar-bedded (upper flow regime) and trough cross-bedded with well-developed tidal bundles and vertical burrows. The shale laminae between the bedding planes are very thin. The sandstone is an upper shoreface and foreshore facies.
From Saqqar-2 to Kahfah-1, the sand/shale ratio and thickness of the sandstone beds gradually decrease. In the deeper part of the basin, the lithofacies of the Quwarah is similar in all cores. In Kahfah-1, 413 ft (125.9 m) of core is composed of interbedded sandstone and shale divided into 40 to 45-ft-thick (12–19 m) coarsening-upward parasequences. The shale at the base of the parasequence is dark-gray, homogeneous, pyritic, fissile and bioturbated and is overlain by an interbedded succession of shale, siltstone and sandstone. Sandstone and siltstone laminae show current ripples, lenticular bedding, and horizontal and vertical burrows. Thin sandstone beds with current ripples, graded bedding, flame structures, load casts, cut-and-fill structures, soft-sediment deformations and small-scale penecontemporaneous faulting indicate rapid deposition by turbidity currents on a wide shelf. The uppermost part of the parasequence consists of 95 percent sandstone and 5 percent shale. The sandstone beds are sharp-based, up to 10 ft (3 m) thick and have tidal bundles and herringbone cross-bedding.
The Quwarah is gradational with the underlying Ra’an in outcrop (Figure 9a) and subsurface. In both cases, the contact was picked at the base of the interbedded sandstone/shale sequence. In contrast, its upper boundary is well defined as it is deeply eroded by glacially formed paleochannels at the base of the Zarqa and Sarah in outcrop and in the shallower parts of the basin (Williams et al., 1986; Vaslet et al., 1987). Figure 9b shows the Quwarah cut by a glacial paleochannel at the base of the Sarah formation.
The Zarqa is not represented in the deeper parts of the basin but the contact between the Quwarah and the Sarah is still sharp, although there is no significant erosion. Here the base of the Sarah represents a subaequous erosional surface. In the absence of the Sarah (such as between channels or in the deepest parts of the basin) the ‘hot shale’ facies of the early Llandoverian Qusaiba Formation directly overlies the Quwarah.
Age and Fossils
The Quwarah was inferred by Williams et al. (1986) to be of late Caradocian to possible Ashgillian age. Based on the occurrence of typical chitinozoans, such as Belonechitina robusta, Armoricochitina nigerica, and Ancyrochitina merga within the Member, Al-Hajri (1995) assigned it a late Caradocian to middle Ashgillian age. Where the ‘hot shale’ facies of the early Llandoverian Qusaiba Formation directly overlies the Quwarah, a hiatus of latest Ashgillian to earliest Llandoverian age is present in the stratigraphic succession.
PART II MEASURED SECTIONS AND INTERPRETATIONS
Four detailed stratigraphic/sedimentologic sections (At-Tariq 1 and 2, and Al-Qar’a 1 and 2) were measured (Figure 1) and interpreted in terms of the depositional environment. Although the distance between At-Tiraq 1 and 2 is only 9 km, rapid lateral facies changes occur in a predictable pattern in the Kahfah member. The Al-Qar’a 1 and 2 sections were measured by Senalp from the prograding Ra’an and Quwarah members.
Lower Qasim Progradational Sequence
At-Tiraq 1 measured section (26°34’35”N., 43°22’54”E.) (
In the At-Tiraq 1 section, the upper 3 m of the Hanadir Member and 21 m of the Kahfah Member was measured. The Khafah is a succession of alternating greenish-gray shale and pale-red sandstone. Its lower boundary with the underlying Hanadir Member is transitional and the contact was placed at the first occurrence of hummocky cross-stratification (HCS) in the sandstone beds. Its upper boundary is an erosional surface formed by glacial paleochannels of the Sarah Formation. The sand/shale ratio, grain size and bed thickness of the sandstones gradually increases upward. Detailed sedimentologic studies indicated that the measured section consists of at least five vertically stacked thickening/coarsening-upward parasequences that range in thickness from 3.70 to 5.50 m. They are separated from each other by marine flooding surfaces. Each parasequence consists of a conformable succession of genetically related lithofacies.
The lower three parasequences are composed of grayish homogeneous slightly bioturbated offshore shales overlain by HCS sandstones (Figure 11a). The thickness of the HCS sandstone units range from 0.20 to 0.90 m. They are light-gray to pale-olive, fine- to very fine-grained, and have sharp and commonly erosive bases. These erosional surfaces are covered by pebble-sized shale rip-ups that are overlain by horizontal parallel laminations up to 5-cm-thick of the upper flow-current regime. The HCS sandstone beds may grade up into bioturbated shale through abundant well-developed interference wave ripples or, more commonly, the top contacts are disturbed by bioturbation.
The geometry and internal arrangement of the laminations in the hummocky cross-stratification is almost identical to that described by Walker (1982, 1984). The internal structure of the HCS sandstones (Figure 11b) consists of curving laminations, both convex-up (hummocks) and concave-up (swales), with maximum dips of about 10° to 17°. The upper 15 to 20 cm of the thicker (at least 40 cm thick) sandstone beds show abundant and well-developed small symmetrical oscillation ripples. The sandstone beds less than 20 cm thick within the parasequences are very fine-grained and contain excellent examples of small-scale symmetrical oscillation ripples.
The fourth parasequence is 5.40 m thick and is a perfect upward-coarsening sequence in which the sandstone beds thicken and the grain size becomes gradually coarser upward. The lower 0.8 m is a greenish-gray, laminated, fissile, slightly bioturbated shale. The overlying middle unit is 2.1 m thick and consists of interbedded shale and very fine-grained laminated sandstone in the lower part, which is capped by very fine-grained, wave-rippled HCS sandstone. The uppermost unit of the parasequence consists of a 2.5-m-thick light-brown, fine-grained, well-sorted, trough cross-bedded sandstone, partly tigillite-burrowed and bioturbated.
The fifth parasequence is 5.5 m thick and represents another coarsening-upward sequence. The lowermost 0.5-m-thick unit of greenish-gray shale and thin laminae of very fine-grained sandstone conformably overlies the fourth parasequence. It is overlain by 0.5 m of well-bedded, HCS sandstone with symmetrical oscillation ripples that, in turn, is conformably overlain by 3.0 m of thick, red to light-brown, very fine- to fine-grained, well-sorted, trough cross-bedded, and tigillite-burrowed sandstone. The uppermost 1.5 m is composed of fine- to medium-grained, characteristically planar-bedded (upper flow regime) and strongly tigillite-burrowed sandstone.
The fifth parasequence is conformably overlain by Unit 6 (Figure 10) that consists of about 3 m of regularly interbedded sandstone and shale. The sandstone is reddish, very fine-grained, hummocky cross-stratified and wave rippled. The top of the unit is an erosion surface at the base of the Sarah Formation within an incised glacial paleovalley.
At-Tiraq 2 measured section (26°35’20”N., 43°28’58”E.) (
In the At-Tiraq 2 measured section (9 km east of At-Tiraq 1), the Kahfah Member is about 18 m thick and conformable on the Hanadir. The boundary was placed where there is a sharp break in the lower slope of Al-Hanadir cuesta. The Kahfah Member is conformably overlain by shales of the Ra’an Member. The lowermost 3.0 m of the Kahfah is not exposed, but bulldozed sandstone blocks and slabs in this interval show hummocky cross stratification and well-preserved wave ripples.
The exposed 15-m-thick section is sandstone and shale, and the well-developed large-scale HCS sandstone facies that is common in the At-Tiraq 1 measured section is absent. The section is composed of pinkish, very fine- to fine-grained, very micaceous, gently dipping, planar bedded (internally planar laminated), and strongly Skolithos-burrowed (Figure 13) sandstone. However, after close examination of this apparently uniform sandstone succession it was possible to subdivide it into four parasequences from 2 to 3.5 m thick.
The parasequences are separated by three black iron-stained surfaces that have abundant diagenetic quartz crystals, well-rounded small quartz pebbles, shale rip-ups, and pelecypod shell fragments. It was established in the field that the surfaces correlate with the flooding surfaces in the At-Tiraq 1 measured section (Figure 14). The correlation shows that rapid facies changes occurred in the 9 km betweenthe sections.
The small pebbles on the iron-stained surfaces are transgressive lag deposits that indicate minor submarine erosion by wave and current action, and non-deposition. Subsequent cementation of these erosion and non-deposition surfaces led to the formation of firm grounds that were easily identified in the field but gradually become less distinct basinward. Although the pelecypod shell fragments on the blackened surfaces are winnowed and reworked by storms they are indigenous and were not eroded from underlying deposits.
Depositional environments of the lower Qasim progradational sequence (Hanadir and Kahfah members)
In well-exposed outcrops along the Al-Hanadir and Jal At-Tiraq cuestas, shales of the Hanadir Member and sandstones of the Kahfah Member form a coarsening-upward progradational sequence (Figure 8b) that resulted from a continuous seaward migration of the coastline.
The Hanadir shales are medium-gray, laminated, fissile, micaceous, and contain abundant trilobites and graptolites. Some graded silty laminae occur in the lower part of the succession, and some thin, graded, very fine-grained sandstone beds in the upper part. The predominantly argillaceous sediments were deposited from suspension in an open-marine shelf environment below storm-wave base. However, the presence of graded siltstone laminae and the rare occurrence of very fine-grained, thinly graded sandstone beds indicate periodic introduction of storm-related low-density turbidity currents.
This shale sequence, immediately overlying the strongly burrowed and bioturbated Saq Formation, represents a rapid marine transgression recognized as an important event over much of the Gondwana platform (El-Khayal and Romano, 1988). Sharland et al. (2001) located MFS O30 within the Hanadir shale sequence and correlated it regionally across the Arabian Plate (Figures 2, 6b; Table 1).
The interbedded shale and siltstone facies of the Hanadir Member pass gradually into the interbedded shale and HCS sandstone facies of the Kahfah. In the At-Tiraq 1 measured section, the Kahfah consists of vertically stacked complete and incomplete beach parasequences. From the base upward within each parasequence the following occur: (1) sandstone beds thicken; (2) sandstone/shale ratio increases; (3) grain size increases; (4) horizontal bioturbation decreases; and (5) vertical burrowing and bioturbation increase.
One complete parasequence (for example, unit 5 in At-Tiraq 1) begins at the base with medium-gray homogeneous or laminated shale, followed by interbedded shale and HCS sandstones. The first appearance of the HCS sandstone indicates a shallowing of the depositional site above storm wave base. Harms et al. (1975) suggested its formation, “by strong surges of varying direction that are generated by relatively large storm waves of rough sea ”. It is generally accepted that storm waves acting below the fair-weather wave base are one of the main agents forming HCS sandstones. The superimposed small-scale symmetrical oscillatory ripples suggest a combination of waning storm-generated unidirectional flow with superimposed oscillatory storm wave action (Dott and Bourgeois, 1982; Walker et al., 1983; Swift et al., 1983).
This interbedded shale and HCS sandstones in the lower part of the parasequences in the At-Tiraq 1 section represent the lower shoreface beach facies (Figure 15a). The upper part of the parasequence consists of tigillite-burrowed, trough cross-bedded sandstones and gently dipping planar-bedded sandstones. The poorly developed trough cross-bedded sandstone facies indicates deposition above the fair-weather wave base in the upper shoreface of the beach. The intensively tigillite-burrowed, gently seaward-dipping (west and southwest), planar-bedded (internally plane laminated) sandstone facies are representative of a beach foreshore environment. The burrowed sandstone is capped in some places by 1- to 2-m-thick, steeply cross-bedded coastal eolian sandstones (Figure 15b). However, in the At-Tiraq 2 measured section, the Kahfah Member is composed of vertically stacked foreshore facies because of a rapid facies change between the two measured sections.
In the At-Tiraq-1 measured section (Figure 10), parasequences 1 to 5 make up a distinct stacking pattern bounded by marine-flooding surfaces and form a typical prograding parasequence set.
The evidence strongly indicates that the Hanadir and Kahfah members of the Qasim Formation were deposited as a progradational sequence in a storm-dominated beach to shelf environment (Figure 15a). These parasequences and progradational parasequence set are very similar to those observed by the authors in the Late Cretaceous (Campanian) Blackhawk Formation in Book Cliffs, north of Price in Utah, USA (Van Wagoner et al., 1990).
Upper Qasim Progradational Sequence
The two Al-Qar’a measured sections are located near to the southern end of the Khashm Al-Madba’ah cuesta (Figure 1). The Ra’an and Quwarah members are exposed and were measured for a total thickness of 26 m (Al-Qar’a 1) and 18 m (Al-Qar’a 2). Although the two sections are less than 2 km apart, they show significant variations in thickness, rock types, sedimentary structures, and vertical lithofacies arrangements. The Al-Qar’a sections are divided into lithological units and no attempt has been made to interpret the successions in terms of sequence stratigraphy.
Al-Qar’a 1 measured section (26°22’20”N., 43°45’21”E.) (
The succession is divided into Units A and B (Ra’an Member) and Units C to F (Quwarah Member). The Ra’an is 17.5 m thick. The base of the section is not exposed, but its upper contact with the sandstones of the overlying Quwarah Member is gradational. Unit A forms the lower 15 m and is composed of dark-gray, homogeneous shale, interbedded with 1- to 2-mm-thick purple silty shale and discrete silt laminae. The section is cut by veins of secondary gypsum (satin spar). The overlying Unit B is 2.5 m thick and consists of alternating yellowish-gray to light-gray shale and siltstone. The shales contain graptolites and trilobites. A conodont layer about 1 cm thick was reported by Manivit et al. (1986) as being locally present near the base of the Member in the Jal al Aqba’ cuesta east of Jal Hanadir.
The overlying Quwarah Member (Units C to F) is 8.5 m thick and represents a coarsening-upward sequence. Its lower contact with the shales of the Ra’an is transitional, but its upper part is deeply incised by the Bukariyah glacial paleovalley of the Sarah Formation. Unit C is 1.0 m thick, thinly bedded, and consists of very fine-grained, well-sorted sandstone in beds ranging in thickness from 5 to 17 cm. Lenticular and flaser bedding, wave-modified current ripples, graded bedding, and bioturbation are common sedimentary structures. Unit D is 1.3 m thick and consists of well-bedded, fine-grained, well-sorted sandstone with abundant wave ripples and burrows. Unit E is 1.0 m thick and is composed of regularly alternating sandstone and shale in almost equal proportions.
The sandstone is very fine-grained, laminated and bioturbated. Unit F has a sharp base scoured into Unit E that is overlain by intraformational mud pebbles. The thickness of Unit F varies from 4 to 4.5 m as its upper part is truncated by the Sarah Formation. It is composed entirely of very fine-grained, well-sorted sandstone. Bedding surfaces are irregular and indicate minor scouring, and thin mud drapes are present between bedding planes in places.
Al-Qara 2 measured section (26°23’15”N., 43°46’27”E.) (
The section contains part of the Ra’an Member and the Quwarah Member. The succession is divided into units A to E. The Ra’an is about 5.5 m thick and consists of units A and B. The base of the section is not exposed. Unit A is 2.5 m thick and is composed of greenish-gray (weathering pale-blue), homogeneous, graptolitic shale capped by 0.2 m of reddish-brown, very fine-grained, horizontally burrowed sandstone. Unit B is a greenish-gray to yellowish-gray, partly homogeneous, partly laminated shale about 3.0 m thick. In the upper part of the unit, there are intervals of 1- to 5-cm-thick, continuous and discontinuous, very fine-grained wave-rippled sandstone.
The upper 12 m of the measured section (units C, D, and E) consists of sandstones of the Quwarah Member (Figure 18). Unit C conformably overlies the Ra’an. It is 1.5 m thick and is composed of light-colored, fine-grained, muddy sandstone (greater than 15 percent mud). Individual beds of sandstone are less than 7 cm thick and are separated by shale laminae. Flaser and lenticular bedding is common and wave ripples are well developed at the top of the cleaner sandstone beds. Bioturbation is common and Skolithos, Gordia, Planolites, Taenidium, Teichichnus, and Didymaulichnus burrows were identified within this unit from recent field work in the Qasim region. They indicate a mixed community dominated by deposit feeders (A. Taylor and R. Goldring, Consultants, oral communication, 2000).
Unit D is 1.5 m thick. Its lower contact is sharp and scoured and its upper contact is gradational. It consists of light-colored, fine- to medium-grained, well-sorted, large-scale cross-stratified and imbricated lens-shaped sandstone bodies (Figures 19a-c). They represent tidal sand waves and dominate Unit D. They are completely enclosed by silt and clayey-silt mud drapes up to 2.5 cm thick that contain abundant shell fragments. These partially or completely isolated sandstone bodies are as much as 0.65 m thick and 3 to 4 m wide. In many cases, the enclosing mud drapes have been completely weathered out. The bottomsets of the tidal sand waves pass down-flow into thinly bedded, wave and current rippled, bioturbated muddy sandstone facies of Unit C (Figure 19b). Their upper surfaces are locally overlain by trough cross-sets representing the migration of trains of megaripples analogous to those described by Homewood and Allen (1981) from Miocene molasse deposits in Switzerland.
The lens-shaped bed forms consist of a group of tidal bundles of foreset laminae deposited by avalanching under the dominant current conditions (Boersma, 1969; Visser, 1980). Each bundle was reworked by a subordinate current of lesser strength to form the asymmetric current ripples that climb up the avalanche slopes (Figure 19b). Following reworking, slack water allowed the fallout of fine-grained sediments (mud drapes). These mud drapes between tidal bundles are well developed in the lower half of the tidal sand waves. They gradually thicken toward bottomsets of the bundles but thin toward the top of the sand waves where they are replaced by erosional surfaces (reactivation surfaces). The foreset laminae within the bundles are generally sigmoidal and tangential to the bedding surfaces. The bundles show thickness and grain-size variations in a cyclic pattern laterally within the sand waves. Thin bundles are fine-grained whereas thicker bundles are medium-grained, indicating that the latter were formed under stronger tidal currents (spring tides) that were able to carry a coarser bed load (Figure 19c). Vertical burrows are abundant in the sandstones and horizontal burrows are common within the mud drapes.
Measured paleocurrent directions from the foreset beds indicate that the dominant current direction was toward the east (and rarely southeast) into the deeper part of the basin. The subordinate current is represented by current ripples on the lower parts of the foreset beds. The evidence is that the tidal bundles were produced by strong ebb-tidal currents.
Unit E is 9 m thick and forms a small steep cuesta (background Figure 19a). Its lower contact is gradational with Unit D but its upper contact is truncated by the Sarah Formation. It is a light-colored fine- to medium-grained, well-sorted sandstone unit with a large suite of sedimentary structures that provides persuasive evidence for a tidal mode of origin. However, the large-scale, lens-shaped cross-bedded sandstone (tidal sand waves) dominant in Unit D (Figure 19a) gradually disappear to be replaced by sandstone bodies up to 30 cm thick that contain large-scale cross-bedding with tidal bundles, horizontal planar bedding and well-developed reactivation surfaces. The reactivation surfaces (Figure 20) are 0.5 to 2 m wide, and sometimes two surfaces merge and become one continuous surface. Large-scale cross-beds are generally overlain by small-scale current ripples that are frequently cut by the reactivation surfaces and planar to undulatory scour surfaces. Tabular cross-laminations up to 10 cm thick that dip in the direction opposite to that of the dominant cross-bedding are evidence of bidirectional sediment transport (herringbone cross-bedding) in Unit E. Some parts of the section are massive and show strongly contorted bedding. Iron-oxide coated, suspension-feeding vertical burrows (mainly Skolithoss) and ?Asterosoma are common and help to distinguish the Quwarah Member from the Sarah Formation.
Depositional environments of the upper Qasim progradational sequence (Ra’an and Quwarah members)
The Ra’an and Quwarah members are conformable and together form the upper coarsening-upward progradational sequence of the Qasim Formation (Senalp and Al-Duaiji, 1996). The presence of graptolites, trilobites and other fossils, together with strong bioturbation indicate that the Ra’an shales were deposited in a marine environment below the effective wave base on a broad continental shelf. Its sharp basal contact with the very shallow-marine and coastal eolian sandstones of the underlying Kahfah marks a rapid regional transgression in response to a global rise in sea level. Sharland et al. (2001) defined this regional event across the Arabian Plate as MFS O40. The flat-pebble bioclastic conglomerate composed of fragmental lingulids, conodonts and fish debris at the base of the Ra’an (Ekren et al., 1986), marks the beginning of onlap onto the Kahfah. The graptolitic shales overlying the conglomerates indicate a rapid deepening of the sea to form an open shelf environment.
The thick section of the Ra’an in subsurface consists of three to four coarsening upward parasequences that can be identified from gamma-ray logs. Each parasequence has interbedded shale and thin, wave-rippled sandstone in their upper part, indicating that the overall transgression had brief periods of offlap when the sand supply into the basin increased. This regional transgression was followed by a major progradation that marked the beginning of deposition of the Quwarah when the sand supply increased significantly and shifted the coastline seaward.
The Quwarah in the Qasim region contains many sedimentary structures that indicate a tidal origin. The most diagnostic are tidal bundles and bundle sequences (Figures 19a–c). Such structures have been described in detail by Boersma (1969), Visser (1980), Terwindt (1981), Middleton (1991), and Nio and Yang (1991). These cross-bedded sedimentary structures are related to the regular increase and decrease of flow velocities during one tidal episode (tidal bundle) and to variations in tidal flow strengths during the lunar month (bundle sequence). Characteristically, they form laterally arranged groups of large-scale foreset beds that are separated by double mud drapes (mud couplets) or single mud drapes at centimeter to decimeter intervals (Nio and Yang, 1991). Each foreset group, representing sandy foresets deposited during a single dominant current stage, makes up a bundle (Visser, 1980) and each bundle is bounded by slack-water mud drapes. A series of bundles laid down by successive dominant currents is described as a tidal-bundle sequence (Figure 19c).
Distinct lateral thickness variations of the sand bundles can be recognized in the bundle sequences observed in the outcrops of the Quwarah Member. This systematic thickness variation within tidal bundles is related to increasing and then waning flow velocities during a tidal episode (a neap-to-spring cycle). Thin bundles correspond to weak dominant currents during neap tide, and thicker bundles to the strong dominant currents of the spring tide. A relatively minor change in tidal current velocity can have a major change in the rate of sand transportation.
Unit D in the Al-Qar’a 2 measured section is composed of large-scale, imbricate lenses of cross-stratified sandstone bodies (tidal sand waves) enclosed in thin mud drapes (Figures 19a–c). These lens-shaped sandstone bodies contain several sigmoidal tidal bundles and bundle sequences. Similar tidal bundles have been described by Clifton and Abbott (1979), Allen (1981), Homewood and Allen (1981), and Allen and Homewood (1984). Kreisa and Moiola (1986) described sigmoidal tidal bundles, gently inclined lateral accretion surfaces and low-angle truncation surfaces that they interpreted as having been formed in shallow subtidal to intertidal channels and estuaries. The paleocurrent patterns of these large-scale bed forms indicate a dominant basinward transport direction.
The overlying Unit E (Figures 19a, 20) is represented by horizontal plane bedding, large-scale megaripples with bundled foresets, herringbone cross-bedding and smaller-scale bed forms. The base of Unit D is sharp and scoured into the strongly burrowed, horizontally laminated muddy sandstone facies of unit C. Unit D and Unit E both pinch out within less than 2 km and are not represented in the Al-Qar’a 1 measured section. This is interpreted as further evidence of their formation in shallow subtidal to intertidal channels and migrating bars.
The large lens-shaped sandstone bodies with well-developed sigmoidal bundles and bundle sequences in Unit D and the laterally accreted tidal bundles in Unit E are believed to have formed as sand waves within the main ebb-tidal channels (Figure 21). In contrast, the horizontally bedded, flat-laminated, bi-modal land-oriented cross-bed sets and current-rippled sandstone facies of Unit E were formed by sand waves in the marginal flood channels and by swash bars (M.O. Hayes, Consultant, oral communication, 1997). Modern analogs of these structures develop in North Sea estuaries at tidal velocities of between 95 and 110 cm/sec (Kreisa and Moiola, 1986).
The Quwarah in the Al-Qar’a 1 measured section represents a continuous coarsening-upward sequence. Although the upper part of the sequence is deeply eroded by the Sarah, it clearly shows that the sandstone becomes cleaner upward, and sedimentary structures indicate shallower water depths. The lenticular, flaser, and wavy beddings occur in the lowermost part of the section, whereas channels with bi-directional cross-bedding, reactivation surfaces, and tidal bundles are common in the upper parts of the section. The sandstone beds are finer-grained but laterally more continuous than those in the Al-Qar’a 2 measured section.
Based on this evidence, the Quwarah in Al-Qar’a 1 is interpreted as forming a tidally influenced regressive barrier-island, and representing the shoreface and lower parts of the beach environment. The upper part of the beach, dune and lagoon facies was eroded by the Sarah glacial event. The regressive nature of the Quwarah is a result of seaward progradation of the coastline when nearshore sediments (beach and shoreface) prograded seaward over offshore sediments. This situation occurs when sediment is abundant and sea level is relatively stable or gently rising.
This brief review indicates that there is abundant evidence for a subtidal origin of the Quwarah as seen in Al-Qar’a 1 and Al-Qar’a 2. The morphological elements of its depositional system are interpreted as tidally influenced barrier-islands, tidal inlets, and associated ebb-tidal deltas (as defined by Hayes, 1969; 1979) in a mesotidal (tidal range of 2 to 4 m) coastline setting (Figure 21). According to Hayes (1979) wave-dominated barriers tend to be long and continuous with few inlets, whereas tidally influenced barriers are short and stunted, with numerous inlets. He defined an ebb-tidal delta as a lobate sand accumulation located seaward of a tidal inlet and deposited primarily by ebb-tidal currents and modified by waves.
DISCUSSION ON THE DEPOSITION OF ARENACEOUS COASTAL SEQUENCES AND THEIR MODERN ANALOGS
Because the intertidal zone is more easily accessible than the subtidal zone, modern intertidal sediments have been studied extensively whereas less attention has been paid to subtidal sediments. However, the preservation potential of the subtidal sediments is generally higher than for their intertidal counterparts. Consequently, most of the tide-dominated features recognized in the geologic record are of subtidal origin and criteria previously developed for the recognition of tidal deposits (Van Straaten, 1954; Evans, 1965; Reineck and Wunderlich, 1968; Klein, 1970a; 1970b; 1972; and Reinek and Singh, 1986) are based on observations within the intertidal and supratidal zones. Therefore, these criteria have limited geologic applications to the understanding of the entire depositional model of tidal deposits.
For decades, geologists have recognized shallow-water tidal facies in the rock record by using sedimentary structures that reflect the periodic reversals of tidal current directions that are separated by episodes of exposure or slack-water conditions (Kreisa and Moiola, 1986). These sedimentary structures, also common in the Quwarah, include lenticular, flaser, and wavy beddings (Reineck and Wunderlich, 1968) that are formed by alternating deposition of bedload sediments and suspended sediments. ‘Herring-bone’ cross-stratification, which results from current reversal, has become standard for interpreting tidal flat and shallow subtidal facies. Tidal processes are strongly influenced by the fluctuation of flow velocities. One of the consequences of such fluctuation is the presence of reactivation surfaces (or erosional surfaces: Klein, 1970a, 1970b), developed within the cross-stratified sets of tidal deposits. Reactivation surfaces can be produced by dominant currents as well as by dominant and subordinate currents (Visser, 1980). Reactivation surfaces related to dominant flow are formed during the dominant current phase(s) of the tidal cycle (Nio and Yang, 1991). They are similar to these formed in any unidirectional flow system, such as a fluvial channel. However, dominant-flow reactivation surfaces occurring with those formed by dominant and subordinate flows are unique features for tidal environments (Nio and Yang, 1991).
The authors greatly appreciate the importance of paleocurrent directions in any sedimentological study and basin analysis. This is one of many tools for finding orientation of the paleoshorelines. We plan to expand this study to other areas and establish regional patterns of sediment transport direction. Furthermore, measuring tidal-bundle thickness/frequency will also be helpful in establishing tidal dynamics for the Ordovician seaway.
From field studies and facies analysis, we conclude that the Middle to Late Ordovician Qasim Formation was deposited as two coarsening- and thickening-upward progradational siliciclastic sequences. Each sequence is characterized by a different set of geologic conditions acting over a broad and gently dipping shallow-marine shelf.
All the members of the Qasim were deposited in coastal to shallow-marine system tracts. There is continuity from the sandstones of storm- and tide-dominated shallow-marine coastal systems to the shales of the shelf/platform system. Basinward, the sand-dominated Kahfah and the Quwarah members are interbedded with the open-marine shales. The thickness and grain size of each sandstone bed decrease basinward. The large-scale cross-stratification in the sandstones are replaced by small-scale hummocky cross-stratification (HCS), and wave and current ripples. Toward the deepest part of the basin, the entire Qasim becomes shale dominated, and the identification of each member based on lithofacies becomes harder. Therefore, the distinction between the storm- and tide-dominated shelf systems is possible only in the shallower parts of the basin.
The lower sequence of the Hanadir and Kahfah members represents a series of storm-dominated, stacked beach parasequences. These are similar to parasequences described in the Blackhawk Formation (Late Cretaceous) exposed in the Book Cliffs, north of Price, Utah, USA (Van Wagoner et al., 1990). The upper progradational sequence consisting of the Ra’an and Quwarah Members is characterized by tide-dominated shoreline facies, similar to those described in Holocene North Sea tidal estuaries, and in the Jurassic Curtis Formation of Utah, USA (Kreisa and Moiola, 1986). Of particular significance is the recognition, for the first time in Saudi Arabia, of tidal bundles within the Quwarah Member. These features are formed only by shallow-water tidal processes and are especially useful for interpreting subtidal depositional environments.
The Middle Ordovician Hanadir and Kahfah members form the lower progradational sequence. The lower part of the Hanadir is composed of pelagic and hemipelagic, graptolite- and trilobite-bearing organic-rich shales. This condensed sequence of organic–rich shales is the result of a regional marine transgression over the Saq Formation due to eustatic sea level rise in the Middle Ordovician (Llanvirn). Toward the upper part, the shales are interbedded with thin siltstone beds deposited by weak, storm-generated turbidity currents. The Hanadir gradually passes upward into the interbedded sandstone and shale sequences of the storm-dominated beach and coastal dune facies of the Kahfah member. This member consists of stacked beach parasequences separated by high-order marine flooding surfaces. The beach parasequences are vertically stacked to form a progradational parasequence set, indicating basinward migration of the coastline due to an increasing rate of sediment supply. The lower shoreface facies is characterized by well-developed HCS sandstones interbedded with bioturbated pelagic shales. The conformably overlying upper shoreface facies is composed of trough-cross-bedded and partly tigillite-burrowed sandstone. The foreshore facies consists of gently dipping planar-bedded sandstone, strongly burrowed. This facies forms the entire Kahfah in the landward direction, but thins gradually in a basinward direction where it forms an insignificant part of the parasequences. The coastal-dune sandstones have been locally preserved in the successions between the foreshore sandstones, where not cut out by the base Zarqa/Sarah unconformity.
The Late Ordovician Ra’an and Quwarah members form the upper coarsening and thickening-upward progradational sequence of the Qasim. The Ra’an is composed of homogeneous and laminated shale and siltstone facies. The lowermost part of the succession is rich in graptolites and trilobites indicating another condensed section as the result of a very slow rate of deposition. The Ra’an was deposited during rapid regional transgression in the Late Ordovician (Caradoc) when a eustatic sea-level rise flooded the underlying shallow-marine/coastal plain deposits of the Kahfah Member. The subsequent rapid influx of coarse-grained clastics derived from the erosion of the rising basement and older formations (for example, the Saq Formation) caused a regressive cycle that culminated in the deposition of the Quwarah.
Tidal influence during the regressive cycle was strong, and is reflected in characteristic sedimentary structures in the sandstones, the most conspicuous features being the presence of large-scale, lens-shaped cross-stratification, enclosed within the fine-grained, burrowed sediments. These large-scale bed forms (tidal sand waves) are composed of sigmoidal tidal bundles and tidal-bundle sequences. They show cyclical variations in foreset thickness and easterly and southeasterly dominated paleocurrent directions that might suggest deposition within the tidal channels of estuaries, and ebbtidal delta settings. The juxtaposition of barrier-island and tidal-channel sequences suggest that barrier island, tidal-inlet and associated ebb-deltas in a mesotidal complex (tidal range of between 2 and 4 m) is a possible interpretation for the depositional environment of the Quwarah.
In the subsurface, each of the members of the Qasim Formation shows great increases in thickness due to the large accommodation space provided by the eastward-dipping continental shelf. Basinward, the sand-dominated Kahfah and Quwarah gradually become more shale dominated, and the sedimentary structures that indicate a storm- and tide-dominated shallow-marine environment gradually become less significant. Small-scale structures such as current ripples, graded bedding and soft-sediment deformations indicate deposition of the thin sandstone beds by weak turbidity currents. In the deeper parts of the basin, the entire Qasim is shale dominated and identification of members based on lithofacies is difficult.
In many modern shallow-marine and nearshore settings, tides and storms are recognized as nearly co-effective agents of sediment transport. However, most ancient shallow-marine depositional systems are interpreted typically as end products of two main processes, either storm- or tide-dominated. The dominance of storm or tides recorded within the stratigraphic sequence of the Qasim Formation reflects the changing paleo-oceanographic conditions.
This paper is published with the permission of the Saudi Arabian Ministry of Petroleum and Minerals and Saudi Aramco. We acknowledge critical discussions with our Saudi Aramco colleagues during the various field trips. Our thanks are also extended to Hassan S. Talu for his advice and help with the original figures. I thank my daughter, Derya Sinem Senalp, for her encouragement and help in typing the manuscript. Our special thanks and appreciation to the editorial and drafting staff of Gulf PetroLink and GeoArabia, and to David Casey of GeoArabia’s Editorial Board, for their continuous help in every stage of its publication.
ABOUT THE AUTHORS
Muhittin Senalp is a Senior Geological Consultant in the Geological Technology Department of Saudi Aramco. He has worked for Saudi Aramco since 1981. He previously worked in Turkey as a field geologist for 16 years and taught advanced sedimentology at the Middle East Technical University in Ankara. He has a BSc in Geology (1965) from the University of Istanbul, and an MSc in Petroleum Geology (1970) and PhD in Sedimentology (1974) from Imperial College, London. Muhattin built multilayer geologic models for the Khafji reservoirs of the Zuluf and Marjan fields (currently used in other simulation models), and has recently completed the Regional Depositional Model of the Unayzah reservoir study. He has published many papers on siliciclastic sedimentology.
Abdulaziz A. Al-Duaiji is a Regional Team Leader in the Geological R&D Division of Saudi Aramco. He joined Saudi Aramco in 1984 as a Wellsite Geologist. He became Northern Area Exploration Geologist in 1985 and a Research and Development Geologist in 1992. Abdulaziz has a BSc in Geology from the King Fahd University of Petroleum and Minerals, Dhahran (1984) and an MSc in Geology from Texas A&M University (1991). His professional interests include the regional and depositional geology of central Saudi Arabia, in particular the Permian Unayzah Formation and the Devonian Jauf Formation.