GeoArabia, Vol. 6, No. 2, 2001 Gulf PetroLink, Bahrain Qasim Formation: Ordovician Storm- and Tide-Dominated Shallow-Marine Siliciclastic Sequences, Central Saudi Arabia Muhittin Senalp and Abdulaziz A. Al-Duaiji Saudi Aramco ABSTRACT 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. INTRODUCTION 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. 233 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/6/2/233/4559788/senalp.pdf by guest on 27 September 2021 Senalp and Al-Duaiji 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 40° 50° 60° 28°N ° ° 42 E43BAQ'A Jal Az-Zarqa Qasim Formation S Zarqa Formation 30° 30° a q HA'IL F At Tiniyat HA'IL o S r a m r a a ARABIAN t i h AL-AJFAR SHIELD o RED SEA n F 20° 20° o r m Jal As- a t 0 500 Saqiyah i o Km Arabian Shield n 40° 50° AL-KAHFAH 27o 27° 050km Jal At-Tiraq QUSAYBA Khashm Ar-Ra'an AL-QUWARAH ° 15’ 43°30’ 44° 27 N To H Qusaybah Jal Al- a’il 020km Aswad Road Map Al-Quwarah Al-Hanadir Al Qar'a Khashm Al-Madba’ah At-Tiraq 1 At-Tiraq 2 26°30’ Uyun Al-Jiwa Al-Qar’a BURAYDAH Al-Qar’a 1 Jabal Saq Al-Qar’a 2 UNAYZAH Buraydah To Riyadh Jabal Saq 42 43° 44° ° Al-Bukayriyah 26 Riyad Al-Khabra Unayzah 26° Figure 1: Simplified geologic map of the Qasim region (modified after Vaslet, 1989). Inset shows the locations of measured sections At-Tiraq 1 and 2 and Al-Qar’a 1 and 2 in the Qasim Formation. 234 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/6/2/233/4559788/senalp.pdf by guest on 27 September 2021 Qasim Formation, central Saudi Arabia STRATIGRAPHY DEPOSITIONAL GENERALIZED FIGURE INDEX ENVIRONMENT LITHOLOGY (this paper) PERIOD Ludlow Sharawra Wave-dominated Member shallow marine Wenlock Qusaiba Member Offshore/Shelf QALIBAH FORMATION MFS S10 mfs Llandovery Glacially/ SARAH periglacially FORMATION incised valley ZARQA Glacial, glacio- FORMATION lacustrine/marine Ashgill Al-Qar’a 1, 2 Quwarah Tide-dominated 16, 17 Member shallow marine 18 20 19a 9b9b 9a 8b 8c Ra'an Member Offshore/Shelf 8a Caradoc MFS O40 mfs 13 15b Kahfah Storm-dominated ORDOVICIAN SILURIAN QASIM FORMATION Member shallow marine 7 1 At-Tiraq At-Tiraq 2 At-Tiraq Llandeilo 11a 10 12 Hanadir Offshore/Shelf Llanvirn Member MFS O30 mfs Arenig Sajir Fluvio-marine Member (braided delta) Type-1 Sequence Boundary (Tremadoc) Risha Member Braided stream SAQ FORMATION CAMBRIAN BASEMENT Figure 2: Generalized lower Paleozoic stratigraphic column of the Qasim and Ha’il regions (modified after Vaslet, 1989). The Figure Index shows the stratigraphic position of photographs and measured sections. MFS=Maximum Flooding Surface; mfs=marine flooding surface. 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). 235 Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/6/2/233/4559788/senalp.pdf by guest on 27 September 2021 Senalp and Al-Duaiji o 3,500' o o 35 E454,000' 50 Risha 3 4,500' 5,000' 0 500 IRAQ 5,500' km N Jalamid JORDAN 5,000' Sirhan o 4,000' IRAN o 30 N Zalma 3,500' 30 N 3,000' EGYPT Farwan Kahfah 2,500' Saqqar 2,000' KUWAIT Uqab ARABIAN GULF Qalibah 1,500' 1,000' Mayyah Zabeirah 500' Qiba Qasim Hunayzil Fadhili RED SEA Jalah Tarafiyah 2,500' Ain Dar BAHRAIN 227 2,000' QATAR o Ain Dar 25 1,500' 196 Sedgum 25o Amar 2 212 LMTN Uthmaniyah 630 SAUDI ARABIA Qasim, Zarqa and Sarah formations 1,000' 51 Haradh Saq Formation RYHN 1 UAE Arabian Shield 500' Key wells penetrate entire Qasim Wells without complete Qasim section Hamdan-1 40o 45o 50o (approx. Isopachs (ft) 21°N.,54°E.) Figure 3: Location of key wells and schematic isopachs of the Qasim Formation. 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.