Qasim Formation: Ordovician Storm- and Tide-Dominated Shallow-Marine Siliciclastic Sequences, Central Saudi Arabia

Abstract

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.