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

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.

Impact of magmatic intrusion on diagenesis of shallow marine sandstones: An example from Qasim Formation, Northwest Saudi Arabia

“…The area falls within the Shaghab quadrangle (Map GM-109C; Sheet 27B) described by Grainger and Hanif (1989). This area includes outcrops of the Middle to Upper Ordovician Qasim Formation (Senalp and Al-Duaiji, 2001;Laboun, 2010), which has been interpreted as the lower part of the Tabuk Formation (Senalp and Al-Duaiji, 2001). The Tabuk Formation was originally defined by Steineke et al (1958), amended by Powers et al (1966), and subsequently discarded following the recognition of regional unconformities within the formation.…”

“…The Qasim Formation has been described by Manivit et al (1986) in the Buraydah quadrangle and by Vaslet et al (1987) in the Baq'a quadrangle (Hail region). This formation has been interpreted to be deposited in shallow marine settings during tectonic quiescence (Senalp and Al-Duaiji, 2001;Laboun, 2010;Craigie and Rees, 2016). The lithology of the Qasim Formation comprises alternating cyclic units of thin-bedded, finegrained sandstones to shale and thick-bedded, massive-to crossbedded sandstones (Powers et al, 1966).…”

“…The lithology of the Qasim Formation comprises alternating cyclic units of thin-bedded, finegrained sandstones to shale and thick-bedded, massive-to crossbedded sandstones (Powers et al, 1966). These cyclic deposits were subsequently assigned as members of the Qasim Formation, consisting of the Hanadir (lower shale), Kahfah (lower sandstone), Ra'an (upper shale or siltstone) and Quwarah (upper sandstone) Members (Powers et al, 1966;Vaslet, 1990;Senalp and Al-Duaiji, 2001;Laboun, 2010;Strother et al, 2015). The studied samples were from the Quwarah Member of the Qasim Formation (Figure 2).…”

“…The depositional environment then switched back to shallow marine, depositing the sandy Kahfah Member. Subsequently, the sequence was then repeated with the deposition of Ra'an Shale and Quwarah Sandstone Members (Powers et al, 1966;Senalp and Al-Duaiji, 2001). The shaly Hanadir and Ra'an Members contain graptolites as diagnostic fossils indicative of deeper, low-energy open marine conditions whereas the sandy Kahfah Member contains Skolithos, indicating shallower, high-energy conditions (Powers et al, 1966).…”

The Age, Origin and Context of Reservoir Sandstones Underlying the Lowermost Silurian “Hot” Shales in the West of the Sultanate of Oman

“…These siliciclastics may be the inner, fluviallydominated portion of estuarine deposits from which accompanying wave-or tide-dominated portions have been removed by subsequent erosion (Dalrymple et al, 1992). Not all estuaries are tidal depending on the valley shape, and micro-tidal conditions appear to have prevailed through much of the Ordovician in the Arabian Plate (Senalp and Al-Duaiji, 2001;Turner et al, 2012;Heward et al, 2018). The westerly to southerly-oriented palaeocurrents are unusual for a lowstand or estuarine deposit in this location, where one might expect channels and valleys to be orientated towards the northeast and the margin of Gondwana (Fig.…”