Lithostratigraphy and depositional history of Upper Ordovician and lowermost Silurian sediments recovered from the Qusaiba-1 shallow core hole, Qasim region, central Saudi Arabia

The Qusaiba-1 shallow core hole was drilled to a total depth of 551ft in the northwest of the Buraydah quadrangle in central Saudi Arabia. The sedimentology of the lowermost 320ft (96.0m) of core in the core hole is described and interpreted. This includes the entire cored section of Ordovician rocks (294ft: 88.2m) as well as the lowest 26ft (7.8m) of mudstones of the overlying Silurian Qusaiba Member of the Qalibah Formation. The Ordovician section is represented, from the bottom of the cored interval upwards, by the Quwarah Member of the Qasim Formation, unconformably overlain by three members (introduced here) of the Sarah Formation. These are, in ascending order, the Sarah Sandstone, Baq'a Shale and Baq'a Sandstone members respectively. The Sarah Formation is overlain at 257ft (Drilled Depth) by the Silurian Qusaiba Member. The Quwarah Member comprises a lower interval (74.2ft: 22.26m) and an upper interval (15.6ft: 4.68m). The lower interval displays three gross depositional facies, namely (i) grey mudstones with syneresis cracks and interbedded thin sandstones, and a very low Bioturbation Index (BI=0–1), representing deposition in the proximal pro-delta to distal delta-front part of a river-dominated delta; (ii) interbedded sandstones and bioturbated, sandy mudstones with variable BI=2–4, representing distal delta-front deposits in a mixed wave- and river-influenced deltaic setting; and (iii) intensely bioturbated sandy siltstone (BI=5–6), representing delta-lobe abandonment facies in a non-deltaic shallow marine (lower shoreface to offshore) setting. The contact between the lower interval and the upper interval of the Quwarah Member is tentatively identified as a transgressive surface of erosion. The upper interval commences with a thin transgressive pebble-lag deposit that is abruptly overlain by an upward-thickening sequence of sandstones with subordinate mudstones, interpreted as a prograding shoreface sequence. The Sarah Formation rests unconformably upon these shoreface deposits of the Quwarah Member. It commences with the Sarah Sandstone member which displays three gross depositional facies. The lowest part comprises a basal disrupted facies which commences with a pebble conglomerate sitting upon a sharp scoured surface. This is abruptly overlain by a grey-green sandy siltstone characterised by intense disruption manifest in severe brecciation and rotation of bedding with abundant evidence of low angle shear planes with slickensides. This basa