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

Abstract The Qusaiba-1 shallow core hole was drilled to a total depth of 551 ft in the northwest of the Buraydah quadrangle in central Saudi Arabia. The sedimentology of the lowermost 320 ft (96.0 m) of core in the core hole is described and interpreted. This includes the entire cored section of Ordovician rocks (294 ft: 88.2 m) as well as the lowest 26 ft (7.8 m) 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 257 ft (Drilled Depth) by the Silurian Qusaiba Member. The Quwarah Member comprises a lower interval (74.2 ft: 22.26 m) and an upper interval (15.6 ft: 4.68 m). 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 basal disrupted facies is a glaciotectonite that represents the initial advance of the South Polar ice sheets in Arabia at the onset of the late Ordovician (Hirnantian) glaciation. Overlying the basal disrupted facies are thick (85.6 ft: 25.68 m) amalgamated sandstones that comprise the massive sandstone facies of the Sarah Sandstone member of the Sarah Formation. These were deposited by high-density sediment gravity flows. The upper part of the Sarah Sandstone member consists of the structured sandstone facies . This comprises medium to coarse-grained sandstones that display sedimentary structures that are readily identified with a variety of high density sediment gravity flows. These two sandstone facies of the Sarah sandstone member are considered by analogy with similar deposits at outcrop to be the product of subaqueous glacial outwash related to a number of phases of glacial retreat during the Hirnantian glaciation in Saudi Arabia. The Sarah Sandstone member is succeeded by a unit identified here as the Baq'a Shale member of the Sarah Formation. The lower 2.4 ft (0.72 m) of this member is represented by a laminated sandy siltstone. This is characterised by a number of fine-grained sandstone laminae that display an anomalously coarse component of dispersed granules and very coarse sand grains. These rocks are interpreted to be glaciogenic stratified diamictites, representing glacial rainout deposits directly associated with initiation of terminal melting of the Hirnantian ice sheets. The succeeding 55.5 ft (16.65 m) of the Baq'a Shale consists of fissile, silty mudstones that contain an abundance of very thin, very fine-grained sandstones. These display a variety of features characteristic of a range of sediment gravity flows, from low density turbidites to (rare) slurried beds. The sandstone beds display a clear “upward-sanding” sequence, interpreted to represent distal-to-proximal progradation of a prodelta wedge into a brackish, marginal marine embayment. This occurred at a time of ongoing rising sea level and very high rates of deposition related to the continued terminal melting of the Hirnantian ice sheets. The Baq'a Sandstone member is the stratigraphically highest unit of the Sarah Formation. It is 36.5 ft (10.95 m) thick and consists of two distinct sandstone units. The lower of these units comprises fine-grained, well sorted sandstone that appears to be massive (structureless) for the most part. In places faint ripple lamination can be discerned. The textural characteristics of this sandstone unit suggest that it represents the highest part of the upward-sanding sequence identified in the underlying Baq'a Shale, and as such was possibly deposited in a delta-front setting. The upper unit of the Baq'a Sandstone member sits abruptly upon the lower sandstone unit and consists of trough cross-bedded, moderately to poorly sorted, fine- to very coarse-grained sandstone. It represents the abrupt introduction of high energy fluvial deposits over the underlying delta-front sandstones. The contact of these fluvial sandstones with the underlying delta-front deposits is tentatively identified with the isostatic rebound unconformity that has been recognised in outcrop near the town of Baq'a in central Saudi Arabia. The lowermost deposits of the Silurian Qusaiba Member in the Qusaiba-1 core have been identified from palynology in a concurrent study as being early Rhuddanian in age, and occupy the 2.5 ft (0.75 m) thick interval that directly overlies the Sarah Formation (Baq'a Sandstone member). That interval commences with a thin grey mudstone that passes upwards into a sandy, carbonaceous mudstone that is moderately to heavily bioturbated displaying a moderately diverse assemblage of trace fossils. The intensity and diversity of bioturbation diminishes upwards, ultimately becoming represented only by prominent Thalassinoides burrows. These are unlined, with sharp walls and are filled with sandstone that is clearly derived from a thin, overlying sandstone bed that is itself heavily bioturbated. The Thalassinoides burrows are considered to represent a Glossifungites Ichnofacies and therefore indicate a significant hiatus at the base of the thin sandstone bed, at 254.5 ft in the core. This ichnological and sedimentological evidence, combined with the new palynological data supports an early Rhuddanian/mid-Aeronian contact at the base of that sandstone. The uppermost 23.5 ft (7.05 m) of core, between 254.5 ft and 231.0 ft, consist of a thin (1 ft: 0.3 m), basal bioturbated sandstone overlain by grey, fissile, graptolitic mudstones that have been demonstrated to be mid-Aeronian in age. It can be shown that these rocks were deposited in a marine shelf setting.