A Novel Seismic and Sequence Stratigraphic Framework for the Progradational Lower Silurian Clastic Wedge in the Arabian Subsurface

Following the well-known latest Ordovician glacial-terrestrial and glacial-marine phase, an early Silurian deglaciation brought mid-to-outer shelf facies, including a condensed Rhuddanian-age basal hot shale, throughout most of the North African and Arabian region (Jones & Stump, 1999; Konert et al., 2001; Sharland et al., 2001). The Lower Silurian hot shale, in particular, corresponds to organic-rich condensed marine facies with up to 8% total organic carbon and is the main source rock for the Paleozoic oil and gas accumulations in the region (e.g., Jones & Stump, 1999; Lüning et al., 2000; Konert et al., 2001). After the earliest Silurian transgression, a very thick (several thousands of feet) coarsening-upward succession of Lower Silurian clastics prograded basinwards, forming shelf edge-scale clinoform-bearing regressive stratal architectures (Jones & Stump, 1999; Grabowski, 2005; Soliman & Balilah, 2015; Balilah, 2018). As a consequence, fluvio-deltaic, shallow- to open-marine (‘shelfal’ and ‘shelf- margin’) sandstone-rich settings developed towards paleo-landward directions. At the same time, deeper-marine (‘delta-toe’ or offshore) clay-prone conditions were established in two rapidly subsiding distal sagging depocenters, developed following the Taconic Uplift (e.g., Jones & Stump, 1999; Konert et al., 2001; Soliman & Balilah, 2015). One of these broad tectonic basins, known as ‘Qalibah Trough’, formed in central and southern Arabia from the Late Ordovician; the latter, known as ‘Taymah Trough’, formed in north-western Saudi Arabia from the Early Silurian (Jones & Stump, 1999). In and around the Qalibah Trough, the Lower Silurian succession (LSS) is an offshore mudstone-rich siliciclastic wedge, showing an overall coarsening-upward log motif and progradational clinoform-bearing seismic architecture (Soliman & Balilah, 2015), representing a part of the AP3 Megasequence of Sharland et al. (2001) (Table 1). Several sandstone-rich members often occur in the middle and upper part of the LSS, corresponding to Rhuddanian and Aeronian ages, respectively (e.g., Miller & Melvin, 2005; Soliman & Balilah, 2015), but their presence and distribution has not been completely clarified. Lower Silurian offshore turbiditic-influenced and lower-shoreface siltstone, silty sandstone, and fine-grained sandstone facies were described from the Tabuk outcrops (Abdlmutalib et al., 2022). Sigmoidal shelf-edge scale clinoforms with ascending trajectories, as well as steeper shelf-edge scale clinoforms with low-angle or descending trajectories were identified by Soliman & Balilah (2015) and Balilah (2018). These authors also speculated on the presence of shelf-edge and slope channels and possible deep-basin submarine fans.