Microfacies Characterization of the Silurian Qalibah Formation (Qusaiba and Sharawra Members), Saudi Arabia

The Lower Silurian Qalibah Formation is a key component of the Arabian Basin petroleum system and hosts the organic-rich Qusaiba Member, a major source rock for Paleozoic hydrocarbon reservoirs in Saudi Arabia. Despite its importance, the micro-scale sedimentological characteristics and depositional processes of the Qalibah Formation remain poorly constrained. This study presents a microfacies analysis integrated with sedimentological and ichnological observations from Qalibah Formation outcrops in the Qassim Region, Saudi Arabia. Five microfacies were identified, from bottom to top: (1) massive claystone with sparse bioturbation (BI 0–1), (2) claystone with starved ripples, (3) massive to graded siltstone, (4) massive to laminated sandstone, and (5) rippled and low-angle laminated sandstone with moderate bioturbation (BI 2–3). These microfacies record suspension-settling, sediment-laden density-flow deposition, and tractional reworking. Trace fossils, including Phycosiphon and mantle-and-swirl structures, occur mainly within event-bed siltstones, suggesting opportunistic colonization following rapid deposition. Sandier microfacies exhibit greater diversity of trace fossils, which disrupt primary sedimentary fabrics. Vertical coarsening-upward facies relationships are interpreted to represent deposition within a deltaic system evolving from offshore to distal prodelta to proximal delta-front environments. These results improve understanding of sediment transport and biogenic modification, and provide insights into heterogeneity in mud-dominated unconventional systems.