Late Hirnantian Clastic Depositional Systems Using Sequence Stratigraphy and Chronostratigraphy
Authors: Alshahrani Saeed , Zuhlke Rainer 
Published: November 2018

Abstract
The Hoban and Baqaa Members are located between the glacial to early post-glacial Sarah Formation (Ordovician) and the Qalibah Formation (Silurian). They have been distinguished as separate stratigraphic units in few outcrops of northern Saudi Arabia based on palynological analyses, graptolites, shell fragments, and their predominantly shallow marine depositional environments. There has been long debate over whether the Hoban and Baqaa Members should be attributed to other upper Sarah Formations or the lower Qalibah Formation. This debate fundamentally concerns the location of the Silurian-Ordovician sequence boundary, which developed globally, in central Arabia. Detailed sedimentological and stratigraphical data of the latest Ordovician to earliest Silurian interval were available from the subsurface. Therefore, the main aim of this project is to provide a detailed sedimentological model for the Hoban and Baqaa Members, both in outcrop-reservoir analogues and in the subsurface. This includes a sequence stratigraphic correlation between outcrops and wells, and general depositional environment maps. The resulting model provides an improved understanding of depositional changes as well as relative and eustatic sea-level changes across the Ordovician-Silurian boundary in Arabia.
The study is based on 16 outcrop sections in three different regions of northwestern and northeastern Saudi Arabia (Tabuk, Tayma, and east of Hail) and 26 shallow and deep well cores. Outcrops and the well are enclosed in a total area of about 340 by 100 km. The Hoban Member has a thickness between 0-40 m (typically 23 m) and consists of two units, which represent the final stages of glaciation. Sediment deformation, internal brecciation, and angular Sarah clasts are widespread. The Baqaa Member, which has a thickness of 15-40 m (typically 31 m), unconformably overlies the Hoban Member and consists of two to three shallowing-upward cycles (high-frequency sequences). Each cycle consists of a basal transgressive part (proximal offshore) and a regressive part (lower to upper shoreface or foreshore, respectively). The top of each cycle is formed by closely stacked submarine hardgrounds (transgressive surface at the base) with ample bioturbation and shell horizons. Hot shales of the Qusaiba Member (Qalibah Formation) conformably cover the hardground of the upper Baqaa shallowing-upward cycle. Palynological data indicates that both the Hoban and Baqaa are late Ordovician in age, while the Qusaiba shales are lower Silurian.
The Hoban Member represents stages of final glacial advance and local sediment mass transport in a peri-glacial environment. The Baqaa Member represents post-glacial flooding with prograding beach and shoreface systems. During peak transgression, intermittent open marine conditions existed. Long-term fully marine conditions were only established in the lowermost Qusaiba Member. From a process-oriented perspective, the Hoban Member constitutes the uppermost member of the (peri-) glacial Sarah Formation, the Baqaa Member as the lowermost member of the post-glacial Qalibah Formation.