Sequence Stratigraphic Interpretation of the Baqa and Uqlah Members at the Post Glacial Ordovician-Silurian Boundary in Saudi Arabia presented by Ahmed Alghuraybi
The Baqa and Uqlah Members are located between the glacially-derived, Ordovician Sarah Formation and the postglacial, Silurian Qalibah Formation. There has long been a debate about whether these members are the lowermost members of the Qalibah Formation or the uppermost members of the Sarah Formation. This debate fundamentally concerns the location of the worldwide Silurian-Ordovician sequence boundary as exposed in Northwest and Central Saudi Arabia. The location of this sequence boundary relative to the Baqa and Uqlah Members has important implications for their regional continuity and the potential distribution of laterally equivalent reservoir units within sequence stratigraphic models. The Baqa was previously defined in the Widyan Basin of Central Arabia, Saudi Arabia; while the Uqlah was defined in the Tabuk Basin northwest of Saudi Arabia. To better understand the sequence stratigraphic location of the Baqa and Uqlah Members in relationship to the Silurian – Ordovician sequence boundary, this study looked at three stratigraphic sections with data comprising: 16 outcrop sections; 14 shallow well cores; and 12 deep well cores. The Baqa Member is typically a coarsening and thickening upward succession consisting of two main lithofacies, representing offshore, and foreshore environments. The Uqlah Member is typically a thinning and fining upward successions representing fluvial to marine to shallow marine environments. Both have a basal contact with the underlying Sarah Fm. and a sharp contact with the shales of the overlying Qalibah Fm. The nature of the top surface of both members indicates a hiatus in deposition and a major flooding surface. Palynological data, indicates that the Baqa and Uqlah are Late Ordovician in age, while the shales directly overlying this marine flooding surface are from lower Silurian. This work suggests that the Baqa and Uqlah Members are generated by post-glacial prograding beach and shallow-marine parasequences within the early part of the post-glacial transgressive phase, prior to the establishment of true marine conditions. The Baqa and Uqlah Members as early transgressive sediments indicates that they should be regarded as part of the post-glacial Qalibah Formation rather than the glacial Sarah Fm. Consequently, the two members are laterally equivalent with a proximal-distal relationship. Lateral variability in thickness is interpreted as infilling pre-existing topography and potentially delays in the marine flooding.
AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018