f Definition and Distribution of the Bab Sequence – The Last Upper Aptian Shu’aiba Sequence in the Bab Basin (Best of Shu'aiba)

The Shu’aiba Formation consists of six depositional sequences. The first five sequences make up a 2nd order supersequence, while the sixth sequence is the lowstand sequence of the next supersequence. The sequence is deposited within the Bab basin as a set of prograding clinoforms. This sequence is here called the Bab Sequence and is equivalent to the “ap6”, “sequence 6”, and the “Upper Shu’aiba sequence 3” of other authors. The sequence boundary at the base of the Bab sequence coincides with the supersequence boundary and is characterized by a major exposure across the Shu’aiba platforms and down-stepped deposition. The onset of clay deposition marks the beginning of the Bab Sequence. Internally the Bab Sequence consists of nine clinoforms (high-frequency sequences) that prograded into the Bab Basin in a series of clinoforms over more than 30km. The most striking observation is the extremely regular and seaward progression of the clinoforms, sub-linear along the southern margin and concentric around the eastern isolated platforms, resulting in a series of almost perfectly parallel clinoforms. The prograding clinoforms can be grouped into three sets. The 4 clinoforms in the first set are relatively wide, with mounded topsets at the margin. They reach a thickness of 120 meters. The top surface of these first clinoforms lies 30 to 40 meters below the top of the previous shelf break (sequence ap5). The 3 clinoforms of the second set are relatively narrow, reach 115 m in thickness and have a slightly steeper slope than the first set of clinoforms. The last two clinoforms (third set) are broader and thinner (75 meters in thickness) and have a very low angle slope. The almost perfect parallel geometry of clinoforms is likely the result of the regular distribution of clay and carbonate mud along the margins by long-shore currents. The lithologic content of the clinoforms varies within the three sets. The topsets of the first set of clinoforms tops are rudist-bearing wackestone – rudstone, the foresets are composed of burrowed argillaceous wackestone, and the bottomset of calcareous shale. In the topsets of the second set of clinoforms the rudist facies is missing and replaced by marl-limestone alternations with occasional skeletal packstone layers. The third set is dominated by clay and marl-limestone alternations. The rudist-bearing topsets form the main reservoirs in the Bab sequence, whereas the clay-rich intervals may provide seals for a stratigraphic play.