Rapid progradation of pre-land-plant Early Silurian shelf-margin clinoforms, Central Arabia 

Recent 3D seismic surveys acquired over a Lower Silurian intracratonic sag basin in central Arabia have revealed for the first time shelf-prism clinoforms 375 meters high with slope gradients of 0.5 degrees. These... [ view full abstract ]

Recent 3D seismic surveys acquired over a Lower Silurian intracratonic sag basin in central Arabia have revealed for the first time shelf-prism clinoforms 375 meters high with slope gradients of 0.5 degrees. These Rhuddanian-age clinoforms crossed a wide shelf (>200 km from hinterland) and filled 800 meters (1.6 km decompacted) of accommodation space in about 3 million years, indicating rapid progradation and sedimentation rates (150 km/My and 0.53 m/Ky, respectively). Nearly all slope sections exhibit major erosional channels with low sinuosity and low width-to-thickness ratio, indicating that a substantial volume of sediment was delivered over the shelf-edge break and presumably bypassed the slope. However, seismic lines and attribute horizon slices indicate no shelf incision and no shelf edge deltas. Within the study area, cores from the shelf section show numerous hummocky cross-stratified thin beds in addition to turbidites. This evidence is used to infer that the Lower Silurian shelf in Arabia was regularly subject to storms which remobilised sediment across the shelf. In addition, some cores show diffusely-bedded successions with distinct reverse to normal grading, interpreted as hyperpycnites. These beds are found in shelf and also in deeper basinal deposits. This suggests that recurring hyperpycnal flows generated at river mouths transported terrestrially-derived sediment beyond the subaqueous realm of deltas, across the shelf, and over the shelf edge. Hyperpycnal flows occur usually during river flooding, which is consistent with a wet-temperate and stormy climate when Arabia was located at latitude 40°S. It is likely that more suspended sediment was being “flushed-out” of the subaerial coastal plain in pre-land-plant times as root systems are known to trap and retain fine grained sediment, Therefore, conditions ideal for generating hyperpycnal flows were attained more easily. Given the rapid progradation, abundance of major slope channels, and the lack of shelf-edge deltas, it is argued these (sandy and muddy) hyperpycnal flows contributed significantly to shelf-margin building even when the shoreline system was some distance from the shelf edge break. This study not only contributes to the growing number of documented examples of long distance cross-shelf sediment transport by processes such as wave resuspension and hyperpycnal flows but adds in the scenario of increased fine grained sediment in pre-land-plant times.