The Timing of Floods and Storms as a Controlling Mechanism for Shelf Deposit Morphology

Deltas and clinoforms are accretionary deposits with subaerial and subaqueous topsets, respectively. Various factors have been proposed that may control their geometry, especially the interaction of sediment supply and physical oceanographic energy. A conceptual model of how this interaction affects shelf sedimentation indicates that sediment from most rivers should form deltaic features in quiescent environments. In more energetic settings, most sediment from small rivers (sediment load of 106–107 t/y) should be dispersed, limiting significant shelf accumulation, but the increased supply from larger rivers (sediment load of 108–109 t/y) could allow for sediment retention on the shelf, leading to clinoform development. However, in the Adriatic Sea, a river delta and a shelf clinoform (i.e., adjacent to the Po River and the Apennine rivers, respectively) have developed from similar sediment supply and oceanographic energy. This suggests that other factors are likely important, particularly differences in the shelf gradient and the timing of floods and storms. The shelf gradient is lower near the Po River, favoring retention of sediment in shallower water as compared to the Apennine rivers. In addition, floods and storms are uncorrelated on the Po River shelf due to the large size of the drainage basin, enhancing sediment deposition in shallow water. For the Apennine rivers, the drainage basin of individual rivers is small, and floods and storms are generally correlated, facilitating offshore deposition of sediment and leading to the development of a shelf clinoform.