The Importance of Sediment Supply and Sequence-stacking Pattern in Creating Hyperpycnal Flows

Abstract

The Eocene Central Basin on Spitsbergen, Neogene offshore eastern Venezuela, and offshore northern Sakhalin are high-sediment supply systems that do not obey standard rules of sequence stratigraphy. These studies show that high sedimentation rates are capable of filling accommodation space created during transgression and highstand. The thick successions of coastal plain deposits in these basins consist of fluvial and estuarine deposits and are interpreted to result from rapid increases and cutoffs in fluvial sediment supply generated by tectonic pulses and climatic cycles. We suggest that the areal positioning of fluvial input points along a margin will give different shelf geometries. A shelf may be fluvial dominated or wave or tide influenced at the same time, which together with multiple fluvial input points creates lateral variations in the depositional architecture. The results from any studied two-dimensional section will therefore be very dependent on which part of the margin is studied, and changes in depositional architecture might be incorrectly interpreted as basinwide sea level changes. Prolonged or periodically high sedimentation rates result in progradation of the fluvial-deltaic system toward the shelf edge. With high sedimentation rates, the sediments reaching the shelf edge are prone to collapse, creating failure back to the contemporary shoreline. The collapse scars will act as conduits, and the fluvial system will be capable of overriding the shelf edge and transporting deposits to the deep water by hyperpycnal flows.