The Importance of Sediment Supply and Sequence Stacking Pattern in Creating Incised Valleys and Hyperpycnal Flows
The Eocene Central Basin on Spitsbergen, and the Neogene offshore eastern Venezuela are high supply basins that does 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 cut offs 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, wave or tide influenced at the same time, which together with multiple fluvial input points create lateral variations in the depositional architecture. The results from any studied 2-D section will therefore be very dependant on which part of the margin is studied, and changes in depositional architecture might be incorrectly interpreted as basinwide sea-level changes.
Prolonged high sedimentation rates combined with a narrow shelf result in progradation of the fluvial-deltaic system towards 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 transport deposits to the deep water by hyperpycnal flows.
The above described lateral variations along narrow shelves thus offers an alternative explanation to the earlier sequence stratigraphic models of the Central Tertiary Basin on Spitsbergen and other high sediment supply margins. Finally, if a long-lived, large canyon once incises the shelf and link up to an efficient feeder system, transport of sediment by hyperpycnal flows into the deep water may occur during both lowstand and highstand, and are not restricted to periods of sea-level fall.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009