THE SIGNIFICANCE OF TRANSGRESSIVE EROSION

Erosional surfaces connected to shoreline transgressions are recognized in the rock record and attributed to shoreface erosion induced by wave action. By contrast, the incised valleys that are controlled by base level variation and stream flow are subjected to climate and basin physiography thresholds. Thus the related erosional processes are confined to the valleys. Transgressional ravinement, however, creates a surface that is dependant on the shoreface topography and wave energy. Erosion of exposed surfaces during sea level low-stand is virtually absent between valleys, while transgressive erosion is widespread and may erode an entire shoreface. This paper examines both lacustrine and marine systems in the present and in the rock record. The sediment volumes that originated form the Holocene transgression in the Delaware and Texas coasts vastly exceed the sediment volumes of incised valleys on the shelf that formed during the last glacial low-stand. Moreover, the time interval which is attributed to these erosional processes amplifies the difference between ravinement surfaces and incised valleys to the favor of the ravinement process. A modern example was observed at the Dead Sea as a result of a 0.8 m lake level rise in lake levels. Comparison between measured profiles before and after the erosion took place point to a 2.5X10^6 cubic/m volume of eroded sediments. A similar scenario is observed in the Holocene section which represents a lake transgression. This event removed by ravinement 10 times more sediments then incised valleys along the basin margins in 1/5 the time. This observation helps quantify the importance of ravinement surfaces in the ancient rock record.