Lowstand alluvial bypass systems: Incised vs. unincised

Abstract

Alluvial systems ranging in age from Miocene to late Pleistocene are observed beneath the southern Java Sea Shelf, offshore northwest Java. A combination of seismic reflection attributes, time slices, and horizon slices extracted from three-dimensional seismic volumes have enabled identification of these alluvial systems. The plan-view expression of these systems ranges from low sinuosity to high sinuosity, and incised to unincised. Widths of individual channels range from 100 to 250 m. Meander belt widths range from 2 to 6 km. In some instances, well-developed minor tributary feeder systems can be observed to be associated with major trunk valleys. Late Pleistocene alluvial systems imaged on the shelf were active during periods of lowered sea level when vast shelf areas were emergent. Of these systems only a select few are characterized by incision. Incision is inferred where trunk channels of fluvial systems are associated with minor, orthogonal, deeply etched tributary channels/valleys. The incised trunk valleys range from 0.5 to 5 km wide and contain channels within them; the incised tributary valleys are an order of magnitude narrower and are characterized by well-developed dendritic drainage patterns. Valley incision, which likely formed within a period of 3-5 k.y., can be traced more than 200 km inboard of the shelf edge. The presence of numerous unincised alluvial systems on marine shelves of the southern Java Sea suggests that valley incision likely characterizes only the lowest of lowstands. To the extent that the Pleistocene can be used as an analog to older sections, we conclude that unincised lowstand alluvial bypass systems can constitute a more common response to sea level lowering than do incised systems.