Lowstand alluvial bypass systems: Incised vs. unincised

Alluvial systems ranging in age from Miocene to late Pleistocene are observ ed beneath the southern Java Sea Shelf, offshore northwest Java. A combinat ion of seismic reflection attributes, time slices, and horizon slices extra cted from three-dimensional seismic volumes have enabled identification of these alluvial systems. The plan-view expression of these systems ranges fr om low sinuosity to high sinuosity, and incised to unincised. Widths of ind ividual 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 c an be observed to be associated with major trunk valleys. Late Pleistocene alluvial systems imaged on the shelf were active during pe riods of lowered sea level when vast shelf areas were emergent. Of these sy stems only a select few are characterized by incision. Incision is inferred where trunk channels of fluvial systems are associated with minor, orthogo nal, deeply etched tributary channels/valleys. The incised trunk valleys ra nge from 0.5 to 5 km wide and contain channels within them; the incised tri butary valleys are an order of magnitude narrower and are characterized by well-developed dendritic drainage patterns. Valley incision, which likely f ormed within a period of 3-5 k.y., can be traced more than 200 km inboard o f the shelf edge. The presence of numerous unincised alluvial systems on marine shelves of th e 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.