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Henry W. Posamentier1
(1) Atlantic Richfield Indonesia, Plano, TX

Abstract: Case studies of lowstand alluvial bypass systems: incised vs. unincised

Alluvial systems ranging in age from Oligocene to late Pleistocene are well imaged beneath the South Java Sea shelf, offshore northwest Java, using 3D seismic volumes. A combination of seismic reflection attributes, seismic interval attributes, time slices and horizon slices have been used to identify these alluvial systems. The plan view expression of these systems ranges from low sinuosity to high sinuosity to braided, and incised to unincised. Widths of individual channels range from 100-250 m. Meander belt widths range from 2-6 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 likely were active during periods of lowered sea level when shelf areas submerged during sea-level highstands became emergent during lowstands. Of these systems only a select few are characterized by incision. Incision is inferred when trunk channels of fluvial systems are associated with minor, orthogonally-oriented, deeply-etched tributary channels/valleys. The incised trunk valleys range from 0.5 to 2 km wide and contain within them channels commonly less than 200 m wide; the incised tributary valleys are an order of magnitude narrower and are characterized by a well-developed dendritic drainage pattern. The presence of numerous unincised alluvial systems on marine shelves of the South Java Sea suggests that valley incision likely characterizes only the lowest of lowstands. Using the Pleistocene as an analog to older sections, we conclude that unincised lowstand alluvial bypass systems likely constitute a more common response to sea-level lowering than do incised systems.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana