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