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AAPG Bulletin, V.
Lowstand alluvial bypass systems: Incised vs. unincised
Henry W. Posamentier1
1Anadarko Canada Corporation, 425-1st Street SW, Calgary, Alberta T2P 4V4, Canada; email: [email protected]
Henry W. Posamentier is the manager of geology for Anadarko Canada Corporation. Prior to joining Anadarko in 2001, he was with Veritas Exploration Services (2000-2001), the Atlantic Richfield Company (1991-2000), Exxon Production Research Company, and Esso Resources Canada, Ltd. (1979-1991) and at Rider University as assistant professor of geology (1974-1979). Posamentier's research interests have been in the fields of sequence stratigraphy and depositional systems analysis, where he has published widely. He has employed an interdisciplinary approach using 3-D seismic data integrated with borehole data to interpret depositional systems and develop basin fill histories. Recently he has focused his efforts on deep-water depositional settings. In 1971-1972, Posamentier was a Fulbright Fellow to Austria. He has served as an AAPG Distinguished Lecturer to the United States (1991-1992), an AAPG Distinguished Lecturer to the former Soviet Union (1996-1997), and an AAPG Distinguished Lecturer to the Middle East (1998-1999).
ARCO Indonesia, Inc., gave permission to publish this article. I appreciate all the computer support I have received from S. Sauvagnac, Teddy N., Jaya, Saptoto, and Lilik P. during the course of this study. I am also very grateful to J. V. C. Howes, Mark Golborne, John Dolson, Brad Macurda, and Neil Hurley for their insightful reviews, which significantly improved the quality of the article.
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.
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