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Seismic Expressions of Prograding Carbonate Bank Margins: Middle Miocene, Maldives, Indian Ocean
Andrei V. Belopolsky,1 Andr W. Droxler2
1Rice University, Houston, Texas, U.S.A.; Present address: BP, Houston, Texas, U.S.A.
2Rice University, Houston, Texas, U.S.A.
We thank the government of the Republic of Maldives and Royal Dutch Shell for providing seismic and well data and Shell EP Technology Laboratory in Bellaire, Texas, for usage of a seismic workstation. We also thank Mitch Harris, Albert Bally, Ed Purdy, John Karlo, and Peter Vail for fruitful discussions of the presented material. Constructive reviews by Gregor Eberli, Evan Franseen, and Steven Dorobek improved the quality of the manuscript. Funding for this research was provided by an NSF Grant OCE-9730954 to A.W.D. and a Rice University Mills Bennett Fellowship and an SEPM Grant-in-Aid to A.V.B.
Carbonate bank margins in the Maldives, a large isolated carbonate platform in the equatorial Indian Ocean, prograded significantly in the middle Miocene. The interpretation of a dense grid of two-dimensional seismic data located prograding margins and imaged the internal architecture of the prograding sequences in detail. Three individual prograding bank margins were positioned tens of kilometers away from each other. The margins prograded from both east and west toward the central seaway. Each prograding complex consists of five depositional sequences bounded by unconformities and correlative conformities. Seismic correlation shows that the deposition of sequences in individual prograding complexes was synchronous and driven by a mechanism of regional scale.
One prograding complex, complex I, is examined in detail in three dimensions on a set of seismic lines. Each prograding sequence is subdivided into two packages: strong- and weak-amplitude reflection packages. Strong-amplitude reflection packages consistently display a basinward shift in onlap and/or downstepping and are interpreted as having formed during falling relative sea level. Weak-amplitude reflection packages are interpreted as having formed during the rise and highstand of relative sea level. Each sequence thus represents a complete sea level cycle. The terminal middle Miocene sequence is characterized by voluminous downslope deposition of sediments interpreted as gravity-flow deposits. This regional event is interpreted as a response to a significant sea level fall at the end of the middle Miocene.
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