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Abstract: Modifications of the Sequence Stratigraphic Model with Emphasis on Passive Margins

Posamentier, Henry - Atlantic Richfield Indonesia, Inc.

This paper addresses three key stratigraphic issues that significantly impact variations on the general sequence stratigraphic theme: 1) recognition of forced regression deposits, 2) the extent of incised valleys, and 3) Type 1 vs. Type 2 sequences.

Forced regression deposition is common and is associated with periods of relative sea level (RSL) fall. Principal forced regression recognition criteria include: 1) a significant zone of separation between successive shoreface deposits, 2) long- distance regression, 3) absence of fluvial and/or coastal plain deposits capping the proximal parts of regressive deposits, and 4) foreshortened stratigraphic sections.

Incised valleys commonly form when RSL fall exposes a seafloor characterized by a relatively steep gradient. The landward extent of incision may be restricted to only the outer shelves as well as the inner shelf coastal prism. Consequently, simple channel fill rather than incised valley fill may be present across significant portions of the middle shelf.

Type 1 and Type 2 sequences were defined originally as stratigraphic sections that did or did not contain lowstand deposits at their base respectively. Type 1 sequence boundaries commonly are expressed as unconformities on the shelf grading to correlative conformities in the offshore. Along strike, a similar unconformable to conformable relationship can be observed where subsidence rates grade from low to high respectively. The area along strike where the sequence boundary is correlatively conformable is said to be characterized by Type 2 sequences. Changing sequence boundary type, and hence sequence type, when going along strike, when no similar change of type is invoked when going along dip seems unreasonable. Consequently, the terms Type 1 and 2 sequence (and sequence boundaries) should be replaced by sequences bounded by unconformable and correlatively conformablesurfaces. The magnitude of the unconformity will dictate whether the sequence is a major or minor one.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil