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Improving the Lowstand Component of the Sequence Stratigraphic Model

Ron Steel, University of Texas, Austin, TX

The term "lowstand" is used both to refer to a sea level position and to strata believed to have been deposited around this sea level position. The lowstand stratal package, like the highstand one, is regressive in character and originally included both the deposits of the falling stage plus the deposits that accumulated during early stages of rise of relative sea level. This same stratal package was later subdivided into units variously termed early and late lowstand (Posamentier), late highstand and lowstand (Van Wagoner) or falling-stage and lowstand deposits (Hunt and Tucker). Each scheme has some advantages.

The falling-stage and lowstand concepts are reexamined in a series of early Eocene shelf-margin clinoforms, exposed in seismic-scale outcrops (10 km - 1 km) on Spitsbergen. These shelf margins illustrate clearly how sandy sediment was delivered across the shelf to the shelf edge, sometimes partitioned farther down onto the deepwater slope and basin floor, and then trapped progressively landward on the shelf during a transgressive transit of the delivery system. In these outcrops, the shelf edge is a remarkable template or datum for examining facies and geometry changes during high and low relative sea levels. Water depth between shelf edge and basin floor was always less than 600 m.

Two main types of sand-prone, lowstand shelf margins are seen, each with evidence of sea level fall below the shelf edge: one where sand is narrowly partitioned via shelf edge and slope channels to basin-floor fans, the other where strike-extensive sandy slope aprons are thickly developed, but terminate before the base of slope. Both show some contrasts with highstand margins. The influence of both supply- and accommodation-dominated systems on the development of shelf margin is discussed.

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