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J. Frederick Sarg1, Lincoln Pratson2
(1) Mobil Technology Company, Dallas, TX
(2) Duke University, Durham, NC

Abstract: Erosional Surfaces in Sedimentary Sequences and the Impact of Basin Slope and Relief on Lowstand Depositional Geometry

Experimental stratigraphy formed in a new laboratory basin and observations in natural systems suggest that certain combinations of sediment supply, basin-margin relief and basin-margin slope can lead to the formation of two bypass surfaces beyond the former shelf edge. These are (i) a basinward dipping surface of onlap, overlain by (ii) a more horizontal surface of erosion resulting in toplap. The former defines a sequence boundary, and the latter develops within the lowstand systems tract when bypassing sediments infill the accommodation space immediately basinward of the shelf edge. This second surface is formed by the continued regression of the shoreline during deposition of the lowstand prograding complex, and is the maximum regression surface (MRS).

The presence and absence of the MRS in natural sequences appears to be linked to a continuum of possible lowstand geometries governed by basin margin relief and slope. One end member is represented by high-relief, steep-sloped, bypass-dominated basins, where sediment supply is too low to infill the accommodation space seaward of the shelf edge. These are characterized by weakly developed prograding complexes that lack an MRS. Intermediate is basins where sediment supply is able to fill much of the space above more moderately dipping slopes. These develop onlapping lowstand basin floor and slope-fill units that provide the undercarriage for deposition of significant prograding complexes containing the MRS. Low relief, low slope settings occupy the opposite end of the spectrum and are characterized by downward shifted shorelines, and a lack of gravity flow bypass to basin areas.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana