--> The Attached Lowstand Systems Tract: An Opposite End Member Lowstand Scenario, by R. B. Ainsworth and S. A. J. Pattison; #90986 (1994).

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Abstract: The Attached Lowstand Systems Tract: An Opposite End Member Lowstand Scenario

R. Bruce Ainsworth, Simon A. J. Pattison

High resolution outcrop studies of Upper Cretaceous strata from the ramp type margin of the Western Interior of Canada and the USA indicate that deposition during falling-stage and lowstand can result in a lowstand coastal sandbody that is essentially attached to the sandbodies of the underlying systems

tract. This observation therefore represents an opposite end member scenario from the Exxon sequence stratigraphic lowstand model in which a sediment bypass surface develops during relative sea level fall and lowstand resulting in the deposition of a detached lowstand coastal sandbody. Lowstand systems tract (LST) deposits are thus classified as detached (LSTd) or attached (LSTa). A major implication of this observation is that subsequent to a relative sea level fall, it is not always necessary to invoke a detached lowstand potential reservoir coastal sandbody lying basinwards of the sandbodies of the underlying systems tract. It is also suggested that in some cases detached lowstand coastal sandbodies cannot be detected in down-dip locations because they were actually never deposited there. Instead the lowstand coastal sandbody may have been deposited as a single parasequence or an aggradational to progradational parasequence set attached to and hence in updip communication with the sandbodies of the underlying systems tract. The same principles may also apply in shelf-edge margin settings when relative sea level falls do not drop base level below the shelf break. In these scenarios the shelf will mimic a ramp margin and geometries superficially resembling the shelf margin systems tract of type-2 sequences may be produced.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994