Abstract

Studies of. outcrops of Upper Cretaceous strata from the Western Interior of Canada and the United States indicate that deposition during relative sea-level fall and lowstand can result in lowstand shorefaces that are attached to or in direct contact with the sand bodies of the immediately preceding sedimentary systems. This observation therefore represents an opposite end member from the Exxon sequence stratigraphic lowstand model, in which a detached lowstand shoreface develops during relative sea-level fall and lowstand. In this scenario, the lowstand shoreface is separated from the underlying shoreface by a zone that is bypassed by the net sediment load during sea-level fall and lowstand (sediment bypass zone). The sediments deposited during relative sea-level fall and lowstand (the lowstand systems tract; LST) can therefore be classified as detached (LSTd) or attached (LSTa). A major implication of this observation is that subsequent to a relative sea-level fall, it may not necessarily be correct to predict a detached lowstand shoreface lying basinward of the sand bodies of the underlying sedimentary system. Instead, the lowstand may have been deposited as a single depositional unit that is attached to or in contact with the sand bodies of the underlying deposits.

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