--> The Sedimentological Expression of a Transgressive Sequence Set, Campanian, Eagle Formation, Bighorn Basin, Wyoming, USA, by R. Fitzsimmons, T. Elliot, and S. Flint; #90986 (1994).

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Abstract: The Sedimentological Expression of a Transgressive Sequence Set, Campanian, Eagle Formation, Bighorn Basin, Wyoming, USA

Roy Fitzsimmons, T. Elliot, S. Flint

Sequence stratigraphy requires the critical evaluation of links between depositional systems. In updip sectors these links are best examined when accommodation space is increasing, that is during transgressive systems tract or transgressive sequence set. The Campanian Eagle Formation is an easterly prograding, clastic wedge. High-resolution sequence stratigraphic analysis reveals two composite sequences defined by regionally widespread, low-order erosional unconformities. The internal stratal architecture of these sequences reveals high-order sequence boundaries developed at the base of incised valleys and sharp based shorefaces.

The transgressive sequence set to the second, low-order sequence is composed of four high-order sequences. Depositional-dip exposure of the succession shows that the succession is dominantly composed of coastal plain depositional systems passing downdip into shoreline/shelf depositional systems.

High-order falls in relative sea level caused incision in updip coastal plain areas. Sediment bypassed through incised valleys to lowstand prograding strandplains, where the basal sequence boundary is expressed as sharp based shoreface/mouthbar.

During the ensuing transgression, sediment was trapped within the incised valleys forming multistorey fluvial/estuarine sandbodies. Down dip, the lowstand strandplain was reworked by migrating tidal inlets. As transgression continued, fine grained, carbonaceous rich strata developed in updip areas, while downdip retrograding storm dominated parasequences were deposited. Highstand systems tracts are poorly preserved due to erosion at the base of the next sequence boundary.

Using these criteria, thick, lithologically complex, seemingly random coastal plain strata can be sub-divided into depositional systems tracts.

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