--> ABSTRACT: Highstand Versus Lowstand Architecture in the Campanian of Wyoming: Variations in High Frequency Sequence Patterns, by Donald J. P. Swift, Brian Parsons, Roy Fitzsimmons; #91020 (1995).

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Highstand Versus Lowstand Architecture in the Campanian of Wyoming: Variations in High Frequency Sequence Patterns

Donald J. P. Swift, Brian Parsons, Roy Fitzsimmons

The "lower Mesaverde" (Eagle Group) of the Big Horn and Wind River Basin begins with a succession of clinoform tongues (Virgelle Sandstone). The Virgelle is truncated by a progressive, third order, subaerial unconformity, which is overlain in turn by backstepping coastal plain deposits (Gebo Formation). Further east, towards the basin center the shelfal, transgressive Shannon Sandstone rests on an erosional surface that correlates with the Eagle unconformity.

The stacking of high frequency sequences changes markedly from west to east in the basin-margin Eagle Group. Virgelle tongues flatten out and are increasingly consumed by the unconformity as they pass westward into Cody Shale, marking the shift from highstand to falling sea level. Above the Virgelle, the thick sandstones, shales and coals that form the fourth order sequences of the Gebo Formation pass westward into thin shales, reflecting the equivalent transition from the slow late transgression to the earlier, faster sea level rise. Thus the highstand stacking pattern of aggradational sandstone cycles of the basin-margin Eagle Group passes basinward into a mid-cycle progradational-retrogradational pattern where the unconformity is a shale-on-shale contact. Aggradational expansion at the lowstand turnaround occurs largely in shale and is difficult to detect, except in the late transgressive Gebo Formation, and the early transgressive Shannon cycles may be transgressive reworkings of similar high frequency progradations.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995