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ABSTRACT: A Comparison of Tectonic and Eustatic Control on Maastrichtian Depositional Sequences, South-Central Wyoming

Kenneth J. McMillen, Robert D. Winn, Jr.

Timing and geometry of Maastrichtian third-order sequences and systems tracts of south-central Wyoming were mostly controlled by rapid basin subsidence resulting from thrust loading of foreland basin crust. The basement of the Red Desert/Washakie basin was affected by thrusting in the Sevier belt to the west and by thrusting along the edge of the Wind River Mountains and Granite Mountains.

A transgressive systems tract consists of the youngest Ericson Formation above a regional unconformity, backstepping Almond Formation nearshore and nonmarine facies, and shelfal mudstones the lower Lewis Shale. Regionally, the onset of transgression is younger from east to west, beginning at 71.5 Ma in eastern Wyoming and at 70.5-71 Ma in south-central Wyoming. The transgression culminated in a condensed shale section evident on gamma-ray logs that occurs throughout the basin. Above the gamma-ray zone, the highstand systems tract of the middle and upper Lewis Shale, Fox Hills Sandstone, and Lance Formation consists of an aggradational/progradational interval overlain by a dominantly progradational clastic unit. Depositional environments consist of delta, slope, and submarine fan in th Lewis Shale, shoreface and shallow marine in the Fox Hills Sandstone, and nonmarine facies in the Lance Formation.

Tectonic control of sequences is indicated by two lines of evidence. (1) The timing of the transgressive, regressive, and condensed section facies does not correlate with the inferred global eustatic curve. The Lewis Shale condensed section occurs at 70.5 Ma and is older than the closest global condensed section at 69.5 Ma. (2) Subsidence, estimated by the amount of delta aggradation of the Lewis, totals 700 m roughly 0.5 Ma and greatly exceeds the 40 m of eustatic rise hypothesized to have occurred at about the same time.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990