--> --> Revisiting the offset-stacked Campanian stratigraphic sequences of S. Wyoming and NE. Utah

AAPG Rocky Mountain Section Meeting

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Revisiting the offset-stacked Campanian stratigraphic sequences of S. Wyoming and NE. Utah

Abstract

The Campanian-Maastrichtian succession in S. Wyoming and NE. Utah is comprised of offset, stacked series of transgressive-regressive stratigraphic sequences. The stratal succession records approximately five eastwardbuilding clastic wedges or 3rd-order (>1 myr) sequences with strongly progradational to backstepping, stacked patterns into the middle of the fourth cycle during Canyon Creek/Pine Ridge formation time. One major transgression occurred at the beginning of the fifth (last) 3rd-order sequence during deposition of the Lewis Shale before the Western Interior seaway completely retreated from the area during Lance/Fox Hills formation time. Superimposed on these 3rd-order cycles are high-frequency (4th- and 5th-order) sequences (20-25 in the Chimney Rock, Rock Springs, Iles and Williams Fork clastic wedges) that appear to increase in frequency through the Campanian into the Maastrichtian (some 20 in the Lewis-Fox Hills clastic wedge). The high-frequency cycles are characterized by tens of meters thick fluvial and deltaic/shoreface tongues that prograded out for hundreds of km’s and intertongued with the marine mudstones. Two Campanian successions were revisited and measured, one relatively proximal section at Minnie’s Gap, WYUT (total ~1,225 m), and a relatively distal section (total ~1,720 m) along the North Platte River in the Haystack Mts, WY. The succession shows an increase in thickness from the proximal to distal ends of the system, and the internal cyclical nature of the depositional pattern is best observed in the marginal marine deposits of the Chimney Rock, Rock Springs, Haystack Mts and Almond formations. The proximal, nonmarine to fluvio-tidal Ericson, Allen Ridge and Pine Ridge formations show isolated-to-amalgamated, channelized sandstone bodies with some brackish-water traces, as well as thinner-bedded sandstones. These are interpreted in terms of an alluvial plain to a coastal plain setting. The marine shoreline-to-shelf associations in both sections show upward-coarsening motifs some 20-60 m thick that reflect either mixed fluvial-tidal processes or mixed fluvial-wave processes. The former shoreline type is more common distally and the latter proximally. Paleoflow measurements indicate a southeast flow direction. Ongoing work is aimed at better understanding the tectonic and eustatic controls on sediment supply and accommodation.