--> --> The Mesaverde Group of S. Wyoming and N. Utah

AAPG Rocky Mountain Section Meeting

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The Mesaverde Group of S. Wyoming and N. Utah

Abstract

The Mesaverde Group in Wyoming and Utah records three large, successively basinward-advancing 3rd-order (> 1 Myr) clastic wedges. These in turn are comprised of numerous 4th-order sequences (< 1 Myr, ~20-25 in the Rock Springs, Iles and Williams Fork clastic wedges), which 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. The Mesaverde Group is capped by the Almond Formation which records the last major transgression of the Western Interior Seaway before its final departure during the Lance-Fox Hills clastic wedge. Two Campanian sections are presented here, one relatively proximal section at Minnie’s Gap/Glades Ridge, WY-UT (total ~1,225 m thick), and a relatively distal section (total ~1,720 m thick) along the North Platte River in the Haystack Mts, WY. They are representative of the offset-stacked architectures that are preserved in the Mesaverde Group. Overall, 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 (Trail, Canyon Creek, and Rusty Members) to fluviotidal (Upper Rusty Member) facies of the Ericson, Allen Ridge (Lower and Marine Members) and Pine Ridge formations show isolated-to-amalgamated, channelized sandstone bodies with some brackish-water traces, as well as thinner-bedded sandstones. Numerous coal seams and a few paleosols occur in these formations, which are interpreted as alluvial and coastal plain settings. The lignite zones are particularly abundant in the Almond Formation. The marine shoreline-to-shelf associations in both sections show upwardcoarsening 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. Many more sections are being measured this fieldwork season to better understand both the 3rd- and 4th-order cycles of the Mesaverde Group.