The Muddy Sandstone (Albian) of the Wind River Indian Reservation, Western Wind River Basin, Wyoming: Underlain by a Lowstand Surface of Erosion or Correlative Conformity?

Costanzo, Lisa A. ¹, Donna S. Anderson² (1) Shell E & P, Rocky Mountain Exploration, Houston, TX (2) Colorado School of Mines, Golden, CO

The Muddy Sandstone (Albian) in eastern and western Wyoming generally was deposited in incised valleys that were formed during a regional retreat of the Western Interior Cretaceous Seaway. However, across the Wind River Indian Reservation (WRIR) in the westernmost Wind River basin, regional analysis shows that the Muddy Sandstone is a widespread unit, not confined to well-defined linear paleovalleys like those to the east. In addition, facies offset across the Thermopolis-Muddy contact decreases to the northwest across the WRIR, suggesting the potential presence of a correlative conformity to the regional lowstand surface of erosion (LSE) generally associated with the base of the Muddy Sandstone.

Regional analysis using four measured sections and four cores tied to 137 wells consisted of correlating and mapping four stratigraphic cycles within the Thermopolis Shale, four cycles within the Muddy Sandstone and two regional bentonites within the overlying Shell Creek/Mowry Shale, which provided a stratigraphic framework to analyze the nature of the contact between the Thermopolis Shale and Muddy Sandstone. The analysis shows that the two uppermost Thermopolis cycles were consistently truncated by the regional LSE to the southwest across the WRIR. The youngest Thermopolis cycle was consistently truncated to a lesser degree to the northwest, confirming the presence of the regional LSE. Paleogeographic maps of facies tract distributions within Muddy Sandstone cycles across the WRIR suggest that the paleovalleys became progressively wider and less incised to the northwest, until they coalesced to form a broad coastal-deltaic plain, which was underlain by the regional, yet low facies-offset LSE.

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana