M. Shane Long¹, Scott M. Ritter², Thomas H. Morris²
(1) Exxon, The Woodland, TX
(2) Brigham Young University, Provo, UT

Abstract: Facies architecture and sequence stratigraphy of the Lower Cretaceous Muddy Formation in the southeastern Bighorn Basin, Wyoming

Within the southeastern Bighorn Basin, the Muddy Formation consists of two members. The lower, designated the Manderson unit, is comprised of 0 to 19 m of fluvial and estuarine sandstones that fill a valley system incised into the top of the Thermopolis Shale. The largest paleovalley trends northwest and is joined by a smaller, southeast trending channel north of the Manderson Field. The sharp Manderson-Thermopolis contact represents a Type I sequence boundary that developed between 97-100 m.y.a. Manderson sands represent backfilling of incised channels during lowstand and early transgressive phases of the Greenhorn cycle.

The upper Muddy, here called the Ainsworth unit, comprises a regionally extensive sandstone couplet (3-7 m) deposited during the transition from transgressive to highstand conditions. These sandstones, separated by a thin shale split, are interpreted as progradational parasequences. The overlying Mowry Shale represents highstand deposition.

Gross isopach maps show that stratigraphic pinch-outs form an important element of the Muddy interval, particularly in the Manderson unit. These maps also show that the thickest Manderson deposits occur well off of established drilling trends in relatively unexplored areas.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana