ABSTRACT: Cretaceous Paleofaults on Basement Block Boundaries Near Hartzog Draw Field, Wyoming

SHURR, GEORGE W., St. Cloud State University, St. Cloud, MN, and S. PARKER GAY, JR., Applied Geophysics, Inc., Salt Lake City, UT

Fault-block patterns in the crystalline basement beneath the Powder River basin of northeastern Wyoming can be mapped using anomaly patterns in aeromagnetic data. At a regional scale, a mosaic of discrete lithosphere blocks is outlined by areomagnetic lineaments. On the western margin of the basin, the major aeromagnetic lineaments correspond with changes in structural style along the flank of the Big Horn Mountains. The basement block mosaic influenced paleotectonism during the Cretaceous, as well as subsequent Laramide tectonism during the Tertiary.

Paleofaults are observed in sedimentary rocks above block boundaries in the crystalline basement. At a local scale, areomagnetic anomalies define small blocks that constitute the margins and corners of regional lithosphere blocks. Well pairs across aeromagnetic features outlining Hartzog Draw field in Campbell County, Wyoming, document paleofaults along the boundaries of small basement blocks. The Cretaceous Mowry, Frontier, and Niobrara Formations are offset. However, higher in the section, bentonite markers in the Cody Shale are not offset.

Paleofaulting after deposition of the Niobrara influenced deposition of a siliciclastic shelf sand ridge in the Shannon Sandstone that produces oil in the Hartzog Draw field. Basement block patterns mapped from aeromagnetic anomalies, therefore, are useful in generation of specific prospects. Not only is there direct structural control associated with faults, folds, and fractures, but also an indirect control exerted on stratigraphy.

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)