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Abstract: Laramide Wrench Faults on the Antelope Arch, Wyoming


The Antelope Arch is the eastward extension of the Wind River Mountains Uplift. Cretaceous and older strata are complexly faulted and have more than 15,000 feet of relief, but most structural details are concealed by gently dipping Tertiary beds.

The Bison Basin Fault, which separates the oil-productive Bison Basin anticline from the Wind River Uplift, is the only previously documented wrench fault in the area. This left lateral fault appears to merge with the Wind River Thrust along which the mountain block was transported southwestward during Late Cretaceous to Eocene time.

Synthesis of outcrop, subsurface, palynology end seismic data indicates at least 4 left lateral, northeast-trending wrench faults east of Bison Basin. An exotic block of Mesaverde and older rocks commenced moving southwestward along the Crooks Creek Fault during late Campanian or early Maestrichtian time while the Lewis Shale and Fox Hills Sandstone were being deposited. Isopachs of net sandstone in the Baculites sp. (weak flank ribs) zone of the Steele Shale indicate 21 miles of southwest transport. Seismically, this fault is a dead zone which locally is nearly one mile wide. Other faults with smaller movements are indicated by key wells which encountered chaotic, overturned and steeply dipping strata.

These faults may break the southeast-plunging axis of the Wind River Uplift into en echelon anticlines. Such structures are future exploration targets.

AAPG Search and Discovery Article #90946©1997 AAPG Rocky Mountain Section Meeting, Denver, Colorado