About This Item

Share This Item

The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 63 (1979)

Issue: 5. (May)

First Page: 833

Last Page: 834

Title: Wrench Faulting and Hydrocarbon Occurrence in Northwestern Powder River Basin, Montana and Wyoming: ABSTRACT

Author(s): B. E. Law, B. E. Barnum

Article Type: Meeting abstract

Abstract:

Detailed geologic mapping in the northwestern Powder River basin of Montana and Wyoming has delineated a west-northwest-trending structurally deformed zone that is related to a primary, left-wrench fault. The southeast end of the zone is located on the west side of the Badger Hills in Wyoming and trends N75°W for about 70 km to the east side of the Big Horn Mountains in Montana. Published aeromagnetic and gravity maps indicate that this structural zone extends across the northern Big Horn and Pryor Mountains and may represent a southeast extension of the Nye-Bowler lineament.

The most prominent structural elements within the zone are en echelon normal faults with surface traces trending N50°E in Paleocene and Eocene rocks. These high-angle tension faults commonly show reverse drag, with the downthrown sides rotated into the fault planes. Some of these faults have a small component of left-lateral strike-slip movement in Upper Cretaceous rocks. Fault displacement in the northern half of the zone is generally down on the southeast side and in the southern

End_Page 833------------------------------

half of the zone is down on the northwest side.

Another conspicuous feature related to the wrench-fault system is the Ash Creek oil field on a small faulted anticline. Isopach mapping of the producing Upper Cretaceous Shannon Sandstone Member of the Steele Shale shows that folding and faulting were initiated prior to deposition, thus providing optimum conditions for early migration of hydrocarbons into the structural trap.

End_of_Article - Last_Page 834------------

Copyright 1997 American Association of Petroleum Geologists