Reactivation of Bedding Planes within Carbonate Thrust Sheets, Sun River Canyon,

Sawtooth Range, Northwestern Montana

By

Emily M. Geraghty and A. John Watkinson

Washington State University, Pullman, WA

Sun River Canyon in the Sawtooth Range of northwestern Montana provides an excellent cross-section of a portion of the Montana disturbed belt. This portion consists of west-dipping thrust faults that emplace packages of Mississippian-age carbonate over Cretaceous shales. Previous research describes the thrust sheets in the region as coherent and essentially undeformed, when, in fact, they contain abundant strain accommodation features due to folding and thrust faulting. Various mesoscale deformation features are present and include bedding plane slip surfaces, wedge structures, stylolites, and veins.

Reactivation of bedding planes during deformation accommodates strain within the thrust sheets. Although many stylolite surfaces remain unreactivated, some may have provided a locus for bedding parallel slip. The field evidence suggests that certainly the bedding planes create an anisotropy for slip and may have initiated in response to compressional tectonic stresses.

The number of slip surfaces and wedge structures increases as the thrust sheets become more steeply dipping. Several mechanisms may be responsible for the increase: 1) flexural slip folding in ramp regions may have been greater in the westerly piggy-back thrust sheets, 2) thrust sheet rotation during foreland propagation caused increased slip on pre-existing slip planes and created more slip planes to accommodate the rotation, 3) partitioning of the strain such that motion was taken up on slip planes throughout the sheet as well as on the larger-scale thrust faults or 4) perturbed local stress fields around the propagating thrust faults allowed for more bedding plane slip surfaces to be in preferred failure orientation.

AAPG Search and Discovery Article #90004©2002 AAPG Rocky Mountain Section, Laramie, Wyoming