Year of Award


Document Type


Degree Type

Master of Science (MS)

Degree Name


Department or School/College

Department of Geosciences

Committee Chair

James Sears

Commitee Members

Marc Hendrix, Paul Wilson


Montana, stratigraphy, triangle zone, structural geology


University of Montana


Henry, Heather, M.S., May 2007 Geosciences Investigation of a triangle zone structure between Augusta and Bowman’s Corners, Lewis and Clark County, Montana Chairperson: James Sears Productive hydrocarbon reservoirs along Canada’s Rocky Mountain thrust belt are contained by a triangle zone, which is comprised of complex antiformal wedge structures that commonly form along the leading edges of foreland fold-thrust systems world-wide. Duplex imbrication within triangle zones create stacked structural traps for hydrocarbons driven toward the surface by the propagation of thrust belts. This geometry is well documented in a series of en echelon structures that stretch approximately 900 kilometers, from northeast British Columbia into southwest Alberta, along the western flank of the Alberta syncline, Canada. Along structural trend, the Rocky Mountain foothills triangle zone has not been well documented south of the Canadian-United States border, into Montana. Select parts of four 7.5-minute United States Geologic Survey quadrangles, in Lewis and Clark County, Montana, were mapped in detail to precisely locate structures associated with triangle zones. The region is dominated by thick packages of upper Cretaceous sediment, including the Saint Mary River Formation, Horsethief Formation, Two Medicine Formation, Virgelle Sandstone, and Telegraph Creek Formation of the Montana Group. These formations were subdivided based on laterally distinct lithologies, allowing for greater structural resolution of the area. Field observations and analysis of geologic maps and cross-sections show passive roof thrusting has raised the Augusta syncline above an east dipping thrust creating disharmonic folding parallel to the Rocky Mountain Front. This is analogous to the Alberta syncline, allowing for a correlation to be made between the Canadian triangle zone and west-central Montana.



© Copyright 2007 Heather Marie Henry