Year of Award

2017

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Department or School/College

Department of Geosciences

Committee Chair

James Sears

Commitee Members

James Sears, James Staub, Andrew Ware

Keywords

Montana Triangle Zone, Montana Structural Geology, Rocky Mountain Front, Craig Anticlinorium, Montana Geology, Montana Petroleum Exploration

Publisher

University of Montana

Subject Categories

Geology | Tectonics and Structure

Abstract

A triangle zone is a structure commonly found at the leading edge of a thrust belt, characterized by a triangular cross-section of thrust faults. In the Montana triangle zone, west-dipping thrust faults intersect east-dipping ones above a regional detachment fault. Most studies of triangle zones focus on the structure in a cross-sectional sense, but little work addresses variations along the trend. The Canadian Rocky Mountain triangle zone has long been a productive petroleum province, so understanding the southern extent of this zone, where it enters Montana, is of interest for economic purposes as well as geologic understanding. Previous studies have concluded that the triangle zone continues into Montana, and this study interprets the triangle zone an additional 25 km farther south. This area is, however, more complex than previously thought.

The triangle zone in Montana undulates vertically in a system of culminations and depressions. The surface geology of the area was mapped in detail in the early 1960’s by the United States Geological Survey (USGS), but the subsurface interpretations were made before triangle zones were identified in the area. Multiple cross sections were created in this study to update these interpretations. Select areas from the USGS Comb Rock quadrangle in Lewis and Clark County, Montana, were studied in detail to locate, measure, and document a culmination of the triangle zone. Erosion of the culmination produced structural windows through the triangle zone, whereas the neighboring depressions preserve the overlying thrust plates. Strain accumulation in shale units, specifically the Marias River Shale, as well as thick igneous sills elevated the culmination. In addition, penetrative deformation styles and disharmonic folding were documented in three formations in the study area: the Telegraph Creek Formation, Marias River Shale, and Blackleaf Formation. This allowed for a new interpretation of triangle zone formation to be made that helps to explain the structurally thickened units that are commonly found near the apex of the blind thrust. This interpretation closely examined the geometries necessary to create a triangle zone and found a thrust angle discrepancy that can be accommodated by extra material being piled up on the thrust flat.

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© Copyright 2017 Shane S. Fussell