Year of Award

2007

Document Type

Dissertation

Degree Type

Doctor of Philosophy (PhD)

Department or School/College

Department of Geology

Committee Chair

James W. Sears

Commitee Members

Julia A. Baldwin, Marc S. Hendrix, Steven D. Sheriff, Anna E. Klene

Keywords

foreland basin, geoscience education, hydrocarbons, Montana, Rocky Mountains, thrust belt

Abstract

The sub-Middle Jurassic unconformity exhumed at Swift Reservoir, in the Rocky Mountain thrust belt of Montana, exposes structures that call for a re-evaluation of the deformation history at this locale. The unconformity separates Late Mississippian Madison Group carbonate (~340 Ma) from the transgressive basal sandstone of the Middle Jurassic (Bajocian-Bathonian) Sawtooth Formation (~170 Ma). Fieldwork established that northwest-trending, karst-widened fractures (grikes) are filled with cherty, phosphatic sandstone and conglomerate of the Sawtooth Formation and penetrate the Madison Group for 4 meters below the unconformity. Clam borings, filled with Sawtooth sandstone, pierce the unconformity surface, some of the fracture walls, and also perforate rounded clasts of Mississippian limestone that lie on the unconformity surface within basal Sawtooth conglomerate. Following deposition of the overlying foreland basin clastic-wedge, the grikes were stylolitized by layer-parallel shortening and buckled over fault-propagation anticlinal crests in the Late Cretaceous-Paleocene fold-and-thrust belt. The model proposes that the grikes record uplift and erosion followed by subsidence as the Rocky Mountain foreland experienced elastic flexure in response to tectonic loading at the plate boundary farther to the west during Early Jurassic; the forebulge opened strike-parallel fractures in the Madison Group that were karstified. The grike system contributes to the secondary porosity and permeability of the upper Madison Group; a major petroleum reservoir in the region. Grikes acted as fluid pathways during basin evolution as seen from the clay mineral assemblage and fluid inclusions contained within the grike fill. Mixed-layer illite-smectite (I/S) indicates that the grikes did not exceed 210∞ C (complete smectite-illite transition). The illite likely resulted from superstaturated fluids flushing through the foreland at the onset Laramide orogeny and may have been coincident with hydrocarbon migration. Hydrocarbon inclusions contained within the grike cements were trapped at temperatures ranging from 110∞-170∞ C; correlative with the clay temperature calculations. Recognition of the fractures as pre-middle Jurassic revises previous models, which related them to Cretaceous fracturing over the crests of fault-propagation folds, substantially changing the understanding of the hydrocarbon system.

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© Copyright 2007 Emily Geraghty Ward