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Development of the Rocky Mountain Foreland Basin: Combined Structural University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 2007 DEVELOPMENT OF THE ROCKY MOUNTAIN FORELAND BASIN: COMBINED STRUCTURAL, MINERALOGICAL, AND GEOCHEMICAL ANALYSIS OF BASIN EVOLUTION, ROCKY MOUNTAIN THRUST FRONT, NORTHWEST MONTANA Emily Geraghty Ward The University of Montana Follow this and additional works at: https://scholarworks.umt.edu/etd Let us know how access to this document benefits ou.y Recommended Citation Ward, Emily Geraghty, "DEVELOPMENT OF THE ROCKY MOUNTAIN FORELAND BASIN: COMBINED STRUCTURAL, MINERALOGICAL, AND GEOCHEMICAL ANALYSIS OF BASIN EVOLUTION, ROCKY MOUNTAIN THRUST FRONT, NORTHWEST MONTANA" (2007). Graduate Student Theses, Dissertations, & Professional Papers. 1234. https://scholarworks.umt.edu/etd/1234 This Dissertation is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for inclusion in Graduate Student Theses, Dissertations, & Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. DEVELOPMENT OF THE ROCKY MOUNTAIN FORELAND BASIN: COMBINED STRUCTURAL, MINERALOGICAL, AND GEOCHEMICAL ANALYSIS OF BASIN EVOLUTION ROCKY MOUNTAIN THRUST FRONT, NORTHWEST MONTANA By Emily M. Geraghty Ward B.A., Whitman College, Walla Walla, WA, 1999 M.S., Washington State University, Pullman, WA, 2002 Dissertation presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Geology The University of Montana Missoula, MT Spring 2007 Approved by: Dr. David A. Strobel, Dean Graduate School James W. Sears, Chair Department of Geosciences Julia A. Baldwin Department of Geosciences Marc S. Hendrix Department of Geosciences Steven D. Sheriff Department of Geosciences Anna E. Klene Department of Geography Ward, Emily M. Geraghty, Ph.D., May 2007 Geology DEVELOPMENT OF THE ROCKY MOUNTAIN FORELAND BASIN: COMBINED STRUCTURAL, MINERALOGICAL, AND GEOCHEMICAL ANALYSIS OF BASIN EVOLUTION, ROCKY MOUNTAIN THRUST FRONT, NORTHWEST MONTANA Chairperson: James W. Sears 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. ii Acknowledgements Many thanks go to my advisor, Jim Sears, who inspired this research. His vast understanding of the Rocky Mountain thrust belt has proved invaluable. Thank you to my committee, all of whom strengthened this dissertation by generously sharing their geological knowledge. Much of this research would not have been possible without help from professionals at other institutions: Dr. Alexandra Skewes (Fluid Inclusion Laboratory, University of Colorado Boulder), Dr. Dennis Eberl (Quantitative X-ray Diffraction Laboratory, USGS Boulder, Colorado), Erin Marsh (Fluid Inclusion Laboratory, USGS Lakewood, Colorado), Dr. Patrick Meere (Dept. of Geology, University College Cork, Ireland), Dr. Martin Feely (Geofluids Research Group, National University of Ireland, Galway) and James Conliffe (PhD Student under Martin Feely, NUI-Galway). Additional thanks goes to Billie Jo Brown (Montana State University) my mentor in educational research, to my professional development colleagues Calvin Weatherwax and Helen Augere (Blackfeet Community College), and to all the Browning Middle School teachers who kindly took time out of their extremely busy schedules to develop earth science education for their students. I also would like to give special thanks to my family, particularly to my husband, Luke Ward, whose incredible support, understanding, and impenetrable positive outlook throughout this process has been immeasurable. This research was funded in part by NSF EPSCoR Grant EPS-0091995, Geological Society of America Research Grant, Rocky Mountain Association of Geologists Norman H. Foster Memorial Scholarship, and Tobacco Root Geological Society Field Scholarship. iii Preface Numerous theoretical and empirical models for basin development of the Rocky Mountain foreland basin suggest that in early-middle Jurassic an uplifted forebulge migrated through the present-day Rocky Mountain region. Models presented by Beaumont (1981) suggest that the Alberta basin development began in the early Jurassic and that the associated stratigraphic and structural relationships within the basin since that time have resulted from foreland basin migration; initiation of the foredeep and forebulge, subsidence of the basin, and propagation of the fold and thrust belt. The numerical models presented by Beaumont (1981) attribute the foreland basin development to terrane accretion occurring on the west coast of North America during the early Jurassic; where the load of the newly accreted terranes caused flexure within the crust, which then propagated eastward with continued thrusting. Empirical models based on the stratigraphic relationships within the Alberta Basin suggest that regional unconformities within the Cretaceous section are tectonically controlled. Tectonic loading causes subsidence and transgressive deposition in the proximal areas of the basin, while the more distal portions of the basin are uplifted (e.g. as a forebulge). Orogenic loading and flexural compensation of the crust is recorded in the Cretaceous stratigraphy and marks the position of the forebulge region during this time (Price, 1981). These models for foreland basin development are important to the oil and gas industry, which rely on these models to predict generation and migration of hydrocarbons within iv the basin. The timing of these events is imperative in order to identify maturation stage of the hydrocarbon in question. In addition, the structural and stratigraphic relationships within the basin will control the migration path of the hydrocarbons and hence are necessary in identifying the location of the hydrocarbon reservoirs. This research (comprised of 7 chapters) will build upon these earlier studies of the thrust belt and foreland basin by testing the validity of the models proposed by Beaumont (1981) and Catuneanu et al. (1997). The research area for this work lies south of the Canadian border, in northwest Montana, where the Rocky Mountain fold and thrust belt is superbly exposed. Of particular interest is the Sawtooth Range, which exhumes Mississippian carbonate rocks in unconformable contact with overlying middle Jurassic sandstones and shales. Furthermore, brittle structures contained within the carbonate rocks suggest that this area may provide a snapshot of early foreland basin development. If the models presented by Beaumont (1981) and Catuneanu et al. (1997) prove viable, the stratigraphic and structural evidence collected in this study should provide more information as to the early-middle Jurassic development of the foreland basin of northwest Montana; affirming what the numerical models predict and supporting the evidence of the empirical models. To test the hypothesis that the structural and stratigraphic relationships at the Swift Reservoir study site within the Sawtooth Range indeed do support the foreland basin models proposed, a variety of investigations were formulated. A combination of structural, mineralogical, and geochemical
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