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Cenozoic Sedimentary Evolution of the Helmville Basin, West-Central Montana

by McCune, Julian Glenn

Abstract (Summary)
Abstract McCune, Julian, Glenn, M.S., Spring 2008 Geosciences Cenozoic Sedimentary Evolution of the Helmville Basin, West-Central Montana Chairperson: Marc S. Hendrix The Helmville basin is an intermontane topographic depression located ~100 kilometers east of Missoula, Montana. The basin occurs within the Rocky Mountain Fold and Thrust Belt and is immediately north of the Lewis and Clark structural Lineament. The Helmville Basin contains sedimentary and volcanic fill of Paleogene and Neogene age, along with abundant Quaternary sediment. The purpose of this study is to document the geologic evolution of the Helmville basin through a combination of field-based geologic mapping and sedimentologic characterization and to assess the nature of geological resources the basin contains. I also seek to test a rift shoulder tectonic model presented by Janecke (1994) for western Montana and Idaho by reconstructing the paleogeography of the Helmville basin and comparing those results against the model predictions. To accomplish these goals, I constructed a 1:24,000 scale geologic map of the study area, measured a well-exposed section of Paleogene and Neogene sedimentary strata, conducted compositional analysis of Paleogene and Neogene sandstone framework grains and conglomerate clasts, and measured paleocurrent directional indicators . The western edge of the study area consists mainly of massive undifferentiated andesite and basalt that is Eocene in age. Oligocene sedimentary strata belong to the Cabbage Patch beds of the Renova Formation, which in the study area consists of well sorted sandstone and fossiliferous mudstone interpreted to represent meandering fluvial and paludal (swamp) floodplain environments. Measured paleocurrent indicators within the Cabbage Patch beds suggest that sediment was transported from south to north. Neogene strata consist mainly of massive gravel of the Sixmile Creek Formation, which is composed dominantly of Proterozoic metasedimentary clasts. Paleoflow was from west to east. The deposition of the Sixmile Creek Formation gravel facies appears to have been controlled by a down-to-west normal fault on the east side of the Helmville basin. Erosional excavation of Neogene gravels and older sediments and volcanics beginning in the late Neogene is suggested by the preserved thicknesses and regional distribution of these sediments across the study area. During the Quaternary, a large lobe of ice originating from the Monture Creek drainage blocked the upper Blackfoot River drainage, impounding a proglacial lake in the study area (glacial Lake Blackfoot). A Gilbert-style delta prograded into this glacial lake from Yourname Creek drainage, located on the western edge of the study area. I infer that the Helmville basin formed as a result of down-dropping along a major southwest-facing normal fault located on the eastern edge of the basin. This fault is mostly obscured by Paleogene gravel but is locally exposed. The presence of aligned tufa mounds within the central part of the basin suggests the presence of at least one additional en echelon fault.
Bibliographical Information:

Advisor:Marc S. Hendrix; Julie Baldwin; Ulrich Kamp

School:The University of Montana

School Location:USA - Montana

Source Type:Master's Thesis

Keywords:geology

ISBN:

Date of Publication:08/07/2008

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