Structure and stratigraphy of the Argenta area, Beaverhead County, Montana Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/zk51vm77f

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  • The thesis area comprises 53 square miles located in Beaverhead County, Montana, and includes the Argenta mining district. Sedimentary and igneous rocks of the area range in age from Precambrian to Recent. The aggregate thickness of the sedimentary rocks is approximately 13,500 feet. The only systems not represented are Ordovician and Silurian. The oldest sedimentary rocks consist of Late Precambrian shallow marine and nonmarine subarkose, argillite, and ortho-quartzite of the Belt Supergroup. These strata are divided into three informal map units. The Paleozoic sedimentary rocks that overlie the Belt strata were deposited in a marine and in part nonmarine environment. The Cambrian system is represented by the Late Cambrian Hasmark Formation, succeeded disconformably by the Late Devonian Jefferson Dolomite and Three Forks Formation. Overlying the Three Forks is a thick sequence of Early to Late Mississippian limestones which were separated into the Lodgepole Formation and Mission Canyon Limestone. The Amsden Formation of Late Mississippian to Early Pennsylvanian age overlies the Mission Canyon with probable disconformity. The Pennsylvanian Quadrant Formation conformably succeeds the Amsden. The Permian Phosphoria Formation is considered to disconformably(?) overlie the Quadrant Formation. Mesozoic strata were deposited in continental and marine environments. The marine Dinwoody Formation of Early Triassic age is disconformably succeeded by the Late Jurassic continental Morrison Formation. The dominantly continental Early Cretaceous Kootenai Formation overlies the Morrison Formation with a gradational and arbitrarily located boundary. Marine strata of the Colorado Group (?) of Early and Late Cretaceous age overlie the Kootenai Formation with probable disconformity. Rocks representing the Tertiary System are dominantly igneous. The Argenta Stock and related plutons consist of quartz monzonite, granodiorite, and latite porphyry of probable Middle or Late Eocene age. The Argenta Stock is a composite pluton comprising quartz monzonite and quartz monzonite porphyry. Lithic welded tuff and an olivine basalt flow of possible Eocene-Oligocene age are found only in the northwest part of the area. Rhyolitic vitric tuff discomformably overlain by a limestone-pebble conglomerate, both of probable Miocene age, are present only in the eastern part of the area. Quaternary strata in the area include both glacial and fluvial deposits. Pleistocene glaciers, representing at least three glacial stades, produced a glacial trough with valley-in-valley profile, and left moraines in the northwest part of the area. Resulting outwash deposits are found in terraces above the present Rattlesnake Creek drainage. Quaternary landslide and alluvial deposits are locally present. The Laramide Orogeny has produced a complex series of folds and faults which locally trend anomalously northeast-southwest. The main structural features are the Kelly Thrust and the Argenta Anticline. Horizontal compression from the west and northwest produced a series of dominantly northeast-plunging anticlines and synclines broken by steep faults in addition to the major structures.
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