Graduate Thesis Or Dissertation
 

Bedrock geology and stratigraphy of the northern parts of the Armstead and Madigan Gulch anticlines, Beaverhead County, Montana

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  • Approximately 18 square miles are included in the study area which lies 17 miles southwest of the town of Dillon, Montana. Strata exposed in the area include metamorphic, sedimentary, and volcanic rocks ranging in age from Precambrian to Recent. The Paleozoic and Mesozoic section is approximately 8000 feet thick and includes conglomerates, sandstones, siltstones, shales, cherts, carbonates, and andesite flows and tuffs. These rocks are divided into 18 formations. Cenozoic strata include basalt flows and volcaniclastic rocks, unconsolidated gravels, alluvium, colluvium, and landslide deposits. The study area, located on the western edge of the Paleozoic-Mesozoic Cordilleran cratonic shelf, was dominated by marine sedimentation in shallow to moderate water depths from Middle Cambrian to Late Jurassic time. Eustatic sea level changes, and epeirogenic movements on the shelf and in adjacent areas, acted alone or in concert to effect numerous episodes of subaerial exposure and erosion on the shelf during this period. Cretaceous rocks consist of thick accumulations of syorogenic conglomerates and sandstones of the Beaverhead Formation. These rocks reflect increasing diastrophism on the cratonic shelf and within the adjacent geosyncline to the west. Laramide deformation culminated within the study area in Late Cretaceous time, forming large, north- to northwest trending folds and the low angle, east-verging Tendoy and Indian Head thrust faults. These thrust faults are thought to involve both the Phanerozoic sedimentary strata and the underlying Precambrian metamorphic basement. Volcanic activity, in the form of andesite flows and tuffs, closely followed the period of maximum deformation. An extensional tectonic regime, which resulted in normal faulting and the formation of block-faulted ranges and subsiding basins, developed in the study area during Oligocene time and continues to the present. The faulting was accompanied, probably in Eocene time, by basaltic volcanic activity.
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