Graduate Thesis Or Dissertation
 

Stratigraphy and depositional environments of the Mississippian Rocks, Garnet Range-Bearmouth area, Granite County, western Montana

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  • The Garnet Range-Bearmouth area of western Montana was the depositional site for basinal, platform slope, shelf, and peritidal Mississippian carbonate rocks. The following stratigraphic units, listed in ascending order, are recognized: the Paine Limestone, Middle Canyon Formation, Mission Canyon Limestone, and McKenzie Canyon Limestone of the Tendoy Group; the Kibbey Sandstone and the Lombard Limestone of the Snowcrest Range Group; and the Quadrant Sandstone. Although the Rattler Gulch section most closely resembles the Mississippian sequence described in the Tendoy Mountains of Montana, the Tendoy Group exposures at Rattler Gulch are older and occupied a position closer to the Transcontinental arch during the Mississippian. The geologic history of the thesis area consists of a major Early to Late Mississippian transgressive-regressive marine eustatic cycle documented by the Tendoy Group, and a less extensive Late Mississippian to Early Pennsylvanian transgresssive-regressive cycle represented by the Snowcrest Range Group and the Quadrant Sandstone. The first eustatic cycle began with a major transgression of the sea onto the Cordilleran platform in the Early Mississippian (late Kinderhookian). This resulted in the progradation of subtidal to lagoonal platform carbonates of the Mission Canyon Limestone over basinal limestones of the Paine Limestone and carbonate slope deposits of the Middle Canyon Formation. This cycle ended with a major regression that began in the Late Mississippian (early Meramecian). It is evidenced by the intertidal and supratidal sediments of the McKenzie Canyon Limestone and by the emergence of the carbonate shelf as a broad karst plain exposed to subaerial weathering. With renewed marine transgression in the late Meramecian, red tidal-flat and lagoonal siliceous sediments of the Kibbey Sandstone filled and covered the McKenzie Canyon Limestone karst. As the transgression continued into Early Pennsylvanian (Morrowan) time, the progressively clearer and deeper waters favored deposition of shallow-water carbonates of the Lombard Limestone. Following deposition of the Snowcrest Range Group, a slow westward regression, concurrent with the development of a rising land source in north-central Idaho, resulted in the deposition of the latest Mississippian-Early Pennsylvanian Quadrant Sandstone throughout western Montana.
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