Stratigraphy and depositional history of the Amsden and lower Quadrant Formations, Snowcrest Range, Beaverhead and Madison Counties, Montana Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gt54ks07t

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  • During the Late Mississippian-Early Pennsylvanian (latest Chesterian-early Morrowan), a thick sequence of shallow -water carbonate-clastic sediments, equivalent to the "Amsden" and lower Quadrant Formations, was deposited in the Snowcrest trough on the western margin of the northern Rocky Mountain Cordilleran platform in southwestern Montana. Today this late Chesterian sequence is exposed primarily within the Snawcrest Range which trends in a northeasterly direction through Beaverhead and Madison Counties, Montana. Conforming to new Late Mississippian nomenclature for rocks in southwestern Montana, the following stratigraphic units, listed in ascending order, were recognized; the Conover Ranch Formation of the Snowcrest Range Group and the Quadrant Formation. In addition, a relatively thick (325 foot average), unnamed, Shallow water carbonate sequence was recognized between the Conover Ranch and Quadrant Formations. Thus, three stratigraphic units, the Conover Ranch Formation, an unnamed limestone unit, and the Quadrant Formation were recognized and comprise the latest Mississippian-earliest Pennsylvanian strata in the thesis area. The Conover Ranch Formation (late Chesterian) represents a low- energy, marginal marine depositional sequence. The formation consists primarily of red shales and siltstones with occasional quartz arenite -limestone conglomerate lenses. The quartz arenite-limestone conglomerate lenses are thought to represent higher energy, subtidal channel deposits. The Conover Ranch Formation rests unconfonnably upon the Lombard Limestone (Big Snowy equivalent) thoughout much of the thesis area. The unnamed limestone unit (latest Chesterian) was deposited in a shallow-water carbonate shelf environment. The unit consists primarily of wackestones, packstones, and mudstones. Occasionally, an organic-rich mudstone interval was recognized and samples were collected for hydrocarbon source-rock analyses. The unnamed limestone unit grades vertically into a lower dolomitic interval of the Quadrant Formation. Only the lower dolomitic interval of the Quadrant Formation (latest Chesterian? -Morrowan) was studied in this thesis. The sequence consists primarily of dolostones and dolomite - cemented quartz arenites which were deposited in a littoral to neritic environment. The origin of the dolomite is considered to be secondary, resulting from post-depositional diagenetic events. In general, the Late Mississippian -Early Pennsylvanian stratigraphic units in the thesis area reflect a single, major transgressive-regressive depositional sequence. The transgression of the sea onto the Cordilleran platform began with the deposition of the Conover Ranch Formation, reached a maximum during deposition of the unnnamed limestone unit, and began to retreat off the platform at the onset of clastic deposition characteriztic of the Quadrant Formation. The overall hydrocarbon source rock potential of the unnamed limestone unit ( "Amsden ") in southwestern Montana is poor.
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