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The AAPG/Datapages Combined Publications Database
Montana Geological Society
Abstract
MTGS-AAPG
Montana Geological Society: 1993 Field Conference Guidebook: Old Timers' Rendezvous Edition: Energy and Mineral Resources
of Central Montana
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Stratigraphic and Structural Summary for Central Montana
ABSTRACT
The central Montana trough, an eastward extension of the Lewis and Clark lane of western Montana, has been an area within the Western Interior shelf of repeated diastrophism throughout the Phanerozoic. Major structural movements occurred during Devonian, post-Mississippian to Middle Pennsylvanian, and Late Cretaceous to Early Tertiary; but lesser tectonic events recurred episodically at many other times in the geological development of the region. Broad, onlapping sequences of sandstone, mudstone, and carbonate rocks have repeatedly blanketed the shelf between periods of non-deposition and erosion. Many of these sedimentary packets are locally thin or are absent above uplifts in the central Montana trough, or they are thicker than elsewhere in the region owing to subsidence near the trough. Epeiric marine deposits of mostly carbonate rocks are notably thick in central Montana because of inundation by the epicontinental sea into the subsiding trough and onto the adjacent shelves, especially during the Mississippian. Subsequent siliciclastic sediments ranging from fluvial, deltaic, and marine deposits, filled a tectonically formed valley during the early Pennsylvanian, but later in the Pennsylvanian the region was overspread by shallow, epeiric marine and eolian deposited sands. There are no Permian through Lower Jurassic strata because of a long period of non deposition and erosion, but continental and shallow-marine deposits of mud, sand and carbonate again covered central Montana as the Late Jurassic to Late Cretaceous seaway inundated the inner part of the Western Interior shelf prior to Laramide diastrophism. Uppermost Cretaceous and younger sediments are primarily of subaerial, continental origin.
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