About This Item
Share This Item
Central Montana has had a complex structural and sedimentary history, especially the area of today's Central Montana uplift.
Precambrian and Cambrian subsidence allowed deposition of 1300+ feet of clastics in an east-west trending through roughly coincident with the present-day uplift.
Pre-Devonian uplift and erosion followed stable depositional conditions during Ordovician time. Ordovician has been eroded from the western one-half of the study area. Silurian is absent from the entire study area.
Central Montana uplift area remained high during Lower, Middle and part of Upper Devonian time. Upper Devonian rocks lap onto the uplift and uppermost Devonian finally covered the area. Pre-Mississippian uplift removed these carbonates and shales completely from a large area of the uplift.
Mississippian system is comprised of the carbonate evaporite Madison Group and clastic Big Snowy Group. Stable conditions prevailed through most of Madison deposition, but central Montana began to subside in Late Madison time. The Big Snowy Group was restricted by continued subsidence which downwarped central Montana into a synclinorium.
Early Pennsylvanian streams draining eastern Montana cut valleys in the Central Montana trough. These valleys were filled as the streams attained old age primarily with Big Snowy Group derived sands and shales. This stream-channel deposit, the Lower Tyler Formation, contains the major reservoirs of central Montana, Middle and Late Pennsylvanian sediments covered central Montana, but all of the Late Pennsylvanian was eroded pre-Jurassic time. Pre-Jurassic folding accentuated Mississippian structure.
Jurassic saw uplift in the Belt Mountain area to the west. Jurassic laps onto this high and thickens eastward as well as in the trough area.
Lower Cretaceous deposition was controlled by uplift to the south and thickens from south to north.
The Laramide revolution upwarped the old trough into the Central Montana uplift and also generally folded the old synclines into anticlines as at the Sumatra trend. Isostatic adjustment at basement fault blocks was the force behind the down up down up movements of central Montana.
End_of_Article - Last_Page 1878------------