PORTER, KAREN W., Montana Bureau of Mines and Geology, Butte, MT, and THADDEUS S. DYMAN and RUSSELL G. TYSDAL, U.S. Geological Survey, Denver, CO

ABSTRACT: Sequence Boundaries and Other Surfaces in Lower and Lower-Upper Cretaceous Rocks of Central and Southwestern Montana

In the Montana portion of the Western Interior sedimentary basin Lower and lower-Upper Cretaceous strata contain several types of conformable and unconformable stratigraphic surfaces that can be related in a sequence stratigraphic framework. Some surfaces are interpreted as sequence boundaries, and others are contained within sequences, such as transgressive surfaces of erosion local scour surfaces and condensed sections associated with maximum flooding. Together these surfaces record the sedimentary response of terrestrial (southwest Montana) and marine (central Montana) environments within the basin to relative sea level changes.

Four sequence boundaries (SB1-SB4) in upper Aptian, Albian, and Cenomanian rocks are recognized and correlated regionally. The oldest, SB1, of late Aptian or early Albian age, is placed within the upper Kootenai Formation at the base of the gastropod limestone interval in southwest Montana; it records a rise in regional base level in terrestrial deposits. In central Montana, SB1 is placed at the top of the Kootenai Formation, where coastal deposits of the Fall River Sandstone onlap Kootenai red beds. SB2, of late Albian or early Cenomanian age, is placed within the upper sandstone unit of the Flood Member of the Blackleaf Formation, and within the correlative Muddy Sandstone, in southwest Montana. It is correlated with a chert-pebble bed in marine shales of the lower Thermopolis Shale in central Montana. SB3 is recognized in the lower Cenomanian Mowry Shale in the Madison Range; its regional relations are presently uncertain. SB4, of early middle Cenomanian age, is recognized at the top of oxidized terrestrial rocks of the Vaughn Member, or in lower Frontier beds, in southwest Montana. It is correlated with a gravel bed recognized in marine shales of the lower Belle Fourche Shale in central Montana.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.