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ABSTRACT: Stratigraphic Sequences and Unconformities in Cretaceous Rocks of East-Central MT

Karen W. Porter, Elizabeth B. Campen, Previous HitWilliamNext Hit A. Coban, Previous HitWilliamTop B. Hansen

Cretaceous strata across east-central Montana are terrigenous clastics, dominantly of marine origin, reflecting complex patterns of sediment source and distribution associated with fluctuating sea levels that may have been both eustatically and tectonically controlled. A composite surface section and a subsurface well log section represent the Bull Mountains basin and Blood Creek synclines and the intervening Cat Creek anticline and Porcupine dome. Stratigraphic terminology reflects lithologic affinities with the Black Hills region.

Lower Cretaceous redbeds of the Kootenai Formation (Aptian) are separated from overlying Fall River Sandstone (Aptjan) marine deposits by a sequence boundary. Overlying rocks of the Thermopolis (Albian) through Niobrara (Coniacian-Santonian) are dominantly marine shales up to 1800 ft (600 m) thick. Strata of late Santonian through Maestrichtian age are represented by approximately 2100 ft (700 m) of marine sandstones and shales of the Telegraph Creek through Fox Hills formations, and by up to 500 ft (166 m) of nonmarine Hell Creek beds, recording three regressive shoreline sequences associated with overall highstand sea-level conditions.

Chert-pebble horizons in the marine section suggest stratigraphic breaks that may correlate with unconformities recognized elsewhere in the Western Interior. Such horizons are currently known within the lower Thermopolis, Mowry, Belle Fourche, Greenhorn, and upper Eagle.

Cretaceous deposits in central Montana are of particular interest because they lie at an approximate axial position within the depositional basin. Thus they provide opportunity to evaluate conditions in the basin center during sea-level fluctuations that produced unconformities on the basin margins.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990