David A. Lopez1
(1) Montana Bureau of Mines and Geology, Billings, MT
ABSTRACT: The Greybull Sandstone, Lower Cretaceous valley-fill on the Crow and Northern Cheyenne Indian Reservations, south-central Montana
The Greybull Sandstone occupies valleys incised into the Lower Cretaceous, non-marine Kootenai Formation during a marine low stand that is recorded basin-wide in the Western Interior sedimentary basin. The Greybull is a part of the sequence that includes the overlying Fall River Sandstone of shoreline and near-shore origin. Together, these two intervals represent part of a transgressive systems tract overlying a lowstand unconformity. On the Crow Reservation, four major Greybull channels have been identified in surface exposures. These valley-fill sandstones are up to 45 m (150 feet) thick, and are up to 6 km (4 miles) wide. Paleocurrent analysis indicates that fluvial transport in this region was generally westward. The source of the sand is presumed to be the Precambrian Sioux Quartzite in the South Dakota-Minnesota area. One Greybull channel can be traced on the surface for about 100 km (60 miles). An abrupt change in transport direction of this channel, from westward to northward, coincides with faults of the Nye-Bowler and Fromberg fault systems. These basement-controlled faults are interpreted to have exerted structural control on the trace of this channel. Surface exposures of channels have been tied to occurrences of Greybull channels in the subsurface, both on the Crow and Northern Cheyenne Reservations. Five major Greybull valley-fill systems have been identified in the subsurface in this region.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado