Comparison of Rudite-Rich Facies in Tertiary Intermontane Basins, Montana and Wyoming

Jean N. Weaver, Romeo M. Flores, John W. M'Gonigle

Rudite-rich deposits are characteristics near the margins of Tertiary intermontane coal basins in the Medicine Lodge basin, Montana, and Powder River basin, Wyoming. The rudites are graded, crudely imbricated, framework to matrix-supported, pebble to boulder-size deposits that have basal erosional contacts. In the Medicine Lodge basin, the rudite-rich facies includes basally scoured conglomeratic sandstone, siltstone, fish scale-bearing mudstone, biohermal carbonate, and coaly carbonaceous shale. Matrix-supported rudites occur in the lower part 3of coarsening-upward sequences. Framework-supported rudites and offset conglomeratic sandstone, which are common in the upper part of these sequences, are interbedded with coal-bearing units along with fish scale-bearing mudstones, gradational to mixed siliciclastics, and biohermal carbonates.

In the Powder River basin, the rudite-rich facies consists of fining-upward sequences. Framework-supported rudites are common in the lower part of these sequences, and conglomeratic sandstones are common in the upper part; matrix-supported rudites are absent. Finer grained, freshwater mollusk-bearing, and coaly sediments bound these coarse facies.

Variations in the facies associations and architectural elements of the rudite-rich facies suggest contrasting depositional models for these two basins. In the Medicine Lodge basin, the facies were deposited in lacustrine fan deltas on stable platforms. Debris flows characterized deposition on the fan-delta fronts. Peat swamps developed and carbonate precipitated in areas adjacent to the fan deltas. In the Powder River basin, the conglomerates accumulated in alluvial fans. Peat swamps developed in interfan and fan frontal areas. Although conglomeratic facies are related to deformed basin-margin fronts, their deposition was also controlled by subsidence and microclimatic settings of the basins.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.