Fluvial and lacustrine-dominated clastic sedimentary rocks as thick as 1,800 m (6,000 ft) comprise the Paleocene Fort Union Formation and the Eocene Wasatch Formation of the western Powder River Basin in northeastern Wyoming and southeastern Montana. The systematic mineralogy of 45 samples of channel-fill sandstone from this sequence reflects the uplift and erosion of the Bighorn Mountains. Samples were collected to study vertical changes in the mineralogy of lower Tertiary sandstones adjacent to the Bighorn Mountains, lateral variations in the composition of the upper Paleocene Tongue River Member of the Fort Union Formation along the eastern front of the mountains, and variations in the composition of equivalent upper Paleocene sandstones of the central and western parts of the basin. Vertical changes in the mineralogy of a succession of Paleocene and Eocene sandstone units adjacent to the Bighorn Mountains most likely were produced by uplift and sequential erosion of the rocks that formerly overlaid the mountains. Uplift probably began in the middle Paleocene, during deposition of the Lebo Member of the Fort Union Formation, and continued into the Eocene. Differences in the mineralogy of the sandstone units along the western edge of the Powder River Basin that correspond to differences in the rock types now exposed along the crest of the Bighorn Mountains suggest that much of the erosional degradation of the Bighorn Mountains occurred during an early Tertiary tectonic episode. Lateral changes in the suite of unstable detrital grains within the Tongue River Member are compatible with facies and paleotransport studies that indicate a substantial eastward flux of detritus of early Tertiary age from the Bighorn Mountains into the central Powder River Basin.

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