Abstract

The sequence of pebble types and pattern of sediment deposition observed in the thick (10,000+ ft) Late Cretaceous and Paleocene Beaverhead and Monida Formations in the Lima area of southwestern Montana permit determination of source areas, approximate carrying capacity and slope of streams transporting debris to the depositional site, and character of the depositional environments. These data lead, in turn, to an accurate reconstruction of the paleogeography in the Lima-Monida area during the Late Cretaceous and early Tertiary.

Directional sedimentary structures and variations in maximum boulder size and pebble-cobble composition suggest the presence of two source areas. Both source areas, the Blacktail-Snowcrest uplift and ancestral Beaverhead Range, were domal uplifts and/or high-angle fault blocks, as indicated by the simple inverted sequence of pebble-cobble lithologic types, sandstone lithologic types, and heavy-mineral suites. Uplift of the source areas preceded emplacement of Laramide low-angle thrust plates, which probably originated as late-stage gravity slides off the ancestral Beaverhead Range. This sequence of Laramide events conflicts in part with the sequences suggested by workers in other areas of the Rocky Mountain thrust belt where synorogenic conglomeratic rocks are present.

The coarseness, graded bedding, channeling, imbrication, bed geometry, and bedding characteristics indicate that the conglomerates of the Beaverhead Formation were deposited as alluvial-fan debris. The cross-bedded sandstone of the Monida Formation on the southeast may represent deposition on the distal parts of alluvial fans or possibly on adjacent fan aprons or floodplains.

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