NEOGENE DISTRIBUTIVE FLUVIAL SYSTEM STRATA AT THE ANCENEY FOSSIL VERTEBRATE LOCALITY, SOUTHWESTERN MONTANA

The depositional environment of the Neogene Anceney fossil vertebrate locality near Bozeman, Montana, is newly interpreted to be part of a distributive fluvial system (DFS). The total thickness of the Neogene portion of the exposure is only about 14 meters, but a coarsening-upward grain size trend and an ascending change in channel morphology from isolated to amalgamated channels within this exposure indicate sediment deposition within a DFS. Isolated channels occur throughout the lower 5.5 meters of the exposure. Paleosols developed within floodplain deposits associated with the isolated channels exhibit mottling with low chroma and are interpreted as hydromorphic paleosols. Both the isolated channels and their associated floodplain hydromorphic paleosols are indicative of distal DFS strata. The upper 8.5 meters of exposure include a debris flow unit and amalgamated channels typical of DFS proximal deposits.

The Anceney vertebrate locality has yielded a varied fossil assemblage of Barstovian age that includes mammals, reptiles, and amphibians. The fossil collection includes both fragmental and relatively intact remains. A largely complete skull of the canid Paratomarctus temerarius was found at the base of a debris flow unit that occurs near the top of the exposure. Rounded to angular fragmental fossil material is scattered throughout the exposure but is concentrated towards the top of the exposure as lag deposits within the amalgamated channels. We interpret the presence of both fragmental and relatively complete fossil material to be indicative of deposition within a DFS.

Of note are earthworm castings that occur near the top of the Neogene part of the exposure. The castings are positioned along crossbedding surfaces within the amalgamated channels. The casting piles are typically approximately 2 to 13 cm in width and up to 10 cm in height. Earthworm castings of this size are little described in the literature and represent a unique feature of this DFS sequence.