SEDIMENTARY GEOLOGY AND TAPHONOMY OF A PLEISTOCENE FOSSIL ASSEMBLAGE FROM A DEBRIS FLOW

Fossil assemblages within debris flows can be the result of a diverse set of sedimentary and taphonomic processes. A Pleistocene fossil locality in Centennial Valley, in southwestern Montana, provides an example where fossils that were likely already in reworked contexts were eroded from older deposits, and then subsequently entrained by the debris flow and transported down-slope. The debris flow (stratum D), consists of a fine-grained (silty, clayey sand) matrix intermixed with micritic limestone and quartz cobbles and fossils (bone, teeth, and tusks). Most of the faunal remains consist of materials from Mammuthus (mammoth), and there are also elements of Equus (horse), Bison (bison) and Homotherium serum (Scimitar cat). The debris flow accumulated on the surface of an arroyo-type channel that was incised into underlying sediments (stratum A). The main deposit of bones and cobbles measured about 150 X 100 X 50 cm and rested unconformably on an irregular, sloping erosional surface. The debris flow is an isolated sedimentary unit, locally restricted and apparently deposited after 19,000 B.P. A luminescence sample from well-sorted sands suggests the sediments directly underlying the debris flow are about 50-76,000 years old. There is also a radiocarbon value of about 49,350 years 14C B.P. from a Mammuthus tooth in stratum A. Based on radiocarbon ages ranging from >52,800 to 19,290 years 14C B.P., the fossil assemblage is temporally mixed. The fossils incorporated into the debris flow are disarticulated, in reworked position, and can exhibit surface damage in the form of striations. The debris flow contains fossils that may have initially been incorporated into older strata within this sedimentary sequence (strata A-C), and then redeposited as a result of mass-movement. This example provides information on the processes of formation and characteristics of fossil assemblages in debris flow depositional conditions.