TAPHONOMY AND TIME RESOLUTION OF BONE ASSEMBLAGES IN A CONTEMPORARY FLUVIAL SYSTEM - THE EAST FORK RIVER, WYOMING

Experiments that recorded the dispersal of 142 bones within a meanderi ng, 2030 m-long reach of the East Fork River, Wyoming over a 13-year p eriod provide a basis for interpreting distribution patterns and time averaging in. fossiliferous channel deposits. Results show that light and porous bones, (e.g., vertebrae, patellae, and phalanges) were tran sported farther than heavy bones (e.g., limb bones and mandibles). Dis persal patterns of bones from individual experimental sets representin g point sources demonstrate that bones became sorted by size and shape within 1 to 2 years and that sorting patterns varied according to ini tial channel position. The combined distribution of bones from all the experimental sets, however was unsorted and generally random, suggest ing that unsorted fluvial bone assemblages reflect multiple bone sourc es and differences in the time at which bones enter the channel. Estim ates of time-averaging of potential and observed natural bone assembla ges in. the East Fork River and the South Platte River range from 10(1 )-10(4) years. The upper Limit for this estimate is controlled by both the age of fossiliferous floodplain deposits that border the rivers a nd by the ability of the rivers to rework these floodplain. deposits. The lower limit reflects either the scarcity of bones in the floodplai n sediments or the inability of the rivers to rework these older bones ; in this case channel bone assemblages should represent only remains from deaths in the channel or remains that were transported into the c hannel from adjoining land surfaces, resulting in. short intervals of time-averaging (10(1)-10(2) years). The East Fork study further sugges ts that sandstone geometry, paleosol development, and the sedimentary context of fossil occurrences cart be used to evaluate time-averaging in ancient fossiliferous channel deposits. Fossil bone assemblages tha t are present exclusively in ribbon-shaped channel deposits associated with weakly developed paleosols and unfossiliferous floodplain deposi ts should represent shorter time intervals than similar fossil assembl ages associated with Sheet sandstones and moderately developed paleoso ls with abundant fossils.