HOLOCENE WEATHERING/EROSION RATES FOR MACROSKELETAL FOSSILS: OBSERVATIONS FROM THE KAIPAROWITS FORMATION (LATE CRETACEOUS), GRAND STAIRCASE-ESCALANTE NATIONAL MONUMENT, UTAH

The residence time of fossil bone on Holocence erosional surfaces has long been used as an argument both for and against the urgency of collecting specimens. Decision makers within federal, state, and local land management agencies may see fossils as immutable parts of an unchanging geologic backdrop as they struggle with the demands of managing social and biological issues. Others argue that fossils are highly ephemeral, even by human standards, and that more effort should be made to document and collect such irreplaceable heritage resources. To date, much of the evidence used on both sides of this argument has been anecdotal. Using lichen growth rates, we have placed constraints on the surface residence time of a variety of bone sizes and morphologies in an array of matrices, soil types, and microtopography as found in Late Cretaceous (Late Campanian) age outcrops of Kaiparowits Formation within Grand Staircase-Escalante National Monument. Preliminary results confirm anecdotal observations that bone preserved in mudstone may last only a few years, as these surfaces are unstable and erode rapidly, exposing fossils to destructive freeze-thaw. Bones eroding from these substrates are invariably lichen free. However, lichen colonies on robust ornithischian long bones in the latter stages of eroding from well indurated medium sandstones in flat terrain suggest that the bones have been exposed for less then 75 years.