GEOMORPHIC PROCESSES PROMOTING MAMMOTH BURIAL AND SKELETAL PRESERVATION IN NORTH CENTRAL NEW MEXICO

Near Abiquiu in northern New Mexico, mammoth skeletal remains were recently discovered in the near-surface deposits of a very small alluvial channel. The channel occupied a depression on the back-tilted top of a Toreva slump block, a highly unusual setting for a mammoth burial. Numerous similar Toreva block-type slumps line the canyon wall, where more competent Triassic Poleo Sandstone overlies weak, clay-rich Salitral Formation mudstone. Slumping was likely promoted by wetter Late Pleistocene climate and snowmelt on the northwest aspect. On the mammoth site slump bench, discontinuous bouldery footslope colluvial deposits show clay films and stage I+ to II carbonate, indicating greater soil development than other local surficial deposits, and were likely most active shortly following slumping due to failure of oversteepened slump scarps. Colluvium also accumulated on the scarp slopes, which we infer to have occurred mostly in the Late Pleistocene due to greater physical weathering, denser vegetation adding stability, and active aeolian transport adding fine sand-silt to the colluvial matrix. The mammoth lies within a debris-flow deposit in the slump depression channel, which is up to ~4 m wide and 1.2 m deep. Ped-face carbonate coatings and soil structure in the debris-flow deposit indicate a greater age than adjacent channel deposits. Runoff from the mesa top likely entrained sediment by incision of gullies in hillslope and footslope colluvium, bulking to a debris flow. The debris flow deposit created a high point in the channel, so that subsequent flow was diverted off the downslope edge of the slump block, protecting the mammoth from later erosion. Following mammoth burial and continuing to the present, deposition of finer footslope colluvium continued in an incremental manner, and alluvial channel activity is evident in relatively well-sorted, stratified alluvial deposits, with local shallow incision of active channels. In the Holocene, reduced vegetation cover, intense monsoonal summer convective storms, and depletion of hillslope sediment storage in gullies may have promoted a change to more dilute flows in channels. Understanding the geomorphic setting and processes at this site may prove useful in locating other atypical preservation sites for Pleistocene megafaunal remains in the Southwest.