THE DYNAMIC GEOMORPHIC SETTING OF THE LATE PLEISTOCENE HARTLEY MAMMOTH SITE: BURIAL AND SKELETAL PRESERVATION IN A SLUMP-BLOCK DEPRESSION NEAR ABIQUIU, NEW MEXICO
Near Abiquiu in northern New Mexico, the skeletal remains of two mammoths were discovered in the summer of 2014 in the near-surface deposits of a very small alluvial channel. The channel occupies a depression on the backtilted top of a Toreva slump block, a highly unusual setting for a mammoth burial. Geomorphological investigation of the site has provided insight into processes leading to burial and preservation of the remains, as well as local environmental change. Field mapping of sediment sources and LiDAR scans of the contributing slope basin and slump bench provided a map of geomorphic features and surficial geologic deposits. To better understand the geomorphic context of the mammoth remains, termed the ‘Hartley Mammoth,’ six soil pits from the mapped surficial geologic deposits were described in the field. Bedrock and soil samples were analyzed using x-ray fluorescence (XRF), loss on ignition (LOI), x-ray diffraction (XRD) and particle size analysis. Bone collagen from a limb fragment returned a calibrated 14C age for one mammoth of about 33 ka. On the mammoth site slump bench, discontinuous bouldery footslope colluvial deposits show clay films and stage l to l+ carbonate, and were likely deposited shortly following slumping due to failure of oversteepened slump scarps. The deposit surrounding the mammoth remains consists of cobbles and small boulders of sandstone supported by a muddy matrix; this texture strongly suggests that the remains were buried by 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 remains from later erosion. Ped-face carbonate coatings (stage I+) in the debris-flow deposit indicate a greater age than the relatively well-sorted and stratified alluvial deposits in the channel above the debris flow. Following mammoth burial, incremental deposition of finer footslope colluvium continued to the present. Overall, field observations, XRF and XRD analyses indicate that despite the ~33 ka age of the mammoth, very little chemical weathering and limited soil development has occurred in the debris flow and other surficial deposits of apparent late Pleistocene age in this dynamic environment
