Paper No. 16
Presentation Time: 8:00 AM-6:00 PM
GEOLOGY OF THE COYOTE CANYON MAMMOTH SITE, BENTON COUNTY, WASHINGTON STATE
GUETTINGER, Maxamilian, Pacfic Northwest National Lab, Richland, WA 99352, LAST, George V., Geosciences Group, Pacfic Northwest National Lab, P.O. Box 999, MS K6-81, Richland, WA 99352-0999 and BARTON, Bax R., Burke Museum of Natural History and Culture and Quaternary Research Center, University of Washington, Box 351360, Seattle, WA 98195-1360, Maxamilian.Guettinger@pnl.gov
The Coyote Canyon Mammoth Site in Benton County, Washington State provides a unique opportunity to unravel the detailed geologic context for a mammoth skeleton found in Pleistocene outburst flood deposits (i.e. the Missoula Floods). The mammoth site contains the partially articulated skeleton of a mammoth (Mammuthus sp.). The purpose of this study was to develop a large (i.e. hectometer to kilometer) scale conceptual model of the site’s geologic history to provide the context for small (i.e. centimeter) scale geologic, paleoecologic, and paleontologic data collected during excavation of the mammoth remains. The primary objectives were to map the nature and configuration of the basalt bedrock surface, evaluate the distribution and possible origins of a basalt ladened conglomerate and possible tephra deposit, and evaluate the history of Ice Age flooding (e.g. the number of ice age floods represented by rhythmites) at the site. This investigation consisted primarily of geologic mapping supplemented by well log data, and visual examination of collected samples.
The uppermost basalt flow beneath the site is believed to be that of the Elephant Mountain Member of the Saddle Mountains Basalt Formation, deposited about 10.5 million years ago. Its surface dips gently to the north along the northern limb of the Horse Heaven Hills anticline. Locally overlying the Elephant Mountain basalt lies a coarse angular basaltic conglomerate interpreted to be of alluvial fan origin (i.e. fanglomerate). This deposit is heavily calcified, suggesting a lengthy period of weathering and subareal exposure. A thin tephra deposit of unknown origin is locally present at the top of the fanglomerate. There is some evidence that the fanglomerate and tephra deposit have been reworked, possibly by Ice Age flood events that deposited a fairly thick (meters) sequence of rhythmically bedded fine-grained sediment. At least four different graded bed sequences overlie the mammoth bone-bed. These graded beds are believed to represent distinct Ice Age flood events. Where undisturbed, the ground surface is blanketed by a thin (~1 m) veneer of loess. Further investigation of the site is needed to better define the number and frequency of Ice Age floods large enough to inundate this site, and their relationship to the death and burial of the mammoth.
