GEOLOGY OF THE COYOTE CANYON MAMMOTH SITE, BENTON COUNTY, WASHINGTON STATE
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.