LITHOLOGY AND PROVENANCE OF ERRATIC PEBBLES FOUND AT THE COYOTE CANYON MAMMOTH SITE, KENNEWICK, WASHINGTON

A cluster of forty ice-rafted erratics were found adjacent to skeletal remains of a Columbian Mammoth at the Coyote Canyon Mammoth Site, Kennewick, Washington. Both the mammoth remains and erratics are entrained in late Pleistocene outburst flood deposits, presumably from Glacial Lake Missoula. The cluster of pebble-size erratic clasts are interpreted as having been deposited by a small, solitary iceberg that may have come from the breakup of an ice dam formed by the Purcell Lobe of the Cordilleran Ice Sheet. Thus, the set of lithologies in this cluster of erratics were expected to reflect the bedrock in contact with the ice dam located at present-day Lake Pend Oreille, Idaho.

Visual examination of the pebble-sized erratics found similar lithologies to those identified by Bjornstad (2014) and Karlson (2006), who had focused on boulder-size erratics. Bjornstad and Karlson both found that 75-80% of the erratics they studied were granitic, while we found that only about 31% of the erratics in our study were granitic (granitoids or altered granitoid). All three studies yielded very similar percentages of quartzite (7-10%). However, neither Bjornstad or Karlson identified any calcareous (e.g. limestone) erratics, while we found that the majority (54%) of the erratics in our study were limestone, interbedded limestone, or marble. To our knowledge, this study provides only the second documented occurrence of limestone (or other calcareous rocks) among erratics studied in relation to these outburst floods. Additionally, we found that eleven of the forty samples (consisting of four different lithologies) could be associated with limestone, plutonic, and Belt Supergroup metasedimentary rocks surrounding Lake Pend Oreille, Idaho (site of the ice dams that created Glacial Lake Missoula).

Bjornstad, B. N. 2014. Ice-Rafted Erratics and Bergmounds from Pleistocene Outburst Floods, Rattlesnake Mountain, Washington, USA. E&G Quaternary Science Journal, v. 63: Number 1: 2014: p. 44-59. DOI 10.3285/eg.63.1.03.

Karlson, R. C. 2006. Investigation of Ice Age Flood Geomorphology and Stratigraphy in Ginkgo Petrified Forest State Park, Washington: Implications for Park Interpretation: Thesis. Central Washington University, Ellensburg, WA.