13 Catastrophic Glacial Outburst Floods in the Pacific Northwest

Monday, 13 May 2002
1:30 PM - 5:30 PM

CHARACTERIZATION AND PROPOSED FORMALIZATION (HANFORD FORMATION) FOR ICE-AGE FLOOD DEPOSITS WITHIN THE PACIFIC NORTHWEST

Paper number: 13-5
Program: THEME SESSIONS
Time: 2:20 PM – 2:35 PM

Authors

Abstract

Cataclysmic floods, associated with the periodic breakup of the Cordilleran Ice Sheet during the Pleistocene, are well known for carving out the Channeled Scabland. Floods deposited detritus in the scablands as well as basins of the Columbia River and tributary valleys downstream. In the Pasco Basin, behind a hydraulic constriction at Wallula Gap, up to 100 m of flood deposits incrementally accumulated beneath the Department of Energy’s Hanford Site. Regionally, all deposits from ice-age flooding can be subdivided into 11 lithofacies, based on textural-structural characteristics. Coarse-grained lithofacies lie proximal to high-energy flood tracts whereas the finest-grained facies lie in backflooded or other slack-water environments.

Cataclysmic flood deposits in the Pasco Basin have been referred to, informally, as the Hanford formation for over 30 years. We propose the expansion and formalization of the Hanford formation to include all cataclysmic ice-age flood deposits in the Pacific Northwest. We propose the name “Hanford Formation”, since at the Hanford Site: 1) 10 out of 11 flood lithofacies are represented, and 2) an extensive data base already exists that includes thousands of borings from over 50 years of geologic investigation. Furthermore, flood deposits are thickest at the Hanford Site with one of the longest records of flooding. Magnetostratigraphic evidence from borings drilled into a giant flood bar indicate flood deposits date back to the early Pleistocene (>780 ka). Regionally, deposits of the Hanford formation can be subdivided into four facies associations. Two types of fine-grained, slack-water flood deposits are recognized, consisting of the Sanpoil (sand-silt rhythmites separated by lacustrine fines) and Touchet (sand-silt rhythmites only) facies associations. Higher energy flood deposits are composed of the Malden (sand-dominated) and Pasco (gravel-dominated) facies associations.