T147 Dynamics of Huge Floods

Tuesday, 12 October 2021
8:00 AM - 12:00 PM
Portland Ballroom 254 (Hybrid Room)

WAVE EROSION OF GEOMORPHIC SURFACES, DEPOSITION OF ERRATICS, MASTODON AND WOOLY MAMMOTH OCCURRENCE, AND DEPOSITION OF MALPASS STRATIGRAPHIC UNIT

Paper number: T147-7
Program: Technical Sessions
Time: 9:25 AM – 9:40 AM

Authors

Abstract

Geomorphic Surfaces have been extensively eroded by wave action of Lake Allison from a Missoula Flood. Side valley alluvium covers paleosols with a Bt horizon. In several counties the paleosol is eroded off so only reddish or brownish clay skins are found in fractures of saprolite or rock. Gullies into the surfaces have filled with very fine textured sediment. If the last Missoula Flood was the largest and occurred between 12.8 ka. to 15.0 ka. years BP, Lake Allison would have had a pool that was tens of miles wide. Such a lake would have allowed a fetch of hundreds of feet. Based on studies of other large lakes, the wave run up could reach 6-10 ft. Waves are continuous 24hrs./day when wind is blowing. Surface occurs over miles on the valley wall of the Willamette Basin. Alen noted the at a 400 ft elevation the Lake Allison pool has roughly 3,000 sq. mi. of water. That much water would take considerable time to erode the valley wall which is estimated to have taken weeks to months to years.

At Irish Bend in Lynn Co., Mastodon or Wolly Mamouth remains were observed by a fisherman falling out of the Willamette Silts. About 200 feet east a farmer collected Mastodon or Wolly Mamouth bones. at top of the Willamette Silts. The same person collected dozens of fist size or smaller erratics.. That person dragged several large erratics, with one up to 8.5 feet long, to an adjacent field. Water had to be deep enough, at roughly 275 ft. msl. elevation, for the ice to carry the large erratics to the southern valley, and temperature cold enough to prevent the ice from melting. A frozen Willamette River and flood plain water would have allowed Mastodon or Wolly Mamouth to travel far south in the Willamette Valley, walking on ice. This interpretation is consistent with research on Younger Drayas (about 12,900 to 11,600 yrs. B.P.), which ia a return to glacial condition. The Willamette River and flood waters were likely carrying a heavy suspended load under these conditions and deposited very fine sediment on flood plain that became the Malpass Stratigraphic unit.

Geological Society of America Abstracts with Programs. Vol. 53, No. 6

doi: 10.1130/abs/2021AM-363818

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