T241 The Geological Signature of Natural Hazard Events II

Tuesday, 24 October 2017
1:30 PM - 5:30 PM
Chelan 5

THE WEST SALT CREEK ROCK AVALANCHE, SAG POND, AND OUTBURST FLOOD: MORPHOLOGICAL AND SEDIMENTOLOGICAL SIGNATURES FROM A SERIES OF EXTRAORDINARY EVENTS

Paper number: T241-11
Program: Technical Sessions
Time: 4:00 PM – 4:15 PM

Authors

Abstract

From May 25, 2014 to May 27, 2016, the West Salt Creek valley in western Colorado underwent a rapid morphological and sedimentological transformation from the emplacement of a fast-moving rock avalanche and the subsequent formation, and partial drainage of a 500,000 m3 sag pond. A May 25, 2014 rock avalanche filled about 5 km of the West Salt Creek valley with hummocky, shale-rich debris up to 40-m thick that blocked West Salt Creek at the head of the valley. Immediately after the rock avalanche, shale in the rock-avalanche deposit began to rapidly slake, giving the deposit a slow-moving, earth-flow like appearance. Ground-water and surface-water flow created a series of about 80 ponds on the surface of rock-avalanche deposits along the path of the former West Salt Creek. The blocked creek at the head of the valley formed a sag pond that existed until May 27, 2016, when runoff from spring snowmelt, and a landslide from the rock-avalanche headscarp, caused the pond to overtop and rapidly release about 150,000 m3 of water. The outburst flood filled the ponds on the deposit, or disconnected them from their water sources, and left a complex record of flow deposits for 13 km down the West Salt Creek and Salt Creek valleys. The outburst flood lasted about 33.5 hours and had a measured peak discharge of about 17 m3/second. Field mapping after the outburst flood revealed repeating sequences of debris flow, hyperconcentrated-flow, and water-dominated (flood) deposits. Debris flows were first in the flow sequence and were generated by extensive erosion and entrainment of rock-avalanche debris. Vertical incision in rock-avalanche deposits was as great as 32 m. When incision ceased, hyperconcentrated flows reworked earlier debris-flow deposits, and later, water–dominated flows reworked the earlier hyperconcentrated-flow deposits. This sequence occurred near the head, in the main body, and at the toe of the rock-avalanche deposit. Our work indicates that caution should be exercised when interpreting valley-fill deposits because they can be emplaced and modified in short periods of time by complex events that could easily be misinterpreted. Field-based observations and mapping are needed to understand the timing and evolution of complex valley-fill mechanisms.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-306710

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