Volcaniclastic interbeds and their possible involvement in initiation of mass wasting in the Big Sink, Oregon

Shannon Othus

Whitman College, Walla Walla, WA 99362

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The Big Sink in the Umatilla National Forest is the largest subarieal landslide in the nation spanning an area close to 3 miles wide and who's complete length from scarp to toe has not been completely recorded. Within my research my plans have evolved to encompass the composition of the interbeds located between the Columbia River Basalt Flows. Thus far my research thus far has been to record the stratigraphy of the interbeds in two outcrop locations in an area located above the Big Sink. The stratigraphy has shown layers of volcaniclastic sediments deposited mostly by volcanic activity in the area but minor layers also show fluvial deposition. The various sediments that have been recorded are volcanilclastic sands with grain sizes of fine to coarse. These layers many times show sedimentary structures such as laminar bedding and cross bedding. Other layers are completely composed of massively bedded ash and reverse graded pumice.

Once I have completely recorded and described the interbed locations I plan to look at their role in the initiation of the mass wasting seen at the Big Sink. To do this I will enlist the help of another senior working in the Big Sink who is recording the dimensions of the landslide and will be trying to describe the movement. Once we have received this information the landslide can be described and the exact mechanics can be studied. It is clear that this movement had to have started with creep but other factors have contributed to its rapid movement. The other student research will also tell if there were other contributing factors to the mass-wasting event such as movement along a nearby fault or infiltration of water and thus a loading of weight that overcame the stability of the slope. Within this information I plan to show how the interbeds I am studying played an integral part in the formation of the Big Sink by creating greater slope instability.

With or without the previously stipulated information I plan to look at the Big Sink as an event that could have had dire consequences had it been developed and inhabited by humans in modern times. To do this I will classify the mass-wasting event to the best of my ability and relate it to other mass wasting events that have occurred in the Columbia River Basalts of the Pacific Northwest. By researching other similar mass wasting events I hope to show how mass wasting can have devastating effects on communities and can prove extremely expensive to state and federal government during cleanup periods. I also hope to show how losses due to mass wasting can hopefully be avoided by studying the geology of an area using techniques that can simulate the geomorphologic processes that are currently working the Pacific Northwest