HYDRODYNAMIC FLOW REGIME BEDFORMS WITHIN THE IMMENSE MEGAFLOOD HYPERPYCNAL DEPOSITS FILLING THE UPPER REACHES OF GLACIAL LAKE COLUMBIA BETWEEN 17 AND 16 KA

We have been mapping out the extent of a single glacial Lake Missoula outburst megaflood deposit between 18 and 16 ka exposed along the upper reaches of glacial Lake Columbia from Lake Pend Oreille to Lincoln on the Columbia River. This immense flood lacustrine deposit filled all the available accommodation space in the central channel of glacial Lake Columbia to a depth of 400 meters. This megaflood deposit displays reverse-to-normal grading and wax–wane vertical stratification sequence typical of hyperpycnal flows and displays bedform changes from proximal to a more distal faces along the flow path in response to both the depth and flow velocity. There are no compelling features that would suggest more than a single flood event is responsible for this immense flood fill sequence.

Three fairly distinct depositional settings exist within the confines of glacial Lake Columbia: the Rathdrum Prairie and Spokane Valley; the central channel along the Spokane and Columbia River basin; and the adjacent basalt prairies exemplified by the Spokane West Plains. Hydrodynamic flow patterns are deduced by a variety of bedforms, exposed within the gravel pits located near Post Falls, Idaho and in the Spokane Valley. This proximal gravel fill reveals immense foreset bed separated by horizontal sets of climbing ripples and antidunes that are boulder-rich, reflecting penecontemporaneous erosion and deposition during hydrologic jumps commonly formed in the transition between flow regimes. Four stacked sequences, each about 100 meters in thickness, can be deduced from surface exposures, well logs and seismic data.

This 400-m-thick megaflood sequence is present along the central channel of glacial Lake Columbia where the more distal waxing lower half is a uniform pebbly granule gravel with down flow directed foreset bedding coupled with an overlying waning planer bedded sand. The West Plains megaflood sequence is 50-m-thick with a grading and coarsening upward bedform succession. Almost all of the cobble or boulder sized clasts are locally derived from scoured rip-up material, including spheroidal basalt and lithic saprolite derived from the regionally weathered surface of the Miocene plateau. This suggests that the sediment-rich density flow was restricted to central channel and overlying clear water high flow portion is responsible.