A FOUR HUNDRED METER THICK HYPERPYCNITE DEPOSITED FROM THE GLACIAL LAKE MISSOULA OUTBURST MEGAFLOOD INTO THE UPPER PORTION OF GLACIAL LAKE COLUMBIA~ 16,000 CAL YEARS AGO

We have been mapping out the extent of a single glacial Lake Missoula outburst megaflood deposit exposed along the upper reaches of glacial Lake Columbia from Lake Pend Oreille to Lincoln on the Columbia River for over a decade. 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 unconformably overlies an older episode with multiple flood events deposited under the penultimate glacial Lake Columbia and incised by fluvial processes prior to the last incarnation of this glacial lake. This landscape was modified by scores of floods occurring over the 3,000 year interval between 16,000 and 13,000.

This megaflood deposit displays reverse-to-normal grading and wax–wane vertical stratification sequence typical of hyperpycnal flows and is here named the Bretz- Pardee hyperpycnite (BPH). Both the waxing and waning phases of the BPH display bedform changes from proximal to a more distal faces along the flow path in response to both the depth and flow velocity. The BPH is described from three fairly distinct depositional settings within the confines of glacial Lake Columbia: 1) the Rathdrum Prairie and Spokane Valley, 2) the central channel along the Spokane and Columbia River basin, and 3) the adjacent basalt prairies exemplified by the Spokane West Plains.

The scoured basin has a gravel fill 400 m thick. The gravels deposited during the waxing phase of the BPH are about 250 m thick, but only the upper 30 to 50 meters are exposed in a series of large open pit quarries from Post Falls to central Spokane Valley. The large bedforms are remarkably similar with steeply downflow foreset beds with cobble size clasts within a granule to angular pebble matrix, with a subhorizontal to porpoising boulder layer near the top of the exposure. The waxing phase is represented by the overlying 150 m thick planar bedded gravel which has subaqueous dune field as the uppermost bedform.

The BPH is 400 m thick in the central channel with the lower half (waxing) being a fairly uniform pebbly granule gravel with climbing ripples overlain by a planer bedded sand deposited during the waning phase. The West Plains BPH is 50 m thick, mostly deposited during the waning phase with a bedform succession very typical of a grading and coarsening upward sequence.