SEDIMENTOLOGICAL ANALYSIS OF THE WEST CRATER LAKE: INSIGHTS INTO PROCESSES OF LAVA DAM FORMATION, LAKE SEDIMENTATION, AND RIVER INCISION

The Owyhee River was dammed by multiple basaltic lava flows during the Quaternary. The youngest lava dam to affect the river was the West Crater lava dam (~70 ka) which impounded a lake that reached ~40 km upstream and filled much of the middle Owyhee River Canyon. By compiling the lake sediment record of the West Crater lake, tephra dates, cosmogenic surface dates, and geomorphic relations, the chronology of the lava dam and associated lake can be constructed. The emplacement of the West Crater lava dam and formation of the West Crater lake basin occurred at ~70 ka. Deltaic and lacustrine sedimentation were likely immediate responses by the river to damming. Some preserved lacustrine sections approach the crest elevation of the lava dam (1030 m). The main 22-m-thick section of lacustrine sediments fines upward from basal ripple-laminated sands to massive clays. Two tephra layers identified as Mt. St. Helens Cw (50 +/- 5 ka) and Cy (46 +/- 5 ka), are located at ~8 m and ~14 m above the base of the section. Extrapolating the sedimentation rate calculated for the interval between the two tephra layers (~2 mm/yr) to the upper ~8 m of section above tephra layer Cy suggests that sedimentation was occurring until at least ~42 ka. Rounded basalt boulders in shallow channels eroded into the top of the dam yielded a mean cosmogenic age of 62 ka, indicating overflow of the lake by at least this time. Rounded gravels capping the lacustrine sequences and deposited in overflow channels on the dam suggest that the West Crater lake filled with sediment prior to dam incision. Erosion of the lava dam and the adjacent canyon wall was likely episodic, as shown by the dated terrace surfaces cut into the lava dam and lake sediments. Flood boulder bars 5 km upstream of the crest of the West Crater lava dam provide a minimum age of 25 ka for the reestablishment of a through-flowing Owyhee River near the elevation of the modern channel. Incision through the downstream portions of the dam may have continued until after 11 ka. Overall, the West Crater lava dam and lake it created persisted for ~25-30 kyr before substantial erosion of the dam began. Dam removal and reestablishment of a new river channel required an additional ~20-30 kyr.