PLEISTOCENE HISTORY OF LAKE MILLICAN, CENTRAL OREGON

Millican Valley is presently a dry, high-desert perched basin in central Oregon, bounded to the north by lava flows and volcaniclastic ridges uplifted by the Brothers Fault Zone, a series of predominantly northwest-trending faults spanning ~300 km through central Oregon. The basin is bounded to the south and west by Quaternary lava flows and alluvial fans shed from Newberry Volcano, and to the east by a ~22 Ma lava dome. During the Quaternary, the basin has partly filled with basaltic-andesite flows as well as alluvial fans consisting of unconsolidated gravels and tuffaceous deposits from Newberry Volcano. Millican Valley is also host to laminated sediments from a brief lacustrine occupation, and extensive dune-fields made up of tephra from the ~6.9 ka eruption of Mt. Mazama. Both alluvial/fluvial and lacustrine systems were active throughout the Pleistocene and were coevally active in Millican Valley during at least one point in time, perhaps forming the northernmost body of water in the Pleistocene Great Basin pluvial system. Eventual linkage to the Columbia River basin via the Crooked and Deschutes Rivers was established in the late Pleistocene by the breaching of a shallow divide at the northern margin of the basin, leading to an outburst flood and nearly complete draining of Lake Millican through the present-day Dry River Canyon.

Pluvial Lake Millican boundaries were defined from a 1310 m elevation paleoshoreline locally identified by beveled ledges of equal elevation littered with gravel along the southeastern edge of the Smith Well lava flow in Millican Valley and from the elevation of a gravel spit near the western lake margin. At this elevation, the lake inundated an area of ~ 48 km2. The presence of freshwater diatoms in lake deposits in conjunction with a reworked tephra tentatively identified as the 27.3 ka Wono tephra (0.97 similarity coefficient) suggests that the lake was full and draining at that time but had not yet breached its outlet. Final linkage with the Colombia River basin, which persists to present time, therefore occurred in latest Pleistocene time.