Characterizing formation processes of Rimrock Lake, a Late Pleistocene–Holocene playa, Harney Basin, south‐eastern Oregon, USA, using an end‐member mixing analysis

Abstract

Playas collect sediments from different transported processes, producing grain-size distributions (GSDs) reflecting changes in environment. End-member mixing analysis (EMMA) was used to derive end-members (EMs) from 256 sediment samples from Rimrock Lake (Harney Basin, Oregon, USA) and a nearby relict channel. EMMA identified six EMs representing deposition in fluvial, lacustrine, littoral and colluvial environments. Stratigraphy was time-correlated using nine radiocarbon dates. Results show that Rimrock Lake existed by $ 18 535 cal a BP. A relict playa surface also was exposed along its margin and indicates lake contraction $ 6190-5990 cal a BP. In the relict channel, a dominant eolian environment is identified at $ 8000 cal a BP. These results represent a shift to a more arid environment in the Harney Basin beginning around the mid-Holocene and are in general agreement with other studies reported in the Great Basin. This study demonstrates the application of EMMA for analyzing complex playa sediments and for linking environmental response across separate but related geomorphic environments. Our results provide a paleoenvironmental record along Harney Basin's western margin between $ 18 500 and 6000 cal a BP and demonstrate the use of small playas in the basin as environmental archives. Figure 2. (A) Modified Google Earth image of Rimrock Lake with geomorphic features, core locations and Trench 1 identified. (B) Modified Google Earth image of Rimrock Draw Rockshelter with excavation units, Trench 2 and channel boundary identified (dashed line).