USING THE LITHOSTRATIGRAPHIC RECORD PRESERVED WITHIN TWO PLAYA WETLANDS TO RECONSTRUCT PALEOENVIRONMENTAL CHANGE AND GEOMORPHIC RESPONSE, HARNEY BASIN, OREGON (USA)

End-member mixing analysis (EMMA) along with GIS analysis and radiocarbon dating were used to investigate the stratigraphic and geomorphic relationship between two playas, Rimrock Lake and Hay Lake. Both playas are < 3km apart and located along the western margin of the Harney Basin. Both playas have similar morphometry, with surface area, perimeter, width, length, and circularity differing < 4%. EMMA was conducted on the grain-size distributions of 455 sediment samples collected from seven cores from Rimrock Lake (n=4) and Hay Lake (n=3). A 5-EM model explained a combined 97.23% of the total grain-size variance and successfully identified two major environments of deposition.

EM 1 and EM 5 are strongly associated with each other and are interpreted as lacustro-aeolian deposition. EM 2 and EM 4 also are strongly associated with each other and are interpreted as fluvio-lacustrine deposition. EM 3 is interpreted as a wet marsh or backshore environment receiving colluvial, fluvial, and aeolian sediments. The normalized difference between the finest and coarsest EMs also was calculated as a visual proxy for comparison of the change in energy within the playas.

Results suggest both playas were established during the Late Pleistocene, ca. 19,500 – 18,500 cal yr BP, and experienced increased fluvial activity during the Early Holocene, between ca. 9000 and 8000 cal yr BP. A distinct and well-preserved portion of the relict Rimrock playa surface also was identified ~82 cm below the margin surface with a radiocarbon date of 6190 – 5990 cal yr BP just below the relict surface, suggesting increased aridity in the Harney Basin during the middle Holocene. Rimrock Lake playa experienced alternating lake levels well into the Late Holocene, after ca. 6000 cal yr BP, even during times of increased aridity, with regional tectonics possibly playing a significant role in diverting ephemeral channels towards Rimrock Lake and away from Hay Lake.