STRATIGRAPHIC RELATIONSHIP AND HISTORY OF TWO LATE PLEISTOCENE AND HOLOCENE PLAYAS IN THE HARNEY BASIN, EASTERN OREGON, USA

Stratigraphic and geomorphic data were used to compare the Quaternary environmental records of Rimrock and Hay Lake playas in the Harney Basin of eastern Oregon, USA. End-member mixing analysis (EMMA) was conducted on grain-size distributions of 455 sediment samples from seven cores. GIS and remote sensing was used to define playa environments in relation to USGS fault data. Eight radiocarbon dates were used for temporal correlation. EMMA identified five end-members (EM) representing two major environments of deposition: a lacustro-eolian and a fluvio-lacustrine. Within Rimrock Lake, the lacustro-eolian deposits are within the upper parts of the cores, while the fluvio-lacustrine deposits are found in the lower portion, with a distinctive and abrupt boundary between them prior to a radiocarbon date of ca. 6500 cal. BP. Within Hay Lake, though, the depositional environments are more interbedded and the fluvio-lacustrine deposits are found near the top of the cores within a narrow interval, bounded above by a date of ca. 8400 cal. BP., while the lacustro-eolian deposits occur throughout. Radiocarbon dates suggest both playas were established during the Late Pleistocene, ca. 19500 cal. BP, and experienced fluvial activity during the Early and Middle Holocene, between ca. 9000 and 8000 cal. BP. Rimrock Lake continued to experience a relatively deep lacustrine environment well into the Late Holocene (after ca. 6000 cal years BP.), with decreased fluvial deposition and increased eolian deposition, even during times of increased aridity evidenced elsewhere. Hay Lake, however, has very little lacustrine sedimentation above the 8400 cal. BP. date, suggesting a loss of sediments due to bypass and deflation. While EM distributions and temporal correlations suggest several possible scenarios of the relationship between the two lakes, tectonics probably played a major role in diverting the ephemeral channel(s) toward Rimrock Lake and increasing its accommodation space due to down-faulting, with GIS results identifying a positive correlation between the playas and fault orientation.