Reconstructing Late Pleistocene And Holocene Paleoenvironments Using Playa-Lunette System Sediments Within The Harney Basin Of Southeastern Oregon, USA

Paleoenvironmental investigations of the Late Pleistocene and Early Holocene in the Harney Basin of eastern Oregon have been limited to date. This Dissertation investigates and links the stratigraphy of Rimrock Draw Rockshelter (35HA3855) (RDR) and two surrounding playas, Rimrock Lake and Hay Lake, located on the western margin of the Harney Basin, in order to identify paleoenvironmental shifts during the last ~20 kyr. An emphasis also is given to demonstrating the potential for playas in the Harney Basin to record changes in paleoclimate, as well as the application of multivariate statistics to interpret paleoenvironments from sedimentary grain size data. Stratigraphy was exposed at RDR and Rimrock and Hay Lakes through back-hoe trenching, coring, and archaeological excavations between 2013 and 2015. A total of 498 sediment samples were collected across the study area from 10 sedimentary profiles representing three different environments: fluvial, eolian, and lacustrine. The measured inorganic grain-size distributions (GSD) of the sediment samples were measured using a Malvern Mastersizer 2000 high-dimensional laser diffraction grain-size analyzer and mathematically unmixed using and end-member mixing algorithm (EMMA) in order to characterize the spatial and temporal paleoenvironmental variations across the study area. EMMA was capable of distinguishing GSDs between littoral, fluvial, and deltaic environments, provided for a more geologically meaningful interpretation of stratigraphy, and characterized the magnitude and relationships of environmental change across the study area. End-member distributions across the study area in conjunction with radiocarbon dating identified several key paleoclimate shifts. Results indicate that Rimrock Lake and Hay Lake playas existed simultaneously by c. 19,000 cal. BP. as springs or marshy areas. Significant fluvial input was identified across the study area between c. 16,000 and 8,000 cal. BP, correlating with lake expansion and lake-margin transgression. A relict playa surface exposed beneath marginal sediments was dated to 6,190 - 5,990 cal. BP. and represents an abrupt and significant lake contraction in response to a shift to a drier climate during the mid-Holocene. Within the relict channel, a shift from a fluvial environment to a marsh environment is identified at c. 10,000 cal. BP., and is followed by a shift to a drier environment with increased eolian activity at c. 8,000 cal BP. A thick layer of Mt. Mazama tephra (c. 7,800 cal. BP) also is found preserved within the relict channel and suggests that the relict channel was consistently dry after deposition. Overall, this Dissertation contributes to a better understanding of paleoenvironmental change during the Late Pleistocene and Holocene within the Harney Basin in the context and vicinity of an important archaeological site. The results presented in this Dissertation also demonstrate the ability of playas in the Harney Basin to record paleoenvironmental changes, and illustrate the utility of end-member mixing analysis to characterize paleoenvironmental these changes through analysis of sediment grain-size distributions