CHARACTERIZING SEDIMENTARY PROCESSES OF A PLAYA-LUNETTE SYSTEM USING END-MEMBER MIXING ANALYSIS (EMMA): A CASE STUDY ALONG THE WESTERN MARGIN OF THE HARNEY BASIN, SOUTHEASTERN OREGON (USA)

Playa-lunette systems collect and mix sediments from multiple geomorphic processes (aeolian, fluvial, lacustrine), providing multimodal grain-size distributions (GSD) that reflect paleoenvironmental variations. End-member mixing analysis (EMMA) is used to mathematically unmix the measured GSDs of 256 sediment samples from Rimrock Lake (late Pleistocene/Holocene playa-lunette system in the northern Great Basin, southeastern Oregon) to characterize the timing, magnitude, and relationships of sedimentary processes shaping its evolution.

EMMA identified six sedimentary end-members (EM) representing 97.76% of the GSD variance, identifying three significant depositional environments: fluvial (EM 1), lacustro-aeolian (EMs 2 and 6), and littoral (EM 5). Additionaly, a relict playa surface was exposed beneath lunette sediments and radiocarbon dated to 6,190 - 5,990 cal. BP., representing a shift to a drier climate causing desiccation of the lake, deflation of the playa floor, and subsequent formation of the lunette dune. The lunette sediments are characterized by the coarsest end-members, EMs 1 and 5, and the finest end-member, EM 6, representing multiple depositional processes: silt and clay aggregates deflated from the playa floor, washed/inblown material along the lake margin, sediment recycling, and alteration by lake-level rise and fall.

Performing EMMA on the playa-lunette GSDs allowed for an unbiased confirmation of the general characterization of paleoenvironmental changes reflected in the playa-lunette system’s sediments. Their spatial-temporal association indicate Rimrock Lake existed by c. 18,535 cal. BP., with lake expansion and lake-margin transgression through the Early Holocene, followed by desiccation through the Late Holocene.