CYCLOSTRATIGRAPHIC ANALYSIS OF A RHYTHMICALLY BEDDED LACUSTRINE DEPOSIT (UPPER HARGAS NUUR FORMATION), KHYARGAS NUUR, GREAT LAKES DEPRESSION OF WESTERN MONGOLIA

The endorheic Great Lakes depression of western Mongolia hosts a series of lacustrine deposits and shorelines representing the growth and shrinkage of lakes in the Pliocene and Quaternary. These lakes reflect climate-driven changes in source hydrology coupled to the topographic growth of the Altai Mountains. We use a 10 m thick section of Pliocene lacustrine mudstone and marl of the Hargas Nuur Formation to determine lake longevity, depositional facies, and water depth changes. These geological characteristics contribute to a broader effort dedicated to answering the question of when the Altai mountains rose to a critical elevation and width capable of intercepting westerly moisture. We apply the well-established proxy of mass-corrected magnetic susceptibility (MS) sampled at regular 11.5 cm intervals of a measured lithostratigraphic section (49.35575º N, 93.11625º E) to document climate-encoded changes in sedimentology. For rhythmically bedded lacustrine deposits exposed at this location, we take the MS data as a proxy for lake depth to track periodic or quasi-periodic lake expansion and contraction. Time series of the MS data are analyzed to generate a power spectrum and the related product-moment correlation coefficient between that spectra and a target spectra defined by Milankovitch periodicities (Laskar04) across a range of test sedimentation rates using Acycle software. We find that the rhythmic bedding tracks with orbitally-driven Milankovitch cycles indicating sediment accumulation rates (SAR) of ~10 – 40 m/Ma and lake longevity of ~ 1 Ma. We speculate that higher SAR values are associated with littoral depositional settings, and lower SAR with deeper lake depositional settings, due to the proximity to sediment sources. Overall, the lithostratigraphy indicates a deepening lake basin over this time, consistent with the growth of the Altai, subsidence of the Altai foreland (Great Lakes basin) and greater interception and storage of westerly moisture in the form of snow and glacial ice in the surrounding highlands. These data are further supported by isotopic and sedimentologic characterization of the measured section. Collectively, they are being used to test the idea that the Altai Mountains became an increasingly important control on meridional circulation in the late Pliocene.