GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 15-11
Presentation Time: 11:00 AM


SCOTT, Jennifer J., Earth and Environmental Sciences, Mount Royal University, 4825 Mount Royal Gate SW, Calgary, AB T3E 6K6, Canada, CHUPIK, Daniel T., School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, DEINO, Alan L., Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, WESTOVER, Karlyn S., Department of Earth and Environmental Systems, Indiana State University, Terre Haute, IN 47809, LUKENS, William E., Department of Geology and Environmental Science, James Madison University, Harrisonburg, VA 22807, KINGSTON, John D., Department of Anthropology, University of Michigan, 101 West Hall, 1085 S. University Ave, Ann Arbor, MI 48109-1107, STOCKHECKE, Mona, University of Minnesota Duluth, Large Lakes Observatory, Duluth, MN 55812; Swiss Federal Institute of Technology (ETH), Zurich, 8200, Switzerland, MINKARA, Karim E., Department of Geosciences, Georgia State University, 24 Peachtree Center Ave, Atlanta, GA 30302, DEOCAMPO, Daniel M., Department of Geosciences, Georgia State University, 24 Peachtree Center Ave NE, Atlanta, GA 30303 and COHEN, Andrew S., Department of Geosciences, University of Arizona, Tucson, AZ 85721

High-resolution sedimentology, deep-tier terrestrial trace fossils, and pedogenic features were used to develop a sequence stratigraphic framework for the 3.3 to 2.6 Ma lacustrine and fluvial–alluvial Chemeron Formation drill core (BTB13) and nearby outcrops from the Kenya Rift Valley. Sequence boundaries and transgressive surfaces delineated in the 227 m-long core divide the succession into discrete packages that represent stacked lake flooding and regressive cycles intercalated with thick alluvial intervals. X-ray fluorescence, diatom paleoecology, and authigenic minerals correspond well to the sedimentology-based environmental interpretations, and help to characterize the changing conditions in the lake basin. The periodicity of stratigraphic surfaces is revealed using absolute dates derived from the untuned Bayesian statistical age model developed for the core. Sequence boundaries and transgressive surfaces are aligned with peaks in insolation for June and March at 30⁰N and the equator, which appears to correspond to whether the lake system was a closed underfilled lake-type (March) or a balanced fill lake-type (June). The Chemeron lake system developed in response to evolution of the rift basin as well as orbital cyclicity at the scales of: (1) 23 ky precession from ~3.2 to ~2.6 Ma; (2) 100 ky and 400 ky high eccentricity at ~3.2 to ~2.9 Ma and ~2.7 to ~2.6 Ma; and (3) possibly 40 ky obliquity from ~3.1 to ~2.9 Ma. Evidence for early post-depositional tectonism at ~2.9 Ma is provided by the stratigraphic cross-cutting relationship of a brecciated and faulted lacustrine horizon with injection features that is overlain by a long-term terrestrial interval. Correlation between the core and outcrops near the drill site confirms that an alluvial system prograded into the basin after this widespread event, roughly corresponding to a period of low eccentricity, before the saline, alkaline and then relatively freshwater lake system recovered through a series of precession-scale cycles modulated by 400 ky eccentricity. This multi-proxy and sequence stratigraphic approach, with a high-resolution age model, permits the distinction between the dominant tectonic and climate controls on this late Pliocene equatorial rift lake succession.