Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Holocene-Aged Lake Sediments From The Southeastern Margin Of Tulare Lake, CA

Abstract

Holocene-aged, high resolution lacustrine stratigraphy from the Tulare Lake basin in the San Joaquin Valley paired with oceanic surface temperature data form the components necessary to inform models that forecast regional precipitation in this important agricultural center. The results of earlier work, based on both trench exposures and core from the northwestern margin of Tulare Lake, are tested here using exposures from the SE margin of the lake. Quaternary lacustrine sediments from three new trench localities within the Tulare Lake Basin were described, sampled for quantitative geochemical and geophysical analyses, and dated using 14C methods on anodonta, gastropods, and bulk organic carbon found within the layers. The highest elevation locality exposed on the sides of a borrow pit near the Pixley National Wildlife Refuge has a more or less uniform grain-size distribution and fossil hash assemblage suggestive of a prolonged shoreline facies. Grain-size analysis yields a typically bimodal distribution for the individual samples and five peaks in both mean and coarse fraction grain size when all the sample sizes are plotted versus depth. The largest peak has a 460 μm mean grain size corresponding to a 500 μm coarse fraction mean grain size. Total organic carbon varies from 0-1.2% by weight and total inorganic carbon varies from 0-1.7%. Radiocarbon dates from these sediments range in age from −1ka to −6k suggesting that the lake reached a level of at least 64 m several times during the late Holocene. Poso Canal, a second locality 4.5 km SW of Alpaugh, CA contains distinct stratigraphic units varying from deepwater clay deposits to shallow water near shore sands similar to that found in the trench exposures in the previous work thus supporting these initial results. Eleven macro fossil samples and seven bulk sediment samples from the second trench locality are currently being processed at the Center for Accelerator Mass Spectrometry at the Lawrence Livermore National Lab. A third locality on the Atwell Island Restoration Project land operated by the Bureau of Land Management, exposes stratigraphy related to a large sand spit that was deposited by long shore drift of Kern River sediments. This exposure contained layers of well sorted fine sands intercalated with fine-grain lake muds that likely reflect changing discharge of the Kern River through time. The lithology consist of well-defined layering characterized by well sorted, clay rich lake silt, very fine grain sand, and fine to medium grain oxidized sand.