SEDIMENTOLOGICAL CHARACTERIZATION OF MULTIPLE SEISMICALLY-DEFINED MASS-TRANSPORT DEPOSITS (MTDS) CORED AT SITE U1551 IN THE GUAYMAS BASIN DURING IODP EXPEDITION 385

The Guaymas Basin, central Gulf of California, is a young pull-apart basin characterized by a mix of biogenic and terrigenous sediments including gravity-mass flow deposits as documented by International Ocean Discovery Program Expedition 385. Mass-transport deposits (MTDs) have been identified across the region based on seismic reflection interpretation. Seismic stratigraphic interpretations also indicate that part of the MTD complex is hosted in the section cored at Site U1551, where thick sandy layers were recovered. We evaluate and confirm the presence of MTDs at Site U551 by augmenting shipboard core descriptions of sedimentary structures and lithology, with X-radiograph images, physical property measurements (XRF and magnetic susceptibility), and sand petrology (e.g., 44 thin sections).Our preliminary analysis shows an older potential MTD (~25 m thick), and a younger potential MTD (~14 m thick). The lower MTD is characterized by homogeneous sand overlying an interval of various thinilty clay intervals with bioturbation. The MTD sand is fine to medium-grained and quartzo-feldspathic with a variety of lithic fragment types. In contrast, the upper MTD interval is characterized by sandy silt and clayey silt intervals with structureless, thin, fine-grained sand layers that are quartzo-feldspathic with minor volcanic lithics. The intra-MTD interval (~9 m thick) is characterized by silty sand and diatom-bearing mud with thin, fine-grained and bioturbated sand layers. Sand composition is consistent with seismic thickness patterns (time maps) which suggest the MTDs originated to the east, near the Yaqui River delta along the Sonora margin of Mexico.