UNIFYING LITHOLOGY, FACIES AND BEDDING CHARACTERISTICS OF THE DSDP LEG 64 AND IODP EXPEDITION 385 SITES, FOR A COMPREHENSIVE GUAYMAS BASIN DEPOSITIONAL MODEL: PRELIMINARY RESULTS

The Guaymas Basin, in central Gulf of California, is a marginal ocean basin characterized by active seafloor spreading and high sedimentation rates. Over the past 60 years, there have been many studies geared towards defining the marine, clastic sediment types and their distributions in the Guaymas Basin. Deep-Sea Drilling Project Leg 64 set out in 1978, designed to answer questions about the fundamental plate-tectonic processes of continental rifting and seafloor spreading, by drilling 5 sites across the basin. With renewed interest in Guaymas Basin tectonics and hydrothermal systems, International Ocean Discovery Program set out on Expedition 385 in 2019, recovering over 4km of middle Pleistocene to Holocene core at 8 new drill sites, complementing prior results from Leg 64.

For our study, shipboard sedimentologic descriptions of units are being used to create new, uniform, lithological columns at a ~decimeter (core) scale for correlation purposes, among all of the Expedition 385 and Leg 64 sites. With the accompanied use of slabbed core images, visual core description sheets, shipboard notes and a normalized classification scheme for all Expedition 385 and Leg 64 lithologies, the uniformly generated stratigraphic columns enable the definition of distinct sediment facies at the sites. An abundance of clastic facies intervals can be seen in certain basinal sites, with possible sources from the Yaqui and Baja California drainage systems. Slope sites near Isla Tortuga show an abundance of pelagic to hemipelagic diatom ooze and diatom clay facies. Other basinal sites from both Expedition 385 and Leg 64 are more complex and exhibit mixed facies; approximately 20-40% of these sites show an abundance of siliceous claystone and sandy intervals, locally altered by hydrothermal activity near mafic sill intrusions that characterize the basin. The variety of sediment packages may be a result of diverse origins of gravity-mass flows within the packages. Further analysis of the sandy gravity-mass flow beds in the basin will aid in deducing whether the facies and compositional patterns are indicative of their triggers, transport mechanisms, and source areas. Such lithological packages and event beds may be used as mappable units or traceable horizons across the basin and eventually help with seismic interpretations.