RECORD OF TERRIGENOUS INPUT AT AN ACTIVE PLATE-MARGIN: STRATIGRAPHIC ANALYSIS OF IODP EXPEDITION 385 SITES U1552 AND U1549, GUAYMAS BASIN, MEXICO

Expedition 385 of the International Ocean Discovery Program (IODP) explored the Guaymas Basin, Central Gulf of California, Mexico by drilling at eight sites, where sediment ages range from the Holocene or the late to middle Pleistocene and sedimentation rates range up to 1km/my. High recovery rates allow for almost continuous stratigraphic representation and potential correlation. We focus on Sites U1549 and U1552, located west of the northern axial graben (9 and 20 km respectively) and near the base of the Guaymas Transform fault slope/escarpment (~9 and ~6 km respectively). Both were drilled in water depths of ~1840m, ~11 km apart, near active methane-rich cold seep communities above deeper sills. Site U1549A cored to a depth of 168 meters below the seafloor (mbsf) with 99% recovery, whereas U1552A reached 107 mbsf with lower recovery (69%) related to the presence of gas hydrates. The average lithology at Site U1552 is more clay-rich consistent with its proximity to the submarine channel extending across the Sonora slope. However, Site U1549 (more proximal to the Yaqui River) is primarily biosiliceous (diatom) mud or ooze, with thicker and more frequent event beds. Both sites have minor but significant clastic terrigenous content (~10%) concentrated in laminae and beds, including depositional units (event beds), of various styles and thicknesses. The recurrence intervals of silt/sand intervals at both sites are on average 1/meter. Their downhole distributions are not uniform however, reaching maxima at similar depths (e.g., ~60mbsf) at both sites. These beds were classified into eight types, from thin distinct sand/silt laminae/beds to very thick event beds (~1 m to >6 m). The latter exhibit scoured, or abrupt basal contacts overlain by thin sandy layers, in turn overlain by laminated to homogenous muddy intervals, locally capped by diatom ooze. Silt to sandy beds could be products of gravity flows or other current processes and thicker “event” beds may be the result of seismic or storm events. There is likely a mix of processes acting in the region. Focusing on sandy to silty event beds may provide insights into the history of basin seismicity and/or storminess and help define key beds for regional correlation. Petrographic data are key to fingerprinting terrigenous source areas. Bioclastic debris such as benthic foraminifera may help discriminate shelf vs. slope sources.