A CLASSIFICATION SCHEME FOR MAFIC VOLCANIC PARTICLES: AN ANALYSIS OF VOLCANICLASTIC INTERVALS IN THE QUATERNARY MARINE SEDIMENTARY SUCCESSION RECOVERED DURING IODP EXPEDITION 385 IN GUAYMAS BASIN, GULF OF CALIFORNIA

Guaymas Basin, located in the central Gulf of California, is an obliquely-rifted continental margin characterized by high sedimentation rates. Seafloor spreading is manifested by robust sill intrusion into the basin’s sedimentary fill with graben development but no magmatic spreading centers. IODP Expedition 385 recovered a remarkable record of Quaternary biogenic and terrigenous sedimentation in Guaymas Basin with sparse, but notable, mafic volcaniclastic intervals. Current subaerial mafic volcanism in the basin is limited to Isla Tortuga, an off-ridge volcano composed of primitive tholeiitic lavas and tuffs. Local submarine volcanism is implied by the presence of cratered and conical features in seabed bathymetry. Mafic volcaniclastic layers/laminae were cored at six sites (U1545-UI550) drilled on the submarine flanks of Isla Tortuga and across the basin floor extending into the eastern graben. We sampled 35 mm- to cm-thick layers dominated by silt- to sand-sized particles. Smear slides of fines show diverse volcanic clast morphologies suggesting potential epiclastic, pyroclastic, and hydroclastic processes. To facilitate volcaniclastic petrofacies identification in smear slide and thin section, we developed a classification scheme for the mafic volcanic particles based on images (e.g., photomicrographs, SEM) from published studies of mafic epiclastic, pyroclastic, and hydroclastic deposits. The scheme includes glassy lava fragments, such as Pele's hair, Pele’s tears, limu o Pele, angular dense fragments (scoria/cinder), and other fluidal quenched glass. Other particle attributes include: 1) degree of vesiculation, 2) degree of crystallinity (vitric, microlitic, lathwork), 3) glass color (tan/brown sideromelane, black tachylite, orange palagonite), and 4) rounding. These clast types and attributes can be related to different modes of clast creation and modification during eruption, erosion, and transport in subaerial, coastal, and/or submarine settings. The types and proportions of volcanic particles in laminae/beds in the cores will facilitate potential inter-site correlation and provide insight into the relative timing and distribution of mafic volcanic eruptions in the basin. This proposed scheme can be applied to other similar basins in the Gulf of California and beyond.