GREYWACKE OVERLYING THE SANTA CRUZ ISLAND VOLCANICS

The Santa Cruz Island Volcanics has been mapped as underlying the Monterey formation on much of eastern Santa Cruz Island, but recent studies conducted by Brelle, et. al. (2018) have documented a m-scale greywacke unit between the Santa Cruz Island Volcanics (SCIV) and the Monterey Formation. Recent field studies by the authors have documented the greywacke unit infilling Dm-scale erosional topography in the underlying SCIV. Bisgaard and Mrofka (2019) mapped additional areas where the greywacke unit underlies the Monterey Formation and mapped outcrops of an intraformational breccia unit at the base of the Monterey Formation. Brelle et. al. (2019) tentatively interpreted the SCIV-greywacke transition as a shift from subaerial lava flows to fluvial deposits, which Bisgaard and Mrofka (2019) concurred with. Both mapping projects were conducted in Scorpion Canyon; the authors feel mapping studies near Prisoners Harbor would further elucidate the processes responsible for the paleo-geomorphology of the volcanics underlying the Monterey Formation. The overlying Monterey Formation records a significant sea level rise, making the underlying erosional contact and greywacke unit important for determining the timing of uplift, clastic input and sea level change along the southwestern edge of North America during the deposition of the Monterey Formation in the southern California borderland basins. The transition from terrestrial lava flows (SCIV) to the deep marine Monterey Formation diatomites represents a significant sea level rise but the erosional contact below the newly mapped greywacke unit indicates a period of uplift and erosion prior to deposition of the Monterey Formation. The presence of Monterey Formation clasts at the base of the Monterey Formation indicates deposition locally began long after deposition of the Monterey Formation elsewhere and active uplift elsewhere provided a source for the clasts. Ongoing high-resolution geologic mapping of canyons on Santa Cruz Island may help answer several questions: 1) was the Monterey Formation deposited in deep marine basins or in broad marine depressions containing low grade slopes and 2) what was timing, tectonic history and depositional history between the SCIV and the Monterey Formation.