Eastern Santa Cruz Island is primarily composed of two Miocene geologic units: the Santa Cruz Island Volcanics and the younger Monterey Formation. The Santa Cruz Island Volcanics consist of andesitic flows and volcaniclastics, and the Monterey Formation is characterized by rhythmically bedded silicious marine mudstones. These geologic units provide a detailed record of the past tectonic settings, deposition, and environmental conditions of Santa Cruz Island, and indicate that the island has experienced a complex geologic history. Formation of the volcanics coincides with a period of transtension along the Pacific-North America plate boundary, which occurred as the boundary transitioned from a subduction zone to a transform margin. In the time since formation, the Santa Cruz Island Volcanics have also recorded significant rotation on Santa Cruz Island. The rocks are currently oriented 76.5° ± 14.5° from the geomagnetic field present at the time of rock formation. The Santa Cruz Island Volcanics indicate Santa Cruz Island experienced a combination of transtension and rotation. The younger Monterey Formation continued recording the history of Santa Cruz Island. The Monterey Formation accumulated in deep, sediment starved basins along the North America-Pacific plate boundary. The influx in the biogenic material that formed the Monterey Formation indicates a change in oceanic circulation and suggests increased upwelling occurred along the Pacific Ocean margin. Formation of the Monterey may have also led to global climate change by preferentially removing δ12C from ocean circulation, which caused an increase in δ13C, more rapid drawing down of CO2 from the atmosphere, and a cooler global climate.
Chapter 3: Miocene Geology of Eastern Santa Cruz Island, California