HIGH RESOLUTION STRATIGRAPHIC ANALYSIS OF THE BASAL MONTEREY FORMATION AND UNDERLYING SILICEOUS UNITS ON SANTA CRUZ ISLAND

The Miocene Monterey Formation, the primary source rock of California’s petroleum resources, has often been interpreted to have been deposited in deep submarine basins between 17Ma and 5Ma. Alternatively, the Monterey Formation may also have been deposited on low-gradient slopes in broad marine depressions. The marine depositional environment was in part the result of the complicated interplay between global and eustatic sea level change and the preservation of facies below the Monterey Formation on Santa Cruz Island may illuminate the magnitude and pace of sea level change preceding deposition of the Monterey Formation. In addition, the sequence of facies below the Monterey Formation may lend more support to one of the depositional models mentioned above. The authors travelled to Santa Cruz Island to measure a high-resolution stratigraphic section of the basal Monterey Formation and the siliciclastic transition zone between the basal Monterey formation and the underlying volcanic unit. The basal Monterey Formation in the study area is composed of meter-scale beds of relatively unaltered diatomite, separated by cm-scale beds of a more siliceous facies. The basal Monterey Formation rests unconformably upon a 16.5 meter thick massive interval of graywacke sandstone, with sporadic stringers of larger pebbles and cobbles. This underlying siliciclastic unit rests, in turn, upon cm-scale beds of andesite. While additional field work remains to be done, we tentatively interpret the volcanic-siliciclastic transition to represent a shift from subaerial lava flows to near shore fluvial deposits. The erosional contact below the diatomaceous Monterey Formation is problematic, as the Monterey Formation obviously records a significant sea level rise. The transition between subaerial andesitic flows and the deep marine Monterey Formation points to a significant eustatic sea level change. However the erosional contact between the Monterey Formation and the underlying transitional siliciclastic unit indicates a period of uplift and erosion preceding deposition of the Monterey Formation. Additional field work and facies analyses will hopefully lend support to one of the two depositional models for the Monterey Formation.