LIMESTONE SEDIMENTATION CONCURRENT WITH SUBMARINE VOLCANISM IN THE CONEJO VOLCANICS, MIOCENE, SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA

Fossiliferous carbonate and terrigenous deposits are intercalated with submarine flows of the Conejo Volcanics in the central Santa Monica Mountains. The limestones are skeletal lime grainstone to wackestone and contain volcanic clasts ranging in size from silt to boulders. They occur on flow surfaces as lenses and as matrix within volcanic pebble to cobble conglomerate, and within flows as fillings in primary fissures and as neptunian dikes. Hard-substrate taxa, notably barnacles, oysters, serpulids, and regular echinoids are characteristic of the limestone, in contrast to the soft-substrate and infaunal taxa characteristic of Cenozoic strata of the Pacific Coast in general.

Clastic strata occur above or lateral to limestone lenses, or as separate beds. Their biota is distinctly different from that in the limestones with infaunal bivalves, inarticulate brachiopods, and vascular plant debris as characteristic components; the dominant limestone taxa are absent or much less abundant. Both types of sediment are mass-flow deposits.

Oxygen-isotopic analysis indicates that the organisms making up the limestones grew at a temperature of approximately 15º C. The biota is indicative of warm-temperate to sub-tropical climate at the southern end of the modern Californian molluscan province. The present equivalent location based on both isotopic and provincial results is off northwestern Baja California. The biota is referred to the barnamol association of temperate carbonates.

Limestone is very rare in the Cenozoic of the Pacific Coast because high relief along the convergent plate margin generated abundant terrigenous sediment that accumulated at a much higher rate than skeletal carbonate, particularly for temperate carbonates, which accumulate at an order of magnitude slower rate than do tropical carbonates. Limestones in the Conejo Volcanics form time horizons that define local sea-floor topography resulting from volcanic accumulation and concurrent sea-floor subsidence.