PALEONTOLOGY OF THE CONEJO VOLCANICS, LOWER TO MIDDLE MIOCENE, LOS ANGELES COUNTY, CALIFORNIA

Skeletal limestone occurs as interbeds and neptunian dikes within the sequence of submarine andesitic / basaltic flows and hyalobreccias of the Conejo Volcanics. They formed as patches on the surfaces of flows during intervals of eruptive quiescence and were buried by breccia deposition; the very irregular lower surfaces of the limestones reflect original relief on the underlying flow.

The Conejo Volcanics Formation is widespread in the Santa Monica Mountains. It is most extensive in the western part, where it is about 1800 m thick; it thins eastward to about 1100 m in the study area north of Malibu Canyon in the central part, pinches out about 4 km farther east, and occurs as isolated patches in the eastern Santa Monica Mountains. Limestone beds are most common in the upper 550 m of the sequence near the eastern edge of the major area of volcanic deposition, where they form lenses up to 2 m thick and 100 m long that either are isolated or occur along horizons traceable for as much as 1800 m. Their lateral extent depends on that of the underlying flow. Neptunian dikes as fissures and irregular pods extend as much as 8 m into the underlying flow rock. Volcanic clasts ranging from sand to cobbles are common, floating within the limestones. Terrigenous sediment is a component of only the uppermost limestone beds.

The skeletal lime grainstone and packstone is essentially autochthonous. In several locations, the epifaunal assemblage in the limestone grades into an infaunal assemblage in the basal 1 m of overlying volcanic sediment. The fauna consists of about 70 taxa. It is largely epifaunal, reflecting the hard volcanic substrate. Barnacles, oysters, pectinids (including Spondylus), regular echinoids, terebratulid brachiopods, Discinisca and serpulid worms are most abundant and characteristic; Nerita, Corbula, and common opercula of turbinid gastropods are noteworthy. The fauna is in marked contrast to the fauna of typical Neogene marine terrigenous strata in California, which is characterized by diverse infaunal bivalves, clypeasteroid echinoids, and a diverse and different assemblage of gastropods, including particularly turritellids and naticids. Assemblages in neptunian dikes differ from those in overlying limestones because of differences in skeletal size, habitat and preservation.