Depositional Paleoecology of Miocene Ostracodes in the Monterey Formation, Laguna Hills, Southern California, U. S. A.

Abstract

Outcrop sections of the lower Monterey Formation in Laguna Hills, southern California, con-tain an unusual early middle Miocene (Luisian Stage) subtropical marine fauna of macroinverte-brates and microfossils. The basal exposures consist of patch reefs constructed by serpulids, oys-ters, and bryozoans interbedded with sands, silts, and muds dominated by bivalves. In the overlying section, turbidite sands yield abundant ostracodes. The purpose of this study is to analyze this ostracode assemblage in relation to the paleoenvironment of deposition. Ten genera and 14 new species comprise the ostracode assemblage recovered at one of the locali-ties where the turbidites had been exposed: Microcytherura sp. (33%), Aurila sp. A (27%), Aurila sp. B (11%), Aurila sp. C (7%), “Cytheretta” sp. (5%), Loxoconcha (Loxoconcha) sp. (4%), Am-bostracon sp. (3%), Loxoconcha (Palmoconcha) cf. L. (P.) polugari Finger (3%), Paracosta cf. P. huddlestoni Finger (3%), Hamanella cf. H. implexa Finger (2%), Neonesidea sp.? ( The ostracodes appear to be a mixed neritic assemblage, and most are preserved as complete carapaces, suggesting that they had been rapidly displaced and buried downslope. This mode of deposition is also evident in the associated foraminifers, which constitute a mixed assemblage of inner-neritic to upper middle-bathyal taxa. Mudstones within the superjacent section are barren of ostracodes but richer in bathyal foraminifers, suggesting that the embayment continued to get wider and deeper during the early middle Miocene. Rapid basin subsidence is a tectonic phenomenon prevalent throughout the early and middle Miocene of California. Further study of basal sedimentary units along the California borderland could uncover additional ostracode assemblages that can be useful in determining the geologic, ecologic, and paleontologic history of the East Pacific Rim.