Diatom biostratigraphy developed in the middle to high latitude North Pacific allows detailed correlations within the middle and upper Miocene Monterey Shale, and provides ties to the international time scale. In contrast, planktonic foraminiferal biostratigraphy is severely limited owing to low species diversity caused by climatic deterioration during the middle and late Miocene. Despite these limitations, fluctuating abundances and morphologic variants of planktonic foraminifers provide paleoclimatic information which, taken in conjunction with high resolution diatom biostratigraphy, provides a means for interpreting the paleoclimatic and paleo-oceanographic history of the California Current and, hence, the Monterey Shale.

Diatom and foraminiferal assemblages were studied from DSDP Sites 173 and 470, from the upper Newport Bay section, and from other selected sections in California. Microfossil assemblages record fluctuations in the intensity of the California Current during warm and cold episodes. During warm to temperate periods, planktonic foraminifers are abundant and exhibit relatively high species diversity and morphotypic variation, whereas diatoms are not very abundant. During colder periods, diatoms are abundant and planktonic foraminifers exhibit very low diversity or may even be absent owing to carbonate dissolution.

Paleoclimatic trends deduced from microfossil and isotope studies of Pacific DSDP material can be recognized in middle and upper Miocene strata from the California area. A cooling trend beginning at about 15.0 Ma and reaching a maximum at about 13.0 Ma is apparent in rocks assigned to the upper Luisian through lowermost Mohnian provincial stages. Diatoms and planktonic foraminifera reveal a further cooling near the middle Miocene-upper Miocene boundary at about 11.5 Ma (middle Mohnian). An inundation of diatoms and near disappearance of foraminifers at about 7.5 Ma (upper Mohnian) corresponds to increased upwelling associated with intensified cooling in the uppermost Miocene.

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