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The diatom assemblages of a 1,600-ft section of diatomites in the Monterey and Sisquoc Formations near Lompoc, California, give evidence of a warming trend within the late Miocene. Frequency changes of individual warm and cold water species, ratios of different ecologic types, and variations in the overall assemblage indicate a general increase in temperature during the period of deposition. The lower third of the section (upper Mohnian Stage) contains a cool-temperate microflora, characterized by relatively high frequencies of Denticula hustedtii Simonsen and Kanaya. A mixed warm- and cold-temperate assemblage predominates through the overlying 1,000 ft of diatomites, and the highest 80 ft (Delmontian Stage?) contains a warm-temperate microflora, with Coscinodiscus excen ricus Ehrenberg, C. lineatus Ehrenberg, Thalassionema nitzschioides Grunow, and Thalassiosira sp.
Diatom evidence for these temperature changes is further supported by that from the fossil fish and sparse Foraminifera of the Lompoc section. The changes may reflect a general climatic warming, but also may result from local variations in current distribution, such as the fluctuations in the cold-water California Current proposed for the late Miocene of southern California. Diatoms may provide valuable paleoecologic evidence for this extensive diatomite facies in California, which lacks the more commonly utilized foraminiferal and invertebrate paleoecologic indicators.
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