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Biochronology and Paleoecology of Fine-Grained Sediments Belonging to "Esmeralda" Formation in Stewart Valley, Nevada

Scott W. Starratt

Fine-grained rocks belonging to the middle Miocene "Esmeralda" Formation in Stewart Valley, Nevada, are rich in diatoms and ostracods as well as domal and subspherical stromatolites. In addition to their use as paleoecological indicators, the diatoms have also been used to date the rocks.

The diatom flora, collected from a 45-m section consisting of thinly bedded to laminated siliceous shales, is dominated by Fragilaria construens (Ehrenberg) Grunow and Melosira granulata (Ehrenberg) Ralfs. This indicates that during the time represented, the waters in the deeper part of the lake were alkaline, clean, and low in salinity (0-5 ^pmil). The lake was eutrophic and probably possessed a seasonal thermocline which was responsible for low-oxygen bottom waters at least part of the year. Varved rocks from another section in the valley tend to support this conclusion. The presence of the diatoms Coscinodiscus miocaenicus Krasske and Coscinodiscus grobunovii Scheschukova-Poretskaya in the deeper water deposits suggests an age of 12-15 Ma. This age is in agreement with that indicat d by mammalian fossils present in closely associated rocks.

The ostracod fauna suggests that the shallow waters of the lake may have been more saline. The domal and subspherical stromatolites indicate that the shallow-water environment was alkaline enough to exclude algae-grazing gastropods. Such gastropods appear in other parts of the lake.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.