The lacustrine Esmeralda Formation (Miocene) of Nevada was studied with the purpose of determining the depositional environment of petroliferous laminated rocks that are interbedded with volcaniclastic sands and localized carbonate units.

Fine-grained petroliferous shales make up a 40-m section at the inferred depositional center of the basin and interfinger laterally with volcaniclastic mudstones, sandstones, and breccias. The areal extent and geometric shape of this unit were determined by detailed geologic mapping and measurement of stratigraphic sections. This unit and other units of the Esmeralda Formation have undergone extensive folding and faulting which produced possible reservoir structures.

The depositional environment is interpreted as beginning as a freshwater lake that evolved through time into a saline lake environment. This basin received volcaniclastic sediments from surrounding volcanic highs. Influx of these sediments slowed during periods of carbonate deposition. Unusual stromatolites and large tufa mounds with pseudomorphs of evaporite minerals are associated with dolostone and limestone layers.

This study provides a depositional and structural context for evaluating the petroleum potential of the Esmeralda Formation in Stewart Valley, as well as an aid in understanding similar Tertiary deposits in the Basin and Range province of Nevada.

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