Depositional Environments of the Miocene Esmeralda Formation, Stewart Basin, Stewart Valley, Nevada
The purpose of this study was to determine specific depositional environments of the Miocene Esmeralda Formation in Stewart Basin (Stewart Valley), Mineral County, Nevada. Previous workers have concentrated on paleontological aspects of the basin and have described in detail the molluscs, mammals and macroflora. Less work has been done on the diatoms and spores and none on the fish, algae, wood or ostracodes. Recently, seven lithostratigraphics units of the Esmeralda Formation have been mapped in Stewart Valley (Clausen, 1983). Detailed lithofacies are described in each of the seven lithostratigraphic units. Lithofacies 1 is composed of porcelanite and diatomite containing plant fossils. Lithofacies 2a and 2b are volcanic conglomerates with fossil wood. Lithofacies 3a, 3b and 3c are varieties of feldspathic sandstones containing molluscs and fish. Thinly-laminated porcelanite, diatomite and dolomite comprise lithofacies 4. Articulated fish, plant and insect fossils are present. Massive and bedded porcelanite and silty diatomite dominate lithofacies 5. Lithofacies 6 is composed of dolomicrite. A diagenetic origin for the porcelanite is proposed. The precursor is interpreted to be a silty qiatomite. Depositional environments are proposed for six time intervals during the Miocene. In time A, limnetic and littoral environments and debris-flows existed. Lahars then covered previous sediments. During time C meandering and braided fluvial systems reworked the lahars and a paludal environment formed. A second limnetic environment then formed and transgressed over the littoral sediments during times D and E. The lake reached a size of 100 km 2 . During times A, D and E lake waters were generally fresh and moderately alkaline with periods of increasing salinity and alkalinity. The last major depositional period (time F) was regressive. The salinity and pH of the lake waters increased favoring the formation of evaporites and zeolites. The Esmeralda Formation in Stewart Basin is an unusual example of a fresh to moderately saline-alkaline lake environment. The stratigraphic relationship of diatomaceous, volcaniclastic, stromatolitic, and dolomitic sediments provide new insights into ancient lake environments.
Geology, Paleontology, Geology; Stratigraphic -- Miocene; Geology -- Nevada -- Mineral County; Paleontology -- Miocene -- Mineral County
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