Stratigraphy and Structure of the Miocene "Esmeralda" Formation in Stewart Valley, Mineral County, Nevada
The Miocene rocks of Stewart Valley, eastern Mineral County, Nevada, consist of volcanic flows and breccias and fluvio-lacustrine sedimentary rocks. They are called the Esmeralda Formation because they contain lithologies similar to those of the Esmeralda type section 100km south; however, the rocks in Stewart Valley were probably deposited in a separate local basin. The lower part of this formation contains small lacustrine claystone deposits (unit 1) that interfinger with volcanic breccias and flows (unit 2). The main fluvio-lacustrine sequence is above the flows and·is divided by the author into five units based on lithology: sandstone and claystone in unit 3, shale in unit 4, claystone and mudstone in unit 5, sandstone and carbonate in unit 6, and sandstone and vitric tuff in unit 7. The total thickness of the formation is about 600m. Younger basalt flows lie unconformably above the Esmeralda Formation in part of the Gabbs Valley Range. Plio-Pleistocene conglomerates and pediment gravels overlie with angular unconformity the Esmeralda Formation in the valley. During the early Miocene, Stewart Valley was downdropped as a graben. On the east side of the valley the Cedar Mountains were uplifted, but not to their present height. On the west side of the valley the Gabbs Valley Range was uplifted along the Battle's Well Fault. However, this fault is mainly a strike-slip fault associated with Walker Lane and most of the displacement on the fault was horizontal. During most of the Miocene the valley was tectonically stable and the main drainage was south. The lake that formed in the valley fluctuated in size and depth, at times connecting with basins to the south and east. In this lake the sediments of the Esmeralda Formation were deposited. At the end of the Miocene, more faulting occurred on the east and southeast sides of the valley and sub-parallel to the valley axis, altering drainage to the north. These faults cut the Miocene fluvio-lacustrine rocks, but not the later Plio-Pleistocene sediments. The upthrown side of these dip-slip faults is usually on the east side, toward the Cedar Mountains. This fault pattern, along with the general eastward dip of the sediments and the steep gravity gradient near the Cedar Mountains, suggests tilted block faulting. The Gabbs Valley Range-Stewart Valley block was tilted east and was dropped relative to the Cedar Mountain block.
Geology, Geomorphology, Stratigraphy, Tectonics and Structure, Geology; Stratigraphic -- Miocene; Geology -- Nevada -- Mineral County; Grabens (Geology) -- Nevada -- Mineral County, Loma Linda University. Geological Sciences Program -- Dissertations
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