Stratigraphy, tephrochronology, and structural setting of Miocene sedimentary rocks in the Middlegate area, west-central Nevada

Abstract

Miocene strata in the Middlegate area, Churchill County, Nevada, consist of the Middlegate Formation unconformably overlain by the Monarch Mill Formation. The Middlegate Formation is about 110m thick and consists of a lower member of fluvial and lacustrine tuffaceous sandstone and conglomerate and an upper member of lacustrine diatomaceous siltstone. Megabreccia landslide deposits are common in the Middlegate Formation. Tephrochronologic studies indicate that the Middlegate Formation is about 15.2 Ma. The Monarch Mill Formation consists of lacustrine, fluvial, and alluvial-fan deposits that are at least 700 m thick in the central part of the Middlegate basin. Coarse, near-source, alluvial-fan deposits of the Monarch Mill Formation interfinger with finergrained fluvial and lacustrine deposits from source areas to the north and east and southeast of the Middlegate basin. Local megabreccia deposits are present in the Monarch Mill Formation in the southern and eastern parts of the basin. Tephrochronologic studies indicate that the Monarch Mill Formation ranges from about 14.7 to 9.8 Ma.The Middlegate Formation is mostly a quiet-water lake deposit that appears to have been deposited in an environment distinctly different from the present-day environment that is dominated by alluvial-fan deposits. It does, however, contain megabreccia deposits, indicative of nearby escarpments that produced far-traveled landslide deposits. The Monarch Mills Formation represents a distinct change in the paleogeographic setting in the Middlegate area. It contains coarse near-source deposits that indicate local high-relief, probably related to the development of faults and topographic relief that led to the presentday structural setting of the Middlegate area. This change occurred at about 14 Ma.

Timing of Cenozoic Extension in the Southern Stillwater Range and Dixie Valley, Nevada

“…While some tilting of Oligocene rocks is almost certainly due to magmatic processes (e.g., resurgent doming following caldera collapse), the consistent tilting of large crustal blocks that include multiple calderas is due to east west extension, and only rapid unroofing from substantial depths would lead to the observed patterns in the cooling data. Rapid middle Miocene extension in the southern Stillwater Range is consistent with ~15–17 Ma extension in the East Range (Figure ) (Fosdick & Colgan, ), with the inference of middle Miocene extension by MacNamee () in the Stillwater Range to the north of our study (Figure ) and with local and regional ages of clastic sedimentary rocks inferred to have been deposited during extensional faulting (Figure ) (e.g., Stewart et al, ; Alm, ; Bristol, ).…”

Reconstructing Miocene Paleogeography of the Middlegate and Eastgate Basins, Nv Through Geologic Mapping and Sedimentary Provenance Analysis

“…Improved knowledge of the paleoenvironment conditions and physiography of these non-marine basins have important implications for our understanding of crustal extension, paleotopography, and paleoclimatic studies in the Western U.S. John et al (2008). Crustal isotope boundaries from Henry and John (2013) New detailed sedimentologic, stratigraphic, geochronologic, and geochemical data from this study in combination with previously collected data (e.g., Barrows, 1971;Stewart et al, 1999;Colgan et al, 2018;Colgan et al, 2019) suggest that Miocene…”

“…Both Middlegate and Eastgate Basins contain extensive exposures of Miocene sedimentary rocks that were studied by Axelrod (1956Axelrod ( , 1985, Barrows et al (1971), Stewart et al, 1999), with study areas from this study: 1) East Middlegate area, 2) Central Middlegate area, 3) South Middlegate area. B) East Middlegate area with sample locations (red square and text).…”

“…Basin and Range extension) than previous work may suggest. Stewart et al (1999) suggested that correlation between Middlegate and Eastgate Miocene basin stratigraphy is plausible, but further work to attempt to study the stratigraphy in terms of lateral extent between both basins has not been done until this study.…”

“…3). Previous work consists of geologic mapping of sedimentary units in Middlegate Basin by Stewart et al, (1999) and Eastgate Basin by Barrows (1971), but little work has been done since to try to correlate the Miocene strata between both basins as deposition of basin fill was inferred to have occurred following basin partitioning. Improved knowledge of the paleoenvironment conditions and physiography of these non-marine basins have important implications for our understanding of crustal extension, paleotopography, and paleoclimatic studies in the Western U.S. John et al (2008).…”