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Reconnaissance map of the Cenozoic geology in the Carlin basin area, Elko and Eureka counties, Nevada [MAP AND TEXT]
Cen geol Carlin basin

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Title: Reconnaissance map of the Cenozoic geology in the Carlin basin area, Elko and Eureka counties, Nevada

Author: Alan R. Wallace
Year: 2020
Series: Open-File Report 2020-02
Format: plate: 34 x 29 inches, color; text: 10 pages, b/w
Scale: 1:50,000

The middle Miocene Carlin sedimentary basin encompasses a large area between the Adobe Range to the east, the Piñon Range to the south, the southern Independence Mountains and Marys Mountain to the west, and Swales Mountain to the north. The town of Carlin is in the southern part of the basin. The geologic map includes detailed to more reconnaissance mapping of Cenozoic units in the main part of the basin, including different facies of the middle Miocene Humboldt Formation. The mapping was part of a broader study of the Miocene and younger paleogeographic evolution of the region. Earlier work obtained numerous 40Ar/39Ar and tephra correlation dates on sedimentary and volcanic units in the basin. The basin connected to the east into the Elko sedimentary basin and to the northwest and northeast into similar smaller basins between present-day mountain ranges. Early sediments, largely fluvial, began to accumulate in the lowlands between the surrounding ranges at about 16.5 Ma. The sediments were derived from Paleozoic sedimentary and middle Tertiary volcanic units in the nearby highlands, and flow patterns indicate a general southward flow towards present-day Pine Valley. The eruption of the Palisade Canyon–Marys Mountain rhyolite flows at the southwest end of the basin at 15.3 Ma blocked the southward flow, and a lake began to form in the basin. As the lake grew in extent, sedimentary units around the fringes of the lake included a mixture of inflowing fluvial sediments mixed with the pyroclastic-fall, ash-rich sediments deposited in the lake. The lake margin expanded, and stratigraphic sections record the progressive transition from fluvial to mixed fluvial and lacustrine, and finally to entirely lacustrine. The volcanic rock dam was breached at about 14.7 Ma, the lake drained, and fluvial sediments blanketed the entire basin for an unknown period of time after that. Sedimentation progressively buried existing highlands and bridged gaps between adjacent basins. For example, the Carlin and Elko basins connected across the southern Adobe Range.

Normal faulting produced numerous, mostly north- to north-northeast-striking faults that cut the sedimentary units and surrounding highlands largely after sedimentation ceased. The largest fault formed in the eastern third of the basin and tilted all of the sedimentary units in the western two-thirds of the basin, as well as the eastern part of Marys Mountain, to the east. Some offset took place during sedimentation. Many other normal faults of smaller extent and offset cut the sedimentary units.

The integration of streams draining the Elko and Carlin basins began after about 9.8 Ma. The streams, which together comprised the early stages of the Humboldt River, flowed regionally southwestward beyond the Carlin basin. As many as thirteen, downward-stepping strath terraces in the Carlin basin record the progressive downcutting into and removal of the middle Miocene sediments. Gravel deposits form a thin veneer on some of the higher terraces. Clasts in those gravel deposits, as well as the overall terrace pattern, indicate southward drainage towards the Humboldt River. The erosion gradually re-exposed the flanks of the surrounding highlands. A brief pause in downcutting allowed the formation of a small lake in the Hemphillian (late Miocene), represented by lacustrine units northwest of Carlin.

The sedimentary rocks of the Carlin basin conceal a large segment of the world-class, late Eocene Carlin gold trend, which extends from the southern Independence Mountains south into the Piñon Range. Sedimentation largely buried the Gold Quarry and Mike gold deposits in the northwestern part of the basin. Later faulting and erosion re-exposed the Gold Quarry deposit, but the Mike deposit remains buried. The basin’s sedimentary units conceal potential Paleozoic host rocks, and the sedimentary facies and post-sedimentation faults shown on the map may help guide interpretations of geophysical and other exploration data in the Carlin basin.
The current map publication was supported by the USGS National Cooperative Geologic Mapping Program under STATEMAP award number G19AC00383.

Suggested Citation:
Wallace, A.R., 2020, Reconnaissance map of the Cenozoic geology in the Carlin basin area, Elko and Eureka counties, Nevada: Nevada Bureau of Mines and Geology Open-File Report 20-2, scale 1:50,000, 10 p.

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Original Product Code: OF20-2