Lake Lahontan: Geology of Southern Carson Desert, Nevada
Report: Lake Lahontan: Geology of Southern Carson Desert, Nevada
This report presents a stratigraphic study of an area of about 860 square miles in the southern part of the Carson Desert, near Fallen, Churchill County, Nev. The exposed rocks and surficial sediments range in age from early Tertiary (?) to Recent. The late Quaternary sediments and soils were especially studied: they furnish a detailed history of the fluctuations of Lake Lahontan (a huge but intermittent late Pleistocene lake) and of younger lakes, as well as a history of late Quaternary sedimentation, erosion, soil development, and climatic change that probably is representative of the northern part of the Great Basin.
The Tertiary rocks are divided into five main map units. The lower three are chiefly lavas: the Miocene or older andesite of Rainbow Mountain and dacite of Rainbow Mountain, and the Eagles House rhyolite of Miocene to Pliocene age. The Truckee formation, which overlies these, consists of tuff, tuffacious sandstone, gravel, diatomite, and limestone, and contains lower Pliocene fossils. The youngest and most extensively exposed unit, the Bunejug formation, is mainly andesitic and basaltic lavas of Pliocene and possibly early Pleistocene age. Generally it overlies the Truckee formation with an angular unconformity, but in the south its lower part may intertongue with the Truckee formation.
All the Tertiary rocks are cut by high-angle normal faults, and commonly are more or less tilted. The degree of deformation increases with age of the rocks, showing that faulting was fairly continuous. The earlier Tertiary deformations were mostly compressional, with notable strike-slip faulting, but Quaternary faulting was dominantly tensional. Few of the older faults are exposed; most exposed faults, date from two climaxes: the first in late Pliocene or early Quaternary time, and the second probably also in relatively early Quaternary time long before Lake Lahontan time. During the interval of relative quiescence between these two climaxes, extensive pediments formed at the edges of the mountains. The main faults were active repeatedly, and as most of them bounded mountain blocks, relief was increased progressively to a maximum at the close of the second climax. Subsequent erosion and sedimentation have been more rapid than faulting and have progressively lowered the mountains and filled the basins.
The Carson Desert, long a major drainage sump of the northwestern Great Basin, contains Quaternary sediment probably more than 1,000 feet thick in places. It is one of the largest and deepest basins of northern Nevada inundated by Lake Lahontan; its floor lies as much as 515 feet below the highest shoreline. The area mapped covers the whole range of lake fluctuations from highest to complete desiccation. The exposed Quaternary deposits, exclusive of volcanics, comprise seven main units which are, from oldest to youngest: (1) lacustrine sediments of pre-Lake Lahontan age; (2) subaerial sediments and soil of late pre-Lake Lahontan age; (3) deep-lake sediments and minor amounts of intertonguing subaerial deposits of early Lake Lahontan age; (4) subaerial sediments, soil, and intertonguing shallow-lake sediments of middle Lake Lahontan age; (3) deep-lake sediments and minor amounts of intertonguing subaerial deposits of late Lake Lahontan age; (6) subaerial sediments and soil of early post-Lake Lahontan age; and (7) subaerial sediments and intertonguing shallow-lake sediments of late post-Lake Lahontan age. Only the deposits of Lake Lahontan and younger age are widely exposed.
Pre-Lake Lahontan history is fragmentary, but Lake Lahontan and post-Lake Lahontan history is fairly complete. A lake older than Lake Lahontan is suggested by a single exposure of lacustrine sediment. After deposition of this sediment, a long interval of lake recession or desiccation ensued, during which all lakes remained at least 420 feet below the maximum level of Lake Lahontan. During early Lake Lahontan time the lake reached its maximum level of 4,380 feet, receded briefly to at least as low as 4,100 feet, then rose again to 4,340 feet. In middle-Lake Lahontan time the basin intermittently was dry and held shallow lakes. During late Lake Lahontan time the lake had 3 maximums and 2 recessions; first it rose to 4,370 feet, then dropped to at least 3,990 feet, then rose to 4,190 feet, then dropped at least to 3,900 feet, and then rose a last time to 3,990 feet. During early post-Lake Lahontan time the basin generally was completely dry, and during late post-Lake Lahontan time five successive small lakes, having maximum depths of 15 to 85 feet, occupied parts of the basin floor.
The writer’s deductions on the lake history give no support to J. C. Jones’ (1925) interpretation of a single lake cycle starting a mere 2,000 year or so ago. These deductions agree, however, with most of Russell’s (1885) and Antevs’ (1945, 1948, 1952) conclusions. The writer’s belief that Lake Lahontan had multiple maximums during both early and late Lake Lahontan times supplements both Russell’s and Antevs’ interpretations; that the lake dried completely in middle-Lake Lahontan time is contrary to Antevs’ interpretation but in line with Russell’s; and that the lake reached its highest level in early Lake Lahontan time is as Antevs inferred, but contrary to Russell’s conclusion.
It now seems possible to correlate the later Quaternary stratigraphic units of the Carson Desert with those of the Lake Bonneville and Sierra Nevada areas, largely by means of soil stratigraphy. The successions in all three areas have similar soil sequences, in terms of relative age and relative development of the soils. The physical record in each area indicates that the soils formed during distinct widely separated intervals, in response to infrequent combinations of climatic factors that induced erosional stability and a more rapid rate of chemical weathering than normal. The most strongly developed soils formed during the main intervals of lake desiccation and deglaciation; weaker soils formed during the shorter recession intervals; soil-profile development at other times was inappreciable. The soil-forming intervals were periodically repeated parts of whole climatic cycles mainly fluctuations in temperature and precipitation that are manifest in the Quaternary sequences of each area. The climatic cycles, and the depositional and soil-forming cycles induced by them, probably were virtually synchronous over the entire Rocky Mountain to Sierra Nevada region because the whole region probably acted as a climatic unit during the Quaternary as it does now. Thus, soils of similar relative age and development in the four areas are assumed to have formed contemporaneously. Because they formed during generally shorter intervals than the intersoil sediments, they are considered to be more precise time-stratigraphic markers. The stronger soils are readily recognizable in the successions of each area and are the most reliable and useful markers; they provide the basic framework for correlation. The sediments and weaker soils intermediate in age between the main soils are correlated by matching those units that record depositional cycles, or parts of cycles, of similar relative age and similar climatic genesis; that is, units recording early lake cycles are matched with each other and with units recording early glacial cycles; likewise, lake-recessional units are correlated with glacial-recessional units.
By this method the Cocoon soil is correlated with the pre-Wisconsin soil of Hunt and Sokoloff (1950) in the Lake Bonneville and Rocky Mountain areas and with the soil of pre-Tahoe age (of Blackwelder, 1931) in the Sierra Nevada; the Churchill soil is correlated with the middle Lake Bonneville soil and with the soil of inter-Tahoe Tioga age (of Blackwelder, 1931) in the Sierra Nevada. The Toyeh soil is correlated with soils of post-Lake Bonneville and post-Tioga time. The deposits intermediate in age between these main soils are correlated as follows: The Eetza formation is correlated with the Alpine and Bonneville formations of the Lake Bonneville area, and with the deposits of the Tahoe glacial stage in the Sierra Nevada. The Wyemaha formation is correlated with subaerial deposits of inter-Bonneville-Provo age in the Lake Bonneville area and with disconformities of inter-Tahoe-Tioga age in the Sierra Nevada. The Sehoo and Indian Lakes formations are correlated with the main part of the Provo formation of the Lake Bonneville area, and with deposits of the Tioga glacial stage in the Sierra Nevada. The Turupah formation is correlated with deposits of Antevs’ altithermal age (2000 to 5500 B.C.) in the Great Basin. The Fallon formation is correlated with deposits of Antevs’ medithermal age in the Great Basin and with deposits of Matthes’ Little Ice Age in the Sierra Nevada.
These correlations, if valid, lead to several general deductions about regional Quaternary events:
1. The fluctuations of Lakes Lahontan and Bonneville were similar and synchronous....
- Roger B. Morrison
- U.S. Geological Survey
- U.S. Government Printing Office, 1964
- Report Number
- USGS Professional Paper 401
- Not Provided
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Roger B. Morrison (U.S. Geological Survey). 1964. Lake Lahontan: Geology of Southern Carson Desert, Nevada. Washington, District of Columbia: U.S. Government Printing Office. Report No.: USGS Professional Paper 401.