Browsing by Author "Burchfiel, B. Clark"
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Item Stratigraphic and structural relations in a portion of the Northwest Spring Mountains, Nevada(1964) Livingston, John Lee; Burchfiel, B. ClarkThe Johnnie Formation is subdivided into five distinct members that are mapped areally for the first time in order to decipher the structurally complex geology within a portion of the Northwest Spring Mountains. Distinctive quartzite and dolomite beds serve as boundaries between the members. The sediments of the Johnnie Formation and overlying Stirling Quartzite were deposited in a nearshore environment in the Cordilleran miogeosyncline which persisted during most of the Paleozoic Era in this part of the Basin and Range Province. Folding and thrusting during the late Mesozoic Laramide Orogeny are responsible for the complex local and regional structural relationships. Two large recumbent folds and related smaller folds represent the earliest effects of deformation; this period of intense folding was accompanied by the development of the Johnnie Thrust, a regional thrust fault which is probably located near the base of the Johnnie Formation, above which the overlying beds have been displaced several miles to the southeast. This interpretation is different from that of T.B. Nolan (1929) who originally described the Johnnie Thrust as occurring at the contact between the Stirling Quartzite and Johnnie Formation. Later or synchronous deformation by right-lateral displacement along the Las Vegas Valley Shear Zone is reflected in the change of trend of cleavage and fold axes in the recumbent folds. Gentle folding unaffected by movement on the shear zone followed. Present deformation is probably continuing along normal faults which have been active since early Tertiary time. This faulting is responsible for the steep topography that has triggered a significant gravity slide involving a large block of Stirling Quartzite which rests with stratigraphic and structural discordance on the lower Johnnie Formation.Item Structural geology of the Gass Peak area Las Vegas Range, Nevada(1965) Ebanks, W. J; Burchfiel, B. ClarkA field-mapping study of the area around Gass Peak, Las Vegas Range, Clark County, Nevada, has demonstrated the presence of a thick sequence of Paleozoic and Precambrian rocks overthrust on younger, Upper Paleozoic beds along the Gass Peak Thrust. Formations recognized in the upper plate of the thrust are the Precambrian Stirling Quartzite, the Precambrian and Cambrian Wood Canyon Formation, the Cambrian, Carrara, Bonanza King, and Nopah Formations, and the Ordovician lowermost Pogonip Group. Rocks in the lower plate are the Devonian Sultan Limestone, Mississippian Monte Cristo Limestone, and the Permo-Carboniferous Bird Spring Formation. The Miocene Horse Spring Formation uncomfortably overlies the Paleozoic rocks. Faulting on the Gass Peak Thrust resulted in approximately 18,000 feet of stratigraphic displacement from west to east. Several large folds and many high-angle reverse faults are associated with the thrust fault. All of these features, the major thrust and smaller related structures, have been rotated westward through 90 degrees by right-lateral strike-slip movement on the adjacent Las Vegas Valley Shear Zone. The "drag" structure and a unique set of faults in the Gass Peak area are related to east-west extension and north-south compression caused by the rotation. Strata above and below the Gass Peak Thrust are similar to strata in the Wheeler Pass Thrust, suggesting the two thrusts are offset equivalents. This implies more than 25 miles of relative horizontal shift between the two faults. Many details of structure are dissimilar between the Wheeler Pass and Gass Peak Thrusts, and if they were once the same feature, they have developed independently after being separated. Structural evidence indicated that displacement on the Gass Peak Thrust occurred before movement along the Las Vegas Valley Shear Zone. There is little evidence in the mapped area for Tertiary block faulting.Item Structural geology of the southern Silurian hills, San Bernardino County, Calif(1971) Abbott, Earl, 1942-; Burchfiel, B. ClarkIn the southern part of the Silurian Hills, seven lithologic units are delineated. They are an earlier Precambrian gneiss complex, the later Precambrian Pahrump Group, a Precambrian (?) metasedimentary complex, a Precambrian (?) metadioritic and metasedimentary complex, the Riggs Formation, basic plutonic rocks, and acidic plutonic rocks. Metamorphism reaches high greenschist to low amphibolite grade with an overprint of contact metamorphism caused by the intrusion of the acidic plutonic rocks. The Riggs thrust fault separates the Pahrump Group, which is in the lower plate, from all of the other units. The upper plate rocks show a more complex folding history than the lower plate rocks. Two quite different types of movement have occurred along the Riggs thrust. The first occurred in late Mesozoic time, was a compressional event, moved in a north or northeast direction, formed a sharp contact with plastic deformation rather than brecciation, and can be seen in the southern Silurian Hills. The second occurred in the Tertiary, was a gravitational event, moved south, formed a chaos structure in the lower plate, and can be seen in the northern Silurian Hills.Item The structural geology of the Shadow Mountains area, San Bernardino County, California(1966) Wilson, Raymond Carl; Burchfiel, B. ClarkWithin the Shadow Mountains area, located in the northeast corner of San Bernadino County, California, a series of allochthonous blocks of Precambrian gneisses rest on Late Tertiary non-marine sediments. The Precambrian gneisses consist of dioritic sills and granitic country rock. The Tertiary sediments consist of Pliocene (?) non-marine gravels, sands, tuffs, bentonitic playa clay, and lenses of massive, recemented dolomite megabreccia up to 500 feet thick. During the Sevier Orogeny, the Mesquite and the Winters thrusts developed in the Mesquite Range and Winters Pass area to the east of the Shadow Mountains, and the Halloran and the Kingston Peak batholiths were intruded south and north of the area respectively. In the early Tertiary (?), the Riggs thrust was emplaced in the Silurian Hills west of the Shadow Mountains, and the Kingston Valley graben formed between the Silurian Hills, the Kingston Range, and the Mesquite Range. During middle and late Tertiary (?), a thick sequence of non-marine sediments were deposited in the Kingston Valley graben. Interbedded with these sediments are a series of thick lenses of dolomite megabreccia which were emplaced by a combination of sedimentation and incoherent gravity sliding. The Goodsprirgs Dolomite exposed in the Riggs thrust plate in the Silurian Hills appears to have been the source of the megabreccia. The allochthonous Precambrian gneiss blocks of the Shadow Vountains area may have been emplaced originally as a single original sheet, 6 miles by 10 miles in area and more than 600 feet thick. The contact surface beneath the gneiss blocks is sharp, smooth, planar, and has a dip of less than one degree. A layer of bentonitic clay 3 inches to 4 feet thick is found immediately under the soles of the gneiss blocks and apparently served as a lubricating layer for the movement of the gneiss sheet. Although they are incompetent, the Tertiary sediments underlying the thin clay layer are virtually undeformed, but the Tertiary beds are overturned at Spring Peak, the one locality where the clay layer is missing. The source area for the allochthonous gneiss sheet was in the vicinity of Winters Pass. Because the underlying Tertiary rocks are undeformed where the clay layer is present, most of the shear stress needed to emplace the gneiss sheet was concentrated upon the gneiss itself. In a gravity slide, this stress would be distributed over the area of the plate, rather than on the thin rear edge as in a compressional thrust. For this and other reasons, the gneiss blocks of the Shadow Mountains area are interpreted as the remnants of a gravity slide block. The theoretical mechanics of gravity slide blocks are also discussed. In general, a gravity slide requires a plane of weak-ness with a dip steeper than its angle of repose, and a horizontal stress less than the overburden stress. Formulas are derived for the necessary horizontal (0-3) / overburden (0-1) stress ratios.