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At least 523 records · Page 29Linked to original sources
USGS1980

Paleozoic and Mesozoic deformations in the central Sierra Nevada, California

Analysis of structural and stratigraphic data indicates that several periods of regional deformation, consisting of combined folding, faulting, cataclasis, and regional metamorphism, occurred throughout the central Sierra Nevada during Paleozoic and Mesozoic time. The oldest regional deformation occurred alono northward trends during the Devonian and Mississippian periods in most roof pendants containing lower Paleozoic metasedimentary rocks at the center and along the crest of the range. This deformation is expressed in some roof pendants by an angular unconformity separating older thrice-deformed from younger twice-deformed Paleozoic metasedimentary rocks. The first Mesozoic deformation, which consisted of uplift and erosion and was accompanied by the onset of Andean-type volcanism during the Permian and Triassic, is expressed by an angular unconformity in several roof pendants from the Saddlebag Lake to the Mount Morrison areas. This unconformity is defined by Permian and Triassic andesitic to rhyolitic metavolcanic rocks unconformably overlying more intensely deformed Pennsylvanian, Permian(?), and older metasedimentary rocks. A later regional deformation occurred during the Triassic along N. 20?_30? W. trends in Permian and Triassic metavolcanic rocks of the Saddlebag Lake and Mount Dana roof pendants, in upper Paleozoic rocks of the Pine Creek roof pendant, and in the Calaveras Formation of the western metamorphic belt; the roof pendants are crosscut by Upper Triassic granitic rocks of the Lee Vining intrusive epoch. A still later period of Early and Middle Jurassic regional deformation occurred along N. 30?-60? E. trends in upper Paleozoic rocks of the Calaveras Formation of the western metamorphic belt. A further period of deformation was the Late Jurassic Nevadan orogeny, which occurred along N. 20?_40? W. trends in Upper Jurassic rocks of the western metamorphic belt that are crosscut by Upper Jurassic granitic rocks of the Yosemite intrusive epoch. Structures of similar age occur in intensely deformed oceanic-lithospheric and syntectonic plutonic rocks of the lower Kings River area, in Jurassic metavolcanic rocks of the Ritter Range roof pendant, and in Triassic metasedimentary rocks of the Mineral King roof pendant. The final Mesozoic deformation occurred along N. 50?-80? W. trends in both high-country roof pendants and the lower Kings River area; structures of this generation are crosscut by relatively undeformed Upper Cretaceous granitic rocks of the Cathedral Range intrusive epoch.

Professional Paper↗
USGS1980

Revised nomenclature and stratigraphic relationships of the Fredericksburg Complex and Quantico Formation of the Virginia Piedmont

The Fredericksburg Complex, in part a migmatitic terrane in northeast Virginia, is subdivided on the basis of lithology, as well as aeromagnetic and aeroradiometric data, into two metamorphic suites. These suites are separated by the northeast-trending Spotsylvania lineament, a rectilinear geophysical feature that is probably the trace of an old fault zone. East of the lineament, the Po River Metamorphic Suite, of Proterozoic Z and (or) early Paleozoic age, consists dominantly of biotite gneiss, generally augen gneiss, and lesser amounts of hornblende gneiss and mica schist. West of the Spotsylvania lineament is the Ta River Metamorphic Suite, composed mostly of amphibolite and amphibole gneiss. However, to the southwest, along its strike belt, the Ta River contains abundant biotite gneiss and mica schist. Both the Ta River and Po River contain abundant foliated granitoid and pegmatoid bodies as concordant tabular masses and as crosscutting dikes; these rocks are considered part of the Ta River and Po River Metamorphic Suites. The amphibolitic Holly Corner Gneiss is interpreted to be a western allochthonous equivalent of the Ta River. Both the Ta River and Holly Corner are considered to be coeval, eastern, distal facies of the Lower Cambrian(?) Chopawamsic Formation. The Paleozoic Falls Run Granite Gneiss intrudes the Ta River Metamorphic Suite and the Holly Corner Gneiss; locally the Falls Run is interpreted to have been transported westward with the Holly Corner after intrusion. The Quantico Formation, in the core of the Quantico-Columbia synclinorium, rests with angular unconformity along its northwest and southeast limbs, respectively, on the Chopawamsic Formation and the Ta River Metamorphic Suite. The Quantico Formation is assigned the same Late Ordovician age and similar stratigraphic position as the Arvonia Slate of the Arvonia syncline. The youngest rocks of the area are the granitoid and pegmatoid bodies of the Falmouth Intrusive Suite. They consist of several generations of chiefly dikes and sills that are intrusive into the Fredericksburg Complex and into the Quantico Formation. Granitoid rocks also form small plutons. The Falmouth is isotopically dated as Carboniferous in age. Some of the metavolcanic rocks of the Evington Group and part of the amphibolite gneiss and amphibolite of the Hatcher Complex, named by W. B. Brown in 1969, are probably coeval with the Chopawamsic Formation and hence equivalents of the Ta River Metamorphic Suite and the Holly Corner Gneiss. The biotitic gneiss and granitoid rocks east of the Spotsylvania lineament in the Dillwyn area are considered to be coeval with the Po River Metamorphic Suite.

Virginia↗
USGS1979

Chemical correlation of some late Cenozoic tuffs of Northern and Central California by neutron activation analysis of glass and comparison with X-ray fluorescence analysis

Glasses separated from several dacitic and rhyolitic late Cenozoic tuffs of northern and central California were analyzed by neutron activation for more than 43 elemental abundances. Eighteen elements--scandiurn, manganese, iron, zinc, rubidium, cesium, barium, lanthanum, cerium, samarium, europium, terbiurn, dysprosiurn, ytterbiurn, hafniurn, tantalurn, thorium and uranium--were selected as most suitable for purposes of chemical correlation on the basis of their natural variability in silicic tuffs and the precision obtainable in analysis. Stratigraphic relations between tuffs and replicate chemical analyses on individual tuffs make it possib1e to calibrate a quantitative parameter, the similarity coefficient, which indicates the degree of correlation for the tuffs studied. The highest similarity coefficient (0.99) was obtained for analyses of two tuffs (potassium-argon dated at about' 6.0 m.y.) exposed in the Merced(?) and Petaluma Formations of Sonoma County, which represent different paleoenvironments, shallow-water marine and fresh water or brackish marine, respectively. Corre1ation of these formations on the basis of criteria other than tephrochronoloqy would be difficult. Results of neutron activation analysis in general confirm earlier correlations made on the basis of analysis by X-ray fluorescence but also make it possible to resolve small compositional differences between chemically simi1ar tuffs in stratigraphic proximity. The Lawlor Tuff (potassium-argon dated at about 4.0 m.y.) is identified at two new localities: in a core sample obtained from a bore hole east of Suisun Bay, and from the Kettleman Hills of western San Joaquin Valley. This identification permits correlation of the uppermost part of the marine Etchegoin Formation in the San Joaquin Valley with the continental Livermore Gravels of Clark, the Tassajara Formation, and the upper part of the Sonoma Volcanics in the cel1tral Coast Ranges of California. A younger tuff near the top of the marine San Joaquin Formation in the Kettleman Hills has been identified at both new 1oca1ities .

Professional Paper↗
USGS1981

Surface geology of the Jeptha Knob cryptoexplosion structure, Shelby County, Kentucky

The Jeptha Knob crytoexplosion structure, described by Bucher in 1925, was remapped in 1973 as part of the U.S. Geological Survey and the Kentucky Geological Survey cooperative mapping program. The knob is in the western part of the Blue Grass region. Hilltops in the rolling farmland adjacent to the knob are underlain by the nearly flat-lying Grant Lake and Callaway Creek Limestones of middle Late Ordovician age, and the valleys are cut in interbedded limestone and shale of the Clays Ferry Formation of late Middle and early Late Ordovician age. Precambrian basement is estimated to be 4,000 ft below the surface. The mapped area is 50 miles west of the crest of the Cincinnati arch; the regional dip is westward 16 ft per mile. The 38th parallel lineament is 50 miles to the south. The structure, about 14,000 ft in diameter, consists of a central area 6,300 ft in diameter of uplifted Clays Ferry Formation surrounded by a belt of annular faults that are divided into segments by radial faults. The grass structure of the Clays Ferry Formation is that of a broad dame, but same evidence indicates that, in detail, the beds are complexly folded. The limestone of the Clays Ferry is brecciated and infiltrated by limonite. The brecciation is confined to single beds, and there is no mixing of fragments from different beds. A small plug of the Logana Member of the Lexington Limestone (Middle Ordovician) has been upfaulted at least 700 ft and emplaced within the Clays Ferry. The central uplift is separated by high-angle and, in places, reverse faults from the belt of annular faulting. The concentric faults in the zone of annular faults are extensional, and the general aspect is of collapse and inward movement. Lenses of breccia are present along many of the concentric faults, but not along the radial faults. At least same of the breccia was injected from below. The youngest beds involved in the faulting are in the Bardstown Member of the Drakes Formation of late Late Ordovician age. The faulted and brecciated beds are overlain by nearly horizontal dolomite and shale of Early and Middle Silurian age. The basal 5 ft of the oldest Silurian unit, the Brassfield Formation, contains calcarenite and calcirudite composed, in large part, of locally derived fragments from the Upper Ordovician formations. The Jeptha Knob structure was formed in latest Late Ordovician or earliest Early Silurian time. At the time of formation, the area was either very slightly above or very slightly below sea level; the sediments were already largely indurated. At the onset of Silurian deposition, the area of the central uplift was probably a broad shallow depression not more than about 15 ft deep, possibly surrounded by a rim of Upper Ordovician rocks or rock fragments. The origin of the Jeptha Knob structure cannot be determined from the available data. Shatter cones and coesite, considered by many to be definitive criteria far origin by impact, have not been found. On the other hand, geophysical studies indicate that there is no coincident uplift of the basement, and there is no certain relation of Jeptha Knob to any obvious structural trend.

Professional Paper↗
USGS1981

Lithostratigraphy of the Silurian rocks exposed on the west side of the Cincinnati Arch in Kentucky

This report discusses the Silurian rocks that crop out in Kentucky west of the axis of the Cincinnati arch. The Silurian System has a maximum thickness in this outcrop area of about 190 feet. It is in most places separated from the underlying Ordovician System by a minor erosional unconformity. It is separated from the overlying Devonian System by a major erosional unconformity along which the entire Silurian section has been removed in some areas. The Silurian is divided into five formations, which are, from lowest to highest: (1) Brassfield Dolomite (or Formation), composed of dolomite in the south and both limestone and dolomite in the north; (2) Osgood Formation, shale and minor dolomite; (3) Laurel Dolomite, dolomite; (4) Waldron Shale, shale and minor dolomite; and (5) Louisville Limestone, calcitic dolomite and dolomitic limestone. The Brassfield is of Early Silurian age; the other formations are Middle Silurian. The main outcrop area of the rocks considered in this re- port extends about 85 miles south from the Ohio River and has a maximum width of about 20 miles. The rocks in this belt generally dip westward about 30 feet per mile. Part of this dip was imparted to the rocks prior to the cutting of the overlying unconformity, but most of it came later. Minor tectonic movement along west-northwest and east-northeast trends probably took place during Silurian time. Thickening of the Brassfield and Osgood southward across the westnorthwest-trending Bardstown monocline and thickening of the Brassfield in the axial zone of the east-northeast-trending Lyndon syncline indicate that movement took place along these structures during or immediately before the deposition of these formations. Several remarkably persistent thin units of dolomite and shale that make up the Osgood, the Laurel, and the Waldron indicate a depositional strike about north. The dolomite was originally mostly fossiliferous limestone deposited in shallow marginal seas. The source of the shale was to the east.

Professional Paper↗
USGS1984

Stratigraphy and structure of the western Kentucky fluorspar district

The western Kentucky fluorspar district is part of the larger Illinois-Kentucky fluorspar district, the largest producer of fluorspar in the United States. This report is based largely on data gathered from 1960 to 1974 during the U.S. Geological Survey-Kentucky Geological Survey cooperative geologic mapping program of Kentucky. It deals chiefly with the stratigraphy and structure of the district and, to a lesser extent, with the fluorspar-zinc-lead-barite deposits. Sedimentary rocks exposed in the district range in age from Early Mississippian (Osagean) to Quaternary. Most rocks exposed at the surface are Mississippian in age; two-thirds are marine fossiliferous limestones, and the remainder are shales, siltstones, and sandstones. Osagean deep-water marine silty limestone and chert are present at the surface in the southwestern corner of the district. Meramecian marine limestone is exposed at the surface in about half the area. Chesterian marine and fluvial to fluviodeltaic clastic sedimentary rocks and marine limestone underlie about one-third of the area. The total sequence of Mississippian rocks is about 3,000 ft thick. Pennsylvanian rocks are dominantly fluvial clastic sedimentary rocks that change upward into younger fluviodeltaic strata. Pennsylvanian strata of Morrowan and Atokan age are locally thicker than 600 ft along the eastern and southeastern margin and in the major grabens of the district where they have been preserved from erosion. Cretaceous and Tertiary sediments of the Mississippi embayment truncate Paleozoic formations in and near the southwestern corner of the district and are preserved mostly as erosional outliers. The deposits are Gulfian nonmarine gravels, sands, and clays as much as 170 ft thick and upper Pliocene fluvial continental deposits as thick as 45 ft. Pleistocene loess deposits mantle the upland surface of the district, and Quaternary fluvial and fluviolacustrine deposits are common and widespread along the Ohio and Cumberland Rivers and their major tributaries. Many mafic dikes and a few mafic sills are present. The mafic rocks are mostly altered mica peridotites or lamprophyres that are composed of carbonate minerals, serpentine, chlorite, and biotite and contain some hornblende, pyroxene, and olivine. Most of the dikes are in a north-north west-trending belt 6 to 8 mi wide and strike N. 20 0 -30 0 W. The dikes dip from 80 0 to 90 0 and are commonly 5 to 10 ft wide. Radioisotopic study indicates that the dikes are Early Permian in age. The district is just southeast of the intersection of the east-trending Rough Creek-Shawneetown and northeast-trending New Madrid fault systems. The district's principal structural features are a northwest-trending domal anticline, the Tolu Arch, and a series of steeply dipping to nearly vertical normal faults and fault zones that trend dominantly northeastward and divide the area into elongated northeast-trending grabens and horsts. Formation of these grabens and horsts was one of the major tectonic events in the district. Vertical displacement may be as much as 3,000 ft but commonly ranges from a few feet to a few hundred feet; no substantial horizontal movement is believed to have taken place. Many cross faults having only a few feet of displacement trend northwestward and are occupied at places by mafic dikes. Faulting was mostly post-Early Permian to pre-middle Cretaceous in age. Many theories have been advanced to explain the structural history of the district. A generally acceptable overall hypothesis that would account for all the structural complexities, however, is still lacking. Useful structural data, such as the structural differences between the grabens and the horsts, have been obtained, however, from the recently completed geologic mapping. Mapping also has more clearly shown the alinement of the Tolu Arch, the belt of dikes, and an unusually deep graben (the Griffith Bluff graben); this alinement suggests that possibl

Professional Paper↗
USGS1984

Lithostratigraphy of Upper Ordovician strata exposed in Kentucky, with a section on biostratigraphy

Ordovician formations above the Lexington Limestone crop out in the Blue Grass region of Kentucky and along the Cumberland River and its tributaries. The formations are all conformable and in places intertongue and intergrade. The major Ordovician units above the Lexington Limestone in the Blue Grass region are: The Clays Ferry Formation, the Kope Formation, the Garrard Siltstone, the Fairview Formation, the Calloway Creek Limestone, the Grant Lake Limestone, the Ashlock Formation, the Bull Fork Formation, and the Drakes Formation. The Clays Ferry Formation is made up of subequal amounts of fossiliferous limestone and shale and minor siltstone; the Clays Ferry is as much as 300 ft thick and intertongues with the Lexington Limestone and the Kope Formation. The Kope Formation resembles the partly equivalent Clays Ferry but has a higher shale content (60-80 percent) and thicker layers of shale; the Kope, as much as 275 ft thick, is mostly restricted to the northern part of the State. The Garrard Siltstone, which consists of very calcitic siltstone and minor shale, overlies the Clays Ferry Formation in the southeastern part of the Blue Grass region; the Garrard, as much as 100 ft thick, feathers out into the upper part of the Clays Ferry in southern central and northern east-central Kentucky. The Fairview Formation is characterized by even-bedded limestone interlayered with nearly equal amounts of shale and minor siltstone. The Fairview crops out in the northern part of the Blue Grass region, where it generally overlies the Kope Formation or the Garrard Siltstone; it grades southward into the Calloway Creek Limestone. The Calloway Creek contains more limestone (generally at least 70 percent) and is more irregularly and thinner bedded than the Fairview. The Grant Lake Limestone is composed of nodular-bedded limestone (70-90 percent), interlayered and intermixed with shale; it overlies the Fairview Formation in the northern part of the Blue Grass region and the Calloway Creek Limestone in the western and central parts. In east-central Kentucky, the Grant Lake is classified as a member of the Ashlock Formation, an assemblage of lithologically distinct units that were combined to facilitate mapping in the southeastern and southern part of the region. The Ashlock consists of the following members, in ascending order: The Tate (calcitic and dolomitic mudstone), the Grant Lake, the Gilbert (micrograined limestone and shale), the Stingy Creek (nodular-bedded mudstone and limestone), the Terrill (dolomitic and calcitic mudstone), the Sunset (micrograined limestone), and the Reba (nodular-bedded limestone and shale). The Bull Fork Formation, which overlies the Grant Lake Limestone, is made up of subequal amounts of thin-bedded highly fossiliferous limestone and shale; limestone makes up about 80 percent of the basal part of the formation and decreases in abundance irregularly upward to only 20 percent of the top part. On the east side of the Blue Grass region, the Bull Fork grades into the Reba Member of the Ashlock Formation; on the west side, it grades into the Grant Lake. The uppermost formation in the region is the Drakes Formation, which in east-central Kentucky consists of the Rowland Member (calcitic to dolomitic mudstone) overlain by the Preachersville Member (dolomitic to calcitic mudstone and dolomite and dolomitic siltstone). In northeast Kentucky, the Drakes is represented by only the Preachersville Member. In most of central and north-central Kentucky, the formation consists of three members: the Rowland at the base (dolomitic mudstone to muddy limestone), the Bardstown (fossiliferous limestone and shale), and the Saluda Dolomite (dolomite, in part calcitic and muddy). In northern north-central Kentucky, the Drakes is represented by only the Saluda Dolomite Member.

Kentucky↗
USGS1983

Stratigraphy of the Silurian outcrop belt on the east side of the Cincinnati Arch in Kentucky, with revisions in the nomenclature

Silurian rocks form a narrow arcuate outcrop belt about 100 mi long on the east side of the Cincinnati Arch in Kentucky. They range from as much as 300 ft thick in the north to a pinchout edge in the south. The nomenclature of this sequence is revised to reflect mappability and lithologic uniformity on the basis of detailed mapping at a scale of 1:24,000 by the U.S. Geological Survey in cooperation with the Kentucky Geological Survey. The Silurian rocks are divided into two parts: the Crab Orchard Group, raised in rank from Crab Orchard Formation and redefined, in the lower part of the Silurian section, and Bisher Dolomite in the upper part of the section. The Crab Orchard Group is subdivided into the Drowning Creek Formation (new name) at the base of the Silurian, overlain by the Alger Shale (adopted herein) south of Fleming County and by the Estill Shale (elevated to formational rank) north of Bath County. The Brassfield Member (reduced in rank from Brassfield Dolomite or Formation) and the Plum Creek Shale and Oldham Members of the former Crab Orchard Formation are included as members of the Drowning Creek; the Lulbegrud Shale, Waco, and Estill Shale Members of the former Crab Orchard Formation are now included in the Alger. The Drowning Creek Formation, 20 to 50 ft thick, is composed mainly of gray fine to coarse-grained dolomite with shale interbeds. The dolomite beds average several inches thick, with bedding surfaces that are locally smooth but generally irregular and are fossiliferous in many places; fossils include brachiopods, crinoid columnals, horn corals, colonial corals, trilobites, pelecypods, and bryozoans. The shale interbeds average several inches thick, except for its Plum Creek Shale Member which is entirely shale and as much as 12 ft thick, and are most abundant in the upper half of the formation. The members of the Drowning Creek intergrade and are indistinguishable in the northern part of the area. The Alger Shale, as much as 170 feet thick, is predominantly grayish-green clay shale with a thin (0.5-3 ft) dolomite member (the Waco, or its northern equivalent, the Dayton Dolomite Member, reduced in rank from Dayton Limestone) near the base. North of Bath County, the Lulbegrud Shale and Dayton Dolomite Members are reassigned to the underlying Drowning Creek Formation, the Estill Shale Member is elevated to formational status, and the Alger is dropped. The Bisher Dolomite, which overlies the Estill Shale in the northernmost part of the Silurian belt, ranges from 0 to 300 ft in thickness and is composed of medium-to coarse-grained, gray, fossiliferous dolomite. The Silurian section overlies Upper Ordovician rocks in apparent conformity, although faunal studies suggest a minor hiatus, and is overlain by Middle to Upper Devonian rocks in a regional angular unconformity that truncates the entire Silurian section at the southwest end of the outcrop belt, where it is nearest the axis of the Cincinnati Arch. All of the units recognized in the Silurian appear to thicken eastward, away from the axis of the arch and towards the Appalachian basin. This, with the presence of isolated remnants of the Brassfield near the axis, suggest that formation of the arch was initiated in Early Silurian time by subsidence of its eastern flank.

Professional Paper↗
USGS1984

Sandstone units of the Lee Formation and related strata in eastern Kentucky

Most of the Cumberland Plateau region of southeastern Kentucky is underlain by thick sequences of quartzose sandstone which are assigned for the most part to the Lee Formation. Much new information has been gathered about the Lee and related strata as a result of the cooperative mapping program of the U. S. Geological Survey and the Kentucky Geological Survey between 1960 and 1978. This report summarizes the age, lithology, distribution, sedimentary structures, and stratigraphic relations of the sandstone units of the Lee within and between each of three major outcrop belts in Kentucky: Cumberland Mountain, Pine Mountain, and the Pottsville Escarpment area. The Lee Formation generally has been regarded as Early Pennsylvanian in age and separated from Mississippian strata in Kentucky by an unconformity. However, lithostratigraphic units included in the formation as presently defined are broadly time-transgressive and range in age from Late Mississippian in parts of the Cumberland Mountain outcrop belt to Middle Pennsylvanian in the Pottsville Escarpment area. Members of the Lee intertongue with and grade into the underlying Pennington Formation and overlying Breathitt Formation. Sandstone and conglomeratic sandstone members of the Lee of Mississippian age found only in parts of the Cumberland overthrust sheet are closely associated with marine rocks; Pennsylvanian members are mostly associated with continental coal-bearing strata. Sandstone members of the Lee are mostly quartz rich and range from more than 90 percent to more than 99 percent quartz. They are relatively coarse grained, commonly pebbly, and in places conglomeratic. The units are southwest-trending linear or broadly lobate bodies. The Lee Formation is as much as 1,500 ft thick in the type area in Cumberland Mountain where it has been divided into eight members. The Pinnacle Overlook, Chadwell, White Rocks Sandstone, Middlesboro, Bee Rock Sandstone, and Naese Sandstone Members are mostly quartzose sandstone and conglomerate. The Dark Ridge and Hensley Members are mostly shale, siltstone, thin-bedded silty sandstone, and coal. The lower three of these members, the Pinnacle Overlook, Chadwell, and White Rocks Sandstone, are assigned to the Upper Mississippian Series because they intertongue with marine reddish or greenish shale and siltstone of the Pennington Formation or equivalent strata that contain a Late Mississippian fauna. The overlying quartzose sandstone members of the Lee commonly have coalified plant remains and impressions of plants and are Early to Middle Pennsylvanian in age; they are generally associated with terrestrial shale and siltstone containing coal beds and pinch out eastward into subgraywacke, siltstone, and shale. Although marine members commonly are bimodal, resultant transport directions for both marine and terrestrial members are southwesterly as determined by crossbedding. Thickness variations of the Middlesboro Member in the Cumberland overthrust sheet suggest that it represents tills of at least three major southwesterly trending paleovalleys. Thickness variations of the Bee Rock Sandstone Member east of Rocky Face fault and the combined Bee Rock and Naese Sandstone Members west of Rocky Face fault suggest that these members represent tills of at least two major southwesterly trending paleovalleys. East of Rocky Face fault, the Bee Rock is generally the uppermost member of the Lee; west of the fault, the overlying Naese is at the top. The Naese may range in age from Early to Middle Pennsylvanian and is partly or wholly equivalent to the Rockcastle Sandstone member of the Lee Formation in the area of the Pottsville Escarpment. The Mississippian-Pennsylvanian systemic boundary in the area of the Cumberland overthrust sheet in most places has been placed at an unconformity at the base of the Middlesboro Member; locally it is projected at the base of shales of the underlying Dark Ridge Member or equivalent strata in the Penningto

Professional Paper↗
USGS1980

Potassium-argon geochronology of the eastern Transverse Ranges and southern Mojave Desert, southern California

More than 200 potassium-argon apparent ages on minerals from crystalline rocks, chiefly from the San Bernardino and eastern San Gabriel Mountains and the southern Mojave Desert, define an area greater than 10,000 km 2 in which the potassium-argon isotopic systematics have been highly disturbed. The disturbance or disturbances appear to have culminated at different times in different parts of the region, ranging from 57 m.y. ago in the eastern San Gabriel Mountains to about 70 m.y. ago in the southern Mojave Desert. The region can be subdivided into three parts on the basis of potassium-argon dating: (1) An inner area of anomalous ages in which the rocks yield apparent potassium-argon ages that indicate complete or nearly complete resetting of the isotopic system. (2) An outer area in which the rocks yield apparent ages that are, or approach, emplacement ages. (3) A zone separating these two areas from which rocks yield discordant apparent ages on coexisting mineral pairs. This discordant zone varies in width from about 6 to 12 km and grades inward to rocks reset to the degree that they yield concordant potassium-argon apparent ages on coexisting mineral pairs and outward toward rocks that yield near-concordant apparent ages. Rocks from the center and the inner parts of the discordant zone yield the most discordant apparent ages. Contouring of the apparent ages defines the extent of the reset region that occurs on both sides of the San Andreas fault. The apparent ages can be contoured across the fault, although the position of the fault is well defined by abrupt deflection of the contours parallel to the fault. The reverse fault bounding the north side of the San Bernardino Mountains mayor may not be reflected by offset contours; correlation of possible offset features across the fault is uncertain. Several northwest-trending faults on the Mojave Desert strongly disrupt the contours but do not show the right-lateral displacements that have been attributed to them on the basis of apparent offsets of geologic features. These faults may have a component of vertical movement, and it is not known what effect this might have on the contours; even a small amount could be profound. The cause of the isotopic disturbance is not well understood, as the area of most complete resetting does not appear to be coincident with any single batholithic mass. The apparent-age contours cross the boundaries of individual plutons, and the configuration of the contours shows no apparent relation to the shapes of plutons or groups of plutons. Even though there does appear to be a lack of correlation between individual plutons or batholith-size collections of plutons, this lack of correlation may be more apparent than real. It is possible that some of the plutons within the area of anomalous ages are part of an extensive batholithic mass of which only the uppermost part is exposed. An alternative interpretation of the anomalous ages is that a continuing post-emplacement heat source generated by continued under thrusting of the Pacific plate beneath North America caused the region to remain at elevated temperatures such that argon retention by minerals datable by potassium-argon methods was not possible for some time after pluton emplacement. As suggested by Coney and Reynolds (1977), possibly the angle of underthrusting was shallower than when the Transverse Range plutons were emplaced. Locally, the youngest ages show a spatial relation to the Vincent thrust fault and its correlatives. If this fault is the cause of the region-wide resetting, it has to have extended under most of the western part of southern California. The configuration of the anomalous-age zone is roughly coincident with the position of the Transverse Ranges, the recently discovered southern California uplift, a relatively shallow high-velocity zone in southern California, and the projection of the Murray fracture zone. The eastward extent coincides roughly with the eastern limit of high seismicity in southern California.

California↗