Geology Reports⌕ Search

SEARCH · Geology Reports

Results for “Names”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 919 records · Page 51Linked to original sources
USGS1947

Delineation of parallel folds and measurement of stratigraphic dimensions

The delineation of parallel folds in structural sections, and the extraction therefrom of stratigraphic information, has generally been done with considerable personal interpretation. If profiles must be drawn, or sections measured, from structural observations used in pairs, this is unavoidable; but superior results may be obtained if more than two observations are simultaneously utilized. The first section of this paper is an exposition of the method of evolute and involutes, which is applicable if three or more observations are available, lying in or close to a profile plane that is normal to the strike of a series of folded rocks. Parallel curves, which in certain sections represent the traces of parallel stratigraphic surfaces, are necessarily involutes that may be generated from one or more evolutes. It is more practical to derive an evolute, and to construct from it a set of parallel curves, than it is to draw such curves directly. Simple graphical methods are given for the construction of evolutes from different sets of structural data, and for the subsequent derivation of parallel curves. An examination of the resulting evolutes and involutes shows that most of them may be represented by the equation y = ax n , if suitable values are assigned to the parameters a and n. The second section of the paper is an exposition of methods that apply to the measurement of stratigraphic thickness, or other stratigraphic dimensions, if structural observations must be used in pairs. Four methods are discussed, which are known as the method of mean strikes and dips, the method of integrated trigonometric functions, the method of skew-line normals, and the method of integrated strikes and dips. The last named of these is a new method, which yields a mean value for the strike or dip, utilizing indirectly the concept of concentric arcs. A formula for mean dip (or mean strike) is derived, which has been computed for all values from 0° to 90°, at intervals of 5°. The results of this computation are given in a chart, which is used for the graphical computation of these values and for interpolation to less than 5°. The mean values of strike and dip that are thus obtained are substituted in any formula for stratigraphic dimensions that applies to a homoclinal sequence of rocks. Under the topic of Errors and Differences, it is shown that the error resulting from the application of the method of evolute and involutes is small and is dependent mainly upon original errors in the determination of strike and dip. When observations are used in pairs, however, the resulting error may be much larger. If certain enumerated conditions are favorable, this error may be 10 per cent or less; but under unfavorable conditions, it may be 100 per cent or more. © 1947, The Geological Society of America, Inc.

Geological Society of America Bulletin↗
USGS1948

Mineralogy and petrology of the currant creek magnesite deposits and associated rocks of Nevada

Cryptocrystalline magnesite occurs as relatively small dense, white masses in a Tertiary volcanic tuff, here named the Currant tuff, in White Pine and Nye counties, 29 miles southwest of Ely, Nevada. The deposits are small, but some of the magnesite is of very high quality with almost no iron or aluminum. However, much of the magnesite is mixed with dolomite and calcite, and a serpentinelike mineral occurs in one of the deposits in sufficient abundance to be of possible use in ceramics. In scattered areas the tuff has been altered by solutions rich in magnesium and bicarbonic acid to dolomite, magnesite, magnesium silicate, and calcite. Mineral and chemical characteristics are the same throughout the entire rock assemblage of flows and tuffs, suggesting that all the rocks have been derived from the same parent magma. The tuff formation, which ranges in thickness from a few feet to over 400 feet, occurs between two groups of volcanic flows here referred to as the lower volcanics and the upper volcanics. The lower volcanics is composed mainly of flows of hypersthene dacite, but in the southwestern part of the area flows of a hypersthene andesite of balsaltic habit occur between the dacite and the overlying Currant tuff. The upper volcanics which overlie the Currant tuff consist of porphyritic quartz latite and an overlying massive latite crystal tuff. The Currant tuff containing the magnesite has both unmodified and reworked tuff members. The formation of hydrothermal dolomite, magnesite, and deweylite is discussed in detail, and the physical chemistry of solutions containing calcium, magnesium, and carbon dioxide is applied in an explanation of the origin of magnesium-rich solutions.

Nevada↗
USGS1953

Beaverhead formation, a Laramide deposit in Beaverhead County, Montana

The name Beaverhead formation is proposed for a thick sequence of conglomerate, sandstone, siltstone, and limestone that crops out over an area of at least 400 square miles in Beaverhead County, Montana, extends southward across the Montana-Idaho boundary, and may extend eastward into Madison County. These rocks are clearly sedimentary by-products of the Laramide orogeny and probably range from late Cretaceous to early Eocene. The Beaverhead formations consists predominantly of conglomerate. In part of the area, an upper and a lower conglomerate member are separated by a middle member of limestone. Where the limestone member is inconspicuous or absent, the upper and lower conglomerate members cannot be differentiated. The most nearly complete and best-exposed known section of the Beaverhead formation, designated the type section, is near the mouth of McKnight Canyon, 6 miles west of Dell, Montana. Here the formation can be divided into four mappable units; elsewhere no more than three units can be recognized. In the McKnight Canyon section, the top and bottom members are dominantly conglomerate, composed of pebbles, cobbles, and subordinate boulders set in a sandy matrix cemented by calcite; breccia beds occur locally. This coarse debris was derived from rocks of Precambrian, Paleozoic, and Mesozoic age and consists largely of limestone and quartzite. The intermediate member consists of two mappable units: a lower thick, massive limestone, locally concretionary, and an upper sequence of interbedded siltstone, sandstone, arkose, limestone, and subordinate conglomerate. At McKnight Canyon, where the base and the top of the formation have been faulted and eroded, the exposed thickness of the section is approximately 9700 feet. The Beaverhead formation rests unconformably on rocks as young as the Colorado group and as old as early Paleozoic, and it probably rests unconformably upon rocks as young as the Montana group and as old as Precambrian. It is unconformably overlain by vertebrate-bearing fluviatile or lacustrine tuffaceous beds of Eocene and Oligocene age. The coarse debris that composes the formation was eroded from nearby mountains that were uplifted in Late Cretaceous, Paleocene, and early Eocene time, and was deposited in basins adjacent to these mountains. The resulting rocks, a product of Laramide orogeny, were later folded and displaced by overthrusting and block faulting. In places, these rocks are overlain by thrust sheets of Paleozoic rocks. © 1953, The Geological Society of America, Inc.

Montana↗
USGS1960

Foothills fault system, western Sierra Nevada, California

A large fault system , here named the Foothills fault system , is the dominant structural feature of the western Sierra Nevada . The steeply dipping to vertical component faults trend northwestward through an area about 200 miles long and 30 miles wide north of 37°30' north latitude. The faulted Paleozoic and Mesozoic rocks are overlapped by unfaulted younger rocks, and the total extent of the fault system is not known. It is probably not limited to the western Sierra Nevada . Faults are marked by belts as much as 4 miles wide of cataclastically deformed and recrystallized rocks and by truncated folds. Along one fault , Upper Jurassic rocks are juxtaposed against Paleozoic rocks for at least 100 miles. The direction of fault movement has not been determined. Net displacement on some of the component faults exceeds 3000 feet and may be measurable in miles. Major faults cut beds of Late Jurassic age and are in turn cut by plutonic rocks of probable Late Jurassic and Middle Cretaceous age. Faults that controlled deposition of quartz veins and gold ore bodies of the Mother Lode belt are apparently younger and structurally less important features superimposed on one of the fault zones of the large system .

California, Nevada↗
USGS1961

Late quaternary history of the snake river in the American Falls region, Idaho

While mapping the American Falls region, we found evidence that contributes to the middle Pleistocene to Recent history of the Snake River, and indirectly to the history of overflow of Lake Bonneville. Middle Pleistocene to recent rocks in the valley are mainly lacustrine and fluvial silts and clays, with some sand, gravel, basalt, and a few thin tuff beds. The formation of terraces can be correlated with events both up- and downstream. The Snake River was at least once, and possibly twice, dammed and diverted by eruptions of basalt, resulting in the formation of lakes and deposition of lacustrine beds. A rather flat-lying, thin, but persistent gravel at the base of one lake bed formation may represent a glacial period, possibly Illinoian, during which the Snake River had a large volume. Overflow of water from Lake Bonneville into the Snake River system, by way of the Marsh Creek-Portneuf valley, laid down a deltaic-fluvial deposit here named the Michaud Gravel. At this time the Snake River, greatly augmented by Lake Bonneville overflow, began to cut channels through and around a lava dam. Terraces between Aberdeen, American Falls, and Pocatello were formed during the existence of the lake in which the Michaud Gravel was deposited and by fluvial processes after drainage of the lake. At one stage in the downcutting, bars of huge basalt boulders were built across the mouths of abandoned spillways. Radiocarbon dating and geologic evidence from the area between Preston and Soda Springs, Idaho, suggest that basalt flows diverted the Bear River into Lake Bonneville, perhaps causing it to overflow. This diversion probably occurred about 33,000 years ago. This dating accords with events in the American Falls region.

Idaho↗
USGS1963

Cathedral Cliffs formation, the early acid Breccia unit of northwestern Wyoming

The name Cathedral Cliffs Formation is proposed for the rocks in the Clarks Fork area of northwestern Wyoming that have long been known by the informal designation "early acid breccia." In the Clarks Fork area the Cathedral Cliffs Formation is composed of tuffs, with lesser amounts of volcanic sedimentary rocks and breccias. Its thickness ranges from less than 100 feet to about 1500 feet but more commonly is 500-900 feet. The formation is tentatively considered to be late early Eocene or early middle Eocene. It is underlain by rocks ranging from Precambrian to early Eocene(?) and is overlain unconformably by the early basic breccia of middle Eocene age. Low-angle detachment faulting, which involved the Cathedral Cliffs Formation but not the overlying early basic breccia, has made recognition and correlation of the formation difficult. Blocks and masses of Madison Limestone of Mississippian age were emplaced locally on its upper surface by the Reef Creek detachment fault. The Cathedral Cliffs Formation and the Paleozoic carbonate rocks beneath it, as well as the Reef Creek fault masses on its surface, were then transported southeastward by the Heart Mountain detachment fault. As movement on the Heart Mountain detachment proceeded, the large fault mass broke up into smaller blocks, which separated as movement continued. Consequently the Cathedral Cliffs Formation was distributed in a pattern which gives the appearance of isolated occurrences and erosional remnants. The detached blocks of the Reef Creek fault on the upper surface of the Cathedral Cliffs also were scattered more widely than by their original movement on the Reef Creek fault. Soon after the fault-transported segments of the Cathedral Cliffs Formation ceased moving they were buried beneath the early basic breccia. The unconformity between the early acid breccia and the early basic breccia is thus substantiated in the Clarks Fork area; in the time interval represented, the Reef Creek and Heart Mountain fault masses were emplaced. The Cathedral Cliffs Formation is correlated with the early acid breccia in northern Yellowstone National Park and the upper part of the Reese Formation as mapped by Calvert west of Gardiner, Montana. The volcanic-source area probably is not in the central Yellowstone Park region, but somewhere to the north. © 1963, The Geological Society of America, Inc.

Wyoming↗
USGS1965

Structural geology of aconcagua province and its relationship to the central Valley Graben, Chile

Aconcagua Province is herein divided into three major structural provinces which, for the sake of simplicity, are named the Coastal Cordillera, Central Valley graben , and Andean Cordillera structural provinces to correspond to the three geomorphic provinces recognized farther south. The coastal structural province includes the Coastal Cordillera which is underlain mainly by layered sedimentary and effusive rocks that strike north and dip homoclinally to the east, range from Triassic to Late Cretaceous in age, and are intruded by Cretaceous granodioritic and dioritic rocks. Igneous and metamorphic rocks largely of Paleozoic age comprise the western coastal margin, Along the eastern edge of the province is the Los Angeles fault zone, a wide, poorly defined band of semiparallel, arcuate faults which show downward displacement to the east and which appear to have resulted mainly from intrusion and uplift on the west. The Coastal Cordillera, therefore, may be considered a large horst with an intrusive granodiorite core. The Central Valley graben is bounded on the west by the Los Angeles fault zone and on the east by the Pocuro fault zone. Between these two fault zones is an area 20-30 km wide in which volcanic rocks of Late Cretaceous age are flat lying to gently folded and block faulted. In places, pipelike stocks of andesitic to dioritic igneous rock intrude the area. The Pocuro fault zone is a prominent lineament that marks the eastern limit of the Central Valley at Santiago and has been traced northward for 150 km. It may, however, have a mappable length of more than 1200 km. Vertical displacement downward to the west has been measured near Los Andes to be at least 2000 m. Consequently, this fault zone may rank among the major faults of the world. The Andean structural province is subdivided into the Las Ollas and Juncal subprovinces. The Las Ollas is a mountainous front range that lies east of the Pocuro fault and extends eastward for about 25 km. Upper Cretaceous volcanic strata are gently warped into broad, open, north-trending folds, with local sharp flexures and faulted mainly by normal faults. The eastern part of the subprovince is cut by a narrow belt of Tertiary plutonic rocks, some of which are associated with porphyry copper deposits. The Juncal structural subprovince extends eastward into Argentina. It is typified by close, overturned folding, vertical bedding, and thrust faulting. In general, structural deformation increases in intensity from west to east, with imbricate overthrusting to the east in Argentina. Present evidence indicates that major graben formation began during early Tertiary (pre-Miocene) time as a result of tensional stress developed by strain release of earlier compressional forces that folded the Andes. Post-Miocene uplift of the central Andean region renewed tensional stress and opened deep-seated, north-striking fractures and reinitiated volcanism. Most of the active volcanos of Chile may be aligned along such fractures.

Geological Society of America Bulletin↗
USGS1967

Stratigraphy and correlation of the precambrian belt supergroup of the southern Lewis and Clark Range, Montana

Several well-exposed and little-deformed Belt Supergroup sections have been studied in the southern Lewis and Clark Range . In the area studied, the Belt thins eastward or northeastward due both to primary sedimentation and to pre-Middle Cambrian erosion. These rocks can now be more precisely correlated with the well-known sections near Bonner, Helena, and Glacier National Park. In the western part of the area, the Missoula Group is thickest and lithologically intermediate between the sections at Bonner and Marias Pass. Formation names from these two sections are applied in the southern Lewis and Clark Range . The thin Belt sequence in the eastern part of the area includes the lower part of the Missoula Group and older formations that may be traced southward into the Helena, Empire, and Spokane Formations of the Helena area. Consequently, the Helena Dolomite of the Helena area, the Siyeh Formation of the Marias Pass area, and the "Newland Limestone" of the Bonner area are probably lateral equivalents.

Montana↗
USGS1970

Eocene age of the Adak ‘Paleozoic (?)’ rocks, Aleutian Islands, Alaska

In 1948, several specimens identified as the plant genus Annularia, a primitive horsetail of Pennsylvanian or Permian age, were found in tuffaceous sandstone exposed near the northern end of Adak Island, Alaska. These beds form the basal part of the Andrew Lake Formation, a newly named sequence of marine sedimentary rocks that is more than 850 m thick, and, in the main, consists of northwest-dipping tuffaceous sandstone, siltstone, shale, and siliceous siltstone and shale interbedded with basaltic flows or penecontemporaneous(?) sills (or both) a few tens of meters thick. This formation rests depositionally(?) on the Finger Bay Volcanics, the massive and intensely altered andesitic and basaltic flows and pyroclastic rocks that form the bulk of Adak Island. Mollusks, foraminifers, sponge spicules, and fish scales and skeletal remains occur in the lower 350 m of the section immediately overlying the basal “Annularia”-bearing beds. Included in this fauna is the pecten Pro-peamussium (cf. P. stanfordensis Arnold), of probable Eocene age; the associated foraminiferal fauna is provincially considered to be of late Eocene (Narizian) age, and the fish scales are similar to those found in the Narizian and Refugian (Eocene and Oligocene) of California. Examination of the matrix surrounding specimens of “Annularia” revealed a substantial dinoflagellate flora—establishing that the “Annularia”- bearing beds are themselves marine units of middle or late Eocene age. The Andrew Lake Formation probably accumulated in a perched basin along the crestal region of an early Tertiary Aleutian ridge. Accordingly, there is no evidence for a Paleozoic Aleutian ridge. There is only scant evidence that the ridge existed in Mesozoic time.

Alaska↗
USGS1971

Mesozoic granitic rocks in northwestern Nevada: A link between the Sierra Nevada and Idaho batholiths

Extensive areas in northwestern Nevada are underlain by granodiorite and quartz monzonite plutons, as well as less common smaller bodies of quartz diorite. Twenty-six K/Ar age determinations on rocks from this suite range from about 175 to 85 m.y., but most of the plutons are between 105 and 85 m.y. old. This Late Cretaceous intrusive epoch extending from 105 to 85 m.y. ago is here named the Lovelock intrusive epoch. Twenty-three whole-rock chemical analyses show that the granitic rocks of northwestern Nevada form a homogeneous differentiation series with a narrow range in major element distribution. The granitic plutons of northwestern Nevada are chemically and petrographically indistinguishable from granitic intrusives of equivalent age in the Sierra Nevada and Idaho batholiths, and form a link between these two major batholiths.

California, Idaho, Nevada↗
USGS1973

Miocene tholeiitic basalts of coastal Oregon and Washington and their relations to coeval basalts of the Columbia Plateau

Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement. Tholeiitic basalt flows and breccias of Miocene age in western Oregon and Washington form three distinct stratigraphic units. Each unit was erupted from coastal vents marked by dikes and sills of the same composition as associated extrusive rocks. The three coastal basalt units are interbedded with predominantly marine sedimentary rocks of middle to late Miocene age. These units are here named, from older to younger, the Depoe Bay Basalt, Cape Foulweather Basalt, and basalt of Pack Sack Lookout. The three units can be distinguished by their petrographic characteristics. The Depoe Bay Basalt is nonporphyritic; Cape Foulweather Basalt has sparse large labradorite phenocrysts; and Pack Sack basalt has labradorite phenocrysts with numerous pyroxene and glass inclusions as well as augite and olivine phenocrysts. Chemical analyses of basalts from these three units show that each has a distinct and uniform composition. The Depoe Bay Basalt is characterized by high SiO 2 content; the Cape Foulweather Basalt has high content of total iron, TiO 2 , and P 2 O 5 ; and Pack Sack basalt is marked by relatively high MgO and CaO content. The Depoe Bay Basalt, Cape Foulweather Basalt, and basalt of Pack Sack Lookout on the coast occur in the same stratigraphic order and are essentially the same ages as three basalt units that erupted on the Columbia Plateau. The plateau-derived units are the Yakima and late-Yakima petrographic types of Waters (1961) and the Pomona flow of Schmincke (1967). The virtual identity in chemical composition of the Depoe Bay Basalt and Yakima-type basalt, the Cape Foulweather Basalt and the late-Yakima–type basalt, and the Pack Sack basalt and the Pomona basalt flow indicate that each pair is consanguineous. Fissure vents for the plateau basalt are located in eastern Oregon and Washington and western Idaho more than 500 km east of the coastal vent areas. Thus, a regional mechanism of magma generation or emplacement is required. Three models of magma genesis considered in this report are: (1) partial melting of the subducted Juan de Fuca plate; (2) partial melting along a nearly horizontal shear zone at the base of the American plate; and (3) partial melting within the asthenosphere and fractionation during ascent of the magma.

Oregon, Washington↗
USGS1974

Buried Triassic basin in the central Savannah River area, South Carolina and Georgia

A basin filled with Triassic red beds, located on the South Carolina–Georgia line ∼32 km southeast of Augusta, Georgia, is buried beneath ∼350 m of Coastal Plain sediments. An extensive aeromagnetic survey, seismic refraction and reflection surveys, and geophysical logs and samples from three wells define the extent and character of the basin. This basin, herein named the Dunbarton Triassic basin, is ∼50 km long, 10 km wide, and trends northeast. The northwest margin of the basin is well defined by the aeromagnetic survey, a seismic reflection traverse, and a well that passed through 485 m of Triassic fanglomerate before entering the crystalline metamorphic rocks below. Near the center of the Triassic basin, a well passed through 902 m of maroon Triassic mudstone and sandstone of fluvial origin without penetrating the bottom of the basin. The permeability of the Triassic rock is extremely low, and water-transmitting fractures were not penetrated. Even slight water-level disturbances in the Triassic wells require many years to recover. Total dissolved solids of the water from the Triassic basin are about twice that in the crystalline metamorphic rocks that surround it. The inferred geologic history of this basin is similar to the history of Triassic basins that crop out in North Carolina.

Georgia, South Carolilna↗
USGS1975

Chino Valley formation (Cambrian?) in northwestern Arizona

A thin persistent unit (maximum thickness 13 m) of probable Cambrian age in the Chino Valley region of northwestern Arizona consists of three laterally equivalent, mutually exclusive lithofacies. These are, from west to east, lithic sandstone, pebble to boulder conglomerate, and dolomite. The unit, named the Chino Valley Formation, is younger than middle Middle Cambrian and older than Devonian in age. A Late Cambrian age is probable, although no fossil evidence supports this conclusion. The lithic sandstone and conglomerate facies were derived from two apparently active source areas and were probably deposited in shallow marine water. Uplift to the south or southwest of the area almost reversed the regionally westward-dipping Cambrian paleoslope and exposed sedimentary rocks to erosion. Sediment derived from this terrain formed the lithic sandstone facies. Synchronous movement initiated erosion of the Mazatzal Quartzite (Precambrian) from which the conglomerate facies was derived. The dolomite facies is inferred to have accumulated on a mud flat in the supratidal zone.

Arizona↗
USGS1976

Obsidian hydration dating and correlation of Bull Lake and Pinedale Glaciations near West Yellowstone, Montana

The ages of the last two glaciations near West Yellowstone, Montana, can be calculated by obsidian hydration techniques that are calibrated by K-Ar dating of obsidian-bearing lava flows. The average age of glacial abrasion of obsidian in the Pinedale terminal moraines is about 30,000 yr, with most age measurements between 20,000 and 35,000 yr. For the Bull Lake moraines, it is about 140,000 yr, with most measurements between 130,000 and 155,000 yr. This age for the Bull Lake moraines is also supported by geologic relations that show that the moraines are older than a rhyolite flow dated by K-Ar as 114,500 ± 7,300 yr old (lσ). These obsidian hydration ages of the Pinedale and Bull Lake Glaciations correlate well with the last two cold intervals of the marine record. The age determined for the Bull Lake Glaciation near West Yellowstone antedates the last interglaciation of the marine record, which is commonly correlated with the Sangamon Interglaciation. Our results suggest correlation of the Pinedale Glaciation near West Yellowstone with much or all of the Wisconsin Glaciation and of the Bull Lake with the late Illinoian. This differs with the commonly accepted correlation of the Pinedale with the late (“classical”) Wisconsin and of the Bull Lake with the early Wisconsin. The correlation of Bull Lake with late Illinoian appears equally or more compatible with traditional criteria for correlation, namely comparative soil development and degree of preservation of morainal morphology.

Wyoming, Montana, Idaho↗
USGS1991

Anomalous folds associated with the east-central part of the Garlock Fault, southeast California

The east-central part of the left-lateral Garlock fault, in southeast California, is associated with three broad folds that trend and plunge northeast. The folds, which lie north, south, and astride the fault, postdate deposition of alluvial gravels that at one time formed a continuous northwest-sloping fan; the gravels rest conformably on lake beds that contain the Lava Creek B ash bed (0.62 Ma). The anticline, named after Christmas Canyon, has vertical relief of as much as 100 m. Topographic profiles suggest that the gravels exposed along the synclinal troughs to its southeast (Pilot Knob Valley syncline) and northwest (Teagle Wash syncline) lie at about the same elevations as they did when part of the originally active fan, implying that the synclines are passive results of the anticline development. If so, the original slope of the middle Pleistocene alluvial sediment sheet was about 0.6° northwest. The age and dimensional controls allow calculation of the rates of tilting and horizontal shortening. Corrected tilting rates vary from about 2.5° to 4.7°/m.y.; the total shortening of about 7.5 m developed at a minimum rate of 12.5 m/m.y.

California↗
USGS1993

A late Pliocene to middle Pleistocene pluvial lake in Fish Lake Valley, Nevada and California

The question of whether a pluvial lake existed in Fish Lake Valley, Nevada and California, has been debated for more than 100 yr. New stratigraphic evidence indicates that a lake did exist in this valley at intervals during late Pliocene to middle Pleistocene time. This lake may have drained northward, or it may have been periodically contiguous with a pluvial lake to the north in Columbus Salt Marsh. Proof of the existence of this lake, informally named Pluvial Lake Rennie, is derived from three principal outcrops of shallow-water deposits, two outcrops of deep-water deposits, and several drilling logs. The deposits contain beds of silicic tephra, which provide age control. On the basis of thickness, grain size, major-oxide chemistry of glass shards, and paleomagnetism, three of the shallow-water deposits, including deltaic(?), beach, and siliceous hot-spring sediments, consist mainly of Bishop ash derived from the 0.77 Ma eruption of the Long Valley caldera. A fourth shallow-water deposit(?) is associated with ∼1 Ma Glass Mountain tephra beds. The exposed deep-water deposits consist of green claystone, siltstone, and fine-grained sandstone containing tephra derived from the eruptions of the ∼2.1 Ma tuff of Taylor Canyon and the ∼2.0 Ma Huckleberry Ridge Tuff. The drilling logs record numerous thick beds of clay and sandy clay inferred to be deep-water lacustrine deposits. Pluvial Lake Rennie fluctuated in size and depth beginning prior to 2 Ma and continuing until sometime alter 0.77 Ma. At about 0.77 Ma, the lake had a highstand at an elevation of ∼1,460 m, covered an area of 400-500 km 2 , and had a maximum depth of ∼250 m. The lake level dropped just after the eruption of the Bishop ash, but the lake may have persisted at a lower level until ∼0.5 Ma. No large, long-lived lake existed in Fish Lake Valley in late Pleistocene time, probably due to the increasing rain-shadow effect caused by the relative uplift of the White Mountains and Sierra Nevada in the Pleistocene. These results indicate that the late middle to late Pleistocene history of Pluvial Lake Rennie is similar to that of Lake Tecopa but is quite different from those of Lake Lahontan and Searles Lake.

California, Nevada↗
USGS1994

Origin, transport, and emplacement of an exotic island-arc terrane exposed in eastern Kamchatka, Russia

The regional stratigraphy of eastern Kamchatka includes an exotic, Early-Late Cretaceous ophiolite and Late Cretaceous island-arc volcanic sequence. Integrating the existing geologic and geophysical data, we examine the origin, transport, emplacement, and postemplacement deformation of the island-arc terrane, which is named the Olyutorsky island arc. Results from several paleomagnetic studies consistently indicate that the island-arc terrane originated >1000 km to the south of where it is presently exposed. Although the formative paleolatitudes of the island-arc rocks approximately correspond to the location of the Izanagi-Farallon subduction zone, the age of the volcanic rocks postdates the cessation of Izanagi-Farallon convergence, thus indicating that an unnamed plate or back-arc basin existed in the northwest Pacific during Late Cretaceous time. We examine two possible models for northward transport of the island-arc terrane to Kamchatka: (1) infra-oceanic transport with the Pacific or Kula plates and (2) coastwise translation of the island-arc terrane after accretion to the Eurasian margin far to the south of Kamchatka. For both models, the dominant Eocene and Miocene deformation ages observed in eastern Kamchatka are used as two possible age limits for the cessation of northward transport. Although the observed paleolatitudes from paleomagnetic data correspond best with the infra-oceanic transport model, the provenance of the Paleogene "transport" stratigraphy indicates a near-shore sediment supply. Our preferred interpretation is that the island-arc terrane (1) accreted onto the Eurasian margin concurrent with cessation of island-arc volcanism (Maastrichtian-Danian) and (2) underwent northward coastwise translation along a major strike-slip fault zone ending by middle-late Eocene time (43-50 Ma). It is unclear whether the ophiolite was exposed during arc-continent collision or whether the ophiolite was obducted onto the island arc prior to collision. A consequence of either infra-oceanic transport or coastwise translation is that an open corridor between the western terminus of the Aleutian Arc and Kamchatka must have existed until middle to late Eocene time. Spreading within the Komandorsky Basin, subduction of sea-mounts, and collision of the Aleutian Arc with Kamchatka are proposed to have instigated the second Miocene phase of deformation, which uplifted and reexposed the island-arc terrane.

Kamchatka Peninsula↗
USGS1996

Recurrent eruption and subsidence at the Platoro caldera complex, southeastern San Juan volcanic field, Colorado: New tales from old tuffs

Reinterpretation of a voluminous regional ash-flow sheet (Masonic Park Tuff) as two separate tuff sheets of similar phenocryst-rich dacite erupted from separate source calderas has important implications for evolution of the multicyclic Platoro caldera complex and for caldera-forming processes generally. Masonic Park Tuff in central parts of the San Juan field, including the type area, was erupted from a concealed source at 28.6 Ma, but widespread tuff previously mapped as Masonic Park Tuff in the southeastern San Juan Mountains is the product of the youngest large-volume eruption of the Platoro caldera complex at 28.4 Ma. This large unit, newly named the “Chiquito Peak Tuff,” is the last-erupted tuff of the Treasure Mountain Group, which consists of at least 20 separate ash-flow sheets of dacite to low-silica rhyolite erupted from the Platoro complex during a 1 m.y. interval (29.5−28.4 Ma). Two Treasure Mountain tuff sheets have volumes in excess of 1000 km 3 each, and five more have volumes of 50–150 km 3 . The total volume of ash-flow tuff exceeds 2500 km 3 , and caldera-related lavas of dominantly andesitic composition make up 250-500 km 3 more. A much greater volume of intermediate-composition magma must have solidified in subcaldera magma chambers. Most preserved features of the Platoro complex-including postcollapse asymmetrical trap-door resurgent uplift of the ponded intracaldera tuff and concurrent infilling by andesitic lava flows-postdate eruption of the Chiquito Peak Tuff. The numerous large-volume pre-Chiquito Peak ash-flow tuffs document multiple eruptions accompanied by recurrent subsidence; early-formed caldera walls nearly coincide with margins of the later Chiquito Peak collapse. Repeated syneruptive collapse at the Platoro complex requires cumulative subsidence of at least 10 km. The rapid regeneration of silicic magmas requires the sustained presence of an andesitic subcaldera magma reservoir, or its rapid replenishment, during the 1 m.y. life span of the Platoro complex. Either case implies large-scale stoping and assimilative recycling of the Tertiary section, including intracaldera tuffs.

Colorado↗