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At least 271 records · Page 15Linked to original sources

USGS2019

Lava lake thermal pattern classification using self organizing maps and relationships to eruption processes at Kilauea Volcano, Hawaii

Kīlauea Volcano’s active summit lava lake poses hazards to downwind residents and over 1.6 million Hawai‘i Volcanoes National Park visitors each year. The lava lake surface is dynamic; crustal plates separated by incandescent cracks move across the lake as magma circulates below. We hypothesize that these dynamic thermal patterns are related to changes in other volcanic processes, such that sequences of thermal images may provide information about eruption parameters that are sometimes difficult to measure. The ability to learn about current gas emissions and seismic activity from a remote thermal time-lapse camera would be beneficial when conditions are too hazardous for field measurements. We apply a machine learning algorithm called self-organizing maps (SOM) to thermal infrared time-lapse images of the lava lake collected hourly over 23 April – 21 October 2013 (n=4354). The SOM algorithm can take thousands of seemingly different images, each representing the spatial distribution of relative temperature across the lava lake surface, and group them into clusters based on their similarities. We then relate the resulting clusters to sulfur dioxide emissions and seismic tremor to characterize ties between the SOM classification and different emplacement conditions. The SOM classification results are highly sensitive to the normalization method applied to the input images. The standard pixel-by-pixel normalization method yields a cluster of images defined by the highest observed SO2 emission levels, elevated surface temperatures, and a high proportion of cracks between crustal plates. When lava lake surface patterns are isolated by minimizing the effect of temperature variation between images, relationships with seismic tremor activity emerge, revealing an “intense spatter” cluster, characterized by unstable, broken-up crustal plate patterns on the lava lake surface. This proof of concept study provides a basis for extending the SOM classification method to hazard forecasting and real-time volcanic monitoring applications, as well as comparative studies at other lava lakes.

Hawaii↗

USGS2021

Middle and late Pleistocene pluvial history of Newark Valley, central Nevada, USA

Newark Valley lies between the two largest pluvial lake systems in the Great Basin, Lake Lahontan and Lake Bonneville. Soils and geomorphology, stratigraphic interpretations, radiocarbon ages, and amino acid racemization geochronology analyses were employed to interpret the relative and numerical ages of lacustrine deposits in the valley. The marine oxygen isotope stage (MIS) 2 beach barriers are characterized by well-preserved morphology and deposits with youthful soil development, with Bwk horizons and maximum stage I+ carbonate morphology. Radiocarbon ages of gastropods and tufas within these MIS 2–age deposits permit construction of a latest Pleistocene lake-level curve for Newark Valley, including a maximum limiting age of 13,780 ± 50 14 C yr B.P. for the most recent highstand, and they provide a calibration point for soil development in lacustrine deposits in the central Great Basin. The MIS 8–age to MIS 4–age beach barriers are higher in elevation and represent a larger lake than existed during MIS 2. The beach barriers have subdued morphology, are only preserved in short segments, and have stronger soil development, with Bkm and/or Bkmt horizons and maximum stage III+ to IV carbonate morphology. Newark Lake reached elevations higher than the MIS 2 highstand during at least two additional pluvial periods, MIS 16 and MIS 12, 10, or 8. These oldest lacustrine deposits do not have preserved shoreline features and are represented only by gravel lags, buried deposits, and buried soils with similar strong soil development. This sequence of middle and latest Pleistocene shorelines records a long-term pluvial history in this basin that remained internally drained for the last four or more pluvial cycles. Obtaining numerical ages from material within lacustrine deposits in the Great Basin can be challenging. Amino acid D/L values from gastropod shells and mollusk valves proved to be a valuable tool to correlate lacustrine deposits within Newark Valley. Comparison of soils and geomorphology results to independent 36 Cl cosmogenic nuclide ages from a different study indicated unexpected changes in rates of soil development during the past ~200,000 yr and suggested that common stratigraphic changes in lake stratigraphy could obscure incremental changes in soil development and/or complicate 36 Cl cosmogenic nuclide age estimates.

Nevada↗

USGS2020

Deglaciation of the Puget Lowland, Washington

Recently obtained radiocarbon ages from the southern Puget Lowland and reevaluation of limiting ages from the Olympic Peninsula in the light of new light detection and ranging (LiDAR) data suggest that the Juan de Fuca and Puget lobes of the Cordilleran ice sheet reached their maximum extents after 16,000 calibrated yr B.P. Source areas for both lobes fed through a common conduit, likely requiring that downstream responses to changes in either source area were similar. Dates for ice-sheet retreat are sparse and contradictory, but they suggest that retreat was rapid. Depositional and geomorphic evidence shows that retreat of the Juan de Fuca lobe predated retreat of the Puget lobe. No recessional end moraines have been identified in the Puget Lowland, in contrast to numerous recessional end moraines constructed by the Okanogan lobe east of the Cascade Range, and in contrast to later ice-sheet retreat in western Whatcom County north of the Puget Lowland. These observations lead to the hypothesis that collapse of the Juan de Fuca lobe, hastened by the instability of a marine-based ice sheet, steepened the ice-sheet surface over the eastern Strait of Juan de Fuca and diverted ice flow upstream of the Puget lobe to the west. Starved of ice, the Puget lobe retreated quickly.

Washington↗

USGS2022

Magmatism, migrating topography, and the transition from Sevier shortening to Basin and Range extension, western United States

The paleogeographic evolution of the western U.S. Great Basin from the Late Cretaceous to the Cenozoic is critical to understanding how the North American Cordillera at this latitude transitioned from Mesozoic shortening to Cenozoic extension. According to a widely applied model, Cenozoic extension was driven by collapse of elevated crust supported by crustal thicknesses that were potentially double the present ~30–35 km. This model is difficult to reconcile with more recent estimates of moderate regional extension (≤50%) and the discovery that most high-angle, Basin and Range faults slipped rapidly ca. 17 Ma, tens of millions of years after crustal thickening occurred. Here, we integrated new and existing geochronology and geologic mapping in the Elko area of northeast Nevada, one of the few places in the Great Basin with substantial exposures of Paleogene strata. We improved the age control for strata that have been targeted for studies of regional paleoelevation and paleoclimate across this critical time span. In addition, a regional compilation of the ages of material within a network of middle Cenozoic paleodrainages that developed across the Great Basin shows that the age of basal paleovalley fill decreases southward roughly synchronous with voluminous ignimbrite flareup volcanism that swept south across the region ca. 45–20 Ma. Integrating these data sets with the regional record of faulting, sedimentation, erosion, and magmatism, we suggest that volcanism was accompanied by an elevation increase that disrupted drainage systems and shifted the continental divide east into central Nevada from its Late Cretaceous location along the Sierra Nevada arc. The north-south Eocene–Oligocene drainage divide defined by mapping of paleovalleys may thus have evolved as a dynamic feature that propagated southward with magmatism. Despite some local faulting, the northern Great Basin became a vast, elevated volcanic tableland that persisted until dissection by Basin and Range faulting that began ca. 21–17 Ma. Based on this more detailed geologic framework, it is unlikely that Basin and Range extension was driven by Cretaceous crustal overthickening; rather, preexisting crustal structure was just one of several factors that that led to Basin and Range faulting after ca. 17 Ma—in addition to thermal weakening of the crust associated with Cenozoic magmatism, thermally supported elevation, and changing boundary conditions. Because these causal factors evolved long after crustal thickening ended, during final removal and fragmentation of the shallowly subducting Farallon slab, they are compatible with normal-thickness (~45–50 km) crust beneath the Great Basin prior to extension and do not require development of a strongly elevated, Altiplano-like region during Mesozoic shortening.

Nevada↗

USGS2022

Downhill from Austin and Ely to Las Vegas: U-Pb detrital zircon suites from the Eocene–Oligocene Titus Canyon Formation and associated strata, Death Valley, California

In a reconnaissance investigation aimed at interrogating the changing topography and paleogeography of the western United States prior to Basin and Range faulting, a preliminary study made use of U-Pb ages of detrital zircon suites from 16 samples from the Eocene–Oligocene Titus Canyon Formation, its overlying units, and correlatives near Death Valley. The Titus Canyon Formation unconformably overlies Neoproterozoic to Devonian strata in the Funeral and Grapevine Mountains of California and Nevada. Samples were collected from (1) the type area in Titus Canyon, (2) the headwaters of Monarch Canyon, and (3) unnamed Cenozoic strata exposed in a klippe of the Boundary Canyon fault in the central Funeral Mountains. Red beds and conglomerates at the base of the Titus Canyon Formation at locations 1 and 2, which contain previously reported 38–37 Ma fossils, yielded mostly Sierran batholith–age detrital zircons (defined by Triassic, Jurassic, and Cretaceous peaks). Overlying channelized fluvial sandstones, conglomerates, and minor lacustrine shale, marl, and limestone record an abrupt change in source region around 38–36 Ma or slightly later, from more local, Sierran arc–derived sediment to extraregional sources to the north. Clasts of red radiolarian-bearing chert, dark radiolarian chert, and quartzite indicate sources in the region of the Golconda and Roberts Mountains allochthons of northern Nevada. Sandstones intercalated with conglomerate contain increasing proportions of Cenozoic zircon sourced from south-migrating, caldera-forming eruptions at the latitude of Austin and Ely in Nevada with maximum depositional ages (MDAs) ranging from 36 to 24 Ma at the top of the Titus Canyon Formation. Carbonate clasts and ash-rich horizons become more prevalent in the overlying conglomeratic Panuga Formation (which contains a previously dated 15.7 Ma ash-flow tuff). The base of the higher, ash-dominated Wahguyhe Formation yielded a MDA of 14.4 Ma. The central Funeral Mountains section exposes a different sequence of units that, based on new data, are correlative to the Titus Canyon, Panuga, and Wahguyhe Formations at locations 1 and 2. An ash-flow tuff above its (unexposed) base provided a MDA of 34 Ma, and the youngest sample yielded a MDA of 12.7 Ma. The striking differences between age-correlative sections, together with map-based evidence for channelization, indicate that the Titus Canyon Formation and overlying units likely represent fluvial channel, floodplain, and lacustrine deposits as sediments mostly bypassed the region, moving south toward the Paleogene shoreline in the Mojave Desert. The profound changes in source regions and sedimentary facies documented in the Titus Canyon Formation took place during ignimbrite flareup magmatism and a proposed eastward shift of the continental divide from the axis of the Cretaceous arc to a new divide in central Nevada in response to thermal uplift and addition of magma to the crust. This uplift initiated south-flowing fluvial systems that supplied sediments to the Titus Canyon Formation and higher units.

Arizona, California, Nevada, Utah↗

USGS2022

A refined assessment of the paleoceanographic and tectonic influences on the deposition of the Monterey Formation in California

Application of updated diatom biochronology to the Monterey Formation and related biosiliceous rocks reveals the imprint of both global paleoclimatic/ paleoceanographic and regional tectonic events. A rise in global sea level combined with regional tectonic deepening associated with the development of the transform California margin resulted in the abrupt onset of deposition of fine-grained Monterey sediments between 18 and 16 Ma. The base of the Monterey does not mark a silica shift in diatom deposition from the North Atlantic to the North Pacific. Rather, a North Atlantic decline of diatoms after ~13 Ma and increasing divergence in nutrient levels between the North Atlantic and North Pacific between ~13 and 11 Ma, coincided with a major enhancement of diatom deposition in the Monterey Formation. A stratigraphically condensed interval of phosphate-rich sediments between 13 and 10 Ma in coastal southern California appears to have resulted from sediment starvation on offshore banks during a period of higher sea level, as inland sections commonly contain thick sequences of diatomaceous sediment. Increasing latitudinal thermal gradients in the latest Miocene, which triggered a biogenic bloom in the equatorial Pacific at 8 Ma, also lead to enhanced diatom deposition in the uppermost Monterey and overlying biosiliceous rocks. Uplift of the California coastal ranges after ~5.2 Ma resulted in an increasing detrital contribution that obscured the presence of diatoms in onshore sediments. Major reduction in coastal upwelling in the early Pliocene at ~4.6 Ma caused a drastic reduction of diatoms in sediments of offshore southern California.

California↗

USGS2022

Tephrochronology of the Miocene Monterey and Modelo Formations, California

Tuff beds have been known in the Miocene Monterey and Modelo Formations since the initial descriptions; however, age control and correlation is predominantly biostratigraphy. Here we combine tephrochronology and biostratigraphy in order to provide numerical age control for eight sedimentary sequences of the Monterey and Modelo Formations from Monterey, California to Orange County, California. We correlate 38 tuffs and tephra beds in the Monterey and Modelo Formations to 26 different dated tuffs found mainly in non-marine sequences in Nevada, Idaho and New Mexico. We also include geochemical data for an additional 19 tuffs in the Monterey and Modelo Formations for which there are no known correlative tuffs and geochemical data for 11 additional tuffs in other units that will add to the Miocene tephrostratigraphy. The identified tuffs range in age from 16 to 7 Ma with 31 tuffs erupted from volcanic centers of the Snake River Plain of northern Nevada to eastern Idaho. Twelve other tuffs erupted from the Southern Nevada Volcanic Field, one from the Sonoma Volcanic Field, north of San Francisco, and the eruptive source of 12 other tuffs is uncertain. These tuffs provide useful correlations of marine sequences deposited at varying depths along offshore Miocene California and possible insight into the distribution of air-fall tephra from so-called super eruptions

GSA Special Papers↗

USGS2023

Construction and modification of debris-flow alluvial fans as captured in the geomorphic and sedimentary record: Examples from the western Sangre de Cristo Mountains, south-central Colorado

Debris-flow alluvial fans are iconic features of dynamic landscapes and are hypothesized to record tectonic and climatic change. Here, we highlight their complex formation and evolution through an exemplary suite of Quaternary debris-flow alluvial fans emanating from the western range front of the Sangre de Cristo Mountains in south-central Colorado, USA. To evaluate the constructive and modifying processes that produce fan form and the associated sedimentary signatures, we applied a combined geomorphologic and sedimentologic approach using sedimentary facies analysis, soils mapping, high-resolution topographic data, and luminescence geochronology to document timing of fan construction and modification. We explored two subsets of fans in the study area: a southern set sourced from the extensively glaciated drainages of the Blanca Peak massif, and a northern set from the unglaciated drainages south of Great Sand Dunes National Park. Both sets of fans have: (1) active and successively abandoned surfaces that show evolving degradation of primary features through modification by secondary processes, (2) associated facies that display distinct characteristics representative of primary depositional and secondary modifying sedimentary processes, and (3) evidence of primary debris flow with subsequent modification by secondary processes. We found that surface geomorphology and facies assemblages in exposed alluvial-fan deposits represent sediment transport processes on both active and abandoned lobes. The link between fan surface morphologies and the sedimentary facies of their deposits provides a basis for an evolutionary process–based interpretation of debris-flow alluvial-fan geomorphology and provides a better understanding of complexities in buried paleosurfaces (intraformational progressive unconformities), surficial deformation, and landform development as recorded in debris-flow fan deposits in the sedimentary record.

Colorado↗

USGS2007

Exhumation and continental strike-slip fault systems: Introduction

Metamorphic rocks adjacent to and within strike-slip faultsystems occur in a wide range of tectonic settings. Detailed studies show that for a number of these locales a significant part of the exhumation occurred during strike-slip fault motion, but the specific processes involved are often cryptic. Although some sites share characteristic features, such as metamorphic rocks exhumed in extensional step-overs within overall transtensional systems, no one common theme emerges from all of the studies. Our understanding of the variables that control continental strike-slip faults' interaction with mid- to lower-crustal structures is still primitive.

Geological Society of America Special Papers↗

USGS1969

Modern coastal mangrove swamp stratigraphy and the ideal cyclothem

The general stratigraphy of the “ideal” cyclothem of Late Paleozoic age can be recognized in a modern succession of sedimentary units underlying the coastal mangrove swamps of southwestern Florida. Because coal deposition is associated with the formation of cyclothems, this stratigraphic similarity has geologic importance with respect to coal formation. The lower part of the succession in Florida consists of nonmarine sediments, the middle part of brackish-water or fresh-water mangrove peat, and the upper part of brackish-water and marine units. This sequence of sediments records a relative rise in sea level. In comparison, the lower part of the ideal cyclothem consists basically of nonmarine units, the central sedimentary member is coal, and the upper units are brackish-water and marine sediments. The ideal cyclothem is thought to have formed in part in a deltaic environment and to record a periodic fluctuation in terrigenous sediment supply and a relative rise in sea level. In contrast, southwestern Florida has essentially no deltas, as most of its paralic sediments are derived from coastal sources. In view of this, the stratigraphic similarity noted above must reflect a partial duplication of sedimentary environments brought about by a relative rise in sea level across a low coastal platform supporting peat-depositing paralic and fresh-water swamps and forests. This conclusion tends to support the point of view that the coal member of some cyclothems formed in a swampy environment penecontemporaneously with a relative rise in sea level. The coal member, therefore, is in part a transgressive unit.

Florida↗

USGS1969

The Cloudy Pass epizonal batholith and associated subvolcanic rocks

The Cloudy Pass batholith, one of several small epizonal Tertiary batholiths in the Northern Cascade Mountains, discordantly intrudes metamorphic rocks of pre-Late Cretaceous age. The batholith is remarkable for its chilled borders, associated porphyry plugs, and intrusive breccias. The main body of the batholith consists largely of labradorite granodiorite. Part of the northeast side of the batholith is bordered by a complex more than half a mile thick of chilled rocks of Hart Lake that consists of separately injected, contrasting layers of porphyry. At lower levels these are an early, outer layer of dacite; a younger, inner layer of dacite and labradorite-by-townite andesite; and a middle, still younger layer of autobreccia compositionally similar to the inner layer. Contacts between layers are complex and consist at lower levels of intermixed zones, suggesting that at these levels the early rocks were still molten when injected by the later rocks. At higher levels the younger rocks split into separate dikes, and contacts between younger and older rocks are sharp, suggesting that at these levels the older rocks had solidified before they were intruded by the younger rocks. The border complex is thought to have been cooled largely by expanding gases which were released from the batholith and which escaped to the surface through this zone. The contact between the labradorite granodiorite and the inner layer is gradational locally, but in most places the granodiorite intrudes the inner layer, and the contact is sharp. Where the middle and inner layers of the complex are absent, the contact of granodiorite and dacite of the outer layer is gradational. Porphyry plugs which puncture the adjacent metamorphic rocks, although more siliceous than the rocks of the complex, consist largely of dacite and are petrographically indistinguishable from dacite of the complex. Intrusive breccias are of two types: one consists of rounded fragments of batholithic rocks in a “marble cake” mixture of calcic quartz diorite and white quartz monzonite and is confined to the core of the batholith; the other consists of fragments of batholithic rocks and gneiss in a matrix of varying proportions of igneous and finely comminuted materials and is confined to the porphyry plugs and gneiss. This second type seems to have been injected explosively, probably accompanying “second boiling” of the labradorite granodiorite. Plagioclases range from high- to low-temperature varieties. The transitional and high-temperature plagioclases are confined to the deuterically least-altered parts of the chilled margins. The low-temperature plagioclase occurs in the batholithic core and the complex of Hart Lake, and is thought to have inverted from an original high-temperature form because of long-continued existence at elevated, although subsolidus, temperatures, or because of the action of volatile constituents, or both. The batholith seems to have made room for itself by lifting its roof, and the complex of Hart Lake developed along the upfaulted eastern side.

Washington↗

USGS1970

Aquilapollenites (Rouse) Funkhouser — Selected Rocky Mountain taxa and their stratigraphic ranges

The genus Aquilapollenites and its type species, A. quadrilobus, are redescribed, and the latter is illustrated. This report summarizes our present information on stratigraphic ranges of selected Rocky Mountain species and varieties of Aquilapollenites. Of the 17 taxa included in this report, 10 are previously named species, 4 are new species, and 3 are new varieties. One species is transferred from the genus Mancicorpus and another from the genus Parviprojectus. The stratigraphic data concerning Aquilapollenites incorporate available information from U.S. Geological Survey pollen collections.

Alaska, Colorado, Montana, North Dakota, South Dak↗

USGS1970

Palynology of the Cretaceous-Tertiary boundary in the northern Rocky Mountain and Mississippi Embayment regions

In both the Rocky Mountain and the Mississippi Embayment regions it is often difficult to distinguish Upper Cretaceous from lower Tertiary rocks on the basis of physical characteristics; the transition can be recognized with relative ease, however, on the basis of abrupt qualitative changes in plant microfossils. Many Cretaceous species vanish and new species appear in the Paleocene. In both regions the Paleocene yields fewer species than does the underlying Cretaceous. A pronounced difference exists between the Late Cretaceous pollen and spore floras of the two regions. The spore-pollen assemblage from the Rocky Mountains is partially characterized by the presence of Aquilapollenites, Proteacidites, Wodehouseia, and an unnamed species of Tricolpites. These taxa are not found in the Late Cretaceous of the Mississippi Embayment region. The embayment region yields Rugubivesiculites, an unnamed species of Araucariacites, and several genera belonging to the Normapolles group that are not found in the Rocky Mountain region. The Mississippi Embayment Cretaceous and Paleocene pollen floras show greater similarities to pollen floras from Europe than to those from the northern Rocky Mountains. The Rocky Mountain pollen floras exhibit closer similarities to the Alaskan and Siberian floras. It is suggested that the floral dissimilarity between the Mississippi Embayment and the northern Rocky Mountain regions may be accounted for by the separation of the Rocky Mountain province from the Mississippi Embayment province during Late Cretaceous time by the great north-trending Cretaceous epeiric sea. It is further suggested that the floral changes across the Cretaceous-Tertiary transition may have been caused by slight climatic changes brought about by uplift or by withdrawal of the tempering influence of the epeiric sea.

Kentucky, Mississippi, Missouri, Montana, Tennesse↗

USGS1970

Introduction

No abstract available.

Special Papers of the Geological Society of Americ↗

USGS1969

Geology and regional metamorphism of some high-grade cordierite gneisses, Front Range, Colorado

Cordierite is common in regional metamorphic gneisses of Precambrian age in the central part of the Front Range. It occurs in discontinuous stratigraphic units that are structurally a minor component, except locally, of the thick succession of biotite gneisses that comprise the widespread Idaho Springs Formation. The rocks have mineral assemblages, that are characteristic of the sillimanite grade of metamorphism. The cordierite occurs in three principal rock types: (1) potassic feldspar-bearing cordierite-garnet-sillimanite-biotite gneiss, (2) cordierite-biotite gneiss, and (3) cordierite-gedrite-biotite gneiss; each type contains several characteristic mineral assemblages. The rock types are gradational and overlap in areal distribution, and mainly owe their diversity in mineralogy to differences in bulk chemical composition. The field relations are consistent with an interpretation that the diverse cordierite rocks were derived from original sedimentary rocks, largely pelitic sediments. The potassic feldspar-bearing cordierite-garnet gneisses were formed from shales that contained more MgO and FeO than the more abundant sedimentary facies that yielded sillimanitic biotite gneisses. Cordierite-gedrite-biotite gneisses contain much aluminum, iron, and magnesium and little sodium and potassium as compared to the other biotite gneisses; they have an extremely low content of minor elements. Although their chemical compositions are unlike those of known modern sediments, the cordierite-gedrite gneisses are considered also to have been derived from sedimentary rocks. The physical properties and chemical compositions of the mineral phases vary somewhat from one rock type to another. Biotite varies systematically in composition, and the changes are closely related to rock type and thus to bulk composition; the MgO/FeO ratios range from 1.7 in the more mafic cordierite-gedrite rocks to 0.49 in potassic feldspar-bearing cordierite-garnet gneisses. Cordierite is magnesium-rich and intermediate in the range of composition of all analyzed cordierites (Leake, 1960); its MgO/FeO ratio is higher in the gedrite-bearing gneisses than in the potassic feldspar-bearing gneisses. The garnets consist dominantly of the almandine and pyrope molecules, and range from 64 to 75 percent almandine and from 14 to 27 pyrope. These crystals are zoned; their rims are slightly more ferrous and less magnesian than their cores. Both monoclinic and triclinic alkali feldspars coexist in the potassic feldspar-bearing cordierite-garnet gneisses. The potassic feldspars contain from 18 to 27 weight percent NaAlSi 3 O 8 . Plagioclase (oligoclase-andesine) is uncommon in the rocks. Gedrite has an MgO/FeO ratio ranging from 1 to 1.2. Associated minor minerals include iron oxides, andalusite, spinel and its alteration product högbomite, and corundum. The mineral assemblages can be correlated imperfectly with episodes of deformation and metamorphism. Relict staurolite and associated garnet occur locally as remnants of an assemblage formed early in regional metamorphism, presumably early in the first period of deformation. The dominant assemblage biotite-cordierite-garnet-magnetite-plagioclase-potassic feldspar-quartz-sillimanite and associated assemblages having fewer phases, were formed during period one and period two deformations, the principal episodes of regional dynamothermal metamorphism in the central part of the Front Range. A minor assemblage andalusite-biotite-magnetite-plagioclase-quartz was formed later, possibly coincident with a third period of deformation, largely cataclastic in effects, which was more local than the earlier deformations and metamorphism. Phase equilibria studies of the assemblage biotite-cordierite-garnet-magnetite-plagioclase-potassic feldspar-quartz-sillimanite and associated assemblages are interpreted to indicate that the cordierite assemblages approach a state of chemical equilibrium. The scatter of points in a distribution diagram can be interpreted in terms of at least two sets of equilibrium conditions that prevailed during the major plastic deformations. Other discrepancies indicating departure from a homogeneous equilibrium can be explained as a result of mosaic equilibrium involving limited diffusion of iron and magnesium for short distances. The mineral assemblages and the compositions of the ferromagnesian minerals in the cordierite rocks of this region are dependent primarily on the bulk composition of the rocks and variations in the mineral species that comprise the rocks and, to a lesser degree, on the grade of metamorphism. Biotite and cordierite are markedly more magnesian in the more mafic cordierite-gedrite-biotite gneiss than in the potassic feldspar-bearing cordierite-garnet-sillimanite-biotite gneiss. Associated microcline gneiss and biotite-sillimanite gneiss that contains muscovite as a primary stable mineral provides a means to define the metamorphic grade in the area of study. It is concluded from analyses of the assemblages with respect to theoretical phase relations in the system SiO 2 -Al 2 O 3 -Na 2 O-K 2 O-H 2 O that at least some of the rocks in the Central City-Nederland area are above the sillimanite-potassic feldspar isograd as defined by Evans and Guidotti (1966). In rocks of appropriate composition, muscovite is a stable phase in assemblages containing potassic feldspar and sillimanite. The cordierite assemblages and associated rocks are inferred to have formed in an environment having a load pressure of 3–5 kilobars (fluid pressure equaled load pressure) and a temperature somewhat in excess of 620° C.

Colorado↗

USGS1982

Regional ground-water flow concepts in the United States: Historical perspective

A number of important ideas, developed during the past 100 years, form the framework of the present understanding of regional ground-water flow. The most important of these ideas are: Differences in topographic elevation provide the principal driving force for regional flow. Flow through confining layers forms an essential element of regional flow systems. Chemical evolution within the flow systems can be used to understand the flow. Moving ground water is an efficient transport mechanism for heat within the Earth. We trace the evolution of these ideas in the United States and demonstrate their influence on the present-day understanding of flow systems with examples taken primarily from the American literature.

Special Paper of the Geological Society of America↗

USGS1982

Collision of astronomically observable bodies with the Earth

There are at present about 1,000 Earth-crossing bodies of asteroidal appearance that have diameters greater than 1 km. It is calculated that on the average about 3 of these bodies impact the Earth every million years. Because there are many more small bodies than large ones, impacts of 10-km-diameter objects, as postulated by Alvarez and others to explain the Cretaceous/Tertiary extinction, occur less frequently. Nevertheless, it is expected that the frequency of impact of these larger bodies will be about once every 40 million years. The cratering record on the Earth and the Moon is in agreement with this estimate. It is likely that bodies as large as 20 km in diameter have struck the Earth during the last 3 billion years. A somewhat smaller but possibly comparable impact rate of active comet nuclei of similar size is also expected. The bodies of asteroidal appearance represent a quasi-steady-state population. Losses by collision and perturbation out of the solar system are balanced by supply on a 10 7 to 10 8 year time scale of new objects: asteroidal fragments and extinct comet nuclei. The relative importance of these two sources is at present uncertain.

Special Papers of the Geological Society of Americ↗

USGS1982

Iridium abundance measurements across the Cretaceous/Tertiary boundary in the San Juan and Raton Basins of northern New Mexico

During the past year we have been measuring trace element abundances and searching for anomalously high iridium (Ir) concentrations in continental sedimentary rocks that span the Cretaceous-Tertiary boundary in the Raton and San Juan Basins of northern New Mexico and southern Colorado. Using neutron activation and radiochemical separations, we have identified anomalous concentrations of Ir in samples from two sites in the Raton Basin: in a drill core at York Canyon, about 50 km west of Raton, New Mexico, and in a road cut near the city of Raton. In both cases the anomaly occurs essentially at the base of thin coal beds, across a thickness span of only a few cm and at the same level at which several species of Cretaceous pollen become extinct and the ratio of angiosperm pollen to fern spores drops sharply. The Ir surface density ranges from 8 to 40 × 10 −9 g cm −2 . In the York Canyon core the Ir concentration reaches a value of 5.6 × 10 −9 g/g of rock over a local background of about 10 −11 g/g; the Pt abundance distribution is similar to that for Ir, while Au reaches its maximum concentration about 10 cm below the Ir peak. Se, V, Cr, Mn, Co, and Zn are about two-fold more abundant at the anomaly zone than in adjacent zones, and mass spectrometric 244 Pu analysis showed the 244 Pu/Ir atom ratio ⩽ 1 × 10 7 . In the San Juan Basin we have located a small Ir spike (55 × 10 −12 g/g over a local background of 8 × 10 −12 g/g) that is accompanied by high concentrations of Co and Mn. It is thought to be due to geochemical enrichment processes.

Colorado, New Mexico↗