USGS Science⌕ Search

SEARCH · USGS Science

Results for “Special Paper”

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 289 records · Page 16Linked to original sources
USGS1984

Late Devonian icriodontid biofacies models and alternate shallow-water conodont zonation

Recognition of differences in the habitats, apparatuses, and ranges of Late Devonian Icriodus and Pelekysgnathus permits refinement of their biofacies interpretations and construction of an alternate icriodontid zonation. Icriodus is a euphotic genus that predominated in most environments during the early Late Devonian (Frasnian) but died out during the early Famennian. Its apparatus consists of platform (I) elements; four larger, acodiniform cones; and two smaller, oneotodiform, scolopodiform, or drepanodiform cones. Pelekysgnathus is a shallow-water genus, which shortly after Icriodus died out, produced somewhat deeper water taxa with triple-rowed I elements that are homeomorphs of Icriodus I elements. Apparatuses for both single-rowed taxa ( Pelekysgnathus ) and triple-rowed taxa ( “Icriodus”) contain oistodiform cones, apparently in place of one or more of the acodiniform cones. Biofacies models for southern Belgium and Utah show that Icriodus lived not only in nearshore environments but ranged into the pelagic palmatolepid-bispathodid (I) and palmatolepid-polygnathid (II) biofacies. Younger “Icriodus”, however, inhabited mainly the deeper subtidal polygnathid-“icriodid” (III) and polygnathid-pelekysgnathid (IV) biofacies. Pelekysgnathus inhabited mainly the polygnathid-pelekysgnathid biofacies and ranged shoreward into the shallow-subtidal clydagnathid (V), scaphignathid (VI), patrognathid (VII), and pandorinellinid (VIII) biofacies, but has not yet been found in the hypersaline antognathid (IX) biofacies (new). The Late Devonian, subdivided by 28 mainly Palmatolepis -based zones from the Lower (Polygnathus) asymmetricus to Upper (Siphonodella) praesulcata Zones in the standard conodont zonation for pelagic biofacies, can be subdivided into nine icriodontid-based zones in nearshore biofacies. In ascending order, these are the Icriodus symmetricus; Lower and Upper Pelekysgnathus planus; Lower, Middle, and Upper “ I .” cornutus ; and Lower, Middle, and Upper “ I .” costatus Zones. Taxonomic revisions involve mainly relegating several previously described species to subspecies and morphotypes and raising some subspecies to species. Two new biostratigraphically significant subspecies, I. iowaensis ancylus n. subsp. and I. alternatus helmsi n. subsp., are recognized to occur both in the western United States and in Europe. Pelekysgnathus brevis n. sp. is described as new on the basis of a Middle Devonian occurrence in Utah.

Arizona, Colorado, Idaho, New Mexico, Nevada, Utah↗
USGS1985

Rate and depth of pedogenic-carbonate accumulation in soils: Formation and testing of a compartment model.

The rate and depth of pedogenic carbonate accumulation in soils formed in Quaternary alluvium may be viewed as a theoretical problem that involves the mutual interaction of several independent and dependent soil-forming variables. We propose a model for carbonate accumulation in which the soil column is defined by a vertical sequence of 1-cm 2 -area compartments, each with a specified texture, bulk density, water-holding content, lithologic and mineralogic composition, soil-air pCO 2 , ionic strength, and temperature. On the basis of these data, rates of carbonate solubility and dissolution within a given compartment are determined. In arkosic to lithic arkosic sandy parent materials, high carbonate solubility (0.137 to 0.212 mg/ml) and the large reactive surface area of eolian calcareous dust result in rapid carbonate dissolution (0.79 to 9.92 × 10 −10 g/cm 2 /sec) that promotes rapid translocation of carbonate by infiltrating water. We derive a group of equations and use them to calculate net carbonate depletion or accumulation in a soil compartment over an interval of time as a function of the independent variables temperature and precipitation. These two variables largely determine or strongly influence soil-water balance, the external carbonate influx rate, and carbonate solubility. The carbonate distribution that our model predicts closely resembles the observed carbonate distribution in soils associated with Holocene deposits forming in arid, hyperthermic to xeric, thermic moisture-temperature regimes in southern California. This modeling indicates that with a mean carbonate influx rate of 1 × 10 −4 g/cm 2 /yr and in a semiarid, thermic climate, the maximum depression of the top of the Cca horizon is attained within only a few thousand years. In contrast, given the same influx rate, our model predicts that a noncalcareous B horizon cannot form in an arid, hyperthermic climate, a conclusion supported by field and laboratory studies of calcic soils in this climate. The influence of glacial-to-interglacial climatic changes on carbonate accumulation can be modeled by calculating latest Pleistocene soil-water balance with the aid of published estimates of full-glacial temperature and precipitation. On the basis of these modeling results, we propose that either of two types of glacial-to-interglacial climatic changes may account for the strongly bimodal, apparently polygenetic carbonate distribution that is observed in late Pleistocene soils of the eastern Mojave Desert of southern California. Such results of compartment-strategy modeling are encouraging and indicate the great potential of combined theoretical and empirical methods for considering pedological problems of interest to Quaternary geologists.

GSA Special Papers↗
USGS1987

Remanent magnetization of rocks of latest Cretaceous and earliest Tertiary age from drill core at York Canyon, New Mexico

At the end of 1980, seven complete cores were recovered from a 30-m (100-ft) interval in the Raton Formation at York Canyon, New Mexico. The interval cored spans the palynologically defined Cretaceous-Tertiary boundary, which is marked by a distinctive noble metal–bearing claystone in the Raton basin. Azimuthal orientation of the cores can be recovered both from the average directions of the most stable components of the remanent magnetization, with a root mean square error of 28°, and from the average direction of secondary components of magnetization removed by thermal and alternating field demagnetization, with a root mean square error of 33°. The natural remanent magnetization of about 95 percent of the core is dominated by a secondary normal polarity component. Polarity of the characteristic magnetization of each core, interpreted from 12 to 14 samples per core run, is reversed. No evidence of normal polarity characteristic magnetization was found in the 30-m (100-ft) interval sampled. The characteristic magnetization probably is a depositional remanent magnetization acquired during chron 29r. The noble metal–bearing boundary claystone in the Raton basin is interpreted to be part of a synchronous global deposit laid down at the end of the Cretaceous period.

New Mexico↗
USGS1987

Dinosaurs, pollen and spores, and the age of the Ojo Alamo Sandstone, San Juan Basin, New Mexico

The Ojo Alamo Sandstone of the San Juan Basin of New Mexico is composed of interbedded conglomeratic sandstone, sandstone, siltstone, and mudstone. Conglomerates are found in the western part of the basin; siliceous pebbles diminish in size both southward and eastward across the basin, becoming rare to nonexistent in the eastern part. There is great variation in the internal stratigraphy of the Ojo Alamo: Individual sandstone or mudstone beds thicken, thin, and pinch out laterally. The thickness of the Ojo Alamo varies from 6 m (20 ft) to more than 122 m (400 ft). The formation varies in composition from one to as many as five sandstone beds with interbeds of siltstone or mudstone. Dinosaur bone has been found within the Ojo Alamo at several sites in the western part of the basin. Paleocene pollen has been found within the Ojo Alamo in the eastern part of the basin. To date, no Cretaceous pollen has been found at or above the stratigraphic level of dinosaur bone within the Ojo Alamo Sandstone. Near Barrel Spring, in the southwest part of the basin, both dinosaur bone and Paleocene pollen have been found. One bone, found at the top of the Ojo Alamo, was loose on the surface, and its significance is therefore equivocal. Dinosaur bone, however, has also been found in place in the upper part of the Ojo Alamo about 1.6 km (1 mi) west of Barrel Spring, at about the same stratigraphic level as Paleocene pollen from a site just east of Barrel Spring. Because there is no apparent unconformity between the highest in-place bone level and the Paleocene pollen level in this area, the Ojo Alamo dinosaurs, if not reworked, are Paleocene in age at this site and probably throughout the San Juan Basin.

Colorado, New Mexico↗
USGS1987

The ages of the continental, Upper Cretaceous, Fruitland Formation and Kirtland Shale based on a projection of ammonite zones from the Lewis Shale, San Juan Basin, New Mexico and Colorado

The Kirtland Shale or Fruitland Formation directly underlies the Cretaceous-Tertiary boundary throughout most of the San Juan Basin of northwest New Mexico and southwest Colorado. These formations have been known to be Late Cretaceous in age since the early 1900s. Now, with the greatly renewed interest in rocks adjacent to mass extinction boundaries, it is important to place more precise ages on such rock units as the Fruitland and Kirtland. Deposition of the Fruitland and Kirtland was closely related to deposition of the underlying marine-regressive Pictured Cliffs Sandstone. Because the Pictured Cliffs was deposited as a strandline sandstone in a subsiding seaway, its stratigraphic expression, when related to a time horizon (the Huerfanito Bentonite Bed), is a series of rising-to-the-northeast, time transgressive, stair steps. Thus, time lines (or horizons) drawn parallel to the Huerfanito cut through the marine Lewis Shale, the strandline Pictured Cliffs Sandstone, and the continental Fruitland Formation and Kirtland Shale. Ammonites have been collected and identified from various stratigraphic levels within the Lewis Shale around the northwest, north, and east sides of the San Juan Basin. These fossils can be tied in to the established ammonite zonation of the Western Interior seaway. Because some of these ammonite zones have been radiometrically dated outside the San Juan Basin, it is possible to project these dated faunal zones from the Lewis Shale along time lines into the Fruitland Formation and Kirtland Shale and thereby estimate the age of those rocks. Based on these projections the part of the Fruitland and Kirtland laterally time-equivalent to the Lewis Shale is estimated to range from 73.2 ± 0.7 Ma to 73.9 ± 0.8 Ma. The average age for this interval based on these dates is 73.5 ± 0.5 Ma; the maximum range of the interval at the 95 percent confidence level is 71.8 to 75.5 Ma. This age range puts these rocks in the Campanian Stage of the upper Cretaceous in the San Juan Basin.

Colorado, New Mexic↗
USGS1986

The non-transferability of a Cretaceous coal model in the San Juan Basin of New Mexico and Colorado

The San Juan Basin of northwest New Mexico and southwest Colorado contains several Upper Cretaceous coal-bearing formations. The coals in these formations were deposited in environments associated with repeated transgressions and regressions of the Western Interior seaway in Late Cretaceous time. A detailed subsurface and surface study of the coal beds in one of these units, the Fruitland Formation, formed the basis for a coal-depositional model (Fassett and Hinds, 1971). This model basically shows that the thickest Fruitland coals formed landward of thickly stacked sandstone beds of the regressive-marine Pictured Cliffs Sandstone. Transferability of the Fruitland coal model was tested by comparing it to another San Juan Basin coal-bearing rock unit, the lower Menefee Formation. Lower Menefee coal deposits were formed in association with the Point Lookout Sandstone, an older regressive-marine unit. The purpose of this comparison was to see if thick lower Menefee coal beds were also concentrated adjacent to thick vertical shoreface-sandstone buildups. This comparison showed that even though thick sandstone buildups were found in the Point Lookout, no thick coal beds were found in the lower Menefee Formation adjacent to them. This test suggests that certain coal depositional models may have limited value as predictive tools and must be used with caution by coal explorationists. Even more importantly, potential coal-bearing areas should never be written off simply because they do not fit a previously described model.

Colorado, New Mexico↗
USGS1988

Amino-acid diagenesis and its implication for late Pleistocene lacustrine sediment, Clear Lake, California

The diagenesis of amino acids in sediments from Clear Lake core CL-80-1 is indicated by changes in amino acid concentrations, compositions, and stereochemistry. Concentrations of total amino acids decrease with depth, but the decrease is not systematic, possibly reflecting a nonuniformity in sedimentary and postdepositional processes affecting the amino acids. Ratios of neutral/acidic amino acids may indicate that the pH of interstitial water is slightly alkaline to slightly acidic and that the organic matter is well humified. Ratios of nonprotein/protein amino acids suggest that some changes in amino acids with depth result from microbial degradations. The extent of racemization of alanine increases with depth; the trends of these data may be explained, in part, by rapid sedimentation within the lake. Agreement between extents of alanine racemization for sediments from equivalent depths in two cores from the lake suggests that diagenetic temperatures are uniform within the sediments of the northern basin of Clear Lake.

California↗
USGS1988

Depositional environments of the Cache, Lower Lake, and Kelseyville Formations, Lake County, California

We describe the depositional environments of the Cache, Lower Lake, and Kelseyville Formations in light of habitat preferences of recovered mollusks, ostracodes, and diatoms. Our reconstruction of paleoenvironments for these late Cenozoic deposits provides a framework for an understanding of basin evolution and deposition in the Clear Lake region. The Pliocene and Pleistocene Cache Formation was deposited primarily in stream and debris flow environments; fossils from fine-grained deposits indicate shallow, fresh-water environments with locally abundant aquatic vegetation. The fine-grained sediments (mudstone and siltstone) were probably deposited in ponds in abandoned channels or shallow basins behind natural levees. The abandoned channels and shallow basins were associated with the fluvial systems responsible for deposition of the bulk of the technically controlled Cache Formation. The Pleistocene Lower Lake Formation was deposited in a water mass large enough to contain a variety of local environments and current regimes. The recovered fossils imply a lake with water depths of 1 to 5 m. However, there is strong support from habitat preferences of the recovered fossils for inferring a wide range of water depths during deposition of the Lower Lake Formation; they indicate a progressively shallowing system and the culmination of a desiccating lacustrine system. The Pleistocene Kelseyville Formation represents primarily lacustrine deposition with only minor fluvial deposits around the margins of the basin. Local conglomerate beds and fossil tree stumps in growth position within the basin indicate occasional widespread fluvial incursions and depositional hiatuses. The Kelseyville strata represent a large water mass with a muddy and especially fluid substrate having permanent or sporadic periods of anoxia. Central-lake anoxia, whether permanent or at irregular intervals, is the simplest way to account for the low numbers of benthic organisms recovered from the Kelseyville Formation. Similar low-oxygen conditions for benthic life are represented throughout the sedimentary history of Clear Lake. Water depths for the Kelseyville Formation of 10 to 30 m and 12 m near the margins of the basin are inferred both before and after fluvial incursions. These water-depth fluctuations cannot be correlated with major climatic changes as indicated by pollen and fossil leaves and cones; they may be due to faulting in this technically active region.

GSA Special Papers↗
USGS1988

Large-scale bedforms in boulder gravel produced by giant waves in Hawaii

Approximately 105,000 yr ago (based on uranium-series dating), waves in a giant wave train swept up to an elevation of about 375 m on the island of Lanai. The waves deposited the Hulopoe Gravel, which near the present shoreline consists of basalt boulders, coral fragments, and calcareous beachrock slabs, and near the upper limit of the deposit consists of sand and shell fragments. The maximum heights of similar but lower deposits on nearby islands, when adjusted for their estimated subsidence due to volcanic loading during the past 105,000 yr, indicate that the source of the wave was about 50 km southwest of Lanai. We hypothesize that failure and downward movement of the huge Lanai submarine landslide created an ocean disturbance, which produced waves that rushed across the Lanai reef and beach, picked up limestone and lava fragments, and deposited them high on the island as the Hulopoe Gravel. Backwash from the waves stripped soil and rock from the islands and carried much of it to the sea. The Hulopoe Gravel is 8 m thick in a gulch 200 m inland from the Lanai shoreline, where it consists of three beds, successively, 2, 4, and 2 m thick. These beds are considered to have been laid down by successive waves in the wave train. Each bed consists of two units: a lower unit of basalt and limestone boulders, cobbles, and sand, and an upper bimodal unit of large basalt boulders with a pebbly sand matrix. These subunits are inferred to be deposited from the runup and backwash of each wave. At the upper surface of the Hulopoe Gravel, basalt boulders averaging 0.5 m in diameter are arranged in dunelike ridges about 1 m high and 10 m apart. Nearby, where young streams have cut into and exposed the lower beds of the Hulopoe, clasts at the boundaries between the beds are locally imbricated and dip landward. We interpret these features as aspects of torrential flow and crossbedding created during the high-speed backwash of the great waves.

Hawaii↗
USGS1989

Tectonostratigraphic terranes and their Paleozoic boundaries in the central and southern Appalachians

Parts of the central and southern Appalachian orogen appear to have evolved away from Proterozoic North America (Laurentia) and to have been accreted to it during the Paleozoic orogenies that collectively formed the orogen. Identifying each tectonostratigraphic terrane is a necessary step in understanding the evolution of the orogen. The terranes in the central and southern Appalachians are delineated, interpreted, and classified with varying degrees of confidence as: (1) Laurentian native terranes , (2) internal continental terranes of the Appalachian orogen, (3) disrupted terranes , (4) possible oceanic crustal remnants, (5) volcanic-arc terranes , (6) a continental terrane of Gondwanaland affinity, and (7) metamorphic complexes of undetermined affinity. The Laurentian native terranes consist of external massifs of Laurentian basement (Grenvillian and older), their rift- and shelf-facies cover rocks, and slope-rise prism deposits. External massifs are present in the Blue Ridge tectonic province, Reading Prong, and Honey Brook Upland. Rocks of the Talladega block are stratigraphically tied to Laurentia and, with the possible exception of the Hillabee greenstone, are also considered native. Offshore, deep-water, post-rift deposits of the Hamburg and Westminster terranes have no direct stratigraphic ties to Laurentia and are considered discrete native (not suspect) terranes . The internal continental terranes of the Appalachian orogen are isolated massifs of Middle Proterozoic (Grenvillian) continental basement and their cover sequences that occur within the metamorphic core of the orogen. These terranes , the Baltimore, Sauratown, and Pine Mountain terranes , could be either structurally isolated outliers of Laurentia or microcontinental fragments of Laurentian crust displaced by rifting or transcurrent faulting and later reassembled. Disrupted terranes in the central and southern Appalachians contain mélange complexes as well as more coherent terrane fragments (volcanic, ophiolitic, or continental) intermingled with the mélange complexes. Those identified include the Jefferson, Potomac, Smith River, Inner Piedmont, Falls Lake, Juliette, and Sussex terranes . The Bel Air-Rising Sun terrane (Baltimore Complex) in Maryland and Pennsylvania is the only terrane named separately as a possible oceanic crustal remnant. Similar mafic and ultramafic complexes are present in all of the disrupted terranes , but are too small to consider as separate terranes . Volcanic-arc terranes include the Chopawamsic, Carolina, Spring Hope, Roanoke Rapids, and Charleston terranes . The only terrane recognized as a continental terrane of Gondwanaland affinity is the Suwannee terrane , which contains rocks believed to correlate with those now exposed in west Africa. Metamorphic complexes of undetermined affinity are terranes that could not be clearly classified on the basis of available data. These include the Milton, Gaffney, Uchee, Crabtree, Goochland, Wilmington, and Hatteras terranes . The Penobscottian, Taconian, Acadian, and Alleghanian Paleozoic compressional events collectively assembled the various terranes into what is now the Appalachian orogen. Only the central and southern parts of the U.S. Appalachians are considered here. The Penobscottian orogeny, about 550 to 490 Ma, amalgamated the Potomac, the Chopawamsic, probably the Bel Air-Rising Sun, and possibly other exotic terranes at some unknown distance from Laurentia. This was followed by the Taconian orogeny, about 470 to 440 Ma, which accreted the previously amalgamated terranes and probably other terranes such as the Carolina terrane to Laurentia. The younger age limit for the Taconian event is partly constrained by Middle and Late Ordovician faunal assemblages in successor basin deposits of the Arvonia Slate and Quantico Formation. The significance of the Acadian orogeny, dated about 400 to 380 Ma in New England, is unclear in the central and southern Appalachians . In the Talladega block of Alabama and Georgia, an Early to Middle Devonian dynamothermal event is firmly bracketed between Early Devonian fossils and K-Ar ages that indicate a thermal peak no later than Middle Devonian time. A regional tectonothermal event and faulting of approximately this age are also suggested by isotopic studies in terranes to the east. The late Paleozoic (Alleghanian) continental collision between Laurentia and Gondwanaland, which formed the supercontinent Pangea, marks the final stage of accretionary history in the Appalachian -Caledonide orogen. Effects evident in the central and southern Appalachian region include: (1) the accretion of the Suwannee terrane and perhaps the Charleston terrane to what is now North America, (2) slicing and shifting of terranes along dextral strike-slip faults, particularly in the eastern Piedmont, (3) westward transport of native and previously accreted terranes in the western Piedmont and Blue Ridge as part of a composite crystalline thrust sheet, (4) deposition of clastic wedges in the Appalachian foreland, and (5) imbricate thrusting and folding of the resultant strata in the Valley and Ridge Province.

Appalachians↗
USGS1989

Slope movements triggered by heavy rainfall, November 3–5, 1985, in Virginia and West Virginia, U.S.A.

Study of slope movements triggered by the storm of November 3–5, 1985, in the central Appalachian Mountains, U.S.A., has helped to define the meteorologic conditions leading to slope movements and the relative importance of land cover, bedrock, surficial geology, and geomorphology in slope movement location. This long-duration rainfall at moderate intensities triggered more than 1,000 slope movements in a 1,040-km 2 study area. Most were shallow slips and slip-flows in thin colluvium and residuum on shale slopes. Locations of these failures were sensitive to land cover and slope aspect but were relatively insensitive to topographic setting. A few shallow slope movements were triggered by the same rainfall on interbedded limestone, shale, and sandstone. Several large debris slide-avalanches were triggered in sandstone regolith high on ridges in areas of the highest measured rainfall. Most of these sites were on slopes that dip 30 to 35° and lie parallel to bedding planes, presumably the sites of least stability.

Virginia, West Virginia↗
USGS1990

Geochemistry of highly fractionated I- and S-type granites from the tin-tungsten province of western Tasmania

The Devonian batholiths of western Tasmania represent a diverse assemblage of highly fractionated intrusions (70 to 77 percent SiO 2 ) that are the products of different source materials. The Housetop batholith exhibits compositional affinities to a fluorine-rich I-type magma. The Meredith batholith also has characteristics indicative of I-type source materials. The Heemskirk batholith is composite, and consists of a volatile (F, B, H 2 O)–rich S-type granite underlying an I-type granite. The Three Hummock Island, Interview River, Sandy Cape, and Conical Rocks plutons probably have an S-type source and are grouped together as the Sandy Cape Suite. Rapakivi texture is common in the Housetop, Meredith, and Heemkirk batholiths. Quartz-tourmaline nodules are found in the Conical Rocks pluton and the S-type portion of the Heemskirk batholith. The Conical Rocks and Interview River plutons yield high initial Sr isotopic ratios of 0.74242 and 0.76009, respectively. The Housetop and Meredith batholiths yield the lowest initial Sr isotopic ratios of 0.71041 and 0.71445, respectively. The S-type portion of the Heemskirk batholith has an initial Sr isotopic ratio of 0.76387. The 40 Ar/ 39 Ar release spectrum and Rb/Sr mineral isochron analyses corroborate previously reported Devonian to Carboniferous age estimates for these batholiths. A relatively low-temperature thermal event (<200°C) caused argon loss from the K-feldspars at about 105 Ma. This heating event is probably related to the continental breakup of Australia from Antarctica. Major-element compositions of the western Tasmanian granites are very similar. The highly fractionated Sandy Cape Suite leucogranites exhibit high Ga/Al ratios typical of A-type granites, but not their extreme Zr, Y, or Ce enrichments. A distinctive feature of the Sandy Cape Suite is the increase in P 2 O 5 concentration with fractionation. The increase in P 2 O 5 with fractionation is apparently due to extremely low Ca activity, which precludes the formation of apatite, thus allowing P 2 O 5 to behave incompatibly in the melt. All of the granitoids have LREE–enriched chondrite-normalized rare earth element patterns. REE fractionation within the individual granitoids can be summarized by two trends: those with LREE >> HREE depletion (Housetop, Meredith, and Heemskirk batholiths), and those with LREE = HREE depletion (Sandy Cape Suite). The first trend is caused by the initial undersaturation of accessory mineral assemblage that resulted from high concentrations of volatiles and/or alkali complexes. The second trend is caused by early saturation of accessory phases and/or refractory accessory phases.

Tasmania↗
USGS1990

Asteroid and comet flux in the neighborhood of Earth

Approximately 90 Earth-crossing asteroids had been discovered through September 1989. Discovery is thought to be complete at absolute V magnitude (H) = 13.2 (the magnitude of the brightest known object, diameter ∼8.1 km), and about 6 percent complete at H = 17.7 (typical diameter about 1 km). The calculated mean probability of collision of Earth-crossing asteroids with Earth is (4.2 ± 1.7) × 10 −9 yr −1 . When multiplied by the estimated population of 1030 ± 470 at H = 17.7, this probability yields a collision rate of (4.3 ± 2.6) × 10 −6 yr −1 for asteroids larger than about 1 km in diameter. At H = 15.8, roughly equivalent to asteroid diameters more than 2 km, the estimated collision rate is ≈7 × 10 −7 yr −1 , and at 8-km diameter, the rate is ≈3 × 10 −9 yr −1 . Comet nuclei with diameters more than 2.5 km are estimated to strike the Earth at the rate of ≈ 10 −7 yr −1 ; comets larger than 10 km in diameter probably strike at a rate ≈10 −8 yr −1 . Impact of asteroids probably dominates the production of craters smaller than 30 km in diameter, whereas comet impact probably forms most craters larger than 50 km. The production rate for craters larger than 20 km in diameter, estimated from the astronomical evidence, is (4.9 ± 2.9) × 10 −15 km −2 yr −1 ; this rate is consistent with the cratering rate estimated by Grieve from the geologic record for the last 120 m.y.

Special Papers of the Geological Society of Americ↗
USGS1990

Geology, geophysics, and geochronology of the Manson impact structure

A problem with the impact hypothesis for the Cretaceous/Tertiary (K/T) mass extinction is the apparent absence of an identifiable impact site. The Manson impact structure is a candidate site because of its size (the largest such structure recognized in the United States); in addition, the largest and most abundant shocked quartz grains at the K/T boundary are found relatively close by, and its age is indistinguishable from that of the K/T boundary. The region of northwest central Iowa that contains the Manson impact structure is covered by Quaternary glacial deposits, which are underlain by Phanerozoic sedimentary rocks (mostly flat-lying carbonates) and Proterozoic red clastic, metamorphic, volcanic, and plutonic rocks. In a circular area about 22 mi (35 km) in diameter around Manson, Iowa, this normal sequence is absent or “disturbed.” Within the structure, three roughly concentric zones of rock associations have been identified: an outer zone of displaced strata, a zone of completely disrupted strata, and a central area in which basement igneous and metamorphic rocks have been uplifted at least 1,220 ft (4,000 m). Gravity, magnetic, and seismic refraction surveys readily identify the central uplift within the structure. Manson is established as an impact structure based on its circular shape, its central uplift, and the presence of shocked quartz within the granitic central uplift. Paleontological evidence, a fission track age, and preliminary 40 Ar/ 39 Ar dating all allowed a K/T boundary age for the Manson structure. Improved 40 Ar/ 39 Ar age spectra may be interpreted in terms of samples that were incompletely degassed during heating due to the Manson impact. An age of 65.7 ± 1.0 Ma was obtained, a value indistinguishable from that of the K/T boundary.

Iowa↗
USGS1990

Plant microfossil record of the terminal cretaceous event in the western United States and Canada

Shortly after the introduction of the extraterrestrial-impact hypothesis of the terminal Cretaceous event (TCE), plant microfossils, which had been used to locate the Cretaceous /Tertiary (K/T) boundary in nonmarine rocks, became critical to its precise identification; they continue to serve in this capacity. The K/T boundary in nonmarine rocks from New Mexico to Alberta is identified by the coincidence of a palynological extinction horizon and an iridium abundance anomaly. Plant microfossils provide evidence of the effects of the TCE and place constraints on theories of its cause. Changes in plant microfossil assemblages within intervals spanning the K/T boundary are evidence of abrupt and permanent changes in terrestrial, floras that were a consequence of the TCE; these changes are essentially independent of lithofacies. Extinction levels varied among major groups of plants (angiosperms, gymnosperms, and pteridophytes), but simultaneously affected different plant communities throughout the region. The abrupt nature of the extinction across western North America is consistent with the impact hypothesis; it is inconsistent with progressive change in paleoclimate possibly being the cause of the terminal Cretaceous extinctions. By causing the extinction of a significant portion of the Late Cretaceous flora of the region, the TCE influenced the development of the modern flora, but its effects appear to have been concentrated in western North America.

Geological Society of America Special Papers↗
USGS1990

Seismic imaging of extended crust with emphasis on the western United States

Understanding of the crust has improved dramatically following the application of seismic reflection and refraction techniques to studies of the deep crust. This is particularly true in areas where the last tectonic event was extensional, such as the Basin and Range province of the western United States and much of western Europe. In these regions, a characteristic reflective pattern has emerged, whereby the lower crust is highly reflective and the upper crust and upper mantle are either poorly reflective or strikingly nonreflective. In the metamorphic-core-complex belt in the western United States, where extension can be as much as an order of magnitude greater than in the more classic continental rift zones, the lower crustal reflectivity thickens and rises, yielding a picture of a crust that is reflective throughout. Synthetic seismic studies have documented that the reflectivity in these regions can be modeled by numerous laminae tens of meters thick and hundreds of meters across, characterized by inter-layered high and low velocities. Two geologic factors are interpreted as contributing to this layered character: ductile strain, responding to stress in the thermally weakened middle and lower crust, and intrusive layering, corresponding to injection of subhorizontal sheets of mantle-derived magmas. These two processes yield a variety of geologic structures, including transposed compositional layering, mylonitic ductile shear zones, and intrusive mafic sheets, all of which occur at the proper scales to cause the prominent reflectivity observed. If metamorphic core complexes are representative of extended continental crust world-wide, then these results suggest that magmatism and ductile flow have also contributed to the evolution of the middle and lower crust in many other areas around the world.

Special Papers of the Geological Society of Americ↗
USGS1991

Upper Triassic Barranca Group; Nonmarine and shallow-marine rift-basin deposits of northwestern Mexico

The 3,000-m-thick Upper Triassic Barranca Group in the Sierra de San Javier in east-central Sonora, Mexico, is composed, in ascending order, of the Arrayanes, Santa Clara, and Coyotes Formations. The Arrayanes and Santa Clara Formations are composed of fluvial and marine-delta deposits of quartzose and arkosic sandstone, conglomerate, shale, and siltstone; the Santa Clara Formation includes minor amounts of coal and tuff. A sharp contact (perhaps an unconformity) separates the Santa Clara Formation from the overlying Coyotes Formation. The Coyotes consists of alluvial-fan deposits of pebble-to-boulder conglomerate. Paleocurrents were southward during deposition of the Arrayanes and Santa Clara Formations and southwestward during deposition of the Coyotes Formation, assuming that no major post-deposition tectonic rotation has occurred. The Santa Clara Formation has been dated paleontologically as Late Triassic; the age of the entire group is unknown, but is commonly assumed to also be Late Triassic. The Barranca Group in the Sierra de San Javier rests unconformably on a sequence of eugeosynclinal chert, argillite, quartzite, and carbonate rock of Paleozoic age, and is unconformably overlain by the Tarahumara Volcanics, which have been dated no more precisely than latest Triassic to earliest Cenozoic. The thick, coarse, and laterally variable deposits of the Barranca Group indicate deposition in a basin, or basins, flanked by areas of high relief. Much of the Barranca in Sonora appears to have been deposited in a single basin, which is delineated by the occurrence of major outcrops of the Barranca Group in an east-west-trending belt about 110 km long and 40 km wide. The elongate shape of this basin and the interpretation of flanking areas of high relief suggests a basin of rift origin. If so, the Barranca Group is part of a broad zone of rift-related Upper Triassic sequences in northern Mexico that apparently formed by transtensional and/or extensional faulting.

Sonora↗