scispace - formally typeset
Search or ask a question

Showing papers in "Geological Society of America Bulletin in 1992"


Journal ArticleDOI
TL;DR: A new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the Izu-Bonin-Mariana (IBM) arc system and then applied to Late Jurassic ophiolites of Cailfornia.
Abstract: A new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the Izu-Bonin-Mariana (IBM) arc system and then applied to Late Jurassic ophiolites of Cailfornia. The model accounts for several key observations about the earliest stages in the evolution of the IBM system: (1) subduction nucleated along an active transform boundary, which separated younger, less-dense lithosphere in the west from older, more-dense lithosphere to the east; (2) initial arc magmatic activity occupied a much broader zone than existed later; (3) initial magmatism extended up to the modern trench, over a region now characterized by subnormal heat flow; (4) early are magmatism was characterized by depleted (tholeiitic) and ultra-depleted (boninitic) magmas, indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth; (5) early arc magmatism was strongly extensional, with crust forming in a manner similar to slow-spreading ridges; and (6) crust production rates were 120 to 180 km 3 /km-Ma, several times greater than for mature arc systems. These observations require that the earliest stages of subduction involve rapid retreat of the trench; we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault. This resulted in strong extension and thinning of younger, more buoyant lithosphere to the west. This extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere. Addition of water and asthenospheric upwelling led to catastrophic melting, which continued until lithosphere subsidence was replaced by lithospheric subduction . Application of the subduction-zone infancy model to the Late Jurassic ophiolites of California provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger Sierran magmatic arc and the Franciscan subduction complex, provides a mechanism for the formation and subsidence of the Great Valley forearc basin, and explains the limited duration of high-T, high-P metamorphism experienced by Franciscan melanges.

756 citations


Journal ArticleDOI
TL;DR: In this paper, a detailed stratigraphic, sedimentologic, paleocurrent, and subsidence analyses were conducted on Mesozoic nonmarine sedimentary sections of the south Junggar, north Tarim, and Turpan basins, Xinjang Uygur Autonomous Region, northwest China.
Abstract: Detailed stratigraphic, sedimentologic, paleocurrent, and subsidence analyses were conducted on Mesozoic nonmarine sedimentary sections of the south Junggar, north Tarim, and Turpan basins, Xinjang Uygur Autonomous Region, northwest China. These three basins have been foreland basins throughout the Mesozoic and Cenozoic eras, as demonstrated by asymmetrically distributed basinwide sediment accumulations, foreland-style subsidence profiles, and a variety of outcrop and subsurface facies data. Mesozoic paleocurrent indicators measured in the south Junggar and north Tarim basins, as well as Mesozoic sandstone compositions from both basins, indicate that the intervening Tian Shan has existed as a positive physiographic feature partitioning the two basins throughout Mesozoic and Cenozoic time. Paleocurrent, facies, and subsurface isopach data suggest that the Turpan basin was established as a discrete feature by the Early Jurassic period. The timing and style of depositional systems within the north Tarim Mesozoic depocenter, the south Junggar Mesozoic depocenter, and the central Turpan basin are remarkably similar. Upper Triassic strata of each basin consist of alluvial conglomerate and associated braided-fluvial sandstone and siltstone which fine upward into lower through Middle Jurassic, locally organic-rich, meandering-fluvial, and lacustrine strata. Upper Jurassic braided-fluvial red beds in each basin are overlain by a distinct pulse of uppermost Jurassicmore » alluvial conglomerate. Lower Cretaceous exposures consist of fine-grained red beds in north Tarim and Turpan and interbedded red and gray shale with local silty carbonates in south Junggar. Upper Cretaceous strata of the north Tarim and south Junggar basins are composed of alluvial conglomerate with associated braided-fluvial sandstone and siltstone. 94 refs., 17 figs.« less

515 citations


Journal ArticleDOI
TL;DR: In this article, the authors present a correlation of 31 sections from the Pacific and Caribbean coasts of Costa Rica and western Panama, using calcareous nannofossils and planktonic foraminifera at both the tops and bottoms of each formation.
Abstract: The final closure of the Isthmus of Panama at ∼3.5 Ma divided the American tropical ocean into two separate and different oceanographic regions. Consequences for the marine biota were profound, but, hitherto, correlation of the Pacific and Caribbean coastal sections has not been precise enough to track biologic patterns. We present here a correlation of 31 sections from the Pacific and Caribbean coasts of Costa Rica and western Panama. Using calcareous nannofossils and planktonic foraminifera at both the tops and bottoms of each formation, we estimate that the Caribbean section ranges from 8.2 Ma to 1.7 Ma; and the Pacific sequence, from 3.6 Ma to <1.7 Ma. These intervals bracket postulated dates for final closure of the Isthmus and provide the first well-dated record of middle and late Pliocene faunas from the region. The Caribbean and Pacific sections include very different environments of deposition, yet there is sufficient overlap and diversity of habitats to permit meaningful biological comparisons. On the Caribbean side, formations tied together by the overlap of the upper Pliocene markers Sphenolithus abies and Pseudoemiliana lacunosa (3.5 Ma to 3.6 Ma) range from very shallow to shallow inner shelf (<200 m) and upper slope (200-800 m). The Pacific coast sections were mostly deposited in a trench slope environment, which is absent on the Caribbean side. These sections fortuitously include abundant thick intra-formational slumps containing shallow-water fauna more appropriate for biological comparison with the Caribbean biota. Similarly, the ∼1.9 Ma to 1.5 Ma interval, well constrained by various taxa, includes middle- to outer-shelf, and inner-shelf to upper-slope deposits on the Caribbean side, and marginal-marine to inner-shelf deposits on the Pacific coast. Using our new biostratigraphic framework to correlate previously poorly constrained mollusc collections, we show that evolutionary divergence of the Pacific and Caribbean near-shore marine faunas had occurred by 3.5 Ma. This strongly suggests that the Isthmus was effectively closed by this time.

467 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used the ages and depths of a series of dated submerged coral reefs off northwest Hawaii to estimate the time of the end of shield building of the seven volcanoes of the island of Hawaii.
Abstract: The seven volcanoes comprising the island of Hawaii and its submarine base are, in order of growth, Mahukona, Kohala, Mauna Kea, Hualalai, Mauna Loa, Kilauea, and Loihi. The first four have completed their shield-building stage, and the timing of this event can be determined from the depth of the slope break associated with the end of shield building, calibrated using the ages and depths of a series of dated submerged coral reefs off northwest Hawaii. The composition of lavas collected adjacent to these reefs helps to define the eruptive history of the various volcanic centers. The island of Hawaii has grown at an average rate of about 0.02 km 2 /yr for the past 600 k.y. and presently is close to its maximum size. Mahukona completed shield building about 465 ka; Kohala, 245 ka; Mauna Kea, 130 ka; and Hualalai, 130 ka. On each volcano, the transition from eruption of tholeiitic to alkalic lava occurs near the end of shield building. The larger volcanic systems (which stood more than 4 km above their shoreline at the end of shield building) change in composition before the end of shield building, and the smaller volcanoes change near or after the end of shield building, or never make the change to eruption of alkalic lava. The rate of southeastern (south 40° east) progression of the end of shield building (and hence the postulated movement rate of the Pacific plate over the Hawaiian hotspot) in the interval from Haleakala to Hualalai is about 13 cm/yr. Based on this rate and an average spacing of volcanoes on each loci line of 40-60 km, the volcanoes require about 600 thousand years to grow from the ocean floor (generally from a point on the southeastern submarine flank of the next older volcano of the same loci line) to the time of the end of shield building. They arrive at the ocean surface about midway through this period.

296 citations


Journal ArticleDOI
TL;DR: In this paper, the authors provided new estimates of the maximum late Pleistocene glacial Lake Missoula flood discharges for two important reaches along the flood route and showed that the peak discharge probably exceeded 17 ± 3 million m3 sec-1.
Abstract: New field evidence and discharge calculation procedures provide new estimates of maximum late Pleistocene glacial Lake Missoula flood discharges for two important reaches along the flood route. Within the Spokane Valley, near the point of release, the peak discharge probably exceeded 17 ± 3 million m3 sec-1. Downstream at Wallula Gap, a major point of flow convergence, peak discharge was about 10 ± 2.5 million m3sec-1. Flow duration was on the order of several days. These are the largest known terrestrial fresh-water flows. Consideration of these discharge values constrains models for the failure of glacial Lake Missoula. The maximum discharges estimated here are larger than theoretical and empirical predictions of maximum subglacial jokulhlaup-style releases for Lake Missoula. We postulate, consistent with geological relations in the glacial Lake Missoula basin and in the Channeled Scabland, that the largest late Wisconsinan Missoula Flood resulted from a cataclysmic failure of the impounding ice dam of glacial Lake Missoula. This large release may have been the result of a complete rupture of the ice dam. Subsequent multiple flows of lesser magnitude may have resulted from repeated subglacial releases from the lake.

228 citations


Journal ArticleDOI
TL;DR: In this article, a finite-element model for ice flow through a glacier cross section with an erosion model was proposed to investigate the development of one of the most striking glacial landforms, the U-shaped valley.
Abstract: The steep-sided valleys and overdeepened basins of alpine landscapes are well-known products of glaciation, yet relatively little is known about how the dynamics of ice flow and glacial erosion interact to give rise to such landforms. By linking a finite-element model for ice flow through a glacier cross section with an erosion model, it is possible to investigate the development of one of the most striking glacial landforms, the U-shaped valley. In addition to providing a detailed understanding of landform development, such modeling provides a way to test current understanding of the controls on glacial sliding and erosion. To simulate valley development, I first model flow through an initial glacier cross section and calculate the glaciological parameters that govern erosion. I then numerically simulate erosion to produce a modified transverse profile, for which a new flow field and erosion pattern are computed. A number of iterations permits examination of the progressive transformation of cross-section form, which can be compared with field data. Model predictions of the cross-section flow field are in close accord with data from the Athabasca Glacier and include marked lateral variations in sliding velocity. With an erosion law dependent on basal velocity, the model predicts the rapid transformation of a V-shaped cross section into a recognizably glacial form over a time period on the order of 10 4 yr and the eventual development of a steady-state, quasi-parabolic glacier cross section. Better agreement with empirical data from glaciated valleys is obtained by including temporal variations in ice discharge, in order to mimic the characteristics of 100,000-yr glacial cycles. The high-discharge phase dominates form development, and, at low discharges, cross-section form is essentially inherited from the central part of the form that developed during the preceding high-discharge phase.

226 citations


Journal ArticleDOI
TL;DR: In this article, the authors define four provenance groups using ternary plots of site means: intraoceanic arc and remnant arc, continental arc, triple junction, and strike-slip-continental arc.
Abstract: Detrital modes for 524 deep-marine sand and sandstone samples recovered on circum-Pacific, Caribbean, and Mediterranean legs of the Deep Sea Drilling Project and the Ocean Drilling Program form the basis for an actualistic model for arc-related provenance. This model refines the Dickinson and Suczek (1979) and Dickinson and others (1983) models and can be used to interpret the provenance/tectonic history of ancient arc-related sedimentary sequences. Four provenance groups are defined using QFL, QmKP, LmLvLs, and LvfLvmiLvl ternary plots of site means: (1) intraoceanic arc and remnant arc, (2) continental arc, (3) triple junction, and (4) strike-slip-continental arc. Intraoceanic- and remnant-arc sands are poor in quartz (mean QFL%Q 75); they are predominantly composed of plagioclase feldspar and volcanic lithic fragments. Continental-arc sand can be more quartzofeldspathic than the intraoceanic- and remnant-arc sand (mean QFL%Q values as much as 10, mean QFL%F values as much as 65, and mean QmKP%Qm as much as 20) and has more variable lithic populations, with minor metamorphic and sedimentary components. The triple-junction and strike-slip-continental groups compositionally overlap; both are more quartzofeldspathic than the other groups and show highly variable lithic proportions, but the strike-slip-continental group is more quartzose. Modal compositions of the triple junction group roughly correlate with the QFL transitional-arc field of Dickinson and others (1983), whereas the strike-slip-continental group approximately correlates with their dissected-arc field.

207 citations


Journal ArticleDOI
TL;DR: Upper Paleozoic to Triassic Chilean granitoids in the Andean Frontal Cordillera between 28°S and 31°S record crustal and mantle conditions at the Gondwana margin during the final assembly and initial breakup of the Pangea supercontinent as mentioned in this paper.
Abstract: Upper Paleozoic to Triassic Chilean granitoids in the Andean Frontal Cordillera between 28°S and 31°S record crustal and mantle conditions at the Gondwana margin during the final assembly and initial breakup of the Pangea supercontinent. This period overlaps the end of Paleozoic terrane accretion and precedes Andean subduction. Integration of new trace-element and isotopic data with other information on the granitoids and the regional geology leads to a tectonic model that has implications for other parts of the Gondwana margin. In the model, the Carboniferous to Early Permian is a period of oblique convergence. Associated Elqui complex granitoids are diverse. Those in the Guanta and Montosa units are predominantly related to subduction processes, whereas those in the Cochiguas and El Volcan units are dominated by melting of the subduction complex and older crust. Progressive oblique collision of the last pre-Pangea terrane (Equis) along the margin resulted in crustal thickening associated with shortening deformation of foreland basinal sedimentary rocks and uplift of the Elqui complex. Subsequent gravitational collapse of the inactive slab and lithospheric delamination resulted in the production of large amounts of basalt, which intruded and melted the crust, producing the post-collisional Ingaguas complex. The Los Carricitos granitoids formed in thickened crust, whereas the Chollay, El Colorado, and El Leon units formed in thinner crust. The Ingaguas complex is part of the Choiyoi granite-rhyolite province, whose formation, similar to that of other Gondwana silicic provinces, was probably accentuated by anomalously hot upper mantle associated with the Pangea supercontinent.

197 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the nature of ground-water flow in alluvial valleys and presented a new classification scheme for allvial aquifers, based on the Darcy flux end-member components, and concluded that the dominant regional groundwater flow component, base-flow or underflow, can be inferred from geomorphologic data.
Abstract: The delineation of ground-water flow in alluvial valley stream-aquifer systems is important in studies of water availability, environmental impact, and aquifer remediation and is required by some states' water laws. Published potentiometric surfaces and head data, however, are not always available. When data are absent, it is commonly assumed that ground water and dissolved solutes flow toward the river. Analysis of published geomorphologic and hydraulic data in 24 alluvial systems and the results of digital simulations indicate that in some cases this assumption is only valid immediately adjacent to an effluent river. Ground-water flow in the remaining portions of an alluvial valley aquifer may be dominated by downstream or underflow components. This paper investigates the nature of ground-water flow in alluvial valleys and presents a new classification scheme for alluvial aquifers. The geomorphologic conditions that may allow underflow to be the predominant regional ground-water flow component are described. In order to classify alluvial aquifers, two Darcy flux end-member components are defined. The baseflow component is that portion of the ground-water flux that moves perpendicular to the river. The baseflow component may flow toward the river or away from the river depending on whether the river is effluent or influent, respectively. The underflow component of the Darcy flux moves parallel to the river and in the same direction as the stream flow. The alluvial stream-aquifer systems are classified based on the predominant regional ground-water flow component as underflow-component dominated, baseflow-component dominated, or mixed. On the basis of the results of this study, we conclude that the dominant regional ground-water flow component, base-flow or underflow, can be inferred in an alluvial valley aquifer from geomorphologic data. These data include the channel slope, the river sinuosity, the degree of penetration (incision through the alluvium) of the river, the width-to-depth ratio, and the fluvial depositional system. The underflow component is demonstrably predominant when the following conditions exist: (1) the channel gradient exceeds .0008, (2) the sinuosity is less than 1.3, (3) the river penetration is less than 20%, (4) the width-to-depth ratio is greater than 60, and (5) the fluvial depositional system is either valley fill or mixed load to bed-load. The underflow component can also be dominant on flood plains where the lateral valley slope is negligible.

195 citations


Journal ArticleDOI
TL;DR: In this paper, the authors investigated the surface morphology of debris-flow fans comprising the bajada along the western slope of Owens Valley, California, and inferred that the frequency distribution of debris flow rheologies, set by source-terrain geology and hydrology, is an important control on fan morphology.
Abstract: We have investigated factors controlling the surface morphology of debris-flow fans comprising the bajada along the western slope of Owens Valley, California. These fans have average slopes of 4°; an extensive network of abandoned, boulder-lined channels; rough, undulatory surfaces near the range front; and smooth distal surfaces. Field relationships and mechanical considerations indicate that the channels of the bajada are products of fluvial incision and not debris-flow scour. This is significant because detailed geomorphic maps indicate that the channels strongly influence the pattern of debris-flow deposition. The locus of debris-flow deposition on a channelized fan surface is set by the interaction of debris flows with the channel system and is controlled by channel size, channel gradient, flow volume, flow hydrograph, and flow rheology. Debris-flow behavior is most directly controlled by variations in bulk-sediment concentration and its influence on flow rheology. Whereas low-sediment-concentration debris flows tend to smooth the surface of the lower fan, spreading into thin sheets and filling channels and surface undulations, repeated deposition of high-sediment-concentration debris flows produces the rugged topography of the upper fan. The texture of the fan surface, rough or smooth, is determined by the relative volumetric importance of these two types of debris flow. In addition, channel avulsions and the associated long-term shifting of depositional loci are driven by in-channel deposition of debris flows with the highest sediment concentrations. These debris flows, therefore, play a critical role in determining both the structure of the channel network and the long-term pattern of deposition on the fan surface as a whole. We infer that the frequency distribution of debris-flow rheologies, set by source-terrain geology and hydrology, is an important control on fan morphology.

183 citations


Journal ArticleDOI
TL;DR: The Quadrilatero Ferrifero is a metallogenic district (Au, Fe, Mn) located in the southernmost Sao Francisco craton in eastern Brazil, which is composed of Archean granitoids, gneisses, and a greenstone belt sequence (Rio das Velhas Supergroup), overlain by a Proterozoic clastic-chemical sedimentary sequence hosting a thick Lake Superior type banded iron formation (Minas Supergroup) as discussed by the authors.
Abstract: The Quadrilatero Ferrifero is a metallogenic district (Au, Fe, Mn) located in the southernmost Sao Francisco craton in eastern Brazil. It is composed of Archean granitoids, gneisses, and a greenstone belt sequence (Rio das Velhas Supergroup), overlain by a Proterozoic clastic-chemical sedimentary sequence hosting a thick Lake Superior-type banded iron formation (Minas Supergroup). A U-Pb study of the main units leads to the following conclusions: granitoid emplacement was dated at 2776 +7/-6 Ma and 2721 ± 3 Ma; two felsic volcanic rocks from the Rio das Velhas Supergroup yielded ages of 2776 +23/-10 Ma and 2772 ± 6 Ma. These data indicate that greenstone-belt volcanism was coeval with granitoid emplacement, a common feature of greenstone-belt terrains, but so far undocumented in the southern Sao Francisco shield. The younger granitoid intrusion marks the stabilization of the shield in Late Archean time. Zircon cores older than 2776 Ma, together with other data, indicate the presence of continental crust 2.88-3.1 Ga old in the Quadrilatero Ferrifero. A detrital zircon from the uppermost formation in the Minas Supergroup (Sabara Formation) is concordant at 2125 ± 4 Ma and provides the first age constraint on the deposition of the Supergroup. Monazite from two pegmatites and titanite from an amphibolite enclave in gneisses from the Bacao Complex, in central Quadrilatero Ferrifero, yielded ages between 2060 Ma and 2030 Ma. These data may constrain the minimum age for the deposition of the Minas Supergroup. In addition, they indicate that the Early Proterozoic Transamazomian orogeny is represented by amphibolite-facies metamorphism and partial melting of Archean crust.

Journal ArticleDOI
TL;DR: In this paper, a model for subduction of the buoyant, aseismic Cocos Ridge and the overriding Caribbean plate is invoked to explain the variation in rates of vertical crustal uplift along a coastal transect from Nicoya to Burica.
Abstract: The Pacific coast of Costa Rica lies within the Central American forearc and magmatic-arc region that was created by northeastward subduction of the Cocos plate beneath the Caribbean plate at the Middle America Trench. From the Peninsula de Nicoya south-eastward toward the Peninsula de Osa and the Peninsula de Burica on the Panamanian border, the Middle America Trench loses its physiographic expression where it intersects the aseismic Cocos Ridge. Interaction between subduction of the buoyant, aseismic Cocos Ridge and the overriding Caribbean plate is invoked to explain the variation in rates of vertical crustal uplift along a coastal transect from Nicoya to Burica. The Pliocene and Pleistocene stratigraphic record and Holocene marine terraces and beach ridge complexes indicate that maximum rates of crustal uplift have occurred on the Peninsula de Osa, immediately landward of the aseismic Cocos Ridge. Crustal uplift rates decrease northwest toward the Peninsula de Nicoya, and to a lesser extent southwest toward the Peninsula de Burica. The late Quaternary stratigraphy on the Peninsula de Osa is subdivided into two major chronostratigraphic sequences from groupings of radiocarbon dates. Crustal uplift rates calculated from these sequences systematically decrease from 6.5 to 2.1 m/ka north-east across the peninsula. Deformation of the peninsula is modeled as uplifted and down-to-the-northeast-tilted fault blocks with an angular rotation rate of 0.03° to 0.06° per thousand years. Although less well constrained, crustal uplift rates on the Peninsula de Nicoya, 200 km to the northwest of the Peninsula de Osa, vary from <1 m/ka for Pliocene and Pleistocene sediments to 2.5 m/ka for Holocene marine terraces. In the Quepos region, 100 km to the northwest of the Peninsula de Osa, calculated uplift rates derived from incision of late Quaternary fluvial terraces range from 0.5 to 3.0 m/ka. On the Peninsula de Burica, only 60 km to the southwest of the Peninsula de Osa, calculated uplift rates range from 4.7 m/ka for a late Holocene marine terrace to 1.2 m/ka for post-late Pliocene deep-sea sediments. The variations in calculated uplift rates on the Peninsula de Osa constrain a dynamic model for subduction of the Cocos Ridge and the resulting uplift of the overriding Caribbean plate. Deflection of the Caribbean plate is modeled using various effective elastic thicknesses as the response of an elastic plate to the buoyant force of the subducted Cocos Ridge. Because the shape of the subducted end of the Cocos Ridge is unknown, two scenarios are evaluated: (1) a radially symmetric ridge with a slope similar to the slope of the flanks of the ridge and (2) a ridge where the subducted end was truncated by the Panama fracture zone. The best-fit model utilizes a truncated ridge that has been subducted during the past 0.5 m.y. ∼50 km beneath the overriding Caribbean plate, which has an effective elastic thickness of 5 km. The model predicts that the highest uplift rate should be ∼3.7 m/ka and occur on the southwest coast of the Peninsula de Osa. The rate of uplift slows considerably to the northeast and indicates that the Peninsula de Osa is tilting to the northeast, which agrees with observations in that region. The predicted uplift rate attributed to aseismic ridge subduction also decreases along the coast both north and south of the Peninsula de Osa, resulting in little uplift that can be attributed to Cocos Ridge subduction in the northwestern portions of the Peninsula de Nicoya.

Journal ArticleDOI
TL;DR: This article used the GCM model to simulate the early Jurassic climate and found that increased ocean heat transport may have been the primary force generating warmer climates during the past 180 m.y.
Abstract: Results from new simulations of the Early Jurassic climate show that increased ocean heat transport may have been the primary force generating warmer climates during the past 180 m.y. The simulations, conducted using the general circulation model (GCM) at the Goddard Institute for Space Studies, include realistic representations of paleocontinental distribution, topography, epeiric seas, and vegetation, in order to facilitate comparisons between model results and paleoclimate data. Three major features of the simulated Early Jurassic climate include the following. (1) A global warming, compared to the present, of 5 °C to 10 °C, with temperature increases at high latitudes five times this global average. Average summer temperatures exceed 35 °C in low-latitude regions of western Pangaea where eolian sandstones testify to the presence of vast deserts. (2) Simulated precipitation and evaporation patterns agree closely with the moisture distribution interpreted from evaporites, and coal deposits. High rainfall rates are associated primarily with monsoons that originate over the warm Tethys Ocean. Unlike the megamonsoons proposed in previous studies, these systems are found to be associated with localized pressure cells whose positions are controlled by topography and coastal geography. (3)Decreases in planetary albedo, occurring because of reductions in sea ice, snow cover, and low clouds, and increases in atmospheric water vapor are the positive climate feed-backs that amplify the global warming. Similar to other Mesozoic climate simulations, our model finds that large seasonal temperature fluctuations occurred over mid-and high-latitude continental interiors, refuting paleoclimate evidence that suggests more equable conditions. Sensitivity experiments suggest that some combination of ocean heat transport increase, high levels of CO2, and improved modeling of ground hydrological schemes may lead to a better match with the geologic record. We speculate, also, that the record itself is biased toward equable climatic conditions, a suggestion that may be tested by comparing GCM results with more detailed phytogeographic analyses.

Journal ArticleDOI
TL;DR: In this paper, the morphology, sedimentary properties, and sequence of recent coarse-grained flood deposits and earlier Holocene alluvial fills were investigated in Thinhope Burn, a small (12km2) catchment in the Northern Pennine uplands, northern England.
Abstract: The morphology, sedimentary properties, and sequence of recent coarse-grained flood deposits and earlier Holocene alluvial fills were investigated in Thinhope Burn, a small (12-km2) catchment in the Northern Pennine uplands, northern England. Twenty-one large flood events are recorded by distinctive cobble-boulder bars, sheets and splays, and boulder berms and lobes. Lichenometric analysis showed that all but one of these floods dated from the mid-eighteenth century. The timing of large floods between 1766 and 1960 corresponds with major hydroclimatic trends evident in northern Britain and northwest Europe over this period. Discharge estimates suggest that flood magnitudes have decreased since the mid-eighteenth century. Channel and flood-plain metamorphosis in late Roman times and in the eighteenth century, following major valley-floor entrenchment (locally as much as 8 m), would appear to have been caused by increased runoff and flood magnitude. This was linked to a shift to a wetter climate with flow augmented by Iron Age and Roman woodland clearance, and drainage of the catchment in more recent times. Results from this study suggest that current models of longer-term Holocene and Pleistocene valley-floor development in the British uplands may need to be re-evaluated.

Journal ArticleDOI
TL;DR: The Burdekin Gorge of northeastern Australia lies within the seasonal tropics and is characterized by high discharge variability as discussed by the authors, which helps to explain the location of boulder bars, high flood levees, small-scale erosional features in the bedrock, and the formation of inner channels.
Abstract: The Burdekin Gorge of northeastern Australia lies within the seasonal tropics and is characterized by high discharge variability. Slackwater sediments and paleostage indicators in the gorge record seven large floods that have occurred during the past 1,200 yrs. These floods range in magnitude from 11,000 to 30,000 m 3 s -1 and are characterized by large downstream variations in hydraulics. The floods generate high values of velocity, boundary shear stress, and stream power per unit area. Downstream fluctuations in these variables help to explain the location of boulder bars, high flood levees, small-scale erosional features in the bedrock, and the formation of inner channels. Boulder bars and flood levees form where shear stress and stream power decrease due to channel widening. Small-scale erosional features, such as potholes and troughs, are best developed at sites of channel constriction and associated increases in shear stress and stream power. The development of the inner channel appears to be controlled by complex interactions between bedrock lithology and structure, and by flow hydraulics. All of the channel features are related to the hydraulics of large floods, which therefore are the dominant controls on many aspects of channel morphology in this bedrock gorge.

Journal ArticleDOI
TL;DR: In this article, the ages of nine major depositional sequences in the eastern Pyrenean foreland have been specified within the context of these chronologic data, beginning with the early Eocene transgression that commenced at 58 Ma.
Abstract: Abundant and clearly exposed relationships between structures and syntectonic sedimentary rocks in the southern Pyrenees make this area particularly suitable for detailed studies of the sequential development of a collisional orogenic belt. During the Pyrenean orogeny, major shortening occurred in the eastern Pyrenees in Eocene and early Oligocene times. Until now, individual tectonic and depositional events have been only loosely defined temporally by biostratigraphic data. In order to provide a more precise chronological framework, four new magnetostratigraphic sections, spanning 8 km of Eocene strata and encompassing 18 m.y., have been developed in the eastern Pyrenees of northern Spain. The ages of nine major depositional sequences in the eastern Pyrenean foreland have been specified within the context of these chronologic data, beginning with the early Eocene transgression that commenced at 58 Ma. The timing of numerous tectonic events that occurred during the subsequent 16 m.y. can also be delineated, including the initial emplacement of the Pedraforca thrust sheet (58-54 Ma), the development of the Pedraforca breakback thrusts (47.5-40 Ma) at a mean shortening rate of 2.4 mm/yr, rotation of the Pedraforca footwall, three intervals of motion along the Vallfogona thrust (42.5-44.0, 40.5-41.5, and

Journal ArticleDOI
TL;DR: In this article, a conceptual model of the Sevier orogen was proposed, consistent with geologic constraints and supported by simplistic rheological arguments, in which westward movement of a mid-crustal extensional allochthon was driven by buoyancy stresses in the over-thickened Internal Zone.
Abstract: Recent geologic studies in the western United States Cordillera provide evidence of extensional deformation in Cretaceous time, during development of the Sevier foreland fold and thrust belt. This evidence comes from the region roughly bound on the east by the fold and thrust belt and on the west by the Mesozoic continental arc. We postulate that extension in this Internal Zone accommodated gravitational collapse of the. evolving Sevier orogen. Major surface-breaking normal faults and extensional basins that characterize other regions of syncompressional gravitational collapse (such as the Himalayan orogen) have not been documented in the Cretaceous record of the Internal Zone. Consequently, if extension was widespread in the Internal Zone, then it had little surface expression. We propose a conceptual model of the Sevier orogen, consistent with geologic constraints and supported by simplistic rheological arguments, in which westward movement of a mid-crustal extensional allochthon was driven by buoyancy stresses in the over-thickened Internal Zone. Bound at the top by the master decollement of the Sevier fold and thrust belt and at the bottom by a west-dipping, normal-sense shear zone, this extensional allochthon would have been effectively decoupled from the upper and lower crust and free to accommodate gravitational collapse during continued convergence across the orogen.

Journal ArticleDOI
TL;DR: In this paper, structural geometries and crosscutting stratigraphic packages unambiguously illustrate a sequence of break-back thrusts that developed synchronously with the growth of an adjacent duplex.
Abstract: Clear demonstrations of detailed sequences of thrust development are rare, particularly with respect to hindward-imbricating successions. Similarly, well-constrained chronologies of the timing of multiple thrust motions and associated rates of shortening are also uncommon. Along the eastern oblique ramp of the South-Central Unit (SCU) of the Pyrenees, well-exposed structural geometries and crosscutting stratigraphic packages unambiguously illustrate a sequence of break-back thrusts that developed synchronously with the growth of an adjacent duplex. The break-back sequence includes both the Sierras Marginales and Montsec thrusts: structures defining two of the major east-west partitions within the SCU. Balanced, sequentially restored structural cross sections indicate that a minimum of 7.5 km of shortening occurred within the imbricate stack. An additional 9 km of contraction was accommodated by the duplex, prior to translation of the duplex and imbricate stack to the south along a detachment within upper Eocene evaporites. Magnetic polarity stratigraphies have been used to calibrate these late Eocene to Oligocene episodes of deformation. These temporal data indicate that Sierras Marginales thrusting began in the study area ∼39.8 Ma and that the Montsec thrust ceased movement ∼36.2 Ma, yielding a mean rate of shortening of 2.1 mm/yr. Coeval shortening within the duplex was ∼2.5 mm/yr, and subsequent translation across the footwall ramp occurred at ∼2.5-4 mm/yr. Despite the partitioning of shortening into distinctively different styles of deformation, the mean rate of contraction remained steady over ∼4 m.y. These new chronologies indicate that the locus of large-scale thrusting did not progress systematically toward the foreland, but shifted episodically within the SCU, such that, during the late Eocene shortening, the Sierras Marginales thrust was active both prior to and south of the Montsec thrust.

Journal ArticleDOI
TL;DR: The authors reconstructs the ice surface morphology of several lobes of the southern Laurentide Ice Sheet and finds that they are thin and low sloping, similar to the distal ends of low-sloping (0.4 X 10 -3 ) but fast moving (500 m/yr) West Antarctic ice streams.
Abstract: Reconstructions of the ice-surface morphology of several lobes of the southern Laurentide Ice Sheet reinforce previous arguments that this sector of the ice sheet was thin and low sloping. Driving stresses, estimated from the geometry of the reconstructed ice surfaces, are 0.7-4.3 kPa for the 14 ka Des Moines Lobe, 0.9-1.2 kPa for the 14 ka James Lobe, 0.9-1.7 kPa for the 18-20 ka Lake Michigan Lobe, 1.8-2.9 kPa for the 15-18 ka Chippewa Sublobe, and 17-22 kPa for the 15-18 ka Green Bay Lobe. Previous estimates of rates of ice-margin advance (450-2,000 m/yr) indicate moderate-to-fast ice velocities for the ice lobes. Reconstructed driving stresses and velocity estimates of the Des Moines, James, and Lake Michigan Lobes are analogous to the distal ends ("ice Plains") of low-sloping (0.4 X 10 -3 ) but fast moving (500 m/yr) West Antarctic ice streams, whose dynamics have been attributed to sliding and/or subglacial sediment deformation by pervasive shear. These reconstructions support recent models of the Laurentide Ice Sheet which include movement by sliding or by subglacial sediment deformation along its southern, western, and northwestern sectors; evidence for either mechanism should be represented in the sedimentologic and geomorphic records. Thin ice in these regions indicates that the Laurentide Ice Sheet contained less ice volume and represented less of an orographic obstacle to atmospheric circulation than has been considered in models of the ice sheet on a rigid bed with steep profiles.

Journal ArticleDOI
TL;DR: In this paper, a series of bottom-hole temperatures (BHTs) measured near the bottom of petroleum exploration wells was used to estimate the equilibrium temperature profiles of shallow (average 600m depth) temperature profiles, which were extrapolated to infinite time to yield equilibrium temperature profile (± 0.1 °C).
Abstract: In conjunction with the U.S. Geological Survey's exploration program in the National Petroleum Reserve, Alaska (NPRA) several high-resolution temperature logs were made in each of 21 drillholes between 1977 and 1984. These time-series of shallow (average 600-m depth) temperature profiles were extrapolated to infinite time to yield equilibrium temperature profiles (±0.1 °C). Thermal gradients are inversely correlated with elevation, and vary from 22 °C/km in the foothills of the Brooks Range to as high as 53 °C/km on the coastal plain to the north. Shallow temperature data were supplemented with 24 equilibrium temperatures (±3-5 °C) estimated from series of bottom-hole temperatures (BHTs) measured near the bottom of petroleum exploration wells. A total of 601 thermal conductivity measurements were made on drill cuttings and cores. Near-surface heat flow (±20%) is inversely correlated with elevation and ranges from a low of 27 mW/m2 in the foothills of the Brooks Range in the south, to a high of 90 mW/m2 near the north coast. Subsurface temperatures and thermal gradients estimated from corrected BHTs are similarly much higher on the coastal plain than in the foothills province to the south. Significant east-west variation in heat flow and subsurface temperature is also observed; higher heat flow and temperature coincide with higher basement topography. The observed thermal pattern is consistent with forced convection by a topographically driven ground-water flow system; alternative explanations are largely unsatisfactory. Average ground-water (Darcy) velocity in the postulated flow system is estimated to be of the order of 0.1 m/yr; the effective basin-scale permeability is estimated to be of the order of 10-14 m2. Organic maturation data collected in other studies indicate that systematic variations in thermal state may have persisted for tens of millions of years. The ground-water flow system thought to be responsible for present heat-flow variations conceivably has existed for the same period of time, possibly providing the driving mechanism for petroleum migration and accumulation at Prudhoe Bay.

Journal ArticleDOI
TL;DR: The Newark basin is a half-graben, bounded on its northwestern margin by a system of mostly normal-slip faults, many of which are reactivated Paleozoic thrust faults and were active at least sporadically during the deposition of the entire basin fill as discussed by the authors.
Abstract: The Newark basin is a half graben, bounded on its northwestern margin by a system of mostly normal-slip faults, many of which are reactivated Paleozoic thrust faults and were active at least sporadically during the deposition of the entire basin fill. As a result of along-strike variations in displacement on the border-fault system, subsidence increased from the lateral edges toward the center of the basin and from the hinged margin toward the border-fault system. Some intrabasinal faults also formed during the deposition of units currently preserved in their hanging walls. Along-strike variations in the density of intrabasinal faults probably reflect changes in the dip angle of the border-fault system; variations in the amount of extension accommodated on these intrabasinal faults resulted in the formation of a transfer fault. Along-strike variations in fault displacement probably produced the transverse folds in the hanging walls immediately adjacent to border faults and major intrabasinal normal faults; some folds formed syndepositionally. Progressively younger strata onlap "basement" rocks along the hinged margin of the basin, and younger splays of the border-fault system propagated into the footwall; the basin therefore grew deeper, wider, and longer through time.

Journal ArticleDOI
TL;DR: In this paper, a detailed geologic mapping and structural analysis of the Main Central Thrust (MCT) in the Langtang National Park region of central Nepal reveals that this segment of the fault zone experienced multiple episodes of south-directed movement, under both brittle and ductile conditions, during the Tertiary period.
Abstract: The Main Central Thrust (MCT) is one of the most tectonically significant structures in the Himalayan orogen. Detailed geologic mapping and structural analysis of the MCT in the Langtang National Park region of central Nepal reveals that this segment of the fault zone experienced multiple episodes of south-directed movement, under both brittle and ductile conditions, during the Tertiary period. Early (mid-Miocene) movement resulted in the development of mylonitic fabrics synchronous with amphibolite-facies metamorphism. The mean orientation of the dominant mylonitic foliation is N28°W, 38°NE. An associated mineral/stretching lineation plunges 40° to N40°E. Kinematic indicators suggest hanging-wall movement to the southwest relative to the footwall along the north-dipping fault. It is not possible to constrain the magnitude of high-temperature displacement on the MCT at the longitude of Langtang. Late-stage structures in the MCT zone at Langtang include a series of imbricate, brittle thrust faults that separate different lithostratigraphic units and correspond to metamorphic discontinuities. We interpret this fault system as a duplex structure. Muscovite 40 Ar/ 39 Ar cooling ages from the MCT zone range from 8.9-6.9 Ma. Because the nominal closure temperature of Ar diffusion in muscovite (approximately 625 K) is higher than the apparent temperature conditions under which late brittle deformation occurred, we suggest that brittle deformation was a latest Miocene-Pliocene phenomenon. Another major Himalayan fault, the Main Boundary Thrust (MBT), was developing to the south of Langtang at approximately the same time. We speculate that brittle faulting within the MCT zone may have initiated as the MCT zone was transported over a ramp in the MBT.

Journal ArticleDOI
TL;DR: In this paper, the authors examined the stratigraphy, structure, and seismicity of the Peter the First Range, an actively deforming foreland basin in Soviet Tadjikistan.
Abstract: This paper examines the stratigraphy, structure, and seismicity of the Peter the First Range, an actively deforming foreland basin in Soviet Tadjikistan. The range represents a highly deformed segment of a large intracontinental basin, the Tadjik Depression, which is being shortened in response to the Cenozoic convergence between two orogenic massifs, the Pamir and the Tien Shan Ranges. The Late Jurassic to Paleogene stratigraphy of the Peter the First Range includes more than 2,000 m of shallow marine to nonmarine sediments deposited under relatively quiescent conditions, close to sea level. In contrast, the correlative units of the northernmost Pamir, directly south of the Peter the First Range, show evidence of proximity to a major, uplifted source area to the south. These Mesozoic-early Cenozoic strata are overlain by thick sequences of alluvial sandstone and conglomeratic units that coarsen upward through the Neogene and record the progressive uplift of the Pamir. Shortening within the Peter the First Range is recorded by kilometer-scale isoclinal, upright-to-over-turned folds, and imbricate, north-verging thrusts that emerge near the northern edge of the range and mark the active tectonic boundary with the Tien Shan. Most of the deformation appears to postdate the accumulation of the Mio-Pliocene molasse. The stratigraphy and structure of the region are best explained by a model of the Mesozoic Tadjik Depression as an intracontinental, foreland basin, developed in a back-arc position with respect to the active Andean or collisional orogens located in northern Afghanistan and the Pamir. Seismicity in the region is dominated by earthquakes extending from the surface to ∼12-km depth, apparently occurring within the deformed sediments of the Peter the First Range. A subhorizontal zone of relatively low activity at this depth may mark the location of a basal detachment that underlies a structurally thickened sedimentary section. This aseismic zone is in turn underlain by a south-dipping belt of seismicity that extends from about 17- to 35-km depth, and which can be traced updip to the seismogenic crystalline basement of the Tien Shan range to the north. We interpret this south-dipping structure as a zone of intracontinental subduction that may be an updip continuation of the enigmatic, intermediate-depth Wadati-Benioff Zone beneath the Pamir Range to the south.

Journal ArticleDOI
TL;DR: In this article, it was shown that a significant part of the "Schistes lustres" complex is made up of syn-orogenic Upper Cretaceous deposits, which is interpreted as a sedimentary consequence of the complete or partial closure of the Tethyan ocean.
Abstract: Recent lithostratigraphic and structural studies made in the Western Alps along the Valais, Savoy, and Queyras "Schistes lustres" transects indicate that there are strong similarities in the nature of the sedimentary series and in the general structural edifice between these areas. The main new conclusion is that a significant part of the "Schistes lustres" complex is made up of syn-orogenic Upper Cretaceous deposits. A first type, present in all three transects, consists of lower Upper Cretaceous metasedimentary rocks, containing oceanic and continental detrital materials, which were deposited on the western edge of the northwest Tethyan paleomargin and in the oceanic realm undergoing tectonization. A second type is made up of Upper Cretaceous calcareous-gritty metaflysch, known only in the Savoy transect, which is interpreted as a sedimentary consequence of the complete or partial closure of the Tethyan ocean.

Journal ArticleDOI
TL;DR: Aminozones C, E, F, G, H, J, and K were used in this article to determine the extent of isoleucine epimerization in marine shells, land snails, and whole-rock samples of bioclastic calcarenites.
Abstract: The Pleistocene stratigraphy of Bermuda offers important information for sea-level history. Corals are rare in Bermuda Pleistocene carbonates, and so many deposits are not appropriate for U-series dating. We have extended the geochronological base in Bermuda by determining the extent of isoleucine epimerization (A/I ratios) in (1) marine shells ("Glycymeris" and others) from deposits of sea-level high stands, (2) land snails ("Poecilozonites") from protosols associated with the eolianites that comprise the bulk of Bermuda, and (3) whole-rock samples of bioclastic calcarenites. The three distinct types of samples provide independent aminostratigraphies (Aminozones C, E, F, G, H, J, and K) that are mutually consistent and in agreement with the mapped lithostratigraphy. Correlation of the amino acid ratios with ages based on previously published U-series coral dates verifies the utility of the apparent parabolic kinetics model for estimation of amino acid racemization ages. Extrapolation of ages based on the model to deposits beyond the range of U-series dating reveals that a significant middle Pleistocene section is present in Bermuda.

Journal ArticleDOI
TL;DR: In contrast to previous conclusions, sheetflood and sieve deposits were not identified anywhere on the fan as mentioned in this paper, and many of the facies depicted in the widely cited "Trollheim vertical profile" are not present on, or in stratigraphic cuts of, the fan after which the profile was named.
Abstract: Widely held, fundamental alluvial-fan facies concepts generated in past literature based on the Trollheim fan, Deep Springs Valley, California, are inconsistent with the actual characteristics of this fan Previous publications have established, mostly without field study, that the Trollheim fan consists of debris-flow, sieve lobe, sheetflood, and channel facies; debris-flow and sieve deposits comprise the proximal area, giving way to channel or sheetflood deposits distally Contrary to these views, clayey and gravelly, matrix-supported, debris-flow deposits dominate all parts of the Trollheim exposures, suggesting that this fan has been built almost exclusively by mass-wasting events Channel-fill deposits constitute only a small component of the fan, and most are restricted to the proximal area where they have an inset relationship with debris-flow sequences The channel facies consists of laterally restricted, clast-supported gravel lenses formed primarily through the winnowing of debris-flow fines by streamflow The most widespread facies on the surface of the fan is a mantle of varnished, matrix-free pebbles, cobbles, and boulders that commonly are distributed in radially oriented lobes or levees characteristic of debris-flow deposits This mantle was produced by the surficial winnowing of the fine-fraction from debris-flow deposits primarily by overland water flow In contrast to previous conclusions, sheetflood and sieve deposits were not identified anywhere on the fan Additionally, many of the facies depicted in the widely cited "Trollheim vertical profile," including ripple-bedded sand, cross-bedded sand, and overbank mud, are not present on, or in stratigraphic cuts of, the fan after which the profile was named

Journal ArticleDOI
TL;DR: The Dun Mountain ophiolite and overlying Maitai Group is discontinuously exposed for 480 km in South Island, New Zealand as discussed by the authors, and high-precision 207 Pb/ 206 Pb dates from plagiogranite zircones are presented for relatively intact ophiola, ophitic melanges, and for more silicicic volcanic-plutonic assemblages in the southern part of the belt.
Abstract: The Dun Mountain ophiolite and overlying Maitai Group is discontinuously exposed for 480 km in South Island, New Zealand. Zircon U/Pb dates from plagiogranite are presented for relatively intact ophiolite, ophiolitic melanges, and for more silicic volcanic-plutonic assemblages in the southern part of the belt where a typical ophiolite association is lacking. Step-wise dissolution experiments on slightly discordant plagiogranite zircon produce more concordant residues that indicate the zircons have lost from ∼1% to more than 5% of their radiogenic Pb relatively recently. High-precision 207 Pb/ 206 Pb dates establish the age of ophiolite formation for at least 350 km along strike to a narrow interval between about 275 and 285 Ma. The zircon U/Pb data confirm correlation of petrologically distinct segments of the Dun Mountain ophiolite and show that mafic-ultramafic ophiolite assemblages and moderately potassic high-level granitoids developed within a short time interval, probably during the extension of a volcanic-arc marginal basin. Thick lenses of polymictic breccia and bio-clastic limestone of the Maitai Group locally rest in depositional contact on relatively intact ophiolite within the Dun Mountain ophiolite. Comparison of inferred biostratigraphic ages from the limestone with the ∼280 Ma ages from the plagiogranites indicate a gap of ∼20 m.y. following ophiolite formation. A granite clast from conglomerate higher in the Maitai Group yielded a near concordant U/Pb date of 265 Ma and provides a maximum age for this part of the sequence. Attenuation of the Dun Mountain ophiolite by extensional faulting and erosion may have occurred during the interval between ophiolite formation and Maitai Group sedimentation. The Dun Mountain ophiolite and overlying Maitai Group are bounded to the west by Triassic and Jurassic volcanogenic sedimentary rocks of the Murihiku terrane, and in turn by the Brook Street terrane, which is interpreted as remnants of an early Permian oceanic arc. A hornblende gabbronorite associated with a layered mafic-ultramafic intrusion in the Brook Street terrane yielded a date of 265 Ma, significantly younger than Dun Mountain ophiolite. Such intrusions may represent the plutonic roots for ankaramitic volcanic rocks that comprise a conspicuous component of the Brook Street terrane, but which are not represented by detritus in the Maitai Group. Biotite granite occurs locally in the Brook Street terrane and is dated at 260 Ma. The absence of any clear stratigraphic correlation or provenance linkage between the Brook Street terrane and Dun Mountain-Maitai terrane suggests strike-slip displacements on intervening terrane boundary faults.

Journal ArticleDOI
TL;DR: In the Hamersley Basin, sand-sized spherules of K-feldspar with spherulitic, vesicular, and other crystalline textures help to define a unique marker bed in the 2.6 Ga Wittenoom Formation.
Abstract: Predominantly sand-sized spherules of K-feldspar with spherulitic, vesicular, and other crystalline textures help to define a unique marker bed in the 2.6 Ga Wittenoom Formation which persists for more than 300 km across the Hamersley Basin of Western Australia. These spherules usually occur as a minor constituent of a single turbidite, but locally they are restricted to centimeter-thick lenses of pure spherules along a single horizon in the argillite close beneath it. In the shallower northeastern part of the Basin, similar spherules (as well as larger, more heterogeneous particles) likewise occur at only one horizon, but here they are a minor constituent of a thick dolomitic debris-flow deposit. As the Wittenoom Formation and the Carawine Dolomite are thought to be stratigraphically equivalent, this debris-flow deposit is believed to be a proximal equivalent of the spherule-bearing turbidite. On the basis of their similarity to microtektites and microkrystites in shape, size, and mode of occurrence, the spherules are interpreted as droplets of silicate melt that were generated and dispersed across the Hamersley Basin by a major bolide impact. Moreover, the mass of the spherules preserved in the Hamersley Basin is of the same order of magnitude as the estimated masses of microtektite glass in major Cenozoic strewn fields, despite the fact that the spherules cover an area which is 2 to 3 orders of magnitude smaller. The layers which host the spherules are interpreted as the deposits of a major sediment gravity flow which exhumed and redeposited most of the spherules after shallow burial. The flow, however, is not believed to have been a direct result of the proposed impact. If the spherule-bearing layers constitute a chronostratigraphic marker, as suggested here, there are important implications for the deposition of carbonates and banded iron formations (BIF) within the Hamersley Basin. Similar spherules also occur at a single horizon ∼250 m higher stratigraphically in the Brockman Iron Formation, suggesting that two impacts occurred near the Hamersley Basin in relatively rapid succession. Correlative strata may be present in the Transvaal Basin of South Africa.

Journal ArticleDOI
TL;DR: In this article, the authors evaluated the Neogene stratigraphic succession beneath the continental shelf off New Jersey using high-quality seismic and well data to construct geologic cross sections that relate seismic geometries to lithofacies in a prograding, tectonically quiescent setting.
Abstract: We have evaluated the Neogene stratigraphic succession beneath the continental shelf off New Jersey using high-quality seismic and well data to (1) construct geologic cross sections that relate seismic geometries to lithofacies in a prograding, tectonically quiescent setting; (2) apply a sequence-stratigraphic approach to subdivide the stratigraphic section and relate the characteristics of the deposition observed sequences to the published Exxon model; and (3) determine the resolution of the available biostratigraphy in the correlation of the offshore New Jersey depositional sequences to the published eustatic cycle chart In the offshore New Jersey area, the well-developed progradational geometries in the Neogene section, the high-quality seismic data, and the relatively abundant lithologic information from wells allow us to use the results of this study to predict lithofacies distribution in similar, but less geologically constrained, settings Depositional sequences are characterized by similar overall stacking patterns within component systems tracts; however, the New Jersey sequences are different in several ways These differences include the lack of type-2 unconformities, an absence of leveed channel deposits within lowstand wedges, very poorly developed transgressive systems tracts, and sandstone-rich, high regressive highstand systems tracts These differences are explained in terms of the limited Neogene accommodation on the Newmore » Jersey shelf that resulted from slow subsidence rates and a second-order Miocene eustatic fall Paleontologic data bracket the ages of the individual sequences but are not detailed enough at present to correlate precisely to the time scale and inferred global sea-level records Based on the available biostratigraphy, however, Neogene depositional sequences from offshore New Jersey have a cyclicity consistent with interpreted third-order eustatic fluctuations on the global cycle chart 18 refs, 6 figs, 1 tab« less

Journal ArticleDOI
TL;DR: In the Geological Society of America Bulletin this paper, Chaimov et al. published a paper entitled "Geological Society Bulletin 1992: A Survey of the World's Top 100 Regions".
Abstract: This paper was published in the Geological Society of America Bulletin. The Geological Society of America retains the copyright to this paper. Geological Society of America, P.O. Box 9140 , Boulder, CO 80301-9140 USA See also: http://www.geosociety.org/; http://atlas.geo.cornell.edu/syria/chaimov_gsa_bull_1992.html