82 resultados para Upper Strata

em Publishing Network for Geoscientific


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At Site 464, a 308-meter-deep hole was drilled on the Hess Rise (39°51.64'N, 173°53.33'E; water depth 4637 m). The upper strata consist of siliceous clay and oozes of Pleistocene, Pliocene, and possibly latest Miocene ages (36 m) and the lower strata are Albian chert, chalk, and marlstone (219 m). The middle part of the stratigraphic section is pelagic brown clay (53 m) which is essentially barren of microfossils except for ichthyoliths. At the base of this middle unit in Core 10 is a recrystallized Cretaceous radiolarian assemblage and an impoverished late Miocene nannofossil assemblage. Fifteen samples from Cores 6 through 10 in the middle unit and three samples from the overlying siliceous clay were examined for ichthyoliths.

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Previous workers identified a magnetically anomalous clay layer deposited on the northern United States Atlantic Coastal Plain during the Paleocene-Eocene thermal maximum (PETM). The finding inspired the highly controversial hypothesis that a cometary impact triggered the PETM. Here we present ferromagnetic resonance (FMR), isothermal and anhysteretic remanent magnetization, first-order reversal curve, and transmission electron microscopy analyses of late Paleocene and early Eocene sediments in drill core from Ancora, New Jersey. A novel paleogeographic analysis applying a recent paleomagnetic pole from the Faeroe Islands indicates that New Jersey during the initial Eocene had a ~6°-9° lower paleolatitude (~27.3° for Ancora) and a more zonal shoreline trace than in conventional reconstructions. Our investigations of the PETM clay from Ancora reveal abundant magnetite nanoparticles bearing signature traits of crystals produced by magnetotactic bacteria. This result, the first identification of ancient biogenic magnetite using FMR, argues that the anomalous magnetic properties of the PETM sediments are not produced by an impact. They instead reflect environmental changes along the eastern margin of North America during the PETM that led to enhanced production and/or preservation of magnetofossils.

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Geochemical behavior of Rb-Sr and K-Ar systems in Upper Vendian clayey rocks of the Russian Platform is under consideredation. The use of additional data on grain size fractions of sedimentary rocks recovered from boreholes drilled in the Gavrilov Yam area made it possible to confirm the previous conclusion on two stages of epigenetic matter transformation (approximately 600 and 400 Ma ago). Distortions are related to transformation of sediments due to interaction in the water-rock system. Interaction degree was more intense in the upper part of the sedimentary section relative to its lower strata. These conclusions are substantiated by materials from boreholes that characterize different types of Vendian sections and different tectonic zones.

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This report contains the occurrence data for dinoflagellate cysts recorded from 163 samples taken from Sites 902 through 906, during Ocean Drilling Program (ODP) Leg 150. The dinoflagellate cyst (dinocyst) stratigraphy has been presented in Mountain, Miller, Blum, et al. (1994, doi:10.2973/odp.proc.ir.150.1994), and was based on these data. This report provides the full dinocyst data set supporting the dinocyst stratigraphic interpretations made in Mountain, Miller, Blum, et al. (1994). For Miocene shipboard dinocyst stratigraphy, I delineated 10 informal zones: pre-A, and A through I, in ascending stratigraphic order. These zones are defined in Shipboard Scientific Party (1994a, doi:10.2973/odp.proc.ir.150.103.1994), and are based on my studies of Miocene dinocyst stratigraphy in the Maryland and Virginia coastal plain (de Verteuil and Norris, 1991, 1992; de Verteuil, 1995). This zonation has been slightly revised (de Verteuil and Norris, 1996), and the new formal zone definitions are repeated below. Each new zone has an alpha-numeric abbreviation starting with "DN" (for Dinoflagellate Neogene). The equivalence between the informal zones reported in Mountain, Miller, Blum, et al. (1994), and the new DN zones is illustrated in Figure 1. For clarity, I delineated both zonations in the range charts that accompany this report (Tables 1-6). De Verteuil and Norris (1996a), using these and other data, correlated the DN zonation with the geological time scale of Berggren et al. (1995). Figure 2 summarizes these correlations and can be used to check the chronostratigraphic position of samples in this report, as determined by dinocyst stratigraphy. A thorough discussion of the basis for, and levels of uncertainty associated with, these correlations to the Cenozoic time scale can be found in de Verteuil and Norris (1996a). The Appendix lists all the dinocyst taxa recorded during shipboard analyses of Leg 150 samples. Open nomenclature is used for undescribed taxa. The range charts and Appendix also include reference to several new taxa that de Verteuil and Norris (1996b) described from Miocene coastal plain strata in Maryland and Virginia. Names of these taxa in Tables 1 through 6 and in the Appendix of this report are not intended for effective publication as defined in the International Code of Botanical Nomenclature (ICBN, Greuter et al., 1994). Therefore, taxonomic nomenclature contained in this report is not to be treated as meeting the conditions of effective and valid publication (ICBN; Article 29).

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The chronostratigraphy of Guandao section has served as the foundation for numerous studies of the end-Permian extinction and biotic recovery in south China. Guandao section is continuous from the Permian-Triassic boundary to the Upper Triassic.Conodonts enable broad delineation of stage and substage boundaries and calibration of foraminifer biostratigraphy as follows. Changhsingian- Griesbachian: first Hindeodus parvus, and first appearance of foraminifers Postcladella kalhori and Earlandia sp. Griesbachian-Dienerian: first Neospathodus dieneri, and last appearance of foraminifer P. grandis. Dienerian-Smithian: first Novispathodus waageni and late Dienerian first appearance of foraminifer Hoyenella ex gr. sinensis. Smithian-Spathian: first Nv? crassatus and last appearance of foraminifers Arenovidalina n. sp. and Glomospirella cf. vulgaris. Spathian-Aegean: first Chiosella timorensis and first appearance of foraminifer Meandrospira dinarica. Aegean-Bithynian: first Nicoraella germanica and first appearance of foraminifer Pilammina densa. Bithynian-Pelsonian: after last Neogondolella regalis, prior to first Paragondolella bulgarica and first appearance of foraminifer Aulotortus eotriasicus. Pelsonian-Illyrian: first Pg. excelsa and last appearance of foraminifers Meandrospira ? deformata and Pilamminella grandis. Illyrian-Fassanian: first Budurovignathus truempyi, and first appearance of foraminifers Abriolina mediterranea and Paleolituonella meridionalis. Fassanian-Longobardian: first Bv. mungoensis and last appearance of foraminifer A. mediterranea. Longobardian-Cordevolian: first Quadralella polygnathiformis and last appearance of foraminifers Turriglomina mesotriasica and Endotriadella wirzi. The section contains primary magnetic signature with frequent reversals occurring around the Permian-Triassic, Olenekian-Anisian, and Anisian-Ladinian boundaries. Predominantly normal polarity occurs in the lower Smithian, Bithynian, and Longobardian-Cordevolian. Predominantly reversed polarity occurs in the upper Griesbachian, Induan-Olenekian, Pelsonian and lower Illyrian. Reversals match well with the GPTS. Large amplitude carbon isotope excursions, attaining values as low as -2.9 per mil d13C and high as +5.7 per mil d13C, characterize the Lower Triassic and basal Anisian. Values stabilize around +2 per mil d13C through the Anisian to Carnian. Similar signatures have been reported globally. Magnetic susceptibility and synthetic gamma ray logs show large fluctuations in the Lower Triassic and an overall decline in magnitude of fluctuation through the Middle and Upper Triassic. The largest spikes in magnetic susceptibility and gamma ray, indicating greater terrestrial lithogenic flux, correspond to positive d13C excursions. Several volcanic ash horizons occur in the Lower Triassic and Olenekian-Anisian boundary. High resolution U-Pb analysis of zircons provide a robust age of 247.2 Ma for the Olenekian-Anisian boundary.

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Numerous sapropels and sapropelic strata from Upper Pliocene and Pleistocene hemipelagic sediments of the Tyrrhenian Sea show that intermittent anoxia, possibly related to strongly increased biological productivity, was not restricted to the eastern Mediterranean basins and may be a basin-wide result of Late Pliocene-Pleistocene climatic variability. Even though the sapropel assemblage of the Tyrrhenian Sea clearly originates from multiple processes such as deposition under anoxic conditions or during spikes in surface water productivity and lateral transport of organic-rich suspensates, many "pelagic sapropels" have been recognized. Stratigraphic ages calculated for the organic-rich strata recovered during ODP Leg 107 indicate that the frequency of sapropel formation increased from the lowermost Pleistocene to the base of the Jaramillo magnetic event, coinciding with a period when stable isotope records of planktonic foraminifera indicate the onset of climatic cooling in the Mediterranean. A second, very pronounced peak in sapropel formation occurred in the Middle to Late Pleistocene (0.73-0.26 Ma). Formainifers studied in three high-resolution sample sets suggest that changes in surface-water temperature may have been responsible for establishing anoxic conditions, while salinity differences were not noted in the faunal assemblage. However, comparison of sapropel occurrence at Site 653 with the oxygen isotopic record of planktonic foraminifers established by Thunell et al. (1990, doi:10.2973/odp.proc.sr.107.155.1990) indicates that sapropel occurrences coincide with negative d18O excursions in planktonic foraminifers in thirteen of eighteen sapropels recognized in Hole 653A. A variant of the meltwater hypothesis accepted for sapropel formation in the Late Pleistocene eastern Mediterranean may thus be the cause of several "anoxic events" in the Tyrrhenian as well. Model calculations indicate that the amount of oxygen advection from Western Mediterranean Deep Water exerts the dominant control on the oxygen content in deep water of the Tyrrhenian Sea. Inhibition of deep-water formation in the northern Adriatic and the Balearic Basin by increased meltwater discharge and changing storm patterns during climatic amelioration may thus be responsible for sapropel formation in the Tyrrhenian Sea.

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Abyssal agglutinated foraminifers allow biostratigraphic correlation of Upper Cretaceous brown zeolitic claystones in Deep Sea Drilling Project Holes 196A and 198A and Ocean Drilling Program Holes 800A and 801 A. Three agglutinated foraminiferal zones subdivide the strata overlying the Campanian to Cenomanian cherts. The lower zone is characterized by Hormosina gigantea, which is a Campanian zonal marker in the North Atlantic Ocean and western Tethys. A major correlation level, which was observed in all holes studied, is based on the acme of evolute Haplophragmoides spp. This acme zone was observed in Sample 129-801A-6R-CC, about 9 m above the first occurrence of H. gigantea in Sample 129-801A-7R-1, 62-67 cm (approximately middle Campanian). The uppermost zone is characterized by dominant Paratrochamminoides spp. and in some instances common Bolivinopsis parvissimus (late Campanian to Maestrichtian). The available biostratigraphic data for the Upper Cretaceous of Sites 196, 198, 800, and 801 are correlated with the biochronologic framework of the North Atlantic, western Mediterranean, and Carpathians. Additionally, we use quantitative estimates of the diversity and abundance of agglutinated foraminiferal species to monitor general faunal trends with time in the western Pacific.

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We reconstruct the latest Paleocene and early Eocene (~57-50 Ma) environmental trends in the Arctic Ocean and focus on the Paleocene-Eocene thermal maximum (PETM) (~55 Ma), using strata recovered from the Lomonosov Ridge by the Integrated Ocean Drilling Program Expedition 302. The Lomonosov Ridge was still partially subaerial during the latest Paleocene and earliest Eocene and gradually subsided during the early Eocene. Organic dinoflagellate cyst (dinocyst) assemblages point to brackish and productive surface waters throughout the latest Paleocene and early Eocene. Dinocyst assemblages are cosmopolitan during this time interval, suggesting warm conditions, which is corroborated by TEX86'-reconstructed temperatures of 15°-18°C. Inorganic geochemistry generally reflects reducing conditions within the sediment and euxinic conditions during the upper lower Eocene. Spectral analysis reveals that the cyclicity, recorded in X-ray fluorescence scanning Fe data from close to Eocene thermal maximum 2 (~53 Ma, presence confirmed by dinocyst stratigraphy), is related to precession. Within the lower part of the PETM, proxy records indicate enhanced weathering, runoff, anoxia, and productivity along with sea level rise. On the basis of total organic carbon content and variations in sediment accumulation rates, excess organic carbon burial in the Arctic Ocean appears to have contributed significantly to the sequestration of injected carbon during the PETM.

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Clasts from the Cape Roberts Project cores CRP-2/2A and CRP-3 provide indications of glacially influenced depositional environments in Oligocene and Miocene strata in the western Victoria Land Basin, Antarctica. CRP-2/2A is interpreted to represent strongly glacially influenced, unconformity bound depositional sequences produced by repeated advance and retreat of floating and grounded ice across the shelf. A similar interpretation is extended to the upper 330 meters of the CRP-3 core, but the lower part of the core records shallow marine deposition with significantly less glacial influence. Clast shape analysis from selected coarse-grained facies throughout the cored interval indicates that most clasts are glacially sourced, with little distinction between diamictite and conglomeratic facies. Three dimensional clast fabric analysis from units immediately above sequence boundaries generally display weak or random fabrics and do not suggest that grounded ice actually reached the drillsite at these intervals. Striated and outsized clasts present in fine-grained lithofacies throughout the cores provide further evidence of sub-glacially transported sediment and iceberg rafting. The distribution of these striated and out-sized clasts indicate that a significant glacial influence persisted through most of the time represented by the cores with glaciers actively calving at sea-level introducing ice-berg rafted glacial debris even in the earliest Oligocene.

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Grain-size analyses by sieve and Sedigraph are presented for 115 samples of core from CRP-3, 12 km off the coast of south Victoria Land. The data provide a useful check on visual core descriptions. The geographic setting for the strata sampled, some 790 m of early Oligocene nearshore marine sediments with a persistent glacial influence, is reviewed, and sediment textures interpreted in that context. Sand textures from the CRP-3 samples in the lower part of the core suggest that deposition was initially primarily wave-dominated, but that at times the influence of the waves was over-ridden by episodes of rapid sedimentation. Sedimentary cycles, recognised in the visual description of the core above 485 mbsf, show an increasing proportion of mudstone in the middle of each cycle above 330 mbsf that is interpreted to record periodic sedimentation in deeper water. Sandstone textures in the lower and upper parts of each cycle are interpreted to record departure from and return to shoreface deposition with changes in sea level. Mudstone textures above 176 mbsf indicate sedimentation below wave base. Many of the textures in both sand and mud samples show the coarse 'tail' characteristic of ice-rafted debris, but others do not, indicating ice-free periods. Many sandstones below c. 200 mbsf have virtually no silt, but significant amounts of clay (6 to 17%) that is thought to be of post-depositional origin.