Browsing by Author "Valley, John W."

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    Marine geology of the western Ross Sea: implications for Antarctic glacial history
    (1982) Meyers, Nathan Cebren; Anderson, John B.; Valley, John W.; Clark, Howard C.
    The stability of the Antarctic Ice Sheet has long been the subject of debate. Recently, basal till has been identified in the Ross Sea, yielding evidence for the expansion of the Antarctic Ice Sheet and its grounding in the Ross Sea. However, the sedimentologic criteria used to identify basal till may be inconclusive. Therefore, one purpose of this study is to support the validity of these sedimentologic criteria. Another goal is to determine if basal tills are present in the western Ross Sea. Marine sedimentologic processes on high latitude continental shelves are not well known. Therefore, surface sediment distributions can be used to infer marine processes active in the western Ross Sea. Three major sediment types, siliceous ooze and mud, sands, and diamictons were identified. Sands include graded volcanic sands, granitic sands, and calcareous shell hash sands. Diamictons include compound glacial marine sediment, basal till, and mass flow diamictons. Siliceous sediment distribution is controlled both by bottom currents and surface currents. Sands are found in near-coastal or shallow waters, or in areas of high sediment supply. The distribution of surface sediments in the western Ross Sea is controlled by thermohaline bottom currents, by surface currents, by the presence of polynyas, and by sediment gravity flow processes. Basal tills were identified as far north as Coulman Island, and as far south as Ross Island, but no basal tills were found in McMurdo Sound. A petrologic analysis of these basal tills reveals that three petrologic provinces can be recognized in the western Ross Sea and four petrologic provinces can be recognized in the central Ross Sea. The boundaries between petrologic provinces can be correlated with geologic provinces in Victoria Land and Marie Byrd Land. Paleo-ice flow paths are reconstructed based on petrologic province boundaries and bathymetry, and indicate that during the last glacial maximum ice flowed into the Ross Sea from both East and West Antarctica. The petrologic data also indicate that sedimentologic criteria used to identify basal tills are indeed valid. The marine sedimentologic record in the western Ross Sea indicates that the ice sheet which grounded there initially eroded the sea floor, and later deposited basal till except in Mcmurdo Sound where flow was restricted. Sharp contacts between basal till and compound glacial marine sediment indicate that decoupling of the Ice Sheet from its base was rapid; gradational contacts between compound glacial marine sediment and siliceous sediments indicate that ice-shelf retreat was slow allowing siliceous sediments to slowly dominate sedimentation.
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    The fluid phase in the granulite facies: evidence from the Adirondack Mountains, N. Y
    (1984) Lamb, William M.; Valley, John W.; Lallemant, Hans G. Avé; Leeman, William P.
    The fluid phase plays an important role in many crustal processes, such as metamorphism, metasomatism, and partial melting, yet little is known about the fluid phase in the deep crust. Early workers assumed that H2O and CO2 were relatively constant with P(H2O) = P(lithostatic) in non-carbonate metamorphic rocks while P(CO2) = P(lithostatic) in carbonates (Turner, 1948). The granulite facies may be an important exception to this traditional view of metamorphic fluids as the lack of hydrous minerals suggests that this may be a metamorphic regime where the fugacity of water is low. Phase equilibria, when applied to fluid buffering reactions, can provide useful information concerning the composition and movement of fluids during the granulite facies metamorphism. The Adirondack Mountains, New York, have been chosen for a study of deep crustal fluids as pressures and temperatures of metamorphism are well known and the area last equilibrated during a single pervasive metamorphism. Two mineral assemblages which buffered the fugacity of H2O (fH2) during the granulite facies metamorphism have been located near the Oregon Dome anorthosite in the Adirondack Mountains, New York. The first assemblage involves the breakdown of amphibole to orthopyroxene, clinopyroxene, quartz and H2O. This assemblage buffered fH2O to low values, with XH2O approximately equal to .1. Another assemblage, which is potentially very useful in the granulite facies, is the reaction of phlogopite and quartz to enstatite, sanidine, and H2O. There are, however, a number of uncertainties which must be evaluated before this assemblage can be applied to calculate fl^O. These include: 1) disagreement between various experiments in the P-T placement of this reaction, 2) a lack of understanding of the relationship of this reaction to more Fe-rich analogues, a problem which is in part due to uncertainties in the determination of the distribution coefficient (KQ) for Mg-Fe between biotite and orthopyroxene, and 3) the possible effects of tetrahedral order or disorder in trioctahedral micas. In spite of these uncertainties this reaction can be applied to calculate the fugacity of water, which is low is these rocks with the average XH2O equal to .1 +/- .1. Two samples contain graphite in addition to an H2O buffering mineral assemblage, making it possible to estimate the fugacity of six fluid species, CO2, H2O, CH4, CO, O2 and H2, if it is assumed that the sum of the partial pressures of these six fluid species is equal to the lithostatic pressure (French, 1966; Ohmoto and Kerrick, 1977). Such calculations indicate that if there was a free fluid phase then-CO2 was the dominant fluid species in these rocks during granulite-facies metamorphism. The other five fluid species were minor. One of the graphite-bearing assemblages is located less then 6 meters from a mineral assemblage which buffered the fugacity of CO2 to low values. This shows that CO2 was not a pervasive fluid in large quantities, but may have been the dominant fluid species in certain rock types. The results of this study indicate that the fugacities of various fluid species can be highly variable and sharp gradients in fluid compositions may exist during granulite-facies metamorphism. Any fluid movement which may have existed in the deep crust would, therefore, be channelized rather than pervasive.
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    The organic geochemistry and water-rock system across a contact metamorphic profile in the Mancos shale near Crested Butte, Colorado
    (1982) Cuddihee, John Lee; Baker, Donald R.; Lallemant, Hans G. Avé; Valley, John W.; Heymann, Dieter
    A Mid-Tertiary igneous intrusion into the Upper Cretaceous Mancos Shale provides an excellent natural laboratory to study the thermal effects of an intrusion on the organic geochemistry and water-rock system across a contact metamorphic profile. The intrusion, is believed to have established a high temperature gradient across a two mile long sampling profile. This study investigated the isotopic and chemical evolution of both kerogen and extractable organic material. Kerogen displayed a net weight loss of 11% due to methane generation. Kerogen showed essentially no carbon isotopic change. Bitumen contents decreased dramatically across the profile with a measurable 1.5 per mil depletion in 12C for the highest temperature samples. The relatively unique time-temperature history of the study area, and the difference in the kinetics of bitumen and kerogen degradation, is believed to be responsible for the difference in behavior of kerogen and bitumen. Oxygen isotope ratios of matrix calcite suggest that the calcite exchanged with connate water or a very small volume of meteoric water. The final oxygen isotopic ratio of calcite is a function of temperature and the water/calcite volume ratio. Finally, a thermal model involving conductive heating due to a subjacent extension of the White Rock pluton beneath the study area is shown to be capable of generating a temperature of 3°C.
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    The temperature/pressure conditions of grenville-age granulite-facies metamorphism of the Oaxacan Complex, Southern Mexico.
    (1983) Mora, Claudia Ines; Valley, John W.; Baker, Donald R.; Lallemant, Hans G. Avé
    Coexisting feldspar thermometry for charnockites and quartzofeldspathic gneisses in the Grenville-age Oaxacan Complex, southern Mexico, yields an average temperature of T=762+5°C at 7kb. Reintegration of coarsely-exsolved perthites yields internally consistent peak temperature estimates. Texturally-homogeneous alkali-feldspar has reequilibrated at low temperature. Unusually extensive solid-solution of albite (Abs to Ab5§) and anorthite (Ang.3 to An^ç) necessitated extrapolation of the Stormer (1975) determinative curves beyond their explicit limits. This results in unreasonably high temperature estimates of up to 975°C for feldspar pairs in which X/yj /^f>.4 and suggests that extrapolation is unwarranted. The distribution of albite between coexisting feldspars is consistent with the shape of the Brown and Parsons (1981) general form for the graphical two-feldspar thermometer. Peak metamorphic pressure was estimated using the Newton and Perkins (1982) anorthite+enstatite=pyrope 2/3 grossular 1/3 + quartz geobarometer which yields P=7.5+lkb. The experimentally determined geobarometer anorthite+ferrosilite=almandine 2/3 grossular 1/3 + quartz (Bohlen et al., 1983) yields P=7.25+lkb. The assemblage dolomite+diopside+forsterlte+calcite restricts the metamorphic temperature to 764°C at PQ) =7kb, calculating from the 5kb experimental reversal of the buffering reaction 3dolomite+ldiopside= 2forsterite+4calcite+2C2 by Kase and Metz (1981). Using the temperature estimated by two-feldspar thermometry, at P=7kb the assemblage buffers the fluid composition to Xco =.8. The estimated pressures and temperatures are consistent with metamorphic conditions indicated by the occurrence of quartz+K-feldspar+garnet +sillimanite+biotite in the pelitic gneisses and orthopyroxene+plagioclase+ garnetjMclinopyroxene in the mafic gneisses of the terrane.