Distribution, Thermal Histories, Isotopic Compositions, and Reflection Characteristics of Siliceous Rocks Recovered by the Deep Sea Drilling Project
Kenneth A. Pisciotto, 1981. "Distribution, Thermal Histories, Isotopic Compositions, and Reflection Characteristics of Siliceous Rocks Recovered by the Deep Sea Drilling Project", The Deep Sea Drilling Project: A Decade of Progress, John E. Warme, Robert G. Douglas, Edward L. Winterer
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A synthesis of deep sea drilling results through Leg 70 indicates that siliceous rocks have been recovered in a variety of lithofacies from almost all major ocean basins. Reconstructed distributions of these rocks delineate original sites of deposition. During the Neogene, these areas included the equatorial Pacific, the Southern Ocean, the southern Bering Sea, the Red Sea, and local centers off Japan, California and northwest Africa. Paleogene occurrences describe a wide equatorial belt in the Pacific, Caribbean, Atlantic, and Indian Oceans, and several smaller regions in the North Atlantic, South Atlantic, and southern Indian Ocean. Cretaceous areas of deposition also included the equatorial Pacific and Atlantic with a few sites off western Australia. By analogy with present circulation patterns and areas of opal accumulation, reconstructed distributions of DSDP siliceous rocks delineate areas of past oceanic divergence. The rate of accumulation of silica, when compared with better known accumulation rates of opal, is a crude index to past productivity in these areas.
Opal-CT and quartz are the principal authigenic silica minerals in siliceous rocks recovered from the deep sea; both are derived predominantly from biogenic silica comprising sponge spicules and the tests of radiolarians and diatoms. Time and temperature are important, though not the only, controls of the transformation rates of silica. Plots of age versus present subsurface temperature of opal-CT and quartz in DSDP siliceous rocks generally match the time and temperature stability fields of these two minerals determined from experiments. Using the experimental results, the transformation history of silica can be estimated. High thermal gradients and rates of burial apparently favor rapid transformations.
Oxygen isotopic compositions of opal-CT and quartz depend upon the temperature of formation and the isotopic composition of the equilibrating fluid. Comparisons of present subsurface temperatures with temperatures computed from isotopic values, in conjunction with isotopic data on interstitial water squeezed from deep sea sediments, suggest the 5,80 values of opal-CT and quartz in deep sea siliceous rocks reflect a restricted range of formation temperatures, generally less than 50°C, and equilibration with interstitial water having a 6,80 range of 0%0 to -3%0.
Siliceous rocks are important seismic reflectors in the western North Atlantic (Horizon A°), in the southern Bering Sea (Bottom Simulating Reflector), and in the northwest Pacific. The strength and continuity of these reflectors are complex functions of degree of silicification and bedding characteristics. Any seismic analysis must treat siliceous rocks as both sedimentary and diagenetic facies because stratigraphic and diagenetic boundaries do not always coincide.
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At the present the Glomar Challenger has drilled over 500 holes over the world ocean, involving hundreds of scientists from dozens of countries. This volume is intended as a review of some of theimportant results from the most comprehensive, ambitious and successful earth-bound geologic project ever undertaken. The symposium upon which this volume originated was held April 4, 1979 at the SEPM/AAPG Annual Meeting in Houston. No comprehensive synthesis of all aspects of the DSDP has appeared, and the topic coverage in this volume is biased towards the sediments and fossils, and their significance for certain aspects of earth history – paleogeography, bathymetry, climatology, oceanography, ecology, environments – all in keeping with the audience of sedimentary geologists.