Diagenesis of Oceanic Carbonate Sediments: A Review of the DSDP Perspective
Robert E. Garrison, 1981. "Diagenesis of Oceanic Carbonate Sediments: A Review of the DSDP Perspective", The Deep Sea Drilling Project: A Decade of Progress, John E. Warme, Robert G. Douglas, Edward L. Winterer
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DSDP cores at selected sites provide a continuous carbonate sediment record characterized by comparative compositional simplicity, vertical homogeneity, and uniformity of diagenetic environment. These circumstances have allowed detailed reconstruction of diagenetic events, most notably those during burial. Burial diagenesis converts carbonate oozes to chalks which in turn may become transformed into dense limestones. The most important variables affecting these first-order changes are burial depth and pore water chemistry, but burial lithification is not related to burial depth in a simple way. Instead, differences in original sediment composition appear to predetermine the depths at which the transformations occur. Such differences in composition, caused by major and minor ocean events, may also be important in developing acoustic reflectors in the sediment pile. Some cementation occurs early during diagenesis; but this is comparatively rare in DSDP cores and apparently took place in chemically reactive sediments, such as those with abundant braarudosphaerid pentaliths. Carbonate sediments have a “diagenetic potential” which results from their preburial history and which in part determines subsequent diagenetic events during burial. Gravitational compaction is significant in the upper 50 to 200 meters of sediment; the dominant processes below these depths are pressure solution and reprecipitation of calcite cement, both of which are highly taxa- and site- specific. This lithification proceeds without external sources of calcium carbonate, thus cementation is calcite conservative. Sr++ and 18O in the carbonate tend to decrease with increasing burial depth, a consequence of solution-reprecipitation. But variations in the composition of the original sediment can cause deviations from these trends.
Lithification of carbonate sediments associated with oceanic basalts appears to be a consequence of the submarine weathering of basalt. Shallow-water carbonate sediments have been infrequently encountered in the DSDP; but recognition of diagenetic features in them has allowed reconstruction of important geologic events such as intervals of subaerial exposure and of extremely rapid subsidence. Dolomite and other authigenic carbonate minerals in oceanic carbonate sediments apparently have formed in quite diverse ways. Some pelagic oozes were dolomitized by saline fluids emanating from underlying evaporites, some by Mg-rich pore waters derived from subaqueously weathered basalt; other dolomites formed as a by-product of the diagenesis of organic matter in sediments. The origin of other dolomite as well as siderite, ankerite, and other authigenic minerals in pelagic carbonates has not been systematically investigated.
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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.