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Calcite cements of subsurface origin have received increased notice in the past several years, particularly in Jurassic Smackover sequences o f the Central Gulf by Moore and Druckman in 1981. Initially, documentation of the subsurface origin of these cements centered about petrographic evidence: fractured grains encased in poikilitic cements as well as pressure dissolved grains covered with cement. Although petrographic data did indicate some burial prior to cement precipitation, burial depth at the time of cementation remained uncertain because of our general lack of knowledge relative to the failure of carbonate grains under confining pressure, elevated temperatures, and changing fluid compositions. Subsequently, study by Klosterman in 1981 and Moore and Druckman in 198 of the two phase fluid inclusions common to these cements indicated that elevated temperatures (85 to 112°C; 185 to 234°F) and saline brines similar to present Smackover fluids may have been present at the time of cement formation. If indeed the precipitation fluids were similar to present Smackover brines (ten times more saline than seawater with a ^dgr18O composition near +5), the -6.5 ^dgr18O composition of most of these cements is compatible with a temperature of 90°C (194°F) because of the strong temperature dependence of oxygen isotope fractionation. Final confirmation of the deep subsurface origin of many of these cements may well rest with their apparent equilibrium with present Smackover brines relative to radiogenic strontium. Most Sma kover brines and associated post composition poikilitic calcite cements analyzed to date show an enrichment in radiogenic strontium well above Jurassic seawater values, whereas adjacent grains have radiogenic strontium compositions near that of Jurassic seawaters. Trace element composition of these deep subsurface cements, particularly relative to total strontium, present an enigma. Although present Smackover fluids have a high Sr/Ca ratio (almost four times greater than seawater), the late subsurface cements that have presumably been derived from these fluids have Sr compositions averaging only 200 ppm, well below the values predicted by using Katz et al's 1972 distribution coefficient (1,350 ppm) or Kinsman's 1969 distribution coefficient (3,000 ppm). These discrepancies certainly indi ate that kinetic and compositional controls over trace element distribution coefficients must be reassessed for the carbonate system in the subsurface environment.
The most common subsurface cement (demonstrably post compaction, coarse, clear, single unzoned crystals of a poikilitic habit) generally represents a very late stage diagenetic event. A second type subsurface cement, much less common than the first, that generally consists of an interlocked mosaic of coarse crystals, usually fills large sheltered voids, such as gastropod molds, that exhibit a complex iron zonation as seen by staining or cathodoluminescence. Isotopic composition of these zoned calcites consistently show a progressive depletion of ^dgr18O of some 4 per mil, from center to crystal termination. The lightest composition overlaps the -6.5 ^dgr18O of the late stage unzoned poikilitic cements described. These calcites, based on petrography and stable iso opic composition, seem to represent an intermediate stage of diagenesis and burial. Dissolution associated with progressive pressure solution during the first stages of burial is the most logical source of the carbonate needed for the calcite precipitation of the zoned cements. A potential source for the carbonate needed to form the late stage poikilitic calcites is deep subsurface calcite
dissolution associated with hydrocarbon maturation-migration.
The understanding of these subsurface cements helps decipher the diagenetic history of carbonate rock sequences during progressive burial and can be particularly helpful in timing hydrocarbon migration.
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