Zonation and geochemical patterns of burial calcite cements; upper Smackover Formation, Clarke County, Mississippi

ABSTRACT Cathodoluminescence (CL), trace element concentrations, and isotope compositions of luminescently zoned poikilitic and mosaic calcite cements of the upper Smackover Formation are studied in the Harmony, Pachuta Creek, East Nancy, Goodwater, and Garland Creek fields in Clarke County, Mississippi. These calcites precipitated during burial in a depth range of 100 m to 3 km, a temperature range of 30-100°C, and a time span of 60 my. The calcium carbonate for calcite cementation was primarily derived locally by pressure dissolution of the Smackover Formation. Luminescent patterns are consistent in all samples within each field. However, calcites from individual fields differ in number of zones and CL intensity. There is no compelling evidence that equivalent zones in separate fields precipitated simultaneously. It is therefore unwise to correlate individual zones or groups of zones as time horizons from one field to another. Magnesium content of all calcites ranges from about 500-1000 ppm in the oldest zones and increases to about 3000-4000 ppm toward the younger zones. The low Mg2+ content of the older zones is the result of precipitation from waters with low Mg2+/Ca2+ ratio, typical of subsurface waters. The increase in Mg2+ toward the younger zones may have resulted from precipitation over time at progressively higher temperatures and the increase in DMg2. from a pore water whose Mg2+/Ca2+ ratio was declining. Strontium varies from 100-300 ppm and does not exhibit systematic variation across zones but increases with increasing Mn2+ content. Low Sr2+ content of the calcites is the result of very slow precipitation at high temperatures from a water with low Sr2+/Ca2+ ratio. Co-variation of Sr2+ with Mn2+ may reflect preferential Sr2+ substitution for Ca2+ in order to reduce lattice distortion caused by the substitution of Mn2+ for Ca2+. Non-luminescing to dull zones contain less than 30 ppm Mn2+, while bright zones have 300-1500 ppm Mn2+. Iron is nearly absent in all zones regardless of CL intensity. Low Mn2+ content of older zones is attributed to calcite precipitation in a reduced environment where Mn2+ was not available. The increase in Mn2+ content in younger zones is the result of importation of Mn2+ through local conduits from the underlying Norphlet Formation, coupled with an increase in DMn2 due to a progressive increase in temperature during burial. Absence of Fe2+ in all zones may have been the result of precipitation of iron-sulfides when Fe2+ - and Mn2+-bearing waters came into contact ith H2S generated by organic maturation of the lower Smackover Formation. 18O decreases and 87Sr/86Sr becomes more radiogenic toward the younger zones, while 13C remains constant. The lower 18O and more radiogenic 87Sr/86Sr composition of the younger zones suggest calcite precipitation at higher temperature and influx of radiogenic strontium from the underlying Norphlet Formation. The absence of any CL quenchers, lack of any CL activators other than Mn2+, and geologic and geochemical considerations suggest that CL intensities were primarily controlled by the Mn2+/Ca2+ ratio of pore waters and temperature.