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Early marine cementation and subaerial leaching of Stuart City rudist reef facies in south Texas yielded a rock with substantial intergranular, moldic, and shelter porosity. Most of this porosity was subsequently filled with a mosaic of blocky calcite, resulting in a tight rock. A detailed study integrating transmitted light and luminescent petrography with electron microprobe and stable isotope analysis reveals that the pore filling occurred in three stages representing distinctly different diagenetic environments. All three stages are seen commonly in single syntaxial crystals, suggesting continuous crystal growth while the environments were changing.
A model consistent with the data would have the following sequence of events: (1) initiation of calcite druse on pore walls in a freshwater phreatic environment, (2) continued pore filling in the phreatic mixing zone, and (3) final pore filling in the "marine" saline phreatic zone. The sequence could have occurred during a single, transgressive submergence of the pore system.
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