The Cretaceous Edwards Limestone in south-central Texas was deposited in alternating shallow-marine, intertidal, and supratidal environments and underwent normal early diagenesis. First-stage calcites include calcitic shell material, unaltered marine micrite, submarine cement, and meteoric phreatic or shallow subsurface cements. The first-stage calcites yield isotopic values of ^dgr¹³C from -1.0 to +3.5 and ^dgr¹⁸O from -6.0 to +3.0.

Dolomite in the Edwards occurs in a variety of forms. These forms range from "dirty" rhombs (1 to 5 µm), petrographically very similar to but isotopically slightly lighter than modern tidal flat dolomites (^dgr¹⁸O of 0.5 and ^dgr¹³C of 3.0 for the most hypersaline dolomites), to perfectly formed rhombs (30 to 40 µm) interpreted as freshwater dolomite (^dgr¹⁸O of -5.7 and ^dgr¹³C of 1.7).

The Edwards was divided into two diagenetic zones by Miocene faulting along the Balcones fault zone. On the upthrown side of the fault, an oxidized freshwater aquifer developed. This water is now saturated with calcite, but undersaturated with dolomite and gypsum. Relatively stagnant brackish and reduced water on the downthrown side of the fault is saturated with calcite, dolomite, and gypsum. Differences in the chemistry of the interstitial fluids in these zones are related to different types of diagenesis.

Second-stage calcites, which can be separated regionally and petrographically from first-stage calcites, formed after faulting and result from reactions between first-stage calcites and dolomites and fresh water introduced after faulting. These reactions can be written generally as: Fresh Water + Dolomite + Gypsum ^rarr Brackish Water + Calcite. Second-stage calcites are as light as -7.5 ppm ^dgr¹³C and -10.0 ppm ^dgr¹⁸O, and these values vary inversely while the ratios of first-stage calcite cements are heavier and vary together. The second-stage calcites are considerably lighter because they have grown in equilibrium with meteoric water containing some organically derived carbon, and sometimes at considerable depth.

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