ABSTRACT

Outcrops of the Seven Rivers, Yates, and Tansill Formations contain widespread evaporites replaced by quartz and calcite. The original evaporites consisted of discrete horizons, scattered nodules, enterolithic layers, and individual crystals or crystal fragments of gypsum and/or anhydrite within a finely crystalline dolomite matrix.

Based on field and petrographic observations, silicification of evaporites proceeded from the exterior to the interior of the nodules. The earliest replacement was by euhedral, black megaquartz that contains abundant fluid inclusions (water and hydrocarbons) and solid inclusions (mainly anhydrite and dolomite). The siliceous replacements were followed by precipitation of equant, blocky calcite spar, which filled pores left by latestage dissolution of evaporites.

The fluid inclusions in the replacive megaquartz are primary, and many contain both hydrocarbons and water. Daughter minerals of halite, gypsum, or possibly antarcticite (CaCl₂·6H₂O) are also found within the aqueous inclusions. Homogenization-temperature data for hydrocarbon and aqueous fluid inclusions average 67.7°C and 67.1°C, respectively. Hydrocarbonbearing and aqueous inclusions are thus interpreted to have formed simultaneously from the same fluids. Eutectic melting and final melting temperatures for aqueous inclusions indicate that the fluids were concentrated brines consisting of CaCl₂ and NaCl. Oxygen-isotope values for the megaquartz replacements averaged 28.4 (SMOW), indica ing precipitation from evaporative waters with an isotopic composition of +2.9 (SMOW).

Evaporite silicification was coeval with or slightly postdated hydrocarbon migration. The fluid-inclusion data provide a record of the fluid temperatures and compositions that prevailed during silica precipitation. These data, coupled with regional stratigraphy and published geothermal gradients, suggest a burial depth of approximately 1.3 km during silicification.

The source of the silica for evaporite replacement is problematic. We postulate, however, that silica may have been derived from dissolution of siliciclastics in back-reef units. Organic acids that form from breakdown of hydrocarbons increase the solubility of quartz by bonding with silicic acid. As these products (and associated brines) moved into cooler oxidizing zones, the organic acids broke down, releasing silica into solution. This organically mobilized silica eventually precipitated as megaquartz replacements of preexisting evaporites.