Oxygen Isotopic Exchange During Near-Surface Diagenesis in a Mississippian Carbonate-Evaporite System

Dana S. Ulmer-Scholle, Peter A. Scholle

The Mississippian (Tournaisian-Visean) Arroyo Penasco Group of northern New Mexico consists of thin interval of carbonate and siliciclastic strata which have undergone complex diagenesis. A thick breccia, the Macho member of the Tererro Formation, caps intensely recrystallized carbonates and evaporites of the Espiritu Santo Formation throughout most of the study area, and is interpreted to result from collapse of carbonates in association with intrastratal dissolution of bedded sulfates during a mid-Visean subaerial exposure. The Macho member clearly predated the deposition of overlying late Visean units.

Oxygen isotopic data for both the calcitized evaporites and "dedolomites" are much more depleted than one would expect for the low paleolatitude marginal-marine setting and near surface diagenetic reactions; the data suggest a higher temperature, burial diagenetic origin. To reconcile the petrographic and field data with the oxygen isotopic information, water/rock interactions were modeled. From the calculations, it was determined that calcites with depleted oxygen isotopic compositions could be generated from dissolution of interbedded dolomite and gypsum units with the sulfate undergoing microbial reduction. These waters could then precipitate calcites with depleted isotopic values, even at low surficial temperatures.

Integration of observational data with the results of water/rock modeling indicates that near-surface evaporite dissolution not only controlled carbonate diagenetic reactions, but also strongly affected their preserved isotopic record. Calcites with depleted oxygen isotopic values can be generated not only at elevated temperatures during burial, but may also occur under surficial conditions in association with evaporite dissolution and sulfate reduction.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995