Dolomitization of Thin-Bedded Calcarenites in Lacustrine Los Monegros, Spain

Jonathan D. Peterson, Lluis Cabrera

The Los Monegros Group (Oligocene) of the southeastern Ebro basin includes lacustrine rocks interfingered with progradational distal alluvial-fluvial sandy calcarenites (predominantly detrital calcites). Cores from three wells penetrating the Oligocene section show partial to complete dolomite replacement of detrital calcite grains and calcite cement. The most extensively dolomitized calcarenite intervals coincide with what has been interpreted to be the marginal limits of the Oligocene lake.

Petrologic analyses show detrital calcites in calcarenites of marginal lake limits were replaced by dolomite, followed by gypsum cementation. Calcite cement with meniscus texture suggests formation within the vadose to upper phreatic zone. Lake-center and distal alluvial-fluvial areas show partially dolomitized calcarenites with no subsequent evaporite cementation. The ^dgr¹⁸O (PDB) isotopic values from micritic limestones associated with the calcarenites range between -4.77 and -13.72. Assuming a precipitation temperature of 30°C, the pore fluid would have a mean ^dgr¹⁸O (SMOW) of -4.20, suggesting light isotopic meteoric water sources.

The ⁸⁷Sr/⁸⁶Sr values from two chemically separated dolomites from rocks interpreted to be the center lake area are 0.707878 and 0.707874. A chemically separated coexisting calcite yields a value of 0.70793. Dolomite values are lower than calcite values, suggesting fluids supplying Mg⁺⁺ had slightly different Sr isotopic concentrations.

Evaporation of evolved lake pore waters up through porous and permeable calcarenites and subsequent mixing with incoming fresh waters along lake margins produced extensively dolomitized calcarenites. Center lake areas relatively protected from evaporitic pumping and meteoric input were less subject to these processes, hence only minor dolomitization occurred.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.