--> ABSTRACT: ^dgr<SUP>18</SUP>O and ^dgr<SUP>13</SUP>C Stable Isotope Geochemistry of Dolomitized Detrital Calcites of the Lost Monegros Group, Southeastern Ebro Basin, Spain, by Jonathan D. Peterson; #91003 (1990).

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ABSTRACT: ^dgr18O and ^dgr13C Stable Isotope Geochemistry of Dolomitized Detrital Calcites of the Lost Monegros Group, Southeastern Ebro Basin, Spain

Jonathan D. Peterson

Isotope values of ^dgr18O (PDB) and ^dgr13C (PDB) from lacustrine limestones and dolomitized lithocalcarenites (predominantly detrital calcites) of the Los Monegros Group (Oligocene) in the southeastern Ebro basin, Spain, were studied to determine the geochemical conditions associated with dolomitization. The most extensively dolomitized

lithocalcarenite intervals correspond to what has been interpreted to be the marginal limits of the paleolake. Stratigraphic and paleogeographic data indicate that lake margins and surrounding mud flats were subject to evaporitic conditions throughout the Oligocene. These conditions were accompanied by seasonal changes in the evolution of the lake. The lack of minerals of hypersaline origin associated with the dolomitized intervals suggests dolomitization occurred in the presence of dilute waters.

Limestones show isotope values of ^dgr18O (PDB) ranging from -4.77 to -13.72. These limestones are interfingered with lithocalcarenites but are not dolomitized. Values of ^dgr18O (PDB) from chemically separated dolomites of the interfingered lithocalcarenites range from -4.57 to -13.01. Assuming a maximum precipitation temperature of 30°C, the pore fluid of formation for the calcite would have a mean ^dgr18O (SMOW) value of -4.20. The pore fluid of formation for the dolomite would be -6.28. These values suggest light meteoric water sources.

Isotopic evidence combined with stratigraphic and paleogeographic information suggests that dolomitization was the result of evaporitic pumping and/or mixing of incoming fresh water and evolved lake waters. Evaporation of evolved pore waters up through porous and permeable calcarenites and subsequent mixing with incoming fresh water along lake margins produced dolomitized lithocalcarenites.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990