ABSTRACT: Geochemistry of Dolomites from Late Miocene Reefs in Southern Spain

OSWALD, ERIK J., Exxon Production Research Co., Houston, TX, and B. PACKMOHR, W. STAUDT, M. A. A. SCHOONEN, R. J. REEDER, and W. J. MEYERS, S.U.N.Y. at Stony Brook, Stony Brook, NY

Geochemical analyses (Oxygen 18, Carobn 13, Na, Sr, SO4, F, Mn, and Cl) of pure dolomite powders from dolomitized late Miocene reef complexes in Mallorca, Las Negras, and Nijar, Spain, testify to dolomitization by marine evaporitic brines mixed to varying degrees with less concentrated seawater. Mallorcan Oxygen 18 values range from +7.0o/oo to +3.9o/oo PDB and correlate strongly with Na abundances (2000 to 500 ppm). Nijar and Las Negras dolomites show similarly high Na concentrations (2900 to 440 ppm), with correspondingly low Cl abundances (generally below 500 ppm) precluding a seawater- or halite-inclusion origin for much of the Na within these dolomites. Samples from all three areas exhibit similar SO4 (6000 to 1000 ppm) and fluorine (1400 to 350 ppm) abundances, which are positiv ly correlated with Na abundances and Oxygen 18 values.

The strong covariance of these variables--particularly with Oxygen 18 values--suggests fluid changes during the dolomitization process that can be quantitatively modeled as mixing of seawater and brines between gypsum and halite saturation. Carbon 13 values (+3.5 to +0.5o/oo) and Mn abundances (500 to 20 ppm) appear to be largely derived from the host rock, and, like Sr abundances (415 to 110 ppm), show no significant covariation with other variables.

In Mallorca, strong spatial variations in Oxygen 18, Na, F, SO4, and Mn are present, with elevated abundances and enriched stable-isotopic values at the platform margins decreasing toward the platform interior. These gradients correspond to petrographic heterogeneities in dolomite cement morphologies and luminescent characteristics. An anhedral to spherulitic, banded luminescent dolomite cement (Zone 1) is always present in samples with elevated isotopic and trace-element signatures and initiates the dolomite cement stratigraphy. TEM analyses reveal submicron inclusions of halite within Zone 1 dolomite and dolomite precipitation from halite-saturated fluids. Zone 1 is transitional with a later, more euhedral nonluminescent cement (Zone 2) that corresponds to geochemical signatures ref ecting seawater or near-seawater fluid compositions. Similar petrographic-geochemical relationships are found at Nijar and Las Negras. These data, combined with margin-preferential dolomite distributions and petrographic and Sr-isotopic timing constraints, suggest regional dolomitization occurred during the Messinian transgression of an evaporitic Mediterranean over previously exposed reefs.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)