ABSTRACT: Dolomitization of an Upper Miocene Reef Complex, Mallorca, Spain: Evidence for a Messinian Dolomitizing Mediterranean Sea

Eric J. Oswald, W. J. Meyers, L. Pomar

A partially dolomitic Tortonian(?) reef complex is superbly exposed along 20 km of sea cliffs in Mallorca, Spain. Dolomite occurs as 10-40-µm euhedral cements and local microcrystalline replacement of red-algal fragments and muds. Distribution of dolomite within the reef complex is controlled by three factors. (1) Proximity to the youngest reef margin. Dolomite abundances decrease from 100 to 0% over a 10-km transect into the platform interior. (2) Facies. Porous reef-crest and slope facies are preferentially dolomitized, and overlying lagoonal and underlying distal shelf mudstones and wackestones are mostly calcitic. (3) Early cementation. Aggradational reef margins contain fibrous marine cements, which precluded precipitation of later dolomite, whereas progradation l margins exhibit no early cementation and are extensively dolomitized. Stratigraphy, dolomite distributions, and petrography suggest that dolomitization occurred as a single event, during initial deposition of the restricted Messinian facies that onlap the final reef slope but while sea level was high enough to cover the reef complex.

Dolomite cements are nonstoichiometric (>57 mole % Ca by microprobe analysis) and have heavy stable isotopes (^dgr¹⁸O= +4.5 to +6.3 ^pmil, ^dgr¹³C= -0.1 to +3.14^pmil PDB), suggesting saline dolomitizing fluids. Transmission electron microscope examination shows primary growth fabrics (growth sector boundaries and microstructural banding corresponding to final crystal faces) and well-developed modulated structures, indicating an absence of recrystallization.

Our model for dolomitization invokes evaporite-modified seawater with elevated Mg/Ca ratios due to extensive basinal gypsum precipitation in a restricted, but not drawn down, Mediterranean Sea. This seawater brine reacted with metastable reefal carbonates, dolomitizing porous and marginal portions of the reef in less than 0.5 m.y. Late Miocene reefs in southern Spain (Las Negras and Nijar) exhibit similar styles and timing of dolomitization and suggest that dolomitization of reefs around the Mediterranean was genetically related to the Messinian salinity crisis.

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