Similarities in the Dolomitization of Upper Miocene Reef Complexes in Mallorca and the Las Negras Area, Spain: Possible Evidence for a Mediterranean Dolomitizing Event during the Messinian

OSWALD, E. J., SUNY at Stony Brook, Stony Brook, NY, E. K. FRANSEEN, Kansas Geological Survey, University of Kansas, Lawrence KS, and W. J. MEYERS, SUNY at Stony Brook, Stony Brook, NY

Reef complexes in the Las Negras area and the island of Mallorca, over 600 km to the northeast, contain dolomite with remarkably similar stratigraphic distribution and petrographic characteristics. In both reef complexes dolomite occurs as 5-50 micro m euhedral cements and microcrystalline replacement of red-algal fragments and muds. In addition, anhedral, moldic-zoned dolomite cements (5-20 micro m) are abundant in the youngest reefs of both areas. All inter-reefal erosional surfaces are cross-cut by dolomite cements. Distribution of dolomite within the reefal sequences is controlled by (1) proximity to the youngest reef margin. Dolomite abundances decrease from 100% to 0% over a 10 km transect into the platform interior in Mallorca. In the fringing reefs at Las Negras, the youngest eefal carbonates are intensely dolomitized, with variable dolomitization in older sequences; (2) depositional facies. Porous reef-crest and slope facies contain the greatest amount of dolomite, whereas overlying lagoonal (in Mallorca only) and underlying distal shelf mudstones and wackestones are more calcitic. In Mallorca, dolomite is sparse in the extensive lagoonal facies, yet abundant in directly underlying reef and reefal-slope facies; (3) early cementation. Fibrous and equant to dogtooth isopachous cements predate dolomitization and are most abundant in aggradational portions of the reefal sequences. Dolomitized prograding clinoforms exposed at Las Negras overlie relatively undolomitized reefal megabreccias containing fibrous marine cements. These allochthonous blocks are surrounde by slope facies, and may represent distal deposits of a now eroded aggradational platform. Progradational margins exhibit very minor cementation and are extensively dolomitized, whereas aggradational margins (in Mallorca) and megabreccias (Las Negras) locally contain abundant early cements, with dolomite minor or absent.

Stratigraphic and petrographic timing support a dolomitization event following exposure of the reefs, during initial deposition of the onlapping Messinian sequence in Mallorca, and before Pliocene deposition in Las Negras. In both areas, dolomitization took place as a Messinian transgression covered the reefs. Evidence for multiple dolomitization events has not been found. Mallorcan dolomites have heavy stable isotopes [O(18) = +4.5 to +6.3%, C(13) = -0.1 to+3.14% PDB), suggesting saline dolomitizing fluids.

One hypothesis for dolomitization invokes brines associated with the deposition of extensive Messinian basinal evaporites. An early Messinian evaporitic lowstand could concentrate marine brines, and with the ensuing transgression bring them in contact with CaCo platforms, dolomitizing them. The similar styles and timing of dolomitization in these two distinct and distant reef settings suggest that dolomitization of late Miocene reefs around the western Mediterranean may have been genetically related to the Messinian salinity crisis.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)