--> Abstract: Dolomite Cementation Before and After Aragonite Dissolution, Upper Miocene Coral Reefs of Mallorca, Spain, by W. C. Ward, E. J. Oswald, and L. Pomar; #90987 (1993).

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WARD, W. C., University of New Orleans, Department of Geology and Geophysics, New Orleans, LA; E. J. OSWALD, Exxon Production Research Co., Houston, TX; and L. POMAR, University of the Balearic Islands, Palma de Mallorca, Spain

ABSTRACT: Dolomite Cementation Before and After Aragonite Dissolution, Upper Miocene Coral Reefs of Mallorca, Spain

After submarine cementation and/or micritization of skeleton margins, the earliest diagenetic event recorded in much of the upper Miocene reef complex of Mallorca is dolomitization. In dolomitized parts of the reef, dolomite cement is common in both intergranular pores and moldic pores left by dissolution of aragonitic constituents. This dolomite cement consists of three major phases: (1) mostly non-luminescent rhombohedral crystals, overgrown by (2) polyhedral or spherulitic crystals having multiple thin bands of bright-yellow to dull-brown cathdoluminescence, overgrown by (2) rhombohedral crystals with thin bands of moderate orange luminescence and broader non-luminescent bands. Type 1 is absent in many places. Composition of the polyhedral/spherulitic phase is Ca<50>Mg<50& t; with 1800 ppm Na; the second rhombohedral phase is Ca<56>Mg<44> with 200 ppm Na. Whole-rock analyses give delta O{18} values from +3.9 to +7 o/oo PDB and delta C{13} values from -0.1 to +3.1 o/oo PDB.

In most of the dolomitized portions of these reefs there is no evidence that dolomite cementation and replacement was preceded by freshwater diagenesis. Indeed, the complicated cathodoluminescence pattern shows that the first-stage dolomite cement precipitated before wholesale dissolution of aragonite. For example, intracoral pores are lined with a succession of dolomite cements, the initial layers of which apparently precipitated on the coral skeleton. Coral-moldic pores, however, have only the younger zones of the cement sequence in the intracoral pores. The petrographic relationships and geochemistry of these dolomite cements indicate that the first episode of dolomite in the Mallorea reefs was precipitated from higher-than-normal-salinity marine water early in the diagenetic histo y, during or before aragonite dissolution. CaCO<3> from aragonite dissolution apparently was consumed by the dolomitization, because there was no early precipitation of calcite in the dolomitized reef.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.