--> ABSTRACT: Reflux Revisited: Evaporative Genesis of Dolomite in a Pliocene Reef Tract, St. Croix, U.S. Virgin Islands, by Ivan Gill, Clyde H. Moore, and Paul Aharon; #91030 (2010)
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Reflux Revisited: Evaporative Genesis of Dolomite in a Pliocene Reef Tract, St. Croix, U.S. Virgin Islands

Ivan Gill, Clyde H. Moore, Previous HitPaulTop Aharon

The samples and data for this project are derived from 22 test holes drilled to a maximum depth of 91 m, and from rock exposures representing the carbonate section through the Miocene.

Dolomite on St. Croix exists in a highly localized region of outcrops and subsurface occurrences in a Miocene-Pliocene reef tract and its related facies. Bioclasts in the dolomitic strata commonly show surprisingly good preservation of microstructure, particularly in large benthic forams and coralline algal clasts. The dolomite is stoichiometrically calcium-rich and exists as euhedral rhombs ranging from 2 to 30 µm in diameter.

The surficial dolomitization occurs in reef, lagoonal, and platform facies that rim the predevelopment shoreline of a coastal lagoon. In the subsurface, dolomitized rock follows the lithified undersurface of the same lagoon. This spatial distribution of dolomitization suggests a causal relationship between the lagoon hydrology and the process of the dolomitization.

The preliminary Sr87/Sr86 isotopic composition of the dolomite is 0.70888, which corresponds to Miocene-Pliocene seawater values, and suggests a Miocene-Pliocene age for the dolomitization event. Stable isotopic values average +2.3 ^pmil for ^dgr13C and +4.1^pmil for ^dgr18O, both relative to the PDB standard. These data imply that dolomitization took place by reflux of Pliocene fluids enriched in 18O. The dolomitization was confined to a fault-bounded region, and affected reef, lagoonal, and platform carbonates.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.