ABSTRACT: Diagenesis of Isla de Mona, Puerto Rice

Luis A. Gonzalez, H. Ruiz, V. Monell

Isla de Mona is a carbonate plateau, 50 mi west of Puerto Rico. The plateau is composed mostly of two Miocene carbonate units, the Isla de Mona Dolomite overlain by Lirio Limestone. These two units consist primarily of back-reef sands with a reefal (boundstones) sequence present in the Lirio Limestone to the southwest.

The Lirio Limestone is heavily karstified with numerous sinkholes in the central part of the island resembling present-day cenotes of the Yucatan peninsula. Numerous large cave openings are exposed along cliffs surrounding the island, with cavern size rapidly decreasing toward the interior. Within the Isla de Mona Dolomite, three different soil development episodes can be identified in the central part of the island, and on the periphery of the island several episodes of vadose diagenesis are identified by the presence of vadose silts and paleotravertines.

Petrographically, both units exhibit selective dissolution of aragonitic skeletal grains with fabric retentive replacement of former high magnesium calcite skeletal components, by calcite in the Lirio Limestone, and by dolomite in the Isla de Mona Dolomite. Petrographic relationships suggest early diagenetic modification by fluids that were substantially undersaturated with respect to aragonite, which preferentially dissolved aragonitic components, near saturation with respect to calcite/high magnesian calcite during dolomitization, and saturated with respect to calcite during calcitization. Stable isotopic composition of individual dolomitized components and dolomite microspar in the Isla de Mona Dolomite define a hyperbolic trend characteristic of composition produced by fluid mixin . The similarity of the karstified Lirio Limestone to present-day Yucatan, the nature and fluid chemistry requirement of the aragonite dissolution and calcite replacement process, the isotopic composition of the dolomite, and the presence of soil horizons strongly suggest diagenetic modification under meteoric-marine fluid mixing.

Stratigraphic variations in isotopic composition, along with field relationships, suggest that dolomitization was achieved by periodic oscillation of sea level during the late Miocene. During lowstands, the exposed central parts of the island developed a freshwater lens and a mixing zone became established. Karstification vs. dolomitization were probably controlled by differences in the amount of dissolved carbon dioxide in the groundwater.

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