EUSTATIC SEA LEVEL-INFLUENCED HETEROGENEOUS DOLOMITIZATION OF THE HOPE GATE FORMATION, DISCOVERY BAY, JAMAICA

As one of four major reef terraces on the north coast of Jamaica, the Pliocene Hope Gate Formation of Jamaica crops out as a coralline biolithite with flanking corraline-molluscan-algal biomicrudites. Previous investigations of limited exposures suggested pervasive dolomitization and led to its serving as the “type” formation for Lynton Land's 1973 model of mixing-zone dolomitization. Observations from newly exposed extensive road cuts at Discovery Bay, Jamaica, shed new light upon the geometry and heterogeneity of the facies and diagenesis of this important dolomite proxy. Macroscopic and petrographic analyses suggest that the depositional environments of the Hope Gate Formation at Discovery Bay ranged between lower reef crest to the upper fore reef zones with incised calcareous algal-molluscan calcarenite channels. Cement mineralogic (XRD) and Mg and Sr analysis (microprobe) matched with cement and fabric petrography suggest a complicated diagenetic history of allochem deposition, cement precipitation, dissolution, replacement, and inversion. Though absolute timing is problematic, the proposed relative paragenetic model of the Hope Gate terrace is one of sea level forcing: Highstand => Deposition and submarine cementation; Lowstand=> Karstification, speleothem precipitation, dissolution (Meteoric) and inversion (Phreatic) of metastable carbonates; Transgression and Stillstand => Dolomitization. Repeated sea level excursions led to increased vugs or dolomitization at the expense of metastable aragonite and Mg-calcite allochems and cements. We conclude that dolomite occurrence within the Hope Gate Formation is not pervasive but heterogeneous (cyclically expanding patches from fabric selective nuclei?), implying the mixing zone model to be more complicated both in space and time than previously thought.