The Pedro Castle Formation (Pliocene) is exposed on the western end of Cayman Brac where it unconformably overlies the Cayman Formation (Miocene). The Pedro Castle Formation, which is 6 to 20 meters thick, is formed of mudstone and wackestone along with lesser amounts of packstone. No evidence of reefal development is observed, although scattered colonies of Porites, Stylophora, and Montastrea are present.
The Cayman Formation is pervasively dolomitized. In contrast, dolomitization of the Pedro Castle Formation varies with height above the basal unconformity. Thus, the basal dolostones pass upward into dolomitic limestones that are then overlain by limestones. The complex pattern of dolomitization may be related to the unconformity at the base of the formation.
The top of the formation is the present-day karst surface that is characterized by rugged phytokarst development. The associated meteoric diagenesis has had a severe impact on the Pedro Castle Formation. Notable features include potholes, pinnacles, terra rossa, rootcretes and solution cavities filled with terra rossa and/or flowstone.
Preliminary stable isotope analysis of the dolomite has produced atypical results when compared to data from similar rocks elsewhere on the Cayman Islands. A marked shift (~-2.0 per mil relative to the Cayman Formation) across the basal unconformity is also evident. Further investigation will focus on the delineation of the facies architecture and the petrographic and geochemical characterization of the dolostones in the Pedro Castle Formation. The results of these analyses may carry important implications for understanding the dolomitization processes that take place on isolated oceanic islands.
AAPG Search and Discovery Article #90925©1999 AAPG Foundation Grants-in-Aid