AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Evaluating Controls on Carbonate Sequence Development: Middle to Late Miocene Ponce Limestone, Puerto Rico
(1) Geology, University of Kansas, Lawrence, KS.
Ponce Limestone exposures along the south coast of Puerto Rico are providing the opportunity to evaluate paleotopographic, sea level, and structural controls on this carbonate system. New ages from strontium isotope data indicate that two sequences identified to date developed from Middle Miocene through Late Miocene time. Exposures of the basal sequence reveal a 20-30 m-thick heterozoan-dominated system, composed of benthic foraminifera (e.g. Miocerites americanus), bivalves (e.g. Myrtaea sp.), gastropods (e.g. Strombus sp., Conus sp.) and solitary corals. Preliminary paleotopographic reconstruction indicates a mostly progradational system. Back stepping geometries associated with high concentrations of in-place Kuphus incrassatus bivalves, likely indicate a relative sea-level rise that punctuated the overall progradation. Abundant burrowing, local trough cross-bedding, and mixtures of abraded and non-abraded bioclasts suggest a subtidal environment, with at least intermittent energy. The presence of some hermatypic coral fragments and abundance of Orvulina universa planktonic foraminifera suggests a downslope environment of a tropical shallow water system. A gradual increase in photozoans near the top of the basal sequence, followed by an abrupt change to an in-place Porites-stick dominated reef indicates shoaling. Some tectonism at this time is indicated by normal faults (~4-6 m of displacement) that end just above the reef horizon. Initial paleotopographic reconstruction shows a depositional slope of at least several degrees during reef development. Tracing across paleotopography for more than 300 m laterally, suggest the reef migrated downslope as sea level fell. Subsequent subaerial exposure is suggested by an erosional surface, brecciation and alteration of underlying rocks. The upper sequence, 15-20 m thick, is mostly composed of coral reef facies, oyster banks, and accumulations of in-place Clypeaster cubensis. Lateral relationships show complex depositional patterns, possibly related to tectonism; however, a progradational reef unit preserved in a high-standing position indicates a significance relative rise in sea level above the basal sequence boundary. Our preliminary results are part of a larger-scale project directed at isolating and quantifying relative contributions of sea level, structure, and local paleogeographic related processes for development of Miocene-Pliocene carbonate systems in the Caribbean.