A NEW SEQUENCE STRATIGRAPHY & RELATIVE SEA-LEVEL CURVE FOR GREEN HOUSE TO ICEHOUSE TERTIARY LIMESTONES, PUERTO RICO

A new sequence stratigraphic framework and relative sea-level curve has been developed for Oligocene through Middle Miocene (and some Pliocene) limestone strata on the north coast of Puerto Rico combining outcrop, core, biostratigraphy, and strontium isotope data. Strontium isotope data, collected from unaltered Kuphus incrassatus tubes, were critical in providing absolute age constraints for: 1) Late Oligocene to Early Miocene (25.97+/-0.32 to 24.07+/-0.16 Ma) strata that include the San Sebastian Formation, Lares Limestone and Montebello Member; and 2) late Middle Miocene (12.07+/-0.53 to 10.91+/-0.34 Ma) strata that include the Cibao Formation, Aguada (Los Puertos) Limestone and Aymamón Limestone.

Results of the study and new chronostratigraphy indicate five carbonate–dominated sequences, each bounded by major sequence boundaries that reflect relative sea-level falls, formed during a transition from greenhouse to icehouse climate conditions. Sequence character reflects this change in climate, with greenhouse sequences (San Sebastian and Lares Limestone) tending to consist of complete systems tracts, and icehouse sequences (Montebello Member, Cibao Formation, Aguada Limestone, Aymamon Limestone, Quebradillas Limestone) that tend to not to have all systems tracts present and exhibit more subaerial exposure surfaces reflecting the change to high-frequency, high-magnitude sea-level fluctuations.

Detailed sedimentologic study and lateral correlation of important surfaces and sequences over reconstructed paleotopography provided the basis for construction of a relative sea-level curve. When calibrated with the chronostratigraphic data, the relative sea-level curve indicates that the five sequences may reflect some control related to second-order eustacy (TB1, TB2, TB3) and perhaps some third-order fluctuations, but that significant departures from the global sea-level curve reflect fourth and higher-order sea-level fluctuations and the influence of local and regional tectonism.