HOLOCENE MARINE TERRACES IN PUERTO RICO: EVIDENCE FOR TECTONIC UPLIFT?

We identified paleo-shorelines and associated marine deposits at elevations of approximately 3-5 m above modern sea level on the island of Puerto Rico in four widely separated locations: Patillas, Salinas, Rincón, and Ramey. Near Patillas (southeastern Puerto Rico), a paleo-seacliff extends up to 200 m inland and has a shore-parallel extent of ~5 km. Pits excavated into the terrace surfaces seaward of these paleo-shorelines exposed fossiliferous marine sand beneath soils. Seven radiocarbon analyses of samples from the Patillas terrace range in age from 400-520 yr BP to 730-900 yr BP (two-sigma ages). Near Salinas (southern Puerto Rico), well-developed paleo-sea cliffs extend up to 800 m inland and at least 8 km parallel to the modern shore. Eleven radiocarbon ages and two U-series ages of samples collected from the Salinas terrace range from 670-860 yr BP to 2740-2870 yr BP. Near Rincón, in northwestern Puerto Rico, ongoing commercial sand extraction activities on a terrace approximately 1 km inland exposed fossiliferous marine sand approximately 3 m above sea level that underlie fluvial and marsh deposits. Radiocarbon analyses of two coral samples yield ages of 1350-1550 yr BP and 1680-1870 yr BP. At the Ramey locality, a well-developed paleo-seacliff extends up to 20 km parallel to the coastline and up to 500 m inland. Sand extraction activities have removed deposits associated with this terrace, but fossil corals left behind on the bedrock platform approximately 3-5 m above sea level yield eight U-series ages ranging from 4000 yr BP to 7000 yr BP. Studies in the Caribbean suggest that sea level has been steadily rising in this region for the last 10 ka and has not been higher than it is today during the Holocene. Glacio-hydro-isostatic models do not predict a relative high sea level for the Caribbean during the Holocene. In addition, the preliminary ages suggest that the marine terraces are not all of the same age. These data suggest Holocene tectonic uplift as a possible mechanism of formation for the terraces.