RIVER PROFILES AND UPLIFT-RELATED RELICT LANDSCAPES OF THE YAUCO, GUAYANILLA, AND TALLABOA BASINS, SOUTHERN PUERTO RICO

The island of Puerto Rico is part of the Puerto Rico/Virgin Islands microplate which lies at the boundary between the larger North American and Caribbean plates. The microplate has experienced uplift in the recent geologic past resulting in the exhumation of Paleogene-Neogene marine carbonates to altitudes of 450+ meters above sea level, Pleistocene marine cut terraces, and widespread relict platformal landscapes, among other geomorphic features.

We focused on the fluvial systems at work in the Yauco (119 km²), Guayanilla (59 km²), and Tallaboa (79 km²) river basins which flow from 1000+ meters above sea level in the Cordillera Central southward over 25-40 km to the Caribbean Sea. River profiles and stream statistics for the three basins were studied to compare potential local uplift-related features with those seen elsewhere across the island. Our GIS analysis shows that numerous knickzones of various size, many of which were field-verified, exist in all three basins. The locations of knickzones do not systematically coincide with mapped geological contacts and are often co-located at equal or similar elevations.

The most prominent set of knickzones exists between 225-280m on the Yauco, Chiquito, Duey, and Guayanilla rivers. These knickzones in the Yauco and Guayanilla systems separate upland regions that are graded to a base-level currently higher than present sea-level, reflecting a significant drop in relative base level (uplift) and that the incipient wave of erosion has not fully propagated up the southern flank of the Cordillera Central. The large knickzone at 340m in Quebrada Barreal of the Tallaboa basin may be a remnant of an earlier pulse of uplift than that responsible the 225-280m set. Knickzones in the headwaters of the Yauco (Quebrada Caracoles, 664m) and Tallaboa (Rio Guayanés, 660m) basins are likely a result of stream piracy, not uplift, as the south coast river systems are much steeper than the Rio Arecibo and Rio Añasco on the north side of the Cordillera Central.

Previous physiographic and geomorphic studies of Puerto Rico have largely ignored the southern draining systems. Our work shows that relict upland landscapes exist to the south of the Cordillera Central and will be useful in the overall understanding of uplift of the PRVI microplate.