Puerto Rico Geology
Project: Timing and Character of the Western Puerto Rico Fault System,
Stratigraphy and Geomorphology
The island of Puerto Rico lies at the interface between the Caribbean and North American plates. This situation has led to the development of complex fault systems and related dynamic topography, which pose natural hazards. Western Puerto Rico is characterized by a system of east-west striking faults that frame parallel topographic features such as the Rio Añasco, Rio Culebrinas, Rio Guanajibo, and Lajas Valleys, in addition to the Cadena San Francisco, Monte Grande, and Sierra Bermeja mountain ranges.
While these pronounced topographic features persist, in most places the faults are not exposed as they have either been buried by alluvium or eroded along footwall escarpments. As the fault systems is an apparent potential seismic hazard, previous studies have attempted to understand this fault system by investigating Quaternary flood plain sediments, subsurface features via geophysical means, and modern plate movement with GPS.
Rather than focusing on the faults in the modern environment, the goal of this proposal is to understand the timing of the origin of the western Puerto Rico fault system and it's most recent faulting style. To accomplish this ambitious goal, it will be necessary to characterize and date deposits that are related to the initiation of the fault system and its subsequent development. These features include isolated carbonate sequences in extreme western Puerto Rico as well as fluvial deposits in the Rio Añasco and Rio Culebrinas systems. Their development is related to the onset of fault activity, and dating these features will frame the timing and character of the system.
This project receives funding from the Puerto Rico Science, Technology and Research Trust and is collaborative with Drs. Wilson Ramirez and James Joyce.
See a promotional video of our project prepared by the PRSTRT.
In the news:
http://prsciencetrust.org/nueve-investigadores-reciben-subvenciones-de-fideicomiso-para-ciencia/
September, 2017 update:
Graduate student Michael Moul and Undergraduate Assistant Nelmary Rodriguez spent summer 2017 in the field identifying and sampling terraces of the paleo-Rio Culebrinas. In addition, they collected modern channel sediment for a satellite research project to be undertaken by Nelmary. Both performed well. Michael took his samples to the USGS Luminescence Laboratory in Denver, Colorado to perform OSL analysis.
Nelmary for scale on mostly unvegetated terrace of the Paleo Rio Culebrinas.
Sampling procedure for OSL in a terrace of the Paleo Rio Culebrinas.
Dark room mineral separation at USGS Luminiscence lab in Denver, Colorado.
May, 2018 update:
In April, MS student Michael Moul took his fossil kuphus and oyster sample powders to the University of North Carolina Department of Geosciences Isotope Geochemistry Laboratory for Strontium Isotope analyses. These analyses will help refine our understanding of the timing of carbonate deposition and associated fault initiation in western Puerto Rico. Preliminary results from one pilot sample showed that some of the faulting may have began at about 15 million years in the past (Vicens et al., 2017). An additional OSL sampling campaign in the Rio Añasco Basin in summer 2018 will complement the Rio Culebrinas samples from 2017; the research team includes Michael, MS Student Yanira Santiago, and Undergraduate Student Luis Espada.
Before traveling to North Carolina, Michael used a micro-drill to extract very fine amounts of powder from fossil samples. These powders were first checked for alteration using Oxygen and Carbon isotope analyses at the University of Puerto Rico Department of Geology Gas Analysis Stable Isotope Laboratory. Samples that appeared unaltered in both thin section analysis and in stable isotope analysis were chosen form Strontium Isotope analysis.
The work at UNC Chapel Hill involved dissolving the sample powders, concentrating Sr using column chemistry, and measuring the ratio of 87Sr/86Sr in each using mass spectrometer.
