Fracture Networks in the San Cristobal Canyon, Puerto Rico from High-Resolution Topography, GIS, and Aeromagnetic Analyses

Daniel Laó-Dávila, Boone Pickens School of Geology, Oklahoma State University
Andrew Buss, Boone Pickens School of Geology, Oklahoma State University
Andrew Katumwehe, Boone Pickens School of Geology, Oklahoma State University
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Abstract

The San Cristobal Canyon (Cañon San Cristobal), a natural protected area, is the deepest terrestrial canyon in Puerto Rico and one of the deepest in the Antilles. It consists of Cretaceous tuffaceous sandstone, siltstone, tuffaceous conglomerates, and flow breccias. One fault was mapped previously, parallel to one segment of the canyon. In other parts of the area, the Usabon River, which flows through the canyon, makes 90° bends at some points and develops step-like meanders at others. This suggests that fractures significantly influence both river flow and canyon formation.
In this study, we aim to address two key questions: 1. Is there geomorphological evidence of active faulting in the area? 2. Is there subsurface evidence of an extensive fracture system present? To explore these questions, we used a 1-meter-resolution digital elevation model and an aeromagnetic dataset to map fracture networks in the San Cristobal Canyon region.
To address the first question, we used QGIS to analyze the canyon's topographic and geomorphological features. We created elevation profiles and developed a geomorphic indicator ranking system. Additionally, we solved a three-point problem to calculate the strike and dip of the geological layers. For the second question, we used Geosoft software to filter the data and analyze rock magnetic signatures. We applied various mathematical filters to the reduction-to-the-pole total magnetic intensity grid to generate magnetic derivatives to delineate lithological boundaries, linear features, basement fabric, and lithological trends.
The results show scarps and drainage patterns that correspond to a fracture set oriented northwest-southeast, which aligns with the river's bends. Additionally, the magnetic data shows, in some places, features that coincide with the lineaments identified from geomorphology. However, no strong evidence of active faulting has been found, suggesting that the canyon formed by uplift and downward erosion of the rock by the river that followed old fractures.

Session

Deformation in the upper crust