DRILLING AND GPR IN THE RíO ICACOS WATERSHED OF THE LUQUILLO CZO, PUERTO RICO: WEATHERING PROCESSES AND ARCHITECTURE OF THE DEEP CRITICAL ZONE

In the Río Icacos drainage, Luquillo Mountains, Puerto Rico, the Río Blanco quartz diorite intrusion oxidizes during weathering to form concentric onion-skin like layers (rindlets) around relatively unweathered bedrock blocks (corestones). These observations, and the general weathering model, are derived from outcrops, but drilling has also revealed sequences of corestones and rindlets in the subsurface. Results from a new drilling project on the East Peak Ridge, a narrow spine separating the Río Icacos and Río Mameyes watersheds, provide additional information on the spatial distribution of rindlets and the geometry of the downward propagating oxidation front near its leading edge. Drilling was preceded by a ground-penetrating radar (GPR) survey at this site and elsewhere within the adjacent Río Icacos and Río Mameyes watersheds, encompassing 3 lithologies: quartz diorite, volcaniclastic, and hornfels grade metavolcaniclastic. On East Peak Ridge, drilling targeted the hornfels lithology but quartz diorite was encountered below the saprolite at ~14 meters below land surface (mbls). Some of the saprolite appears to be derived from the hornfels, however, suggesting that we drilled through the geologic contact. Drilling revealed three incipient quartz diorite corestones separated by thick zones of highly weathered material. Rindlets had not formed at depths shallower than ~38 mbls. Evidence of oxidation, noted by the existence of red fractures, was found within the saprolite and in the hard rock. A red fracture at 34 mbls was recovered from within the saprolite above the third incipient corestone. The oxidative fracture in the saprolite may act as a conduit in the low permeability material, allowing oxygen to reach the deeper hard rock where fractures are becoming visible (38 mbls). Preliminary results suggest that GPR works well on regolith derived from quartz diorite and volcaniclastic lithologies. On the hornfels, the signal becomes quickly attenuated. The thickness and mineralogy of the saprolite may control the propagation of the oxidation front. Outcrop, drilling, and GPR allow us to couple geological observations with geochemistry to formulate a conceptual model for the architecture of deep weathering in the Río Icacos watershed, and at its margins which may be controlled by lithological differences.