SAN CARLOS BASIN: A GEOLOGIC AND GEOPHYSICAL STUDY, NORTHERN COSTA RICA AND SOUTHERN NICARAGUA

We present data and interpretations on the tectonic and geologic history of the San Carlos Basin, northern Costa Rica and southern Nicaragua, based on geologic field observations, petrography, age determinations and geophysical data starting from off shore Santa Elena Peninsula across the San Carlos Basin to the Rio San Juan region near San Isidro, Costa Rica. Oil and gas exploration in the central San Carlos Basin identified Upper Mantle serpentinized peridotite basement in the Tonjibe 1 wellbore, (depth~1985 m) underlying laminated to massive bedded siliceous shales, cherts, ash fall tuffs/volcanic glass beds and volcaniclastic sandstones of the Cenomanian-Campanian, Loma Chumico Formation. Several surface locations of serpentinized peridotite occur near San Isidro and, associated with, radiolarites, near Sabolos, Nicaragua. The interpreted “tectonic nappe” on the Santa Elena Peninsula is also composed of serpentinized peridotite that may have been southward thrusted over the Aptian-Albian deep water radiolarian cherts and volcanites of the Santa Rosa Accretionary Complex (SRAC) and later intruded by basaltic magmas.

The two regions have been characterized as part of the southern Chortis Block basement composed of Upper Mantle serpentinized peridotites. Field observations of the peridotites, their regional geologic settings, and the San Carlos basin structure constrain nature, origin and development. Regional subsurface components are studied using available well log, seismic, gravity and aeromagnetic data provided by Suncor, UTIG, GEOMAR and the USGS respectively. It appears that a dissected southern Chortis region is the most logical conclusion to explain the variability between the two regions.