CAMPANIAN-MAASTRICHTIAN LIMESTONE WITH LARGER FORAMINIFERA FROM PEÑA BRUJA ROCK (SANTA ELENA PENINSULA)

Abstract

We report on a Upper Cretaceous (Campanian-Maastrichtian) limestone sample collected by P. Denyer and a group of students during a field trip of the course Geología de Campo II, of the Escuela Centroamericana de Geología (UCR). The sample was taken of the islet known as Peña Bruja, located near to the shore of Naranjo Beach, Santa Rosa National Park, in the southeast of the Santa Elena Peninsula in Northern Costa Rica (Fig. 1), Lambert coordinates 352.2E/308.3N. Unfortunately only one small sample was furnished due to the difficult access of the cliff and the high waves and strong currents around the rock. The sample was analyzed by Claudia MoraBaumgartner at the Institut of Geology of University of Lausanne. A polished thin section of 30 mm thickness was used for the observation under a petrographic microscope and for cathodoluminescence microscopy. Cathodoluminescence (CL) of the polished surface is obtained by the exposure to an electron beam produced by a cold cathode with an acceleration potential of 15 kV, in a vacuum chamber with a dilute He-atmosphere at 0.2 Torr. The CLequipment is attached to a conventional light microscope. Different CL-intensities in calcite correspond to variation in trace concentrations of Mn+2. The field trip was sponsored by the project # 113-90-071, Análisis geo-estructural comparado de Costa Rica financed by the Vicerrectoría de Investigación (UCR). We thank administration of the Área de Conservación Guanacaste for their valuable help. Dr. Jorge Cortés (CIMAR) gave logistic support during the field trip.