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.
