37. A TECTONIC CROSS-SECTION OF THE COSTA RICAN PACIFIC LITTORAL AS A KEY TO THE STRUCTURE OF THE LANDWARD SLOPE OF THE MIDDLE AMERICA TRENCH OFF GUATEMALA1

Pre-upper Senonian basement of Costa Rica crops out in the Santa Elena and Nicoya peninsulas. From south to north and from base to top the basement includes the Esperanza, Matapalo, and Santa Elena units. The Esperanza unit is Albian-Santonian in age and consists mainly of pillow basalt and massive basalt flows. The Matapalo unit includes Callovian-Cenomanian radiolarite and exhibits a massive basalt flow, pillow basalt, and dolerite basement. The Santa Elena unit contains ultramafic and mafic rocks in which harzburgite is the major component. The most important tec-tonic features of the Nicoya Complex are the large Santa Elena and Matapalo nappes. Nappe emplacement was from north to south during late Santonian time. The sedimentary cover of the Nicoya Complex comprises (1) the Campanian El Viejo Formation, which consists of shallow-water sediments to the north on Santa Elena Peninsula and the Campa-nian-Maestrichtian Sabana Grande Formation of deep-water origin to the south on Nicoya Peninsula; (2) Paleocene sediments, which indicate deposition in a deep-water environment and which comprise the Rivas, Las Palmas, and Samara formations; and (3) post-upper Eocene(?) sediments, which consist of the shallow-water Barra Honda and Montezuma formations. Two unconformities are the major geologic features of the upper Senonian-Tertiary sequence of Costa Rica. The lower one is at the base of the Sabana Grande Formation and marks a major change in the geologic conditions (basalt is scarce in the Campanian-Tertiary series); the upper unconformity at the base of the Barra Hond