BARRA HONDA (UPPER PALEOCENE – LOWER EOCENE) AND EL VIEJO (CAMPANIAN-MAASTRICHTIAN) CARBONATE PLATFORMS IN THE TEMPISQUE AREA (GUANACASTE, COSTA RICA)

ABSTRACT: In this preliminary paper we report on age, microfacies, microfossils and structural setting of platform limestones, that crop out in the Río Tempisque area (Guanacaste, Costa Rica). The limestones can be attributed to two distinct carbonate platform systems: 1. The El Viejo Platform, dated as late Campanian – Maastrichtian, which is rep-resented by slope facies in the Cerros Ballena and Cebollín (near Filadelfia, Peninsula de Nicoya). 2. The Barra Honda Platform, of Upper Paleocene – Lower Eocene age, that extends from the classical outcrops at Cerro Barra Honda in the South, to the Palo Verde region in the North. At Cerro Cebollín, carbonate-rich turbitites and mass flow deposits rest unconformably on basalts of the Nicoya Complex s. str. Pelagic limestone clasts with planktonic foraminifera, and abundant rudist fragments document a late Campanian- Maastrichtian age of the sequence. Basalt and serpentinite lithoclasts witness erosion of an oceanic base-ment, such as the Santa Elena Ultramafic Unit. The Cerro Cebollín sequence is interpreted as a slope deposit that received abundant material form shallow carbonate platforms of the El Viejo Formation that crops out in the northern Nicoya and Santa Elena Peninsulas. At Cerro Barra Honda, Cerro Quebrada Honda and other classical outcrops of the Barra Honda Formation microfa-cies range from restricted to open marine. Most facies contain Ethelia alba a Squamariacean algae reported for the first time from Central America. Restricted micritic facies from Lomas las Pozas contain Thalmannita madrugaensis