CALIZAS LUTETIANAS DEL ARCO INTERNO PALEÓGENO DE COSTA RICA

Abstract

We present new sedimentologic and biostratigraphic data on the shallow-water limestone sequen-ce of the quebrada Concha, the type locality of the Eocene Parritilla Formation in central Costa Rica. This unitappears at the base of the about 4.500 m thick sedimentary sequence of the Cenozoic Valle Central basin, un-conformably overlying the Nicoya ophiolite complex. The Parritilla limestone contain a rich larger foraminife-ral fauna from the early Middle Eocene (Lutetian) represented by well-preserved and diverse specimens of theAmphistegina-Helicostegina-Lepidocyclina(Polylepidina) phylogenetic line. On the basis of the high biostrati-graphic resolution of the analyzed foraminiferal assemblage, we define the quebrada Concha sequence as a Lu-tetian type locality of Costa Rica and erect a new Eocene chronostratigraphic unit: the Parritillian. From a sedi-mentologic perspective, the Parritilla limestones represent a bioclastic carbonate sequence deposited on the south-ern border of the inner-arc basin in an open shallow-marine environment with locally restricted water circula-tion. In the study area, the uplifted ophiolite complex probably formed part of a neritic arc platform surroundingthe volcanic edifices. Although Lutetian carbonate sedimentation began in response to a rapid marine transgres-sion at ~49 Ma over the exposed basement, the carbonate development itself terminated abruptly with the depo-sition of the volcaniclastic sequence of the Caraigres Formation, resulting from the Eocene volcanic activity inadjoining areas. As suggested by these voluminous volcaniclastic deposits conformably overlying the Parritillalimestone sequence, the strong subsidence of the Lutetian arc platform was probably related to emergent ande-sitic edifices in the inner arc region of central Costa Rica.