ABSTRACT:We present new sedimentologic and biostratigraphic data on the shallow-water limestone sequence of the quebrada Concha, the type locality of the Eocene Parritilla Formation in central Costa Rica. This unit appears at the base of the about 4.500 m thick sedimentary sequence of the Cenozoic Valle Central basin, unconformably overlying the Nicoya ophiolite complex. The Parritilla limestone contain a rich larger foraminiferal fauna from the early Middle Eocene (Lutetian) represented by well-preserved and diverse specimens of the Amphistegina-Helicostegina-Lepidocyclina(Polylepidina) phylogenetic line. On the basis of the high biostratigraphic resolution of the analyzed foraminiferal assemblage, we define the quebrada Concha sequence as a Lutetian type locality of Costa Rica and erect a new Eocene chronostratigraphic unit: the Parritillian. From a sedimentologic perspective, the Parritilla limestones represent a bioclastic carbonate sequence deposited on the southern border of the inner-arc basin in an open shallow-marine environment with locally restricted water circulation. In the study area, the uplifted ophiolite complex probably formed part of a neritic arc platform surrounding the volcanic edifices. Although Lutetian carbonate sedimentation began in response to a rapid marine transgression at ~49 Ma over the exposed basement, the carbonate development itself terminated abruptly with the deposition of the volcaniclastic sequence of the Caraigres Formation, resulting from the Eocene volcanic activity in adjoining areas. As suggested by these voluminous volcaniclastic deposits conformably overlying the Parritilla limestone sequence, the strong subsidence of the Lutetian arc platform was probably related to emergent andesitic edifices in the inner arc region of central Costa Rica.
RESUMEN:Presentamos un estudio sedimentológico y bioestratigráfico de las calizas eocenas de la quebrada Concha, localidad tipo de la Formación Parritilla, ubicada en el sur del Valle Central de Costa Rica. Es una secuencia bioclástica de plataforma carbonatada de 150 m de espesor, depositada discordantemente sobre rocas cretácicas del Complejo de Nicoya, formando la base de la secuencia sedimentaria cenozoica de la cuenca intra-arco del Valle Central. Las asociaciones de macroforaminíferos de las calizas analizadas se caracterizan por especies de la línea filogenética Amphistegina-Helicostegina-Lepidocyclina (Polylepidina), típicas del Eoceno Medio, especialmente de la parte basal e inferior (Lutetiano). Con base en la alta resolución bioestratigráfica de esta asociación de foraminíferos, consideramos la quebrada Concha como localidad tipo del Lutetiano e introducimos una unidad cronoestratigráfica para el Eoceno Medio, la cual denominamos Parritillano, sinónimo del Lutetiano en Costa Rica. Sedimentológicamente las calizas de Parritilla representan acumulaciones bioclásticas de organismos bentónicos, depositadas sobre el basamento ofiolítico levantado en un ambiente marino somero y abierto, localmente bajo condiciones restringui...