LARGE PALEOBATHYMETRIC CHANGES INDICATE RAPID PLEISTOCENE TECTONIC UPLIFT OF THE PACIFIC MARGIN OF WESTERN PANAMA – EASTERN COSTA RICA

The aseismic, anomalously light and thick Cocos Ridge is a physiographic feature which currently subducts in a northeast direction beneath the Central American volcanic arc. Subduction began during the early Pleistocene and was recorded by the marine deposits and faunas of the Burica and Armuelles formations of the Pacific coast of western Panama to eastern Costa Rica. The study area is located on the Burica Peninsula which has exposed a thick sedimentary sequence of about 2600 m of turbidites of the Burica Formation and about 500 m of marine conglomerates, siltstones and lithic arenites of the Armuelles Formation.

Tectonic response of the sedimentary regimes is recognized by changes of deep benthic foraminiferal assemblages of the Burica Formation on the east coast of the Burica Peninsula (e.g., Stilostomella spp., Melonis pompiloides, Plectofrondicularia advena) which are characteristic of lower bathyal depths (>2000m water depth), to inner neritic (0-50m) assemblages of the Armuelles Formation (e.g., Ammonia beccarii, Buliminella elegantissima) on the Rabo de Puerco River.

The biochronology from calcareous nannofossils of the Burica and Armuelles formations indicates that the accumulation of these units took place entirely during the Pleistocene, probably from early to middle Pleistocene. These data suggest a high sedimentation rate during the tectonic uplift and filling of the sedimentary basin. Our interpretation supports the idea that the uplift and initial subduction of the Cocos Ridge were about twice as fast and young as previously thought.