Evolution of the slope landward of the Middle America Trench, Nicoya Peninsula, Costa Rica

Abstract

Summary The Nicoya Peninsula of Costa Rica represents an uplifted portion of the trench slope break landward of the Middle America Trench, and is composed of the Nicoya Complex and its sedimentary cover. The Nicoya Complex is upper Mesozoic oceanic crust, showing the effects of continued igneous activity and deformation probably due to the Late Cretaceous initiation of subduction in an oceanic region. The sedimentary cover reflects uplift and progressive deformation throughout the evolution of this intra-oceanic arc-trench system. Sediments of the Nicoya Complex are open-ocean deposits, comprising locally derived sedimentary breccias made up of recycled Nicoya Complex material as well as more typically pelagic radiolarian cherts, black shales and deep-water limestones. The basal unit of the sedimentary cover is the mainly hemipelagic Campanian Sabana Grande Formation, which was probably deposited on the juvenile trench slope. In the Sabana Grande Formation, basal siliceous mudstones give way up-section to the foraminifer-rich calcareous mudstones, representing the passage of the sediment surface up relative to the CCD. Interbedded conglomerates and sedimentary breccias reflect recycling of Nicoya Complex material, apparently by uplift and erosion of the forearc basement. The Sabana Grande Formation is overlain by the ?Campanian to Palaeocene Rivas and Las Palmas Formations which consist of thick, mainly volcanogenic, thin- to thick-bedded turbidites and massive and/or pebbly sandstones characteristic of mid-fan facies associations. These rocks were probably deposited in a forearc basin situated landward of a structural high. In contrast, Palaeocene turbidites which crop out to the SW and seaward of the proposed high are thin-bedded, associated with redeposited hemipelagic mudstones, and interpreted as trench slope deposits. This thin-bedded turbidite/hemipelagic mudstone unit is in turn overlain by Palaeocene or Eocene calcareous mudstones and Eocene siliceous mudstones, both of which are interpreted to represent continued deposition on the trench slope, respectively above and below the CCD. These ?Palaeocene and Eocene mudstones are overlain unconformably by Eocene to ?Quaternary shallow-water clastic and carbonate deposits, reflecting rapid Eocene tectonic uplift. The general trend of shallowing in depositional environments through time is paralleled by an overall decrease in intensity of structural deformation up-section, indicating progressive deformation in this forearc terrane.