The Sandino Forearc Basin of western Nicaragua and northwestern Costa Rica (Central America) provides a Campanian to Pliocene sedimentary record. The study of the onshore part of the basin in northwestern Costa Rica reveals for the first time the occurrence of upper Paleogene shallow-marine siliciclastic and carbonate sequences. These sequences have remained undescribed so far and are grouped herein into two new lithostratigraphic units-the upper Eocene Junquillal Formation (Fm.) and the upper Oligocene Juanilla Fm. The upper Eocene Junquillal Fm. is characterized by storm-related, arenitic to conglomeratic deposits comprised in metric, massive amalgamated beds. The shallow shelfal environment of deposition is attested by the presence of hummocky and swaley cross-stratifications. The lithologies of the Junquillal Fm. were previously considered to be part of the underlying, deep-water turbiditic deposits of the Eocene Descartes Fm. The deposition of the Junquillal Fm. is indicative of tectonic uplift that forced regression, which affected the southeastern part of the Sandino Forearc Basin during the late Eocene. The upper Oligocene Juanilla Fm. unconformably overlies the Junquillal Fm. and occurs as a 25-m-thick, 700-m-wide outcrop on Isla Juanilla. It is composed essentially of nodular, coral framestones exhibiting massive, closely packed corals in growth position that are associated with coralline red algae and Larger Benthic Foraminifera (LBF). A late Oligocene age of the reef is attested by LBF assemblages occurring in two different facies. The Juanilla Fm. coral reef is a unique exposure, characterized by extensive constructed coral framework, and which has no equivalent in the Oligocene geological record of Central America. The reef grew on a short-lived, siliciclastic-poor tectonic high, which developed in relation to a lower Oligocene, basin-scale folding event in the Sandino Forearc Basin.
