SHALLOW MARINE PALEOENVIRONMENTS AND BENTHIC FORAMINIFERA AT THE EDGE OF THE EARLY MIOCENE CENTRAL AMERICAN SEAWAY

The Culebra Formation crops out along the banks of the Panama Canal and consists of early Miocene, fossiliferous sediments that accumulated at the western edge of the Central American Seaway, which separated North and South America until about 4 Ma. The benthic foraminiferal faunas of the Culebra Formation are used to document changes in marine habitats and water depths, and to assess the oceanic affinity of the marine biota that occurred within the seaway during that time interval.

The lower part of the Culebra Formation, which overlies the Las Cascadas Agglomerate, contains nearshore assemblages with species of Elphidium, Ammonia and Rosalina, and marsh assemblages that include Trochammina and Miliammina. Assemblages from the coral-rich Emperador Limestone Member are the only ones that contain carbonate-associated taxa, particulary miliolids. Foraminiferal assemblages of the upper part of the Culebra Formation include species of Cibicides, Bolivina, Uvigerina, Cassidulina and Bulimina, and indicate a deeper, bathyal paleoenvironment (~200–400 m). Therefore, changes in foraminiferal species indicate a transition from coastal marsh and nearshore environments to reefal habitat, and then bathyal marine conditions. This indicates that the general shallowing of the Central American Seaway included intervals of considerable deepening. The taxa are of Pacific or cosmopolitan affinity, and have a strong association with assemblages of California. This biogeographic pattern agrees with the hypothesis that the early Central American arc was in the Pacific Ocean rather than near its present position.