REVISED MIOCENE STRATIGRAPHY OF THE PANAMA CANAL BASIN AND ITS BEARING ON THE PENINSULA OF PANAMA

Before there was an Isthmus of Panama, there was a Peninsula of Panama. Here we show that the Peninsula of Panama existed in southern Central America as early as 19 Ma, based on new lithostratigraphic, biostratigraphic and strontium chemostratigraphic analyses of the Gaillard Group exposed along the Gaillard Cut of the Panama Canal. Land mammals found in the Miocene Cucaracha Formation have similar body sizes to conspecific taxa in North America, indicating that there existed a dry-land connection with North America that allowed ready gene flow between populations during this time. Previous stratigraphic studies of the Panama Canal Basin have suggested that the Cucaracha Formation lies stratigraphically between the shallow-marine Culebra Formation and the shallow-to-upper-bathyal La Boca Formation, the latter containing the Emperador Limestone (Woodring, 1964; Stewart et al., 1980; Woodring, 1982; Graham et al., 1985). If this stratigraphic arrangement is correct, then we would conclude that the Peninsula of Panama was short-lived (1-2 m.y.) in the early Miocene, having been submerged in part by the transgression represented by the overlying La Boca Formation. We find no evidence for this stratigraphic model, however; but instead, our data show that the La Boca Formation is significantly older (23.07 to 20.62 Ma) than both the Culebra (19.83-19.12 Ma) and Cucaracha formations (Hemingfordian to Barstovian age). The Emperador Limestone is also significantly older (21.24-20.99 Ma). What has been called the “La Boca Formation” (with the Emperador Limestone), we re-interpret as being the lower part of the Culebra Formation. Our new data sets demonstrate that the main axis of the volcanic arc in southern Central America more than likely existed as a peninsula connected to North America for much of the Miocene, which has profound implications to our understanding of the tectonic, climatic/oceanographic, and biogeographic history related to the formation of the Isthmus of Panama.