MIOCENE FORAMINIFERAL BIOFACIES ALONG THE CARIBBEAN COAST OF NORTHWEST PANAMA

One of the last straits in Central America that connected tropical Atlantic and Pacific waters was through the Panama Canal Basin, central Panama. The strait was closed in the middle Miocene, as shown by terrestrial deposits of the underlying Cucaracha Formation (central Panama Canal Basin), and was reopened by late middle to late Miocene time when sediments of the lower Gatun Formation were deposited in the northern part of the basin. The Gatun Formation is informally divided into lower, middle and upper parts, and foraminifera from all parts have primarily Caribbean associations. Overlying the Gatun Formation is the uppermost Miocene Chagres Formation, the youngest formation of the Panama Canal Basin. Foraminifera from the type Chagres Formation have primarily Pacific associations.

New foraminiferal collections were made from outcrops previously mapped as either undifferentiated volcanics or Miocene lutites, silts and conglomerates. Analyses of similarity between the foraminifera and those from different facies of the Panama Canal and Bocas del Toro basins are used to identify changes in biofacies along the 180 km of Caribbean coast between the basins. Twenty-two inner-middle neritic benthic foraminiferal assemblages from the lower, middle and upper parts of the Gatun Formation, and twelve assemblages from the middle neritic Rio Indio section and outer neritic/upperbathyal type section of the Chagres Formation are compared statistically to the newly collected assemblages from the lowermost Gatun Formation (east of the Panama Canal), and the coastline between Gobea (west of the Panama Canal) and Bocas del Toro. The paleoenvironments and biogeographic associations of the foraminiferal biofacies are incorporated into reconstructions of the history of uplift and Atlantic-Pacific connections, and to infer formational boundaries.