MIOCENE DEPOSITIONAL ENVIRONMENTS OF THE SOUTHEASTERN GAILLARD CUT, PANAMA CANAL, REPUBLIC OF PANAMA

Understanding the temporospatial distribution of paleoenvironments in the Panama Canal Basin is necessary to unravel the tectonic evolution of the Isthmus of Panama, especially in light of new evidence for early closure of the Central American Seaway. The La Boca Formation of the Gaillard Cut, Panama Canal, was originally defined as early Miocene marine strata that overlies or is the lateral marine facies equivalent of the terrestrial Cucaracha Fm. The former stratigraphic relationship would imply Miocene intervals of subaerial exposure were short-lived and the Panama Canal Basin was covered by a shallow seaway for most of the Miocene. Recent work has questioned the validity of the La Boca Fm, making an argument, based on strontium isotope age estimates, that the La Boca Fm should be subsumed in the older marine strata of the Culebra Fm. This work did not include strata from the southeastern half of the Gaillard Cut, which encompasses the type locality of the La Boca Fm. Recent excavations made by the Panama Canal Authority have produced new exposures of rock once only accessible via cores. Here, we summarize the distribution of depositional environments in 5 fault-bounded sections exposed along a 6.5 km transect (i.e., the Pedro Miguel Locks to the new Pacific Locks) and evaluate the validity of the La Boca Fm in the southeastern Gaillard Cut. Three out of the 5 sections exhibit indicators of terrestrial depositional environments and represent stages of a marine regressive transition from a restricted estuarine environment (upper Culebra Fm) to a fluvial channel belt (Cucaracha Fm) and subsequent deposition of subaerial volcaniclastics (Pedro Miguel Fm). The remaining 2 sections represent shallow shelf environments and exhibit similarities, in terms of fossils, lithology, and sedimentary structures, with the upper Culebra Fm. The lack of direct stratigraphic association with the Cucaracha or Pedro Miguel Fms prevents us from definitively attributing these strata to either the Culebra or La Boca Fms, and strontium isotope age estimates will be necessary for unequivocal identification. We conclude there are no marine strata overlying the Cucaracha Fm in the southeastern Gaillard Cut, and if the La Boca Fm is valid, it is most likely a lateral marine facies geographically restricted to the Pacific side of the Panama Canal Basin.