GEOLOGICAL RECONNAISSANCE OF THE MIDDLE-LATE MIOCENE ALHAJUELA FORMATION (PANAMA): IMPLICATIONS FOR SHOALING OF THE CENTRAL AMERICAN SEAWAY

The shoaling and closure of the Central American Seaway (CAS) during the late Neogene had a major impact on local ecosystems, oceanographic currents, and global climate. Constraining the spatial and temporal context of CAS shoaling, however, is hindered by ambiguity in the stratigraphic relationships and depositional environments of marine sedimentary exposures in Panama and adjacent regions. Here, we present new information from the Alhajuela Formation (Lago Alajuela, central Panama) with implications for the presence of the CAS in the Panama Canal and Lago Alajuela Basins.

The age and stratigraphic relationships of the Alhajuela Formation are problematic due to its geographically-restricted exposures and lack of contacts with other age-constrained formations in Panama Canal Basin. Recent discoveries of a vertebrate assemblage from basal strata of the Alhajuela Formation suggest a middle-late Miocene age contemporary with the proposed onset of shoaling, yet the stratigraphy exposed in Lago Alajuela exhibits a transgressive transition from nearshore facies to shallow shelf environments. Matrix-supported conglomeratic horizons containing volcanic clasts, fossil terrestrial mammals, wood fragments, and marine vertebrates and invertebrates exhibit erosional contacts with underlying bioturbated marine sandstones with exclusively marine fossil content. Other mollusc-bearing sandstone units exhibit thin conglomeratic beds and lenses. We interpret these lithological sequences as representing conglomerate deposition in nearshore environments via subaqueous debris-flow and current-reworking processes, respectively. Higher in the stratigraphy, sediments comprise massive, carbonate-cemented, tuffaceous sandstone with exclusively marine fossil content, interpreted as shallow shelf deposits.

This transgressive sequence in the Alhajuela Formation is consistent with the overall trend in paleobathymetry recently reported for the late Miocene Gatun Formation, which is either contemporaneous or slightly younger than the Alhajuela Formation. We conclude that the Panama Canal Basin did not serve as a major conduit for the mixing of Pacific-Caribbean waters. Rather the sedimentary record in this basin records an increase in accommodation space during the middle-late Miocene.