PALEOENVIRONMENTS OF THE UPPER CUCARACHA FORMATION (EARLY MIOCENE, PANAMA): EVIDENCE FROM PEDOFACIES AND FLUVIAL DEPOSITS

The Cucaracha Formation (early Miocene, 19-18 Ma), exposed along the Panama Canal, preserves important information about the paleoenvironments and depositional conditions in the neotropics prior to the formation of the Isthmus of Panama. Recent discoveries of new fossil assemblages at various Puente Centenario localities have prompted a new detailed lithostratigraphic study of the upper Cucaracha Fm. and the distribution of depositional paleoenvironments. The new section, bound by a welded tuff marker bed at its base and an upper contact with the Pedro Migual Fm. basalts, includes 2 highly fossiliferous fluvial units and several paleosols of various maturities. The lower fluvial unit consists of fining-upward sequences of clast-supported conglomerate to coarse sandstone and contains fragments of petrified wood and reworked lignite deposits. The upper fluvial unit is composed of a single fining-upward sequence of matrix-supported conglomerate, trough crossbedded medium sand, and weakly pedogenically-altered mudstone. The matrix-supported conglomerate contains abundant macrofloral elements. The lowest maturity paleosols, interpreted as poorly-drained fluvisols, are concentrated stratigraphically near the lower fluvial unit, typically interbedded with decimeter-thick lignites. These paleosols are greenish-gray in color, exhibit limonite mottling around rhizoliths, and contain rare slickensides and vertebrate fossils. The highest maturity paleosols, interpreted as well-drained cambisols, are concentrated near the upper fluvial unit and occasionally exhibit erosional upper contacts with laterally-restricted interbedded sand/mudstone lenses. The hematite-rich B horizons of these paleosols can reach 1.5 meters of thickness and contain extensive limonite mottling and abundant gray rhizohaloes. Previous studies of the upper Cucaracha paleosols and paleoenvironments have suggested the presence of short-lived brackish water settings. Based on the revised section described here, however, we interpret the upper Cucaracha as representing lateral facies changes within a lowland fluvial system. Specifically, our section exhibits a transition from a braided stream system with adjacent freshwater wetland deposits to a seasonally-dry forest with abundant, small tributary channels.