Fluvial-gravitational processes and preservation of forest vegetation in a volcanic area: the Rancahué formation (Oligocene), Neuquén Andes, Argentina.

Abstract

The siliciclastic deposits of the Rancahué Formation (Oligocene, Aluminé Basin), formed in the eastern side of the Andean Cordillera (70ºW y 39ºS) are studied to evaluate the relationship among different depositional systems in volcanic settings and to disclose the processes that contributed to the preservation of a forest developed at the foot of a volcanic relief. In the alluvial volcano-sedimentary record of the Rancahué Formation an upland petrified forest with 33 logs in life position was identified. This flora, with Nothofagoxylon as the main taxon, was developed under temperate and humid climatic conditions. Three facies associations were defined in the study region. Facies association A was accumulated in subaerial talus and foot-talus slopes, and is composed of monomictic (basaltic) boulder and cobble conglomerates formed by debris floods, hyperconcentrated flows, avalanches, landslides and debris flows. The source of these sediments is local and is related to the growth of an intrabasinal basaltic volcanic edifice. Facies association B consists of cross-stratified, plane bedded and massive conglomerates, pebbly sandstones and coarse-grained sandstones accumulated in a regional braided fluvial system with source areas located in the Andean Cordillera. Facies association C is characterised by fine-grained sediments accumulated in subaerial and subaqueous environments, with periodic contribution of ash falls and development of palaeosols. The forest rooted in deposits of facies association C, and the logs preserved in life position are located within the overlying coarse-grained volcaniclastic deposits of facies association A. The burial and preservation of this vegetation was favoured by the highly aggrading rhythm and comparative low erosive power of the gravitational flows that operated in alluvial and colluvial fans of the riparian hydrogeomorphic zone. Conversely, abundant transported and broken trunks included in the deposits of facies association B clearly indicate the strong erosive power of the currents of the associated braided fluvial system.