The Valcheta Petrified Forest (Upper Cretaceous), northern Patagonia, Argentina: A geological and paleobotanical survey

Abstract

The Valcheta Petrified Forest is located in the northeast of the Río Negro Province, Argentina. This work provides a geological characterization of the site and a survey of the fossil logs exposed on the surface and their systematic study. The fossiliferous levels bearing the silicified logs are here referred to the Allen Formation (middle-upper Campanian – lower Maastrichtian, Malargüe Group). These deposits crop out in patches in the study area, and have been deposited on a paleorelief carved in Eopaleozoic rocks of the Nahuel Niyeu Formation being mostly covered by thin layers of Quaternary sediments. The Valcheta Petrified Forest was developed on the margins of a braided fluvial system. Numerous silicified logs have been located at the site. They consist of large axes, not found in life position, with monopodial growth, some of which retain the bases of lateral branches and flared-like base. The estimated height and ages of these trees suggests that it consisted of a mature forest. No other associated vegetative or reproductive structures have been found. Anatomically, a single xylological type has been distinguished and assigned to Podocarpoxylon mazzonii (Petriella) Müller-Stoll et Schultze-Motel, (Podocarpaceae). This taxon has been previously identified from other localities of the Upper Cretaceous to the Danian of Patagonia although never preserved in such complete tree grow architecture. This shows that, along others conifers, cycads and palms, P. mazzonii would have constituted an important element of the arboreal strata in the North Patagonian forest communities during the Campanian–Danian interval producing monotypic patches in some areas.