Abstract

Resumen. The Cretaceous stratigraphy of the Ultima Esperanza District, southern Chile, is exposed parallel to the adjacent fold-thrust belt of the Patagonian Andes and reflects the evolution of the Magallanes retroarc foreland basin. The final filling of the basin is known as the Dorotea Formation which represents a delta system of Maastrichtian–Danian age. The unit consists of shallow-water shelfal and deltaic strata and forms the topset of large-scale southward-progradating shelf-and-slope clinoforms. The Dorotea Formation contains marine invertebrates, plants, and rare marine reptilians (e.g. mosasaurs). In addition, abundant dinosaur remains, among them several partially articulated hadrosaurs, were recently recorded from the Dorotea Formation by INACH expeditions carried out in 2012 and 2013 to the El Puesto locality in the Río de las Chinas valley, at approximately 4.2 km southeast of the border to Argentina. The new findings represent the southernmost known documented dinosaurs in South America and are associated with well preserved plant remains, including the oldest South American Nothofagus leaf imprint. Here we reconstruct the depositional environment and sedimentary facies of the dinosaurand Nothofagus-bearing strata. Based on three stratigraphic columns with total thicknesses of 485 m, 211,5 m and 211 m, depositional sequences at El Puesto are divided into four cyclic units of 65–303,5 m thickness. Each unit is separated by morphologically resistant sandstone and conglomeratic ridges up to 14,5 m thick which crop out on top of fine grained sediments several to up to 129 m thick. These units reveal upward-coarsening lithologies characteristic of a southward progradation of the deltaic succession. Lithofacies include mudstone, shale, fine- to mediumgrained structureless sandstone, irregular and cross-bedded as well as trough cross-stratified medium- to coarse-grained sandstone, and horizontally bedded conglomerate. These lithologies represent fine-grained prodelta or delta-plain environments and underlie wave-dominated delta-front mouth bar sandstone or delta lobe sandstone, subaqueous delta-plain distributarychannels and interdistributary deposits. Conglomerate layers are interpreted as fluvial deposits of a braided river, or stream-channel alluvial fans. Storm deposits in a shoreface or foreshore position were inferred from coquinite layers and calcareous concretions containing abundant shell fragments. Panopea (Panopea) inferior WILCKENS 1905, Panopea sp. and Pterotrigonia (Rinetrigonia) windhauseniana (WILCKENS 1921) indicate shallow-water marine conditions and a late Cretaceous age, while Modiolus sp. indicates brackish environments for the uppermost part of the measured sections, for which a Paleogene age is indicated by Venericardia sp.