Stratigraphic evolution of a fluvial-eolian succession: The example of the Upper Jurassic-Lower Cretaceous Guara and Botucatu formations, Parana Basin, Southernmost Brazil

The Guard and Botucatu formations comprise an 80 to 120 in thick continental succession that crops out on the western portion of the Rio Grande do Sul State (Southernmost Brazil). The Guard Formation (Upper Jurassic) displays a well-defined facies shift along its outcrop belt. On its northern portion it is characterised by coarse-grained to conglomeratic sandstones with trough and planar cross-bedding, as well as low-angle lamination, which are interpreted to represent braided river deposits. Southwards these fluvial facies thin out and interfinger with fine- to medium-grained sandstones with large-scale cross-stratification and horizontal lamination, interpreted as eolian dune and eolian sand sheets deposits, respectively. The Botucatu Formation is characterised by large-scale cross-strata formed by successive climbing of eolian dunes, without interdune and/or fluvial accumulation (dry eolian system). The contact between the Guard and the Botucatu formations is delineated by a basin-wide deflation surface (supersurface). The abrupt change in the depositional conditions that took place across this supersurface suggests a major climate change, from semi-arid (Upper Jurassic) to hyper-arid (Lower Cretaceous) conditions. A rearrangement of the Parana Basin depocenters is contemporaneous to this climate change, which seems to have changed from a more restrict accumulation area in the Guard Formation to a wider sedimentary context in the Botucatu Formation. (C) 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.