LAUSR

Abstract A sedimentological-architectural review, and a palaeohydrological study was performed in a 380 m thick succession of the Bajo Barreal Formation (Upper Cretaceous) in the Cerro Ballena anticline, Golfo San Jorge Basin, Patagonia. The studied exposure shows, at the base, isolated sandstone channel belts of small scale and low connectivity within siliciclastic floodplain strata (Section A, 185 m thick). It changes upwards into sheet-like, sandstone channel fills with larger interconnectivity that form channel-belt complexes intercalated with thinner floodplain fines of volcaniclastic origin (Section B, 195 m thick). The succession contains seventeen lithofacies and twelve architectural elements that provide information on the alluvial organization of channel and overbank deposits. Palaeoflow data of 218 fluvial channels (Az. 187°; n = 3777) indicate the presence of a depocenter located southward. Seven fluvial styles were identified: 1) multistorey bodies dominated by downstream accretion, 2) multilateral, broad sheets dominated by downstream accretion, 3) narrow sheets dominated by lateral accretion, 4) ribbon or sheet-like bodies infilled by resedimented volcaniclastics, 5) ribbon-shaped fixed channels with attached splays, 6) unconfined, isolated sheet complexes, and 7) unconfined, moderately amalgamated sheet complexes. The variable geometry and style of palaeo-rivers in coeval levels, and dominance of avulsion deposits support that the fluvial system was anabranching, and locally anastomosed. Using palaeohydrological parameters estimated from cross-bed set thickness, a climatic shift toward wetter and more humid climatic conditions from Section A to Section B was demonstrated. We find support to assess the climate as the main external forcing factor on the alluvial organization and stacking density of the Bajo Barreal Formation.