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The Cretaceous Chubut Group in the Golfo San Jorge Basin (Patagonia, Argentina) comprises up to 6000 in of continental sediments. At the base of the succession, an extensive saline-alkaline lacustrine unit (Pozo D-129 Fm) grades laterally towards the A basin margin into the fluvial Matasiete Fin (Hauterivian?-Aptian). The Matasiete Fin comprises up to 650 in of siliciclastic and pyroclastic deposits in its type area, where three Members have been distinguished. The following facies associations were identified: 1) single fluvial channels; 2) multistorey fluvial channels; 3) proximal floodplain; 4) distal floodplain; 5) pyroclastic (ash-fall and ground-surge) deposits. Additional observations in the time-equivalent Pozo D-129 Fm provide evidence for 6) shallow lacustrine, and 7) deep-lacustrine facies associations. The floodplain association contains paleosols with abundant carbonate concretions and fine pyroclastic tuffs. Paleosols are mostly vertisols, characterized by deep desiccation cracks, slickensides and carbonate nodules. These observations, coupled with the occurrence of shallow-water oolitic grainstones in the lacustrine Pozo D-129 Fin indicate a semiarid environment. Tree trunks up to I in in diameter and 15 in long, preferentially located near channel margins, provide evidence for vegetated riverbanks. The exposures of the Matasiete formation in the San Bernardo Fold Belt show individual channel belts containing straight, meandering and braided channels. Evidence of changes in fluvial style attributable to base-level control has not been observed. The most dramatic changes in fluvial architecture, which were comparatively short-lived, were provoked by intermittent pyroclastic deposition on the floodplain. The reduced infiltration capacity of the floodplain and the concomitant increase in sediment load increased runoff, as well as volume and flashiness of the discharge, which favoured the development of shallow, multichannel rivers. The fluvial system abruptly changed from single, ribbon channels to a braided system, which notably increased the width/thickness ratio of sandbodies. (c) 2007 Elsevier B.V. All rights reserved.