Fluvial styles, palaeohydrology and modern analogues of an exhumed, Cretaceous fluvial system: Cerro Barcino Formation, Cañadón Asfalto Basin, Argentina

Abstract

Superb surface exposures of a fluvial palaeochannel-belt 2-5km wide and 13km long occur in Cretaceous sedimentary rocks of the Cerro Barcino Formation (Las Plumas Member), Chubut Group, Cañadón Asfalto Basin, Argentina. The exceptional plan-view exposures of fluvial sandbodies allowed identification of a large number of WNW-ESE sinuous ridges that represent high-sinuosity, braided and low-sinuosity fluvial palaeochannels, crevasse and chute palaeochannels, and crevasse-splay deposits. Predominance of low-sinuosity palaeochannels and low values of the width/thickness ratio of most them suggest high stability of their margins, probably controlled by the cohesiveness of tuffaceous floodplain deposits. Morphological and sedimentological observations provide evidence of abrupt lithofacies changes in palaeochannel fills, fine-grained intercalations in multi-storey palaeochannels and occurrence of different fluvial styles, implying temporal/spatial variations in palaeodischarge and/or slope conditions. Comparison of the near-identical plan view morphology of the Cretaceous palaeochannel belt with the nearby Chubut River, plus the comparison of palaeohydrological data of the exhumed palaeochannels with hydrological data from this modern analogue, gives evidence of a very similar fluvial behaviour. This provides an integrative tool to be employed in the study of other ancient fluvial successions, including the reconstruction of palaeofluvial hydrological parameters in planetary geology.