Distribution of palaeosols and deposits in the temporal evolution of a semiarid fluvial distributary system (Bauru Group, Upper Cretaceous, SE Brazil)

Abstract

The stratigraphic and sedimentological knowledge of the Bauru Group (Upper Cretaceous, SE Brazil) is still generally insufficient and controversial. A sedimentological and palaeopedological study allowed to interpret the south-eastern portion of the Bauru Group according to the model of a fluvial distributary system. This work has two objectives: (1) to include palaeosols in the interpretation of a fluvial distributary system and (2) to give detailed information on the sedimentological and stratigraphic features of the SE portion of the Bauru Group in order to support biostratigraphical, taphonomic and palaeoecological studies. In the south-eastern portion of the Bauru Group, three genetic stratigraphic units were described and interpreted, here informally called lower, intermediate and upper units. The lower unit is constituted of muddy sandstone salt flat deposits and sandstone sheet deltas deposits and is interpreted as a basinal part of a fluvial distributary system. The intermediate unit is formed of very fine to fine-grained sandstone-filled ribbon channel and sandy sheet-shaped beds, suggesting a distal or medial portion of a fluvial distributary system. The upper unit does not match with the present models of the fluvial distributary system because mostly constituted of moderately developed, well-drained, medium- to fine-grained sandstone palaeosols, which testify pauses of sedimentation to the order of 104years. Preserved features of sedimentary structures suggest that the parent material was formed by occasional catastrophic unconfined flows. This unit may represent the most distal portion of a fluvial distributary system generated by retrogradation of the alluvial system due to aridification of the climate. The upper unit may be interpreted also as proximal portion of fluvial distributary system if considering the coarser-grained and the well-drained palaeosols. However, the absence of channel deposits makes this interpretation unconvincing.