Transition of ephemeral palustrine to aeolian deposits in a continental arid – semi-arid environment (Upper Cretaceous Bauru Basin, Brazil)

Abstract

In this work, we examine the transition from an ephemeral palustrine to a desertic sand sheet depositional system, corresponding to the Araçatuba Formation and Vale do Rio do Peixe Formation, respectively. These Upper Cretaceous formations form the lower part of the sedimentary sequence that fills the intracratonic Bauru Basin in the South American Platform (southeast Brazil). The transition between the two formations is through paleosols located at the top of the Araçatuba Formation, which have different spatial and temporal evolutions. In the inner, southern, and western parts of the Araçatuba Formation area, the paleosols are well developed, while in the outer, northern and eastern parts they are less developed. In the inner area, the mature paleosols are in sharp contact with the overlying deposits of the Vale do Rio do Peixe Formation, while in the outer area, immature paleosol successions in which sedimentary and pedogenetic features are interlayered represent the transition. Ghosts of sedimentary structures in the outer paleosols allow us to attribute the variations in paleosol development to different sedimentary inputs. In the outer part, a slow but continuous supply of sediment caused continuous rejuvenation of the soils, whereas in the inner portion the absence of sedimentary input allowed very mature soils to form. The transition between Araçatuba and Vale do Rio do Peixe formations may thus be explained by a sudden increase in sedimentation in the area, probably triggered by tectonic uplift in northern and eastern parts of the Bauru Basin, causing the progradation of alluvial fans of the Marília Formation, which supplied sediments to the sand sheet depositional system of the Vale do Rio do Peixe Formation.