Factors of paleosol formation in a Late Cretaceous eolian sand sheet paleoenvironment, Marília Formation, Southeastern Brazil

Abstract

The Marília Formation, which crops out in southeastern Brazil, is interpreted as a Late Cretaceous eolian sand sheet area. The sedimentary succession, ca 110 m thick, is characterized by alternating strata of eolian deposits and paleosols. The paleosols constitute more than 66% of the thickness, and are an important element in the interpretation of the paleoenvironmental controlling factors which affected the soil formation in this eolian sand sheet. In this paper six paleosol profiles are described and assigned to two pedotypes: Itaja and Apore. The Itaja pedotype is constituted of a sequence of five superimposed polygenetic profiles which show different phases of clay illuviation and carbonate precipitation, and it has been classified as Aridisol. The Apore pedotype overlies the Itaja pedotype. This pedotype shows an increase in leaching, CIA-K ratios, reddening, and illuvial clay features in respect to Itaja, and has been classified as Alfisol. The analysis of the factors which controlled the soil formation revealed that both pedotypes formed on a stable landscape probably covered by a community of low stature plants, in which the soils had sufficient time to develop very mature profiles. The changes in soil-forming processes were driven principally by variations in available soil moisture from precipitation. Using depth-to-carbonate functions from Bk and CIA-K proxy from Bt horizons of the Itaja pedotype, mean annual precipitation (MAP) estimates range from 240 to 1078 mm/yr, respectively. These contrasting climatic conditions resulted in the superimposition of arid or semi-arid with more humid climates and determined the considerable change in the pedogenic features, with many horizons showing the interlacing of calcite and clay features. In the Apore pedotype CIA-K proxy from Bt horizons estimates that averaged MAP was around 900 mm/yr, and the prevalent humid conditions can be attested by soil properties and abundance and depth of root traces. Our results show that the diversity in pedotypes is mainly attributed to differences in paleoclimatic conditions during Maastrichtian time, and that eolian sedimentation was restricted to periods of harsh arid conditions, as in semi-arid climates, with MAP estimates around 240 mm/yr, the formation of soils with well developed calcic horizons was possible.