Palaeosols as stratigraphic proxies and palaeosurface markers in distributive fluvial systems

Abstract

Palaeosols can be used as stratigraphic proxies, palaeosurface indicators, and palaeorelief reconstitution tools, as they reflect a complex interrelationship between sedimentation, erosion, non-deposition, and pedogenesis. The objective of the article was to estimate the relative time of formation of palaeosols of the Marília Formation, to determine the periods of non-deposition and non-erosion, indicative of stabilities on the surfaces, and to interpret the profiles as markers of palaeosurfaces in distributive fluvial systems (DFS) of the Late Cretaceous of Brazil. Eleven sections of the Marília Formation (S1, S2, S3, S4, S5, S6, S7, S8, S9, S10, and S11) were described, comprising forty-three profiles. The pedotypes classification (polygenic, compound, and cumulative) and the determination of palaeosols' relative formation time and residence time were based on the specialized literature. The reconstitution of pedosedimentary cycles considered the dynamics of sedimentation, erosion, and pedogenesis. Eight sedimentary cycles and 45 pedosedimentary cycles were interpreted. All the Maastrichtian pedosedimentary cycles of the Bauru Basin corresponded to depositional cycles interspersed with erosive and pedogenic processes. Palaeosols with argillic (Btk, Bt, Bss) and calcic (Bkm) horizons, developed from fluvial sandstones, evolved in processes predominantly after the deposits, with later erosive processes. However, although most of the profiles developed in periods between deposits and erosion, pedogenic processes also occurred concomitantly with the sedimentary cycles, favouring the formation of polygenetic profiles, which presented the longest pedogenic cycles in chronological terms. Palaeosols with argillic horizons can be important stratigraphic landmarks. They represent stopping moments in erosion and sedimentation of up to 1.374 My, thus consisting of the closing of high-frequency depositional cycles. Although palaeosols can demonstrate relief inversion and pedotypes with Bt-horizons can indicate ancient regional surfaces, the complete reconstitution of the topography becomes more possible at the local level, in a palaeoslope approach, due to the variability of pedofacies.