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Landscape and depositional controls on palaeosols of a distributive fluvial system (Upper Cretaceous, Brazil).
(2020) Soares, Marcus Vinícius Theodoro; Basilici, Giorgio; Lorenzoni, Paolo; Colombera, Luca; Mountney, Nigel Philip; Martinelli, Agustin Guillermo; Mesquita, Aquila Ferreira; Marinho, Thiago da Silva; Vásconez García, Richard Guillermo; Marconato, André
The stratigraphic record of distributive fluvial systems is commonly characterised by frequent and complex interstratification of palaeosols among channel and overbank deposits. However, current models focus primarily on sedimentation and pay only limited attention to palaeopedogenesis, thereby failing to incorporate important palaeoenvironmental and stratigraphic information. This study proposes a pedosedimentary model for distributive fluvial systems that depicts and accounts for two palaeopedogenetic trends: one downdip, in relation to distality from the fan apex, and one along-strike, in relation to distance from active channel belts. Palaeosols are reported in detail from an Upper Cretaceous succession of the Bauru Basin, southeastern Brazil, through the application of macro-, micromorphological and geochemical studies, combined with facies and architectural-element analyses of sediments. In the downdip palaeopedogenetic trend, the proximal zone of the depositional system is characterised by a dominance of well-drained Inceptisols that develop on amalgamated channel fills; in the medial zone, Inceptisols occur interlayered with overbank deposits containing Entisols and poorly drained Vertisols. The distal zone preserves more mature and poorly drained Inceptisols developed on deposits of overbank and sporadic distal channel fills. These pedotypes show an increase in maturity and hydromorphism, moving away from the apex to the fan toe. This is likely linked to (i) the progressive approach of the topographic surface to the water table, and (ii) the average increase in distance to an active channel belt in distal locations. The along-strike palaeopedogenetic trend culminates in poorly developed palaeosols in floodplain regions that correspond to topographic depressions located between channel belts and which were subject to recurrent floods. Because palaeopedogenesis in the floodplain region is penecontemporaneous to sedimentation, pedotypes show an increase in maturity, bioinduced calcification and hydromorphism with distance from the active channels; they pass laterally from Entisols and Inceptisols near active channels, to Vertisols away from active channels. Conversely, following avulsion, abandoned channel belts remain as topographically elevated alluvial ridges located at some distance from the newly active channels and positioned above the water table and this leads to the development of better drained and better developed Inceptisols relative to pedotypes of the floodplain region. Overall, both palaeopedogenetic trends demonstrate the overriding controls of topography, sedimentation rate and parent material on pedogenesis, with only minor climatic influence. This work offers a novel pedosedimentary model for distributive fluvial systems and highlights the palaeoenvironmental significance of palaeosol trends, providing new constraints for the recognition of distributive fluvial systems in the rock record.
Sedimentology of a distributive fluvial system : the Serra da Galga Formation, a new lithostratigraphic unit (Upper Cretaceous, Bauru Basin, Brazil).
(2021) Soares, Marcus Vinícius Theodoro; Basilici, Giorgio; Marinho, Thiago da Silva; Martinelli, Agustin Guillermo; Marconato, André; Mountney, Nigel Philip; Colombera, Luca; Mesquita, Aquila Ferreira; Vasques, Julia Tucker; Abrantes Junior, Francisco Romero; Ribeiro, Luiz Carlos Borges
The Bauru Basin of SE Brazil is a large (ca. 370,000 km2 ) Upper Cretaceous intracratonic feature, important for its fossil remains and of particular value as a source of regional palaeoclimatic information. Historically, lithostratigraphic reconstructions have been performed mainly for successions of the central and southern parts of the basin, resulting in a lithostratigraphic scheme that is not applicable to the northernmost regions. In particular, the northeastern deposits of the Marília Formation (Serra da Galga and Ponte Alta members) reveal lithological, stratigraphic, and palaeontological differences from southeastern and northwestern counterparts (Echapor~a Member). Nevertheless, these deposits are considered as a single lithostratigraphic formation in the literature. To address this problem, this study demonstrates how the northeastern deposits of the Marília Formation do not show affinity to the rest of the unit. A more suitable lithostratigraphic model is proposed for the northeastern succession as a distinct and independent unit. Lithofacies and palaeopedological analysis, combined with lithostratigraphic mapping of the northeastern deposits, reveal 11 distinct lithofacies and three pedotypes over an area of 450 km2 . Sedimentary facies and pedotypes were assigned to six interbedded architectural elements: (a) type 1 channel fill, (b) type 2 channel fill, (c) type 3 channel fill, (d) interchannels, (e) palaeosols, and (f) calcrete beds. The succession is interpreted as a distributive fluvial system with overall direction of flow to the NNW, and which developed under the influence of a semiarid climate regime. This contrasts with deposits of the southeastern and northwestern Marília Formation, previously suggested to be of fine-grained aeolian affinity with interbedded poorly channelised deposits assigned to an aeolian sand sheet environment. By revising the existing lithostratigraphic scheme for the northeastern deposits, and contrasting them with laterally equivalent strata, this work demonstrates how the previously named Serra da Galga and Ponte Alta members reveal a unique set of lithological, architectural, and genetic signatures that permits to separate them from the Marília Formation. Finally, a new lithostratigraphic classification for the unit is proposed: the Serra da Galga Formation, whose deposition relates to an ancient distributive fluvial system.

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