Climate‐induced sediment‐palaeosol cycles in a Late Cretaceous dry aeolian sand sheet: Marília Formation (North‐West Bauru Basin, Brazil)

Abstract

AbstractAeolian sand sheets, which are characterized by low relief surfaces that lack dunes, are common in arid and semi‐arid climatic settings. The surface of an aeolian sand sheet can either be stable and subject to pedogenetic effects, or unstable such that it is affected by deflation or sedimentation. The Marília Formation (Late Cretaceous) may be interpreted as an ancient aeolian sand sheet area, where alternating phases of stability and instability of the accumulation surface have been recorded. Detailed field studies were carried out in several sections of the Marília Formation, where cyclic alternations of palaeosols and aeolian deposits were evident, using palaeopedological and facies analysis methods, supported in the laboratory by the analysis of rock samples, cut and polished in slabs, thin sections, scanning electron microscope images and X‐ray diffraction data from the clay minerals. The deposits comprise three lithofacies that, in order of abundance, are characterized by: (i) translatent wind‐ripple strata; (ii) flood deposits; and (iii) ephemeral river channel deposits. Palaeosols constitute, on average, 65% of the vertical succession. Three types of palaeosols (pedotypes) are recognized: (i) Aridisols; (ii) Entisols; and (iii) Vertisols. Erosional surfaces due to aeolian deflation divide the top of the palaeosol profiles from the overlying aeolian deposits. The palaeoenvironmental interpretation of the deposits and the palaeosols allows the depositional system of the Marília Formation to be defined as a flat area, dominated by aeolian sedimentation, with subordinate ephemeral river sedimentation, and characterized by a dry climatic setting with occasional rainfall. The climate is the main forcing factor controlling the alternation between episodes of active sedimentation and periods of palaeosol development. A climate‐controlled model is proposed in which: (i) the palaeosols are indicative of a stable surface that is developed during the more humid climatic phases; and (ii) the erosional surfaces and the overlying aeolian sediments attest to periods of deflation and subsequent sedimentation, thereby increasing the availability of sediment during the drier climatic phases. The ephemeral fluvial deposits mark the more humid climatic conditions and contribute to the lagged sediment influx caused during the drier periods by the erosion of previously stored sediment.