Upper Cretaceous palaeosols as basin analysis proxies: Bauru Group (Brazil) and Los Llanos Formation (Argentina)

BASILICI, GIORGIO; SOARES, MARCUS VINÍCIUS THEODORO; MARTINELLI, AGUSTÍN G.; MARINHO, THIAGO S.; MARCONATO, ANDRÉ; FIORELLI, LUCAS E.; HECHENLEITNER, E. MARTÍN; FERREIRA MESQUITA, ÀQUILA

Praga

Congreso; 35th IAS Meeting of Sedimentology; 2021

International Association of Sedimentologists

Palaeosols constitute powerful palaeoenvironmental and stratigraphic proxies because during their development they record climate, biology, parent material, time, sedimentation rate and topography. In this respect, palaeosols are key tools to define palaeoenvironmental conditions and to unravel the alternation between sedimentation and topographic stability, thus giving fruitful information on type and frequency of the sedimentary processes. The Cretaceous continental successions of South American make a compelling case due to its rich record of palaeosols.Using field and laboratorial methods for facies analyses and palaeopedological studies, this study demonstrates how palaeosols can give useful insights on causes, characteristics and periodicity of the sedimentary processes in two Upper Cretaceous South America sedimentary successions: the Los Llanos Formation (Argentina) and the Bauru Group (Brazil).The Los Llanos Formation is a sandstone unit, deposited in a complex system of bajadas. The succession has 69% of its thickness constituted of cumulative Inceptisols, which suggest low development of the pedogenesis due to autogenic high and regularly continuous sedimentary input. The Bauru Group is a sandstone unit formed by several distributive fluvial systems. In the SE portion of the Bauru Group, 89% of the thickness is formed of compound Inceptisols alternated with subaqueous sandstone deposits generated by sheet floods on the distal portion of a distributive fluvial system. The development of these palaeosols was controlled by autogenic periodic and paroxysmal sheet floods, whose recurring event was of the order of 103 years. In the NW portion of the Bauru Group the palaeosols (~60% in thickness) are constituted of Aridisols and Alfisols, alternated to aeolian sand sheet deposits. These alternations signify long periods (in the order of 105 years) of topographic stabilisation and pedogenesis in semi-arid or humid climate alternated with arid conditions, which brought the aeolian sedimentation. In this case, allogenic factors controlled the palaeosol/deposit alternations. In the NE portion of the Bauru Group, compound palaeosols are Entisols, Inceptisols and Vertisols. Inceptisols are located above channel deposits in higher topographic position where the sedimentation rate was reduced. Entisols and Vertisols are positioned in interchannel depressed areas where the sedimentation rate was higher. Sudden depositional episodes covered the soils generating compound profiles. Autogenic factors associated to the depositional morphology controlled the type and distribution of the palaeosols.Comparative study of these deposits and palaeosols allowed unravelling frequency, type and magnitude of the sedimentary processes, which can be integrated into the theoretical and applicative analysis of sedimentary basins.