Sediments and palaeosols of Cretaceous desert depositional systems (Bauru Group, SE of Brazil, and Los Llanos formation, W of Argentina): proxies for palaeoenvironmental interpretation and stratigraphic benefits

Abstract

Palaeosols are relatively common in continental and paralic sedimentary successions. Despite that, they are ignored or described as massive sediments and interpreted as chaotic debris flows or diagenetic changes. This superficiality considering palaeosols cannot be justified studying continental and paralic sedimentary successions because they constitute very important proxies of palaeoenvironmental controlling factors and useful stratigraphic tools. This new project wants to be a continuation of a research concluded in 2010 (Fapesp Project n. 2007/00140-6), whose study area was the northern portion of the Cretaceous Bauru Basin. Using relationships between palaeosols and deposits we interpreted this sedimentary succession representing a desert aeolian sand sheet where instability phases of the topography (erosion/sedimentation processes) alternated with stability phases (pedogenesis). Palaeosols have been determinant in discriminating the palaeoenvironmental conditions and the stratigraphic architecture of the unit. This new project proposes to study the eastern area of the Bauru Basin (São Paulo State) and the Los Llanos Formation (La Rioja, Argentina). The Bauru Group, exposed near the town of Marilia, displays different lithological and palaeopedological aspects than we observed in our previous studies. Thus, we think that new data on the evolution and architectural features of the Bauru Group may be obtained from this study area. The Los Llanos Formation is a Cretaceous unit with aeolian deposits and palaeosol formed in arid climate. This sedimentary succession, located 900 km towards south and 1600 km towards west from Bauru Basin study area, look like very similar to the Bauru Basin succession and its study and comparison with the Bauru Basin will allow: i) to verify the existence and meaning of the alternations of stability and instability phases of the topographic surfaces in sedimentary desert palaeoenvironments; ii) to verify the use of palaeosol as palaeoenvironmental and stratigraphic proxies; iii) to generate a palaeoclimatic reconstruction of Latin America during the Cretaceous for a area between 20-30° of latitude. To realise this project, sedimentological and palaeopedological studies in the field and laboratory will be integrated. Main objective will be verify the relevance of the palaeosols, when studied in conjunction with sediments, as palaeoenvironmental and stratigraphic proxies. To reach this result we have to follow this secondary objective in Bauru Group and Los Llanos Formation: i) to interpret the factors that control the depositional and palaeopedogenic processes; ii) to define a stratigraphic geometry using the principles of the depositional architecture and sequence stratigraphy in aeolian and desert systems. (AU)