Continental systems tracts of the Brazilian Cretaceous Bauru Basin and their relationship with the tectonic and climatic evolution of South America

Abstract

The application of sequence stratigraphy concepts to continental deposits lacking the referece provided sea level has been a challenge, mainly because the temporal relationships between stratigraphic surfaces and systems tracts depend on the tectonic and climatic evolution of the area. Using the concept of accommodation space (A) and sediment supply (S), we identify specific stacking patterns of aeolian, lacustrine, fluvial and alluvial systems that correspond to the particular tectonic and climatic evolution of the southeastern portion of South America. With the end of the Early Cretaceous volcanism (133 Ma), the southeastern portion of South America underwent tectonic restructuring, which generated basins that encompassed continental sedimentary sequences. The tectonic events responsible for the accumulation of these sequences occurred during two primary phases. The first phase is related to Early Cretaceous thermal subsidence, which was more pronounced in the regions where the thickest Serra Geral Formation basaltic successions are found, resulting in the formation of Bauru Basin. The second phase was related to the Late Cretaceous uplift in southeastern Brazil as a result of magmatic/volcanic activity associated with the Trindade Mantle Plume. Stratigraphic analysis based on well-logs and outcrops and aided by petrographic studies identified three sequences that are bounded by regional unconformities that record important changes in the Bauru Basin's tectonic and paleoenvironmental conditions. The unconformity K-0 is related to the origin of the Bauru Basin in the Early Cretaceous. The Early Cretaceous Sequence 1 (Caiu a Group) is interpreted as a second-order sequence, formed by aeolian and fluvial deposits and constituting a Fluvial-Aeolian Systems Tract. Unconformity K-1 that was generated in the Late Cretaceous (Cenomanian -Campanian?) is related to the tectonic evolution of the basin and source area. Overlying Unconformity K-1, lacustrine, fluvial and alluvial deposits display progradational characteristics of the two-third-order sequences: Sequences 2A and 2B, constituted by the Fluvial-Lacustrine and Alluvial Systems Tracts, respectively, and separated by the Unconformity K-1A. Sedimentological characteristics, paleosols and stratigraphic architecture, suggest that A/S ratio was neutral in the late stage of the Sequence 1, whereas in the Sequence 2 there was an increase (Sequence 2A) followed by a decrease in the A/S ratio (Sequence 2B). Aeolian facies and paleosol P1 (Sequence 1), fluvial-lacustrine facies and hydromorphic soils (Sequence 2A), and alluvial facies and Paleosol P2 (Sequence 2B), indicate climatic changes in the South American during the Cretaceous. The stratigraphic framework, subaerial unconformities and paleosols provide key elements for subdividing of the Brazilian continental sequence into third-order sequences and systems tracts, for identification of allocyclic and autocyclic patterns in time and space.

The mineralogy of paleosols in the Marília Formation and their importance in the environmental evolution of the Maastrichtian of the Bauru Basin in southeastern Brazil

“…This discordance is signaled by a much evolved paleosol (Geosol Santo Anastásio, as in Fúlfaro et al 1999). Batezelli (2010Batezelli ( , 2015 and demonstrated, using outcrops and well-logs data, that the two groups are not contemporary (Fig. 2), and proposed chronostratigraphy to understand the basin.…”

“…Authors such as Fernandes & Coimbra (1996, 2000 and Fernandes (2004) state that the two groups are contemporary. Other authors (Fúlfaro & Perinotto 1996, Paula e Silva et al 2005, Batezelli 2010, 2015 put the Caiuá in the lower portion of the basin, separated from the Bauru Group with a stratigraphic discordance. This discordance is signaled by a much evolved paleosol (Geosol Santo Anastásio, as in Fúlfaro et al 1999).…”

“…1) (Riccomini 1995, 1997, Fernandes & Coimbra 2000, Batezelli 2003, Batezelli et al 2007, Batezelli, 2010, 2015.…”

“…The Bauru Group in the State of São Paulo consists of Araçatuba, Adamantina (Vale do Rio do Peixe according to Fernandes & Coimbra, 2000) and Marília (Echaporã Member) formations, from its base to the top (Batezelli, 2003(Batezelli, , 2010(Batezelli, , 2015. According to Batezelli (2003Batezelli ( , 2010Batezelli ( , 2015, the Araçatuba Formation was formed in a lacustrine environment (playa-lake) that served as the base level for the river system (Adamantina and Marília formations). The filling occurred because of the progradational advance of an alluvial system dominated by a braided river that gave rise to the Marília Formation.…”

“…The sedimentary evolution of the alluvial system was marked by periods of fluvial sedimentation and eolian reworking, interspersed with periods of non-deposition (Batezelli 2010, 2015. During non-deposition times the floodplain would be covered by vegetation, and as such, soils could develop.…”