Estratigrafía y ambientes deposicionales de la Cuenca Bauru (Cretácico superior, Brasil)

LUIZ ALBERTO FERNANDES, ARMANDO MÁRCIO COIMBRA

Abstract


The Upper Cretaceous Bauru Basin developed in the south-central part of the South American Plate resulting from the thermo-mechanical subsidence processes which followed the break-up of Gondwana and the opening of the Southern Atlantic Ocean (Figs. 1 to 3). In this inland basin it was developed an essentially sandy, 300 m thick siliciclastic, alluvial and aeolian dominated sequence (red beds), the so called Cretaceous Suprabasaltic Sequence -¡.e. "Secuencia Suprabasáltica Cretácica" (SSC). This sequence presently occurs in a very extensive area of 370,000 km2 and unconformably overlies the mainly basaltic Neocomian Paraná Volcanic Floods (Sena Geral Formation), from which it is separated by an erosive surface (Figs. 4 and 5). The sequence deposits extend between 18OS and 24OS latitudes and 47OW and 56OW longitudes covering parts of the states of SEO Paulo, Paraná, Mato Grosso do Sul, Minas Gerais and Goiás in Brazil. It also occurs in northeastem Paraguay. A stratigraphic review of the whole SSC, based on studies developed mainly on the southwestern part of the basin is shown in this paper which also deals with the structural, paleoclimatic and paleogeographic settings of the basin. The sedimeniological features, depositional environments and paleogeographic distribution of the defined stratigraphic units is analyzed, as well as their general resulting implications on the paleobiological record (Figs. 5 to 7). The sequence is formed by two synchronous groups, the aeolian dominated Caiuá Group and the alluvial dominated Bauru Group. and was deposited in an asymmemcal closed basin, which underwent climatic conditions ranging from semi-and at its margins to dessertic in the inner basin zones. Alluvial sedimentation in the Bauru basin was simultaneous with the progressive uplift of its margins, constituted by tectonic uplifts which separated the basin from other neighbouring cretaceous basins. Life in the Bauru Basin developed best in areas with largest water supply, such as in the broad braid plains, where ephemeral carbonate water shallow lakes and ponds developed. Reptiles (mainly dinosaurs, crocodiles and turtles) dwelled there. Increasing aridity resulted in the gradual reduction in the paleobiological diversity as recorded in the inner, central basinal aeolian dominated units, with an almost complete lack of fossils in the sand sea deposits of the central paleodessert.

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