The Ibaté paleolake in SE Brazil: Record of an exceptional late Santonian palynoflora with multiple significance (chronostratigraphy, paleoecology and paleophytogeography)

• Autor(a) principal: Arai, Mitsuru [UNESP]
• Data de Publicação: 2018
• Outros Autores: Dias-Brito, Dimas [UNESP]
• Tipo de documento: Artigo
• Idioma: eng
• Título da fonte: Repositório Institucional da UNESP
• Texto Completo: http://dx.doi.org/10.1016/j.cretres.2017.11.014; http://hdl.handle.net/11449/175605
• topic: Anacolosidites eosenonicus palynozone; Bauru Group; Paleoclimatology; Palynology; Paraná Basin; São Carlos Formation

The Cretaceous Bauru Group of the Paraná Basin in Brazil is a widespread continental unit well known for its vertebrate and invertebrate fossiliferous content. The São Carlos Formation is an exception among its distinct and dominantly reddish siliciclastic units, which are otherwise devoid of palynomorphs. It includes an unique unit, the Ibaté Bed, which corresponds to the lower interval of the Fazenda Nossa Senhora de Fátima Member (FNSF Mbr) and is represented by a dark grey organic silty-argillaceous rhythmite with a rich palynoflora. These rocks are the main focus of this study. Samples were collected from the type section of the formation that is located in the central area of the São Paulo State. Identified palynomorph types are as follows: 32 pteridophyte spore taxa, 31 gymnosperm pollen taxa and 64 angiosperm pollen taxa. Acritarchs, prasinophytes and phytoclasts were also recognized. Seven new taxa are proposed: Anacolosidites eosenonicus sp. nov., Leiosphaeridia ibateensis sp. nov., Stellatia gen. nov., Trichomites brevifurcatus gen. et sp. nov., Trichomites duplihelicoidus gen. et sp. nov., Trichomites simplex gen. et sp. nov. and Tricolpites joelcastroi sp. nov. In quantitative terms, the ephedroid pollen grains – e.g., Equisetosporites, Gnetaceaepollenites and Steevesipollenites – are predominant. Their occurrence associated with Classopollis grains suggest a warm climate tending towards dryness. Some levels are relatively rich in pteridophyte spores, suggestive of contribution from lakeside vegetation. The permanent presence of freshwater is attested by conspicuous microalgal remains (Botryococcus, Pediastrum, Staurastrum and prasinophycean phycomata), recognizable via fluorescence microscopy. Significant amounts of microalgal remains in some samples suggest episodic blooms in the lake, probably induced by eutrophication. The Ibaté Bed corresponds to a depositional scenario involving a distal lacustrine environment with low-oxygen conditions in its bottom waters. As observed in offshore Brazilian basins, the presence of Anacolosidites eosenonicus sp. nov., combined with the absence of Steevesipollenites nativensis, indicates a late Santonian age for the bed (ca. 84 Ma). Other associated index-palynomorphs are compatible with this time assignment, which is reinforced by the presence of carbonized sclereids that are associated with the “Great Santonian Wildfire” recorded in coeval marine offshore strata of the Campos and Santos basins. The FNSF Mbr is stratigraphically related to the uppermost part of the Adamantina and Uberaba formations of the Bauru Group. Among all known Brazilian Late Cretaceous palynological continental records, this is the richest one ever detected. Although Late Cretaceous in age, the composition of the Ibaté Bed palynoflora reflects better the mid Cretaceous ASA Paleophytogeographic Province.