Fossildiagênese de tafocenoses de vertebrados quaternários de depósitos de tanque do Nordeste do Brasil

Abstract

Tank deposits of northeastern Brazil have a long been studied, with currently many areas, including taphonomy, being the focus of said studies. While taphonomic studies have been done in those deposits, they have mainly focused in the biostratinomic aspects, resulting in a lack of fossildiagenetic data from these deposits. This study is focused in four tanks (, Lagoa do Santo, Currais Novos County, Rio Grande do Norte State; Riacho Verde, Ouro Branco County, Rio Grande do Norte State; Camp Alegre, Taperoá County, Paraíba State; and Lagoa do Rumo, Baixa Grande County, Bahia State), where a fossildiagenetic analysis of vertebrate bone thin sections collected from said tank deposits will be performed, as well as the interpretation of the fossildiagenetic data by means of statistical analysis tools and the comparative analysis between locations. The statistical analysis of the data shows that all four locations have some common characteristics: Permineralization with oxide, carbonate, quartz and pyroxene, Replacement by oxide and carbonate, Fracturing by desiccation, Fracturing under subaquatic conditions and Histological Indexes 3 and 4.. These common characteristics could be a possible result of the tanks being geographically close and thus being subjected to similar paleoclimactic conditions. The statistical data also highlights striking differences, with special mention being given to the Lagoa do Rumo tank, where “Prolonged Desiccation”, “Permineralization Index between 70 and 80”, “Replacement Index between 80 and 90” and “HI 5” characteristics are dominant, while also being rare in the other three locations. The differences observed were interpreted as being the result of the relationship of the tank deposit with both the geological characteristics of the surrounding area (such as the composition of the surrounding rocks), as well as with the nearby drainages. The comparison between the observed mineralogy and the geological characteristics in the area has made it possible to identify a possible source for the detrital minerals in the Riacho Verde tank, as well as a possible paleocurrent, a previously nonexistent data. Based on the data obtained it became possible to deduce the following: (i) permineralization and substitution are the main fossildiagenetic processes occurring in the tanks; (ii) the locations were subjected to similar processes but with different intensities; (iii) the drainages have strong influence in the manner in which the detrital minerals occur as well as being vectors for the transportation of Fe2+ ions; (iv) every location has shown evidence of time-averaging; (v) three of the location have varying values for the concentration of Fe2+ ions and this results in the intercalation of oxide and carbonate in the pores; (vi) the osteocyte lacunae are preferential locations for the deposition of oxide and this opens up the discussion about the influence blood has in the formation of oxide; (vii) every location has been affected by seasonality since they have evidence of desiccation and high water availability; (viii) every location has fractures but each location has a different process dominating and this results in different patterns dominating in each tank; (ix) in the Riacho Verde tank there is evidence of variation in the acidity levels in the sedimentation location.