Concreções calcárias da formação Santana, bacia do Araripe: uma proposta de classificação.
Résumé
The calcareous concretions of the Santana Formation have been known since 1800 thanks to the work of João da Silva Feijó (1800). Their study was promoted internationally by the report of von Spix and von Martius (1823-1831), and their richness in fossil fishes led Silva Santos to name them “ichthyoliths” in 1950. Here we propose a classification for the various types of calcareous concretions that occurred in the Santana Formation (Late Cretaceous, Araripe Basin). We consider their geographic distribution and we propose a terminological uniformization for the descriptions of that material. This will serve future statistical, geographical, geochemical and taphonomical studies that, altogether, will improve our understanding of the Santana ‘Lagerstätten'. The classification criteria are: the number and position of the fossils in the concretions, the dimensionality of the macrofossils, the relationship matrix/fossil, the shape of the concretions, as well as their lithological structure and texture. The contents in fossils is classified as afossiliferous, unifossiliferous and multifossiliferous; the relative position of the fossils in the concretions can be described as superior, medium, inferior, traverse, dispersed or in parallel planes; the preservation type is characterized by adjectives such as “three-dimensional” and “compressed”; the percentage of matrix in relation to the macrofossil is expressed as abundant matrix, regular, scarce and short. The shape of the concretion can be subspherical, ellipsoidal, oval-elongated and irregular; the sedimentological structure of the matrix is classified as laminated and little laminated, while its texture is either a fine matrix or a very fine matrix.