Controlled excavations in the Romualdo Member of the Santana Formation (Early Cretaceous, Araripe Basin, northeastern Brazil): stratigraphic, palaeoenvironmental and palaeoecological implications

摘要

The Romualdo Member of the Santana Formation (Araripe Basin, northeastern Brazil) is famous for the abundance and the exceptional preservation of the fossils found in its early diagenetic carbonate concretions. However, a vast majority of these Early Cretaceous fossils lack precise geographical and stratigraphic data. The absence of such contextual proxies hinders our understanding of the apparent variations in faunal composition and abundance patterns across the Araripe Basin. We conducted controlled excavations in the Romualdo Member in order to provide a detailed account of its main stratigraphic, sedimentological and palaeontological features near Santana do Cariri, Ceará State. We provide the first fine-scale stratigraphic sequence ever established for the Romualdo Member and we distinguish at least seven concretion-bearing horizons. Notably, a 60-cm-thick group of layers (“Matrac?o”), located in the middle part of the member, is virtually barren of fossiliferous concretions. Moreover, a sample of 233 concretions shows that (i) the stratigraphic distribution of the concretions is very heterogeneous and their density varies from 0.8 to 15 concretions/m3; (ii) concretions have a preferential, bimodal orientation pattern (major NW–SE axis and secondary ≈N–S axis) throughout the section, suggestive of permanent palaeocurrents of low energy; (iii) few concretions yield the well-preserved vertebrates that have made the Romualdo Member so famous, and those are mainly restricted to four stratigraphic horizons; (iv) only six fish taxa were recovered, the most common being Vinctifer and Tharrhias, followed by Cladocyclus, whereas Brannerion, Calamopleurus (=Enneles) and Notelops are rare. No tetrapod was found in the sample; (v) there is a strong stratigraphic control in the distribution of these taxa and one can distinguish at least three major assemblages at the same locus. These are, from older to younger, a Tharrhias-dominated assemblage, an assemblage dominated by Tharrhias and by Cladocyclus, and a Vinctifer-dominated assemblage. The stratigraphic sequence of these assemblages also corresponds to their ranking in terms of diversity (richness and evenness); (vi) previous accounts on the taxonomic composition and relative abundance of fossils from the Romualdo Member were severely biased toward well-preserved and exotic fossils. They are therefore inappropriate for drawing palaeoecological inferences. The factors responsible for the variations in faunal composition and abundance patterns across the Araripe Basin remain largely unknown, and we hypothesize that climate and/or palaeogeography might be the major forcing agents. Only fine-scale stratigraphic and palaeontological investigations have the potential to solve this issue. In turn, this work marks the first step of an expanded research program that aims at explaining the spatio-temporal relationships between palaeocommunities and their palaeoenvironment in the Araripe Basin during Aptian/Albian times.