Controlled Excavations In the Romualdo Member of the Santana Formation (Early Cretaceous, Araripe Basin, Northeastern Brazil): Stratigraphic, Palaeoenvironmental … (original) (raw)
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 (bMatracãoQ), 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/m 3 ; (ii) concretions have a preferential, bimodal orientation pattern (major NW-SE axis and secondary cN-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
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