Evolução singenética e diagenética de uma seção do membro crato (Cretáceo inferior) da Bacia do Araripe em Nova Olinda-CE

Abstract

The Crato Membre is one of the main stratigraphic units of the Araripe Sedimentary Basin, renowned mainly for the laminated limestone rich in diversified fossils of Albian-Aptian age, worth a title offossil Konservat-Lagerstütte. In the same unit, less famous massive layers of some tabular, decimetres-thick fine sandstones occur interlayered with green shales and limestones, composing an mixed sedimentation. In this study, the main goal was to perform facies analysis integrated to petrography, mineralogy and chemical analyses of the terrigenous and carbonatic rocks in a continuous geologic section of the Crato Membre, on the locality of Massapê, in Nova Olinda city (Ceará, Brazil). Five depositional systems were recognized: marly shale (Fm), laminated calcilutite (Cl), almost-massive arenite (Aqm), siltstone with climbing ripples (Scc), and columnar evaporite (EC). The petrographic analyses allowed characterising the carbonatic rocks as pseudo-microsparitic calcilutites, and the terrigenous sandstones as quartz-wackes. Particularly, the deposition of the sandstones in the Crato Membre must have occurred episodically, fed by immature fluxes from high source areas near towards a lacustrine basin that, for its turn, was subjected to periods of expansion and retraction of the lake levei. The diagenesis on the rocks must have been mild, with little evidence of mechanic compaction and incipient chemic transformation. There are suspicions on favouring occurrence of a strictu sensu hydrothennal event in the Aptian-Albian interval of the Araripe Basin, chiefly by the presence of silicified calcilutites and the occurrence of nontronite in the shales. The presented data corroborate to an interpretation of a gradual transition between Barbalha and Santana formations, with a retrogradational sedimentary fill in the period reported for the Araripe Basin.