Multi-proxy approach of the mixed carbonate-siliciclast Crato Formation: sedimentary evolution, paleogeography and tectonics

Author(s)

Filipe Giovanini Varejão

• Document type: Doctoral Thesis
• Press: Rio Claro. 2019-08-21.
• Institution: Universidade Estadual Paulista (Unesp). Instituto de Geociências e Ciências Exatas. Rio Claro
• Defense date: 2019-08-02
• Advisor: Lucas Veríssimo Warren

Abstract

The Aptian evaporitic transitional megasequence constitutes important register of the equatorial margin rifting, development of the seaway between the South and Central Atlantic oceans, and deposition in the interior basins of the NE Brazil. In this context, highlights the up to 90-m-thick mixed carbonate-siliciclastic succession from the Crato Formation, Araripe Basin. This unit is worldwide renowned for the exceptional preservation of terrestrial assemblages in laminated limestones and as one of the most important Cretaceous Konservat- Lagerstätte of the world. The occurrence of such well-preserved fossils boosted paleontological and sedimentological research focused in the origin of the laminated limestones. The Crato Formation is part of a transgressive-regressive depositional sequence of which the Barbalha (fluvial-lacustrine) and Ipubi (playa lake evaporites) formations are part. The unit is considered a lacustrine system with deltaic deposition in the shallow waters and carbonate precipitation in the basinal areas. However, these interpretations have been questioned in the last years, mainly by the discovery of microbial activity in the limestone intervals. Considering this context, the present research used a multi-proxy approach (sedimentology, taphonomy, ichnology, sequence stratigraphy, organic geochemistry, stable istotopes, and analysis of sin-sedimentary deformation structures) for the interpretation of the mixed deposits of the Crato Formation. The application of multiple technichs is considered essential to support a credible interpretation of the depositional environments, and to characterize the stratigraphic positioning, and definition of the preservational mechanisms of one of the most important Cretaceous Konservat-Lagerstätte in the world. The data presented herein points to marginal marine deposition during parts of the Crato Formation. At least two transgressive phases were recognized by typical facies associations and occurrence of dinocysts and marine ichnofossils. The regressive phases are highlighted by the occurrence of laminated limestones with distinctive continental isotope signature. At the top of the first regressive phase, microbial laminites, stromatolites, oncoids, and thrombolites, attests to shallow water deposition at the Konservat-Lagerstätte layers. Microbial mats of this interval were observed recovering fossils and potentializing mineralization of soft tissues, in a process known as “microbial entombment”. These evidences and interpretations are divergent from the old interpretations for the Crato Formation and represent significative inovations in the comprehension of mixed sedimentation in the unit. (AU)

• FAPESP's process: 16/13214-7 - Multi-proxy approach of the mixed carbonate-siliciclast Crato Formation: sedimentary evolution, paleogeography and tectonics
• Grantee: Filipe Giovanini Varejão
• Support Opportunities: Scholarships in Brazil - Doctorate