Clay mineralogy and lithogeochemistry of lutites from the Lower Cretaceous Crato Member, Araripe Basin, NE Brazil: Implications for paleoenvironmental, paleoclimatic and provenance reconstructions

Abstract

The Aptian Crato Member of the Araripe Basin is a 50-m-thick succession of laminated limestones alternated with siliciclastic rocks. This unit has been receiving attention worldwide mainly because it is one of the most famous fossil Konservat-Lagerstätten, where were identified well-preserved paleofauna and paleoflora. Clay mineralogy and lithogeochemical data have been widely used in paleoenvironmental, paleoclimatic and provenance reconstructions. However, these techniques were not systematically used to study the Crato Member rocks. This study reports the mineralogical and lithogeochemical characterization of the Crato Member lutites looking for paleoenvironmental, paleoclimatic, and provenance reconstructions. Facies analysis, X-ray diffractometry, lithogeochemical, and scanning electron microscopy studies were carried out on thirteen Crato Member lutite samples in the Três Irmãos quarry, near Nova Olinda County. Regional arid paleoclimate conditions were identified using clay mineral assemblages and two paleoenvironmental models with subtle paleoprecipitation and provenance differences were interpreted. In the lower Três Irmãos quarry, a tidal-dominated delta mouth bar facies succession coincident with a clay mineral association with lesser kaolinite contents was identified, which indicates drier conditions. Lithogeochemical data suggests provenance from Archean felsic rocks. In the upper Três Irmãos quarry, a marginal paleolagoon facies succession coincident with a clay mineral association with higher kaolinite contents was recognized, which indicates wetter conditions. Lithogeochemical data indicate provenance from mixed Post-Archean to Archean felsic-mafic rocks. The paleolagoon water was interpreted as brackish and sudden paleosalinity increases indicated seawater inputs, which were also responsible for raising the paleolagoon water table in the marginal paleolagoon facies succession, which presented paleoenvironmental proxies suggesting a more expressive marine character.