Microfacies and diagenetic evolution of the limestones of the upper part of the Crato Formation, Araripe Basin, northeastern Brazil

Abstract

This paper presents the results of a petrographic and diagenetic study of the laminated limestones of the upper part of the Aptian to Albian Crato Formation, northeast of Brazil. The applied techniques were optical microscopy, cathodoluminescence and scanning electron microscopy (SEM) coupled to a wavelength-dispersive spectrometer (WDS). Petrographic analysis has revealed that most of the laminated limestones are calcilutites with a dominance of a micritic matrix, indicating a low-energy depositional environment. Microstructures such as microfaults, microfractures, microslumps, and loop bedding were observed. Based on textural, structural and paleontological features, seven microfacies were recognized: massive limestone, limestone with parallel laminations, limestone with undulated laminations, limestone with slumps, limestone with loop bedding, limestone with ostracods and limestone with peloids. In addition, the processes of cementation, dissolution, replacement, recrystallization and compaction, which are related to different diagenetic stages, were also recognized. The diagenetic constituents found in the sections include calcite, pyrite, silica and sulfates. We can conclude that a large part of the microstructures (microfaults, microfractures, microslumps and loop bedding) can be related to local seismicity, probably due to the reactivation of the Patos Shear Zone. The diagenetic constituents indicate an early to late diagenesis (eogenetic, mesogenetic and telogenetic stage).

La plataforma carbonática de la Formación Tibasosa Superior, Cretáceo Inferior, Cuenca Cordillera Oriental, Firavitoba-Boyacá, Colombia

“…However, stylolites in quartz mainly found at depths exceeding 1.5 km (Tada and Siever, 1989), explaining the presence of this feature in detrital quartz of limestones from the Upper Tibasosa Formation, concordant to the age and stratigraphic position of this unit in the Basin. High luminescence in CL could be associated with the relative high Mn/Fe ratio, which is frequent under reducing conditions during the early stage of burial diagenesis, also evidenced by the formation of framboidal pyrite at temperatures of about 200°C (Wilkin and Barnes, 1997;Cabral et al, 2019) Scholle, 2003). The telodiagenetic events restricted to the iron oxides and hydroxides formation, moldic porosity, and evidence of chemical weathering in outcrops due to the dissolution of carbonates.…”

Mixed siliciclastic–carbonate sedimentation in an evolving epicontinental sea: Aptian record of marginal marine settings in the interior basins of north‐eastern Brazil

“…Plastic deformation, loop-bedding structures (see also Cabral et al, 2019), syneresis cracks and breccia in laminites, and gypsum laminae forming ascending structures are interpreted as the product of intense evaporation. The increase in the intensity of evaporation towards the top and the association with tidal flats at the base points to terrestrial evaporitic facies overlapping marginal marine deposits.…”

“…Indeed, Cabral et al . (2019) reported the effect of early to late diagenesis in this interval. Relatively low carbon isotope values coupled with high TOC content may suggest DIC sourced from the oxidation and biological degradation of organic matter (Armstrong & Brasier, 2005).…”

Revealing the diversity of amber source plants from the Early Cretaceous Crato Formation, Brazil

“…The Crato Formation represents mainly lacustrine carbonates and siliciclastic sediments, which are interfingered, at least at its top, by possible fan delta deposits [14]. Six informal limestone units (C1-C6) are distinguished [15], of which C6 at the top of the formation is commercially important and is excavated as "Pedra Cariri" in several quarries between the towns Nova Olinda and Santana do Cariri, Ceará [16]. Fossil biota are therefore almost exclusively recorded from the C6 limestone, which is the case with our material.…”