Interpretações paleoambientais sobre a macroflora cretácea da formação Serra do Tucano, Bacia do Tacutu - RR

Abstract

The Tacutu Basin, of Juro-Cretaceous age, located in the northeastern portion of the state of Roraima, Brazil, with extension to Guyana, comprises a sequence cretaceous of fluvial deposits that compose the Serra do Tucano Formation. Recent studies provided to the discovery of new fossiliferous records for the Serra do Tucano Formation. This material consists of fragments of vegetal remains and occurs in the Murici II, Arizona and Morro da Frente outcrops, all located in the municipality of Bonfim, state of Roraima. At present, 139 samples of sandstone are cataloged in the sub-collection of Paleobotany of the Laboratório de Paleontologia da Amazônia - LaPA, at the Instituto de Geociências of the Universidade Federal de Roraima. Aim of this research is to interpret the conditions that contributed to the preservation of macroflora from Serra do Tucano Formation. The methodology was based on field expeditions to describe the sedimentological characteristics and laboratory analyzes, such as: petrographic analyzes, X-ray diffraction, scanning electron microscopy with dispersive energy spectroscopy (SEM / EDS) and faciological study. Based on the petrographic study, the rocks were classified as quartzoarenites, with a framework rich in quartz and muscovite grains, and in a subordinate form, with hematite, goethite and alkaline feldspars changed to clay minerals. The identified diagenetic events were: dissolution of the feldspar to the argilomineral kaolinite; presence of iron oxide / hydroxide in the depositional environment; incipient mechanical compaction; processes of oxidation, weathering and leaching. The faciological study identified 10 facies, three of them fossiliferous, with characteristics of deposits of channel overflow. The fossiliferous level I has the highest concentration of plant fragments, being composed of pteridophyte and gymnosperms. The taphonomic features show that the plant material was transported and selected before burial, and that the presence of iron in the depositional environment contributed to the preservation. The presence of this macroflora in the Serra do Tucano Formation allowed us to assign the youngest possible age to the Aptian unit, besides recording favorable environmental conditions the presence of riparian and floodplain vegetation.