Fósseis de vegetais da Formação Pedra de Fogo:: aspectos taxonômicos, mineralogia e composição química

Abstract

In the surroundings of Bielândia-TO, the Pedra de Fogo Formation (Upper Permian of the Parnaíba Basin) fossilized plants were collected, associated with a sedimentary succession, consisting of basal sandstone, overlaid by siltstones and claystone at the top, with nodules and levels flint continuums along its entire length. Morpho-anatomical studies, based on the identification of the type of vascular system (stele) preserved, of 17 specimens of fossilized plants revealed the presence of 9 stems, 4 roots and 2 petioles, and 2 specimens were not identified due to few anatomical elements. preserved. In general, the vascular system of the stems consists of masses of vascular bundles (xylem), limited by a reinforcing parenchyma wall, immersed in a fundamental parenchyma matrix. Traces of phloem were observed. Externally, it is surrounded by a layer of adventitious roots, constituted by vascular bundles immersed in fundamental parenchyma. This configuration is characteristic of the Psaroniaceae family, represented in this work by the genera Psaronius and Tietea, whose stele morphology allowed the identification of the species Psaronius brasiliensis and Tietea singularis. The roots exhibited a protostele-like stele and the petiole showed incipient structuring in the form of concentric rings. Only 1 specimen represented the Taxaceae family, due to the presence of sclereids and tracheids. Studies of 34 thin sheets, representative of transversal and longitudinal cuts to the preserved structure of the plant, showed the presence of granular and prismatic crystalline quartz, together with microcrystalline quartz, with emphasis on the chalcedony variety, both fibroradial and in fringes. In XRD, analyzes of 17 total samples and 17 isolated fractions of the identified tissues, the diffractograms drew the quartz pattern and small reflections of anatase, calcite and hematite. In the infrared spectra these samples recorded diagnostic bands of quartz (1166, 1082, 798, 800, 694, 511 and 462 cm-1) and strictly bands of organic matter (close to 3000 cm-1) and also the doublet 3700 and 3622 cm-1, along with bands 1103, 1035, 1011 and 915 cm-1 characteristic of disordered kaolinite. Quartz crystallinity is medium and low, with values ​​close to 5.0 (calculated index, by XRD, according to Murata & Norman, 1976). SEM images reveal two distinct phases: one rich in microcavities, typical of parenchyma and parenchyma cell contacts, and another more massive, predominant in vessels and fundamental parenchyma. Electron microprobe analyzes show practically pure silica composition (99.73% SiO2) at values ​​of 76.37% SiO2. The highest values ​​of the main impurities were: Al2O3=5.95%; Fe2O3=2.45%; CaO=0.22%; Na2O=0.19 % and TiO2=1.52 % and are coincident with sclerenchyma samples, where kaolinite and hematite appear more frequently. The identified plants characterize a Permian flora, typical of swamps, where environmental changes, which made the pH more alkaline, promoted the death of the forest together with the permineralization of silica in the plant tissues, allowing the preservation of minute histological details. The presence of organic matter, related to methyl and methylene, also confirmed the origin of the process as infiltration and impregnation, developed at relatively low temperatures (below 200° C).