Caracterização químico-mineralógica de sedimentos associados e hidrogeoquímica de águas subterrâneas, ocorrentes na Formação Codó-MA

Abstract

Previous studies have described for the Codó Formation (Lower Cretaceous, Aptian-Albian) the presence of carbonates and gypsum, interbedding of sandy and muddy sediments, carbonate and bituminous sediments, pyrite concretions, levels with plant remains, ostracoids and gastropods, characterizing a shallow to very shallow water environment, with good movement. The depositional model was initiated by a dominant river system, but progressing to a lagoon system. These restricted circulation phases also provide the accumulation of carbonaceous shale facies. The upper shales were deposited in a transit phase to lacustrine conditions of the Itapecuru Sequence. The present study describes and interprets analytical results of 19 sediment samples, corresponding to different lithotypes, represented by shales, limestones, gypsum and calcite, collected in profiles of the two deposits being explored near Codó (MA). 70 samples of natural waters were also studied (river, rainwater accumulated in the pockets of land of the deposits, excavated wells and tubular wells), collected in four distinct seasonal periods (beginning of the rainy season, rainy season, dry season and end of dry season). ). The results of the sediment analyzes were distributed from the average chemical composition of shales, limestones, gypsum and calcites, based on total rock analysis, in addition to the characterization of surface mineralogy, identification of the main clay minerals in the shale horizons and limestones and surface geochemistry through the behavior of major and trace elements in lithological profiles. Among the main minerals identified, the presence of kaolinite, montmorillonite (smectite), illite, chlorite and quartz in the shales, and in the limestones, kaolinite, montmorillonite (smectite) and illite stand out. Shales are associated with arkoses due to the abundance of freshwater kaolinite, as they are rich in montmorillonite. As for the silica content, it was found to be common (silica content below 58%) and medium (silica content around 58%). Regarding the CO2 contents, the shales are calcitic, with an average of 2.72%. Sr and Ba enrichment in the studied sediments, recorded that these trace elements are indicators of the marine environment, confirming for the Codó Formation, what has already been characterized in previous studies. The low contents of organic carbon in the limestones are traces of calcareous algae, the main skeletal materials, which accompany the carbonate rocks and the results of organic matter (contents below 1%) confirm that the shales studied do not belong to the class of pyrobetuminous shales. The collected waters are physiologically located in areas of the Itapecuru, Codó and Corda aquifers and the analyzes of these waters were evaluated based on physical, physical-chemical, chemical and biochemical parameters, and the discussion of the results made it possible to classify these waters according to the total ionic concentrations , study the quality of water for human consumption, interpret the values ​​of chemical and biochemical parameters, which are indicators of pollution, and characterize them for irrigation and livestock conditions. The classified surface waters are of the sodium bicarbonate and calcium sulfated types, while the groundwater are of the sodium bicarbonate, calcium bicarbonate and calcium sulfated types. These waters are associated with shales and limestones as shown by the significant concentrations of Ca, Na, Mg, Cl, HCO3, SO4 and average pH values. A small sample recorded significant values ​​of SO4, concluding that these ions originated in gypsum and bassanite, minerals present in the studied area.