A importância da formação barreiras como área-fonte dos sedimentos dos manguezais de Marapanim com base na assinatura de minerais pesados

Abstract

Heavy minerals are important indicators of sedimentary processes, also helping in the identification of source areas. The Marapanim River estuary, located on the northeast coast of Pará, was chosen as a working area to demonstrate that the sediments and soils of the Barrier Formation (Tertiary) are the main source area for the mudflat, over which the region's mangroves developed. The research was based on the study of heavy minerals and Energy Dispersive Spectrometry (EDS) analysis of the Rare Earth Elements (RWE) present in zircons from these deposits. Six sediment samples were collected from the Barreiras Formation, 19 core samples (3 m) from the mudflat and 50 bottom sediment samples (Pettersen dredge) from the Marapanim River. The samples were dried, disaggregated, grain size fractions separated and minerals weighed (by bromoform) in the ranges 0.250-0.125mm and 0.125-0.062mm. Grain identification and counting was done under a petrographic microscope. Some grains were observed and analyzed with the aid of scanning electron microscope (SEM/EDS) for analysis of zircons and textural aspects. The main heavy minerals occurring in the Barrier Formation, mudflat and bottom sediments are: colorless zircons (super abundant), rounded, subrounded, subhedral, prismatic and angular fragments; pink zircons (common) in rounded, subhedral and angular forms and brown zircons (rare), bipyramidal prismatic and subhedral. Steurolites irregular, subrounded, as well as angular with colors varying from yellow-orange to light yellow. Rounded, subrounded, and prismatic tourmalines (most frequent) with blue-green, green, greenish-brown (brown), and brown colors. Colorless cyanites typically exhibiting flattened and elongated grains with tabular shapes (long and short) and rutile presenting as grains in angular shapes with progressively rounded, irregular and subhedral edges in colors ranging from blood red to orange-yellow. In the fine fraction, most significant for this study, zircon, staurolite, tourmaline, cyanite and rutile make up an average of 35%, 32%, 21%, 9% and 3%, respectively, in the sediments and soils of the Barreiras Formation; 35%, 24%, 30%, 9% and 2%, in the sediments of the muddy plain and 38%, 31%, 17%, 11% and 3%, in the bottom sediments. The main heavy minerals identified in the Barreiras Formation resemble those of the mudflat and bottom sediments in typology, proportions and textural aspects. Furthermore, the semi-quantitative analyses of the ETR in the zircons of the Barreiras Formation indicated the same distribution patterns for these elements in the mudflat and bottom sediments, corroborating the results of the mineralogical analysis. The strong relationship between mineralogy and chemical composition of the sediments demonstrates the importance of using heavy minerals as reliable indicators for provenance studies in the region under consideration.