O Neógeno da Plataforma Bragantina e da parte leste da Bacia do Marajó, norte do Brasil: Paleoambiente, Proveniência e relação com a evolução do Proto-Amazonas

Abstract

During the lower Miocene, tectonic stability on the Bragantina Platform favored the accumulation of carbonate deposits, later suppressed by a siliciclastic system associated with the Barreiras Formation. Previous works suggest that this one came from the sedimentary plume of the proto-Amazonas, while others indicate an independent evolution. Based on the above, this study applied provenance techniques to indicate the tectonic compartments that contributed as source rocks of the Barreiras Formation, discuss the influence of the proto-Amazonas in the suppression of the carbonate platform and understand the paleogeographic context. The study area covers the municipalities of Ourém (OU), Capanema (CA), Castanhal (CS), Santa Isabel do Pará (SI), Outeiro (OT), Mosqueiro island (IM), Salinópolis (SA), Aricuru (AR) and Mocooca (MO). Facies analysis techniques were performed with paleocurrent measurements, and 12 samples were collected, containing around 500 g of sediment each. Clasts were collected for morphological characterization in Ourém and Mosqueiro island. For the unconsolidated sediment, sieving was performed targeting the 250 - 125 µm and 125 - 63 µm fractions. Oxalic acid (50 g.L-1) was applied to these fractions and the light and heavy minerals were separated by density in bromoform (2.8 g/ml). Twenty-four petrographic thin sections of light minerals were prepared for cathodoluminescence (CL) and 24 thin sections of heavy minerals for identification of the mineralogical assembly, analysis of shape and elaboration of graphs. From the results of heavy minerals, it was possible to group the values with data from previous studies and apply the Inverse Distance Weighting method. The results indicate the presence of 10 facies, which were grouped into two associations, covering conglomerates, sandstones and pebbles. The clasts presented exclusively quartzous composition, predominating subangular to rounded shapes. The CL indicates a predominance of 45% of quartz with dark blue luminescence, 28% with light blue luminescence and 26% with violet luminescence. The results of heavy minerals indicate the presence of staurolite (54.45%), zircon (20.29%), tourmaline (10.02%), kyanite (7.19%), rutile (5.37%) and sillimanite (2.67%). Values above 50% of the sum of the percentages of zircon, tourmaline and rutile indicate an increase in maturity towards the coastal portion, while the distribution maps show a predominance of staurolite in the Bragantina Platform and an increase in kyanite values following those of tourmaline and zircon in the Marajo Basin. From the faciological interpretations, we were able to improve paleoenvironmental reconstruction consisting of alluvial fans, intertwined and meandering rivers, sandy plains, muddy plains and mangroves. The CL values and the high concentration of staurolite reflect sources of metamorphic rocks for the Bragantina Platform that come from southeastern Pará, possibly from the Gurupi Belt and São Luís Craton; while the CL values for the Marajó Basin, linked to the values of kyanite, zircon and tourmaline, indicate a strong influence of plutonic and metamorphic sources from southern Pará, such as the Tocantins-Araguaia Belt, Grajaú Basin and Sub -Cametá Basin. In general, the proto-Amazonas had no influence on carbonate suppression, the main phenomenon being responsible for the progradation of sediments, from metamorphic rocks with smaller contributions of plutonic and volcanic rocks, from the evolution of tectonics in the south and southeast of Pará.