Provenance and diagenesis of Guama Sandstone, northeastern Para, Brazil: A Silurian link between the Amazonas and Parnaiba basins

摘要

The Guama Sandstone, exposed in small isolated outcrops in the northeastern region of the State of Para (Brazil), which are surrounded by Cenozoic sediments, represents a Silurian relic of the Amazonas and Parnaiba basins connected during the Paleozoic and separated only since the Triassic period through the development of the Marajo Basin. Previous outcrop-based studies showed that the Guama Sandstone is a fine to medium-grained quartzarenite, with high compositional and textural maturity, which was deposited in a coastal marine setting (foreshore/shoreface), correlatable with the environment of the Silurian Nhamunda Formation of the Amazonas Basin. The principal primary sources of the sandstone are associated with rocks formed during the Transamazonico and Brasiliano Cycles according to isotopic dating of zircon. In order to better understand provenance and diagenesis of the Guama Sandstone core samples of a 50 m deep bore hole, located 6 km north of the town of Sao Miguel do Guama, have been studied. The sandstone is usually bioturbated and massive, but planar stratification (foreshore) and cross bedding (shoreface), the latter in the lower part of the profile, are also present indicating a transgressive to regressive coastline. In addition to quartz, the Guama Sandstone is composed of the following detrital accessory minerals: zircon, tourmaline, rutile, epidote, mica, ilmenite and chromite while kamacite occurred only sporadically indicating, at least for the latter, origin from Fe-Ni-meteorites. Provenance data obtained from the composition of zircon and tourmaline grains, as well as from internal textures of zircon, point to primary igneous and metamorphic source rocks of the Guama Sandstone. While zircon is principally of magmatic origin, tourmaline grains are derived dominantly from metapelitic, metapsammitic and granitoid rocks. However, pre-existing sediments probably represent, to a lesser extent, the ultimate direct source as suggested by the well-rounded grains of the quartz arenite and worn tourmaline overgrowths. The diagenetic phases of the sandstone include syntaxial quartz overgrowths, microquartz, kaolinite booklets, illite and anatase. The quartz overgrowths are pseudo-zoned and engulf parts of the kaolinite booklets, These may be surrounded by microquartz which usually replaced quartz overgrowths. Illite, which is rare, is probably the product of replacement of kaolinite, and anatase may be related to partial dissolution of ilmenite. The main diagenetic processes including precipitation of quartz and kaolinite began during eodiagenesis, in shallow burial conditions, and proceeded through mesodiagenesis as suggested by multi-episodic quartz zoning and formation of illite. The early precipitation of kaolinite is related to alteration of feldspar and mica caused through flushing of meteoric water during forced marine regression and uplift, probably still in Silurian time. The sources for quartz cementation may include kaolinitization of feldspar and pressure dissolution, the latter associated to concavo-convex and sutured grain contacts, although these types of contact are relatively rare. Therefore, external silica-rich solutions likely occurred.