Petrografia da sequência sedimentar da Ilha Sudeste do arquipélago de São Pedro e São Paulo, Atlântico Equatorial

Abstract

The St. Peter and St. Paul archipelago is characterized as a set of islands and islets situated in the Equatorial Atlantic Ocean, representing the top of a ridge, with 90 km long, 25 km wide and 3,800 m high, composed of peridotitic rocks of mantle origin, called the Peridotitic Ridge of St. Peter and St. Paul or Atobá Ridge. Over the basement of peridotitic composition there is a sedimentary cover of a low expressivity, relatively thin and without continuity. It is possible to individualize three groups of main rocks deposited under particular conditions: carbonate fillings in fractures, conglomerates filling Plunge pool and a sedimentary sequence forming a bench, limited to the Southeast Island, with a thickness that reaches up to 6 meters. The present research aimed at the diagenetic study of this outcropping sedimentary sequence in the Southeast Island, which is essentially formed by clastic sediments from the rocky substrate and fossils, with beachrocks characteristics. Eleven thin section were made, representing the outcrop profile, providing support for the reaffirmation of the existence of two sedimentary units: Atobá and Viuvinhas. The basal unit is called Atobá: it has a framework composed essentially of lithoclastic, conglomeratic sediments, showed poor selection and low porosity and ascending granodescence, with more rounded pebbles at the base and increasing in angularity to the top. The Viuvinhas unit is formed by rocks of lithoclastic-carbonic hybrid composition in a composite and stratum mixture. Two main lithofacies in the composition of the unit are individualized, one essentially carbonate, poorly selected and with high porosity, and another lithofacies of medium and moderate granulometry selection, presenting contribution of carbonate material in the composition of the framework. Basement blocks may also occur in all lithofacies, resulting from the active seismic activity in the region that causes erosion in the cliffs of the islet. The cementation is formed exclusively by magnesian calcite and aragonite, in the main morphologies microcrystalline, fringe, botroidal and crypto-crystalline cuticle and fill almost all porous spaces of the rock with a characteristic cementation of the active phreatic marine zone. The study found that although the rocks studied have characteristics also present in beachrocks, they cannot be described as "true beachrocks".