Stratigraphy and sedimentology of the Santa Bárbara and Redonda islands, Abrolhos Archipelago, Southern Bahia

Abstract

The Abrolhos Archipelago is located offshore of the Mucuri Basin in the southern part of the Bahia State and is formed by five islands, which are mainly composed of igneous rocks. However, in two of the islands, Santa Barbara and Redonda, sedimentary rocks constitute a significant part of the exposures representing, together with the igneous rocks, a rare evidence of the Brazilian marginal basins filling. This study aims to characterize the exposed sedimentary rocks, which are about 45 m thick, to provide a local stratigraphic framework and to understand its depositional setting. In order to achieve the goals of the study, the rocks were described, measured, stacked and sampled. Based on facies descriptions and a correlation of four stratigraphic surfaces, three unconformities and a siltstone marker used as a datum, we divided the succession into three depositional units: lower, middle and upper. The units are limited by unconformities and composed by a particular facies association. Sandstones are the most abundant fa-cies, but siltstones, conglomerates and heterolithics are also common. The sedimentary facies are interpreted as deposits of low and high density gravity flows, also including slumps and debris flows. Facies related to an unstable depositional context, probably influenced by tectonism, volcanic activity or slope oversteepening, compose the lower unit and are also important in the upper unit. Otherwise, the middle unit is composed by unconfined deposits developed in a more stable setting. Considering the presence of a set of features and sedimentary structures like sandstone lobes, the occurrence of sandstones beds with hummocky cross stratifications, siltstones enriched in plant fragments and the abundance of large burrows, we propose that the studied sediments were deposited in a marginal setting, represented by a shelf environment fed by a deltaic system during rivers in flood. The exposed sediments were intruded by igneous rocks and contain fragments of them, suggesting a contemporaneity between igneous activity and sediments deposition. They have been dated between Paleocene-Eocene based on preliminary nannofossils analysis and geochronological (Ar-Ar method) dating of igneous samples.