The Paramirim Aulacogen

Abstract

The Paramirim aulacogen occupies the region of the northern São Francisco craton that extends from the northern Espinhaço range to a large plateau known as the Chapada Diamantina in the northeastern Brazilian state Bahia. Corresponding to a long-lived and partially inverted rift-sag basin, the aulacogen hosts and exposes the most complete section of the cratonic cover complex accumulated between the Statherian and Edicaran periods. The sedimentary and volcanic rocks of the basin fill successions are grouped into two major lithostratigraphic units, the Espinhaço and São Francisco supergroups. Exhibiting remarkable facies differentiation between their occurrences in the northern Espinhaço range, on the west, and Chapada Diamantina, on the east, these units are associated with Statherian granitic rocks, as well as Statherian, Calymmian and Tonian mafic dykes and sills. The Espinhaço Supergroup comprises five unconformity bounded sequences, which record alternating mechanical and flexural subsidence pulses that took place between 1775 and 900 Ma. Frankly dominated by quartz-rich sandstones, deposited in alluvial, aeolian and shallow marine systems, the supergroup reflects continuous recycling of sediments in the large depocenters of the aulacogen. The Neoproterozoic São Francisco Supergroup is composed of three distinct sequences, deposited between 900 and 500 Ma, among them the glaciogenic Bebedouro Formation and the shallow marine carbonates and pelites of the Salitre Formation. The overall geologic architecture of the aulacogen is dominated by structures generated during its contractional inversion in the Edicaran/Cambrian boundary. The regional structure of the aulacogen corresponds to a NNW-trending basement cored uplift, flanked on both sides by systems of reverse faults, thrusts, folds, and strike-slip shear zones. Structures related to the basin generation processes are preserved only in a small area of the central segment of the aulacogen. The dominant NNW-oriented fabric elements interact or are overprinted by E--W-trending structures in the northern and southern ends of the aulacogen. These structures represent the Neoproterozoic Brasiliano orogenic fronts that propagated from the marginal belts toward the craton interior.