Luís Alberto Dávila Fernandes¹, Rualdo Menegat², Gerson Caravaca³, José Manuel Marques Teixeira de Oliveira⁴

(1) Federal University of Rio Grande do Sul/CNPq, Porto Alegre - RS, Brazil
(2) Federal University of Rio Grande do Sul, Brazil, 91.501-970 Porto Alegre, Brazil
(3) Federal University of Rio Grande do Sul
(4) Agência Nacional do Petróleo

ABSTRACT: Strike-slip control and depositional sequences of the late-orogenic Piquiri Basin

The Piquiri Basin crop out in the central domain of the Dom Feliciano Belt. Its evolution is related to reactivation of the neoproterozoic NE-trending strike-slip shear zones. Four large-scale sedimentary sequences separated by flooding surfaces were recognised by stratigraphic and provenance studies. They comprise a 4 km-thick sequence of conglomerates and sandstones steeply dipping towards NW. The first is represented by up to 0.6 km-thick fining-upwards cycle of conglomerates and breccias of proximal and intermediate facies of a fan-delta system. Sequences II and III (up to 1.5 and 0.6 km of thickness, respectively) are composed by two coarsening-upwards cycles of rhythmites and matrix- to clast-supported conglomerates, representing facies of distal to proximal fan-deltas. Fragments of quartzite and acidic metavolcanics derived from the neoproterozoic supracrustal sequences and clasts of sandstones and conglomerates represent cannibalised rocks. Sequence IV, with up to 1.2 km of thickness starts with facies of distal fan-delta at the base and is followed by conglomerates and conglomeratic sandstones. Deformation features of these sequences include rotation of clasts, pinch-and-swell structures in clasts of granites and porphyritic acidic volcanic rocks as well as development of a low-temperature schistosity in matrix-supported breccias. Other mesoscopic-scale and compressional structures are conjugate kinks, upright chevron folds and reverse faults. Although ascribed to collisional processes, the relative ages, conditions of development and shortening accommodated by these structures suggest that they were more likely a product of reactivation of the NE-trending neoproterozoic shear zones.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado