O Cambro-Ordoviciano da Provincia Borborema

Abstract

This paper describes some volcano-sedimentary deposits that characterize the time interval from Neoproterozoic IE to the early Ordovician, typical of the Borborema Province, Northeast Brazil. They are usually designated molassic deposits and comprise coarse-grained immature rocks with common lateral gradation to pelitic lithologies of continental environments. The volcanism varies, but is mostly acid to intermediate. Two major types of basins may be identified: The first type is connected with the formation and evolution of pull apart depressions along the major shear zones (or lineaments) as part of the escape-tectonics phase of the Borborema Province (LL type basin). The second type is formed by foreland (foredeeps) and intradeeps (distal rifts) related to the late and post-tectonic evolutionary processes of the Sergipano Belt, a "miogeoclinal" orogen positioned at the northeast border of the São Francisco Craton. In both cases, the preservation of these deposits - usually scarce and scattered throughout the province - was only possible due to the immediate protection by younger Paleozoic covers (Silurian and younger) of syneclises and Mesozoic rifts (generated above these syneclises). Presently, the thickest and largest occurrences of these deposits do not crop out as they are beneath the Paleozoic basins according to geophysical and other subsurface data. Some of these Eo-Paleozoic rifts have been pointed out as precursor rifts for the development of the Parnaiba Syneclise.