The Ordovician-Silurian tectono-stratigraphic evolution and paleogeography of eastern Parnaiba Basin, NE Brazil

Abstract

The Phanerozoic Parnaiba Basin (PB) occupies 600.000 km2 in northeastern Brazil and preserves one of the largest Ordovician-Silurian sedimentary record of western Gondwana, represented by the Serra Grande Group (SGG). This unit marks the initiation of the depositional history of Parnaiba Basin and comprises from the base to the top, the Ipu, Tianguá and Jaicós formations. Therefore the tectono-stratigraphy and paleogeography of SGG is key to unveil the genesis of PB and also the Ordovician-Silurian evolution of Gondwana. Here we present new sedimentological and tectono-stratigraphic surface and subsurface data from the eastern Parnaiba basin. Seven facies associations are described within the SGG: alluvial fan, braided river channel, outwash fan, lodgment till, shoreface, offshore and delta. They are assigned to each formation of SGG and, together with seismic and well data interpretation results, supporting a depositional model for this group. The lowermost portion of the Ipu Formation is represented by immature conglomerates in a scarp-related alluvial fan system, filling up NE-SW-elongated grabens aligned to the crustal scale Transbrasiliano Lineament. This implies that the early sedimentation of the Parnaiba Basin could relate to post-Brasiliano tectonic reactivations following the amalgamation of Gondwana. Upwards, the Ipu Formation grades to a braided river system. Its uppermost portion contains a glacial deposit, here named as the Ipueiras tillite and discontinuously preserved across the basin. This glacial record might be correlated to the Hirnantian Glaciation, a global episode of about 2 m.y. during the Ordovician-Silurian transition (c.a. 443.8 Ma), well documented in other Gondwanan basins from Africa (e.g. the Taoudeni and the Tindouf basins) and South America (e.g. the Amazonas and the Paraná basins). Above it, a 15 m thick layer of marine black shales represent the base of the Tianguá Formation and is interpreted as the maximum flooding event of Serra Grande Group. To the top and laterally, this unit grades to shoreface and deltaic facies associations and is coeval to the global deglaciation transgressive event of Early Silurian. Finally, the overlying Jaicós Formation is composed by braided river deposits in a regressive cycle, possibly related to isostatic rebound, closing the deposition of the Serra Grande Group. Regarding the paleocurrent pattern of this unit, the main northwestwards direction suggests a paleotopographic high in the central portion of western Gondwana.