Sequence stratigraphy of a lagoonal estuarine system—an example from the lower Permian Rio Bonito Formation, Paraná Basin, Brazil

Abstract

Detailed stratigraphic analysis of the Early Permian (Artinskian/Kungurian) succession of the intracratonic Paraná Basin in southernmost Brazil has revealed that the study region, previously interpreted as formed by a barrier island system in a shallow, wave-dominated sea, actually originated mainly in a mixed (tide and wave) influenced estuary setting. Four main depositional systems have been recognized: alluvial fan, fluvial-dominated delta, lagoonal estuary and barrier/shoreface. The regional correlation of the lithofacies within the different depositional systems has led to a high-resolution-stratigraphic framework, with three third-order depositional sequences. Sequence boundary SB1 marks the base of the Permian succession above the crystalline basement, sequence boundary SB2 is characterized by fluvial sediments overlying marine shales and sandstones. Sequence boundary 3 (SB3) has a different signature reflecting differential subsidence: some areas clearly experienced temporary regression and basinward shift of facies, while in others the transgression rapidly reworked the regressive sediments and left only a thin veneer of pebbly sandstone, the typical signature of a transgressive surface coinciding with a sequence boundary. Within depositional sequence 2, the main target of the study because it has economically important coal seams, seven parasequences are recognized, two forming the lowstand systems tract of the sequence, four forming the transgressive systems tract and one parasequence forming the highstand systems tract. Two parasequence limits are erosional transgressive surfaces, as indicated by the occurrence of a substrate-controlled ichnofacies ( Glossifungites) and a veneer of intraclasts composed of nodules (chert?), shell fragments and muddy rip-up clasts. The base-level variations had a strong tectonic component, as indicated by (1) the shift in shoreline configuration from east–west to a north–south direction after unconformity SB3 formation, (2) the transgressive reworking of regressive sediments in the southern part of the study area indicating differential subsidence and consequent differential accommodation available for the sediments and (3) the angularity displayed by the strata adjacent to the sequence boundary SB3.