Facies relationships recorded in a Late Paleozoic fluvio-deltaic system (Paraná Basin, Brazil): Insights into the timing and triggers of subaqueous sediment gravity flows

Abstract

This paper aims at documenting the spatial and genetic relationships between fluvial and subaqueous gravity-flow deposits in an ancient fluvio-deltaic system exposed in the eastern border of the Paraná Basin, southern Brazil. The examined interval comprises the Pedra Alta sandstone and associated deposits, which are included in the glacially-influenced, Late Paleozoic Itararé Group. Five facies associations were identified and comprise the following coeval depositional settings from the more proximal to the more distal: A) braided fluvial/delta-plain, B) channel-mouth zone, C) delta-slope, D) distal marine glacially-influenced and E) non-cohesive density-flow deposits. These facies associations form a conformable, shallowing-upward succession about 200m thick, whose stacking pattern is interpreted as a postglacial progradation. The stratigraphic architecture suggests a genetic correlation between high-energy fluvial discharges (possibly related to meltwater release) and coarse-grained deposits (non-cohesive density-flow deposits) accumulated on the delta foreset and bottomset. Mass-transport deposits were more likely triggered by sediment load increase on the delta slope due to high sediment supply, rather than by relative sea-level fall. The stratigraphic evolution can be reconstructed considering an initial stage characterized by a grounded ice margin and the deposition of mudstones with ice-rafted debris (glacial stage). This stage was succeeded by a second phase in which a fluvial system formed as a result of ice-margin retreat farther to the SE (postglacial stage). This glacial-postglacial stratigraphic succession is different from that recognized by previous workers in the Itararé Group, a conclusion that impacts greatly in reservoir prediction and modeling in the Paraná Basin.