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

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 Parana Basin, southern Brazil. The examined interval comprises the Pedra Alta sandstone and associated deposits, which are included in the glacially-influenced, Late Paleozoic Itarare Group. Five facies associations were identified and comprise the following coeval depositional settings from the more proximal to the more distal: A) braided fluvial/deltaplain, 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 200 m 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 triggeredby 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 Itarare Group, a conclusion that impacts greatly in reservoir prediction and modeling in the Parana Basin. (C) 2016 Elsevier B.V. All rights reserved.