On the origin of the Itararé Group basal nonconformity and its implications for the Late Paleozoic glaciation in the Paraná Basin, Brazil

Abstract

Outcrop studies of the Itararé Group in the Alfredo Wagner area, along the eastern margin of the Paraná Basin in Brazil, reveal a partially exhumed, NNW-SSE oriented glacial paleovalley. The Carboniferous Rio do Sul Formation represents the basal interval of the glacially influenced Itararé Group, and rests directly on Precambrian basement rocks. This nonconformity is thought to represent an areal scouring surface, as indicated by the series of glacially-carved linear depressions that were incised into the glaciated bedrock during glacial advances. These large-scale erosional landforms differ in their extent by orders of magnitude, and include wide, deep troughs as well as narrower, shallower gouges. Glacial troughs and gouges usually display smaller-scale, subglacial erosion landforms, such as polished and grooved striations on their walls and floors. These indicate the NNW motion of a warm-based, grounded glacier. In addition, large-scale erosional features represent local topographic lows within the major glacial paleovalley. These local lows are large enough to contain part of the succeeding deglacial deposits, which thin toward and onlap the wall of these depressions. We grouped these deglacial deposits into six genetic facies associations (FA-1 to -6) within three fining-upward glacial cycles. FA-1 is composed of black shale, and represents an initial marine flooding event. It can be correlated with the regionally widespread Lontras Shale. FA-2 is composed of sandstones and mudstones associated with flood-derived hyperpycnal flows, and occasionally with surge-type turbidites. FA-3 is represented by plastered or injected, matrix-supported conglomerates emplaced as lodgment tills. FA-4 comprises matrix- and clast-supported conglomerates, sandstones, and mudstones, attributed to ice-rafting and concentrated flows in grounding-line fan facies. This unit also contains ice-keel dragging features, carved into soft substrate. FA-5 is composed of dropstone-bearing black shale, related to a second major marine flooding event within the paleovalley. FA-6 comprises sandstones associated with channelized, surge-type turbidites, forming the second turbidite system in the succession. As these deposits are confined to a paleovalley, we infer that sedimentation was influenced by topographically controlled ice streams, instead of non-confined, widespread ice lobes as previously suggested. Reconstructing South America and southern Africa in their positions during the LPIA suggests that the glacial valleys then present in the Namibia area may have extended further, into the eastern margin of the Paraná Basin. In this scenario, confined outlet glaciers would have flowed out from the Namibian Windhoek ice cap, and terminated along the eastern margin of the Paraná Basin.