A review on late Paleozoic ice-related erosional landforms in the Paraná Basin: origin and paleogeographical implications

ABSTRACT: The Late Paleozoic Ice Age is recorded in the Paraná Basin as glacial deposits, deformational features and ice-related erosional landforms of the Itararé Group. Erosional landforms are often employed to build paleogeographic models that depict the location of ice masses and paleo ice-flow directions. This paper provides a review of the literature and new data on micro- to meso-scale ice-related, erosional landforms of the Paraná Basin. Examined landforms can be placed into four broad categories based on their mode of origin. Subglacial landforms on rigid substrates occur on the Precambrian basement or on older units in the Paraná Basin. They include streamlined landforms and striated pavements formed by abrasion and/or plucking beneath advancing glaciers. Subglacial landforms on soft beds are intraformational surfaces generated by erosion and deformation of unconsolidated deposits when overridden by glaciers. Ice-keel scour marks are soft-sediment striated/grooved landforms developed by the scouring of free-floating ice masses on underlying sediments. Striated clast pavements are horizons containing aligned clasts that are abraded subglacially due to the advance of glaciers on unconsolidated deposits. Only those erosional landforms formed subglacially can be used as reliable paleo ice-flow indicators. Based on these data, the paleogeography of the Paraná Basin during the Late Paleozoic Ice Age fits into a model of several glacial lobes derived from topographically-controlled ice spreading centers located around the basin instead of a single continental ice sheet.

摘要：晚古生代冰期（Late Paleozoic Ice Age）在巴拉那盆地（Paraná Basin）的地质记录体现为伊塔雷组（Itararé Group）的冰川沉积、变形构造以及与冰作用相关的侵蚀地貌。侵蚀地貌常被用于构建古地理模型，以还原冰体的分布位置与古冰流方向。本文对相关文献进行了综述，并补充了巴拉那盆地微观至中观尺度冰相关侵蚀地貌的新数据。经研究的侵蚀地貌可依据成因机制划分为四大类别：发育于刚性基底之上的冰下地貌，分布于前寒武纪基底或巴拉那盆地的古老地层单元中，包括由冰川前进过程中冰下磨蚀和/或拔蚀作用形成的流线型地貌与擦痕面；发育于软质层之上的冰下地貌，为冰川推覆未固结沉积物时，经侵蚀与变形作用形成的层内构造面；冰龙骨冲刷痕是自由漂浮冰体冲刷下伏沉积物所形成的软沉积物擦痕/槽沟地貌；砾石擦痕面则指冰川在未固结沉积物上推进时，经冰下磨蚀作用形成的含定向砾石的地层层面。仅冰下形成的侵蚀地貌可作为可靠的古冰流指示标志。基于上述数据，晚古生代冰期巴拉那盆地的古地理格局符合“多冰川舌源自盆地周边受地形控制的冰扩散中心”的模型，而非单一大陆冰盖（continental ice sheet）模式。