Sedimentary record of a fluctuating ice margin from the Pennsylvanian of western Gondwana: Paraná Basin, southern Brazil

Abstract

The Paraná Basin is a key locality in the context of the Late Paleozoic Ice Age (LPIA) because of its location east of the Andean proto-margin of Gondwana and west of contiguous interior basins today found in western Africa. In this paper we document the sedimentary record associated with an ice margin that reached the eastern border of the Paraná Basin during the Pennsylvanian, with the aim of interpreting the depositional environments and discussing paleogeographic implications. The examined stratigraphic succession is divided in four stacked facies associations that record an upward transition from subglacial to glaciomarine environments. Deposition took place during deglaciation but was punctuated by minor readvances of the ice margin that deformed the sediment pile. Tillites, well-preserved landforms of subglacial erosion and glaciotectonic deformational structures indicate that the ice flowed to the north and northwest and that the ice margin did not advance far throughout the basin during the glacial maximum. Consequently, time-equivalent glacial deposits that crop out in other localities of eastern Paraná Basin are better explained by assuming multiple smaller ice lobes instead of one single large glacier. These ice lobes flowed from an ice cap covering uplifted lands now located in western Namibia, where glacial deposits are younger and occur confined within paleovalleys cut onto the Precambrian basement. This conclusion corroborates the idea of a topographically-controlled ice-spreading center in southwestern Africa and does not support the view of a large polar ice sheet controlling deposition in the Paraná Basin during the LPIA.

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

“…The linear features trend N2E and the presence of clasts at the end of some grooves indicate a paleo ice-flow direction towards the northeast. The same outcrop was later reexamined by Vesely et al (2015), who recognized a small-scale flute (Fig. 7C) that corroborates the subglacial origin and the previously indicated paleo ice-flow.…”

“…This suggests that advancing glaciers were effective in sculpturing the pre-Itararé substrate and in influencing deposition during early stages of deglaciation due to fluctuations of the ice margins (e.g. Vesely et al 2015). The only exception is the clast pavements reported by Rocha-Campos et al (1976) and Rocha-Campos et al (1968) that are within diamictites of the middle to upper Itararé Group and that testify a possible younger advance of glaciers over the basin.…”

“…Ice-marginal deposits in southern Paraná state (Vesely et al 2015) and in some localities of Rio Grande do Sul state (Tomazelli & Soliani 1997) present characteristics that suggest deposition in continental settings (subglacial tillites overlain by glaciofluvial or glaciolacustrine facies), similar to the present day outlet glaciers from Iceland that terminate on land (e.g. Kjaer et al 2008).…”

“…van der Wateren 1994, Roberts & Hart 2005), including folds, faults, boudins and foliation. These structures can also be used to give ice-flow kinematics and are documented in the late Paleozoic succession in Paraná Basin (Vesely et al 2015, Aquino et al 2016, however they are not discussed in the present paper. In glacial marine or lacustrine environments blocks of floating ice (icebergs or sea/lake ice) may scour the bottom sediments producing ice-keel scour marks that are shallow to deep scours containing parallel sets of grooves and ridges (e.g.…”

“…Furthermore, this area also has deposits confined in paleovalleys (Visser 1987), which reinforce the existence of elevated land and indicate topographically constrained ice flow towards the Paraná Basin. The angular unconformity separating Devonian from Carboniferous strata in eastern Paraná Basin (Ponta Grossa Arch area) and the absence of pre-Carboniferous Paleozoic units in western Namibia are indicative of latest Devonian to Mississippian uplifting east of the present border of the Paraná Basin (Vesely et al 2015). This tectonic event was probably the cause for uplifting the land above the equilibrium-line altitude, leading to net snow accumulation and glacier growth (e.g.…”