Late Paleozoic glacial deposits of Brazil: Paraná Basin
Published:January 01, 2008
A.C Rocha-Campos, Paulo R dos Santos, José R Canuto, 2008. "Late Paleozoic glacial deposits of Brazil: Paraná Basin", Resolving the Late Paleozoic Ice Age in Time and Space, Christopher R. Fielding, Tracy D. Frank, John L. Isbell
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In the early Pennsylvanian, glacial lobes draining from an ice sheet probably centered on the northern highlands of Namibia (Windhoek ice sheet) reached the eastern margin of the marine-flooded Paraná Basin in southeastern and southern Brazil. Here, the glacier lobes extended at least 50 km across the depositional surface and terminated in a grounded tidewater system. The late Paleozoic paleolatitude of 40°–50°S of the Paraná Basin is consistent with a temperate warm-based regime for the glaciers. Accumulation of glacially influenced sediments in the main northern depocenter of the rapidly subsiding cratonic depression may have been preceded by deposition of proglacial sandy deposits (Lagoa Azul Formation; terrestrial?). Glacier displacement seems to have been controlled mainly by glacial-estuarine or attenuated fjord-like coastal inlets. The glacial lobes initially moved over highly eroded Precambrian to early Paleozoic crystalline rocks, then middle Paleozoic sedimentary rocks, and eventually over their own deposits. Multiple advance-retreat phases left a carpet of subglacial and meltwater deposits both on land and along the basin margin, although the former deposits have been almost completely eroded away. Continued, albeit less intense, tectonic subsidence of the basin led to southward displacement of the depocenter and more widespread sediment deposition. An ice cap centered on the Rio Grande do Sul shield was drained by lobes that flowed along radially distributed preglacial valleys. On the western basin margin, reduced numbers of lobes not connected to a recognizable major ice mass (or masses) seem to have moved toward the southeast. Destabilized glacigenic debris (mainly sand, diamicton, and mud) accumulated proglacially on the basin ramp and moved downslope by mass-flow processes, resulting in thick and laterally extensive packages of sand and diamicton interbedded with laminites and shales (Lagoa Azul, Campo Mourão, and Taciba Formations). Ubiquitous dropstones in laminites and in massive silty-clayey diamictons point to deposition by rain-out from sediment plumes and icebergs. Repetitive sedimentary cycles consisting of alternations between limited terrestrial facies and intraformational evidence of subglacial processes indicate that glacial conditions in the Paraná Basin persisted for 17–27 m.y. Glacial deposits are overlain by shallow-marine deltaic, coal-bearing sandstone of the Rio Bonito Formation and equivalent beds.