Tunnel-valley fills in the Paraná Basin and their implications for the extent of late Paleozoic glaciation in SW Gondwana

Abstract

Tunnel valleys are subglacial incisions formed mainly by meltwater erosion on soft sediment overridden by glaciers. Despite being common in the Pleistocene and Ordovician glacial record, where their fills constitute excellent aquifers and hydrocarbon traps, tunnel valleys are poorly recognized in late Paleozoic strata. Other types of glacial and non-glacial valleys, on the other hand, have been widely reported from Carboniferous and Permian glacial successions, most cut into the basement or older sedimentary rocks and interpreted as glacial valleys eroded by outlet glaciers draining high-relief settings. The degree of glacial influence on the formation of these valleys has strong implications for the volume of ice during late Paleozoic glaciations. In this paper, we report the Lapa sandstone, an exhumed valley-fill succession encased in Pennsylvanian glacial rocks of eastern Paraná Basin, southern Brazil, and its subsurface counterparts. The Lapa sandstone forms a slightly sinuous ribbon several tens of kilometers long, 1500 m wide and 100 m thick composed essentially of glaciofluvial facies. By comparing the external geometry and the sediment fill of the valley with several types of erosive-based, elongated sediment bodies, we conclude that the Lapa formed as the infilling of a tunnel valley cut beneath an ice margin that advanced to the north onto poorly consolidated, mud-rich sediments. Valley fills previously mapped in the subsurface area to the west are considered to have been formed by the same mechanism and during the same glaciation event. The Lapa tunnel valley and its subsurface counterparts are the first example reported from the late Paleozoic glacial record in South America, but similar features are also expected in any formerly glaciated basin, and their recognition may help in estimating the extent of glaciation across the region and guiding exploration for hydrocarbon traps.