Superfícies estriadas em arenitos do Grupo Itararé produzidas por gelo flutuante, sudeste do estado do Paraná

Abstract

Permocarboniferous glacial pavements has already been found in several places in southern Brazil. In the southeast of Parana State there are many occurrences, as the well-known striated surface produced over Devonian basement rocks in the locality of Witmarsun. However, most of the pavements are in fact soft-sediment striated surfaces preserved in the lower part of the glacial Itarare Group itself. At the base of a deglacial sequence exposed in the valley of the Salto River near the Palmeira town, several soft-sand striated surfaces are distributed in four different stratigraphic levels inside cross-stratified sandstones interpreted as deposits of subaqueous outwash fans. Scour marks indicate paleoglacial movement towards the azimuth 341o, coherent with the advance of glaciers postulated in regional paleogeographic reconstructions for Permocarboniferous of south Brazil. The surfaces are discontinuous, sometimes slightly concave and associated with linear grooves, and the preservation was favored by thin laminae of fines. These characteristics and the fact that they are not covered by coarse facies like diamictites, allow us to interpret them as soft-sand striated surfaces produced by floating ice. The highly consistent orientation of striations and grooves suggests that they were formed near the grounding zone of an ice shelf in a marine environment, but the surfaces could be ice keel scour marks produced by free-floating ice masses.