PRELIMINARY CHARACTERIZATION OF A DYNAMIC GLACIAL MARGIN: CONGLOMERATES AND DIAMICTITES FROM RIO DO SUL FORMATION, PRESIDENTE GETULIO, SOUTH BRAZIL

The Itararé Group (Paraná Basin, Brazil) has one of the most extensive records of Neopaleozoic glaciation. However, there is few strata where it’s possible to identify subglacial and/or glacio-marginal sedimentation. At the Presidente Getúlio locality (south Brazil) the basal/intermediate portion of the Rio do Sul Formation (upper portion of the Itararé Group) has some features that could indicate more proximity of the glaciers that differs from previous interpretations for in this portion of the basin.

The studied interval are 40-to-60m and are composed by conglomerate, diamictite, and sandstone facies. Previous interpretations describe this interval as submarine gravitational deposits channeled into the slope fed by distal glacio-influenced deltaic systems. The methodology used in this study was the faciological analysis. In total, 40 outcrops were described. In addition paleocurrents measurement were taken, and five detailed sedimentological log were raised.

The conglomerate facies have a sandy matrix ranging from medium-to-very-coarse-grained sandstone, and can containing dispersed clasts that vary from granule to boulder (up to 6m). Many of the clasts are striated, faceted, and polished, sometimes found aligned, forming possible clasts pavements. The predominant sandstones facies are sandstones with climbing-ripple lamination. Diamictite deposits present two main facies. Massive diamictite facies have a sandy-muddy matrix with dispersed clasts that vary from granule to boulder. Many of these clasts are interpreted as lonestones. It’s common the occurrence of thin deformed sandstone layers sandwiched to the diamictite deposits with folds, faults and boudins. The ressedimented diamictite facies predominate in the upper part of the sucession and are marked by a sandy-muddy matrix that has cobbles and boulders of sandstone and conglomerate as intraformal blocks.

The sedimentation in the interval appears to be dominated by gravitational flows. However, several characteristics observed, such as striated cobbles and boulders, lonestones and the occurrence of clast pavements, leads us to infer that the origin of these gravitational flows is much more proximal to the ice margin than previously described. The occurrance of lonestones and clast paviments can suggest ice-rafting and subglacial process respectively, which would be an evidence of the close presence of the ice. Due to its chaotic character, the upper portion seems to be close associated with gravitational mass flows in a preliminary analysis.