Esker-like features in the Itararé Subgroup (late Paleozoic), Northern Paraná Basin, Brazil

ESKER-LIKE FEATURES IN THE ITARARÉ SUBGROUP (LATE PALEOZOIC), NORTHERN PARANÁ BASIN, BRAZIL^*

MARCOS R. ARAUJO AND JOSÉ R. CANUTO^**

Instituto de Geociências, USP, São Paulo, SP.

Presented by ANTONIO C. ROCHA-CAMPOS

Sediments occurring as discontinuous domed, elongated forms, 50 to 200 m in length, 20 to 50 m in width, and about 20 m in thickness, showing intense deformation, under the Lapa sandstone, are interpreted as eskers.

Three facies have been recognized: a) conglomerate of rounded pebbles and boulders in a massive, poorly sorted sandy matrix, sometimes cross-bedded, with lineated and imbricated clasts indicating NW transport, with normally graded sandy layers fragments included. The conglomerates show 1 m in thickness and lenticular geometry; b) two types sandstones: the first one is medium to fine-graded, massive, sometimes cross-bedded, showing NW transport, in layers up to 80 cm thick with tabular geometry; the second one is medium to fine-graded, well sorted, massive, sometimes showing gradacional stratification, with thicknesses of 10 to 20 cm; c) massive tabular diamictite, of decimetric thickness, silty-sandy matrix with dispersed clasts, intercalated and interfingered in the base of its layers with sands and conglomerates and with sandstones in its tops. The layers are in a vertical way and deformed, and parallel to the direction of the channel of the Lapa sandstone.

The basal contact of the ruditic succession is erosional on older shales, while the upper contact also erosional, defines the base of the Lapa sandstone.

The glacial context of the time suggested that the sandstones and conglomerates were deposited by fluvio-glacial processes and the diamictites (cohesive debris flows) flowed from the channel walls and interfingered with the sandstones and conglomerates.

The first phase of deformation caused folding with the differential melting of underlying ice below the sediments. Later, the total collapse of the sedimentary stack, due to melting of still existing ice in some parts, associated with the unstable settling of the sediments caused by the previously differential melting, would have led to a second deformation phase, facilitated by the still plastic state of the sediments, resulting in the refolding of the first phase folds. Deformation by compaction is rejected because the overlying Lapa sandstone is not deformed.

The above features suggest that the sediments correspond to an esker. The deposit is part of the lowstand system tract of the 3^rd order sequence S₂ described by Canuto et al. (2001).— (December 14, 2001) .

Publication Dates

• Publication in this collection
  09 Oct 2002
• Date of issue
  Sept 2002