Late Paleozoic glaciotectonic structures in northern Paraná Basin, Brazil

Abstract

Fluviatile–deltaic sediments containing thin coal layers, and glaciofluvial sediments of the Itararé Subgroup (late Paleozoic) cropping out near Cerquilho, northern Paraná Basin, exhibit post-depositional deformation such as recumbent and drag folds, shear fractures, faults and shear lamination. The deformation occurs in an interval of 3–4 m thick and is confined between horizontal strata. The deformed strata are directly overlain along a horizontal erosional/tectonic contact surface by a silty–sandy, massive, 1 m thick, clast-poor tillite. Deformation shows a consistent geometry indicating a predominant vergence toward the SSW having been developed under a horizontal stress oriented from NNE to SSW. The deformation is interpreted to be of glaciotectonic origin associated with an ice readvance in the area. The glacier moved toward the SSW on relatively soft, deformable sediments, depositing the subglacial till on top of them. The tillite shows parallel, upglacier-dipping shear fractures at its base and wedge-like intrusions into the underlying sandstone, probably formed during its subglacial deposition. The tillite is overlain by cyclic (braided stream) fluviatile, sandstone beds. The top of these shows deformations (small recumbent folds and reverse faults) below another clast-poor, silty–sandy tillite. This could represent another glacial readvance with subglacial deposition of till. Occurrence of glaciotectonic deformation overlain by subglacially deposited tillites in the Cerquilho area indicates glacier readvances and permanence of the glacial influence in the upper part of the Itararé Subgroup. Absence of incorporated material from the underlying deformed sediments in the tillite and the style of the deformations suggest an unfrozen, water-saturated, mostly unconsolidated substratum. These conditions may have generated instability and fast ice flow of the glacier. They also suggest that the sediment may have most likely failed in this manner under rapidly applied stresses associated with a surging behavior of the retreating glacier.