José Roberto Canuto

Patterns of late Palaeozoic deglaciation in the Paraná Basin, Brazil

Abstract The Itarare Subgroup (Middle Carboniferous-Early Permian) of the Parana Basin of southeastern Brazil contains the thickest, most extensive and one of the longest records of late Palaeozoic glaciation in all of the Gondwana supercontinent. Glacially controlled deposition started as early as the Middle Carboniferous when lobate termini of ice streams from the Windhoek Ice Sheet (WIS), and Rio Grande do Sul and Asuncion ice caps moved into the Parana Basin. Lobes reached sea-level and advanced grounded at least as far as 200 km into the basin, stabilizing as tidal/intertidal margins. Glaciation and glacial sedimentation occurred penecontemporaneously with the transgressive phase of the late Palaeozoic tectonic cycle of the Parana Basin, while Gondwana was gradually moving away from the South Pole. Evolution involved complex interaction of tectonic, palaeogeographic and palaeoclimatic factors. Initial flooding of the basin by an epicontinental sea in the Middle Carboniferous may have provided moisture necessary for the growth of glaciers. Conversely, episodes of sea-level rise probably affected the stability of marine glaciers. The palaeoclimatic setting for the WIS and other ice masses at the margins of the Parana Basin was mostly temperate. Multiple ice advances are evidenced by subglacial tillites resting on striated and/or glaciotectonized basement and deformed glacigenic sediments identified at different stratigraphic levels in the Itarare Subgroup. Glaciation reached its maximum expansion sometime between the Late Carboniferous and Early Permian, gradually diminishing and vanishing in the late Early Permian. The reconstructed palaeogeographic evolution favors a model of irregular melt back of ice masses, particularly along the eastern margin of the Parana Basin, in analogy with the deglacial pattern of the Pleistocene Laurentide ice sheet, in North America. Retreat of marine glaciers was mostly associated with relatively simple accumulations, or complex build-ups of ice contact/proglacial facies associations in glacial estuarine embayments. Fast retreat was mainly controlled by decoupling associated with sea-level rise and intense calving. In the terrestrial environment, more fragmentary evidence shows that retreat may have involved several mechanisms including downwasting of stagnant/inactive ice margins and destabilization by surging of glaciers moving over deformable sediments. Highstand sea-level following ice wastage is often marked by widespread deposition of relatively thin marine beds. At least four stratigraphically different basinwide marine “horizons” are recognized. Isostatic rebound of deglaciated margins of the Parana Basin is recorded by transition from deep marine beds to shallowing/coarsening upward, prograding sequences of littoral/fluvio-deltaic facies basinwards, sourced from raised basin margins. Eventually, upon emersion, these constituted platforms for interglacial and postglacial peat development. Although glaciation lasted for a long time, glacial sedimentation alternated with relatively long periods of intense and widespread subaerial and subaquatic (submarine) reworking of previously deposited glacigenic sediments and deposition of “normal”, non-glacial sediments, that comprise the bulk of the Itarare Subgroup sequence. Thin coal beds, which locally rest on fossil soils, denote climatic amelioration during these interglacial phases.

Late Paleozoic glaciotectonic structures in northern Paraná Basin, Brazil

Abstract Fluviatile–deltaic sediments containing thin coal layers, and glaciofluvial sediments of the Itarare Subgroup (late Paleozoic) cropping out near Cerquilho, northern Parana 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 Itarare 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.