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Journal of the Geological Society

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AMS and grain shape fabric of the Late Palaeozoic diamictites of the Southeastern Paraná Basin, Brazil

¹ Instituto de Geociências, USP, 05508-080 São Paulo, Brazil (e-mail: archan@usp.br)
² Instituto de Geociências, Universidade Estadual Paulista, IGCE, 13506-900 Rio Claro, Brazil
³ Laboratoire de Planétologie et Géodynamique, Université de Nantes, 44322 Nantes, France
⁴ Instituto de Astronômia, Geofísica e Ciências Amosféricas, USP, 05508-900 São Paulo, Brazil
⁵ Universidade Federal do Rio G. Norte, DFTE, 59078-970 Natal, Brazil

Diamictites interbedded with marine shales and turbidites onlap the eastern border of the Paraná Basin (Southern Brazil). These poorly sorted sediments were deposited during the Permo-Carboniferous glaciation, and their matrix-supported clasts show no preferred orientation. These massive rocks have been studied using anisotropy of magnetic susceptibility (AMS) and grain shape fabric. Hysteresis loops and thermomagnetic measurements show that AMS depends mostly on the paramagnetic clays, but fine ferromagnetic particles also contribute to the anisotropy. The coarse silt to sand grain preferred orientation study supports the use of AMS in describing the diamictite fabric, at least regarding the orientation of the foliation. AMS and grain shape data reveal subhorizontal to weakly inclined magnetic and grain shape foliation parallel to the regional bedding. The magnetic lineations are normally scattered within the foliation plane in agreement with the oblate AMS ellipsoids found in these rocks. Both fabric patterns are consistent with deposition by subaqueous mudflows that were resedimented downslope, with clastic supply from continental sources. The off-vertical grain shape foliation poles suggest that the deposition of diamictites was controlled by the depocentre topography of the Rio do Sul sub-basin.