Early Silurian glaciations in Brazil
Abstract The opening of the Brazilian Paleozoic basins during the Late Ordovician was followed by a tectonic stabilization in the Silurian. At the time of the Early Silurian glaciations, deposition of continental sandstones prevailed in the basins. Offshore environments were present in the Amazonas Basin and, to a smaller extent, in the Parana Basin. The ice sheets advanced over fluviatile and littoral sediments, except for the south margin of the Amazonas Basin, where the glacial diamictites immediately overlie marine facies. The ice produced much meltwater, even when advancing, and this was probably climatically controlled. The ice movements were from the northwest at the northern margin of the Amazonas Basin, and towards the northeast at the southern margin. In the Parnaiba Basin and its outliers, the Jatoba Basin and Cariri Valley, the ice came from southeast, and probably also from east. Further to the south, in the Parana Basin, it came from the north and northeast. Four glaciations have been dated to the Early Silurian. The oldest is dated as latest Ashgill and/or earliest Llandovery in the Parana Basin, as evidenced from tillites in the upper Rio Ivai and lower Vila Maria Formations (including the former Iapo Formation). It is contemporaneous with a glaciation in the Cape Basin, South Africa. The next glaciation is dated as early middle Llandovery ( gregarius graptolite Zone) and is present in the Amazonas Basin and the Andean Belt, and probably also in the Parnaiba Basin and its outliers. The two youngest tillites were widespread in the northern Brazilian basins (Amazonas, Parnaiba and Jatoba basins and Cariri Valley), as well as in the Andean Belt, and are dated as early late Llandovery (possibly starting already in the late middle Llandovery) and latest Llandovery to earliest Wenlock. The latter tillite is the most widespread one. Assuming these glaciations are related to polar positions, the apparent migration of the South Pole ice cap must have been quite rapid, from central North Africa in late Ashgill towards central Brazil in the middle-late Llandovery. The glaciation in the Parana Basin took place up to 45° from the late Ashgillian South Pole. The cool climate continued during the Silurian, but no new glaciations are known before latest Devonian.