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Abstract

Sea-level rise after the Hirnantian glaciation resulted in the global inundation of continental shelf areas and the widespread formation of early Silurian black shales. Black shales that were deposited on shelves receiving drainage from earlier glaciated areas have high uranium (U) contents because large-scale glacial erosion brought rocks with leachable U to the surface. In contrast, black shales receiving drainage from non-glaciated areas that had lost leachable U earlier have low U contents. Early Silurian U-rich shales formed only on shelf areas that had not been separated from earlier-glaciated mainland Gondwana by oceanic lithosphere. Therefore, early Silurian U-rich black shales within the Variscan orogen provide direct evidence that these areas had not been separated from mainland Gondwana, but were part of the same, contiguous shelf. This implies that the Rheic Ocean was the only pre-Silurian ocean that opened during the early Paleozoic extension of the peri-Gondwana shelf.