lain by the Holmwood Shale and Irwin River Coal Measures (Fig. 4). The Nangetty Formation occurs on a major erosional unconformity on striated Precambrian crystalline basement. This formation contains dropstones and boulders (up to 6 m in diameter). Cobbles and pebbles display facets and striations, and these are found scattered throughout sandy silt and shale beds (Mory et al. 2005). According to Eyles et al. (2002), so-called “glaciation” accompanied graben formation in southwestern Australia, beginning in the Carboniferous. This graben complex ran down the tectonically active western margin of southwestern Australia (Norvick 2004). So-called “glacial” sediments include the Mosswood Formation in the Bunbury Trough, the Shotts Formation of the Collie Sub-basin, and the Nangetty Formation of the northern Perth Basin (Fig. 4). E. Late Paleozoic drainage and missing sediment There is evidence of Late Paleozoic drainage in various parts of Gondwana. In South Africa’s Witibank Coalfield, paleovalleys in the Permian coal seams are up to 5 km in lateral extent (Cairncross et al. 1988). Late Paleozoic paleovalleys in South America rest nonconformably on Precambrian basement and do not have any evidence of glacially-influenced deposition (Fedorchuk et al. 2019). It has been inferred that westward transportation of detritus occurred via “glacial” valleys on the Yilgarn Craton and deposited into the western margin of Australia (Norvick 2004). Valleys (now paleovalleys) formed on the craton due to water draining towards the ocean. Figure 5 shows the example of the Avon River paleodrainage system in southwestern Australia, with its many large and extensive (hundreds of kilometers long) paleovalleys that in today’s semi-arid zones contain salt lakes. These vast valleys cut across crystalline basement rocks of the Archean Yilgarn Craton. This system has been inferred to have initiated in the Permian (Freeman 2001). This drainage system may have been active in Late Permian times when sea level was low (Schopf 1974; Hallam 1992). The paleovalleys in southwestern Australia have a distinctly differFigure 4. Perth Basin, southwestern Australia. A: Map showing locations of the basin’s Permian coalfields (after Le Blanc Smith and Mory 1995). B: Permo-Carboniferous lithostratigraphic column (after Mory et al. 2008). DICKENS Flood Waters Lead to Seafloor Spreading 2023 ICC 452
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