ent shape in cross-section to valleys incised by glaciers (Fig. 6). Most paleovalleys on southwestern Australia’s Yilgarn Craton are missing the steep sidewalls and U-shaped valley (top cross section) which are normally expected of a valley carved by a glacier (bottom cross section). Inset valleys are generally less than a kilometer wide, and occupy a small proportion of the broad flat primary valleys. They are completely concealed and must be found with geophysical methods using the difference in gravitational or electrical properties with the surrounding bedrock (Braimridge and Commander 2005). The Permo-Carboniferous “glaciation” is said to have been the most significant geomorphic event on the Yilgarn Craton leading to paleodrainage sytems, and it was followed by subsequent repeated infillings into the Tertiary (Finkl and Fairbridge 1979). The evidence for significant Late Paleozoic terrestrial erosion on the vast Yilgarn Craton of Western Australia (Thomas 2014) raises the question of the eventual depositional site for these volumes of detritus (Sircombe and Freeman 1999). However, to date there is no known evidence of where the huge volume of eroded sediment went to. Several methods have been used to estimate the thickness of sediments eroded. These methods include coal rank (Lowry 1976), vitrinite reflectance (Le Blanc Smith 1993), and thermal modelling based on apatite fission track data (Olierook et al. 2019). These estimates of eroded thickness have ranged up to several kilometers. F. Late Paleozoic coal deposits Major coal deposits are found in strata from Mid-Carboniferous to end Permian, as well as Late Triassic to Cenozoic. It is a remarkable fact that no coal seam has yet been discovered in Early Triassic strata (hence the term “Coal Gap”) (Fig. 7) and coal seams in Middle Triassic strata are scarce and thin (Retallack et al. 1996). Over the passage of the so-called Late Paleozoic “Ice Age”, and together with fluctuations of both physical and chemical conditions operating on Earth at that time, thick and geographically extensive coal deposits began to form (Gastaldo et al. 2020). -A broad chain of large coalfields of Carboniferous age is found in the northern hemisphere. It extends from eastern Northern America, through Europe, the Russian Federation and south into China (Fig. 7). In addition, a chain of Permian coalfields is found in the more southern (or Gondwanan) continents—Australia, India, southern Africa, South America and Antarctica (Shao et al. 2020) (Figs. 7 and 8). A massive volume of organic debris was deposited and subsequently buried in Carboniferous-Permian strata. Extensive foreland and cratonic basins, formed in association with the Pennsylvanian−Permian tectonism, ensured the subsidence requisite for long-term preservation of buried organic matter (Nelsen et al. 2016). China’s Paleozoic coal-bearing basins have been inferred to be mainly large epicontinental sea basins. The shallow sea was the most important coal-forming sedimentary environment. Coastal delta and delta-detrital coast systems were important coal-forming sedimentary environments in the Late Palaeozoic. (Li et al. 2018). Such deltaic deposits host much of the world’s coal reserves. Detailed mapping of sediment increments between regionally widespread Pennsylvanian coal marker horizons in the Illinois basin shows radiating shoe-string sand bodies similar to those in the modern Mississippi delta (Selley 1988). There are numerous published records which provide evidence for Figure 5. Avon River paleodrainage system, southwestern Australia (after Freeman 2001). Figure 6. Representation of a cross section of paleovalleys in (a) southwestern Australia compared to (b) a valley incised by a glacier (after Heilbronn et al. 2018). DICKENS Flood Waters Lead to Seafloor Spreading 2023 ICC 453
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