bedding directions in cross-bedded sandstones of the Collie Coal Measures indicate palaeo-currents, and so sediment transport, was almost exclusively from the south. This direction is consistent across the area and throughout the sequence, and suggests that the present Collie Basin is a remnant of a much greater area of sedimentation (Wilde and Walker 1977). A map and cross-section through the Collie Basin are shown in Fig. 9. G.Triassic drying Extensively documented in the geoscientific literature is the view that there was a time of drying from at least the later Permian and Triassic, particularly in continental interior locations. In the Permo-Triassic, it has been inferred that land area became larger and the overall climate became warmer and drier. The Early to Middle Triassic corresponds with a first-order low sea level stand during the Mesozoic and the time of maximum continental emergence (Ford and Golonka 2003). Drying is attested by lithological, paleogeographic and floral indicators. 1. Lithologies Terrestrial clastic deposits with evaporites and red beds are widespread in Triassic strata. These types of sediments occur in South America, western Europe, southwestern US, maritime Canada, northwestern Africa, and South Africa. Rift basins developed in the central and northern Atlantic region (Ford and Golonka 2003). The occurrence in Triassic strata of calcrete, gypsum, anhydrite, laterite, bauxite, red beds, lacustrine deposits and alluvial deposits are together lithological indicators of drying non-marine depositional environments (Chumakov and Zharkov 2003). Calcrete is a calcium-rich hardened layer in or on a soil. It forms today on calcareous materials as a result of climatic fluctuations in arid and semiarid regions. Calcite is dissolved in groundwater and, under drying conditions, is precipitated as the water evaporates at the surface (Brittanica. Calcrete). Arid landscapes in outback Australia today have calcrete (Chen et al. 2002). Sedimentary calcium sulphate, commonly known as gypsum, is found in nature in different forms, mainly as the dihydrate (CaSO4·2H2O) and anhydrite (CaSO4). They are products of partial or total evaporation of inland seas and lakes (Karni and Karni 1995). I have collected gypsum crystals from near the surface of modern dry salt lakes in the semi-arid Eastern Goldfields region of Western Australia. Figure 8. Timetable of Carboniferous to Jurassic environments of Gondwana’s Antarctica, South Africa, Greater India and Australia. The lacuna, or absence of sedimentary deposition, indicates a time of erosion. This is consistent with erosion beginning in an East Antarctica paleo-upland (Tewari and Veevers 1991), followed by deposition of diamictites (“glacials”) in Gondwanan basins. Deposition of the first coal measures (Coal I) occurred as sea level fell to the end-Permian minimum (Hallam 1984). Later there is the Early Triassic Coal Gap and the occurrence of Triassic redbeds. Red pigment replaced black at the gross change in environment and biota at the Permian-Triassic boundary (after Veevers and Tewari 1995). DICKENS Flood Waters Lead to Seafloor Spreading 2023 ICC 455
RkJQdWJsaXNoZXIy MTM4ODY=