percontinent has been inferred from lithologies and flora (Chumakov and Zharkov 2003). Drying conditions are indicated by the wider paleolatitudinal spread of evaporite deposits in the Triassic compared with Permian strata (Retallack et al. 1996). Sedimentary evaporites are rocks precipitated from saturated surface or near-surface brines by hydrologies driven by solar evaporation (Warren 2006). 3. Flora The onset of the Coal Gap at the Permian-Triassic boundary was a time of extraordinarily low sea level as determined by both sequence stratigraphy and the percentage of marine sedimentary cover (Hallam 1984; Haq and Schutter 2008; Peters 2011). Marine shelfal habitat area was reduced and a great area of land emerged. This included an enormous inner land with an extreme and arid climate, where many plant species which could resist heat and aridity became more prevalent (Chumakov and Zharkov 2003). The proportion of plant types buried in post-Paleozoic strata is very different to those of the Paleozoic. Post-Paleozoic plants are dominantly plants that need less water compared with plants that were dominant in the Paleozoic strata (Fig. 7). Gymnosperms dominated, including many different forms of conifers. In addition, the woodrich conifer and angiosperm-dominated source floras of Cenozoic coals differ radically from the mostly nonwoody floras of the Carboniferous (Nelsen et al. 2016). Worldwide Jurassic coals have a higher proportion of flora generally better suited to drier land environments than flora of the previously mentioned Permian coals (Retallack et al. 1996; Orem and Finkelman 2003). The proportion of plant types buried in Jurassic strata is totally different to those of the Permian. For example, the Jurassic vegetation of Australia was predominantly made up of abundant southern conifers (Turner et al. 2009). Conifers do not grow in swamps, but in drier environments. 4. Mass extinction A review has concluded that that of all the causal factors proposed to account for mass extinctions, marine regressions associated with sea level fall is the one that correlates best throughout the Phanerozoic (Jablonski 1986). As previously mentioned, the end Permian has been inferred to have had the lowest sea level of the Paleozoic and even of the entire Phanerozoic. This would imply significant marine habitat loss. The Paleozoic terminated in a complex environmental catastrophe and mass extinction of life. This sharp paleobiological division led Phillips (1840) to introduce the term Mesozoic (middle life, with Triassic at the base) between the Paleozoic (old life, ending with the Permian) and Kainozoic (now Cenozoic; recent life, after the Cretaceous). The latest Permian to earliest Triassic strata record the progressive disappearance of up to 80% of marine genera, pronounced negative carbon-isotope and strontium-isotope anomalies, massive flood basalts of the Siberian Traps, widespread anoxic oceanic conditions, a major sea-level regression, and exposure of shelves, a “Chert Gap” and “Coal Gap,” and replacement of reefal ecosystems with microbial-dominated carbonate precipitation. The majority of ecosystems did not fully recover until the early Middle Triassic (Gradstein et al. 2020). H. Post-Paleozoic seafloor spreading 1. Successive phases of continental rifting and seafloor spreading Initial continental breakup began in the Early Triassic (Muller et al. 2019). It is remarkable that this coincided with a time of sea level low and continental drying, as previously described. The rifting continued and intensified in the Late Triassic. Rifting and breakup of Pangea is inferred to have taken place in stages (Fig. 10) (de Lamotte et al. 2015). The initial rifting included the development of rift basins about the north Atlantic and eastern Indian Oceans. The central North Atlantic and Australian Northwest Shelf Figure 10.Stages of seafloor spreading (after https://www.britannica.com/ science/plate-tectonics/Development-of-tectonic-theory. Accessed 8 February 2022) • Mid-Triassic is before modern-day oceans opened up significantly. • Upper Jurassic shows the initial rifting between West and East Gondwana, and separation of the Americas. • Mid-Cretaceous shows the progressive isolation of Gondwanan landmasses and northward migration of India. • By the Oligocene, Australia and Antarctica had separated. Figure 11.Change of paleocoastline length through time. Note the abrupt increase in the Mesozoic (after Kocsis and Scotese 2021). DICKENS Flood Waters Lead to Seafloor Spreading 2023 ICC 457
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