These include distribution of major fossilizable invertebrate and algal groups; marginal marine environments characterized by reduced diversity compared with fully marine environments; interpretation of continental and marginal marine environments and facies (alluvial, aeolian, lacustrine, glacial, deltaic and coastal plain deposits); recent phosphorites occurrence at depths of a few hundred metres within 400 of the equator; evaporite-bearing continental red beds; continuous pelagic deposition and widespread marine anoxicity; and heavier carbon-isotope values in marine calcareous organisms and organic matter (Hallam 1992). Hallam’s highest first-order sea level peak is in the Ordovician. This is consistent with the peak in marine carbonate dominance that occurs during the Great American Carbonate Bank and is unrivalled in the Phanerozoic of North America. More than 80% of the sedimentary rock packages in the Late Cambrian and Early Ordovician there are marine carbonates. The second-highest first-order sea level peak of the Phanerozoic is inferred to be in the Cretaceous. This is consistent with the Cretaceous being a time of flooding of low-lying parts of stable cratonic areas of the world, but not completely covering the continents. This was during a time of seafloor spreading, with hot expanded midocean ridges displacing water onto land to form interior seaways such as the Western Interior Seaway in North America, the Eromanga Sea in Australia, trans-Asian and trans-African interior seaways. At the Paleozoic-Mesozoic boundary a marked first-order sea level low is inferred. This is consistent with a great volume of literature providing evidence for a time of widespread drying from at least the later Permian and Triassic, particularly in continental interior locations. I use the terms primary (or first order) receding waters and primary (or first order) marine transgression. This is because the first order global sea level curve may be overprinted by less areally extensive tectonic effects in various regional areas giving secondary and higher order, higher frequency sea level changes. Higher order cycles of sea level may relate to more regional or local causes including orogenic (mountain building) and tectonic events (uplift, downwarp and thrusting), basin filling as sediments were eroded from uplifts, timing and rates of plate motions, mid-oceanic ridge growth rates and subsidence of mid-ocean ridges. Laterally-extensive sandstone beds in the late Paleozoic and Mesozoic of North America can be related to energetic runoff as mountains were built during the Late Paleozoic Sonoma Orogeny and Alleghanian Orogeny, and Mesozoic rifting of Atlantic Ocean and Cordilleran orogenies (Dickinson et al. 1983). Each of today’s continents may have had distinctive regional tectonism and associated higher order sea level changes. The Early Devonian regional marine regression of eastern North America and Europe is an example of a higher order sea level change associated with the Acadian orogeny. Smaller unconformity-bounded units include Carboniferous “cyclothems” which have been referred to as fourth-order sea level cycles (Ross and Ross 1987). APPENDIX C MEGASEQUENCES REFLECT REGIONAL TECTONISM I commend Dr Tim Clarey and co-workers for gathering the data of the dimensions, especially volumes (from mapped areal extents and thicknesses), of the megasequences around the world. However, I respectfully disagree with the assertion that the lateral extent, volume, and thickness of the megasequences may indicate the height of sea level (Clarey and Werner 2018). Regional tectonism and higher sedimentary volumes I believe that regional tectonism (along with non-marine versus marine indicators of the formations themselves) needs to be taken into account. The greater dimensions, especially of the later three megasequences, are associated with times of greater regional tectonism at a time of seafloor spreading, together with mountain-building on active continental margins. With greater topographic relief due to tectonism (such as mountain-building), and with rain, the more the erosive runoff and the greater volume and thickness of sediment deposited. Key examples follow (consistent with Fig. 21): Africa – From the Jurassic onwards (Zuni and Tejas megasequences), Gondwana rifted resulting in the African plate as we now know it, with most tectonic activity controlled by extension and hotspot activity (Dirks et al. 2003). To a first approximation, Africa has experienced a single, long-duration cycle of erosion since the start of the Jurassic (Burke and Gunnell 2008). South America – From the Jurassic (Zuni and Tejas megasequences) tectonic activity was caused by the breakup of Gondwana. Intracratonic basins received lithic fill derived from recently formed reliefs (mountain building such as the Andes) (Mabesoon et al. 1981). North America – From the Jurassic (start of Zuni megasequence), Pangea breakup was underway (including opening of the Atlantic Ocean) along with Cordilleran mountain building (Dickinson 2004). Marine carbonates dominated the first three megasequences (to the Mid-Carboniferous) but from Mid-Absaroka, clastics dominated (Peters 2006). Note: Neoproterozoic sedimentary strata on the western margin of North America can be of the order of 10 km in thickness (Dickens and Hutchison 2021b). Such strata are a good indicator for the uniquely enormous erosion and peneplanation of the supercontinent, rather than an indicator of high sea level at that time. Europe – This continent has a complex tectonic and sedimentary history. The Kaskasia megasequence is associated with the Variscan fold belt. The Absaroka megasequence is associated with the beginning of the splitting of Pangea and the formation of new rifts and their filling. Zuni and Tejas megasequences are associated with the transtensional zone between Europe and Africa since the Jurassic, when the Atlantic Ocean opened (Plant et al. 2005). Asia – The first three megasequences (to Mid-Carboniferous) may represent shallow seas whereas the last three megasequences have high volume (Clarey 2022) at a time of continental drift and detritus runoff from continental mountains. Tejas has the highest volume due to runoff from the Himalayan mountains (Clarey 2022) which arose when India collided with Asia. “Tectonic setting is the principal controlling factor of lithology, chemistry, and preservation of sediment accumulations in their depocenters, the sedimentary basins.” (Veizer and Mackenzie 2014, p. 402). The preservation of the sedimentary record is a function of tectonic setting, with sediments on continental crust surviving back into the Precambrian, while the continuous record of passive margin sediDICKENS Flood Waters Lead to Seafloor Spreading 2023 ICC 474
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