OVERVIEW: Neoproterozoic-Paleozoic geography and tectonics: Review, hypothesis, environmental speculation
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The ever-changing distribution of continents and ocean basins on Earth is fundamental to the environment of the planet. Recent ideas regarding pre-Pangea geography and tectonics offer fresh opportunities to examine possible causative relations between tectonics and environmental and biologic changes during the Neoproterozoic and Paleozoic eras. The starting point is an appreciation that Laurentia, the rift-bounded Precambrian core of North America, could have been juxtaposed with the cratonic cores of some present-day southern continents. This has led to reconstructions of Rodinia and Pannotia, supercontinents that may have existed in early and latest Neoproterozoic time, respectively, before and after the opening of the Pacific Ocean basin. Recognition that the Precordillera of northwest Argentina constitutes a terrane derived from Laurentia may provide critical longitudinal control on the relations of that craton to Gondwana during the Precambrian-Cambrian boundary transition, and in the early Paleozoic. The Precordillera was most likely derived from the general area of the Ouachita embayment, and may have been part of a hypothetical promontory of Laurentia, the “Texas plateau,” which was detached from the Cape of Good Hope embayment within Gondwana between the present-day Falkland-Malvinas Plateau and Transantarctic Mountains margins. Thus the American continents may represent geometric “twins” detached from the Pannotian and Pangean supercontinents in Early Cambrian and Early Cretaceous time, respectively—the new mid-ocean ridge crests of those times initiating the two environmental supercycles of Phanerozoic history 400 m.y. apart. In this scenario, the extremity of the Texas plateau was detached from Laurentia during the Caradocian Epoch, in a rift event ca. 455 Ma that followed Middle Ordovician collision with the proto-Andean margin of Gondwana as part of the complex Indonesian-style Taconic-Famatinian orogeny, which involved several island arc-continent collisions between the two major continental entities. Laurentia then continued its clockwise relative motion around the proto-Andean margin, colliding with other arc terranes, Avalonia, and Baltica en route to the Ouachita-Alleghanian-Hercynian-Uralian collision that completed the amalgamation of Pangea. The important change in single-celled organisms at the Mesoproterozoic-Neoproterozoic boundary (1000 Ma) accompanied assembly of Rodinia along Grenvillian sutures. Possible divergence of metazoan phyla, the appearance and disappearance of the Ediacaran fauna (ca. 650–545 Ma), and the Cambrian “explosion” of skeletalized metazoans (ca. 545–500 Ma) also appear to have taken place within the framework of tectonic change of truly global proportions. These are the opening of the Pacific Ocean basin; uplift and erosion of orogens within the newly assembled Gondwana portion of Pannotia, including a collisional mountain range extending ≈7500 km from Arabia to the Pacific margin of Antarctica; the development of a Pannotia-splitting oceanic spreading ridge system nearly 10 000 km long as Laurentia broke away from Gondwana, Baltica, and Siberia; and initiation of subduction zones along thousands of kilometres of the South American and Antarctic-Australian continental margins. The Middle Ordovician sea-level changes and biologic radiation broadly coincided with initiation of the Appalachian-Andean mountain system along >7000 km of the Taconic and Famatinian belts. These correlations, based on testable paleogeographic reconstructions, invite further speculation about possible causative relations between the internally driven long-term tectonic evolution of the planet, its surface environment, and life.This paper contains 50 maps which have been designed for use by the geologic community in preparing paleogeographic, biogeographic, climatologic, and tectonic reconstructions of the Paleozoic periods. Seven maps for each of seven Paleozoic intervals are included, plus a suture map showing the outlines of the Paleozoic continents in their present positions. The intervals chosen are the Late Cambrian (Franconian), Middle Ordovician (Llandeilian-earliest Caradocian), Middle Silurian (Wenlockian), Early Devonian (Emsian), Early Carboniferous (Visean), Late Carboniferous (Westphalian CD), and Late Permian (Kazanian). The paleomagnetic information used to orient the continents is given. For each interval, three types of maps are included, one locality map with place names labelled, four paleogeographic maps with our interpretation of the distribution of mountains, lowlands, shallow seas, and deep oceans, and two outline maps for those who prefer to make their own paleogeographic interpretations. Several projections are used-Mercator, Mollweide, and stereographic polar-to suit the various requirements of paleogeographic work.
Devonian
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Devonian
Late Devonian extinction
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Currently available Palaeozoic palaeomagnetic data from Gondwanan continents can be interpreted in terms of either (a) a migration of the pole from northern Africa to southern Africa between Ordovician and late Palaeozoic times, or (b) a rapid excursion of the pole from northern Africa to southwest of South Africa during late Ordovician to early Silurian times, followed by a return to central Africa in late Devonian times, thereafter continuing southward again. With respect to this uncertainty, pertinent stratigraphical evidence from western Gondwana includes the distribution of glacial deposits and cold-water and warm-water faunas. This record, although meagre and to some extent contradictory, appears to favour a drift history consistent with the second (b) of the APW alternatives that involves a rapid southerly excursion of the pole by early Silurian times.
Devonian
Excursion
Late Devonian extinction
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Stratigraphic and geographic distribution, systematic description, S. flexuosa, Arinskian (lower Permian), Western Australia
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