Changes

Late Ordovician paleogeography ([[:file:M106Ch01Fig03.jpg|Figure 3]]) is represented by Cocks and Torsvick,<ref name=Cocksandtorsvik_2002 /> Robardet,<ref name=Robardet_2003>Robardet, M., 2003, The Armorica “microplate": Fact or fiction? Critical review of the concept and contradictory palaeobiogeographical data: Palaeoeography, Palaeoclimatology, Palaeoecology, v. 195, p. 125–148.</ref> and Ruban et al.,<ref name=Rubanetal_2007>Ruban, D. A., Al-Husseini, M. I., and Yumiko, I., 2007, Review of Middle East Palaeozoic plate tectonics: GeoArabia, v. 12, no. 3, p. 35–56.</ref> and shows that most continental blocks/terranes were located in the southern hemisphere except for Siberia and Tarim, which were entirely within the northern hemisphere. South China lay across the equator. The major oceans were not too large to prevent biotic exchange; thus the biota is quite cosmopolitan.

Late Ordovician paleogeography ([[:file:M106Ch01Fig03.jpg|Figure 3]]) is represented by Cocks and Torsvick,<ref name=Cocksandtorsvik_2002 /> Robardet,<ref name=Robardet_2003>Robardet, M., 2003, The Armorica “microplate": Fact or fiction? Critical review of the concept and contradictory palaeobiogeographical data: Palaeoeography, Palaeoclimatology, Palaeoecology, v. 195, p. 125–148.</ref> and Ruban et al.,<ref name=Rubanetal_2007>Ruban, D. A., Al-Husseini, M. I., and Yumiko, I., 2007, Review of Middle East Palaeozoic plate tectonics: GeoArabia, v. 12, no. 3, p. 35–56.</ref> and shows that most continental blocks/terranes were located in the southern hemisphere except for Siberia and Tarim, which were entirely within the northern hemisphere. South China lay across the equator. The major oceans were not too large to prevent biotic exchange; thus the biota is quite cosmopolitan.

The collision of Avalonia and Baltica occurred during Late Ordovician time, as documented by paleomagnetic, tectonic, isotope, and faunal data.<ref name=Cocksandtorsvik_2002 /> <ref name=Robardet_2003 /> Baltica, after the accretion of Avalonia, was positioned at intermediate latitudes NW of the Northern Gondwana margin and could have deflected the South Equatorial current southward. The Iapetus Oceanic lithosphere was subducting beneath the Laurentian active margin and its width was decreasing. The Rheic Ocean was several thousands of km wide with Perunica in the northern part, having probably detached from NW Gondwana in mid-Ordovician time. According to von Raumer and Stampfli,<ref name=Vonraumerandstampfli_2008 /> the Rheic Ocean was subducting beneath the Peri-Gondwanan blocks placed along the northern margin of Gondwana. The Panthalassic Ocean was very large and mostly covered the northern hemisphere. Cocks and Torsvick (2002) suggest this ocean was comparable in size to today’s Pacific. Peri-Gondwanan blocks were located along the Gondwanan margin at high to intermediate southern latitudes. However, some faunal data (e.g., Villas et al.<ref name=Villasetal_2002>Villas, E., Hammann, W., and Harper, D. A. T., 2002, Foliomena fauna (Brachiopoda) from the Upper Ordovician of Sardinia: Palaeontology, v. 45, p. 267–295.</ref>) suggest lower latitudes, as well as the existence of peripheral blocks detaching from North Gondwana. The position and the architecture of the Armorica composite plate is still discussed (e.g., Robardet<ref name=Robardet_2003 />). Nysæther et al.<ref name=Nys&aelig;theretal_2002 /> suggested that by the Late Ordovician, it is possible that part of Armorica had rifted off the NW Gondwanan margin; however, Robardet (2003) casted doubts on the reliability of the paleomagnetic data on which the evolution of the Armorica was based and proposed a different scenario in which the southern European blocks remained attached to the northern Gondwanan margin from Ordovician to Devonian<ref name=Robardet_2003 /> ([[:file:M106Ch01Fig09.jpg|Figure 9]]).

The collision of Avalonia and Baltica occurred during Late Ordovician time, as documented by paleomagnetic, tectonic, isotope, and faunal data.<ref name=Cocksandtorsvik_2002 /> <ref name=Robardet_2003 /> Baltica, after the accretion of Avalonia, was positioned at intermediate latitudes NW of the Northern Gondwana margin and could have deflected the South Equatorial current southward. The Iapetus Oceanic lithosphere was subducting beneath the Laurentian active margin and its width was decreasing. The Rheic Ocean was several thousands of km wide with Perunica in the northern part, having probably detached from NW Gondwana in mid-Ordovician time. According to von Raumer and Stampfli,<ref name=Vonraumerandstampfli_2008 /> the Rheic Ocean was subducting beneath the Peri-Gondwanan blocks placed along the northern margin of Gondwana. The Panthalassic Ocean was very large and mostly covered the northern hemisphere. Cocks and Torsvick (2002) suggest this ocean was comparable in size to today’s Pacific. Peri-Gondwanan blocks were located along the Gondwanan margin at high to intermediate southern latitudes. However, some faunal data (e.g., Villas et al.<ref name=Villasetal_2002>Villas, E., Hammann, W., and Harper, D. A. T., 2002, Foliomena fauna (Brachiopoda) from the Upper Ordovician of Sardinia: Palaeontology, v. 45, p. 267–295.</ref>) suggest lower latitudes, as well as the existence of peripheral blocks detaching from North Gondwana. The position and the architecture of the Armorica composite plate is still discussed (e.g., Robardet<ref name=Robardet_2003 />). Nysæther et al.<ref name=Nys&aelig;theretal_2002 /> suggested that by the Late Ordovician, it is possible that part of Armorica had rifted off the NW Gondwanan margin; however, Robardet<ref name=Robardet_2003 /> casted doubts on the reliability of the paleomagnetic data on which the evolution of the Armorica was based and proposed a different scenario in which the southern European blocks remained attached to the northern Gondwanan margin from Ordovician to Devonian<ref name=Robardet_2003 /> ([[:file:M106Ch01Fig09.jpg|Figure 9]]).

Global climate deteriorated at the end of Ordovician time, resulting in the Hirnantian glacial episode. The glaciation is documented by sedimentary evidence and isotopic data<ref name=Brenchleyetal_1994>Brenchley, P. J., Marshall, J. D., Carden, G. A. C., et al., 1994. Bathymetric and isotopic evidence for a short-lived Ordovician glaciation in a greenhouse period: Geology, v. 22, p. 295–298.</ref> and lasted about 0.5–1 million years. Peri-Gondwanan and Gondwanan glacial deposits occur in North Africa (where a N-S high was present in Egypt<ref name=Schandelmeierandreynolds_1997>Schandelmeier, H. and Reynolds, P. O., eds., 1997, Palaeogeographic-palaeotectonic atlas of north-eastern Africa, Arabia, and adjacent areas: Balkema, Rotterdam, 160 p. 17 pls.</ref>), South America, Arabia, and South Africa, and periglacial features are known also from Armorica and Avalonia. Several interpretations have been offered on the distribution of the ice caps during the Hirnantian glaciation (a single large ice cap vs. a number of smaller ice caps), as summarized in Veevers.<ref name=Veevers_2004 /> This glaciation followed a period of climatic amelioration along the Northern Gondwana margin, evidenced by deposition of temperate bioclastic limestones and pelmatozoan-bryozoan mud-mounds, which overlie a very thick terrigenous succession of Early-Middle Ordovician age. The change from pre-Hirnantian “greenhouse” climates to Hirnantian “icehouse” conditions was rapid and was not preceded by any climatic fluctuations, which might have helped acclimatize the biota to the climate change.<ref name=Brenchleyetal_1994 /> If the pre-Hirnantian benthos was widespread in epicontinental seas and inland basins, the Hirnantian shelly fauna (e.g., Sutcliffe et al.;<ref name=Sutcliffeetal_2001>Schandelmeier, H. and Reynolds, P. O., eds., 1997, Palaeogeographic-palaeotectonic atlas of north-eastern Africa, Arabia, and adjacent areas: Balkema, Rotterdam, 160 p. 17 pls.</ref> Jin and Copper<ref name=Jinandcopper_2008>Jin, J., and Copper P., 2008, Response of brachiopod communities to environmental change during the Late Ordovian mass extinction interval, Anticosti Island, eastern Canada: Fossils and Strata, v. 54, p. 41–52.</ref>) was mostly restricted to the continental margins, due to the sea-level drop caused by the glaciations in the latest Ordovician. The Hirnantian glaciation seems to have occurred during times of very high levels of the greenhouse gas CO2 (14–18 times the present atmospheric value). Brenchley et al.<ref name=Brenchleyetal_1994 /> considered that the onset of glaciation was the result of an early Hirnantian increment in burial rates of organic carbon acting as a major sink for the atmospheric CO2. However, according to Villas et al. (2002), the accumulation of great volumes of carbonates in the pre-Hirnantian late Ordovician served as the sink of the atmospheric CO². At the end of the Hirnantian, the ice cap melting caused a rapid, eustatic sea-level rise and the development of low-oxygen conditions on the shelves.<ref name=Rongandharper_1988>Rong, J.-Y., and Harper, D. A. T., 1988, A global synthesis of the latest Ordovician Hirnantian brachiopod faunas: Transactions of the Royal Society of Edinburgh Earth v. 79, p. 383–402.</ref> <ref name=Owenandrobertson_1995>Owen, A. W., and Robertson, D. B. R., 1995, Ecological changes during the end-Ordovician extinction: Modern Geology, v. 20, p. 21–39.</ref> The end of the glaciation was followed by the deposition of organic-rich shales (Lower Silurian “hot shales”) which represent the most important source rocks in North Africa and one of the major in the Arabian peninsula.<ref name=Luningetal_2000>Luning, S., Craig, J., Loydell, D. K., Storch, P. B., and Fitches B., 2000, Lower Silurian ‘hot shales’ in North Africa and Arabia: Regional distribution and depositional model: Earth Science Review, v. 49, p. 121–200.</ref>

Global climate deteriorated at the end of Ordovician time, resulting in the Hirnantian glacial episode. The glaciation is documented by sedimentary evidence and isotopic data<ref name=Brenchleyetal_1994>Brenchley, P. J., Marshall, J. D., Carden, G. A. C., et al., 1994. Bathymetric and isotopic evidence for a short-lived Ordovician glaciation in a greenhouse period: Geology, v. 22, p. 295–298.</ref> and lasted about 0.5–1 million years. Peri-Gondwanan and Gondwanan glacial deposits occur in North Africa (where a N-S high was present in Egypt<ref name=Schandelmeierandreynolds_1997>Schandelmeier, H. and Reynolds, P. O., eds., 1997, Palaeogeographic-palaeotectonic atlas of north-eastern Africa, Arabia, and adjacent areas: Balkema, Rotterdam, 160 p. 17 pls.</ref>), South America, Arabia, and South Africa, and periglacial features are known also from Armorica and Avalonia. Several interpretations have been offered on the distribution of the ice caps during the Hirnantian glaciation (a single large ice cap vs. a number of smaller ice caps), as summarized in Veevers.<ref name=Veevers_2004 /> This glaciation followed a period of climatic amelioration along the Northern Gondwana margin, evidenced by deposition of temperate bioclastic limestones and pelmatozoan-bryozoan mud-mounds, which overlie a very thick terrigenous succession of Early-Middle Ordovician age. The change from pre-Hirnantian “greenhouse” climates to Hirnantian “icehouse” conditions was rapid and was not preceded by any climatic fluctuations, which might have helped acclimatize the biota to the climate change.<ref name=Brenchleyetal_1994 /> If the pre-Hirnantian benthos was widespread in epicontinental seas and inland basins, the Hirnantian shelly fauna (e.g., Sutcliffe et al.;<ref name=Sutcliffeetal_2001>Schandelmeier, H. and Reynolds, P. O., eds., 1997, Palaeogeographic-palaeotectonic atlas of north-eastern Africa, Arabia, and adjacent areas: Balkema, Rotterdam, 160 p. 17 pls.</ref> Jin and Copper<ref name=Jinandcopper_2008>Jin, J., and Copper P., 2008, Response of brachiopod communities to environmental change during the Late Ordovian mass extinction interval, Anticosti Island, eastern Canada: Fossils and Strata, v. 54, p. 41–52.</ref>) was mostly restricted to the continental margins, due to the sea-level drop caused by the glaciations in the latest Ordovician. The Hirnantian glaciation seems to have occurred during times of very high levels of the greenhouse gas CO2 (14–18 times the present atmospheric value). Brenchley et al.<ref name=Brenchleyetal_1994 /> considered that the onset of glaciation was the result of an early Hirnantian increment in burial rates of organic carbon acting as a major sink for the atmospheric CO2. However, according to Villas et al. (2002), the accumulation of great volumes of carbonates in the pre-Hirnantian late Ordovician served as the sink of the atmospheric CO². At the end of the Hirnantian, the ice cap melting caused a rapid, eustatic sea-level rise and the development of low-oxygen conditions on the shelves.<ref name=Rongandharper_1988>Rong, J.-Y., and Harper, D. A. T., 1988, A global synthesis of the latest Ordovician Hirnantian brachiopod faunas: Transactions of the Royal Society of Edinburgh Earth v. 79, p. 383–402.</ref> <ref name=Owenandrobertson_1995>Owen, A. W., and Robertson, D. B. R., 1995, Ecological changes during the end-Ordovician extinction: Modern Geology, v. 20, p. 21–39.</ref> The end of the glaciation was followed by the deposition of organic-rich shales (Lower Silurian “hot shales”) which represent the most important source rocks in North Africa and one of the major in the Arabian peninsula.<ref name=Luningetal_2000>Luning, S., Craig, J., Loydell, D. K., Storch, P. B., and Fitches B., 2000, Lower Silurian ‘hot shales’ in North Africa and Arabia: Regional distribution and depositional model: Earth Science Review, v. 49, p. 121–200.</ref>