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Phanerozoic Tethys region (view source)
Revision as of 23:52, 17 February 2016
, 23:52, 17 February 2016no edit summary
During Albian time the paleoceanographic setting was characterized by a major anoxic event that can be traced all across the Albian seas.
During Albian time the paleoceanographic setting was characterized by a major anoxic event that can be traced all across the Albian seas.
[[file:M106Ch01Fig10.jpg|thumb|300px|{{figure number|10}}
[[file:M106Ch01Fig10.jpg|thumb|300px|{{figure number|10}}Global paleogeography (top) and major depositional settings in the southern margin of the Tethys (below) at the time of the Cretaceous-Paleocene boundary (about 65.5 Ma).]]
===K-T Boundary (65.5 Ma)===
At the end of Cretaceous times the present-day continents were completely defined (Figure 10). Only the northernmost Atlantic Ocean was not completely opened (rifting was still active between Greenland and Canada). Africa detached from Antarctica and India, which began its northern flight that would eventually lead to the Himalayan orogeny. The Alpine Tethys and related basins, which linked the Central Atlantic Ocean with Neo-Tethys, were in a convergent plate regime. The large and complex puzzle of blocks of Adria, Greece, and Turkey were approaching the southern margin of Eurasia, after the almost complete subduction of the Neo-Tethys Ocean. The collision between these complex assemblages of different microplates would produce the Alpine-Dinaric and Turkish orogenic belts. In the Alpine area the lower plate was represented by Eurasia, whereas east of the Alps Laurasia represented the upper plate. This change can be ascribed to the different age and origin of the subducting oceanic crust (Alpine Tethys in the Alps, Neo-Tethys in the east). The possible occurrence of minor oceanic basins (Vardar, Pindos, and Lycian oceans; Stampfli and Borel, 2002) north of the Mesogea Ocean between the Alpine Tethys and Neo-Tethys accounted for the presence of multiple verging subduction zones. To the north of the subduction-collision belt it was still possible to recognize the occurrence of backarc basins, from the Black Sea to the Caspian Sea. The progressive closure of the Neo-Tethys also affected the evolution of the passive margin of Arabia, where the Peri-Arabian Massif high delivered sediments both northward (toward the Neo-Tethys) and southward. The origin of this high was related to the approach of the lower plate (Arabia) to the southern margin of Laurasia (represented by the Sirjan blocks of central Iran) or, alternatively, to an intra-oceanic subduction zone (Stampfli and Borel, 2002). The southern margin of Arabia was probably represented by a transform separating this plate from India. South of the Peri-Arabian Massif, on the Arabian plate, sedimentation was represented by prevailing deep-sea clastics and shallow-water carbonates passing to a large coastal plain with deposition of alluvial sediments. Extensional basins with deep-sea carbonates (Sirt Basin) developed along the northern passive margin of Africa and into the Sirt gulf. Rift basins (filled by continental clastic deposits) were also present across the interior of central-eastern Africa to the south.
End-Cretaceous time recorded the last of the Big Five mass extinctions (e.g., Ward, 1990; Bambach et al., 2004), so drastic and so close in time to leave a biogeographic imprint even on modern biota (Krug et al., 2009). Extinctions happened both in the sea (marine reptile, cephalopods, foraminifers, brachiopods, sharks) and on land (dinosaurs, pterosaurs, some bird groups, marsupial mammals). However, the pattern of this extinction is still disputed, with some groups interpreting gradual decline before the K-Pg boundary, and others catastrophic die-off (Ward, 1990; Benton and Little, 1994; MacLeod et al., 1997). One of the first proposed causal mechanisms was a major asteroid impact (Alvarez et al., 1980; Ocampo et al., 2006) with proposals of craters, such as Chicxulub Crater, Yucatan (Hildebrand et al., 1991). Among other suggested triggering mechanisms are global warming and flood basalts (Deccan Traps) (Courtillot, 2005).
[[file:M106Ch01Fig11.jpg|thumb|300px|{{figure number|11}}Global paleogeography (top) and major depositional settings in the southern margin of the Tethys (below) at the time of the Eocene-Oligocene boundary (about 34 Ma).]]
===Eocene-Oligocene Boundary (about 34 Ma)===
The opening of the Atlantic and Indian Oceans was coupled with the movement of Africa toward the southern boundary of Eurasia and with the gradual closure of the Neo-Tethys Ocean (Figure 11). The rapid northward flight of India was responsible for the continental collision and the development of the Himalayas, following the complete closure of the eastern Neo-Tethys. The former complex puzzle of microplates that was present north of the Mesogea and south of the Neo-Tethys was sandwiched in the collision zone along an area stretching from the Alps to India (e.g., Dercourt et al., 1993; Barrier and Vrielynck, 2008; Moix et al., 2008). This time-transgressive collision gave rise to the orogenic belts from the Alps to Himalaya, including the Serbo-Pelagonian area, the Pontides, and the Taurus. North of the collision belt, basins such as the Carpathian Flysch Basin, the Black Sea, and the Caspian developed. After the collisions, recorded by this complex assemblage of microplates, the continued compressional regime related to the counterclockwise rotation of Africa produced the development of the northward subduction of the Mesogea Ocean (evolving into the Eastern Mediterranean Basin) below the newly accreted terranes on the southern border of Eurasia. The Peri-Arabian Massif was approaching Laurasia, which initiated development of the Zagros deformation front (Barrier and Vrielynck, 2008). The emerged area of the north-eastern side of the Arabian plate can be interpreted as the peripheral bulge of the lower plate. The docking of Arabia to Eurasia led to partial separation between the Indian Ocean to the east and the Eastern Mediterranean Basin to the west. The Arabian plate was significantly uplifted, so that the former shelf area was almost entirely exposed. Sedimentation (shallow marine carbonate passing to deep-water clastics, Kirkuk Basin) was reduced to a narrow belt along the future Mesopotamia and Persian Gulf. Northern Africa was still characterized by a passive margin facing north toward the Eastern Mediterranean Basin. Deep marine clastics were deposited in the Sirt Gulf, whereas continental deposits accumulated in present-day Egypt, Libya, and Sudan. Close to the time of the Eocene-Oligocene boundary, intense magmatic activity was recorded in the Afar area (Afar Traps). Volcanics were also deposited along the western margin of the Arabian Plate, where a rift valley, in which alluvial-lacustrine sediments were deposited, marked the beginning of the opening of the future Red Sea. A complex network of rift basins developed along the future Aden Gulf, and volcanic activity was recorded within the orogenic belts of southern Eurasia (mainly Lut Block, Central Iran, and Armenia; Barrier and Vrielynck, 2008).
==Paleogeography and petroleum plays==
The paleogeographic and tectonic evolution of the southern Tethys area during the Phanerozoic plays an important role in determining the distribution of the source rocks and reservoirs as well as the origin of stratigraphic and tectonic traps, both strictly related to the geodynamic evolution of the area. This recognition of these controls on hydrocarbon resources and accumulation of oil and gas along the Tethyn margin has been the major thrust of the publications of many geologists, including Murris, 1980; Beydoun, 1986; Beydoun, 1988; Beydoun, 1991; May, 1991; and Sharland et al., 2001.
The giant fields of North Africa, Arabia, and the Middle East reflect episodes of enhanced primary productivity with high export production and storage of this organic matter in the sedimentary successions of different types of sedimentary basins.
The factors that control primary productivity are light intensity, nutrient inputs (nitrate and phosphate), and climate (Begon et al., 1996). Today maximum productivity in the oceans is recorded on the inner shelf of the continental platforms and in ocean upwellings because of high nutrient concentration and relatively clear water (Haines, 1979; Barnes and Hughes, 1982). The paleogeographic configurations of the late Paleozoic-Mesozoic time interval is dominated by E-W oceans, particularly in North Africa, Arabia, and the Middle East, where they extended mainly from the equator to the tropics; indeed, for most of this interval the Tethyan Seaway was present at very low latitudes north of Africa and Arabia, indenting the Pangea supercontinent. Paleocurrent models for a general Pangea configuration (e.g., Kutzbach et al., 1990; Kiessling et al., 1999; Winguth et al., 2002, 2005) envisage a westward-flowing equatorial surface current which, upon reaching the continental shelves of the western Tethys Seaway, deflected southeastward and northeastward; in the meanwhile, a deep water circulation brought cold waters from high latitudes to the equator. Ocean upwellings of these cold and nutrient-rich bottom waters were created by monsoonal wind circulation (Crowley et al., 1989; Parrish, 1993; Peyser and Poulsen, 2008) along the Gondwanan margin and in the lee of continental blocks scattered in the Paleo- and Neo-Tethys, as well as at the equatorial divergence zone.
The combination of extended continental platforms in the proximity of a nutrient-delivering supercontinent and developed ocean upwellings caused the increase of primary productivity during favorable climate conditions, particularly at low latitudes where light intensity was higher and rate of mineralization (hence greater nutrient supply) more rapid. Storage of increased production as organic matter in the sediments was in turn enhanced by high sedimentation rates and availability of accommodation space.
The latitude and relative position of the Pangea and the Tethys therefore favored the deposition of source rocks, whereas the continuous and time-transgressive generation and evolution of different sedimentary basins controlled the creation of reservoirs and traps (such as those related to tectonic inversion of extensional structures), leading to the impressive concentration of oil and gas fields in this area.
The distribution of giant oil fields is related to the nature of the sedimentary basins. According to Mann et al. (2003), most of the giant oil and gas fields known until 2000 are related to continental passive margins facing the major ocean basins (34.66%), continental rifts and overlying sag basins (especially failed rifts at the edges or interiors of continents; 30.90%), and collisional margins produced by terminal collision between two continents (19.73%). These types of basins are common in the succession of North Africa and the Middle East. Due to the geodynamic evolution of this area, rift basins (mainly formed due to the opening of the Tethys oceans and to the extensional events affecting North Africa) rapidly evolved to passive margins (e.g., evolution of the peri-Gondwanan blocks) and then to active margins, with the development of collision-related basins (e.g., foredeep related to the accretion of the peri-Gondwanan blocks to the southern margin of Eurasia).
As a consequence, different types of sedimentary basins were continuously created by the movement of continental blocks, so that at any time different basin types can be recognized (e.g., divergence on the southern side of the Tethys and convergence on the Asian margin) in North Africa and the Middle East. Therefore, favorable conditions for the development of petroleum plays were almost continually present. Though there are some differences in tectonic evolution across the margin plate scale, correlations of stratigraphy and thus petroleum systems are possible (Murris, 1980; Beydoun, 1986, 1988, 1991; May, 1991; Sharland et al., 2001).
When considering the distribution of the giant oil and gas fields, two major groups of sedimentary basins can be identified: one in North Africa mainly dominated by rift, sag, and passive margins, and one in the Middle East, where oil fields are mainly preserved in sag and passive margin and collision-related basins (Al-Husseini, 2000; Ziegler, 2001). In the latter sector, oil fields are clustered in two major sets: Eastern Arabia-Persian Gulf-Zagros and Caspian Sea.
North Africa experienced several stages of alternating passive margin (e.g., Devonian, Carboniferous) and rift settings, related to different geodynamic events (e.g., effects of the Tethys opening in late Paleozoic) which recurred in this area. Rift and passive margin stages are commonly separated by local or regional compressional events (e.g., Cretaceous) with reactivation/inversion of extensional structures (Boote et al., 1998). Favorable environmental conditions controlled the deposition of major source rocks, such as the Hot Shale during the Silurian. This unit reflects the environmental changes (warming and sea-level rise) that postdated the Ordovician glaciations (Figure 3). The history of repeated rifting, sag stage, and folding favored the creation of a large number of stratigraphic and tectonic traps that store, at different stratigraphic levels, several giant fields (Boote et al., 1998; Macgregor, 1998). The presence of a wide, shallow-water shelf in an arid environment (Triassic-Jurassic) led to the deposition of thick and widespread salt layers, which represent an effcient seal at the regional scale (Boote et al., 1998).
If in North Africa the basins are mainly related to rift basins followed by passive margin, the accretion of the peri-Gondwanan blocks to the southern margin of Eurasia led to the formation of a major concentration of giant fields along the northern passive margin of the peri-Gondwanan blocks and in the overlying peripheral basins related to their collision. In the southern Caspian Sea area, the giant fields are mainly stored in collision-related basins (Mann et al., 2003) whose origin was controlled by the docking of the peri-Gondwanan along the southern margin of Eurasia (e.g., Cimmerian orogeny). A similar origin is suggested for the Northern Caucasus Basins (Mann et al., 2003), whereas a complex history (from cratonic backarc extension and rifting followed by a sag basin stage) is recorded in the Pricaspian Basin (Weber et al., 2003).
In the Arabian peninsula, Mann et al. (2003) identified three basin types preserving giant fields: continent–continent collision for the elongate fields along the Zagros Mountain front; passive margin basins of the southern shore of the Neo-Tethys (central Arabian peninsula and Persian Gulf area); and continental rifts with overlying sag basins on the eastern Arabian Peninsula. Source rocks and reservoirs are present at different stratigraphic levels, reflecting a complex interaction of depositional and tectonics events (Fox and Ahlbrandt, 2002; Pollastro, 2003).
==Conclusions==
In the Tethys region, the evolution of North Africa and the Arabian Plates are intimately involved with the occurrence of hydrocarbons in both regions. In the Early Paleozoic, paleogeography was characterized by the breakup of Rodinia and by the re-arrangement of the major continental plates in the Pangea supercontinent. During the assemblage of Pangea, a major role was played by the transformation from Pangea B to Pangea A during Permian time by means of dextral motion of Laurasia relative to Gondwana, which changed the relative position of the Paleozoic and Mesozoic domains facing the east-west oriented Tethys Gulf (Muttoni et al., 2009a, b).
Since late Paleozoic time, the southern margin of the Tethys was affected by the time-transgressive opening of the Neo-Tethys, which gave origin to a complex mosaic of peri-Gondwanan terranes. They gradually collided, during Mesozoic and Cenozoic times, with the northern margin of the Tethys, as the oceanic lithosphere of the Paleo-Tethys Ocean was subducted below Laurasia. Collisions were distributed irregularly along the northern margin of the Tethys. The spreading of the Neo-Tethys balanced the subduction of the oceanic lithosphere along the northern margin of the Paleo-Tethys, preserving the Tethys Ocean until the beginning of Cenozoic time. The subduction of the Paleo-Tethys led to the accretion of microplates that today characterize the Middle East outside of Arabia. Accretion started in Triassic time with the Cimmerian orogeny and persisted up to today, with the collision of Arabia along the Zagros suture. The present day relationships among orogenic belts are further complicated by the presence of important strike-slip movements, which accommodated the different convergence rates among plates, from the Alps to the Himalayas.
The Mesozoic and Cenozoic evolution of the Tethys Oceans was also affected by the plate reorganizations caused by the breakup of Pangea. The opening of the Atlantic Ocean further complicated the geodynamic settings of the Laurasian and Gondwanan margins due to the changes in stress fields during different stages that characterized the breakup of Pangea. In particular, the movement and rotation of Africa, controlled by the opening of the central and southern Atlantic oceans, heavily controlled the relative motions among the numerous plates (which suffered alternatively both extensional and compressional tectonic regimes) in the Tethys. The present-day setting of south Mediterranean and Middle East regions is therefore the result of the global reorganization derived from the closure of the Tethys Ocean(s) and the time-transgressive opening of the Atlantic Ocean.
Different and sometimes incompatible reconstructions exist for some intervals and, as data can be either contradictory and/or scarce, it is frequently a matter of interpretation which of several alternative reconstructions should be favored, and which should be discarded. For these reasons, paleogeographic maps should always be viewed as a work in progress, and should be continually revised and reconsidered in the light of new data. Also, differences mainly increase with the detail of the maps. Therefore, the maps presented here constitute just one of the possible solutions and do not aim to represent definitive paleogeographic reconstructions. In our opinion, these maps generally honor the available data, which mainly summarize long, complex, and multidisciplinary studies from regions that are not always easily accessible (sometimes for political reasons).
The distribution of giant oil and gas fields in North Africa, Arabia, and the Middle East is the result of the interplay between the paleogeography of oceanic and continental areas, which favored the creation of source rocks, and the geodynamic evolution of Pangea and the Tethys Oceans during the Phanerozoic. There is a range of different basin types, with a dominance of rift and sag basins, passive margin basins, and collision-related basins, often evolving from one type to another. The interplay between sedimentation and tectonics controlled both basin development and post-depositional deformation, favoring the creation of a large number of structural and stratigraphic traps that store a significant percentage of the world’s oil and gas reserves.
==See also==
==See also==