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An introduction to Mediterranean deep-sea biology* - page 3 / 32

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Pliocene and Quaternary, from 5 Ma to the Present.

With the emergence of the modern plate tecton- ics theory, it was attempted to assess the structure of the Mediterranean basins within this new context (Maldonado, 1985; Rehault et al., 1985; Vanney and Genneseaux, 1985; Roca et al., 1999; Séranne, 1999). Specifically, following the a criteria of tec- tonic stability, Vanney and Gennesaux (1985) subdi- vide the Mediterranean area into three geomorphic styles:

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    Relatively stable regions such as the Pelagian

Sea and the Gulf of Sirte in the central Mediter- ranean Sea and the Nilotic Front in the eastern Mediterranean Sea. This geomorphic setting results from deformation of the northern boundary of the African plate and alternation of periods of high clas- tic sediment supply during humid conditions and periods of construction of carbonate platforms dur- ing arid conditions.

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    Unstable convergent regions, with an irregular

and complex configuration related to the collision between the African and European plates. Four regions are included in this style: the Mediterranean Ridge, the Arc-Trench system, the Aegean Sea and the Adriatic Sea.

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    Rifted cenozoic basins of the western Mediter-

ranean and the Tyrrhenian basins formed by intru- sion of oceanic crust and its cooling and contraction.

Morpho-sedimentology

From the sedimentological point of view the interesting feature of the geologic structure of marine basins is that their lower and lateral bound- aries are formed of sediments. Furthermore, in all areas the depositional pattern is closely related to the surrounding topography, the sediment input and the oceanographic conditions (Maldonado, 1985). The importance of each of these factors differs for each depositional province, and each province is charac- terised by a series of depositional processes and con- trolling factors: 1) terrigenous shelves that predom- inate off major deltas, although there is a relative presence of calcareous-rich areas; 2) continental slopes dominated by the interplay between mass- gravity flows and hemipelagic settling; 3) areas with submarine canyons characterised by rapid sediment bypass to deeper basin zones; 4) base-of-slope deposits described as submarine fans and other types of fans; and 5) bathyal or basin plains with abundant deposits of hemipelagic muds on structural highs and protected areas, and turbiditic muds.

From the publication of the book “The Mediter- ranean Sea: A Natural Sedimentation Laboratory” by D. J. Stanley in 1972 to the present-day study of the sedimentology and stratigraphy of the Mediter- ranean basins we can provide a series of hot-spots that focus the interest of specialists: a) the Messin- ian salinity crisis; b) the presence and significance of sapropel layers; c) megaturbidites and their occurrence in the Mediterranean Sea; d) the phe- nomenon of ‘‘mud volcanism’’ with respect to degassing of deeply buried sediments; e) the role of catastrophic sediment failures in the configuration of continental margins; f) volcanism and its influ- ence and use in the stratigraphic record; and g) the relationship between oceanographic structures and sedimentation. Some of these subjects will be intro- duced in the following sections in a more physio- graphical context.

Continental shelves

Mediterranean shelves represent less than 25% of the total area. The continental shelves are modest around most of the Mediterranean Sea (Got et al., 1985). The main continental shelves are the Tunisian Plateau located off the Gulf of Gabes, between Tunisia and Libya, and the North Adriatic Sea off the Po delta. Excluding the Tunisian Plateau, which has a structural origin, the main wide shelves are related to the sediment supply from the main Mediterranean rivers. Examples of these are the Valencia shelf related to the Ebro river, the Gulf of Lions related to the Rhone river, the Adriatic shelf related to the Po river and the shelf in front of the Nile river. The offshore boundary of continental shelves, the shelfbreak or shelfedge, is defined by the change of gradient to the basin centre. This off- shore limit is situated between 100 and 150 m depth, but reaches 260 m in the widest distance of the Nile continental shelf.

The narrow type of shelves is related to mountain belts surrounding the Mediterranean basin. These restricted shelves include the whole southern side of the Mediterranean basin, from Morocco to Turkey and Greece, only disrupted by the Tunisian Plateau and the Nile shelf. At the northern side of the basin, the southern shelves off Spain and the Balearic islands, the Corsica and Sardinia promontory and the southern shelves of the Italian peninsula are also included in the narrow shelves.

Sediments on the continental shelves show a configuration called a “sedimentary prism”. The

MEDITERRANEAN DEEP-SEA BIOLOGY 9

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