Imaging the Seascapes of the Mediterranean

Imaging the Seascapes of the Mediterranean BY DAVID AMBLÀS, MIQUEL CANALS, GALDERIC LASTRAS, SERGE BERNÉ, AND BENOÎT LOUBRIEU he Mediterranean Sea is a relative newcomer to Earth’s landscape. Due to its complex tectonic history, this mid-latitude sea is composed of a cluster of basins. Their seascape is in most cases dominated by geologically young structures, but Talso by sedimentary processes. Among the latter, sedimentary processes related to the dynamics of the largest rivers in the Mediterranean (Ebro, Rhône, Po, Danube, and Nile) stand out. This overview article illustrates the main sedimentary processes and their products contributing to shape the Mediterranean seascape within a source-to- sink approach. To highlight this approach, this article mainly focuses on one of the EUROSTRATAFORM project study areas: the northwestern Mediterranean. Th is article has been published in Oceanography, Volume 17, Number 4, a quarterly journal of Th e Oceanography Society. Copyright 2003 by Th e Oceanography Society. All rights reserved. Reproduction of any portion of this article by photo- 144 Oceanography Vol.17, No.4, Dec. 2004 copy machine, reposting, or other means without prior authorization of Th e Oceanography Society is strictly prohibited. Send all correspondence to: [email protected] or 5912 LeMay Road, Rockville, MD 20851-2326, USA. STRATA FORMATION ON EUROPEAN MARGINS Oceanography Vol.17, No.4, Dec. 2004 145 146 Oceanography Vol.17, No.4, Dec. 2004 STRUCTURAL BACKGROUND OF OVERALL PHYSIOGRAPHY OF THE MEDITERRANEAN THE MEDITERRANEAN After a complex tectonic history of The Mediterranean Basin occupies an thrusting and rifting involving the Eur- area of almost 2.6 million km2, with an asian, African, and Arabian plates, the average water depth of approximately northward motion of Arabia fi nally 1,500 m. The Mediterranean Sea is com- closed the link between the eastern parts posed of a series of small basins (Figure of the Mediterranean and the remnants 1) involving a large number of conti- of the Tethys Ocean in the late Miocene, nental fragments. These basins, which around 9 million years ago. From this time onwards, the Mediterranean devel- David Amblàs is Ph.D. Student, GRC Geo- oped the enclosed basin physiography ciències Marines, Universitat de Barcelona, that is seen today. Spain. Miquel Canals (miquelcanals@ub. The Mediterranean Sea is a geo- edu) is Professor, GRC Geociències Marines, logically active region as shown by its Universitat de Barcelona, Spain. Galderic seismic and volcanic activity, and the Lastras is Associate Professor, GRC Geo- uplift of present-day landmasses. The ciències Marines, Universitat de Barcelona, landscape thus continues to develop in Spain. Serge Berné is Senior Research a highly dynamic manner, which has Scientist, Institut français de recherche pour important consequences for this region’s l’exploitation de la mer (IFREMER), Plouzané continental margin. Some of the main Cedex, France. Benoît Loubrieu is Research geological structures in the Mediterra- Assistant, Institut français de recherche pour nean are clearly refl ected in its present l’exploitation de la mer (IFREMER), Plouzané seafl oor morphology. Cedex, France. Figure 1. Topography and bathymetry of the Mediterranean region at one minute (latitude by longitude) grid resolution produced from the GEBCO Digital Atlas published by the British Oceanographic Data Centre on behalf of the International Ocean Commission, International Hydrographic Organization, and British Oceanographic Data Centre (2003). Th e Mediterranean Sea is composed of several individual basins. Th e hypsographic curves for these basins tell us about the morphogenetic processes that have shaped them. Color bars represent the distribution of the areas at 500 m depth intervals, and white dots illustrate the depth cumulative curves. As we observe in the hypsographic curves, we can group the Mediterranean basins into: (a) Shelf basins, with depths usually less than 500 m (Adriatic Sea, Aegean Sea, Marmara Sea, Strait of Sicily); (b) Margin-dominated basins, with a balanced areal distribution of continental shelf, slope, rise, and abyssal plain sections (western Mediterranean basins in general, and the Black Sea); (c) Deep basins, predominated by abyssal plains (Tyrrhenian Sea, Ionian Sea, and Levantine Basin). Th e physiographic distribution of each basin mainly results from the interplay between tectonic and sedimentary evolution processes. Th e whole Mediterranean hypsographic curve shows an overall balanced distribution of the main submarine physiographic provinces. Bold black lines show the location of the topographic profi les (Figure 2) along the main river systems, from their catchment areas to the deeper parts of the associated submarine continental margins. EbR: Ebro River; RoR: Rhône River; PoR: Po River; DaR: Danube River; NiR: Nile River. Oceanography Vol.17, No.4, Dec. 2004 147 are very different in terms of dimen- westernmost end of the western Mediter- time Alps, where mountain slopes drop sions, physiography, and geologic evolu- ranean area there is the only connection almost straight into the sea. Larger con- tion, can be grouped into western and of the present Mediterranean Sea to the tinental shelves, more than 50 km wide, eastern sets, separated by the relatively global ocean through the Gibraltar Strait. are present off the Ebro and Rhône shallow Strait of Sicily. The Black Sea is This passage, only 14 km wide, exerts an Rivers mainly due to the progradation an isolated basin adjacent to the eastern important control on the water circula- of deltaic systems (Figure 2). The con- Mediterranean Sea. tion in the Mediterranean Sea and has tinental shelf off the north of Tunisia is The western Mediterranean has an largely determined its singularity. also wide, but in this case, it is because area of approximately 0.9 million km2, The continental shelves tend to be of structural control. Bathyal plains oc- and it includes the Alboran Sea, the Al- narrow off the southern and northern cupy large areas in the western Mediter- gerian-Balearic Basin, the Catalano-Ba- Iberian Peninsula, the Balearic Islands, ranean. These are located between the learic Sea, the Gulf of Lions, the Ligurian Corsica, Sardinia, the western Italian Balearic Islands, north of Africa and Sar- Sea, and the Tyrrhenian Basin. At the coast, northern Africa, and the Mari- dinia with depths reaching 2,800 m, and Figure 2. Topographic profi les along the main river systems of the Mediterranean, from catchment areas to the deeper parts of the associated continental margins. See Figure 1 for location. Th e profi les have been drawn in a way that they extend onshore and off shore from coastline for ease of comparison. Yellow rect- angles show the extent of the present-day emerged deltaic plains. Because of the length of the Nile and Danube Rivers, which are over 6,650 km and 2,850 km, respectively, their upper courses and headwaters are not represented. Th e Ebro, Rhône, Po, Danube, and Nile Rivers are characterized by large hydrographic basins and by large associated deltas that have prograded for tens to hundreds of kilometers. Th e submarine shape of these fl uvial-infl uenced margins is highly variable. Th e enclosed, shallow character of the Adriatic Basin largely determines the large-scale geometry of deposits contributed by the Po River. Th e Ebro, Rhône, Danube, and Nile Rivers, which develop without major physiographic constraints, have developed prograding continental shelves and slopes leading to wide continental rises. Note the diff erences between the sizes of the emerged deltaic plains. 148 Oceanography Vol.17, No.4, Dec. 2004 in the Tyrrhenian basin with depths up Because of its geographic and geologic to 3,430 m. These bathyal plains formed variability, the Mediterranean may be during the thinning of continental crust that took place in the rifting phases when used as an almost perfect laboratory for the Balearic Islands, Corsica, and Sardin- studying landscaping processes on ia rotated away from the Eurasian conti- nent, and the Tyrrhenian Sea opened. submarine environments. Conversely, the eastern Mediterra- nean has a highly varied physiographic character. It includes the Strait of Sicily, basin are narrow off Peleponnese, Crete, into different units that are interlinked the Adriatic Sea, the Ionian Sea, the Le- and southern and northern Turkey. by the fl ux of sediment: emerged lands, vantine Basin, the Aegean Sea, and the However, they are well developed par- continental shelf, continental slope, con- Marmara Sea. The eastern Mediterra- ticularly to the east of Tunisia, in the area tinental rise, and basin fl oor. nean occupies an area of approximately directly under the infl uence of depos- Taking a general overview of the 1.7 million km2. These seas and basins its from the Nile River delta, and in the Mediterranean region topography we are underlain by oceanic crust under Adriatic, where large portions are shal- note that, apart from the deltaic zones their main area, and thinned continental lower than 100 m (Figure 2). The Aegean of large rivers, the coastlines are mostly crust below their peripheries. This sec- Sea is also relatively shallow, a result of surrounded by mountain ranges (Figure tor is more diffi cult to describe in terms its relatively young crust rather than high 1). Only the coastal plains from eastern of physiography than the western basins. sediment input. Basin fl oors are deeper, Tunisia to the Sinai Peninsula are free of There are important tectonic structures but smaller, than those in the western mountains. The existence of surround- that determine the morphology and basin. Maximum depths are up to 4,200 ing mountains largely determines the development of the basins. The main m in the Ionian Abyssal Plain and 3,200 seascape of the Mediterranean since it structures clearly visible in the bathym- m in the Herodotus Abyssal Plain. makes the basin boundaries very steep, etry of the eastern Mediterranean are The Black Sea is an inland sea segre- thus impeding the development of settle- the Hellenic Trench and the Mediter- gated from the eastern Mediterranean, ments.

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