Underwater canyons

Keywords Coasts Shelf Continental slope Continental footstep Underwater canyons Deep-sea floor Bottom sediments... 

Fig. 1 Topography of the coasts and floor of the Black Sea. Bottom relief 1 shelf a accumulative, b abrasive 2 continental slope a accumulative, b stepwise 3 floor of the basin 4 continental footstep 5 underwater canyons 6 bars a sandy, b marginal 7 morphological boundaries a distinct, b fuzzy. Coast types 1 landslide 2 abrasive 3 abrasive-accumulative 4 accumulative 5 lagoonal 6 deltaic...

The steepest and narrowest portions of the continental slope are confined to the Crimean coast, the Adler segment of the Caucasian coast, and to the regions off Trabzon and Zonguldak of the Anatolian coast. These parts of the continental slope are dissected by series of underwater canyons. 

Virtually over its entire extension, the continental slope of the Black Sea is dissected by numerous faults and underwater canyons. These canyons, confined to tectonic dislocations (fracture zones or grabens), are later transformed by turbidity flows, which use them as channels for the transport of mineral particulate matter from the near-shore zone to the foot of the continental slope. At the places of discharge of turbidity flows, alluvial fans are formed, which may be cut by runoff channels [10]. 

The tops of underwater canyons are confined to river mouths and form a complicated branched pattern of tributaries, which cross the shelf edge and join the main channel of the canyon within the continental slope. 

The largest of the canyons known in the Black Sea is the Danube Canyon it has a length of 220 km and eight tributaries up to 58 km long. The greatest number of tributaries (69) is distinguished in the underwater canyon system off the Bosporus Strait. 

The Danube underwater canyon system is crowned by a thick alluvial fan. The relative height of this topographic feature reaches 500 m at a width changing from 40 km in its upper part to 60 km at its base. The alluvial fan is advanced by 100 km in the southeastern direction into the western depression of the sea. At the center of the fan, one observes an underwater valley rimmed by high (up to 300-400 m) near-channel bars [11]. 

A similar pattern is also observed in the Kerch-Taman portion of the continental slope, where the system of underwater canyons features a length of the main valleys up to 280 km and a great number of secondary channels (52). 

The morphologies of the different underwater canyons have much in common. They feature V-shaped or U-shaped transverse profiles. With depth, the width of the canyons increase from 150-200 m at the shelf edge to to... 

The continental footstep of the Black Sea occupies an intermediate position between the continental slope and the floor of the central depression at depths from 1100-1200 to 1800-2000 m. Morphologically, it is represented by a slightly inclined plain that borders the base of the continental slope. It is a kind of accumulative tail formed owing to the merging of numerous alluvial fans near the mouths of underwater canyons and to the sedimentation matter supplied from the shelf and continental slope due to the sediment runoff and landslide processes. 

In the Pleistocene-Holocene, the Black Sea underwent a series of intensive transgressions and regularities. At this stage, the topographic pattern inherited by recent marine-coastal features was formed, accompanied by the appearance of the shelf, marine terraces, and systems of underwater canyons on the continental slope. 

FIGURE 2.10 A schemaKc of ocean topography. The continental shelf, which may contain submarine canyons, extends from the shoreline to the deep ocean, abruptly ending at the continental slope. From the foot of the continental slope extends a more gentle continental rise. The deep ocean contains such features as trenches, active or extinct underwater volcanoes (seamounts and guyots), and mid-ocean ridges. 

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