Morphology and evolution of an anastomosed channel network where saline underflow enters the Black Sea



The Bosphorus Strait accommodates two-way flow between the Aegean and Black Seas. The Aegean (Mediterranean) inflow has speeds of 5 to 15 cm sec−1 in the strait and a salinity contrast of ∼12‰ to 16‰ with the Black Sea surface waters on the shelf. An anastomosed channel network crosses the shelf and in water deeper than 70 m is characterized by first-order channels 5 to 10 m deep, local lateral accretion bedding, muddy in-channel barforms, and a variety of sediment waves both on channel floors and bar crests, crevasse channels entering the overbank area and levée/overbank deposits which are radiocarbon-dated in cores to be younger than ∼7·5 to 8·0 ka. This channel network accommodates the saline density current formed by the Mediterranean inflow. The density contrast between the density underflow and the ambient water mass is ∼0·01 g cm−3, similar to the density contrast ascribed to low-concentration turbidity currents in the deep sea. Channel-floor deposits are sandy to gravelly with local shell concentrations. Low-relief bedforms on the channel floor have relatively straight crests, upflow-dipping cross-stratification, heights 1 to 1·5 m and wavelengths 85 to 155 m. Bankfull flows are subcritical, so these probably are not antidunes. Bar tops are ornamented locally with mudwaves having heights 1 to 2 m and wavelengths ∼20 to 100 m; these are potentially antidunes formed under shallow overbank flows. Towards the shelf edge, the degree of channel bifurcation increases dramatically and bar tops are dissected locally by secondary channels, some of which terminate in hanging valleys. Conical mounds on the shelf (possibly mud volcanoes or sites of fluid seepage) interact with the channel network by promoting accretion of muddy streamlined macroforms in their lee. This channel network may be one of the largest and most accessible natural laboratories on Earth for the study of continuously flowing density currents. Although the driver is salinity contrast, the underflow transports sufficient sediment to form levée wedges and large streamlined barforms, and presumably transports sediment into deep water.