Morphology of a Saline Gravity-Flow Channel on the SW Black Sea Shelf from Multibeam Bathymetry, and Preliminary Monitoring of Transiting Currents
R. N. Hiscott1, A. E. Aksu1, R. D. Flood2, D. R. Parsons3, J. Peakall3, and D. Mouland1
1Memorial University of Newfoundland, St. John's, NL, Canada, [email protected], [email protected], [email protected]
2Stony Brook University, Stony Brook, NY, [email protected]
3University of Leeds, UK, [email protected], [email protected]
Seawater of Aegean (Mediterranean) origin exits the Bosphorus Strait north of Istanbul and forms a persistent underflow across the Black Sea shelf. The density contrast between the underflow and the ambient water is ~0.01 g cm-3, like that of low-concentration turbidity currents in the deep sea. The underflow has created a prominent seafloor channel system.
Immediately north of the Bosphorus exit there is a 20–30 m-deep single channel which hooks to the left forming a prominent bend. Before reaching the middle shelf, channel relief decreases and some of the saline underflow spills overbank into 2–3 secondary crevasse channels. Although flow confinement is reduced on the middle shelf, it increases farther seaward as levees become more prominent and channel courses become better defined. An anastomosed channel network accommodates the underflow on the outer shelf and has been described by Flood et al. (2008 Sedimentology). There, first-order channels are 5–10 m deep and are associated with local lateral-accretion bedding, muddy in-channel streamlined barforms, a variety of sediment waves, and levee/overbank deposits younger than ~7.5–8.0 ka.
This channel network is one of the largest and most accessible natural laboratories on Earth for the study of channelised density currents. The shallow water depth and quasi-continuous discharge provide a better opportunity to monitor flow conditions than in turbidity-current channels, even those created by the discharge of mine tailings (e.g., Hay 1987 J. Geophys. Res., Normark and Dickson 1976 Sedimentology, Normark 1989 J. Sed. Petrol.). An acoustic Doppler current profiler (ADCP) towed across the 20–30 m-deep bend recorded a maximum velocity of ~1 m s-1. There, the saline underflow is density-stratified, exhibits outer-bank super-elevation, and the sense of secondary flow is apparently opposite to that in rivers (cf. Corney et al. 2006 Sedimentology). Future 3D flow monitoring will use the British autonomous vehicle AutoSub3.
AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009