Gravity Flow Evolution During Transition From By-pass to Deposition: Results From High Resolution Seafloor Observations of a Modern Sand Lobe System

Akhmetzhanov, Andrey M.¹, Neil H. Kenyon¹, E. L. Habgood¹, C. O'Byrne¹, Russell B. Wynn¹, M. K. Ivanov¹, UK Taps Group¹ (1) Southampton Geological Centre, Southampton, United Kingdom

An 8 km long active sand lobe system has been discovered in a small deepwater intraslope basin, at a water depth of 1500 m, in the Gulf of Cadiz, eastern Atlantic Ocean.

The lobe system was mapped in its entirety using a multibeam echosounder, a deep-towed high-resolution sidescan sonar (100 kHz) and a seismic profiler (5 kHz). The resulting plan view pattern is spectacularly detailed, with source, by-pass and depositional areas resolved on a meter scale.

The source area is represented by a small valley which cuts into a sand-rich sequence. Multiple scours and displaced slabs of sand are recognised on acoustic images on the slopes and floor of the valley, indicating frequent failures of outcropping sands.

The valley opens onto a gentle slope with a gradient of about 1-2°, across which a system of incised channels develops. Features observed on acoustic data indicate seabed erosion and widespread overbanking, which highlights the turbulent state of by-passing gravity flows.

The depositional area on the basin floor comprises a complex bifurcating system of narrow aggradational sinuous channels with characteristic marginal levees. According to their morphology, and by comparison with subaerial analogues, they are formed by quasi-steady sand-rich hyperconcentrated gravity flows. Channel plugging and avulsion are the dominant processes responsible for the formation of the distributary pattern.

The diversity of seabed features observed during this integrated study shows that turbulent flows initially develop on high gradient slope and then evolve into hyperconcentrated sandy debris flows upon reaching the flat basin floor.