Along-slope and down-slope sediment transport processes in the Gulf of Cadiz

• Main Author: Habgood, Edward Leslie
• Format: Thesis
• Language: English
• Published: University of Southampton 2003
• Subjects: North Atlantic
• Online Access: https://eprints.soton.ac.uk/464983/; https://eprints.soton.ac.uk/464983/1/904788.pdf

A study of a 9000 km 2 region of the seafloor of the Gulf of Cadiz between 35 o 30'N and 36 o 20'N by 6 o 30'W by 8 o 15'W, using an integrated geophysical and sedimentological dataset, gives new insights into sediment deposition from thermohaline bottom currents and sediment gravity flows. In this area, the Mediterranean Outflow (MO) begins to mix with North Atlantic waters and separates into along-slope geostrophic, and down-slope ageostrophic, components. Interpretation of TOBI and SeaMap sidescan sonar reveals changes in bedform morphology across the study area which indicate a decrease in the MO's peak velocity from >1 m s -1 to <0.5 m s -1 as it spreads along and down the slope. Piston coring reveals that the associated sediment waves form a continuum from sand waves to muddy sand waves to mud waves. Coring also reveals much sandier 'contourite' facies than previously documented. From the sidescan sonar imagery, a series of down-slope oriented channels, formed by the MO, are observed where the MO starts to descend the continental slope at a water depth of ~700 m. These channels are up to 40 km long, have gradients of < 0.5 o , a fairly constant width of ~ 2 km and a depth of ~ 75 m. Sand waves move down the channels and they have mud-wave covered levees similar to those seen in turbidite channel-levee systems, although the channel size and levee thickness do not decrease downslope as in typical turbidite channel systems. The channels terminate abruptly where the MO lifts off the seafloor. Below several of these bottom current channels, are sediment gravity flow channels with lobes that terminate in intra-slope basins. Each gravity flow system has a narrow, slightly sinuous channel, up to 20 m deep, feeding a depositional lobe up to 7 km long. The lobes are composed of dendritic distributory channels. Massive, fine sands with very low clay content ~2.0 wt%, have been recovered from the lobes and are interpreted to have been deposited by hyper-concentrated stratified sediment gravity flows.