EXPERIMENTAL TURBIDITY CURRENTS ENTERING DENSITY-STRATIFIED WATER - ANALOGS FOR TURBIDITES IN MEDITERRANEAN HYPERSALINE BASINS

Experimental turbidity currents entering two-layer density-stratified water behave differently from similar currents flowing over the same t opography into non-stratified water. Experiments were designed as anal ogues for flows entering Mediterranean hypersaline pools. In both the hypersaline pools and the experiments, the water density changes abrup tly across a pycnocline. Turbidity currents generated on a platform at the level of the pycnocline behaved in one of three ways as they flow ed from the platform into deeper stratified water. (1) When the bulk d ensity of the current was less than the dense water layer, the current spread at the pycnocline. The head of the current advanced rapidly wh en it lost contact with the bed. Grains settling out of the current fe ll through the dense water layer forming an extensive deposit. In natu re this behaviour will lead to 'turbidites' with sharp but non-erosive bases, strongly developed grading and no traction features. (2) When the bulk density of the current was greater than the dense water layer , the current continued as an underflow, plunging into the deeper wate r. Sedimentation lowered the bulk density of the current and the low-d ensity interstitial fluid caused the head to loft. Low-density interst itial fluid convected from the body of the current, lofting particles into the water column. These particles were hydraulically sorted durin g upward transport and subsequent settling to the floor. The resulting turbidites had a more limited extent than the deposits of either non- lofting underflows or interflows. By inference from the experiments, n atural deposits of this type may have local (proximal) erosion and tra ction features at the base and strongly graded tops. (3) In some of th e currents with high bulk density, the rising turbid water reached the pycnocline and spread at that level as a secondary interflow. The tai l of the turbidity current, which was less dense than the head and bod y of the current, flowed above the pycnocline adding momentum to the s econdary interflow. The thin non-erosive graded deposit from the secon dary interflow may extend beyond the deposits of the primary underflow . In all three cases (but more pronounced in Gases 2 and 3) the intera ction of the current with the pycnocline displaced that surface and ge nerated a wave that was reflected back and forth from each end of the pool. The waves remobilized sediment on the ramp.