AAPG Geoscience Technology Workshop

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Settling Velocity of Sands in Hypersaline Solutions. An Experimental and Modeling Approach. The Messinian Paradigm


During the last decades, subaqueous mass-transport processes (slides, slumps, debris flows, and turbidites) and sediment flows (debris flows, turbidity flows, fluidized sediment flows and grain flows) closely associated with sub-marine fans are well documented. It is proven that sediment flows can transport large quantities of sand prone material in deep water. They are separated by time duration into short-term events (minutes to several hours or days, earthquakes, high sand deposition), intermediate-term events (100s to 1000s years, tectonic events, intermediate sand deposition) and long-term events (1000s to 1000000s of years, low stands, low sand deposition). Logically, such phenomena should have occurred during the Messinian Salinity Crisis in a saline-hypersaline solution with very different results regarding the sediments deposition. It is generally accepted that waters of evaporitic basins get stratified. An existing example of a stratified evaporitic basin is the Dead Sea. Also, stratified waters exist today in well protected deep basins in the Mediterranean. These brine waters emerge from Messinian salt dissolution. During the Messinian, the Mediterranean basin was isolated from the Atlantic because the narrow connection of Gibraltar was entirely closed. This unique event turned the whole Mediterranean into a hypersaline giant because of the negative water budget. Sea-level drawdown because of high evaporation rates forced the great rivers (Rhone and the Nile) of the era to create enormous deep canyons. The rivers, in their effort to adapt to the new base level, transported vast amounts of clastic materials that were deposited in low stand deltas. These newly created low stand deltas would reasonably be unstable, creating sediment flows in a stratified column of water. The purpose of this study is to investigate the alteration in settling velocity of sands in hypersaline solutions and correlate the results with the presence of clean sand sheets far away from their source area in the deep setting. This study presents a mathematical solution about the settling velocity of sand grains in fresh water, sea water and hypersaline (brine) water. Results have been tested in experimental laboratory conditions by using the simulated settling speed of different grain sizes to extract some useful conclusions on the settling conditions in hypersaline environments. This was achieved by using a digital video recorder in 30 frames/second (1 frame / 0.02 sec), measuring the distance traveled by the sand particles in a tube illuminated by a laser beam. Τhe radiation from the laser beam was reflected by the sand particles making them easily visible. The experiments lead to the conclusion that settling velocity in brine-water produces better shorting of coarse and medium-sized sands. Settling speed of the fine sand fractions is about 50% slower in hypersaline water than in fresh and sea-water. On the contrary, settling speed of coarse sand fractions is only 20% slower in hypersaline water than in fresh and sea-water.