QUATERNARY TURBIDITES IN A NERITIC ENVIRONMENT: WELL CNR VE 1, VENICE, ITALY

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Abstract
Turbidites are generally deep‐sea deposits. By contrast, the turbitdites which form a large part of the nearly 1000m Quaternary section, cored in well CNR Venice 1, do not seem to have been deposited at sea depths much greater than 200m. Sea depth, determined mostly by ecological analyses of faraminifera, gradually decreases as the basin was filled with sediments. In the upper part of the well, above 302 m, turbidite sedimentation was replaced by alternating fluvitile and very shallow marine deposition.In well CNR Venice 1, turbite characteristics vary with the sea depth and the gradient of sea slopes. If near riveer mouths, the slope gradient is sufficient, sediments debouching from rivers readily form turbity currents. The displaced fauna in sand and in shale turbidites is fresh‐water. When gradients decrease, sediments are distributed along the coast and form submarine deltas or coastal shelves. If these shelves are limited by a sandy slope, turbidity currents may originated on this slope. Displaced fauna in the sand turbidites is marine littoral. If sediments of the slope limiting the coastal shelf are silt and clay, muddy turbidity currents can originate provided the gradient is 1% or more.A critical factor of sedimentation of Venice neritic turbidites is the very fine grain‐size of the sands. Turbidity current suspensions probably were stable as they could be supported by a moderate bottom turbulence. Suspensions therefore were not likely to settle rapidly when the gradient decreased to form fan‐like deposits. They flowed to the bottom of the basin where they spread and made their thickest deposits.Turbidites similar to Venice Quaternary turbidites may be common in ancient neitic basins, where gradients were sufficiently steep. These turbidites can form good petroleum reservoirs as well as impervious shale beds and permeability barriers.