Abstract

The "spaced stratification" that characterizes some thick deep-water sandstones consists of a stack of layers 5-10 cm thick that are inversely graded at their base and structureless at their top. These layers have been attributed to deposition from a succession of collapsed inertia layers (traction carpets). Prior to collapse, each layer was believed to have been maintained by shear from the overriding turbidity current. This depositional model is based on the mistaken assumption that the distribution of shear stress in such a traction carpet is indistinguishable from the distribution of shear stress in a grain flow on a steep slope. Instead, shear stress in an externally sheared traction carpet should be approximately constant from the top to the base of the layer, and deposition should be a gradual process, leaving no record of sequentially active inertia layers. Inertia layers that do form under large turbidity currents are probably never as thick as 5-10 cm, based on available data from shear-cell experiments. Instead, inertia layers are probably about 1 cm thick, similar to the thickness of the inversely graded basal part of each layer in field examples of "spaced stratification". The rest of each inversely graded stratum probably forms due to rapid fallout from suspension. The basal, inversely graded nearly equal 1 cm is believed to record strong shear that may accompany impingement on the bed of large turbulent eddies. Hence, "spaced stratification" may not be a structure produced beneath steady flows, but may record strongly fluctuating hydrodynamic conditions and vigorous burst/sweep cycles in large turbidity currents.

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