Supercritical bedforms in deep-marine lower-slope, base-of-slope and proximal basin-floor setting, Middle Eocene Ainsa Basin, Spanish Pyrenees

Abstract:

There is an increasing appreciation that seafloor gradients in many ancient deep-marine clastic environments are likely to have been steep enough to promote upper flow regime (Froude Number >1) processes and deposits. This issue was addressed by Komar (1971) who concluded that for a reasonable friction factor, f = 0.02, turbidity currents would be supercritical on slopes > 0.5°, a value exceeded on many basin-margin slopes and on the upper parts of submarine fans. Many deep-water depositional systems, therefore, probably contain significant volumes of deposits that have been misinterpreted using the classic Bouma T_a-T_e divisions for a waning flow rather than as upper flow-regime (supercritical) bedforms. Using outcrop examples from lower-slope, base-of-slope and proximal basin-floor settings in the Middle Eocene Ainsa Basin, Spanish Pyrenees, linked with recent experimental and theoretical work, this paper considers the range of deposits that can be reinterpreted as upper-flow regime bedforms in what might be termed "supercritical clastic systems". Such sedimentary structures involve a very wide range of grain sizes and include discontinuous and irregular sandwaves, backset (upstream-inclined) bedding in mounded gravels and sands and scour-and-fill structures. Irrespective of varying relative base level in the staging area for such flows, supercritical clastic systems show complex facies relationships.

Komar, P.D. 1971. Hydraulic jumps in turbidity currents. Geological Society of America Bulletin, 82, 1477–1488.