Plate 57

Traction-plus-fallout structures: vertical sequence in flood deposits

Waning flows carrying a great amount of suspended sediment are usually associated with catastrophic phenomena like gravity-driven turbidity currents, storm or tsunami waves at sea, fluvial floods. Tractive transport represents only a moment, a phase in the development of mechanical and hydraulic conditions promoted by such surgelike events. They strike a tranquil bottom with concentrated energy and pass by like a train, leaving a residual turbulence in their tail. During their passage, the local energy decreases, more or less rapidly, after the initial peak. A surgelike, catastrophic flow leaves a quite distinct signature in its deposit, especially if it is turbulent. An example is a turbidite layer, which we first met in plate 8. Another is presented here, and derives from a fluvial flood as the one in plate 56.

Flood waters made a breach in a levee of a channel; water passing through this breach, or crevasse, formed a secondary current that expanded in the adjacent alluvial plain. The flow expansion (jet effect) caused a slowing down of the current and a loss of transport capacity. Sediment was thus abandoned in a deposit called crevasse splay.  Deposition can be preceded in these cases (as in other cases of catastrophic flows) by erosion, which is reflected in a sharp base of the bed. Coarse sand and gravel can be strewn on this erosional surface, as a part of the bed load can escape from the main channel through the crevasse (when levees are overtopped, instead, only suspended load gets out). Finer sand and then mud can follow, thus forming a graded bed.  The grading of the grain size and the sequence of traction-plus-fallout structures (analogous to the "Bouma sequence" of turbidites, see plate 8) constitute the signature of the waning current. Plane-parallel laminae are here on top of coarser textured, low-angle laminae, and are followed by ripple-drift cross-lamination. Above the white undulated surface, which is the top of the event, another bed starts directly with tractive structures (dune cross-bedding).

Gravel pit in Recent alluvial deposits of the Jarama river, near Madrid, Spain.