Plate 54

Climbing bed forms: ripple-drift lamination produced by currents

Bed load and suspended load can give both separate and combined deposits. Distinct beds often alternate in time on the same place, as we have already seen in many examples in the first section. On the other hand, traction and fallout can cooperate in accumulating a single, almost simultaneous deposit. When bottom traction of coarse material is accompanied by deposition from suspensions, tractive structures still form but acquire some peculiar features that allow us to recognize the role of the suspended load. They are called traction-plus-fallout structures.

Traction and fallout can act together but are also competitive mechanisms: the more particles settle, the more hampered are their movements on the bed. In other terms, with a few particles in suspension, one gets purely tractive structures; with a high particle concentration and a high rate of sedimentation, traction tends to be suffocated. The picture shows an intermediate condition, where tractive laminae are well developed and fallout subordinate. The bed is a turbidite sandstone, i.e., the product of a catastrophic flow, and it can be assumed that most of this sand was transported in a turbulent suspension: to keep sand suspended, a high turbulence is needed, which implies a high flow velocity.

The continuous addition of sand to the bed makes it accrete while ripples are migrating on it. How this sand is distributed determines the geometry of the resulting structure (lamination). If the sand is deposited on the lee side of ripples only, while the stoss side is kept free or eroded, every ripple advances by climbing on the back of the adjacent one. An incremental ramp thus forms, whose cumulative expression is a diagonal surface, dipping opposite to the flow. Such surfaces develop from each ripple crest; they are parallel and sometimes more pronounced than the foreset laminae, thus giving a false impression of inclined bedding. This effect is not apparent in this picture: surfaces of climbing, as they are called, are sharper in plate 8.

To the right of the pen, climbing ripples "degenerate" into oversteepened forms (see also plate 118); this means that sand grains locally stuck together and responded as a coherent mass to current drag.

Other examples of climbing ripple cross-lamination can be seen in color photos 7 and 16. In photo 7, the sand is very fine and mixed with silt, and dark hues in the lower foreset and bottomset laminae indicate concentrations of vegetal matter and clay.

Marnoso-arenacea Formation, northern Apennines.