Print this pageErosional Depressions Lacking Fluvial Deposits: Incised Valleys or Tidal-Ravinement Scours?
DALRYMPLE, ROBERT W.
Incised valleys are defined as lying on a sequence boundary because they are cut by fluvial action during a relative sea-level fall. However, many elongate, erosional features in the stratigraphic record are interpreted as valleys even though they lack fluvial deposits (and hence evidence of a downward shift of facies) at their base. Four modern examples provide potential analogues for such features: 1) some of the Scheldt estuaries in the Netherlands; 2) isolated, empty "deeps" in the south-central North Sea; 3) elongate, surficial depressions offshore from Torres Strait; and 4) a channel cutting the delta front of the Fly River delta. The present form of all of these features was created by tidal erosion during transgression, either at the coast or on the shelf. In some cases tidal scour has removed the easily eroded fill of a pre-existing, fluvially-cut valley, but in many instances, the erosion is unrelated to an older valley. These features are as large as valleys (incision depth 5-20 m; length >10 km; width >2-5 km), are associated with regional erosion surfaces, and (will) have an onlapping fill; therefore, they might be regarded as valleys although many are not. In cases where fluvial deposits have either been removed (it was a once valley) or were never present (the feature has a tidal-ravinement origin), estuarine or shelf sediments now lie directly on the erosion surface. Thus, erosional features lacking fluvial deposits should not be interpreted automatically as incised valleys. The presence of a downward shift of facies remains the most important criteria for the correct identification of incised valleys and sequence boundaries.