Unusual sedimentary structures in the Oregon coastal dunes

Abstract

The Oregon coastal dunes have evolved in a temperate, rainy climate with a bimodal onshore wind regime. The south-west winds are the strongest of the two major wind groups, and coincide with the winter rainy season. These winds cause wet sand accumulations on north-facing slipfaces, that eventually fail in the form of huge slump plates. The plates have distinctive features that should make it possible to identify them in ancient rocks. The rainy climate maintains a high water table in the dunes. This water table emerges in places to form freshwater interdunal ponds and sabkhas that also leave distinctive deposits. The alternation of the strong south-west winds during the rainy winter season with the persistent but weaker north-west winds of summer causes episodic advance (winter) and scour and retreat (summer) of the large ‘oblique’ or transverse ridge dunes for which the area is famous. An upwind to downwind exposure of a transverse ridge dune carved by Tenmile Creek revealed a complex internal structure consisting of north-dipping sets of cross strata formed by south-west winds separated by gently sloping bounding surfaces formed by wind reversals. This internal structure is similar to that hypothesised for compound bedform or ‘draa’ deposition in unidirectional wind regimes, because the appearance is that of multiple bedforms migrating down the lee slope of a larger bedform. This pattern of bedform development may be relatively common, as bimodal and complex wind regimes are common throughout the world. The internal structure of the Tenmile Creek dune provides a modern example of bounding surfaces in a draasized dune that are formed by wind reversals. As has already been pointed out bounding surface models cannot always be predicated on continuous forward movement of bedforms. The draa model of compound bedforms may possibly be expanded to include single bedforms with multiple slipfaces in time due to seasonal changes in wind regime.