ABSTRACT

Many of the straighter, less rocky parts of the southern Oregon coast are characterized by nearshore bars that extend obliquely out from shore and migrate alongshore. A typical oblique bar is attached to the foreshore at its upcurrent end, nearly parallel to shore through most of its length, and bowed seaward into a rip-channel-mouth bar at its downcurrent end. The main part of an oblique bar is separated from the foreshore by a longshore trough that curves seaward into a rip channel. The net wave-induced currents flow obliquely shoreward over the bar, parallel to shore through the longshore trough, and seaward through the rip channel and over the rip-channel-mouth bar. Medium-scale crossbedding formed by the migration of megaripples in the direction of net water flow is the dominant nternal structure in most of the nearshore. Small-scale crossbedding formed by wave-ripple migration is dominant in the inner offshore, and planar bedding is dominant on the foreshore.

Most or all of the bar deposits would be destroyed if the coast prograded slowly but continuously. The vertical sequence produced during progradation should be characterized, in ascending order, by inner offshore deposits, possibly deposits of the lower seaward slope of the bar, a subhorizontal erosion surface corresponding to the deepest part of the rip channel, rip channel deposits, longshore trough deposits, deposits of the foreshore-trough transition, and foreshore deposits,

Subhorizontal erosion surfaces probably occur in deposits formed by the slow progradation of most kinds of barred nearshore systems, but they should not occur in deposits formed by the progradation of a non-barred nearshore system. The nature of the deposits produced by the progradation of a barred nearshore system should vary with orientation of the bars relative to the shoreline, number of bars, wave energy, and available grain sizes.