Abstract

The stratigraphic record on a continental shelf commonly contains sediment introduced by background deposition, modified by extreme events, and reworked by the activities of benthic organisms. If the mean frequency of extreme events times the mean thickness of the storm-affected layers is large relative to the rate of background deposition, then the storm deposits will be amalgamated. If this product is also large compared to the rate of reworking of these sediments by marine organisms, or if the mean storm deposit thickness is greater than the depth of reworking, then the significant stratigraphic signature will be of the extreme events. The situation where a storm-generated stratigraphy dominates is typical of the dynamic region just seaward of the offshore bar. Early in major storms, sediment is transported toward this bar crest from both sides, causing the bar to grow in height; eventually, however, during the events that leave a stratigraphic signature, the largest waves begin breaking seaward of the bar top. Relatively rapid shoaling on the offshore side causes the largest of these waves to plunge, taking material from the bar face into suspension and creating a field of turbulence that partly supports the suspended sediment. The result is a bulk-fluid-density-induced pressure gradient that overwhelms the mean onshore bottom flow responsible for building the bar in the first place, producing a pulsating density current with a net motion in the offshore direction. The largest waves break at random locations on the offshore side of the bar, resulting in an erosional topography characterized by irregular swales that are quickly draped with material settling out of suspension. Calculations using conditions typical of the northern California to southern Washington shelf indicate that scours several tens of centimetres in depth with spacings of several metres would be draped with a scries of layers, each of which ranges from a millimetre to nearly a centimetre in thickness. The predicted strati-graphic record would be similar in style and scale to the hummocky stratification found in Cenozoic and Cretaceous rocks exposed along this coast.