Abstract

Where repeated sharp changes of sedimentation are inescapable, periodic cycles are commonly invoked to preserve uniform, orderly variations from some supposed norm. The sedimentary record rarely reflects such uniformity, however, as sedimentologists have gradually realized. Magnitude versus frequency of processes has received little attention in sedimentology. Recurrence interval, recovery time, and preservation potential are critical factors for evaluating their significance for the sedimentary record. Large-magnitude processes, which represent positive deviations from the norm and are rare on the human time-scale, must be significant over geologic time. Everyday processes may have obliterated much of the evidence. The sedimentological importance of rare events is difficult to assess because the record of such events may be very subtle. This is especially true if a deposit has been thoroughly bioturbated or if the record of an event is simply an erosional surface. It has now become possible to evaluate quantitatively ancient episodic sedimentation using modern-process rates as well as refined biostratigraphic and isotopic dating. I predict that such evaluation will necessitate revision of our favorite depositional models, which have become so important for exploration as well as for research.--Modified journal abstract.

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