Orders of stratigraphic sequences are being used loosely and with widely varying definitions. The orders seem to be subdivisions of convenience rather than an indication of natural structure. It is proposed that, at least at time scales of 103–106 yr, sequences and systems tracts are scale-invariant fractal features in which units bounded by exposure surfaces and units bounded by flooding surfaces are about equally likely. Support for this conceptual model derives from the well-known facts that (1) sediment architecture is largely scale invariant over a wide range of scales in time and space; (2) first-order trends of sea-level movements and sedimentation rates are fractal on all geologically relevant time scales; and (3) detailed studies in carbonates indicate approximately equal abundance of flooding surfaces and demonstrable exposure surfaces among unit boundaries. Finally, traces of prograding shelf edges that step up and step down in response to sea-level changes are demonstrated in this study to have a fractal nature.

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