ABSTRACT

Clastic shelf sequences are generally considered to prograde uniformly, producing relatively simple Waltherian facies stacking patterns. Modeling of depositional kinematics by applying sediment mechanics indicates that the Waltherian view of progradation is not correct. Rather, clastic shelf sequences prograde by the building of successive new shelf profiles culminating in the capture of the shoreface environment and construction of new barrier island systems offshore of the previous shoreface. This results in localized pods of grainsize facies with a characteristic spacing equivalent to the spacing of the barrier islands.

The spacing between barrier island systems is determined by water temperature, grainsize distribution, and relative rates of sediment accumulation and shelf subsidence. The instantaneous stable profile of a shelf is created by the interaction of areal distributions of dynamic grain stabilities with the statistical distribution of grainsizes. Addition of sediment disrupts the equilibrium and results in the establishment of a new stable shelf profile, with the eventual result that a new shoreface is built and the prior shoreface abandoned. Increasing the water temperature or shelf subsidence rate increases the resulting barrier island spacing, whereas increasing mean grainsize or sedimentation rate decreases the spacing.

Nonlinear progradation provides an explanation for the deviations from Waltherian layer-cake facies stratigraphy seen in many sections. The deviations from layer-cake stratigraphic packages have led to rapid-eustacy models (such as the "punctuated aggradational cycle") to account for "missing" shoreface facies. Our model, on the other hand, shows that continuous progradation produces the observed variable facies stacking and that the locations of the facies can be accurately predicted.