STACKING THE SIGNAL: HOW TO FIND THE PURE EUSTATIC CURVE

The “holy grail” of sequence stratigraphy has been a pure eustatic sea-level curve with known amplitudes. Whether this may ultimately be obtained is unknown, but Carboniferous and Permian cyclothems may provide a statistical basis for a semiquantitative answer.

In the past, the amplitude of relative sea level changes has been estimated by depositional relief and areal coverage. Comparison of relative onlap was made to adjacent cycles. This is subject to local effects of tectonism and sedimentation. In the Pennsylvanian, long series of cyclothems (effectively wave trains) have been evaluated by Heckel and others. By applying the principle of stacking wave series, as is used to remove noise from seismic signals, it should be possible to back out the eustatic signal (tectonism and local sedimentation are noise). The wave trains can be compared between biostratigraphic “pinning points”. Since this is based on statistics, larger sample sizes, from multiple basins, would improve the answer. Higher frequency cycles, locally expressed, should stack out. While the relative onlap edge of sea-level cycles is often not available, relative amplitude may be determined by biofacies, following the principles of Isrealsky (1949). Deeper water, particularly with increasingly low-oxygen conditions, exerts physical control on the distribution of organisms. This provides a semiquantitative measurement of relative water depth and the maximum depth reached, producing a wave train with relative amplitudes. Evaluation of the rate of sea level change and the relative amplitudes of low stands, although more difficult, should also be possible. Distribution of facies belts and the expression of lowstand deposits should provide clues.

Even a semiquantitative eustatic curve permits the recognition of local tectonism and sedimentation. This in turn permits improvements in the timing and magnitude of those processes, and results in improved basin modeling for maturation and migration models. It also provides a basis for exploration models, providing a framework for projecting where organic-rich facies should be and where suitable conventional and unconventional reservoir facies should occur.