|University of Georgia||Geology Department||Stratigraphy Lab||Steven Holland|
Many of sequence stratigraphic surfaces can serve as useful time-markers. Parasequence boundaries are commonly useful correlation horizons in local studies. Because individual parasequences may look so similar, long-distance correlation of parasequence boundaries is prone to error and must be checked with other means of correlation.
For depositional sequences, the transgressive surface and maximum flooding surface can also be useful correlation markers, at least within a basin. Because the transgressive and maximum flooding surfaces are defined by changes in stacking patterns (from progradational to retrogradational and from retrogradational to progradational, respectively), they are sensitive to regional changes in sediment supply and long-term accommodation driven by differences in subsidence rate. Consequently, correlation of these two surfaces over long distances becomes increasingly less reliable.
The sequence boundary has attracted the most attention as a potentially correlatable and chronostratigraphically significant surface. By chronostratigraphically significant, it is meant that all rocks overlying the sequence boundary are younger than all rocks below the sequence boundary, throughout its extent. Although this is true along a cross-section parallel to depositional dip, it is less certain along strike or in different sedimentary basins. Clearly, tectonically produced sequence boundaries will be of different ages in different basins. Early studies suggested that eustatically generated sequence boundaries coincide with time of maximum rate of fall in eustatic sea level and are therefore chronostratigraphically significant. However, more recent studies suggest that the timing of the sequence boundary can vary from the time of maximum rate of fall to the time of the lowest position of eustatic sea level. In particular, faster tectonic subsidence rates and higher rates of sediment supply may cause the timing of the sequence boundary to be delayed. If these modeling results are correct, then the sequence boundary could differ in age by as much as 1/4 of the duration of a eustatic cycle.
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