ORIGIN AND APPLICATION OF THE PARASEQUENCE CONCEPT

Abstract:

The term “parasequence” first appeared in the scientific literature in a 1985 conference abstract by John Van Wagoner. The proposed parasequence concept was the result of a long period of evolution of sequence stratigraphy, with an increasing focus on smaller-scale stratal units. The concept was developed predominantly on the basis of observations made on Cretaceous coastal successions exposed along the Book Cliffs, Utah, and nearby outcrops. Since 1985, the definition of a parasequence has evolved, but key diagnostic elements remain well established and consist of: (1) a shallowing-upward facies succession, which is (2) relatively conformable, that is, without significant internal unconformities, and (3) bounded by flooding surfaces and their correlative surfaces. A key additional element for identifying parasequences in strata dominated by shallow-marine facies is the interpretation that the succession embodies overall shoreline progradation, typically under conditions of normal regression, and, if present, the deposits of subsequent flooding and retrogradation of the shoreline.

The parasequence concept is useful for constructing a physical chronostratigraphic framework, calibrating well logs, correlating outcrop, core, and well-log data, mapping lithofacies distributions, understanding depositional processes, and building quantitative models of depositional environments. Parasequences are the building blocks of larger-scale units of the sequence-stratigraphic hierarchy: parasequence sets, corresponding systems tracts, and depositional sequences. The concept is applied at a wide range of temporal and physical scales, since scale is not part of the definition. However, even sedimentologists with specific expertise in paralic successions interpret different numbers of parasequences in the same dataset. Notably, there are consistent differences in the way the parasequence concept is applied to different datasets; for example, parasequences tend to be interpreted as systematically smaller and produced over shorter periods of geological time in recent (e.g., late Quaternary) successions or higher-resolution datasets versus ancient settings with lower-resolution data.

We suggest that the parasequence definition should remain somewhat flexible, adaptable to circumstances, and open to interpretation, taking into account the goals of the investigation and available data. It is thus critical that, for each application of the parasequence concept, the specific recognition criteria and process interpretation of a parasequence be stated clearly.