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The Case for the Regressive Systems Tract


Edwards, Marc B.

Houston, TX


Until the advent of sequence stratigraphy (publications in the late 1980's), geologists commonly subdivided the clastic units of the Gulf Coast Basin into regressive and transgressive components. Sequence stratigraphy divided the regressive component into two separate units: the highstand systems tract (HST) and the lowstand systems tract (LST).

According to sequence stratigraphic theory the HST and the LST were separated by a sequence boundary, with two implications: 1. the LST of a particular sequence was younger than the HST of the preceding sequence, and 2. the two systems tracts contained facies that were not genetically related.

In the years following the introduction of sequence stratigraphy (early 1990’s), it was noted in several publications that in areas where sediment supply was active during a period of overall sea level fall, a single widespread sequence boundary did not form. Since the model necessitated the existence of a sequence boundary, stratigraphers argued over where the sequence boundary should be placed. To try to deal with this problem, the concept of “forced regression” was introduced, but this did not resolve the fundamental problems.

Uncritical application of the original (eustatically forced) sequence stratigraphic paradigm requires geoscientists to identify stratigraphic components without any basis in fact or theory. Where analysis of the data indicates that updip and downdip facies are genetically related a sequence model may be inappropriate or misleading. In these cases, the term regressive systems tract is preferred, as it removes the need to identify a conceptual artifact: the chronostratigraphically and geometrically significant sequence boundary.