Edwards 2002 : The Case for the Regressive Systems Tract With Examples From the Tertiary and Pleistocene of the Northern Gulf Coast Basin. Trans. Gulf Coast Assoc. Geol. Socs. v. 52, 243-255.

The full paper is available online at the SEARCH AND DISCOVERY web site.

The standard Exxon Sequence framework - 69k - click to enlarge

Until the advent of sequence stratigraphy (late 1980's), geologists commonly subdivided the silici-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).






The regressive systems tract - 83k - click to enlarge

According to sequence stratigraphic theory, the HST and theLST are separated by a sequence boundary, with two implications: (1) the LST of a particular sequence is younger than the HST of the preceding sequence, and (2) the two systems tracts contains facies that are not genetically related.





A Yegua palgeogreographic map

In the years following the introduction of sequence stratigraphy (early 1990's), it was noted in several publications that in areas where sediment input 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.

A lower Miocene stratigraphic dip section - 52k - click to enlarge

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.


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©by Marc B. Edwards
Consulting Geologists, Inc