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The Case for the Regressive Systems Tract with Examples from
the Tertiary and Pleistocene of the Northern 
Gulf
Coast Basin*
by
Marc B. Edwards1
Search and Discovery Article #40063 (2002)
*Adapted for online presentation from article of the same title by the same author at the GCAGS 52nd Annual Convention, Austin, Texas, October 30-November 1, 2002. Appreciation is expressed both to the author and to GCAGS.
1Marc B. Edwards Consulting Geologists, Inc., Houston, Texas (www.marcedwards.com; [email protected])
Until the advent
of sequence stratigraphy (late 1980s), geologists commonly subdivided the
siliciclastic 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 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 contain facies that are not genetically related. In the
years following the introduction of sequence stratigraphy (early 199Os), 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 houndary,
stratigraphers argued over where the sequence boundary should he placed. To try
to deal with this problem, the concept of “forced regression” was introduced,
hut 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 he 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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uLate Pleistocene shelf to shelf margin uSequence stratigraphy of Late Pleistocene uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin uSequence stratigraphy of Late Pleistocene uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin uSequence stratigraphy of Late Pleistocene uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin uSequence stratigraphy of Late Pleistocene uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin uSequence stratigraphy of Late Pleistocene uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin uSequence stratigraphy of Late Pleistocene uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin uSequence stratigraphy of Late Pleistocene uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin uSequence stratigraphy of Late Pleistocene uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin uSequence stratigraphy of Late Pleistocene uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin uSequence stratigraphy of Late Pleistocene uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin uSequence stratigraphy of Late Pleistocene uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin uSequence stratigraphy of Late Pleistocene uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin uSequence stratigraphy of Late Pleistocene uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin uSequence stratigraphy of Late Pleistocene uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin uSequence stratigraphy of Late Pleistocene uLate Pleistocene shelf margin to slope uTertiary depositional systems
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Thumbnails with Abbreviated Captions
Click here for sequence of the phases of progradation and minibasin initiation.
Figure 1. Depositional elements and significant surfaces in a shelf to
shelf margin setting characterized by high sediment
Building paleogeographic maps (which represent, by implication, a time
interval) requires the ability to delimit, both vertically and
laterally, a corresponding rock-stratigraphic interval. Sequence
stratigraphy provides principles for recognizing time-significant
stratigraphic surfaces, thereby improving stratigraphic prediction.
However, observations on Cenozoic Another property of a sequence boundary that is unsatisfactory in a The main purpose of this paper is to argue that the above assumptions do not apply to regressive sequences formed in a high-sediment-supply regime where sediment is delivered to the shoreline during sea level fall. This argument applies as well to revised sequence-stratigraphic models (the “forced regression” described by Posamentier et al., 1992; Hunt and Tucker, 1992; Kolla et al., 1995; Hunt and Tucker, 1995; Kolla et al., 2000) which replaced the single sequence boundary with a family of sequence boundaries of the next highest order (shorter duration). One of the most confusing aspects of sequence stratigraphy is the multiple use of the key terms. Thus a rock unit can be variably interpreted as: (1) having been deposited during a particular time interval, (2) having been deposited during a particular phase of the sea level cycle, or (3) having been deposited in a particular paleogeographic location. Without additional clarification by the user, confusion can result. I view the sequence-stratigraphic model as unambiguously including both chronostratigraphic and sea level connotations: “Sequence stratigraphy is the study of genetically related facies within a framework of chronostratigraphically significant surfaces” (Van Wagoner et al., 1990, p. l), and “. .Vail proposed eustasy as the predominant driving mechanism for sequence evolution (Vail et al., 1977)” (Van Wagoner et al., 1990, p. 2). This paper argues that fundamental tenets of the sequence-stratigraphic
model violate a significant body of observations on Pleistocene and
older depositional units in the
Late Pleistocene Depositional Systems-Shelf to Shelf Margin Late Pleistocene depositional systems are a useful starting point for
improving reconstructions of depositional and stratigraphic aspects of
older systems. Several deltas along the northern The first comprehensive regional database and interpretation of the
Pleistocene of the northern Studies of the late Pleistocene Mississippi deltas by Morton and Suter
(1996) were conducted to compare the development of these deltas to the
predictions of the sequence-stratigraphic model. These authors reached
several conclusions that are pertinent here. Their first key conclusion
was that the “strata1 patterns are continuous from shelf phase to
shelf-margin phase of The second key conclusion of Morton and Suter (1996) concerned the comparative properties of the highstand and lowstand deltas: “Strata1 patterns and lithologic relationships clearly demonstrate that potential reservoir facies of lowstand shelf-margin deltas include distributary mouth bars, channel fills, and reworked shoreline and shelf sands that are not constrained by an incised valley or former shelf margin and are comparable in stratigraphic position and paleogeographic location to sand bodies of the highstand systems tract model” (Morton and Suter, 1996, p. 529). Thus highstand and lowstand deltas can only be distinguished by their context with external phenomena, not by their internal properties. The third key conclusion reached by Morton and Suter (1996) involves the
placement of the lower boundary of the depositional sequence. They
stated: “At the depositional lowstand shoreline
Sequence Stratigraphy of Late Pleistocene Depositional Systems--Shelf to Shelf Margin The discussion by Morton and Suter (1996) on the issues surrounding the choice of surface to serve as a sequence boundary is only one of numerous viewpoints on this topic. Figure lA, modified from Morton and Suter (1996, their Figure 4D) shows the key features and surfaces of a shelf-margin delta. Note the updip sequence boundary at the base of the fluvially incised valley. As an erosion surface, this surface terminates downdip in the erosional base of the distributary channels. Note that the downdip portion of the sequence boundary, as defined by Morton and Suter (1996) exists as a correlative conformity, in particular the zone composed of the downlap surface of the prodelta clinoforms and the condensed section represented by the prodelta clinoform toes. A problem arises when the correlative-conformity downlap surface is traced
updip to an area where the overlying section contains an incised For the purpose of placing genetically related sediments in the same stratigraphic unit, it would seem desirable to retain the lower boundary along the correlative conformity, except where this is removed locally by valley incision. But this essentially places the sequence boundary along (or very close to) the flooding surface, which is clearly not in accord with sequence stratigraphic practice. I conclude that in areas of continuous sediment supply during a sea level fall, there is no geologically coherent way to designate a sequence boundary (a single surface that is chronostratigraphically significant) that separates updip from downdip deposits (Figure 1C). The above considerations do not apply in areas where sedimentation was shut
off for a significant portion of the time during a sea level fall. These
relationships can be illustrated in a strike section Two diagrams are included to contrast the stratigraphic architecture and chronostratigraphic interpretation of late Pleistocene deltas described above according to the sequence-stratigraphic model (Figure 2) and the modified Morton and Suter (1996) model (Figure 3). The latter figure was based upon regional studies of Tertiary strata, described below. In the sequence-stratigraphic model, there is a moment of time when the entire sequence boundary existed as a physical surface. In contrast, in the regressive-systems-tract model, the deltaic package cannot be subdivided into non-genetically related parts by a unique surface that existed over the region at one moment of time. An alternative sequence-stratigraphic approach places a higher order sequence boundary between each successive downstepping event (Mitchum and Van Wagoner, 1991; Posamentier et al., 1992; Kolla et al., 2000), so that a fourth order cycle would be subdivided by fifth order sequence boundaries and their correlative conformities.
Late Pleistocene Depositional Systems-Shelf Margin to Slope Just as the sequence boundary serves as a crucial boundary between the
highstand and lowstand systems tracts on the shelf to shelf margin, this
surface also extends down the slope into the basin. Within the
sequence-stratigraphic paradigm, the transport of coarse sediment into
deep water occurs along the basinal segment of the sequence boundary
(e.g. Van Wagoner et al., 1990). Recent studies of sediment-gravity-flow
systems of the northern In general terms, Beaubouef and Friedman (2000) agree: “The southern limit of the Texas-Louisiana Shelf is rimmed by thick shelf margin deltas interpreted to have formed during the last Wisconsin glacial event These fluvio-deltaic systems provided the source for large volumes of sediment transported to the deep basin” (p. 41). However, referring to the minibasins in front of the Trinity-Brazos shelf margin delta, they stated “. the presence of an older, buried canyon beneath the deltas can not be precluded, as that region of the subsurface is not imaged by the high-resolution seismic data” (p. 52). These authors emphasized the uncertainties that remain: “although the source of sediment delivered to these basins is well known, the exact mechanisms of sediment gravity flow initiation and transport to the slope is not known” (p. 52). On the other hand, Morton and Suter (1996) emphasized that “. . . no incised valleys or submarine canyons breach the paleoshelf margin, even though incised drainages were present updip.. .” (p. 505), apparently implying that sediment gravity flows originated on shelf-margin-delta clinoforms (as shown in their Figure 4), and then continued downslope into minibasins. The late Pleistocene Mississippi canyon that strongly eroded a narrow corridor of the shelf and shelf margin did not form until after the Mississippi deltas had reached the shelf margin (Coleman et al., 1983). Additional work is required to resolve the important issues raised by Beaubouef and Friedman (2000), as cited above. The large-scale relationships between stratigraphy and structure in the
The outer shelf, shelf margin, and upper slope area can be defined in terms of two fundamental and complementary regimes that define the stratigraphic relationships around the important interface between shallow water (topset, or shelf) and deep water (foreset, slope) settings (Figure 4). The first regime is characterized by shelf-margin progradation associated with complex systems of growth faults (Winker and Edwards, 1983). Individual fourth-order regressive cycles expand by up to ten times across individual faults. The second regime is shelf-margin retrogradation caused by large-scale collapse that removes thousands of feet of section, truncating deeper structures, and tilling with a complex of slide blocks and gravity flows, capped with deltas (Morton, 1993; Edwards, 2000). Collapse events affect wide areas of the shelf margin along strike, unlike submarine canyons that excavate narrow dip-oriented erosional features. Retrogradational shelf-margin collapse is related to high rates of sediment supply, rapid progradation of the shelf margin and progradation over a buried shelf margin (see Ross et al., 1994). There is no clear evidence that these features form as a direct response to relative sea level fall, although there is abundant evidence for high-frequency sea-level falls manifested by incised valleys and other phenomena (e.g. Edwards, 1990, 1994). An important issue, but beyond the scope of this paper, concerns the relative importance and character of the delivery of sands to the slope via shelf-margin deltas (progradational) versus shelf-margin erosional features (retrogradational). In seismic data, the diagnostic features of the retrogradational collapse are the basal collapse discontinuity, which truncates updip strata, and the internal till, which commonly has an internal clinoform signature. The features are similar in appearance, respectively, to the sequence boundary and lowstand prograding wedge unit of the sequence stratigraphic model (e.g. Edwards, 1990, 1991), although in plan view the collapse is strike continuous rather than dip elongate, as pointed out above. The collapse events usually truncate several regressive cycles, and the fill contains one to several regressive cycles. Therefore, the alternating pulses of progradation and retrogradation would be considered third order according to sequence-stratigraphic terminology (e.g. Mitchum and Van Wagoner, 1990). The relative importance of regional drainage switching, climate, eustasy, and hinterland tectonics in influencing third-order cycles still does not appear to be well understood in the Paleogene. In the Nevertheless, a number of regional stratigraphic studies conducted on the
Marine flooding surfaces and their associated transgressive shales and “highstand” condensed sections are the best, and often only, regional markers for correlating depositional cycles (e.g. Van Wagoner et al.; 1990, their Figs. 22 and 33). The cycles formed in a range of waveto river-dominated coastal plains and shorelines, often in relation to well-defined depocenters and depoflanks (Fisher and McGowen, 1967; Galloway, 1989a, b; Edwards, 199 1; Morton et al., 1988, 1991). Depending on facies, cycles can be traced along dip for tens of miles. The downdip extensions of these cycles are either truncated by collapse erosional events or thickened dramatically by growth faulting. Cycle expansion is commonly associated with specific, predictable facies changes (Edwards, 1995a). Farther downdip, correlation usually breaks down in the slope environment. Regional studies illustrate examples of the distribution of lithofacies, depositional environments, and stratigraphic architecture for the various units mentioned above. These include the river-dominated depocenter and depoflank of the Yegua (Edwards, 1990, 1991), the wave-dominated interdeltaic bight of the Frio (Edwards, 1986; Galloway and Morton, 1989), growth-faulted river-dominated, wave-influenced Wilcox deltas (Edwards, 198 l), and the river-dominated lower Miocene (Edwards, 1994). A key element of depocenter versus depoflank setting is the rate of sediment supply. In a depocenter the sediment supply “hose” is on most of the time; small relative changes in sea level move the hose around, but do not shut it off. In contrast, in the depoflank, the hose is on only a small part of the time, and genetically unrelated sediment bodies may become juxtaposed. Another contrast is the subsidence rate, which in the depocenter has an average rate that may be several times that of the depoflank. The relationship between third- and fourth-order events is shown in a dip stratigraphic cross section in the lower Miocene of southwest Louisiana (Figure 5). This section connects 20 wells and is based on the integrated analysis of almost 2,000 wells in a 1,500-mi2 area (Edwards, 1994). In this section, the higher resolution stratigraphy is provided by flooding surfaces. Two shelf-margin collapse events are identified, of which the larger earlier one represents a third-order event, as does the group of regressive cycles that occur in this part of the lower Miocene. Sediment gravity flows locally overlie the collapse unconformities. I will selectively cite the literature in order to make several points within the scope of this paper. These concern the distribution and properties of channel-form features, and the delivery of sand to the shelf margin. Typically, sequence stratigraphy recognizes channel-form features to be either distributary channels or incised valleys (e.g. Van Wagoner et al., 1966, their Figure 26). This model of extremes led to an interpretation of the middle Miocene sandstones as having been either lowstand incised valleys or highstand shelf and mouth bar sands (Van Wagoner et al., 1988, their Figure 33). In a study of the lower Miocene (Edwards, 1994), I displayed a series of maps of selected channel-form systems to illustrate the variations shown both in terms of external geometry and relation to adjacent facies. The conclusion reached, that erosion surfaces within a regressive cycle were possibly formed at different times, is consistent with the model proposed by Morton and Suter (1996; see discussion above). The observed wide variety of channel-form geometries (Edwards, 1994) is not consistent with the sequence-stratigraphic model of channel versus valley. Although the study of the lower Miocene mapped and analyzed the section
using fourth-order stratigraphic features, another The depocenter is characterized by an area of amalgamated and coalesced mouth-bar sands along the shelf margin, with mappable associated channels. Thus, many of the channels are surrounded by mouth bars. Some of the channels, however, appear to be associated with downdip shelf-margin deltas, without adjacent mouth-bar deposits (bays of Figure 6). In contrast, in the depocenter flanks, channels can be mapped across the shelf and are associated with downdip shelf-margin deltas. In several cases channels cannot be mapped across the entire shelf but only inferred from limited updip well control. The occurrence of shelf-margin deltas in the same depositional cycle is consistent with sand bypass across the shelf. These relationships are similar in many regards to the late Pleistocene shelf and shelf margin deltas described earlier in this paper. At times, science goes to great lengths to adapt new data to existing
explanations. Past examples include complex solar system models designed
to remain geocentric, and the original grouping of the Burgess Shale
invertebrate fauna in existing phyla (Gould, 1989). How important are
the underlying premises if the whole paradigm shifts as refinements are
introduced? Is an underlying principle effectively buried by subsequent
developments, or does it remain a permanent, integral property of the
paradigm? For
During relative sea-level fall, a high-sediment-supply delta will deposit a single coherent, integrated deposit that can be considered a regressive systems tract. There is no surface comparable to the sequence boundary of the sequence-stratigraphic model that separates this delta into updip highstand and downdip lowstand systems tracts. In order to honor the genetic integrity of the delta deposit, cycle boundaries should be placed at flooding surfaces. Reorganization of depocenters is more likely to occur across flooding surfaces rather than between flooding surfaces. Subsurface maps of regressive cycles bounded by flooding surfaces will clearly show the positions of incised valleys, using either net sand or log facies data, in the depoflank areas, due to their contrast with adjacent facies. In the depocenters, the discrimination of incised valleys from distributary channels is often either not possible, or not desirable, for sound theoretical reasons. It is possible that some degree of “forcing,” or sea-level fall,
accompanied many
I would like to acknowledge Tucker Hentz’s careful review of the manuscript. Numerous oil and gas companies have supported the regional studies that underlie many of the observations made and conclusions reached in this paper. I appreciate the encouragement and support they have given to me.
Beaubouef, R. T., and S. J. Friedman, 2000, High
resolution seismic/sequence stratigraphic framework for the evolution of
Pleistocene intra slope basins, in P. Weimer, R. M. Slatt et al., eds.,
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Formation of the Texas Gould, S. J.,1989, Wonderful Life. W. W. Norton and Co. NY. 347 p. Hunt, D., and M. E. Tucker, 1992, Stranded parasequences and the forced regressive wedge systems tract: deposition during base-level fall: Sedimentary Geology, v. 81, p. l-9. Hunt, D., and M. E. Tucker, 1995, Stranded parasequences and the forced regressive wedge systems tract: deposition during base-level fall-reply: Sedimentary Geology, v. 95, p. 147-160. Kindinger, J. L., 1988, Seismic stratigraphy of the Mississippi-Alabama shelf and upper continental slope: Marine Geology, v. 83, p. 79-94.
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Figure
1. Depositional elements and significant surfaces in a shelf to shelf
margin setting characterized by high sediment 
Figure
2. Strata1 patterns and chronostratigraphy according to the standard
Exxon sequence-stratigraphic model.
Figure
3. Strata1 patterns and chronostratigraphy for the regressive systems
tract described in this paper, modified from the concepts of Morton and
Suter (1996).
Figure
4. Schematic cartoon to illustrate the large-scale (third-order) phases
of shelf-margin progradation and slope-minibasin initiation, alternating
with phases of shelf-margin retrogradation.
Figure
5. Stratigraphic dip section hung on the Lower Miocene S. davisi
marker.
Figure
6. Depositional-environment map of a major regressive sequence of the
Yegua Formation, middle Eocene of southeast Texas and southwest
Louisiana.
