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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.
uLate Pleistocene shelf to shelf margin
u uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin
u uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin
u uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin
u uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin
u uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin
u uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin
u uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin
u uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin
u uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin
u uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin
u uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin
u uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin
u uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin
u uLate Pleistocene shelf margin to slope uTertiary depositional systems
uLate Pleistocene shelf to shelf margin
u uLate Pleistocene shelf margin to slope uTertiary depositional systems
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Thumbnails with Abbreviated Captions
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Figure 1. Depositional elements and significant surfaces in a shelf to
shelf margin setting characterized by high sediment input and falling
sea level. A. Reconstruction of late Pleistocene shelf to shelf margin
Mississippi deltas showing depositional components and stratigraphic
boundaries, modified from Morton and Suter, 1996, their Figure 4D. B.
The dip section edge of the block in (A), showing the nomenclature of
Morton and Suter (1996). C. Same stratigraphic units as (B) hut with
additional designations for key surfaces. D. Strike view suggesting how
a depocenter, as emphasized in the above diagrams, would contrast with
the depoflank, where the sea level fall would yield a Figure 2. Strata1 patterns and chronostratigraphy according to the standard
Exxon
Figure 5. Stratigraphic dip section hung on the Lower Miocene S. davisi
marker. The section is about 30 mi. long and connects 20 wells.
Facies/environment
interpretations are based on integration with dense, nearby Figure 6. Depositional-environment map of a major regressive Building paleogeographic maps (which represent, by implication, a time
interval) requires the ability to delimit, both vertically and
laterally, a corresponding rock-stratigraphic interval. Another property of 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 One of the most confusing aspects of This paper argues that fundamental tenets of 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 Gulf Coast Basin shelf margin provide insights into key features and unresolved issues. Of particular note are studies of the late Pleistocene Mississippi deltas (Morton and Suter, 1996) and the late Pleistocene (Mobile River) Lagniappe delta (Kindinger, 1988; Sydow et al., 1992; Sydow and Roberts, 1994; Winn et al., 1998; Kolla et al., 2000). Another key area of progress has been the issue of how and when sediment is transported to the slope and basin floor. Recent studies have aimed at linking sands in slope minibasins to their updip sources (e.g., Winker and Booth, 2000, Beaubouef and Friedman, 2000). The first comprehensive regional database and interpretation of the
Pleistocene of the northern Gulf of Mexico is the landmark work of
Berryhill et al. (1986). Numerous seismic lines in this atlas illustrate
not only the incised fluvial systems on the shelf, but the widespread
distribution of packages of basinward dipping clinoforms that strongly
indicated that delta sediments were deposited (and preserved) while sea
level fell to the shelf margin during Pleistocene glacial periods. This
would counter the 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 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
The discussion by Morton and Suter (1996) on the issues surrounding the
choice of surface to serve as a A problem arises when the correlative-conformity downlap surface is traced
updip to an area where the overlying section contains an incised fluvial
channel (Figure 1). A 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 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 through a deltaic depocenter and a flank area, updip of the shelf margin (Figure 1D). This shows how the sandy deltaic package that is deposited during the sea level fall changes facies to marine mud overlain by the subaerial exposure surface. Farther along strike, this mud may be locally eroded by a true incised valley that was bypassing sediment across the exposed shelf. The absence of deltaic deposits fringing the incised valley suggests that the valley formed by headward gully extension on the emergent shelf, until an updip drainage was captured. Two diagrams are included to contrast the stratigraphic architecture and
chronostratigraphic interpretation of late Pleistocene deltas described
above according to the An alternative
Late Pleistocene Depositional Systems-Shelf Margin to Slope Just as the 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 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
Gulf of Mexico basin, where the depositional substrate is highly mobile
salt and shale, are 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 In the Gulf Coast Basin individual regressive pulses are stratigraphic
events that are commonly referred to as fourth-order sequences (e.g.
Mitchum and Van Wagoner, 1990). Several papers presented under the
“ Nevertheless, a number of regional stratigraphic studies conducted on the
Gulf Coast Basin provide a basis for delineating several key features of
depositional cycles or sequences in the region (e.g. Edwards, 1980,
1990, 1991, 1994; Galloway et al., 1982; Van Wagoner et al., 1990; Xue
and Galloway, 1995). Unfortunately, relatively few of the published
studies combine sufficient detail (correlation and mapping to at least
“fourth-order” cycles), adequately dense 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
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 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, Although the study of the lower Miocene mapped and analyzed the section
using fourth-order stratigraphic features, another example that may be
more compelling is a regional study of the middle Eocene Yegua and Cook
Mountain Formations that spans the entire depocenter and flanks
(Edwards, 1990, 1991). This study was based on about 4,500 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 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 example, the astronomical system based on the positioning
of the Earth at the center of the Solar system had to eventually be
swapped out, in its entirety, for a Solar-centric system (although
lessons learned about various orbital geometries could be retained for
use in different systems). What does
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 It is possible that some degree of “forcing,” or sea-level fall,
accompanied many Gulf Coast Tertiary high-frequency (“4th order”)
cycles. In this case, “valley” features, difficult to identify in
depocenters, would be important targets for exploration in depoflanks,
where their detection would require time-scale stratigraphic
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.
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