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Origin and Significance of Retrograde Failed Shelf Margins; Tertiary Northern Gulf Coast Basin
Marc B. Edwards*
Search and Discovery Article #40021, 2001
Adapted for online presentation from an article of the same title by the same author published in Gulf Coast Association of Geological Societies Transactions, v. 50, 2000, p. 81-93. Appreciation is expressed here to both Marc Edwards (www.marcedwards.com) and GCAGS (www.gcags.org).
*Consulting Geologists Inc., 5430 Dumfries, Houston, Texas 77096.
The unusually high rates of sedimentation and subsidence in the Gulf Coast Basin are occasionally overwhelmed by large catastrophic collapse of the shelf margin that relocates the shelf margin landward behind the headwall of a strike-parallel slump scar. Unique reservoir and trapping opportunities are created by these genetically related processes: instantaneous (relatively speaking) creation of the collapse, emplacement of slump blocks into the collapse, possible uplift of the collapse scar headwall due to isostatic rebound, and transport of sediment gravity flows into and across the collapse scar.
A review of the distribution of documented retrograde failed shelf margins suggests that many more remain to be discovered in the subsurface of the Gulf Coast Basin. It is recommended that careful geological modeling be combined with a regional perspective and 3-D seismic in order to discover new exploration opportunities in this hyper-mature basin.
Figure 1. Simple schematic diagrams to show the effect
of contemporaneous instability in a prograding shelf margin on cycle thickness
and subsurface transport of shallow water sediments to greater depths due to
enhanced subsidence rates. Above: prograding stable shelf margin; below:
prograding unstable shelf margin. No attempt has been made to describe the lower
slope setting, or the role of salt in creating accommodation space on the slope
for updip growth fault systems.
Figure 2. Features of a typical retrograde failed shelf
margin in which slump blocks are a small part of the fill. In this and some of
the following figures, the profile of the previous shelf and shelf margin are
used to project the approximate position of a filled collapse scar. The collapse
discontinuity is primarily an unconformity in which high relief headwall
truncates updip strata. Isolated slump blocks of updip strata rest on the
collapse unconformity. Gravity flow sands locally rest on the collapse
unconformity and slump blocks. The collapse scar is filled mostly with deepwater
shales, and is finally capped by prograding shallow water deposits.
Figure
3. Features of a typical retrograde failed shelf margin in which slump blocks
are a large part of the fill. The most obvious discontinuity is the
post-collapse unconformity, but the collapse detachment is equally important.
The latter surface has never been exposed at the sea floor, and therefore is a
structural rather than stratigraphic feature. Note the possibility for
stratigraphic inversion and sub-discontinuity traps.
Figure
4. Features and recognition of slump blocks. Slump blocks occur in clusters or
isolated above the collapse unconformity. Blocks are shown as rotated slivers,
but may also have a plate-like form. Internally, stratification is parallel
rather than divergent. Some wells encounter missing section at both the top and
base of the blocks, and shallow water faunas in the blocks are overlain by
deepwater faunas in the shales above.
Figure
5. Plan view of a hypothetical retrograde failed shelf margin immediately
following the collapse phase. The headwall is arcuate, and isolated slump blocks
litter the collapse unconformity on the left. On the right, strata were torn
apart from the headwall, but did not continue to slide into the basin. The
rollover is largely postdepositional. A schematic dip line through this region
is shown in Figure
6.
Where the slab joins the headwall, rate of translation is lower, and a typical
growth fault situation is attained.
Figure
6. Schematic dip line through a tear-apart between a translation slab and the
stationary headwall of the collapse scar. Note the parallel strata in the slab
that reflect its rapid post-depositional movement. 
Figure
7. Plan view of a hypothetical retrograde failed shelf margin during the
sediment gravity flow phase. Channels (or valleys) on the shelf are sensitive to
local surface gradients, and may follow the downthrown sides of faults. If the
headwall area is uplifted, then channels will skirt this region, and this may be
reflected in the distribution of channels and downslope equivalent slope fans.
As sediment gravity flows accelerate down the headwall, current erosion is
likely, but will be constrained by bottom irregularities such as remnant slump
blocks and growth faults.
Figure
8. Schematic dip section through slope fan along the basal part of a collapse
fill. Reactivation of a growth fault has offset the collapse unconformity and
caused reversal of regional basinward dip because of the greater subsidence rate
downthrown to the fault. This can be a favorable position for the preservation
of coarse sediment gravity flows, which would otherwise be bypassed into deeper
water further down slope.
Figure
9. Map of the northern Gulf Coast Basin showing selected erosional features.
Those which are considered to be retrograde failed shelf margins are the middle
Wilcox of south Texas, Yegua/Cook Mountain and Hackberry of southeast Texas and
southwest Louisiana, Abbeville Lower Miocene of south Louisiana, and Neogene
features on the Texas shelf. References are shown in Figure 10.
Figure
10. Distribution in time of the features shown in Figure
9.
Position in time is
approximate. As a proxy for sea level, the reconstructed oxygen isotope curve of
Abreu and Anderson (1998) is shown.
Figure
11. A stratigraphic dip section through the middle Wilcox in south Texas showing
an excellent example of highly consistent updip stratigraphy abruptly replaced
downdip by a thick wedge of deep water shale. The shale overlies displaced
productive blocks of shallow water strata that correlate perfectly with parts of
the updip section. The depiction of slump blocks is schematic.
Figure
12. A stratigraphic dip section through several wells near the headwall of the
Hackberry collapse in south Louisiana. Unlike the example in the middle Wilcox
in south Texas, gravity flow sandstones are abundant and represent a major
reservoir type.
Figure
13. A stratigraphic dip section through several wells across the headwall of the
lower Miocene (Abbeville shale wedge) in southwest Louisiana. In this example
the rotated blocks downdip from the headwall are very thick compared to the
deepwater wedge that covers them.
Figure 14. Schematic dip sections illustrating the
relationship of shelf margin collapse to retrogradation. (A) An initial episode
of shelf margin progradation is followed by retrogradation and renewed
progradation. During this time a condensed section is deposited above the
previous shelf and shelf margin. (B) When the active shelf margin progrades to
the previous shelf margin, steep slopes trigger collapse of the active shelf
margin, which excavates headward along the weak condensed section. (C) Through
time, the collapse cannibalizes the earlier shelf and shelf margin deposits and
transports them down slope into deep water. Details of slump blocks and slope
aprons are not shown.Contents
Synopsis of Prograding Unstable Shelf Margin
Retrograde Failed Shelf Margins—Key Features and Genetic Model
Slump Blocks and Sliding Slabs
Distribution of Retrograde Shelf Margins in Northern Gulf Coast Basin
South Texas “Lobo” Lower Wilcox
Middle Texas Gulf Coast Lower Wilcox Canyon Complex
Burgos Basin Reklaw and Basal Queen City
SE Texas and SW Louisiana Yegua / Cook Mountain
Why Do Retrograde Failed Shelf Margins Occur?
Introduction
Much has been made in recent years of the role that sea level fluctuations play in influencing the organization of depositional systems and distribution of sedimentary facies in a basin. The northern Gulf Coast Basin is however characterized by both enormous variations in sediment supply over various time scales and a structurally highly active substrate for sediment accumulation (e.g., Worrall and Snelson, 1989; Diegel et al, 1996).
Whereas the development of the continental shelf and upper slope are associated with large-scale progradation (e.g., Winker and Edwards, 1983), the explicitly cyclic nature of the Tertiary section attests to alternating periods of retrogradation, when the shoreline retreats landward. On a regional scale, shoreline retreat is also accompanied by retrogradation of the shelf margin. Most explanations of shelf margin retrogradation invoke a decrease in sediment supply or rise in relative sea level, but there are several examples in which shelf margin collapse or failure was the key mechanism.
Retrograde failed shelf margins can have a regional extent and require a large-scale perspective to be recognized and applied to exploration and development. In today’s domestic U.S. environment of “close-in” exploration driven by analysis of local 3-D seismic surveys, with only limited resources devoted to integration of well, biostratigraphic and whole core studies, retrograde shelf margins may be overlooked. It is therefore worthwhile to review some of those that have been identified, and consider their significance to the contemporary oil and gas company.
An additional key issue concerns their role in the delivery of sand beyond the continental slope. Most continental margin models attribute sand bypass to deep water via deeply eroded submarine canyons, with some influence of sea level. Can the recognition of retrograde failed shelf margins help downdip sand prediction?
The role of slumping in forming unconformities in the Gulf Coast subsurface was first appreciated by Bornhauser (1966) who recognized in the subsurface the action of submarine currents, but not sliding and slumping of sedimentary material. Subsequently, Edwards and Tuttle (1989) and Edwards (1990, 1991) described the regional shelf margin collapse in the Yegua that can be mapped for at least 200 miles along strike. Similar processes were suggested in studies of the middle Frio Hackberry (DiMarco and Shipp, 1991 and Cossey and Jacobs, 1992). This was followed by Morton’s (1993) description of additional features offshore Texas in the middle Miocene and Pliocene.
Synopsis of Prograding Unstable Shelf Margin
In a stable prograding shelf margin, basin subsidence can create space for the accumulation of sediments as vertically stacked regressive cycles. (In contemporary sequence stratigraphic jargon, a distinction is made between highstand and lowstand systems tracts based on eustasy-driven deposition models, a distinction that cannot always be made in data sets. I therefore lump them together as “regressive systems tract” when inferences about coastal onlap trends cannot be reconstructed, or are a secondary issue). These regressive cycles typically have high continuity and correlatability on the shelf, although their internal facies distribution can be very variable. Cycle thickness, compensated for compaction and regional subsidence, is related to the water depth into which the system prograded (Fig. 1).
In the unstable shelf margin of the Gulf Coast Basin, translation of the continental slope, driven by gravitational sliding (Winker and Edwards, 1983), spreading (Worrall and Snelson, 1989) and salt tectonics (Diegel et al., 1995) creates localized areas of very high subsidence rates that are usually developed across a series of adjacent growth faults; accordingly, the thickness of individual cycles can increase by a factor of 10 or more in a basinward direction (e.g., Edwards, 1995). In a regime where the rate of accommodation increase is dominated by subsidence, the thickness of cycles bears less relation to the depth of water or eustatic fluctuations. A consequence of localized high subsidence rates at the shelf margin is that accommodation space associated with slope progradation is instead filled largely with shallow water sediments (Fig.1). It is common for thousands of feet of shallow water deposits to accumulate in water no deeper than a few hundred feet.
Retrograde Failed Shelf Margins—Key Features and Genetic Model
Integrated study of seismic, well log, biostratigraphic and whole core data has supported a complex model for the formation of retrograde shelf margins that form by failure. As with many models, although the processes of formation are recognized, the instigating trigger is more contentious. The following discussion describes some of the important features, generally following the geological order in which they form.
The distinguishing genetic element of a retrograde failed shelf margin is that the process of collapse is instantaneous compared to the normal processes of sedimentation and deformation that characterize the adjacent strata.
The key geometric feature of a retrograde failed shelf margin is a large-scale regional discontinuity surface that in updip areas steeply truncates updip strata but is sub-horizontal in downdip areas. The surface is extensive along strike, and thereby contrasts with submarine canyons. Overall, the surface can be associated with the removal of thousands of feet of section. In areas of structural complexity, the surface can be difficult to identify; it is especially difficult to distinguish it from a slowly evolving, low-angle detachment fault or salt weld. The area of the landward termination of the surface is the easiest to recognize, as there is a relatively high angle truncation of updip strata. Nevertheless, this surface is occasionally misinterpreted as a normal fault.
A spectrum of retrograde shelf margins are considered in this article. Two end-member examples are shown in schematic diagrams. In the most easily recognized type of collapse (Fig. 2), the older, autochthonous strata below are exposed at the sea floor. Slumped material from the headwall is only sparsely preserved on the floor of the excavated collapse scar.
At the other extreme (Fig. 3), and more difficult to recognize, are collapse scars in which slump blocks and chaotic slide sediments are a significant product of the collapse event. As discussed below, both styles of slides can occur in different regions of a collapse that formed during the same event. Thus, the regional discontinuity (using the term to mean a surface that separates unrelated sets of rocks) can take on the appearance of an unconformity (stratigraphic origin) in some places, and a detachment (structural origin) in others. Thus, any of the terms, discontinuity, unconformity and detachment, can be used to describe the same surface, depending on the context.
In terms of paleoenvironments, as determined from faunal paleobathymetry, in updip areas the unconformity abruptly separates updip shallow water deposits from downdip deep water deposits. Thus the deep water shales form a thick wedge that is distinct from adjacent shallow water deposits. The wedge expands downdip to thousands of feet thick, and can be traced along strike for several to many tens of miles. The basal discontinuity is formed by mass wasting of the continental margin, and the term slump, or collapse discontinuity conveys the sense of its origin. The collapse expands headward in a general landward direction because the steep bounding slope destabilizes the adjacent sediments, which can spall off as blocks, or liquefy and then be retransported down slope.
In general it is not helpful to apply the term sequence boundary because there is no clear connection to a causal mechanism related to sea level changes (see also discussion by Morton, 1993).
Slump Blocks and Sliding Slabs
Additional properties of retrograde failed shelf margins are very important to differentiate them from other features, and to understand the implications to lithology prediction. Remnant slump blocks that rest on the unconformity (Figs. 4 and 5) are a key feature because they help identify the presence of the failed shelf margin in both log and seismic data. In log data, a diagnostic feature is the occurrence of thin intervals of interbedded sands and silty shales (presumed shallow water deposits) that occur below thick, monotonous, fine-grained shales (presumed deep-water origin), and above inferred missing section. It is important to note that block rotation results in greater apparent thickness, which could be confused with expansion due to growth faulting. The dipmeter is particularly useful here (e.g., Cossey and Jacobs, 1992). In rare cases, complex slump block movements can even produce repeated section. In some cases, chaotic slides may be preserved downdip from rotated block zones (Fig. 3).
Comparison of several subsurface examples in the Gulf Coast shows that the thickness of the layer of rotated blocks and the thickness of the overlying shale wedge vary widely. To some extent this may reflect the physical properties of the sediments removed by mass wasting. A lower weak layer is exploited as the headwall migrates updip. The depth of this layer beneath the sea floor is the main factor that determines the depth of erosion. Above this weak layer, is a layer of comparatively cohesive sediment that can withstand the stresses of downslope sliding, at least for a while. The thickness and cohesiveness of these sediments determines the dimensions of the slump blocks, at least prior to any subsequent erosion by bottom currents.
In seismic data through downdip areas, the strata below and above the unconformity may show subparallel reflectors, and the unconformity may not be apparent; hence the presence of isolated patches of inclined reflectors suggests the presence of a regional collapse unconformity. Sometimes the discontinuity surface may be evident, but a structural detachment interpretation is also a valid alternative. In this case, the presence of small rotated blocks, especially without fanning or divergence of internal stratification is valuable evidence supporting a collapse origin.
A plan view of a hypothetical retrograde failed shelf argin emphasizes the variability of the collapse scar along strike (Fig. 5). One side of the collapse scar has an arcuate form that merges into the continental slope. The other margin is represented by areas of sediment that slid rapidly basinward, but did not move down the continental slope because they are anchored on one side. The rapid sliding of these slabs causes a tear, or opening to form between their updip edge and the headwall of the collapse. Because these blocks move so quickly, the rollover that formed is not characterized by stratal divergence, but rather shows sub-parallel strata plunging into the collapse discontinuity (Fig. 6). Note that because the collapse base is never actually exposed to the sea floor in this position, this bounding surface can really be considered to be a structural low-angle detachment fault, rather than a stratigraphic surface. The multiple roles of one surface requires that care be used when applying standard nomenclature.
Farther along strike (Fig. 6), as the rate of sliding decreases, a normal growth fault situation is present, in which rates of deformation and sedimentation allow gradual expansion of strata updip into the headwall fault.
The immediate removal of hundreds to thousands of feet of sediment would be expected to result in crustal rebound that would extend to the nearby adjacent areas. With regard to the timing of such uplift, consider that isostatic rebound during and after deglaciation occurs during the ensuing thousands of years. Models of passive margins that have experienced deep erosion suggest that crustal rebound would result in the development of pronounced unconformities, mimicking the effects of a relative sea level fall (McGinnis et al., 1993). The erosion through the uplifted headwall region creates features that, following subsidence and burial, will resemble incised valleys. Uplift of the headwall area of a retrograde failed shelf margin would profoundly affect the delivery of coarse sediment from updip sediment sources.
What are some of the impacts of such uplift? The reversal of surface gradients would temporarily direct fluvial transport to flanking areas (Fig. 7). Such sediment would eventually make its way along strike until a route is found into deep water. Sediment wasting along the headwall and erosion by bottom flows may cut channels that eventually breach the crest of the headwall and capture updip fluvial channels. Such capture would abruptly introduce clastics down the headwall into the floor of the collapse slump scar.
As suggested above, the mass wasting phase can be followed by deposition of sediment gravity flows. The presence of a submarine depression bounding an updip area across a steep slope is an ideal setting for attracting sediment and generating sediment gravity flows. Such flows can be sourced either by local reworking of the adjacent collapse unconformity, or by rivers and valleys captured by the collapse scar. The latter are more likely to provide larger quantities of coarser sediment. Whle the collapse is gradually filling with sediment, any effects of crustal rebound as described above would be countered by subsidence due to loading.
Sediment gravity flows could be constrained by bathymetric obstacles such as slump blocks and active growth faults. Slump blocks arranged in a series of blocks can create small troughs that fill to the spill point before flows continue downdip. Contemporaneous faulting can also effect deposition on slope aprons. Due to greater subsidence rates on their downthrown sides, basinward dipping growth faults decrease basinward sea-floor gradients. This enhances the preservation of coarse sediments (Fig. 8). The local preservation of sand is also favorable for the development of stratigraphic traps. For example, preservation of sand on the downthrown block but not the immediately adjacent upthrown block would reduce the likelihood of leakage updip across the fault.
Distribution of Retrograde Shelf Margins in Northern Gulf Coast Basin
Relatively few publications have described features that are unmistakably of the type covered in this paper. This is partly because most recent published studies are insufficiently regional in scope, and because older publications did not recognize them as a distinct feature.
Features that appear to be of this type include middle Wilcox from South Texas, middle Frio (Hackberry) of southeast Texas and southwest Louisiana, middle Miocene of the Texas OCS and Pliocene of the Texas OCS (Figs. 9 and 10). Additional features whose origin is uncertain include the Lower Wilcox (“Lobo”) of South Texas, and various “embayments” in the upper Frio and Miocene of south Louisiana. The middle Wilcox Yoakum canyon and nearby lower Wilcox Hallettsville canyon are shown for reference. The following sections are brief, and the reader is guided to the references cited to obtain more information.
South Texas “Lobo” Lower Wilcox
Little has been published of a regional nature about the large slump complex of shallow water strata that is referred to as Lobo. An approximate outline is provided by Stricklin (1996), and recent work provides seismic data of a characteristic field (Miller and Baker, 1998). Slumping was post-depositional, and the slump blocks are bounded by an erosion surface above and a detachment surface below. The slump complex is offset in places by post-depositional growth faults. Although the slide has been penetrated by hundreds of wells, it has not been described from a regional perspective.
Middle Texas Gulf Coast Lower Wilcox Canyon Complex
Several authors have described numerous channels/valleys and canyons that occur in the lower part of the lower Wilcox (Devine and Wheeler, 1989; Chuber and Begeman, 1982). These are much more limited along strike than the features emphasized in this paper, but they are noteworthy because they may indicate that strike elongate collapse features exist elsewhere along this trend but have not yet been identified.
A middle Wilcox retrograde failed shelf margin occurs in Zapata County in South Texas (Fig. 9). Figure 11 is a schematic stratigraphic dip section datumed on a marker near the base of the upper Wilcox. The interpretation is based on the correlation of numerous well logs in the area and is consistent with available seismic data. The cross section shows the abrupt downdip appearance of a thick shale interval that lacks coarsening-upward signatures of stacked deltaic (shelf or platform) cycles that are evident updip. At the base of the shale wedge, some wells display part of the section that is present updip, other wells lack this section at all, and the shale directly overlies older strata.
In this example, the log evidence is clearly consistent with the model described above. The implications for exploration and development are: (1) updip of the collapse, structural traps are probably required because of the high continuity of the depositional units; and (2) within the collapse, 3-D seismic is required for exploration and development because the complexity of the distribution of isolated slump blocks on the collapse unconformity precludes reservoir prediction but supports the likelihood of stratigraphic traps. Additional areas of inquiry are whether or not gravity flow reservoirs occur in this example and whether other parts of the Wilcox section have similar features.
Burgos Basin Reklaw and Basal Queen City
Two extensive erosion surfaces identified as “unconformities” occur in the Reklaw and basal Queen City formations in the Mexican part of the Rio Grande Embayment (Fig. 9; Vazquez et al., 1997). These surfaces, which have been studied using extensive well control and 3-D seismic data appear to be examples of retrograde failed shelf margins, but little data has been published. Seismic data published by Vazquez et al. shows part of the post-collapse fill to be clinoforms.
SE Texas and SW Louisiana Yegua/Cook Mountain
The collapse unconformity of a retrograde failed shelf margin effectively separates the older pre-failure Cook Mountain from the younger post-failure Yegua. Edwards and Tuttle (1989) and Edwards (1990, 1991) described the features of this surface using extensive well control, seismic, and biostratigraphic data. Additional information on parts of the collapse in updip areas has been provided more recently by Ewing and Vincent (1997) in the middle Texas Gulf Coast and Swenson (1997) in southeast Texas.
Ewing and Vincent argued that a fall in relative sea level was involved in causing both the failure and the bypass of sediment across the shelf into the collapse scar. An important contribution was their mapping of channels (incised valleys) on the updip retrograde shelf edge that supposedly fed sand-bearing slope fans in the updip part of the collapse scar.
The late Oligocene middle Frio Hackberry interval has been described extensively in the literature (Paine, 1966; DiMarco and Shipp, 1991; and Cossey and Jacobs, 1992). The cross section included here (Fig. 12) uses logs to reconstruct the stratigraphic relationships at the edge of the collapse scar. The relatively updip well shows a generally continuous upward-coarsening trend distributed over numerous stacked thin depositional cycles. These cycles correlate extremely well in the adjacent well control. Note the abrupt lateral change to featureless shale. Below the shale, some wells contain a section of blocky sediment gravity flow sands. Where present, these overlie transported blocks of the updip section. Wells through these blocks show expanded section, which is an apparent thickening due to block rotation (established by dipmeter logs). At the top of the shale wedge, there is a very thin, upward-coarsening trend back into shallow water environments.
The uniform fine-grained lithology and deepwater faunas that characterize the lower part of thick shale wedge suggest that most of it is deposited as a hemipelagic drape above the gravity flows. Seismic data (DiMarco and Shipp, 1991) show the presence of a downlap surface forming the lower boundary of a clinoform package that fills the collapse; see Fig. 12, progradational cap. Thus the basal gravity flow sandstones can not be considered toesets of the shallow water progradational cap at the top of the shale wedge.
Study of the Miocene trends has been tied to the concept of the “embayment,” especially in south Louisiana. The term has traditionally referred to arcuate, sub-regional, growth-faulted areas with large increases in thickness downdip. The deep, expanded sections are in part composed of shales containing deep water faunal assemblages. Stuckey (1964) succinctly points out that in the embayments, deepwater shales have no intermediate facies to updip shallow water equivalents. This description closely matches the collapsed shelf margin architecture described in this paper. Unfortunately, most cross sections displayed in earlier field studies and sub-regional studies of the Gulf Coast subsurface tend to project growth faults downward indefinitely, reflecting the quality of the seismic data available at the time. More recent studies with better seismic have also made similar interpretations, suggesting that the concepts proposed in this paper should be helpful in exploiting recent seismic data.
Three major embayment margins are depicted by Stuckey (Figs. 9 and 10), the lower to middle Miocene Abbeville, Cib. op. and Harang. Relationships for the deep water Abbeville shale in southwestern Louisiana demonstrate the abrupt downdip termination of continuous updip strata and the equally abrupt updip termination of the regional deepwater shale wedge (Fig. 13). The regional context for this section has been illustrated previously by Edwards (1994). In his Figure 4, the collapse detachment is labeled “detachment,” and the post-collapse unconformity is labeled “collapse erosion surface.” Are the other embayments described by Stuckey also retrograde failed shelf margins?
Why Do Retrograde Failed Shelf Margins Occur?
It is important to distinguish immediate triggering causes from long-term processes that set up favorable conditions for the triggers to work. Sea level changes can act as a triggering mechanism, but if they were an underlying cause, then retrograde failed shelf margins would be as common as sequence boundaries. A more likely underlying cause is the presence of a weak stratal zone along which sediment collapse can propagate up dip. Good candidates for this include either shales associated with a transgression of the shoreline and retrogradation of the shelf margin (condensed section) or perhaps elevated pore pressures associated with salt flowage and other types of deformation. A series of schematic diagrams (Fig. 14) illustrate the former process.
Retrogradation of the shelf margin was also suggested as a mechanism for achieving sufficient slope gradient to allow bypass and delivery of sediment to the lower slope (Ross et al., 1994). This model also emphasized the role of relative sea level rise (as pointed out by Morton, 1993) as opposed to the sea level fall required by sequence stratigraphic models. This model concluded that periods of shelf margin progradation alternated with periods of shelf margin erosion and degradation. Large collapse scars appear to represent a particular style of shelf margin degradation that is favored by certain subsurface conditions, as mentioned above.
Morton (1993) provided a discussion of numerous other factors which might contribute to the formation of retrograde failed shelf margins.
Retrograde failed shelf margins tend to form during renewed progradation of the shelf margin, following major shelf margin retrogradation when a weak shale layer is subjected to destabilizing gravitational stress.
The process takes place instantaneously, compared to the normal rates of sedimentation and subsidence that characterize the adjacent strata. This allows recognition of two key types of surface, a basal collapse detachment, and a post-collapse unconformity, which in many cases merge into the same discontinuity surface.
A variety of reservoir types and trapping situations are associated with this feature. Updip strata are truncated by the collapse, confounding predictive models, whereas reservoirs can be found in slump blocks and slope aprons. The collapse discontinuity also sets up subjacent truncation traps.
This type of shelf margin has important implications to reservoir prediction, and undoubtedly many such margins remain unidentified in the Gulf Coast Basin subsurface. The combination of careful geological modeling, a regional perspective, and integration with 3-D seismic data is the recommended approach to exploration in this hyper-mature region.
Deanne Schlanger helpfully reviewed the manuscript, and Norm Rosen offered numerous editorial improvements.
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