Chapter 6 Figures

Figure 1-Structural summary map of the northern Gulf of Mexico Basin. Black areas are shallow salt bodies. Tick marks are on the downthrown side of major growth faults: black = seaward dipping; red = landward dipping (counter-regional); blue = thrust faults.

Figure 2-Tectono-stratigraphic provinces of the northern Gulf of Mexico Basin. Locations of profiles described in the text are indexed by figure numbers (in red).

Figure 3-Structural summary map of the northern Gulf of Mexico Basin. Tectono-

stratigraphic provinces are color coded as in Figure 2 and faults as in Figure 1.

Figure 4-Artificial illumination display of seafloor structure, Louisiana slope. Data are from the NOAA multibeam bathymetric survey. Vertical exaggeration is 10 times; illumination is from the west-southwest. Colors from gray, brown, light blue, to dark blue indicate shallow to deep water. OCS protraction areas are outlined in black: KC = Keathley Canyon, GB = Garden Banks, GC = Green Canyon, WR = Walker Ridge, MC = Mississippi Canyon, and AW = Atwater.

Figure 5-Seismic time profile across the Sigsbee escarpment, western Louisiana slope. Eocene(?) (E) and middle Cretaceous (K) reflectors are pulled up beneath the relatively fast allochthonous salt layer (green). There is no evidence of a foldbelt here. See Figure 2 for location.

Figure 6-Slope minibasins, offshore Louisiana. Two deep basins are separated by a shallow basin perched above allochthonous salt. Events marked "M" are multiple reflections. See Figure 2 for location.

Figure 7-Seismic profile from the middle slope of Louisiana. Down-to the-south normal faults at the north end are linked to thrust structures at the south end. Sliding perched basin is beginning to subside into shallow salt (green). See Figure 2 for location.

Figure 8-Seismic profile from the shelf edge of Louisiana. Shelf margin sedimentation and associated listric growth faults collapsed the north end of a salt body to produce a weld (green) just south of a counter-regional salt feeder. The southern end of the salt body remains near the seafloor with shelf margin sediments onlapping and thinning onto the southern flank.

Figure 9-Uninterpreted (top) and interpreted (bottom) seismic profile across an organized roho system, western Louisiana outer shelf, showing roho reflections along the detachment for Pliocene-Pleistocene listric growth faults. A north-dipping counter-regional salt feeder is interpreted at the north end of the subhorizontal salt weld (green). Pl A, B, C = three successive Pliocene-Pleistocene levels. See Figure 2 for location.

Figure 10-Reconstruction of depth-converted seismic profile based in part on Figure 9. Pl. A, B, C = three successive Pliocene-Pleistocene levels. Large extensions are balanced by reduction of a shallow allochthonous salt body (green). Inverted triangles represent the approximate position of the paleoshelf margin based on paleontologic interpretation of depositional environments. The base of salt through time is based only on the structural relief above salt. Reconstruction by the PREP method (see Chapter Appendix).

Figure 11-Interpreted seismic profile showing basinward-thinning strata above the Pliocene-Pleistocene salt weld, western Louisiana outer shelf. See Figure 2 for location.

Figure 12-Reconstruction of depth-converted seismic profile based in part on seismic profile in Figure 11. Basinward-thinning strata above the weld restore to an onlapping configuration on the south flank of a presently evacuated salt body. Reconstruction by the MESH method (see Chapter Appendix). Pl. A, B, C = three sucessive Pliocene-Pleistocene levels.

Figure 13-Uninterpreted (top) and interpreted (bottom) seismic profile across the Oligocene-Miocene detachment, onshore southern Louisiana. Listric normal faults with greatly expanded Miocene strata (upper colorless interval) sole above through-going Eocene and older strata. Large counter-regional faults beneath the detachment are interpreted to be feeders for allochthonous salt since evacuated from the detachment. See Figure 2 for location.

Figure 14-Reconstruction of depth-converted interpretation of seismic profile in Figure 13 (top) to the end of Oligocene time (bottom). Backstripping of expanded Miocene deltaic sediments leaves a basinward-thinning wedge of Oligocene sediments (purple) above the detachment. Assuming no salt withdrawal since the end of Oligocene at position 1, backstripping indicates 1.1 km of excess subsidence in this area, certainly an overestimate. The change in sediment overburden thickness from the end of Oligocene to the present is 7 km. Change in water depth (Dwd) is less than 500 m. Isostatic balance estimates salt withdrawal of greater than 2 km of salt. If change in water depth is less or excess thermal subsidence is less, then more salt withdrawal is required for isostatic balance. (See discussion on subsidence and salt withdrawal in Chapter Appendix). Reconstruction is by the PREP method.

Figure 15-Isostatically balanced reconstruction of the cross section in Figure 14 assuming withdrawal of salt (green) from the autochthonous salt level. This interpretation is rejected in favor of an allochthonous salt model (Figure 16). Reconstruction above detachment is by the PREP method.

Figure 16-Isostatically balanced reconstruction of the cross section in Figure 14 assuming a two-stage evolution: (1) extrusion of salt (green) into a canopy near the seafloor by Eocene-Oligocene time (bottom section) and (2) evacuation of allochthonous salt by prograding Miocene depocenters to produce a salt weld (middle two sections). In this model, counter-regional faults below the detachment are interpreted as collapsed salt bodies that acted as feeders during the salt emplacement. Reconstruction above detachment is by the PREP method.

Figure 17-Interpreted seismic profile across the Oligocene-Miocene detachment province and mid-shelf salt dome-minibasin province. Strata above the salt detachment (green) form a series of expanded wedges that thin basinward above more isopachous subdetachment Eocene and older strata, which are deformed by counter-regional faults. Well symbols indicate wells that penetrate salt at the level of detachment. Pl = lower Pleistocene, UM = upper Miocene, MM = middle Miocene, LMA and LMB = lower Miocene, UF = Oligocene upper Frio, MF = Oligocene middle Frio, E = Eocene, UK = Upper Cretaceous, and LK = Lower Cretaceous. See Figure 2 for location. This vertically exaggerated profile is shown at true scale in Figure 35 (folded insert).

Figure 18-Structure contour map on Eocene subdetachment horizon illustrating subdetachment minibasins and counter-regional faults interpreted as feeders for a now evacuated allochthonous Paleogene salt canopy.

Figure 19-Reconstruction of a stepped counter-regional system formed by evacuation of an isolated allochthonous salt body (Schuster, 1995). UM = upper Miocene, MM = middle Miocene, LM = lower Miocene, Pg = Paleogene. See Figure 2 for location.

Figure 20-Evolution of salt withdrawal fault systems above allochthonous salt (gray) as shown by a series of true-scale depth sections from the present-day Louisiana Gulf Coast. The present-day structures from (a) deep water to (e) onshore represent evolutionary stages as the continental margin prograded across allochthonous salt. These deep-water examples are analogs for the early history of fully developed fault systems onshore and on the inner shelf.

Figure 21-Uninterpreted seismic profile (top) and depth-converted interpretation (bottom) of the Vicksburg detachment system, onshore southern Texas. The strong reflector at the base of the rotated section is the interface between faster Vicksburg Formation rocks and slower Eocene shales (dull green). Offsets of this reflector are interpreted to be fault slices of Eocene shales on the hanging wall of the detachment. See Figure 2 for location.

Figure 22-Reconstruction of cross section in Figure 21. The restored base of the Vicksburg section remained subhorizontal through time. Slices of Eocene shales (dull green) were stripped off the footwall and carried eastward along the detachment. This sliding system does not directly involve salt withdrawal. Reconstruction is by the MESH method. Vx A, B, C indicate four successive Vicksburg levels.

Figure 23-Reconstruction above a salt-based detachment in the onshore southern Louisiana Frio trend. Contrast the characteristic reconstructed geometry of a basinward thinning wedge onlapping a now evacuated allochthonous salt body with a shale-based detachment system (Figure 22). Reconstruction is by the MESH method. Sections from bottom to top represent successive stages of evolution from Oligocene to present day.

Figure 24-Interpreted seismic profile (top) and detailed drawing (bottom) across the Oligocene-Miocene detachment province on the south-central Texas shelf. The middle Miocene perched detachment (center) is interpreted as an analog for the Vicksburg shale-based detachment; it is connected downdip to the deeper Oligocene-Miocene detachment interpreted as an extensive salt weld. UM = upper Miocene, MM = middle Miocene, LM = lower Miocene, OL = Oligocene. See Figure 2 for location.

Figure 25-True-scale reconstruction of cross section in Figure 24. Perched detachment (center) restores as a sliding surface above a basinward-thinning wedge that collapsed onto the developing salt weld above Eocene and older strata. Salt is shown in black. Reconstruction is by the MESH method. See Figure 2 for location.

Figure 26-Uninterpreted regional seismic profile (top) and interpreted drawing (bottom) across the Wilcox and upper Eocene fault systems, onshore southern Texas. YEG = Eocene Yegua, QC = Eocene Queen City, Wx = Eocene Wilcox,

LK = Lower Cretaceous, BS = base of Jurassic Louann salt. See Figure 2 for location.

Figure 27-Reconstruction of the southern Texas Wilcox fault system above autochthonous salt (black). This end-member model assumes large extension during Wilcox deposition, but a continuum of models, trading the width of Upper Cretaceous salt walls for Tertiary extension, is geometrically possible. LK = Lower Cretaceous, UK = Upper Cretaceous, UWx = Upper Eocene Wilcox. Sea water is stippled. Reconstruction is by the MESH method.

Figure 28-Shaded relief map of the structure on a Lower Cretaceous horizon, southern Texas. Lower Cretaceous strata are absent from prominent depotroughs for Wilcox deposition. Bracket symbols indicate generalized traces of Tertiary growth faults. LK = Lower Cretaceous. See inset map for location.

Figure 29-Seismic time-structure map of Lower Cretaceous horizon near the Mexican border showing offset of collapse edge (outlined in Figure 30) along the trend of basement faults (purple) that offset the base of the Louann. Northwest-trending anticline represents reactivation of the fault system during Laramide compression related to the Coahuila foldbelt in northeastern Mexico.

Figure 30-Pre-Louann basement structure in the same area as Figure 29 showing basement faults (purple).

Figure 31-Reconstruction of a regional cross section across the Wilcox, upper Eocene, and Frio fault systems, onshore central Texas. The Wilcox fault system is perched above autochthonous salt (black). The Frio-age growth faults are interpreted to sole into an evacuated Paleogene canopy now forming the Oligocene-Miocene salt-based detachment that continues offshore (Figure 24). LK = Lower Cretaceous. See Figure 2 for location.

Figure 32. Five times vertically exaggerated frame section (showing only reliable correlations and observed seismic geometries) across western Louisiana, from the Lower Cretaceous margin to the abyssal plain. A true-scale section is shown in Figure 35 (folded insert). Salt is shown in black. WxFS = Wilcox fault system, SDMB = salt dome-minibasin province, OMB = Oligocene-Miocene detachment province, PPD = Pliocene-Pleistocene detachment province, TSMB = tabular salt-minibasin province, LK = Lower Cretaceous, UK = Upper Cretaceous, P-E = Paleocene-Eocene, UE = upper Eocene, LO = lower Oligocene, UO = upper Oligocene, LM, LMB, LMA = lower Miocene, MM = middle Miocene, UM = upper Miocene. See Figure 2 for location.

Figure 33-Speculative cross section for model I and reconstruction based on frame section (Figure 32). Reduced and squeezed from true-scale original; only selected stages are shown. Salt is shown in black. Base of Sigsbee salt mass is connected directly to the autochthonous Louann salt layer. This model is rejected in favor of model II (Figures 34, 35). Reconstruction is by the PREP method.

Figure 34-Speculative cross section for model II and reconstruction based on frame section (Figure 32). Reduced and squeezed from true-scale original; only selected stages are shown. Salt is shown in black. This model, which we prefer, connects the base of the Sigsbee allochthonous salt to the Paleogene canopy level. Reconstruction is by the PREP method.

Figure 36(a)-Comparison of seismic profiles across the Sigsbee salt body (green), southwestern Louisiana slope. Profile used for the frame section and reconstructions (Figures 32, 33, 34, 35) shows the base of the Sigsbee salt sheet reaching as deep as a reflector tentatively correlated to the Eocene (top of brown interval).

Figure 36(b)-Profile farther northwest shows the base of salt (green) continuing above through-going Eocene(?) and older abyssal plain strata (brown interval) for an additional 30 km northward. Apparent structure on the base of salt and subsalt strata is due to the velocity contrast of salt and sediment.

E = Eocene, K = Cretaceous. See Figure 2 for locations.

Figure 37-Alternative rejected reconstruction of onshore and inner shelf portions of frame section (Figure 32) assuming withdrawal of salt from the autochthonous level rather than a Paleogene salt canopy. Reconstruction is by the PREP method.

Table 1-Summary of Alternative Reconstructions in Model I (Figure 33) and Preferred Model II (Figures 34, 35)

Event or Feature Model I Model II

Sigsbee salt overthrust initiated Before Paleogene During Paleogene

Allochthonous salt extrusion On upper part of Paleogene slope only On entire Paleogene slope

Miocene slope sedimentation Around down-building, leaning stocks In slope minibasins on alloch-

thonous salt

Feeders for allochthonous salt on outer shelf Rooted directly to Louann salt level Rooted to allochthonous

Paleogene canopy

Figure A-1-One-dimensional isostatic balance including salt withdrawal and paleobathymetry. Excess subsidence is defined by Y'.

Figure A-2-Magnitude of the subsidence problem in southern Louisiana. Backstripping of 6 km of sediments (column A) requires about 2.7 km of change in water depth (column B). Using a reasonable estimate of 200 m change in water depth still leaves 1.4 km of unaccounted excess subsidence (column C) that could be balanced by 400 m of excess thermal subsidence and 4 km of salt withdrawal (column D).

Figure A-3-Graph of subsidence versus age using restored decompacted thicknesses above basement at the south end of Figure 14 (onshore southern Louisiana). Dashed lines are extrapolated to Jurassic time based on depth to the Louann salt level. The backstripped excess subsidence curve is interpreted to represent thermal subsidence, very nearly flat by 50 Ma. The large anomaly in Miocene time (last 20 m.y.) is interpreted to be the result of salt withdrawal.

Figure A-4-Graph of backstripped excess subsidence versus age for three positions indicated in Figure 14. Dashed lines are extrapolated to Jurassic time based on depth to the Louann salt level. No subsidence anomaly is apparent at position 1, but successive positions southward (2 and 3) show a large anomaly of excess subsidence following the prograding depocenters. This anomaly is interpreted as a wave of salt-withdrawal subsidence.