Guglielmo, Giovanni, Jr., Ge, H., B. C. Vendeville, and M. P. A. Jackson 1998, Progradation Over Salt in the Santos Basin, Brazil: A BEG hypertext multimedia publication on the Internet at: http://www.beg.utexas.edu/agl/animations/AGL98-MM-009.
PROGRADATION OVER SALT IN THE SANTOS BASIN, BRAZIL
GUGLIELMO, G., Jr., GE, H., VENDEVILLE, B. C., and JACKSON, M. P. A.
Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas 78713-8924 USA
An animation showing an restoration of a seismic section from the Santos Basin illustrates progradation over salt. The deformation includes continuous and discontinuous salt welding, apparent downlaps, passive diapirism, distal diapir burial, faulting and diapir subsidence.
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DESCRIPTION AND INTERPRETATION
This animation shows a restoration (Ge et al, 1997) of a time-migrated seismic section (Mohriak et al, 1996) from the Cabo Frio Region, Santos Basin, offshore Brazil. It illustrates how a prograding wedge over salt strata may create apparent downlaps and discontinuous salt welds.
The SE-NW seismic section at the beginning of the animation runs down the continental slope and was depth-converted before restoration. Salt is likely to have expelled by the prograding wedge toward the left (seaward). Salt is also likely to have been expelled out of the plane of the section as the depocenter shifted eastward and progressively northward (Mohriak et al, 1995).
Aptian salt forms three low diapirs separated by a long, flat weld. The subsalt basement is fairly flat. A giant rollover anticline appears to downlap onto the weld. Demercian et al (1993) inferred that the rollover formed during Late Cretaceous extension of the thin-skinned Cabo Frio "fault," traditionally viewed as a landward-dipping listric growth fault 300 km long. The "fault weld" created a lateral gap in the lowermost Albian overburden of 25-50 km (Demercian et al, 1993; Szatmari et al, 1994) and narrower gaps for sediments as young as Maastrichtian. In contrast, using physical modeling, Szatmari et al (1994) attributed the rollover to the bending of Upper Cretaceous prograding wedges as the underlying salt was evacuated; the listric fault bounding the rollover was thought to have hindered further progradation. Mohriak et al (1996), on the other hand, scrutinized several hypotheses and favored a combination of progradation and extension.
Our restoration illustrates deformation driven entirely by progradation without any regional extension. The deformation during continental progradation can be described in 4 stages. Color bars indicate age shown in the legend. The animation lasts 12 seconds and represents approximately 110 million years of deformation.
Stage 1 – Albian to Campanian (0-15% time elapsed) - Continuous welding and apparent downlaps: The load of prograding sediments squeezes salt seaward (to the left) and the tabular source layer ahead of the wedge uniformly inflates to form a salt plateau. Prograding strata onlap the salt plateau. Overlying strata then rotate counterclockwise, turning onlaps into apparent downlaps. Overburden ground onto the basement creating a trailing, almost continuous, salt weld.
Stage 2 – Maastrichtian to Paleocene (15-59% time elapsed) - Discontinuous welding and passive diapirism: Continuity of the basal salt weld is broken by low-relief diapirs representing remnant pockets of salt trapped after they become rapidly overriden by prograding shale. Distal aggradation on the extreme left restricts salt from migrating basinward. There, salt was mounded up by encroaching sediments from both sides, forming a passive diapir that grows by downbuilding.
Stage 3 – Middle Miocene to Present (59-65% time elapsed) - Distal diapir burial: Further sedimentation covers the diapir and prevents further growth. Flanking strata from withdrawal basins. Distal overburden grounds onto the basement and prevents further salt flow toward the basin. However, salt is free to flow along the strike of the salt wall.
Stage 4 – Future sediments (65-100% time elapsed) - Faulting and diapir subsidence: Laterally shifting depocenters or along-strike variations in salt-diapir geometry can cause underlying salt to migrate along strike and the diapir to sag. To accommodate sagging, a landward-dipping fault in the diapir roof. The faulting and subsidence of the landward flank of the diapir (1) produce counterclockwise rotation of the future (yellow) sediments, (2) reactivate rotation of Maastrichtian to Present sediments nearby as the gentle withdrawal basin on the landward side of the diapir deepens to form a major expulson rollover syncline, and (3) ground these sediments onto the basement increasing the area of the regional weld. This hypothetic construction in the future actually exists near this section, as illustrated by the seismic section in Demercian et al (1993, their Figure 10).
POTENTIAL STRUCTURAL TRAPS FOR HYDROCARBONS
Stage 1: Counterclockwise rotation would result in landward migration of any hydrocarbons within each reservoir in strata of the overburden. However, hydrocarbons might not be generated from in situ source rocks so early in the burial history. Even so, welding at the base of the overburden could allow hydrocarbons from a subsalt source to migrate through welded windows into suprasalt reservoirs. This migration potential progressively increases as the welding area increases basinward.
Stage 2: Residual pillows formed during phases of unusually rapid progradation (1) would interrupt or retard the counterclockwise rotation and resulting landward migration of hydrocarbons in the overburden, and (2) may locally seal subsalt or laterally migrating hydrocarbons. Typical diapir-related traps could form near the distal passive diapir.
Stage 3: Domed strata next to and above the diapir may trap hydrocarbons that do not migrate landward during the rotations in stage 1. Enormous seaward expansion of seaward-dipping strata indicate the likelihood of numerous pinchout traps if sands are present. Grounding of the most distal overburden (seaward of the diapir) onto the basement could enhance vertical migration from below the salt.
Stage 4: As in previous stages, counterclockwise rotation could enhance landward migration and welding could increase vertical migration. The fault could serve as seal or conduit for hydrocarbons in the roof of the passive distal diapir.
Demercian, S., Szatmari. P., and Cobbold, P. R., 1993, Style and pattern of salt diapirs due to thin-skinned gravitational gliding, Campos and Santos basins, offshore Brazil, Tectonophysics, v. 228, p. 393–433.
Ge, H., Jackson, M. P. A., and Vendeville, B. C., 1997, Kinematics and dynamics of salt tectonics driven by progradation: AAPG Bulletin, v.81, p. 398–423.
Mohriak, W. U., Macedo, J. M., Castellani, R. T., Rangel, H. D., Barros, A. Z. N., Latg, M. A. L., Mizusaki, A. M. P., Szatmari, P., Demercian, L. S., Rizzo, J. G., and Aires, J. A., 1995, Salt tectonics and structural styles in the deep-water province of the Cabo Frio region, Rio de Janeiro, Brazil, in Jackson, M. P. A., Roberts, D. G., and Snelson, S., eds., Salt tectonics: a global perspective: AAPG Memoir 65, p. 273–304.
Szatmari, P., Guerra, M. C. M., and Pequeno, M. A., 1994, Genesis of large "rift" by flow of Cretaceous salt in the south Atlantic Santos basin—Brazil (abs.): Geological Society of London Salt Tectonics Meeting, September 14–15, 1994, Programme and Abstracts, p. 35–36.
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This animation was released to the AGL's Consortium in October 1996. This article is published by permission of the Director, Bureau of Economic Geology, The University of Texas at Austin.