Guglielmo, Giovanni, Jr., D. D. Schultz-Ela, and M. P. A. Jackson 1997, Raft tectonics in the Kwanza Basin, Angola: an animation. A BEG hypertext multimedia publication on the Internet at: http://www.beg.utexas.edu/indassoc/agl/animations/AGL96-MM-003/index.html.

RAFT TECTONICS IN THE KWANZA BASIN, ANGOLA: AN ANIMATION

GUGLIELMO, G., Jr., D. D. SCHULTZ-ELA, and M. P. A JACKSON

Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas 78713-8924 USA

ABSTRACT

An animation showing a palinspastic reconstruction of a seismic section from the Kwanza Basin, Angola, illustrates the evolution of rafts (fault blocks isolated by extreme extension) above a décollement of salt. The deformation involves reactive, active, passive, subsiding, and remnant diapirs; pre-rafts and true rafts; half-grabens; and salt welds. The animation shows structures that are analogous to hydrocarbon-bearing basins in both sides of the South Atlantic, Gulf of Mexico, and Red Sea, among other areas.

ANIMATION

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INTRODUCTION

Raft tectonics is the most extreme form of thin-skinned extension above a décollement of salt. Interactions between rafts (Burollet, 1975) and underlying mobile salt during gravity gliding and spreading may result in unusually complex structures. For example, rafts may (1) glide downslope far from their place of origin, (2) separate from each other, creating gaps where sediments accumulate, (3) subside owing to displacement of underlying salt, creating overlying synkinematic basins, (4) juxtapose with other fault blocks or onto the basement, where the resulting expulsion of intervening salt layers creates gaps in the stratigraphy, or (5) split salt walls into multiple isolated walls or diapirs. These processes may occur simultaneously and interact with each other. As a result, final structures in a seismic section such as in the figure above may disguise a complex history. This animation attempts to clarify this history for structures in the Kwanza Basin, Angola, in the western divergent continental margin of Africa. Similar structures are mirrored in the South American eastern margin (Campos Basin), Nordkapp Basin in Norway, eastern Mediterranean Basin, the Gulf of Mexico, and the central and southern Red Sea.

 

DESCRIPTION AND INTERPRETATION

This animation shows a restoration of a seismic section perpendicular to a rifted continental margin. The time-migrated section was roughly depth converted and restored by hand (Jackson and Cramez, 1989, Duval and others, 1992) to introduce the process of salt welding. Subsequently the section was computer restored (Schultz-Ela, 1992) to introduce the process of diapiric fall induced by extension (Vendeville and Jackson, 1992). The same computer restoration forms the basis for this animation.

Whereas the basement remains the same length, the overburden (colored layers) extended by gravity gliding over a salt layer (purple). A fold and thrust belt formed downdip (not visible, to the left of the animation) to accommodate extension. The section "Potential structural traps for hydrocarbons" suggests how structural traps could be created or destroyed. The animation lasts 16 s. Percentages of elapsed time listed below show the approximate age and duration of each stage with respect to the entire history. Deformation can be described in 6 sequential stages.

Stage 1 (0–7% time elapsed)—Pre-rafts and reactive diapir: Early extension triggers a reactive diapir. Fault blocks in the overburden (pre-rafts) are still in mutual contact.

Stage 2 (7–12% time elapsed)—Active diapir: This diapir emerges actively through the dark-blue overburden.

Stage 3 (12–27% time elapsed)—Rafts and passive diapir: During deposition of the green layer, the diapir grows passively in the widening gap between two isolated fault blocks, now forming rafts (A). Rafts are free to rotate and tilt above a cushion of mobile salt.

Stage 4 (27–43% time elapsed)—Depocenter and sagging diapir: The subsiding rafts constrict the rate of salt flow from the source layer toward the diapir. However, regional extension continues to widen the diapir. Thus, when salt supply is too slow to fill the widening diapir to its current height, this diapir has to subside. Consequently, during deposition of the yellow layer, the diapir accommodates a local depocenter (B) above it.

Stage 5 (43–68% time elapsed)—Half-graben: The depocenter above the diapir evolves into a half-graben (C) bounded on the right by a major growth fault (D). The half-graben hangingwall depocenter consistently rotates clockwise. The age of the oldest sediment in the depocenter core (B) indicates the time when the original diapir began to subside. Flatlying sediments are continuously deposited synkinematically. Progressive rotation of the half-graben tilts these strata, creating apparent downlaps (E). The diapir shrinks in cross-sectional area, perhaps partly by dissolution while the salt was shallow, but most likely by flow out of the plane of section when the salt was deeply buried. The final width of the half-graben is roughly proportional to the amount of extension since rafting began.

Stage 6 (68–82% time elapsed)—Remnant diapirs and salt welds: Ultimately the diapir splits into two smaller remnant diapirs (F), and the Tertiary depocenter (yellow to brown) welds directly on the pre-salt Cretaceous strata (white) producing a stratigraphic gap of 60-90 Ma (Duval et al., 1992). The weld (G) is recognized by discordance or local inversion above it. For the remaining elapsed time (82–100%), the cross section transforms back into the seismic section.

 

POTENTIAL STRUCTURAL TRAPS FOR HYDROCARBONS

Each stage of deformation described above could create or destroy structural traps for hydrocarbons. The following assessment is based on the two-dimensional section and would depend on structural or stratigraphic closure in the third dimension and on the existence of reservoirs and hydrocarbons.

Stage 1Reactive diapirs could produce typical diapir-related hydrocarbon traps.

Stage 2Salt-wall emergence compartments reservoirs in the roof of the reactive diapir.

Stage 3Subtle tilting of the rafts at this stage or throughout the deformation history could redirect the flow of oil (a) away from the diapir to be trapped by sealing extensional faults within each raft, or (b) toward the diapir to accumulate at the diapir-raft interface.

Stage 4Diapir subsidence may create hydrocarbon-trapping turtle structures in the adjacent rafts.

Stage 5Clockwise rotation of the raft on the right may direct hydrocarbons toward the half-graben. The clockwise rotation of the half-graben (C) could cause hydrocarbons within the half-graben to migrate seaward away from the growth fault (D), to be trapped against the structurally higher left boundary of the half-graben. The age of the oldest sediment in the depocenter core (B) suggests a minimum age for the beginning of this migration.

Stage 6—Diapir subsidence could induce counterclockwise rotation of the left raft and counterclockwise rotation of the right raft. These rotations could direct hydrocarbons away from traps adjacent to the diapirs toward turtle structures within the rafts.

 

REFERENCES

Burollet, P. F., 1975, Tectonique en radeaux en Angola: Bulletin de la Société Géologique de France, v. XVII, p. 503–504.

Duval, B., Cramez, C. and Jackson, M. P. A., 1992, Raft tectonics in the Kwanza basin, Angola: Marine and Petroleum Geology, v. 9, p. 389–404.

Jackson, M. P. A., and Cramez, C., 1989, Seismic recognition of salt welds in salt tectonics regimes, SEPM Gulf Coast Section Tenth Annual Research Conference Program and Abstracts, Houston, Texas, p. 66–71.

Schultz-Ela, D., 1992, Restoration of cross-sections to constrain deformation processes of extensional terranes: Marine and Petroleum Geology, v. 9, p. 372–388.

Vendeville, B. C., and Jackson, M. P. A., 1992, The fall of diapirs during thin-skinned extension: Marine and Petroleum Geology, v. 9, p. 354–371.

 

ACKNOWLEDGMENTS

Amanda Masterson Stephen Seni, Stephen Laubach, and Andrew Scott improved the manuscript. This animation was previously released to the AGL's Industrial Associates and is published by permission of the Director, Bureau of Economic Geology, The University of Texas at Austin. 


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