Allochthonous Salt Sheet During Progradation

Guglielmo, Giovanni, Jr., Vendeville, B. C., and Jackson, M. P. A., 1998, Map View of Evolution of a Complex Allochthonous Salt Sheet During Progradation: A BEG hypertext multimedia publication on the Internet at: https://www.beg.utexas.edu/agl/animations/AGL98-MM-007.

MAP VIEW OF EVOLUTION OF A COMPLEX ALLOCHTHONOUS SALT SHEET DURING PROGRADATION

GUGLIELMO, G., Jr., VENDEVILLE, B. C., and JACKSON, M. P. A.

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

ABSTRACT

An animation showing an overhead view of a physical model illustrates the evolution of a complex allochthonous salt sheet during progradation. The deformation includes box folding, graben and reactive diapirs, slumping, multiple salt breakouts, salt canopies, rafts, stacked salt flows, diapir subsidence, diapir roof dismemberment, arcuate grabens with listric faults, salt welds, and grounding and colliding rafts. Rafts may be free floating, fully grounded, or partially grounded.

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ANIMATION

DESCRIPTION AND INTERPRETATION

This animation shows a series of photographs taken during deformation of a scaled physical model. The animation illustrates a map view of the complex evolution of an allochthonous salt sheet during progradation. Illumination is from the right. The green line in the opening photo represents the location of a cross section that was previously restored and animated.

Throughout the experiment, synkinematic sediments (simulated by dry sand) prograde from left to right across a flat-topped horizontal salt basin or sheet (simulated by viscous silicone animated in brown). Sediment progradation is shown as changes in color and texture of sand in the left side of the animation. The apparent seaward strains of sand surface during the first 25% of the animation are an artifact of the animation process.

The animation lasts 12 seconds and represents approximately 54 hours of the original experiment. The resulting deformation can be described in nine sequential stages, which are named consistently with the stages in the animated cross section to facilitate cross-referencing. Percentages of elapsed time listed below show the approximate age and duration of each stage with respect to the entire animated history. When many processes happen quickly and simultaneously (e.g., at salt breakouts and flows), this duration was increased in the animation for clarity.

Stage 1 and 2 (0–20% time elapsed)—Box folding: At the onset of deformation, the weight of overlying prograding sediments pressurizes the salt and squeezes it seaward (to the right). The pressurized salt lifts and deforms the prekinematic layer into a seaward-verging box fold, which forms by bending not buckling. The frontal hinge of this fold is sinuous in map view owing to uneven rates of salt flow to the fold's core.

Graben and reactive diapirs: A linear backgraben forms on the landward hinge of the box fold and is progressively covered by prograding sediments. An analogous foregraben forms on the seaward (right-hand) hinge of the box fold. The grabens are delineated by striated depressed zones. These grabens locally weaken and thin the overburden, causing differential loading, which triggers reactive diapiric walls below each graben. These walls are visible only in cross section.

Stage 3 (20–30% time elapsed)—Slumping of forelimb: Both linear grabens widen and deepen. These grabens, like the hinge of the box fold, are curvilinear in map view. The forelimb of the box fold steepens, slumps, and sheds pale-brown sediments onto a fan at its base.

Stage 4 and 5 (30–60% time elapsed)—First breakout of salt: The weight of prograding sediments continuously increases salt pressure, including within the box fold. The rear diapir continues to rise reactively. The reactive diapir underlying the foregraben pierces actively and emerges along zones of greatest extension at the two seaward salients of the foregraben, and at the lateral edges of the model owing to edge effects. Salt expelled from the foregraben (breakout 1) flows seaward down and over the resedimented forelimb. Traction exerted by the extrusions folds the partially collapsed forelimb into a flap in the form of an isoclinal recumbent syncline underlying each salt flow (seen only in cross section). In map view, as the extrusive salt spreads radially, we infer that the synclines propagate, northward and southward, away from each salt breakout site. Extrusive salt spreads radially as lobes in the abyssal plain like piedmont glaciers.

Canopies: These salt flows coalesce laterally along sutures to form a canopy with a scalloped leading margin (allochthonous sheet 1). The lobate front of this canopy resembles that of the Sigsbee Scarp in the abyssal Gulf of Mexico.

Rafts: The roof and backlimb of the box fold (comprising the original prekinematic layer) are passively carried by the underlying salt sheet. They extend and segment into rafts that migrate seaward as overthrusts . The overthrusted rafts above the isoclinal syncline create a triple stratigraphic repetition of the prekinematic layer, a repetition visible only in cross section. Rafts may later ground and collide as described in Stage 9.

Stacked salt flows: Sedimentation buries the backgraben, but the diapir below the backgraben locally evolves to a passive stage, emerging and extruding near the northern breakout point (breakout 1a), creating an additional seaward salt flow that (1) overlies the flow of Stage 4, (2) overflows and disrupts fault blocks of the backgraben, and (3) drags overlying, prograding sediments seaward. The new salt flow covers early detached rafts, which become entirely engulfed by salt.

Stage 6 (60–70% time elapsed): Further sedimentation (thick prograding brownish-gray strata on the left and bluish-purple strata on the right) buries the allochthonous rafts and the salt extrusions.

Stage 7 and 8 (70–75% time elapsed)—Renewed breakouts and roof dismemberment: Continuous pressure due to seaward progradation reactivates salt extrusions (1) at shallowly buried former breakout sites in the ruins of the original box fold and (2) in grabens above the flowing salt canopy. Salt flow from the former breakout sites covers newly deposited strata downslope and the roof of a few rafts. This new breakout causes local folding and slumping downslope from, and next to, the breakout point (not visible at the scale of the animation).

Arcuate grabens with listric faults: More extension due to spreading of salt flow seaward causes differential loading and thinning of the overburden, triggering buried reactive diapirs (emergent at A) and large seaward-dipping listric faults at linear and arcuate grabens (B).

Subsidence of diapir: These subsidence-related structures were rapidly covered by sediments and are only visible in cross section. Sedimentary loading and salt expulsion seaward through breakouts 1, 1a, and 2 enhance subsidence of the rear diapir. Both flanks subside and tilt toward the diapir as overlying strata locally thicken above its crest. Continuous rotation of the left footwall ultimately produces a structure that resembles a counter regional growth fault along the left flank of the diapir at the left end of the animation window. However, no slip occurred along this pseudo fault. To the left of the animation, diapir subsidence created a broad salt-expulsion rollover anticline crested by a late, linear, keystone graben system visible in map view (C).

Stage 9 (75–100% time elapsed)—Salt welds and diapirs: The weight of overlying sediments, combined with overthrusting, join strata at the breakout conduits created in Stages 4 and 7, restricting flow of salt, and eventually producing salt welds (only visible in cross section). Small diapirs at the seaward edge of the basin continue to evolve and some extrude (A).

Grounding and colliding rafts: Rafts can be free floating, fully grounded, or partially grounded:

Free floating rafts are carried by flow of underlying salt (D). They translate downslope, rotate about a vertical axis, can reach high speeds, have negligible influence on the direction of salt flow, and actively impinge against and deform neighboring rafts.

In contrast, grounded rafts are no longer supported by underlying salt so become stationary obstacles (E). They influence salt flow directions and only passively collide with impinging free-floating rafts. Grounded rafts are usually bounded by a basal or lateral fault weld.

Partially grounded rafts share characteristics of both free-floating and grounded rafts (F): (1) They collide both passively and actively with other rafts and (2) their displacement paths both change and are changed by underlying salt flow. Partially grounded rafts are liable to rotate about an axis fixed on their grounded parts.

Salt quickly flows downslope from primary breakout vents and can carry rafts for great distances (D). Thus, the proportion of free-floating rafts in the center and front of salt lobes tends to be high.

In contrast, at the sides of each lobe, the salt is thinner, and flows more slowly commonly obliquely to the downslope direction. So rafts move slowly and are more likely to ground fully or partially.

If salt flows are multiple and have shifting directions, rafts may evolve through several grounding modes.

Rotated and offset structures (bedding, paleocurrent trends, piercing lines, etc.) on a regional scale could be used as markers to determine the rotation and translation of individual rafts to constrain palinspastic reconstructions. These markers would be most effective if raft rotation history were brief and simple, so orientation markers could still be related to their original position.

Raft collision could induce local deformation (reverse faulting, buckling, tilting, rotation, etc.) that would disrupt reservoirs, change hydrocarbon migration paths, or form new closures.

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ACKNOWLEDGMENTS

This animation was released to the AGL's Consortium in March 1996. This article is published by permission of the Director, Bureau of Economic Geology, The University of Texas at Austin.