Origin of “Drag” FoldsOrigin of “Drag” FoldsBordering Salt DiapirsBordering Salt Diapirs

D. D. Schultz-ElaD. D. Schultz-ElaBureau of Economic GeologyBureau of Economic Geology

John A. and Katherine G. Jackson School of GeosciencesThe University of Texas at Austin

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Applied Geodynamics Laboratory

Industrial Associates

Amerada Hess Corporation

Anadarko Petroleum Corporation

BHP Petroleum (Americas)

BP Amoco Production

Burlington Resources

Chevron USA Production

Conoco

ENI - AGIP

Enterprise Oil

Exxon Mobil

Marathon Oil Company

PanCanadian Petroleum

Petroleo Brasileiro

Phillips Petroleum Company

TotalFinaElf

Woodside

Bureau of Economic Geology

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MOTIVATIONMOTIVATION

(after Davison et al., 2000)

Drag folds

Withdrawal folds

Contraction folds

Modeling of:

Not:

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PREKINEMATIC MODEL

Model simulates cross section through an existing salt wall. Geometry and density contrast favor vigorous salt rise . Passive lines track displacements. Salt is eroded back to overburden elevation during rise.

Sym

met

ry li

ne

Symmetry line

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STRONG OVERBURDENSTRONG OVERBURDEN

“Strong” overburden: no pore pressure, normal friction angle (31°), low cohesion (0.1 MPa). Relatively rigid subsidence and tilting. Deformation only at top corner; diapir spreads near surface. Dashed lines show original overburden position and restored salt surface.

Vertical displacement t = 266 kaMax: -173 m

Min: -275 m

Insignificantuplift

Current top saltrestored to t=0

Salt

Overburden

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OVERPRESSURED OVERBURDENOVERPRESSURED OVERBURDEN

Pore pressure/Total pressure () = 0.9.

More drag deformation:

• Deformed zone wider and deeper.

• More spread of diapir top.

But only minor folding, even with extremely weak rock.

Vertical displacement

t = 132 ka

-100 m

-200 m

Minor localuplift

Overburden = 0.9

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OVERPRESSURED OVERBURDEN, SALT LAYERSOVERPRESSURED OVERBURDEN, SALT LAYERS

Vertical displacementt = 170 ka

200 m

-350 m

Overburden = 0.9

Highly overpressured overburden (=0.9) alternating with salt layers.

Substantial folding.

Folding increases upward.

More lateral, not vertical, deformation near surface .

Highly overpressured overburden requires very weak interbeds to drag fold across wide zone.

Weaksaltlayers

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Smooth-walled diapir for comparison.

IRREGULAR DIAPIR SHAPEIRREGULAR DIAPIR SHAPE

Vertical displacement

t = 245 ka200 m

Significant drag folding of overburden protrusions.

Most folding near top surface.

Uplift above regional.

-200 mIrregular initialsalt

contact

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TILTED DIAPIR, OVERPRESSURED OVERBURDENTILTED DIAPIR, OVERPRESSURED OVERBURDEN

Vertical displacement

t = 283 ka

Diapir spreads near surface, shallow folding in very weakest overburden

Footwall uplift at base

Protrusions tend to fold, especially if shallow

? Folding increases upward, unlike observed “drag” folds

300 m

-350 m

15° diapir tilt

Hang

ingw

all

Foot

wal

l

Overburden = 0.9

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2° slopeOverburden = 0

Overburden density increases following normal shale compaction curve. Strong rock, 60 25-m-thick layers (groups of 4 shown), 1 mm a-1 aggradation. Salt flow driven by overburden relief, slope break always located 3 km from left boundary

(vertical dashed line). Emergent salt eroded until layer 56. Thick source layer – no effects of limited salt supply.

Slopebreak

25-m-thick layer

First layer

Second layer2° slope DepositErode

1 km3 km

Salt

Salt

SYNKINEMATIC, INCREASING DENSITYSYNKINEMATIC, INCREASING DENSITY

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SYNKINEMATIC, INCREASING DENSITYSYNKINEMATIC, INCREASING DENSITY

Older layers onlap far, create long, gently tapered wedges.

• Easily folded by salt rise, “drape” folds. Younger layers onlap steeper salt contact, create short thick wedges.

• Narrower folds, older folds static. Strength proportional to thickness, so older layers form broad, high-amplitude folds.

500 m

1000 m

Long onlap onto rising salt crest

Decreasing onlap;onto preceding layer

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SYNKINEMATIC, INCREASING DENSITYSYNKINEMATIC, INCREASING DENSITY

Salt contact steepens to vertical, then is overturned.

Width of folded zone narrows.

Diapir would spread next; no onlapping flap to fold.

Most folding ends.

Diapir crest rolls upward against sediment, stretches even as crest narrows.

Crest would stretch and disaggregate any cover, slumping of steep parts.

1500 mNarrow and shallowzone of folding

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SYNKINEMATIC, PULSED DEPOSITIONSYNKINEMATIC, PULSED DEPOSITIONSheath < 2 m, eroded

50-m every sixth layer starting at layer 41; same average aggradation rate (twice the thickness, twice the rise time).

First thick layer (green) stretches upward into sheath, blocks onlap of later layers.

Sediment pulses create long thin flaps easily carried upward with salt.

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SYNKINEMATIC, THICK LAYERSSYNKINEMATIC, THICK LAYERS

Strong rock, 18 100-m-thick layers, 1-mm a-1 aggradation.

Thicker layers onlapped farther; oldest layers had long thin tapers.

Slope break at 5 km, longer model.

No salt erosion less onlap, steeper diapir walls.

Horizontal2° slopeSlopebreak

Overburden = 0

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SYNKINEMATIC, THICK LAYERSSYNKINEMATIC, THICK LAYERS

Bulge in early wedge forms crane’s head, blocks later progradation. Latest layers contract horizontally as diapir spreads. Shortened layers adjacent to greatly thinned and stretched uplifted layers.

Stretched and thinned older layersContracted youngest layers

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Very weak overburden Strong overburden

SYNKINEMATIC, VERY WEAK LAYERSSYNKINEMATIC, VERY WEAK LAYERS

Very weak overburden develops patterns similar to model having strong overburden, except:

• Wider folded zone.

• More horizontal stretching of overburden as salt flows toward diapir: more tectonic thinning of wedge, early layers carried higher.

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SYNKINEMATIC SEDIMENTATIONSYNKINEMATIC SEDIMENTATION

(after Johnson & Bredeson, 1971)

Maximum drag folding where sediment onlapped farthest across salt:

• salt rise aggradation rate

• sediment pulse (sand prone?)

• beneath depositional hiatus (more rise)

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True drag folding of prekinematic overburden into wide drag zones only possible in highly overpressured, anisotropic sedimentary rock.

CONCLUSIONSCONCLUSIONS

Zones of increased folding onlaps over or protrusions into salt.

Maximum folding near free top surface —overburden rotates upward and outward as diapir spreads.

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CONCLUSIONSCONCLUSIONS

Synkinematic sedimentation onlapping a narrowing diapir continually adds new shallow layers greatest potential for folding.

“Drag” folding most likely by synkinematic sedimentation during downbuilding; shear from salt drag has much less effect. Flap folds possible even in strong rock.

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Flap folds decrease in width and amplitude upward as diapir steepens.

CONCLUSIONSCONCLUSIONS

Drag folds in prekinematic strata increase in width and amplitude upward.

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Increased net aggradation rate: onlap , salt rise , burial.

Decreased net aggradation rate: onlap , salt rise , spread.

Equal rates: thick layers onlap farther and have more uplift time.

Vigorous salt rise may decrease folding potential.

Folding most likely for episodic or variable deposition rates.

Depositional hiatus increases time for folding of underlying layers.

CONCLUSIONSCONCLUSIONS

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CONCLUSIONSCONCLUSIONS

Models may show just one instance of a commonly cyclical process.

Cyclical variations in rates of salt rise and sediment deposition could form a series of stacked flap folds separated by salt flanges or slumps that record times of salt spreading and possible overturning of underlying slumped and stretched flaps.

• Unconformity-bounded packages, called halokinetic sequences by Giles and Lawton (2002), observed in La Popa Basin diapirs, Mexico

?