Origin of “Drag” Folds Bordering Salt Diapirs

1. Origin of “Drag” FoldsBordering Salt Diapirs D. D. Schultz-ElaBureau of Economic Geology John A. and Katherine G. Jackson School of Geosciences The University of Texas at Austin

2. Bureau of Economic Geology Applied Geodynamics Laboratory Industrial Associates Amerada Hess Corporation Enterprise Oil Anadarko Petroleum Corporation Exxon Mobil BHP Petroleum (Americas) Marathon Oil Company BP Amoco Production PanCanadian Petroleum Burlington Resources Petroleo Brasileiro Chevron USA Production Phillips Petroleum Company Conoco TotalFinaElf ENI - AGIP Woodside

3. Drag folds Withdrawal folds Contraction folds MOTIVATION Modeling of: Not: (after Davison et al., 2000)

4. PREKINEMATIC MODEL Symmetry line Symmetry line • 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.

5. “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. STRONG OVERBURDEN Insignificant uplift Max: -173 m Vertical displacement t = 266 ka Min: -275 m Overburden Current top salt restored to t=0 Salt

6. Pore pressure/Total pressure (l) = 0.9. More drag deformation: Deformed zone wider and deeper. More spread of diapir top. But only minor folding, even with extremely weak rock. OVERPRESSURED OVERBURDEN Vertical displacement Minor local uplift t = 132 ka -100 m Overburden l = 0.9 -200 m

7. Highly overpressured overburden (l=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. OVERPRESSURED OVERBURDEN, SALT LAYERS Vertical displacement t = 170 ka 200 m Overburden l = 0.9 Weak salt layers -350 m

8. Significant drag folding of overburden protrusions. Most folding near top surface. Uplift above regional. IRREGULAR DIAPIR SHAPE Vertical displacement 200 m t = 245 ka Irregular initial salt contact -200 m • Smooth-walled diapir for comparison.

9. 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 TILTED DIAPIR, OVERPRESSURED OVERBURDEN Vertical displacement t = 283 ka 300 m Footwall Hangingwall -350 m Overburden l = 0.9 15° diapir tilt

10. 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. SYNKINEMATIC, INCREASING DENSITY First layer Slope break 2° slope Overburden l = 0 25-m-thick layer 3 km 1 km Salt Second layer Deposit 2° slope Erode Salt

11. 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. SYNKINEMATIC, INCREASING DENSITY Decreasing onlap; onto preceding layer 1000 m Long onlap onto rising salt crest 500 m

12. 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. SYNKINEMATIC, INCREASING DENSITY Narrow and shallow zone of folding 1500 m

13. 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. SYNKINEMATIC, PULSED DEPOSITION Sheath < 2 m, eroded

14. 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. SYNKINEMATIC, THICK LAYERS Slope break 2° slope Horizontal Overburden l = 0

15. 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. SYNKINEMATIC, THICK LAYERS Stretched and thinned older layers Contracted youngest layers

16. 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. SYNKINEMATIC, VERY WEAK LAYERS Very weak overburden Strong overburden

17. Maximum drag folding where sediment onlapped farthest across salt: salt rise £ aggradation rate sediment pulse (sand prone?) beneath depositional hiatus (more rise) SYNKINEMATIC SEDIMENTATION (after Johnson & Bredeson, 1971)

18. Zones of increased folding ðonlaps over or protrusions into salt. CONCLUSIONS • True drag folding of prekinematic overburden into wide drag zones only possible in highly overpressured, anisotropic sedimentary rock. • Maximum folding near free top surface —overburden rotates upward and outward as diapir spreads.

19. Synkinematic sedimentation onlapping a narrowing diapir continually adds new shallow layers  greatest potential for folding. CONCLUSIONS • “Drag” folding most likely by synkinematic sedimentation during downbuilding; shear from salt drag has much less effect. Flap folds possible even in strong rock.

20. CONCLUSIONS • Flap folds decrease in width and amplitude upward as diapir steepens. • Drag folds in prekinematic strata increase in width and amplitude upward.

21. CONCLUSIONS • 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.

22. CONCLUSIONS ? • 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