Soft-sediment deformation in deltaic sediments

Transcription

1 Soft-sediment deformation in deltaic sediments Alie: Background about soft sediment deformation Mary: Sandbox modeling Nick: What we ll see in Ireland Becca: Modern analogs

2 Soft-Sediment Deformation Deformation that occurs during or soon after sediment deposition while sediment is still partially or fully saturated with water.

3 Liquefaction often referred to simply as liquefaction, is the process by which saturated, unconsolidated soil or sand is converted into a suspension during an earthquake Fluidization Similar to liquefaction Happens via interaction with a gas or liquid

4 A slump is a mass movement process of slope failure, in which a mass of rock or unconsolidated material drops along a concave slip surface. A slide is similar, but have less internal deformation and generated by relatively shallow sliding

5 Water escape structures Mainly vertical motion, water escapes pores mud and sand dykes and volcanoes, collapse depressions, mud diapirs and small-scale loading

6 Folds Local, common Faults and microfaults Occur within slumps Internal shear structures Similar to faults, but horizontal and only occur in slumps Late stages

7 Soft-sediment deformation in deltaic systems Sandbox models Key Points Soft-sediment deformation in delta systems occurs in an environment that changes through time. Why sandbox models? Allow us to control the parameters in a simplified system. Analog models show us that deformation style is related to sedimentary environment extension and compression occur in predictable places within the delta Key factors: pore water, compaction, grain-size; mobile-plastic layer (buoyancy, differential loading) Causes: force of gravity; paleoseismicity?

8 Soft-sediment deformation in deltaic systems Experimental setup McClay et al Video: Syn-Sedimentary Deformation of a Fan

9 Soft-sediment deformation in deltaic systems McClay et al Time = 1.5 hr Load = 2.4 cm Delta-top grabens are the first structures to form Delta-toe thrust faults are not yet formed Delta-top graben system Delta slope Delta-toe bulge

10 Soft-sediment deformation in deltaic systems New grabens develop on the delta-top and delta-slope (they are synkinematic) McClay et al Time = 3 hr Load = 3 cm Delta-toe folds and thrusts begin to form Delta-top graben system Delta slope Delta-toe fold-thrust belt

11 Soft-sediment deformation in deltaic systems Final state McClay et al Delta-top graben system grows as mud diapirs rise (driven by buoyancy forces) Time = 19 hr Load = 5.5 cm Delta-toe fold-thrust belt is now well-developed Delta-top graben and diapir system Translated delta slope Delta-toe fold-thrust belt

12 Soft-sediment deformation in deltaic systems Sandbox models Conclusions What is it about deltaic systems that is conducive to soft-sediment deformation? Sediment is saturated; alternating mud/sand Delta slope (heavy things move downhill) Can we correlate sedimentary environment with the style of structural deformation?... Yes. Delta-top: extension Delta-toe: compression Mud diapirs (mud and sand volcanoes): multiple environments What does the process look like over time? Thank god for sandbox experiments with known parameters New structures develop in sediments over time (i.e. new faults cut across old ones, or new sediments conceal existing faults)

13 Growth Faults

14 Outcrop of Growth Fault

15

16

17 What s a Bouma? Play a major role in the soft sediment deformation in the Gull island formation. Flow patterns were controled by sliding and slumping.

18

19 The Mississippi River Delta -gravitationally driven system with movement along mobile salt/shale detachment -shallow deformation caused by high rates of fluvial deposition and mud-flows -greater occurrence of plastic flow deformation due to large amounts of organic matter that release methane within deltaic sediment

20 The Nile River Delta

21 A link! If there is time. DC1/00012-Animation02.gif Hypothetical evolution of the Mississippi River delta through the next glacial maximum. Apart from the plausible guess that the next cycles of delta growth will be in the coastal bight west of the present delta, locations of deltas and channels are purely speculative for the purpose of showing the complexity of future geology and the relationship between eustasy and delta formation. Pliocene and older deposits are shown in peach, Pleistocene deposits in cream, and Holocene deposits in light gray. Recent deltas at each 10 ka interval are shown brightly colored, and older delta complexes are in subdued tones.

22 References Dutch, S.I., May 2006, The Earth Has a Future, Geosphere, v. 2, p Martinsen, O.J., 1989, Styles of soft-sediment deformation on a Namurian (Carboniferous) deltaslope, Western Irish Namurian Basin, Ireland, in Whateley, M.K.G., and Pickering, K.T., eds., Deltas: Sites and traps for fossil fuels: Geological Society of London Special Publication 41, p McClay, K., Dooley, T., Zamora, G., 2003, Analogue models of delta systems above ductile substrates: Geological Society, London, Special Publications, v. 216, p Stanley, J-D, 2003, Nile Delta margin: failed and fluidized deposits concentrated along distributary channels, Géomorphologie : relief, processus, environnement, n. 4, p Wignall P, Best J. Sedimentology and kinematics of a large, retrogressive growth-fault system in Upper Carboniferous deltaic sediments, western Ireland. Sedimentology [serial online]. December 2004;51(6): Available from: Academic Search Premier, Ipswich, MA. Accessed April 25, 2012.