Soft-Sediment Deformation in Deltaic Deposits

Soft-Sediment
Deformation in Deltaic

Deposits

Gideon Bartov
Luke O’Sadnick
Mauricio Perillo
Amanda Peters
Pragnyadipta Sen

Outline

• Introduction to soft-sedimentary
deformation

• Structures
• Mechanism of formation of structures
• Analogue modeling
• County Clare Examples

Features for Identifying
Penecontemporaneous Structures

1. Convolute laminations are intrastratal

• bounding layers do not share the same
deformation

2. Usually truncated by an erosion surface

3. Folds are formed more easily due to:
- less consolidated sediment
- can deal with the space issue

4. Folds that are not greatly modified can be
identified by an axial surface that is cut
by earlier cleavage traces

Specific Penecontemporaneous
Structures

• Crinkly laminae

• Harmonic Crenulations

• Contorted to disrupted cross-strata

• Clastic intrusions

• Reworked, structureless and sometimes graded
beds and slumped or collapsed zones.

Salt Structures

• Form due to density differences between salt
and overlying strata.

Shale Structures

• Form in response to differential loading of
overpressured shales

• Regional and counter-regional faulting

Main mechanisms

• Gravity driven

– Density reversal
– Slumping

• Liquefaction
• Shear stress
• Differential loading

Gravity Driven

• Load structure: Sinking of heavier
sediments into lighter sediments.

• Ball and Pillow structure
• Slump



Liquefaction

• Sand volcanoes
• Clastic dikes

Shear Stress

• Convolute Lamination
• Folds

Differential Loading

• Diapirs

Why Analogue Models?

• Scarce exposures
• Dewatering of exposures
• Seismic data drawbacks
• Numerical data drawbacks
• Overdependence on salt tectonics

Methods

Model 1: Raft Structure

Model 2: Single Differential Load

Model 3: Two Stage Differential
Loading

Model 4: Variable Differential
Loading





Slumped Basin Margin, Gull Island Formation, Gull Island, Co Clare, Ireland
http://strata.geol.sc.edu/Deepwater/Clare-Basin-Clastics.html

References

• Blatt, H., Middleton, G., and Murray, R., 1972, Origin of Sedimentary
Rocks, Prentice-Hall.

• Ghosh, S.K., 1993, Structural Geology, Fundamentals and Modern
Developments, Pergamon Press.

• Lucchi, F.R., 1995, Sedimentographica: A Photographic Atlas of
Sedimentary Structures, Columbia.

• McClay, K., Dooley, T., Zamora, G., Eds., Rensbergen, V.P., Hills,
P., Maltman, A.J., and Morley, C.K., 2003, Analogue models of delta
systems above ductile substrates in Subsurface Sediment
Mobilization: Geological Society of London Special Publication, v.
216, p. 411-428.

• Morley, C.K., Eds., Rensbergen, V.P., Hills, P., Morley, C.K., and
Maltman, A.J., 2003, Mobile Shalerelated deformation in large deltas
developed on passive and active margins: Geological Society of
London Special Publication, v. 216, p. 335-357.