Dear Ali,

I think slump folds are forming in uncosolidated sediments which are solidified later. 

Strike-slip fault? It's already a vertical section. Which is the fault surface? Help me to understand you.

The researcher who downvoted anonymously the question, perhaps can explain us why.

Hi, do you have other pictures? Some information o the lithology of the limestone and surrounding rocks?

Dear Jerome,

I don't have further information except maybe from the following: There are no surrounding rocks, the successions of these limestones reaching 2-3 km at least thick. Their age varies from Jurassic to Eocene. They are mostly beige-coloured, they often have pyritic nodules and some times are consisted by monomictic limestone breccias. The thickness of the strata varies from cm scale up to several metres.

What do you have in mind?

Dear Ioannis,

from your descriptions I assume that the structure under consideration occurs within a normal (i.e. not inverted and not otherwise folded or thrusted) stratigraphic section. Hence I would rather assume that the structure is indeed a slump fold that formed shortly after sedimentation and before the overlying sediments were deposited. Otherwise it would be difficult to explain why only a certain level of the sequence has been subjected to folding. You mention also monomictic limestone breccias; such breccias may also develop by means of slump movements and other kinds of soft-sediment deformation shortly after sedimentation. What is a little bit confusing for me are the boudinaged layers which you indicated on the photograph: are they really boudins (i.e. a tectonically stretched competent layer within a less competent country rock)? If they were indeed boudins, this could be an indication for real tectonic deformation. On the other hand, the supposed boudins could also be just sedimentary concretions.

To sum up: it's difficult to judge from a photo alone, but taking into account your additional comments, I would suggest that the structure is a slump fold.

With best regards from Switzerland

Dominik 

Dear Dominik,

Τhank you for your excellent and skilled answer. I'll take into account all your suggestions.

Best regards

Ioannis Alexandridis

Hmmm...I wonder if it is not a fold illusion due to a kind of cross beding structure. Hard to say from one picture.

Need other pictures from other angles.

It's not, my dear Jerome! I know it's very difficult to say from one picture, but I must say that I observed a perfect shaped hinge, and additionally there were no cross-cutting relations between "limbs". Unfortunately, I don't have other pictures from this outcrop.

Kind regards

Ioannis Alexandridis

Dear Ioannis,

Its hard to decide according to pictures, giving limiting imagination. Anyway, slumps should be accompanied by sediment deformations and fractionation up to sedimentary breccias. What is important if you deal with limestones, is the filling of voids and cavities - if they are filled by calcite, the deformation is most probably post-compactional, probably of tectonic nature. Sedimentary cavities use to be filled rather by unconsolidated sediment...

Dear Ioannis,

The interpretation of 'boudinage' can safely be excluded due to the fact that the white lenses just occur inside the competent host rock, that do not show any deformation and pinches. Real boudinage axis consequentely should follow a regional trend of fold axis.

I do agree to Dominik's comprehensive response.

Kind regards

Guenter

This reminds me of a mass transport deposit (MTD), or some other mass wasting feature. Probably (?) created when before sediment was consolidated. Essentially I'm thinking this is a gravity-driven movement of sediments en masse, with the folded layer potentially the decollement zone or surface. Remember that depending on your frame of reference, a slump can look like a normal fault, but will also look like a thrust with compression features when you are near the toe. Geologic context of the basin should lead you to an educated/informed decision.

Hope this helps.

Dear Joannis,

Does your exposure give a profile view of the fold? This picture can also be a nearly axis-parallel cross-cut of a sharp, cylindrical, recumbent fold. Of course, when You measured the dip of the limbs, and have other evidences from the area about existence and geometry of folding in general which does not correspond to the geometry of this fold, it is much more probable that it is the product of soft-sediment deformation. Boudin-like structures can also form in soft-sediment state; do they have a constant orientation which can be identified with a b axis of the folding?

Dear Nemeth,

The section is not perpendicular to the b-axis of the fold, but from the measurements I found that it's isoclinal (

Dear Joannis,

maybe they are not boudins but only cherty nodules and lenses, consistent with a slope environment and hence with slumping.

Ciao

The replies to your question seem reasonable. I have a similar question, but don't think I can provide enough information to get a clear answer. The photo was taken in late September 2014 on Block Island, Rhode Island, USA, 250 km east of New York City which is part of a glacial terminal moraine for the last glacial maximum. The cliff is 25 or 30 m-high. I am on the beach. A few hundred meters to the right there is a weathered visitors information sign with geology that says there are thrust faults driven by glacial forces (certainly not tectonics; only 20,000 years old sediments!). But, instead of a thrust, could be a slump. Is the only feature like this along about 1 km of cliff that I looked at. I was looking at the cliff because I am working on Oligocene-Miocene glacial features in Ross Sea, Antarctica. An eroded 400 km x50 km trough is buried to between 3 and 6 km depth, and the resolution of the seismic reflection data is so low that nothing on the photo would be imaged.

Dear Christopher,

Maybe this structure isn't so clearly a slump, because a glacier like those existed in the area, is a great external-stress provider for the newly formed moraine. The stress from a glacier, it's not of course a tectonic stress, but as far as I know, slump folding is mainly associated with "external" forces such as weight, although weight alone cannot provide itself the final strain image we observe, but it's helped by the slope angles or in general, by the dynamic setting of the basin.

Maybe if you could define if the "fold" isn't or it is strictly associated with the glacier, then you could call it a slump fold or not.

Of course, I can't say that this structure is sedimentary, but mostly, I can't say neither it's tectonic. Very interesting single-feature though, such as your work in Antarctica.

Best regards

Ioannis Alexandridis

Ioannis,

the clue to your question has been addressed in a magnificently illustrated paper that was published some years ago by Tim Debacker, Manuel Sintubin & coworkers on the Journal of Structural Geology. I cannot remind the exact citation, but it should be easy for you to make a quick seacrh. Otherwise, with a bit of time, I can provide you the reference needed. You may also want to take a look at the references that they quote in their beautiful contribution!

Best wishes,

Enrico Tavarnelli

A quick comment... boudinage can occur during soft-sediment deformation. It only requires that there be competency contrast between different sedimentary layers during the deformation.

soft sediment deformation along slope may be responsible for development of such structures.The resulting geometry is based on the slope morphology and the position of the fold to the slope.

As noted by previous contributors, slump folds are frequently isoclinal and potentially sheath-like. Boudinage (on a variety of scales) is also generated within unlithified sediments. The key to this problem of separating 'soft-sediment' and 'tectonic' structures is to look carefully at the nature of the contact with the overlying undeformed sequence. Truncation along this surface can develop either through a) later tectonic detachments or, b) erosive contacts during continued sedimentation (suggesting the structure is soft-sediment). Some of these issues have been discussed in the attached publication and references therein.  

Dear professor Alsop,

Thank you for your response. I have study a vast part of your work considering slump folds but mostly sheath folds because of my studies in the area of greek Rhodope massif. They are the most enlightening within the relevant literature.

Ioannis Alexandridis

I've studied similar structures in different sections: Slumps like yours - if I read the picture clear enough - occur usually in heterolithic muddy sediments of lacustrine & marine environments with gentle paleoslopes. The sediment must have been still plastically deformable and therefore possess enough cohesion. Such muddy (clay + silt, carbonates and/or siliciclastics) sediments usually have low permabilties, slow / insufficient porewater drainage and are very sensitive to loading, shaking (seismites), boyancy by gas production and the like. The first mentioned processes lead to an increase in porewater pressure, followed by loss of shear resisivity and failure > slump. Such mass movements are commonly accompanying by sediment remobilisation and resedimentation > look for graded beds, resedimentet intraclast (micro-)breccias and the like in the paleo-downstream direction of your structure.

Dear Ioannis,

generally my opinion and interpretation of this section follows Dominik Letsch. I believe too that this structure represent a synsedimentary slump deposit.

My question for you concern the so called "boudinage" : Are you sure that this is really a boudinage ? Which kind of material are these boudins??  Layers and nodules e.g.  of chert may look like in a very similar way.........maybe you can give some more informations??

best regards

Andreas

Dear Andreas,

I am now convinced that the term "boudinage" which I have used, is wrong. At first place I place it into the picture as a description for the form which I saw and not for the possible cause. This led to a misinterpretation from my side and an over-show of something clearly sedimentary.

But let's think: the fact that the chert nodules have a such boudin-like shape, could easily and equally be a product of the tension which acted during syndepositional folding (then we could talk for "small-scale" tectonics in the vicinity of a slump itself) and/or be product of the sedimentological procedures.

I hope I gave you a better explanation

Best regards

Ioannis Alexandridis

Also syn-sedimentary slump in my opinion...suggested by non-folded but slightly deformed layers in the same bed just above the slump, which seem to "mantle" the slump "topography". Any indications on the facies/depositional environment?

Boudinage can indeed occur during soft-sediment deformation - it simply reflects extension and stretching of unlithified beds within the slump. Remember contraction and extension should balance within a slump (although in detail they may overprint one another). Cristophe Matonti is correct to point out that if beds above 'mantle the topography' then we are dealing with surficial slumps. Check out the photos in Fig. 3 of the attached paper that illustrate examples of this.

Thanks

ian

Dear Ioannis,

thank you very much for your explanations concernin the term "Boudinage"

best regards

Andreas

Dear Ioannis,

from the image it looks like anintra-formational fold, indeed and it looks like a slump fold or layer-parallel shear zone or detachment in soft sediments.

However, you mentioned that the socalled boudins are appearing in a chert layer. Did you consider that the boudin or lense-shaped chert nodules are an diagentic featurer of precipitation and growth of chert nodules in a confined layer? Difficult to see in the photo. Nodualr layers (chert, gypsum, carbonates) easily can look like "boudinage" as well.

Cheers, Jürgen

Dear Ioannis,

In addition to the suggestions already given: I have studied similar features in profundal lacustrine oil shales. If no extensional-contractional features (+breccias) are visible, that would be typical of slumps as mentioned earlier, it is possible to create similar features intrastratally (i.e. under shallow burial depth) in heterogeneous deposits, with contrasting rheologies. From the picture it seems that only the thin-bedded part is folded - that was susceptible to deformation. Is the bed above have an erosive base or it might deformed (slightly) with the folded interval?