Dear Dilshad,

I think it is algae mat could you send it to my email so I may double check it? [email protected]

Regards,

Thanks Rzgar,

Done, but bear in mind that the age of the formation is more than 720 My.

with best wishes,

Dilshad.

would you compare it to this image.

The different between them are: your figure is patchy while mine are more look like flakes. 

Hi Dilshad,

Spencer (1971) describes “flake-breccia beds” in the Islay Limesone, derived from desiccation of underlying stromatolitic growth-layers. However, the flakes in his example are generally curled and exhibit clast-supported fabrics, unlike your pictured example.

Your mottled limestone appears to be a matrix-supported pseudoconglomeratic micrite. The clasts range from subspherical to tabular (elongate, but generally uncurled), are angular to moderately rounded, with isolated tabular examples showing occasional edgewise orientations.

“Autoconglomeratic” textures like these in lime mudstone record soft-sediment disruption caused by selective early post-depositional cementation, compaction and fluidization, hydraulic disaggregation, and appear to involve broadly coeval brittle fracturing and ductile deformation.

I see no evidence at outcrop scale for stromatolitic precursors – however, thin section analysis may provide evidence for algal involvement in the formation of the intraclast component.

Check out the work of S.K. Chough, and Jitao Chen who have both published examples of this type of soft-sediment process in lime mudstones.

Hope this helps

Regards

Graham

Spencer, Anthony Mansell (1971) "Late pre-Cambrian glaciation in Scotland." Geological Society of London, Memoir v.6; p5-102

I suggest a structure in algal life position

Dear Graham,

Many thanks for your comment, it is really appreciated. Before I post the photos here, I read the memoir that you mentioned above. It is really a good piece of work; luckily, I have a hard copy of that memoir. Also, as you pointed out the description there about mudcracks and curl flakes with these photos here, are a bit unlike. I just want to check if there is any other opinion or if someone has a different idea. However, mudcracks usually concentrated at the top part of the bed, but this mottled structure is going through the bed from base to top.

Thanks for suggesting above scientific articles; I will check with them.

Best regards,

Dilshad.

Dear Miloud,

Thanks for your comment, but what do you mean by algal life position? Can you be more precise please?

with best wishes,

Dilshad.

It may be burrow structure or algale

Hi So: Thanks for your comment.

The age is Cryogenian, and at that time we don't have an organism to leave the trace fossil. But what do you mean by algae? Please can you be more specific? Or can you suggest any article?

Best regards,

Dilshad.

Hi Dilshad,

If you want to follow up on the limestone pseudoconglomerate/soft-sediment deformation/fluidization idea, check out the following papers

Chough, S. K., et al. "Autoconglomeration of limestone." Geosciences Journal 5.2 (2001): 159-164.

Chen, Jitao, et al. "Limestone pseudoconglomerates in the Late Cambrian Gushan and Chaomidian Formations (Shandong Province, China): soft‐sediment deformation induced by storm‐wave loading." Sedimentology 56.4 (2009): 1174-1195.

and then review the papers which have cited them, particularly those of AJ Van Loon.

If the processes described above are applicable to your cryogenian lime mudstones it may be significant - I do not think they have been hitherto recognized in the Islay Limestone. 

Cheers,

Graham

Hi Graham,

Your comments are really helpful, thank you. However, I searched more about your first suggestion in former comment. I found they likely to be thrombolite than limestone pseudoconglomerate/soft-sediment deformation/fluidization idea. Because the sizes of the flakes are up to 15 mm, and commonly they are 2-5 mm. Nevertheless, based on the articles that you suggested, the size of clasts within pseudoconglomerate is more than 2cm. Here are papers that I checked with and slightly convinced with thrombolite:

Feldmann, Mark, and Judith A. McKenzie. "Stromatolite-thrombolite associations in a modern environment, Lee Stocking Island, Bahamas." Palaios 13.2 (1998): 201-212.

Shapiro, Russell S., and Stanley M. Awramik. "Favosamaceria cooperi new group and form: a widely dispersed, time-restricted thrombolite." Journal of Paleontology 80.03 (2006): 411-422.

Kennard, John M., and Noel P. James. "Thrombolites and stromatolites: two distinct types of microbial structures." Palaios (1986): 492-503.

Johnson, J., and J. P. Grotzinger. "Affect of sedimentation on stromatolite reef growth and morphology, Ediacaran Omkyk Member (Nama Group), Namibia." South African Journal of Geology 109.1-2 (2006): 87-96.

Especially article nos. 1 and 2 are showing various types of microbalit structures (Thrombolite). Also, figure 6 and 7 (pp. 415-416) in the second article is quite similar in shape and size with what I have got.

I  appreciate your helps and interesting comment. Thank you again.

with best wishes,

Dilshad.

Hi Dilshad,

Many thanks for the very interesting references  - I agree with you that the more homogenous mottled areas of your outcrop share some features with the “clotted” thrombolite structures described in the Shapiro paper – however the bulk of the outcrop, to my eye, exhibits a clastic texture which is clearly seen in closeup (Detail 1). As I said in my original post, thin section work may reveal a microbialitic origin for some proportion of the clasts, particularly the tabular ones.

With regards to the autobrecciation model, there appears to be no lower limit to clast size – the literature mentions clast classes ranging from “a few mm” up to 10’s of cm. The point is that the fluidization process disrupts whatever precursor mechanical layering provides a contrasting rheology – this could be on a mm scale (even involving, I suppose, microbialite laminites).

Another distinctive feature of the autobrecciation process is the occurrence of “edgewise” tabular clasts, isolated, or in imbricate “stacks”, particularly towards the top of the disrupted unit. Detail 2 highlights apparent stacks of tabular clasts at high angle to the overall bedding (coin diameter is 25mm).

Either way, an interesting outcrop – any sense of lateral extent of the deposit and relation to the underlying lithology?

Best Wishes

Graham         

Hi Graham,

Thanks for your help and comment. At the end, I got few thin section photographs. They are all similar, but I am attaching one of them here (The best one, because others are blurred). It is under 10X.

Dear Dilshad,

Nice image, the elongated dark brown has similar shape to what I found within Jurassic and Upper Miocene rocks.

Regards,