I found these soft-sediment sedimentary structures a month ago working in the Colombian Pacific coast and I think we might be looking at actual seismites, but I´m not an expert. The above and below lithology is mudstone and the convoluted one in the middle is sandstone, all Neogene. The area is well known for its seismic activity. Any thoughts?
I know seismites from the Kathmandu Basin, Nepal, and from the Lisan Fm. along the Dead Sea Transform Fault, Jordan . In both case the convolute lamination and the contacts to the footwall and hanging wall beds is very sharp caused by sudden tremors in an otherwise tectonic quiescent basinal environments. But I must be outspoken both environments are continental basins with a pronounced lacustrine facies. The present structures seem to me very unlikely to have formed by seismic activity even if the volcanic and tectonic processes might have have had an impct on these structures
You can see these figures in my papers about the Nepalese and Jordanian lake sediments available on RG and to be accessible upon request.
DILL, H.G., KHAREL, B.D., SINGH, V.K., PIYA, B., BUSCH, K. and GEYH, M. (2001) Sedimentology and paleogeographic evolution of the intermontane Kathmandu Basin, Nepal, during the Pliocene and Quaternary. Implications for formation of deposits of economic interest.- Journal for Asian Earth Sciences, 19: 777-804
HGD
Hi Tomas,
seimistes is a generic term for Soft Sediment Deformation Structure (but also brittle deformation ) link to an earthquake, so Seismite is already a conclusion. Indead earthquake is a triggering mechanism, whereas Soft Sediment Deformation Structure are the observation and consequence of the liquefaction.
Sticking to your outcrop, we can see a Convolute bedding (a type of Soft Sediment Deformation Structure) but nothing else. The trigerring mechanisn is unknow at this stage.
You wrote that it is a seismic are, so it is possibly link to an earthquake but you have to found other clues to clearly establish a seismic origin of such deformation.
Best regards.
read: https://www.sciencedirect.com/science/article/pii/S2095383616300530
Thanks both for your comments. I´ll look for the suggested paper, Dr. Dill. You seem very certain that the structures unlikely formed by seismic activity, is it because the contacts are not as sharp as the ones that you encontered in Nepal? I wonder if the area is not as quiescent but rather active, how would a "bigger that usual" event would look in the geologic record.
I´ll be going back to the area early next year to look for more data that leads to the understanding of the cause of theses structures.
I think the best way is to map the outcrops from the decimeter scale to dekameter scale. I know only two areas where earthquakes left their imprints as convolute lamination and bedding both of which significantly differ from what you presented in your photographs. I can only send you the above paper on request because I saved all my digital prints on the Researchgate Server.
HGD
Thanks Dr. Gill. I already requested you the paper, looking forward to reading it. Reagards.
Hello
In this case you can check is there any injection of sand from source bed to the upper one. Seismites such as sand dyke is relevant evidence of seismic trigger. Another possibility is the paleo liquifaction structure. All of these should be check in the field.
Thanks for the specific features to look for, I will definitely look for more evidences next time I visit the outcrop. Best!
Dear Tomas,
additionally to the comments of the colleagues above, you should focus on the primary origin of the beds. Are mudstones surrounding the deformed horizon deposited as offshore sediment? Is there any remnant of stratification or other structures in the deformed sands? Trough cross-stratification, ripples of HCS?
Such convolute bedding can result from multiple triggers, e.g., similar structures can be found associated with storm beds (tempestites). The paleoenvironmental context will help you to resolve the likely cause, but you will always have several options, just the probability can be constrained by a detailed analysis of primary and secondary structures and the overall settings.
Best wishes and good luck!
Michal
Thanks Michal, I´ll keep in mind your comments when I have a chance to re-visit the outcrop.
Best!
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