It should look like a spiral circles if it will be formed by rotation during the collapse of the delta front (it's low viscosity). But actually it is concentric circles. It's not oncoid or concretion. What's the process to form a structure like this?
The picture was taken from a piece of drill core of the upper Triassic Yanchang formation in the Ordos basin, central China. The sedimentary settings were deltas-deep lake. And there are a lot of soft sediment deformation structures both in drill cores and outcrops in the Ordos basin.
Dear Renchao!
In my opinion you first should determine if this is really a sedimentary structure due to a process of sedimentation or if it is only a change of colour due to different oxidation conditions.....
best regards
Andreas
Dear Andreas, Thank you evry much for your kindly answer!
But there are still some doults. Since the drill core was burialed ca 1500m and never underwent any weathering condition. A lot of pyrite concretions co -exist within the sandstone. And the different colors of circles are caused by different contents of mud. So I wonder the reason for different oxidation conditions.
I guest that argillaceous haloes might be formed by pulsing water escape flushing. But I am not sure about it. ...
All my wishes and best regards!
Renchao
Is this just a 3 D soft sediment overfold that is now being sectioned?
Dear Nicholls, thanks for your answer. Yes, it might be a 3D soft sediment dorformation structure that is now being sectioned showing geometriceffect.
It's the outside of a piece of core. Now， I get it. Thank you again!
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