Tuesday 19 October 2010

Sedimentary features query


Can anyone identify these please?
Let us know what you think either on 'comments' on this blog or by email.
Please ask if you would like a better image of any of them.

9 comments:

Anonymous said...

The lower right one (and possibly the left one) appear to be Liesegang banding -- a diagenetic structure due to deposition of iron oxides by groundwater flow through permeable rocks like sandstones. I can't see any kind of detail on the upper right one -- maybe tubes of some kind? If you can post a larger copy of the image, that would almost certainly help people identify it.

Nick said...

Ripple-like Structures - might be 'tepee structures' (would really need cross-sections through the structures to be sure).
Tube structures - not familiar with this; pictures shows well-formed tubes, but also less-well formed ones that suggest this is some sort of concretionary feature.
Web structures - I've seen / heard of stuff like this before: these are 'box' concretions - the framework is composed of limonite, and the host sediment that lay within the 'boxes' (looks like it was sand - some of the sand remains as a coating on the limonite) has since been removed to leave just the frame. I think they form when the host sediment becomes lithified enough to develop joints and then iron-bearing solutions fill in the joint spaces.

Dave said...

Ripple structures: these appear to be in a stream
so may be more likely to be erosion features than
sedimentary structures.
Tube structures - I did see something similar in Spain and that was travertine which had deposited around tree branches/roots and grass. The woody bits then rotted away to leave tubes around where the plant material had been.

Alan said...

Web structures I suggest it is to do with ground water with chemicals in solution passing through the rock during diagenesis and the chemicals are precipitated in bands (like liesegang rings), probably iron compounds. These are now better cemented and harder and have been weathered out by differential weathering and erosion.

Richard said...

The structures in the one photo look like iron-rich leached solutions which get so far through the sand (like ink through blotting paper) before they reach an osmotis-like limit then they condense out at a sort of wave-front.

Edie said...

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Charles said...

Web structures I seem to recall seeing structures like this on the Northumberland coast where the Millstone Grit (?) is exposed. The sandstone is a mass of little 'boxes' or cells where the edges are formed from ironstone impregnating the sandstone. This has hardened the edges so that the sandstone inside the 'boxes' is softer and eroded more easily causing the edges to stand up.
Tubular structures these seem more like lava tubes - I have seen something similar on Iceland - Lanzarote, but if they are truly sedimentary structures, the picture suggests dewatering tubes in a sandstone like Millstone Grit where the same process has occurred as in the weblike structures ie. the tube walls have been hardened by impregnation with ironstone.
Ripple-like structures - could this be ripples in a sandstone similarly hardened as the other two?

Isobel said...

Web-like structures - these are 'box' concretions - the framework is composed of limonite, and the host sediment that lay within the 'boxes' (looks like it was sand - some of the sand remains as a coating on the limonite) has since been removed to leave just the frame. I think they form when the host sediment becomes lithified enough to develop joints and then iron-bearing solutions fill in the joint spaces.'

Anonymous said...

Here's a very similar picture to the one I identified as Liesegang banding.