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GeneralMiocene lakebed silicified breccia from central Arizona, USA

26th Mar 2020 22:48 GMTJohn Christian

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Photo 1:

I found some silicified, brecciated and folded structures from Miocene lake deposits in central Arizona, USA. The first photo shows  a laminar layer that has a wrinkled or cracked appearance. Does anyone know what they are and how they were formed? Stromatolites occur in these lake sediments.

I am going to try to add more photos to this thread in another post since I cannot insert another one right now. Anyone know how to add more photos to one post? 

Is there some sort of limit for photos attached to one post? Number of photos? Size of photos?

26th Mar 2020 22:53 GMTJohn Christian

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Photo 2: Silicified structures.

26th Mar 2020 22:56 GMTJohn Christian

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Photo 3: Edge of photo 2.

27th Mar 2020 12:59 GMTTom Tucker

I'm not sure what your material is composed of, but I really like Specimen No.2.  Interesting stuff.

27th Mar 2020 17:54 GMTGregg Little

A search of the internet (stromatolite + Miocene + Arizona) has a wealth of information and probably covers the area that you sample comes from.

Photo #1 shows features from that time and environment; hot, arid, saline, prone to drying out and subject to vulcanism. Lake beds dried out causing structures like shrinkage cracks, desiccation polygons, mineral casts, etc.

Photos #2 & #3 are horizontal and vertical sections through a stromatolite bed. Stromatolites grow in undulating and dome-like structures which can mimic folds seen in a tectonic setting. Any breccia and folding here is likely depositional caused by processes like solution and collapse, sediment draping, desiccation, etc. The huge rhyolitic ash fall, 1 kilometer or 0.6 miles thick during mid Oligocene to mid Miocene, provided the silica-rich waters for preserving these structures.

27th Mar 2020 18:57 GMTJohn Christian

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Thank you Gregg. There are volcanic rocks near this lake deposit that probably provided silica.

See photo of stromatolite preserved in chalcedony that I found nearby. 19mm wide penny for scale.

27th Mar 2020 19:02 GMTJohn Christian

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Here is a piece of silicified palm wood with calcite crystals that I also found in the Miocene lake deposits.  13.5 cm long.
 
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