Comments on: Geopuzzle (updated)

By: Why I Blog: Brian Romans (Clastic Detritus)

Why I Blog: Brian Romans (Clastic Detritus) — Wed, 23 Jun 2010 16:19:10 +0000

[...] example, when a reader asked me to help identify and interpret an interesting rock I posted it as a “geopuzzle” and the community helped answer the question. Afterwards, the reader e-mailed me thanking me for [...]

By: Geology Links for July 6th, 2009 | The Geology News Blog

Geology Links for July 6th, 2009 | The Geology News Blog — Tue, 07 Jul 2009 01:53:20 +0000

[...] Geopuzzle (updated) « Clastic Detritus [...]

By: Phil

Phil — Sat, 04 Jul 2009 06:45:28 +0000

Nice rock.

I’m just a casual observer with an interest.

As for Andrew Wanting “a T-shirt of that rock.”

There were made in the mid 70′s as both T-shirts and casual shirts. Having owned one, believe me, you get tired of it really fast. After about one month, I gave it away.

As for the rock it self, (remember I have no training in this), I was guessing the blues were copper sulfate, the reds or orange-rust, as some form of iron.

But what do I know?

By: Saranne Cessford (Notablogger)

Saranne Cessford (Notablogger) — Thu, 25 Jun 2009 16:09:51 +0000

No-one has suggested my first thought – marble with less soluble bands [?quartz veins parallel the banding in the rock]. The folds are not only ptygmatic and disharmonic, it seems to me there has been a huge amount of material removed by pressure solution in the not-white layers. Marbles come in all sorts of colours too. I’ve seen similar in the marbles on the facade of the Basilica in Venice. The folds in adjacent white bands often have opposite sense – like this () or this )(.

FWIW
Notablogger.

By: BrianR

BrianR — Wed, 24 Jun 2009 02:52:27 +0000

I’ve updated the post above to try and summarize the comment thread to this point.

By: christie

christie — Tue, 23 Jun 2009 16:29:25 +0000

Ditto what Kim said – also the orange laminated layers seem to have a spaced axial planar cleavage – probably pressure solution, implying that the folding and constriction occurred at low T (greenschist or lower facies). However, I wouldn’t rule out that the cleavage could have been overprinted by a higher T healing event (post deformational) since it doesn’t seem to form a dominant fabric in the rock.

By: Kim

Kim — Tue, 23 Jun 2009 14:52:29 +0000

I’ve seen biotite-grade rocks in Vermont with similar structures. So, no, it doesn’t require high temperatures to get those kinds of structures. (And high deformation doesn’t mean high metamorphic grade – some structures are only possible at high temperatures and some only at lower temperatures, because of the different deformation mechanisms that dominate rocks at different combinations of temperature and differential stress. But metamorphic grade is primarily a temperature effect, and the metamorphic pressures that cause higher pressure minerals to be stable are an order of magnitude greater than the differential stresses that cause deformation. Sorry – pet peeve. Metamorphism and deformation are different processes, even if they occur together, and sorting out exactly what happened and why is difficult and fascinating.)

And it doesn’t take much in the way of sulfides to allow rocks to weather with that kind of iron staining. I’ve seen rocks without much pyrite, but with a lot of ilmenite (and graphite – generally reduced redox conditions) that weather like that. In fact, the shade of orange on the rock surface is more typical of a weathered surface than it would be of a BIF. (BIFs tend to be more red/black or red/silver in their layering.)

By: BrianR

BrianR — Tue, 23 Jun 2009 14:15:28 +0000

suvrat … yeah, I always thought BIFs just needed the appropriate conditions to form, which may have been more prevalent in Archean/Proterozoic, but wasn’t necessarily tied directly to age. I’m learning a lot from this thread.

By: suvrat

suvrat — Tue, 23 Jun 2009 11:59:30 +0000

sure there are Paleozoic BIF's. they may not form the extensive thick formations of classical late Arcaean early Proterozoic, but smaller deposits are known. Silver Fox was probably referring to these deposits. here is another link on Paleozoic "BIF".

By: christie

christie — Tue, 23 Jun 2009 08:00:53 +0000

I’m with Andrew on this one. Maybe we share a regional bias but that is almost certainly low greenschist facies turbidites. I would bet that the ptygmatically folded layers are bedding-parallel quartz veins. I have seen very similar in the late Neoproterozoic Malmesbury Group in Cape Town area. I don’t have a problem with the reddish color. The Merced Falls formation (low gs turbidites/slates) in the sierra foothills takes on a similar patina because it’s full of pyrite to weather. Definitely not BIF – all the ones I’ve seen have strong differential weathering between fine banded layers of chert-rich vs. Fe-rich mineralogy