Geopuzzle (updated)
I’ve been so busy lately that I completely forgot to do a bit of research for a reader that e-mailed me a photograph of some interesting (and beautiful) rocks.
So, I decided to make it a geopuzzle and let the collective intelligence of the blogosphere discuss, debate, and ultimately solve.
photo courtesy of a reader -- click on it for larger version
Here is what the reader says about the rock in an e-mail to me:
The rock is about 4 feet across and is near the Martial Glacier outside Ushuaia, Argentina.
Alright … have at it!
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UPDATE (6/23/09): This update was posted after the comments below to try and capture the general consensus on this particular geopuzzle. Everyone mentioned and admired the quartz ptygmatic folds (the white squiggly layer). Kim pointed out the parasitic folding — which, correct me if I’m wrong, is the term for the self-similarity of fold geometries of different wavelengths (note how the thinner layers are folded tightly within a longer-wavelength fold). In terms of what kind of rock this is, looks like most would agree that these were sedimentary rocks (evidenced by the layering) that were deformed (evidenced by the folding) and likely metamorphosed somewhat.
I have done some work in this general region and there are extensive outcrops along the Andean orogen composed of Paleozoic (~250-550 million yrs ago) meta-sedimentary rocks. While we can’t be absolutely certain that’s what this rock is just from this one photo … if I had to make the call, that’s what it would be. Or, another way to look at it … now we have a solid hypothesis to test. We just need to get down to Tierra del Fuego and sample this thing. Field trip!
Clastic Detritus 
My first reaction is that there are some lovely ptygmatic folds in that thin white layer just left of center. I’ve seen folds like these in metamorphic rocks all the way from greenschist facies slates to migmatitc gneisses. Not sure that’s much help, but it certainly is a pretty rock. Would make a nice piece of courtyardite.
Knowing you – although it isn’t your photo – I’d have to guess ptygmatically folded turbidites, though I don’t know what all that Fe-oxide is from. Beds/layers look laminated. But I don’t know much about the area.
It’s a great example of parasitic folds. (And of the effect of layer thickness on fold wavelength.) And I would guess greenschist facies, quartz-rich but originally fine-grained metasediments, based on color and behavior of the layers.
banded iron formation?
My first guess guess, looking at this in reader, was that it looked like the fine intraformational folding you can get when anhydrite rehydrates in situ to gypsum… but the color, texture and luster don’t look right. Then I was thinking a metamorphosed and ptygmatically folded BIF… but I don’t know that I’ve ever seen bright white layers of quartz like that. Quartz veins, yes, but not interbeds. Then I noticed the location… I’ve been to Ushuaia, and from what I recall, most everything I saw there was schisty in nature, with some dikes. There does seem to be sort of a rhythmic nature to the bedding, so my money is on Silver Fox’s answer. Though as she points out, those are some mighty unusual colors for turbidites, metamorphosed or not.
this is awesome …
Suvrat, the reader in fact wondered about a BIF (banded iron formation) interpretation for these rocks in his email to me. I really don’t know too much about BIFs … but I’ve seen other examples of metasedimentary rocks with the look of BIFs.
Knowing a bit about the regional geology of Patagonia, the basement in this area is composed of Paleozoic metasedimentary rocks interpreted to be from an older accretionary wedge. I’m guessing that’s what these are from … there aren’t any Precambrian rocks (that I know of) around here … does that exclude a BIF interpretation. Are BIFs attributed solely to Precambrian? Maybe Chris Rowan or Lab Lemming can chime in on that since they’ve worked old rocks.
Brian, BIFs solely precambrian. The conditions for their formation were no longer met in the Paleozoicum. At least I would be highly surprised to find a true Cambrian BIF.
My first thought was BIF – but the interlayers that aren’t red would then presumable have to be part of the silicate facies and might include something like riebeckite.
BIF’s do occur in Chile.
Not a BIF at all, no way. Just a lovely mashed and remashed metasediment. The blue/green pelitic layers appear to have iron stains, pointing to sulfide content and hence a marine (euxinic?) source. The combination with psammitic layers (the white wiggly quartz) is quite turbidite-y. The whole is at a low retrograded metamorphic grade–I say retrograded because the deformation ought to have pushed it to pretty high grade, not just chlorite/greenschist.
I want a t-shirt like that rock.
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
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”.
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.
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.)
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.
I’ve updated the post above to try and summarize the comment thread to this point.
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.
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?
Alternative understanding of folding in metamorphic rock:
Authigenic rock forms in underwater oceans of Oort cloud dwarf planets by precipitation in which quartz membranes are precipitated at the ice-water boundary where silica solubility is lowest, as more mafic mineral grains precipitate lower in the ocean. If the quartz membrane has a degree of mechanical competency, perhaps due to slime bacteria, the membrane may remain partially intact as it becomes waterlogged and sinks onto the sedimentary core below. As the larger circumference membrane from the ice-covered surface of the ocean maps onto the smaller circumference core it crumples in ptygmatic folds. The larger frequency folding occurs during the subsequent diagenesis of the sedimentary core.
Lithification follows and metamorphism may even occur if the overlying ocean has time to freeze solid, exerting tremendous pressure on the underlying core due to expansion of the water during freezing.
Ptygmatic folding occurs more frequently in smaller authigenic (sedimentary) gneiss-dome dwarf-planet cores. Authigenic gneiss dome mantle rock (quartzite, carbonate rock and schist)precipitates from the hydrothermal fluids issuing from diagenesis of the underlying gneissic sediments, explaining the difference of the overlying mantle rock from the underlying migmatite gneiss.
Supposed metamorphic folding occurs exactly like the folds in raisins as the hydrothermal aqueous fluids are expelled from the sedimentary grape during diagenesis. And gneiss-dome-sized dwarf planets often freeze solid, undergoing metamorphism, before accreting onto larger dwarf-planet ‘platforms’ in the Oort cloud, such as the Appalachian dwarf planet platform that impacted Earth at 443 Ma, causing the End Ordovician extinction event (so Silurian and younger Appalachian rock is terrestrial). ‘Circumferential folding’ (grape-to-raisin folding) does not occur in sedimentary rock on Earth (example: Grand Canyon) because of Earth’s giant diameter, since the before and after circumference change in Earth’s diameter of sedimentary layers that shrink during diagenesis is immeasurably small.