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Highly Allochthonous
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Chris Rowan is a geologist specialising in the dark arts of paleomagnetism, and getting people to pay him to travel to exotic destinations for fieldwork. Having drilled up New Zealand during his PhD, he is now a post-doc at the University of Johannesburg.
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Geopuzzle #10
Category: geopuzzling
Posted on: April 4, 2008 11:05 AM, by Chris Rowan
Comments
First quick thought before I go out to look at dirt road stream crossings bleeding sediment (and affecting water quality) and figure out how to fix them - it looks like some sort of breccia. Without a specimen and hand lens (ooh, it's been too long since I did this), I can't figure out what the brecciated rocks and matrix are. They look vaguely volcanic but I really can't say for sure.
Posted by: coconino | April 4, 2008 11:29 AM
I've made custard that looks like that. It even had the pen in it.
Posted by: Bob O'H | April 4, 2008 1:13 PM
Looks like something I've seen pictures of, a brecciated basalt with carbonate cement. The light cement weathers out faster than the dark basalt. In the lower left corner is what might be a large crystal of calcite. If I'm right the next question is tectonic or erosional breccia?
Posted by: bpatey | April 4, 2008 1:22 PM
From the image you gave us, it looks like some sort of breccia. But it could also be a collapsed magma chamber with dark country rock included into light magma.
Since I live in Southern California and don't know where the picture is taken, I have to go on just what I see in the image.
But my first thought was a breccia. The carbonate cement mentioned by bpatey makes sense.
Posted by: socalmike | April 4, 2008 3:34 PM
Not at all anything close to a geologist...
My guess is iron ore in sedimented loam.
Posted by: Jeb, FCD | April 4, 2008 3:59 PM
hmmm... it's not a fault breccia - because the edges of the clasts are variable in sharpness, angularity, etc... the light-coloured matrix material appears in veins in the clasts, some looking a bit more organized than others, but joint-like. hydrothermal breccia? i'm guessing very old. baked but not stirred.
the accompanying vegetation would give away the general location to a botanist...
Posted by: christie | April 4, 2008 5:51 PM
I vote hyaloclastite, and not just because that's such a cool word.
Posted by: Tuff Cookie | April 4, 2008 5:57 PM
Well, it's a breccia. Volcanic clasts - prob basalt - and I'm voting for some sort of reworked laterite as the matrix.
Posted by: PaulG | April 4, 2008 7:35 PM
Some sort of volcanic breccia with basalt clasts? Sorry, tuff cookie, but I don't think it looks anything like a hyaloclastite.
Posted by: ScienceWoman | April 6, 2008 1:22 PM
I vote for some kind of breccia, too. Not fault breccia, and all one clast type, so I vote for something volcanic, too.
Unless this is actually a picture of a stone wall in Hawaii, and that's basalt cemented together with, ummmm, cement.
Why are there T-shaped fractures in it? I've been looking at it, trying to see if the fracture pattern looks like something resulting from fluid pressure, but I keep seeing all these T shapes, and they're messing with my sense of fracture mechanics. (Which, granted, is not as good as it could be.)
Posted by: Kim | April 6, 2008 3:53 PM
All right then, how about pillow lava fragments in hyaloclastite? I've seen pillow basalts in Texas that looked like this (cemented-together fragments), and the people at the Geological Survey of Canada have a pretty similar photo here (scroll down to "Eruptions Under Ice"). They call it "pillow breccia".
Posted by: Tuff Cookie | April 7, 2008 2:30 PM
It's a blue pen.
But seriously....
A conglomerate of lateritic rock stuck in calcrete.
Posted by: Greg Laden | April 7, 2008 6:23 PM
It's peculiar in lacking all small clasts of the brown rock. It's a clast-supported breccia with no rounding of the clasts. It contains exactly two pure rock types. It hasn't been transported, so it's not an olistostrome. It looks like it was very gently cracked. I have two guesses: it's a dissolution breccia, mudstone in oolitic limestone, or it's a volcanic breccia, andesite in ashfall tuff. But it's gotta be some weird Archean thing, right?
Posted by: Andrew | April 7, 2008 8:18 PM
The longer I look at it the more I think it was human made. The matrix could be a weathered kind of concrete or mortar with basaltic rocks placed into it. Iron richs basalt could explain the red weathering of the matrix if left outside long enough.
But I'm not good at telediagnosis.
Posted by: Mathias | April 8, 2008 7:37 AM