Geopuzzle #9 : 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, and South Africa in his first post-doc, he now works at the University of Edinburgh.

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Geopuzzle #9

Category: geopuzzling
Posted on: March 28, 2008 11:10 AM, by Chris Rowan

After a bit of a hiatus, I return to the geopuzzling fray with this photo:

Your task is easy enough: interpret the features in the image above.

Update: Answer here.

Find more posts in: Environment

Comments

OK - I see ripple marks (hard to tell but they look symmetric) and mud-cracks - shoreline environment - shallow water carbonate platform??

Posted by: David | March 28, 2008 12:05 PM

I agree. Mudcracks on the right and ripplemarks on the left. Hard to judge whether the ripplemarks are symmetrical or asymmetrical at this distance. Probably a terrestrial to shallow marine environment.

Posted by: Ron Schott | March 28, 2008 12:54 PM

Why the different colours?

Posted by: Bob O'H | March 28, 2008 1:22 PM

Bob O'H:

Why the different colours?

A lot of dark rocks weather to orange colors, due to iron oxidation.

But that makes me curious: why is that rock so dark? I've seen similar sedimentary structures in sandstone/mudstone sequences (interpreted as anything that could get wet and dry), but usually the sandstones are lighter-colored (whites and yellows and tans and pinks). (Shales and mudstones are often dark.) Is the mineralogy of that sediment weird? Is this a pre-oxygen-atmosphere sediment?

Posted by: Kim | March 28, 2008 2:22 PM

I agree with the above ... even looks like some ripples on a different layer going a different direction (?).

Not knowing anything I'd put it in some shallow-marine, or potentilly tidal flat type of environment.

Are those definitely mudcracks? The seem to not have the typical polygon geometry ... ??

Posted by: BrianR | March 28, 2008 2:54 PM

I agree that the ripples and mud cracks indicate a shoreline of some sort that clearly fluctuated with time (hence subaerial to produce mud cracks but subaqueous to produce ripples). If the color differences are not due to diagenetic staining by other (probably iron) minerals...well, the fact that there is a tendency (though not a perfect one, and made difficult to discern by shadowing) for the dark colors to be on only one side of the ripples...might these be mud drapes? If so, it would represent flaser bedding, meaning that the environment was tidal flat...?

Posted by: Jerry D. Harris | March 28, 2008 6:54 PM

I'm not a geologist. The rippled 'steps' look pretty close to what I have seen in the fossil cliffs near Parrsboro, Nova Scotia, on the Bay of Fundy (the colour is common as well). In fact, I have several nice variously rippled fossil shoreline bits, complete with eel-grass looking fossil imprints and worm(?)snail(?) trails. I haven't seen petrified mudflats there, but I've seen the present flats dry on a hot day, between tides, pretty much like that.

And it's a nice photo, too.

Posted by: Bee | March 28, 2008 7:49 PM

Are the lower ripple marks also rippled or stepped by a cross-cutting pattern, or is that just a weathering effect? I'd sure like to see an enlargement!

Posted by: Silver Fox | March 28, 2008 9:05 PM

Lower right upper layer seems to be rippled mudstone or sandstone formed by moving shallow water. Right is mudflat with drying cracks later filled by lighter color sediments also by shallow water or possibly blowing sand. I've seen this same rock type on the Colorado Plateau but couldn't say what formation or age.

Posted by: Chas | March 28, 2008 11:06 PM

it's god's thumbprint! finally, clear evidence of the hand of god!

Posted by: djlactin | March 29, 2008 2:18 AM

Chris, you have the coolest photos. We've got ripples going two different directions plus mud cracks, so clearly it's an environment that is sometimes wet and sometimes dry - a shoreline. Or an ephemeral lake bottom?

But as pointed out above, the rocks are dark. Given where in the world you are, I like Kim's question about a pre-oxygen atmosphere environment. I just can't think of any sandstones that dark, and I've never seen clear ripples in mudstone.

Posted by: ScienceWoman | March 29, 2008 6:12 PM

Mmmmm, ripples and mud cracks. [Homer Simpson-like drooling] I agree with many of the commenters about the different ripple directions being tidal or barrier island influenced deposits and the coloring looks like the different forms of iron. I've seen iron concretions with similar colors and would presume similar processes are occuring here. But, what tidal environment is that rich in Fe? I tend to follow Kim's logic, knowing your location in S. Africa, it might be some pre-oxygen atmosphere deposit. Otherwise, it speaks highly of a different water chemistry where FeO is the dominant cement instead of Fe2O3 or any of it's iron oxy-hydroxide-hydrate precursors. It's also possible that it was originallly hematite cement at the surface, but later burial allowed reducing diagenetic fluids to migrate through and converted it to FeO. I'm interested to hear the answer Chris!

Posted by: Mel | March 29, 2008 7:50 PM