Geology Question of the Week

Exodus · « **on:** 07/07/2004 01:40:51 »

The first question of the week:

YOU ARE OUT WALKING AND DISCOVER A RED SEDIMENTRY ROCK (E.G. A SANDSTONE) WHAT CAN A RED COLOURED ROCK SUCH AS THIS TELL YOU ABOUT THE EARTH'S CLIMATE WHEN IT WAS LAID DOWN?

AND FOR BONUS MARKS....

IF THIS ROCK CONTAINS SMALL GREEN AREAS, WHAT MIGHT HAVE LED TO THEIR FORMATION?

chris · « **Reply #1 on:** 09/07/2004 02:28:35 »

Well, it's red, so it must have some iron III oxide (Fe2O3) in it. And for it to have formed the oxide, there must have been some oxygen in the air / soil / sediment at the time...

I've no idea about the green inclusions - are they copper ?

Chris

"I never forget a face, but in your case I'll make an exception"
- Groucho Marx

Exodus · « **Reply #2 on:** 23/07/2004 00:30:02 »

Thats not bad... the redness is indeed down to the oxidation of iron and actually suggests that the environment was Warm and fairly dry...

The green inclusions are in fact the passage of old roots which have rotted underground and have subsequently caused reducing conditions. This in turn lead to iron reduction which is a charachteristic green colour...

So Overall, a warm dry environment with trees!

chris · « **Reply #3 on:** 28/09/2004 17:49:41 »

Come on Exodus, where's the next geology question of the week ?

"I never forget a face, but in your case I'll make an exception"
- Groucho Marx

Kiss · « **Reply #4 on:** 13/10/2004 11:32:04 »

I have visited a "early Ordovician red rock" locality a few month ago which is given to be a fresh water sedimentary system under hot climatic conditions. Two days collecting but I found no fossils there (which is not a surprise) but I've seen many sedimentary figures suggesting a shallow environment (ripples-marks) and probable dry periods (polygonal mud-cracks). All the sequence is almost reddish color, but a few layers are bright green. That seems to fit with the reducing condition period hypothesis exposed by Exodus. I'll buy that.

Exodus · « **Reply #5 on:** 13/10/2004 11:48:06 »

Kiss, the interesting thing was that there were what appeared to be round nodules of green rather than layers which had been formed by roots burrowing through the sands and producing reducing conditions upon breakdown.

I saw some great ripple marks whilst in the South Western Alps on some finer grained sandstones. One particular bedding plane had a trackway of bird footprints!

I guess the lack off fossils was down to the environmental setting of the time, it could well have been a braided river deposit? or it could have even been an ephemeral lake?

Kiss · « **Reply #6 on:** 13/10/2004 11:56:23 »

Honestly, when observing the basis of the sequence, it could be something like a melting glacier (brech+conglomerate) and I agree for the inner lake idea but really, the thickness suggest a long term pool.

Exodus · « **Reply #7 on:** 13/10/2004 12:52:16 »

Sounds interesting, where abouts was it?

Kiss · « **Reply #8 on:** 13/10/2004 13:10:35 »

I should be able to put a picture in this board. I've done some. See that this afternoon.
The location is Western France (Britanny).

Exodus · « **Reply #9 on:** 13/10/2004 15:35:10 »

Pictures will be great. I'm not too knowledgeable about the geology of North Western France i'm afraid, maybe i should look into it, never really thought about it!

So Kiss, are you a geologist?

Kiss · « **Reply #10 on:** 13/10/2004 15:42:23 »

Oh no Exodus [

], I'm just a simple student but I love fossils and geology. I'm not a specialist huh [:I] sorry I'm so dumb superficial in this field. Very difficult disciplines.

Ok I will post some pics to have a look on. Must find how to host them first.

Exodus · « **Reply #11 on:** 13/10/2004 15:44:53 »

quote:
Originally posted by Kiss

Oh no Exodus [], I'm just a simple student but I love fossils and geology. I'm not a specialist huh [:I] sorry I'm so dumb superficial in this field. Very difficult disciplines.

Ok I will post some pics to have a look on. Must find how to host them first.

don't be so harsh on yourself!!! sounds like you are keen! nice to have another rock lover on here... not many of us... [

]

Exodus · « **Reply #12 on:** 13/10/2004 15:51:32 »

if you want a site to host them then go to

http://www.photobucket.com

its easy and free, then highlight the picture's details and input them in between the insert image code which may be found on the message toolbar!

Kiss · « **Reply #13 on:** 14/10/2004 07:06:18 »

Greatings,

It's my first picture posting. A friend of mine was kind enought
to open a guestfolder in her photopocket account.

The best upper red sandstone+shale exposure at Brehec (Western France).
Left is a volcanic dyke (bubble lava), center a 1/4 meter rejection fault.
The light green reducing episode is very well differencied.
Above, there are compact grey shales sequence.

Ripple-marks on the shale (my pic gives scale).
IMHO, the wavelenght of this sedimentation figure should indicate a very shallow water deposit.
A shame, there are no vertebrate foot-print and no vegetation evidence,
because of the early age of these rocks ! The whole succession is azoïc.

Exodus · « **Reply #14 on:** 14/10/2004 22:59:55 »

Really nice pics, post any other you get!!!

Notrenchgeology · « **Reply #15 on:** 18/01/2005 20:47:06 »

I would suggest to you that truly, the red color is indeed from iron (III) oxide, but that the green color would be from one of three possible sources, depending upon the size and orientation of the "mottles" that are present.

The first possibility that leaps to mind is the presence of glauconite, which as a semi- or pseudo- clay mineral, depending upon whether the mineral is ordered or disordered in molecular organization, lends the green color to so-called "greensand" deposits. Glauconite also can be a part of ancient dolomite deposits, such as the Bonne Terre Formation dolomites (Upper Cambrian) we have here in Missouri, USA.

The second possibility may be illite, or an illitic shale deposits, which would be consistent with shallow-water or tidalite deposition.

The final possibility would be coloring due to the presence of bivalent nickel. Nickel (II) sulfide, or millerite, is known to occur as acicular needles in limestones. But what happens when these weather out of formation? The bivalent nickel can become part of any shaly layers or laminae that are present that have any remaining anionic charge. This has also been known to happen in the Cedar Valley Formation (Devonian) of the central United States.

Good question!

Notrenchgeology

Bass · « **Reply #16 on:** 15/09/2005 22:49:26 »

Great picture!
Reminds me of several sequences of siltstones and argillites (St. Regis Fm. and Missoula Group) in the Proterozoic Belt supergroup rocks in Montana and Idaho.
I agree with Notrenchgeology- the red is due to oxidized iron. The green, however, is probably due to reduced iron. My guess, by your comments, is that these rocks may be PreCambrian (lack of fossils). Evidence for alternating oxidizing/reducing conditions is common in Precambrian rocks.

Prediction is difficult, especially the future. -Niels Bohr

Bass · « **Reply #17 on:** 16/11/2005 01:26:56 »

Here is a photo taken during a recent trip to a famous park- I call it "the scream". The feature is about 15 meters tall and is outdoors.

Identify the type of rock in the photo and how this feature was formed?

Prediction is difficult, especially the future. -Niels Bohr

neilep · « **Reply #18 on:** 16/11/2005 03:16:34 »

I haven't a clue but what a fantatstic picture...and 'the scream' is very apt.....it looks salty !

I presume it's Yellowstone Park eh ?

Men are the same as women.... just inside out !!

Bass · « **Reply #19 on:** 17/11/2005 00:44:58 »

Neil
You hit the hot-spot right on the head, Yellowstone it is.
More later

Prediction is difficult, especially the future. -Niels Bohr

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