Tuesday, October 28, 2008

What is this Layering?

This is a limestone, maybe a dolomitic limestone, probably Devonian, with some layering. What is the layering? Is this soft-sediment deformation or tectonism or both or... ?

I really want to know! Any opinions?


Anonymous said...

Looks to me like an algal laminite (peritidal/supratidal/shallow lagoon, etc.). Similar looking rocks are not uncommon here in the Devonian and Carboniferous of the Alberta Rockies. If so, the waviness would be a soft-sediment feature (and/or a dessication/shrinkage feature), rather than tectonic.

Is the picture upside down? Or could the laminae be out of place (part of a boulder) or overturned? I copied and pasted your photo into Photoshop and tweaked up the contrast a bit to get a better look at the laminae, and they seem to make more sense when the image is flipped 180 degrees. I'm sure I can see scour surfaces with truncated laminae on several horizons, for example the dark band just above your finger tip (below the finger tip if the picture is inverted).

Great blog, BTW--I just found it while surfing some links.

--Howard (Geologist, Calgary, AB, Canada)

Lockwood said...

The rocks look more shattered than could be explained by simple lithification & jointing (a number of tiny-offset microfaults, 1-2 mm, mostly up to the left), but I don't think that has anything to do with the bedding lamination patterns. Carbonates are not so much my thing, but I do agree with Howard- the patterns I'm seeing would make more sense if the sequence was overturned from the way it's displayed in the picture.

BrianR said...

Dang, those are some cryptic beds! I think Howard is on the right track ... and I agree w/ the possible upside down thing. I'm fairly carbonate-challenged myself.

The only Devonian I've looked at in Nevada was near Area 51 ... looking at evidence for the Alamo impact event several years ago. That was fun. The answer to all the geological mysteries was in Area 51, of course ... where we couldn't go.

Silver Fox said...

Everyone agrees that the photo is upside down - and it looks that way to me, now that I look at it more closely! It's from a piece of float. I was pretty excited about it when I posted the photo, so didn't look that closely. I've seen a lot of this kind of layering in some limestones around here.

Howard, thanks for the detailed description of the deposistional environment. I'm glad you found my blog - thanks for the compliment. I once knew a Howard who worked some in Nevada a long time ago and then worked in limestones in CA. If you might be that person, maybe you could email me?

Lockwood, I think maybe I see some shatter-like textures in the lower part of the photo - there is a lot of post-depositional brecciation in other pieces of float nearby, along with some veining, so it wouldn't be impossible at all.

Brian, that would have been fun down near Area 51. That was probably after they closed the Groom Range off; before that, you could climb some mountain and take a peek!