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Friday Field Foto #60: Chunk of microbialite in mudstone

August 22, 2008

My recent trip to the Canadian Rockies to look at some Neoproterozoic turbidites is still very fresh in my mind so it will be part of this week’s Friday Field Foto series.

First of all … when is the Neoproterozoic? Note the red line on the figure below, which is Chris Rowan’s handy-dandy time scale (read interesting discussions about this here and here).

One of coolest things about studying ancient sedimentary deposits is thinking about the material that makes up the sediment. Where did it come from? What can it tell us? More often than not, this means taking a look at the composition and/or age of sand grains or the bulk geochemistry of the mudstone. If you’re lucky enough to have material bigger than sand-sized grains then it gets a bit more fun because of the potential to see internal features.

The photo below shows a chunk of carbonate within mudstone (note toe of boot for scale at bottom). Click on it for a higher-resolution version.

Clast of laminated carbonate in turbidite succession (© 2008

Clast of laminated carbonate in turbidite succession (© 2008

The photo below zooms in a bit so you can see the laminated structure. A lamina is simply a very thin sedimentary bed (less than a couple centimeters thick). The laminae are not very distinct, but if you squint your eyes you should be able to see a crude layering (slightly tilted to the right).

Algal-laminated carbonate (© 2008

Algal-laminated carbonate (© 2008

So, what is this rock all about?

Well … to be perfectly honest, I did not spend any time researching this specfic rock. Everything I say here is from the researchers that were showing us around (if someone is truly interested in these specific rocks I will track down some real references … let me know in the comments).

The basic interpretation is that the crude lamination originates from microbial mats. The ‘living’ mats trap and bind sediment, successive layers stack over time, and when the organic matter decays this laminated structure remains. What’s cool is that these chunks that were transported off the shelf into the marine basin are all that’s left of it. That is, in this area, only the slope and basinal deposits have been preserved … the shelfal environments were eroded away long ago.

One thing to note is that although the age of the sedimentary succession is ~600 Ma (give or take several million years), the age of the clast itself is not very well known. In this case, we were told that it’s been interpreted to be generally equivalent in age.

Happy Friday!


9 Comments leave one →
  1. August 22, 2008 6:37 am

    I’m wondering a couple things. Are the harder, tiny pieces in the carbonate chunk chert? Has there been any shearing in the carbonate to produce the texture? The laminae seem swirly – or something. If there isn’t any shearing, how can you tell the difference?

    I’ve seen what appear to be foliated cherty limestones, and one reason I attribute the texture to metamorphic foliation is because I’ve nearby seen cherty limestones that don’t show that texture – it’s similar to your picture, but perhaps on a slightly larger or coarser scale. Now I’m wondering if there isn’t something else, like mats, involved. I’ll have to get a picture sometime and post it – obviously you can’t tell me about rocks you haven’t seen! ;)

  2. August 22, 2008 7:11 am

    Silver Fox … there has been a bit of deformation/metamorphism in these rocks (during Mesozoic convergence, I believe) but not to the extent you refer to (at least not in this spot). The orientation of the structures in these chunks are also not consistent w/ the orientation of deformation in the ‘country rock’. But, again, I’d stress that I don’t have a reference for this at my fingertips … I could be wrong.

  3. August 22, 2008 8:47 am

    Is this inferred to be a dropstone? Snowball Earth related?

    Which way is paleo-up in the photo?

    Cool clast… very cool.

  4. August 22, 2008 9:02 am

    Callan … stratigraphic up for the succession is towards the top of the photo.

    This clast (and others like it) are not interpreted as dropstones. They are interpreted as clasts w/in submarine debris-flow deposits.

    In terms of snowball Earth, there is a formation underlying this thick turbidite succession called the Old Fort Point Formation … apparently that formation has been studied w/in the context of snowball Earth ideas. If you the formation name some interesting looking papers come up.

  5. CamArchGrad permalink
    August 22, 2008 9:28 am

    I would actually disagree with the penecontemporaneous age of the carbonate. All throughout the Canadian cordillera there are relict successions hinting at much earlier wilson cycles (Pre- Grenville or greater than 1 ga), such as the Muskwa Assemblage, the Werneke Group, and the Belt-Purcells.

    The Windermere is primarily a grit/arkose/clastic sequence, while the assemblages above all have significant carbonate components, hence the clast above is probably from one of these older sequences. Near to the area of McBride (geologically) is the Muskwa assemblage which could serve as the source of the clast.

  6. August 22, 2008 9:40 am

    CamArchGrad … that very well may be. What’s interesting about this succession though is that in between the coarse-grained intervals, the sediments become more carbonate-rich. This pattern repeats several times. The interpretation that we heard was that the carbonate ‘factory’ was turned on during relative highstands in sea level and then subsequently ‘turned off’ during siliciclastic delivery. Additionally, in the more carbonate-rich intervals there are calciturbidites present that suggest episodic shedding of platform material into the deep sea.

    But, like I said, I haven’t looked into specific papers so if you have links to papers that discuss the origin of these carbonates, please put in this comment thread.

  7. CamArchGrad permalink
    August 22, 2008 10:10 am

    There hasn’t been much work on the sourcing of those clasts, so there isn’t any published material on the net. It would make a great project to provenience the source of these clasts.

    I take back some of what I earlier said. Checking my sources there are some carbonate beds in the Windermere, (though usually blue-grey in colour).

  8. August 22, 2008 10:12 am

    It would be an interesting project!

  9. August 31, 2018 8:58 am

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