Posted by: Alan Pitts | September 14, 2011

Dalradian Metasedimentary Continued

More from my mapping area in the beautiful Connemara region of Ireland.

A few days ago, I wrote about the Lakes Marble formation, a folded marble and interesting member of Ireland’s Neoproterozoic Dalradian Super Group. Today, I’m continuing the discussion with another member from the DSG and more details about the Connemara metamorphic complex.

Here is once again our celebrated interpreter of local geology, a truly distinguished Irish gentleman, Dr Martin Feely (along with another gentleman who is apparently less confident in his choice of headgear for the day), standing on top of a folded quartzite.

Here is a pretty “blocky” fold I found nearby, with a great example of a rounded hinge and straight limb fold.

Here is what I mean.

And another tighter fold.

This is the Bennabeola formation, a “predominately clean” orthoquartzite containing more than 90% quartz, which was metamorphosed from sandstone deposits. This unit is more resistant to weathering than its local lithological neighbors, so it stands “proud” forming the hills in the core of the Connemara Antiform. I say “predominately clean” because while the description above makes the Bennabeola seem relatively trustworthy and safe, it is anything but.  I can say after several days of mapping this unit, there are enough gradational changes and shifts to make even the most enduring mapper want to break their map-board in two and launch their Brunton into the nearest, deepest bog.  Usually, that was a good time to break for lunch.

We are on the western coast of Ireland in County Galway, looking at the metamorphosed sedimentary rocks of the Dalradian Supergroup. This is to refresh everyone’s memory, or to introduce the concept to those who didn’t read the last post.

Here is once agin the Geologic map of Ireland, simplified from the original produced by the GSI.

In the last post, I discussed how the larger anticlinal structure was super-imposed onto the landscape in the shape of the lakes. Here are some similar large-scale structures seen in the Bennabeola quartzite hills.  This one should be easy to spot.

As a proponent of both Irish geology and improving safety on Irish roadways (if you’ve ever driven in Ireland, you know this is not a joke), I’m happy to report that the Irish Department of Transport found a clever way to warn motorists of hazardous road conditions, while simultaneously drawing their attention to some killer folds in the quartzite hills.  Somehow I feel like these two concepts are at odds (if you are a geologist/geology student and drive a car, you know this is also not a joke).

Though cleverly positioned, it is not entirely correct.  Anyone can clearly see  a “Z” fold showing a dextral sense of shear. I took the liberty of making the correction for them.

All joking aside.

The Bennabeola is stratigraphically below the Lakes Marble and together with a package of other sedimentary rocks form the DSG. This package was laid down on the Laurentian passive margin, after the break up of Rodinia around 600 million years ago.  After deposition, these rocks experienced a series of 4 separate deformation events.  These events started with the Grampian Orogeny (we call the Taconian) in the mid to late Ordovician and ended with the Hercynian Orogeny (we call the Alleghanian) which produced the Connemara Antiform.

Well wait just a minute… that kind of sounds like a Cambrian package of sedimentary rocks deposited on the Laurentian passive margin after the break up of Rodinia I know!   The Chilhowee group from my beloved home region, The Blue Ridge Province in Virginia. While the Chilhowee was less impacted (or not at all) by the Taconian Orogeny (what they call the Grampian), was majorly impacted by the Alleghanian, which formed the anticlinal structure and westward thrusting which defines the Blue Ridge today. Not that these two rock units are the exact same, because they are not, but they share a similar story. Like cousins with a similar upbringing on opposite sides of town, these two formations are alike in maybe more ways than they are different.

This was a serious “ah ha” moment for me personally, relating rocks of my home region to global geologic processes.

Ah HA !

Making the connection.

Image from University of Michigan school of Public Health.

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Responses

  1. Great post and nice to see some pictures of Connemara; I did my PhD there.
    Did you visit any of the syntectonic gabbros while you were out there? They are a big part of the tectonic and metamorphic picture.
    Dawros or Currywongaun are nearest the road and good for student groups. Anyway, if you missed them, I did a post recently that will give you a flavour: http://all-geo.org/metageologist/2011/08/accretionary-wedge-%E2%80%93-37-sexy-geology/

    • Hi and Thanks !

      Wow thats pretty cool, I was only there for the 4 week mapping portion of my field camp, but what an amazing place to do a Phd !
      We spent most of out time in Connemara looking at the metamorphics, though we did see a few igneous intrusive units too. Im going to have to look back through my field book, I only remember seeing the Omey granite and perhaps the Outhergard (sp?) granite, which I hope to blog about soon.
      But thanks for the feedback and link ! I enjoyed reading your post.


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