Rock n Roll fault rocks: a pictorial journey aka my Namibian field season

So my last post was a tiny tiny update. I should write more about what I’ve been up to the past month and a half. First off, this has been an incredible adventure. Namibia is amazing. This was my second time to Africa (South Africa and Namibia). I’m currently living in the Geo department flat at UCT. Thank you to the department staff, and Tanya and Jacq for helping Ben and I get settled.

EDIT: Click any of the pictures below for a larger view!

I know, I know, you want to see rocks. Bear with me. Let me give you some context.
So where in Namibia was I? Where the hell is Namibia!?

Let’s zoom in:
-24.262224, 16.2432

Naukluft Nappe Complex
That’s about 200km South-Southeast of the capital city of Windhoek as the crow flies. 

The Naukluft Nappe Complex is a thrust fault system associated with the Damara Orogen (~550Ma) that emplaced meta-sedimentary rocks in a roughly Southeast vergence on undeformed basin sediments. This area was originally studied by two German geologists, Henno Martin and Herrmann Korn, in the 1930s. These two geologists would later live two years in the Namib desert hiding (and mapping!) during World War 2 (blag post on that coming soon). They divided the complex into three different nappes, though more recent mapping has established five nappes. Korn and Martin also made the observation of a fault rock they describe as the “Unconformity Dolomite”. You know how geologists love to name stuff. First it was “Unconformity Dolomite”, then other authors called it “Lubricating Layer”, then other authors called it “Sole Dolomite”. From now on let’s agree to stick with the sole dolomite convention.

Carrying on, this sole dolomite is the a layer of rock that separates the footwall sediments, blue limestone and shale, from the hanging wall, which is a charlie foxtrot of dolostone, quartzites, and shales. Want a photo of what this thrust looks like? Check out the header for this blog! Oh, you’re too lazy to scroll back up? Okay, here you go:

Naukluft Thrust

and at another location:

Naukluft Thrust

Did you spot the fault yet?

I have so many pictures of this thing it is ridiculous. Tons of panorama shots too. Sadly, the GigaPan uploader can’t get through the UCT firewall, so we’re all going to have to wait until I’m back in Montreal to see those panoramas. It’ll be worth the wait. I’m getting ahead of myself.

So what does this sole dolomite look like? It’s the brown-tan bed on the blue limestone in the above photos, but let’s move a little closer…

Ben (Dr. Choff) for scale

The limestone at the fault margin is mylonitized. Enough teasing, let’s look at the rock.

Slicks!

As you can see there is a foliated zone of dolomite at the fault surface. How about that rock above it though? That’s called “Gritty Dolomite.” It is a “cataclasite-like” fault rock that has dolomite phenocrysts, lithics, and silica. It looks like this:

Gritty dolomite with silica banding.

But sometimes it looks like this:

Discrete gritty dolomite layer between sole dolomite (below) and a dolomite breccia (above).

Or even this:

Uhm, what? Laminated and folded (flow-folds?) gritty dolomite. Some silica banding.

But sometimes it does this:

Footwall limestone clasts in a gritty dolomite / sole dolomite breccia. A brown silica cortex surrounds footwall clasts.

And how about up on the ridge (picture from this blog’s header and picture #2)… BAM!

That is a RocknRoll breccia if I’ve ever seen one.

Sometimes the gritty dolomite will inject upsection/downsection off of the fault, sometimes looking like this, often found with neocrystallized dolomite, sometimes doubly-terminating neocrystalline quartz on the surface:

This rock is from part of a clastic gritty dolomite injection.

And these photos are only on the eastern side of the nappe. Okay, okay, so WHAT were we doing out there? We were mapping this fault with a pair of these:

Ben desires more satellites.

The Trimble GeoXH. We set one up as a base station. The other is a rover. We walked the fault, making observations, taking measurements, ect. Once home I load the rover file with the corresponding base station file into the Trimble TerraSync software and presto! Centimeter GPS accuracy! As a first order question, I’ll be looking at how the geometry of the basal fault relates to the type of fault rock observed, injections, and make interpolations of the fault surface.

In the eastern side of the Nappe typical fault dip is betwee 15-25 degrees, but varies widely. Now on the West side…

Tsams Ost locality

Can you spot the fault? Hmm… that looks a bit different. Typical fault dip here is ~3 degrees. The foot wall is typically blue shales, and there is no gritty dolomite to be found. We do observe the occasional footwall shale injecting up into sole dolomite…

Footwall shale injecting up into sole dolomite.

There’s also some fantastic folding in the hanging wall.

Hanging wall folds.

I feel like I could continue on and on posting pictures, so I’m going to force myself to stop. I’ll put up a gallery of photos on my Google+ page in the near future. I’m incredibly eager to get back to Montreal to look at the GPS data and start piecing this puzzle together.

Special thanks to Ben Mapani for his invaluable assistance and advice. Thanks to Ben Melosh, Jodie Miller, Clint Isaacs, Rangers of Tsams Ost, and my advisor Christie Rowe. An extreme thank you goes out to the Naukluft 9 park staff for their generous hospitality and assisting with charging of our GPS batteries.

Desert graffiti. 

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Back on the grid

I’ve returned from my field work in Namibia. Currently I am at UCT where I will be doing SEM work,  sample prep and writing. Expect updates in the future + tons of field photos. Until then…

Naukluft Thrust, Namibia