21 January 2011

Friday fold: scenes from new 55

Posted by Callan Bentley

On Monday, driving back from a cross-country skiing trip last weekend in West Virginia’s Canaan Valley, Lily allowed me to pull over to check out a previously-un-checked-out outcrop on the north margin of “New 55,” the pork barrel boondoggle spawn of Robert Byrd, late senator of West Virginia. This enormous road, far out of proportion to its actual usage, is simultaneously an environmental disaster and a superb collection of Valley & Ridge outcrops. It is rich in exposures of both primary sedimentary structures and deformational structures associated with Appalachian mountain-building. Like this one:

My photographer took a closer in shot, too:

Although I feel like I could have gotten some more direction as to the exact position of my arms. Fortunately, I’m such a pro at Photoshop that I can easily resolve such issues in post-production:

…Ahh, yes. That’s better…

Today’s fold is an asymmetric plunging anticline in the Hampshire Formation, strata laid down in the Kaskaskia Sea, a late Devonian to early Mississippian epeiric sea that covered much of North America while the Acadian Orogeny was playing out on the “east” coast. The young mountains shed off sediments, which “west”ward flowing rivers carried to the Kaskaskia. There the sediments accumulated, layer upon layer.

During the late Paleozoic Alleghanian Orogeny, convergence with Africa buckled up the strata of the Valley & Ridge province, folding and thrust faulting them.

Here’s some lovely ripple marks a few meters away, formed as Kaskaskia wavelets sloshed about, arranging little ridges of sediment:

Can you tell what I’m pointing at here?

It’s a patch of slickensides on the bedding plane, indicating that these sedimentary layers of sandstone acted as relatively stiff, coherent sheets during deformation, and interleaved layers of shale slipped and slid, allowing the layers to shift a bit relative to one another, and allowing the overall folding to occur. This shifting over pre-existing mechanical layers (the bedding planes) creates a palimpsest of structure: tectonic structures superimposed on primary structures.

The bedding and ripple marks are late Devonian; the slickensides formed during shortening of the Valley & Ridge province and the thickening of the young Appalachian mountains during the late Paleozoic. Let’s call them Pennsylvanian in age. It offered me a nice little geological “time-taste” to break up a long drive home. A snack of perspective…

Bonus structure! Pencil cleavage:

Happy Friday, everyone!