28 February 2013

Unconformity seen in the field yesterday

Posted by Callan

I was out in the field all day yesterday with Dan Doctor (USGS) and Alan Pitts (University of Camerino). We were puzzling over some deformational features, but what I wanted to show you today was something we saw at the end of the day (hence the subdued lighting): a classic angular unconformity, exposed in a quarry wall east of Front Royal:

The bedding of the Harpers Formation (Cambrian mud + sand layers) are upright and folded. A nice anticline / syncline pair can be seen at right, with the anticline cut by a small fault:

These strata must have been deposited more or less horizontally, then buried deep in the Earth. Later, they were deformed (folded, faulted, and tilted) during a tectonic event, most likely the late Paleozoic Alleghanian Orogeny, and then erosion chewed its way down to them, eventually exposing the tilted strata at the surface. The surface regime shifted from erosion to deposition, and a layer of well-rounded river cobbles were deposited atop the erosional surface. These where subsequently buried in sand and clay.

The exposure we observe here was made by excavation equipment in the quarry. It is more than a hundred feet above the level of the modern Shenandoah River. So at some point AFTER the events described in the previous paragraph, the Shenandoah cut down anew, chewing through these old terrace deposits and down into the bedrock beneath.

We chuckled to see that the teeth on the excavator bucket have approximately the same spacing as the width of the sand and mud layers in the Harpers.

What’s the age of the terrace gravel deposits? That’s a great question, and one I’d love to know the answer too. They could conceivably be as old as Cretaceous, based on regional erosional history, and the deposition of the basal layers of the Coastal Plain during the Cretaceous east of here. The level of chemical weathering suggested to Dan that they were younger, maybe Paleogene. We would love to have a cosmogenic nuclide date on this unit!

All photos in this post by Alan Pitts.