Conococheague Formation, Part 1

Since VMNH received the remarkable Boxley stromatolite a few months ago, I have read several hundred pages on both stromatolites and the Conococheague Formation, the rock unit exposed in the Boxley Blue Ridge Quarry. As part of my follow-up on the specimen, Brett, Tim, and I returned to the quarry last week to take a closer look at the rocks there.

The first thing that you notice on entering the Blue Ridge Quarry is that the rocks are, to put it technically, “really messed up.” This quarry is bounded on all sides by a series of thrust faults associated with the uplift of the Blue Ridge Mountains. The Conococheaque here is part of an allochthon, a body of rock that has been carried to a new location by faulting (these rocks originally formed further to the east). The diagram below (based largely on Decker et al., 1985) very roughly shows the relationships of the rocks in that area:

The heavy lines are all thrust faults. The white rocks are the Conococheague Formation. The green is the Cambrian Elbrook Formation, which is actually older than the Conococheague, but has been thrust over it. The red is the (older) Cambrian Rome Formation, while the blue represents much younger Devonian rocks; all of the Cambrian units here were thrust over the Devonian during the formation of the Appalachian Mountains.

As is visible in the photos at the top and below, the Conococheague was caught up in between these faults and intensely folded. There are a whole series of overturned folds like the one below.

It’s easy to get lost in these rocks, so here is the same image with one bed marked in red, so the fold is more obvious:

Note that in an overturned fold like this, the rocks in the upper part of the fold are upside-down. This is one way to get inverted sequences of sedimentary rocks, in which the younger rocks are on the bottom.

In spite of these rocks being so heavily deformed, they contain a bunch of information about the environment when the rocks were originally forming. In Part 2, we’ll look at some of the sedimentary features in the Conococheague.

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