19 February 2014

Foreknobs Formation: a new outcrop with a possible submarine mass transport deposit

Posted by Callan

Today, we continue with the story of the field trip I took last week out to Corridor H, the new superhighway in West Virginia that is practically unused, and decorated with multistory roadcuts of spectacular Valley and Ridge sedimentary sequences, and their attendant structures.

From the putative Hampshire Formation exposures with their uncharacteristic marine incursion strata, we moved downhill (and, down-section) to examine a road-cut of what we inferred to be Foreknobs Formation. The left (uphill/youngest) part of this roadcut had some interesting stuff going on:


Whereas most of the strata in the Valley & Ridge are relatively coherent, the red blobby layer caught our eye as being odd. That clued us in to the layer overlying it, which we think might be a big shaley mass transport deposit: a weak pile of sand and mud that slumped, pre-lithification.


Note how the layer in question varies in thickness from the lower right to the upper left.

You’ll note there’s a lot of massive quartz sandstone in this outcrop, although there are occasional breaks in that monotony, such as this black shale layer seen at the far left of the outcrop (penultimate stratum in this sequence):


But while shale is all well and good, I’m more interested in the structural story in the gray shale / sandstone layer:



Here’s Alan pointing at some key features in the unit, which appear to be dismembered (boudinaged?), folded competent sandstone layers, surrounded by incompetent shale.


Let’s examine a few of these chunks in more detail… 1, 2, and 3…


First, let’s look at fold #1, just to the right of Alan’s pointing finger:



Then, there’s fold #2:



…And fold #3:



What do you think – is this a “broken formation” sort of scenario (i.e., the precursor to a tectonic mélange), where the deformation is tectonic (related, presumably, the Alleghanian thrusting and faulting), or is this a primary sedimentary feature (a submarine slump, a mass transport deposit)? I’m curious to hear your thoughts.