by Philip S. Prince, Virginia Tech Active Tectonics and Geomorphology Lab
While the “frog legs gorge” post was supposed to be a bit funny, the real purpose of it was to link outcrop patterns with geologic structure. This follow-up post tries to put the frog legs’ underlying structure into a broader context within the Appalachian Valley and Ridge, a deeply eroded fold-thrust belt with locally distinct structural styles. The frog legs gorge is located in a part of the Valley and Ridge in which a ~2,000 ft (600 m)-thick section of sedimentary rock is able to fault and fold relatively independently of the much thicker sedimentary sequences above and below it.
The cross section above from Kulander and Dean (1986) in intended to communicate the general idea of independent faulting and folding within different parts of the overall layer sequence. The frog legs occur in an exposure of the thin yellow zone, which contains lots of sandstone, along with some limestone and shale. The black lines show closely spaced faults within this zone. These faults and their associated folds don’t extend upwards or downwards into the layer sequences above and below, so their wavelength is short (3,300 ft/ 1 km or less). This pattern can be reproduced in a sand model using granular materials of varying strengths, as seen in the Aguaragüe anticline post.
All of the layers in the model are faulted and folded, but if you look closely, it is not possible to link faults from the deep dark blue layers to faults in the uppermost light blue layers. The white stuff in the middle is weak microbeads, and these effectively disconnect the uppermost sequence from the lowermost sequence. The thin yellow layer in the middle is quite brittle, but it is most along for the ride within the weak zone. This model would need to be deeply eroded to expose its inner portions and produce an outcrop pattern like what is seen in today’s Valley and Ridge.
This style of faulting can be seen on the model’s surface while it is deforming. Because the uppermost light blue layer can slide somewhat freely from layers below, it produces small and closely spaced surface folds due to its thickness. When the entire layer pack faults at once, the surface fold is much broader. This video shows deformation of the model above, as well as an additional experiment:
I call this model setup the “Oreo Cookie” because it involves a middle weak zone sandwiched between stiffer, more brittle layers above and below. In my experience, the results most closely resemble the reality of this part of the Valley and Ridge when the uppermost layer sequence is both the strongest and the thickest part of the layer pack. Some of the stylistic features that form in this model setup can be compared to real structural features and the landforms they produce.
The second experiment in the video link above produced slightly different results because it used slightly different sand (my materials are color coded by their relative strength). In many ways, the second model matches the structural surroundings of the frog legs gorge more closely because it developed a huge, broad syncline on its hinterland side (see the light blue syncline in the real Kulander and Dean section). This is the broad, downward-flexed zone of pink and red layers on the right side of the model below.
Significant erosion of this model would create an outcrop pattern that is dominated by the lower portions of the pink/red layers where they are flexed downward into the syncline. A similar feature in the real Valley and Ridge creates a broad zone of high and rugged mountain topography today.
Another section from this model produced a higher displacement frontal thrust fault. Again, this thrust does not cut straight through the entire layer pack. Instead, its displacement is distributed onto several smaller faults within and above the weak white layers. I don’t even try to draw them here, but you can see that the yellow/green layer is not offset like the dark blue below it. Changes in thickness of the weak white layer is the result of displacement of the frontal thrust fault being redistributed throughout the section.
Like many other models I have posted on this blog, these experiments produce folds and fault patterns that are limited to specific sections within the overall stratigraphy. If these models were made with uniform sand, they would produce 4 or 5 thrust sheets that each carry the entire layer pack and bring deep layers to shallow depths. With the addition of microbeads, the result is an upper and lower sheet, each of which is internally faulted and folded. Each of the disconnected horizons contains its own set of faults and folds, and they often don’t line up vertically in the section. You could stand on a surface anticline that is underlain by a syncline at depth, and vice versa. The tendency of the microbeads to “flow” to a certain degree allows them to fill in the space to make the fold mismatches possible.
I really like the monster syncline under the words “Upper Sheet” above because it resembles the real Whip Cove Syncline (landform 2 above) of Virginia and West Virginia. Today, this broad expanse of Devonian- and Mississippian-aged sedimentary rocks produces a significant mountain range due to its size and erosional resistance. The area is one of surprisingly unbroken wilderness because the topography is extremely rugged and the soil is not good for agriculture. The huge dark swath running through the middle of the image below is about 10 miles (16 km) across, and stretches for many 10’s of miles along the trend of the Valley and Ridge.