April 30, 2018
Written by Philip S. Prince, Virginia Division of Geology and Mineral Resources
The explanation for Cove Mountain’s shape lies in the fold structures within the fault-bounded Cove Mountain block. The ages of rock units associated with the “2” shape on the geologic map in the last installment show that the upper part of the “2” is a slightly overturned anticline cored by Cambrian-aged carbonates, while the angular base of the “2” is a tight, overturned syncline cored locally by middle Devonian Millboro shale. This kilometer-scale overturned anticline-syncline pair defines the southwest end of the Cove Mountain block. The anticline is in the immediate hanging wall of a thrust fault, the Cove Mountain Fault, and the tightly “creased” character of the syncline results from its position in the immediate footwall of a major regional thrust/detachment fault, the Catawba Branch Fault. The pairing of these folds is essential to producing the pattern.
At this point, it’s necessary to say a bit about landscape evolution and which portion of the stratigraphy actually supports Cove Mountain topography. The geologic map image of the first installment showed that only the upper Ordovician and Siluro-Devonian units actually express the “2” outcrop pattern. The upper Ordovician section in this area is dominated by shales, which are frequently calcareous and unable to support rugged topography. The Devonian shales to the north and east of Cove Mountain are not as prone to weathering, but they are physically weak and equally unable to support high, steep topography. The Siluro-Devonian, on the other hand, is dominated by quartz arenites that are often greater than 95% quartz and typically extensively recrystallized. These are among the hardest and most chemically-resistant rocks in Appalachia, and they are extremely robust supporters of topography. As a result, the “2” shape is controlled by the outcrop pattern of the Siluro-Devonian interval and its strong contrast with calcareous shale below and weak Devonian black shale above. The Siluro-Devonian sandstone section is a bit less than 1,000 (300 m) thick in this area, but its erosional resistance is so much greater than neighboring units that it completely dominates topography within a section thousands of feet thick. It is therefore possible to visualize the “2” structure as a fold pair in a single stratigraphic unit, which makes the rest of the explanation much simpler!
An important consideration in understanding Cove Mountain is that the anticline-syncline pair is plunging to the northeast…and plunging steeply, at that. Plunging folds are folds whose axes or hinge lines are not horizontal, meaning the fold is tilted. Erosion of a non-plunging anticline will produce parallel outcrop belts, which in the humid-temperate Appalachians would produce parallel mountain ridges. Plunging folds produce closed curve shapes in the landscape, which result from both the limbs and hinge zone of the fold being intersected by the land surface at once. In the case of Cove Mountain, plunge is steep to the northeast and is localized to the southwest end of the structure. The “2” results from the erosional surface cutting across the steeply plunging anticline-syncline pair. The simple visual below gives an idea of how this happens. The Play-Doh represents the shape of the Siluro-Devonian interval, which is luckily the only one you need to think about to figure out the topography!
Some interesting expressions of the plunging anticline model can be seen in the field as well as in digital topography. The upper left tip of the “2” is segmented by gaps in the ridge, but these gaps are not simply the result of erosion. The ridge-forming sandstone is actually segmented into small pods in this area, and the beds do not extend downward into the subsurface. The gaps have formed where the sandstone is absent, and a walk up the creeks flowing out of the gaps makes this very clear. The sandstone pods appear to represent the “smeared” forelimb of the anticline, which is now visible due to the steep overall plunge of the structure.
A close look at digital topography reveals the abrupt cut-off at the base of one large sandstone outcrop, which is indeed conspicuously absent from the bedrock stream below. This location also provides an interesting look at the physical of fold-thrust belt structures. The tiny “pod” of sandstone is a very minor part of the overall structure, but it still looks like huge cliffs to a person on the ground!
Digital topography greatly enhances the geologist’s ability to interpret structural details like the forelimb of the Cove Mountain anticline. From the ground, the nature of the contact of the Silurian sandstones with surrounding units is difficult to discern. Outcrop is not constant, tree growth is dense, and colluvium mantles many areas to an extent that makes it difficult to see where bedrock stops. The physical size of the Silurian “pod” outcrop is still large enough to prevent viewing it all at once; the picture below gives a sense of what it looks like from the ground.
Walking the stream channels in the ridge gaps makes it clear that sandstone is limited to the ridge crest in the segmented zone, but this geometry is difficult to visualize for an on-the-ground observer. When field observations are combined with inspection of digital topography, however, the overall context of the features starts to make more sense. Throw in some basic physical conceptual models, and interpretation continues to pick up speed!
The next chapter will talk about the formation of plunging folds. Two models are suitable for explaining the Cove Mountain structure, but even detailed field mapping cannot prove which one is correct…
This post was originally published on The Geo Models blog.