October 12, 2018
By Philip S. Prince, Virginia Division of Geology and Mineral Resources
This is another small material test model that ended up producing a cool result. The area of interest is circled in red here–a triple thickness of the lowest white layer in the stratigraphic sequence. Note that this is a nearly flat triple thickness; these are not hinterland-dipping imbricates. The pile is slightly anticlinal due to rotation of the thrust sheets after their low-angle cutoffs moved onto the flat glide plane.
So, how do you end up with a big pile of the same unit, stacked on top of itself? The model uses laterally spaced weak layers at progressively shallower depth in the section (the picture below illustrates this). Large thrust flats in these weak layers between ramps in strong layers are the only way to end up with the “triple stack” geometry.
The full section initially slides on the deepest decollement, then ramps onto the next flat decollement, producing imbricate thrust sheets. These sheets then climb onto the next shallow flat decollement, in this case tripling one particular stratigraphic interval (the white stuff between green and purple). As with most things related to structural geology, explaining this in words takes too many prepositions and isn’t effective. The video below does a better job of illustrating it.
So…how does this relate to the title? If you were a geologist standing on the triple stack zone, you might assume that the stratigraphic sequence you mapped nearby (green to purple to gray marker layers, moving downward) would hold true underneath you. Therefore, you would encounter the gray stuff at a predictable depth if you started to drill. This holds true in some settings, but in the presence of weak stratigraphic layers acting as decollements, the game gets switched significantly. West of Blacksburg, Virginia and Virginia Tech, a large structure called the Bane Dome reflects some aspects of the structural style seen in the model, and once sparked an interesting debate about this same interpretive challenge. The model is not a perfect representation, but the triple stack zone serves to provide some basic context.
In the 1940s, understanding of the decollement concept had not yet been thoroughly developed. In 1948, a wildcat oil well was drilled (by cable tool) in the middle of the Bane Dome, which can clearly be recognized as an anticline from surface outcrop data and stratigraphic knowledge. Imagine the model eroded to the level shown here.
On the land surface, a geologist would be aware of the anticlinal structure as well as relative position in the section due to units exposed nearby on dipping thrust sheets. If you’re deep in the white section below green, why wouldn’t you expect to drill through purple and end up the gray stuff in the near subsurface?
Obviously the folks working on the well did. Drilling quickly passed through the prospective interval with no shows and past a well-known middle Cambrian marker horizon. Assuming that the internal volume of the Bane Dome was taken up by deeper stratigraphy and then crystalline basement, the decision was made to stop drilling as no potentially productive units would remain further down section. The gray dolomites in which the well was abandoned certainly looked like what should occur below the Cambrian marker…but there’s lots of gray dolomite in the Valley and Ridge.
Based on the assumed lower Cambrian identity of the gray dolomite, the deep stratigraphy/basement-core model for the dome was subsequently written up and published. 30 years later, however, cuttings from the base of the well were re-examined due to renewed interest in gas prospectivity in the region and improved understanding of external thrust belts. Conodonts indicated that the well was, in fact, abandoned in Ordovician gray dolomite that was younger than the Cambrian marker near the top of the hole (check out Perry et al., 1979). Rocks in anticline cores should get older with depth, so this was an interesting result. The data indicated that a package of stratigraphy had been thrust onto a flat decollement, doubling a large portion of the section, and this “double stack” appeared to be subsequently folded by another deep thrust imbricate (see the black and white cross section 2 figures down). This arrangement is only possible with lengthy flat portions of thrusts; if faults dip over their entire trajectory, such a geometry cannot arise. The model does a decent job of illustrating the flat concept, and if the model faulted where indicated below, we’d end up with as good a miniature Bane Dome as you might hope to make.
Because no drilling has ever reached basement in this area (basement was encountered at ~17,000 feet to the southwest in Russell County, Virginia), the slightly folded flat-on-flat geometry cannot be conclusively determined. Even so, evidence for this style is sufficient for interpretations today to use a stack of “slices” of the Cambro-Ordovician carbonate section to fill in the space underneath the Bane Dome.
Interestingly, this stacked slice interpretation is supported by Bouguer Gravity Anomaly data.
Present interpretation puts ~16,000 feet of almost entirely carbonate rock under the Bane Dome. Carbonate bedrock is slightly more dense than sandstones and shales or continental crustal basement, and a big pile of carbonates shows up as a gravity high. This high is clearly visible on gravity maps, and extends southwest along the Narrows Thrust Sheet towards Burke’s Garden, the Bane Dome’s “little brother.”
Now for the rivers. It’s not a coincidence that the New River drains a tremendous portion of the Blue Ridge and Valley and Ridge through the Bane Dome into the Appalachian Plateau. North and south of the Dome, imbricate thrust sheets exposed repeated ridge-forming sandstone layers, forcing trellis drainage patterns that are structurally confined. The Bane Dome, on the other hand, offers a vast expense of erodible and easily-solutioned carbonate rock and thus a mechanically-favorable course.
Given the scale of the dome and its geometry with structural projection, this mechanically-appealing course would have existed in the area for some time during Appalachian exhumation. Sandstone outcrop should have been absent from the dome core for at least the last 2 km of exhumation, which should represent several million– likely 10’s of millions– of years of erosional history in this passive margin setting. Breaching into the carbonate stack might even have occurred during Alleghanian shortening (refer back to the black and white cross section). The structure has therefore long represented the easiest way for external base level to get into the Valley and Ridge and Blue Ridge, and the path is only going to get broader and easier with deeper exhumation. The same concept applies to Burke’s Garden. With deeper exhumation, this dome will produce a larger and larger valley zone, potentially altering drainage patterns in the geo-future (assuming passive margin processes persist long enough!).
This post was originally published on The Geo Models blog.