UMR Journal -- V. H. McNutt Colloquium Series

UMR Journal -- V. H. McNutt Colloquium Series


Byron N. Cooper


The Appalachian folded belt of southwestern Virginia exemplifies most of the structural and stratigraphic features that are considered typical of the Appalachians as a whole. The folded belt which is only 36 miles wide along the valley course of New River is generally well-defined on the southeast by the western foothills of the Blue Ridge and on the northwest side by a sharp structural front beyond which the beds are relatively gently folded.

Despite the fact that Paleozoic shelf successions of the Central Interior region are known to have controlled the thickness and facies of sedimentary formations within them and to have evolved as a result of differential crustal subsidence, the interpretations of many Appalachian geologists either specify or imply that Appalachian sediments accumulated in flat sheets exhibiting only regional variations in thickness and facies. As detailed work in many sections of the Appalachians has shown, there are actually two types of stratigraphic variations: one that is broadly regional, and another that is local and commonly structurally controlled. There is a pronounced tendency for structural geologists to ignore the relations between stratigraphy and structure and to visualize the latter as evolving after all the Paleozoic succession had been deposited.

A currently popular hypothesis of Appalachian geology interprets Appalachian folds and thrusts as “thin-skinned” and to have formed by decollement or Abscherung along a great master sole thrust formed during late Paleozoic time. Inherent in the decollement hypothesis is an implied requirement that if the idea applies everywhere it must hold anywhere in the Appalachians. Perhaps the strongest endorsement of the “thin-skinned” hypothesis for Appalachian structure is that advanced by Gwinn who envisions the sole fault extending not only all the way across the Appalachian folded belt but also across the Appalachian Plateau and under the deepest part of the Pennsylvanian basin of westernmost West Virginia. Gwinn even envisions this entire width of Paleozoic rocks to have been involved in a decollement movement that was motivated by gravity.

In the best of the more recent summaries on the inferred structural evolution of the Appalachian folded belt. King and Ferguson (1960) related all the existing thrusts and folds in northeasternmost Tennessee to one late Paleozoic deformation. Gwinn’s interpretation of Appalachian structural evolution requires that all the major structures formed at about the same time.

The absence of a major thrust within, or at the base of, or at the top of the Cambrian Rome Formation in the Bane anticline of Giles County, Virginia, is indicated by both surface geology and subsurface information. This is strong evidence against regional decollement as the basic cause for Appalachian folding as recently championed by R. L. Miller, Rodgers, Gwinn, and others.

There is convincing stratigraphic and petrographic evidence that the great Holston Mountain thrust of Virginia and northeasternmost Tennessee was initiated in mid-Champlainian time. Tear faults along the northwestern border of the overthrust block produced a salient “finger” of overthrust rocks that points directly at localized occurrences of Middle Ordovician polymictic conglomerates nestled in the trough of the South Knobs syncline. These conglomerates, as exposed near Avens Ford Bridge over South Holston Lake, are composed of clasts out of the same lithofacies as those composing the northwestern portion of the Holston Mountain thrust block.

The stratigraphic range of the clasts in the conglomerates near Avens Ford Bridge clearly implies local exhumation of a succession of strata at least 10,000 feet thick, either by folding or faulting, in order for the association of polymictic clasts to have been achieved. The logical source for these polymictic pebbles and cobbles is the advancing Holston Mountain thrust block. It may be possible to establish the Middle Ordovician generation of that thrust block by K/Ar radiometric dating of selected portions of the overthrust sheet which includes some crystalline rock and some Late Precambrian rhyolites.

The supposed superficial nature of Appalachian folding is further denied by some of the typical structural features of the region, including that studied by King and Ferguson in northeasternmost Tennessee. The once flat thrust surfaces have been folded sufficiently to develop closures of thousands of feet in them. Because the folding of the thrust faults involved both overriding and overridden beds, the later stress conditions had to affect a greater thickness of the strata than during thrusting. Folding of thrust sheets strongly suggests the dominance and persistence of vertically acting forces even after thrusting had ceased. Numerous klippen and windows associated with a number of major Appalachian .thrusts aid in ascertaining the dimensions of vertical movements after displacement of thrust sheets.

As Bailey Willis (1893) showed, the pattern of Appalachian folds was determined by differential axial subsidence in the Appalachian trough, which led to development of “synclines of deposition” each of which in its own way affected the succession of sediments deposited within itself. Synclines are the dominant fold type from Pennsylvania to Alabama. The general structure of the folded Appalachian belt is a synclinorium or geosynclinorium.

Studies of the stratigraphy and petrography of the strata within a given major structure will provide a basis for determining when the structure had its inception. In a general way, the Appalachian trough seems to have grown in width by progressive addition of synclines of deposition which formed at the expense of the outer margin of the foreland shelf. Some of the great synclines of deposition, such as the Hurricane Ridge syncline of Virginia and West Virginia, developed in Mississippian time, whereas some farther east probably developed in the earlier parts of the Paleozoic.

Decollement deformation, such as illustrated by the Cumberland block, indicates how some Appalachian thrusts worked their way upward through successions primarily composed of thick, competent strata with only a few shaly zones. Transposition of thrust sheets does not necessarily depend upon a shale acting as a decollement zone. The Holston Mountain-Iron Mountain thrust illustrates a folded, once-flat thrust that cut the basement rocks. If the Middle Ordovician age of this thrust can be determined radiometrically, a strong probability favored by the relations of the mid-Champlainian polymictic conglomerates to the overriding block, a major break-through in understanding Appalachian structure will have been achieved. If all Appalachian thrusts did not originate at about the same time, after all the Appalachian sediments had been deposited in the Appalachian trough, then the hypothesis of regional decollement as the mechanism that created all the folds and faults would be untenable.

Based upon his own observations in the Appalachian region, the writer believes that basement has been involved in Appalachian folding since the time that the Paleozoic sediments in the Appalachian geosyncline began to accumulate. Differential axial subsidence of the floor of the Appalachian trough, which affected an increasingly wider belt from early to late Paleozoic time, set the pattern for folds and faults as seen today. Dominance of vertical forces over “tangential” forces in the tectonic history of the Appalachians is strongly affirmed by the common occurrence of remnants of thrust sheets preserved in the axial portions of indigenous synclines whose strata reflect thickness and facies control by the major structure of which they are a part. Such conditions, which are not uncommon, imply that differential down warps, which started early in many instances, lasted even after overthrusting. Thrusting, therefore, might be conceived as one of the transient consequences of deposition of a thick succession of strata that were being differentially downwarped during and after their accumulation.

In pursuing the unraveling of the tectonic history of the Appalachian folded belt, countless minor adjustment structures preserved within relatively thin successions need careful analysis. The down- slumped portion of the adjustment structures the writer has seen over the greater part of the Appalachian region are “down” as referenced to the particular major structure in which the beds containing a minor adjustment structure occur. Deep cuts along modern highways offer a great reservoir of information on adjustment structures.

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