ABSTRACT
The carbonates of the Vermont Valley and the overlying rocks of the Taconic sequence have been generally believed to lie on the east limb of a major unfaulted syncline (Middlebury Synclinorium). In this view a westward dipping, north-south trending basal Taconic overthrust must be exposed somewhere along the eastern margin of the Taconic Range.
In contrast, this study based on detailed mapping at the north end of the Taconic Allochthon suggests that a folded overthrust surface is only locally seen in fensters; more commonly the basal obduction surface of the Taconic Allochthon has been truncated by later cross-cutting thrust faults. In the study area these newly recognized east dipping faults are the "Whipple Hollow Fault" and the "Proctor Fault". These two faults postdate the initial Taconic overthrust and must belong to the Champlain thrust system of which another fault forms the frontal thrust of the Taconic thrust belt towards the west. Where exposed the basal thrust is characterized by a thick melange unit and isolated slivers of shelf carbonates. The melange underlies wide areas which had previously been mapped as part of the authochthonous shelf sequence. Other areas which were also believed to represent these authochthonous phyllites have been identified as characteristic black pelitic Taconic lithologies. The redefined Taconic rocks also include typical green and greenish gray phyllites of the Cambrian Bull Formation. It indicates that the Middle Ordovician shale unit which conformably to unconformably overlies the carbonate shelf west of the Taconic Range is not yet, or perhaps only locally, exposed at the present erosion level of the Vermont Valley. In addition the easternmost equivalents of this unit must have been tectonically removed and incorporated into the basal melange. Where a lithological correlation with the Taconic stratigraphy is not possible the remaining black phyllites are probably Middle Ordovician in age; still these phyllites have been incorporated into the overlying melange and must be viewed as allochthonous. The rocks of the study area underwent a progressive deformation path which can be subdivided into three different stages (D1 to D3). During each stage a characteristic thrust system (T1 to T3) was active. D1-deformation describes the stacking of the Taconic lithologies in an accretionary imbricate fan thrust environment (T1). This early deformation is associated with a prominent slaty cleavage (S1). Large-scale F1-folds, if present, would be strongly refolded and tightened so that they are not detectable. Obduction of the composite T1-thrust stack onto the carbonate shelf resulted into a small-scale imbricated T2-marble/phyllite schuppen structure. The propagation of this duplex has produced large-scale folds. F2-structures are associated with a main regional crenulation cleavage (S2). Progressive shortening during D3-deformation culminated in foreland directed thrust faults (T3). A second crenulation cleavage (S3) is related to this late thrusting event. The regional application of the observed thrust geometry strongly suggests that the Middlebury Synclinorium is unlikely to be an unfaulted structure. In particular this study suggests major north/south trending, eastward dipping late thrust faults for the entire length of the eastern Taconic margin and the Vermont Valley. The "higher Taconic slices" are also believed to be related to this period of thrusting; this out-of sequence imbrication by later faults explains the existing "stacking controversy" among Taconic geologists.

Bierbrauer, K., 1990. The geology of Taconic thrust sheets and surrounding carbonates of the west central Vermont marble belt, north of Rutland, Vermont.  Unpublished MSc. thesis, State University of New York at Albany. 105 pp., +ix; 1 folded plate (map)
University at Albany Science Library call number:  SCIENCE Oversize (*) QE 40 Z899 1990 B54

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