USING PHOTOGRAMMETRY TO ANALYZE STRUCTURES IN A TECTONIC SLIVER IN THE FOOT WALL OF THE CHAMPLAIN THRUST, SHELBURNE, VERMONT

In the Champlain Valley Belt of west-central Vermont, the Champlain Thrust juxtaposed ferruginous quartzites and dolostones of the Lower Cambrian Monkton Fm (hanging wall) with calcareous shales of the Late Ordovician Stony Point Fm (foot wall = Parautochthon), during the Ordovician Taconian Orogeny. Internal to the Parautochthon in this area, a tectonic sliver of dark gray limestone is found that likely correlates with the Cumberland Head Fm.

Structural analysis of a ~100 m long outcrop was accomplished through a combination of photogrammetric techniques and field measurements. We used photogrammetry software to construct photo mosaics of 1) the entire outcrop, 2) each structural package, and 3) across the tectonic boundaries between structural packages. A preliminary structural synthesis is described below. It is important to point out that structures with the same relative age, determined using cross cutting relationships, may have developed diachronously across the outcrop.

The “sliver” of Cumberland Head Fm consists of 5 structural packages that are bounded by 4 faults. The oldest planar structure in each package is bedding (S₀), which appears as a millimeter-scale, dark and light, compositional banding. Superposed on S₀ at high angles, is a penetrative spaced pressure-solution cleavage S₁ that, where associated folds are absent, is thrust-related. Southeast-trending slickensides formed on fault-parallel zones of calcite veins in each of the 4 faults.

In packages 1-3 (numbered from west to east), S₁ is truncated by, and/or rotated into parallelism with, the overriding east-dipping thrust faults. These faults (A+B) are deformed by open folds (F₂) with a local axial planar spaced cleavage (S₂). The fault C boundary between packages 3 and 4 truncates S₂. Although the S₁ cleavage is openly folded (F₃) and fans on the west side of package 4, it shears out on the east side to form a ~vertical fault contact with package 5. Within package 5, S₁ is deformed by tight reclined F₃ folds, with gently south-plunging axes.

This poster shows photo mosaics of different scales that are integrated with field data to document the structural history of the Shelburne outcrop. Our approach illustrates the utility of using photogrammetric techniques in the structural analysis of complexly deformed outcrops.