Date of Award
Master of Science (MS)
The Geology of the Whippoorwill Corners Area, NW Adirondacks, NYS
In the northwestern Adirondack Mountains of New York State lies a 110 km long, 0-5 km wide topographic and lithologic boundary which divides the Adirondack Highlands from the Northwest Lowlands. Structurally this boundary is apparently a highly strained zone of intensely foliated and lineated rocks. Within the zone mineral grains are highly recrystallized and of fine grain size with respect to rocks outside the zone, quartz becomes undulose, and mineral assemblages frequently exhibit retrograde characteristics. Despite the apparent high strain no demonstrable offset may be seen across the zone. Furthermore, finite strain indicators such as lineation and foliation orientations show no variation inside and outside the zone. Likewise, sense of shear indicators, which include rotated augen and multiple foliations, and two heretofore undescribed indicators: hornblende fabrics and oblique secondary quartz ribbons, rarely show asymmetry. The rare asymmetries which can be found are inconclusive, with 67% showing a northwest side down, or normal fault motion. This decided lack of the standard evidence for a simple shear zone suggests that the entire region has suffered a very high strain such that changes in finite strain can not be accurately measured, or, perhaps more probably, that the region has undergone a late progressive.coaxial deformation.
Grain Boundary Bands
Grain Boundary Bands (GBB) are small, elongate rims which may be observed on feldspar grains in many northwest Adirondack quartzofeldspathic rocks. These albitic bands exhibit a strong preferred orientation within a given rock, although the significance of the orientation is enigmatic. They are probably a late feature, as they are unstrained and free of the dirt and inclusions which are common in their host grains. They appear in optical continuity with the host plagioclase or potassium feldspar and are found only in contact with other feldspars. A sharp change in composition exists across the boundary of the band and its host, and the boundaries themselves generally appear correspondingly sharp. Occasionally, however, the boundary may be cuspate and less distinct, and it may exhibit faint-trails at a high angle to the walls. It appears that the GBB are produced by a dilational mechanism, and it is thought likely that grain boundary sliding and diffusive mass transfer play important roles. This indicates that grain boundary sliding may be active in coarser grain rocks (.15 to .2 mm diameter) than previously thought possible. Furthermore, this is the first direct observation of a microstructure which might be attributed to grain boundary sliding in rocks.
Hall, Peter C., "Some aspects of deformation fabrics along the Highland/Lowland boundary, northwest Adirondacks, New York State" (1984). Geology Theses and Dissertations. 32.
Plate 1 - Geological Map of the Whippoorwill Corners Area, Northwestern Adirondack Mountains, New York State (uncoloured outcrop geological map scale 1:12,000)
hallpl2xsec.pdf (382 kB)
Plate 2 - Cross Sections of the Whippoorwill Corners Area, NW Adirondacks (cross-sections scale 1:12,000)
hallpl3ocnomap.pdf (1603 kB)
Plate 3 - outcrop map of the Whippoorwill Corners Area, NW Adirondacks, NYS (uncoloured outcrop number map scale 1:12,000)