ABSTRACT
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, P.C., 1984. Some aspects of deformation fabrics along the
Highland/Lowland boundary, northwest Adirondacks, New York State.
Unpublished MSc. thesis, State University of New York at Albany.
124 pp., +viii; 3 folded plates (maps)
University at Albany Science Library call number: SCIENCE
Oversize (*) QE 40 Z899 1984 H34
thesis (scanned
text) - 18.5MB pdf file
Plate 1 - uncoloured outcrop geological map
(scale 1:12,000) pdf file 2.4 MB
Geological
Map
of the Whippoorwill Corners Area, Northwestern Adirondack
Mountains, New York State
Plate 1 - cross-sections (scale
1:12,000) pdf file 0.4 MB
Cross
Sections
of the Whippoorwill Corners Area, NW Adirondacks
Plate 3 - uncoloured outcrop
number map (scale 1:12,000) pdf
file 1.6 MB
outcrop
map
of the Whippoorwill Corners Area, NW Adirondacks, NYS
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Geological Sciences Program, University at Albany