ABSTRACT
The geology of the eastern limb of the Green Mountain
Anticlinorium
consists of a series of Paleozoic metasedimentary rocks with
lithologic
boundaries arranged in a remarkably straight trend approximately
parallel
to the axis of the Green Mountains. Published reports of the area,
consisting
largely of reconnaissance mapping, have treated this complex
series of
polyphase deformed rocks as an essentially upright autocthonous
sedimentary
sequence. Boundaries between rock units have, for the most part,
been
assumed to be primary in origin, as have various structural
elements
within
the rock units. More recent work in selected areas within the
Ludlow
Quadrangle
has revealed problems in the application of stratigraphic
techniques
toward
field mapping in these medium grade metamorphic rocks.
One of the main problems is the consistent misidentification of
demonstrably
secondary mesostructures as primary sedimentary features by early
workers.
Many rocks in the area have been shown to contain secondary
structures
which mimic such primary features as bedding and conglomeratic
pebbles
(Gregg and Nisbet, in preparation) and which have in fact been
mistaken
for these structures in the past. In addition to problems arising
from
incorrect interpretation of mesostructures, other problems have
resulted
from the failure of some workers to distinguish various fold
groups on
the basis of overprinting relationships,
This investigation deals with a subarea within the Ludlow
quadrangle
where detailed structural mapping was performed by the author from
1971
to 1974. The central feature of the area is the ultramafic zone
consisting
dominantly of serpentinized ultramafic rock masses up to 1 km long
with
minor zones of talc carbonate rocks around the boundaries, This
ultramafic
zone is situated along the boundary of two rock units, the
Moretown
member
and: the Cram Hill member of the Missisquoi Formation. Although
these
units
have been called members of the same formation by early workers,
the
author
has found a number of striking contrasts in the deformational
features
of the units. For example, the Cram Hill phyllites have been
involved
in
only two phases of deformation, while the Moretown gneisses
contain
structures
from at least one earlier deformational phase. The Cram Hill
phyllites
contain layering which may be of sedimentary origin and which is
moderately
deformed in most cases. The Moretown member has been severely
deformed,
and all trace of initial layering is obliterated. In addition to
these
contrasts the author has shown a discordancy between early
layering and
the contact between the units which is probably of tectonic
origin.
Because.
of these contrasts the author considers that there is little basis
for
classifying the various rock types by the "Formational" and
"Member"
designations,
and that future structural work in the Central Vermont area will
result
in the abandonment of the stratigraphic nomenclature now being
applied.
During field work and thin section examination the author observed
a number of tabular garnets in the rocks of the Moretown member.
In
most
cases the garnets were formed during the earliest deformational
event
and
were deformed into tabular shape by later deformation. The
deformation,
however, was not the typical flattening assumed by most workers,
but a
slicing process in which segments of garnets from an initially
equidimensional
crystal are sheared parallel to rock layering. A number of
examples of
partially sliced crystal sections were observed on mesoscopic and
microscopic
scales.
Gregg, W.J., 1975. Structural Studies in the Moretown and Cram
Hill
Units near Ludlow, Vermont. Unpublished MSc. thesis, State
University
of
New York at Albany. 118pp., +vii, 2 folded plates (maps).
University at Albany Science Library call number: SCIENCE
Oversize
(*) QE 171 G64X
MS thesis scanned
text
pdf
(11 MB);
Plate 1 - (coloured outcrop
map,
scale 1:4,800 - pdf file 10 MB)
Geological
map
of southeast Ludlow Township, Vermont
Plate 2 - (patterned non-outcrop
map,
scale 1:4,800 - pdf file 4 MB)
Geological
Map
of
the
Moretown and Cram Hill areas near Ludlow, Vermont
the same map, but with sample
locations
Return to MS Theses completed in the Geological Sciences Program, University at Albany