ABSTRACT
Stratigraphies previously proposed for the Taconic sequence of southern
Washington County, eastern New York, have incorrectly defined and positioned
lower Cambrian black slate units. The proper sequence of Cambrian (?) through
Cambrian lithologies is (terminology from Jacobi, 1977): Bomoseen green
wacke, Truthville green slate, Browns Pond black slate, Mettawee purple
and green slate, and West Castleton and Hatch Hill black slates. This and
the entire Taconic sequence is conformable within the western Giddings
Brook slice. The detailed lithostratigraphy reported by Jacobi (1977) and
Rowley (1980) in northern Washington County can be followed at least some
45 kilometers to the south (Fort Miller, Schuylerville, Cossayuna and Cambridge
7½' quadrangles) in the westernmost portions of the allochthon.
Within some units, variations between "sub-domains" can be recognized.
Lithologic characteristics suggest that each sub-domain records deposition
at slightly different positions (distal/proximal) on the Cambro-Ordovician
North American continental rise. These stratigraphic sub-domains correspond
with areas of internally coherent structure (fold and cleavage orientation,
sense of structural facing). Together these define an assemblage of imbricate
thrust sheets within the western Giddings Brook slice.
The basal Taconic thrust and thrusts internal to the allochthon cut
obliquely across fold axes, hinge lines and cleavage in the allochthon
and the underlying Hudson flysch. Allochthon emplacement clearly post-dates
the earliest regional slaty cleavage development and two generations of
large-scale tectonic folding. Tectonic slivers and fault rocks (Bald Mountain
Terrane) present along the basal Taconic thrust initially formed as the
allochthon ramped over the Cambro-Ordovician North American continental
shelf. The fold-thrust mode of deformation continues in the underlying
flysch sequence of the Hudson River lowlands. The progressive development
of structures observed within these units is analogous to the formation
of structures within the leading edge of a subduction-accretion assemblage.
Halite single crystals loaded dry and then placed in brine are preferentially
dissolved at surfaces adjacent to regions of high plastic strain. Riecke's
Principle satisfactorily accounts for the observed distribution of dissolution
rates in these and the circular hole experiments performed by Sprunt and
Nur (1977), but plastic strain probably dominates over elastic strain in
the free energy calculations. Faster dissolution of crystalline material
with high defect concentrations is the mechanism suggested to be responsible
for this phenomenon.
Although pressure gradients may dominate over strain energy terms in
chemical potential calculations along grain-to-grain contacts, the role
of permanent strain should be considered in the total physicochemical process
of diffusive mass transfer (pressure solution). Of particular importance
is knowledge of the rate limiting step in the transfer sequence, which
may occur at grain boundary-pore fluid junctions. There the kinetics of
dissolution/precipitation may play a significant role.
The thermodynamic and kinetic criteria for equilibrium between states
are not truly equivalent. This becomes an important consideration when
forward and reverse reactions of a given change of state occur by different
reaction pathways, as may be the case for dissolution/precipitation at
a strained solid-fluid interface. The possibility then arises that solution
transfer mechanisms exist which are driven by kinetic rate differentials
rather than the energy change between solid and solution state. A complete
theoretical analysis of any model designed to represent solid-fluid interactions
during deformation will require a knowledge of state energy levels, reaction
mechanisms and their associated activation energies, and the various parameters
of diffusion within each state and along phase interfaces. Not only must
the effects of deformation through fluid-assisted diffusive mass transfer
be considered, but also the role of fluid phases in the plasticity of crystalline
material. These two general processes may be easily confused in both experimental
analogues of geologic systems and the naturally occurring rocks themselves.
Bosworth, W.B., 1980. Structural geology of the Fort Miller, Schuylerville
and portions of the Schaghticoke 7½' quadrangles, eastern New York,
and its implications in Taconic geology; and experimental and theoretical
studies of solution transfer in deforming heterogeneous systems. Unpublished
PhD dissertation, State University of New York at Albany. 237pp., +xv;
4 folded plates (maps)
University at Albany Science Library call number: SCIENCE MIC
Film QE 40 Z899 1980 B68
Copies of this PhD dissertation can be ordered
from Proquest UMI
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