Pre-thrust normal faults and post-tectonic micas in the Taconic Range of west-central Vermont
Nicholas W. Hayman 1997
A thesis presented to the Faculty of the University at Albany, State University of New York in partial fulfillment of the requirements for the degree of Master of Science
College of Sciences and Mathematics, Department of Earth and Atmospheric Sciences
Advisors: W.S.F. Kidd and W.D. Means

New geologic maps show that the Champlain Thrust System traces continuously between Shoreham and Benson, Vermont. The Champlain Thrust System consists of at least three internally imbricated thrust slices in continental shelf facies quartzites and carbonates. From north to south there is a structurally controlled thinning of the Champlain Thrust System, and a climbing in stratigraphic level by the thrusts. Within the Champlain Thrust System are a set of across-strike structures which create offsets in the thrusts and the surrounding lithic map unit boundaries. These structures function as lateral ramps in the thrust system geometry and bound thrust duplexes. Often there are changes in the stratigraphic level across these offsets which cannot be explained by the thrust geometry. These are interpreted to be to be pre-thrust normal faults, and a subsidiary (trench normal) set to the normal faults predicted by the model of Bradley and Kidd (1991), wherein normal faulting occurs in the continental crust in response to lithospheric flexure with the onset of collision. Trench parallel normal faults bounded grabens containing shales, and horsts of carbonate rocks. These horsts and grabens are now reflected in the thrust system's geometry. The Mettawee River Fault, an east-side-down normal fault, juxtaposes an intermixed belt of Middle Ordovician shelf facies shale and Pre-Cambrian continental rise facies slates and arenites, and Middle Ordovician flysch and melange, against the Champlain Thrust System. This structure could be late Taconic (orogenic) to post-Taconic in age, and have amounts of throw of tens to hundreds of meters. East of the Mettawee River Fault is the westernmost Taconic allochthon, the Sunset Lake Slice. This is a roughly north-south trending belt of green slates interspersed with lenses of continental rise facies quartzites. The Sunset Lake Slice is bound to the east by the Taconic Frontal Thrust, a late, out of sequence thrust which transports an eastern belt of intermixed Ordovician and Taconic shales and melange, as well as an eastern thrust belt of shelf facies carbonates and shales. This belt of carbonates and shales is litholologically identical to the rocks found in the Champlain Thrust System but structurally dissimilar. The separation of these rocks from their facies equivalents in the Champlain Thrust System is suggested to be due to their topographically higher position on the paleo-continental shelf, the topography being largely (pre-thrust) normal fault controlled.
"Cross-micas" are observed mica grains from the slates at the Cedar Point Quarry, W. Castleton, Vt.. The Cedar Point cross-micas cross-cut the boundaries of a late crenulation cleavage which has the morphology of micro-kinks at the microscopic scale. This cross-cutting relationship suggests that these grains grew later than the deformation which produced the micro-kinks, and therefore later than the deformation which produced the slaty cleavage. If this is the case, then perhaps many, or all of the mica grains in the slate grew at this later time. The investigation to gain insight into the timing of the development of the cross-micas included producing a structural map and cross-section of the Cedar Point Quarry which may be useful for related future studies. Field based observations provide insight into the deformation history recorded at Cedar Point. The cross-micas, and the surrounding micro-structure of the slate are documented with photomicrographs. The observation that some micro-kinks are enriched with opaque minerals (oxides and sulfides), whereas other, adjacent, micro-kinks are enriched with phyllosilicates, is evidence that there was some amount of mass transfer, presumably due to solution processes, after the formation of the micro-kinks. Electron microprobe data was attained from the slates in the hopes of confirming whether the cross-micas represent mica growth which was localized along the micro-kinks (and thus in the absence of a widespread metamorphic/mica growth event), or mica growth which was widespread throughout the slates. It was found that there are two groups of muscovite in the slate, a high K and a low K muscovite. There is also a group of interlayered muscovite and chlorite. There is no obvious relationship between structural setting, such as cross-micas, and the mineralogical variation, though with future work, such a relationship may be established.

Hayman, N.W., 1997. Pre-thrust normal faults and post-tectonic micas in the Taconic Range of west-central Vermont. Unpublished MSc. thesis, State University of New York at Albany. 179 pp., +xiii; 3 folded plates (maps)
University at Albany Science Library call number:  SCIENCE Oversize (*) QE 40 Z899 1997 H396

thesis (scanned text) - 16.6MB pdf file

    Plate 1 - Bedrock geologic map, Shoreham-Whiting region, west-central Vermont
                (coloured outcrop map, scale 1: 25,000) - 0.8 MB pdf file
    Plate 2 - Bedrock geologic map, Orwell-Benson region, southwest Vermont
                (coloured outcrop map, scale 1: 25,000) - 0.9 MB pdf file
    Plate 3 - Structural map of the Cedar Point Quarry, West Castleton, Vermont
                (structural data map with color annotation; scale 1:250) - 4.9MB pdf file

    Plate 1+2 merged - Geology of the Shoreham-Benson area, Vermont
                (coloured outcrop map and cross-sections, scale 1: 42,400) - 1.5 MB pdf file

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