Detailed mapping and structural investigations in an area bound to the west by the Grenvillian basement of the Adirondack Mountains and to the east by the allochthonous slates of the Taconic Mountains led to the distinction of a Western Undeformed Zone and an Eastern Deformed Zone. The former is composed of the gently east-dipping shelf sequence of upper Cambrian to lower Ordovician clastics and carbonates which unconformably overlie the Precambrian basement. The latter is characterized by lower to middle Ordovician carbonates, striking roughly north-south and dipping predominantly to the east, which are overlain by well-cleaved shales and slates. The stratigraphic sequence is thought to represent the tectonic stages of super-continent rifting (Bird & Dewey, 1970; Rankin, 1976), thermal subsidence of the passive margin and ocean transgression (McKenzie, 1978), and obduction of slope/rise shales and slates (Rowley & Delano, 1979; Rowley & Kidd, 1981).
The only published geologic maps which include the entire study area are the Vermont Centennial Map (Doll et al., 1961) and the Geologic Map of New York State (Fisher et al., 1970), both at a scale of 1: 250,000. While these maps and other previous studies (Rodgers, 1937; Cady, 1945; Zen, 1961; and Fisher, 1984) correctly identified the rocks in the area as faulted and deformed, the extent and nature of deformation was heretofore unrealized. In addition, some strata and structures were either not identified or were misinterpreted (i.e. Unit 1-def and the Root Pond Thrust).
In contrast, this thesis identifies four major east-dipping thrust faults in the Eastern Deformed Zone; from west to east they are the Temple Road, Shaw Mountain, Root Pond, and Forbes Hill Thrusts. Regional geology suggests east over west thrusting driven by convergent plate motion during the medial Ordovician Taconic Orogeny (Chapple, 1973; Rowley & Kidd, 1981). I propose that the thrust faults in the field area represent an imbricate duplex system formed by the foreland propagation of thrusts during Taconic Allochthon emplacement. Movement along normal faults is interpreted to be a response to the loading and ensuing flexural extension of the subducting slab. The sequential development of the structures in the field area, as well as the processes causing their formation, are discussed in detail in the final chapter.
Based on stratigraphic and structural evidence, regional structures are correlated with those in the field area. Furthermore, supported by evidence from Coney et al., (1972), I propose that the Temple Road, Shaw Mountain, and Root Pond Thrusts represent the southern extension of the Champlain Thrust System, the location of which was previously unknown this far south.
Granducci, J.L., 1995. Stratigraphy and structure at the southern
end of Lake Champlain in Benson, Vermont. Unpublished MSc. thesis,
State University of New York at Albany. 106 pp., +xi; 2 folded
University at Albany Science Library call number: SCIENCE Oversize (*) QE 40 Z899 1995 G73
Granducci MS thesis (scanned
text pdf - 12.6MB)
Plate 1 - Geology of Benson, VT
(coloured outcrop map, scale 1: 10,000) - 17 MB pdf file
Plate 2 - Geologic cross-section of Benson, VT
(uncoloured geological cross section, scale 1: 10,000) - 1MB pdf file