STRUCTURE OF EASTERN SHOREHAM, VERMONT: A COMPLEX PORTION OF THE TACONIAN FORELAND THRUST BELT

The eastern part of Shoreham, Vermont and immediately adjoining areas represents one of the most complex and best exposed portions of the Taconian foreland thrust belt. Despite the relatively dense outcrop and the presence of the primary Beekmantown reference sections, the structure has defied the attempts of geologists for more than 150 years. After 30 years of recursive mapping and structural analysis, I have come up with an internally consistent and retrodeformable structural model for this complex area.

Significant elements of the structure, including the Pinnacle and Orwell thrusts and the younger-over-older Shoreham thrust, have been known for more than a century. Several additional thrust sheets have been mapped, including additional breaching thrusts and a significant backthrust cutting across the deformed pile. Stratigraphic terminations within individual thrust sheets and stratigraphic relations illuminated by palinspastic restoration indicate the presence of submarine canyons dissecting the carbonate shelf at the time of deformation. The presence of the canyon segments accounts for otherwise rootless thrust sheets and unbalanceable structure.

Deformation proceeded westward and downward by the successive detachment of thrust sheets, beginning with the Pinnacle thrust sheet which emerged onto the shelf surface (still submerged) and proceeded westward across much of the remaining thrust belt. Breaching thrusts formed where later-forming thrusts encountered earlier formed thrust sheets that had passed off of raised portions of the bank and into canyon segments. The backthrust formed when the accreted thrust sheets and accompanying foreramp hangingwall syncline encountered the western wall of a major transverse canyon segment.