NEW BEDROCK MAP AND CROSS-SECTIONS OF VERMONT: STRUCTURAL AND STRATIGRAPHIC CONSTRAINTS FOR NORTHERN VERMONT

Four E-W cross-sections in northern VT (with contributions from R. Stanley, B. Doolan and J. Kim) highlight differences between the new map and the 1961 Centennial Geologic Map of Vermont. Taconian to Acadian faults, some reactivated, in the pre-Silurian rocks include: the Champlain/Logan’s Line, Hinesburg, Underhill, Prospect Rock (PRF), and Coburn Hill (CHT) thrusts, the Brome and Honey Hollow back thrusts, the Burgess Branch (BBFZ)/Baie Verte Brompton/St Joseph, and the Eden Notch faults. In contrast to the 1961 map, the new map shows fault-bounded lithotectonic packages with internal stratigraphy. West of the Green Mtn. anticlinorium (GMA), west-directed thrusts splay from east of the Ripton anticline. Faults transition to neutral zones, then back thrusts, south to north. Age control is based on the Middle Ordovician conodont-bearing West Bridgewater Fm. unconformably at the top of the rift clastic section. In the GMA and east, anastomozing Acadian faults alternately cut out and expose sections of relatively flat Taconian thrust slices. The Ottauquechee Fm. and interlayered green phyllites of the Mt. Abraham, Jay, and Stowe Fms. occur in thrust slices which span from west of the GMA to the Moretown contact. Ultramafic rocks, including the Belvidere Mt. Complex (serpentinite and metamorphic sole) with an Ar/Ar age of 505 ma (Laird et. al., 1984), are rootless, fault-emplaced ophiolitic rocks rather than Ordovician intrusives. The Tillotson Peak Complex is a blueschist-eclogite fault-bounded complex. The complexes are nested between schists of the Green Mtn. – Sutton Mtn. anticlinorium and overlying phyllites of the PRF. Additional slices of ultramafic rock and dike-bearing metasediments are prevalent east of the BBFZ. The Umbrella Hill Fm. (conglomerate and schist), pillow lavas, and slaty breccia are along strike with the St. Daniel Fm. which overlies the ophiolites in Quebec. The Umbrella Hill Fm. occupies a similar structural position. The contact of Silurian –Devonian rocks with the underlying Ordovician section is mapped as an unconformity in Vermont, with local Acadian offset. In Quebec, the contact is the La Guadeloupe fault. A mapped fault of similar age in VT is the CHT which transports Ordovician Cram Hill and Moretown Fms. and Silurian (Aleinikoff, pers. comm.) intrusives to the west.