Northeastern Section - 42nd Annual Meeting (12–14 March 2007)

Paper No. 9
Presentation Time: 4:00 PM


THOMPSON, Peter J., Earth Sciences Dept, Univ of New Hampshire, Durham, NH 03824,

Bedrock mapping at 1:24,000 in the town of Woodstock confirms published cross-sections that portray a deformed E-verging nappe. Woodstock is situated on the east flank of the Green Mountain anticlinorium, in a structural saddle between the N-plunging Chester dome and S-plunging Pomfret dome. The domes deform two sets of folds, one that deforms a foliation and an older one that deforms only bedding.

The Silurian Waits River Formation, with characteristic interbedded garnet +/- kyanite schist and punky-weathering sandy marble, underlies much of the town. Graded beds in the upper schist-dominated part of the Waits River confirm a topping direction east toward the Standing Pond Volcanics. The schist and quartzite beds east of the Standing Pond are similar to rocks mapped as Devonian Gile Mountain Formation around the Pomfret dome; minor limestone beds do not warrant reassignment to Waits River, as proposed for the adjacent Hartland quadrangle. Discontinuous metavolcanic horizons are common east of the Standing Pond, but absent in the Waits River of the mapped area.

The Gile Mountain, in the synclinal core of a large nappe, plunges south from the Pomfret dome beneath the Standing Pond and Waits River, which are on the refolded, overturned limb. Informal members of the Waits River, based on varying proportions of schist and marble, have been mapped out to show that the Standing Pond connects at depth from the Woodstock area to metavolcanics in a synclinal isocline on the north flank of the Chester dome in Reading. (The metavolcanics cannot be traced on the ground due to the intervening Ascutney pluton.) A greatly thinned post-Ordovician section on the underlying dome suggests (Acadian W-directed?) décollement above the Moretown.

The question of how and when the Vermont sequence (with clockwise PTt metamorphic path and E-verging structures) and the New Hampshire sequence (with counter-clockwise PTt path and W-verging structures) were juxtaposed must take into account these opposing senses of vergence. One possibility is that the Woodstock nappe was co-eval with early Quaboagian backfolds in NH that deform Acadian W-directed structures. Steep spaced cleavage formed as the late Quaboagian domes rose to deform all older structures. Exhumation thus was accomplished first by backfolding and continued through doming.