Bedrock Geologic Framework of the Mirror Lake Research Site, New Hampshire

By William C. Burton, Thomas R. Armstrong, and Gregory J. Walsh

ABSTRACT

The geology of the Hubbard Brook watershed in the vicinity of Mirror Lake, New Hampshire consists of metasedimentary rocks of the Silurian Rangeley and Perry Mountain Formations that have been intruded by Devonian granite and Cretaceous-Jurassic lamprophyre dikes. The metasedimentary rocks were affected by at least three episodes of deformation in the Devonian Acadian orogeny. The dominant regional foliation is second-generation (S2) and formed during metamorphic conditions that produced sillimanite-muscovite and sillimanite-kspar mineral assemblages. Both lithologic contacts and the S2 foliation trend east-west to northeast and dip steeply northward in the vicinity of the CO and FSE well fields. Dikes of Devonian granite cut contacts and S2 and appear to be syn-D3. The well fields and the nearby I-93 roadcut appear to be in a zone of more abundant granitic rocks than areas farther west up the watershed, where granite dikes and migmatite are rare. Steeply-dipping fractures in outcrops near the eastern boundary of the Hubbard Brook watershed show a northeast preferred orientation, in agreement with subsurface fracture orientations in the CO well field, whereas the area to the northwest of the well fields has fractures with a northwest preferred orientation. Both lithologic and fracture-orientation data suggest that the geology of the Mirror Lake research site is not representative of areas elsewhere in the watershed.