BEDROCK GEOLOGY OF THE JEFFERSON 7.5’ QUADRANGLE, NEW HAMPSHIRE

The purpose of this study was to produce an updated, detailed bedrock map of the Jefferson NH 7.5’ quadrangle at the scale 1:24,000 as a part of the USGS/NHGS StateMap program. This work builds upon the previous detailed mapping of the adjacent Mt Washington East and Mt. Dartmouth NH 7.5' quadrangles.

This study area contains older Ordovician rocks of Jefferson Dome, one of the Oliverian gneiss domes of western New Hampshire, and younger Jurassic rocks of the Pliny Range Caldera Complex that cut the older rocks. From oldest to youngest, new mapping has identified the following units: 1) Ordovician hornblende gneiss (+/-amphibolite) transitioning to K-feldspar-rich orthogneiss with increasing proximity to the Pliny ring dikes; 2) Silurian (new age 430.6±2.6 Ma) syenite; 3) a Jurassic (?) basalt dike correlative to the Mill Brook Dike; and the following Pliny Complex rocks: 4) Jurassic diorite; 5) Jurassic porphyritic quartz monzodiorite; 6) Jurassic hornblende quartz syenite; 7) Jurassic quartz monzodiorite; 8) Jurassic pink biotite granite (new age 188.3±1.0 Ma) that is essentially coeval with; 9) Jurassic Conway granite (new age 187.3±1.1 Ma); and 10) a newly discovered Jurassic rhyolite (new age 184.9±2.3 Ma).

The syenite is not visibly deformed and cuts the strongly foliated Ordovician gneiss suggesting that some deformation in the Jefferson Dome may actually be older (Ordovician?) than previously thought. All of these units were in turn intruded by the series of near contemporaneous Jurassic ring dike intrusive and extrusive rocks of the Pliny Range Caldera Complex. Despite the complexity of the Jurassic intrusions, it is possible to map the Ordovician and Silurian units through the ring dikes. Foliations of xenoliths within the ring dike shattered zone are only slightly deflected if at all from the regional orientation. This suggests the ring dike intrusion and caldera collapse was not so forceful as to reorient the country rock blocks.

The newly discovered rhyolite may be the source of the long sought after Jefferson Rhyolite, a stone used to make the characteristic fluted spear points used by Paleoindians 13,000-11,000 years ago and documented at numerous archaeological sites in the Northeast. XRF analyses will be performed to compare the rhyolite geological samples with artifacts identified as Jefferson Rhyolite.