BEDROCK GEOLOGY OF THE MOUNT DARTMOUTH 7.5’ QUADRANGLE, NEW HAMPSHIRE

The Mount Dartmouth 7.5’ Quadrangle, located in the White Mountain National Forest of Northern New Hampshire was last mapped at 1:62,500 by Billings et al. (1946). The quad was then re-interpreted by Lyons et al. (1997) as a part of the Bedrock Geologic Map of New Hampshire. When the Billings' map was integrated into the NH map, many of the finer details were lost due to the larger scale used. The purpose of this study was to produce an updated, more detailed bedrock map at a scale of 1:24,000 and a cross section of the quad. This study focuses on the assessment of the brittle and ductile structures that contributed to the deformation processes that shaped the quad, as well as defining and evaluating the igneous rock types and metamorphosed stratigraphy of the area. The quad was mapped as part of a coordinated EdMap/StateMap project with the NHGS over a seven week period in the 2014 field season at a scale of 1:12,500 using digital mapping techniques. From youngest to oldest the new mapping has identified the following: 1) the 7.8 km long, 100 meter wide, N-S striking Mill Brook Dike Zone and associated brittle joints (Cretaceous?); 2) the Jurassic Conway Granite and Cherry Mountain hornblende syenite; 3) three distinct phases of the Devonian to Carboniferous Bretton Woods granite, including the dominant medium-grained two-mica granite, a pegmatite zone, and flow-aligned porphyritic granite; 4) the Silurian Rangeley formation, sub-divided into the dominant migmatitic gray gneiss with internal layers of rusty, non-rusty, and calc-silicate pod-bearing migmatized schists; 5) the Ordovician Jefferson Oliverian Dome with three new units: a coarse-grained and also a porphyritic, hornblende-bearing alkali feldspar granite, and a porphyritic biotite granite; and 6) a near complete lack of any Ammonoosuc Volcanics with the exception of an unusual belt preserved in the interior of the Dome. A major normal (?) fault, manifest by a belt of interconnected silicified pods, strikes diagonally northeast across the quadrangle and forms the fundamental break between the Bretton Woods granite and Rangeley Formation to the SE and the Oliverian Dome rocks to the NW. The Rangeley Formation preserves a penetrative foliation that is likely a D₁ Acadian fabric. The dome shows different foliations, one related to doming and a later shear zone fabric.