BEDROCK GEOLOGIC MAP OF THE MOUNT MOOSILAUKE, 7 ½′ QUADRANGLE, NEW HAMPSHIRE

The shapes of the mountains and valleys in the Mount Moosilauke quadrangle are controlled by the geometry of the underlying bedrock. Biotite gneiss (Bethlehem Granodiorite), sillimanite schist (Littleton Formation) and porphyritic granodiorite (Kinsman Granodiorite) are all resistant rock units that tend to form ridges and hills. Millions of years of erosion have removed thousands of feet of material since the rocks were first formed. The deep valleys drained by Tunnel Brook and Little Tunnel Brook may be in part controlled by steep faults and associated less-resistant rocks. The map presents the results of detailed mapping at 1:24,000 accomplished from 2020 to 2022 as part of the Statemap program.

The main new contributions to our understanding of the local bedrock geology are refinement of the configuration of faults, the recognition of a granofels-rich member in the Littleton associated with metavolcanics, which holds up Moosilauke's summit ridge, and confirmation of an east-facing, 4000 m thick section of Littleton Formation without large-scale folds. Overall, the dominant lithology of the Littleton consists of laminated quartzitic schist characterized by a turbidite-dominated, presumably deep-marine sedimentary protolith. The metasediments in the Monadnock septum are part of the Bronson Hill stratigraphic sequence that lies atop Oliverian domes. New U-Pb analyses of samples from Kinsman Granodiorite near Kinsman Notch, and from Bethlehem Granodiorite 13.5 miles southwest in Wentworth, suggest that both have a crystallization age of ~407.3 Ma. These results are very similar to published radiometric ages for volcanics in the Littleton (e.g. 407 +/- 2 Ma, Rankin, Tucker and Buchwaldt, 2013) and therefore require that these plutons were emplaced along Acadian thrust faults or shear zones, rather than intruding the Littleton directly. The western contact of the Kinsman may be at the same structural level as the Brennan Hill fault, which in the Monadnock area separates the Bronson Hill sequence in the footwall from the Central Maine sequence.