MAPPING BEDROCK GEOLOGY IN NORTHERNMOST NEW HAMPSHIRE

As STATEMAP projects, we mapped bedrock geology of the 7.5’ Pittsburg, Lake Francis, Cowen Hill, Metallak Mountain and Greeley Brook quadrangles from 2019-2023. We mainly mapped with iPhones (Avenza and Stereonet Mobile apps) and checked with compasses. Improved base maps, Lidar, GPS, zircon dating, GIS tools, recent Quebec (QC) studies, and geochemical analyses were key to this effort.

The field area is on the eastern edge of the Connecticut Valley – Gaspé Trough and is significantly affected by glaciation. West of the Monroe Fault are the pre-Acadian, Siluro-Devonian Waits River, Gile Mt (GM), and Ironbound Mt (IBM) formations with fine-grained metasedimentary rocks (and the coarser Halls Stream grits (HSG)) with minor pillowed basaltic andesite and diabase. East of the Monroe Fault are pre-Acadian Siluro-Devonian metasedimentary rocks with variable volcaniclastic, basalt and mafic intrusive rocks of the Frontenac Formation (FF). Farther east, the Deadwater Ridge Fault separates FF with less from FF with more mixed extrusive rocks plus biotite granite.

Twelve maximum depositional ages (MDA) from detrital zircons and seven zircon crystallization ages (CZ) range from ~388-434 Ma and ~403–432 Ma respectively. A HSG sample with a ~unimodal zircon population yielded an MDA age of ~391 Ma and a nearby GM pillow basalt had a CZ age of ~403 Ma. Two samples from HSG along strike are in analysis to test these ages. One sample from a graded graywacke (IBM) is in analysis.

Two major Acadian events (D1 and D2) are expressed as folds (F1, F2) and cleavages (S1, S2) and as high-angle reverse faults. Locally, evidence suggests a D3 event. The orientation and style of D1 and D2 folds and cleavages are remarkably similar throughout the area, including on opposite sides of the Monroe Fault (NH) and correlative Bella Fault (QC). Bedrock was regionally metamorphosed to greenschist facies (chlorite to biotite zone near Late Devonian intrusive rocks).

Geochemical data from igneous rocks (our study and published data) were used to evaluate tectonic environments and genetic relationship of igneous units. Much of the basalt data are consistent with in-plate tholeiites, but other interpretations are possible (e.g., MORB and/or plate margin basalts). Geochemical differences exist between the Canadian Clinton River and Frontenac basalts.