UPDATED SILURO-DEVONIAN GEOLOGY IN THE INDIAN STREAM REPUBLIC (1832 – 1835)

Bedrock geology of three 7.5’ quadrangles in northernmost NH was mapped as part of the NHGS-USGS StateMap project – 2 more maps are in progress. The field area includes the eastern edge of the Connecticut Valley Synclinorium and the westernmost edge of the Bronson Hill Anticlinorium. Previous NH bedrock maps (1970s-1990s) were at a scale of 1:250,000 (except locally). The adjacent Quebec geology was mapped in detail recently (Perrot, 2019).

New analytical data, LA-ICP-MS zircon crystallization (7 CZ) and detrital zircon (12 DZ) ages as well as 19 whole rock geochemical analyses, aided the mapping. Most metasediments and metaigneous rocks have maximum depositional ages (MDAs) or CZ ages in the Early Devonian. Laurentian ages are preserved in metaigneous rocks across the area.

The western field area consists of N-NE-trending metasedimentary rocks (graywackes, shales, variably calcareous siltstones, debrites, volcaniclastics) of the Ironbound Mountain (IBM), Gile Mt. (GM) and Waits River formations with sparse meta-igneous rocks. Pillow basalt (GM) is dated at 403 ± 6 Ma. Tuffaceous sediments (IBM) have mostly euhedral zircons with a DZ age of 391 ± 4 Ma. These Devonian ages are close to depositional ages. Mean MDA for other IBM and GM samples range from 413 – 425 Ma and 388 – 426 Ma respectively, which may represent sediment provenance ages. The stratigraphy correlates well across the border. The major structure is the NE-trending Beaver Brook Anticlinorium, which plunges shallowly to the north but locally “porpoises”. The rocks exhibit multiple foliations and folding events. The Monroe Thrust Fault separates GM and IBM from the Silurian Frontenac (FF) Formation to the east. The MDAs are similar in all formations, although the older DZ populations differ. The structural features are also similar.

The eastern field area consists of N-NE-trending FF metaigneous (mafic and felsic volcanics, diabase and granite), and metasedimentary rocks (volcaniclastics, siltstones, shales). The FF MDA ages are: 408 ± 6 Ma, 415 ± 5 Ma, and 434 ± 4 Ma. Farther east are additional thrust faults, and a normal fault that separates FF from possible GM. One biotite granite (411 ± 7 Ma, 424 ± 4 Ma) is similar in age and composition to a nearby ignimbrite (411 ± 3 Ma). The East Inlet Pluton has the same age as nearby felsic volcanics (416 ± 4 Ma).