THE BRONSON HILL ARCH, UPPER CONNECTICUT VALLEY, NORTH OF 43º 52.5'

The Connecticut River inexactly follows the faulted contact between the Connecticut Valley trough (CVT) (west) and the Bronson Hill arch (BHA) (east). The BHA includes the Middle Ordovician to earliest Silurian Bronson Hill magmatic arc (Ammonoosuc Volcanics [here mostly felsic], Partridge Formation, and Oliverian plutons) active between about 469 and 443 Ma. The arc is built on the continental margin Albee Group whose southern limit of exposure coincides roughly with the southern limit of the more granitic Oliverian plutons. The Ammonoosuc Volcanics significantly overlaps in time black pelites of the traditionally overlying Partridge Formation. West of Groveton, NH, the Albee grades upward through sedimentary breccia into volcanogenic coticule- and magnetite-bearing chert, probably correlative with the Ammonoosuc to the east. Deformed pre-Silurian rocks of the BHA are cut by mafic dikes (many sheeted) and complexes dated at 419±1 Ma, indicating late Silurian crustal extension and sagging of the CVT. The Silurian section (upper Llandovery to Pridoli) is thin to absent across the BHA. The thicker turbiditic Emsian Littleton Formation includes two newly dated rhyolites that together with a newly discovered rhyolite in the Gile Mountain Formation (CVT) indicate an age of about 407 Ma for volcanism in the CVT and BHA.

Evidence for Taconian deformation includes truncation of folded foliation by late Silurian dikes, intrusion of folded strata and an annealed metamorphic fabric by a dated (~444 Ma) offshoot of the Lost Nation pluton, and the southwest plunging pre-Silurian Moore Reservoir anticline truncated by the Silurian unconformity at the base of the northwest limb of the Acadian Walker Mountain syncline. The Piermont allochthon (mostly rocks of the Silurian Rangeley Formation thrust from the east) is restricted to the 25 km² Bean Brook slice (BBS) of Moench rather than extending north into Quebec. The hypothesized Foster Hill sole fault in its type locality is a stratigraphic contact within the Ammonoosuc Volcanics and elsewhere mostly coincides with the stratigraphic contact between the Albee and Ammonoosuc. Moreover, the Rangeley Formation of the BBS is not the Rangeley as mapped in the BHA to the north. Silurian mafic dikes do not cut the BBS, but do cut both in place and hypothesized transported rocks to the north.