Reevaluation of the piermont-frontenac allochthon in the upper connecticut valley: Restoration of a coherent boundary mountains-bronson hill stratigraphic sequence

Abstract

The regional extent and mode and time of emplacement of the Piermont-Frontenac alloch thon in the Boundary Mountains-Bronson Hill anticlinorium of the Upper Connecticut Valley, New Hampshire-Vermont, are controversial. Moench and coworkers beginning in the 1980s proposed that much of the autoch thonous pre-Middle Ordovician section of the anticlinorium was a large allochthon of Silurian to Early Devonian rocks corre lated to those near Rangeley, Maine. This ~200-km-long allochthon was postulated to have been transported westward in the latest Silurian to Early Devonian as a soft-sedi ment gravity slide on a hypothesized Foster Hill fault. New mapping and U-Pb geo chronology do not support this interpretation. The undisputed Rangeley sequence in the Bean Brook slice is different from the disputed sequence in the proposed larger Piermont-Frontenac alloch thon, and fi eld evidence for the Foster Hill fault is lacking. At the type locality on Foster Hill, the postulated "fault" is a stratigraphic contact within the Ordovician Ammonoosuc Volcanics. The proposed Foster Hill fault would place the Piermont-Frontenac allochthon over the inverted limb of the Cornish(?) nappe, which includes the Emsian Littleton Formation, thus limiting the alleged sub marine slide to post-Emsian time. Mafi c dikes of the 419 Ma Comerford Intrusive Complex intrude previously folded strata attributed to the larger Piermont-Frontenac allochthon as well as the autochthonous Albee Formation and Ammonoosuc Volcanics. The Lost Nation pluton intruded and produced hornfels in previously deformed Albee strata. Zircons from an apophysis of the pluton in the hornfels have a thermal ionization mass spectrometry 207Pb/206Pb age of 444.1 ± 2.1 Ma. Tonal ite near Bath, New Hampshire, has a zircon sensitive high-resolution ion micro probe 206Pb/238U age of 492.5 ± 7.8 Ma. The tonalite intrudes the Albee Formation, formerly interpreted as the Silu rian Perry Mountain Formation of the proposed alloch thon. Collectively, these features indicate that the large Piermont-Frontenac allochthon gravity slide of Silurian-Devonian strata, as previously proposed, cannot exist. Alloch thonous rocks are restricted to a 25 km2 klippe, the Bean Brook slice, emplaced by hard-rock thrusting in the post-Emsian Devonian. The Albee Formation, the oldest unit in the study area, is older than the Late Cambrian tonalite at Bath. The correlation and apparent continuity along strike to the northeast of the Albee Formation with the Dead River Formation suggest that the Albee Formation, like the Dead River Formation, is of Ganderian affi nity and that the Bronson Hill magmatic arc in the Upper Connecticut Valley was built on Ganderian crust. The Dead River Formation is unconformably overlain by Middle and Upper Ordovician volcanic units; the unconformity is attributed to the pre-Arenig Penobscottian orogeny. Some of the pre-Silurian deformation in the Upper Connecticut Valley may be Penobscottian rather than Taconian. New stratigraphic units defi ned herein include the pelitic Scarritt Member of the Albee Formation, the Ordovician Washburn Brook Formation consisting of synsedimentary breccia and coticule, chert, and ironstone, and the Devonian-Silurian Sawyer Mountain Formation, probably correlative with the Frontenac Formation. The Partridge Formation is partially coeval with the Ammonoosuc Volcanics. © 2013 Geological Society of America.