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 207 Pb/ 206 Pb age of 444.1 ± 2.1 Ma. Tonal ite near Bath, New Hampshire, has a zircon sensitive high-resolution ion micro probe 206 Pb/ 238 U 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 km 2 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...

Geochronology of the Oliverian Plutonic Suite and the Ammonoosuc Volcanics in the Bronson Hill arc: Western New Hampshire, USA

“…3; Lyons et al, 1997;Ratcliffe et al, 2011;. Rocks of the Bronson Hill arc are informally considered part of the New Hampshire sequence (Billings, 1935(Billings, , 1937White and Jahns, 1950;Rankin et al, 2013). From the base up, the stratigraphy of the New Hampshire sequence consists of the Albee Formation (Late Cambrian or older), Ammonoosuc Volcanics (Upper and Middle Ordovician), Partridge Formation (Upper Ordovician), Clough Quartzite (Lower Silurian), Fitch Formation (Upper Silurian-Lower Devonian), and Littleton Formation (Lower Devonian).…”

“…Based on the work of Rankin et al (2013) in the area of Billings' type localities near Littleton, New Hampshire, the stratigraphy of the Ammonoosuc Volcanics (from oldest to youngest) includes (1) rusty sulfidic slate, felsic tuff, and other metasediments; (2) metasiltstone, phyllite, and volcaniclastic rocks; (3) metadolomite and siltstone; (4) metarhyolite tuff and siltstone; (5) meta-andesite, basaltic tuff, and pillow lavas; (6) metarhyolite tuff, lapilli tuff, and lava; (7) metafelsic and mafic volcanics, volcaniclastic rocks, and metasediments. The lower Partridge Formation overlaps with the upper Ammonoosuc Volcanics (Rankin et al, 2013) and consists of interbedded metavolcanics rocks in the lower Partridge Formation and rusty sulfidic schist and slate interlayered with metarhyolite in the upper Ammonoosuc Volcanics. Near Plainfield, New Hampshire, the contacts between the Partridge Formation and the Ammonoosuc Volcanics are generally sharp, but may be gradational over a few meters .…”

Evolution of the Early to Middle Ordovician Popelogan arc in New Brunswick, Canada, and adjacent Maine, USA: Record of arc-trench migration and multiple phases of rifting

“…A similar unconformity is present in the central and northern parts of the Miramichi inlier in New Brunswick (van Staal and Fyffe, 1995b). In New Hampshire, a Penobscot arcrelated tonalite that intrudes the lower part of the Albee Formation has yielded an age of 492.5 ± 7.8 Ma (Rankin et al, 2013). The Albee Formation is the same unit as the Dead River Formation (Rankin et al, 2013); both are correlatives of the Woodstock and Miramichi Groups and hence form part of the Gander margin (see also Reusch and .…”

“…In New Hampshire, a Penobscot arcrelated tonalite that intrudes the lower part of the Albee Formation has yielded an age of 492.5 ± 7.8 Ma (Rankin et al, 2013). The Albee Formation is the same unit as the Dead River Formation (Rankin et al, 2013); both are correlatives of the Woodstock and Miramichi Groups and hence form part of the Gander margin (see also Reusch and . This interpretation is supported by isotope data, including Pb isotopes in massive sulfide deposits, from the Ammonoosuc volcanic rocks, which display signatures typical of Ganderia, similar to their coeval counterparts in Maine and New Brunswick (Slack et al, 1991;Dorais et al, 2012).…”

“…Several independent lines of evidence (e.g., stratigraphy, isotopes, and zircon geochronology) show that these units can also be traced across the international border into the United States (Figs. 1A and 1B), where they are known under a variety of names (Tucker et al, 2001;Moench and Aleinikoff, 2002;West et al, 2004;Hussey et al, 2010; Reusch and Rankin et al, 2013). The Grand Pitch, Chase Brook, Hurri cane Mountain, and Dead River Formations in northern Maine (Figs.…”