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Reevaluation of the Piermont-Frontenac Allochthon in the upper Connecticut Valley; restoration of a coherent Boundary Mountains-Bronson Hill stratigraphic sequence

Douglas W. Rankin, Robert D. Tucker and Yuri Amelin
Reevaluation of the Piermont-Frontenac Allochthon in the upper Connecticut Valley; restoration of a coherent Boundary Mountains-Bronson Hill stratigraphic sequence
Geological Society of America Bulletin (November 2012) 125 (5-6): 998-1024


The regional extent and mode and time of emplacement of the Piermont-Frontenac allochthon 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 autochthonous pre-Middle Ordovician section of the anticlinorium was a large allochthon of Silurian to Early Devonian rocks correlated to those near Rangeley, Maine. This approximately 200-km-long allochthon was postulated to have been transported westward in the latest Silurian to Early Devonian as a soft-sediment gravity slide on a hypothesized Foster Hill fault. New mapping and U-Pb geochronology 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 allochthon, and field 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 submarine slide to post-Emsian time. Mafic 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 (super 207) Pb/ (super 206) Pb age of 444.1 + or - 2.1 Ma. Tonalite near Bath, New Hampshire, has a zircon sensitive high-resolution ion microprobe (super 206) Pb/ (super 238) U age of 492.5 + or - 7.8 Ma. The tonalite intrudes the Albee Formation, formerly interpreted as the Silurian Perry Mountain Formation of the proposed allochthon. Collectively, these features indicate that the large Piermont-Frontenac allochthon gravity slide of Silurian-Devonian strata, as previously proposed, cannot exist. Allochthonous rocks are restricted to a 25 km (super 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 affinity 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 defined 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.

ISSN: 0016-7606
EISSN: 1943-2674
Serial Title: Geological Society of America Bulletin
Serial Volume: 125
Serial Issue: 5-6
Title: Reevaluation of the Piermont-Frontenac Allochthon in the upper Connecticut Valley; restoration of a coherent Boundary Mountains-Bronson Hill stratigraphic sequence
Affiliation: U. S. Geological Survey, Reston, VA, United States
Pages: 998-1024
Published: 20121121
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 91
Accession Number: 2013-007413
Categories: StratigraphyStructural geology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: Includes appendices
Illustration Description: illus. incl. 4 tables, geol. sketch map
N44°00'00" - N44°30'00", W72°04'60" - W71°40'00"
Secondary Affiliation: Australian National University, AUS, Australia
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201305
Program Name: USGSOPNon-USGS publications with USGS authors
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