Abstract

Several recent papers have accepted the idea that there is a large overthrust, the "Piermont allochthon," in west-central New Hampshire. In my opinion, such an overthrust does not exist. For the reasons given below, I believe that stratigraphic and structural proof of this hypothesis are lacking and that other interpretations can fit the known facts as well or better.

Careful mapping combined with routine observation of primary geopetal features and supported by U-Pb zircon age determinations prove the existence of the Piermont allochthon in a 200-km-long belt that includes the type area of the Ordovician Albee Formation of M. P. Billings near Littleton, New Hampshire. The Albee Formation of Billings is divided into seven stratigraphic units that are correlated with one Upper Ordovician(?) formation, one Lower Silurian(?) formation, three Silurian formations, one Upper Silurian(?) formation, and one Lower Devonian formation previously mapped in the Central Maine trough. In the Littleton area, the allochthon is bounded on the west by the Monroe fault, and on the east by the Foster Hill sole fault, which coincides with the contact that Billings mapped between his Albee Formation and the overlying Ordovician Ammonoosuc Volcanics. This contact must be a fault, because stratigraphic sequences on opposite sides face (young) toward, and are truncated against, one another. Where exposed, the Foster Hill fault is a sharp, pre-metamorphic boundary with only local evidence of shearing.

Regional geologic relationships indicate that the Piermont allochthon has traveled several to many tens of kilometers relative to the southeast adjacent Bronson Hill-Boundary Mountains anticlinorium. For reasons given elsewhere, however, my previous interpretation that the allochthon is an Acadian thrust sheet is probably not tenable.

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