A Late Silurian (Pridolian) age for the Eastport Formation, Maine: a review of the fossil, stratigraphic, and radiometric-age data

Abstract

The Eastport Formation is part of a larger volcaniclastic sequence (the Coastal Volcanic Belt) that extends from Massachusetts, USA, into New Brunswick, Canada. It represents a portion of Avalonia that was accreted to Laurentia during the Acadian orogeny. For more than thirty years the age of this formation has been regarded as Early Devonian (Lochkovian or Pragian) based on a new and yet undescribed ostracode genus. However, careful analysis of the fossil data indicates a Late Silurian (Pridolian) age for the fauna, and shows that the evidence used to assign a Devonian age should have been regarded as provisional. This interpretation is additionally strengthened by local stratigraphy and recent radiometric dating, both of which support a Late Silurian age for the Eastport Formation. RÉSUMÉLa Formation d'Eastport fait partie d'une séquence volcanoclastique plus importante (ceinture volcanique Côtière) qui s'étend du Massachusetts, aux États-Unis, jusqu'à l'intérieur du Nouveau-Brunswick, au Canada. Elle représente une partie du continent avalonien qui s'était accrétionné au continent laurentien pendant l'orogenèse acadienne. Pendant plus d'une trentaine d'années, on a considéré que cette formation remontait au Dévonien précoce (Lochkovien ou Praguien) en se basant sur un nouveau genre d'ostracode encore non décrit. Une analyse attentive des données relatives au fossile révèle toutefois que la faune remonte au Silurien tardif (Pridolien) et montre que les observations utilisées pour l'attribution d'un âge dévonien auraient dû être considérées comme provisoires. Cette interprétation se trouve par ailleurs renforcée par la stratigraphie locale et une datation isotopique récente, qui appuient toutes deux la datation de la Formation d'Eastport à l'époque du Silurien tardif.[Traduit par la rédaction.]

Geochemistry of Silurian-Devonian volcanic rocks in the Coastal Volcanic belt, Machias-Eastport area, Maine: Evidence for a pre-Acadian arc

“…It pinches out to the southeast, where its stratigraphic position is occupied by rocks within the Eastport Formation (Gates and Moench, 1981). The Eastport Formation, of Pridoli age (Churchill-Dickson, 2004), contains a variety of volcanic rocks, including basalt, basaltic agglomerate, dacite and rhyolite tuffbreccia, and ash-fl ow deposits, as well as rhyolitic vitrophyre with interbedded shales and conglomerates (Gates and Moench, 1981). Volcanic rocks of the Eastport Formation erupted onto tidal fl ats or subaerially, in contrast to the largely submarine eruptions of the underlying units (Gates and Moench, 1981).…”

“…The Coastal Vol canic belt in the Machias-Eastport area (Fig. 1B) consists of a thick sequence of well-exposed Silurian to Late Devonian marine to subaerial volcanic and sedimentary rocks (Gates, 1975(Gates, , 1978Berry and Osberg, 1989;West et al, 1992;Bradley et al, 2000;Churchill-Dickson, 2004). This sequence is at least 8 km thick in the Eastport area.…”

“…Based on ages, rocks from the Machias-Eastport area are divisible into two groups: a Silurian assemblage that includes the Dennys, Edmunds, Leighton, and Eastport Formations, and rocks from the Late Devonian, post-Acadian Perry Formation. Ages for these units are based upon fossil assemblages in interbedded sedimentary layers, as summarized by Gates and Moench (1981, and references therein), except for the Eastport Formation, wherein fossils have been restudied and re assigned to the latest Silurian (Pridoli) instead of the Early Devonian (Churchill-Dickson, 2004). The Quoddy Formation, assumed to contain the earliest volcanic rocks in the Coastal Volcanic belt (Bastin and Williams, 1914;Gates, 1975), was not part of this study.…”

Microvertebrates from the Silurian–Devonian boundary beds of the Eastport Formation, Maine, eastern USA

“…The Silurian succession in the coastal volcanic belt of Maine exhibits strong similarities to European Silurian sequences, a fact that troubled pre-continental-drift workers (Churchill-Dickson 2004). Previously the age of this succession had been regarded as wholly Early Devonian (Lochkovian or Pragian), based principally on an undescribed ostracod, which Berdan (1990) considered to belong to a quadrilobate beyrichiacean form allied to Cornikloedenina; but it is more like Carinokloedenia from the Pragian of Podolia and the Lochkovian of northern France.…”

“…Relatively few middle Paleozoic fish remains are known from Maine (Churchill-Dickson 2004). Bastin and Williams (1914) mentioned a fish spine in a calcareous nodule from the Eastport Formation, and the acanthodian Nosto lepis sp.…”