INTRODUCTION

1 The Eastport Formation is part of an extensive group of mid-Paleozoic volcanic and sedimentary units known collectively as the Coastal Volcanic Belt. This belt stretches from Massachusetts, through eastern Maine and into New Brunswick and represents a portion of Avalonia that was accreted to Laurentia during the Acadian orogeny (Berry and Osberg 1989; Ludman et al. 1993; Fyffe et al. 1999). The Eastport Formation represents the youngest, syntectonic unit in the Coastal Volcanic Belt. There has been significant interest in accurately dating it in an effort to better understand the dynamics of the orogeny, specifically as these dynamics relate to its overall duration (e.g., Osberg et al. 1989; Bradley et al. 2000; Tucker et al. 2001). In addition, correlation with the Eastport Formation has also been used to date the uppermost strata (Seal Cove Formation) within the North Haven and Vinalhaven volcanic sequence of the Penobscot Bay region, Maine (Brookins et al. 1973; Gates 2001), as well as Silurian and Devonian sequences – particularly the Canadian portion of the Eastport Formation - in New Brunswick (Pickerill and Pajari 1976; Fyffe et al. 1999).

2 For more than thirty years, researchers have consistently cited ostracode occurrences to date the Eastport Formation of Maine as Early Devonian (e.g., Gates 1975; Berry and Osberg 1989; Bradley et al. 2000; Fyffe et al. 1999). However, a critical review of the ostracode data shows that it was never robust enough to assign a definite Devonian date to the formation, and that the age assignment should have been noted as provisional at best. In fact, a more complete analysis of the fossil and stratigraphic evidence indicates a Late Silurian, rather than a Devonian, age for the Eastport Formation. It is notable that the early authors also regarded the formation as Late Silurian based on biostratigraphic correlations with European faunas (Bastin and Williams 1914). Recent radiometric dating (Miller and Fyffe 2002; Van Wagoner and Dadd 2003) also support a Silurian age for the Eastport Formation.

HISTORICAL WORK

3 The formations of the Coastal Volcanic Belt in eastern Maine were first described by Jackson (1837) and more formally by Shaler (1886). With the prospect of potential coal-bearing beds in the area, the U. S. Geological Survey dispatched a geologist (E. S. Bastin) and paleontologist (H. S. Williams) to study the formations and their faunas in more depth. From this work, a folio of the area was produced (Bastin and Williams 1914), as well as several other publications discussing the taxonomic and biostratigraphic significance of the faunas (e.g., Williams 1912a, b; 1913).

4 While working with the fossils, Williams became acutely aware that the faunas more closely resembled European specimens than those from North America (Williams 1912a, b). This presented a serious conundrum for him. With the theory of plate tectonics nearly fifty years away, Williams had no way to satisfactorily explain why American faunas should more closely resemble their trans-Atlantic counterparts than those on the same continent. In fact, this apparent anomaly became so troubling to him that he devoted an entire address to it to the Geological Society of America in 1912, later published in 1913. The Eastport fauna, particularly the molluscs, he correlated with the Downtonian of Great Britain, which Williams considered firmly in the Silurian (Bastin and Williams 1914; p. 10). He assigned a Late Silurian age to the Eastport Formation based on that correlation (Williams 1912a, b; Bastin and Williams 1914), an assignment that remained for nearly sixty years before being revisited.

FOSSIL EVIDENCE

STRATIGRAPHIC RELATIONS

11 A summary of the stratigraphic relations, age assignments and supporting age data for the Coastal Volcanic Belt formations in Maine is given in Table 2. The Dennys, Edmunds, Leighton, Hersey and Eastport formations seem to represent a continuous sequence with no obvious hiatus at any of the contacts. The Quoddy and Dennys formations are separated by a fault, whereas the Perry Formation overlies the Eastport, Hersey and Leighton formations with an angular unconformity (Gates 1975).

12 The nature of the boundary between the Eastport and Hersey formations is gradational, and there are places where the two formations have been identified as age equivalents (Gates 1975). The Hersey Formation has been convincingly dated as Pridolian based on the presence of the ostracode Nodibeyrichia (Martinsson 1970; Berdan 1971; 1990; Copeland and Berdan 1977; Berdan 1983; Siveter 1989). Therefore, the portions of the Eastport Formation that are known to grade into the Hersey Formation, and have been identified as age equivalents, must also be assigned a Pridolian age. While it could be argued that the units stratigraphically above the Nodibeyrichia-bearing beds within the Hersey and Eastport formations may be Devonian, there is no evidence to convincingly suggest this. Therefore, the more parsimonious approach would be to regard the entire Eastport Formation as Pridolian age until compelling evidence is found to suggest otherwise.

13 The Early Devonian age that is commonly assigned to the Hersey Formation is based on the fact that it interfingers with the assumed Devonian age Eastport Formation. The Hersey Formation, in addition to Nodibeyrichia, contains two undescribed ostracode taxa (Carinokloedenia? and cf. Zygobeyrichia) reminiscent of similar forms in Devonian age deposits of Podolia (Berdan 1971; Gates 1975). This brings up the concerns expressed previously about the use of undescribed species for biostratigraphic correlations. Also, as discussed previously, the presence of Nodibeyrichia (Pridolian) in both the Hersey and Eastport formations provides well-accepted Silurian constraints to the units. In view of these facts, the Devonian age designation for the Hersey formation should likewise be discontinued.

14 Although the formations are arranged vertically in Table 2, there is preliminary evidence to suggest that some of these units may actually represent age-equivalent facies. This is the case for the Eastport and Hersey formations, and may be the case for part of the Leighton Formation. Stratigraphic evidence includes the interfingering of the Hersey and Eastport formations and the replacement of the Hersey by the Eastport in more south-erly reaches, with the Eastport Formation apparently conformably overlying the Leighton Formation (see Gates 1975). Fossil data, including the Nodibeyrichia-Carinokloedenia? association in both the Hersey and Eastport formations, and the possible presence of Eurymyella within the Leighton Formation, are also suggestive of age-equivalency among the different formations. Charlotte Mehrtens (University of Vermont) and the author are presently engaged in field work detailing the sedimentology, stratigraphy, and paleoecology of these formations in an effort to gain a better understanding of their lateral and vertical (e.g., space and time) relationships, as well as the environmental parameters active during their deposition.

RADIOMETRIC DATES

15 There have been several attempts to obtain radiometric dates on the Eastport Formation. Fullagar and Bottino (1970) reported an age of 408 ± 3 Ma for the formation based on Rb-Sr dating of volcanics within it. They also dated a dike that cut through the Eastport and obtained an age of 400 to 420 Ma (Fullagar and Bottino 1970).

16 The Red Beach pluton, which intrudes the Eastport, was dated by both Rb-Sr and U-Pb methods. The Rb-Sr work produced an age of 385 ± 6 Ma, recalculated (Spooner and Fairbuirn 1970), while the U-Pb analysis yielded an age of 415 ± 6 Ma (Jurinski 1990). Both dates were considered highly suspect and ultimately dismissed by Bradley et al. (2000). Furthermore, Bradley et al. (2000) noted that Rb-Sr dates have consistently proven to be too young in the region, an observation that also casts considerable doubt on Fullagar and Bottino's work.

Table 1. Faunal listing for the Eastport Formation, Maine. The numbers in parentheses correspond to the references following the listing. See bibliography for full citations. Paleontological workers responsible for taxonomic identifications are listed within the references. Explanation of abbreviations used in references: B (bivalves), Br (brachiopods), Eu (eurypterid), F (fish), G (gastropod), O (ostracode) and Ph (phyllocarid). The brachiopod listings have been updated by A. J. Boucot (personal communication, 2001). Quotation marks indicate that the taxa are probably something other than what was originally identified.

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Table 2. Summary of formations within the Maine sequence of the Coastal Volcanic Belt. Age criteria are based largely on fossil evidence, although radiometric dates are also listed. The types of contacts between each set of formations, and references to that information, are also provided. Full citations are listed in reference section.

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17 In New Brunswick, Miller and Fyffe (2002) have obtained radiometric dates from zircons in Coastal Volcanic belt units underlying the Eastport Formation. Previous analyses of these units relied heavily on the presence of the brachiopod, Salopina, to indicate a Late Silurian age (see Miller and Fyffe 2002 for discussion). However, the recent radiometric ages obtained from the volcanic rocks in those units indicated an Early Silurian (Llandoverian), rather than a Pridolian, age. From this, Miller and Fyffe (2002) suggested that the Eastport Formation, which overlies these Llandoverian units, may in fact predate the Early Devonian. Their paper suggested a Ludlovian or Pridolian age.

18 Recently, there have been renewed efforts to obtain U-Pb dates of the Eastport Formation itself using laser ablation, LAM-ICPMS, methods (see Sylvester 2001 and Jackson et al. 2003 for an overview of this technique and its various applications). Van Wagoner and Dadd (2003) used this technique to date felsic tuffs and rhyolite flows within the Eastport Formation and obtained dates ranging from 419 ± 6 to 436 ± 6 Ma. A paper describing the dating techniques, exact localities and horizons of the two units from which the dates were obtained, and their geological significance is currently in preparation (N. A. Van Wagoner, personal communication, 2004). Although there is definite disparity between the two dates (one places the Eastport in the Late Silurian, the other in the Early Silurian), they do suggest a firm Silurian date.

CONCLUSION

19 The age of the Eastport Formation has the potential to provide important information about the timing and duration of the Acadian orogeny, as well as significant information to be used in regional correlations. For over thirty years, the date of the Eastport Formation has been regarded as Early Devonian based on poorly constrained fossil data – principally the unnamed ostracode, cf. Carinokloedenia. Instead, age assignments should have been based on more robust fossil data, notably the presence of Nodibeyrichia, in addition to the stratigraphic evidence that convincingly links portions of the Eastport Formation with the Hersey Formation of known Pridolian age (Table 1). This data, combined with the recent radiometric dating and stratigraphic work of Miller and Fyffe (2002) and Van Wagoner and Dadd (2003), strongly favour a Late Silurian (Pridolian) age for the Eastport Formation over a Devonian one. A Silurian age for the Eastport Formation has important implications for regional stratigraphy and will require that previous correlations, particularly the relationship between the Seal Cove and Eastport formations, and the Maine and New Brunswick portions of the Eastport Formation, be re-examined in light of these findings.