8 Stratigraphic interpretations of Cambrian to Middle Ordovician basement and overlying cover successions in Maine and adjacent New Brunswick have evolved over several decades, and revisions continue today (e.g., Marvinney et al. 2010; Ludman 2013; Bradley and O’Sullivan 2016; Ludman et al. 2017). The area covered in this paper (Fig. 3) was for many years known only from reconnaissance mapping. It was surrounded by well-established stratigraphic successions in western, central, and northeastern Maine, but its relationships with the rocks of southern Maine and southwestern New Brunswick were uncertain on the two most recent Maine bedrock maps (Doyle and Hussey 1967; Osberg et al. 1985). More than 30 years of fieldwork since the 1985 Maine bedrock map was published have resolved long-standing correlation problems and provided a lithofacies framework for the detrital zircon studies reported here (summarized by Ludman et al. 2017; Ludman 2013), including:

1. The Central Maine and Aroostook-Matapedia successions accumulated in a single depocenter – the CMAM basin – sourced from emergent highlands to the west (Boundary Mountains, Lobster Mountain, Weeksboro–Lunksoos Lake) and to the east (Miramichi), and by NE-to- SW axial currents.
2. The Fredericton trough was a two-sided basin that received sediment from post-Middle Ordovician Miramichi and St. Croix highlands to the west and east, respectively. No similar highland is present in southwestern Maine, and relationships between Fredericton and Central Maine strata there are uncertain.
3. Silurian lithofacies patterns adjacent to the Cambrian-Ordovician Weeksboro-Lunksoos Lake and Munsungun belts indicate that these areas also supplied post-Late Ordovician sediment to the CMAM basin.
4. Sedimentation in the Fredericton trough ended in the late Silurian with onset of Salinic deformation but continued in CMAM through the Early Devonian.
5. Acadian, Alleghanian, and post-Alleghanian folding and faulting have distorted original basin geometries and drastically telescoped the basin sediments.

9 Recent geologic mapping has refined the stratigraphy in the Fredericton trough in New Brunswick from that described by Ruitenberg and Ludman (1978), and provided fossil age control for some of the units (see Table 2; summarized in Dokken et al. 2018; Fyffe 1991, 1995; Fyffe and Riva 2001; Fyffe et al. 2011). Distinctive units along the northwest flank of the Fredericton trough in New Brunswick apparently are not present in Maine, perhaps due to facies changes, removal by faulting, or a combination of the two.

10 The basement tracts in the study area were clearly emergent and served as major internal Ganderian sediment sources for the cover rock basins following accretion of Ganderia to Laurentia (Ludman et al. 2017). It was hoped that detrital zircon age spectra might identify input from sources external to Ganderia – Laurentia and/or the approaching Avalonian plate. Our current interpretation of regional correlation is summarized in Table 2. Because of the complexity of this table, only Period and Series chrono-stratigraphic divisions are shown.

11 Previous detrital zircon studies in Maine and New Brunswick have helped to constrain depositional ages and determine provenance of rocks related to this study. Bradley and O’Sullivan (2016) reported detrital zircon age spectra from several proximal units on the west flank of the CMAM basin in western Maine. Discrepancies between ages of the youngest zircon grains and stratigraphic ages inferred from regional correlations suggest that some units may be as much as 15 m.y. younger than previously thought. McWilliams et al. (2010) demonstrated that the Connecticut Valley-Gaspé trough, containing the northwesternmost cover succession (Fig. 1), was derived from highlands flanking the basin. Studies in southwestern Maine, adjacent New Hampshire, and Massachusetts identified both Laurentian and Ganderian sources for rocks correlated with CMAM and Fredericton successions in an area where there is no basement block separating the two basins (Sorota 2013). Hussey et al. (2016) cited paleocurrent indicators as evidence for an eastern (peri-Gondwanan) source for rocks in southwestern Maine and southeastern New Hampshire correlated with the Fredericton trough succession.

12 Some detrital zircon work has been done in southwestern New Brunswick on stratigraphic units continuous with those sampled in this study. Fyffe et al. (2009) compared detrital zircon age spectra from pre-Silurian terranes in southern New Brunswick and coastal Maine to assess their tectonic settings and refine their provenance and accretionary histories. Pre-Silurian units related directly to this project include the Baskahegan Lake Formation, oldest unit in the Miramichi terrane, also reported here, and the Calais Formation in the lower part of the St. Croix terrane. Current work in New Brunswick has focused on rocks of the Fredericton trough in an attempt to clarify the tectonic setting and provenance of its thick turbidite succession (Dokken et al. 2018).

13 Detrital zircon studies designed to distinguish terranes and reconstruct relationships among peri-Gondwanan fragments resulting from the breakup of Rodinia are described in the Discussion section, below.

15 The Pocomoonshine gabbro-diorite intruded folded strata of the Fredericton trough, and thus provides a minimum age for both deposition in that depocenter and onset of Salinic deformation in eastern Maine and New Brunswick (West et al. 1992). A ⁴⁰Ar/³⁹Ar hornblende cooling age of 422.7 ± 3 Ma (West et al. 1992) has been the regional standard for defining the onset of Salinic deformation in Maine and New Brunswick, and a sample from near the northern margin of the Pocomoonshine gabbro-diorite was collected to compare the U–Pb zircon crystallization age of the pluton with the previously determined hornblende cooling age.

16 Samples were collected from the thick volcanic Olamon Stream Formation at the top of the Miramichi succession (OS-1) and from a tuff horizon (KMV-1) at the base of the Kendall Mountain Formation, youngest unit in the St. Croix terrane, to determine their eruptive ages, clarify stratigraphic ranges of underlying units, and constrain the timing of the Ordovician tectonism. A cryptocrystalline tuff from the Smyrna Mills Formation was also collected but proved to have insufficient zirconium to attempt zircon separation and dating.

17 Ten samples of cover rock were collected for detrital zircon dating; the stratigraphic problems addressed by these samples are detailed below. Eight of these samples are interpreted to have been deposited in the initial Late Ordovician – Middle Silurian sedimentary regime in the CMAM basin [(Mayflower Hill (MH) and Hutchins Corner (HC) formations; Vassalboro Group (VG)], and Fredericton trough [(Flume Ridge (FR) and Appleton Ridge (AR)], and two in the subsequent Late Silurian to Early Devonian regime in CMAM [(Madrid Formation (MA)].

18 Four additional sedimentary rock samples were collected from basement terranes; three from the Miramichi Baskahegan Lake (BL-1, BL-2) and Bowers Mountain (BM) formations and one from the Kendall Mountain (KM-1) Formation. Nine of the fourteen detrital zircon sample sites are from chlorite-grade rocks, two from higher grades of regional metamorphism (HC-1 at biotite grade, AR-1 at andalusite + staurolite grade), and three from contact aureoles with biotite-cordierite assemblages (MH-1, BM-1, and KM-1).

27 The Miramichi terrane extends southwestward for more than 300 km from northern New Brunswick into Maine where it narrows and terminates abruptly at the surface in the Greenfield area (Fig. 1; Ludman 2018). The Bottle Lake pluton divides the terrane in Maine into northern (Danforth) and southern (Greenfield) segments (Fig. 3). The two segments have similar stratigraphies, with the turbiditic Baskahegan Lake Formation at the base of each, overlain by a dominantly pelitic, partially euxinic, unit and capped by a thick volcanic section. The upper parts of these sections are sufficiently dissimilar to require different nomenclature (Fig. 7).

28 The Baskahegan Lake Formation comprises turbiditic quartzofeldspathic and quartzose sandstone with varying amounts of interbedded slate. The base of the overlying Bowers Mountain Formation in the Danforth segment is mostly black sulfidic shale with quartz arenite that pass upward into interbedded gray shale and siltstone. The equivalent Greenfield Formation in the Greenfield segment contains some black shale but also has numerous manganese-rich mudstone/siltstone horizons and a distinctive manganiferous iron formation. A thick succession of dacitic, rhyodacitic, and andesitic tuffs and flows form the tops of both segments. The Stetson Mountain volcanic rocks (Danforth segment) are typically fine grained ashfall tuffs with subordinate crystal tuffs and sparse coarser-grained ashflow tuffs, whereas the equivalent Olamon Stream volcanics (Greenfield segment) are coarser (including a few agglomerates), and have more flows and crystal or lithic tuffs.

29 Correlation with fossil-bearing units outside the study area suggests a Cambrian to Middle Ordovician age range for the Miramichi succession in Maine. The lower limit is based on correlation of the Baskahegan Lake Formation with the Grand Pitch Formation in the Weeksboro–Lunksoos Lake terrane (Fig. 1), which contains the Early to Middle Cambrian trace fossil Oldhamia (Neuman 1984). Ichnofossils in New Brunswick (Pickerill and Fyffe 1999) indicate that the Baskahegan Lake Formation extends into the Early Ordovician but is older than the overlying late Tremadocian Bright Eye Brook Formation (Fyffe et al. 1983). The upper age limit of the Miramichi succession in Maine is set by poorly constrained “Lower to Middle Ordovician” graptolites in the Stetson Mountain Formation (Larrabee et al. 1965). Until this study there had been no direct evidence for the age of the Bowers Mountain Formation, nor radiometric age control for the volcanic section in Maine.

30 Igneous zircon was collected from a felsic lava in the Olamon Stream Formation (OS-1) to directly date this formation and, indirectly, the correlative Stetson Mountain Formation. Detrital zircon from sandstone near the base of the Bowers Mountain Formation (BM-1) and from Baskahegan Lake sandstones in both segments (BL-1, BL-2) was dated in an attempt to constrain the ages and ranges of these formations.

31 Olamon Stream Formation: The first radiometric age of Miramichi volcanism in Maine is provided by zircon from the Olamon Stream Formation (OS-1). The sample, originally thought to be in place, proved to be part of a train of extremely coarse glacial debris derived from outcrops of identical rock about a kilometre to the north of the sampling site. The rock is a crystal-rich felsic lava flow containing abundant 2–8 mm quartz and feldspar phenocrysts in a buff-weathering, very fine-grained matrix (Fig. 8a). Zircon from the Olamon Stream felsic lava is fairly coarse (~100– 200 µm), euhedral, and displays fine, concentric oscillatory zoning (Fig. 8b), suggesting an igneous origin. A few grains contain partially resorbed cores; two analyses of cores yielded ages of about 660 and 550 Ma, thought to represent inheritance of Neoproterozoic xenocrystic zircon from an unknown source.

32 U–Pb isotopic data of 15 analyses yield a concordia age of 469.3 ± 4.6 Ma (Fig. 8c), at the boundary between the Lower and Middle Ordovician (Floian–Dapingian), and is interpreted as the eruption age of the felsic lava. This age for the youngest formation in the Miramichi succession also constrains the range of the underlying units and is the maximum age for subsequent Ordovician deformation (see Discussion below).

33 Bowers Mountain and Baskahegan Lake formations: Figure 9 is an expanded excerpt from Figure 5a, highlighting the Late Neoproterozoic to Early Cambrian ages of the youngest detrital zircon grains in the four basement units sampled. The absence of Silurian zircon clearly distinguishes all four units from the younger cover rocks. The latest Neoproterozoic to earliest Cambrian age of the youngest zircons in the Baskahegan Lake Formation is consistent with its Cambrian-Early Ordovician age described above, and with zircon ages from the formation in New Brunswick (Fyffe et al. 2009). Unfortunately, the youngest zircon ages in the Bowers Mountain and Kendall Mountain formations are not statistically robust enough to establish a youngest age population as they were found in only one grain each (Fig. 9).

34 Stratigraphic ages of units in the St. Croix terrane shown in Table 2 are based on fossils in the lower and uppermost formations of the succession. Tremadocian graptolites at Cookson Island indicate an Early Ordovician age for the upper part of the Calais Formation, above a thick basal pillow basalt member. Graptolites in black shale interbedded with quartz arenite in the Kendall Mountain Formation suggest a Late Ordovician (Caradoc) age (Fyffe and Riva 1990) but there is no age control for the intervening Woodland Formation.

35 The age and stratigraphic affinity of the Kendall Mountain Formation have been debated. Fyffe et al. (2009) proposed that this formation lies conformably below the Digdeguash Formation, but Ludman (1990) mapped a terrane-bounding fault between the two formations and interpreted the Kendall Mountain as the youngest formation in the St. Croix terrane. This study attempted to resolve this issue by (1) acquiring a detrital zircon age spectrum for the Kendall Mountain (KM-1) to compare with those of the Digdeguash (Fyffe et al. 2009; Dokken et al. 2018) and basement units, and (2) dating a tuff horizon (KMV-1) just above the contact with the underlying Woodland Formation.

36 Kendall Mountain Formation: Zircon was extracted from a white, cryptocrystalline felsic rock (KMV-1) interpreted as an ashfall tuff interbedded with thinly laminated black shale and siltstone near the base of the Kendall Mountain Formation (Figs. 10a–b) in the St. Croix terrane. These grains are fine (~50–100 µm in length), euhedral, and display fine, concentric oscillatory zoning, indicative of an igneous origin (Fig. 10c).

37 U–Pb isotopic data for 16 grains yield a concordia age of 477.4 ± 3.7 Ma (Fig. 10d). This age is interpreted as the time of eruption of the felsic tuff and thus dates the base of the Kendall Mountain Formation as Lower Ordovician (Floian–Tremadocian). It is considerably older than the Late Ordovician (Caradoc) fossil-based age for the formation but is compatible with detrital zircon ages from overlying quartz arenites (see below).

38 Contributions of zircon ages to basement stratigraphy: Figure 11 shows the relationships between detrital and volcanic zircon ages obtained in this study with the stratigraphic ages of their host rocks. In the Miramichi terrane, ages of the youngest zircons in the Baskahegan Lake Formation are statistically significant and consistent with its proposed Cambrian to Early Ordovician age. They set maximum Late Neoproterozoic and Early Cambrian depositional ages for the Miramichi succession in the Greenfield and Danforth segments, respectively but constrain the age of the formation only slightly. The age of the youngest Bowers Mountain detrital zircons could potentially have limited the upper range of the Baskahegan Lake Formation but the single 485 Ma age is statistically insignificant. The 469.3 ± 4.6 Ma age for the Olamon Stream lava dates one horizon within the thick Miramichi volcanic pile. It also limits the age of the Bowers Mountain Formation to the Early Ordovician, between that eruptive age and the Cambrian–Early Ordovician ichnofossils in the underlying Baskahegan Lake Formation (Pickerill and Fyffe 1999).

39 In the St. Croix terrane, ages of volcanic and detrital zircons in the Kendall Mountain Formation improve understanding of Cookson Group stratigraphy in three ways. First, the 477.4 ± 3.7 Ma age of the volcanic horizon just above the base of the formation narrows the range of the underlying Woodland Formation to a narrow span in the Tremadocian, bracketed between the Kendall Mountain volcanic horizon and the Tremadocian fossils in the Calais Formation (Cumming 1967; Fyffe and Riva 1990). Second, if the fossil-based Caradocian age of the Kendall Mountain is correct, the range of the formation would span much of the Ordovician Period, from Tremadocian through Caradocian (Fig. 11, right column). Finally, the detrital zircon age spectrum for Kendall Mountain quartz arenite (KM-1) differs significantly from that of the adjacent Digdeguash Formation (Dokken et al. 2018) from which it is separated by a terrane-bounding fault (Ludman 1990; Mohammadi et al. 2017), but is similar to those of the Miramichi basement units, confirming the basement rock status of the Kendall Mountain.

43 Figure 13 shows the current interpretation of Fredericton trough stratigraphy in eastern Maine and New Brunswick. The Flume Ridge Formation underlies more than 75% of the Fredericton Trough in Maine and is divided into western and eastern facies. The western facies, close to the Miramichi terrane, is characterized by coarser grains, more abundant lithic fragments and pelite, thicker beds, and lower carbonate content than the eastern facies exposed in the central and eastern parts of the trough. Fossils have not been found in Maine but provide good age control for units in the western part of the Fredericton trough in New Brunswick (Fig. 13) and for at least part of the Digdeguash Formation on the east flank of the basin (Dokken et al. 2018).

44 Zircon from calcareous wackes of the Flume Ridge eastern (FR-1) and western (FR-2) facies were dated for comparison with data from New Brunswick (Dokken et al. 2018) and to broaden the geographic distribution of zircon ages from this widespread formation. The Digdeguash Formation was not sampled in this study; however, zircons from the mid-coastal Appleton Ridge Formation (AR-1) were dated to test its proposed equivalence with the Digdeguash (e.g., Hussey et al. 2010; Berry et al. 2016).

45 Flume Ridge Formation: The age of the Flume Ridge Formation had previously been bracketed between early Silurian graptolites (Upper Rhudannian; ca. 440 Ma) in the underlying Digdeguash Formation (Fyffe and Riva 2001) and the 421.9 ±2.4 Ma Pocomoonshine gabbro-diorite that intrudes folded Flume Ridge and Digdeguash strata. The youngest zircon grains in FR-1 and FR-2 are ca. 430 Ma, providing a maximum age for deposition of the formation and narrowing its range to 430–421.9 ± 2.4 Ma (Wenlock to Pridoli).

46 Digdeguash Formation: The Digdeguash Formation lies conformably and gradationally below the Flume Ridge in eastern Maine (Ludman 1990). Rhuddanian graptolites from an uncertain stratigraphic level indicate an early Llandoverian age for part of the Digdeguash (Fyffe and Riva 2001) but the 430 Ma youngest Flume Ridge zircons suggest that it may be as young as Wenlockian. The lower range of the Digdeguash is unknown, but is limited by its youngest detrital zircons, a small population around 490 Ma (late Cambrian; Dokken et al. 2018).

47 Age of the Appleton Ridge Formation and relationship to the Digdeguash Formation: The Fredericton cover succession is truncated to the southwest by the ca. 384 Ma Deblois pluton (Ludman et al. 1999) and ca. 380 Ma Lucerne pluton (Wones and Ayuso 1993) (Fig. 3). Large roof pendants in these plutons mapped as Bucksport Formation are very similar to the Flume Ridge Formation, and Appleton Ridge lithology and bedding southwest of these plutons are similar to those of the Digdeguash Formation (Fig. 14). The Buck-sport and Appleton Ridge formations thus appear to extend the Flume Ridge–Digdeguash succession of the Fredericton trough to the west shore of Penobscot Bay (Fig. 14).

48 The age of the Appleton Ridge Formation is uncertain, listed on recent maps as Silurian–Ordovician(?) (West 2006; Pollock 2012). Results of this study (Fig. 12, AR-1) constrain this age range to Middle Ordovician–latest Silurian, that is, between the prominent youngest detrital zircon peak at 469 Ma on Figure 12 (the Floian–Dapingian boundary) and the age of 422 ±2 Ma (Pridoli) from the North Union tonalite gneiss (Tucker et al. 2001) that intruded the Appleton Ridge Formation. It is noteworthy that Pollock (2012) interpreted the Appleton Ridge as being younger than the Bucksport Formation.

49 Detrital zircon age spectra for the Appleton Ridge and Digdeguash formations are similar in some ways but differ in others (Fig. 15). The most significant difference is a prominent zircon population in the Appleton Ridge Formation at around 1 Ga (Fig. 15), suggesting a source that apparently did not contribute to the Digdeguash Formation. Both have zircon populations at around 465 Ma and 600 Ma, suggesting common sources – most likely Miramichi volcanic rocks and peri-Gondwanan sources, respectively – but in different proportions. Additional data are needed to determine whether these differences demonstrate that the Appleton Ridge and Digdeguash formations are not equivalent or that the differences are the result of bias in the zircon grains dated.

50 Samples were collected from several unfossiliferous sandstone units in the CMAM basin (Fig. 3). Osberg (1988) concluded that the stratigraphic succession in the axial part of the basin near Waterville (W in Fig. 3) is a “sandwich” in which the thin-bedded, dominantly pelitic Waterville Formation is overlain (Mayflower Hill) and underlain (Hutchins Corner) by similar thick-bedded, variably calcareous sandstone units (Table 2). The Waterville Formation contains middle Silurian (Wenlock) graptolites (Ruedemann, cited in Perkins and Smith 1925) and early Silurian (Llandovery) trace fossils (Orr and Pickerill 1995).

51 Where sandstone is demonstrably younger than the Waterville Formation, Marvinney et al. (2010) recommended that the name Mayflower Hill Formation be used, following Osberg’s (1968) original name. Three samples met this condition and are assigned to the Mayflower Hill Formation. Two lie in the same outcrop belt, demonstrably above a Waterville Formation equivalent (Smyrna Mills Formation) — MH-1 east of Lincoln (L on Fig. 3), and MH-3 at Howland dam southwest of Lincoln. MH-2 was collected 5 m above basal black phyllite at the contact with the underlying Waterville Formation near the eponymous Mayflower Hill in Waterville (W) (Stop 5 of Ludman and Osberg 1987). Osberg (1988) named the sandstone below the Waterville Formation the Hutchins Corner Formation, and HC-1 was collected from rocks interpreted as the Hutchins Corner Formation from Freedom village, Maine, at the east edge of the belt.

52 Where the position of a sandstone unit relative to the Waterville Formation is uncertain, or where the Waterville is absent, as in the broad swath of sandstone between Waterville and the Miramichi terrane (Fig. 3), Marvinney et al. (2010) recommended using the less specific term Vassalboro Group for the unit (Table 2). One sample meets this criterion and is designated as Vassalboro Group, undifferentiated (VG-1) in this paper and Appendices A and B.

53 Two samples of rocks mapped as the Madrid Formation (MA-1, MA-2) were collected in the northern part of our study area to explore possible differences between detrital zircon age spectra from the early and late stages of CMAM sedimentation. Their assignment to the Madrid is highly probable but they are separated by other units from known Madrid belts to the west on which the stratigraphic succession is based (Osberg et al. 1985).

54 The small number of zircon grains that yield the youngest ages is insufficient for determining definite maximum ages in most CMAM samples without further analyses, but their general temporal distribution is suggestive, as shown in Figure 16. Even though the 1σ errors reported in Appendices C and D span the short middle and late stages of the Silurian (see Appendix A), it is likely that the Madrid, Mayflower Hill, Hutchins Corner, and undifferentiated Vassalboro Group samples are, indeed, Silurian. If additional dates in the future corroborate those obtained in this study, all the sampled CMAM rocks would be younger than the Waterville Formation fossils. This result might be problematic in particular for the Hutchins Corner sample (HC-1) which is currently interpreted to be stratigraphically below the Waterville (Osberg 1988). If further detrital zircon work confirms it to be Wenlock, then its relation to the Llandovery fossil age of the Waterville will need to be revisited.

57 Several lines of evidence indicate that most CMAM and Fredericton sediment was recycled from Ganderian sources with little input from external terranes. First, the Miramichi and several other Ganderian basement terranes in Maine were emergent highlands by the late Ordovician (Hirnantian) immediately after accretion of the main body of Ganderia to Laurentia, and were clearly sources of sediment for both CMAM (Bradley and Hanson 2002; Bradley and O’Sullivan 2016; Hopeck 1998; Pankiwskyj et al. 1976) and the Fredericton trough (Fyffe 1995; Ludman et al. 2017).

58 Second, although the leading edge of Ganderia had docked with Laurentia by the late Ordovician (van Staal et al. 2009), there would have been significant obstructions during the early Silurian to eastward transport of Laurentian sediment to the two depocenters. To reach CMAM, Laurentian sediment would have had to traverse the deep-water Connecticut Valley-Gaspé trough (Tremblay and Pinet 2005, 2016) and bypass the exhumed Boundary Mountains, Munsungun, and Weeksboro-Lunksoos Lake basement terranes. To reach the Fredericton trough, the sediment would have had to traverse the entire deep-water CMAM and then bypass the Miramichi terrane (Ludman et al. 2017). Tightly folded rocks of CMAM today span a width of 160 km, but the undeformed depocenter was at least three to five times wider (Bradley et al. 2000) and would have been a formidable barrier to transport of Laurentian sediment.

59 It would also have been difficult for detritus to reach the Fredericton trough or CMAM from a peri-Gondwanan source southeast of Ganderia (most likely Avalonia). Starting at about 430 Ma, that sediment would have had to traverse the Eastport-Mascarene arc-trench system created by subduction of oceanic lithosphere that separated Avalonia from composite Laurentia + Ganderia (Llamas and Hep-burn 2013).

60 In the absence of external sources, Ganderian basement terranes were most likely the primary sources of CMAM and Fredericton sediment. Hopeck (1998) demonstrated that exhumed Miramichi basement contributed sediment to CMAM in Maine by Hirnantian times, just a few kilometres from the New Brunswick border. It would therefore have been available as a source for the Fredericton trough immediately to the southeast as proposed by Fyffe (1995).

61 The updated geologic map of eastern and east-central Maine (Fig. 3) shows that samples FR-1, FR-2, and AR-1 were deposited in the Fredericton trough. Significant zircon populations in these rocks between ca. 475 and 455 Ma and ca. 422 and 435 Ma are interpreted to have been derived from two volcanic episodes. The older group corresponds to eruptions in the Popelogan arc and Tetagouche back-arc basin early in the evolution of Ganderia as described above (Wilson et al. 2015). Their recycling into the cover rocks is evidenced by abundant Miramichi volcanic clasts in both CMAM (Hopeck, 1998; Ludman et al. 2017) and the Fredericton trough (Fyffe 1995), and the 469.3 ± 4.6 Ma age of the Olamon Stream lava reported here.

62 The younger group was interpreted above as having come directly (i.e., not recycled) from Silurian pyroclastic eruptions in the Eastport-Mascarene volcanic arc. This continental arc formed by melting of, and deposition on, Ganderian lithosphere during westward subduction of a seaway separating Avalonia from composite Laurentia + Ganderia (Llamas and Hepburn 2013). Thus, although they are the product of the Acadian orogenic cycle, these zircons, like those of the older group, are Ganderian.

63 Dokken et al. (2018) reached different conclusions about the Fredericton trough, most importantly that (a) the Miramichi terrane was not exhumed until Wenlock times; (b) there is no peri-Gondwanan debris in the northern part of the Fredericton trough; and (c) Laurentian sediment is abundant in all units in the northern part of the Fredericton trough and overwhelmed peri-Gondwanan sediment in the southern part by the middle Silurian.

64 Evidence cited above contradicts these assertions and in addition:

1. Dokken et al. (2018) reasoned that the Miramichi terrane could not have been a sediment source because it was undergoing deep crustal, high-grade metamorphism during late Ordovician to early Silurian subduction (van Staal et al. 2008) and was not exhumed until Wenlock time. The timing and intensity of Miramichi metamorphism in northern New Brunswick are not in question, but neither can be applied to the full > 350 km extent of the terrane. The entire Maine portion of the Miramichi terrane experienced no greater than lowest greenschist conditions during all late Ordovician through Devonian deformation events. Thus, while the northernmost Miramichi terrane was being subducted to deep crustal levels (van Staal et al. 2008), the southern part, from at least the Maine-New Brunswick border to Greenfield, was at very shallow crustal levels, and, as shown above, had been exhumed well before the Wenlock
2. Dokken et al. (2018) reported that peri-Gondwanan detritus is absent from the Rhudannian Hayes Brook Formation and arrived later in the younger Burtts Corner and Flume Ridge formations, signaling exhumation of the Miramichi terrane. However, they attributed the dominant 465-480 Ma zircon populations in all three formations to distant peri-Laurentian arcs rather than to local Ganderian (i.e., peri-Gondwanan) Popelogan and Tetagouche eruptions. For reasons given above, a Laurentian or peri-Laurentian source is unlikely.
3. As explained above, it is unlikely that Laurentian sediment contributed significantly to the Fredericton trough at any time in its history.

65 Basement rock barcodes contain zircon populations at ca. 800 Ma (BL-2, KM-1) and ca. 2.0 to 2.5 Ga (BL-2, BM-1, KM-1) that are absent from or rare in the cover rocks (Fig. 5). These suggest a source or sources not tapped by the younger strata, but without the sedimentologic evidence that clarified the source of the cover rock sediment, it is impossible to distinguish between the immediate and ultimate provenance of these basement zircon populations.

66 Detrital zircon age spectra have become an important tool for identifying terranes formed during the breakup of Rodinia and reconstructing where in Rodinia those fragments originated. Most northern Appalachian studies focus on distinguishing Laurentia from Ganderia, Avalonia, and Meguma, the other Rodinian fragments with which it was reunited during the Paleozoic (Dorais et al. 2014; McWilliams et al. 2010; Wintsch et al. 2007). Others seek to determine whether Ganderia broke off directly from Gondwana or from Amazonia (van Staal et al. 2012; Fyffe et al. 2009). Interpretations are based on matching local and regional detrital zircon age spectra with ages of tectonic, plutonic, and volcanic zircon-producing events unique to individual fragments (e.g., Nance et al. 2008).

67 Figure 17 compares the Ganderian signature obtained from basement and cover rocks in this study with the compilation by Nance et al. (2008) of events in the eastern Laurentian, Amazonian, and West African cratons. The continuous range of zircon ages between 600 Ma and 1.8 Ga from the study area is most similar to the compilation of Amazonian events (Nance et al. 2008), supporting the earlier conclusions of van Staal et al. (2012) and Fyffe et al. (2009). However, the viability of a West African connection has been raised by recently reported detrital zircon ages from the Mauritanide orogen (Bradley 2018) indicating similarities with Amazonian events.

68 Although the ages reported here from the Miramichi and St. Croix terranes improve understanding of stratigraphy and provenance of the basement rocks, they contribute little to refining the ages of Ordovician orogenic events. Deformation of the St. Croix terrane occurred between ca. 453 and 442 Ma, bracketed between middle Caradocian fossils in the Kendall Mountain Formation (Fyffe and Riva 1990) and Upper Rhudannian fossils in the Digdeguash Formation (ages after Cohen et al. 2013).

69 Deformation of the Miramichi terrane in Maine occurred at about the same time (between ca. 453 and 445 Ma), bracketed between deposition of “Llanvirnian to Caradocian” black shale in the Stetson Mountain Formation (Larrabee et al. 1965) and Hirnantian limestone of the Carys Mills Formation (Rickards and Riva 1981). The 469.3 ± 4.6 Ma zircon age of the Olamon Stream lava suggests a range for eruption of the upper volcanic component of the Miramichi suite (ca. 469–453 Ma) but does not refine the timing of Late Ordovician deformation.

70 The U–Pb zircon age for the post-Salinic Pocomoonshine gabbro-diorite (421.9 ± 2.4 Ma) overlaps the 423 ± 3 Ma ⁴⁰Ar/³⁹Ar hornblende cooling age of West et al. (1992) and is the minimum age of the Salinic event. This is consistent with the ca. 450–430 Ma span proposed for the Salinic event in New Brunswick (Wilson et al. 2015), but the presence of ca. 431 and 433 Ma zircon grains suggests that the Salinic might have lasted somewhat longer in eastern Maine. The overlap of the zircon crystallization and hornblende cooling ages shows that the pluton cooled rapidly at shallow crustal levels.

71 Detrital zircon ages of the cover rocks provide insight into conditions between Late Ordovician and Silurian (Salinic) tectonism. An enormous volume of turbidites was deposited during this interval in the area between western Maine and central New Brunswick (Ludman et al. 2017). Detrital and plutonic zircon ages suggest that the transition from post-Middle Ordovician sedimentation to Salinic deformation in the later part of that time span is best described as “busy”, with several events in a short time span: (1) crystallization of ca. 430 Ma zircon during Eastport-Mascarene volcanism; (2) incorporation of the zircon in the kilometres-thick Flume Ridge Formation; (3) lithification of a thick pile of Flume Ridge + Sand Brook + Digdeguash turbidites; (4) intense isoclinal folding; and (5) post-folding emplacement of the Pocomoonshine gabbro-diorite at (421.9 ± 2.4 Ma).

75 Fossil and geochronologic evidence indicate that turbidite sedimentation in the Fredericton trough began shortly after Late Ordovician deformation, by the Rhudannian or earlier, and continued until Salinic deformation and emplacement of the Pocomoonshine gabbro-diorite at ca. 422 Ma. In the eastern part of the study area, the Fredericton trough was separated from CMAM during Silurian time by a Miramichi highland that shed sediment into both basins (Ludman et al. 2017). This timing is compatible with the Salinic foredeep model cited above, with subsidence of the Fredericton trough caused by tectonic loading associated with thrust sheets of the Brunswick subduction complex (Fyffe et al. 2011; Wilson et al. 2015). This model is also supported by estimated pre-deformation width and lateral extent of the Fredericton trough comparable with those of modern foredeep basins (Allen and Homewood 1986; De-Celles 2012). In addition, estimates of Fredericton trough sedimentation rates are comparable to those of modern deep marine depocenters based on published thicknesses of Fredericton trough formations, decompacted following curves of Kominz et al. (2011) and the time frame discussed above.

76 Sedimentation began in CMAM at approximately the same time as in the Fredericton trough (Ludman et al. 2017), but its origin is less certain and its evolution was more complex, with two distinct phases of sedimentation. The initial phase of deep-water turbidite sedimentation began on both west and east flanks of the basin by late Ordovician time, as evidenced by the Quimby Formation in western Maine (Bradley and O’Sullivan 2016) and Carys Mills (Hirnantian), Daggett Ridge, and Mill Priviledge Brook formations in eastern Maine (Ludman et al. 2017). The eastern flank of the CMAM basin in mid-coastal Maine is unknown due to truncation by faulting (Tucker et al. 2001).

77 Initial CMAM sedimentation thus preceded the onset of Acadian subduction and deformation by millions of years, so the CMAM basin could not have originated as an Acadian foreland basin as is commonly proposed (e.g., Bradley and O’Sullivan 2016; Hatcher 2010). Tremblay and Pinet (2005, 2016) suggested that the northwesternmost Ganderian cover rock basin, the Connecticut Valley-Gaspé trough (Fig. 1), probably formed by intraplate extension and proposed a similar origin for the western part of the CMAM basin (“Merrimack trough” in their paper).

78 The size of the CMAM basin is also problematical for a foreland basin. Although its current ~160 km deformed width is comparable to that of modern deep marine fore-land basins such as the Hidaka Trough (northern Japan; Noda et al. 2013) and the Western Taiwan foreland basin (Yu and Chou 2001), CMAM experienced intense Acadian shortening. Bradley et al. (2000) estimated that its current outcrop width is only 30% of its undeformed extent but did not consider (1) an episode of recumbent folding, (2) two generations of thrusting now recognized in the basin (Tucker et al. 2001), (3) blind thrusts imaged in seismic reflection profiling (Doll et al. 1996), nor (4) thrusting of the east flank of the basin over its Miramichi source (Ludman et al. 2017). The degree of crustal shortening was probably greater than 70% and CMAM was possibly as much as 800–1 000 km wide, far wider than modern marine foreland basins.

79 The second sedimentary regime began in the Late Silurian and continued through the Early Devonian (Bradley et al. 2000), adding approximately 9 km of sediment to the CMAM basin after sedimentation had ceased in the Fredericton trough (Bradley and O’Sullivan 2016). Cogent paleocurrent, paleontological, and geochronological evidence show that the CMAM basin had become an Acadian foredeep during this time span, and that its sediment was sourced uniformly from an Acadian forebulge to the southeast (Bradley et al. 2000).