The Salinic Orogeny is defined to encompass tectonic interactions that affect all elements of Ganderia involved in the closure of the Tetagouche–Exploits back-arc basin between the Late Ordovician and Early Devonian. Hence, the D1 and D2 deformations in the Miramichi Highlands and Elmtree Inlier of northern New Brunswick are Salinic events, and onlap of Lower Silurian rocks onto exhumed parts of the Brunswick Subduction Complex represents the earliest (Salinic A) of three Silurian unconformities in the region. Upper Ordovician to Lower Silurian rocks of the Matapédia successor basin contain widespread evidence of Middle Silurian tectonism (e.g., disconformities, angular unconformities, and fold interference patterns) created by Devonian overprinting of Silurian folds lacking axial planar cleavage (Salinic B). Recent U–Pb radioisotopic dating of chemically abraded zircon from rhyolite just above the Salinic B unconformity has yielded an age of 422.3 ± 0.3 Ma; combined with late Early Silurian fossil ages just below the unconformity, this indicates a ca. 5 million year Middle Silurian hiatus. Finally, Upper Silurian (Ludfordian) rocks are locally disconformably overlain by polymictic conglomerates that form the base of the Devonian section (Salinic C). All Silurian rocks in northeastern New Brunswick have historically been included in the Chaleurs Group; however, unconformities and local stratigraphic variations (especially compared with the type locality) support the introduction of new higher rank names in New Brunswick. Hence, the Quinn Point Group is introduced to incorporate Lower Silurian rocks, the Petit Rocher Group to include Upper Silurian sedimentary rocks in the Nigadoo River Syncline, and the Dickie Cove Group for Upper Silurian volcanic rocks in the Charlo – Jacquet River area. Upper Silurian rocks west of Campbellton that are contiguous with the Chaleurs Group in Quebec, will remain part of the Chaleurs Group.
Many advances have been made in recent years in unraveling the complex and protracted accretionary history of the northern Appalachian orogen (e.g., van Staal et al. 1998, 2003, 2009). Accreted terranes include assorted volcanic arc and back-arc sequences that evolved within and marginal to the Iapetus oceanic tract, as well as microcontinents that rifted from the Laurentian and Gondwanan cratons in the early stages of the opening of Iapetus. On the “eastern” Gondwanan margin of Iapetus, early Paleozoic terranes include Avalonia and Ganderia; Ganderia is characterized by a late Neoproterozoic basement and encompasses the Gander Zone and adjacent part of the Dunnage Zone (i.e., the Exploits Subzone) as defined by Williams (1979) (van Staal et al. 1998, 2009). The Exploits Subzone comprises volcanic arcs and back-arc crust that formed within or marginal to the leading (“western”) edge of Ganderia; in northern New Brunswick, the Exploits is represented by remnants of the Popelogan–Victoria arc and by rocks deposited in the associated Tetagouche–Exploits back-arc basin (e.g., van Staal et al. 1998, 2003, 2009). Collision of the Popelogan–Victoria arc with Laurentia occurred in the Sandbian–Katian (time scale of Walker and Geissman 2009) and is regarded as the terminal pulse of Taconic orogenesis. Continued convergence was accommodated by stepping-back of northwest-dipping subduction beneath the Laurentian margin during the Katian to Telychian (Late Ordovician to late Early Silurian; ca. 450–430 Ma), which led to closure of the Tetagouche–Exploits back-arc basin (van Staal 1987, 1994; van Staal et al. 1990, 1998, 2003, 2009). By the late Telychian, full entry of the buoyant segment of the Gander margin into the subduction zone signaled closure of the back-arc basin, and Ganderia–Laurentia collision. In northern New Brunswick, evidence of Tetagouche–Exploits back-arc subduction consists of a nappe of Middle Ordovician ophiolite forming part of the Fournier Group (van Staal and Fyffe 1991; van Staal et al. 2008), and a Late Ordovician – Early Silurian subduction complex (Brunswick Subduction Complex) characterized by the local development of blueschist (van Staal et al. 1990, 2003). Tectonic events occurring between the Late Ordovician (i.e., post-late Taconic) and Late Silurian are collectively attributed to the Salinic Orogeny. Around 440–430 Ma, the Mascarene arc and backarc complex developed on the southern trailing margin of Ganderia above the northwesterly subducting Avalonian plate. Inversion of the Mascarene back-arc basin at ca. 421 Ma signals the collision of Avalonia with Ganderia (composite Laurentia) and onset of the climactic Acadian Orogeny (e.g., van Staal et al. 2009; Reusch and van Staal in press).
Of these three orogenic events, the impact and and dynamic significance of the Salinic Orogeny has not been recognized until relatively recent times. For example, detailed structural analysis and dating of syndeformational S1 phengites in early Paleozoic rocks in the northern Miramichi Highlands (Currie and van Staal 1999; van Staal et al. 2003) revealed conclusively that the early penetrative deformation in this area postdated the Taconic Orogeny, rather than being Taconic-related as believed by previous workers (e.g., Helmstaedt 1971; Skinner 1974). Furthermore, Middle Paleozoic (post-Taconic) rocks of the Matapédia successor basin (see section titled “The Chaleurs Group: regional setting and stratigraphy”) exhibit Acadian (Devonian) structures that clearly overprint an earlier generation of folds in many places; however, an axial planar cleavage has not been observed to accompany the older folds, at least in New Brunswick (e.g., Wilson et al. 2004). A Silurian unconformity or disconformity in various parts of northern New Brunswick has been implied or suspected for some time. For example, Alcock (1935; pp. 33, 52) noted an erosional interval based on the presence of Early Silurian limestone pebbles in what he took to be Devonian conglomerate at Belledune and east of Quinn Point (now recognized as the Late Silurian South Charlo Formation). Irrinki (1990) reported limestone clasts from the Early Silurian La Vieille Formation in the overlying South Charlo Formation, and a possible Silurian disconformity was suggested by Greiner (1966). Lee and Noble (1977, in their Fig. 1) reported that the upper part of the La Vieille Formation is locally missing and suggested an unconformity between the La Vieille and the Late Silurian Bryant Point Formation, but went on to discount the idea of an unconformity as “lacking evidence”. Walker and McCutcheon (1995) report that the Late Silurian Simpsons Field Formation “conformably to disconformably” overlies the La Vieille, and that the Bryant Point Formation locally overlies the La Vieille and Upsalquitch formations “unconformably or disconformably”. In spite of this, the significance and the regional extent and duration of a Silurian hiatus were not seriously considered, and the evidence for an apparent hiatus was not explained. The purpose of this paper is to review the various manifestations of Salinic tectonism in northern New Brunswick, report new geochronological data that constrain the duration of the Middle Silurian Salinic hiatus, and examine the implications of Salinic tectonism for group-level nomenclature of Silurian rocks in this part of the province.
Chaleurs Group: regional setting and stratigraphy
The stratigraphic framework and evolution of the Matapédia successor basin in northern New Brunswick has been described by Wilson et al. (2004). The history of the successor basin can be considered to begin with the Sandbian–Katian collision of the Popelogan arc with Laurentia, which is reflected in a Late Ordovician disconformity at the top of the Balmoral Group in the Popelogan Inlier (Fig. 1; van Staal 1994; van Staal et al. 1998; Wilson et al. 2004). Late Ordovician to late Early Silurian (late Katian to Telychian) sedimentation in the Matapédia successor basin occurred in a forearc setting with respect to subduction of the Tetagouche–Exploits backarc basin (van Staal and de Roo 1995; van Staal et al. 1998) and sparse Early Silurian arc magmatism (Wilson et al. 2008 and references therein). Following the Silurian collision between Ganderia and Laurentia, Matapédia sedimentation occurred in a retroarc foreland setting with respect to migration of the Avalonian (Acadian) orogenic wedge (e.g., Malo 2001; Wilson et al. 2004).
The Matapédia successor basin is commonly regarded as comprising three structural zones, namely, from northwest to southeast, the Connecticut Valley – Gaspé Synclinorium, Aroostook–Percé Anticlinorium, and Chaleur Bay Synclinorium (Rodgers 1970; Fig. 1). The oldest rocks are in the Aroostook–Percé Anticlinorium, where late Katian to ca. Aeronian sedimentary rocks include lower siliciclastic (Grog Brook Group) and overlying calcareous (Matapédia Group) turbidite sequences that represent a gradual infilling of the (Salinic) fore-arc basin. East of the Aroostook–Percé Anticlinorium, the Chaleur Bay Synclinorium is underlain by Aeronian to lower Lochkovian rocks of the Chaleurs Group and Lower Devonian rocks of the Dalhousie and Tobique groups and Campbellton Formation. The focus of this paper is the Chaleurs Group, which conformably overlies the Matapédia Group on the west side of the Chaleur Bay Synclinorium, and unconformably overlies the Fournier Group (Tetagouche–Exploits oceanic crust) on the southeastern margin of the Synclinorium (Alcock 1935; Helmstaedt 1971; Walker et al. 1993; Walker and McCutcheon 1995; Fig. 1).
The type locality of the Chaleurs Group is situated between Port-Daniel and Gascons on the south shore of the Gaspé Peninsula, Quebec. “Chaleur group” was first referred to in print by Ami (1900), and the current terminology was established by Burk (1964); (because the geographic component of a rock-stratigraphic name should conform to the usage recognized at the type locality, the French term “Chaleurs” has been adopted in New Brunswick). At the type locality, the Silurian depositional record is interpreted to be complete and conformable, and includes, in ascending order, the Clemville, Weir, Anse Cascon, Anse à Pierre–Loiselle, La Vieille, Gascons, West Point, and Indian Point formations (Schuchert and Dart 1926; Northrop 1939; Burk 1964; Bourque 1975; Bourque and Lachambre 1980; Bourque et al. 1995, 2001; Fig. 2). Silurian rocks throughout the Gaspé Peninsula are all assigned to the Chaleurs Group, although local nomenclature has been introduced for individual formations in the central and northern parts of the peninsula (Burk 1964; Bourque 1975). The apparent absence of a Silurian unconformity in the Port-Daniel area has had important consequences for New Brunswick, because this section is the template that has historically been used as the framework for the Chaleurs Group on the south side of the Baie des Chaleurs. However, a Silurian unconformity has been recognized elsewhere in the Gaspé Peninsula, that is, north of Baie d’Escuminac in the Restigouche Syncline (Bourque and Lachambre 1980; Bourque et al. 2000; Fig. 2).
In New Brunswick, Young (1911) made an initial attempt to divide Silurian rocks in the area into formations. Several subsequent workers, notably Alcock (1935, 1941), Skinner (1974), Greiner (1966, 1970), Noble (1976), Lee and Noble (1977), St. Peter (1978), Irrinki (1990), and Walker and McCutcheon (1995) made additions or revisions to the stratigraphic nomenclature. In some cases correlation with the Gaspé section was attempted; however, except for distinctive carbonate units, such as the La Vieille and LaPlante formations, such correlations are not easily made, as pointed out by Greiner (1966, 1970) and Skinner (1974). The names of individual formations of the Chaleurs Group in New Brunswick were introduced by numerous workers, and incorporated by Walker and McCutcheon (1995) into the stratigraphic scheme that is currently in use (Fig. 2), although modifications have recently been made by Wilson and Kamo (2008).
The complex Silurian history of the Chaleur Bay Synclinorium is reflected in (i) locally abrupt lateral and vertical facies changes related to differential uplift and eustatic sea level changes (Bourque 2001; Bourque et al. 2000, 2001); (ii) two episodes of intraplate magmatic activity, in the Late Silurian (Gorstian to early Pridolian) and Early Devonian (early Lochkovian to early Emsian); and (iii) various expressions (and local absence) of Silurian tectonism (e.g., Malo and Bourque 1993; van Staal and de Roo 1995; Malo and Kirkwood 1995; Malo 2001).
The Salinic Orogeny in New Brunswick
The Salinic Orogeny is herein defined to encompass all tectonic events that involved Ganderian rocks between initiation of Tetagouche–Exploits basin closure during sinistral oblique convergence of Ganderia and Laurentia, and the onset of the Acadian Orogeny in the Early Devonian. The first such event accompanied development of the Brunswick Subduction Complex and was characterized by Late Ordovician to Early Silurian D1 thrust faulting and D2 isoclinal folding in the Miramichi Highlands and Elmtree Inlier (van Staal 1994; van Staal and de Roo 1995). Unconformities produced by the onlap of Silurian beds during exhumation of the Brunswick Subduction Complex are consequently Salinic, and are referred to herein as Salinic “A” unconformities, for example, where Silurian rocks overlie metamorphosed rocks of the Fournier and California Lake groups at the northern margin of the Miramichi Highlands and the margins of the Elmtree Inlier (Fig. 1).
A Salinic “A” disconformity also exists at the northern end of Limestone Point (Fig. 1), where a narrow (<10 m) interval of unnamed limestone containing early Rhuddanian conodonts (G. Nowlan, written communication, 1988) is overlain successively by ∼10 m of conglomerate assigned to the Weir Formation, and by calcareous sandstone and interbedded limestone of the late Telychian Limestone Point Formation. The disconformity at the lower limestone–conglomerate contact, therefore, represents a large segment of Early Silurian time. The beginning of this hiatus is roughly coeval with (but not necessarily genetically related to) incorporation of the California Lake microcontinental block in the Brunswick Subduction Complex at ca. 442 Ma (van Staal et al. 2003).
The late Early Silurian terminal collision of Ganderia and Laurentia, herein designated a Salinic “B” event, is manifested in diverse tectonic effects observed in Katian to Telychian rocks of the northern Appalachians. These effects range from erosional disconformities (Boucot et al. 1964; Roy 1980; Wilson et al. 2004), to extensional block-faulting and folding without axial planar-cleavage (Malo and Kirkwood 1995; Malo 2001), to shearing and folding with axial planar cleavage (Hibbard and Hall 1993; Dunning et al. 1990; Cawood et al. 1995). Yet in some parts of northern Maine (Roy and Mencher 1976) and the Gaspé Peninsula (e.g., the type area of the Chaleurs Group; Bourque and Lachambre 1980) continuous deposition is claimed to have occurred throughout the Silurian. The first reference to a Silurian unconformity in the northern Appalachians was by Boucot (1962), who noted a hiatus in the fossil record in the Houlton area of northeastern Maine (Fig. 1 inset) and attributed it to a “Salinic disturbance”, presumably named for the Salinan stage of the North American Silurian system. Across the border, in west-central New Brunswick, St. Peter (1982) has also reported an erosional unconformity between the Silurian Smyrna Mills Formation and the Early Devonian Wapske Formation. However, Salinic tectonism in the Maine – New Brunswick border area involved more than just uplift and erosion: northwest of Woodstock, New Brunswick, Rast et al. (1980) described Acadian folds superimposed on an older generation of folds in the Late Ordovician to Early Silurian Carys Mills Formation (now known as White Head Formation); both generations of structures are spectacularly well-developed on the North Branch Meduxnekeag River at Oakville near the Maine border (Fig. 1 inset, Fig. 3).
Similar relationships are recognized elsewhere in northern New Brunswick: in the Kedgwick area, the distribution of the Grog Brook and Matapédia groups in the Aroostook–Percé Anticlinorium provides clear evidence of overprinting of early, northwest-trending, macro-scale folds that lack cleavage, by steeply plunging Acadian folds accompanied by a penetrative, northeast-trending, axial cleavage (Carroll 2003; Wilson et al. 2004; Fig. 4). Steeply-plunging Acadian folds have also been reported within the Matapédia Group in the Grand Falls area (Wilson 1990). East of Grand Falls, a basal polymictic conglomerate marks the disconformable contact between the Early Devonian Cameron Mountain Formation and underlying Early Silurian Hazeldean Formation; the latter is characterized by doubly plunging Acadian folds (Wilson 1990). Near Belledune on Chaleur Bay, the distribution of Lower Silurian rocks of the Weir and La Vieille formations demonstrates that Acadian folds are superimposed on an older generation of northwest-trending folds without axial planar cleavage (Fig. 5). Elsewhere, clasts of the Telychian – early Sheinwoodian La Vieille Formation occur in Upper Silurian (Gorstian) rocks of the South Charlo Formation along South Charlo River (Fig. 6) and at Belledune (Fig. 7). At Black Point (Fig. 1), a thick layer of limestone cobble–boulder conglomerate between the La Vieille Formation and the Bryant Point Formation (Fig. 8) signifies post-La Vieille uplift and erosion in that area.
Some of the best field evidence for the Salinic Orogeny is observed adjacent to the Elmtree Inlier (Fig. 1), where deformation is manifested as an angular unconformity rather than an erosional disconformity. On the western margin of the Elmtree Inlier, Lower Silurian rocks of the Weir and La Vieille formations have a moderate to steep easterly dip towards the unconformable contact with Ordovician rocks of the Fournier Group. Sedimentary structures indicate that, at least locally, these beds are overturned (Wilson 2010). Farther west, the Lower Silurian rocks are overlain by Upper Silurian volcanic rocks of the Bryant Point and Benjamin formations, and Lower Devonian sedimentary rocks of the Dalhousie Group; these strata dip and young to the west, indicating that overturning of the Lower Silurian sequence occurred during the Middle Silurian (Sheinwoodian–Homerian). On the east side of the Elmtree Inlier, on the Chaleur Bay coast at Limestone Point, the Salinic unconformity is expressed as a metre-thick layer of limestone breccia–conglomerate and a ca. 20° bedding discordance between Early Silurian limestones of the La Vieille Formation and overlying sandstones of the Simpsons Field Formation (Dimitrov et al. 2004; Fig. 9); the Simpsons Field has been shown to contain Gorstian–Ludfordian fossils (Potter 1965). Also at Limestone Point, moderately to steeply plunging minor folds (Fig. 10) are developed in all Lower Silurian rocks; these structures are absent in Upper Silurian and younger rocks, again suggesting that they are related to Acadian overprinting of an earlier (Salinic) deformation.
The Salinic “B” unconformity is typically developed at the top of the La Vieille Formation or its facies equivalent Limestone Point Formation (Fig. 1), but exceptions occur where Salinic erosion has removed all of the La Vieille – Limestone Point and dissected the underlying Upsalquitch Formation, or even (rarely) the White Head Formation of the Matapédia Group (Wilson et al. 2004). In most areas, Early Silurian rocks are disconformably overlain by Late Silurian mafic volcanic rocks of the Bryant Point Formation or polymictic conglomerate of the South Charlo Formation. In the Nigadoo River Syncline (Fig. 1), the La Vieille Formation is disconformably overlain by the Late Silurian Simpsons Field Formation, which has been shown to be contemporaneous with the South Charlo and Bryant Point Formations (Wilson and Kamo 2008).
A third, late Silurian to earliest Devonian hiatus (Salinic “C”) is observed in the Charlo–Jacquet River and Pentland Brook areas (Fig. 1). On the west limb of the Jacquet River Syncline, volcanic rocks of the mainly Ludfordian Benjamin Formation are overlain by a discontinuous but locally thick layer of polymictic conglomerate at the base of the Dalhousie Group (Wilson et al. 2004; Wilson and Kamo 2008). Similarly, at Pentland Brook, the Simpsons Field Formation is overlain by a substantial thickness of polymictic conglomerate at the base of the Tobique Group (Wilson and Burden 2006; Wilson and Kamo 2008). However, these are only local phenomena, as continuous deposition occurred across the Silurian–Devonian boundary elsewhere in the Chaleur Bay Synclinorium (Wilson and Kamo 2008; Fig. 1). This hiatus immediately post-dates Late Silurian volcanism and precedes an earliest Devonian transgressive cycle (e.g., Wilson et al. 2004), suggesting a waning of the thermal anomaly responsible for volcanism, and possibly a link to coeval D3 extensional collapse documented in the adjacent Miramichi Highlands (van Staal and de Roo 1995).
Although it is clear that most or all of the Middle Silurian section is missing throughout northern New Brunswick, the duration of this hiatus has not been quantified until now. Fortunately, about 10 km south of Dalhousie, the base of the dominantly mafic Bryant Point Formation contains rhyolite flows just above the Salinic unconformity. A sample of this rhyolite (sample O/16e-177; Fig. 1) was collected to establish its age of emplacement, and hence the upper limit of the Salinic hiatus. In addition, a sample of rhyolite (sample O/9f-398) was collected from the top of the Benjamin Formation south of Blue Mountain (Fig. 1) to determine the temporal range of Late Silurian volcanism, and to help constrain the timing of the latest Silurian – earliest Devonian (Salinic C) disconformities in the Pentland Brook and Jacquet River areas (Wilson et al. 2004; Wilson and Kamo 2008).
Samples O/16e-177 and O/9f-398 were crushed, pulverized, and passed over a Wilfley table. The resulting heavy mineral concentrates were reprocessed on the Wilfley table until a significantly reduced sample size of ∼5–10 g was achieved (from rock samples weighing ∼8–12 kg). The small heavy mineral concentrates were rapidly processed through mineral separation procedures (i.e., magnetic separation and reduced volumes of methylene iodide of ∼2–8 mL) and did not require the use of the heavy liquid “bromoform.”
U–Pb analysis was carried out by isotope dilution – thermal ionization mass spectrometry (ID–TIMS) methods on zircon grains that were pretreated by chemical abrasion (Mattinson 2005). Zircon crystals were placed in a muffle furnace at 1000 °C for 60 h to repair radiation damage, that is, by thermal annealing of the crystal lattice, followed by partial dissolution in Teflon capsules using 50% HF and 7N HNO3 at 195 °C for 6 h for sample O/16e-177, and 16 h for sample O/9f-398, which also contained 6N HCL. Mattinson’s chemical abrasion procedure (modified here) has the advantage of penetratively removing alteration zones where Pb loss has occurred so that only pristine unaltered zircon is analysed, thus improving concordance. After selecting the zircons, their dimensions were measured, and the weights of each grain were calculated, followed by cleaning in 8N HNO3 and ultra-clean acetone prior to dissolution. A mixed 205Pb–233U–235U spike (the ET535 tracer solution provided by the EARTHTIME NSF project; spike calibration error approximately 0.1%) was added to the Teflon dissolution capsules during sample loading. Zircon was dissolved using ∼0.10 mL of concentrated HF and ∼0.02 mL of 7N HNO3 in teflon bombs at 195 °C (Krogh 1973) for five days, and redissolved in ∼0.15 mL of 3N HCl. U and Pb were isolated from the zircon solutions using 50 µL anion exchange columns using HCl, dried in dilute H3PO4, and deposited onto outgassed rhenium filaments with silica gel (Gerstenberger and Haase 1997). U and Pb were analyzed with a VG354 mass spectrometer using either a Daly detector in pulse-counting mode, or multiple Faraday collectors in multidynamic collection mode. The dead time of the measuring system for Pb was 22.8 and 21.5 ns, and for U, 20.8 and 21.5 ns, for O/16e-177 and O/9f-398, respectively. The mass discrimination correction for the Daly detector is constant at 0.05% per atomic mass unit. Amplifier gains and Daly characteristics were monitored using the SRM982 Pb standard. Thermal mass discrimination corrections are 0.10% per atomic mass unit for Pb; U fractionation was corrected internally on a ratio-by-ratio basis. The total amount of common Pb for each analysis was attributed to laboratory Pb, therefore, no correction for initial common Pb from geological sources was made. Decay constants are those of Jaffey et al. (1971). All age errors quoted in the text and table and error ellipses in the concordia diagrams are 2σ. Plotting and age calculations were done using Isoplot/Ex 3.00 (Ludwig 2003).
In geologically young zircons, the 238U–206Pb isotopic system is more reliable and precise because of the much greater abundance of 238U and its more accurately known decay constant (Schoene et al. 2006; Mattinson 2010). Therefore, the ages presented herein refer exclusively to the 206Pb/238U ages. For sample O/16e-177, four fractions of single and multiple (2–3) fragments of euhedral, elongate, platy zircon were chemically abraded prior to dissolution. These yielded data that are concordant within the decay constant uncertainties (Fig. 11a; Table 1). Three analyses provide overlapping data and have a weighted mean 206Pb/238U age of 422.3 ± 0.3 Ma (mean square weighted deviation (MSWD) is 0.8). One datum is slightly older and plots outside the analytical uncertainty of the cluster, having a 206Pb/238U age of 423.8 ± 0.4 Ma (2σ). It is assumed to have crystallized during an earlier phase of volcanism and was inherited from an older volcanic edifice, possibly entrained during eruption at 422.3 Ma, or earlier in the magma chamber.
In sample O/9f-398, abundant zircon grains are generally small, euhedral, equant to 2 : 1 prisms, but a number of larger euhedral to slightly rounded crystals and fragments are also present. Most grains are hazy (pitted surfaces?), and many show some degree of alteration. Three larger fragments and one equant euhedral grain were selected for analysis; all grains were chemically abraded prior to dissolution. The data for the four grains (Fig. 11b; Table 1) are concordant, equivalent, and have a weighted mean 206Pb/238U age of 419.69 ± 0.33 Ma (2σ). This age is interpreted as the best estimate for the time of eruption of the rhyolite.
From these data, the duration of the Salinic hiatus can be established as 5–6 million years, ranging from the early Sheinwoodian (ca. 427–428 Ma) to mid-Gorstain (422 Ma), and the duration of Late Silurian volcanism is demonstrated to be 2.6 million years.
Discussion: Silurian stratigraphic nomenclature
New U–Pb ID–TIMS zircon data confirm a 5–6 million years Middle Silurian hiatus throughout northern New Brunswick, a period characterized by the development of erosional disconformities, angular unconformities, and northwest-trending folds. This interval coincided with a switch from sinistral-oblique convergence of Ganderia and composite Laurentia, to dextral-oblique convergence of Avalonia (van Staal and de Roo 1995; Malo 2001; van Staal et al. 2009). Paralleling the change in geodynamic setting was a transition in depositional environments from an Early Silurian (Salinic) forearc basin to an Acadian foreland basin following Salinic forearc basin inversion (Malo 2001; Wilson et al. 2004). It follows that, for the regional stratigraphic framework to more accurately reflect tectonic history, Upper Silurian rocks in northern New Brunswick should not be included in the Chaleurs Group. This is especially true of the Upper Silurian section in the Charlo – Jacquet River area, which consists of a within-plate, subaerial volcanic edifice that is bounded by disconformities. Furthermore, to be consistent with the North American Stratigraphic Code (Article 19g: “When a unit is divided into two or more of the same rank as the original, the original name should not be used for any of the divisions”) the Lower Silurian section in northern New Brunswick has also been assigned a new higher rank designation. It is proposed that Upper Silurian sedimentary rocks in the Nigadoo River Syncline be assigned to the Petit Rocher Group (new name), the Upper Silurian volcanic and volcaniclastic succession in the Charlo – Jacquet River area to the Dickie Cove Group (new name), and Lower Silurian rocks to the Quinn Point Group (new name). However, the original usage and application of “Chaleurs Group” in the Gaspé Peninsula should remain intact, as this area hosts the type section and the name enjoys precedence and a long history dating back to the turn of the 20th century. West of Campbellton, Upper Silurian sedimentary rocks are contiguous with the West Point and Indian Point formations on the Québec side of the Restigouche River; these units are defined at the Chaleurs Group type locality and should, therefore, remain part of the Chaleurs Group. The proposed nomenclatural revisions for New Brunswick are illustrated on a geological map (Fig. 12) and on a stratigraphic correlation chart (Fig. 13).
The Quinn Point Group is named for a prominent headland on the coast of Chaleur Bay about halfway between Belledune and Jacquet River (Fig. 12). At this location, the Quinn Point Group consists of the Weir and La Vieille formations, which are exposed almost continuously for 1800 m. At the west end of the section, the Weir Formation is juxtaposed against Lower Devonian rocks by a fault along Armstrong Brook. East of Quinn Point, the La Vieille Formation is overlain (although the contact is not exposed) by poorly exposed conglomerate of the South Charlo Formation; this conglomerate is reported to contain clasts of La Vieille limestone (Alcock 1935, p. 52), demonstrating the disconformable nature of the contact. Just south of Pointe Rochette, underlying about 100 m of Weir conglomerate along the northern margin of the Nigadoo River Syncline (Fig. 12), the oldest rocks in the Quinn Point Group are calcareous sandstones and interbedded calcarenites assigned to the Madisco Brook Formation (Wilson 2011). These rocks were included in the Clemville Formation by Walker and McCutcheon (1995); however, the Clemville in the Gaspé Peninsula is regarded as early Rhuddanian in age (Boucot and Bourque 1981; Bourque and Lachambre 1980), whereas the Madisco Brook Formation is significantly younger, as indicated by a Telychian conodont assemblage (with some elements similar to those in the Anse à Pierre – Loiselle Formation, which underlies the La Vieille Formation in the Gaspé Peninsula; A.D. McCracken, pers. comm. 1993) and a 429.2 Ma U–Pb (zircon) date from a thin bed of felsic tuff at the top of the unit, just below the Weir Formation (Wilson et al. 2008; Fig. 12). It is recommended that the Clemville Formation be abandoned in New Brunswick.
West of the Black Point – Arleau Brook Fault (Fig. 12), the La Vieille and Limestone Point formations are underlain by deeper water rocks of the Upsalquitch Formation, which consists of grey, thin-bedded, typically calcareous siltstones deposited in an upper slope to shelf environment (St. Peter 1978; Wilson et al. 2004). The Upsalquitch Formation gradationally overlies calcareous turbidites of the Matapédia Group (e.g., St. Peter 1978; Wilson et al. 2004).
The Dickie Cove Group disconformably overlies the Quinn Point Group in the Charlo – Jacquet River area (Fig. 12); it comprises volcanic rocks of the Bryant Point and Benjamin formations, and coarse-grained volcaniclastic rocks of the South Charlo and New Mills formations. At its type section, and in scattered other locations, the South Charlo Formation forms the base of the Dickie Cove Group; however, the South Charlo is absent in many areas, so the Bryant Point Formation commonly disconformably overlies either the La Vieille or Limestone Point formation. The New Mills Formation overlies the Bryant Point Formation and underlies the Benjamin Formation, which forms the upper part of the Group. The Dickie Cove Group is disconformably overlain by the Dalhousie Group on the northwest limb of the Jacquet River Syncline (Figs. 12, 13); because of poor exposure, it is uncertain whether a disconformity also exists on the southeast limb of the syncline.
The Petit Rocher Group is restricted to the Nigadoo River Syncline, including its western extension on the south side of the Rocky Brook – Millstream Fault (Fig. 12), and consists of the Simpsons Field, LaPlante, and Free Grant formations. The type section of the Petit Rocher Group is the same as that defined for the Petit Rocher Formation of Noble (1976), on the Chaleur Bay coast between Little Elmtree River and the unconformable contact with the La Vieille Formation north of Point Rochette. The Petit Rocher Formation was abandoned by Walker and McCutcheon (1995). Noble (1976, p. 541) reported a “late Llandovery C4 to Wenlock and possibly Ludlow” age for the unit and stated that it is a lateral facies equivalent of the La Vieille and Limestone Point formations. However, it is believed that the Llandovery–Wenlock (Telychian–Sheinwoodian) age assignments were based on fossils from rocks that were incorrectly included with his Petit Rocher Formation, as the rocks herein assigned to the Petit Rocher Group are unfossiliferous in the area examined by Noble (1976). The Petit Rocher Group unconformably overlies Ordovician rocks on the south limb of the Nigadoo River Syncline, and disconformably overlies the Quinn Point Group on the north limb. To the west, it is conformably and gradationally overlain by the Devonian Greys Gulch Formation (Tobique Group; Fig. 12).
The Silurian stratigraphic record in northern New Brunswick contains evidence of up to three unconformities–disconformities that can be attributed to tectonism associated with convergence and terminal oblique collision of Ganderia and Laurentia (the Salinic Orogeny). The earliest of these (Salinic “A”; ca. 435–440 Ma) is represented by onlap of Silurian rocks (mainly Weir Formation) onto Ordovician rocks (mainly Fournier Group) that were deformed in the Brunswick Subduction Complex and exhumed in the Early Silurian. The effects of Salinic “B” tectonism, associated with the terminal late Early Silurian Ganderia–Laurentia collision, are widespread in the Matapédia successor basin, and consist of northwest-trending folds without cleavage in Lower Silurian rocks, and a post-Telychian erosional hiatus that preceded deposition of subaerial, within-plate volcanic rocks of the Bryant Point and Benjamin formations. Discussion of the possible causes (kinematics, stress regimes, etc.) for the northwest-trending folds is speculative at this point and in any event is beyond the scope of this paper; however, it is clearly a topic for future study. The duration of the Salinic “B” hiatus, based on the U–Pb zircon age of rhyolite at the base of the Bryant Point Formation, is 5–6 million years (422–ca. 428 Ma). Upper Silurian rocks are locally disconformably overlain by polymictic conglomerates that form the base of the Devonian section (Tobique and Dalhousie groups); this depositional break (Salinic “C”, ca. 416 Ma) may be associated with local crustal isostatic adjustments related to slab breakoff (cf. van Staal et al. 2009).
This active tectonic history is matched by a complex Silurian stratigraphic record that varies from place to place; all Silurian rocks have in the past been assigned to the Chaleurs Group, but most strata cannot be readily correlated with the type area of the Chaleurs Group in the Gaspé Peninsula, where the Silurian stratigraphic record is reported to be continuous. In particular, Upper Silurian sections in northern New Brunswick are markedly different from each other and from the type area, supporting the introduction of new higher rank names. In the Nigadoo River Syncline, a transgressive sequence of near-shore to fully marine sedimentary rocks is assigned to the Petit Rocher Group. In the Charlo – Jacquet River area, 422.3–419.7 Ma subaerial volcanic rocks are bounded by disconformities below and above; these rocks are assigned to the Dickie Cove Group. Lower Silurian sedimentary rocks, including near-shore to shallow-water rocks around the Elmtree Inlier, and deeper water, thin-bedded turbidites in the Restigouche–Upsalquitch area, are assigned to the Quinn Point Group. The Quinn Point Group unconformably to disconformably underlies the Dickie Cove and Petit Rocher groups. Upper Silurian rocks west of Campbellton that are contiguous with units of the Chaleurs Group defined in the type area, will remain in the Chaleurs Group.
Careful and insightful reviews by Les Fyffe and Jim Walker greatly improved the draft manuscript. The paper has also benefitted from comments and suggestions by journal referees Cees van Staal and Alex Zagorevski. Thanks to Cees van Staal and Brendan Murphy for encouraging this submission to the Ward Neale special issue.
1 This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.
- Received April 4, 2011.
- Accepted May 30, 2011.
- Published on the NRC Research Press Web site at http://cjes.nrc.ca on December 12, 2011.
- Published by NRC Research Press