Church Point formation

12 In the northwestern area of Meguma, most of the Goldenville Group consists of grey, medium- to thick-bedded metasandstone, locally interlayered with green, cleaved meta-siltstone, and rare black slate (Fig. 4a). This thick (>8000 m) metasandstone-dominated unit has been named the Church Point formation (White 2008b). The formation is well exposed along the coast from Salmon River to Cranberry Point (Fig. 2), which can be designated as the reference section, following the criteria established in the North American Commission on Stratigraphic Nomenclature (2005). The base is not exposed but the lowermost strata are seen in the core of the Black Point anticline. Large (up to 50 cm – long axis) brown carbonate-rich concretions are locally abundant. Metasandstone beds typically are laminated to cross-laminated or display convolute laminations near the top but the lower parts are devoid of sedimentary structures, possibly due to fluidization by water escape. Sole marks such as flute and load casts are locally common. Such beds have been described elsewhere in the Goldenville Group and interpreted to be the result of high-concentration turbidity currents (e.g., Waldron and Jensen 1985; Waldron 1987). Limited paleocurrent data from the Church Point formation indicate southward paleoflow (Waldron et al. 2009), in contrast to flow to the northeast reported elsewhere in the Goldenville Group (Schenk 1981; Waldron and Jensen 1985; Waldron 1988). The stratigraphically lowest and highest meta-sandstone beds in the formation yielded concordant detrital zircon ages of 544 ± 18 and 529 ± 19 Ma, respectively, limiting the maximum depositional age of the base and top of the formation (Waldron et al. 2009), so that the unit could be as old as the Ediacaran of the uppermost Neoproterozoic.

13 The metasandstone is typically very fine- to medium-grained and poorly sorted. Although the original mud matrix has recrystallized to a mixture of sericite, chlorite, and epidote, subangular to rounded sand-sized detrital grains are present still in many samples. However, locally where regional or contact metamorphism is at higher grades, the matrix contains abundant coarse muscovite and biotite, and individual sand-sized grains exhibit granoblastic textures. The composition of the metasandstone is dominantly feldspathic wacke with subordinate quartz- and lithic wacke and rare feldspathic arenite (White 2010). Coarse sandstone to granule conglomerate beds occur locally in the lower half of the Church Point formation where they form lenses (1 to 2 m by 10 to 20 cm thick) with irregular, erosive bases and flat tops, and probably represent channel fills. They are matrix supported, poorly to well sorted, and massive to normally graded. Lithic fragments are dominantly polycrystalline quartz (quartzite) with subordinate volcanic clasts of pilotaxitic basalt and andesite, porphyritic rhyolite and dacite, and recystallized felsic crystal tuff (White and Barr in press). Rare plutonic fragments are tonalitic in composition (quartz and euhedral plagioclase).

14 About 4 km up-section in the Church Point formation near High Head, on the southeastern limb of the Black Point anticline (Fig. 2), a thick (<1 km) interval of greyish-green, parallel-laminated, (rarely cross-laminated), magnetite-rich, metasiltstone to slate (Fig. 4b) has been termed the High Head member (White et al. 2005a, b). The base of the member is marked by an abrupt decrease in abundance of metasandstone beds whereas the upper contact is marked by an increase in metasandstone beds. The High Head member metasiltstone locally contains a distinctive trace fossil assemblage ranging from irregular to meandering trails on bed surfaces (Fig. 4c), including the Early Cambrian Oldhamia ichofossils (White et al. 2005b). These trace fossils suggest a Nereites ichnofacies indicative of a deep-water environment (Gingras et al. in press). The matrix has typically recrystallized to a mixture of sericite, chlorite, and epidote with scattered grains of quartz and plagioclase. The composition of the metasandstone is dominantly feldspathic wacke (White 2010). The high magnetite content has resulted in a relatively high aeromagnetic response with respect to the rest of the Church Point formation, allowing the High Head member to be traced under glacial till into inland areas where it pinches out (Fig. 2). This dominantly fine-grained succession likely was deposited from suspension in a relatively quiet abyssal plain setting, although the presence of local cross-laminations indicates the existence of near-bottom currents.

Display large image of Figure 4

15 The High Head member includes several massive meta-sandstone beds in its central part. They are up to 3 m in thickness, displaying parallel lamination and normal grading with coarse-grained sand lenses and tabular cross-bedding. Like the metasandstone in the rest of the Church Point formation, these beds are interpreted to be the result of episodic high-concentration turbidity currents. The youngest detrital zircon extracted from the uppermost metasandstone bed has an age of 537 ± 15 Ma, indicating the maximum depositional age and consistent with similar (within error) ages obtained from the base and top of the formation (Waldron et al. 2009).

16 Although a few minor faults cut the member, a complete stratigraphic succession is exposed along the coast at High Head (Fig. 2) where the type section is defined (e.g., Gingras et al. in press).

Bloomfield formation

17 The upper part of the Goldenville Group in the northwestern area (northwest of the Chebogue Point shear zone; Fig. 2) is the metasiltstone-dominated Bloomfield formation (Bloomfield member of White et al. 1999, 2001 and Horne et al. 2000). This unit is exposed on the limbs of the regionally extensive Black Point anticline. It is typically 200 to 400 m thick on the southeastern limb, but increases to 1000 m thick on the northwestern limb. The variation in thickness is interpreted to be due to transposition and related thickening by the Cape St. Marys and Chebogue Point shear zones (Culshaw and Leisha 1997; White et al. 2001).

18 The Bloomfield formation consists of mainly distinctly banded maroon and green, thin- to medium-bedded meta-siltstone to slate (Fig. 4d). Some of the thin metasiltstone beds display small-scale cross-lamination. Along strike to the southwest where regional metamorphism is at biotite and garnet grade, the metasiltstone is more phyllitic to schistose and light greyish-green. Thin (<10 cm thick) metasandstone layers are rare and are typically very fine grained and poorly to moderately sorted. The composition of the metasandstone is dominantly feldspathic wacke (White 2010). Deposition of the Bloomfield formation may have been from dilute silty to muddy turbidity currents.

19 A complete section of the Bloomfield formation is not exposed. The best exposures, however, are in an abandoned quarry near Bloomfield and along the coast at Cranberry Point (Fig. 2). The section at Cranberry Point could be designated as the type section because a sharp contact between metasiltstone of the Bloomfield formation and underlying massive metasandstone of the Church Point formation is well exposed. However, the grade of metamorphism is sufficiently high that the original sedimentary features have been obliterated. In contrast, in the abandoned quarry the contact with the underlying Church Point formation is not exposed but grade of metamorphism is low and the original lithologies and textures are better preserved. The thick section exposed in Meteghan River on the northwestern limb of the Black Point anticline could also be considered as a type section but at this location the rocks are more deformed and structurally complex.

20 Although originally assigned to the lowermost part of the Halifax Group, regional mapping has resulted in reassignment to the Goldenville Group (White 2008b) (see Discussion).

Acacia Brook formation

21 The Acacia Brook formation (Acacia Brook member of White et al. 1999) is the basal unit in the Halifax Group (see Discussion) in the northwestern area and forms two distinct areas of outcrop: Cape St. Marys and Bear River (Fig. 2, 3). As measured near Bear River, the formation is about 820 m thick and consists of grey to dark grey, laminated slate with minor, thin (<5 mm thick) beds and lenses of light grey meta-siltstone, as well as beds of cross-laminated fine- to medium-grained metasandstone up to 20 cm thick. The metasandstone is dominantly feldspathic wacke but is more quartz-rich than the metasandstone beds in the High Head member and Bloomfield formation (White 2010). Sulphide minerals (py-rite, pyrrhotite, and arsenopyrite) are locally abundant. Trace fossils are rare. The slate-metasiltstone and metasandstone are interpreted as low- and high-concentration turbidites, respectively. The abundance of graphite and sulphide minerals suggest deposition under anaerobic sea-floor conditions during a period of basin-wide stagnation (Waldron 1987, 1992).

22 The outcrop along Sissiboo River, east of Weymouth (Fig. 2), provides the best exposure of the Acacia Brook formation and is designated the type section. Here the contact is sharp and conformable with the underlying Bloomfield formation (Horne et al. 2000). However, farther to the southwest the lower contact with the Bloomfield formation is tectonic and marked by the Cape St. Marys and Chebogue Point shear zones (Culshaw and Leisha 1997; White et al. 2001).

Bear River formation

23 Like the Acacia Brook formation, the Bear River formation (Bear River member of White et al. 1999; Sissiboo member of Horne et al. 2000) outcrops in two areas (Fig. 2). It forms the upper unit in the Halifax Group west of the Chebogue Point shear zone, separated by an inferred regional disconformity from the overlying Late Ordovician and younger White Rock and Torbrook formations (Fig. 3). The most continuous section of the Bear River formation exposed along the lower part of Bear River (Fig. 2) in the shallow southeastern limb of the Black Point anticline is designated as the type section (White et al. 1999). Here the minimum thickness of the formation is about 1370 m. The Bear River formation consists predominantly of light to dark grey, well laminated, cleaved metasilt-stone, with minor, thin (<5 cm thick) slate beds (Fig. 4e). These metasiltstone beds are a characteristic feature of this formation and are typically discontinuous, and lenticular with cross-laminations. Grey, massive, fine-grained metasandstone beds, up to 50 cm thick, are locally present. Like metasandstone in the older units, its composition is dominantly feldspathic wacke (White 2010).The metasiltstone and metasandstone typically contain abundant detrital muscovite. Carbonate-rich lenses and beds (up to 5 cm thick) locally occur. Pyrite and arsenopyrite are less common than in the underlying Acacia Brook formation but locally occur in beds up to 2 cm thick. The lower contact of the Bear River formation is gradational and placed above the highest occurrence of thick cross-bedded metasand-stone of the Acacia Brook formation.

24 Primary sedimentary structures are common in the Bear River formation and include cross-laminations and graded-bedding, groove, tool, and load marks, ripples, and small-scale slump features. Fossil occurrences are generally sparse, although trace fossils (burrowing and grazing) and bioturbated layers are common. Trace fossils are interpreted to be of the Zoophycos ichnofacies formed in mainly silty turbidites in an oxygen-rich, upper slope to muddy outer shelf environment (e.g., Schenk 1997). Early Tremadoc acritarch microfossils were reported by Doyle (1979) and confirmed by Palacios et al. (2009). The upper part of the Bear River formation locally contains the Early Ordovician graptolite Rhabdinopora flabelliformis (White et al. 1999) (Fig. 4f).

Moses Lake formation

25 The lowermost strata exposed in the southeastern area were named the Moses Lake formation by White (2008b) and occur in the cores of north- and northeast-trending anticlines in the west in and around the town of Wedgeport (Fig. 2). There, the formation is exposed in scattered outcrops and because of its relatively high aeromagnetic response with respect to the over-lying Green Harbour formation, can be traced into till-covered inland areas as well as into the offshore (White 2003). Although the base of the formation is not exposed, the minimum thickness is about 800 m. A designated type-section is difficult to erect given the scattered distribution of outcrop and lack of continuous exposures. However, the outcrops in and around Moses Lake are indicative of the lithologies in this formation.

26 The Moses Lake formation consists mainly of grey, thinly-to medium-bedded metasandstone, interlayered with minor green, cleaved metasiltstone. The composition of the metasandstone ranges from feldspathic to quartz wacke with rare quartz arenite (White 2010). Beds of black, magnetic slate with abundant sulphide minerals (pyrite, pyrrhotite, and arsenopyrite) are a distinctive feature of the formation. Large (up to 50 cm – long axis), light-grey to buff pink, calc-silicate concretions and thin calc-silicate beds (<5 cm) are locally abundant in the metasandstone. Sedimentary structures are common and include cross-laminations, graded bedding, and ripple marks. The clastic lithologies are typically spotted with biotite and cordierite as a result of contact metamorphism by the Wedgeport Pluton (MacLean et al. 2005). The very fine-grained, well-sorted metasandstone-slate package has characteristics typical of classical distal tubidites (e.g., Mutti 1992).

Green Harbour formation

27 The Green Harbour formation (New Harbour member of O’Brien 1988; Green Harbour member of White 2005) is the most extensive unit in the southeastern area and occurs in the cores of northeast-trending anticlines (Fig. 2). Although a complete, uninterrupted section does not exist, the good exposures along the coast of Green Harbour are designated as the type section (Fig. 2). The base of the formation is placed at the top of the most prominent magnetic slate bed in the underlying Moses Lake formation that coincides also with an upward increase in metasandstone grain size and feldspar content (White 2010). The formation is about 4000 m thick and is dominated by grey, thick-bedded, medium-grained metasandstone (Fig. 5a). The metasandstone is locally interbedded with minor green, cleaved metasiltstone and rare black rusty slate. Large (up to 50 cm – long axis) brown carbonate-rich concretions and beds (<10 cm thick) are locally abundant (Fig. 5b). Most metasandstone beds are internally featureless but typically laminated to cross-laminated and rippled in the uppermost 10 cm. The composition of the metasandstone ranges from dominantly feldspathic to quartz wacke with subordinate feldspathic to quartz arenite (White 2010). Like metasandstone in the Church Point formation, the metasandstone displays de-watering structures and was likely the product of deposition from high-concentration turbidity currents (Waldron 1987, 1992; O’Brien 1988; Schenk 1997). Rare bedding-plane grazing trace fossils are present. Paleocurrent data indicate a northwestward paleoflow (Waldron 1987, 1988; Waldron et al. 2009), in contrast to the more north-directed paleoflow reported by Schenk (1981). The Green Harbour formation displays uniform low aeromagnetic response throughout the area (King 2006).

28 The Lake Rossignol member was recognized recently as a distinct unit in the upper part of the Green Harbour formation in the northeastern part of the southern area (White 2006). The type section of the member is exposed in scattered out-crops on the shores of Lake Rossignol; however, a quarry near Middlewood displays the typical lithologies of the unit. It is approximately 1 km thick, and consists of rhythmically layered (2 to 3 m thick) grey, fine-grained metasandstone (Fig. 5c). Many of the beds have conglomeratic bases with thin (<20 cm) well laminated metasiltstone at the top. The composition of the metasandstone ranges from dominantly feldspathic to quartz wacke with subordinate feldspathic to quartz arenite (White 2010). Large, up to 2 cm wide, pyrite cubes are locally abundant in the metasandstone. The base of the unit appears to be erosional into underlying metasandstone of the Green Harbour formation and is composed of poorly sorted, granule to cobble metaconglomerate in a coarse-grained metasand-stone matrix. The top of the unit is not exposed. The Lake Rossignol member displays relatively uniform high aeromagnetic response throughout the area but is not laterally extensive and ‘pinches-out’ to the southeast (King 2006). Like metasand-stone units in the rest of the Green Harbour formation, these beds are probably the result of high-concentration turbidity currents (e.g., Waldron 1992) but their coarser grain size suggests that they are more closely related to channelized debris-flows.

Display large image of Figure 5

Government Point formation

29 In areas of low metamorphic grade, the Government Point formation is typically composed of grey, thinly to thickly bedded metasandstone, rhythmically laminated green to grey-ish-green metasiltstone, and black slate (Fig. 5d). As in the Green Harbour formation, calc-silicate nodules are common in the metasandstone and are locally manganese-rich. In the Kejimujik area the formation is dominantly well-laminated green to greyish-green to purple metasiltstone with minor metasandstone. In this area manganiferous staining along bedding planes and fractures is common. At higher regional metamorphic grade and around the margins of plutons, the more pelitic beds are composed of andalusite-garnet-sillimanite schist, granofels, and migmatite. In the Kejimujik area rare coticules were observed in contact metamorphic aureoles. The composition of the metasandstone ranges from dominantly feldspathic to quartz wacke with subordinate feldspathic to quartz arenite (White 2010). A bioclastic limestone near the top of the formation in the area mapped by O’Brien (1988) farther to the east yielded a Middle Cambrian trilobite faunule (Pratt and Waldron 1991). A pelmatozoan-bearing calc-silicate nodule at a similar stratigraphic level to the west was reported by White (2006). Grazing and burrowing trace fossils (Fig. 5e) are abundant (Waldron 1987; White 2005, 2006). Rare quartz-pebble conglomerate beds form deeply scoured channels that can be traced locally for 10’s of meters along strike. They are probably the result of high-concentration turbidity currents (e.g., Waldron 1987). The type section is well exposed along the coast at Government Point where the maximum total thickness of the formation is about 1750 m.

30 The contact between the Government Point formation and underlying Green Harbour formation is marked by an increase in more silty interbeds and thinning of the massive metasand-stone beds over an interval 100 to 300 m and the contact is placed where the silty interbeds exceed 30–40%. The contact also coincides with a higher positive aeromagnetic response that can be regionally traced in the southeastern area. Overall, the Government Point formation is characterized by thin, alternating bands of high and low aeromagnetic response that reflect the high magnetite content of the interlayered silty beds.

31 To the northeast in the Mahone Bay area (Fig. 2) the equivalent stratigraphic level was assigned to the Tancook member (O’Brien 1985; Waldron 1987) whereas in the Lahave Islands area (Fig. 2) the lower 100 to 300 m was assigned to the Rissers Beach member and the upper part to the West Dublin member (O’Brien 1985). These members, including the overlying Moshers Island member (see Discussion section) were included in the informally defined Green Bay formation of O’Brien (1985).

Moshers Island formation

32 The Moshers Island formation (Snug Harbour Sandstone of O’Brien 1986; Moshers Island member of O’Brien 1988) is a thin (<300–500 m wide) unit of green to greenish-grey to grey to purple, well laminated metasiltstone to slate, inter-layered with minor, fine-grained, 1 to10 cm-thick metasand-stone beds (Fig. 5f). A characteristic feature in this unit is the presence of steel-blue manganiferous nodules and laminations. At slightly higher metamorphic grades these nodules and laminations form thin (up to 10 cm thick), pink coticule beds and lenses due to the growth of spesssartine garnet (Fig. 5g). Carbonate laminations and concretions are locally present and some contain rare ooid-like structures. Rare burrowing and grazing trace fossils have been observed. Some of the best exposures of the formation are on Moshers Island, herein designated as the type section, but the most complete section is located on Big Tancook Island (e.g., O’Brien 1985; Waldron 1987). The contact with the underlying metasandstone-dominated Government Point formation is gradational. However, following the criteria established by O’Brien (1988) the contact is placed at the top of the highest decimetre-thick metasand-stone in the underlying Government Point formation. Hence, this unit is included in the Goldenville Group, rather than in the Halifax Formation as done by earlier workers (O’Brien 1986, 1988; Waldron 1992; Schenk 1995). An additional argument in favour of including the unit with the Goldenville Group is its sharp and easily mappable contact with the over-lying Cunard formation (described below), from which it is lithologically distinct (see Discussion for further details).

33 Sedimentary structures are generally absent, which led Waldron (1987, 1992) to suggest deposition in relatively deep, generally quiet-water conditions. However, the presence of ooids suggests that the water depth may not have been great, unless the ooids were transported from shallower depths. Metasandstone beds in this unit have not yet been sampled for petrographic analysis.

Cunard formation

34 The Cunard formation (Cunard member of Hall 1981 and O’Brien 1986, 1988) consists predominantly of black to rust-brown slate with thin beds and lenses of minor black meta-siltstone. Cross-laminated, fine- to medium-grained, poorly sorted, pyritiferous metasandstone beds, up to 30 cm thick, are locally present (Fig. 5h). The composition of the metasand-stone is dominantly feldspathic wacke with subordinate quartz wacke (White 2010). It locally contains abundant pyrite, arsenopyrite, and pyrrhotite beds up to 5 cm thick. Trace fossils are rare. The formation is about 2250 m thick. The best exposures are along the Atlantic coast in and around the Ovens area farther to the east where O’Brien (1988) established the type section.

35 Like the stratigraphically equivalent Acacia Brook formation in the northwestern area, the slate-metasiltstone and metasandstone are interpreted as low- and high-concentration turbidites (e.g., Stow et al. 1984). The abundance of graphite and sulphide minerals suggest deposition under anaerobic sea-floor conditions during a period of basin-wide stagnation (cf., Waldron 1992; Schenk 1997). However, as in the underlying Moshers Island formation, ooid-rich thin beds are present which suggests that some parts may have been deposited in more shallow water or that these micro-structures were transported into deep water.

36 The basal contact of the Cunard formation with the underlying Moshers Island formation is exposed in several localities. It is conformable, sharp, and placed at the first appearance of black rusty slate.

Feltzen formation

37 The overlying Feltzen formation (Feltzen member of Hall 1981 and O’Brien 1985, 1986, 1988) occurs in the cores of northeast-trending synclines in the Mahone Bay, Sherbrook Lake, and Maitland Bridge areas (Fig. 2). It is composed of light grey to blue-grey slate, rhythmically interlayered with laminated to thinly bedded, fine-grained metasandstone. The best exposure of this formation is near Blue Rocks (Fig. 2) in the area mapped by O’Brien (1988) and this location is considered the type section. However, excellent outcrops occur in the Mersey River near Maitland Bridge although access is restricted as many of the outcrops are in the Kejimkujic National Park. In contrast to the Cunard formation, abundant worm burrows are a characteristic feature of the Feltzen formation (O’Brien 1988), as well as numerous other trace fossils. The lower boundary with the Cunard formation was considered gradational, and placed at the first occurrence of black slate (e.g., O’Brien 1988). This position also correlates with the disappearance of sulphide minerals. The stratigraphic top is not exposed, and therefore an estimated minimum thickness is about 2 km. Rare graptolites indicate an Early Ordovician age (e.g., Cumming 1985). Like the Bear River formation, the Feltzen formation likely formed as mainly silty turbidites in an oxygen-rich, shelf environment (e.g., Schenk 1997).