The Early Silurian Gun River Formation of Anticosti Island, Eastern Canada: a Key Section for the Mid-Llandovery of North Ame

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The Early Silurian Gun River Formation of Anticosti Island, eastern Canada: A key section for the mid-Llandovery of North Ame....

Article in Newsletters on Stratigraphy · November 2012 DOI: 10.1127/0078-0421/2012/0024

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The user has requested enhancement of the downloaded file. Newsletters on Stratigraphy, Vol. 45/3, 263–280 Article Stuttgart, November 2012

The Early Silurian Gun River Formation of Anticosti Island, eastern Canada: A key section for the mid-Llandovery of North America

Paul Copper1, Darrel G. F. Long2, and Jisuo Jin3

With 8 figures

Abstract. The 85–100 m thick carbonate succession of the Gun River Formation is revised on the basis of nearly continuous cliff sections measured on the northeast coast, and a re-analysis and re-measurement of the stratotype sections on the southwest coast of Anticosti island, Québec. The Gun River Formation (Llan- dovery: lower Aeronian) is herein formally divided into four members, from the base up, the Lachute, In- nommée, Sandtop and Macgilvray members. The formation characterizes a generally deepening phase on the northwestern shelf of Laurentia, bordering the Iapetus ocean in this region. Since the base of the formation is located ca. 150 m above the O/S boundary, it provides fresh data as to the nature of the Early Silurian trans- gression, radiation, and re-invasion of shelf communities following the Late Ordovician mass extinctions. The Gun River strata were deposited in a mid- to distal, tropical, very gently sloping (Ͻ 1–3 m per km), carbonate ramp or shelf, some 40–80 km offshore from the stable Laurentian craton, and at water depths of 30 to 60 m. Much of the formation consists of rhythmically deposited, light grey micrites (with fissile muddy partings), and minor calcareous mudstones, punctuated by abundant hardgrounds, intraformational conglomerates, and shell-beds. The fauna of brachiopods and corals is generally of low diversity, marking a turnover from a latest Rhuddanian community (Merrimack–lower Gun River Fm) to a more diverse mid- Aeronian fauna (top Gun River–lower Menier Fms) when large pentamerids become abundant. This indi- cates gradual changes rather than abrupt invasions, sustained deposition, and lack of major sedimentary gaps between stratigraphic units. The absence of shallow water reefs, skeletal stromatolites, and ooid or oncoid shoals, also suggests deposition near or periodically below the storm wave base on a carbonate platform in southern tropical latitudes at 20°–25° S.

Key words. Gun River Formation, stratigraphy, Early Silurian, Anticosti Island, eastern Canada

1. Introduction and background Telychian (Llandovery, Early Silurian). This is one of the most complete, thickest, most richly fossiliferous The stratigraphic succession exposed on Anticosti and diagenetically unaltered carbonate sections in the Island, in the Gulf of St Lawrence, contains strata world across the Ordovician/Silurian (O/S) boundary, ranging in age from the late Katian and Hirnantian serving as an alternate boundary stratotype sequence (Late Ordovician) through to as young as the mid- (Barnes 1988, 1989). These ca. 800 m thick strata, out-

Authorsʼ addresses: 1 Corresponding author: Paul Copper, Loupicoubas, 46220 Prayssac, France. E-Mail: [email protected] 2 Darrel G. F. Long, Department of Earth Sciences, Laurentian University, Sudbury, P3E 2C6, Canada. E-Mail: dlong@laurentian.ca 3 Jisuo Jin, Department of Earth Sciences, Western University, London, N6A 5B7, Canada. E-Mail: [email protected]

© 2012 Gebrüder Borntraeger, Stuttgart, Germany www.borntraeger-cramer.de DOI: 10.1127/0078-0421/2012/0024 0078-0421/2012/0024 $ 4.50 264 P.Copper et al. cropping for 240 km along strike (Fig. 1), represent a A to F in a stratigraphically upward order. The fossils continuous marine succession, primarily of storm-in- collected by Richardson, and later reported in various fluenced carbonate ramp to mid-shelf facies (Long papers by Billings (1862, 1865, 1866), provided the 2007). There was a very low paleoslope over this dis- initial biostratigraphic backbone to date the rocks. tance, as for example in the overlying upper Menier Richardson’s (1857, pp. 218–226) ʻDivision Dʼ en- Formation, the East Point reefs reached sea-level some compassed Lower Silurian strata, but its basal beds 80 km offshore (Copper and Jin 2012). Sporadic patch were referred later by Schuchert and Twenhofel (1910) reef complexes are distributed within the overall se- to the underlying Becscie Formation, ca. 110 ft (34 m), quence, but reefs are absent in the Gun River Fm, above what is today usually regarded as the top of the despite reports of such by Twenhofel (1928), and oth- Ordovician System. The upper part of Division D cor- ers. Erosional gaps, scour channels, ball-and-pillow responds to the Gun River Fm of Schuchert and Twen- structures, intraformational flat pebble conglomerates, hofel (1910) and up to the East Point coral patch reefs and broken, reworked coral skeletons may be attrib- of the Menier Formation above (Fig. 1; see Copper and uted to subtidal storm and hurricane effects, and in part Jin 2012, for revisions). Thus Richardson’s Division D to bio-erosion (Long 1997, 2007). Tsunamis, initiated spanned not only the Gun River strata of Schuchert by faulting in the Appalachian high to the south and and Twenhofel (1910), but also those beneath and east, may have influenced sedimentation, but this is above that formation. Logan et al. (1863), in part sum- difficult to distinguish from the influence of intense marizing Richardson’s work, separated the Anticosti storms (Long 2007). Submarine hardgrounds and fir- section into four divisions, with ʻDivision 2ʼ contain- mgrounds are common, associated with carbonate sed- ing rocks that may be assigned to the Gun River For- iment starvation. Siliciclastics are very rare in the Sil- mation in current usage. They identified a south coast urian of Anticosti, and occur primarily in the eastern section stretching from the Otter River (= Laloutre outcrop belt in the late Ordovician units exposed (Cop- River) to near the Jupiter River, consisting of ʻash-grey per and Long 1990). Siliciclastic grain content in the slightly bituminous limestoneʼ, through unit 9, ʻash- Gun River carbonates and shales is trivial. grey, smoke-grey, and reddish grey, slightly bitumi- Richardson (1857) was the first to map the geology nous limestoneʼ, a total of 320 ft (98 m) of strata. This and measure the strata of Anticosti Island: he divided is fairly close to the total thickness of our newly meas- the sedimentary sequence into six units, labeled from ured Gun River sections on the SW coast.

Fig. 1. Geological map of Anticosti Island (inset Gulf of St Lawrence) showing the Gun River Formation underlain by the Merrimack, and overlain by the Menier formations. (Modified from Copper and Jin 2012.) The Early Silurian Gun River Formation of Anticosti Island, eastern Canada 265

Fig. 2. Stratigaphic ranges of common fossil groups in the Aeronian Gun River Formation (Lower Silurian), Anticosti Is- land.

The Gun River Fm was established by Schuchert called Baie Innommée). Thus these authors under-es- and Twenhofel in 1910 as part of their ʻAnticostian timated total thicknesses of the north and south coast Series’ with a ʼClinton assemblageʻ, representing part sections. Undoubtedly this was due to very difficult of the Early Silurian (Llandovery) of eastern North boat access on the stormy north coast, and inaccessi- America (Fig. 2). The term Anticostian has given way ble or poor outcrops at the time. to the standard British ʼLlandoveryʻ over time. Ac- Later, Twenhofel (1928) reiterated that the lower- cording to Schuchert and Twenhofel in 1910, the Gun most outcrops of the formation were to be found at River Fm of the Anticosti sequence basically repre- ʻSte Anne and Ste Mary’s cliffs’ (on the south coast): sented ʼRichardson’s Division D, excepting D1ʻ. however, the rocks exposed around Ste Marie River Schuchert and Twenhofel (1910, p. 708–713) consid- [modern name] and also those at Pointe Anne (Ste ered the Gun River Fm to include strata outcropping Anne’s Cliff), west of the Ste Anne River, are now from ʼSaint Anne’s Cliff to within one mile [1.6 km] known to belong to the underlying recessive Merri- of Jupiter Riverʻ along the southwest coast, and about mack Formation and the resistant Becscie Formation 135 ft thick (ca. 41 m). The base of the formation is not (Copper and Long 1989). The most westerly exposures exposed on the south coast, and the overall thickness of the Gun River Formation are those on the tidal flats is very difficult to measure as the upper contact of the ca. 300 m E of the Ste Anne River, where Petryk Gun River Fm is poorly exposed along shore. (1981b) has placed his lower contact. As will be shown About 150 km away, on the northeast coast, from in this paper, the sections of Schuchert and Twenhofel ʻWreck Beach to East Cliffʼ, Schuchert and Twenhofel (1910) on the opposing coastlines include strata of (1910) correlated their Gun River Formation with that different ages, and are only partly correlative: this has of the south coast, estimating a local thickness of created some confusion in subsequent correlations. 297 ft (ca. 91 m). By including the upper strata along They also remarked that “coral reefs are a marked fea- Goéland Cliffs (Falaises des Goéland) to East Point, ture of the lower half of this formation” [on the south- they unfortunately added about 55 m of their overlying west coast], perhaps referring to coral concentrations Jupiter Fm to the Gun River Fm (an error continued or horizons (meadows) found within the Gun River well into the 1980s). In addition, they also appear to Fm. We have found neither reefs per se within the Gun have included part of their upper Becscie Fm (= Mer- River Formation, nor dense coral concentrations that rimack Formation of current usage) in Gun River stra- may be confused for such. Reefs previously identified ta of the north coast along the old ʻWreck Bayʼ (now as defining the upper boundary of the Gun River Fm on 266 P.Copper et al. the northeast coast (e. g. Bolton 1972, Petryk 1981b), ʼBecscieʻ Fm below and ʼJupiterʻ Fm above. With begin more than 55 m above the base of the overlying more precisely defined boundaries at the base and the Menier Fm relative to the southeastern type section top (Copper and Long 1989, 1990), we were able to (Copper 1989, Copper and Long 1990). The original measure only 85–90 m for the Gun River Formation on overall thickness of the formation suggested by Schu- the NE coast. On the south coast the thickness may ap- chert and Twenhofel (1910) was stated to be 400 ft (ca. proximate 70–80 m, as part of the section is covered. 122 m), but that included substantial parts of the old This difference in measurement is readily understand-

Fig. 3. Geological and topographic map of the southwest coast in the Gun River area. Note the upper contact (exposed around Ranga Creek), and the estimated lower western contact (covered). The Early Silurian Gun River Formation of Anticosti Island, eastern Canada 267 able, since on the northeast coastline Schuchert and hofel units overlap with strata of the underlying and Twenhofel (1910) included within the Gun River Fm overlying formations. The recessive weathering shaly units belonging to the underlying Becscie Fm (on the units within the lower half of the formation are both sides of Fox Bay), the Merrimack Fm (the Rhyn- covered by Quaternary sediment exposed on the SW chotrema fringilla-Fenestrirostra glacialis beds along coast, although these shales are recognizable inland at Baie Innommée), and the overlying Jupiter Fm (the road outcrops and along the exposed cliffs of the NE ʼTriplesia ortoniʻ beds of the Goéland Mbr, and the coast of the waterfall at the mouth of Ruisseau de la coral patch reefs of the East Point Mbr). If these strata Chute [N. B., this is not the Rivière à la Chute just east are deducted from the total, the partial measurements of Southwest Point]. of disconnected outcrops by Schuchert and Twenhofel Using accessible sections along the SW coast and (1910) amount to about 300 ft [almost identical to the inland, Bolton (1961) defined the lower contact of Richardson measurements in 1857]. The error in the the Gun River Formation as transitional, identifying measurement by Twenhofel and Schuchert can be ex- localities along the Jupiter River and in road sections plained by their statement that they had not examined leading to the Jupiter. He used a basal 1 ft (30 cm) bed ʼfully 200 ft of strata, which have not been seen except of ʼgrey argillaceous limestoneʻ and 16.5 ft (5 m) of re- from a distanceʻ. sistant-weathering, light grey, semi-lithographic lime- Twenhofel (1928) also subdivided the Gun River stone as a marker for the lowermost unit of the forma- Fm into four lithic and faunal ʻzones’, based on infor- tion on the Jupiter River. This was said to be visible mation from the incomplete sections of the SW coast in the Jupiter River below ʼ24 mileʻ bridge (Bolton from the Ste Anne cliffs to Cape Macgilvray (Fig. 3). 1961). The Merrimack-Gun River contact is directly However, it is likely that Twenhofel’s Zone 1 belongs exposed, and accessible, on the road leading east from partly to the Becscie Fm, considering that the cliffs to 24 mile bridge (Copper and Long 1989). The top of which he referred (Ste Anne and Ste Marie’s) can be the formation was said by Bolton (1961) to be ʼan assigned to that formation, and that he missed the un- intraformational conglomerate about 6miles [10 km] exposed softer shales of the Merrimack Fm. His total from the mouth of the Jupiter Riverʻ, which we assume thickness was estimated at 343 ft (ca.105 m). We sug- here lies somewhere within the upper part of the gest that the section varies in facies towards the SW Macgilvray Mbr of the Gun River Fm, east of the coast from the northeast, and that this estimate may be Gun River and west of the Jupiter River mouth, along reasonable. The lower contact of the Gun River Fm the coast. This conglomerate is absent at the exposed with the Merrimack Fm below is not exposed on the upper contact at Cape Macgilvray (the type section) southwest coast because of its recessive nature and and at Cap aux Goélands, and we have been unable to glacial till cover, but is precisely definable on the use it as a marker bed. Intraformational conglomerates northeast coast at Merrimack Bay (Copper and Long are abundant and repetitive in the lower Menier Fm, 1989, p. 67), where it fully outcrops. and in nearly all units of the Gun River Fm, and in the On the SW coast (Figs. 3–4), the top of the Gun underlying Merrimack Fm, and are thus not reliable River Formation crops out at Cape Macgilvray, as far markers for a formational boundary. We use the basal as Ranga Creek, which has its mouth about 150 m W bed of unit 1 of the more recessive overlying Menier of Cape Macgilvray. The lower part of the overlying Fm (Goéland Mbr) as an upper marker bed: this is a Menier Fm and its basal contact is exposed on both relatively resistant-weathering Pentamerus coquinite coasts, and was measured bed-by-bed by Copper and associated with Favosites clusters and stromato- Long (1990, see Fig. 3) at Cape Macgilvray and Ran- poroids at Gull Cape, that breaks up more easily than ga Creek. On the NE coast at Cap aux Goélands, the the resistant-weathering micrites of the Gun River Fm hard grey limestones of the Gun River Fm are overlain below (Fig. 5). The shelly faunas of the upper Gun by the alternating rhythmic calcarenites and shales of River Fm (particularly the diagnostic genus Kulum- the Menier Fm (refer below to Figs. 7a, c). The inland bella (Jin and Copper 1998), and undescribed species exposures of the Gun River Fm, extending more than of Joviatrypa and Pentamerus are also distinct from 150 km along strike and provisional roads, also pro- those of the lower Menier Fm. vide a wealth of lithic and faunal data. The four ʻzones’ Bolton in 1972 determined a thickness of 450 ft identified by Twenhofel (1928) do not appear to be (137 m) for the Gun River strata on the SW coast. He readily recognizable either faunally or lithically across elaborated on the lithology of the formation, mention- the island, and are not used herein. Indeed, the Twen- ing the semi-lithographic nature of much of the for- 268 P.Copper et al.

Fig. 4 (a–h). Southwest coastal outcrops, (a–c) the grounded remnants of the Fayette Brown, an iron ore carrier, at high tide facing ʻHannah Cliff (of Twenhofel, 1928), flanking the lower two members of the Gun River Fm outcropping as bluff faces 5–7 m high; (d) bluffs at Cape Macgilvray exposing the Macgilvray Mbr; (e) contact (yellow line) of the lowest unit of the overlying Menier Fm with the Gun River Fm at Ranga Creek (see Fig. 3); (f–h) resistant hummocky cross stratified carbonates tempestites, locally disturbed to produce ball-and-pillow units in the Sandtop Mbr at Hannah Cliff faces. The Early Silurian Gun River Formation of Anticosti Island, eastern Canada 269 mation, megaripples, and intraformational conglomer- Petryk (1979, 1981a, b) provided brief descriptions ates. He correlated the fossils with those of ʻClintonʼ of the Gun River Formation as part of a comprehen- beds of northeastern US, and determined a middle C2 sive mapping program of Anticosti. He listed five to early late Llandovery age. A number of the ʻGun zones and measured 146 m of Gun River strata. He in- Riverʼ fossils illustrated in accompanying plates cluded in the Gun River Fm the same East Point reefs, (Bolton 1972), however, come either from the Goé- and the shaly Goéland Member, that Schuchert and land Mbr of the Menier Fm above, or the Merrimack Twenhofel (1910) had incorrectly identified from the Fm below, and do not occur in the Gun River strata NE coast (and Richardson in his Division D in 1857). herein re-defined based on the original Twenhofel If the ca. 65 m of the Goéland and East Point members determinations. A description of isolated outcrops of is removed, Petryk’s measurement of the Gun River Gun River strata in the central part of the island, near beds, sensu stricto, would be about 91 m, equivalent to the upper reaches of the Jupiter and Chaloupe rivers, our measurements of the type section on the SW coast. followed later in Bolton (1975). Petryk (1979) used the top of the last distinctive shale

Fig. 5. Geological and topographic map of the northeast coast of Anticosti from Gull Cliff westwards to Lachute River, and measured type sections of the members. 270 P.Copper et al. The Early Silurian Gun River Formation of Anticosti Island, eastern Canada 271 of the ʻBecscie Fmʼ (the Merrimack Fm of Copper and then evaluated across the island and compared with Long 1989), as the base of the Gun River section in those of the SW coast type sections. Our first field the Baie Innommée section, and the Merrimack/Gun work on the Gun River sections commenced in 1966. River contact exposed in the E bank of the Jupiter River below 24mile bridge. This concurs with our evaluation. The base of the Gun River Fm was not 2. Paleontology and identified at Baie Innommée by Schuchert and Twen- Biostratigraphy hofel (1910), but was placed at the top of the last shaly recessive unit of the Merrimack Fm by Copper and Twenhofel (1914) believed that corals ʻplayed a Long (1989), in the section exposed directly below the greater role in the Gun River Formationʼ than other lip of the falls of Ruisseau de la Chute, which plunges older formations on Anticosti, and assigned ʻlarge over the cliff at the shore (Figs. 5, 8). The top of the reefs’ at Ste Anne Cliff to the lower Gun River Fm. formation was defined by Petryk (1979, 1981a, b) as These are non-reefal random coral concentrations of the top of the East Point reefs on the NE coast [our the upper Becscie and Merrimack formations, al- East Point Mbr of the Menier Fm]. The top of the Gun though biostromes or solitary coral carpets have been River Fm, as described by Schuchert and Twenhofel found in the Gun River outcrops cited by Twenhofel (1910), is exposed at the type section along Cape (1928). In comparison with the Jupiter and Ellis Bay Macgilvray on the SW coast, which faunally and litho- formations, faunal diversity of the Gun River Forma- logically matches the section at Cap aux Goélands, tion is low. Twenhofel’s (1914) extensive faunal list 7 km W of East Point, and along the Jupiter Firetower for his Gun River Formation is misleading because it road area in the island centre. includes also the underlying Gun River and Merri- To date the Gun River Fm has been covered only as mack as well as the overlying Menier formations on scattered and composite sections, and studied mainly the northeast coast. Calcimicrobes (Wetheredella, Gir- from partly exposed outcrops in the type area on the vanella and Rothpletzella) and rhodophytes (e. g. southwest side of the island, and along the Jupiter solenoporids) have not been discovered anywhere River in the west central part of the island (e.g., within Gun River strata. Cyclocrinitids (Chlorophyta) Fahraeus and Barnes 1981). In the SW coast area, the are abundant to nearly bed-forming in the Sandtop resistant-weathering limestones of the Gun River Fm Mbr, but less common lower and higher in the forma- are exposed discontinuously from the tidal flats about tion. Stromatoporoids are small and uncommon (most- 300 m east of the mouth of Rivière Ste Anne, to about ly Clathrodictyon, possibly rare Camptodictyon): a 25 km eastward to the uppermost beds of the tidal flats few occur in the lower two members in the SW coastal jutting out at Cape Macgilvray. The sedimentology of sections, but very few were found in the northeast the Gun River Fm was provided by Long and Copper Macgilvray Mbr (Nestor et al. 2010). (1994) and Long (2007), and is not repeated here. This Favositid and heliolitid tabulate corals (coralla nor- paper attempts to fill in the gaps by documenting a mally hemispherical and Ͻ 10 cm in diameter) are suite of measured sections from the poorly accessible common to abundant locally, forming in situ clusters northeast coast, most of which were completed from in the upper 10 m of the formation on the northeast offshore by zodiac boat over the period from 1980– coast (Fig. 6d). Probably the most characteristic corals 1993, when weather permitted. These sections were (especially the Sandtop Mbr, less commonly in the

Fig. 6. Lithology and bedding plane surfaces of the Gun River Formation. (a) typical coral meadow concentrations of silicified solitary rugosans (diameter of calyx ca. 1.5 cm), as seen in the Sandtop Member, locality A671 (12E/6, 88600:82500); (b) bedding plane view of Favosites clusters, Sandtop Member, locality A1017 (12F/4, 88000:54800); (c) intraformational conglomerate with firmground altered to hardground, with pyritised lining to burrows and bedding planes, reworked micrite clasts and shelly infill, characteristic of the Sandtop and Macgilvray members, thickness of slab ca. 10 cm (locality A727, 12E8, 61380:5680); (d) mottled texture of micritic firmground contact showing deeply penetrating burrows and burrowed backfill from overlying carbonate sediment (Sandtop Mbr, polished surface normal to bedding, thickness shown ca. 12 cm, locality A728 (12E/6, 61100:56220); (e) bedding plane view of Kulumbella-Coolinia brachiopod shellbed, near top of Macgilvray Member, locality A1156; (f) intraformational flat-pebble conglomerate with ripped-up micritic clasts, Sandtop Member, locality A727 (see Fig. 6c); (g) soft-bodied algae preserved in association with the Kulumbella-Coolinia shellbed, near top of Macgilvray Member, locality A1374. 272 P.Copper et al.

Macgilvray Mbr) are small solitary corals (lengths nor- Community occurs just below the first appearance of mally Ͻ 5 cm and diameters Ͻ 1.5 cm): these are usu- Pentamerus, dominated by the exotic flat-shelled Ku- ally silicified and locally abundant (Fig. 6a), with the lumbella, Coolinia, and soft-bodied algae (Fig. 6g), cup corals Helicelasma, Brachyelasma, and Crassilas- with minor small-shelled Hyattidina and Mendacella, ma being most typical (McLean and Copper, in press). and small favositids and delicate crinoids (Jin and Cop- Colonial rugosans are rare. Small colonies of the colo- per 1998, Jin 2008). Globally, Kulumbella is confined nial rugosan Palaearaea (Ͻ 4 cm in diameter) occur to a narrow zone of mid-Aeronian age. in the uppermost Macgilvray Mbr at Cap aux Goélands Small smooth atrypids (unnamed genus) occur in on the NE coast, and in eastern road outcrops, fore- Innommée Mbr, but the smooth atrypid Cerasina from shadowing the abundance and large size of this genus the Merrimack Fm below is absent (Copper 1995). in the East Point Mbr of the Menier Fm (McLean and Ribbed atrypids, such as Sypharatrypa sp. and ʻGota- Copper, in press). No other colonial rugosans are trypaʼ sp., first appear on Anticosti in the Innommée known in the Gun River Fm to date, unlike their com- Mbr. Joviatrypa n. sp., and a new small spirigerinid mon presence in the formations above and below. genus resembling Spirigerina cf. jupiterensis first ap- Brachiopods are the most diverse faunal elements in pear in the Macgilvray Mbr (Copper 2001), marking the Gun River Fm, with generally a more restricted the first return of spirigerinids since the earliest Rhud- ʻMerrimackʼ fauna at the base (Lachute Mbr), and a danian (lower Becscie Fm). Diversity is lowest in the more cosmopolitan ʻMenierʼ fauna in the top Mac - Lachute Mbr, with remnants of the largely endemic gilvray Mbr. The most abundant are athyrids such as Fenestrirostra fauna of the Merrimack Fm. (Jin 1989, Hyattidina (Sandtop to Macgilvray members), and Jin et al. 1990). Diversity is greatest in the Macgilvray Meristina (Innommée Mbr), rhynchonellids (e. g., Mbr, showing a transition to the Menier Fm fauna Fenestrirostra pyrrha, Lachute Mbr, Jin 1989, Jin et al. above. Several brachiopod genera and species remain 1990) and higher in the section, Stegerhynchus (Jin and undescribed. Copper 2004), strophomenids (Brachyprion, Coolinia, For the crinoids, the Gun River Fm was dominated Sandtop and Macgilvray members: Dewing 1999, by Dendrocrinus, and then in the Macgilvray Mbr, Eu- 2001), and abundant dalmanellids in the soft grey- myelodactylus, Stupatocrinus and Laurocrinus were green shale units (Mendacella: Innommée Mbr, Li added (Ausich and Copper 2010). and Copper 2006). Pentamerids make an evolutionary Conodonts identified by Fahraeus and Barnes switch and replacement from the Virgiana shell-beds of (1981, p. 170) initially suggested that both the Becscie the Becscie and Merrimack formations below (Jin et al. and Gun River formations (which could not be distin- 1996), to a stricklandiid-pentamerid assemblage in the guished on the basis of conodonts) belonged to the up- Macgilvray Mbr of the upper Gun River strata above per part of the Icriodus discreta – I. deflecta Zone, (Jin and Copper 2000). Rare Stricklandia lens occur in (mid Aeronian = early Fronian sensu Aldridge 1972). the topmost Merrimack beds, and Stricklandia gwelani This implied that the Rhuddanian succession on Anti- appears in the top of the Gun River Fm, alongside costi was highly compressed into a few meters. Later Pentamerus (Jin and Copper 2000). Pentamerus and Barnes (1988) modified this to show that Icriodella Joviatrypa form thin shell-beds and clusters only in the deflecta, and possibly Icriodella discreta were present uppermost few meters of the Macgilvray Mbr, as seen in the uppermost Gun River strata, but the exact along the Dauphiné River and at Gull Cape, and occur sampling levels were not specified, using Petryk’s there in a deeper water Stricklandia-cyclocrinitid com- and Bolton’s former boundaries for the Gun River Fm, munity setting (Copper 1997). A distinct Kulumbella the localities sampled by Barnes (1988), and earlier by

Fig. 7. Helicopter and coastal cliff photos of the northeast coast in the Cap aux Goélands to Sandtop area covering the upper units of the Gun River Fm: (a) looking N along coastal bluffs from Goélands Cape (*) at the boundary of the Gun Riv- er and Menier formations; (b) Cape Sandtop area, with ʻSandtopʼ in upper right, overlooking the Macgilvray Mbr; (c) Cap aux Goélands showing contact of lowermost Goéland Mbr and uppermost Macgilvray Mbr (*); (d) coastal bluffs due east of Sandtop and outcrops of Macgilvray Mbr; (e) Sandtop cliff marking contact of Macgilvray and Sandtop members; (f) westerly extent of Sandtop with both South (S) and North (N) Sandtop creeks in background; upper part of cliff is the Macgilvray Mbr; (g) Sandtop Cliff with base of Macgilvray Mbr marked below lowermost shelly and intraformational conglomerate resistant ledge (*). The Early Silurian Gun River Formation of Anticosti Island, eastern Canada 273 274 P.Copper et al.

Fahraeus and Barnes (1981) probably belonged to the gap is mostly filled by the new road section which lower two members of their Jupiter Fm, and not the traverses Ruisseau de La Chute, cutting steeply into Gun River Fm. This means that I. discreta may be the south bank (Fig. 5). Sections along the SW coast present only in the Menier Fm. Nowlan (1983) as- were also measured from the first outcrops E of the signed the Gun River Fm. to the ʻDistomodus’ ken- Laloutre River (lower contact not exposed), and, with tuckyensis Biozone: this species ranges from the covered intervals, intermittently outcropping to Cape Rhuddanian through Aeronian (Becscie through Me- Macgilvray (upper contact exposed at the mouth of nier formations), and is thus not useful for precise age Ranga Brook, and on the tidal flats at low tide: see constraints. Zhang et al. (2004) suggested that Icriodus Fig. 3). deflecta occupied a nearshore siliciclastic-carbonate For convenience, each member on the NE coast was facies and that Rexroadus kentuckyensis was in an further subdivided into units, using recognizable local offshore belt, based on the Sandtop and Jupiter River marker beds which could be traced along strike and sections. The conodonts of the Gun River Fm are of bedding planes in the cliffs, such as recessive shaly early Aeronian age, assigned to the Oulodus jeannae units (the base of which was used to mark some units). through Ozarkodina clavula zones (Zhang and Barnes Otherwise, resistant-weathering ledges (usually more 2002a, b, c), confirming the brachiopod correlations. thickly or irregularly bedded grainstones, shell beds, Using chitinozoans, Soufiane and Achab (2000) sug- intraformational conglomerates, or discontinuous ball- gested that the Gun River Fm spans the Conochitina and-pillow units (SW coast only) were used to mark electa and Ancyrochitina ramosasina zones that match the tops of other units. Such markers may not be rec- the Aeronian Raikülla Baltic regional stage (though in ognizable inland unless they are fossiliferous, or form their Fig. 7, they correlate the Gun River Fm as Rhud- resistant bedding planes. danian). Graptolites are scarce, but they have been re- Much of the Gun River Fm consists of rhythmical- covered from the deeper water facies of the Sandtop ly bedded, hard lithographic limestones or micrites Mbr, where they occur in abundance locally on the with thin fissile muddy partings, or thin mudstones northeast coast. These include Paraclimacograptus cf. less than 5 cm thick. This lithology is especially typi- innuitus (identification by M. Melchin), a species with cal of the lower Lachute Mbr and the Sandtop Mbr a relatively long range. Copeland (1981) correlated the in the upper half of the formation (Figs. 7–8). These Gun River Fm into the Herrmannina selwyni and Bre- rhythmites have been interpreted as storm deposits by vibolbineossia clarki ostracode zones of Aeronian age. Long (2007), based on bed-by-bed observations. In the SW coastal sections, there are periodic, chaotically bedded units with displaced blocks, overturned coral 3. Lithology and stratigraphic units heads, ball-and-pillow collapse structures, and submarine erosional features, probably caused by storm, hur- Long and Copper (1994) and Long (2007) provided ricane or typhoon induced disturbance by long-period detailed sedimentological discussions of the Gun waves (Figs. 4f–h). This suggests that the south coast River Formation, together with tentatively proposed ramp was shallower [or had soft mud to induce gravi- members. In the present study, the lithostratigraphy of ty-related sediment deformation]. On the NE coast the formation will be summarized and the members only a single ball-and-pillow unit was discovered formalized. The strata are described from the base (Lachute Mbr, top of unit 3), and the less disturbed upwards. The total thickness is estimated to vary from more micritic strata, with fewer grainstones, suggest ca. 85 m (NE coast) to ca. 100 m (SW coast). Though that sedimentation generally took place in waters be- Petryk (1981b) reported a siliciclastic-carbonate facies low normal storm wave base (the reverse of that sug- on the northeast coast, there are only rare, isolated gested by Zhang et al. 2004). Some of the ball-and- quartz grains in the carbonates, unlike their counter- pillow units and scours may have been induced by parts in the Ellis Bay Fm (Long 1997). The Gun Riv- tsunamis, propagated from the Gaspe Peninsula to the er Fm is divided into four members, based on the sec- south. A direct seismic influence can be rejected as tion measured in the cliffs along the NE coast from the pillow rich horizons are laterally discontinuous, the base of the falls at Ruisseau de la Chute to Cap extending for a maximum of 250 m along strike. Cor- aux Goélands (Figs. 5–6). From Point Merrimack to relations between the SW and NE coasts are not pre- the first outcrops on the western edge of Sandtop Bay cise: the members appear traceable to the central part there is a short covered interval of about 20 m. This of the island from the NE coast. The Early Silurian Gun River Formation of Anticosti Island, eastern Canada 275

3.1 Lachute Member small hemispherical favositids and dalmanellids again become common. Corals are otherwise rare in the NE The Lachute Member is approximately equivalent to section. By contrast, favositids are common (and some the lower two members identified by Petryk (1981b, quite large, Ͻ 40 cm diameter) in the SW sections. p. 27), that he identified as mainly lime mudstones with shaly partings, and measured as 12 m thick, along 3.2 Innommée Member the tidal flats east of Rivière Ste Anne. The remaining The generally recessive-weathering Innommée Mem- Gun River strata (sensu stricto) were lumped as a ber is exposed on the upper lip of the falls and up- 105 m thick Member 3 by Petryk (ibid.). Bolton (1961, stream from the mouth of Ruisseau de la Chute on the 1972) did not divide the Gun River Fm into units. NE coast, and at Merrimack Point (Fig. 8a). Partial The uppermost bed of the underlying Merrimack Fm exposure is also present on the south bank of the river is the last recessive mudstone bedset exposed near the in outcrops created by roadwork in 1990. Only 5 m of base of the falls of Ruisseau de La Chute (where the the member has been measured at the falls and at member gets its name: Copper and Long 1989, p. 68, Point Merrimack, and a further 15–20 m lies covered fig. 3A). This recessive bed is visible as a notch located or poorly exposed from Merrimack Point to the first above a 2–3 m thick step at the base of the falls above outcrops southward along the coast, marking the over- the beach. The La Chute drops over the steep cliffs for lying member. The base of the member can be defined some 15 m: the river cannot be ascended at these falls by soft bluish-grey calcareous shales, weathering re- (Figs. 8a, e–g). Most of the cliff at this point consists of cessively. Such blue-grey shales are typical of the the Lachute Member, which is 14 m thick. The section member as a whole and are repeated up section. Lo- is continuous along the coast until Point Merrimack cally the mudstones may be 1–3 m thick with almost (Figs. d–f), where the next member crops out. The top no limestone interbeds (Chaloupe River area, central of the Merrimack Fm at 24 mile bridge road east of Anticosti). The dominant lithology of the member is the Jupiter River also has a sharp shale/limestone break thinly and irregularly bedded, commonly burrowed (locality A1242, 12E/11, 69300:95420). This contact is calcisiltites to calcarenites, that are brown to bluish- not exposed on the southwest coast. The Lachute Mbr grey weathering. The top of the recessive member is consists primarily of hard, resistant weathering, light to not exposed, but the overall thickness is estimated at medium grey, irregular to nodular, thinly bedded mi- 12–15 m. crites with muddy partings usually less than 1–2 cm Fossils are predominantly the dalmanellid brachio- thick. Mudstone partings tend to decrease towards the pod Mendacella, which occur as whole or disarticulat- top of this member, though there are some distinctive ed shells in shell beds, either within the calcarenites thin recessive marker beds used to define some of the or in the soft muddy recessive beds (Li and Copper units within the member. Some of the thicker micrite 2006). Common also are orthoconic nautiloids, and ledges weather resistantly, and are sometimes also bur- local dense concentrations of a smooth, small atrypoid rowed and more brownish in colour. Calcarenite ledges shell (un-named genus), abundant large, undescribed are very rare. Calcareous shales form 5–10% of the Meristina (in the Chaloupe River area), small athyri- member. The top of unit 3 within the NE coast section dides such as Hyattidina, small rhynchonellides (Ste- is marked by sandy calcarenite with locally developed gerhynchus), and coquinas of the strophomenide ball-and-pillow structures. Coolinia (see Dewing 1999). Atrypide brachiopods The most typical fossil in the Lachute Mbr is the are scarce but include early ʻGotatrypaʼ. There are no brachiopod Fenestrirostra pyrrha (Billings, 1866), the spiriferides. Trilobite pygidia and cephalic fragments, last surviving species of the genus on Anticosti (Jin et and graptolite thecae may be locally common. Tabu- al. 1990). As a guide fossil for the unit, it can locally late and solitary rugose corals, and bryozoans are rare be collected in great numbers, weathering out from to absent, suggesting a deeper water BA5 setting. shaly partings between the hard grey micrite beds, or as rare shell lenses. This species is accompanied by 3.3 Sandtop Member Rhynchotrema parviseptatum Jin, 1989, another hold- over from the Merrimack Fm. Small favositids, stro- The Sandtop Member has a minimum measured thick- pho menids (Dewing 1999), and dalmanellid brachio - ness of 26 m, but the total estimated, including a cov- pods (Li and Copper 2006) are also present. These in- ered interval, is 40–45 m. It is best exposed from the dicate a BA4 setting. Near the top of this member, shoreline of Sandtop Bay west of North Sandtop Creek 276 P.Copper et al. The Early Silurian Gun River Formation of Anticosti Island, eastern Canada 277

(Figs. 7f, 8b–c). There the first outcrops begin due land outcrops, (Salmon River to Jupiter upstream near north and west of the first cliffs, to the shoreline out- Lake Wickenden) to SW end of the island (e. g. at crops and bluffs defining the southern parts of Sand- Gun River), this part of the formation contains a rich top cliff. A few more meters are exposed at the base silicified solitary rugose coral fauna (Fig. 6a: Cya - of Merrimack Creek draining into northern Sandtop thactis sp., Dinophyllum hannah, Streptelasma anti- Bay (Fig. 8c). The basal units crop out on the upper costiensis according to Bolton 1981, or Helicelasma ledges of the south bank of Ruisseau de la Chute at the and Brachyelasma in McLean and Copper (in press). mouth, where they have been exposed in ditches by Small favositids (Favosites and Paleofavosites) and road construction. Upstream on Ruisseau de la Chute, heliolitids (Propora) are present. Dalmanellids and the lower contact is not exposed, but there are long the locally abundant rhynchonellid Gnamptorhynchos bluffs of the Sandtop Member 2–3 km upstream from selliseptalicum Jin, 1989 also occur commonly as sili- the mouth of the river (see Fig. 5). cified shells. The lack of large-shelled pentamerids The Sandtop Mbr is predominantly a highly rhyth- and atrypids suggests a BA4 setting. mic sequence of thinly, planar to weakly nodular bedded, medium grey to light grey micrites intercalated 3.4 Macgilvray Member with grey shaly partings to very thin shaly units (usually Ͻ 2 cm thick (Fig. 8c). The lower part of the mem- This uppermost member crops out with similar facies ber is very soft and recessive weathering, producing and faunas at opposite ends of the island, about 150 km more abundant blue-grey fissile shales and rapidly apart. It is 18–19 m thick at the complete section meas- disintegrating slabs of thin, soft, platy lithographic mi- ured from the south end of Sandtop Cliff to the upper crites. The micrites become harder and more resistant contact at Cap aux Goélands. The base of the member weathering towards the top of the member, with a is the underside of the first 5–10 cm thick calcarenite- concomitant decrease in thickness of muddy partings. coquinite ledge above the underlying member (see Calcarenite ledges or shell beds are rare. Local pyritic above, and Fig. 7g.). The top is the base of unit 1 of the blebs or nodules collect around some fossils, and pyri- Menier Fm (Goéland Mbr of Copper and Long 1990, tized or dark grey encrusted hardgrounds are common Jin and Copper 2012). The base of the overlying Menier (Fig. 6c). The upper part of the member is defined at Fm is a very prominent thickly bedded, but usually the base of a 5–10 cm thick calcarenite-coquinite shaly, recessively weathering Pentamerus coquinite of marker ledge visible in the southern part of the Sand- broken shells, containing stromatoporoids (Nestor et al. top Cliffs (Fig. 7g). This appears to be the first contin- 2010). Inland around Schmitt Creek, and the Dauphiné uous coquinite above the member, as no distinct co- River, the basal Goéland Mbr is shaly and rich in small quinites were found lower down in this area. There are brachiopods such as Hyattidina, or marble-shaped numerous intraformational conglomerates in the cen- Paleofavosites. At Cape Macgilvray, the upper Gun tral to western outcrops of the member (Figs. 6b–c). River contact is exposed on the tidal flats that form the The member is only sparsely fossiliferous on the headland, and in the rocks exposed at the mouth of NE coast, with lowest diversity for the formation. Macgilvray Creek as far as Ranga Creek (Figs. 4d–e). The strophomenide Coolinia may be locally common On the poorly exposed southwest coast, the basal Goé- (Dewing 1999). Orthoconic nautiloids and a few rare land Mbr unit is only sparsely fossiliferous. small horn corals are present. Cyclocrinitids appear in The Macgilvray Mbr contains fossiliferous shell this member, sometimes in dense clusters. Almost no beds, calcarenites interbedded with micrites, and soft other fossils were collected on the NE coast except weathering shaly units that make the unit more reces- some undiagnostic graptolites (see above). In the in- sive, except at the very top where the lithology again

Fig. 8 (a–g). Views of the Sandtop Bay and Baie Innommée sections of the Gun River Fm: (a) Ruisseau de la Chute falls, facing Baie Innommée, with lip of falls in Lachute Mbr, with overlying contact Innommée Mbr (*); (b) South Sandtop Creek exposing lower Sandtop Mbr overlain by Macgilvray Mbr up the creek; (c) cliff of Sandtop Mbr on the northwest side of North Sandtop Creek, looking north; (d) base of the Lachute Mbr overlying recessive Merrimack Fm at Merrimack Point; (e) Baie Innommée with Merrimack Point in the upper left (east) and Ruisseau de la Chute falls in the middle; the falls plunge over ca. 5–6 m of the Lachute Mbr; (f) View northwest at Lachute falls with contact recessive Innommée Mbr base at as- terisk; (g) Ruiseau de la Chute with resistant Lachute Mbr forming the river bed, with overlying Innommée Mbr exposed on the north bank. 278 P.Copper et al. returns to hard weathering, thin, platy to irregular divided, from base to top, into the Lachute, Innommée, weathering micrites with only minor shaly partings. Sandtop and Macgilvray members. These strata are for- Grainstones, calcarenites, coquinites and intraforma- mally subdivided into four members, correlated across tional conglomerates usually stand out as resistant the island. Corals are locally common but not reef- ledges that periodically interrupt the succession forming, and they rarely formed more than thin mead- (Figs. 7c, g). These suggest that the member represents ows of solitary rugosans, or bedding plane clusters of a shallowing-upward succession. small colonial hemispherical tabulate corals. Colonial The Macgilvray Member has the greatest faunal di- rugosans are rare and small, as are stromatoporoids. versity within the formation, and its brachiopods have Shell-beds of athyridides, dalmanellids, and stropho- a precursory nature resembling those in the overlying menides are locally common, and large-shelled pen- Menier Formation. The unit is typified by rare Strick- tamerides are confined to the top few meters of the for- landia gwelani, the large, flat, thin-shelled stricklandi- mation. The sedimentological and faunal data indicate id Kulumbella, Pentamerus cf. palaformis, which oc- a storm influenced, broadly shallowing-upward suc- cur in the top few meters of the member at both Cape cession for the Gun River Formation. Macgilvray and South Sandtop Creek (Jin and Copper 1998, 2000). The Kulumbella-Coolinia shell beds con- Acknowledgements. All three authors acknowledge the tain abundant soft-bodied ʻbrownʼ algae (Fig. 6g) and field, laboratory and student assistance supported by Dis- delicate, intact crinoids, suggesting preservation by covery Grants of the Natural Sciences and Engineering Re- search Council of Canada. Attributions: Copper and Long rapid burial in relatively deep water. The first appear- jointly measured the NE coast sections; Long measured the ance of the common atrypids Joviatrypa, Dihelictera, SW coast section; all sections were sampled by Copper and Limbatrypa and ʻSpirigeriinaʼ jupiterensis (Twenhofel Jin for their faunas and lithology. 1928) are noted here on Anticosti (Copper 2001). Common to abundant small Hyattidina junea (Billings 1866) mark bedding planes locally. The rhynchonellid Platytrochalos peninversus Jin 1989 is present in soft 5. References blue-grey shales, just below the upper formational Aldridge, R. J., 1972. Llandovery conodonts from the Welsh contact along the Jupiter River. The age-diagnostic Borderland. Bulletin of the British Museum (Natural His- Eocoelia lineage is absent in the Mcgilvray member tory), Geology 22, 125–231. and makes its first appearance in the deeper water Ausich, W.I., Copper, P., 2010. The Crinoidea of Anticosti facies of the upper Goéland Mbr of the Menier Fm. Island, Québec (Late Ordovician to Early Silurian). Pa- Cyclocrinites becomes more common in this member laeontographica Canadiana 29, 157 p. than before. In the upper 6 m there are nests of hemi- Barnes, C. R., 1988. Stratigraphy and paleontology of the spherical favositids, and lenses of Joviatrypa, and Ordovician-Silurian boundary interval, Anticosti Island, Quebec, Canada. Bulletin British Museum Natural Histo- strophomenids such Protomegastrophia philomela ry (Geology) 43, 195–219. and Brachyprion leda (see Dewing 1999). This sug- Barnes, C. R., 1989. Lower Silurian chronostratigraphy of gests a shallowing-upward depositional environment Anticosti island, Québec. In: Holland, C. H., Bassett, M. G. at both ends of the island for the upper portions of the (Eds.), A global standard for the Silurian System. National formation, probably at depths within the Stricklandia Museum of Wales, Geological Series 10, 100–108. BA3, as identified previously by Cocks and Copper (in Billings, E., 1862. On some new species of fossils from the Johnson et al. 1981: for localities 13, 15–17). Quebec Group: Containing descriptions and figures of new or little known species of organic remains from the Silurian rocks. Geological Survey of Canada, Palaeozoic Fossils 1, 57–168. 4. Conclusions Billings, E., 1865. Notice of some new genera and species of Palaeozoic fossils. Canadian Naturalist and Geologist, The lower Aeronian Gun River Formation is character- New Series 2, 432–452. ized by rhythmic, thin-bedded micrites, intercalated Billings, E., 1866. Catalogue of the Lower Silurian fossils by calcareous shales, with irregularly spaced skeletal of the island of Anticosti, with descriptions of some new genera and species. Geological Survey of Canada Publi- packstones (calcarenites and coquinites), representing cation 427, 1–93. an oscillating transition from the underlying deeper wa- Bolton, T. E., 1961. Ordovician and Silurian formations of ter Merrimack Formation to the overlying more fossil- Anticosti island, Quebec. Geological Survey of Canada iferous Menier Formation. The formation is here sub- Paper 61 – 26, 1–18. The Early Silurian Gun River Formation of Anticosti Island, eastern Canada 279

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