Sami Et Al., (1992). Episodic Sedimentation on a Silurian, Storm-dominated Carbonate Ramp, Anticosti...

Sedimentology (1992) 39,355-381

Episodic sedimentation on an early Silurian, storm-dominated carbonate ramp, Becscie and Merrimack formations, Anticosti Island, Canada

T E R R Y S A M I * a n d A N D R E D E S R O C H E R S

Ottawa-Carleton Geoscience Centre, Department of Geology, University of Ottawa, Ottawa, Ontario, Canada K I N 6N.5

ABSTRACT

The 150-160 m thick lowermost Silurian (Rhuddanian) Becscie and Merrimack formations of Anticosti Island, Canada, represent continuous deposition on a shallow, open marine carbonate ramp. Several rock types are identified : (a) laminated and homogenous mudstone ; (b) laminated and homogenous packstone ; (c) argillaceous mudstone and packstone ; (d) calcareous shale ; (e) laminated calcisiltite; (f) medium- to fine-grained grainstone; and (g) bio/intraclastic rudstone. These rock types are arranged into five distinct lithofacies: (LFI) calcareous mudstone-shale; (LF2) laminated-homogenous mudstone; (LF3) calcareous grainstone-shale ; (LF4) laminated mudstone-grainstone ; and (LF5) laminated calcisiltite-grainstone. The sequence reflects deposition on a low-energy, muddy, carbonate to argillaceous ramp subject to short-lived, episodic high-energy storms. These events produced fining-upwards storm units 5-80 cm thick, or tempestites, consisting of: a sharp scoured base overlain by intra/bioclastic rudstone grading upwards into medium-grained grainstone, finely laminated calcisiltite and mudstone, or shale. These are interbedded with low-energy, fairweather mudstones and calcareous shales.

Deposition progressed from a carbonate mud-dominated ramp in the Becscie Formation to an argillaceous mud-dominated ramp in the Merrimack Formation. Lateral tempestite proximality trends and lithofacies distribution indicate that the Anticosti Basin deepened to the south-east into the Iapetus Ocean and shallowed towards a SW-NE-orientated shoreline to the north-west. Vertical tempestite proximality trends and lithofacies changes identify third-order eustatic sea-level changes. After an initial deepening at the base of the formation, a shallowing-deepening event dominated the sequence. Several higher order fluctuations, defined by lithofacies and tempestite proximality trends, are superimposed on these changes. The fluctuations identified with the aid of tempestite proximality trends are of an order of magnitude higher than those identified by either lithofacies or palaeontological methods.

INTRODUCTION

The Becscie and Merrimack formations (Copper & Long, 1989), exposed on Anticosti Island, Canada, are of particular interest because the Ordpician- Silurian boundary lies at or near the base of thy: wholly subtidal Becscie Formation. Whereas a large'body of work has been produced on the palaeontology and biostratigraphy of the sequence on Antidbsti Island, few workers have addressed the sedimentology (but see Petryk, 1981a; Long & Copper, 1987b; Copper & Long, 1989). The biostratigraphic studies of Copper

*Present address : Department of Geological Sciences, Queen's University, Kingston, Ontario, Canada K7L 3N6.

& Long (1989) and Jin et al. (1990) have provided a framework with which the sedimentology of the Becscie and Merrimack formations can be meaning- fully interpreted. .

The Becscie and Merrimack formations provide a unique opportunity to examine a continuous sequence of shallow-water, subtidal carbonate rocks straddling the Ordovician-Silurian boundary. The lack of evidence to suggest emergence of the platform during deposition of the Becscie and Merrimack formations (Sami, 1989), during a period of sea-level fluctuation (Johnson et al., 1981), suggests that depbsition was on the mid to distal portions of the platform. This study

355

356 T. Sami and A . Desrochers

attempts to recognize the type of carbonate platform which was evolving in the Anticosti Basin during the early Silurian and its responses to variations in sea level. An understanding of the evolution of the Anticosti Basin during this interval can provide further information useful in unravelling the palaeogeography of eastern North America, and in particular the Anticosti Basin.

This paper is divided into two parts, the first dealing with the lithofacies and sedimentology of the Becscie and Merrimack formations and the second describing tempestite proximality trends and basin palaeogeography.

GENERAL SETTING A N D STRATIGRAPHY

Location

Anticosti Island is located in the Gulf of St Lawrence, eastern Canada, about 30 km south of the north shore of Quebec and 75 km north-east of the GaspC peninsula (49"04'-49"57'N, 64"32'--614 1 ' W). The island is 222 km in length and 56 km at its widest point (Fig. 1). Situated on the eastern part of the St Lawrence platform, strata were largely unaffected by the stresses of the Ordovician Taconic Orogeny. The sequence on Anticosti Island consists of up to 1100 m of undeformed, fossiliferous limestone, shale and minor siliciclastic sediments (Petryk, 1981 a). The strata strike NW-SE with an average dip of 2" SW.

Regional palaeogeography

Ranging from late Ordovician (Ashgill) to early Silurian (Llandovery/Wenlock?), the exposed sequence on Anticosti Island represents one of the best exposures of continuous shallow-water deposition across the Ordovician-Silurian boundary in North America (LespCrance, 1981 ; Barnes, 1988). During the early Silurian, the Anticosti Basin was situated at palaeolatitudes of 15--20"S (Ziegler et al., 1977, 1979; Scotese et al., 1985), placing it within a warm, humid zone at a time when much of the Laurentia craton was covered by shallow, epeiric seas (Ziegler et al., 1977). The basin was located on the north-west margin of the Iapetus Ocean, which is believed to have been open from the late Proterozoic until possibly the late Silurian to early Devonian (Ziegler et a/., 1977; Mason, 1988). The strata on Anticosti Island were

Fig. 1. Location of Anticosti Island (cross-hatched) showing distribution of the Becscie Formation (no ornament) and significant outcrop localities (modified from Petryk, 1981a).

deposited at a time of global sea-level fluctuations possibly associated with periods of glaciation or climatic change in North Africa (Hambrey, 1985). The Late Ordovician to Early Silurian was marked by the onset of a eustatic sea-level rise related to melting of the North African continental ice sheet (Ziegler et a/., 1979). The palaeobathymetric changes indicated by Ordovician and Silurian strata have been interpreted as probably of glacio-eustatic origin by Petryk (1981b), whereas Long & Copper (1987b) inferred Taconic and post-Taconic load-induced subsidence to account for the thickness of the Anticosti Basin sequence.

Although the Anticosti Basin experienced several sea-level lowstands, there is no evidence of supratidal or intertidal exposure, with deposition being restricted to shallow, open-marine subtidal conditions. The

Episodic sedimentation on a storm-dominated carbonate ramp 357

sequence is characterized by an abundance of lime mudstone/packstone, diverse open marine biota, whole fossils, upward-fining units, patch reefs and widespread bioturbation. No evidence of slope break, reefal margin or slope and deep basin sediments has been reported.

Previous work

Richardson (1 857) and Billings (1 857) were the first to map and subdivide the sequence on Anticosti Island based on lithostratigraphy and biostratigraphy respectively. Schuchert & Twenhofel (1910) introduced formal formation names based on lithostratigraphic work on coastal exposures. This set up a framework on which Twenhofel (1921, 1928) continued to elaborate. Bolton (1961, 1970, 1972) mapped the interior of the island and refined Twenhofel's subdivisions based on lithostratigraphy and biostratigraphy, together with Copeland (1970, 1973, 1974). Petryk's (1981a,b) work involved reinterpretation of formation boundaries and subdivisions based on extensive lithostratigraphic and basin analysis. John- son et al. (198 1) proposed sea-level fluctuations for the sequence using brachiopod communities, based on Copper's (1981) stratigraphic work on the east of the island. This work on the sedimentology and stratigraphy was continued by Long & Copper (1987a, b). Copper & Long (1989) re-examined the upper two members of the Becscie Formation and formally assigned them to the newly erected Merrimack Formation. In addition, there is a large body of work on the biostratigraphy of the sequence on Anticosti Island, particularly with respect to faunal changes across the Ordovician-Silurian boundary (Lesptr- ance, 1981 ; Barnes, 1988; Jin et al., 1990).

Stratigraphy

According to Schuchert & Twenhofel (1910), the Anticosti Island sequence consists of six formations distributed between two groups, the Upper Ordovi- cian VaurCal and Ellis Bay formations in the Jolliet Group, and the Lower Silurian Becscie, Gun River, Jupiter and Chicotte formations in the Anticosti Group (Fig. 2).

Petryk ( 1 98 1 a) proposed a revised, informal fourfold lithostratigraphic subdivision for the Becscie Forma- tion. Copper & Long (1989) have incorporated the upper two members into the Merrimack Formation on the basis of lithological and palaeontological characteristics. This study follows Copper & Long's

Fig. 2. Stratigraphic subdivisions and nomenclature of the Anticosti Island sequence with the Becscie and Merrimack (MK) formations in detail, showing age and lithofacies relationships. Lithological legend applies to Anticosti Island sequence only. Lithological symbols for the Becscie and Merrimack formations are shown in Fig. 3. The Anticosti Island sequence is modified from Petryk (1981a). The stratigraphy of the Becscie and Merrimack formations is from Sami (1 989).

(1989) stratigraphy for the Merrimack Formation and divides the Becscie Formation into two members. These two members are similar to Petryk's (1981a) members 1 and 2 but with a reassessment of the boundary between them.

The Becscie and Merrimack formations contain 150--160 m of predominantly limestone,, with calcareous shale and siliciclastics being locally important


Table 1. Description of rock types recognized in the Becscie and Merrimack formations.

Rock type Constituents Bedding Basal Sedimentary structures Fossils Bioturbation contact

Homogenous mudstone

Laminated mudstone

Homogenous packstone

Laminated packstone

Argillaceous mudstone/ packstone

Calcareous shale

Laminated calcisiltite

Fossiliferous grainstone

(continued)

Micrite/microspar Tabular Argillaceous clays ( < 2%) 2-1 5 cm Fossils (up to 3.0 cm,

?Peloids

Micrite/microspar Tabular Quartz silt (


Rock type Constituents Bedding Basal Sedimentary structures Fossils Bioturbation contact

Bio/intraclastic Fossil fragments (up to Tabular to Sharp to Normal grading Tabular corals, Rare to none rudstone 20 cm, 5-85%) lenticular erosional Imbrication stromatoporoids, Borings in

Intraclasts (0.1-30.0 cm, Up to 40 cm Planar to Geopetal structures brachiopcds mudstone l&90%, mudstone, channelled Infiltration textures intraclasts packstone, calcisiltite, grainstone, rounded, tabular)

Quartz silt (< 1%. up to 5% locally)

Fossiliferous grainstone matrix

Calcite cement and minor micrite matrix (i 2%)

Fig. 4. Fine-grained rock types. (a) Laminated mudstone (burrowed) overlying homogenous mudstone (Becscie Formation, Member 1, LF2, Cap Henri). The contact is sharp and the laminated unit shows well-defined grading and lamination cut by Chondrifes burrows. Scale bar = 2 cm. (b) Homogenous mudstone (light grey) interbedded with laminated packstone and calcisiltite (dark grey; Becscie Formation, Member 1, LF2, R. aux Saumons). Mudstone contains a few fossils and shows some burrows. The upper contacts of the mudstone are sharp and locally show scours. Scale bar = 5 cm. (c) Laminated packstone (top) grading up from laminated calcisiltite (base; Merrimack Formation, LF3, R. Jupiter). The packstone shows lamination defined by parallel orientation and relative abundance of fossil material. Scale bar = 3 cm. (d) Homogenous packstone (light grey) interbedded with laminated calcisiltite and packstone (dark grey; Becscie Formation, Member 1, LF2, Cap Henri). The packstone cont.ains large, parautochthonous brachiopods, showing geopetal cement fill, in a micrite matrix. Scale bar = 5 cm.


(Sami, 1989). Although the thickness of the full sequence is relatively uniform across the island, the thicknesses of the formations and members show marked east-west variations (Fig. 3).

The contact between the Ellis Bay Formation and Becscie Formation is placed at the top of the biohenns in the Laframboise Member of the former (Long & Copper, 1987a), and below the lowest Occurrence of lime mudstones characteristic of the latter. Petryk (1981 a) suggested using an oncolite-rich, platform bed as the contact in the absence of bioherms, but difficulties in establishing a time marker on this unit across the island makes such a definition inconsistent. The base of the Becscie Formation is transitional over a few metres above the contact, consisting of interbedded mudstones and coarse, encrinitic, reef-derived grainstones. The Ordovician-Silurian boundary is placed within the Laframboise Member of the Ellis Bay Formation (Cocks & Copper, 1981 ; Barnes, 1988; McCracken & Nowlan, 1988). The remainder of the

Fig. 3. Location of composite sections showing distribution and thicknesses of recognized lithofacies. Lithologies are discussed in the text. Covered intervals of less than 5 m are not shown. For details on section localities see Sami (1989).

Becscie Formation is Rhuddanian in age (Copper & Long, 1989; Fig. 2).

Member 1 (including the transition zone) of the Becscie Formation varies from 22 to 3-6 m thick from west to east and is composed mainly of lime mudstone with minor fossiliferous grainstones (Fig. 3). The member 1 to member 2 transition is marked by an increase in abundance and thickness of medium to coarse grainstone units and the appearance of hummocky cross-stratified beds. This transition is commonly gradational and difficult to map lithologically. The boundary is placed 22 m above the Ellis Bay to Becscie formations contact at Cap Henri. Member 2 of the Becscie Formation varies from 98 to 115 m in thickness from west to east and is composed of lime mudstones, grainstones and rudstones. It is terminated by the onset of significant calcareous shale beds of the Merrimack Formation (Fig. 3). The Becscie to Mer- rimack formations boundary is placed at the base of Petryks (1981a) member 3 of the Becscie Formation, above a rippled, skeletal lime grainstone, in agree- ment with Copper & Long (1989). The Memmack Formation is 29 m thick at Jupiter River and is not completely exposed at any other locality. The Merri- mack Formation is characterized by thick, grey-green calcareous shale beds and minor lime mudstones and grainstones. The upper contact with the clean, dense lime mudstone of the Gun River Formation is sharp and easily recognized in most sections.

Copper & Long (1 989) and Jin et al. (1990) attribute the Becscie and Merrimack formations to the A1-A3 and A4 graptolite zones of the Rhuddanian respectively (Fig. 2).

ROCK TYPES AND LITHOFACIES

Rock types

Eighty-two sections were measured spanning the island from west to east; these were subsequently combined into four composite sections representing the four main regions of outcrop : Cap Henri to Rivihe Cailloux, Riviere Jupiter, Riviire aux Saumons and Riviere Prinsta to Reef Point (Fig. 3). Detailed stratigraphical sections and their locations can be found in Sami (1989).

Thesesectionscontain a varietyofrock types which, despite their gradational relationships, are attribut- able to severaldistinct end-members (Table 1, Figs 4 & 5).


Fig. 5. Medium- and coarse-grained rock types. (a) Laminated calcisiltite, from a hummocky cross-stratified unit, showing well-developed horizontal lamination (top, bottom) which truncates, and is truncated by, shallow-dipping lamination (centre; Becscie Formation, Member 2, LF4, R. aux Saumons). Scale bar= 2 cm. (b) Medium- to fine-grained grainstone overlying laminated calcisiltite with a sharp, erosional base (Becscie Formation, Member 2, LF4, R. aux Saumons). Grading is poorly developed within the abundant, disarticulated fossil material. Lamination is poorly defined. Scale bar= 3 cm. (c) Coarse intraclastic rudstone showing a diversity of elongate intraclast types, including laminated packstone and calcisiltite. and irregular (possibly bored), dense mudstone intraclasts in a coarse- to fine-grained grainstone matrix (Becscie Formation, Member 2, LF4, R. Prinsta). The orientation of some clasts suggests an imbricated fabric. Scale bar=4 cm. (d) Coarse bioclastic rudstone showing abundance of large disarticulated fossils in calcite cement (Becscie Formation, Member 2, LF4, R. Jupiter). Scale bar = 3 cm.

Bed morphology and sedimentary structures

The different rock types listed in Table 1 comprise a complete sedimentological package. This package, or parts of it, is found throughout the studied sections. It is characterized by a transition from coarse- to fine- grained rock types that produced a gradational stacking, from base to top, of: bio/intraclastic rudstone, medium- to fine-grained grainstone, laminated calcisiltite, packstone and mudstone. This fining- upward sequence is interbedded with homogeneous mudstones, packstones and calcareous shales (Figs 6 & 7). A complete sequence is rarely preserved at any one locality, but occurs more commonly as a partial or

truncated sequence containing a wide array of sedimentary structures.

Graded units

Graded units, 5-40 cm thick, are common throughout most of the Becscie and Merrimack formations (Fig. 7). Locally, grains show a polymodal distribution of shell material, fine sand, and silt to mud so that units commonly have sharp internal contacts and the beds are subdivided into discrete layers (Fig. 7b). The vertical sequence consists of: (i) a sharp erosional base, commonly showing sole marks such as grooves,


DENSE MUDSTONE/PAU(STONE

NODULAR MUDSTONE/SHALE

LAM I N ATED MU DSTON E / PACKSTONE

WAVE-RIPPLE@ CAL.CISILTITE

LTlTE WITH UB - NATIONS,

GRADED MED- TO FINE- GRAINED GRAINSTONE

GRADED BIO/ INTRACLASTIC RUDSTONE

SCOURED BASE AN@ COARSE LAGS

DENSE UNLAMINATED, BIOTURBATED MUDSTONE AND PACKSTONE

tN Sin/ FOSSILS BIOCLASTS

BURROWS

Fig. 6. Idealized Becscie Formation tempestite sequence, showing vertical relationships between observed rock types and general fining-upwards trend. Dense mudstone and packstone represent the interbedded fairweather rock types. HCS = hummocky cross-stratification, SCS = swaley cross- stratification.

tool marks, and gutter casts and/or lenticular shell/ intraclast lags; (ii) massive or graded sand. often with reworked shell material; (iii) planar or low-angle lamination, possibly hummocky cross-stratification; (iv) wave ripple cross-lamination and wave or interference ripples ; (v) a bioturbated, muddy interval with gradational top. Other features include lenticular bedding, escape traces, ball-and-pillow structures, hardgrounds and condensed horizons.

Hummocky units

Hummocky beds are characterized by the presence of hummocky cross-stratification (HCS) within a vertical sequence (Fig. 8a,b). Beds containing HCS, as defined by Harms et al. (1975), and swaley cross-stratification (SCS), as described by Tillman (1989), are common within the Becscie Formation and rare within the Merrimack Formation. HCS and SCS occur in laterally continuous to lenticular calcisiltite beds which commonly show amalgamation (Fig. 8b). HCS and SCS units have wavelengths of 1-4 m, amplitudes of 10-40 cm and occur in a three-dimensional pattern of hummocks and swales (Fig. 8a). These beds are characterized by sharp, erosional lower bounding surfaces, parallel laminae which dip at angles of 5- 15" in random directions, internal truncation surfaces, interference or wave-rippled tops, and commonly occur within graded units. SCS is similar to HCS but with a greater percentage of preserved swales. In the Becscie and Merrimack formations, a genetic distinction between HCS and SCS, as made by Tillman (1989), is not observed, with the two bedforms commonly being interbedded and showing similar bed thickness and grain-size distributions.

Gutters and gutter casts

Gutters and gutter casts (Whitaker, 1973) can be found throughout the Becscie Formation, and in coarser parts of the Merrimack Formation, on extensive, well-exposed bedding plans (Fig. 8c). Vary- ing widely in morphology and substrate-fill combina- tions, gutters occur most abundantly in mudstone and fine grainstone or calcisiltite substrates, with any of the observed rock types composing the fill.. Borings and/or burrows and colonization of gutter surfaces by attached organisms (tabulate corals, stromatoporoids and bryozoans) can represent episodic activation and fill (Goldring & Aigner, 1982), indicating that the event which infilled a gutter may be distinctly different from that which formed the gutter. Gutters vary from several tens of centimetres to several metres in length and from 5 to 50cm in width. They are linear to sinuous and commonly bifurcate. The larger gutters show greater (> 1 m) spacing, whereas narrow gutters may be only centimetres apart. All observed occurrences form guttered horizons which are laterally traceable for several tens of metres. Gutters and gutter casts are common on storm bed bases and reflect the selective erosion of cohesive sediments by high- velocity currents (Johnson & Baldwin, 1985).

Episodic sedimentation on a stormdominated curbonuri. rump 363

Fig. 7. Tempestite packages. (a) Graded tempestite unit showing a gradual transition from medium-grained intraclastic rudstone to calcisiltite (Becscie Formation, Member 2, LF4, Reef Point). Sharp scoured base truncates a thin calcisiltite unit overlying a nodularly bedded mudstone. The tempestite is overlain by nodular, argillaceous mudstone and calcareous shale. (b) Graded tempestite bed showing two distinct waning flow events. Irregular scoured surface developed in a faintly laminated mudstone and overlain by a graded, laminated, medium-grained grainstone/calcisiltite tempestite (Becscie Formation, Member 2, R. Prinsta). Discoloration of the carbonate mudstone surface suggests the formation of a hardground or firmground. Irregularities in the surface are infilled by mudstone intraclasts (derived from the underlying mudstone) and the overlying grainstone and calcisiltite. Sample is 15 cm in length.

Interference ripples on the flanks of three-dimensional, hummocky surfaces. The pattern of ripple-crest sets represents the presence of multidirectional oscillatory currents, as the two sets are believed to have formed simulta- neously (Allen, 1982).

Interference ripples (Allen, 1982) in the Becscie and Merrimack formations occur on the tops of laminated mudstone/packstone, calcisiltite and fine grainstone units, commonly capping HCS beds (Fig. 8a). Their wavelengths range from 5 to 10 cm, with ripple-crest sets being perpendicular to slightly oblique. Ripples are rectangular to inequant-hexagonal in form, and Shell lags and condensation horizons

locally grade laterally into symmetrical wave ripples of similar wavelengths. This is particularly common

Shell beds of 5-45 cm thickness and' poor lateral continuity are common within biodastic rudstones of


Fig. 8. Sedimentary structures. (a) Hummocky bedform with hummocky cross-stratification (HCS) and interference-rippled top exposed on wave-cut platform (Becscie Formation, Member 2, LF4, Reef Point). Pencil is 15 cm long. (b) Three distinct amalgamated units (Becscie Formation, Member 1, LF5, R. aux Cailloux). The basal unit contains well-developed swaley cross-stratification. The coarse-grained bio/intraclastic rudstone of the middle unit possesses a channel morphology. The upper unit is an HCS calcisiltite with a hummocky surface. Hammer is 34 cm in length. (c) Exhumed, guttered calcisiltite tempestite surface (Becscie Formation, Member 2, LF4, R. aux Saumons). Calcisiltite fill is preserved in cliff section (foreground). Hammer is 34 cm long. (d) Asymmetrical coarse-grained ripple showing internal cross-lamination and asymmetrical form (Becscie Formation, Member 2, LF4, Reef Point). Pencil is 15 cm long.

the Becscie Formation. Locally, these units seem to be controlled by the distribution of depressions or channels. Dott (1983) interpreted them as dynamic shell lags, formed by transport, concentration and winnowing of sands by high-energy events. Lenses of unlaminated packstone, 2- 5 cm in thickness, containing in situ faunas are common in the Merrimack Formation and Member 1 of the Becscie Formation where thin grainstones and calcisiltites are covered by thick lime mudstone or calcareous shale units. Most of these packstones are interpreted as condensation horizons or static shell lags formed through the repeated reworking of quiet-water sediments by rare,

more energetic events (Gebhard. 1982) and/or suffo- cation and rapid burial of in situ faunas (Dott, 1983).

Coarse-grained ripples

Symmetrical and asymmetrical, coarse-grained ripples (CGR; Leckie, 1988) with wavelengths of 45- 85 cm and amplitudes of 5- 10 cm are common in fossiliferous grainstones and bio/intrarudstones of the Becscie and lower Merrimack formations (Fig. 8d). They are slightly sinuous to straight, show no bifurcation and range from symmetrical to slightly asymmetrical. The internal stratification of asymmetr-


ical CGR consists of poorly defined low-angle cross- beds. Coarse fossil debris may be locally concentrated in the ripple troughs. Symmetrical, coarse-grained ripples are inferred to share a close hydrodynamic association, and possibly a common origin with HCS (Swift et al., 1983; Nattvedt & Kreisa, 1987; Leckie, 1988).

Sharp basal contacts

Sharp contacts are common at the base of graded and hummocky units of the Becscie and Merrimack formations. These contacts are typically regular to slightly irregular and of variable lateral extent, but are locally highly sculptured and irregular. The sharpness of these surfaces may be due to erosion prior to deposition, or to an abrupt change in grain size (depositional). Discoloration and possible borings occur locally on underlying dense, unlaminated mudstone, which commonly occurs as irregular intraclasts in overlying beds (Fig. 7b). The irregular surfaces are probably the result of high-energy scouring and accentuation of burrowed sediments, due to selective erosion of the less cohesive burrow-fills. The retention of intricate surfaces within intraclasts suggests early lithification and possible formation of firm- or hardgrounds. Although borings within the mudstone are difficult to confirm, because of its fine-grained texture, the presence of numerous sharp-walled excavations indicates some cohesiveness of the substrate.

Lithofacies

Five distinct lithofacies can be identified within the Becscie and Merrimack formations, based on variations in the nature of the sedimentary package and associated sedimentary structures. The contacts between the lithofacies are gradational (LF2-LF4, LF4- LF5, LF3-LF1) to abrupt (LF4-LF3). These lithofacies are described in Table 2.

SEDIMENTOLOGY AND DEPOSITIONAL ENVIRONMENTS

Sedimentary package

The erosive to sharp nature of the basal contacts of the sedimentary package, the abundance of reworked intraclasts and well-preserved bioclasts, the vertical stacking of rock types and their associated sedimen-

tary structures are all typical of tempestites; beds having been deposited during single storm events, as defined by Aigner (1982). The complete sedimentary sequence is therefore interpreted as being deposited by waning currents associated with storm sedimentation. The homogenous, fine-grained interbeds are interpreted as being the result of intervening fairweather sedimentation.

Dott & Bourgeois (1982) and Walker et al. (1983) proposed an ideal tempestite sequence containing a sequence of hummocky cross-lamination, planar lamination, cross-lamination and bioturbated mudstones in a distinct vertical zonation reflecting the changes in storm processes active at time of deposition. HCS and SCS appear to be diagnostic of inner shelf storm deposits, between fairweather and storm wave bases, where fairweather reworking is minimal and therefore where their preservation potential is greatest (Ham- blin & Walker, 1979).

Most of the tempestites show the effects of both oscillatory (wave ripples and cross-lamination) and unidirectional (gutter casts, tool marks) components of flow within the same unit. Evidence of unidirectional traction transport is rare, but climbing ripple lamination occurs locally.

The restriction of bioturbation to bed tops, the presence of escape traces, infiltration textures and predominantly parautochthonous to autochthonous shell assemblages in graded units support deposition from a single event (Kreisa, 1981 ; Kreisa & Bambach, 1982). Shell material is abundant, representing in situ reworking, winnowing or current deposition (Johnson & Baldwin, 1985). This shell material is commonly well preserved due to rapid burial and the resulting lack of biogenic degradation. Storm flows are episodic and locally short lived; as such they are not effective agents of shell fragmentationso that storms preserve, rather than destroy, loose fossils by burying and protecting them from fairweather processes and early diagenesis on the sea floor (Kreisa, 1981).

The quartz silt is closely associated with the tempestite units and generally absent in the fairweather units suggesting that silt-grade siliciclastic influx was storm controlled. The laminations are also restricted to tempestite units. The fairweather units commonly contain autochthonous or parautochthonous faunas and show extensive bioturbation, whereas tempestite beds are dominated by disarticulated allochems and only minor burrowing. Hardgrounds and firmgrounds (Fig. 7b) and early marine cements occur both at the tops of tempestite units and within fairweather beds and contributed to the widespread


Table 2. Description of lithofacies identified in the Becscie and Merrimack formations.

Lithofacies Rock types Bedding/structures Biota Lateral trends Vertical trends Stratigraphic range

Calcareous mudstone- shale (LFI)

Homogenous, laminated mudstone (LF2)

Calcareous grainstone- shale (LF3)

Laminated mudstone- grainstone (LF4)

Calcareous shale

Argillaceous abundant In situ

Lenticular to nodular High abundance (30-70%) Solution seams Low diversity

mudstone/ corals, packstone (20-50%) stromatoporoids,

Laminated packstone and calcisiltite (10- 20%)

Laminated and unlaminated mudstone ( < 5%)

Unlaminated

Laminated

Unlaminated/

mudstone (40-50%)

mudstone (40-50%)

laminated packstone ( - 5%)

Laminated calcisiltite grainstone ( - 5%)

Argillaceous content (< 1%)

Laminated mudstone, calcisiltite, fossiliferous grainstone, bio/ intraclastic rudstone (40-50%)

Unlaminated argillaceous mudstone, packstone (20-30%)

Calcareous shale (up to 30%)

Tabular, laterally

Irregular due to continuous

sediment loading and solution seams

packstone in thin, discontinuous lenses

Laminated units have wave-rippled tops locally

Coarse-grained ripples show orientated fossils in troughs

Unlaminated

Nodular to lenticular Poor lateral continuity

Hummocky cross- stratification

Coarse-grained ripples

Channels Gutters

brachiopods in fossil-rich horizons

Low abundance Low diversity In situ corals, brachiopods

nautiloids, crinoid stems, bryozoans, on bed tops

Orientated

High abundance High diversity In situ corals, stromatoporoids

Laminated Tabular to lenticular High abundance calcisiltite, bioi Poor lateral High diversity

Thickness increases Argillaceous 1 to the east from 3.3 to 8.0 m increases

content

upwards with a decrease in non- argillaceous units

decreases upwards

Fossil abundance

LJ. Merrimack Fm. Units L- O (Jupiter R.) Units 10-13 (Baie InnommCe; Copper & Long, 1989)

Thickness decreases Mudstone units Member 1, to the east from 22.0 decrease Becscie Fm. to 3.6 m upwards in

abundance as calcisiltite units increase

Fossil abundance increases upwards

Thickness varies from 20.0 to 25.0 m with no marked east-west trend

stromatoporoid colonies decrease in size eastwards

Shale and nodular bedding increase in abundance to the east as laminated calcisiltite and mudstone decrease

Coral and

In situ fossils and argillaceous content increase upwards as coarse-grained laminated units decrease in abundance

Thickness increases Unlaminated to the east from 98.0 mudstone

intraclastic continuity Locally tol15.0m decreases in grainstone, Nodular to irregular hiostromesof coral Unlaminated abundance as fossiliferous mudstone and mudstone decreases coarser-grained grainstone, Hummocky cross- stromatoporoids in abundance to the units and shale laminated stratification west as coarser increase mudstone, Channels grained units packstone (60-90%) Gutters increase

mudstone, ripples packstone (1040%) Coarse-grained

Argillaceous content ripples (


Lithofacies Rock types Bedding/structures Biota Lateral trends Vertical trends Stratigraphic ranee

Laminated Laminated calcisiltite- calcisiltite, grainstone fossiliferous (LF5) grainstone, up to

20% quartz silt

Bio/intraclastic (-100%)

grainstone (i 5%)

Amalgamated units High abundance Thickness decreases Unit thickness Member 2, Tabular to lenticular High diversity to the east from 15-0 decreases Becscie Fm. High thickness In situ to 0.0 m upwards (Jupiter R. variation coral, Calcisiltite and Cap Poor lateral stromatoporoids abundance decreases Henri-R. aux continuity rare to the east as Cailloux)

Hummocky cross- mudstone appears stratification

Channels Coarse basal lags Normal grading

formation of intraclasts with subsequent storm- sion with only minor distal, muddy tempestites generated scouring. present. Lower Stricklandia to Clorinda brachiopod

communities indicate approximate water depths of Lit hofacies 75-120 m (Copper & Long, 1989), placing deposition

Calcareous mudstoneeshale facies (LFI) of LFl effectively below normal storm wave base (Fig. 9). Fairweather sedimentation was dominated The calcareous mudstone-shale facies represents by settling of fine-grained siliciclastic sediment from predominantly fairweather deposition from suspen- suspension, with very little carbonate mud being

INNER MIDDLE SHELF _ I _ OUTER +I I< SHELF I SHELF t

lil . . . . . . . . . . . . . . SANDY TEMPESTITES MUDDY TEMPESTITES

FAIRWEATHER MUDSTOf PACKSTONE

FAIRWEATHER CALCAREOUS SHALE

LF5 LF4 LF3 LF2 LFI

Fig. 9. Depositional profile for the Becscie and Merrimack formations showing lateral relationships between lithofacies types. Of particular note is the overlapping of the depositional environments of LFl, LF2, LF3 and LF4 (modified from Handford, 1986). FWWB=fairweather wave base; SWB=storm wave base.


deposited. Energy levels are interpreted as having been very low, with the exception of occasional high- energy storm events which introduced sediments from shallower water. Diagenetic effects, enhancing non- homogeneous bioturbation, are important in defining the nodular bedding within LFl, as evidenced by the pervasiveness of solution seams and microstylolites (Wanless, 1979). The upward decrease in fossil content, increase in argillaceous mud and decrease in tempestite frequency indicate a deepening-upward trend during deposition of LF1. Carbonate mud sedmentation was re-initiated with deposition of the overlying Gun River Formation.

Laminated-homogenous mudstone facies (LF2)

The strataof this facies alternate between homogenous mudstones and packstones of fairweather origin and laminated mudstones and packstones representing distal tempestites (Fig. 10a). This corresponds with

the upper portion of the complete tempestite sequence (Fig. 6). The presence of in situ cyclocrinitid algae sets a lower depth limit of approximately 100 m (Beadle & Johnson, 1986). Most of the beds are interpreted to have been deposited below storm wave base (Fig. 9). The argillaceous content of the lime mudstones is very low but increases towards bedding contacts with the appearance of microstylolites, resulting in thin calcareous shale interbeds. This suggests that the calcareous shale interbeds are diagenetically enhanced bedding planes. The intense bioturbation and presence of in situ fossils within the homogenous mudstones and packstones indicate that they are depositional and not diagenetic.

Low-energy conditions prevailed during deposition of LF2, resulting in sedimentation due to the settling of carbonate mud and minor argillaceous mud from suspension. The sharp bases and normal grading of laminated units suggests physical sedimentation from low-energy, episodic, waning flows, with little or no

Fig. 10. Laminated-homogenous mudstone (LF2) and laminated mudstone-grainstone (LF4) lithofacies. (a) Cliff exposure of LF2 (Becscie Formation, Member 1, Cap Henri) showing laterally continuous bedding. Strata are dominated by irregularly bedded, homogenous mudstones and regularly bedded, laminated mudstones and packstones and thin interbedded calcareous shale horizons. Tape measure is 90 cm long. (b) Cliff exposure of LF4 (Becscie Formation, Member 2, R. aux Cailloux). Intraclastic rudstones, medium-grained grainstones and laminated calcisiltites are interbedded with thick, nodularly bedded homogenous mudstones. Lens cap is located on a packstone-filled depression between two hummocks of a hummocky cross: stratified tempestite which is gradational from an intraclast base. Lens cap is 55 mm in diameter.


erosion. Sediment transport was not extensive as evidenced by the lack of shallow water faunas, but sufficient to produce a slight difference between fairweather and storm-derived mudstones. The most noticeable difference in the latter is the presence of lamination and quartz silt. The 2 m thick transitional zone at the base of LF2 marks an abrupt deepening from the shallow subtidal environments of the Lafram- boise Member of the Ellis Bay Formation to the deeper subtidal environment of Member 1 of the Becscie Formation, and consists of a transition from stromatoporoid-coral bioherms, through encrinitic, reef-derived grainstones, to mudstones. This is followed by a gradual shallowing which is marked by an increase in tempestite units, from distal to increasingly proximal in nature, continuing up into the laminated calcisiltite-grainstone facies.

Calcareous grainstone-shale facies (LF3)

The calcareous grainstone-shale facies is transitional between the laminated mudstone-grainstone and calcareous mudstone-shale facies, but is distinct enough from both to be separated. The lower part of LF3 resembles the laminated mudstone-grainstone facies and the upper part of LF3 resembles the laminated-homogenous mudstone facies, but with an increased argillaceous content, both within and between beds. In terms of depositional environments, LF3 overlaps LF2 and LF4, but within a siliciclastic rather than carbonate-dominated regime (Fig. 9). Approximate water depths inferred from Virgiana to Stricklandia brachiopod communities (Copper & Long, 1989) were 40-75m (overlapping LF2 and LF4). The coarser rock types at the base of LF3 were deposited above storm wave base (equivalent to LF4), and locally show the complete tempestite sequence. The fine-grained rock types abundant in the upper part of LF3 were deposited below storm wave base (equivalent to LF2), and represent fairweather settling from suspension. Low- to very low-energy conditions were episodically interrupted by high- to moderate- energy storm conditions. These vertical trends suggest that LF3 represents an upward-deepening interval.

Laminated mudstone-grainstone facies (LF4)

The fairweather mudstones and packstones interbedded with the full range of tempestite rock types in this facies suggest a low-energy shelf subject to high- energy, erosive, storm currents (Fig. 9). The complete tempestite sequence is present, from coarse base to

muddy top (Figs 6,7 & lob). The abundance of wave- formed structures and excellent preservation of tempestites indicate deposition above storm wave base but below fairweather wave base (Walker, 1985). Approximate water depths range from 30 to 70m based on relation to other lithofacies, being shallower than LFl, LF2 and LF3, but deeper than LF5. There is an increase in bed thickness and coarseness, and a decrease in bioturbation and in situ fauna relative to lithofacies representing deeper environments (LF1- 3). The abundance of intraclasts indicates the pervasiveness of early lithification in this environment or possibly a derivation from adjacent shallower environments. The abundance of erosive features (gutters, channels, intraclasts) produced a very irregular depositional surface, resulting in the lenticular bedding morphologies (Fig. lob). Overall there is a shallowing- upwards followed by a gradual deepening-upward vertical trend. Up to four secondary shallowing- upwards intervals can be identified within LF4, each capped by occurrences of LF5 in the westernmost Becscie Formation exposures (Fig. 3).

Laminated calcisiltite-grainstone facies (LF5)

Equivalent to Dott & Bourgeois' (1982) M-cutout and lag-type storm units, this facies represents the shallowest subtidal environments observed in the Becscie and Merrimack formations. Deposition occurred above storm wave base and probably near fairweather wave base (Fig. 9), in approximately 10-30 m of water (Walker, 1985). This facies is entirely composed of tempestite units, mostly from the basal portion of the complete package, signifying very high energy (Figs 6 & 8b). The lack of intraclasts may suggest that their formation was restricted to deeper parts of the ramp or that they are effectively removed. Alternatively, the periods between storms may have been too short to allow the required early lithification. The absence of interbedded mudstone is probably due to removal by erosion rather than lack of deposition. In the measured sections, strata of LF5 decrease in thickness strati- graphically upward, indicating a deepening-upward trend.

Palaeocurrent and palaeoslope indicators

Palaeocurrent indicators are abundant in the Becscie and Merrimack formations [Fig. 11) and include: (i) orientated elongate fossils, (ii) gutters and gutter casts, (iii) symmetrical wave and interference' ripples, (iv) symmetrical CGR and (v) asymmetrical CGR. Two


GUTTERS AND GUTTER CASTS

N

@ x-107- SYMMETRICAL WAVE RIPPLES

ELONGATE FOSSIL ORIENTATION

INTERFERENCE WAVE RIPPLES

N - SYMMETRICAL MEGARIPPLES

ASYMMETRICAL MEGARIPPLES CROSS-LAMINATION

SLUMP BED AXES

Fig. 11. Palaeocurrent rose plots for various parameters. The number of measurements is given at the centre of each plot.

dominant palaeocurrent directions are recognized : a unidirectional current heading of 90-100" and a combined unidirectional/bidirectional heading of 20/ 200"-40/220". The east-west variations in the two dominant palaeocurrent directions are minor (Fig. 11) with the unidirectional indicators (asymmetrical CGR) showing a slight clockwise rotation westwards and the bidirectional indicators (gutters, elongate fossils, symmetrical/interference wave ripples, symmetrical CGR) a slight anticlockwise rotation westwards. There are no discernible vertical trends, with palaeocurrent values overlapping throughout the sequence. The range of features represents both unidirectional and oscillatory current structures, suggesting a combined flow regime for the Becscie and Merrimack formations. Palaeoslope indicators (e.g. slump bed axes) indicate a downslope heading of 108". These indicators will be discussed in more detail below.

Depositional environments

The abundance of lime mudstone/packstone, diverse open marine biota, whole fossils, fining-upwards storm sequences, bioturbation, and the absence of any preserved slope breaks, reef margin or slope and basin sediments corresponds to Read's (1985) definition of a homoclinal ramp with an inferred palaeoslope of 1- 2'. Analysis of the five lithofacies suggests that fairweather deposition occurred on a low-energy, muddy to argillaceous carbonate ramp, 10-120 m deep (Fig. 12). Two main depositional processes were involved : the settling of fines from suspension during fairweather periods, and the high-energy physical sedimentation of tempestites from waning flows and from suspension during episodic storms. These short- lived, high-energy events caused extensive re-mobili-

INNFR . . .. . -. .

INNER CRlNOlD SHOALS/WKS 2 RMnP

Fig. 12. Block diagrams illustrating the inferred distribution of depositional environments and their associated lithofacies for: (above) a carbonate mud-dominated ramp (Becscie Formation) and (below) an argillaceous mud-dominated ramp (Merrimack Formation). A crinoid shoal/bank environment is inferred as a source of abundant carbonate sand in the sequence and is not preserved within the Becscie or Merrimack formations. For lithological symbols see Fig. 9 (modified from Handford, 1986).


zation of sediments and basinward transport of both sandy and muddy sediments. The quartz silt is exclusively found in storm units and represents material introduced onto the ramp but derived from another environment, most probably due to longshore transport as in Long & Coppers (1987b) model of the Ellis Bay Formation.

Evidence for storm-induced sedimentation is extensive, including: HCS and SCS, amalgamation, grading, interference ripples, gutters and subtidal intraclasts (Kreisa, 1981 ; Einsele & Seilacher, 1982). The presence of both oscillatory and unidirectional current-generated features suggests a combined flow storm regime (Swift, 1985; N~rttvedt & Kreisa, 1987).

The observed lithofacies reflect ramp subdivisions and their changing depths and related processes (Figs9 & 12). In order of decreasing depth, the depositional environments of these lithofacies are as follows:

(LFl) rare, thin, distal tempestites interbedded with fairweather shales; deposition by settling from suspension on the outer ramp at water depths of 75-120 m;

(LF2) thin, distal tempestites interbedded with fairweather lime mudstones; deposition by settling and episodic waning flow on the middle to outer ramp at water depths of 70-100 m;

(LF3) thin to thick, argillaceous tempestites interbedded with fairweather shales and argillaceous lime mudstones; deposition by settling and episodic waning flows on the middle to outer ramp at water depths of 40-75 m;

(LF4) medium to thick, coarse-grained tempestites interbedded with fairweather lime mudstones; deposition by settling and episodic high-energy waning flows on the inner to outer ramp at water depths of 30-70 m;

(LF5) thick, amalgamated tempestites ; deposition from high-energy storm waves and currents on the inner ramp at water depths of 10-30 m.

The distribution of these lithofacies is illustrated in Figs 9 & 12. The crinoid shoals/banks illustrated in Fig. 12 are inferred from the abundant crinoid material within the observed sediments and are not preserved within the Becscie and Merrimack formations on Anticosti Island. Crinoid-rich facies higher in the sequence (Chicotte Formation) on Anticosti Island are believed to be analogous. The vertical succession of these lithofacies in conjunction with the tempestite proximality data can be used to infer relative sea-level changes based on a muddy carbonate

ramp model. The lithofacies, when all are present, occur in the following sequence from base to top (Fig. 3): LF2, LF4, LF5, LF4, LF3 and LF1. The transition zone at the base of LF2 coincides with the latest Ordovician to earliest Silurian sea-level rise from a shallow subtidal environment to the outer ramp environment of LF2. The transition from LF2 ~ to LF4 marks a shallowing-upward event, or regres- sion; this is continued with the transition of LF5. The shallowing-upward trend marks a shift from deep- water, outer ramp, carbonate mud sedimentation into shallow-water, inner ramp, tempestite-dominated sedimentation. The upward shift from LF4 to LF3 and, subsequently, LF1 represents a deepening- upward or transgressive event from the shallow inner ramp to an argillaceous mud-dominated, deep, outer ramp. The overlying lime mudstones of the Gun River Formation mark a re-establishment of muddy, deep- water carbonate sedimentation. Superimposed on this deepening-shallowing-deepening sequence are the secondary shallowing-upward intervals represented by LF5, also seen in the second-order proximality fluctuations discussed below.

TEMPESTITE PROXIMALITY PARAMETERS AND TRENDS

Introduction

Tempestite units may differ in appearance from an ideal sequence due to incompleteness and amalgamation. These variations are easily observed and may be used to plot trends through storm-influenced sequences. Aigner (1985) suggested that variations in tempestite morphology are useful for establishing proximality trends, these being defined by changes in distance from shore and depth-related parameters. Factors which affect the sedimentary record of storms include : storm duration, storm intensity, storm nature, depth of the deposit and distance of the deposit from the source of sediment or shoreline (Allen, 1982).

The parameters commonly used to infer proximality reflect the sedimentological character of tempestites and include : grain size, unit frequency, bed thickness, amalgamation occurrence, presence of cross-lamination, degree of bioturbation, bedding style and fossil assemblage. Variations in these parameters produce both lateral and vertical trends within a sequence and are meant to reflect relative depth and proximality changes. This enables tempestites to be described as relatively proximal to distal in nature, with no

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indication of absolute depths or depth changes. Proximality trends have been used to infer relative sea-level changes, basin morphology and sediment dispersal patterns (Aigner, 1985; Brett er al., 1986; Baarli, 1988; Brookfield & Brett, 1988; Driese, 1988; Easthouse & Driese, 1988; Gagan et al., 1988). However, bathymetric profiles inferred from vertical sequences are more complex and less straightforward in interpretation because many parameters used are not exclusively controlled by depth and distance to shore. As such, care should be taken in assuming that a vertical sequence can be translated into a bathymetric profile and the subsequent construction of tempestite-based sea-level curves.

Variations in tempestite sequences which are used to infer proximality trends are summarized in Aigner (1985) for recent and ancient sediments. In general, nearshore tempestite sequences are characterized by amalgamated, erosionally bounded, bioclastic tempestite beds, 5-130cm thick with no fairweather interbeds. Proximal tempestite sequences contain a large number of complete tempestite beds, HCS, are 5-100cm in thickness and are interbedded with fairweather beds. Distal tempestite sequences are composed primarily of fairweather units interbedded with very fine-grained tempestite beds 4-10 cm thick, having erosive or non-erosive bases, internal planar lamination grading up rarely into cross-lamination, and contain predominantly parautochthonous fauna.

Proximality parameters

Several proximality parameters were used in the present study : (a) tempestite frequency per metre; (b) maximum tempestite thickness per metre; (c) mean tempestite thickness per metre; (d) combined tempestite frequency and mean thickness per metre; (e) combined intraclastic tempestite frequency and mean thickness per 5 m; and (f) percentage tempestites per metre. Other parameters (bioturbation, cross-lamination) were tested but did not provide any definitive results. These parameters were plotted against the four composite stratigraphic sections and the results are shown in Fig. 13.

Lateral trends

The frequency plots (Fig. 13a) show a slight decrease eastwards, indicating a possible decrease in proximality to the east. Tempestite frequency, however, is not solely related to proximality, but also depends on the thickness of the tempestites, as this will affect the

number that may fit in a given length. Proximality increases with bed thickness (Fig. 13b,c), but as beds become thicker there are less per metre, giving an anomalously low frequency. This apparent anomaly can be corrected for by combining the two parameters (frequency and mean thickness) to give a better indication of proximality (Fig. 13d).

These combined parameters display similar but more pronounced east-west trends than the individual plots. These trends show an eastward decrease in proximality as all the parameters decrease eastward. The proximality peaks are more numerous and more clearly defined to the west, reflecting larger and more frequent fluctuations in energy. Many of the pronounced shifts of these parameters to the west reflect the presence of LF5. The intraclastic mean frequency plots (Fig. 13e) are different in appearance from the previous plots in having a lower resolution, as they are plotted for every 5 m rather than for every 1 m. These plots reveal only the coarser proximality fluctuations. They do not exactly match the other curves because intraclasts are absent in the shallowest and the deeper lithofacies, but are good proximality indicators for the median water depths. They similarly indicate an eastward decrease in proximality.

The percentage plots (Fig. 130 show the best definition of proximality peaks. Up to 60 distinct proximality fluctuations may be distinguished within these plots, showing an average spacing of 2-3 m. The percentage of tempestite units decreases to the east, indicating decreasing proximality. In summary, all proximality parameters indicate an eastward deepening. Some parameters (Fig. 13a,d,e) show an apparent increase in proximality from the Cap Henri section to the R. Jupiter section at several stratigraphic intervals, in contrast to the overall observed trend of eastward deepening. This slight discrepancy is an artefact of the preservation of proximality indicators. Erosion and amalgamation of tempestites in shallow water depths overprint proximality indicators so that the most proximal indicators commonly occur slightly basinward of the shallowest tempestites, producing anomalous trends.

Vertical trends

In order to interpret the vertical proximality trends, the various proximality plots were combined cumula- tively into a single curve for each section (Fig. 14). The vertical fluctuations are used to infer lateral shifts of depositional environments with time, anologous to the lateral trends. The vertical sequence is similar for


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Fig. 14. A comparison of the combined tempestite proximality trends for each of the four composite sections showing vertical and lateral changes. Each plot shows increasing proximality to the right. All plots are to the same horizontal scale for comparison.

the four sections and consists of an upward increase in proximality followed by a decrease in proximality. The main difference between the sections is that in the east, shallowing-upwards trends begin stratigraph- ically lower than in the west (Fig. 14). More than 20 distinct, smaller fluctuations are superimposed on this overall trend. Both lateral and vertical variations in tempestite proximality demonstrate trends similar to those interpreted from the lithofacies distributions.

Proximality fluctuations

Up to three orders of tempestite proximality fluctuation can be distinguished in the combined curves (Fig. 15) with the help of time-series analysis (Miller & Kahn, 1962). The three orders reflect different processes which produce changes in tempestite proximality on different scales.

The third-order curves, represented by the proximality curves (Figs 14 & 15), display 20-25 distinct fluctuations. Adopting a time span of approximately 5 Ma for the duration of the Rhuddanian (Palmer, 1983), these fluctuations may be occurring with a period of 80-100 ka, corresponding to fifth-order eustatic sea-level curves of Vail et a / . (1977). This period of fluctuation is similar to that attributed to

some Milankovitch sea-level variations (Read & Goldhammer, 1988), and commonly used in the modelling of cycles (Read et al., 1990). With such a rapid fluctuation it becomes difficult to separate the influences on tempestite proximality of depth and distance changes from those due to changes in storm intensity, duration and direction. The third-order curves may therefore not represent true sea-level fluctuations. A clear distinction between these factors influencing tempestite proximality is not possible at this stage.

Up to five proximality fluctuations can be identified in the second-order curves (Fig. 16), corresponding to the occurrences of LF5 in western sections, giving an average cycle period of approximately 1 Ma (fourth- order cycles of Vail et a/., 1977). Given such a period, these fluctuations may represent eustatic or non- eustatic changes in sea level. Insufficient data on sea- level fluctuations at this resolution in other Early Silurian basins prevent any conclusions on whether the fluctuations are intrinsically (locally) or extrinsi- cally (globally) controlled.

The first-order proximality curves show a gradual shallowing-deepening cycle, occurring aver a period of 5 Ma (third-order cycles of Vail et al., 1977), following the initial deepening at the base of the


150 -

100 -

m - e3 2 -

50 -

0 3rd Order 2nd Order 1 st Order

Fluctuations Fluctuations Fluctuations

Fig. 15. Breakdown of combined proximality trends into decreasing orders of fluctuation. Curves are derived directly from the proximality curves by time-series smoothing. The example shown is taken from the R. Jupiter section.

W E

0

Cap Henri- R. Jupiter R. aux R. Prinsta- R. aux Cailloux Saumons Reef Point

Fig. 16. Correlation of second-order proximality fluctuations between the four composite sections. Up to five distinct events can be recognized. These events are shifted strati- graphically downwards in the two eastern sections and the fifth event is not represented due to incompleteness of the sections and the inadequacies of time-series analyses in dealing with covered intervals. Proximality increases to the right within each curve.

Becscie Formation (Fig. 17). These curves correlate closely with published North American Early Silurian third-order eustatic sea-level curves (Johnson, 1987). This suggests that this period of proximality fluctuaton may result from eustatic sea-level changes.

DISCUSSION

Argillaceous versus carbonate mud sedimentation

The transition from muddy (carbonate) to argillaceous (siliciclastic) and subsequently back to muddy (carbonate) deep-water sedimentation in the Becscie- Merrimack-Gun River succession marks a significant change in the nature of fairweather sediments. Assuming a sedimentological control such as depth of deposition, there should be no significant overlap between carbonate and argillaceous deep-water lithofacies. A substantial overlap, however, does exist between LF1, LF2, LF3 and LF4, as evidenced by palaeobathymetric and lithological indicators, suggesting that the controls may be related to sediment

Cap Henri- R. Jupiter R. aux R. Prinsta- Johnson R. aux Cailloux Saumons Reef Point eta/. I981

Fig. 17. Correlation of first-order proximality fluctuations of the four Becscie Formation composite sections and a published Early Silurian eustatic sea-level curve based on brachiopod communities (Johnson et al., 1981). A,-*, A3 and A4 represent graptolite zones of the Rhuddanian stage. One fluctuation is observed following an initial deepening event (poorly defined due to scale). Proximality increases to the right, sea level falls to the right. Eastern sections show I poor resolution due to incompleteness of the sections and presence of covered intervals.


source and/or processes. The deep-water carbonate muds were probably derived from shallower-water production on what is commonly referred to as the subtidal carbonate factory (James, 1984) and introduced into deeper water from suspension. The introduction of these fines into suspension may be related to storm or fairweather processes. As long as the zone of carbonate production is at optimal depths then the influx of carbonate mud to the outer ramp is sufficient effectively to dilute siliciclastic input and produce a lime mud sequence. If the marine transgres- sion is rapid enough and sediment production cannot keep up, the supply of carbonate mud to the outer ramp will fall drastically, and an argillaceous mud- dominated outer ramp sequence will result. The authors propose that the shift from a muddy (carbonate) outer ramp at the base of the Becscie Formation to an argillaceous outer ramp at the base of the Merrimack Formation was due to rapid sea-level rise from deposition of the uppermost Becscie Formation onwards. When carbonate production finally caught up, during deposition of the lower Gun River Formation, a muddy carbonate outer ramp was re- established.

Whilst the presence of quartz silt may be explained by longshore drift, the origin of the argillaceous mud is more problematic. A duality of sources, as in the Lower Silurian of Tennessee (Driese, 1988), cannot be positively identified. The paleogeographical placement of Taconica (see Fig. 18) in relation to the Anticosti Basin and its role as a sediment source for

this basin are poorly understood. The occurrence of calcareous shale as thin interbeds throughout the predominantly carbonate portions of the Becscie and Merrimack formations may suggest that the argillaceous mud, like the quartz silt, is being introduced across the platform through longshore drift, and only becomes an important rock type when carbonate production decreases.

Shelf palaeogeography and dynamics

LF2 and LF5 thin to the east, whereas LFl and LF4 thicken to the east; insufficient information is availa- ble on LF3 to identify any thickness trend (Fig. 3). The absence of shallow-water LF5 in the east and the increased thickness of deep-water LF1 in the east are indicative of an eastward deepening, whereas the thinning of LF2 to the east is related to the loss of tempestite units and a decreased supply of lime mud in the deeper waters of the outer ramp. The apparent thickening of LF4 to the east might be related to a decrease in erosion of tempestites and an increase in accommodation space with increasing depth as is common on many storm-dominated ramps, where maximum accumulation is slightly offshore (Aigner, 1985).

These lithofacies variations, together with lateral tempestite proximality variations and palaeoslope indicators, suggest that the Anticosti Platform deepened south-eastward in the Early Silurian during deposition of the Becscie and Merrimack formations,

Combined Flow

lapetus Ocean

Fig. 18. Palaeogeographical reconstruction of the Anticosti Basin during deposition of the Becscie and Merrimack formations, showing the average storm track and induced combined flow regime, superimposed on ramp subdivisions. The ramp deepens south-eastwards towards the partially open Iapetus Ocean. Modern coastlines are represented by broken lines.


indicating that the palaeoshoreline lay to the north- west. Palaeocurrent indicators suggest that unidirectional currents were orientated to the south-east, and that combined unidirectional/oscillatory currents were orientated north-eastwards. Storm tracks, inferred from palaeoreconstructions and climatic models (Ziegler et a[., 1977; Barron, 1989), would have moved from east to west with a poleward deflection. This path agrees with the oscillatory palaeocurrents observed. In response to the induced pressure gradients offshore-directed unidirectional bottom currents and longshore combined-flow, geostrophic currents were generated, ESE- and NE-directed respectively. This information is summarized in a palaeogeographical reconstructionof the Anticosti Basin (Fig. 18) showing average storm track and storm flow dynamics for the earliest Silurian (when the Becscie and Merrimack formations were deposited). The ramp is subdivided into inner, middle and outer parts, deepening into the Iapetus Ocean to the south-east and shallowing towards a proposed shoreline to the north-west. The orientation of the shoreline is based on palaeoslope indicators, slump bed axes, and the orientation of the sedimentary structures : gutters subparallel to shoreline (Aigner, 1985) and CGR crests subparallel to shoreline (Leckie, 1988). The shoreline orientation is approximate and may vary slightly in either sense. Assuming a maximum water depth of 120 m for the deepest lithofacies (LF1) and a minimum of 10 m for the shallowest lithofacies (LF5), the inferred shoreline may be located as close as 35 km at maximum lowstand and as distant as 400 km at maximum highstand. Actual values are probably more conserv- ative, ranging from 50 to 200 km. Similar estimates provide for a water depth difference of about 60 m between the inner and outer ramp.

Storm nature

Inferring palaeolatitudes of 15-20"s (Ziegler et al., 1977; Scotese et al., 1985) from Early Silurian palaeocontinental reconstructions, the Anticosti Basin was situated in a zone strongly influenced by cyclonic storms. This climatic interpretation is based on the modern latitudinal distribution of cyclones and mid- latitude winter storms (Barron, 1989). Assuming that their distribution was similar in the Early Silurian, then the dominant storm type producing tempestite beds on the Anticosti Platform was cyclonic. Circula- tion models for the Early Silurian (Ziegler et al., 1977) show storm tracks rotating anticlockwise in the southern hemisphere. Cyclones would start near the

equator and travel westward and poleward. Cyclones crossing the Anticosti Basin would probably be generated on the west coasts of Kazakhistania or Baltica, depending on the size of the Iapetus Ocean, and cross the basin from east to west. Palaeocurrent data measured in the Becscie and Merrimack formations agree with such a path (Fig. 18).

Barron (1989), however, contends that simple latitudinal distinction of storm types is not adequate enough to permit a genetic interpretation of storm- related deposits, maintaining that climate and geography produce substantial variability in the generation rates and distribution of severe storms. Although the Anticosti Basin lies within what might presently be considered a cyclone-dominated zone, Early Silurian palaeogeography could have produced a different distribution of storm types.

SUMMARY

Several limestone rock types, organized into five distinct lithofacies, are present in the Lower Silurian (Rhuddanian, A,-A,) Becscie and Merrimack formations. The lithofacies suggest that rocks of these formations were deposited on a low-energy, muddy, carbonate to argillaceous ramp subject to episodic high-energy storms. Palaeocurrents and sedimentary structures demonstrate that the resultant storm units were deposited within a combined flow regime, probably as the result of cyclonic storms. The tempestites exhibit proximality trends which prove useful in describing palaeobathymetric fluctuations and inferring basin geometry.

The lateral proximality trends, lithofacies changes and palaeoslope indicators imply that the Anticosti Basin deepened into the partially opened Iapetus Ocean to the south-east and shallowed towards a proposed, NE-SW-trending shoreline to the north- west. The vertical proximality fluctuations and lithofacies changes enable the recognition of palaeobathymetric and possibly climatic fluctuations. The Becscie and Merrimack formations together .exhibit a first- order (third-order of Vail et al., 1977) eustatic deepening-shallowing-deepening cycle, with the first deepening episode producing a lime mud-dominated ramp and the second an argillaceous mud-dominated ramp. This shift from a dominantly carbonate to a mixed carbonate/argillaceous and subsequently back to a carbonate-dominated environment suggests important changes in sediment supply controlled by the ' rate of relative sea-level fluctuations. Superimposed


on this cycle are up to five second-order (fourth-order of Vail et al., 1977) relative sea-level fluctuations and problematic third-order (fifth-order of Vail e t al., 1977) fluctuations which may reflect either palaeobathymetric or climatic changes.

The resolution of fluctuations observed with tempestite proximality trends is an order of magnitude higher than those identified by either lithofacies or palaeontological methods. With further investigation into the factors controlling tempestite proximality, these trends may become powerful tools in the analysis of storm-dominated basins.

ACKNOWLEDGMENTS

This paper evolved from an MSc thesis by T.S., supervised by A.D. a t the University of Ottawa. Critical reviews by N. P. James, 0. A. Dixon, B. Rust and two anonymous reviewers greatly improved earlier versions of this paper. The manuscript subsequently benefited from careful reviews by S. G . Driese and R. L. Brenner and comments by B. Arnott. SEPAQ provided access to the field area. D. Brisebois of the Quibec Department of Energy and Mines provided access to, and help with, the core. L. Bacon provided able assistance during the field-work. E. Hearn provided much needed assistance during the photographic preparations and drafting of some figures. The study was supported by Operating Grant no. A1891 from the Natural Sciences and Engineering Research Council to A.D. NSERC, the CSPG and the AAPG provided funds to the senior author during the study. Many thanks are due to the residents of Anticosti Island for their help and support during the two field seasons.

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(Manuscript received 22 July 1991 ; revision received 2 December 1991)

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