Depositional environments of a thick Proterozoic sandstone: the (Huronian) Mississagi Formation of Ontario, Canada

Department of Geology, University of Western Ontario, London, Ont., Canada N6A 587

Received May 3, 1977

Revision accepted September 9, 1977

The Mississagi Formation is a thick (up to 3.4 km) Proterozoic arenite sequence that forms part of the Huronian (lower Aphebian -2.2-2.5 Ga) succession of the north shore of Lake Huron, Ontario. The formation is characterized by planar and to a lesser extent trough cross-stratified medium to coarse feldspathic arenites, with only minor amounts of argillite and conglomerate. Although the formation lacks any regular systematic cyclicity, both thinning upward and fining upward sequences can be recognized in some sections. Paleocurrent roses for individual outcrops are typically unimodal, although some bimodal distributions are recognized. The bulk of the formation is interpreted to be the result of deposition in a fluvial environment, principally from bed load and mixed load streams. These rivers were probably marked by a braided stream pattern in which channels were characterized by intermediate to low sinuosities and high width to depth ratios. Regional paleocurrent and petrographic trends indicate that two major river systems were operative. One system flowed east and south from the Sault Ste. Marie - Elliot Lake region to meet a second, southwesterly flowing system originating in the Cobalt Plain. These systems met in the southern Huronian area, from where the coalescing river systems flowed south.

La formation de Mississagi est une sequence d'arenite Cpaisse Uusqu'a 3.4 km) du Proterozoique qui forme une partie de la succession de I'Huronien (Aphebien inferieur, -2.2-2.5 Ga) sur la rive nord du lac Huron, en Ontario. La formation se caracterise par des arenites feldspathiques de granulometrie moyenne grossiere en lits planaires et jusqu'a un certain point avec stratification croisee en auges et par des quantitks mineures d'argillite et de conglomerat. Bien que la formation manque de cyclicite systematique reguliere, on reconnait dans certaines sections des sequences qui s'amincissent et deviennent plus fines vers le haut. Les rosettes de paleocourants pour des affleurements individuels sont typiquement unimodales, bien qu'on reconnaisse quelques distributions bimodales. On interprtte le plus gros de la formation comme le resultat de dep6t dans un milieu fluviatile de materiaux provenant de la charge de fond et de la charge mixte des cours d'eau. Ces rivieres possedaient probablement un reseau anastamose de chenaux qui se caracterisaient par des sinuosites intermediaires a faibles et des rapports largeurlprofondeur tleves. Les paleocourants regionaux et les tendances petrographiques indiquent que deux systemes fluvjaux majeurs ont exist&. Un systkme fluvial s'ecoulait vers I'est et le sud en provenance de la region de Sault-Ste-Marie - Elliot Lake a la rencontre d'un second systeme s'ecoulant vers le sud ouest avec sa source dans la plaine de Cobalt. Ces systemes fluviaux confluaient dans le sud de la region huronienne pour ensuite couler vers le sud.

[Traduit par le journal] Can. J . Earth Sci., 15, 190-206 (1978)

Introduction

The depositional environment of the Mississagi Formation has been ascribed variously to fluvial, shallow-water, fluvial-deltaic and marine envi- ronments. The first detailed investigation of the formation (McDowell 1957), undertaken in the El- liot Lake - Blind River area, indicated deposition in a fluvial environment. Pienaar (1963) interpreted the same rocks as indicating a nearshore or deltaic environment. Following this, the majority of publi- cations appear to favour a fluvial origin (Ginn 1965; Card 1967, 1968; Young 1968; Peters 1969; Roscoe 1969; Frarey and Roscoe 1970; Casshyap 1971; Meyn 1972, 1973; Card et al. 1973; Chandler 1973; Parviainen 1973; Young 1973). The fluvial interpre-

tation was questioned by Pettijohn (1970a ,b) , who noted that Huronian sandstones do not show the usual attributes of fluvial (meandering) stream de- posits in that they lack fining upward cycles; he also claimed that deposition in a braided stream envi- ronment was improbable. Pettijohn (1970a,b) suggested that Huronian cyclicity recorded regres- sive marine cycles with little or no evidence of emergence. Palonen's (1971, 1973) study of the Mississagi Formation in the Lake Panache area seemed to confirm Pettijohn's scepticism. Palonen (1971, 1973) concluded that the Pecors and Missis- sagi Formations did indeed constitute a normal re- gressive marine sequence, citing as evidence pos- sible marine sedimentary cycles and bimodal paleocurrent distributions. When his arguments in -

'Present address: Institute of Sedimentary and Petroleum a marine (Pettijohn 1970a) Geology, Geological Survey of Canada, 3303-33t-d s t . N.w., were countered by Roscoe and Frarey (l970), Petti-

- Calgary, Alta., Canada T2L 2A7. john (1970b) suggested that the origin of Huronian

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sandstones could be resolved only by detailed ! studies of lithology, stratigraphy and paleocur- ' rents. This paper stems from an attempt (Long

1976) to apply such methods on a regional, rather than local, scale in order to resolve the controversy that surrounds the interpretation of depositional environments of the Mississagi Formation.

Geological Setting The Mississagi Formation is part of the Huronian

(lower Aphebian) succession of the north shore of Lake Huron, Ontario, Canada (Table 1). Huronian rocks crop out in a broad belt between Sault Ste. Marie and Noranda (Fig. 1 inset), occupying a structural position that straddles the Superior and Southern tectonic provinces (Card et al. 1972). The southeastern termination of recognizable Huronian rocks is the contact of the Superior and Southern Provinces with rocks of the Grenville structural province. Highly metamorphosed equivalents of the Huronian may be present within the Grenville Province (Quirke and Collins 1930; Frarey and Cannon 1969), but have been altered to such an extent that sedimentological studies are impracti- cal. Within the main outcrop belt, metamorphic

I TABLE 1. Huronian Formations (modified after Robertson et a!. (1969))

Nipissing Diabase (e 2.1 Ga old) ................. ; Intrusive contact.. ................ t Cobalt Group

1 Bar River Fm. H Gordon Lake Fni.

Lorrain Fm. Gowganda Fm.*

U

I Unconformable to conformable contact

I Quirke Lake Group

Serpent Fm. 0 Espanola Fm.

Bruce Fm.*

N Local disconformable contact I I Hough Lake Group

Mississagi Fm. Pecors Fm.

A Rarnsay Lake Fm.*

Local disconformable contact

i Elliot Lake Group?

1 McKim Fm. Matinenda Fm.* ?

................... Unconformity ................... Archean (== 2.5 Ga old)

'Contains rocks of glaciogenic origin. tIncludes local volcanic members.

grade ranges from subgreenschist in the north and northwest, to almandine-amphibolite grade in the vicinity of the Grenville front (Card et al. 1972). The southern limit of the Huronian is obscured by Paleozoic and later cover, but is placed below Man- itoulin Island on geophysical evidence, augmented by scant borehole data (Van Schmus et al. 1975). Collectively the Huronian sequence forms an apparently southerly thickening wedge of plat- form-type sedimentary rocks at the southern mar- gin of the Superior structural province. Although analogies have been suggested (Dietz and Holden 1966; Lumbers 1971, 1975; Card and Lumbers 1976; Van Schmus 1976), this wedge (the Southern Province) cannot be fitted readily into conventional geosynclinal-plate tectonic models developed from Phanerozoic examples (Card et al. 1972) and may represent a simple intracratonic, fault-bounded trough (Parviainen 1973, p. 35 1).

The Huronian sequence consists of an appar- ently cyclical association of conglomerates, silt- stones and sandstones (Roscoe 1969; Frarey and Roscoe 1970) deposited between 2.5 and 2.15 Ga ago (Van Schmus 1965). The conglomeratic parts of this sequence (Ramsay Lake, Bruce and Gow- ganda Formations) appear to have been deposited in a glacial to paraglacial environment (Young 1966, 1970; Roscoe 1969; Parviainen 1973), pos- sibly in middle to high latitudes (Morris 1977; Sy- mons 1975). Details of individual formations and prior history of investigation of the area are sum- marized in papers by Collins (1925), Roscoe (1969),

4hbertson et al. (1969), Robertson (1973) and Long (1976). The structural, stratigraphic and metamor- phic setting were described by Card et al. (1972).

Stratigraphy of the Mississagi Formation The Mississagi Formation is a rather monoton-

ous sequence of feldspathic and subfeldspathic arenites (Table 2), which includes only minor amounts of argillite (less than 0.25%) and conglom- erate. A grain size increase, from medium to medium-fine sand grade at the base to medium and coarse sand grade at the top, is noted in some sections but is not a ubiquitous feature of the for- mation. Correlations within the formation, based on grain size changes, sedimentary structures and petrography, were attempted but found to be un- tenable (Long 1976). The contact with argillaceous rocks of the underlying Pecors Formation is in most places conformable, and may be either transitional over afew metres, or sharp. The base of the overly- ing Bruce Formation is commonly erosive and may be locally conformable or disconformable. The local and regional stratigraphy of the Mississagi Formation is described in detail by Long (1976).

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192 CAN. J. EARTH SCI. VOL. 15, 1978

FIG. I . Location, distribution and thickness (in metres) of the Mississagi Formation. A to E are locations of areas in Table 2.

TABLE 2. Average composition of Mississagi sandstones (range in brackets) based on counts of 500 or more points per slide. Framework grains are recalculated to 100%. N = number of samples examined. For analysis of individual samples see Long

(1976, Appendix 3)

Framework grains Matrix

Quartz + Rock frag. Carbonate Area* chert Feldspar + acc. Ratiol Ortho- Epi- cement N

-

A: original Huronian area

B: Quirke Syncline

C : south of the Chiblow Ant.

D : northern outliers

E: southern Huronian

F: Cobalt Plain

All of the Mississagi Fm.

14.8 2.6 (8-29) (0-5) 55:28:17 5.6 1.3 0.4 33

17.3 4.8 (4-55) (1-10) 69:15:16 8.5 1 .0 tr. 56

13.8 3.6 (5-38) (0- 1 3) 58:15:17 7.6 0.6 0.1 64

14.8 1.2 (9-31) (0-2) 65:10:25 3.4 0 .0 0.0 6

20.5 2.2 (3-46) (0-9) 55:27:18 7.2 0.1 0.1 41

22.8 8.2 (8-33) (1-21) 28:21:51 14.8 ti-. 0 .2 25

17.1 3.9 (3-55) (o-21) 57:19:24 8.1 0.6 0.1 225

*See Fig. 1 for location. ?Ratio of granitic, to volcanic plus hypabyssal, to sedimentary plus metasedimentary rock fragments.

Thickness of the formation (in metres) based on a mation, especially in areas of low metamorphic compilation in Long (1976) is shown in Fig. 1 . grade. Recrystallization and growth of new miner-

als in areas of higher metamorphic grade, and Sedimentary Structures and Associations cataclastic deformation in areas of more intense

Sedimentary structures are abundant and gener- deformation both tend to obscure sedimentary ally well preserved in rocks of the Mississagi For- structures, especially in the finer grained rock

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LONG 193

types. Structures such as cross-stratification are preserved locally in coarse-grained units, even in rocks of almandine-amphibolite facies. General bedding characteristics also may be recognized in the higher grade metamorphic rocks were there is an alternation of different rock types.

Cross-stratijication Cross-stratification (Figs. 2,3) is the most impor-

tant type of sedimentary structure in the Mississagi Formation. It is best developed in the sandstones, but is also present in the mud-grade clastics. Indi- vidual cross-beds are present on all scales from a few millimetres to over 4 m (average = 41 cm). Over 87% (by volume) of the formation is made up of planar cross-stratified sets. Average thickness of these (45 cm) is significantly greater than that of associated trough cross-stratified sets (18 cm). Ripple cross-stratification (i.e. less than 5 cm scale) makes up about 2% of the measured units (or 0.2% of the formation).

Within individual sets, grain size is essentially homogeneous, with minor fluctuations emphasiz- ing individual cross-strata. Inhomogeneities ex- ceeding 2 $ units (cf. Allen 1963a, p . 100) occur in some planar sets, especially those characterized by coarse sand grades. Two styles of inhomogeneity are recognized. In the first, individual foresets are graded, in some instances from granule to fine sand grade (Fig. 2C). Individual cross-strata are planar and range in thickness from a few millimetres to 10 cm. Sets of this type are generally thick (up to 4 m). In the second type (cf. McDowell 1957, Fig. 8), individual foresets are also graded and may be pla- nar or planar curved. In addition to the grading within individual cross-strata, grading occurs within sets, to produce higher concentrations of coarse material (in some cases to pebble grade) towards the base of the set. A variant of the second type, developed in a few trough cross-stratified sets, results in the formation of pebble trains (not laterally extensive) that parallel the lateral bound- ing surface of the sets.

Plane Bedding Plane and to a lesser extent, wavy bedding is

present in rocks of the Mississagi Formation up to conglomerate grade. Field observations indicate that plane bedding is characteristic of less than 5% of the formation and is most abundant in the finer clastic grades, being an almost ubiquitous feature of the mudstones.

The small to medium pebble conglomerates that occur sporadically throughout the section in the Quirke Syncline and areas to the west are charac- teristically planar bedded. These conglomerates usually occur as thin (one pebble thick) layers,

which may have formed as lag deposits during mig- ration of coarse-grained ripple trains in a lower flow regime environment. Plane bedding developed in the sand grade clastics is commonly laminated to thinly laminated (terminology of McKee and Weir (1953)) in cosets ranging from a millimetre to sev- eral centimetres. Units of over 20 cm are compara- tively rare. Typically plane bedded sandstone units occur in intimate association with solitary planar cross-stratification and, to a lesser extent, separat- ing grouped planar cross-stratified units. In such situations they generally exhibit poorer sorting than the associated cross-stratified units, in that they contain greater abundances of clay grade material (matrix phyllosilicates). This gives rise to their greenish appearance in outcrop and hence their description in many reports as 'silty partings'. Well-developed parting lineations were observed at only one locality.

Other Structures In addition to the structures described above,

desiccation cracks have been reported (Parviainen 1973, p. 188) from sericitic beds between cross- stratified sets, but are not common. Possible sec- ondary evidence of desiccation is the presence of small mudstone fragments in some of the cross- stratified sets. These 'mud chips' rarely exceed 2-3 cm maximum apparent dimension. Card et al. (1975) reported scattered argillite fragments of up to 15 cm in the upper part of the formation near Lake Panache, and interpreted these as reworked mud flakes.

Other structures include small-scale slumping in some of the larger planar cross-stratified sets, load casts, flames and local development of convolute lamination. Deformation structures are common in units immediately below the contact with the Bruce Formation. Many sandstone units appear to be de- void of internal structure. In most cases this may be due either to a homogeneity of grain size and com- position or obliteration of primary structures by shearing. Clastic dykes are present locally on a scale from afew centimetres to several metres wide (Eisbacher 1970; Robertson 1971) and can be traced through several tens of metres of section. Dyke fill includes sandstone, muddy sandstone and pebbly sandstone. Margins of these dykes are in most instances sharp, possibly indicating that dyke formation occurred subsequent to at least partial lithification of the enclosing sandstone.

Composite Structures and Associations The existence of any regular systematic ar-

rangement of sets of sedimentary structures or as- sociations of structures (composite structures) was not readily apparent during field examination of

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FIG. 3 . (A, B, C) large- and (D) small-scale trough cross-stratification in rocks of the Mississagi Formation. (E) Trough cross-stratification superimposed on a solitary planar cross-stratified set; note the transitional boundary between these forms, indicating avalanching of smaller scale, sinuous crested, ripples over a large scale transverse bar front. (F) Compound, large scale planar cross-stratified unit. The minor discordance (arrow) of foresets may indicate that lateral accretion (possibly at a bar margin) occurred during deposition of this (rare) compound set.

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196 CAN, J . EARTH SCI. VOL. 15. 1978

M ISSlSSAGl A L L D A T A ( = 610 TRANS11

I I

L O C A L M O D E L S

TD. 156:TEN M I L E LAKF

T-COFFEE LAKE

DIEPPE Tp.

24

I SALE 7 ,Tp.

~"~ I

FIG. 4. Model cycles based on Markov chain analysis of selected vertical sequences through the Mississagi Formation. The numbers represent the 'greater than random' probability of occurrence ( X 100) of specific transitions. Only those transitions in which the probability of occurrence is 'greater than random' (Miall 1973) are indicated. A = conglomerate. B = trough cross-stratified sandstone. C = planar cross-stratified sand- stone. D = plane bedded sandstone, muddy sandstone. E = plane bedded mudstone, sandy mudstone. F = irregular erosive contacts (sharp planar contacts not included).

rocks of the Mississagi Formation. Possible fining upward and thinning upward sequences can be recognized in some sections. Large-scale coarsen- ing-upward cycles were not recognized.

Analysis of vertical sequences in terms of transi- tions from one bed type to another using a 'method 1' or embedded Markov chain analysis (Miall 1973) permits construction of a model cycle. Results of such analysis are given (Fig. 4) for grouped data from the whole formation and for individual sec- tions. Only those transitions with a 'greater than random' probability of occurrence (Miall 1973) are indicated in the model cycles. The formation model

indicates a cyclical repetition of facies that involves a reduction in both grain size and scale of major sedimentary structures toward the top of the cycle. This model can be considered as hetero-polymeric (in the sense of Phillips (1836), cited in Duff et al. (1967)) in that it does not record a constant repeti- tion of the same facies sequence. Comparison with the local models indicates that the formational model is not universally applicable, although there is a general cyclicity in which planar cross- stratified sets give way in vertical sequence to trough cross-stratified units. Sequences resembling these model cycles are present in the formation, but are not readily apparent because of the general homogeneity of grain size and dominance of planar cross-stratified units.

Interpretation The plethora of paleoenvironmental interpreta-

tions of the Mississagi Formation can be reduced to the basic question of whether the formation is pre- dominantly marine or nonmarine; the various lines of evidence that can be used to make this distinc- tion are discussed below.

Sedimentary Structures and Associations The geometry of the solitary planar sets is best

explained as having been formed by construction and migration of solitary banks (sand waves) with straight or curved leading edges (Allen 1963a ,b; Harms et al. 1975). Composite sets may have been constructed by repeated migration of such banks or by migration of large-scale asymmetrical ripple trains with essentially straight crests (Allen 1963~). Large-scale trough cross-stratified units most likely were produced by migration of large-scale asymmetrical ripples (or dunes) with curved crests (Allen 1963a,b; Harms et al. 1975). Alternate mechanisms (Allen 1963a) include repeated cut and fill, and the action of migrating eddies. The geometry of smaller scale forms (ripple cross- lamination) can be explained in terms of ripple crest geometry, flow conditions, grain size and aggrada- tion rate (Allen 1963a ,b; Harms et al. 1975).

Unfortunately, the types of cross-stratification present in the Mississagi Formation cannot be con- sidered absolutely diagnostic of any specific envi- ronment, although the abundance of various forms might be considered indicative of deposition in a fluvial environment (cf. Heckel 1972, Fig. 6; Picard and High 1973, Table 3). Solitary banks, respon- sible for the deposition of planar cross-strati- fication are common in modern rivers (Sundborg 1956; Allen 1963a; Coleman 1969), especially braided rivers with sandy beds (Ore 1964; Smith 1970, 1971~). They are, however, also common in

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estuaries, beaches and shallow off-beach environ- ments. Likewise, trains of ripples, megaripples and dunes with both straight and sinuous crests are common in fluvial environments (Allen 1963a; Visher 19656; Coleman 1969), but are present also in the open sea (Allen 1963a; Imbrie and Buchanan 1965).

The absence of abundant, laterally extensive mudstone units, symmetrical and asymmetrical ripple marks (Pettijohn et al. 1972, p. 494), flaser bedding (Reineck and Wunderlich 1968; Terwindt and Breusers 1971) and distinct tidally induced structures such as abundant reactivation surfaces (Boersma 1969; Klein 1970) all point to a nonmarine origin for the Missis'sagi Formation.

Cyclicity A distinct cyclical association of sedimentary

rock types is not readily visible in outcrops of the Mississagi Formation. As Pettijohn (1970~) re- marked, there is a general absence of well- developed fining-upward cycles considered typical of deposits of meandering streams (Allen 1965, 1970a ; Visher 1965a, 1972; Duff et al. 1967). Palo- nen (1971, 1973) claimed to have identified a minimum of 30 'marine' sedimentary cycles in a measured section through the Mississagi Forma- tion in the vicinity of Lake Panache. Individual cycles (6-76 m thick) were considered by Palonen (1971, 1973) to be characterized by increasing grain size and better sortingupward, these features being related to deposition in an environment of increas- ing energy. The cycle figured by Palonen (1973, Plate 1A) was examined in order to determine the applicability of the marine model. If the lower 6 m of this section (?equivalent to Palonen's (1973) unit B) are disregarded, no appreciable decrease or in- crease in grain size is present, nor is any pro- nounced increase or decrease in the scale of sedimentary structures observed. If the finer grained units (A and B) were considered as the uppermost member of the underlying 'cycle' (no erosive scour is noted by Palonen (1973) at the base of his unit A), then this cycle could equally be considered to fine rather than coarsen upward. Coarsening-upward cycles, as might be expected in shallow shelf environments (Asquith 1970; Brenner and Davies 1974), and prograding sandy shorelines (Clifton et al. 1971; Reineck and Singh 1973, Figs. 453,462) are not a conspicuous feature of any sec- tion measured through rocks of the Mississagi Formation during this study.

Analysis of vertical sequences through the for- mation (in terms of first-order embedded Markov chain analysis) indicates a tendency toward reduc-

tion in scale of cross-stratification, associated with a general reduction in grain size in model cycles at both the local and formational levels (Fig. 4). It might be argued that these small-scale cycles rep- resent deposits of single storm events. However, suspension clay laminae are not present in intimate association with even the smaller scale sedimen- tary structures of the arenaceous units; nor are hummocky cross-stratified units (Harms et al. 1975, p. 88), such as may be generated by strong storm surge waves, present in either the siltstones or sandstones of the Mississagi Formation. Where conglomerates (?lag deposits resting on an erosive surface) are present, they generally are superseded by planar or trough cross-stratification. Planar cross-stratification is intimately associated with thin units of plane-bedded sandstone, which may be produced either as thin lags below migrating ripples, megaripples or sandwaves, or as topset beds analogous to those produced in laboratory deltas (Jopling 1963, 1965). Planar bedding also may be expected to form independently of larger scale structures under both upper and lower regime flows for sands coarser than 0.65 mm (Allen 1970b, Fig. 2.6) and upper regime flows for finer grades. Parting lineation was not observed in association with any of the thin planar bedded units associated with large-scale cross-stratification (?perhaps due to the character of the outcrops). Its absence may indicate deposition under lower regime flows, rather than in the upper plane-bed phase (Harms et al. 1975, p. 50). Alternate mechanisms for the pro- duction of planar or pseudo-planar stratification include migration of low relief (less than 1 cm) sand waves (Smith 19716) and migration of small-scale ripples in shallow water, under lower regime flows (McBride et al. 1975). Superimposition of smaller scale (trough) cross-stratification on the larger scale planar form (cf. Fig. 3) may indicate coexistence of such forms, shallowing or reduction in flow vel- ocities. The plane-bedded mudstones and sandy mudstones may represent deposition from suspen- sion under still lower flow velocities.

The general upward tendency toward smaller bed forms in finer grain sizes is a characteristic feature of deposits of fluvial environments (Potter 1967; Pettijohn et al. 1972, p. 456; Miall 1977).

Directional Attributes of Cross-bedding Measurements on over 2500 cross-stratified units

were made from rocks of the Mississagi Formation. Vector data were corrected for tectonic dip (Potter and Pettijohn 1963) and treated using the method outlined by Curray (1956) to obtain vector mag- nitude, direction and intensity of grouped sets at

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CAN. J. EARTH SCI. VOL. 15, 1978

FIG. 5 . Paleocurrent roses, based on grouped data fron

the outcrop, section and formational level. No cor- rections have been made for axial plunge of folds or possible intrastratal deformation (cf. Pettijohn 1957; Ramsay 1961; Young 1968). The degree of dispersion (or variability) of tilt corrected vector data was examined both qualitatively, by visual examination of current roses, and quantitatively using variance values (Long 1976). The sig- nificance of individual modes, plotted in 30" inter- vals, was tested using the method suggested by Tanner (1955) in which a mode is considered sig- nificant only if it contains the mean (= n/ 12, where 30" classes are used) plus one standard deviation (or more) of the observed vector information.

Palonen (1971, 1973) recognized well-developed bimodal distributions in rose diagrams constructed from paleocurrent measurements from the Missis- sagi Formation in the vicinity of Lake Panache, and from similar observations made in areas to the west by Casshyap (1968) and Young (1968). Separation of the principal modes in rose diagrams illustrated by these authors is from 90 to 120". The bimodal character of the rose diagrams was interpreted by Palonen (1971, 1973) as an indication of two princi- pal current directions operating alternately within a marine environment. An apparent counter- clockwise rotation of the principal modes, from east to west within the southern part of the Huro- nian outcrop belt, was interpreted by Palonen (1973) to indicate deposition in a marine ernbay- ment, open to the southeast and affected by a pat- tern of counter-clockwise currents analogous to the present circulation pattern in the Gulf of Mexico. The essentially unidirectional character of grouped data obtained by McDowell (1957) and Pienaar

n measured sections through the Mississagi Formation.

(1963) from the Quirke Syncline area was com- pared by Palonen (1973) to expected sand wave distributions in a tidal flat environment. Palonen (1973) suggested that, in such an environment, sedimentary structures generated by incoming tides would be largely destroyed, and only those produced by the stronger ebb tides preserved.

The observed directional variance of sedimen- tary structures is to some extent a function of the interval through which sampling is carried out (Pot- ter and Siever 1956) for as this is increased so is the scale (or hierarchical rank of Miall (1974)) of the system investigated. Consequently, it is not sur- prising that visual examination of rose diagrams constructed at the sectional level (Fig. 5) leads to the recognition of many polymodal distributions. Paleocurrent roses based on individual sets are in- variably unimodal. If roses are based on grouped sets of data from single outcrops where 15 or more measurements were recorded, only 17% show more than one principal mode (separated by more than 30") which can be considered significant using the test proposed by Tanner (1955). Of these, only one is trimodal (three modes at 60" apart) and only one bimodal bipolar. The remaining ten bimodal distributions have modes preferentially clustered at separations of approximately 60" or 120". Analysis of outcrops at which five or more mea- surements were recorded (167 sites) produced a similar percentage (16%) of polymodal distribu- tions. Three sites have bimodal-bipolar distribu- tions. Three have trimodal distributions and 20 have bimodal distributions with modes preferen- tially clustered at separations of either 60" or 120".

Unimodal distributions, as observed in 84% of

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the outcrops where five or more measurements were made, are a characteristic feature of fluvial paleocurrent distributions (Potter and Siever 1956; Pelletier 1958; Yeakel 1962; Selley 1967, 1968; Klein 1967, Table 1; Mrakovich and Coogan 1974, Fig. 9). In marine environments, diffuse (Wermund 1965; Spencer 1971) and unimodal distributions (Klein 1967; Pryor 1971) might be expected, but the majority of deposits should exhibit strong bimodal- ity, with two modes approximately 180" apart due to tidally induced reversal of current systems (cf. Tanner 1955; Hulsemann 1955; Allen 1963b; Sedimentation Seminar 1966; Hrabar et al. 1971; Banks 1973). Similar bimodal opposed paleocur- rent distributions may be expected in tidal flat envi- ronments (Klein 1967; Selley 1967,1968), estuaries (Klein 1967; Selley 1967, 1968; Land and Hoyt 1966) and lacustrine environments (Picard and High 1972).

Bimodal current distributions with modes less than 180" apart (based on measurements of asym- metric ripple marks) have been recorded from rocks deposited in a shallow marine environment by Picard and High (1968). Similar bimodal dis- tributions of cross-bedding azimuths are a common feature of fluvial systems being recognized in four out of seven fluvial units studied by High and Picard (1974). Polymodal distributions in rocks de- posited in fluvial environments could be explained by local divergence of current systems within streams, perhaps augmented by changes in stream patterns during rising and falling flood stages, and by shifting of stream systems (producing multi- storey sand bodies). Alternately bimodal distribu- tions may be related to the complex geometries of some bed forms, such as linguoid bars (Collinson 1970; Boothroyd and Ashley 1975, Fig. 19) as might be produced by modification of transverse bars at high flow stage (Smith 1972a, p. 625; Cant and Walker 1976). Bimodal distributions, with modes up to 180" apart, have been recorded by Ore (1964, Plate 12) in his studies of modern and ancient braided stream deposits. In most distributions examined, Ore (1964, p. 12) found pronounced bimodality, the two modes commonly being separ- ated by 90-180". Ore (1964) considered that the bimodality of cross-stratification distributions re- sulted from transverse flow directions at the downstream end of bars and from sinuosity of transverse bar fronts. Straight channels were characterized by two modes of about 90" apart, with separation increasing with increases in chan- nel sinuosity. High and Picard (1974) found pro- nounced bimodality (with subequal modes at 50-60" on either side of channel direction) in planar cross-stratified sets, while reporting essentially

unimodal populations from trough cross-stratified units. High and Picard (1974) considered that their observations of bimodality of planar cross- stratification in modern stream deposits might be a function of the small size of the streams they examined. They noted that any secular changes in channel orientation would tend to obscure a bimodal pattern, producing unimodal distributions with large scatter. Apparent bimodal distributions may result from sampling technique when foreset inclinations of trough cross-stratification, rather than trough axis are measured (Young 1968; Dott 1973).

From the above, it can be argued that the domi- nance of unimodal distribution patterns favours a fluvial origin for sandstones of the Mississagi For- mation. Bimodal distributions do not of necessity indicate marine origin, as they are a common fea- ture of braided stream deposits. It is interesting to note that, in two of the three sites where bimodal-bipolar distributions were observed, this was due to development of oppositely directed unimodal distributions of planar and trough cross- stratification. In no section examined was a vertical sequence of alternating, diametrically opposed cross-stratification observed. Such a sequence of herring-bone type cross-stratification would be ex- pected in a tidally influenced environment.

Texture Sandstones of the Mississagi Formation are typi-

FIG. 6. Plot of the coarsest (C) versus median (M) grain size (in micrometres) of rocks ofthe Mississagi Formation. P - Q = bed load, and Q - R = graded suspension segments of Passega's (1957) basic tractive current pattern.

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200 CAN. J. EARTH SCI. VOL. 15, 1978

cally moderately well sorted, medium and coarse arenites with well-developed contact frameworks. The abundance of mud grade material forming the matrix of these texturally immature (Folk 1968) rocks does not argue well for a marine origin, as separation of mud and sand grades by extensive winnowing would be expected in such an environ- ment. Examination of the CM pattern (Fig. 6) de- rived from combined thin section and field exami- nation of 265 samples reveals a poorly defined pat- tern that differs from CM patterns produced by Passega (1957, 1964) for continental shelf deposits, beaches and turbidites. The pattern compares well with the bed load (P-Q) and graded suspension (Q-R) segments of Passega's basic tractive current pattern (Passega 1957, Fig. 12; Royse 1968, Fig. 1). The zones of uniform suspension (S-R) and pelagic suspension are not well developed. Similar deficiencies exist in CM patterns of braided stream deposits in alluvial fan environments (Bull 1962, 1972) and can be explained by lack of favourable conditions (i.e. flood plains, protected channels) for deposition of finer grained sediments from sus- pension.

Mineralogy Sandstones of the Mississagi formation are im-

mature, subfeldspathic and feldspathic arenites (Table 2) suggesting derivation from an extensive area of well-exposed granitic and gneissic rocks, for the most part as first-cycle sediments. Minor second-cycle material in the sequence may have been derived from older Huronian sediments and Archean metasediments. The lack of quartz aren- ites in the formation might be considered tenta- tively as evidence against deposition in a marine environment as quartz arenites can be expected, even as first-cycle sediments, in a stable marine platform environment (Visser 1975).

The high matrix content of sandstones of the Mississagi Formation may be indicative of exten- sive postdepositional, in situ weathering of labile minerals in a continental environment (cf. Pettijohn et al. 1972, p. 181; Mousinho de Meis and Amador 1974; Hester 1974). A secondary origin is suggested by the presence of discrete patches of matrix (epimatrix of Dickinson (1970)). Patches of epima- trix are especially conspicuous in the coarser- grained sandstones where they may be composed almost entirely of white micas (muscovite sericite-illite) or have cores of rotted feldspar. Generation of phyllosilicates from feldspars as a result of hydrothermal alteration in this case can be largely rejected as there is no local spatial relation-

ship between intrusive rock bodies and feldspar alteration on a regional scale, nor is similar altera- tion characteristic of other Huronian sandstones.

Fluvial Model for Deposition of the Mississagi Formation

Comparison with modern sandy braided rivers and river deposits (Ore 1964; Collinson 1970; Smith 1970, 1971a,b, 1972a) suggests that the bulk of the Mississagi Formation was deposited in a distal braided stream environment characterized by dominance of transverse (cross-channel) bars. Studies of the Platte River by Smith (1970) suggest that formation of longitudinal (along channel) bars is favoured by the presence of coarse, poorly sorted sediment, a suggestion verified by the abun- dance of such forms in proximal gravelly and mixed gravelly sandy braided streams and their deposits (Doeglas 1962; Williams and Rust 1969; McDonald and Banerjee 1971; Rust 1972; Church 1972; Cos- tello and Walker 1972; Eynon and Walker 1974; Church and Gilbert 1975). Abundance of finer grained, better sorted material (sand) favours the formation of transverse or modified transverse (lin- guoid) bars. The principal downstream changes in bed forms of the Platte River are an increase in abundance of transverse bars within stream chan- nels (reflected by an increase in planar cross- stratification) and a corresponding reduction in cross-channel topographic relief (Smith 1970). Similar changes were predicted by Ore (1964, p. 13) from his studies of ancient and modern braided stream deposits.

The majority of structures observed in the Mis- sissagi Formation can be explained by in-channel processes active in a distal sandy braided stream environment. Planar cross-stratification is gener- ated principally by the migration of transverse (Ore 1964; Smith 1970, 1971a,b, 1972a) and linguoid bars (Collinson 1970). Migration of sandy straight- crested megaripple trains (cf. Coleman 1969) also may contribute significantly to the production of grouped sets of planar cross-stratification. Trough-cross- stratification is generated by migrat- ing sinuous-crested bars and dunes. The cyclic association of forms predicted by first-order em- bedded Markov chain analysis (Fig. 4) of the Mis- sissagi Formation can be explained by contem- poraneous migration of dunes on top of transverse bars or by later superimposition of dune forms dur- ing falling water stage (cf. Coleman 1969). The relatively thin muddy units were deposited mainly by vertical accretion as overbank flood deposits and as slack water deposits in the main channels.

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LONG

Such muddy layers were prone to erosion by aeo- lian activity (deflation) and later stream activity during rising flood stages and hence rarely are pre- served. The poorly expressed cyclicity observed in the field is consistent with a braided stream envi- ronment. Smith (1970, Fig. 17) was not able to detect any consistent cyclicity in (proximal) braided stream deposits of the Shawangunk Con- glomerate, and related the principal changes in lithology and sedimentary structures to velocity and depth variations accompanying normal changes in discharge and migration of bars within braided stream channels.

Deposition of the Mississagi Formation from ephemeral streams can be rejected on the basis of observed abundance and lateral continuity of pla- nar cross-stratified units, and absence of complex sequences of small-scale bed forms expected in such an environment (Williams 1971; Karcz 1972; Picard and High 1973). The deficiency ofpreserved desiccation features can be explained by the vul- nerability of such facies to later erosion during stream channel migration. Lack of abundant mudstone intraclasts that might be expected from reworking of desiccation features can be explained by the rapid rate of attrition of such clasts under conditions of fluvial transport (Smith 1972b). Where mud clasts are present, they probably were deposited within a few tens or hundreds of metres of their source (Smith 1972b, p. 382).

The absence of abundant, irregular scoured sur- faces in rocks of the Mississagi Formation can be explained in terms of the noncohesive character of the stream banks. In the absence of organic (plant) binding, the banks, composed of medium and coarse sand, would have been destroyed rapidly during rising flood stage. Support for this argument comes from experimental studies by Wolman and Brush (1961), backed by observations of natural stream channels with noncohesive banks. In their flume experiments, Wolman and Brush (1961) showed that, in sands of 0.67 mm (close to the medium diameter of the Mississagi Formation), it was possible to raise water depth by only one and a half times the depth required to initiate bed move- ment before the increase in flow was rapidly com- pensated by broadening of channels. As no vegeta- tion was present to inhibit bank migration, increase in flow consequently would have led to the produc- tion of beds with high width to depth ratios. The dramatic effect of vegetation on bank stability has been illustrated by Smith (1976), who has shown that nonvegetated sediment can be 20 000 times less stable than comparable vegetated banks containing

16-18% by volume of plant roots. Sheet erosion during rising flood stage would explain readily the general deficiency of marked erosion surfaces in the Mississagi Formation.

The lack of well-defined fining-upward cycles (Allen 1965, 1970a; Visher 1965a, 1972; Duff et al. 1967) precludes interpretation of much of the Mis- sissagi Formation as deposits of meandering streams, as does the deficiency of extensive mudstone units (vertical accretion deposits) and complete absence of distinctive lateral accretion sets (epsilon cross-stratification of Allen (1963a), which would indicate point bar formation (Moody-Stuart 1966).

Schumm (1968) spectulated that, in pre- vegetation times, the formation of meandering type streams and the resulting generation of fining- upward sequences would have been hindered by the lack of cohesive banks and increased runoff would have resulted from the lack of vegetative fixation. Schumm (1968) proposed a subdivision of rivers into three types based on the dominant mode of sediment transport. These types were referred to as suspended load, mixed load and bed load streams. Schumm (1968, 1972) proposed that the three types could be distinguished on the basis of the percentage of silt and clay in the bank perim- eter. On this basis, streams of the Mississagi Formation can be considered as bedload (<5% matrix) and mixed load (5-20% matrix). Minor sequences, containing abundant siltstone units, in the upper parts of the formation west of Sudbury, and in the lower parts of the formation south of Lake Panache, may represent deposition from sus- pended load streams. Schumm (1968, Fig. 3) re- lated channel sinuosity to bed load characteristics, such that rivers of the Mississagi Formation could be considered to have low to intermediate sinuosities (- 1.1 to 1.4). Similarly, by using the criteria proposed by Moody-Stuart (1966), includ- ing the lack of fine grained channel fill (vertical accretion deposits) and lack of epsilon cross- stratification, streams of the Mississagi Formation would have been characterized by sinuosities of less than 1.3. Mia11 (1976) has proposed the sinuos- ity can be estimated using the maximum angular range of mean channel azimuth calculated by using a ten point moving average of weighted cross- bedding azimuths as measured in continuous verti- cal sequences. This angle (8) is utilized to calculate sinuosity using the relation (Miall 1976, eq. 1)

Sinuosity = 1

1 - (0/252)2

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CAN. J . EARTH SCI. VOL. 15. 1978

FIG. 7. Paleocurrent distribution inferred from cross-stratification in the Mississagi Formation. Top: Vector mean directions ofgrouped sets in individual sections, with number of readings indicated. Supplementary information from Card et al. 1973 (Station A) and Chandler 1969 (Station C). Bottom: Moving average map based on average vector mean direction in four township blocks, weighted by vector intensity (L of Curray (1956)).

Only three sets of data recorded from the Missis- sagi Formation could be utilized in the way suggested by Miall (1976). Values calculated were S = 1.09, S = 1.01 and S = 1.04 for three sets of data of 17,16 and 39 cross-beds respectively. Using the relationship provided by Schumm (1972, p. 104), these values of sinuosity relate to streams with width to depth ratios of 75: 1, 100: 1 and 90: 1. It can be seen that these calculations are consistent with a sandy fluvial model involving bed load streams with low sinuosity and high width to depth ratios. Whether these calculated values can be applied regionally to the whole formation requires further detailed paleocurrent investigation.

Conclusions

On the basis of observed characteristics, the greater part of the Mississagi Formation appears to have been deposited from bed load and mixed load streams with low to intermediate sinuosity. These streams probably were characterized by high width-to-depth ratios and braided channel pat- terns. Channels migrated rapidly over an aggrada-

tional fluvial plain leading to destruction of over- bank flood deposits and consequent formation of thick multi-storey sand bodies.

The general pattern of stream flow during the time of deposition of the Mississagi Formation can be estimated roughly using a paleocurrent distribu- tion map (Fig. 7). Two major stream systems were probably active: one flowing toward the east (with minor contributions from the north and south) in the original Huronian and Quirke Syncline areas (A and B of Fig. 1) and being diverted to the south over a line roughly corresponding to the Murray Fault; the second, with its origin in the northeast, crossing the Cobalt Plain area (F of Fig. 1) and being di- verted, possibly by rising highlands in the vicinity of the Sudbury structure and the Grenville Front (cf. Young 1968). The two stream systems met in the southern Huronian Area, from where they probably flowed south. Clastic material contri- buted by each of these two major stream systems is markedly different (Long 1976), the western source providing mostly granitic material and the north- eastern system, a mixed supply of granitoid and greenstone rock fragments (Table 2).

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I Acknowledgments

Research for this paper was undertaken as part of the requirements of a Ph.D. degree at the Univer- sity of Western Ontario, supervised by G. M. Young, C. G. Winder and W. S. Fyfe, and the paper completed during tenure of an N.R.C.C. postdoc- toral fellowship at the Institute of Sedimentary and Petroleum Geology under the supervision of J. D. Aitken. I wish to thank G. M. Young for his con- stant support and advice during the formulative stage of this study, and G. M. Young, J. D. Aitken and A. D. Miall for their valuable comments on the original manuscript version of this paper. Financial assistance from National Research Council of Canada grants to G. M. Young is gratefully acknowledged.

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