Epiclastic sedimentation and stratigraphy in the North Spirit Lake and Rainy Lake areas: A comparison

Precambrian Research, 12 (1980) 227--255 © Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands

227

EPICLASTIC SEDIMENTATION AND STRATIGRAPHY IN THE NORTH SPIRIT LAKE AND RAINY LAKE AREAS: A COMPARISON

JOHN WOOD Ontario Geological Survey, Ministry of Natural Resources, 77 Grenville Street, Toronto, Ont. M5S 1B3 (Canada) (Received July 28, 1979; revision accepted October 30, 1979)

ABSTRACT Wood, J., 1980. Epiclastic sedimentation and stratigraphy in the North Spirit Lake and Rainy Lake areas: a comparison. Precambrian Res., 12: 227--255. In the North Spirit Lake greenstone belt within the Sachigo geological subprovince epiclastic metasediments interlayered with mafic and intermediate to felsic metavolcanics unconformably to disconformably overlie a sequence of ultramafic to felsic metavolcanics and quartz-magnetite iron-formation. Polymictic conglomerate of mixed volcanic and plutonic provenance overlies the unconformity. In the northern part of the belt it is followed by cross-bedded sandstone, mudstone and marble: alluvial fan, braided fluvial, and lacustrine deposits. In the southern part of the belt the basal conglomerate is overlain by channelled submarine fan deposits: inverse to normally graded and normally graded conglomerate; sandstone showing dish structure, Bouma cycles, and flame structures; mudstone and quartz-magnetite iron-formation. The submarine fan deposits are separated from the alluvial fan deposits by an area of non-deposition. In the Wabigoon geological subprovince near Rainy Lake polymictic conglomerate of volcanic provenance uncorformably overlies volcanic rocks, a gabbro-anorthosite body, and a granitic subvolcanic intrusion. This alluvial fan deposit is followed upwards by braided river deposits: parallel laminated or low-angle cross-stratified sandstone and trough cross-bedded sandstone. Siltstone and magnetite iron-formation associated with the sandstone may be lacustrine deposits. To the south in the adjacent Quetico subprovince are medium-grade submarine fan sandstones. They are separated from the alluvial fan deposits to the north by a fault. Within the context of present tectonic concepts the North Spirit Lake area is comparable to a marginal basin whereas the Rainy Lake area shows characteristics more compatible with an arc--continent interface.

INTRODUCTION Sedimentary rocks within the Superior Province (Archean) of Northwestern O n t a r i o o c c u r in t w o d i s t i n c t a s s o c i a t i o n s . O n e is w i t h v o l c a n i c r o c k s in w h a t are generally called greenstone belts, The greenstone belts, of low metamorphic grade, are separated from each other by areas of granitic rocks and to-

228

gether with the granitic rocks constitute greenstone subprovinces e.g., the Wabigoon Sunprovince (Fig. 1 ). In the other association, in sedimentary subprovinces, volcanic rocks are almost always absent, and the sedimentary rocks are of high metamorphic grade e.g., the Quetico subprovince (Fig. 1). Individual greenstone belts have been the subject of detailed study for many years. Within the belts the emphasis has been on volcanic rocks because of associated economic ore deposits. Only recently has attention been given to the depositional environments of sedimentary rocks within greenstone belts (e.g., Donaldson and Jackson, 1965; Ojakangas, 1972a; Turner and Walker, 1973; Donaldson and Ojakangas, 1977). Sedimentary rocks within the sedimentary subprovinces as distinct from the greenstone subprovinces have not been studied in detail. The purpose of this paper is to: (a) outline the stratigraphy of one area within a greenstone belt and of another area that covers part of a greenstone subprovince and part of a sedimentary subprovince; (b) outline the depositional environments of epiclastic metasedimentary rocks within these areas; (c) assess the implications of stratigraphy and sedimentation as related to the broader aspects of Archean crustal development. All of the supracrustal rocks within the two areas discussed are metamorphosed; to avoid repetition, specific rock names have the prefix 'meta' omitted. 86 °

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229 NORTH

SPIRIT L A K E A R E A

The North Spirit Lake greenstone belt (Figs. 1,2) lies wholly within the Sachigo subprovince. Within Ontario the boundary between the Sachigo and Berens River subprovinces is a fault zone which has been traced from the southern end of the North Spirit Lake Belt (Wood et al., 1976a, b), northwest through the Favourable Lake area (Ayres et al., 1972) and into Manit o b a where in places the boundary is marked by cataclastic rocks (Ermanovics and Froese, 1978). Some of the discrete faults associated with this zone are shown in Fig. 2.

Stratigraphy In the North Spirit Lake greenstone belt a metavolcanic-metasedimentary sequence unconformably to disconformably overlies a sequence of metavolcanics that range from ultramafic to felsic in composition (Figs. 2 and 3). In Fig. 2 the older rocks lie to the northeast of the unconformity and are considered correlative with those supracrustal rocks north of the North Spirit Lake Fault (the east--west fault underlying North Spirit Lake). Except for minor interflow sediments of volcanic provenance epiclastic rocks are absent from the lower sequence. The upper sequence stands in marked contrast to the lower sequence in that ultramafic metavolcanics are absent and epiclastic metasedimentary rocks abundant. To elucidate stratigraphyand facies relationships a series of stratigraphic sections, localities marked in Fig. 2, are illustrated in Fig. 3. The sections are drawn so that the observer is looking west. Section B is a composite from t w o localities. The base of Section E is a fault (Fig. 2), inferred positions of the unconformity, komatiitic rocks and felsic rocks below the unconformity are indicated. The onset of felsic to intermediate volcanism within the upper sequence is the closest approximation to a time marker within the upper sequence. It has been used in Fig. 3 as the datum line for inter-section comparison. In Section C bedding in iron-formation and in the ultramafic metavolcanics below the unconformity parallels bedding in the metasediments immediately above the unconformity which therefore sensu stricto is a disconformity. Prior to the onset of felsic--intermediate volcanism the locality represented by Section D was positive in relief relative to the areas on either side (Sections A, B, C, E; Fig. 3). Mafic metavolcanics which are present in Sections A and B (Fig. 3) are absent in Section C (Fig. 3). The metasediments above the mafic metavolcanic rocks have a considerable thickness in Section B (Fig. 3) whereas in Section A the thickness is much less. From the latitude of Hewitt Lake south the greenstone belt is composed dominantly of volcanic rocks (Fig. 2). These features suggest that the main area of sedimentation was in the north and the volcanic activity was to the south.

230 The uppermost group of metasediments in the greenstone belt is best exposed in the South Bay of North Spirit Lake (Fig. 2 and Sections D and E, Fig. 3). Probable equivalents of these metasediments occur in other parts of the belt, e.g. immediately to the northeast of the lake north of MacDowell Lake (Fig. 2); however, faulting and folding in this locality preclude precise stratigraphic positioning and thus the tops of Sections A and B (Fig. 3) are not shown. In summary, the main points of the stratigraphy are: (a) An u n c o n f o r m i t y to disconformity separates a lower metavolcanic sequence from an upper metavolcanic-metasedimentary sequence. (b) The upper sequence exhibits changes in rock types and thicknesses from place to place as summarized in Fig. 3. (c) There is a polarity to the upper sequence; in the north it is principally sedimentary, in the south it is principally volcanic.

Epiclastic metasediments For purposes of description, the epiclastic rocks in this upper sequence can be discussed in four groups based on geographical distribution. The first three groups consist of those sediments stratigraphically below the felsic to i n t e r m e d i a t e metavolcanics. The first group is exposed on the northernmost islands in North Spirit Lake and on the mainland immediately to the south of " l a k e " in Fig. 2, The second group consists of the sediments that outcrop in the vicinity of Makataiamik Lake. The third group encompasses those metasediments in the vicinity of Hewitt Lake. The metasediments in the core of the syncline near the South Bay of North Spirit Lake lie stratigraphically above the felsic-intermediate metavolcanics and constitute the fourth group.

North Spirit Lake Group The metasedimentary sequence (thickness 0.8 km) exhibits an upwardfining trend; the rocks at the base are conglomerate, and pass upwards (southwards) into sandstones and subsequently into fine-grained clastic metasediments and into carbonates near the top. Due to faulting the conglomerate at the base of Section E is strongly foliated. Maximum clast size is about 1 m and size sorting poor. Original framework clast shape cannot be determined because of flattening and vertical elongation. Mafic clasts have been deformed more than felsic clasts. The matrix of this conglomerate is a lithic feldspathic wacke. Bedding was not detected in this part of the sequence. Further to the east, on the end of the peninsula just south of the " L a k e " of North Spirit Lake (Fig. 2) on the west side of Peridotite Bay (not marked) is a grey-black conglomerate containing felsic volcanic clasts that is considered to represent debris of rocks immediately below the unconformity. This conglomerate grades upwards into a conglomerate similar to that at the base of Section E. Point-counts of the latter

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235 conglomerate and the conglomerate at the base of Section E are given in Fig. 4a and 4b respectively. The relatively high proportions of mafic-ultramafic and mafic clasts in the conglomerate from Peridotite Bay and the mafic-felsic biomodality of the clast population (Fig. 4a) are considered to be a function of proximity to the unconformity. The conglomerate unit depicted in Section E fines stratigraphically upwards, however,bedding in the conglomerate can only be discerned where sandstone beds are intercalated with the conglomerate. The lowermost sandstone beds are thin. Along with an increase in thickness of sandstone beds stratigraphically upwards, is a decrease in grain size and increase in proportion of silicic framework clasts in the conglomerate. The uppermost 210 m of conglomerate in Section E (Fig. 3) consists of thin cobble to pebble conglomerate beds and interbedded sandstone. The preponderance of granitic, felsic metavolcanic and vein quartz clasts and the quartzo-feldspathic nature of the conglomerate matrix and interbedded sandstone, give this part of the sequence a light coloured weathering surface. The sandstone beds in the lower part of this 210 m of conglomerate and sandstone show only horizontal lamination. However higher in the section cross-bedding is visible, and in the overlying sandstone trough cross-beds are abundant. Figure 5 illustrates a cross-bedded sandstone with conglomerate interbeds. The trough aspect of the cross-beds is readily apparent as is the clast flattening within the conglomerate interbeds. The sandstone overlying the conglomeratic part of the sequence is granular with quartz grains up to 3 mm, but becomes finer-grained upwards. The rocks are arenites composed of quartz, feldspar, and felsic metavolcanic fragments. Extensively cross-bedded sandstones are arkoses containing minor muscovite, calcite, biotite and chlorite. Most of the sandstones are bimodal with size modes at 0.2 mm and 0.03 mm. On the mainland south of the " L a k e " of North Spirit Lake there are several small outcrops of quartz arenite. The units overlying the cross-bedded sandstone on the islands in North Spirit Lake are mudstone and marble (simplified to marble in Section E, Fig.3). They are very poorly exposed. Some of the mudstone is black, very finegrained and pyritiferous. The marble is calcareous and has variable amounts of fine-grained epiclastic detritus.

Makataiamik Lake Group The base of the group north of Makataiamik Lake lies disconformably on komatiites. The top southwest of Makataiamik Lake is attenuated by a fault. In the extreme southern part of the greenstone belt northeast o f MacDowell Lake (Fig. 2) the group is apparently conformably overlain by intermediate pyroclastic rocks. The conglomeratic part of the group shows a mappable decrease in size of the framework clasts in a southwesterly direction away from the disconformity. There is a comparable decrease in framework clast size from the locality of Section C (Fig. 2) towards the locality of Section B (Fig. 2). The transition from conglomerate to sandstone stratigraphically

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238

Fig.5. Trough cross-bedded sandstone from an island in North Spirit Lake, section E, Figs. 2 and 3. Note the intense foliation, apparent in the interbedded conglomerate, at an angle to the bedding. upwards or towards the south is gradual, and at any one locality the two may be interbedded. All of the metasediments in this group are characterized by the occurrence of fuchsite, derived from the weathering of ultramafic rocks below the u n c o n f o r m i t y . A boulder to granule clast-supported conglomerate with a wacke matrix overlies the disconformity. Maximum diameter of felsic clasts as exposed on horizontal o u t c r o p surface is 0.5 m., whereas m axi m um clast length in a vertical plane is 2--3 times the m a x i m u m horizontal dimension. Felsic clasts weather o u t of the rock, are light coloured, are generally the largest clasts in the r o ck and on horizontal surfaces are well r o u n d e d with a high sphericity. T h ey were used for dimensional descriptions, since clasts of mafic metavolcanics although constituting an i m p o r t a n t part of the framework, have been flattened and e x t e n d e d more than the felsic clasts and their boundaries are difficult to define. Within the constraint of o u t c r o p size, no bedding was observed in this conglomerate unit. Some 2 km westnorthwest of the northwest end of Makataiamik Lake the conglomerate described above is some 300 m thick, and shows a decrease in m a x i m u m clast size stratigraphically upwards. It is overlain by a very distinctive black and white conglomerate. The black colour is due to mafic and ultramafic f r a m e w o r k r oc k fragments and the matrix, the white colour to clasts o f felsic metavolcanics, chert, granitic rocks, and quartz. All of the clasts are well r o u n d e d and range in size from a few millimeters t o about 30 cm

239

with a size mode at 8 cm (Carter, 1976). The matrix consists of albite, quartz, actinolite, amphibole, clinopyroxene, carbonate, sphene, malachite, azurite, chalcopyrite and magnetite. Compositions of the framework clasts of the basal conglomerate and the black and white conglomerate are illustrated in Figs. 4c and 4d (data from Carter, 1976). The large population of felsic clasts, particularly coarse porphyritic ones, in the conglomerate overlying the disconformity is noteworthy. In contrast to the conglomerates of Figs. 4a, b and d, the clasts do not reflect the geology of the lower sequence. This is considered to be a result of the depositional environment and will be further discussed under "Interpretation". Immediately above the mafic metavolcanics southwest of Makataiamik Lake (Fig. 2) are polymictic clast-supported conglomerates in which bedding is expressed by ill-defined layers of differing clast size. Stratigraphically up-

Fig.6. Inverse- to n o r m a l l y - g r a d e d c o n g l o m e r a t e f r o m s o u t h w e s t o f M a k a t a i a m i k Lake. T h e base o f t h e b e d is n e a r t h e b o t t o m o f t h e p h o t o , t h e t o p o f t h e b e d is n o t visible. M a n y o f t h e clasts are angular.

240 wards where sandstone interbeds first occur the conglomerates are either normally graded, inverse to normally graded, or inversely graded and show sedimentary imbrication. Within the scope of o u t c r o p exposures t h e y are channelled. Matrix-supported conglomerates, although rare, are present. Bedding thicknesses range from several m to a few cm. Figure 6 illustrates an inverse to normally-graded conglomerate bed. At the base o f the bed just above the b o t t o m of the p h o t o , fram ew ork clasts are in the order of several cm in diameter. T h e y increase to about 10 cm beside the scale card, whence t he y rapidly decrease to granule size near the top of the photo. The top of this bed is n o t visible. Not u n c o m m o n l y , a conglomerate grades upward into sandstone, as illustrated in Fig. 7 which shows the upper p o r tio n of th e conglomeratic part and the overlying sandstone part of a conglomerate-sandstone bed. Particularly noticeable in the sandstone in this p h o t o are dish structures, with, above the dish structures, parallel lamination.

Fig.7. Dish structures in the sandstone part of a conglomerate sandstone unit. Southwest of Makataiamik Lake.

241

The t o p of the bed illustrated is eroded by the overlying unit. Inversely-graded conglomerate was observed at one locality only (Fig. 8). The largest clasts are coarse, granitic, and occur at the t o p of the bed. The conglomerate is overlain by a normally-graded granule conglomerate which because of the sudden large break in framework clast size is not considered to be a product of the same depositional episode that resulted in the inverselygraded conglomerate. Matrix supported conglomerate is comprised of cobble and pebble size clasts in a normally-graded granulte to sand-sized matrix.

Fig.8. Inversely-graded bed, top towards left of photo. Note the well-rounded elasts. Southwest of Makataiamik Lake.

The proportion of sandstone to conglomerate increases stratigraphically upwards until conglomerate dies out. Quartz-magnetite iron-formation occurs at this stratigraphic level. The grain size and textures of the sandstones change with increasing stratigraphic height; those interbedded with the conglomerates may be graded, massive, or equigranular with dish structures, some show parallel lamination at the t o p of the beds. Stratigraphically upwards the sandstone beds lose their massive character, dish structures are no longer present, and the beds exhibit the a, b, c and e subdivisions of the B o u m a sequence. Flame structures are commonplace.

242

Hewitt Lake Group The Hewitt Lake Group is c om pos e d almost exclusively of sandstone and mudstone. Conglomerate constitute only a b o u t 1% of the sequence. Near the west end of Hewitt Lake there are outcrops of arkose and quartz arenite; apart f r o m these, the rocks are immature wackes which show various combinations of the c o m p o n e n t s of the Bouma sequence. On the southwest shore of North Spirit Lake there are some thick units; Fig. 9 shows an a, b unit some 30 cm thick near which there are a, b units up to 3 m thick. The thickest unit was examined for evidence of amalgamation but none was found.

Fig.9. Close up of an a--b turbidite bed. The a interval is only slightly graded. The parallel laminated 'b' part of the bed can be seen at the right of the photo.

South Bay Group The South Bay Group is poor l y exposed due to water cover and lack of o u t c r o p on land areas. Around the periphery of South Bay the rocks are mostly conglomerate or thick, graded, wacke beds with parallel laminated tops. Some thin graded sandstone-mudstone outcrops are present. In the central part of the Bay almost on the synclinal axis is an island with the thinnest (2 mm) graded beds seen within the greenstone belt. The sediment is clay-sized and black-coloured and must have been form ed in a very low energy environment. Deposition of the lower part of the group overlapped in time with the intermediate-felsic pyroclastic volcanic activity, the result being hybrid volcano-sedimentary rocks. Near the base of the indisputably sedimentary part of the sequence is a quartz-magnetite iron-formation.

243 RAINY LAKE AREA

Stratigraphy The Rainy Lake Area situated on the Ontario-Minnesota border has featured prominently in the history of Precambrian geology, due primarily to the fieldwork of A.C. Lawson. In his first report on the area in 1888 Lawson described two series of metamorphic rocks--the Keewatin (comprised of metavolcanic rocks) and the Coutchiching (comprised of metasedimentary rocks). These he interpreted to be older than granitic rocks known as Laurentian and grouped all three as Archean. His conclusion that the Coutchiching was older than the Keewatin was not favourably accepted (Adams et al. 1905), and as a result Lawson revisited the Rainy Lake area in 1911. In a subsequent memoir (Lawson, 1913) he defined two new series the Seine Series and the Steeprock Series and showed that granitic rocks previously called Laurentian were of two distinct ages. Within the context of the geology of the Rainy Lake Area, Lawson reaffirmed the Coutchiching to be older than the Keewatin, and both to be older than the Laurentian; he considered the Seine and Steeprock to be younger than and deposited unconformably on the Laurentian and older rocks, and he demonstrated that the youngest rocks were granitic, to these he assigned the name "Algoman". Far from solving the Keewatin-Coutchiching problem, the recognition of two new series and the separation of the Laurentian from the Algoman started further controversy. For example T a n t o n (1936) in mapping the Mine Centre area, previously mapped by Lawson, considered the Laurentian of Lawson (1913) to be intrusive into the Seine Series--a clearly contradictory viewpoint. More recently Ojakangas (1972b) considered the Seine Series to be interbedded with greenschists (Keewatin) and not deposited on a major unconformity, as suggested by Lawson. The main stratigraphic problem--the position of the Coutchiching relative to the Keewatin still exists. Attempts to sort out relationships in the last two decades using available radiometric dating techniques, by Goldich et al. (1961), Hart and Davis (1969), and Peterman et al (1972) have been unsuccessful. Recently, Poulsen (1979) working in the Rice Bay dome on the north side of Rainy Lake, some 25 km east of Fort Frances where Lawson affirmed Coutchiching metasediments in the centre of the dome to be surrounded by and therefore overlain by Keewatin metavolcanics, has shown that the sequence in Rice Bay is inverted and suggests that the dome is in fact part of a nappe structure. In 1976 and 1977 the author in addition to doing detailed mapping around Mine Centre (located on Highway 11 some 61 km east of Fort Frances) studied the geology of the general region of Rainy Lake as far south as Namakan Lake, and of the area east of Mine Centre. The results of the detailed mapping are shown in Fig. 10. Two major faults, the Quetico Fault and the Seine River

244 Fault, separate t h r e e areas o f differing lithology, s t r u c t u r a l style and metam o r p h i c grade. N o r t h o f the Q u e t i c o Fault are m e t a v o l c a n i c m i g m a t i t e and granitic intrusions. B e t w e e n t h e Seine River Fault and t h e Q u e t i c o F a u l t is a s e q u e n c e o f low-grade m e t a v o l c a n i c rocks ( K e e w a t i n of Lawson), i n t r u d e d by a n o r t h o s i t e and gabbro which in t u r n is i n t r u d e d b y granitic rocks ( L a u r e n t i a n o f Lawson) all o f which are u n c o n f o r m a b l y overlain b y conglom e r a t e and s a n d s t o n e (Seine Series o f Lawson). The u n c o n f o r m i t y b e t w e e n the Seine Series and u n d e r l y i n g r o c k s (Fig. 10) can be observed on one cont a c t only. All o t h e r c o n t a c t s o f t h e Seine Series are fault contacts. S o u t h o f the Seine River Fault are m e d i u m - t o high-grade m e t a s a n d s t o n e s and meta-

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Fig.10. Geological map of the Mine Centre area. The village of Mine Centre is located south of "Fault" in Quetico Fault. The letters A, B, and C relate to localities in Fig. 11.

245

siltstones folded into isoclinal east-west folds. No volcanic rocks were observed in this part of the sequence. The Seine River Fault (which merges with the Quetico Fault to the east) separates the Wabigoon subprovince from the Quetico Subprovince (Mac Kasey et al., 1974). The sandstones south of the Seine River Fault in the Quetico Subprovince will be referred to as the "Quetico Metasediments" in this paper. They were correlated by Lawson with the Coutchiching metasediments, which lie in the Wabigoon Subprovince north of the Seine River Fault. Rocks of the Seine Series occur in a number of different parts of the Mine Centre area (Fig.10). The most northerly contact of the Seine metasediments with the metavolcanic rocks and subvolcanic intrusive rocks is an unconformity, which is exposed just south of the northeastern end of Bad Vermillion Lake and north of Highway 11 some 5 km east of Mine Centre. The locality near Bad Vermillion Lake is described in detail by Lawson (1913). Conglomerate overlies subvolcanic granitic rock and felsic metavolcanics; the angular framework clasts and the matrix of the conglomerate reflects very closely the underlying rock types. Quartz which is scarce in most conglomerate outcrops is abundant where the conglomerate immediately overlies the quartz-rich granitic rock. Where the unconformity is exposed east of Mine Centre the conglomerate overlies intermediate metavolcanic rocks. The conglomerate composition so closely reflects that of the underlying rocks that the contact in places can be outlined only with difficulty. Ojakangas (1972b) working on the American side of Rainy Lake disagrees with Lawson's (1913) interpretation that the Seine Series lay unconformably on older rocks. There are differences in their maps, and Ojakangas (1972b, p. 168) states that the feldspathic quartzite and conglomerates (Seine Series) are interbedded with greenschist and cannot therefore be younger rocks deposited upon an unconformity. That Lawson is correct regarding an unconformity is irrefutable in the area depicted in Fig. 10. As shown in Fig. 10 all of the Seine contacts are faulted except the most northerly one. The faults are marked by lineaments and extensive shearing. The sandstones, most of which are composed of intermediate volcanic detritus, where sheared are very difficult to distinguish from sheared intermediate volcanic rocks. This may account for some of the differences between Lawson's (1913) map and Ojakangas' (1972b) map. In addition, faulting may have resulted in the interlayering of sedimentary and volcanic rocks outlined by Ojakangas.

Epiclastic metasediments Within the Mine Centre area (Fig. 10) the epiclastic metasediments between the Quetico Fault and the Seine River Fault, Lawson's (1913) Seine Series have five readily distinguishable facies: conglomerate, interbedded conglomerate-sandstone, sandstone, siltstone, and quartz-magnetite iron formation. Between the unconformity and the first fault to the south, coarse boulder to granule conglomerate occurs at the base of the sequence. The framework clasts are well rounded and reach a maximum diameter in the order of 1 m.

246

Three histograms of clast types from the b o t t o m , middle and top of the conglomerate sequence are shown in Fig. l l A - - C . The locations of the outcrop areas used are shown in Fig. 10. The histograms show that metavolcanic rocks constitute by far the most important segment of the framework clast population. Medium coarse grained felsic {granitic} rocks are virtually absent. There is a suggestion from Fig. l l A - - C that the clasts at the top of the unit are compositionally more silicic than those near the base, this may be a function either of increasing distance from source or of greater depth of weathering and erosion in the source area that allowed coarse-grained felsic rocks to be put into the sedimentary cycle or possibly both. Bedding in most exposures cannot be identified except where there are sandstone beds between conglomerate beds. Such sandstones beds show either parallel lamination or low angle cross-stratification. Framework clast sizes in the conglomerate decrease with increase in stratigraphic height, and where sandstone forms an important part of the sequence the conglomerate is cobble size or smaller.

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248 Where s a n d s t o n e occurs w i t h o u t c o n g l o m e r a t e it is t r o u g h c r o s s - b e d d e d {Fig. 12). T h e t r o u g h axes o f cross-beds (Fig. 12) m e a s u r e d in t h e core of t h e syncline on t h e n o r t h s h o r e of Shoal L a k e (the s o u t h e r n m o s t lake in Fig. 10) w h e n p l o t t e d o n a rose-diagram, give a v e r y t i g h t u n i m o d a l c u r r e n t d i r e c t i o n (Fig. 13). This d i r e c t i o n is r e p r e s e n t a t i v e o f o n e locality only. T h e mass o f

Fig.12. Trough cross-bedded sandstone, Seine Series. Shoal Lake--the lake southeast of Bad Vermillion Lake.

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249

the gabbro-anorthosite-granodiorite b o d y centred on Bad Vermillion Lake appears to have caused deflection of fold axes during deformation, so that the current directions may have been slightly rotated within a horizontal plane. The sandstone compositions reflect those of the conglomerates. Most of the rocks are litharenite or lithic subarkose; arkose occurs only at the highest stratigraphic levels. The metasediments in the fault block just to the north of the Seine River Fault are correlative with these just described, but their relative stratigraphic positions are not known because of the faulting and the lack of a stratigraphic marker horizon c o m m o n to both. Conglomerate is subordinate to sandstone in the southern area. The framework clasts are of pebble and cobble size. The clast compositions are more felsic than to the north; felsic volcanic rock fragments, granitic clasts, and quartz clasts are the most abundant. The sandstone is more quartzo-feldspathic than to the north, arkose is more abundant, and cross-bedding is common. Outcrops are lichen covered and three-dimen-

Fig.14. Metasandstone from the Quetico metasedimentary belt. The beds are graded, the more argillaceous tops contain biotite porphyroblasts. South of the Seine River Fault.

250 sional exposures suitable for paleocurrent measurements are scarce and small. There are probably several folds within this fault block since at different localities cross-beds give top directions to the north or to the south. Towards the south of the fault block sandstone gives way to a feldspathic siltstone in which quartz-magnetite iron-formation occurs. Any structures that existed in the siltstone have been obliterated by the intense shearing associated with the Seine River Fault and the fault just to the north of it. The sediments south of the Seine River Fault are a m o n o t o n o u s sequence of sandstones that may show no internal structures, or a, b or abe Bouma sequences. Fig. 14 shows a metamorphosed graded sandstone-mudstone bed. These rocks are much more quartz-rich than sediments of the Seine Series, and a quartz arenite was identified at one locality. The proportion of mud to sand varies from bed to bed and also from locality to locality. Near the Seine River Fault the metamorphic grade is low, but it increases gradually and consistently southwards. Garnet and andalusite are present in rocks at the southern end of the area shown in Fig. 10. Further south adjacent to Rainy Lake cordierite and fibrolite are present. South of Rainy Lake the metasediments are migmatitic (see Southwick, 1972). INTERPRETATION R a i n y L a k e area

Because that part of the Rainy Lake area represented in Fig. 10 represents a much clearer picture than does the North Spirit Lake area from the viewpoints of both stratigraphy and sedimentation, it will be discussed first. The conglomerates, sandstones and mudstones between the Quetico Fault and Seine River Fault clearly lie unconformably on volcanic and intrusive rocks. With the possible exception of one 50 cm-thick intermediate porphyritic rock unit that may be crystal tuff, volcanic rocks are not present within the sedimentary sequence. The sedimentary pile which consists of poorly-sorted conglomeratic detritus that compositionally shows a close relationship to immediately underlying rock types, followed b y better-sorted conglomerates with interbeds of sandstone with low-angle or parallel bedding, followed by pebbly sandstones and trough cross-bedded sandstones, strongly suggests an alluvial fan merging into a braided fluvial environment. The paleocurrent directions from cross-beds (Fig. 13) suggest a southwesterly paleocurrent, a result compatible with the greater compositional maturity of rocks in the southern sedimentary fault block north of the Seine River Fault. The hinterland from which the sediments were derived was composed almost exclusively of volcanic rocks. The lack of granitic detritus would suggest a shallow level of erosion. (Although the Bad Vermillion Lake gabbroanorthosite and granitic bodies are unconformably overlain by the rectasediments, the abundance of mafic sills in the volcanic rocks (Fig. 10) and the difficulty in distinguishing between spherulitic extrusive and intrusive

251

felsic rocks west of Bad Vermillion Lake strongly suggests that the gabbroanorthosite and granitic bodies, although large, were emplaced at a high crustal level.) These sediments are what would be expected as first cycle sediments derived from an uplifted volcanic terrain. The siltstone and thinly-bedded quartz-magnetite iron-formation in the southern block of metasediments indicates quiet, low energy possibly lacustrine depositional conditions. The Quetico metasediments show all of the classical features of turbidites, they are therefore considered to be deep-water sediments. Since it may be assumed that they were not reworked after final deposition, the quartz in the rocks must be a product of reworking prior to final sedimentation or must represent the product of a quartz-rich source area. The metasediments of the Rice Bay Dome are not directly correlative with the Quetico metasediments because of the intervening Seine River Fault. They are however lithologically similar to the Quetico metasediments. Ongoing work by K.H. Poulsen (pers. comm., 1979) may serve to clarify their relationships to the Quetico metasediments and to rocks of the Seine Series. The relationship of the Seine Series to the Quetico metasediments also is unknown. Conceivably the Seine Series may be in part a terrestrial equivalent of the submarine Quetico metasediments. A study of the boundary of the Quetico and Wabigoon Subprovinces (especially in those areas where it has not been complicated by faulting e.g., west of Lake Nipigon), would probably provide much information on land-sea transitions, and the nature of the crustal interaction that resulted in the style of deformation, in particular large scale faults and proposed nappe structures that typify this subprovincial boundary. N o r t h Spirit L a k e area

The terrestrial submarine environments suggested for epiclastic metasediments in the Rainy Lake area find ready analogues in the North Spirit Lake area. The North Spirit Lake Group is comparable to the Seine Series, and the Hewitt Lake Group and South Bay Groups are comparable to the Quetico metasediments. The North Spirit Lake Group is considered to represent in ascending stratigraphic order: an alluvial fan environment, (the conglomeratic part of the sequence) grading into a braided fluvial environment (the cross-bedded sandstone part of the sequence) grading into a lacustrine environment, (the fine-grained, thinly-bedded mudstone with carbonate part of the sequence). The most notable difference between this Group and the Seine Series is the quartz-rich characteristics of the sandstone in the North Spirit Lake Group, and the presence of granitic (equigranular medium-grained felsic) clasts in the North Spirit Lake conglomerates (compare Fig. 4b with Figs. l l a - - c ) . The sandstones and mudstones of the Hewitt Lake Group and South Bay Group compare broadly with the Quetico metasediments in that they are all turbidites. The main difference between the two areas is that whereas mono-

252 tony pervades the Quetico metasediments, diversity typifies rocks of the Hewitt Lake and South Bay Groups. This diversity is considered to be a function of the active volcanic environment that existed during sedimentation and a source area that ranged in composition from ultramafic to felsic. It is n o t e w o r t h y that the South Bay Group (the sediments above the uppermost volcanics in Fig. 3, Sections C, D and E) overlie rocks of the North Spirit Lake Group (the sediments beneath the volcanic rocks in Section E). This indicates that water level had risen considerably in the time interval between sedimentation of the North Spirit Lake Group and sedimentation of the South Bay Group. Where it is associated with epiclastic metasediments, quartz-magnetite ironformation occurs with epiclastic rocks of deep-water type. The sediments of the Makataiamik Lake Group are distinctly different from any others described in this paper. The typical turbidites and the pillowed mafic volcanic flows in the sequence suggest a submarine environment. The depositional model which provides the most satisfactory explanation of the sediment association of this group is that of Walker (1975, 1978) for a submarine fan environment. Stratigraphic height and decreasing grain size within the group equates with distance from the head of the fan in Walker's model. In a steady state situation a coarsening-upward trend would be expected. The observed fining-upward trend is most likely a result of continuing subsidence allied with rising water level during deposition as proposed above for the South Bay Group. In Walker's model the coarse-grained poorlysorted rocks such as those in the lowermost part of the Makataiamik Lake Group correspond to deposits in the feeder channel to the fan. The various types of graded conglomerate and graded stratified conglomerate constitute the upper fan deposits whereas the pebbly and massive sandstones together with the proximal turbidites constitute the mid-fan deposits (Walker, 1975, 1978). In a detailed study of the Cap Enrage Formation Hein (1979) has indicated deposition on three main topographic levels in a braided submarine valley system: main channels characterized by coarse conglomerates; marginal terraces and bars characterized by fine conglomerate; and high terraces characterized by massive sandstones, turbidites and shales. This concept fits well with the observation that in the Makataiamik Lake Group at one stratigraphic level a number of different sediment types, that should be separated down-fan and therefore stratigraphically, occur together. In the organized conglomerates it was noted that in those beds comprised of volcanic fragments the clasts are angular (Fig. 6) whereas in those beds with granitic clasts the clasts are rounded (Fig. 8). This raises the question of whether at least two feeder channels were supplying detritus of different compositions and textural maturity or whether at least two sources of sediment were being fed into the one feeder channel. DISCUSSION In both the North Spirit Lake and Rainy Lake areas, as suggested in the preceding section, there are terrestrial and submarine environments. However

253 there is no documentation of intervening facies. In the Rainy Lake area the Seine River Fault occupies the position where a shoreline facies would be expected. This does n o t preclude the possibility that a shoreline facies did exist, and a tectonic-sedimentary reconstruction along the Wabigoon-Quetico interface may reveal this facies. For example at Steep R o c k Lake near Atikokan there are stromatolitic limestone and oolitic iron-formation, both probably shallow marine deposits. The relationships of the rocks at Steep Rock Lake to rocks of the Seine Series, to adjacent Archean supracrustal rocks, and to adjacent granitic rocks, have never been satisfactorily explained. Reconstruction of the stratigraphy along the Quetico Fault according to the suggestions of MacKasey et al. {1974) puts the Seine Series rocks such as those at Mine Centre, to the east of Atikokan. It is possible, given the southwesterly paleocurrent direction in the Seine Series metasediments, that rocks of the Steep R o c k Group could be facies equivalents of the Seine Series metasediments. Much work remains to be done along the Wabigoon Quetico Subprovincial boundary before the full picture of structural relationships and hence facies relationships become apparent. In the North Spirit Lake greenstone belt an obvious shoreline facies was not observed. However, compositionally mature rocks such as arkose and quartz arenite are present, as are limestones and other carbonates. Conceivably such a facies may be of restricted areal extent. A possible modern analogue of the style of sedimentation in the North Spirit Lake belt is the Yallahs Fan Delta in southeastern Jamaica {Wescott and Ethridge, 1978). The Yallahs River has a high gradient and flows directly from the Blue Mountains into the sea where it has built a lobate fan delta characterized by braided channels of conglomeratic sediment, abandoned channels of m u d d y sand, natural levees of silty sand and freshwater ponds surrounded by mangrove swamps. Since the subaqueous delta drops off at an angle of 20o--30 ° into the Yallahs basin, the beach is restricted. Where a beach facies should be present in the North Spirit Lake area, outcrop density is low. The quartz-rich character of the sediments in the North Spirit Lake area has been noted b y several authors. Donaldson and Jackson (1965), first drew attention to the petrography of the sediments and Donaldson and Ojakangas (1977) described quartz arenite clasts in conglomerates of the South Bay Group and suggested modes of origin of the quartz arenite. Wood (1977, 1980) has noted quartz arenite in outcrop at several localities. Pending detailed study of these quartz arenite localities, and based on the quartz-rich nature of even the turbidites, the author currently considers that the crustal stability required to form such quartzose rocks preceded the sedimentation of the rocks in the greenstone belt. Recent geochronological work on rocks of the North Spirit Lake area (Nunes and Wood, 1979) has s h o w n that there is a time difference of almost 300 Ma between the volcanic rocks below the unconformity (3013 Ma) and a subvolcanic stock equivalent in age to the volcanic rocks above the unconformity (2730 Ma). Such an incredibly large time interval in the evolution of the greenstone belt would allow ample time

254 for the intensive w e a t h e r i n g or s e d i m e n t recycling required to p r o d u c e the very q u a r t z o s e sediments. Within the p r e s e n t - d a y c o n c e p t s o f plate t e c t o n i c s , the N o r t h Spirit Lake area shows m a n y o f the characteristics o f a marginal basin, whereas the sedim e n t s o f the Seine Series in the R a i n y Lake area are m o r e akin t o t h o s e t h a t w o u l d be shed f r o m an island arc c o m p l e x . The style o f d e f o r m a t i o n along the W a b i g o o n - Q u e t i c o subprovincial b o u n d a r y is suggestive o f an arc-contin e n t collision interface and s h o u l d be f u r t h e r studied since it is relevant n o t only t o structural g e o l o g y b u t also s t r a t i g r a p h y s e d i m e n t o l o g y and e c o n o m i c geology. ACKNOWLEDGEMENTS The a u t h o r t h a n k s his assistants n o t a b l y J. Dekker, H.L. Gibson, D.A. Hunter, J. Jansen, P.J. Keay, D. P a n a g a p k o , P.G. Pintus and M. R a u d s e p p for their help and ethusiasm during field w o r k a n d G. Finn for d r a u g h t i n g the figures. C.E. B l a c k b u r n o f the O n t a r i o Geological Survey has p r o v i d e d m a n y stimulating discussions o n t o p i c s o f A r c h e a n g e o l o g y and m a d e constructive c o m m e n t s on this text. This p a p e r is published with the permission o f the Director, O n t a r i o Geological Survey. REFERENCES Adams, F.D., Bell, R., Lane, A.C., Leith, C.K., Miller, W.G. and Van Hise, C.R., 1905. Report of the special committee for the Lake Superior region. J. Geol., 13: 89--104. Ayres, L.D., 1978. Metamorphism in the Superior Province of northwestern Ontario and its relation to crustal development. In: J.A. Fraser and W.W. Heywood (Editors), Metamorphism in the Canadian Shield. Geol. Surv. Can., Pap., 78--10: 25--36. Ayres, L.D., Raudsepp, M., Averill, S.A. and Edwards, G.R., 1972. Geological compilation of the Favourable Lake--Berens Lake area, Ontario. Ontario Div. Mines, Map 2262, Geol. Compilation Set. Carter, T.C., 1976. Archean sedimentary rocks and associated mineralization in the North Spirit Lake Area, northwestern Ontario. Thesis, University of Western Ontario, 90 pp. (unpublished). Donaldson, J.A. and Jackson, G.D., 1965. Archean sedimentary rocks of the North Spirit Lake area, northwestern Ontario. Can. J. Earth Sci., 2: 622--647. Donaldson, J.A. and Ojakangas, R.W., 1977. Orthoquartzite pebbles in Archean conglomerate, North Spirit Lake, northwestern Ontario. Can. J. Earth Sci., 14: 1980--1990. Ermanovics, I.F. and Froese, E., 1978. Metamorphism of the Superior Province in Manitoba. In: J.A. Fraser and W.W. Heywood (Editors), Metamorphism in the Canadian Shield. Geol. Surv. Can. Pap., 78--10: 17--24. Goldich, S.S., Nier, A.O., Baadsgaard, H., Hoffman, J.H. and Krueger, H.W., 1961. The Precambrian geology and geochronology of Minnesota. Minnesota Geol. Surv., Bull., 41:193 pp. Hart, S.R. and Davis, G.L., 1969. Zircon U-Pb and whole rock Rb-Sr ages and early crustal development near Rainy Lake, Ontario. Geol. Soc. Am. Bull., 80: 595--614. Hein, F.J., 1979. Greve de la Pointe and St. Simon sur Mer--a Late Cambrian deep-sea braided submarine valley sequence. In: G.V. Middleton {Editor), Cambro-Ordovician, submarine channels and fans, L'Islet to Saint-Anne-Des Monts, Quebec. Geol. Assoc. Can., Field Trip Guide, pp. 14--25.

255 Lawson, A.C., 1888. Report on the geology of the Rainy Lake region. Geol. Sure. Can., Annu. Rep., 3 : 1 8 2 pp. Lawson, A.C., 1913. The Archean geology of Rainy Lake restudied. Geol. Surv. Can., Mem., 4 0 : 1 1 5 pp. MacKasey, W.O., Blackburn, C.E. and Trowell, N F . , 1974. A regional approach to the Wabigoon-Quetico belts and its bearing on exploration in northwestern Ontario; Ontario Div. Mines, M.P. 5 8 : 2 9 pp. Nunes, P.D. and Wood, J., 1979. A 300 m.y. hiatus in the stratigraphy of the North Spirit Lake greenstone belt, northwestern Ontario. GAC Annu. Meeting, Quebec, 1979, Progr. with Abstr., 4: 70. Ojakangas, R.W., 1972a. Archean volcanogenic greywackes of the Vermillion District, northwestern Minnesota. Geol. Soc. Am. Bull., 83: 429--442. Ojakangas, R.W., 1972b. Rainy Lake Area. In: P.K. Sims and G.B. Morey (Editors), Geology of Minnesota: A Centenial Volume. Minn. Geol. Sure., St. Paul, pp. 163--171. Peterman, Z.E., Goldich, S.S., Hedge, C.E. and Yardley, D.H., 1972. Geochronology of the Rainy Lake Region, Minnesota-Ontario. In: B.R. Doe and D.K. Smith, (Editors), Studies in Precambrian Geology and Mineralogy (Gruner Volume). Geol. Soc. Am. Mem., 135: 193--215. Poulsen, K.H., 1979. Polyphase deformation of Archean Rocks at Rainy Lake, Ontario. In technical sessions and abstracts for the 25th Annual Institute on Lake Superior Geology, Duluth, Minnesota, 32. Southwick, D.L., 1972. Vermillion Granite-Migmatite Massif. In: P.E. Sims and G.B. Morey (Editors), Geology of Minnesota: A Centennial Volume. Minnesota Geol. Sure., St. Paul, 108--119. Tanton, T.L., 1936. Mine Centre Area, Rainy River District, Ontario, Geol. Surv. Can., Map 334A (scale 1:31.680). Turner, C.C.,and Walker, R.G., 1973. Sedimentology, stratigraphy, and crustal evolution of the Archean greenstone belt near Sioux Lookout, Ontario. Can. J. Earth Sci., 10: 817--845. Walker, R.G., 1975. Generalized facies models for resedimented conglomerates of turbidite association. Geol. Soc. Am. Bull., 86: 737--748. Walker, R.G., 1978. Deep-water sandstone facies and ancient submarine fans: models for exploration for stratigraphic traps. A.A.P.G. Bull. 62: 932--966. Wescott, W.A. and Ethridge, F.G., 1978. Depositional environments of Yallah's fan delta, southeastern Jamaica. A.A.P.G.--S.E.P.M. Annu. Conv., 1978, Oklahoma City, Progr. with Abstr., p. 123. Wood, J., Gibson, H.L. and Carter, T., 1976a. Mattson-MacDowell Lakes Area (Eastern Half), District of Kenora (Patricia Portion). Ontario Div. Mines, Prelim. Map P.1184, Geol. Set. (scale 1:15,840). Geology 1975. Wood, J., Gibson, H.L. and Carter, T., 1976b. Mattson-MacDowell Lakes Area (Western Half), District of Kenora (Patricia Portion). Ontario Div. Mines, Prelim. Map P.1183, Geol. Ser. (scale 1 : 15,840) Geology, 1975. Wood, J., 1977. Geology of North Spirit Lake Area, District of Kenora (Patricia Portion), Ontario Div. Mines Gr. 1 5 0 : 6 0 pp., accompanied by Map 2362 (scale 1:31,680) Wood, J., 1980. Geology of Hewitt Lake Area, District of Kenora (Patricia Portion). Ontario Geological Survey, G.R. 186: 122p. Accompanied by Map 2408 (scale 1: 31,680).