Late proterozoic rifting, glacial sedimentation, and sedimentary cycles in the light of windermere deposition, Western Canada

Palaeogeography, Palaeoclimatology, Palaeoecology, 51 (1985): 231--254

231

Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands

LATE PROTEROZOIC RIFTING, GLACIAL SEDIMENTATION, AND SEDIMENTARY CYCLES IN THE LIGHT OF WINDERMERE DEPOSITION, WESTERN CANADA

G. H. EISBACHER

Geologisches Institut Universit~'t Karlsruhe, Kaiserstrasse 12, 7500 Karlsruhe 1 (Federal Republic of Germany) (Received December 8, 1983; revised version accepted December 10, 1984)

ABSTRACT

Eisbacher, G. H., 1985. Late Proterozoic rifting, glacial sedimentation, and sedimentary cycles in the light of Windermere deposition, Western Canada, Palaeogeogr., Palaeoclimatol., Palaeoecol., 51: 231--254. The upper Proterozoic Windermere Supergroup of western Canada (800--570 Ma) contains the depositional record of a widening rift system along the evolving passive continental margin of North America. Concomitant fault tectonics affected the cratonic (older than 1750 Ma) crust and superimposed middle Proterozoic pericratonic troughs (dating from about 1600 to 800 Ma). The depositional history of the Windermere Supergroup in Canada (and probably also in the adjacent United States) can be understood in terms of three major shoaling-upwards cycles which are best exposed in the Mackenzie Mountains. The first cycle (Rapitan cycle) contains abundant evidence of faulting contemporaneous with glacial sedimentation. A predominant "proglaeial" siltstone facies is overlain by an "ice-marginal" diamictite complex which represents the glaciomarine grounding-line environment of a fluctuating land-based ice sheet. The second cycle (Hay Creek cycle) commences with transgressive black shale or limestone-laminite which grade upsection into a varied and cyclic shallow-water shelf assemblage of clastics and carbonates; the top o f this cycle is composed of abundant carbonate olistostromes and oligomietic diamictites which are capped by a thin but regionally persistent shallow-water dolostone. The third cycle (Sheepbed-Backbone Ranges cycle) again commences with black silty shale and is topped by a shallow-water marine to nonmarine dolostone-sandstone succession o f sub-Cambrian age. Possible equivalence of these three cycles with similar cycles in the Adelaide Basin o f Australia and in the Sinian successions of China suggests that the three basins could have developed along the margins of a complex rift system that led to the opening of an early Paleozoic Pacific Ocean (?) basin. INTRODUCTION

Glaciomarine depositional processes and thickness changes in the resulting sedimentary strata along continental margins are influenced greatly by the tectonic setting. Indeed, contemporaneous tectonics and related mass movements commonly mask the glacial nature of many ancient glacigenic sediments. This holds true particularly for poorly exposed or severely deformed 0031-0182/85/$03.30

© 1985 Elsevier Science Publishers B.V.

232

Precambrian successions (Schermerhorn, 1974). However, where conditions of exposure and preservation permit the detailed study of both depositional and tectonic settiv~s an analysis of contemporaneous tectonics may actually provide clues to the origin of the various diamictites and associated facies. In addition, t h e sedimentary record of marine basins located far beyond the grounding lines of waxing and waning ice sheets or piedmont glaciers should reflect eustatic rise and fall of sea level brought on by glacial retreats and advances (Crowell, 1978; Chumakov, 1981; BjCrlykke, 1983). A fall in sea level should be expressed by widespread progradation of shallow-water or non-marine facies, while a rise in sea level should be expressed by a rapid transgression of deep-water facies over shallow-water facies. The upper Proterozoic basins of the circum-Pacific cratons probably afford an opportunity to explore more fully the relationships between glaciation(s), cyclic sedimentation, and contemporaneous tectonics. MIDDLE AND LATE PROTEROZOIC BASINS OF WESTERN CANADA Remnants of middle and upper Proterozoic sedimentary basins are preserved on the craton of western North America and within the fold-thrust belt of the Mesozoic Cordilleran Orogen. The cratonic crust underlying the western Canadian Shield is generally older than 1750 Ma and is covered over wide stretches by two middle Proterozoic pericratonic successions deposited between about 1600 Ma and 800 Ma. The first pericratonic succession, deposited between about 1600 Ma and 1300 Ma, consists of thick fluvial to marine~turbiditic clastic wedges and intercalated stromatolitic carbonate complexes which prograded from basements highs of the western Canadian Shield towards deep intracratonic depressions. In general, preserved sedimentary complexes are youngest in the westernmost outcrop areas where they locally suffered penetrative deformation, metamorphism and intrusion at about 1300--1250 Ma (Harrison, 1972; Elston and Bressler, 1980; Delaney, 1981; Kerans et al., 1981; McMechan, 1981; Ramaekers, 1981). The second pericratonic succession was deposited between about 1200 and 800 Ma ago. Locally, a spectacular angular unconformity separates the first from the second pericratonic assemblage (Eisbacher, 1981). Subsidence of the second set of basins probably followed an incipient rift event related to the emplacement of a major NW-trending basaltic dike swarm on the Canadian Shield at about 1200 Ma (Mackenzie Dike Swarm). Platformal quartzites, shales, and partly stromatolitic shelf carbonates predominate (Aitken et al., 1978; Elston and Bressler, 1980; Eisbacher, 1981; Young, 1981, 1982). Stratigraphy, sedimentology, and total thickness of up to 15 km of these two middle Proterozic successions suggest that they were laid down in large depressions underlain by strongly attenuated continental crust. Occasional basalt flows are interstratified with laterally persistent and mature sedimentary formations. No glacial or glacially influenced environ-

233 ments have been recognized in these two rock assemblages which together span a time interval of about 1000 million years. However, in the time span between about 800 and 570 Ma, the deposition of the Upper Proterozoic Windermere Supergroup reflects a pronounced tectonic accentuation of a west~facing basin hinge. Faults cutting the middle Proterozoic sedimentary strata and underlying cratonic basement rocks developed into an upper Proterozoic rift system and controlled subsequent progradation of the Cordilleran miogeocline whose shape greatly influenced the arcuate geometry of the later fold-thrust belt of western Canada and the United States (Stewart, 1976; Eisbacher, 1981). SETTING OF THE WINDERMERE BASIN IN THE MACKENZIEMOUNTAINS (NORTHWESTERN CANADA) Tectonic style and pattern of sedimentation related to the incipient upper Proterozoic rift system along the western North American craton is most completely preserved in the Mackenzie Mountains (Fig.l). In contrast to most other Proterozoic outcrop areas of the North American Cordillera, the succession in the Mackenzie Mountains is not penetratively deformed. Regional late Mesozoic--Paleogene deformation caused folding and faulting of the NE-tapering Proterozoic--Paleozoic sedimentary wedge and produced a NE-bulging arcuate fold belt. Major anticlines are cored by middle l>roterozoic clastics (Tsezotene Formation and Katherine Group) or carbonates (Little Dal Group); narrow intervening synclines contain lower Paleozoic platform carbonates. The late Proterozoic Windermere Supergroup is exposed mainly along a NW-trending belt straddling the Plateau Thrust, the principal thrust fault of the Mackenzie Mountains (Fig.l). This composite SW-dipping structure is soled by an evaporite-shale unit of the middle Proterozoic Little Dal Group and extends for a distance of about 350 km along strike. The Plateau Thrust plate and its subsidiary panels expose different facies of the Windermere succession and individual units are best defined in this region (Eisbacher, 1978, 1981). The Plateau Thrust marks a broad hinge zone for both the Windermere Supergroup and the overlying Paleozoic carbonate formations. Most of the units display a distinct platformal facies (Mackenzie Platform on the northeast) or basinal facies (Selwyn Basin on the southwest). Because lower Paleozoic carbonate formations onlap and overstep the late Proterozoic units of the Mackenzie Platform along the Plateau hinge the main facies transitions are well documented only along a relatively narrow outcrop belt (Fig.2). A few stratigraphic transects show that most formations thicken dramatically on the southwest of the hinge zone, and that the thickness of the Windermere Supergroup probably exceeds 4--5 km at a distance of only 25 km to the southwest. For ease of description the facies of the Windermere Supergroup are therefore discussed in terms of a "central facies" downdip from the hinge zone, a "proximal facies" updip from the hinge

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fault zone along which the middle Proterozoic Wernecke block has been brought into juxtaposition with younger rocks by major mid(?)-Paleozoic dislocation and/or rotation (Eisbacher, 1983). Regionally, differentiation into proximal, transitional, and central facies holds for down-basin transects across the northwestern, northeastern, and southeastern margins of the basin; a steady progradation of sediment depocentres towards the southwest is, in general, confirmed by paleocurrent trends in the upper parts of the Windermere Supergroup. T H E T H R E E L A T E P R O T E R O Z O I C CYCLES IN W E S T E R N C A N A D A

A generalized restoration of the individual formations is shown on a cross section across the Plateau hinge zone (Fig.2). Rift-related sedimentation began after deposition of the upper Little Dal Group dolostone unit (LDu). Along a NW-trending belt oligomictic dolostone conglomerate, sharpstone breccias, red mudstones, evaporites, and intercalated basaltic laves with a composite thickness of several hundred meters make up the Redstone River Formation (RR) and kindred marine to non-marine redbed units; these mark local uplift and subsidence along a new fault-controlled basin margin. It is assumed that basaltic sills, which occur within the Tsezotene Formation (Ts) and which have been dated at 770 Ma, are roughly contemporaneous with

236 this incipient stage of rifting (Armstrong et al., 1982). The Coppercap Formation (Cc) overlies Redstone River redbeds in transitional contact and consists of turbiditic limestone or cherty dolostone. The Sayunei (Sa) and Shezal (Sh) formations are proglacial and ice-marginal facies respectively of the first shoaling-upward cycle (Rapitan Group) of the Windermere Supergroup. The Rapitan Group rests conformably or unconformably on subjacent carbonate or redbed formations; it is in turn overlain abruptly and conformably by shale or limestone laminites of the basal Twitya Formation (Tw). The bulk of the Twitya Formation consists of quartzo-feldspathic sandstones or silty shale and grades upward into a regressive dolostone-limestone--sandstone--olistostrome complex, the Keele Formation (Ke). Twitya and Keele formations together comprise the second shoaling-upwards cycle (Hay Creek Group) of the Windermere basin. The Keele Formation is overlain in sharp but again conformable contact by the black silty shale of the Sheepbed Formation (Sb) which grades into or is overstepped unconformably by shallow-water marine and nonmarine clastics of the Backbone Ranges Formation (BR) of sub-Cambrian to Cambrian age. Sheepbed and Backbone Ranges formations together comprise a third unnamed shoalingupward cycle of the Windermere basin. FIRST CYCLE (RAPITAN GROUP)

The proximal and transitional facies of the glacigenic Rapitan Group are controlled by contemporaneous N--NE-trending faults that cut obliquely across the NW-trending Plateau hinge zone. Detailed mapping has shown that this tectonic relief resulted in a variety of onlap and offlap unconformities, rapid thickness changes, and local intraformational angular unconformities (Eisbacher, 1981). High-standing fault panels with thin Rapitan units alternate with downdropped panels or half-grabens with thick sections composed mainly of sedim e n t gravity flow deposits. Maximum outcrop thicknesses along the Plateau hinge zone range between 500 and 900 meters. Thickness of the Rapitan Group in the subsurface of the Selwyn Basin is unknown. In general, fine grained deposits of a proglacial setting can be differentiated from diamictites of ice-marginal settings. The term "proglacial" is used here in a broad context and refers to a large region in front of glacier ice; "ice-marginal" is used to denote a depositional realm close to the area where debris is being released directly from glacier ice. Use of these terms concurs with recent INQUA consensus (Schluchter, 1982, p. 24). Systematic downdip variations southwest of the feather, edge of the Rapitan outcrop locally permit the distinction of three facies belts: a proximal, a transitional, and a central facies belt. The proximal facies (Figs.3 and 4) consists of massive diamictite complexes whose total thickness varies from a few tens of meters to about 250 meters. The basal contact with the carbonate substratum is generally

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Fig.3. Three sections of the glacigenic Rapitan Group, illustrating proximal (1), transitional facies (2), central facies (3). Sections (1) and (2) have an erosional top. Boxes astride the sections indicate relative proportion of carbonate (block pattern), quartzite (dots), and volcanics + granitoids (v -- pattern) in the diamictites. For location of sections see Fig. 1.

abrupt, scoured, and locally polished. The lowermost diamictite sheets are composed o f angular to subrounded carbonate fragments distributed evenly in a matrix of dolomitic-calcareous sand (Fig.5). Colour of the matrix ranges from gray to buff, reflecting the colours of the predominant carbonate clast lithologies o f the substratum. Upsection the diamictite sheets show crude bedding on a 1--10 m scale, are interlayered locally with pink or red mudstone intervals, and clasts are more diverse in composition including cherty

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siltstone, quartzite, greenstone, and sporadic extra-basinalgranitoids. M a n y of the flatiron-shaped resistant stones (i.e. cherty siltstone) are striated (Fig.5). Sedimentary structures in the diamictite matrix are either absent or have been obliterat£~i by intergranular sof~sediment flow. The proximal deposits are therefore interpreted as ice-marginal faciesincluding subglacially ernplaced massive tillites(lodgement tiUites). The transitional facies, (Figs.3 and 4) immediately downdip from the massive diamictite, generally attains a thickness of 200--500 m and consists of three units. The basal unit resembles the lower diamictite units of the proximal facies. Diamictite clasts are mainly subrounded to angular, are composed of carbonate, and only rarely display striated surfaces. They are derived directly from underlying dolostone--limestone formations. Locally, channel deposits of oligomictic sharpstone conglomerates replace or overlap the basal diamictites. The coarse basal units grade upwards into a middle unit of maroon siliciclasticrhythmites, with fine grained B o u m a CDturbidites alternating with iron-rich argillite on a 5--10 c m scale. A few oligomictic conglomerate channels and stringersof diamictite interrupt the monotony of the rhythmite facies. Occasional lonestones of extrabasinal provenance (greenstone, gneiss) embedded in siltstone laminites probably qualify as glacigenic dropstones (Fig.6); locally "till pellets" (sand clasts) occur as well (Eisbacher, 1981, p. 32). However, the regional distribution,of dropstones and tillpellets is uneven and their volume relative to the total volume of fine grained sediment is negligible.The upper part of the middle m e m b e r hosts beds of banded jaspilite--hematiteiron formation 5--40 m thick, averaging 30--40% Fe203. Dropstones of extrabasinal origin and lenticular dropstone laminites are interbedded with the iron formation (Young, 1976; Yeo, 1981). The uppermost unit of the transitional facies belt consists of clast-rich or clast-poor diamictites whose matrix generally displays

239

Fig.5. Crudely stratified to massive tillite of the proximal facies of the Rapitan Group; length of staff 1.5 m. Bottom: Polished and striated cherty dolostone clast from proximal Rapitan facies; diameter about 10 cm.

240

Fig.6. Extrabasinal dioritic greenstone clasts (dropstones) in siltstone rhythmite of the Sayunei Formation.

parallel or cross lamination. In addition, a variety of clast-free shales, siltstones, and cross bedded sandstones alternate with diamictite layers on a 5--10 m scale. Extrabasinal clasts, particularly greenstone, are common components of these structured diamictites; the dark green chloritic matrix indicates extensive erosion of basic volcanics (and feeder dikes) in the glaciated sediment source area Since the transitional facies was probably deposited into fault-controlled proglacial troughs of the Windermere rift system numerous gravitational instabilities along steep subaqueous slopes and along contemporaneous fault scarps produced gravity mass flows that scoured into or merged with background sediment deposited as dilute silty turbidites. In general the depositional environment seems to have been devoid of major fluvial or deltaic progmdation; it is possible that an extensive cover of sea ice and a cold climate on land prevented substantial fluvial action. The upsection change from proglacial rhythmites to structured diamictites with crossbedded sandstones and clast-free shales suggests shoaling of the basin. In the central facies (Figs.3 and 4), southwest of the Plateau hinge zone and downbasin from the transitional facies belt, the Rapitan cycle can be divided into two distinct formations: the Sayunei Formation and the overlying Shezal Formation. The Sayunei Formation ranges in thickness from a few metres to more than 500 meters. It rests conformably on, onlaps unconformably against, or

241

Fig.7. Top: Onlap unconformity of proglacial Sayunei Formation (Sa) against Coppercap dolostone (Cc); note the location of basal stratigraphic contacts of Shezal (Sh) and Twitya (Tw) formations. Locality is south of Section 3 in Fig.1. Bottom: Diamictite of the Shezal Formation in the central facies belt; rounded block on the lower left is composed of granitic gneiss and derived from the Canadian Shield; note scaly siltstone matrix.

242 oversteps the subjacent carbonate formations (Fig.7). The Sayunei Formation consists of predominantly fine grained siliciclastic rhythmites made up of 1--10 cm couplets of Bouma CD-turbidites and maroon or green argillite. Parallel or cross laminated siltstone beds show wispy, slumped, or undulating tops. Paleocurrents derived from cross laminae in siltstones and fine grained graded sandstones display variable paleoflow patterns, but indicate general SE--NW flow in the southern outcrop belt and N--S flow in the northern outcrop belt (Yeo, 1981). Interbedded with the rhythmites are occasional beds of lithic arenites, oligomictic intrabasinal sharpstone channels, and dropstone laminites. However, some of the thickest sections of Sayunei rhythmite are devoid of coarse components. The banded hematite-jaspilite iron formation near the top of the Sayunei Formation is generally only a few meters thick and was locally eroded prior to deposition of coarse mass flow deposits that define the base of the Shezal Formation. Basaltic volcanic activity, possibly related to the deposition of the Sayunei iron formation member, occurred in the western part of the Wernecke Block (Young, 1982). The Sayunei Formation can be broadly interpreted as proglacial and clearly correlates with the middle unit of the transitional glacial facies. Because dropstones, till pellets, and diamictite lenses are rare, the vigorous depositional processes inferred for most of the transitional facies belt do not appear to have extended far into the central facies belt. Either high subaqueous relief along the fault-controlled coast prevented the advance of the grounding line of glaciers towards the deeper parts of the basin, or a stable sea ice cover restricted the dispersal of ice-rafted debris (see Fig.8 for a possible model). The Shezal Formation ranges in thickness from about 100--300 mr Its base is characterized by a change from maroon to red sharpstone rudites of intrabasinal provenance to pink, green, or buff diamictites containing a variety of rounded extrabasinal clasts. Locally, where the underlying Sayunei rhythmites are thin or entirely absent, the overstepping Shezal diamictites are massive and contain a fair number of striated stones and dolomitic megaclasts with diameters up to several meters. Where fully developed the Shezal Formation consists of bedded diamictites with a great variety of clast sizes, shapes, and compositions. The diamictite sheets are generally no more than a few meters thick and commonly separated from each other by clastfree shale, siltstone, or sandstone beds. The silty--sandy matrix of the diamictites exhibit vestiges of sedimentary structures such as cross laminations, water escape structures, and slump folds. The matrix is also pervaded by a scaly structure reminiscent of incipient shear fractures (Fig.7). The feldspathic sandstones are crossbedded or parallel laminated; graded bedding or Bouma sequences characteristic of turbidites are rare. Within a single section or over short distances along trend diamictites range from tightly packed monomictic dolostone conglomerates near the base to loosely dispersed polymictic assemblages with slabs of shield-derived gneisses

243

elsewhere. Oversize clasts commonly occur in clusters and only rarely display glacially striated surfaces. The top of the Shezal Formation is marked by an abrupt but conformable contact between the highest of the diamictite sheets and a black shale or grey limestone laminite member of the overlying Twitya Formation. The Shezal Formation is interpreted as a water-lain ice-marginal facies that prograded over the proglacial Sayunei rhythmite facies (Fig.8). It is as yet Unknown what caused the basinwide progradation. One of the most striking aspects of this progradation is that it occurred shortly after the deposition of the probably volcanogenic banded iron formation and that over wide areas mass movements seem to have removed the iron formation below the base of the Shezal diamictites. The stratigraphic relationship between Sayunei and Shezal formations suggests also that contemporaneous tectonic movements were less intense during deposition of the Shezal Formation than during deposition of the Sayunei Formation. The alternation Shezal

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Fig.8. Interpretation of the depositional environments of the central facies of the Rapitan Group. During deposition of the Sayunei Formation the basin floor underwent deformation. Land based ice masses were fringed by a broad marine proglacial belt which received dilute turbidites, dropstone lenses, and gravity mass flow deposits. Lack of major deltaic progradation into the basin suggests widespread sea-ice cover in front of cold-ice glaciers. During deposition of the Shezal Formation a strong fluctuating grounding line of a land-based ice shelf created a variety of clast-rich and clast-poor diamictites and intercalated clast-free sandstone-shale units.

244 between clast-rich and clast-poor diamictites, bedded sandstone, siltstone, and clast-free shale units suggests, however, highly variable depositional processes. Rafting of oversized debris by sea or berg ice was probably still a significant mechanism in the deposition of coarse stone clusters or individual boulders into current-winnowed silty beds. However, some laterally persistent clast-rich diamictite phases are probably deposits of melt~out processes at or near the front of grounded shelf or glacier ice. Actualistic sedimentary models proposed recently for ice shelves and tidewater glaciers, although diverse in outlook, concur that most englacial debris is released within a relatively narrow zone near where the subaqueous grounded glacier ice becomes detached from its bed ("grounding line") or where the glacier terminus undergoes rapid retreat (Molnia and Sangrey, 1979; Boulton and Deynoux, 1981; Drewry and Cooper, 1981; Orheim and Elverhoi, 1981; Powell, 1981; Vorren et al., 1983). Depending on the depositional slope in front of the grounding line the glacigenic debris and basal tills can be reworked by sediment gravity flow processes (Anderson et al., 1980; Wright and Anderson, 1982). Only a minor proportion of the Shezal Formation, as presently exposed along the Plateau hinge zone, contains evidence of sediment gravity flow; however, gravity flow deposits are probably more common in subsurface sections southwest of the Plateau hinge zone. In an overall sense the Shezal Formation represents the basinward advance of depositional environments associated with glacial grounding lines: in analogy with the young history of the Antarctic ice-shelves (HoUin, 1962; Mercer, 1983) the general advance was probably a response to relative lowering of sea level. Whether this relative lowering of sea level was due to a worldwide increase in the volume of land-based glacier ice or whether it was due to regional doming concomitant with volcanic and tectonic activity at the margins of the rift basin can not be resolved at present. The increasing compositional diversity of clasts near the top of the Shezal Formation indicates that towards the end of the Rapitan glaciation the dimensions of the ice sheet were such that far-traveUed debris derived from volcanics, exhumed sills, and crystalline basement were thoroughly mixed with debris from more local carbonate and quartzite sources. Termination of the glaciation was abrupt: there is only insignificant sedimentary reworking of the highest diamictite sheets and a virtual absence of transitional dropstone laminites in the highest parts of the Shezal Formation. S E C O N D C Y C L E (HAY C R E E K GROUP)

The second shoaling-upwards cycle of the Windermere Supergroup begins with pyritic shale, fine grained silicichstics, or limestone turbidite-laminites of the basal Twitya Formation. As pointed out above, the contact between the Shezal diamictite and the basal fine grained Twitya sediments is sharp, conformable, and locally an onlap. Along the Plateau hinge zone basal black shale or limestone laminite members are up to 100 m thick and grade into

245

slump-folded laminated siltstones and fine grained feldspathic sandstones. Analysis of slump folds shows that paleoslopes faced predominantly to the southwest towards the centre of the subsiding Selwyn Basin. Along the basin margin shallow-water carbonate complexes thus had become established immediately after deglaciation. In the Wernecke block up to 400 m of shallow-water dolostone represent most of the Twitya Formation (Eisbacher, 1981). However, along most of the Plateau hinge zone the Twitya Formation consists of a prograding clastic shelf-slope assemblage that is 500--1000 m thick. These siliciclastics bypassed local carbonate platforms and, in the deeper parts of the Selwyn Basin, fed an extensive turbidite trough. In its highest parts the Twitya Formation changes gradually into the cyclic shallow-water Keele Formation. The Keele Formation varies regionally from a predominant carbonate bank facies to a predominant shallow-water clastic facies. Both carbonate and clastic facies consist of upward shoaling cycles 5--20 m thick. Total thickness of the Keele Formation ranges from a few tens of metres to more than 400 m. Contemporaneous faults associated with the outer parts of the Keele carbonate platforms are responsible for abrupt thickness changes and for the development of carbonate olistostromes (Eisbacher, 1978). At the top of the unit extensive erosional channeling into platformal carbonates caused widespread redeposition of oligomictic carbonate rudites. Along most of the Plateau hinge zone the olistostromes and rudites are overlain by a thin (10 m) but laterally persistent dolostone ("Tepee" member). Where investigated the olistostrome members are composed almost entirely of dolostone and limestone clasts embedded in fine grained carbonate matrix; no extrabasinal or striated stones have been found in these rudites. The capping "Tepee" dolostone is characterized by desiccation features, extensive recrystallisation, and patchy brecciation; it is probably of intertidal origin and closely resembles the socalled "cap dolomites" described from the top of upper Proterozoic plat~ formal tillites in other parts of the world (e.g. Deynoux, 1980; Coats, 1981). The "Tepee" dolostone is overlain conformably by black silty shale of the Sheepbed Formation. In general, the depositional environment of the Keele Formation thus reflects regional shoaling of the prograding miogeocline. Although a direct influence of ice has yet to be demonstrated for the deposition of the Keele Formation, its internal cyclicity, and the overall shoaling followed by widespread transgression could be related to eustatic sea level changes, regional glaciation and deglaciation elsewhere (see below). THIRD CYCLE The third upper Proterozoic shoaling-upwards cycle in western Canada consists of the widespread shale and siltstone succession of the Sheepbed Formation which grades upwards into or is overlain unconformably by subCambrian clastics and dolostone of the Backbone Ranges Formation. The

246 Sheepbed Formation consists of a dark monotonous silty shale; in its lower parts shale is interbedded with minor limestone turbidites; in its upper parts it hosts siliciclastic turbidites. The total thickness of the Sheepbed Formation in outcrop is up to 900 m. The Backbone Ranges Formation consists of predominantly nonmarine or shallow-water marine feldspathic sandstones and minor dolostone. Cros~ bedded fluvial sandstones, red mudstones, and intertidal quartzites of a proximal facies, exposed along the Plateau hinge zone, change into turbiditic siltstones and fine grained sandstones towards the centre of the Selwyn Basin. Paleocurrents indicate paleoslopes which face towards the centre of the Selwyn Basin. Feldspar contents of up to 20% in the sandstone of the basal Backbone Ranges Formation suggest that their provenance was, at least partly, the Canadian Shield. Trace fossils and regional stratigraphic correlations indicate that the upper part of the Backbone Ranges Formation includes the upper Proterozoic--lower Cambrian boundary beds. In basinal sections the stratigraphic equivalent of the upper Backbone Ranges Formation is overlain conformably by fossiliferous early Lower Cambrian carbonate strata (Fritz, 1980, 1982). In general, the depositional environment of the third cycle reflects the transition from an open and partly starved shelf to a prograding shallowwater marine, tidal, and braided-stream setting. COMPARISON AND POSSIBLE CORRELATIONWITH OTHER CIRCUM-PACIFIC LATE PROTEROZOIC CYCLES. DISCUSSION AND CONCLUSION Glacial deposits have been documented from other upper Proterozoic sections of the North American Cordillera and along late Proterozoic cratonic hinge zones of the circum-Pacific realm. Glacigenic diamictites and rhythmites, similar to the Rapitan Group of the Mackenzie Mountains, have recently been described in detail from eastern Alaska (Young, 1982). There the glacigenic rocks form part of the upper Tindir Group, which was probably deposited along a complex continuation of the basal Windermere hinge zone and was later disrupted by tectonic displacements. As far as is known the Tindir clastics are the northwesternmost preserved equivalents of the basal upper Proterozoic Windermere Supergroup in North America. Correlation of Windermere Supergroup strata of the north-central Canadian Rocky Mountains has recently been established with a fair degree of confidence (Fig.9). Exposed in the hanging wall of major thrust sheets the succession starts with grey siltstone rhythmites (Herrick Pass rhythmites) that grade upwards into polymictic diamictites with measured thicknesses of up to 600 m. The deformed diamictites contain abundant extrabasinal igneous clasts and zones of dropstone laminites (Mt. Vreeland diamictites). The diamictites are overlain by dark silty or calcareous slates which grade upwards into thickbedded, feldspathic granule turbidites (Miette or

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248 Misinchinka "Grits"). These turbidites, in turn, grade into a regressive shallow-water dolostone--sandstone unit (Byng Formation). Finally, turbiditic slates above the Byng Formation pass into a thick sub-Cambrian succession of shallow-water feldspathic sandstones (Gog Group). The three shoaling cycles identified in the Mackenzie Mountains are thus clearly recognizable in the northern Canadian Rockies, some 900 km to the south. Recent re-examination by the author of the Toby Formation of southern British Columbia (Canada) and northern Washington (U.S.A.) has confirmed the strong glacial aspect of this unit which, together with basaltic volcanics, forms the base of the Windermere Supergroup (Aalto, 1971). Relatively undeformed sections of the Toby Formation in the Purcell Mountains consist of a lower siltstone member with dropstones and quartz--chert conglomerate lenses and an upper diamictite member with sporadic striated stones (Eisbacher, 1981, p. 37). The top of the Toby diamictite in these sections is well defined, and is marked by siltstone or limestone laminites deposited along a continental slope; the fine grained units pass regionally into thick successions of basinal feldspathic turbidites (Fig.9). The turbiditic succession contains a conspicuous interval of prograded carbonate bank and clastic deposits (Cunningham Formation) which possibly occupy a stratigraphic position similar to that of the Keele (or Byng) Formation elsewhere in the Canadian Cordillera. A correlation of the Toby Formation with the first glacial cycle of the Mackenzies (Rapitan Group) is therefore favoured. In the Utah--Idaho region of the western United States upper Proterozoic glacial and glaciomarine deposits have been studied and documented in detail within a regionally coherent stratigraphic framework (Christie-Blick, 1982; Crittenden et al., 1983). Two glacial diamictite units occur near the base of a succession and could be broadly equivalent to the basal Windermere strata of Canada. A clastic succession overlying the basal glacials contains a regressive arkose--conglomerate--dolostone unit up to 500 m thick (Maple Canyon Formation) which is capped by a thinly laminated dolostone; towards the top the upper Proterozoic succession consists of shale and siltstone which grade into thick (up to 2000 m) and increasingly pure quartzites assigned to a latest Precambrian or early(?) Cambrian age (Crittenden et al., 1971). It is therefore possible that the three shoaling cycles of the northern Canadian Cordillera have counterparts in the western United States as suggested in Fig.9. In eastern California a thick upper Proterozoic succession also contains the record of a glaciation which occurred contemporaneous with volcanism and basin-margin tectonism. The age of these glacial deposits is only poorly constrained (Miller et al., 1981). In the Adelaide basin of eastern Australia the upper Proterozoic succession is characterized by basal glacials (Sturtian tillites) overlain by shale, silt~ stone, and carbonates (Tapley Hill Formation). The Tapley Hill Formation in turn grades upwards into arkosic sandstones and glacigenic diamictites

249

(Marinoan tillites)which are overlain by a widespread "cap" dolomite and shale (Nuccaleena and Brachina formations). The Brachina shale--siltstone shoals upwards into sub-Cambrian quartzites and dolostones (e.g. Pound Group with Ediacara fauna). The Pound Group is overlain disconformably by fossiliferous Cambrian strata (Coats, 1981; Preiss and Forbes, 1981; Rutland et al., 1981). The age of the Sturtian glaciation is about 800--740 Ma, that of the Marinoan glaciation about 680--690 Ma. The upper Proterozoic depositional pattern of the Adelaide basin thus mimics roughly the three cycles proposed for the Windermere basin, the difference being that the top of the second cycle (Keele Formation) in Canada has not yet yielded any direct evidence of glaciation. In both the Adelaide and Windermere basins the sub-Cambrian third cycle is characterized by progradation of feldspathicclasticsand widespread shoalingof the depositional environment along the basin hinges. In the upper Proterozoic Sinian basin(s) of South China two glaciations are known: the. basal Changan glaciation (assigned an age of about 800--760 Ma) and the Nantuo glaciation(assignedan age of about 680 Ma). Changan and Nantuo glacialdeposits are separated from each other by an intervening feldspathic sandstone--shale succession several hundreds of metres thick (Fulu Formation). Elsewhere in China a sub-Cambrian "Third Glaciation" has been reported (Yongji, 1981; Shih-fan, 1981). It is suggested here that the three glacialintervalsof China might correlateroughly with the top of the three shoaling-upwards cycles of western North America and Australia (Fig.10). The existence of possibly three glacio-eustaticsedimentary cycles in upper Proterozoic assemblages of the circum-Pacific realm has, in the author's opinion, considerable significancewith respectto the originand geodynamic evolution of the early Pacific basin. In western North America initiationof C A N A DA

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250

upper Proterozoic sedimentation was roughly contemporaneous with a rift event that can be inferred from basaltic volcanism, sedimentary facies, and mappable contemporaneous faults (Stewart, 1976; Eisbacher, 1981; Elston and McKee, 1982; Young, 1982). Rifting commenced about 800--750 Ma ago and was associated locally with considerable deformation. However, it is n o t yet clear where and at what stage of the late Proterozoic--early Paleozoic basin evolution subsidence of attenuated continental crust outboard from the asymmetrically prograding miogeocline gave way to formation of oceanic crust. The similarity of the tectonic setting and the possible equivalence of the three glacio-eustatic cycles in the Windermere, Adelaide, and Sinian basins provide a hint that in late Proterozoic time the cratonic blocks of North America, Australia, and South China might have been relatively close to each other and that they bordered a system of rift basins which transected older cratonic crust and sedimentary troughs; probably much later the rift matured into a wider paleo-Pacific oceanic(?) basin. In the Adelaide Basin of eastern Australia, late Proterozoic rifting probably created an E-facing miogeocline (Von der Borch, 1980; Preiss, 1 9 8 3 ) - - a possible equivalent to the W-facing North American miogeocline. Subsequent tectonic events consolidated Australia's position with respect to the Paleozoic Gondwana continent (Rutland et al., 1981). Paleomagnetic work seems to suggest that a close spacial relationship and low paleolatitudes still prevailed for South China and Australia in latest Proterozoic and early Cambrian time (Lin et al., 1983). Low paleolatitudes also have been suggested for the early Paleozoic of North America (Smith et al., 1973; Scotese et al., 1979). Watts (1982) and others have argued recently that rapid transgression and sedimentary onlap along coastal plains of passive continental margins mark a change in flexural rigidity of the crust following the rift-drift transition. It is entirely possible that the three Windermere cycles have a more fundamental cause in rift events of global significance each of which was followed by broad crustal flexure. BjCrlykke (1978) and Nystuen (1982) for example have shown that over wide areas the latest Proterozoic Varangian Glaciation of northern Europe (about 650 Ma) also followed a profound rift event. Thus, late Proterozoic glaciations might have been closely linked to deformation associated with rift events. Evidently, this problem will have to be resolved by more accurate radiometric age controls than those available at present. Figure 11 is the crude image envisaged here for a possible configuration of the upper Proterozoic rift basins along eastern Australia, South China, and western North America at about 800--700 Ma. Sedimentation in complex and widening rift basins seems to have been controlled tectonically by subsidence of thinned continental crust and climatically by three glacio-eustatic cycles that also affected the adjacent cratonic platforms. Regardless of the validity of the paleogeographic concept outlined here, the refinement of subsidence dynamics and the dating of concomitant igneous pulses within the Windermere, Adelaide, and Sinian realms eventually may lead to a better understanding of the dynamics of late Proterozoic glacial episodes as well.

251

Fig.11. Hypothetical juxtaposition o f the upper Proterozoic Windermere (North America), Adelaide (Australia), and Sinian (South China) basins across a maturing rift system that preceded the paleo-Pacific basin at about 800--700 Ma ago. The younger accretionary borders that mark the present continental outline are shown for geographical perspective; cross-ruled patterns indicate areas underlain by Precambrian cratons and pericratonic basins. ACKNOWLEDGEMENTS

The author wishes to acknowledge critical reading of the manuscript by Drs. R. Trompette, R. B. Campbell, and M. Deynoux. REFERENCES Aalto, R . K . , 1971. Glacial marine sedimentation and stratigraphy of the T o b y Conglomerate (Upper Proterozoic), southeastern British Columbia, northwestern Idaho, and northeastern Washington. Can. J. Earth Sci., 8: 753--787. Aitken, J . D . , Long, D . G . F . and Semichatov, M . A . , 1978. Progress in Helikian stratigraphy, Mackenzie Mountains. Geol. Surv. Can., Pap. 78(1A): 481--484. Anderson, J. B., Kurtz, D . D . , Domack, E.W. and Balshaw, K . M . , 1980. Glacial and glacial marine sediments of the Antarctic continental shelf. J. Geol., 88: 399--414. Armstrong, R . L . , Eisbacher, G . H . and Evans, P.D., 1982. Age and stratigraphic-tectonic significance of Proterozoic diabase sheets, Mackenzie Mountains, northwestern Canada. Can. J. Earth Sci., 19: 316--323. Bj~brlykke, K., 1978. The eastern marginal zone of the Caledonide orogen in Norway. Geol. Surv. Can., Pap. 78(13): 49--55. Bj~brlykke, K., 1983. Glaciations and sea level changes. In: M. Deynoux (Editor), Abstracts and introduction to the field excursion of Symposium Till Mauretania '83. Univ. Poitiers, p.'9. (abstract). Boulton, G. S. and Deynoux, M., 1981. Sedimentation in glacial environments and the identification o f tills and tillites in ancient sedimentary sequences. Precambrian Res., 15: 397--422. Christie-Blick, N., 1982. Upper Proterozoic and Lower Cambrian rocks of the Sheeprock Mountains, Utah: Regional correlation and significance. Geol. Soc. Am. Bull., 93: 735--750.

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Young, G. M., 1976. Iron-formation and glaciogenic rocks of the Rapitan Group, Northwest Territories,Canada. Precambrian Res., 3: 137--158. Young, G.M., 1981. The Amundsen Embayment, Northwest Territories,relevance to the Upper Proterozoic evolution of North America. Geol. Surv. Can. Pap., 81(10): 203--218. Young, G. M., 1982. The late Proterozoic Tindir Group, east-centralAlaska: Evolution of a continental margin. Geol. Soc. Am. Bull.,93: 759--783.