Neoproterozoic strata of the southern Canadian Cordillera and the isotopic evolution of seawater sulfate

Pretnnlbriun Resenrth ELSEVIER

Precambrian Research 73 ( 1995 ) 71-99

Neoproterozoic strata of the southern Canadian Cordillera and the isotopic evolution of seawater sulfate Gerald M. Ross a, John D. Bloch"'l, H. Roy Krouse b "Institute of Sedimentary and Petroleum Geology, 3303 33rd Street N. V~, Calgary, Alberta T2L 2A 7, Canada bDepartment of Physics, University of Calgary, Calgary, Alberta, T2N IN4, Canada

Received 15 January 1994; revised version accepted 10 April 1994

Abstract

Neoproterozoic strata of the southern Canadian Cordillera comprise metasedimentary rocks that are impressive in their thickness (6-9 km), areal extent (35,000 km 2 minimum) and predominance of sedimentary rocks of deep-water affinity. The stratigraphy can be subdivided into two broad successions that record the response to rifting and subsequent formation of a passive margin during the breakup of western Laurentia. Syn-rift strata include glaciogenic rocks that likely correlate with earlier Neoproterozoic glaciations elsewhere (e.g. Sturtian). Dating of associated granitic and volcanic rocks in Canada suggests that these units were deposited between 762 Ma and 728 Ma. Post-rift strata are interpreted as the deposits of an elongate terrigenous-sourced turbidite system that formed at the base of the continental slope and shoaled upward through slope into platformal facies. Evidence of a second Neoproterozoic glaciation (Varanger equivalent) is recognized in the post-rift turbidite succession as a regionally mappable condensed interval formed during post-glacial sea-level rise. Deep-water carbonate deposits punctuate the record of dominantly siliciclastic slope sedimentation and hold promise for future C-Sr isotope chemostratigraphy. Coeval shallow-water facies are rarely preserved in the southern Canadian Cordillera owing to erosional removal during uplift associated with a younger rift event that preceded Cambro-Ordovician passive margin formation. The Kaza Group (sand-rich basinal turbidites) and the overlying Isaac Formation (mud-rich slope facies) are post-rift strata in the Cariboo Mountains that contain abundant pyrite as disseminated, large (up to 5 cm) crystals and finely crystalline stratabound layers. Isotopic analysis of pyrite separates from these two units demonstrates a broad range in ~34S values (49 and 53%0, respectively), typical of sulfides formed by the bacterial reduction of seawater sulfate. There is a distinct shift of ca. + 8%0 in median c~34Svalues and + 15%o in the maximum-minimum values from pyrites in the Kaza Group into the overlying Isaac Formation. The magnitude of the shift, in addition to its stratigraphic position above Sturtian age glacial deposits, suggests that this excursion in the sulfide record overlaps with the large sulfate sulfur excursion observed in late Neoproterozoic to early Cambrian evaporites. We suggest that the Kaza Group and Isaac Formation pyrites are the deep-water, 32S-enriched, complement to the sulfate record. Burial of 32S-enriched pyrite in continental margin strata, such as exemplified by the Windermere Supergroup, likely occurred on a global scale in response to widespread passive margin formation during the breakup of western Laurentia with resultant enrichment of residual seawater sulfate in 34S. Thus open marine sulfate reduction and pyrite burial is interpreted as the driving mechanism by which Neoproterozoic sulfate became enriched in 34S and requires microbial reduction of a substantial volume of the global sulfate reservoir (ca. Present address: Scealu Modus, 2617 Cutler Ave N.E., Albuquerque, New Mexico, 87106, USA. 0301-9268/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10301-9268 ( 94 ) 00072-7

72

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

25%). The coupled process of sulfate reduction and organic matter oxidation implies that the process of sulfate reduction on this scale would have had a substantial impact on the evolution of the oxygen reservoir in the Neoproterozoic.

1. Introduction The terminal Neoproterozoic stratigraphic record, from ~ 7 5 0 to 545 Ma (Bowring et al., 1993 ), preserves evidence of remarkable changes in Earth's global climate, tectonic patterns, atmosphere and hydrosphere chemistry and, ultimately, life. This is evidenced by the presence of two global glacial episodes, the break-up of a supercontinent that may have created tens of thousands of kilometres of new passive margin (Bond et al., 1984) coeval with the assembly of Pan-African Gondwanaland (Hoffman 1991 ), large apparent variations in the isotopic composition of seawater (Derry et al., 1992) and the appearance and extinction of Earth's first metazoans (Knoll and Walter, 1992 ). To gauge the magnitude of the changes on the Neoproterozoic Earth, geochemists have been analyzing the isotopic compositions of inorganic mineral precipitates within marine sedimentary rocks. This has led to models that consider both the mass balance and the flux between reservoirs as a means of quantifying chemical changes in the oceans and atmosphere (Veizer et al., 1983; Asmerom et al., 1991; Derry et al., 1992; Hayes et al., 1992 ). For the most part, the Neoproterozoic sedimentary reservoirs that have been investigated comprise cratonal and shelf sedimentary assemblages, a consequence of greater preservation potential but a feature that may impart a bias to our view of secular variations in Neoproterozoic ocean chemistry. The southern Canadian Cordillera represents an anomaly in the record of Neoproterozoic sedimentation around the Pacific Rim; it is characterized by a thick ( > 6 km) assemblage of deep-water sedimentary rocks deposited on and below the continental slope of a passive margin. As such it represents a largely uninvestigated geochemical reservoir that augments the data derived from the analysis of shallow-water shelf successions. The purpose of this paper is two-

fold. First, we will present a stratigraphic and tectonic overview of Neoproterozoic rocks of the southern Canadian Cordillera. Second, we will present the results of a recently completed study of the sulfur isotopic composition of authigenic pyrite that is common in this succession of deepwater rocks.

2. Regional setting The Canadian Cordillera is an arcuate orogenic belt along the Pacific margin of Canada that records a protracted history of plate interaction that culminated in convergence and crustal thickening during the Mesozoic (mid-Jurassic to early Cenozoic) (Monger and Price, 1979; Monger, 1989). Neoproterozoic rocks of the Windermere Supergroup comprise part of the thick continental margin wedge succession of the Rocky Mountain and Omineca belts that includes the Mesoproterozoic Belt-Purcell Supergroup and the lower Paleozoic carbonate shelf succession. The Windermere Supergroup outcrops along the length of the Cordillera although it is restricted largely to the western parts of the Rocky Mountains and the Omineca belts (Fig. 1). The most extensive exposures of the Windermere in western Canada are in the Mackenzie Mountains in the northern Cordillera (see Narbonne and Aitken, 1995 ) and throughout much of the southern Canadian Cordillera. These two regions are interpreted to preserve different paleogeographic segments of the same Neoproterozoic passive continental margin (Fig. 2). The Mackenzie Mountains preserves shelf, shelf-edge and slope facies whereas the southern Cordillera preserves a predominantly deep-water basinal and slope succession. The two regions can be linked on the basis of the strong sea-level signal associated with the second Windermere (Varanger equivalent) glacial event (Ross, 199 lb). In the southern Cordillera the Windermere Supergroup comprises a thick succession of dominantly siliciclastic units that are up to 9 km

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

4TS.

CARIBOC MOUNTAIP

tDILLERA

PURCELl MOUNTAIP 49 +

73

1987; McDonough and Simony, 1988a). The lack of biostratigraphic control within this thick succession of lithologically similar rocks combined with the masking effect of high-grade metamorphism and ductile deformation has fueled controversy concerning stratigraphic correlations within high-grade successions and between high- and low-grade Windermere strata (Struick, 1986; Pell and Simony, 1987; Dechesne and Kubli, 1988 ). Nonetheless, extensive regions of structurally simple and low-grade Windermere strata are present and well exposed in the Purcell Mountains, western Rocky Mountains and particularly in the northern Cariboo Mountains (Fig. 3 ) and it is the Neoproterozoic record from these regions that is the focus of this paper.

3. Stratigraphic setting

Fig. 1. Distribution of Neoproterozoic strata of the Windermere Supergroup in western North America. Representative stratigraphic columns from the Mackenzie Mountains, the southern Cordillera and the western United States were used to construct Fig. 2.

thick and exposed over an area of at least 35,000 km 2, without palinspastic restoration. The succession is everywhere allochthonous and has been transported on Mesozoic thrust faults. The effect of Mesozoic metamorphism and deformation on the Windermere has been dramatic. Estimates of shortening in the southern Cordillera indicate at least 170 km of shortening due largely to thrust imbrication (Price, 1981) and perhaps as much as 300 km if the effects of ductile deformation in the Omineca Belt are considered (Brown et al., 1993). The metamorphic grade reaches upper amphibolite in parts of the Omineca Belt (Shuswap Complex; Fig. 3) and thrust imbrication and locally complex folding have required structural studies to precede stratigraphic studies (Simony et al., 1980; Murphy,

Neoproterozoic rocks in the southern Canadian Cordillera comprise a regionally widespread and locally thick succession of clastic and subordinate carbonate rocks that record deposition on a west-facing passive margin (Ross, 1991 a; Fig. 2 ). The stratigraphic succession can be viewed in a somewhat simplistic perspective as a two-component system (Fig. 4), similar to that proposed by Stewart (1972) for the Neoproterozoic rocks of the western United States. The older units are interpreted to record rifting and are of limited lateral extent, show evidence of locally dramatic facies changes and normal fault activity during deposition, and locally contain magmatic rocks (Aalto, 197 l; Lis and Price, 1976; Glover, 1978; Leclair, 1982; Root, 1987). The rift facies rocks are overlain by regionally persistent stratigraphic units characterized by deep-water sedimentary facies that shoal locally into platformal facies. Shoaling occurs over a stratigraphic thickness of 5-7 km and is interpreted to record shelf progradation in response to a diminishing rate of thermally driven subsidence of the margin. This interpretation implies that the Windermere passive margin faced into an open ocean basin and is consistent with emerging evidence for the timing of break-up of

74

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

SOUTHERN CORDILLERA BASIN AND SLOPE

MACKENZIEMTS. SHELF EDGE

WESTERN U.S. CLASTIC SHELF

~

SHALE/MUDSTONE

~

GRIT

( ~

LIMESTONE/DOLOMITE

Q

BELT-PURCELL SUPERGROUP

@

RIFT-FILL~MAINLY DIAMICTITE AND MAFIC VOLCANIC ROCK

CONTINENTAL CRUST

Fig. 2. Schematic cross-section for the west-facing Windermere margin of western North America based on telescoping the stratigraphic record of the western U.S. (dominantly shallow-water facies), the Mackenzie Mountains (shelf and shelf-edge) and the southern Cordillera (basinal, slope and shelf facies; from Ross, 1991b). Question marks beneath the southern Cordillera indicate uncertainty in the distribution of continental basement.

Laurentia (Dalziel, 1991; Hoffman, 1991 ; Moores, 1991; Ross, 199 la). This is an important conclusion because it implies a priori that the isotopic data we have collected may impact on understanding oceanic processes rather than just local basin evolution. The tectonic significance of the Windermere has been a much debated issue. Bond and Kominz (1984) used back-stripping techniques to demonstrate that the Windermere could not have been the rift event that led directly to subsidence of the lower Paleozoic passive margin. Subsequently, Ross ( 1991 a) and Ross et al. (1992) expanded on suggestions of previous workers and argued that the Windermere represents a passive margin formed when Australia rifted away from western North America, leaving a proto-Pacific Ocean in its wake. A second rift event must then be responsible for the Cambro-Ordovician passive margin. This second rift event is evidenced by local rift facies arkose and mafic volcanic rocks that lie unconformably on the Windermere Supergroup but are overlain by the widespread quartz arenites at the base of the Paleozoic passive margin (Devlin and Bond, 1988; Lickorish, 1992; Kubli and Simony, 1992). Uplift associated with this second rift event has created a sub-

stantial amount of sub-Cambrian erosion that cut down towards the east and may have removed the shallow-water Windermere record throughout the southeastern part of the Cordillera (Ross, 1988 ). Consequently the thickest and most complete section of the Windermere (particularly post-rift strata) is preserved in the northern Cariboo Mountains of British Columbia, the westernmost occurrence of Windermere strata in the southern Cordillera. A more detailed overview of Windermere stratigraphy has been presented elsewhere (Ross et al., 1989, and references therein). The purpose here is not to replicate that discussion but to concentrate on recent developments that have implications for global and regional correlation of Neoproterozoic events and their age constraints. The following discussion will be organized into four parts: ( 1 ) glaciogenic units, commonly used as a basis for interregional correlation and a widely recognized component of the Neoproterozoic record worldwide (Hambrey and Harland, 1981); (2) magmatic rocks, which are widespread and well-dated in the Cordillera and provide constraints on the age of rifting; (3) turbidite systems, a widespread and thick facies characteristic of the Windermere in the southern

G.M. Ross et aL / Precambrian Research 73 (1995) 71-99

75

oup

Carl MI

rl

3elt

~ry

Fig. 3. Simplified geologic map for the southeastern Canadian Cordillera showing the distribution of Windermere strata. Capital letters within the stippled Windermere Supergroup show known locations of the second glacial marker (Old Fort Point Formation) discussed in the text (from Ross and Murphy, 1989 ).

Cordillera; (4) carbonate units, which comprise deposits of both deep- and shallow-water affinity and will serve as the chemostratigraphic targets for future research. The stratigraphic nomenclature in the region is complex and thus will not be emphasized.

3.1. Glaciogenic units Until recently, only a single glaciogenic interval had been recognized in the Canadian Cordillera, the classic Toby Formation in the south (Aalto, 1971) and the Rapitan Group in the north (Young, 1976); both of these occur at, or near, the base of the Windermere succession and are attributed to the Sturtian glacial episode

( ~ 750 Ma). Aitken ( 1991 ) has described a second glaciogenic unit, the Ice Brook Formation, which occurs at least a kilometre above the Rapitan Group in the Mackenzie Mountains and is the first clear evidence of a second Windermere glacial event (e.g. the Varanger episode; ~ 590610 Ma) in western North America. A thick sequence of coarse boulder diamictites also occurs within Windermere strata well above the Toby Formation in the Monkman Pass area (Fig. 1; Eisbacher, 1981; McMechan and Thompson, 1985). Recently, McMechan (1990) and Hein and McMechan (1994) referred to this unit as the Mount Vreeland Formation and demonstrated its glaciogenic origin as well as its strati-

76

G.M. Ross eta/. /Precambrian Research 73 (1995) 71-99

MACKENZIE MTS.

MONKMAN PASS

CARIBOOPURCELLS

CAMBRIAN

POSTRIFT

RIFT

]

5LAIe/~l-lALt GRIT/ SANDSTONE

~

VOLCANICS

[]

EVAPORITE

Fig. 4. Comparative stratigraphic columns for Windermere strata the length of the Canadian Cordillera showing inferred rift and post-rift strata and suggested correlations of the second glacial unit (Ice Brook-Mount Vreeland-Kaza Group) and cap carbonate (Teepee-basal Framstead-Old Fort Point). Sections from the Mackenzie Mountains and Cariboo-Purcells preserve a basal unconformity with the Windermere Supergroup resting on older sedimentary rocks and, in the latter case, Paleoproterozoic, and locally Neoproterozoic, igneous basement. The base of the Windermere section in the Monkman Pass area is a structural contact. Sources of data: Mackenzie Mountains, Narbonne and Aitken, 1995: Monkman Pass, Hein and McMechan, 1994; the CaribooPurcells, Ross et al., 1989.

graphic position well up in the Windermere succession. The Toby Formation is a lithologically heterogeneous unit that includes coarse diamictite, conglomerate, breccia, pelite, cryptmicrobial carbonate, local greenstone both as discrete flows and matrix in the conglomerates, and cross-bedded to massive sandstone. It ranges in thickness from 0 to nearly 2500 m near the Canada-U.S. border (Aalto, 1971; Glover and Price, 1976; Fig. 3). The top of the Toby contains a laterally discontinuous carbonate unit that may be analogous to cap carbonates observed above the Rapitan Group and elsewhere (Root, 1987; Warren and Price, 1992). In the Purcell Mountains the Toby is conformably overlain by pelites and grits that are part of a regionally extensive post-rift turbidite system. The glaciogenic origin of the Toby has been a topic of debate. Aalto (1971) and Eisbacher (198.1) provided good evidence of glaciogenic

sedimentation. During a study of the Toby in the type area, Root (1987) was able to document that the variable thickness changes in the Toby were due to synsedimentary normal faulting, a similar conclusion to that presented by Lis and Price (1976) who inferred up to 6 km of normal fault movement. It is likely that parts of the Toby are glaciogenic, whereas other parts are fluvial or shallow-marine deposits that accumulated under the influence of active tectonism. The Toby is largely restricted to the Purcell Mountains in the southernmost Cordillera (Fig. 3 ) but diamictites at the base of the Windermere have been documented in the high-grade metamorphic rocks of the northern Shuswap Complex and southern Cariboo Mountains and may be Toby correlatives (Simony et al., 1980; Murphy et al., 1991; McDonough and Morrison, 1990). The coarse diamictites of the Mount Vreeland Formation occur in the western part of the Rocky

G.M. Ross et al. /Precambrian Research 73 (1995) 71-99

Mountain belt in northeastern British Columbia and form spectacular cliffs reflecting the massive character of this unit which is up to 1200 m thick (Fig. 5A). It is composed of massive to crudely bedded diamictite with subordinate sandstone interbeds. Boulders within the diamictites are dominantly sedimentary in composition but also include diabase and granitic boulders, the latter derived from crystalline basement. Dropstones, in addition to clast composition, grain size and poor sorting, constitute the best evidence of a glaciogenic origin. Significantly, McMechan (1990) has shown that the Mount Vreeland diamictites do not occur at the base of the Windermere but are underlain by at least 400 m of pelite and siltstone. In addition, the Mount Vreeland diamictites can be mapped laterally to the southwest where they undergo a facies change into the coarse turbiditic grits of the middle part of the Windermere in the southern Cordillera. The Mount Vreeland is overlain by a thin cap carbonate which is a finely laminated dull gray limestone. This is in turn overlain by black shales that contain carbonate olistoliths that are very similar to the Teepee dolomite that occurs above the diamictites of the Ice Brook in the Mackenzie Mountains (Aitken, 1991 ).

3.2. Magmatic units Magmatic rocks (Fig. 6) generally occur near the base of the Windermere succession and include both extrusive and intrusive rocks of mafic composition and granitic rocks, the latter being particularly common in the southern and central Cordillera. Many of these units have been dated with modern isotopic techniques but there are still problems in addressing intrabasinal correlations. For example, in the northern Cordillera, a rhyolite flow in the upper part of the largely volcanic Mount Harper Group is dated at 751_+76 Ma (Roots and Parrish, 1988) but it is uncertain exactly how the dated unit correlates with Windermere strata to the east in the Mackenzie Mountains. In the Mackenzie Mountains themselves, three igneous bodies have been dated. A mafic sill (Tsezotene sill) that intrudes strata beneath the Windermere (Mackenzie

77

Mountains Supergroup) is 779 Ma ( U - P b baddeleyite; Heaman et al., 1992 ), as is a diorite plug that is in fault contact with the basal Windermere Coates Lake Group. A leucogranite cobble in the Sayunei Formation (lower Rapitan Group) has been dated at 755+ 18 Ma (G.M. Ross, unpubl. data), thus providing a maximum age for the Rapitan Group. If the correlations between Neoproterozoic strata of Victoria Island (Shaler Group) and the Mackenzie Mountains are correct (e.g. Jefferson and Young, 1989; Rainbird et al., 1992), then the base of the Windermere may be younger than 723 Ma, which is the age of basalts in the Natkusiak Formation at the top of the Shaler Group, thought to correlate with subWindermere basalts in the Mackenzie Mountains Group (Heaman et al., 1992; Rainbird et al., 1992). In the southern Cordillera, mafic rocks of the Irene Formation overlie and are interbedded with the Toby Formation. The Irene Formation is up to 1600 m thick and is composed of massive, pillowed, and fragmental volcanic flows, ruffs, tuff breccias and sills (Glover, 1978). There are no ages for the Irene Formation but the equivalent unit in northeastern Washington state (Huckleberry Volcanics) has been dated at 762 _+44 Ma by the S m - N d isochron method (Devlin et al., 1989 ). Granitic rocks that range in age from 728 to 740 Ma occur in structural culminations and slices in the western Main Ranges and Omineca Belt. They are generally leucocratic and alkaline in composition, have no known extrusive equivalents and are overlain unconformably by Windermere strata. Recent U - P b zircon dating of leucogneiss sills in the Malton Complex give a Neoproterozoic age of 736 Ma (M. McDonough, pers. commun., 1992). These gneisses are overlain unconformably by possible Toby equivalent conglomerates and diamictites, thereby implying that the Toby Formation must be younger than 736 Ma, consistent with ages for the Rapitan Group in the Mackenzie Mountains. Magmatic rocks also are common in mediumto high-grade metamorphic rocks of the Shuswap Complex where they comprise a substantial part of an informal map-unit known as the SPA (semipelite-amphibolite) within the Mica Creek

78

G.M. Ross et al. / Precambrian Research 73 (1995) 71- 99

.

?

Cp

•

i

Fig. 5. (A) Cliff face of thickly bedded diamictite and rare sandstones of the Mount Vreeland Formation• Height of cliff is ca. 250 m. (B) Amalgamated sheets of Ta arkosic grits with thin Tde interbeds, characteristic of the Kaza Group turbidite system. Person for scale at base. (C) Cliff face of Isaac Formation containing massive mudflows (MF) with olistoliths, thin-bedded allodapic carbonate ( L S ) and thin-bedded silty turbidites ( S T ) . People (circled) for scale• (D) Rhythmic ripple-laminated siltstones and green pelites of unit CST (Old Fort Point Formation equivalent) of the Kaza Group on an overturned fold limb south of Dore River, Cariboo Mountains. Stratigraphic units shown along the skyline correspond to those shown in Fig. 8 except for unit KU which is undivided upper Kaza Group. (E) Cobble with former aragonite crystals (now composed of low magnesium calcite) in lowstand wedge conglomerate that overlies the Old Fort Point equivalent• (F) Transition from upper slope of the Isaac Formation (I) to outer ramp (C~;) and platformal (CP) carbonates of the Cunningham Formations. This section is over 500 m thick.

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

79

Mount Harper Group ow Ma d Parrish,

1988)

;roup ~bble in Sayunei Fro. M a (Ross, u n p u b l ,

data)

Ike Group

~g ~a ( J e f f e r s o n

and

Parrish,

1989)

rs Range gneiss Ma

,Ick et al.

1984)

Group kllan leucogneiss -17 M a

hough and Parrish, 1991)

~t C o p e l a n d

syenite gneiss

r-36 M a sh a n d S c a m m e l h

1988)

~9° Huckleberry Volcanlcs 762+/-44 M a (Sm-Nd Isochron) (Devlln et al. 1989)

Fig. 6. Distribution and ages of dated magmatic rocks in the W i n d e r m e r e Supergroup o f the Canadian Cordillera. All ages are U - P b zircon or baddeleyite dates except for the S m - N d isochron for the Huckleberry Volcanics. Windermere strata of the Miette and Ingenika groups unconformably overlie 736 M a a n d 728 M a rocks, respectively.

succession (Simony et al., 1980; Sevigny, 1987, 1988 ). The SPA is interpreted to represent early Windermere (post-Toby) magmatic activity but has not been dated and the stratigraphic position of this unit is still the source of much debate (Struick, 1986; Pell and Simony, 1987; Dechesne and Kubli, 1988 ). Igneous material is also abundant in high-grade metasedimentary gneisses that mantle the basement culmination in the Monashee Complex (Scammell and Brown, 1990); unfortunately their age and stratigraphic position relative to low-grade Windermere units are unknown.

3.3. Turbiditesystems The stratigraphy of the Windermere Supergroup in the southern Cordillera is dominated by thick-bedded, coarse-grained arkosic sandstones and granule conglomerates referred to as grits. The recognition of widespread shale and carbonate markers within the grits (described below) provides a means of correlating these rocks between thrust sheets and, to a degree, at different metamorphic grades (Aitken, 1969; Ross and Murphy, 1988). It has also provided a key element of continuity within the grit basin such that it can be concluded that these sedimentary rocks

80

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

comprise part of a single depositional system of substantial dimensions. This conclusion is supported by detrital zircon geochronology which demonstrates a throughgoing provenance link between geographically and stratigraphically widespread components of the Windermere grits (Ross and Bowring, 1990; Ross and Parrish, 1991 ). The present outcrop area characterized by this provenance signature and depositional style is ca. 35,000 km2; estimates based on palinspastic considerations would make it nearly 50,000 km 2 (minimum), comparable in size to modern fans such as the Astoria, Magdalena or Monterey (Barnes and Normark, 1985) and making the Windermere succession one of the largest known ancient turbidite systems. Turbidite system deposits reach thicknesses of up to 6 km and include both grit-rich and mudrich intervals. In the Cariboo Mountains, the gritrich intervals comprise a stacked, acyclic succession of thick-bedded massive coarse sandstones, superficially similar to "sandy mid-fan" facies described by Hiscott (1980), with subordinate mudstone and thin-bedded "classic" Bouma turbidites (Fig. 5B). This is overlain by a mud-rich interval in which coarse grits and conglomerates occur as discrete sheets, up to 80 m thick (Fig. 5C). In the western Rockies the turbidite interval is dominated by discrete grit sheets within silty mudstones (Carey and Simony, 1985; Arnott and Hein, 1986; Root, 1987; McDonough, 1989; Kubli, 1990) which locally display cyclicity more typical of channel-overbank turbidite systems. Two scales of topographic control have contributed to the facies patterns observed in the grits and mudstones. Hummocky seafloor topography, produced by a combination of slump/ mass wasting processes that gullied the seafloor slope and aggradational processes, have served to localize deposition into lobate geometries, where mappable (Ross et al., 1989). Syndepositional normal faults, although difficult to recognize because of later contractional deformation, restricted coarse sand and conglomerate deposition into regional northwest-trending troughs (Root, 1988; McDonough, 1989; G.M. Ross, unpubl, data). The northwest trend inferred for

these intrabasinal lows mirrors the overall basinal geometry which, based on paleocurrent data, regional map patterns and provenance data all suggest being a longitudinal (southeast to northwest) dispersal system. Detrital zircon data indicate sediment derivation from a distinct source in the basement of southeastern British Columbia and southwestern Alberta, with virtually no input from crystalline rocks to the east of the basin (Ross and Bowring, 1990; Ross and Parrish, 1991 ). It is concluded that the Windermere grits comprise an elongate, longitudinal basinfloor dispersal system that flowed to the northwest and was flanked by mud-dominated slope facies to the east-northeast. The monotony of the sandstones and mudstones of the Windermere turbidite system is broken by a distinct and unique stratigraphic marker unit originally referred to as the Old Fort Point Formation (Charlesworth et al., 1967; Aitken 1969; Ross and Murphy, 1988). This thin interval ( 80-120 m) is characterized by varicoloured mudstones and rhythmic limestone-marl and mudstone-marl couplets that culminate in a carbonaceous, sulfidic interval 10-15 m thick (Fig. 5D). Ross and Murphy (1988) suggested that this unit records a major sea-level rise that effectively stopped the influx of sands and conglomerates characteristic of the Windermere grit system. It is thus analogous to condensed intervals formed during transgressions in modern settings (e.g. Loutit et al., 1988). The origin of a relatively large and apparently unique sea-level rise has been tributed to melting of continental ice sheets during the waning stages of the second Neoproterozoic (Varanger) glacial episode (Ross, 1991 b). Thus the Old Fort Point Formation in the southern Cordillera correlates with the Teepee dolomite of the Mackenzie Mountains and correlative cap carbonates that overlie the second Windermere (Ice Brook) glaciation (Fig. 4). The physical key to this correlation is the finegrained, lithographic nature of the Teepee dolomite (Aitken, 1991 ) and the local occurrence of crystal rosettes of calcite pseudomorphic after aragonite. These features also are present locally in carbonate clasts within the lowstand wedge that caps the Old Fort Point Formation in the

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

81

! 1 1(32

~

GRIT and PEBBLY SANDSTONE

m

PELITE and MUDSTONE

LIMESTONE and DOLOMITE

Fig. 7. Composite stratigraphic column for Neoproterozoic and lower Paleozoic rocks in the northern Cariboo Mountains. The stratigraphic interval investigated in this study is shown in detailed column to the right; gray bars show the distribution of pyrite samples analysed. OFP=OId Fort Point Formation; ICI and IC2=deep-water carbonate units in the Isaac Formation.

southern Cordillera (Fig. 5E). This correlation should be testable using isotope chemostratigraphy as the Teepee dolomite exhibits distinct Cand Sr-isotopic compositions (e.g. Narbonne et al., 1994). 3.4. Carbonate rocks

Two types of carbonate rocks are preserved in the Windermere Supergroup: ( 1 ) sheets of thinto thick-bedded sandy to silty limestones and conglomerates of inferred aUodapic origin (Ross, 1988; Ross et al., 1989), and (2) local platformal successions characterized by sedimentary structures typical of shallow-water deposition (Poulton, 1973; Campbell et al., 1973; Teitz and Mountjoy, 1989 ). The deep-water carbonates are composed invariably of limestone, although some

of the clasts may be dolomite. At least three intervals have been recognized and it appears from regional and reconnaissance studies that the upper two units (IC1 and IC2 in Fig. 7) can be traced throughout the southern Cordillera and they thus hold tremendous potential for isotope chemostratigraphic analysis. The deep-water carbonates form lenticular deposits that range from 10 to 250 m in thickness and are interbedded with mudstones and turbiditic sandstones. The lenticular morphology likely reflects the channelling effects of a gullied seafloor topography. The carbonate successions display a remarkable suite of lithologies that can include thin-bedded limestone-marl rhythmites, medium-bedded sandy black limestones and pebbly sands with Bouma Tb and Tc structures, and coarse conglomerates and breccias. Signifi-

82

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

cantly the conglomerates contain clasts of shallow-water derivation (stromatolites, oolitic, intraclastic and pisolitic packstones and grainstones); coated grains are common matrix components. These features suggest that the carbonates were derived from actively prograding carbonate platforms. In addition, the carbonates are associated with an influx ofterrigenous sandto pebble-sized siliciclastic sediment, implying that the progradation of the carbonate may be linked to a regional fall in sea level. At present, it is not possible to establish a depositional link between regionally extensive allodapic carbonate sequences and platformal sequences. This has led to the suggestion that the platforms that fed these allodapic complexes are not preserved, perhaps due to erosion along the sub-Cambrian unconformity (Ross, 1988). Carbonate rocks are associated with several platforms that occur in the upper Windermere Supergroup in the southern Canadian Cordillera including the Yellowhead platform (Teitz and Mountjoy, 1989), the "Upper Carbonate Division" (Poulton, 1973), and the Cunningham Formation (Campbell et al., 1973; Ferguson and Simony, 1991 ). It has not been possible to trace these platformal units basinward into the laterally extensive deep-water deposits. The Yellowhead platform is up to 400 m thick, dominated by dolostone and is rimmed by stromatolites that enclose a back-barrier cyclic peritidal facies containing vadose pisolites and coated grains, and a very limited forereef talus facies. In contrast, the Cunningham Formation in the Cariboo Mountains is composed predominantly of limestone and records the evolution from a ramp to rimmed platform over a stratigraphic thickness of more than 600 m (Fig. 5F). The "rim" is composed of cross-bedded ooliticintraclastic grainstones, encloses a mixed siliciclastic-carbonate lagoonal facies and has a substantial apron of allodapic debris.

of correlation in unfossiliferous sections (Christie-Blick et al., 1988 ). Three prominent eustatic signals are inferred within the Windermere of the southern Cordillera and it appears that each can be traced regionally: Old Fort Point Formation and the two allodapic carbonate intervals. The shale- and carbonate-rich condensed interval (Old Fort Point Formation) within the Windermere grits is regionally extensive throughout the southern Canadian Cordillera and is inferred to have resulted from a major sea-level rise which effectively shut off delivery of elastic sediment to the entire Windermere turbidite system. The interpretation that the sea-level rise was in response to melting of the second Windermere glaciation (Ross, 1991b; Aitken, 1991; Young, 1992) implies that this unit correlates with the second Neoproterozoic glacial event, a globally widespread lithostratigraphic interval commonly called the Varanger glaciation (Knoll, 1991 ). The recognition of this correlation is key because it ties the southern Canadian Cordillera succession into a global scheme, and allows the sulfur-isotopic data to be evaluated with respect to coeval biologic and tectonic events. The deep-water carbonates that occur above the grit marker in the Isaac Formation of the Cariboo Mountains are interpreted to represent lowstand wedges that record progradation of an active carbonate platform and elastic shoreline into the basin during sea-level fall. As such these units may represent regionally extensive units. Mapping by the senior author in the western Rocky Mountains confirms the presence of both these allodapic carbonates as regional markers. The challenge will come in determining how these relate to specific platforms, particularly those preserved in the Mackenzie Mountains; future studies of isotope chemostratigraphy hold tremendous potential in this regard. 5. Stratigraphy of the Cariboo Mountains

4. Event stratigraphy and eustatic signals Sequence boundaries and related, mappable surfaces are gaining wider application as a means

The Cariboo Mountains expose a structurally unbroken fold train of Windermere strata along a ca. 75 km cross-strike section. Rugged terrane, periglacial exposure and the relatively simple

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

structure make it an ideal region to examine details of post-rift Windermere stratigraphy. Although the region underwent metamorphic recrystallization in the Middle Jurassic and the Early Cretaceous (?), the metamorphic grade in the northern Cariboos is generally below the first appearance of biotite, but above the first appearance of chlorite porphyroblasts, and considered to be at lower greenschist grade. The Cariboo Mountains contain the thickest and most complete post-rift stratigraphic succession in the southern Canadian Cordillera and can be divided into four stratigraphic intervals (Kaza Group, Isaac, Cunningham and Yankee Belle formations; Fig. 7), the lower two of which are locally rich in pyrite (Campbell et al., 1973) (Fig. 7 ). The lowermost interval (Kaza Group ) records basinal sedimentation on an extensive submarine sandy turbidite system. It is overlain by the Isaac Formation, a succession of mudstones, coarse sandstones, conglomerates and limestones that were deposited on a slope setting. The Isaac Formation is overlain in turn by the Cunningham Formation, composed dominantly of oolitic-intraclastic limestone, and the Yankee Belle Formation, a mixed siliciclasticcarbonate assemblage. These two units were deposited in a shallow-marine, high-energy shelf setting. The Kaza Group is a thick (2-3 km) succession of coarse clastic sedimentary rocks that are dominated by sediment gravity flow deposits, e.g. massive to normally graded Bouma Ta beds interbedded with thin "classical" turbidites. Individual turbidite beds range in thickness from 10 to 900 cm, display little vertical cyclicity and likely were deposited by coalescing depositional lobes (Ross et al., 1989). Pebbly to sandy arkose predominates. Interbedded thin turbidites composed of thin sandy Bouma Tbc beds with green to dull gray pelite, comprise relatively mud-rich facies, up to 20 m thick. The overlying Isaac Formation is composed predominantly of millimetre to centimetre-scale laminated silty blue to gray mudstones with thin sandy turbidite beds and variable proportions of coarse clastic material. Coarse arkosic sediment occurs as composite sheets of turbiditic grit up

83

to 80 m thick. Limestones occur locally and range in thickness from 10 to 250 m. They are dominated by thin- to medium-bedded Bouma Tbc turbidites interstratified with massive calcarenites, conglomerates and breccias interpreted as deep-water allodapic carbonates (Ross, 1988; Ross et al., 1989). The Isaac Formation also contains massive mudflow breccias, olistostromes and intraformational slump deformation (Fig. 5C). These features are interpreted as evidence of deposition in a slope setting. A particularly well-exposed section within the Kaza Group that includes the Old Fort Point Formation was chosen for detailed sedimentologic and isotopic investigation because of the apparent extremes in sedimentation rates inferred from the sedimentology and the abundance of pyrite (Fig. 8). The lower part of the section (unit CGT, informal field term for "chloritic grit" ) consists of medium to thick beds of turbiditic sandy to granular arkose interbedded with greenish-coloured sandy pelite. In the middle of unit CGT (ca. 120 m) there is an abrupt decrease in the abundance of shale interbeds and the section is composed dominantly of amalgamated sand beds. Unit CST ("chloritic siltstone") abruptly overlies the unit CGT and marks the base of the Old Fort Point Formation (ca. 200 m). Unit CST is composed of cm-scale beds of ripple-laminated sandy limestone (Bouma Tc beds) overlain by green pelite. Average bed thickness within unit CST decreases upsection; it is capped by a 10-15 m thick unit of black pyritic pelite (unit BS; "black schist") with interbeds of pyritic siltstone and limestone (278 -290 m). Unit BS marks the top of the Old Fort Point Formation and is overlain abruptly by more limestone-pelite rhythmite (unit CGp; "calcareous grit, pelite facies"), a local pelite-rich facies that grades laterally into the sandy turbidite beds typical of unit CG ("calcareous grit"). In this section, the pelites are overlain abruptly by medium- to thick-bedded sandy calcareous turbidites (Bouma Tbc beds) and thin pelites (Bouma Tde). The lithologic changes in this section are interpreted in terms of variations in sedimentation rates driven in part by the effects of sea-level

84

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

LSVV

325

CG

j

CGp HST

4

BS

]

/

275

I

CST

TST

255 .....

+45m

....

] oxic interval ?1....

"~

i

2OO

i

150

% /

LSW

/

Z

/

CGT

100

t

50

lo 0

metres

Ji ~ ~ ~ E

~

-3'0-20-10

0 l0

1 ~ 3 4 S C DT

2'0

0/00

Fig. 8. Measured section through middle to upper Kaza Group transition, including the Old Fort Point (OFP) equivalent described by Ross and Murphy ( 1988 ). Letter designations to right of the column are informal map units. Sulfur isotope data from analysed pyrites are shown along curve on the right; each point represents a single analysis. L S W = l o w s t a n d wedge; TST= transgressive system tract; H S T = highstand systems tract. The 45 m break in the section is simply for ease of plotting--it consists of monotonous green silty pelites of unit CST. This part of unit CST is distinctly non-pyritic although all of the compositional features appear to be identical to pyrite-bearing parts of this unit. Hence the suggestion that this represents an oxic interval formed during sea-level rise.

fluctuations. The coarse sandy turbidites of unit CGT are interpreted as depositional lobe facies deposited as a lowstand wedge during sea-level fall (Ross et al., 1989). Abrupt termination of coarse sand deposition at the base of unit CST is interpreted as the onset of rising sea-level conditions and establishment of a transgressive sys-

tems tract. The black pelite interval (unit BS) is interpreted as a condensed interval formed during sea-level highstand and maximum flooding of the coeval shelf. The overlying unit CG (and CGp) are interpreted as the base of the lowstand wedge deposited during sea-level fall. Clasts of dolomite with crystal rosettes similar to those

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

that occur in the Teepee dolomite occur locally in this unit. By analogy with Phanerozoic exampies, the lowstand wedges of units CGT and CG are interpreted as recording the highest rates of sedimentation, whereas the transgressive systems tract (unit CST) and condensed interval (unit BS) record the lowest rates of sedimentation (Loutit et al., 1988). Pyrite occurs throughout the section with the exception of the basal part of the transgressive systems tract (basal part of unit CST; ca. 200 m). Pyrite reappears within unit CST (255 m), although there is no obvious change in lithology, suggesting that changes in dissolved oxygen levels, rather than Fe-limitation, are responsible for the absence of pyrite. If the sea-level rise recorded in the Old Fort Point Formation was in response to melting of continental ice sheets during the waning stages of the second Neoproterozoic glacial event, then the infusion of oxygenated water associated with this event may have led to brief oxygenation of bottom waters and development of unfavorable conditions for pyrite authigenesis.

6. Pyrite occurrences

The stratigraphic units examined in this study were the Kaza Group and the Isaac Formation (Fig. 7). Pyrite occurs almost ubiquitously throughout the entire thickness of both units, in amounts generally < 1%. It occurs in a variety of morphologies, many of which may not reflect primary sedimentary or early diagenetic processes. Coarse euhedral crystals (2-50 mm diameter), are the most common form, especially in sandy facies within both the Kaza Group and Isaac Formation (Figs. 9A, 9B). Coarse crystals may be mantled by intergrown fibres of quartz and ferroan carbonate that form pressure shadows (Fig. 9C). The presence of pressure shadow structures suggests that the formation of these crystals, probably by recrystallization of large nodular pyrite, predates regional deformation and coeval metamorphism. In contrast, pyrite within the shale-rich units locally occurs as fine stratiform aggregates parallel to bedding and ap-

85

parently has not undergone substantial recrystallization and grain size increase (Fig. 9D). The two different morphologies can occur in interbeds within a single outcrop. A key concern in this study is whether the original depositional or very early diagenetic isotope compositions of the pyrites are preserved, given the metamorphic history and grade of the rocks, as well as the unusual large crystal size. Numerous studies have addressed the metamorphism of sulfides and their interaction with silicates and oxides during prograde metamorphism of pelitic rocks (Froese, 1977; Ferry, 1981; Mohr and Newton 1983; Tracy and Robinson, 1988). Although calculated metamorphic fluid compositions indicate that under some conditions the proportion of H2S in the fluid phase can be high (XH2s >-40) (Tracy and Robinson, 1988), the general consensus is that there is no evidence of significant transport of sulfur species during metamorphism (e.g. Mohr and Newton, 1983). In addition, empirical evidence suggests that the original isotopic compositions of the sulfides, and coexisting sulfates, can persist to high metamorphic grades (Ohmoto and Rye, 1979; Tracy and Robinson, 1988; Whelan et al., 1990) suggesting that in sulfide-rich protoliths the isotopic composition of the fluid phase is buffered by the composition of the rock sulfides rather than the reverse. For these reasons we believe that the isotopic data that we report represent primary values unchanged by metamorphic processes. This is supported strongly by first-order correlations of pyrite isotopic compositions with sedimentary facies (Fig. 8). For the purposes of isotopic investigation, detailed measured sections were made through selected areas of both the Kaza Group and the Isaac Formation. Sections were measured and sampled in recently deglaciated periglacial alpine regions characterized by complete exposure and excellent preservation of fresh pyrite. The advantage of coupling the isotopic investigation to measured lithologic sections is that it provides constraints on the lithologic and/or facies control of pyrite isotope compositions, as exemplified by the section in Fig. 8. Although Fe-C-S systematics and similar whole-rock geochemical

86

-_-

G.M. Ross et al. / Precambrian Research 73 (1995) 71- 99

4

A

D Fig. 9. Examples of pyrite occurrences in the Cariboo Mountains. (A) Coarse pyrite crystals on a bedding plane in silty mudstone of the Isaac Formation. Black pencil in upper right for scale. (B) Cubic pyrite crystals within ripple-laminated sandstone and blue slates of the Isaac Formation. Matchstick, 4 cm, for scale. (C) Large pyrite crystal in sandy pelite of section A297 (Fig. 8 ) showing fibrous pressure shadows of quartz, suggesting a precleavage-premetamorphic origin of this morphology. Penny for scale. (D) Stratiform layer of finely crystalline pyrite within black slate of the Isaac Formation. Matchstick is 4 cm.

data are commonly used in studies of Phanerozoic strata and recent sediments (e.g. Anderson et al., 1987; Dean and Arthur, 1989; Calvert and Karlin, 1991; Lyons and Berner, 1992), no attempt was made to undertake similar analyses in these rocks largely because of the heterogeneous distribution and locally coarse grain size of pyrite. Very large samples would have to have been collected in order to derive representative chemical data. In addition, the metamorphic grade of the rocks suggests that present TOC values (total organic carbon content) and isotopic composition are not representative of the organic matter present during early pyrite authigenesis.

7. Analytical techniques Pyrite was prepared for isotopic analysis by hand-picking fresh grains under a binocular microscope from lightly crushed whole-rock samples. This procedure guaranteed high sample purity and avoided the effects of contamination by organic carbon or carbonate. Pyrite grains were then hand-crushed in an agate mortar to produce a virtually pure pyrite powder for isotopic analysis. Mortar and pestles were cleaned in dilute nitric acid to avoid contamination between samples. S02 was evolved from the pyrite powders after

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

the method of Ueda and Krouse (1986) and analyzed on a Micromass 602 mass spectrometer. All reported values are in standard delta (~) notation relative to Canon Diablo troilite (CDT). Replicate analyses of unknowns and laboratory standards are generally better than 0.2%o.

8. Results of isotopic analyses The results of the isotopic analyses are presented in Table 1 and Figs. 8, 10 and 11. Some 36 samples were collected through the detailed measured section in the Kaza Group and the isotopic data for pyrites are plotted adjacent to the section in Fig. 8. Throughout units CST, BS and CG, pyrite occurs as large cubic euhedra and less so as stratiform concentrations, generally within shaly beds. The large cubes occur irregularly through sandbeds. Isotopic analysis of closely spaced pyrites exhibiting these contrasting morphologies indicates that there is virtually no difference in the composition of the two forms. In addition, detailed analysis of a large single crystal of pyrite indicated less than a 1%o difference in the sulfur isotopic composition from rim to rim indicating that the large crystals are isotopically homogeneous. This result contrasts with studies that demonstrate differences in the isotopic composition of sulfides as a function of morphology and timing of precipitation (Raiswell, 1982; Strauss and Schieber, 1990) but is similar to the results of Maynard ( 1980 ) who reported that similarly sized nodules of pyrite in Devono-Mississippian shales display internal isotopic homogeneity. The highest observed ~345 values in this section are + 10%0 and the lowest values are - 31%o. The extreme depletions and range of values are characteristic of sulfide derived from bacterially mediated sulfate reduction (e.g. Chambers and Trudinger, 1979). Examination of the section also shows an important correlation between inferred sedimentation rates and isotopic composition of pyrite. The heaviest sulfide sulfur occurs in the interval of the section characterized by the highest rate of sedimentation, the sand-

87

Table 1 Results of isotopic analyses of pyrites from the K a z a Group and Isaac Formation S a mpl e

Unit

Lithology

~345

CST CST CST CST BS BS BS BS CGT CGT CGT CGT CGT CGT CGT CGT CGT CGT CGT CGT CGT CGT CGT CGT CGT CGT CGT CGT CGT CGT CGT CGT CGT CGp CGp CGp CG CG KU KU KU KU BS CG KU KU KU KU KU KU KU

silty mdst silty mdst silty mdst

mdst

- 18.1 - 30.9 - 31.9 - 31.6 -25.8 -25.3 - 27.7 - 27.0 - 9.7 - 9.9 - 8.4 - 3.4 - 3.3 - 3.1 -0.3 - 0.2 -0.8 -0.2 - 1.3 - 2.2 - 1.7 -0.7 - 0.0 2.2 1.5 9.2 14.5 11.9 12.7 12.3 13.1 8.3 4.1 - 25.4 - 22.2 - 20.0 - 5.2 - 3.7 - 2.0 6.8 - 0.2 - 14.2

silty mdst

- 19.1

grit grit mdst mdst mdst mdst mdst mdst

5.1 11.8 - 10.6 7.8 11.3 6.8 7.7 11.8

Kaza Group G 8 7 - P 1A-D $87-33 S87-34A-B $87-35 $87-29 $87-30 $87-31A $87-32 A297-A A297-B A297-C A297D A297-E A 297-F A297-J A 297-K A297-L A 297-M A 297-N A 297-O A297-P A297-Q A297-R A297-S A297-T A 297-U A297-W A297-X A297-Y A 297-Z A 297-A A A297-BB A 297-C C A297-FF A297-GG A297-HH A297-11 A297-JJ A236-A A236-B A236-D A236-E A288 A 294 A235-B A235-C A235-E A 235-F A235-G A 235-H A235-1

limestone carb m d s t limestone carb m d s t carb mds t grit

silty mdst silty mdst silty mdst grit grit grit

silty mdst sandstone

silty mdst sandstone sandstone sandstone sandstone

silty mdst silty m d s t

silty mdst grit sandstone sandstone sandstone grit

silty mdst silty mdst silty mdst mdst

silty mdst silty mdst cc s a n d s t o n e cc s a n d s t o n e

sandy mdst limestone

silty mdst

G.M. Ross et a/. / Precambrian Research 73 (1995) 71-99

88 Table 1 (continued)

Sample

Unit

Lilhology

53aS

Sample

Unit

Lithology

(~34S

A235-K G88-15 G88-17A G88-17B G88-17C G86-37 G86-41 G86-67 G86-73 G86-79 G86-86 G86-114 G86- t 16 G86-135 G87-31

KU KU KU KU KU CST BS KM CST BS BS CST KU KU KU

mdst grit grit grit grit sandy mdst mdst mdst sandy mdst mdst mdst mdst mdst mdst grit

2.8 2.9 - 6.2 -5.3 - 5.9 9.4 -22.7 2.3 - 29.8 -25.2 -23.5 - 29.2 19.1 5.4 - 28.8

IS IG IG IS IG IG IS IC 1 ICI IC2 IC2 IC2 IC2 IC2 IC2 IC2 IS IS 1S IS IS IS IS IS IG IG IS IS IC2 IC2 IC2 IC2 IC2 IC2 1C2

mdst grit grit mdsl grit grit mdst grit grit mdst cc mdst cc mdst cc mdst limestone limestone mdst mdsl mdst mdst mdst mdst mdst mdst mdst grit sst mdsl sst mdst mdst mdst cc sandstone silty mdst silty mdst silty mdst

A 176E A 176F A 176H A 194 A 194A A 195 A206A A207 A209 A210 A212A A212B A216A A216C A216D A216E A216F A216G A2 l 6H A2161 A216J A216L A216M A216N A2160 A225 A226 A227 A228A A235L A235M A235N A2350 A235P A235Q A235R A235S A235T A236F A236G A236H A2361 A236J A236K A236M A2360 A236P A236R A236S A243 A245A A245B

IC2 1C2 IC2 IC2 1C2 IC2 IC2 IC2 IC2 IS IS IS IC2 IC2 IC2 IC2 IC2 IC2 IC2 IC2 IC2 IC2 IC2 IC2 1C2 IG IS IS IS IG IS IS IS IS IS IS IS IS IS IS IS IS IS IS ICI IS IS IS IS IC2 IS IS

mudstone sandy mdst limestone limestone mudstone sandstone cc mdst limestone cc mdst mudstone mudstone limestone limestone limestone limestone limestone limestone limestone limestone cc mdst mudstone limestone limestone limestone m udstone mudstone mudstone limestone mudstone sandstone mudstone siltstone mudstone mudstone mudsione cc m u d s t o n e mudstone mudstone siltslone siltstone limestone mudstone siltstone cc siltstone cc mudsione limestone siltstone silty mdst mudstone limestone limestone mudstone

3.8 6.7 15.5 18.7 l 6.6 17.0 16.6 25.1 23.8 15.8 8.5 7.7 7.8 19.0 13.7 13.3 19.1 10.7 - 15.3 - 12.6 - 3.0 11.2 29. I 19.7 23.9 27.2 l 2.0 7.9 5.2 3.7 11.8 - 4.5 - 12.7 - 18.6 3. I 0.0 - 5.5 22.3 17.3 11.9 t 1. 5.0 -4.8 13.4 13.7 0.6 10. l 0.0 -5.4 6.7 14.6 9.5

Isaac Formation G86-120 G86-121 G86-126 G86-128 G86-129 G86-130 G87-16 G88-21A G88-21B G88-25 G88-26 G88-28 G88-29 G88-34A G88-34B G88-34C A117-A A117-B A117-C A117-D A117-E A117-F A126A A145 A 146A AI46B A157A A 157B A163 A165A AI65B A 176A A176B A 176C A 176D

8.6 19.2 11.2 2.8 35.4 10.6 7.2 - 5.8 - 5. I 14.9 18.3 6.2 10.6 23.5 23.8 20.1 13.5 4.8 12.4 -0.4 11.7 - 1.9 6.9 11.4 6.8 6.6 -4.3 - 5.2 0.9 0.4 0.1 - 3.9 - 1.0 3.3 4.2

89

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

Table 1 (continued)

9

Sample

Unit

Lithology

534S

A245C A247 A248A A248B A249 A256B A256C A262 A306 G87'46 $87-3 $87-4 $87-7 $87-18 $87-21A $87-22 $87-25A $87-28

IS IS IC2 IC2 IG IG IG IS IS IC1 IS IS IS IS IS IS IS IS

sandy mdst mudstone mudstone limestone mudstone grit limestone mudstone cc mudstone cc sandstone mudstone mudstone mudstone mudstone mudstone mudstone mudstone mudstone

9.2 2.6 2.0 5.7 1.3 7.0 7.1 6.0 - 3.9 - 13.4 - 7.3 -6.6 - 5.9 - 7.5 - 6.6 - 5.9 - 3.6 - 5.3 -

All results are reported in delta (5) notation relative to Canon Diablo standard. Formation abbreviations Kaza Group: CGT=chloritic grit; CST=chloritic siltstone; BS=black schist; Cgp= calcareous grit (pelite facies ); CG = calcareous grit; KU ---upper Kaza Group, undivided. Abbreviations Isaac Formation: ICI and IC2=lower and middle Isaac carbonates, respectively; 1S= Isaac slate; IG = Isaac grit. Lithologic descriptions: all lithologies are inferred premetamorphic compositions; cc =calcareous: carb = carbonaceous; mdst = mudstone. rich lowstand facies and in particular, the interval with the highest s a n d s t o n e - s h a l e ratio in unit C G T ( 1 10 to 160 m; Fig. 8). The most 34S-depleted values occur in the interval o f the section inferred to have the lowest rate o f sedimentation, the upper part o f the transgressive systems tract and the m a x i m u m flooding surface (upper unit CST and unit BS, respectively; 2 5 5 - 2 9 5 m ) . In addition, the 534S values b e c o m e more positive again with the r e s u m p t i o n o f turbidite sedim e n t a t i o n within the lowstand wedge o f unit C G (ca. 305 m ) . We take these observations to suggest that sedimentation rate, and hence diffusion-controlled sulfate availability, was a prim a r y control on the isotopic c o m p o s i t i o n o f the pyrite. Similar conclusions have been reported by previous workers for the N e o p r o t e r o z o i c N o n e such Shale (Schwarcz and Burnie, 1973), De-

8 )," L) Z LU

ku ~ I.u

7 6

4 3 2 1 0 -30

-20

-10

O

10

20

30

40

(~34S CDT 0/0 0

"7 L 6

z4-

~

,

,

,

,

q

,

, I

.......

/

i

t

,

E

,

i

i

i

I B_ _ L

r

', ',, ',, ',, :

,

,

V

i

m=-5.5

,,~21O

-30

-20

-10

0 ~34ScDT

10

20

30

,40

0/00

Fig. 10. (A) Histogram plot of sulfur isotope analyses for the Kaza Group. (B) Histogram plot of sulfur isotope analyses for the Isaac Formation. v o n o - M i s s i s s i p p i a n shales o f N o r t h America ( M a y n a r d , 1980) and for the Cretaceous o f Alberta (Bloch and Krouse, 1992). A compilation o f all the data for the Kaza G r o u p is shown in histogram form in Fig. 10; the stratigraphic interval represented by these data, which include the analyses in Fig. 8., is shown in Fig. 7. The data show a b r o a d range o f values from - 3 1 to + 18%o (a range o f 49°/o0) with a m e a n value o f - 5 . 5 % o . Sections measured and sampled through the Isaac F o r m a t i o n (e.g. sections A117, A216, A235 and A236; Table 1 ) did not show a simple correlation between isotopic c o m p o s i t i o n and facies. However, a histogram plot for the data collected for the Isaac F o r m a tion shows a similar pattern to that for the Kaza G r o u p with a broad range of values from - 18 to +35O/oo (a range o f 53%o), with a mean value o f +2.0%o. It is i m p o r t a n t to note that the Isaac F o r m a t i o n is characterized by higher values than

90

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

75

I

I

I

I

f

,~ = ~o41 .: "Ope,na syst, e m " ° r ~ ..-... 50

It3 O o o

6"

25

09 ~o

o

-25

0.0

0.2

0.4

0.6

0.8

1.0

Fraction of Reduced S042 Fig. 11. Rayleigh distillation model demonstrating the evolution of the isotopic composition residual seawater sulfate, cumulative H S - and instantaneous H S - as a function of the fraction of sulfate reduced. Model is calculated for a starting seawater sulfate composition of + 25%0 with kinetic isotope effect (fractionation factor) of 1.04. The fraction of sulfate that is reduced is controlled by the rate of bacterial reduction versus the rate of diffusion of sulfate into the reaction area. Note that when all the sulfate is reduced (e.g. "closed system" ), the composition of the cumulative H S - is nearly the same as the starling sulfate composition (dashed line ). If the sulfate reservoir is considered as a global budget, then approximately 32% of the global sulfate reservoir would have had to be reduced in order to drive the residual seawater sulfate values to +30%0 (boxed area). This value of 32% assumes a one for one molar exchange of oxidized and reduced species (i.e. a perfectly efficient sulfide retention system ) and a fixed volume for the global sulfate reservoir. In addition, if the kinetic isotope effect was higher (1.05-1.06, for example) then the percentage of the reservoir consumed would have been less (modified from Bloch and Krouse, 1992 ).

the Kaza and does not show the extreme 34S-depletion observed in the Kaza Group. However, the spread of values is remarkably similar, ca. 50000.

Sweeney and Kaplan, 1980; Hayes et al., 1992 ). These include: ( 1 ) the magnitude of the kinetic isotope effect, e.g. the ratio of isotopic rate constants, k32/k34, which is noted range from 1.00 to greater than 1.06 (Rees, 1973); ( 2 ) t h e isotopic composition of the sulfate reservoir; (3) the percentage of sulfate reduction which in turn relates to the rate of diffusion (or flux) of sulfate into the reaction area and diffusion of product H S - out of the reaction area, relative to the rate of sulfate reduction; (4) the rate of H S - oxidation relative to the rate of sulfate reduction; and possibly, (5) the effects of metamorphic recrystallization. In addition, because the timing of sulfide precipitation relative to isotopic evolution of the sulfide ( H S - ) can influence the measured pyrite values, controls such as organic matter reactivity and Fe availability are also important. Organic matter type and reactivity (e.g. Gautier, 1987) cannot be evaluated in this study due to the advanced degree of thermal alteration of the rocks. However, there is no reason to suspect that the type and reactivity of organic material was different between the sediments of the Kaza Group and Isaac Formation. In addition, Fe limitation is not considered to be a factor in these rocks which presently are "Fe-saturated" and contain chamosite and ankerite-siderite in addition to ubiquitous pyrite. We suggest that diffusion limited sulfate availability, resulting from differences in sedimentation rate and porosity occlusion, and the isotopic composition of seawater sulfate, are the main factors that have controlled the isotopic composition of sulfides in the Windermere Supergroup.

Rayleigh distillation model 9. Controls on pyrite isotopic composition The isotopic composition of pyrite may result from a variety of processes, some of which are difficult to evaluate in the present study and are still the topic of lively debate, even for Phanerozoic systems. Many authors have discussed factors influencing the S-isotope composition of authigenic sulfides formed during bacterial sulfate reduction (e.g. Chambers and Trudinger, 1979;

The range in sulfur isotopic compositions for both the Kaza Group and Isaac Formation sulfides are similar, ca. 50%o, although there is a consistent shift of 15°0 in the most enriched and depleted values in each unit as well as an 8%o shift in the median values (Fig. 10). The range of values within each unit may be explained by a Rayleigh distillation model in which the range of values is a reflection of relative rates of diffusion of reactants and products. At one extreme, a system characterized by slow diffusion rate (with re-

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

spect to sulfate) would approximate "closed system" behavior such that the rate of sulfate reduction far exceeded the rate of sulfate replenishment (Fig. 11 ). At the other extreme, a rapid rate of sulfate diffusion would correspond to an "open system" where a virtually unlimited supply of sulfate is available, relative to the rate of reduction. With respect to Fig. 11, the instantaneous H S produced by the bacterial reduction of sulfate will have an isotopic composition that will be depleted by an amount corresponding to the kinetic isotope effect. At this instant, the residual sulfate becomes enriched in 34S as 325 is preferentially fixed as a sulfide. If there is a rapid rate of diffusion of fresh sulfate into the reaction area (sediment or water column ), the sulfate isotopic composition will remain close to starting composition, i.e. seawater values, provided that the produced H S - is retained in the sediment. If the rate of diffusion is slow relative to the rate of sulfate reduction, the isotopic composition of the residual sulfate can become very enriched in 345. Under conditions of very sluggish diffusion and high H S - retention, the isotopic system approaches a "closed system" (Zaback et al., 1993 ). In this case, the isotopic composition of the sulfide that is produced from the complete reduction of all available sulfate will approach the starting composition of the sulfate, e.g. seawater values. We interpret the range of values in the histogram plots (Fig. 10) as reflecting the effects of diffusion-limited sulfate availability. Under conditions of unrestricted access of sulfate into the sediment and low H S - retention (Fig. 11 ), the isotopic composition of the sulfides will be the most depleted in 345, -- 31%0 and - 180/oofor the Kaza Group and Isaac Formation, respectively. Under conditions of restricted access of sulfate to the sediment, the isotopic composition of the sulfide will approximate the value of the starting composition of seawater sulfate, which is inferred to be the most 3aS-enriched pyrites that we measured for these two units, + 19%o and +35%o. This assumes an approximation of "closed system" behaviour where the kinetic isotope effect is minimal. The most 34S-depleted

91

values represent an open system end-member and imply a kinetic isotope effect of about 1.05. This value is above the generally accepted value ( 1.04; e.g. Hayes et al., 1992) for sedimentary systems in a normal ocean model. The apparent higher kinetic isotope effect may imply that some sulfate reduction occurred in the water column where diffusional effects are negligible, i.e. a euxinic water column. For the purpose of discussion, we make the following tentative interpretations: ( 1 ) The Kaza Group and Isaac Formation record a broad range of sulfide sulfur t~34S values which are typical of those produced by the bacterial reduction of sulfate in the sedimentary environment. Detailed study in a single measured section shows a direct correlation between pyrite ~345 isotopic composition and inferred sedimentation rates. This implies that sedimentation rates, and the consequences for product-reactant diffusion rates, exerted a primary control on the pyrite isotopic compositions. In addition, this correlation suggests that the measured isotopic values are original synsedimentary to early diagenetic values that have not been altered by metamorphism. (2) The range of compositions is similar for both the Kaza Group and Isaac Formation and suggests that conditions under which sulfate reduction occurred were similar for both units. The most 3as-depleted compositions are interpreted as representing the theoretical maximum depletion predicted for "open system" behavior with a high rate of diffusion of sulfate into the system relative to the rate of reduction. The most 3asenriched compositions are interpreted as approaching the isotopic composition of seawater sulfate. (3) The difference between the most negative and most positive ~34S values is ca. 50%0 in both units examined and implies a k32/k34 value of ca. 1.05. While this value is within recognized values for microbially mediated sulfate reduction, it is higher than the average values observed in Phanerozoic open marine sediments where the isotopic composition of seawater is better constrained. This suggests that euxinic conditions may have been persistent in the Windermere

92

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

basin and perhaps globally (e.g. Hayes et al., 1992; B e r r y et al., 1992).

SULFIDE

SULFATE

o

10. Implications for Neoproterozoic seawater sulfate

500

10.1. Constraints on seawater sulfate C~

Trends in the isotopic composition o f dissolved marine sulfate through geologic time have been d e t e r m i n e d by analyzing evaporite minerals precipitated from seawater (Claypool et al., 1980). The pre-Phanerozoic record is difficult to constrain because o f the poor preservation o f evaporites. The data points used to construct the N e o p r o t e r o z o i c part o f the sulfate curve are both sparse and poorly constrained in terms o f absolute age o f the samples (Figs. 12 and 13 ). However, analyses from Precambrian evaporites suggest that a m a j o r e n r i c h m e n t in 34S occurred s o m e t i m e during the late N e o p r o t e r o z o i c as recorded by a + 15%o shift to more positive isotopic values in the latest N e o p r o t e r o z o i c to early Cambrian, originally referred to as the Y U d o m ski event and recognized by the very positive sulfate 634S excursion recorded in C a m b r i a n evaporites o f the Irkutsk Basin o f Siberia (Holser, 1977; Strauss, 1993). T h e change in both m e d i a n and most 34S-enriched values o f sulfides from the Kaza G r o u p into the Isaac F o r m a t i o n is very similar in magnitude ( + 15) to the shift observed in the Neoproterozoic sulfate record (Fig. 12; Strauss, 1993). While it is not yet possible to precisely correlate the sulfate-bearing stratigraphic units with the sulfide-bearing units, the eustatic signal o f the second N e o p r o t e r o z o i c glacial event provides a starting point. Available correlations, shown schematically in Figs. 4 and 13, suggest that at the present limit o f resolution, the K a z a Isaac interval, and the associated shift in isotope compositions, overlaps with the 34S e n r i c h m e n t o f sulfate recorded in shelf strata. If this correlation is correct, it suggests that the Kaza G r o u p and Isaac F o r m a t i o n are an analogue for the depositional record o f the "missing" 32S required to balance the positive shift in the sulfate record

"-" I 0 0 0

t.iA ® <

19 1500

1

2000

i

,

-40-30-20

"10

10 20 30

8% (%0)

Fig. 12. Plot of sulfate and sulfide isotopic data for interval from 2.0 Ga (from Holser et al., 1988) with new data from this study. The sulfatecurve, shown with uncertainties, is from Claypool et al. (1980). The sulfide curve assumes a fractionation factor of 1.04 and the shaded region shows the uncertainty. The plotted histograms show the numbers of analyses versus isotopic composition for sulfides from sedimentary strata. 1= Onwatin Slate, Sudbury, Ontario; 2=black shales from Outukumpu, Finland; 3= MacArthur Basin, Australia; 4=Adirondack Mountains, New York; 5=Nonesuch Shale, Michigan; 8=Kupferschiefer, central Europe (see Holser et al., 1988 for data sources). Histograms 6 and 7are from the Kaza Group and Isaac Formation, respectively, of this study. Note the composition of the Kupferschiefer which exceeds the 40%ofractionation and is typical ofeuxinic basins, similar to the range of values observed in this study. Note also that the range of sulfide values in the Kaza Group and Isaac Formation fit well with sulfate values at this time and theoretical depleted sulfide values. (e.g. Donnelly et al., 1990; Hayes et al., 1992; Derry et al., 1992 ). Furthermore, we suggest that the record o f enhanced 32S burial in continental margin strata of the southern Cordillera may serve as a model for the mechanism that changed Neoproterozoic seawater sulphate, a theory originally proposed by Nielsen ( 1965 ).

10. 2. Seawater modification Holser (1977) presented a model to accomm o d a t e isotopic shifts in seawater sulfate in the

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99 I0 500

15

20

25

i

,,

30

35

-8-6-4

t

,

~

,

~

-2 0 ,

i

,(

,

2

4

6

i,

I,

i

8 ,

i

10 ,

C

~Varanger

Er

650

75( Rap0ta 80( 85(: Mo

5~'S (%oCDT)

~'3 C (% opDB)

Fig. 13. Modified Claypool curve including new data reported in Hayes et al. ( 1992 ) and Strauss ( 1993 ) ( 1 = Bitter Springs; 2=Redstone River; 3=Minto Inlet; 4=Bakoye; 5 = Yudomski) and new age constraints for the Rapitan Group (<755 Ma; G.M. Ross, unpublished data, but possibly younger than 728-723 Ma) and the Minto Inlet evaporites ( > 7 2 3 Ma; Heaman et al., 1992). Age constraints on the Varanger glaciation are quoted as 610-590 Ma (Knoll and Walter, 1992 ). The ~13C curve for marine carbonates is shown for comparison (from Derry et al., 1992). The stratigraphic interval examined in this study, based on the correlation with the Varanger glacial episode, is shaded

Phanerozoic and suggested that in order to generate the marked shifts to more positive ~345 values, substantial sulfide formation, via bacterial sulfate reduction, must have occurred just prior to these major shifts. In each case of 34S enrichment, Holser ( 1977 ) postulated that a restricted basin underwent extreme changes in the isotopic composition of trapped seawater sulfate as a consequence of burial of light sulfur in the form of pyrite. Subsequently, the modified seawater in these basins would "spill" into the open ocean, thus causing the global shifts inferred from the marine sulfate record. We propose an alternative to this model and suggest that the Neoproterozoic sulfate shift was driven by burial of 32S in lower continental margin sedimentary prisms, of which the southern Cordillera can be considered an example. Holser (1977) considered sulfide formation and burial in a "normal" marine environment as an alternative but concluded that for the Phanerozoic record the rises in 345 were too precipitous, implying a very rapid rate of isotope change, to have occurred under normal circumstances. With respect to the Neoproterozoic,

93

the age constraints on the YUdomski event are very poor compared to Phanerozoic events (e.g. Strauss, 1993). As shown in Figs. 12 and 13, the rise in 834S values is recorded in sedimentary rocks that span an interval of time of more than 160 million years. In the context of these limited chronologic constraints we present the following model realizing that improved time constraints could change our conclusions. In modern settings, the highest rates of sulfate reduction are observed in continental margin sedimentary assemblages, fueled in part by high primary productivity that results in abundant organic matter for anaerobic respiration (e.g. Canfield, 1991 ). Thus the area of the oceans underlain by continental margin assemblages would be an important constraint on the potential volume of sulfate reduction that could be accommodated. From this perspective, the original distribution of Neoproterozoic continental margin sediments, as exemplified by the southern Canadian Cordillera, is an important, albeit difficult, parameter to reconstruct, owing to the effects of younger tectonic activity. Slope and basinal facies preserved in the southern Canadian Cordillera, amount to a strike length of 600 km. However, coeval strata of generally shallowwater aspect are widespread in the Pacific Rim and include the entire coast of western North America, eastern Australia, Antarctica and the northwest Pacific. Assuming that passive margin strata were likely widespread after the breakup of western Laurentia at ~ 700 Ma, then these regions of shallow-water sedimentary rocks can be interpreted as the inboard portions of formerly more extensive continental margins that faced into the proto-Pacific Ocean. This results in a strike distribution of 8000 km, more than an order of magnitude greater than the presently preserved strike length of deep-water Windermere strata. This estimate does not consider the contribution of sulfate reduction and burial of light sulfur in Neoproterozoic circum-Atlantic basins or basins closed by younger Pan-African collisions. It is likely that continental margin sedimentary rocks of Neoproterozoic age were formealy more widespread but are not widely preserved.

94

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

With respect to a simple Rayleigh fractionation model (Fig. 11 ), if the "fraction of sulfate reduced" is considered as representing the global sulfate reservoir, then about 25-30% of the seawater sulfate reservoir has to be consumed in order to effect a + 15%0 shift in residual sulfate t~345 values, assuming a kinetic isotope effect of 1.04. A larger isotope effect ( 1.05-1.06 ), as might occur in a euxinic setting, would reduce the volume of the sulfate reservoir consumed. In either case, the 25-30% is a minimum value based on a one for one molar exchange and does not consider inefficiencies in the process such as reoxidation of H2S, incomplete sulfate reduction and rates of diffusion of 3aS-enriched sulfate back into seawater, etc. (e.g. Zaback et al., 1993). If, for example, only 10% of the reduced sulfate is sequestered as pyrite then a much larger volume of seawater sulfate has to be reduced. This simple calculation does not consider the flux of fresh sulfate into seawater through continental weathering, but is intended to provide a appreciation for the volume of the "standing crop" of sulfate that would have to be reduced to arrive at the observed isotopic values. A by-product of sulfate reduction is the oxidation of organic carbon according to the generalized reaction: 2 C H 2 0 + 5042- = H2 S + 2 H C O ;

According to this reaction, two moles of organic carbon are oxidized for each mole of sulfate that is reduced. Thus the large volumes of sulfate reduction required to produce the 34S enrichment observed in the evaporite record will have a substantial impact on organic matter burial during this time interval. Neoproterozoic marine carbonates typically show a strong enrichment in ~3C, which is interpreted as a consequence of burial of large quantities of organic carbon, most likely in an anoxic setting (Derry et al., 1992). According to the correlations shown in Fig. 13, the 34S shift overlaps with a pronounced excursion to relatively negative ~2C-enriched values coincident with the second glaciation. While it is tempting to ascribe the marked excursion to negative 6~3C values to oxidation of 12C-enriched organic matter during

sulfate reduction, this contrasts with the apparent long-term inverse behavior of the sulfur and carbon isotope records (Strauss, 1993) and the correlation of both of the negative 613C excursions with Neoproterozoic glacial episodes (Derry et al., 1992). It would appear then that the carbon isotopic record does not "see" the effects of organic matter oxidation implied by the sulfate record. In most models of Phanerozoic CS evolution, the burial flux between reduced and oxidized species is interpreted to be balanced in order to maintain constant atmosphericpo2 (e.g. Veizer et al., 1980; Berner and Raiswell, 1983). The apparent increase in the burial of both reduced sulfur and carbon during the Neoproterozoic ( ~ 700-550 Ma), as implied by the isotope records, appears to be in discord to these models. One explanation is that perhaps, unlike Phanerozoic models, atmospheric oxygen levels during the Neoproterozoic were not constant and increased in response to the coeval burial of large volumes of reduced carbon and sulfur. The net result of an increased burial flux of reduced species would be a marked increase in the oxidizing power of the environment and a net increase in atmospheric Po2- Significantly, the sulfate sulfur shift overlaps with, and outlasts, the first appearance of metazoans and is coeval with the major diversification of metazoans, considered by some in response to increased levels of atmospheric oxygen (Narbonne et al., 1994). Thus, burial of bacterially fractionated sulfur in the Neoproterozoic would have compounded the effect that organic matter burial had on the accumulation of atmospheric oxygen and support the comments of Holser et al. (1988) who, in reference to the activity and potential significance of bacterial sulfate reduction, concluded that "the effect of this variation in bacterial activity [implied by the variations in the sulfate sulfur record ] on the interconnected cycles of carbon and oxygen probably has been considerable".

11. Conclusions

The Neoproterozoic record of sedimentation preserved in the southern Canadian Cordillera

G.M. Ross et aL / Precambrian Research 73 (1995) 71-99

includes a substantial thickness of strata of deepwater affinity deposited on a continental margin that faced into the paleo-Pacific Ocean. The deep-water record is generally under-represented in the literature, particularly from a geochemical perspective, and thus these rocks provide valuable insight into the evolution of the deep-water geochemical reservoir in the Neoproterozoic. The sedimentary succession is interpreted to record two major tectonic phases: syn-rift sedimentation, dated at 762-728 Ma, and thermal subsidence on a passive margin. We suggest that both of the major Neoproterozoic glaciations are expressed in the Windermere, thus providing a lithostratigraphic baseline for regional and extrabasinal correlation. Post-rift Windermere strata were deposited on an extensive submarine turbidite system that flowed northwest, parallel to the base of the continental slope. Diminished rates of thermal subsidence during lithospheric cooling led to progradation of slope, and in some areas, platformal facies westwards into the basin, producing a "shoaling" sequence on the order of 6 km thick. Uplift associated with a younger rift event, which led to formation of the CambroOrdovician passive margin, eroded the shallowwater record from much of the southern Canadian Cordillera. Isotopic data from the analysis of authigenic pyrite that is common in sedimentary rocks of the Windermere continental slope indicate a large component of 32S-enrichment, largely unknown from Neoproterozoic rocks but a necessity to balance the isotopic composition of sulfate recorded from evaporites. The isotopic compositions span a broad range of ca. 50%o in both the Kaza Group and Isaac Formation, indicative of sulfide formation by bacterial sulfate reduction strongly influenced by diffusion-controlled sulfate availability and perhaps also indicative of a euxinic marine basin. The most 34S-enriched values of the pyrites are inferred to approach the isotopic composition of seawater, preserved as a consequence of the rate of reduction exceeding the rate of sulfate diffusion (i.e. effectively a "closed system" ). Available stratigraphic correlations suggest that

95

the passive margin strata examined in this study overlap with the major shift to positive 34S-enriched values in the Neoproterozoic evaporite record. The 32S-enriched reservoir in the southern Canadian Cordillera may be used as an analogue for a process that could have occurred on a global scale as a consequence of passive margin formation following the break-up of western Laurentia. We thus suggest that the sequestering of 32S as sulfide in Neoproterozoic continental margins led to the observed enrichment in 345 in seawater sulfate and our calculations suggest that perhaps 25-30% of the worlds oceanic sulfate would have been reduced in order to produce this shift. The by-product of this volume of sulfate reduction would have been oxidation of large amounts of 12C-enriched organic carbon. Perhaps it is not coincidental that this coupled process of sulfate reduction and organic matter oxidation, which impacts strongly on the global oxygen budget, overlaps with the first appearance, and especially the diversification, of metazoans, notably in deep-water sedimentary strata. It is important to temper these speculations with the reality that the stratigraphic, and hence isotopic, correlations, at this point are strictly lithostratigraphic in nature and that absolute age constraints are limited. The latter problem is a deficiency of paramount importance, making the modelling of isotopic shifts from a kinetic and reservoir flux perspective (e.g. Holser, 1977; Derry et al., 1992) very difficult. However, the results of this study indicate that the geochemical record from deep-water sediments is an uninvestigated component of the Neoproterozoic Earth that may have had an important influence on how we model environmental change in this time.

Acknowledgements Special thanks to those who assisted us with the laboratory work, particularly Jay Timmerman and Curtis Evans (mineral separates), Nenita Lozano and Jesusa Pontoy-Overend from the Stable Isotope Laboratory at the University of Calgary which receives support from the Nat-

96

G.M. Ross eta[. /Precambrian Research 73 (1995) 71-99

ural Sciences and Engineering Research Council of Canada. Critical reviews and on-going discussions with Roger Macqueen, Andy Knoll, Jay Kaufman and Guy Narbonne improved the manuscript. Geological Survey of Canada Contribution Nr. 11794.

References Aalto, R.K., 1971. Glacial marine sedimentation and stratigraphy of the Toby Conglomerate (Upper Proterozoic), southeastern British Columbia, northwestern Idaho and northeastern Washington. Can. J. Earth Sci., 8: 753-787. Aitken, J.D., 1969. Documentation of the sub-Cambrian unconformity, Rocky Mountain Main Ranges, Alberta. Can. J. Earth Sci., 6:193-200. Aitken, J.D., 1991. The Ice Brook Formation and post-Rapitan Late Proterozoic glaciation, Mackenzie Mountains, Northwest Territories. Geol. Surv. Can., Bull., 404, 43 pp. Anderson, T.F., Kruger, J. and Raiswell, R., 1987. C-S-Fe relationships and the isotopic composition of pyrite in the New Albany Shale of the Illinois Basin. Geochim. Cosmochim. Acta, 51: 2795-2805. Arnott, R.W. and Hein, F.J., 1986. Submarine canyon fills of the Hector Formation, Lake Louise, Alberta: Late Precambrian synsedimentary rift deposits of the proto-Pacitric miogeocline. Bull. Can. Pet. Geol., 34: 395-406. Asmerom, Y., Jacobsen, S.B., Knoll, A.H., Butterfield, N.J. and Swett, K. 1991. Sr isotope variations in Late Proterozoic seawater: implications for crustal evolution. Geochim. Cosmochim. Acta, 55: 2883-2894. Barnes, N.E. and Normark, W.R., 1985. Diagnostic parameters for comparing modern submarine fans and ancient turbidite systems. In: A.H. Bouma, W.R. Normark and N.E. Barnes (Editors), Submarine Fans and Related Turbidite Systems. Springer-Verlag, New York, pp. 13-15. Berner, R.A. and Raiswell, R., 1983. Burial of organic carbon and pyrite sulfur in sediments over Phanerozoic time: a new theory. Geochim. Cosmochim. Acta, 47: 855-862. Bloch, J. D. and Krouse, H.R., 1992. Sulfide diagenesis and sedimentation in the Albian Harmon Member, western Canada. J. Sediment. Petrol., 62: 235-249. Bond, G.C. and Kominz, M.A., 1984. Construction of tectonic subsidence curves for the early Paleozoic miogeocline, southern Canadian Rocky Mountains--implications for subsidence mechanisms, age of breakup and crustal thinning. Geol. Soc. Am. Bull., 95:155-173. Bond, G.C., Nickeson, P.A. and Kominz, M.A., 1984. Breakup of a supercontinent between 625 Ma and 555 Ma: new evidence and implications for continental histories. Earth Planet. Sci. Lett., 70: 325-345. Bowring, S.A., Grotzinger, J.P., Isachsen, C.E., Knoll, A.H., Pelechaty, S. and Kolosov, P., 1993. Calibrating rates of early Cambrian evolution. Science, 261:1293- 1298.

Brown, R.L., McDonough, M.R., Crowley, J.L., Johnson, B.J., Mountjoy, E.W. and Simony, P.S., 1993. Cordilleran transect, Rocky Mountain foreland and Omineca hinterland, southeastern British Columbia. Geological Association of Canada--Mineralogical Association of Canada, Joint Annual Meeting 1993, Field Trip B-9 Guidebook, 125 pp. Calvert, S.E. and Karlin, R.E., 1991. Relationships between sulphur, organic carbon, and iron in the modern sediments of the Black Sea. Geochim. Cosmochim. Acta, 55: 2483-2490. Campbell, R.B., Mountjoy, E.W. and Young, F.G., 1973. Geology of the McBride map-area, British Columbia: Geol. Surv. Can. Pap., 72-35, 105 pp. Canfield, D.E., 1991. Sulfate reduction in deep-sea sediments. Am. J. Sci., 291: 177-188. Carey, A.J. and Simony, P.S., 1985. Stratigraphy, sedimentology and structure of the Late Proterozoic Miette Group, Cushing Creek area, British Columbia. Bull. Can. Pet. Geol., 33: 184-303. Chambers, L.A. and Trudinger, P.A., 1979. Microbiological fractionation of stable sulfur isotopes: a review and critique. Geomicrobiol. J., 1: 249-293. Charlesworth, H.A.K., Weiner, J.L., Akehurst, A.J., Bielenstein, H.U., Evans, C.R., Griffiths, R.E., Remington, D.B., Stauffer, M.R. and Steiner, J., 1967. Precambrian geology of the Jasper region, Alberta. Res. Counc. Alta., Bull., 23, 74 pp. Christie-Blick, N., Grotzinger, J.P. and von der Borch, C.C., 1988. Sequence stratigraphy in Proterozoic successions. Geology, 16: 100-104. Claypool, G.E., Holser, W.T., Kaplan, I.R., Sakai, H. and Zack, 1., 1980. The age curves of sulfur and oxygen isotopes in marine sulfate and their mutual interpretation. Chem. Geol., 28: 199-260. Dalziel, I.D., 1991. Pacific margins of Laurentia and East Antarctica-Australia as a conjugate rift pair: evidence and implications for an Eocambrian supercontinent. Geology, 19: 598-601. Dean, W.E. and Arthur, M.A., 1989. Iron-sulfur-carbon relationships in organic-carbon-rich sequences, I. Cretaceous Western Interior seaway. Am. J. Sci., 289: 708-743. Dechesne, R. and Kubli, T., 1988. Revised Hadrynian correlations, southeastern British Columbia. Geol. Soc. Am., Abstr. Prog., 20: A402. Derry, L.A., Kaufman, A.J. and Jacobsen, S.B., 1992. Sedimentary cycling and environmental change in the Late Proterozoic: evidence from stable and radiogenic isotopes. Geochim. Cosmochim. Acta, 56:1317-1329. Devlin, W.J. and Bond, G.C., 1988. The initiation of the early Paleozoic Cordilleran miogeocline: evidence from the uppermost Proterozoic-Lower Cambrian Hamill Group of southeastern British Columbia. Can. J. Earth Sci., 25: 119. Devlin, W.J., Bruekner, H.K. and Bond, G.C., 1989. New isotopic data and a preliminary age for volcanics near the base of the Windermere Supergroup, northeastern Wash-

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

ington, U.S.A. Can. J. Earth Sci., 25:1906-1911. Donnelly, T.H., Shergold, J.H., Southgate, P.N. and Barnes, C.J., 1990. Events leading to global phosphogenesis around the Proterozoic/Cambrian boundary. In: A.J.G. Notholt and I. Jarvis (Editors), Phosphorite Research and Development. Geol. Soc. London, Spec. Pubh, 52: 273-287. Eisbacher, G.E., 1981. Sedimentary tectonics and glacial record in the Windermere Supergroup, Mackenzie Mountains, Northwest Territories. Geol. Surv. Can., Pap., 8027, 40 pp. Evenchick, C.A., Parrish, R.R. and Gabrielse, H., 1984. Precambrian gneiss and Late Proterozoic sedimentation in north-central British Columbia. Geology, 12: 233-237. Ferguson, C.A. and Simony, P.S., 1991. Preliminary report on structural evolution and stratigraphic correlations, northern Cariboo Mountains, British Columbia. Current Research, Part A, Geol. Surv. Can., Pap., 91-1A: 103-110. Ferry, J.M., 1981. Petrology ofgraphitic sulfide-rich schists from south-central Maine: an example of desulfidation during prograde regional metamorphism. Am. Mineral., 66: 908-930. Froese, E., 1977. Oxidation and sulphidation reactions. In: H.J. Greenwood (Editor), Application of Thermodynamics to Petrology and Ore Deposits. Mineral. Assoc. Can., Short Course, 2: 84-98. Gautier, D.L., 1987. Isotopic composition of pyrite: relationship to organic matter type and iron availability in some North American Cretaceous shales. Chem. Geol., 65: 293303. Glover, J.K., 1978. Geology of the Summit Creek Map-Area. Unpublished Ph.D. thesis, Queen's University, Kingston, 138 pp. Glover, J.K. and Price, R.A., 1976. Stratigraphy and structure of the Windermere Supergroup, southern Kootenay Arc, British Columbia. Current Research, Part B, Geol. Surv. Can., Pap., 7-1B: 21-23. Hambrey, M.J. and Harland, W.B. (Editors), 1981. Earth's Pre-Pleistocene Glacial Record. Cambridge University Press, Cambridge, 1004 pp. Hayes, J.M., Lambert, I.B. and Strauss, H., 1992. The sulfurisotopic record. In: J.W. Schopf and C. Klein (Editors), The Proterozoic Biosphere: A Multidisciplinary Study. Cambridge University Press, Cambridge, pp. 129-132. Heaman, L.M., LeCheminant, A.N. and Rainbird, R.H., 1992. Nature and timing of Franklin igneous events, Canada: implications for a Late Proterozoic mantle plume and the break-up of Laurentia. Earth Planet. Sci. Lett., 109:117131. Hein, F.J. and McMechan, M.E., 1994. Proterozoic-Lower Cambrian strata of the Western Canada Sedimentary Basin. In: G.D. Mossop and I. Shetsen (Editors). Geological Atlas of the western Canada Sedimentary Basin.Canadian Society of Petroleum Geologists, Calgary, pp. 57-68. Hiscott, R.N., 1980. Depositional framework of sandy midfan complexes of the Tourelle Formation, Ordovician, Quebec. Am. Assoc. Pet. Geol. Bull., 64: 1052-1077.

97

Hoffman, P.F., 1991. Did the breakout of Laurentia turn Gondwanaland inside-out? Science, 252: 1409-1412. Hofmann, H.J., Narbonne, G.M. and Aitken, J.D., 1990. Ediacaran remains from intertillite beds in northwestern Canada. Geology, 18:1199-1202. Holser, W.T., 1977. Catastrophic chemical events in the history of the ocean. Nature, 267: 403-408. Holser, W.T., Schidlowski, M., Mackenzie, F.T. and Maynard, J.B., 1988. Geochemical cycles of carbon and sulfur. In: C.B. Gregor, R.M. Garrels, F.T. Mackenzie and J.B. Maynard (Editors), Chemical Cycles in the Evolution of the Earth. Wiley, New York, pp. 105-174. Jefferson, C.W. and Parrish, R.R., 1989. Late Proterozoic stratigraphy, U-Pb zircon ages, and rift tectonics, Mackenzie Mountains, northwestern Canada. Can. J. Earth Sci., 26: 1784-1801. Jefferson, C.W. and Young, G.M., 1989. Late Proterozoic orange-weathering stromatolite biostrome, Mackenzie Mountains and western Arctic Canada. In: H.H.J. Geldsetzer, N.P. James and G.E. Tebbutt (Editors), Reefs, Canada and Adjacent Areas. Can. Soc. Pet. Geol., Mem., 13: 72-80. Knoll, A.H., 1991. End of the Proterozoic Eon. Sci. Am., 265: 64-73. Knoll, A.H. and Walter, M.R., 1992. Latest Proterozoic stratigraphy and Earth history. Nature, 356: 673-678. Kubli, T., 1990. The Geology of the Dogtooth Range, Northern Purcell Mountains, British Columbia. Unpubl. Ph.D. thesis, University of Calgary, Calgary, Alta. 324 pp. Kubli, T.E. and Simony, P.S., 1992. The Dogtooth High, northern Purcell Mountains, British Columbia. Bull. Can. Pet. Geol., 40:36-51. Leclair, A.D., 1982. Preliminary results on the stratigraphy, structure, and metamorphism of central Kootenay Arc rocks, southeastern British Columbia. Current Research, part A, Geol. Surv. Can., Pap., 82-1A: 45-49. Lickorish, W.H., 1992. Structure of the Porcupine Creek Anticlinorium and Tectonic Implications of the Lower Gog Group of the Rocky Mountain Main Ranges. Unpublished Ph.D. thesis, University of Calgary, Calgary, Alta., 160 pp. Lis, M.G. and Price, R.A., 1976. Large-scale block faulting during deposition of the Windermere Supergroup (Hadrynian) in southeastern British Columbia. Current Research, Part A, Geol. Surv. Can., Pap., 76-1A: 135-136. Loutit, T.S., Hardenbol, J., Vail, P.R. and Baum, G.R., 1988. Condensed sections: the key to age dating and correlation of continental margin sequences. In: C.K. Wilgus, B.S. Hastings, C.G.St.C. Kendall, H.W. Posamentier, C.A. Ross and J.C. Van Wagoner (Editors), Sea-Level Changes: An Integrated Approach. Soc. Econ. Paleontoh Mineral., Spec. Publ., 42:183-216. Lyons, T.W. and Berner, R.A., 1992. Carbon-sulfur-iron systematics of the uppermost deep-water sediments of the Black Sea. Chem. Geol., 99: 1-27. Maynard, J.B., 1980. Sulfur isotopes of iron sulfides in Devono-Mississippian shales of the Appalachian basin: con-

98

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

trol by rate of sedimentation. Am. J. Sci., 280: 772-786. McDonough, M.R., 1989. The structural geology and strain history of the northern Selwyn Range, Rocky Mountains, near Valemount, British Columbia. Unpublished Ph.D. thesis, University of Calgary, Calgary, Alta., 290 pp. McDonough, M.R. and Morrison, M.L., 1990. Ptarmigan Creek (83D/10) West-half, British Columbia, 1:50,000 scale. Geol. Surv. Can., Open File, 2305. McDonough, M.R. and Parrish, R.R., 1991. Proterozoic gneisses of the Malton Complex, near Valemount, British Columbia: U - P b ages and Nd isotopic signatures. Can. J. Earth Sci., 28: 1202-1216. McDonough, M.R. and Simony, P.S., 1988a. Structural evolution of basement gneisses and Hadrynian cover, Bulldog Creek area, Rocky Mountains, British Columbia. Can. J. Earth Sci., 25: 1687-1702. McDonough, M.R. and Simony, P.S., 1988b. Stratigraphy and structure of the Late Proterozoic Miette Group, northern Selwyn Range, Rocky Mountains, British Columbia. Current Research, Part D, Geol. Surv. Can., Pap., 88-1 D: 105113. McMechan, M.E., 1990. Upper Proterozoic to Middle Cambrian history of the Peace River Arch: evidence from the Rocky Mountains, Bull. Can. Pet. Geol., 38A: 36-44. McMechan, M.E. and Thompson, R.I., 1985. Geology of southeast Monkman Pass area (93I/SW), British Columbia. Geol. Surv. Can., Open File, 1150. Mohr, D.W. and Newton, R.C., 1983. Kyanite-staurolite metamorphism in sulfidic schists of the Anakeesta Formation, Great Smoky Mountains, North Carolina. Am. J. Sci., 283: 97-134. Monger, J.W.H., 1989. Overview of Cordilleran geology. In: B.D. Ricketts (Editor), The Western Canada Sedimentary Basin. Canadian Society of Petroleum Geologists, Calgary, Alta., pp. 9-32. Monger, J.W.H. and Price, R.A., 1979. Geodynamic evolution of the Canadian Cordillera--progress and problems. Can. J. Earth Sci., 16: 770-791. Moores, E.M., 1991. Southwest U.S.-East Antarctic (SWEAT) connection: a hypothesis. Geology, 19: 425428. Murphy, D.C., 1987. Suprastructure/infrastructure transition, east-central Cariboo Mountains, British Columbia: geometry, kinematics and tectonic implications. J. Struct. Geol., 9:13-29. Murphy, D.C., Walker, R.T. and Parrish, R.R., 1991. Gold Creek gneiss, southeastern Cariboo Mountains, British Columbia: age, geological setting, and regional implications. Can. J. Earth Sci., 28: 1217-1231. Narbonne, G.M. and Aitken, J.D., 1995. Neoproterozoic of the Mackenzie Mountains, northwestern Canada. In: A.H. Knoll and M.R. Walter (Editors), Neoproterozoic Stratigraphy and Earth History. Precambrian Res., 73: 101121 (this volume). Narbonne, G.M., Kaufman, A.J. and Knoll, A.H., 1994. Integrated chemostratigraphy and biostratigraphy of the upper Windermere Supergroup (Neoproterozoic),

Mackenzie Mountains, northwestern Canada. Geol. Soc. Am. Bull., 106: 1281-1292. Nielsen, H., 1965. Schwefelisotope im marinen Kreislaufund das ~34S der friiheren Meere. Geol. Rundsch., 55: 160172. Ohmoto, H. and Rye, R.O., 1979. Systematics of sulfur and carbon isotopes in hydrothermal ore deposits. In: H.L. Barnes (Editor), Geochemistry of Hydrothermal Ore Deposits, 2nd. ed. Wiley, New York, pp. 509-567. Pell, J. and Simony, P.S., 1987. New correlations of Hadrynian strata, south-central British Columbia. Can. J. Earth Sci., 24:302-313. Poulton, T.P., 1973. Upper Proterozoic "Limestone Unit", northern Dogtooth Mountains, British Columbia. Can. J. Earth Sci., 10: 292-305. Price, R.A., 1981. The Cordilleran foreland thrust and fold belt in the southern Canadian Rocky Mountains. In: K.R. McClay and N.J. Price (Editors), Thrust and Nappe Tectonics. Geol. Soc. London Spec. Publ., 9: 427-448. Rainbird, R.H., Darch, W., Jefferson, C.W., Lustwerk, R., Rees, M., Telmer, K. and Jones, T.A., 1992. Preliminary stratigraphy and sedimentology of the Glenelg Formation, lower Shaler Group and correlatives in the Amundsen Basin, Northwest Territories: relevance to sedimenthosted copper. In: Current Research, Part C, Geol. Surv. Can., Pap., 92-1C: 111-120. Raiswell, R., 1982. Pyrite texture, isotopic composition and the availability of iron. Am. J. Sci., 282: 1244-1263. Rees, C.E., 1973. A steady-state model for sulfur isotope fractionation in bacterial reduction processes. Geochim. Cosmochim. Acta, 37:1141-1162. Root, K.G., 1987. Geology of the Delphine Creek Area, Southeastern British Columbia: Implications for Paleozoic and Proterozoic of the Cordilleran Divergent Margin. Unpublished Ph.D. thesis, University of Calgary, Alta. Root, K.G., 1988. A Windermere Supergroup submarine feeder channel deposited in a halfograben, southeastern British Columbia. Geol. Soc. Am., Abstr. Prog., 20: A402. Roots, C.F. and Parrish, R.R., 1988. Age of the Mount Harper volcanic complex, southern Ogilvie Mountains, Yukon. In: Radiogenic Age and Isotopic Studies, Rep. 2. Geol. Surv. Can., Pap., 88-2: 29-36. Ross, G.M., 1988. The cryptic platform to the Windermere grit system. Geol. Soc. Am. Abstr. Prog., 20: A402. Ross, G.M., 1991a. Tectonic setting of the Windermere Supergroup revisited. Geology, 19:1125-1128. Ross, G.M., 1991b. Meltdown of the second Windermere glaciation: a working hypothesis. Geol. Assoc. Can. Abstr. Prog., 16: AI07. Ross, G.M. and Bowring, S.A., 1990. Detrital zircon geochronology of the Windermere Supergroup and the tectonic assembly of the southern Canadian Cordillera. J. Geol., 98: 879-893. Ross, G.M. and Murphy, D.C., 1988. Transgressive stratigraphy, anoxia and regional correlations in the Late Proterozoic Windermere grit system of the southern Canadian Cordillera: Geology, 16:139-143.

G.M. Ross et al. / Precambrian Research 73 (1995) 71-99

Ross, G.M. and Parrish, R.R., 1991. Detrital zircon geochronology of metasedimentary rocks in the southern Omineca Belt, Canadian Cordillera. Can. J. Earth Sci., 28: 1254-1270. Ross, G.M., McMechan, M. and Hein, F., 1989. Proterozoic history: birth of the miogeocline. In: B.D. Ricketts (Editor), The Western Canada Sedimentary Basin. Canadian Society of Petroleum Geologists, Calgary, Alta., pp. 79104. Ross, G.M., Parrish, R.R. and Winston, D., 1992. Provenance and U - P b geochronology of the Mesoproterozoic Belt Supergroup (northwestern United States): implications for age of deposition and pre-Panthalassa plate reconstructions. Earth Planet. Sci. Lett., 113: 57-76. Scammell, R.J. and Brown, R.L., 1990. Cover gneisses of the Monashee terrane: a record of synsedimentary rifting in the North American Cordillera. Can. J. Earth Sci., 27:712726. Schwarcz, H.P. and Burnie, S.W., 1973. Influence of sedimentary environments on sulfur isotope ratios in clastic rocks: a review. Miner. Deposita, 8: 264-277. Sevigny, J.H., 1987. Field and stratigraphic relations of amphibolites in the Late Proterozoic Horsethief Creek Group, northern Adams River area, British Columbia. In: Current Research, Part A, Geol. Surv. Can., Pap., 87-1A: 751756. Sevigny, J.H., 1988. Geochemistry of Late Proterozoic amphibolites and ultramafic rocks, southeastern Canadian Cordillera. Can. J. Earth Sci., 25:1323-1327. Simony, P.S., Ghent, E.D., Craw, D., Mitchell, W. and Robbins, D.B., 1980. Structural and metamorphic evolution of the northeast flank of the Shuswap Complex, southern Canoe River area, British Columbia. In: M.D. Crittenden, P.J. Coney and G.H. Davis (Editors), Cordilleran Metamorphic Core Complexes. Geol. Soc. Am. Mem., 153: 445-462. Stewart, J.H., 1972. Initial deposits of the Cordilleran geosyncline: evidence of Late Precambrian ( < 8 5 0 m.y.) continental separation. Geol. Soc. Am. Bull., 83: 13451360. Strauss, H., 1993. The sulfur isotopic record of Precambrian sulfates: new data and a critical evaluation of the existing record. Precambrian Res., 63: 225-246. Strauss, H. and Schieber, J., 1990. S sulfur isotope study of pyrite genesis: the Mid-Proterozoic Newland Formation,

99

Belt Supergroup, Montana. Geochim. Cosmochim. Acta, 54: 197-204. Struick, L.C., 1986. Imbricated terranes of the Cariboo gold belt with correlations and implications for tectonics in southeastern British Columbia. Can. J. Earth Sci., 23: 1047-1061. Sweeney, R.E. and Kaplan, I.R., 1980. Stable isotope composition of dissolved sulfate and hydrogen sulfide in the Black Sea. Mar. Chem., 9:145-152. Teitz, M.W. and Mountjoy, E.W., 1989. The Late Proterozoic Yellowhead carbonate platform west of Jasper, Alberta. In: H.H.J. Geldsetzer, N.P. James and G.E. Tebbutt (Editors), Reefs, Canada and adjacent areas. Can. Soc. Pet. Geol. Mem., 13:103-118. Tracy, R.J. and Robinson, P., 1988. Silicate-sulfide-oxidefluid reactions in granulite-grade pelitic rocks, central Massachusetts. Am. J. Sci., 288A: 45-74. Ueda, A. and Krouse, H.R., 1986. Direct conversion of sulfide and sulphate minerals to SO2 for isotope analyses. Geochem. J., 20:209-212. Veizer, J., Holser, W.T. and Wilgus, C.K., 1980. ~3C/~2C and 34S/32S secular variations. Geochim. Cosmoehim. Aeta, 44: 579-587. Veizer, J., Compston, W., Clauer, N. and Schidlowski, M., 1983.87Sr/86Sr in Late Proterozoic carbonates: evidence for a "mantle event" at 900 Ma ago. Geochim. Cosmochim. Acta, 47: 295-302. Warren, M. and Price, R.A., 1992. Tectonic significance of stratigraphic and structural contrasts between the Purcell Anticlinorium and the Kootenay Arc, east of Duncan Lake (82K). B.C. Geol. Surv. Branch, Geol. Fieldwork 1992, Pap. 1993-1: 9-16. Whelan, J.F., Rye, R.O., deLorraine, W. and Ohmoto, H., 1990. Isotope Geochemistry of a Mid-Proterozoic Evaporite Basin: Balmat, New York. Am. J. Sci., 290: 396-424. Young, G.M., 1976. Iron formation and glaciogenic rocks of the Rapitan Group, Mackenzie Mountains, Yukon and Northwest Territories, Canada. Preeambrian Res., 3:137158. Young, G.M., 1992. Late Proterozoic stratigraphy and the Canada-Australia connection. Geology, 20:215-218. Zaback, D.A., Pratt, L.M. and Hayes, J.M., 1993. Transport and reduction of sulfate and immobilization of sulfide in marine black shales. Geology, 21 : 141-144.