8. Upper Devonian to Middle Jurassic assemblages

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Southerly paleoflow (J.G. Abbott, pers. comm., 1984) may ... 8.17) occur locally (Abbott, 1982a). ...... part of the Maude Formation is locally rich in hydrocar- bons.
Contents

Chapter 8 UPPER DEVONIAN TO MIDDLE JURASSIC ASSEMBLAGES PART A. ANCESTRAL NORTH AMERICA S.P. Gordey, H.H.J. Geldsetzer, D. W. Morrow, E. W. Bamber, C.M. Henderson, B.C. Richards, A. McGugan, D. W. Gibson, and T.P. Poulton

Summary The pre-Late Devonian Cordilleran miogeocline consisted of extensive shallow-water platforms upon which carbonate-clastic deposits accumulated. They were flanked to the west by deep-water environments where shale and carbonate accumulated (Rocky Mountains Assemblage). Clastic sediments were largely craton-derived. During the Late Devonian sedimentation patterns changed dramatically as turbiditic, chert-rich clastics, derived from the west and north, flooded the northern Cordillera (Earn and Imperial assemblages). Shale (Besa River Assemblage) was deposited far out onto the miogeocline and Interior Platform; the carbonate front of the Rundle Assemblage retreated far to the east and south of its Middle Devonian position. By mid-Mississippian time the clastic influx waned and normal marine shelf carbonate and clastic sedimentation resumed, once again with clastics derived from the craton. Devono-Mississippian plutonism occurred only in northernmost Yukon Territory, and volcanism was restricted to central Yukon and south-central British Columbia. Pre-Late Mississippian folding occurred in northern Yukon but elsewhere deformation is expressed only by local high-angle faults and disconformities. Devono-Mississippian tectonism in the northern Yukon involved uplift and granitic intrusion in Frasnian to Early Mississippian time, resulting in a n upward shoaling and southward-prograding clastic wedge. The sequence consists of shale a t the base, flyschoid sediments near the middle, and partly fluvial-deltaic strata a t the top. Deformation migrated southward from the area of uplift until the clastics themselves were folded prior to the mid-Carboniferous. The source of Devono-Mississippian sediments in the central Cordillera was uppermost Precambrian quartzose clastics and lower Paleozoic chert from the western Gordey, S.P., Geldsetzer, H.H.J., Morrow, D.W., Bamber, E.W., Henderson, C.M., Richards, B.C., McGugan, A., Gibson, D.W., and Poulton, T.P. 1991: Part A. Ancestral North America; in Upper Devonian to Middle Jurassic assemblages, Chapter 8 of Geology of the Cordilleran Orogen in Canada, H. Gabrielse and C.J. Yorath (ed.); Geological Survey of Canada, Geology of Canada, no. 4, p. 219-327 (h Geological Society of America, The Geology of North America, v. (3-2)

miogeocline. Western coarse clastics are typified by chertquartz wacke and arenite and chert-pebble conglomerate; feldspar and volcanic detritus locally amount to a few per cent. All of the clastics were deposited in submarine fan environments. Numerous facies changes occur over short intervals. The lack of compressional structures, the presence of local volcanics of possible rift type, and the occurrence of syn-sedimentary faults that controlled thickness and facies, favour local block uplift of the outer miogeocline as a consequence of regional extension or strike-slip faulting. It is also possible, however, that the block uplifts and clastic wedges were related to tectonism farther west expressed by the Devono-Mississippian granitic plutons and volcanics in the Kootenay Terrane. The eastern fine clastic facies consists of shale and siltstone. Its southeastern limit, the carbonate-shale boundary, fluctuated widely, possibly reflecting tectonic activity to the west. In the southern Cordillera (south of GOON) the DevonoMississippian clastic record is minimal; the westernmost preserved miogeoclinal strata are shallow-water carbonate. After a n initial period of late Middle Devonian erosion, shallow water settings within a southward-spreading epicontinental sea led to the establishment of a broad carbonate platform (Rundle Assemblage). During the midFrasnian this became the foundation for extensive, northeasterly trending linear reefs that appear to have developed as a result of differential rates of deposition after partial suffocation of organic growth by anoxic terrigenous clastics. A westward prograding apron of shale gradually buried the reefs, but as this clastic supply waned and the seas regressed in the late Frasnian impure carbonates developed over a wide area. The brief late Frasnian regression led to local emergence and the influx of silt into briefly-established peritidal carbonate environments. At the beginning of Famennian transgression, westerly derived sand filled in topographic irregularities and was succeeded by the development of a broad carbonate platform which persisted until a latest Devonian influx of black euxinic shale. Carboniferous rocks occur in two belts. An eastern facies of marine platform and ramp carbonates and shallow marine to continental detrital clastics is succeeded to the west by a basinal shale facies. The eastern facies comprises several assemblages (Rundle, Lisburne and Mattson assemblages), each separated by regional unconformities marking Late Carboniferous and Permian episodes of

Contents CHAPTER 8

erosion. A sub-permian unconformity reflects marked tectonism along the ancestral Aklavik Arch. Carboniferous deposits of the eastern facies in the Rocky and southern Mackenzie mountains consist of Middle Tournaisian to Upper Visean miogeoclinal carbonate platform and ramp deposits (Rundle Assemblage), overlain by widespread, Upper Visean and Lower Namurian, supratidal, deltaic and shallow-marine siliciclastics (Mattson Assemblage). This succession is disconformably overlain by Bashkirian and Lower Moscovian carbonate and siliciclastics (lower Ishbel Assemblage) which are incompletely preserved beneath a regional sub-permian disconformity. In northern Yukon the eastern facies consists of uninterrupted Middle Visean to Moscovian carbonate (Lisburne Assemblage) and minor basal siliciclastics (Mattson Assemblage). Contemporaneous deltaic and shallow-marine deposits (Mattson Assemblage) are restricted to the eastern part of the belt, along the flanks of the Richardson and Barn mountains. The eastern carbonate-clastic facies overlies and passes westward into a succession of basinal deposits (CPo Assemblage) across a broad, transitional zone which temporally varied in position. Within the basinal succession, marked thinning occurs toward the west, because of starved basin conditions. In contrast to the Carboniferous, Permian strata (upper Ishbel Assemblage) consist mainly of shallow-marine to basinal siliciclastics and silty to sandy carbonate. Extensive carbonate buildups are lacking, and neither continental deposits, nor well-defined basinal shale facies have been recognized. A regional unconformity marks the base of the Permian succession and separates i t from strata ranging in age from Early Silurian to latest Carboniferous. In the Rocky and Mackenzie mountains a regional, intraPermian disconformity separates a thin, Upper Artinskian to Wordian assemblage of chert and siliciclastics from underlying Asselian to Lower Artinskian siliciclastics and carbonate. In northern Yukon and northwestern District of Mackenzie, a thick possibly uninterrupted Asselian to Wordian clastic succession (Jungle Creek Assemblage) is present on the flanks of the ancestral Aklavik Arch. No Permian deposits younger than Wordian are preserved b e n e a t h widespread sub-Triassic a n d younger disconformities. The Triassic rocks of the miogeocline comprise a westward thickening sequence of marine siliciclastics, of cratonal origin, and lesser carbonate (Spray River Assemblage). Many paralic and marine environments developed during several transgressive-regressive episodes and their deposits are preserved extensively in a belt extending from the southeast Yukon to the International Boundary (49"N). Similar strata in the central and northern Yukon are only locally preserved. The Triassic ended with regression and probable erosion prior to Jurassic marine transgression. The Jurassic record, dominated by shallow marine shelf sandstone and shale (also Spray River Assemblage), is one of progressive but intermittent flooding, a phenomenon noted on a global scale. Extensive marine transgressions preceded Sinemurian, Toarcian, late Early Bajocian, Bathonian, Callovian (approximately), and Late Oxfordian or Kimmeridgian deposition in both the northern Yukon and southwestern Alberta. In both regions sediments were shed from the craton. The first indication of westerly derived clastics related to the onset of Mesozoic orogeny are

of Late Jurassic age, and most younger Jurassic rocks (Kootenay Assemblage) of western Alberta and eastern British Columbia were shed eastward into the rapidly subsiding foredeep flanking the developing Rocky Mountain orogen. In the Yukon and District of Mackenzie there is no indication of western clastic input until the Early Cretaceous.

Introduction In the following discussion the Upper Devonian to Middle Jurassic assemblages of the Canadian Cordillera are treated with respect to their association with ancestral North America (Part A) and with each of the Cordilleran terranes (Part B). Like the other assemblage chapters the rock stratigraphic sequences are identified according to their assignment to the tectonic assemblages (see the Tectonic Assemblage Map, Map 1712A, in pocket). In Part A Upper Devonian carbonate and Upper Devonian to Mississippian clastic assemblages are discussed separately insofar as each represents fundamentally different tectonic settings. The former reflects continued, dominantly platform and reef carbonate settings which were established on the miogeocline during the Middle Cambrian. With significant interruption in the northern Cordillera during the Late Devonian and Mississippian, these environments continued to the end of the Paleozoic e r a , following which Triassic to Middle J u r a s s i c miogeoclinal sedimentation became increasingly clastic. In the northern Cordillera i m ~ o r t a n twestern sources supplied thick sequences of terrigenous clastics to the miogeocline in Late Devonian and Early Mississippian time. The terranes of the western Cordillera contain assemblages which are dominantly oceanic to island-arc in origin. It was during this interval when most of the assemblages for many of the terranes accumulated and thus the definition and characterization of the terranes is a n important part of this chapter. Stratigraphic evidence for the time and type of terrane linkages is described as are alternative views when such evidence is equivocal.

Upper Devonian carbonate strata of the Foreland Belt (Rundle Assemblage) H.H.J. Geldsetzer and D. W. Morrow Upper Devonian strata of the southern Foreland Belt (Fig. 8.1) and in the subsurface beneath the adjacent western Interior Plains comprise carbonate and mixed carbonate and siliciclastic sediments. Along the eastern margin of the belt the thickness of mid-Givetian to Famennian rocks varies between 250 and 1000 m (Fig. 8.2). Several important petroleum reservoirs occur within Upper Devonian reefs of the Interior Platform; moreover, Upper Devonian carbonates are among the dominant contributors to the magnificent scenery of the Canadian Rockies. Like the discussion of Lower and Middle Devonian strata (see Table 7.1) the Upper Devonian succession of the southern Foreland Belt is divided into distinct sequences separated by thin intervals or discontinuities across which stratigraphic character changes markedly (Fig. 8.3). In ascending order these are the Fairholme sequence of late Givetian to mid-Frasnian age, the Ronde-Kakisa sequence

Contents UPPER DEVONIAN TO MIDDLE JURASSIC ASSEMBLAGES

GSC

Contour Interval In metres

LEGEN0 0-250 750-1000

Figure 8.1. Distribution of Upper Devonian to Middle Jurassic sedimentary and volcanic rocks of the Canadian Cordillera.

of late Frasnian age and the Famennian Palliser sequence. The Fairholme sequence is separated from the Eifelianlower Givetian Hume-Dunedin sequence by the Watt Mountain hiatus and represents the beginning of the classical "Kaskaskia Sequence" of Sloss (1963). The overlying Ronde-Kakisa sequence, bounded i n t h e south by unconformities, represents a brief transgressive pulse within a regressive interval and was followed by deposition of the Palliser sequence, the final transgressive carbonate phase of the Devonian period. The Late Devonian cratonic platform and margin were segmented into several tectonic elements (Fig. 8.4). During most of Late Devonian time the Peace River Arch was emergent and the record of previous Paleozoic deposition there was largely removed. The Watt Mountain Formation, comprising between 20 and 75 m of nonmarine red and green shale, sandstone, limestone breccia, limestone and dolomite is the product of this erosion that formed a clastic apron, 240 km wide, about the

2SO-500

1IIIIIIII[11000-1250

500-760

1250-1500

Figure 8.2. lsopach map of mid-Givetian to Famennian of the Devonian System. Thickness variations are more irregular than for Lower and Middle Devonian strata (see Chapter 7).

arch during late Givetian time. Extending southward from the Peace River Arch the West Alberta Ridge remained positive until a widespread marine transgression began in the late Givetian (Taghanic Onlap). On the crest of the ridge up to 36 m of middle or upper Givetian thinly bedded and laminated dolomite with collapse breccias of the Yahatinda Formation (Fig. 8.3) fill topographic depressions on the erosion surface underlain by Cambrian and Ordovician carbonate (Aitken, 1966). During the Late Devonian the Golden Embayment on the west side of the West Alberta Ridge ceased to exist and, south of the Peace River Arch, the cratonic platform extended westward to the limits of preservation (Fig. 8.4). During the Late Devonian a n influx of external clastics from northern and western sources (see next section) limited the extent of carbonate sedimentation to areas largely south of 60°N. The only important carbonate deposition in the northern

Contents CHAPTER 8

Western Hay River ~ l a torm f

Conodont Zones

I

Overlying unit

I

Cretaceous

Cretaceous

I

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Northwest Alberta Platform l ~ e s aRiver

West-central Alberta Platform I

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Peechee Cairn Borsato

Waterways

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Cambrian

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Figure 8.3. Correlation chart of upper Middle and Upper Devonian formations with standard conodont zones. These are grouped into three sequences: the Fairholme, Ronde-Kakisa, and Palliser. Locations of regions A and B are indicated in Figure 8.4.

cratonic platform after the Watt Mountain hiatus was a brief late Givetian transgression which produced reef buildups such a s the Ramparts Formation (more than 200 m thick) on the Peel Platform. Time-equivalent carbonate units to the south are the Slave Point Formation (150 m thick) along the western edge of the Hay River Platform and the early stage of the Swan Hills reefs (44 m thick) south of the Peace River Arch (Fig. 8.5).

Fairholme sequence (Upper Givetian to mid-Frasnian) The Fairholme sequence of the Rocky Mountains south of Peace River Arch (Fig. 8.3, columns 4-6) is represented by the Fairholme Group of late Givetian and Frasnian age. The group includes the western exposed part of an extensive reef domain which developed on the southern Alberta Platform (Fig. 8.5,8.6) and embraces carbonate and clastic facies which are laterally intergradational (Fig. 8.3, 8.7; Plate 16). The carbonate components can be grouped into a basal platform succession which locally extends upsection into biostromal buildups or platform reefs, and into carbonate

ramps which developed above the buildups and project laterally above and into clastic facies. Minor components are clusters of bioherms or reef mounds along the edge of the carbonate ramps and on the foreslope of reef margins. The lower platform succession, represented by the Flume Formation, comprises dark grey dolostone, commonly characterized by stromatoporoids, corals and chert nodules. I t is about 30 m thick on the West Alberta Ridge and thickens to about 130 m on the western slope of the ridge and to about 250 m in the southernmost Rockies where equivalent rocks are assigned to the Hollebeke Formation (Price, 1965). The basal part of the thicker sections has a yellowish or brownish grey-weathering clastic component of quartzose sandstone and silty or argillaceous carbonate a s much a s 36 m thick in the central Rockies and 57 m thick in the southern Rockies. Along the Southern Rocky Mountain Trench up to 60 m of grey fossiliferous dolomite and quartzose sandstone of the Starbird Formation conformably overlie MiddIe Devonian mixed carbonate and clastics of the Harronate and Mount Forster formations. The Starbird is equivalent to the Flume-Hollebeke platform succession (Norford, 19811;

Contents UPPER DEVONIAN TO MIDDLE JURASSIC ASSEMBLAGES

its terrigenous components may have been derived from western sources (Reesor, 1973). The biostromal buildups or platform reefs rest directly on the carbonate platform succession. Thicknesses increase generally from 200 m in the Front Ranges westward to 365 m. The lower part, assigned to the Cairn Formation or Cairn and Borsato formations, consists largely of dark grey dolomitic stromatoporoid biostromes (Fig. 8.7) and may include the underlying Flume Formation where the Flume and Cairn cannot be differentiated (Plate 16). The Cairn grades upward into light grey carbonate of the Southesk Formation which, in turn, includes the upper

part of the reef (Peechee Member), post-reef carbonate ramps (Grotto and Arcs members) and post-Fairholme sequence carbonate (Ronde Member, uppermost member of the Fairholme Group). The upper light grey part of the reefoid Peechee Member of the Southesk Formation is

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Figure 8.4. Late Devonian tectono-sedimentological elements of the eastern Cordillera. The western margin of the Hay River and Alberta platforms are outlined by the western edge of carbonate units and westward thinning of clastic units. Locations A and B designate areas for columns in Figure 8.3.

x'

2

':

*

( c e r t a i n , approximate, assumed1 Eroded Edge

Figure 8.5. Distribution of facies during Late Givetian and Early Frasnian time (Correlations among named units not necessarily implied). The West Alberta Ridge was inundated and reefs such as the Swan Hills Reefs began to develop in south-central Alberta.

Contents CHAPTER 8

Figure 8.6. Distribution of facies during Middle Frasnian time (Correlations among named units not necessarily implied). This was a time of maximum reef development in the southern Rocky Mountains and Interior Plains. Clastics shed from northerly (Imperial) and westerly (Earn) sources were dominant in the north. Legend accompanies Figure 8.5.

dominated by laminated limestone grading laterally into stromatoporoid biostromes a t the reef margin. Commonly, along the foreslope of reef margins, carbonate tongues of the lower reef (Cairn Formation) project outward across the underlying platform succession (Fig. 8.7) and change facies to dark grey laminated lime-mudstone of the Maligne Formation which, in turn, becomes argillaceous and nodular basinward; the Maligne Formation reaches up to 40 m in thickness near reef complexes (Fig. 8.7). Carbonate ramps developed above most reefs and are characterized by light grey, commonly thick-bedded limestone consisting of grainstone and minor mudstone, to packstone assigned to the Arcs Member. These ramps extend outward across clastic-filled basins and grade laterally into nodular fossiliferous limestone of the upper Mount Hawk Formation which is equivalent to the Nisku Formation in the subsurface to the east. The lower part of

this ramp facies occurs only locally and consists of dark grey, coral-bearing, slightly argillaceous carbonate of the Grotto Member. Clusters of bioherms or reef mounds are found along the foreslope of reef margins and appear to be dominated by various algae (Mountjoy and Riding, 1981). Other bioherms occur along the edge of carbonate ramps (Fig. 8.8). Their dominant lithology is lime-mudstone with prominent stromatolitic structures. These buildups are probably equivalent to similar Nisku bioherms in the subsurface (Krause, 1984). The clastic facies occurs as basal, relatively coarse clastics a t the erosion surface below the Fairholme sequence, as a fine argillaceous component in the initial anoxic sediments (Perdrix Formation) above the carbonate platform succession and between the reefs, and as a thick silty and argillaceous basin fill (lower Mount Hawk Formation) which aggraded the reef topography (Fig. 8.3,8.7). The basal clastics of the lower part of the carbonate platform (Flume, Hollebeke) were discussed above. The anoxic argillaceous lime-mudstone or calcareous shale of the Perdrix Formation are distinguished by their black colour, high organic content and distinct radioactive signature. Toward the reef complexes the Perdrix grades into the Maligne Formation and in the subsurface to the east the unit correlates with the Duvernay Formation (Fig. 8.3, 8.7). Thickness varies from zero a t reef margins to more than 100 m in distant basin areas. The Perdrix is temporally equivalent to the Cairn and Peechee reefs. The basin fill of the lower Mount Hawk Formation consists of very thin-bedded to laminated, calcareous silty shale varying in colour from dark grey to greenish grey, suggesting mostly oxygenated conditions. The unit overlies the Perdrix, Maligne or extensions of the Cairn formations. In distant basinal settings the Perdrix and lower Mount Hawk have similar lithologies and are difficult to separate whereas in near-reef complexes the lower Mount Hawk is characterized by abundant limestone bands. Thickness of the basin fill varies from zero a t reef margins to over 300 m in basinal areas. Along the margins of reef complexes reefderived debris beds serve to separate the Perdrix and Mount Hawk formations (Fig. 8.7) and demonstrate that probably all of the lower Mount Hawk sediments represent a stage of post-reef infilling (Mountjoy and Mackenzie, 1973). The upper Givetian-lower Frasnian carbonate facies of the Fairholme sequence resulted from clear, warm and shallow-water conditions within a n epicontinental sea which advanced southward across the cratonic platform (Taghanic Onlap). The cyclic nature of reef buildups attests to repeated fluctuations in sea level. During the early stages of the transgression terrigenous material, presumably derived mostly from local sources, was reworked by the advancing sea and incorporated in the lower part of the carbonate platform. In the southeastern Foreland Belt the relative proportion of this material decreases to where the lower Hollebeke Formation is only moderately argillaceous. However, westward from there, grey, stromatoporoidal platform carbonates are interbedded with quartzite (Starbird). During the early Frasnian platform carbonates broadly coalesced and spread eastward (Flume, upper Hollebeke). The platform lithologies grade upsection

Contents UPPER DEVONIAN TO MIDDLE JURASSIC ASSEMBLAGES

LEGEND STROMATOPOROIDS

1 LAMlNlTE

AMPHIPORIDS

m STROMATACTIS 0 CH TIC DEBRIS FL& @ CHERT OR SlLlClFlCATlON

BRACHIOPOOS CORALS

DEPOSlllONAL ENVIRONMENTS

1

I SHALLOW SUBTIDAL

GASTROPODS ECHINODERMS BIOHERMS

O FENESTRM FABRIC

@ SOLUTION CAVITIES bx SOLUTION BRECCIA

Figure 8.7. A stratigraphic cross-section of the reef to off-reef relationship of the southwest margin of the Upper Devonian Ancient Wall Reef Complex. Inferred depositional environments are shown in the inset diagram. Location of Ancient Wall Reef Complex is shown in Figure 8.6.

from brachiopod-bearing mudstone and wackestones a t the base to stromatoporoid-coral biostromes above. During mid-Frasnian time the broad carbonate platform became the foundation for extensive, northeasterly trending linear reef tracts (Fig. 8.6) which are commonly attributed to a rapid rise in sea level (Cook, 1972; Mountjoy, 1980; Moore, i n press). Because the growth r a t e of bioconstructed, stromatoporoid-coral biostromes probably far exceeded the rate of epeirogenesis or long-term eustatic sea level change (Schlager, 1981), it is suggested that the sudden influx of a dysaerobic water mass with a minor very fine terrigenous component (Maligne and Perdrix formations) terminated organic growth, particularly stromatoporoids, in areas of comparatively low relief. Elevated areas were bypassed, thus allowing for continued upward reef growth (Geldsetzer, 1984). The much lower r a t e of accumulation of black, anoxic argillaceous mudstones a s compared to bioconstructed carbonates led to significant topographic relief, which, along the Ancient Wall reef complex northwest of Jasper (Fig. 8.6, 8.71, exceeded 150 m. Andrichuck (1961) suggested that the location and orientation of the northeasterly trending linear reef tracts in south-central Alberta was due to a northeasterly trending

hinge zone related to faults in the Precambrian basement. These trends are closely parallel with northeasterly trending magnetic anomalies beneath the Interior Platform (see Fig. 2.30). Reef growth continued along the western reef domain (Rocky Mountains) contemporaneously with burial of those of the eastern domain (Leduc reefs beneath the Interior Plains) by a westward prograding apron of green-grey shale (Ireton Formation). An interruption of sea level rise resulted in the exposure of the northwestern reef domain (Ancient Wall), local brecciation of the upper reef and probably numerous reef-derived debris flows. These foreslope breccias interfingered with the lowest beds of the westward-prograding basin fill which had advanced into the western reef domain (lower Mount Hawk-upper Ireton) and finally infilled the western reef topography (Fig. 8.7). In response to a gradual reduction of terrigenous supply the shale of the lower Mount Hawk changed upward into fossiliferous nodular wackestone of the upper Mount Hawk. Near and above most reef complexes these rocks grade laterally and upward into deeper water wackestone of the Grotto Member and finally into shallow-water grainstone of the Arcs Member of the Southesk Formation.

Contents CHAPTER 8

Ronde-Kakisa sequence (uppermost Frasnian) In the central and southern Rockies the Ronde-Kakisa sequence is represented by the Ronde Member of the Southesk Formation and the Simla Formation (Fig. 8.3, 8.9). The Ronde is a variable unit, up to 55 m thick, and characterized by peritidal silty dolostone and interbedded calcareous siltstone; desiccation cracks and teepee structures are present. The Simla, up to 70 m thick, is a fossiliferous grainstone that grades upward into massive, resistant, light grey stromatoporoid limestone. Both units commonly are underlain by red, green and grey siltstone of the Calmar Formation (up to 20 m thick). In northern regions (Fig. 8.91, the ~ a k i s Formation, a equivalent to the Simla, consists of silty dolomitic limestone with prominent coral and stromatoporoid biostromes.

The sequence developed during a short regressive interval in the late Frasnian. Following the infilling of interreef basinal areas (Mount Hawk), a lowering of sea level led to the emergence of the reef-capping grainstones (Arcs, upper Fairholme sequence) and their erosion above some reefs. Silt, probably derived from northern (Peace River Arch), eastern and southern sources, spread across the southern Alberta platform and established widespread coastal floodplain conditions (Calmar) which evolved into a peritidal setting with local salinas (Ronde). In areas of more open circulation fossiliferous limestone was deposited (Simla Formation, and its eastern subsurface equivalent, the Blue Ridge Member of the Graminia Formation; n: -

O n\

r ~ g 0. . 3 ) .

--

Figure 8.8. Upper Frasnian carbonate coral-bearing mud mounds (or bioherms) in the argillaceous limestones of the Mount Hawk Formation at Winnifred Pass near Kvass Creek in the southern Rocky Mountains of Alberta (about 40 km northwest of the Ancient Wall Reef Complex). Photo by A. Geldsetzer. GSC 205235(2)

Contents UPPER DEVONIAN TO MIDDLE JURASSIC ASSEMBLAGES

In the Rocky Mountains the Palliser sequence is represented by the Sassenach and overlying Palliser formations (Fig. 8.3, 8.10; Plate 17). The Sassenach Formation (McLaren and Mountjoy, 1962) overlies an erosion surface above most of the former reef domain, infilled remnant depressions along the western margin of the reef domains, and is thus highly variable in thickness. At its type section it is more than 180 m thick and consists of a lower member (148 m) of dark grey to greenish grey, variably calcareous silty mudstone containing allochthonous reef debris in its

basal beds. The upper member (32+ m) consists of sandy, medium- to coarse-grained limestone and calcareous sandstone. The Palliser overlies the Sassenach with regional conformity and is divisible into two members. The lower, Morro Member, locally as much as 530 m thick in the central Rockies (Geldsetzer, 1982)comprises a massive, cliff-forming, grey limestone, commonly bioturbated and characterized by pellet intraclast lime-mudstone. The Costigan Member consists of up to 75 m of fossiliferous, platy, and siliceous limestone and dolostone. The Palliser Formation forms the lowermost of a characteristic tripartite sequence of limestone (Palliser), recessive calcareous shale and argillaceous limestone (Exshaw-Banff) and

Figure 8.9. Distribution of facies during Late Frasnian time (Correlations among named units not necessarily implied). This time marked the disappearance of the reef complexes of Alberta. Legend accompanies Figure 8.5.

Figure 8.10. Distribution of facies during Famennian time (Correlations among named units not necessarily implied). Carbonate deposition again dominates in Alberta but reefs are not present. Legend accompanies Figure 8.5.

Palliser sequence (Lower and Middle Famennian)

Contents CHAPTER 8

cliff-forming massive grey limestone (Rundle) that distinctively characterize several thrust sheets in the Front Ranges of the Rocky Mountains. The Frasnian-Famennian boundary, which in the southern Foreland Belt occurs a t or just below t h e Sassenach-Ronde contact (Fig. 8.31, and in the north a t the Kakisa-Trout River contact, coincides with a sudden change in flora and fauna which is recognized worldwide. The cause was likely a major ocean turnover event (Geldsetzer et al., 1987)possibly as a result of a bolide impact (McLaren, 1982). The effect on carbonate sedimentation was profound, as it marked the end of most species of rugose coral (Pedder, 1982) and stromatoporoid faunas, the prime reef builders. The Palliser sequence marks the last, widespread Devonian transgression on the cratonic platform of the miogeocline. During the early Famennian a western source supplied terrigenous clastics (lower Sassenach) to wide depressions along the western margin of the former reef domain which had not been totally infilled by upper Frasnian Mount Hawk shale (Fig. 8.10). Following aggradation of these remnant depressions silty carbonate and mudstone spread eastward onto the cratonic platform (upper Sassenach). This was followed by a period of widespread clean carbonate deposition (Palliser), and, to the southeast in the Interior Plains, the accumulation of evaporite-dominated sediments (Stettler). Towards the north carbonate passes laterally into argillaceous carbonate and shale (Kotcho, Tetcho and Trout River formations; Fig. 8.10) and thence into siliciclastics (Besa River, Fort Simpson, Earn, Ford Lake, Nation River, Imperial and Tuttle formations). The Palliser sequence was succeeded by black, bituminous shale of the lower Exshaw Formation of late Famennian age, which accumulated in a n anaerobic environment during the final stage of the Palliser sequence whereas the upper Exshaw represents the regressive episode preceding the early Carboniferous transgression.

Devonian-Mississippianclastics of the Foreland and Omineca Belts S.P. Gordey In pre-Late Devonian time the Cordilleran miogeocline consisted of an extensive shallow-water carbonate-clastic platform flanked to the west by deep-water shale and carbonate. Until pre-Late Devonian time, clastic sediments were derived from the craton to the east. In the Late Devonian this regime abruptly changed with the influx of turbiditic, chert-rich clastic sediments of westerly and northerly derivation. By the mid-Mississippian sediment incursion from the west and north waned and marine clastic-carbonate deposition, with clastic input from the craton was re-established (Fig. 8.11). The tectonic setting of the clastics is imperfectly understood. Plutonism and metamorphism are recorded only in northernmost Yukon (Norris and Yorath, 1981; Norris, 1981a,b) and volcanism only in central Yukon (Gordey, 1981b; Mortensen, 1982).Except in northern Yukon where pre-Late Mississippian folds and a n angular unconformity are known (Bell, 1974; Norris and Yorath, 1981) structures include only local high-angle faults and associated disconformities. Parts of the clastic succession have been variously interpreted as reflecting rifting and continental

separation (Tempelman-Kluit, 1979a,b), strike-slip tectonics (Eisbacher, 1983) and uplift of unknown western source terranes through thrusting or orogenesis (e.g. Gabrielse, 1976). For description the clastics are discussed in terms of three assemblages (Fig. 8.1 1A): (1)the Imperial Assemblage (Givetian to mid-Tournaisian) of northern Yukon provenance, (2)the Earn Assemblage (Frasnian to Visean), representing coarse clastic strata of westerly derivation, and (3) the Besa River Assemblage representing a mixture of the eastern and southern fine clastic fringes of the Imperial and Earn assemblages as well as fine clastics of possibly eastern cratonal derivation.

Imperial Assemblage In northern Yukon Territary the segmented carbonate platform regime, present for much of the Early and Middle Devonian, was succeeded in Givetian time by shale and chert which hosted local carbonate reef buildups. In the Frasnian to Early Mississippian there was a profound change in depositional regime as northerly derived clastics inundated the area. The source seems to have lain within a n uplifted region in northern Yukon that was cored by a t least partly coeval plutons (Fig. 8.11A). The following is largely summarized from regional syntheses by Pugh (1983) and Norris (1985). Five formations constitute the late Middle Devonian to Carboniferous Imperial Assemblage; in ascending order these are the Hare Indian (recognized in eastern areas), Canol, Imperial, Tuttle and Ford Lake formations. Equivalent clastics in the Brooks Range of Alaska include

Figure 8.11A. Facies distribution of Devono-Mississippian clastic strata in the Canadian Cordillera. Red line shows informal subdivision into three assemblages, a northern assemblage (Imperial) whose provenance was northern Yukon, a southern assemblage (Earn) of coarse clastics of generally western provenance and an eastern assemblage (Besa River) of fine clastics of mixed northerly, westerly and possibly cratonal provenance. The eastern limit of sandstone and conglomerate is shown by small circles. Large arrows indicate regional paleoflow, from non-cratonal source areas (wavy lines). Small arrow shows local paleoflow, possibly from east or northeast cratonal source area. Also shown are Devonian plutons in northern Yukon (crosses; 1-Ammerman, 2Sedgewick, 3-Hoidahl, 4-Fitton, 5-Old Crow, 6-Schaeffer, and 7-Dave Lord plutons). Letters B to E locate sections in Figure 8.12. Barbed line shows limits of accreted terranes. Cretaceous-Tertiary offsets along Tintina Fault (450+ km) (Roddick, 1967; Tempelman-Kluit, 1979a; Gabrielse, 1985) and Kaltag Fault (100 km) (Norris and Yorath, 1981) are not restored. Five main areas of Earn sequence discussed in the text are also shown (diagonal lines). Fig. 8.118. Fluctuation in position of carbonate-shale boundary during the Late Devonian and Early Mississippian (heavy wavy lines-land area; dashed line-shale; dashed pattern-carbonate). From Bamber and Waterhouse (1971), Basset and Stout (1967), Belyea (1964), Geldsetzer (1982), Grayston et al. (1964), Lenz (1972), Morrow (1984), Macauley et al. (1964), Pugh (1983), H. Gabrielse (pers. comm., 1985) and published and open file maps of the Geological Survey of Canada.

Contents

UPPER DEVONIAN TO MIDDLE JURASSIC ASSEMBLAGES

Contents CHAPTER 8

A great variety of fossils, including conodonts, ostracodes, ammonoids and brachiopods indicate a Givetian age. Thick reefoid carbonates of the Ramparts Formation occur locally within the Hare Indian (Fig. 8.5, 8.11B). The Canol Formation (110-225 m) consists of dark grey, noncalcareous shale which weathers grey, rust-brown, green, or jet black. This conspicuous dark shale marks the end of Devonian carbonate sedimentation throughout the northern Cordillera and reflects an abrupt change from shallow water to much deeper water marine sedimentation (Fig. 8.5). Immediately preceding deposition of the Canol, the eastern, southeastern and southern parts of the area were uplifted and eroded leading to a sub-can01 hiatus of

the Kanayut Conglomerate (Fig. 8.12), the overlying Kayak Formation and the underlying Noatak and Hunt Fork formations. The basal Hare Indian Formation (150-180 m) comprises very dark brownish grey or black, bituminous, in part noncalcareous and calcareous shale, locally called the Bluefish member (10-30 m). Dark grey, calcareous shale with less than 10% interbedded calcareous siltstone and argillaceous or silty limestone form the upper part of the formation. The lower beds represent deposition in shallow euxinic water whereas upper portions may reflect open marine conditions. The lower contact of the Hare Indian is conformable with Devonian carbonate (Hume Formation).

A

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PERMIAN

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GSC Figure 8.12. Time-stratigraphic diagram illustrating upper Paleozoic formational nomenclature in northern Yukon and parts of Alaska. Note that Nation River Formation belongs to the Earn Assemblage, Imperial and Tuttle formations to the Imperial Assemblage. Shaded areas indicate those formations mentioned in text. Approximate locations are plotted in Figure 8.10 (B - 68"55'N, I38O55'W; C - 67"05'N, 13g030'W; D 66"28'N, 136'32'W). M=Middle, U=Upper, Fr=Frasnian, Fa=Famennian, T=Tournaisean, V=Visean, S=Serpukhovian. Columns derived from A-Moore et al. (in press), Nilsen (1984); B-Norris (1981a, 1983), Sable (1977); C-Norris (1981b, 1983), Pugh (1983); D-Norris (1983), Norris (1985); E-Nilsen et al. (1976), Churkin (1973), Norris (1985, Fig. 3).

Contents UPPER DEVONIAN TO MIDDLE JURASSIC ASSEMBLAGES

variable magnitude. Palynomorphs and conodonts show the Canol is Late Givetian in age. The Imperial Formation (540-1690 m) comprises mostly brown to grey-green weathering, fine grained shale, siltstone, sandstone and minor limestone. In the area of its type section in the eastern Mackenzie Mountains Imperial strata represent alternating nearshore and offshore marine sediments derived from a n eastern source area (Hills and Braman, 1978; Chi and Hills, 1974). To the north and west, however, the Imperial is predominantly turbidite (see e.g. Fig. 8.13, 8.14). There, load and flute casts and tool marks indicate southerly paleoflow. The thickest sections are near the lower Peel River area (Fig. 8.13). The lower contact is sharp, and conformable with the underlying Canol. The age of the Imperial a s indicated by brachiopods, palynomorphs and conodonts ranges from Early Frasnian to Early Famennian. An alternating succession of coarse- and fine-grained clastic rocks, the Tuttle Formation (870-1420 m) overlies the Imperial, and represents the coarsest part of this clastic assemblage. Like the Imperial the Tuttle Formation is thickest in the lower Peel River area and consists of alternating chert-pebble conglomerate, very poorly sorted quartz and chert sandstone, siltstone, dark greyish brown shale and rare coal (Fig. 8.15). The conglomeratic material consists of white, buff, grey, yellow, orange and pale green chert. In some places conglomerate clasts also include black chert, white feldspar(?) and fragments of Imperial sandstone. White, black and green chert may have been derived from northern Yukon where chert of these colours, particularly a conspicuously banded green variety, is common. The basal contact of the Tuttle with the Imperial is locally paraconformable (Braman, 19811, in other places the contact appears diachronous, becoming younger to the south. Two depositional models for the Tuttle have been proposed. Lutchman (1977) interpreted the formation as a southward advancing clastic sequence of fluvio-deltaic origin. Hills and Braman (1978) interpreted some Tuttle rocks as a turbidite sequence, within which load casts, flute casts and tool marks indicate a southerly paleoflow. Palynomorphs from the Tuttle indicate a latest Late Devonian to mid-Tournaisian age. The Tuttle is overlain conformably by the Ford Lake shale (700 m) a t one locality (Pugh, 19831, and elsewhere unconformably by Cretaceous, and locally Permian strata. Where the Tuttle is absent the Ford Lake rests directly on the Imperial Formation (Pugh, 1983). The Ford Lake consists of black pyritic shale, orthoquartzite, thinly bedded siliceous siltstone, local chert-rich sandstone, and black silty shale and black, bedded chert (Fig. 8.10, 8.11B). I t ranges in age from Late Devonian to Visean and is a t least partly equivalent to the Tuttle Formation. The lower Ford Lake may represent a cycle of marine transgression and reduction in clastic supply following progradation of the Imperial-Tuttle clastic wedge. Its upper part may represent marine regression prior to deposition of overlying carbonate of the Hart River and Calico Bluff formations (Fig. 8.12). Paleozoic igneous intrusions in the northern Yukon r a n g e from g r a n i t e through q u a r t z monzonite t o monzodiorite, and occur as isolated stocks or cupolas to batholiths covering hundreds of square kilometres (Norris and Yorath 1981; Norris, 1983). Although all have been

grouped as Devonian (Norris, 1981a,b), the wide range in radiometric ages (406 to 220 Ma collectively for Fitton, Sedgwick, Ammerman, Dave Lord, Schaffer, and Old Crow intrusions), intense weathering and hydrothermal alteration suggest that some of these ages have little significance. Maximum radiometric values for each pluton, which may indicate the minimum age of plutonism, range from 354-406 Ma (earliest Devonian to Early Mississippian). The nature of late Paleozoic tectonism in the northern Yukon is obscured by poor exposure, and the lack of precise age of plutonic events. The contact between the clastic wedge sediments (Imperial, Tuttle) and younger rocks is commonly unconformable. Bell (1974) indicated that the age of the hiatus becomes younger from northwest to southeast and the time represented by it decreases in the same direction. Locally, a n angular unconformity separates Imperial beds from Permian strata (Fig. 8.16; Norris, 19681, and pre-Permian broad, gentle, possibly northeast plunging folds have been documented (Bell, 1974). In northernmost Yukon (British and Barn mountains), the thin Lower Mississippian Kekiktuk Formation (20 m), a nonmarine quartzite pebble and cobble conglomerate, forms the base of the transgressive Lisburne Assemblage (discussed below) that rests with angular unconformity on deformed argillite and quartzite of the Precambrian Neruokpuk Formation, and shale as young as Early Devonian (Fig. 8.11B, 8.12; Norris and Yorath, 1981; Norris, 1981a; Lenz and Perry, 1972). Similar relationships have been described in adjacent Alaska (Romanzof Mountains), where pre-Kekiktuk strata are metamorphosed to greenschist facies (Sable, 1977). In northern Ogilvie Mountains deposition was continuous from Devonian to Carboniferous time (Fig. 8.12, column D). In the northern Yukon Paleozoic tectonism involved uplift and granitic intrusion in Frasnian to Early Mississippian time resulting in the accumulation of a n upward shoaling and southward prograding clastic wedge. The clastics, derived from pre-Kekiktuk erosion of strata in northern Yukon, consist of shale a t the base, flyschoid sediments near the middle, and partly fluvial-deltaic strata a t the top. Deformation migrated southward from northernmost Yukon until the clastic wedge was itself gently folded prior to the Early to mid-Carboniferous. Although compressional tectonism seems likely, other models, such as deformation related to strike-slip faulting (Bell, 1974) have been suggested. In the Brooks Range of northern Alaska, Upper Devonian and Lower Mississippian(?) fluvial sediments of the Kanayut conglomerate form one of the coarsest, thickest (to 2600 m), and most laterally extensive nonmarine sequences in North America (Nilsen, 1984; Moore et a]., in press). These proximal clastic wedge sediments prograded to the southwest above prodeltaic muds of the Hunt Fork shale and nearshore marine sands of the Noatak Formation (Fig. 8.12) from a source area in northern Alaska and northernmost Yukon; the same source as that inferred for the correlative, but dominantly flyschoid Imperial and Tuttle formations. Perhaps the more distal Imperial clastics traversed a relatively narrow highland-marginal shelf where the development of a broad delta plain was correspondingly limited. Middle and Upper Devonian marine and nonmarine strata form a widespread clastic wedge in the Canadian

Contents CHAPTER 8

Figure 8.13. Frasnian and Famennian lithofacies, Imperial Assemblage. Diagram from Norris (1985). lsopachs (red) of the Imperial Formation from Pugh (1983).

Contents UPPER DEVONIAN TO MIDDLE JURASSIC ASSEMBLAGES

Figure 8.14. Chevron-folded beds of sandstone, siltstone and shale of the Upper Devonian Imperial Formation in the northern Richardson Mountains. Photo by A.W. Norris. GSC 1 17380

Arctic Islands. These clastics, whose sedimentary transport was to the south and west, were deformed during the Early Mississippian Ellesmerian orogeny (Embry and Klovan, 1976; Kerr, 1981). Deltaic, easterly derived Imperial Formation strata in eastern Mackenzie Mountains may represent a southwestward extension of this wedge. Although the timing of development and style of DevonoMississippian structure in the Arctic Islands is like that in the northern Yukon, a direct link is concealed beneath the Beaufort Shelf (a similar inference is drawn with respect to Ordovician and Silurian strata in Chapter 7).

Earn Assemblage Although clastic strata of the Earn Assemblage are widespread, relatively little is known of them except in local areas. Their description is presented through synopses of five localities (Fig. 8.17). Equivalent clastic rocks in the Nation River area in east-central Alaska, also are briefly described, although the source and tectonic relationships of these to either the Earn or Imperial assemblages are unknown.

Macmillan Pass area The Earn Group, containing the coarse clastics includes the Portrait Lake and Prevost formations (Fig. 8.17; Abbott, 1982, 1983; Carne, 1979; Gordey, 1981a; Gordey et al., 1982a; McClay, 1984). The Portrait Lake Formation (40880 m), of Early Devonian to mid-Famennian age, comprises blue-black weathering, thin-bedded siliceous shale and chert. The base of t h e formation is markedly diachronous and the lower part changes facies eastward to shallow-water platform carbonate where the formation overlies a sub-Frasnian(?) unconformity of uncertain lateral extent. The upper part of the formation, probably of Frasnian age, contains the oldest westerly derived clastic detritus. It occurs in a 10-300 m thick member of massive chert-pebble conglomerate and laterally equivalent but thinner muddy chert-quartz sandstone a n d pebbly mudstone. The conglomerate is remarkably uniform in

composition and contains rounded to angular pebbles and cobbles of grey, black, white or green chert and minor quartz sandstone in a clean matrix of quartz and chert sandstone. Contacts with enclosing rocks are sharp. Important deposits of stratiform lead-zinc (Tom, Jason properties; McClay, 1984) are associated with a widespread unit of baritic shale and barite of Frasnian age (Dawson and Orchard, 1982). The Portrait Lake locally is overlain abruptly and unconformably by thin-bedded ripple cross-laminated siltstone, sandstone and shale of the Itzi Formation whereas a t most other localities it is unconformably overlain by the Prevost Formation. Meagre conodont data suggest that the sub-Itzi unconformity is local and moreover that the Itzi may be a tongue within the upper Portrait Lake. Southerly paleoflow (J.G. Abbott, pers. comm., 1984) may indicate a cratonal or eastern source for the Itzi Formation. The Prevost Formation (up to 900+ m), comprises brown-weathering shale, quartz-chert sandstone and chertpebble to cobble conglomerate. Most exposures consist of up to several hundred metres of mainly shale and siltstone which commonly contain thick intervals (+200 m) of coarse clastics (Fig. 8.18). Conglomeratic mudstone is a distinctive local lithology, a t one place a t least 70 m thick, and containing cobbles of quartz sandstone and chert and rare large boulders of shale. Prevost conglomerate is composed of from 25% to 100% varicoloured chert and 5% to 20% fine- to coarse-grained quartz sandstone in a generally clean sandy matrix. Clasts are mostly well rounded pebbles to small cobbles, but large cobbles and boulders occur locally. Prevost sandstone consists mostly of subequal proportions of chert and quartz, a maximum of one per cent feldspar, rare siltstone and shale fragments, and trace tourmaline, zircon and detrital muscovite (Fig. 8.19A, 8.20). Wackes are slightly less abundant than arenites. Sedimentary structures in the conglomeratic and sandy facies include thickly parted massive beds, local graded bedding, rare Bouma sequences, isolated large shale clasts, zones of chaotic (slumped)bedding, and large groove casts. Parting lineation, groove and prod casts, and rare flute casts occur locally in interbedded thin-bedded, fine grained sandstone. Sedimentary features in predominantly shale successions include thin, graded siltstone beds, and thin to thick, rare interbeds of sandstone showing graded bedding, rare Bouma sequences, and large shale clasts. The Portrait Lake Formation was deposited in a lowenergy, sub-wavebase setting with, a t most times, a low rate of coarse clastic influx and accumulation of siliceous shale and chert. The chert-pebble conglomerate, chertquartz arenite, chert-quartz wacke and pebbly mudstone were deposited from sediment gravity flows along a n easttrending, possibly fault controlled submarine fan. The overlying Itzi Formation may have been deposited as a shallow marine shelf tongue of northerly and possibly craton-derived sediment. Sedimentary features of the Prevost suggest deposition of thick conglomeratic clastics as sediment gravity flows within submarine channels. The fine clastics, including relatively uncommon classical turbidite are both lateral and distal equivalents of the coarse facies. Uplift and erosion of the outer miogeocline, including Upper Proterozoic a n d Cambrian gritty quartzose

Contents

.... ......................................

-

......................

w ( w & - ~ , a m

&mid-..

~ a n l r r ~ m

R a ~ c l g e I n m R W k m s o l ~ ..a54 Ma

r-fwmam

....

Lecpwbis(ml"......*.

Figure 8.15 Uppermost Upper Devonlan and Lower Carboniferous Ilthofacles, lrnperlal Assemblage Dlagrarn from Norrls (1985). lsopachs (red) of the Tuttle Forrnatlon from Pugh (1983).

Contents UPPER DEVONIAN TO MIDDLE JURASSIC ASSEMBLAGES

calcareous sandstone capped by 10 m of clean white medium grained quartz sandstone. Overlying this succession is 25 m of conglomerate and siltstone with clasts up to 5 cm in diameter and composed of vitreous grey, medium grained quartz sandstone and lesser dark grey chert. The uppermost exposed strata is well bedded argillaceous limestone. In the southeast part of the area strata occurring conformably beneath the lowest conglomerate consist of siliceous shale, reefal limestone, and chert which, in turn, conformably to unconformably overlie black shale and siltstone of the Road River Formation (Fig. 8.17). Farther to the southeast the conglomerate is replaced by shale and siltstone with about 25% of the sequence comprising interbeds of sandstone to conglomerate. The coarse clastic beds, commonly from 10-75 cm thick, are regularly interbedded with shale, normally graded and their bases exhibit well preserved flute and tool casts (Fig. 8.21). Similar to northwest sequences, quartz sandstone clasts are more common a t higher stratigraphic levels. Easternmost strata consist entirely of shale. Deposits of bedded barite lead k zinc are particularly abundant in strata of Frasnian age (MacIntyre, 1983; Jefferson et al., 1983).Barite is traceable regionally as nodular and bedded varieties within siliceous mudstone. The southeastern clastic facies, with its graded and sole-marked turbiditic sandstone and conglomerate, represent medial to distal submarine fan deposits. Sole markings indicate east-northeast paleoflow (Fig. 8.11A;Gabrielse et al., 1977). The thick conglomerate member to the northwest may reflect a submarine channel that supplied detritus to the fan. The sub-conglomerate unconformity above non-turbiditic calcareous sandstone, together with the younger capping carbonate, suggest the channel traversed a shallow shelf. The composition of the clastics indicates derivation from a sedimentary terrane underlain by chert and quartz sandstone. The upward increase in quartz sandstone may reflect progressive deepening of the erosion level in the source area. Local unconformities combined with variable stratigraphic position of underlying strata suggest that high-angle faults were active during deposition (shown diagrammatically in Fig. 8.17). These also may have influenced the development and distribution of barite lead f zinc deposits.

+

Figure 8.16. Angular unconformity between sandstone, siltstone and shale of the Upper Devonian Imperial Formation and overlying sandstone of Permian age, northern Richardson Mountains. Photo by A.W. Norris. GSC 205235-KK

sandstone and Ordovician and Silurian chert, provide the necessary constituents for the Prevost and Portrait Lake clastics. A recycled sedimentary source for the quartz (nonchert) detritus is indicated by the high degree of roundness and sphericity of quartz, low amounts of feldspar, and quartz sandstone cobbles within Prevost conglomerate. Given the east-southeast paleoflow for the Prevost clastics (Fig. 8.11A; Gordey, 1979) the nearest possible source where appropriate lithologies have been eroded is 200 km distant. Syn-depositional faults with normal and possibly minor strike-slip displacement (shown diagrammatically in Fig. 8.17) occur locally (Abbott, 1982a). Such faults probably formed a graben controlling clastic dispersion of both the Portrait Lake and Prevost formations, as well as localizing important stratiform barite lead zinc mineralization. Pre-Imperial(?) folding a t one locality (McClay, 19841, could be related to local faults.

+

*

Warneford River area In northeastern British Columbia Devonian-Mississippian strata have not been formally subdivided (Jefferson et al., 1983; MacIntyre, 1983; Gabrielse et al., 1977). In the northwest part of the area (Fig. 8.11A) blocky, well bedded pebble and cobble conglomerate with minor interbedded white, vitreous, crossbedded quartz sandstone rests unconformably on Ordovician black graptolitic shale of the Road River Formation (Fig. 8.17). Conglomerate clasts range from 5-10 cm across, are well rounded, and consist of dark to light grey chert and vitreous white quartzite. Overlying the conglomerate is well bedded

+

Pelly Mountains area The Earn Assemblage in the Pelly Mountains consists of the informal "black slate", "felsic volcanic" and "cherty tuff' units (Fig. 8.17; Tempelman-Kluit, 1977, pers. comm.; Gordey, 1981b; Morin, 1977; Mortensen, 1982). The "black slate" unit is dominated by black, fissile, laminated slate that contains rare dark greywacke ( .? pAvu0Nsm-E .... . LOWER AND UPPER T R I A S S I ~ Cache Creek Group SlllclaaUca end chert MARBLE CANYON SUBTERRANE MID-AND UPPER PERMIAN C d e Creek Olwp Marble Canyon

rn

I

Formation

M A R E SUBlERFUNE UPPER PENNSYLVANIAN-TO UPPER TRIASSIC caane creek m u p Mblange

LOWER AND MIDDLE JURASSIC Aahwoft Formatlo11

Figure 8.71. Subterranes and facies belts in the southern type area of the Cache Creek Terrane.

Contents CHAPTER 8

with the main Permian carbonate, are of Early, Middle and Late Triassic ages (M.J. Orchard, pers. comm., 1985). The western Pavilion Subterrane is underlain by argillite, siltstone, radiolarian chert and local but notable fault slices (?) of volcaniclastic rocks, more massive volcanics, and small carbonate pods (Trettin, 1961; Monger and McMillan, 1984). Melange and broken formation, characteristic of t h e assemblage i n t h e Bonaparte Subterrane, is only locally present. The unit ranges in age from Early Triassic to Middle Jurassic (F. Cordey, pers. comm., 1987) with volcanogenic rocks in the younger part.

Stikinia Stikinia comprises well stratified Lower Devonian to Middle Jurassic volcanic and sedimentary strata (Asitka, Stuhini, Lewes River, Hazelton and Takwahoni assemblages) and plutonic rocks which are probably comagmatic with the volcanics. Major units include unnamed upper Paleozoic rocks of the Stikine region, the Permian Asitka Group, Upper Triassic western Takla, Stuhini, and Lewes River groups, and Lower and Middle Jurassic Hazelton and Laberge groups. The rocks are best exposed around the periphery of Bowser Basin. Their continuation south of latitude 53"N is not clear. On its east side Stikinia is in fault contact with the Cache Creek Terrane. Relationships on its western margin generally have been obscured by Cretaceous and Tertiary plutonism and metamorphism in the Coast Belt, although near Prince Rupert (lat. 55"N) Stikinia appears to be structurally emplaced over the Alexander Terrane. Near Atlin (lat. 5g0N), i t is in faultcontact with possible late Precambrian metamorphic rocks included in the Nisling Terrane.

Upper Paleozoic assemblages The stratigraphic relationship of the oldest rocks, which include Lower Devonian carbonate, to other Paleozoic strata in the Iskut River area is not known (P.B. Read, pers. comm., 1984). The oldest rocks, clearly in stratigraphic continuity with younger strata, are exposed between the Stikine and Iskut rivers (Souther, 1972). There, argillite and cherty argillite grade upward into mafic to intermediate tuff and breccia which is interbedded with and overlain by approximately 1000 m of massive- to medium-bedded crinoidal calcarenite and calcarenitic limestone containing foraminifera, corals, bryozoa and brachiopods of Late Mississippian age (Mamet, 1976) (Fig. 8.72). The relationship between Mississippian and overlying Permian strata is uncertain, but the disappearance of a t least 1500 m of Mississippian strata over a distance of 5 km along trend, and the paucity of recognized Pennsylvanian strata, suggest a major post-Mississippian-pre-Permian unconformity. The overlying succession consists of mafic volcanic rocks grading up into argillite which, in turn, is succeeded by nearly 600 m of carbonate ranging from Early Permian (Wolfcampian) to Late Permian (Early Guadalupian) in age. The carbonate is the most distinctive Paleozoic unit in Stikinia, and, where completely preserved, consists of a lower, dark, thinly bedded, pyritic, argillaceous, micritic to calcarentic limestone containing fusulinids, small corals and bryozoa. It grades upwards into pale grey thin-bedded to medium-bedded cherty calcarenitic limestone. The uppermost member is white to cream, locally dolomitic,

Figure 8.72. View of the Mississippian succession of Stikinia, near the divide between Mess and Sphaler creeks, southern Telegraph Creek area, northwestern British Columbia. Wellbedded carbonate overlies fine grained argillite, cherty argillite and local volcaniclastic rocks and is overlain by Permian volcanic rocks. Photo by J.W.H. Monger. GSC-205235-R

calcarenite and dolomitic calcarenite containing bryozoan, brachiopods, crinoids and fusulinids. Some or all of these characteristics are retained in scattered exposures along the west side of the Intermontane Belt for a distance of over 500 km, from north of the Stikine River to south of Terrace. In the Chutine area (lat. 58"N), Permian strata are overlain with apparent conformity by calcareous and silty argillite containing Middle Triassic fossils (Souther, 1959). The Paleozoic stratigraphic succession on the east side of the Bowser Basin is different from that on the west. The Asitka Group (Lord, 1948; Monger, 1977a) consists of a lower subdivision of argillite, chert, tuffaceous and argillaceous carbonate containing abundant Early Permian fusulinids, overlain by a probably partly subaerial basalt, brightly coloured rhyolite and breccia. An upper member of basalt, chert, tuffaceous limestone and calcareous tuff contains Early Permian sponges, corals, bryozoa and brachiopods (Rigby, 1973; Monger and Ross, 1971). Southwest of the Kutcho Fault near latitude 58"N a polydeformed sequence of feldspathic chlorite schist, sericitic and calcareous phyllite, massive rhyolite, chert and carbonate is in part, a t least, of Mississippian (Visean) age. The succession is probably more than 500 m thick with the thickest carbonate unit about 25 m thick (Thorstad, 1980). Along the Stikine River north of the Bowser Basin Lower Permian massive grey limestone up to 100 m thick overlies grey phyllite and is overlain by mafic metavolcanic rocks and green phyllite. Ribbon chert is present locally. The sequence is overlain unconformably by Lower and Middle Triassic rocks (Read, 1983). The limited chemical data on Paleozoic volcanic rocks from Stikinia provide little insight into their tectonic setting. Speculatively, they represent a suite of arc-related rocks. Much carbonate, particularly that of Mississippian age in northwestern Stikinia and Asitka Group carbonate, is clearly intercalated with the volcanics and may represent fringing banks around volcanic highs. The apparently

Contents UPPER DEVONIAN TO MIDDLE JURASSIC ASSEMBLAGES

sheet-like Permian carbonate of western Stikinia, however, is remarkably uniform over a great distance, which suggests deposition on a relatively uniform and stable substrate. Relationships between Paleozoic and younger strata differ in the various parts of Stikinia. In northwestern British Columbia an episode of deformation and metamorphism separated deposition of Paleozoic to Middle Triassic rocks from Upper Triassic and younger strata (Souther, 1971). In northwestern Spatsizi, Telegraph Creek and Tulsequah areas, upper Paleozoic strata in places are more deformed and metamorphosed than contiguous Lower and Middle Triassic rocks, and in the Tulsequah area the older rocks as well as foliated quartz diorite have northerly structural trends which differ from the later northwesterly structures produced in Middle Jurassic time (Souther, 1971). Farther south in the Oweegee Mountains Upper Triassic strata lie disconformably on Permian rocks (Monger, 1977b) and near Terrace, Lower Jurassic strata lie apparently disconformably on both Permian and Upper Triassic rocks. On the east side of the Bowser Basin, in McConnell Creek area, Lower Permian rocks of the Asitka Group are disconformably overlain by Upper Triassic argillite and siltstone comprising the basal part of the western Takla Group (Fig. 8.73).

Lower Mesozoic assemblages: Triassic rocks Lower Mesozoic assemblages of Stikinia include Triassic volcanogenic strata of the western Takla, Stuhini and Lewes River groups, and Lower to Middle Jurassic volcanics and sediments of the Hazelton Group and the Takwahoni facies of the Laberge Group, together with comagmatic plutonic rocks. Upper Triassic rocks extend continuously along the eastern and northern parts of Stikinia. They typically comprise mafic to felsic volcanics, predominantly volcaniclastic rocks, but include flows and associated sediments. On the east side of Stikinia, in the McConnell Creek area, the Upper Triassic (Upper Carnian to Lower Norian) western Takla Group consists of basal siltstone and argillite, overlain by about 3000 m of pillow and massive augite porphyry basalt and trachybasalt, grading upwards into subaerial flows of similar composition. The flows grade laterally into, and interfinger with, coarse- to fine-grained volcaniclastics which, distally, are intercalated with argillites. Overlying rocks are submarine to subaerial andesitic volcaniclastics and mudstone (Fig. 8.74; Monger, 1977a). The volcanics are transitional between alkaline and subalkaline basalt and andesite, and resemble volcanics in the eastern part of the Nicola Group in Quesnellia. As in Quesnellia Upper Triassic rocks contain numerous copper deposits and showings. The sequence is disconformably overlain by Lower Jurassic volcanogenic strata of the Hazelton Group. Near the Stikine River north of the Bowser Basin basal Mesozoic sediments of Early and Middle Triassic age unconformably overlie Carboniferous and Permian rocks. The sedimentary succession, as much as 250 m thick, comprises siltstone, greywacke, pebble conglomerate and shale. Graded bedding is well developed and the rocks probably represent turbidites. Intercalated with the sediments are augite porphyry volcanics which are overlain by a thin sedimentary member. The upper sequence, of

Figure 8.73. Disconforrnity between Lower Permian volcanics and chert of the Asitka Group and overlying Upper Triassic argillite and siltstone of the Takla Group, on east side of Stikinia, McConnell Creek area, north-central British Columbia. Photo by J.W.H. Monger. GSC 205235-Q

Late Triassic age, is a t least 1000 m thick and contains a lower augite porphyry unit, a middle coarse-bladed plagioclase porphyry and a n upper maroon weathering feldspar porphyry, breccia and agglomerate unit. A pyroxenite body in the volcanic succession has been dated by the K-Ar method a t about 230 Ma. Conglomerate a t the base of the volcanics locally contains boulders of the underlying Cake Hill pluton, a monzodiorite body which has been dated by the K-Ar method a t 220-230 Ma (Anderson, 1983). In northwestern Stikinia, near the Taku River similar Upper Carnian and Lower Norian siltstone and argillite are interbedded with pillow basalt, forming a sequence about 1500 m thick. It is overlain, possibly unconformably, by up to 1000 m of andesite and dacite breccia and a n uppermost massive limestone of Late Norian age (Souther, 1971; Monger, 1980). Possibly the younger sequence can be correlated with the foliated, volcanic-sedimentary Kutcho Formation in the Cache Creek Terrane (Fig. 8.75, Thorstad and Gabrielse, 1986). These rocks are disconformably overlain by Upper Pliensbachian conglomerate of the Takwahoni Formation of the Laberge Group. To the north in southwestern Yukon the Lewes River Group is up to 3000 m thick. Along the eastern flank of the Coast Belt it is composed of coarse andesitic fragmentals, conglomerate, and local augite porphyry flows but grades eastward into volcanic sandstone, greywacke, argillite and, in places, prominent reefoid masses containing Carnian and Norian fossils. The western part of the Lewes River Group is overlain disconformably by Lower Jurassic conglomerate of the Laberge Group. Eastward the contact between the two groups is apparently gradational (Wheeler, 1961; Reid and Tempelman-Kluit, 1987).

Lower Mesozoic assemblages: Jurassic Strata Lower Jurassic and lower Middle Jurassic rocks of Stikinia (Early Sinemurian to Early Bajocian) characteristically include the products of contemporaneous volcanism

Contents CHAPTER 8

DEWAR E A K

W A R PEAK

7

SAVAGEMOUNTAIN

SUSTUT PEAK

SUSTUT-ASITKA RIVERS

SlKANNl RANGE

Formation boundaflea

MAJOR TRIASSIC AND JURASM ROCK UNITS,

G E N E R A L LITHOLOGY

AGE

UcCONNELL CREEK MAP A R M AND

Mulne and nmrmdne, m l n l y rrdlmrnUry rocks,

FORMATIONS OF THE TAKLA OROUP

m h a voloanlca

Qmy and red, M n r and n m mrlnn,

htrrboddod rodlmrntuy end voleank rocdm

R d , nonmnrlne tuff, bbn~~ln, wnghnwnte, locnl flows ranalng from bmalt to rhyontm In mporltlon

MOOSEVALE FORMATION

I

2. Red and green nmmulne and marlnn. voloanb breaola, omglomerate, srmdstons, tuff, argllka

m

Figure 8.74. Stratigraphic relationships of the Upper Triassic western Takla Group, McConnell Creek map area (from Monger, 1977a).

Contents UPPER DEVONIAN TO MIDDLE JURASSIC ASSEMBLAGES

COAST

I

INTERMONTANE

BELT I

OMINECA

BELT

Middle Jurasslo to Lower Cretaceous

Lwvcrr and Mddle Jurarwb

Upper Trlassk: Upper Carnlan and Naian

Upper Middle and

(?I Lower u p w Trlasaic

Mlsslsslppian to Middle Trlassk

Paleazoio; o l d r Lower PaieozOIC in 08

may Include Jurassb strata In SE part of area

parVdistal mlogeoclinsl; youngs

part vdcanogenlc

Figure 8.75. Stratigraphic relationships of lower Paleozoic to lower Mesozoic strata, northwestern British Columbia and south-central Yukon. Proximal Jurassic strata were clearly derived from Stikinia; distal Jurassic strata may overlie the Cache Creek Terrane.

Contents CHAPTER 8

(Hazelton and Takwahoni assemblages; Fig. 8.75-8.80). The volcanics are typically calc-alkaline pyroclastics deposited in both marine and nonmarine environments commonly as coarsely layered volcanic breccia, coarse poorly sorted tuffs, well bedded air-fall and reworked tuff and tuffaceous sediments. Flow rocks are generally subordinate but may predominate locally. They include rhyolite flows and rhyolite domes, minor vesicular basalt, and andesite flows. The volcanics, unless altered by later metamorphic effects or by intrusions, are commonly bright shades of red, green, mauve cream, or grey. Zeolitic altera-

Figure8.77 Distribution, lithology and nomenclatureof Lower Pliensbachian rocks. Legend accompanies Figure 8.76.

Figure 8.76. Distribution, lithology and nomenclature of Sinemurian and Hettangian rocks.

tion is widespread but appears to be concentrated around volcanic centres. Interbedded with the volcanic rocks are sedimentary and volcanogenic sedimentary rocks that are probably subordinate in volume but locally may represent most of the time between Sinemurian and Bajocian. Conglomerate, shale, siltstone, and tuffaceous siltstone are the most common sediments, with great variations depending on provenance and basin conditions. Greywacke and minor sandstone, although important, are only dominant in a few areas. Limestone is rare, occurring only as thin lenses and beds in shale sequences or as bioclastic lenses in volcanic piles. Massive limestone units are unknown. Jurassic rocks define particular tectonic elements and environments of deposition (Fig. 2.5). These elements from north to south are: a ) the Whitehorse Trough, mainly developed on the Cache Creek Terrane but also on bordering parts of Stikinia and Quesnellia and extending through the southern Yukon into northern British Columbia to about latitude 5B0N, b) various volcanic belts lying between latitudes 58"N and 56"N along the northwest, north, and northeast margins of the younger Bowser Basin, and

Contents UPPER DEVONIAN T O MIDDLE JURASSIC ASSEMBLAGES

Figure 8.79. Distribution, lithology and nomenclature of Toarcian rocks. Legend accompanies Figure 8.76.

the type Hazelton volcanic area lying between 56"N and 53"N. The extreme southern part of Stikinia is very poorly known because of extensive Tertiary cover but apparently includes Jurassic volcanics and sediments. The Stikine and Skeena arches with related intrusive complexes and smaller plutons influenced all Jurassic tectonic and depositional history. The Whitehorse Trough lies northwest of the Stikine Arch. Through and between the two arches was a shale basin, ancestral to the younger Bowser Basin. All of the basins, arches, and volcanic belts are probably interrelated a s components of one complex island arc terrane. Between the elements, however, stratigraphic continuity appears to be lacking and thus these elements may have been tectonically dislocated during or after their formation. The Takwahoni Formation is mainly a southwesterly derived proximal facies deposited on and derived from Stikinia whereas the Inklin Formation is both a northeastto easterly derived proximal facies (from Quesnellia, in part) and a southwesterly originating distal facies deposited on both the Cache Creek Terrane and Stikinia. Both formations are components of the Laberge Group. C)

The Takwahoni Formation (Fig. 8.78-8.80) consists of several kilometres of interbedded coarse cobble to pebble conglomerate, greywacke, siltstone and shale with rare flows and airfall tuff. The formation ranges in age from Late Pliensbachian to Early Bajocian and mainly overlies Upper Triassic volcanic rocks. The conglomerates, coarsest and thickest to the west and southwest, contain abundant volcanic and plutonic clasts (Fig. 8.81), some of which are more than a metre in diameter. The source area was a n uplifted part of Stikinia coinciding with the Stikine Arch and the region along the boundary between the Coast and Intermontane belts (Fig. 8.78; 2.4). These areas had been the sites of Upper Triassic to Lower Jurassic island arcs southwest and west of the Whitehorse Trough. Lithologies of the granitic clasts can be related to the Stikine and Klotassin granitic suites which presumably formed the roots of the arcs. The Inklin Formation is up to 3 km thick and consists predominantly of interbedded, turbiditic massive greywacke shale and siltstone (Fig. 8.76-8.79). In contrast to the more massive character of the Takwahoni Formation the Inklin i s commonly strongly foliated, particularly i n i t s

Contents CHAPTER 8

Figure 8.81. Conglomerate in the Takwahoni Formation on the Whitehorse Trough southwest of Whitehorse, Yukon Territory. Photo by J.O. Wheeler. GSC 205235-X

Figure 8.80. Distribution, lithology and nomenclature of Aalenian and Bajocian rocks. Legend accompanies Figure 8.76.

southeastern exposures. Local conglomerates contain abundant clasts of limestone derived from erosion of the Upper Triassic Sinwa limestone and quartz porphyry from the Upper Triassic Kutcho Formation both in the Cache Creek Terrane (Thorstad and Gabrielse, 1986). The strata range in age from Hettangian? to Pliensbachian in its southwestern exposures and from Hettangian to Early Bajocian in the northeast. Although the formation has been interpreted as a northeast-prograding submarine fan (Bultman, 1979) and also as a distal equivalent of the Takwahoni Formation (Souther, 1971) it has now been shown that in the Atlin and Whitehorse areas most of the Inklin is older than the Takwahoni. Moreover, the Inklin Formation apparently becomes coarser towards t h e northeastern part of the Whitehorse Trough and thus the sediments may have been derived from Quesnellia. Some volcanics, such as the Nordenskiold dacite in the Yukon and correlative volcanics in the Atlin area of northern B r i t i s h Columbia a r e i n t e r c a l a t e d w i t h Lower Pliensbachian and younger sediments.

The Inklin and Takwahoni formations are everywhere separated by faults, especially along the King Salmon Fault, and it is probable that considerable shortening has taken place across the Whitehorse Trough. Sequences of Lower to Middle Jurassic, dominantly volcanic rocks of the Hazelton Group are typically exposed along and near the Skeena Arch (Fig. 2.5). Correlative rocks, also assigned to the Hazelton Group, occur around the western, northern and eastern margins of the Bowser Basin. Except along the Skeena Arch where meagre chemical data suggest eastward subduction under an island arc (Tipper and Richards, 1976) there is little information as to the tectonic settings and relationships of the various volcanic centres. It is clear from biostratigraphic and isotopic age data and the distribution of volcanic rocks around the Bowser Basin that the group cannot represent a single, simple island arc in its present form. The Hazelton Group on the Skeena Arch and along its flank has been subdivided into four formations with many facies and members. The most widespread and prominent volcanic unit is the Telkwa Formation of Sinemurian age (Fig. 8.76; Plate 20) and deposited unconformably on Triassic rocks; the base is marked by a coarse polymictic conglomerate with clasts of volcanic rocks, Permian limestone and, locally, granitoids. Typically the Telkwa Formation consists of reddish, maroon, purple, grey and green pyroclastic and flow rocks of marine and nonmarine origin. Zeolitic alteration has been extensive around coeval plutons and may reflect paleo-hotspring systems contemporaneous with volcanism. Thickness varies greatly but is in excess of 400 m, particularly southwest of Smithers. Lithologically similar rocks in the Terrace area, however, are of Triassic age. The Nilkitkwa Formation comprises as much as 1000 m of interbedded shale, greywacke, andesitic to rhyolitic tuff and breccia, and minor limestone; it is mainly marine and ranges in age from Early Pliensbachian to Middle Toarcian (Fig. 8.77-8.79). Its local but distinctive basaltic flows and aquagene tuffs have been traced northward to where coeval rocks are entirely shale which disappear beneath younger strata of the Bowser Lake Group. The

Contents UPPER DEVONIAN TO MIDDLE JURASSIC ASSEMBLAGES

formation occupies the eastern part of Stikinia between 56"N and 53"N latitude but may be thinner or covered in other areas to the south. The Smithers Formation (Fig. 8.801, 500 m to 800 m thick, is a light grey to brown, greenish grey to drab grey succession of greywacke, siltstone, tuffaceous shale, volcanic breccia, and rare volcanic pebble conglomerate. Typically the sediments are immature and poorly sorted. The formation is widespread north and south of the Skeena Arch. Along the east side of the Bowser Basin it contains shaly, silty and sandy members. The age of the formation is variable but in most areas i t is Early Bajocian; in a few areas, the more shaly members apparently are as old as Middle Toarcian and as young as Late Bajocian. The Whitesail Formation south of the Skeena Arch in the Whitesail Lake area (Fig. 8.801, is of latest Toarcian to Aalenian age. In the type area, the formation consists of 600 m of rhyolitic tuff and breccia, shale, tuffaceous shale and siltstone that have yielded a n abundant marine fauna. In the Smithers and Terrace areas rhyolite domes and rhyolitic dykes are correlated on the basis of radiometric dates. Also south of the Skeena Arch and extending to the southern extremity of Stikinia is a n area of moderate to low relief in which Cretaceous and younger volcanic and sedimentary rocks cover the Jurassic strata. The meagre stratigraphic record includes Middle Bajocian strata like the Smithers Formation, Pliensbachian strata like the Nilkitkwa Formation, and coarse volcanic breccia similar to the Telkwa Formation. Sedimentary rocks of undetermined age may be Jurassic or they may be Cretaceous. The Toodoggone volcanics along the northeast side of the Sustut Basin (Fig. 8.76, 8.79) are a succession of subaerial, intermediate, calc-alkaline to alkaline, predominantly pyroclastics, as much as 700 m thick, of Early Jurassic age. The volcanics unconformably overlie Upper Triassic volcanic rocks and are unconformably overlain by sediments of the Sustut Group. Typical lithologies include ash-flow and air-fall crystal-lithic tuffs with quartz, biotite, hornblende and plagioclase phenocrysts (Diakow, 1984). Trachyandesite flows with potassium feldspar phenocrysts form a distinctive and widespread member. Radiometric ages of volcanic rocks and related plutons range from 179Ma to 207 Ma (Panteleyev, 1984; Diakow, 1985; Gabrielse et al., 1980)with a distinct cluster around 200 Ma. The Toodoggone volcanics may, therefore, in large part be correlative with the Telkwa Formation in the type Hazelton region. The younger radiometric dates and one collection of ammonites indicates that rocks as young as Toarcian may be present. T h e Toodoggone volcanics h o s t e p i t h e r m a l , amethystine quartz-gold-silver veins. Spectacular alunitebearing alteration zones have been related to Early Jurassic, shallow hydrothermal systems. The Cold Fish volcanics along the southwest side of t h e S u s t u t Basin (Fig. 8.77) a r e mainly of Early Pliensbachian age and comprise more than 700 m of subaerial to submarine felsic to mafic lava and tuff intercalated with minor shale and limestone. The volcanics are arc-related, bimodal rhyolite (mostly tholeiitic) and basalt to trachyte (transitionally tholeiitic to alkaline; D.J. Thorkelson, pers. comm., 1987). Along the Stikine Arch Toarcian and Bajocian volcanic rocks occur but their relationships to the Cold Fish

volcanics are unknown. Toarcian sediments including conglomerate, greywacke, sandstone, siltstone and minor limestone, up to 400 m thick, are overlain by volcanic breccia a n d flows. They r e s t unconformably on Pliensbachian, Sinemuriad?) and Upper Triassic rocks including granitic rocks of the northern flank of the Hotailuh batholith on Stikine Arch. Locally, the volcanics are overlain by Aalenian(?) and Bajocian flows, breccia and tuff, commonly in distinct centres. North of Stikine River and east of the Hotailuh batholith Bajocian volcanics are intercalated with chertpebble conglomerate, sandstone and siltstone which may represent a proximal facies of the Bowser Lake Group. South of the Stikine Arch the volcanics pass into tuffaceous sediments and eventually into shale and siltstone of the Spatsizi Group (Fig. 8.77,8.80,8.82; Thomson et al., 1986). Several porphyry-related copper deposits occur in probable Bajocian volcanics along the northwest margin of the Bowser Basin. Volcanic and sedimentary rocks of Early to Middle Jurassic age are widespread along the west side of the Bowser Basin. In the Iskut River area, Lower Jurassic conglomerate with boulders and cobbles of volcanic and granitic rocks lies unconformably on Upper Triassic volcanics of the Stuhini Group (Souther, 1972).The Lower Jurassic succession comprises more than 1000 m of tuffaceous siltstone and sandstone interbedded with interbedded fragmental volcanic rocks of basalt-andesitic composition. Fossils demonstrate the presence of Hettangian, Upper Pliensbachian and Upper Toarcian stages. About 1000 m of black shale with minor sandstone and grit of Late Toarcian to Middle Bajocian age separates the underlying Lower Jurassic rocks from a n overlying sequence, as much as 2000 m thick, of Bajocian(?) submarine, basalt-andesite pillowed lavas and associated flows, dykes and sills. This unit grades laterally into sequences of tuff-breccia, tuff, volcanic sandstone and conglomerate which is overlain, possibly conformably by chert-pebble conglomerate of the Bowser Lake Group. Farther south, in the Stewart area, a heterogeneous unit of Lower Jurassic (Pliensbachian?) intermediate to felsic flows, tuff and volcaniclastic rocks, important as a host for precious metal deposits, overlies Upper Triassic (Norian) volcanics (R.G. Anderson, pers. comm., 1986). The volcanics may be as much as 1000 m thick locally but are much thinner away from volcanic centres (Alldrick, 1985). Overlying the volcanics is a distinctive unit of buff, fossiliferous sandy limestone 1to 3 m thick which contains a prolific Toarcian fauna. The uppermost formation of the Jurassic to Middle Jurassic succession is a widespread marker unit of alternating siliceous shale, flinty welded tuff and radiolarian chert of Bajocian age. This formation is probably correlative with similar strata a t the top of the Lower and Middle Jurassic succession exposed as inliers in the northern part of the Bowser Basin and along its eastern margin. Overlying strata are Bathonian shales of the Bowser Lake Group. The stratigraphy in several small inliers within the northern Bowser Basin and in localities along its eastern margin suggests that much of the basin may be underlain by fine grained clastic rocks of Early and Middle Jurassic age. As noted above Lower Pliensbachian volcanics of the Nilkitkwa Formation grade into shale northwest from the

Contents CHAPTER 8

SE SECTION SECTION z Y

SECTION X

MAP AREA I

-Teloceros Chondronros Sonninim D~Wceros

II

LOWER BAJOCIAN

LIme8tOns TUlt Acid Voloanlca

-

Lioceratoidss r o t o p ~ c e mp he+iceros Leptakonmr

Unconformlty detlnsd

--uu

............... 76

..........................

~~~d

5 o