The Paleozoic Western Craton Margin

Chapter 5

The Paleozoic Western Craton Margin Andrew D. Miall Department of Earth Sciences, University of Toronto, Toronto, ON, Canada

Chapter Outline Introduction Historical Background The Rifted Margin of Laurentia Southern Canadian Rocky Mountains and Great Basin Yukon Territory and Northwest Territories The Sauk Sequence and The Cambrian-Ordovician Shelf-To-Basin Transition The Kicking Horse Rim and Burgess Shale of The Southern Canadian Rocky Mountains Northern British Columbia Yukon and Northwest Territories Great Basin: Nevada, Utah, Idaho Middle Ordovician-Early Devonian (Tippecanoe Sequence) Northern Canada Great Basin Grand Cycles

239 241 244 244 248 249 249 252 252 252 252 258 260 255

Lower To Upper Devonian (Kaskaskia-i Sequence) Northern Canada Peace River Arch Ancestral Uinta Uplift Great Basin Devonian-Mississippian Arc Collisions and Termination of Parts of The “Passive” Laurentian Margin (Kaskaskia-ii Sequence) Great Basin Western Canada Pennsylvanian-Permian (Absaroka i and ii Sequences) Triassic-Jurassic: Termination of The “Passive” Continental Margin Conclusions Acknowledgments References

256 258 259 259 260

260 260 260 262 263 264 264 264

INTRODUCTION The western continental margin is particularly well exposed and has been extensively studied in four main areas (Fig. 1), for which the stratigraphy is summarized in Fig. 2: (1) The Great Basin, which includes most of Nevada, and extends from the eastern limit of the Basin-and-Range tectonic province, the Wasatch Line, east of Salt Lake City, Utah, to the eastern margins of the Sierra Nevada, California. The rocks of the Paleozoic continental margin of Laurentia are particularly well exposed in the tilt blocks of the Basin and Range within the Great Basin of Nevada, plus adjacent areas of Utah and Idaho. A symposium on Paleozoic paleogeography edited by Stewart and Suczek (1977) and the Cordilleran volume of the Decade of North American Geology project (DNAG) (Burchfiel et al., 1992) provide basic reference sources for this area. It should be noted that the eastern limit of Basin-and-Range extension was determined by the western margin of unthinned continental crust, and the Great Basin was named for a physiographic area within the Basin-and-Range tectonic province, which is characterized by internal drainage. (2) The Rocky Mountains of Banff, Jasper, and Yoho national parks, Canada. Useful syntheses of the Western Canada Sedimentary Basin, which include much information on the ancient continental margin, were compiled by Ricketts (1989) and by Mossop and Shetsen (1994). The Decade of North American Geology volumes on the Canadian Cordillera (Gabrielse and Yorath, 1992) and the Sedimentary Cover of the Craton (Stott and Aitken, 1993) are also essential sources.

The Sedimentary Basins of the United States and Canada. https://doi.org/10.1016/B978-0-444-63895-3.00005-X © 2019 Elsevier B.V. All rights reserved.

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240  The Sedimentary Basins of the United States and Canada

FIG. 1  Location map.

(3) The Mackenzie Mountains of the Northwest Territories, north of the 60th parallel, for which the DNAG volumes are the essential references. The regional tectonic history of this area is discussed in detail by Cecile et al. (1997). (4) The mountainous areas of northern Yukon and adjacent areas of the Northwest Territories. Again, the DNAG volumes plus Cecile et al. (1997) are the most useful sources. This region includes large areas of “pericratonic terranes” (Fig. 1), that is, terranes that are interpreted as originally part of Laurentia, but which have been displaced at some time during the Phanerozoic.



The Paleozoic Western Craton Margin Chapter | 5  241

FIG. 2  Correlation table, showing selected stratigraphic names. (Time scale from www.stratigraphy.org; Sloss Sequences from Sloss (1988), with ages revised according to www.stratigraphy.org. Nevada: Miller et al. (1992); Alberta and British Columbia: Bond and Kominz (1984) and Ricketts (1989); Northwest Territories, Yukon: Gabrielse and Yorath (1992).)

HISTORICAL BACKGROUND The application of plate-tectonic concepts to the interpretation of the ancient geological record, beginning in the late 1960s, has provided many significant breakthroughs in our understanding of complex geologic regions. The western margin of the North American continent was among the earliest tectonic provinces to be understood using these new ideas. Stewart (1972) and Burchfiel and Davis (1972) suggested that, during the late Precambrian and early Paleozoic, the margin of the ancient continent extended through the central portion of the Cordilleran orogen (Fig. 1). The miogeoclinal character of the western continental margin beneath Alberta and British Columbia was recognised by Monger and Price (1979) and compared to an Atlantic-type margin. Interpretation of the rocks of coastal belts extending from Yukon Territory to California had to await the development of terrane concepts in the late 1970s (see Chapters 10 and 11). Belts of upper Precambrian and lower Paleozoic strata, reaching thousands of meters in thickness, extend NNW-SSE through the foreland region of the Rocky Mountains from the Yukon to Idaho, and on southwesterly through central Nevada (Fig.  3). Stratigraphic and facies patterns of these rocks suggest an interpretation in terms of a “passive,” extensional

242  The Sedimentary Basins of the United States and Canada

FIG. 3  Isopach of upper Precambrian to Lower Cambrian strata on the western continental margin. (Redrawn from Stewart (1972).)

continental margin that was subsequently deformed and uplifted by Cordilleran orogenesis. Stewart (1972) and Stewart and Suczek (1977) made an explicit comparison to the modern Atlantic continental margin of eastern North America (see Chapter 15), including (1) evidence of rift faulting and subsidence in the late Precambrian, which is compared to the Triassic “Newark” rift basins of New England and Atlantic Canada; (2) fault-bounded basins containing great thicknesses of westerly derived upper Precambrian clastic strata, including several intervals of glacially derived diamictites; (3) basaltic intrusions and lava flows of late Precambrian age, which can be compared to similar Triassic-Jurassic igneous activity in the Newark basins; (4) gradual onlap of lower Paleozoic strata onto the continental margin during the thermal subsidence phase of margin development; (5) lower Paleozoic shallow-water strata of “miogeoclinal” character, consistently thickening from the craton as they are traced westward into the deformed foreland belt, similar to the pattern of modern sedimentation on the Atlantic continental shelf off the United States; and (6) a transition, where preserved, into deeper-water, largely clastic



The Paleozoic Western Craton Margin Chapter | 5  243

facies toward the west, suggesting a comparison with continental slope and basinal facies of the deeper parts of the modern Atlantic margin. Bond and Kominz (1984) and Bond et al. (1985) applied the methods of backstripping to the ancient continental margin and confirmed the appropriateness of the model of crustal stretching and thermal subsidence. As discussed in following paragraphs, the development of the margin is now known to include more than one rifting event and the probable presence of crustal detachment faults. We now know that the rocks to the west of the early Paleozoic continental margin consist of a belt of amalgamated terranes as much as 500 km wide, attesting to the accretionary growth of the continent since the early Paleozoic. From the late Precambrian until the Late Devonian, the entire western margin functioned as a passive or extensional margin (Figs. 7, 9, 11, and 12 in Chapter 1). Then, commencing in the latest Devonian or Mississippian, the Antler arc began to collide with the continental margin in the region of what is now central Nevada (Fig. 13 in Chapter 1), emplacing the Roberts Mountains Allochthon above the miogeoclinal margin (see also Chapter 11). In Nevada, this was followed by extensional and transcurrent deformation associated with the “Phase two” tectonism that primarily affected the southern part of the continent (Chapters 7 and 8), and then the Permo-Triassic Sonoma Orogeny. There is some evidence, discussed shortly, that the Antler Orogeny affected some or all of the Canadian Laurentian margin, although there is also evidence that much of the Canadian part of the western continental margin, extending from Oregon to Yukon, remained in an extensional regime until the Early Jurassic (Gabrielse and Yorath, 1992). The evidence is controversial because it comes from rocks that have been extensively deformed by post-Mississippian tectonism. A west-facing arc (the Nicola or Quesnellia arc) had developed off the coast from Oregon to central British Columbia by the late Paleozoic or Triassic (Figs. 15 and 19 in Chapter 1), but until the Early Jurassic this appears to have been an extensional arc, with a backarc basin (Slide Mountain terrane) located to the east. The continental margin of a back-arc basin functions tectonically and stratigraphically much like a passive continental margin. Extensional tectonism of the continental margin associated with this plate-tectonic regime included subsidence of the Liard Basin, Prophet Trough, and the Peace River Embayment. The evolution of the western continental margin, as exemplified by the section within what is now British Columbia, is shown in Fig. 4, based on the work of the Canadian Lithoprobe project. Collision and subduction-related basins of the North American Cordillera are described by Ricketts (Chapter 10) and by Ingersoll (Chapter 11).

FIG. 4  Schematic tectonic development of the Canadian Cordillera, showing the evolution of the western craton margin from a rift setting (A), through a west-facing extensional margin (B), to an orogenic collage, within which the original extensional margin setting has been obscured by accretionary tectonics (C, D). BP—Belt-Purcell Basin; CDF—Cordilleran deformation front; GSLsz—Great Slave Lake shear zone; MHB—Medicine Hat Block; OT—Outer terranes; Q—Quesnellia; S—Stikinia; WH—Windermere High; YT—Yukon-Tanana (Cook et al., 2012).

244  The Sedimentary Basins of the United States and Canada

THE RIFTED MARGIN OF LAURENTIA Southern Canadian Rocky Mountains and Great Basin As noted previously, the modern Atlantic margin of the United States was used as an analog for the first plate-tectonic interpretation of the Precambrian–Lower Paleozoic margin of Laurentia. Subsurface data from the Atlantic margin also provided the basis for the first quantitative models for subsidence and sedimentation on passive or extensional, “Atlantic-type” continental margins (Watts, 1981; Watts et al., 1982). The procedure, which is now standard, by which rates of subsidence and sedimentation are calculated and related to tectonic and isostatic controls was first developed using these data (Steckler and Watts, 1978). This procedure, called “backstripping,” was applied to an investigation of the evolution of the ancient continental margin of British Columbia by Bond and Kominz (1984), in order to explore the appropriateness of a Watts-type Atlantic-margin model for these rocks. Stewart and Suczek (1977) constructed a subsidence curve for the continental margin of southern Nevada and also interpreted the result in terms of an Atlantic margin–type model. Bond and Kominz (1984) introduced one important modification to the backstripping method, which was required by the major difference in rock type between the Jurassic to modern sediments of the Atlantic margin and the PrecambrianCambrian rocks of British Columbia. On the Atlantic margin, the succession is primarily clastic. The sediments there have high initial porosities, which are gradually reduced by burial compaction. This compaction results in thickness reductions during burial, which can be compensated for in the subsidence calculations by using the local empirical relationship between porosity and depth. However, most of the miogeoclinal succession on the ancient British Columbia margin consists of carbonate sediments which typically undergo early cementation and lithification. Bond and Kominz (1984) constructed a different porosity-depth curve for these rocks based on studies of early carbonate diagenesis and tested against data from a well drilled off the Florida margin through a predominantly carbonate section. This allowed them to “delithify” the strata as a first step in the backstripping procedure. The oldest sedimentary rocks within the miogeoclinal belt are the Belt-Purcell Supergroup of Helikian age (1.8–1.5 Ga). These constitute an immense thickness of largely clastic deposits up to 15 km thick. They were formed in rift and epicontinental basins within the Rodinia Supercontinent and underwent a deformational event about 1300 Ma (Fig.  4A). These rocks predate the Grenville orogeny (1.3–1.0 Ga), the final event in the assembly of Rodinia, and therefore are unrelated to the formation of the western Laurentian margin. The Belt-Purcell Supergroup is overlain with angular unconformity by the Windermere Supergroup, of Helikian (800–575 Ma) age and up to 9 km in thickness (Gabrielse, 1972; Fig. 4B). Extensive mafic intrusives and extrusives are related to deep crustal fractures that are thought to have been active during sedimentation. Approximately 6 km of Lower Paleozoic miogeoclinal strata lie above these rocks and are spectacularly exposed in the Rocky Mountains of Banff, Jasper, and Yoho national parks. A succession of shallow-water, mainly carbonate, sediments extends from the craton westward to near the Alberta–British Columbia border, where a major facies change takes place, over what Aitken (1971) termed the Kicking Horse Rim (named after the famous mountain pass of the same name). The Paleozoic section thickens and changes facies westward into deeper water facies, the details of which are described in a later section. Rocks of Cambrian to earliest Silurian age are well represented, corresponding to the Sauk I to Kaskasia II sequences. The major sequence-bounding unconformities of Sloss (1963) can be recognized within the succession. A major angular unconformity occurs near the middle of this succession: rocks of the Tippecanoe II sequence, corresponding to most of the Silurian System, are largely absent from the southern Canadian Rocky Mountains. Bond and Kominz (1984) constructed a restored stratigraphic cross-section through the southern Rocky Mountains, approximately along the transect followed by the Trans-Canada Highway. Fig. 5 shows the Precambrian to lowest Silurian portion of this cross-section (above which there is a major unconformity). Their subsidence curves suggest that this interval corresponds to the main thermal-subsidence phase of an extensional margin. Rapid thermal subsidence appears to have commenced at some time between the latest Precambrian and the mid-Early Cambrian (600–550 Ma). A thin unit, the Hamill Group, which spans this age range and occurs in the southern Rocky Mountains of British Columbia, is cited by Bond and Kominz (1984) as providing support for this interpretation. It consists of “coarse-grained arkosic sediments and scattered mafic lavas,” an association that is suggestive of a continental terrane undergoing rifting (Bond and Kominz 1984, p. 167). The development of oceanic crust corresponding to the initiation of Panthalassa, would have followed shortly after the deposition of this unit, initiating the thermal subsidence of the adjacent margin (Fig. 4B). In British Columbia, the increase in thickness of the Lower Paleozoic section from the miogeoclinal carbonate bank to the deep-water shale basin may indicate a greater rate of subsidence of thinned continental crust at the outer margin of Laurentia. In this interpretation, the Kicking Horse Rim is located at approximately the boundary between the normal and thinned basement created by the crustal extension that commenced in the late Precambrian.



The Paleozoic Western Craton Margin Chapter | 5  245

FIG. 5  Reconstructed stratigraphic cross-section through the Precambrian–Lower Paleozoic section of southern Alberta and British Columbia. This section represents a palinspastic reconstruction. Locations of two of the major thrust sheets are indicated for reference purposes. The MacConnell Thrust is the easternmost of the thrusts to bring Paleozoic strata to the surface and defines the front of the physiographic Rocky Mountains west of Calgary. (Redrawn from Bond and Kominz (1984).)

Stewart and Suczek (1977) had earlier developed a similar interpretation for the corresponding continental margin of Nevada. They constructed a subsidence plot (Fig. 6) that they interpreted as indicating an exponential pattern of subsidence. A succession of terrigenous sediments of late Precambrian age extending from Washington to southern California (Fig. 7) is interpreted as the product of rifting and, subsequently, of erosion of the thermal uplift that typically precedes continental separation. They interpreted the beginning of rifting as taking place at about 900–800 Ma, with the commencement of postrift thermal subsidence starting near the end of the Precambrian. Watts (1981, 1989) and Watts et al. (1982) showed that as the flexural rigidity of the continental crust increases and the outboard sediment load increases following continental separation and drift of the continent away from the sea-floor spreading center, the flexural hinge at the edge of the continental crust migrates gradually cratonward, resulting in gradual onlap. This is clearly shown in the map of the western United States (Fig. 7) and in the cross-section of western Canada (Fig. 5) constructed by Bond and Kominz (1984). In a subsequent study, Bond et al. (1985) extended their analysis to a series of locations between Yukon and Utah and confirmed the general pattern of thermal subsidence commencing between about 600 and 555 Ma, indicating the widespread initiation of a passive margin along the western margin of Laurentia. The general similarity of the stratigraphy along

246  The Sedimentary Basins of the United States and Canada

FIG. 6  Subsidence plot for the uppermost Precambrian to Devonian strata of the western United States. At the time this plot was constructed, dating of the oldest rocks in the succession was uncertain, and two positions are shown for the commencement of subsidence. (Redrawn from Stewart and Suczek (1977).) 124

120

116

112

108

104

100

48 48 Lower upper Cambrian

44 44

40

n bria am

bri an

am

rC

dle C

rian

mb

a er C

pp

le u

d Mid

p

r up

e Upp

n

bria

am er C

36

Lo we r

mid

Low e

36

Upp er Cam middl bria e n

40

> 100 m Terrigenous Cambrian strata

32 32

120

116

112

108

104

FIG. 7  The extent of predrift sediments on the western margin of Laurentia in the United States (stippled area) and the onlap pattern of Cambrian strata. (Adapted from Stewart and Suczek (1977).)



The Paleozoic Western Craton Margin Chapter | 5  247

the length of the western continental margin as far north as 60°N suggests that the rifting and thermal subsidence model applies to the entire western Laurentian margin, although Cecile et al. (1997) argued that the pure-shear model of Bond and Kominz (1984) is not appropriate for the northern Cordillera, as summarized in following text. The length of time between the deposition of the oldest of the presumed rift-related Windermere Supergroup (800 Ma) and the initiation of sea-floor spreading (660–550 Ma) is not a problem. Although early models for extensional margins considered rifting as if it were “instantaneous” for the purposes of calculation (e.g., McKenzie, 1978), in fact it is not at all uncommon for the predrift rift stage to extend for as long as 200 million years and to include several discrete rifting episodes, as has been documented for the margins of the North Atlantic, between Newfoundland, Spain, Greenland, and the European margin west of Britain (e.g., Surlyk et al., 1981). Cecile et al. (1997) offered a similar argument for the length and complexity of the extensional tectonism of the northern Cordilleran margin. Deep-crustal reflection profiling, of the type pioneered by Oliver (1982), led Lister et al. (1991) to recognize that a range of tectonic mechanisms occur at extensional continental margins. To the original pure-shear model of McKenzie (1978) has been added a range of simple-shear models based on the recognition that the rifting and separation of some continents takes place across one or more master detachment faults that penetrate the entire crust at a high angle. In the absence of seismic data (which are not available for the study area) structural trends and isopach patterns may be quite distinctive. Many characteristics of the Paleozoic continental margin of British Columbia and the territories to the north indicate that crustal separation along this portion of the margin may have followed a pattern of simple-shear evolution. The western edge of the Alberta–British Columbia portion of the margin is characterized by zones of uplift, including the West Alberta Arch and the Macdonald Platform that, during the Cambrian and Ordovician, functioned as positive elements over which developed widespread sub-Devonian unconformities. These and other indicators, discussed by Cecile et al. (1997), suggest that the margin in this region may have functioned as the upper plate margin above an east-dipping detachment fault (Fig. 8). Lithoprobe data from the Canadian Cordillera provide some evidence for crustal extension, but do not point to the presence of any major detachment faults (Cook et al. 2012, p. 23). Conversely, a reconstruction of the Paleozoic margin derived from SNorCLE line data offered by Welford et al. (2001, Fig. 13), is consistent with this interpretation. Aeromagnetic studies have indicated the probability of mantle underplating and crustal thickening in northeastern British Columbia (Saltus and Hudson, 2007), exactly where a simple-shear model of upper-plate architecture would predict (Lister et al., 1986). Structural and isopach trends suggest that Paleozoic sedimentation and tectonism were influenced by a series of ­northeast-southwest–trending lineaments that probably originated as faults or terrane boundaries in the Precambrian ­basement. The most important of these, which has been named the Liard Line, crosses the continental margin at about the latitude of the British Columbia–NWT border (Fig. 1). As discussed in following paragraphs, this line may have functioned as a major “transfer fault” during the early Paleozoic. Another element of the Bond and Kominz (1984) model is also worth commenting on. They compared a modeled twodimensional cross-section of the continental margin with an actual, restored cross-section (Fig. 9). Note that the restored section exhibits a much greater than predicted thickness of the marginal flexural wedge. This probably indicates that both thermal subsidence and crustal thinning were underestimated in the backstripping calculations. The thin wedge of Middle to Upper Cambrian strata extending across the craton is probably the result of the episode (or episodes) of eustatic sea-level rise (or dynamic-topography subsidence) that were responsible for the development of the Sauk II and Sauk III sequences. Following this episode of high sea level, there was a widespread regression. Rocks of Middle Ordovician to Early Devonian age are almost entirely absent from western and central Alberta, owing to long-lived uplift, or to repeated episodes of sedimentation and erosion of the West Alberta Arch (or Ridge). Rocks of this age are, however, widespread north of the 60th parallel.

FIG. 8  Simple-shear model of extensional continental margin development, after Lister et al. (1991) and Cecile et al. (1997). The British Columbia– Alberta portion of the continental margin may have been an upper-plate margin, whereas the area north of 60°N may have functioned as a lower-plate margin, the reversal in facing direction taking place at a transfer fault corresponding to the Liard Line (Cecile et al., 1997).

248  The Sedimentary Basins of the United States and Canada

FIG. 9  Below: restored cross-section through the craton and miogeocline of western Canada; above: two-dimensional numerical model of a flexural continental margin, based on the data and delithification procedures used for this particular margin. Note (1) the gradual onlap of the craton with time; (2) the considerably greater thickness of strata in the continental-margin wedge than predicted by the flexural model, and (3) the thin unit of Middle to Upper Cambrian strata that extend for hundreds of kilometers into the cratonic interior. Inverted “v’s” are well locations from which thickness data have been derived (Bond and Kominz, 1984).

Yukon Territory and Northwest Territories Yukon Territory includes a large area of the continental margin that has been interpreted as a pericratonic terrane (Gabrielse and Yorath, 1992). This area, the Cassiar Platform, is thought to have been part of the original Laurentian continent during the Paleozoic. However, it lies immediately to the west of the Rocky Mountain Trench–Tintina Fault, one of the longest and most significant of the Cenozoic strike-slip faults that affected the Cordillera during the Cenozoic. Displacement along this fault is unknown, and it is therefore not known what the original relationship of the Cassiar Platform is to ancestral North America. Cecile and Norford (1993, p. 135) cited estimates of dextral displacement ranging from 400 to 750 km, the greater of which would place Cassiar Platform outboard from southern Alberta. Between the Tintina Fault and the craton is an area characterized by a complex array of Paleozoic basins and arches, including the large Selwyn Basin, in which deep-water (basinal) facies are widespread. Cecile et al. (1997) suggested that this part of the continental margin may have been the lower-plate margin of a continental separation above a west-dipping detachment fault (Fig. 8). The “pericratonic” character of the western part of the Paleozoic margin in this area may be attributed to the crustal attenuation and block faulting characteristic of a lower-plate margin. As noted previously, south of the 60th parallel the structural geology of the continental margin is consistent with that of an upper plate margin, and Cecile et al. (1997) suggested that the Liard Line may have functioned as a transfer fault, across which the detachment fault reversed dip direction. A structural embayment in the northeast corner of the Selwyn Basin shows evidence of at least two episodes of rifting, one of Lower to Middle Cambrian to age and the other of Late Early Ordovician to Middle Ordovician age, indicating repeated episodes of crustal extension, similar to that which characterized the Grand Banks off Newfoundland during the Jurassic to Early Cenozoic (Cecile et al., 1997). North of the Cassiar Platform is the north-south–oriented Richardson Trough, a very long-lived crustal depression that functioned continuously as a deep-water environment for most of the early and mid-Paleozoic, from at least the Middle Cambrian to the Middle Devonian. This trough is located close to the pole of rotation around which the Canada Basin opened in the Cretaceous (Fig. 22 in Chapter 1). Prior to that event, Richardson Trough was aligned with the Hazen Trough of the Canadian Arctic Islands, a similarly long-lived deep-water basin that was succeeded in the late Paleozoic by the Sverdrup Basin (Chapter 14). It seems likely that Richardson Trough–Hazen Trough represents a very ancient lineament in the Laurentian crust that was established by rifting in the late Precambrian or early Paleozoic. A western extension of the Laurentian continent is the Yukon Platform, which is located to the west of Richardson Trough, and extends a short distance across the Alaska border, where it is truncated by faults associated with Cordilleran terrane accretion.



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THE SAUK SEQUENCE AND THE CAMBRIAN-ORDOVICIAN SHELF-TO-BASIN TRANSITION Regional isopach maps (Cook and Bally, 1975) reveal a relatively continuous blanket of Middle and Upper Cambrian strata (Sauk II-III sequence) extending along the miogeoclinal margin, from Sonora, Mexico, through Nevada and British Columbia to Yukon. As discussed in following paragraphs, these strata onlap extensively onto the miogeocline and craton. They are truncated eastward by a sub-Early Devonian (sub-Kaskaskia) unconformity.

The Kicking Horse Rim and Burgess Shale of The Southern Canadian Rocky Mountains A transect across the Main Ranges of the Rocky Mountains on either side of the Trans-Canada Highway, just west of the Alberta-British Columbia border, provides the best exposed and most well-documented cross-section through the ancient Paleozoic continental margin. Geological reconnaissance through this region was facilitated by the construction of the Canadian Pacific Railway, which was completed in 1885. In 1909, Charles Walcott, carrying out geological reconnaissance on horseback in the mountains above the railway, made the first discovery of the exotic, prolific fauna in what came to be known as the Burgess Shale, of Middle Cambrian age. His repeated visits to this area resulted in a very large collection being assembled at his home base, the Smithsonian Institution in Washington, DC, and the beginning of the formal description of the Paleozoic sedimentary rocks of the area (Walcott, 1928). Detailed reexamination of this area during the systematic mapping of the Rocky Mountains by the Geological Survey of Canada situated these fossils and their host strata in a modern stratigraphic and paleogeographic context (Fritz, 1971; Aitken, 1971). Researchers from the Royal Ontario Museum (Toronto), and elsewhere, have continued detailed studies of the Burgess Shale fauna. Aitken (1971) synthesized the stratigraphy of the Main Ranges of the Rocky Mountains just west of the AlbertaBritish Columbia border, an area lying mostly within Yoho National Park. He demonstrated that across a belt about 15 km wide, oriented northwest-southeast, Cambrian and Ordovician strata undergo a major facies change from cratonic, shelf facies, consisting predominantly of carbonates and mature clastics in the east, to a mainly deep-water clastic succession to the west (Fig. 10). The belt within which the facies change takes place appears to have remained more or less stable in position from Middle Cambrian to Late Ordovician time. As noted previously, Aitken (1971) termed this belt of facies change the Kicking Horse Rim (Fig. 1). The entire Cambrian-Ordovician section totals about 3400 m in this area (Aitken, 1993a, b). Further, the stratigraphy of the Cambrian section suggests the former existence of three major paleogeographic belts: (1) An inner detrital facies, deposited toward the center of the craton and characterized by shallow-water sandstone, siltstone, and shale. Sandstone increases in importance toward the shoreline on the inner margin of the belt; glauconite is a common accessory. (2) A middle, carbonate shoal facies. Clastic units are rare, but the carbonates may contain grains of clay and quartz sand and silt. (3) An outer detrital belt of thin-bedded sandstone, mudstone, and carbonate. The existence of these three broad facies belts was first suggested from study of the Cambrian section in the Great Basin of Nevada (Robison, 1960; Palmer, 1960), and found wide application in the study of the Lower Paleozoic rocks of the Canadian Rocky Mountains. Fig. 11 illustrates one version of this model as applied to the Middle Cambrian strata of the Rocky Mountains. The inner detrital belt, subdivided into an inshore basin and a shoreline area, corresponds to the interior of the craton, beyond the scope of this chapter. The Kicking Horse Rim corresponds to the carbonate shoal area, and the open basin of Fig. 11 corresponds to the outer detrital belt, an area that had also been termed the Robson Trough in earlier studies. Although this latter term no longer seems appropriate, with the recognition that the deep-water area is an ancient continental margin, open to the Panthalassa Ocean to the west, the terms Robson Basin and Columbia Basin have been used for the early Paleozoic areas of deep-water sedimentation in British Columbia that are immediately adjacent to the Laurentian margin (Fig. 1). Aitken (1971) demonstrated that in places the facies change from the carbonate to the outer detrital belt is a gradual one, as if down a ramp; in places it is marked by major slumps and slides; and in the vicinity of the famous Burgess Shale fossil localities, the facies change takes place across a distinct fossil escarpment about 200 m high (Fig. 12). The intense interest in the Burgess Shale fauna has led to detailed mapping and repeated reexaminations of this area by numerous geologists (e.g., McIlreath, 1977; Aitken and McIlreath, 1982; Fletcher and Collins, 1998). This work has confirmed that the escarpment was a contemporary feature of the submarine landscape during the Middle Cambrian. Careful biostratigraphic study (primarily of the trilobites), commencing with Fritz (1971), demonstrated that the rocks

FIG. 10  Changes in thickness and facies of the Cambrian-Ordovician section of Yoho National Park, Canada, as summarized by Aitken (1971).

FIG. 11  Paleogeographic model for the western craton margin during Middle Cambrian time, in the area of the Rocky Mountains of Yoho Park, Canada, showing the major facies belts. Grand Cycles are discussed in the text (Aitken, 1989). (Modified from Aitken (1978).)

unconformity

Polypleuraspis Subzone

NARAO MEMBER

Odaray shale member

Emerald lake oncolite member

N Bathyuriscus - Elrathina Zone

F

O

R

M

A

Stephen formation

Paradox limestone member

Waputik member

Cathedral rim

T

I

O

Marpole limestone member (4.5 subcycles)

? Pagetia walcotti Subzone

Tonkinelle Subzone

Eldon limestone formation

?

Pagetia walcotti Subzone

Tonkinelle Subzone

The Paleozoic Western Craton Margin Chapter | 5  251



“Phyllopo bed”

S S

Campsite cliff shale member

B

?

Polypleuraspis Subzone

Glossopleure Zone

?

Yoho river limestone member

U

R

G

E

S

Pagetia bootes Subzone

Wash limestone member

Glossopleura Zone

Cathedral limestone formation

Walcott quarry shale member

H

A

L

E

Raymond quarry shale member

Kicking horse shale member

Takakkaw tongue deep-water slope limestone

Core Facies

Basin/platform

dolomite with cryptalgal bioherms

peritidal limestone

FIG. 12  The relationship between the Burgess Shale and the Cathedral Limestone in the vicinity of the original “Walcott Quarry,” on the flank of Mount Field, immediately to the north of the Trans-Canada Highway, near Field, Yoho National Park, British Columbia (Fletcher and Collins, 1998).

beneath the Burgess Shale, within the deep basin, the Takakkaw Limestone Tongue, are the same age as the rocks of the Cathedral Rim, at the top of the Cathedral Limestone Formation. The Burgess Shale therefore consists of somewhat younger Middle Cambrian sediment, derived by transport of fine-grained clastic detritus across the edge of the escarpment and then banked up against, and eventually burying, the scarp face. There is very little evidence of direct derivation of sediment from the scarp itself. The Wash Limestone Member contains thin carbonate debris flow beds that probably represent a local collapse of the scarp face. The rocks at this margin consist of reef-flat fenestral, stromatolitic, and thrombolitic carbonates with oolites and grainstones, but they are pervasively dolomitized and have lost much of their primary fabric. At the time the Burgess Shale accumulated, the Cathedral Rim was probably not an actively growing reef construction (Aitken, 1989). The most popular model for the deposition of the Burgess Shale is that it represents a very quiet-water environment in the lee of the scarp face. This would account for the preservation of the unusual and abundant soft-bodied fauna. There is no evidence for deposition by turbidity currents, in the form of graded bedding or sole markings. However, there is also virtually no evidence of bioturbation of the soft sediments, even within the beds where the prolific fauna has been studied in detail. This is one of the lines of evidence that led Gostlin and Miall (2005) to dispute the conventional interpretation, that the organisms lived in the deep basin, where their remains are now found. Instead, it was suggested that the organisms lived in the shallow waters above and behind the Cathedral Rim and were swept over the scarp edge by occasional storms. They are therefore, for the most part, not preserved in their original living positions. However, this interpretation has not met with widespread acceptance.

252  The Sedimentary Basins of the United States and Canada

Beyond (west of) the classic Burgess Shale locations, in the Columbia Basin, outcrops of the basinal facies, The Chancellor Group, are sparse. Aitken (1993a, p. 111) reported that the group “consists of shale, laminated siltstone, ribbonbedded, more or less argillaceous and silty lime mudstone, and limestone-shale couplets. Debris flow breccias and large olistoliths are present at several levels.” The group totals about 1600 m in thickness.

Northern British Columbia To the northwest of the classic Yoho Park locations discussed already, in the Robson Basin of northern British Columbia, a conformable succession of upper Precambrian to Lower Cambrian quartzites is present. These have been assigned to the Hamill Group in southeastern British Columbia, and to the Gog Group, in areas along strike to the northwest. The latter is up to 2200 m thick. The Middle and Upper Cambrian section in this area is similar to that comprising the middle carbonate shoal facies of areas further south. Mixed shallow-water carbonate and fine clastic units predominate. The total Cambrian-Early Ordovician section in the Robson Basin reaches 6300 m in thickness (Fritz et al., 1992). A description of the passage from miogeoclinal to deep-water facies is not available for this area.

Yukon and Northwest Territories Strata of earliest Early Cambrian age are not present in Richardson Trough and Yukon Platform (Fritz et  al., 1992). Sedimentation in Richardson Trough commenced with the Illtyd Formation, a pure carbonate more than 600 m thick, succeeded by the Slats Creek Formation, a marine to nonmarine clastic unit, up to 1500 m thick, containing volcanics. Extensional faulting appears to have established the identity of the trough at this time. The Slats Creek is followed by the Road River Formation, a unit up to 3 km thick that appears to represent virtually continuous, deep-water sedimentation in local basins of a predominantly muddy facies for tens of millions of years, until the mid-Devonian. Carbonate deposition took place on adjacent platform areas. On Cassiar Platform, carbonate sediments with archaeocyathid bioherms of Lower Cambrian age are present. The Middle Cambrian is absent and strata of Late Cambrian age are poorly known.

Great Basin: Nevada, Utah, Idaho A terrigenous detrital sequence spanning the Precambrian-Cambrian boundary is widespread in the Great Basin (Stewart and Suczek, 1977; Poole et al., 1992). A quartzite unit occurs at the base, consisting of cross-bedded sandstone, commonly conglomeratic, interstratified with siltstone and argillite. Units of limestone and dolostone are also present, locally reaching 600 m in thickness. The clastic deposits contain algal remains and (in southern Nevada and eastern California) archaeocyathids, and are characterized by shallow-water sedimentary structures, including bimodally oriented crossbedding, flaser, and lenticular bedding, and are interpreted as tidal in origin. West of the continental margin, in Nevada, the sequence is locally more than 6 km in thickness. It thins to the east, and a 300-m isopach was used by Stewart and Suczek (1977) to differentiate a “cratonal” facies. Middle and Upper Cambrian strata constitute a “carbonate sequence” in the Great Basin. This is a westerly thickening succession, ranging from less than 200 m within the miogeocline, to as much as 1500 m in western Nevada. The facies belts erected by Palmer (1960) and Robison (1960) for this area can be traced south from Canada into the western United States and northern Mexico, although the inner detrital belt is only present well within the miogeocline, in Montana and Arizona (Stewart and Suczek, 1977). The middle carbonate belt consists of a variety of shallow-water limestones, containing stromatolitic boundstones, oolites, oncolitic and pellet grainstones, and wackestones. The outer detrital belt shows a westerly increase in shales, graded bedded grainstones and wackestones, slump structures and intraformational conglomerates, and the local presence of chert. This indicates a gradation westward into basinal slope facies.

MIDDLE ORDOVICIAN-EARLY DEVONIAN (TIPPECANOE SEQUENCE) Strata of Middle Ordovician to Early Devonian age (Tippecanoe Sequence) are rather more patchily distributed than those of the underlying Sauk Sequence, owing to epeirogenic warping (Fig. 13). The West Alberta Ridge or Arch was active during the mid-Paleozoic, from the Middle Ordovician to the Early Devonian. Cecile and Norford (1993) suggested that the Peace-Athabasca Arch (shown with its alternative name, Peace River Arch, in Fig. 13) was also active as a low-relief sediment source. Rocks of this age are absent over most of western Alberta and adjacent areas of British Columbia. The two



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FIG. 13  Isopach of the Tippecanoe Sequence (Middle Ordovician to Early Devonian). (Adapted from Cook and Bally (1975), Sloss (1988), and Aitken (1993a). The extent of shale facies is from Miller et al. (1992).)

arches effectively separated the continental margin into two broad depositional areas: (1) northern Canada, from northern British Columbia to Yukon, and (2) the Great Basin and a miogeoclinal embayment to the Sweetgrass Arch of the cratonic interior. Aitken (1993b) attributed the West Alberta Arch and other mildly active positive features to changing patterns of intraplate stress acting across the craton in response to plate-tectonic forces acting at the continental margins. Cecile et al. (1997) interpreted this arch as an uplift associated with an upper-plate continental margin above an east-dipping detachment fault that developed during late Precambrian to early Paleozoic continental separation. Root (2001) interpreted the arch as the forebulge of a foreland basin that developed in response to crustal loading by terrane accretion to the west. This is

254  The Sedimentary Basins of the United States and Canada

discussed further, in following paragraphs. Sedimentation was presumably continuous along the deeper parts of the continental margin, between Idaho and British Columbia, but these rocks have not been preserved (or have yet to be identified) within this area of the Cordilleran orogen.

Northern Canada The Mackenzie Mountains area of the Northwest Territories, and Yukon Territory, consisted of a large number of platforms, basins, and embayments that underwent repeated epeirogenic movement during this period. Stratigraphic thicknesses are very variable; they exceed 900 m in the Liard Depression, which is described as a basin that underwent abnormally rapid subsidence during this period (Cecile and Norford 1993, p. 143). The Liard Depression, Richardson Trough, and the Selwyn Basin are characterized by shale, cherty shale, and dolomitic mudstone and siltstone. The eastern margin of this facies (Fig. 13) defines an approximate boundary to the deeper-water, basinal margin of the Laurentian continent. The broad extent of Tippecanoe deposits in Yukon and Northwest Territories, north of the Liard Line, is noteworthy. They extend eastward across the entire craton to the margin of the Precambrian Shield. This is part of the evidence suggested by Cecile et al. (1997) for their interpretation of this part of the continental margin as a lower-plate margin above a west-dipping detachment fault. Typical of the many platform carbonate units that have been mapped in this area is the Mount Kindle Formation of the Mackenzie Mountains. This unit, consisting of thick-bedded, fossiliferous dolostone, ranges between 100 and 400 m in thickness. Many other units, too numerous to describe in this brief chapter, have been erected for the localized and variable facies belts of the northwest. Cecile and Norford (1993) have described the Lower Paleozoic platform succession as consisting of a suite of three transgressive-regressive cycles. Cycle B, of which the Mount Kindle Formation constitutes the greater part, corresponds approximately to the Tippecanoe Sequence, although there appears to be no evidence of the end-Ordovician regional unconformity within this unit that elsewhere separates the Tippecanoe Sequence into two subsequences (Fig. 2).

Great Basin Cyclic sedimentation patterns and the presence of local widespread disconformities indicate a similar pattern of regional tectonic and global sea-level control in Nevada to that of Northern Canada. For example, Fig. 14 shows a cross-section through the Middle Ordovician section from Utah to Nevada. “The Whiterockian carbonate shelf of western Utah, Nevada, and southern California developed during a single offlap-onlap cycle, 12 m.y. in duration, while most of the North American continent was exposed to subaerial weathering” (Poole et al. 1992, p. 22). Graptolitic back shales of the Kanosh Shale were deposited in Utah, while the shallow-bank and tidal-flat carbonate deposits of the Antelope Valley Limestone were forming to the west. Banks of sponges and algae appear to have formed barriers, behind which the graptolitic muds were deposited. The Eureka Quartzite, which follows, consists of great thicknesses of relatively pure quartz sand thought to have been derived by erosion of preexisting quartz sandstone deposits within the cratonic interior. Upper Ordovician strata of the continental-slope and rise facies are preserved within the Roberts Mountain allochthon and are thought to have been thrust eastward from an originally off-shelf setting during the Antler Orogeny. They consist of strongly deformed shaly, siliceous rocks containing planktonic and nektonic fossils. A widespread dolostone facies up to 300 m thick (Red River, Bighorn formations) extends across the miogeocline to the east. Silurian and Lower Devonian strata are well represented in the Great Basin (Poole et al., 1977), thinning eastward onto the flanks of the Transcontinental Arch. On the miogeocline margin, Silurian rocks total as much as 600 m in thickness and consist mainly of shallow-water dolomites and sandy dolomites. A distinct facies change into slope deposits has been mapped near the east edge of the Roberts Mountains Allochthon. There, the shelf dolomites become cherty westward, and grade laterally into the laminated limestone and silty limestone of the Roberts Mountains Formation. Sparse fossils of a shelly fauna are present in the shelf dolomites. In the slope deposits fossils are more abundant and consist of both the shelly and graptolitic facies. Lower Devonian strata also contain a distinct thickness and facies change passing from the craton margin to the continental slope. Cratonic sedimentary rocks are predominantly near-shore and shallow subtidal to intertidal dolomites and sandy dolomites. In the southern part of the shelf a cherty, argillaceous dolomite occurs near the top of the Lower Devonian. Westward, these rocks grade into slope deposits of thick-bedded detrital limestone and thin-bedded sandy to argillaceous limestone with subordinate beds of mudstone and dolomite. The section thickens in the same direction, from 100 to 300 m on the shelf, to locally as much as 1000 m on the outer shelf and slope.



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FIG.  14  Middle Ordovician facies relations from the miogeocline of Utah to the continental margin of southwestern Nevada and adjacent areas of California (Poole et al., 1992).

GRAND CYCLES The Cambrian-Ordovician stratigraphy of Laurentia is strongly cyclic. The concept of the Grand Cycle was first proposed based on studies in Nevada (Robison, 1960; Palmer, 1960), but the ideas were more fully explored and developed in application to the Canadian portion of the continental margin, along the Kicking Horse Rim (Aitken, 1966, 1978), and have now been widely applied to cyclic Cambrian deposits throughout Laurentia (e.g., Cowan and James, 1993; see also Chapter 3). Eight Grand Cycles have been identified in the Cambrian to Middle Ordovician stratigraphy of the Canadian Rocky Mountains (Aitken, 1993a, p. 109). Five of these are shown in Fig. 15. Cycle 1, of Early Cambrian age, corresponds approximately to the Sauk-I sequence of Sloss (1988). The next four cycles, of Middle Cambrian and earliest Late Cambrian age, represent a higher-order cyclicity within the Sauk-II sequence. The top three cycles (Upper Cambrian–Middle Ordovician) overlap from the Sauk III into the basal Tippecanoe I Sequence. The boundaries of these two younger Sloss subsequences do not appear to correspond to the Grand Cycle boundaries, suggesting that eustatic sea-level change does not provide a complete explanation of the cycle driving processes, at least for the upper three cycles. Aitken (1978) identified two broad types of Grand Cycle: “Stephen-type cycles” were deposited when the inshore basin behind the Kicking Horse Rim was confined behind a narrow and discontinuous rim of intertidal to supratidal carbonate shoals, permitting easy tidal exchange between the inshore basin and the open ocean, and “low-energy” conditions in the inshore basin. “Sullivan-type cycles” were deposited when the inshore basin was confined behind a wide, practically unbreached carbonate-shoal complex. Limited tidal exchange across the shoal complex combined with favorable orientation and scale of the inshore basin resulted in locally high tidal range and consequent high tidal energy at the continental margin. The classic “Stephen-type” cycle commences with “flaggy” lime-mudstones, with mudstone partings of the basal Stephen Formation. Feeding burrows and trilobite fossils are abundant. There are minor internal shale-limestone cycles a few meters thick. Facies trends suggest a gradual shallowing of the depositional setting with time. In the middle of the cycle, the proportion of shale increases, and there are minor rippled laminae of quartz siltstone and calcisiltite. Brachiopod fragments are common. Interbedded units of thrombolites and stromatolites appear, together with lime-mud partings with ripples. The Stephen-Eldon contact is transitional, with shales becoming rarer, while limestones become predominant.

256  The Sedimentary Basins of the United States and Canada

FIG. 15  Schematic stratigraphic cross-section of the Cambrian rocks of the western continental margin, extending from Jasper National Park southeastward to Banff National Park, and then westward across the Kicking Horse Rim (Fritz et al., 1992). Six of the Grand Cycles of Aitken (1966, 1978) are highlighted, but the top of Cycle 5, which is of Upper Cambrian age (Lyell Formation), is not shown. (Reproduced with the permission of the Minister of Public Works and Government Services Canada, 2007 and Courtesy of Geological Survey of Canada.)

Higher in the Eldon Formation, a mottled facies with dolomitized burrows becomes characteristic. Occasional interbeds of pellet and intraclast grainstone occur. This type of cycle is interpreted as the product of marine transgression. Initially a peritidal carbonate shoal complex existed at the Kicking Horse Rim, but this was gradually drowned with increasing turbulence, permitting the transportation of detrital material westward into the deeper parts of the basin. Sea levels then gradually fell, again, permitting a reestablishment of the carbonate rim. The type example of the Sullivan-type cycle begins with an abrupt transition from the grainstones of the Waterfowl Formation into the shale of the Sullivan Formation. This lithology is somewhat calcareous and sparsely fossiliferous, with fragments of trilobites, brachiopods, and echinoderms. Interbedded with the shale are units of dolomitized skeletal grainstone and packstone, and cryptalgal limestone. There is an upward transition into the predominantly carbonate Lyell Formation, most of which consists of shoaling-upward carbonate subcycles constituting packages of carbonate conglomerate, cryptalgal laminate, and calicisiltite laminate. Aitken (1978) suggested that the Lyell Formation represents a carbonate shoal complex some 400 km across, with oolitic carbonate sands accumulating in tidal banks comparable to those developing at the present day in the Persian Gulf and the Bahamas. Both cycle types are interpreted as the product of cycles of eustatic sea-level rise and transgression, resulting in the migration or drowning of the carbonate shoal at the margin of the craton. However, as noted previously, detailed biostratigraphic correlation does not support a precise correspondence between the younger three cycles and the Sloss sequences, suggesting that in the southern Rocky Mountains accommodation changes may have also been influenced by other processes, such as epeirogenesis driven by dynamic topography.

LOWER TO UPPER DEVONIAN (KASKASKIA-I SEQUENCE) By Early Devonian time, a clear distinction can be made in the Great Basin between continental margin strata, deposited on the shelf or slope of Laurentia, and arc-related rocks of the approaching Antler arc (Poole et al., 1977). Fig. 16, which summarizes the thickness distribution and facies of mid-Devonian strata, is provided here as an indication of the general paleogeographic configurations during Kaskaskia-I sedimentation. On the western continental margin, the initial Kaskaskia transgression during the Early Devonian generated a cratonic seaway that extended all along the western margin of Laurentia (Cook and Bally, 1975). The West Alberta Arch was transgressed by Middle Devonian time, and ceased to be recognizable as a distinct cratonic arch. Meanwhile, however, the Peace River Arch became active. Some evidence for its presence as a modest sediment source exists in late Precambrian and early Paleozoic strata, and by the Mid-Devonian a distinctive basal clastic facies was being deposited in erosional hollows across the uplifted Arch (O’Connell, 1994). The Ancestral Uinta Uplift was also active as a clastic sediment source during the early-Late Devonian (Poole et al., 1977). This uplift and the Peace River Arch are transverse elements; that is, they are oriented at a high angle to the continental margin. More or less simultaneous movement on these two unusual tectonic elements may indicate a response to a temporary change in intracontinental intraplate stress patterns.



The Paleozoic Western Craton Margin Chapter | 5  257

FIG. 16  Schematic paleogeography and generalized facies trends, Middle Devonian of the western Laurentian margin, corresponding to the middle part of the Kaskaskia-I sequence. Position of the continental margin is generalized—in northern Canada areas of uplift and subsidence and of deep- and shallow-water sedimentation underwent regular change owing to active local tectonism. Isopachs are shown where data are consistent enough to indicate local trends. Broad facies characteristics are indicated by black and white ornamentation. Areas lacking ornamentation are areas characterized mainly by shallow-water carbonate sedimentation. (Data for the United States is from a map detailing the Frasnian and Lower Fammenian (lower Upper Devonian) paleogeography of the Great Basin, from Poole et al. (1992, Plate 3). Data for Canada is from maps showing thickness and facies trends for the midGivetian to Famennian, from Gordey et al. (1992). Locations of the two cross-sections of Fig. 17 are shown.)

258  The Sedimentary Basins of the United States and Canada

Slope and basin deposits are well represented in Yukon, where the Selwyn Basin encompassed a large area of thinned, subsided, and downfaulted continental crust. Across the Tintina fault lies the Cassiar Platform, a pericratonic terrane of originally Laurentian affinities but probably displaced northwards hundreds of kilometers from its original location. Along the margin between the Peace River Arch and the northern Great Basin, little evidence of slope and basin facies is preserved, this facies belt having been metamorphosed and upthrust, or eroded, during Cordilleran tectonism (this is discussed further in following text).

Northern Canada Kaskaskia strata have been subdivided by Fritz et al. (1992) and Gordey et al. (1992) into sequences, corresponding to allogroups, as defined by the North American Commission on Stratigraphic Nomenclature (1983). Although this provides for convenience in classification and description, the actual complexity of the stratigraphy in this large area suggests that the region was affected by many local episodes of tectonism that have obscured large-scale trends. A summary of the stratigraphy and broad facies relationships is given in Fig. 17. Miogeoclinal sedimentation on the Mackenzie Platform was characterized by a succession of largely carbonate units, consisting variously of dolomite, limestone, sandy limestone, siltstone, and shale. Reef barriers formed at several different times during the Devonian (the extensive petroleum-producing Devonian reefs of the Alberta Basin are beyond the scope of this chapter). Evaporites formed behind a shelf-margin barrier at several times (e.g., Bear Rock Formation, Elk Point Group). Dramatic facies changes into deeper water, predominantly clastic facies, have been mapped along the eastern

FIG.  17  Stratigraphic cross-sections through the Devonian system of western Canada. Gray shading indicates predominantly clastic sediments. Locations are shown in Fig. 16 (From Morrow and Geldsetzer, in Fritz et al., 1992). (Reproduced with the permission of the Minister of Public Works and Government Services Canada, 2007 and Courtesy of Geological Survey of Canada.)



The Paleozoic Western Craton Margin Chapter | 5  259

­ argin of Selwyn Basin. Morrow and Geldsetzer (in Fritz et al. 1992, p. 204) noted that Lower Devonian (Siegenian to m Emsian) strata consist of black graptolitic shale, crinoidal limestone, argillaceous limestone, dolomitic sandstone, and units of fine-grained quartzite up to 100 m thick. Carbonate and siliciclastic units were deposited as debris flows and turbiditycurrent deposits derived from the platform to the east. In Selwyn Basin and Richardson Trough, calcareous shale, limestone, and dark, siliceous shale of the road River Formation and other, similar units were deposited through much of the Early and Middle Devonian. Between the late Middle and the early Late Devonian, carbonate sedimentation largely ceased in northern Canada. Tectonism in northwest Yukon and adjacent areas of Alaska, including granitic intrusion and uplift, generated uplifted orogens, from which sediment was shed south and east across the entire Selwyn Basin and Mackenzie Platform area. The northern craton was also uplifted and underwent exposure and erosion at this time. The dominance of clastic sedimentation began in Yukon in the early Middle Devonian. In the Mackenzie Platform area, carbonates of the Hume Formation were succeeded in the mid-Devonian by the calcareous shale-limestone succession of the Hare Indian formation, then by black organic shale of the Canol Formation, and finally by the thick, fine-grained clastic succession of the Imperial Formation. This unit, which reaches thicknesses of more than 2 km at the northern margin of the Mackenzie Platform, consists of shale, siltstone, fine-grained sandstone, and minor limestone, derived from cratonic sources to the east (and possibly ultimately from Caledonian sources on the eastern margin of Laurentia—see Chapter 19). It was deposited in shallow-marine nearshore to offshore environments. To the west and north, in the area of Richardson Trough, the Imperial formation consists of deeper-water deposits, predominantly turbidite sandstone, derived from orogenic sources in NW Yukon and Alaska. By the early Late Devonian, clastic sedimentation had spread to southern Alberta, where it constitutes the Ireton Shale, an important organic source rock and reservoir cap rock (Gordey, in Gordey et al., 1992). A succession of Upper Devonian to mid-Mississippian rocks more than 700 m thick is present on the Cassiar Platform (Gordey, in Gordey et al., 1992). These consist mainly of siliceous mudstone and fine-grained siltstone. Debris-flow conglomerates are also present, containing abundant clasts of silicified and sandy carbonates thought to have been derived from the nearby miogeocline. The facies suggest a rapid subsidence of the platform, and the indication of a nearby miogeoclinal clastic source suggests that the platform at this time was functioning as a rift basin adjacent to the continental margin.

Peace River Arch This craton-margin tectonic element has been characterized by anomalous tectonic episodes relative to the adjacent craton for much of the Phanerozoic (O’Connell, 1994). It became paleogeographically significant during the Middle and Late Devonian, when it was uplifted and subject to significant erosion. A clastic unit, known as the Granite Wash, blanketed the uplift and interfingered with marine carbonate, shale, and evaporite units to the north, east, and south. The Granite Wash is up to 100 m thick over the crest of the arch and is composed of material derived from erosion of the uplift, predominantly Precambrian granitic and metasedimentary rocks, indicating deep erosion of the earlier Paleozoic cover. The deposit fills fault-bounded rift basins across the arch. At the margins of the Arch it interfingers with estuarine and fluvial sand bodies and the Arch is encircled by a series of fringing and patch reefs spanning the Middle to early Upper Devonian. Together with the Tathlina Uplift and the West Alberta Arch, these areas of craton-margin uplift served as barriers to marine circulation across the continental interior during the Middle Devonian, leading to restricted-marine environments and the deposition of the Elk Point and other widespread evaporite deposits across eastern Alberta and Saskatchewan. Early in the Late Devonian the Peace River Arch became passive and was onlapped by the marine rocks of the miogeocline.

Ancestral Uinta Uplift This is another unusual east-west oriented tectonic element, in that it shed synorogenic sandstone and conglomerate during the early part of the Late Devonian from a local uplift on the inner shelf into what was otherwise a broad miogeoclinal sea characterized by deposition of carbonates, now mostly dolostones (Poole et al., 1992). Transport directions recorded in the Stansbury Formation are predominantly eastward.

Great Basin As noted previously, most of the miogeocline margin of the Great Basin area was undergoing shelf carbonate sedimentation during the mid- to Late Devonian (Fig. 16), although the distribution of rocks of this age is now limited, owing to deep pre-Mississippian erosion (Poole et al., 1992).

260  The Sedimentary Basins of the United States and Canada

The Pilot Shale (late Upper Devonian) of eastern Nevada is interpreted as a “protoflysch” reflecting the early influence of the approaching Antler orogen (Poole et al., 1977). It consists of siltstone, carbonaceous siltstone, and mudstone with interbeds of turbidite and debris flow deposits. Sedimentation of passive-margin type on the ancient continental margin in the Great basin area was brought to a close by the Antler Orogeny in latest Devonian or earliest Mississippian time. This and other arc-related tectonic episodes and related sedimentary units are discussed by Ingersoll (Chapter 11).

DEVONIAN-MISSISSIPPIAN ARC COLLISIONS AND TERMINATION OF PARTS OF THE “PASSIVE” LAURENTIAN MARGIN (KASKASKIA-II SEQUENCE) The Antler Orogeny emplaced the Roberts Mountains allochthon on the continental margin, above a thrust belt extending from southeastern California to central Idaho, in the latest Devonian or Early Mississippian, permanently changing the tectonic character of the western continental margin of that area (Chapter 11). Within Canada, the change to a convergent margin may have taken place much later, although the evidence is unclear and controversial. At least part of the margin, that part lying within eastern British Columbia, may have remained a “passive” margin until the Triassic or Early Jurassic. As discussed shortly, there is some evidence from the northwestern United States and southern British Columbia that stratigraphic units deposited within the Late Devonian-Early Mississippian time span are comparable in sedimentary and tectonic setting to the Antler “flysch” of Nevada. Further to the north, the Kootenay Terrane of west-central Yukon and adjacent areas of Alaska has been interpreted as a west-facing arc of Upper Devonian and Early Carboniferous age (Richards, 1989). The name Prophet Trough has been assigned to the belt of downfaulted continental margin rocks of latest Devonian and Carboniferous age extending from southern Yukon to northern Idaho, and possibly linking up with the Antler foreland basin (Richards, 1989; Gordey et  al., 1992). Rift faulting in these rocks could be extensional faults related to passivemargin or backarc rifting, or it could indicate crustal down-warping as a result of contractional loading by the approaching Kootenay arc. Within this model, the pericratonic Cassiar Platform would be interpreted as a retroarc foreland basin or a backarc basin, depending on whether the arc was contractional or extensional. Gabrielse and Yorath (1992, p. 691) noted that no contractional structures comparable to those associated with the Roberts Mountains allochthon have been identified in northern Canada, but that “it is conceivable that the rifting, volcanism, uplift and sedimentation in the northern Cordillera was linked to tectonism to the west in the Kootenay terrane.” Rocks of Mississippian to Early Pennsylvanian are well represented along the western margin. Fig.  18 provides an isopach map for the Lower Mississippian. Contours cannot be drawn for some areas of occurrence, such as the Prophet Trough, because of structural complexities.

Great Basin A foredeep developed in front of the Antler orogen on the outer continental shelf. Fig. 18 shows the maximum areal extent of this foreland basin, which was reached during the Late Mississippian (Miller et al., 1992, Plate 4C). Shallow-water carbonates interfingered with siliciclastic deposits derived from the emerging Roberts Mountains Allochthon. Deep-water slope and submarine-fan deposits prograded cratonward (eastward) during the Early Mississippian, and in the Late Mississippian graded up into deltaic deposits as the foredeep filled with sediment and became shallower. By this time, it appears that contractional tectonism of the Antler Orogeny had ceased (Miller et al., 1992). On the craton, shallow-water limestone and sandstone accumulated, with a depocenter in the area of the present Uinta uplift. A regression commenced in the mid-­Mississippian, terminating the Kaskaskia sequence, and much of the North American craton became exposed as a vast karst plain.

Western Canada Carbonates and shales of late Devonian and Mississippian age, including such units as the Palliser and Rundle formations, constitute some of the most spectacular mountain ranges of the Rocky Mountains in Banff and Jasper national parks. These are almost entirely cratonic in origin. In the western main ranges, close to the Alberta–British Columbia border, the basal Banff Formation, of early Mississippian age, grades westward and stratigraphically downward into the Besa River Formation, of latest Devonian and earliest Mississippian age (Gordey et al., 1992). The Banff Formation is a limestone, dolomite, and silty carbonate unit of shelf-margin and slope origin, and the Besa River Formation consists of basinal shale and dolomitic shale. A similar transition has been identified at several places along the eastern margin of the Prophet Trough, for example near the 60th parallel and in the Eagle Plain. These shelf-slope-basin assemblages are almost everywhere progradational, with shallower water facies gradually advancing basinward (westward).



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FIG. 18  Distribution of strata of “pre-Chesterian” strata on the western continental margin, showing major contemporaneous paleogeographic elements. The time interval used in this map corresponds to the lower half of the Mississippian (Tournaisian and lower Visean). (Isopachs from Cook and Bally (1975); additional Canadian data from Gabrielse and Yorath (1992); US data from Miller et al. (1992).)

In the Mississippian, the Peace River Arch underwent collapse and became a basin, termed the Peace River Embayment (Fig. 18). From here northwards the Rundle Group, of late early to mid-Mississippian age, shows a westward transition from a shelf and slope carbonate facies into a 200 m thick unit of spicule-rich carbonate grainstones, called the Prophet Formation, and thence into the basal Besa River shales. Large-scale submarine erosional channels are common in these slope deposits. This is overlain by the thick deltaic sandstone-shale succession of the Stoddard Group and Mattson Formation, which filled the Peace River Embayment and the Liard Basin.

262  The Sedimentary Basins of the United States and Canada

The Cassiar terrane contains a succession about 400 m thick of Mississippian deposits, consisting of a lower sandstoneshale succession and an upper argillaceous, thick-bedded limestone. As noted previously, the evidence for arc- or terrane-related tectonism and sedimentation off the Canadian portion of the western Laurentian margin during the Late Devonian–Mississippian is controversial. There is no conclusive evidence in Yukon for the approach and collision of the Kootenay arc with Laurentia, although this has been postulated by several authors (e.g., Ricketts, 1989). The classic characteristics of a foreland basin—a detrital sedimentary wedge thickening away from the craton and showing evidence of derivation from the west, were not claimed for Prophet Trough in this earlier study (in contrast to the very clear evidence for this tectonic setting for the Antler foreland basin of Nevada: see Chapter 11). However, an alternative model for this basin as a backarc basin behind an extensional or neutral arc (in the terminology of Dewey, 1980) would appear to be consistent with the evidence. Several studies have pointed to analogies between deformed sedimentary units in British Columbia and the northwest United States to the Antler “flysch” of Nevada, including paleotransport directions and petrology. Recent studies of detrital zircon are providing invaluable evidence for provenance, thereby helping to clarify contemporaneous basin settings and paleogeography; but deformation and overprinting by later orogenic activity has complicated attempts at paleotectonic reconstruction. The Earn Group of the Selwyn Basin Yukon (Figs. 16 and 17) consists of chert-rich sandstone and conglomerate, alkalic volcanic rocks, chert, shale, and stratiform barite. Coarse clastic strata have been interpreted, on the basis of paleocurrent indicators, to have been derived from an unknown western landmass (Gordey et al., 1987; Smith et al., 1993). Detrital zircon and Hf isotope data from Devonian-Mississippian strata of east-central Idaho suggest a western source from an early Paleozoic arc built on Proterozoic crust, with potential provenance regions in outboard basement complexes of the Eastern Klamath, Northern Sierra, and Quesnellia terranes (Beranek et al., 2016). In an earlier reconstruction, based on an extensive review of the sedimentary and structural evidence from western Canada, Root (2001) suggested that the Columbia Basin/Prophet Trough of eastern British Columbia and the adjacent West Alberta Ridge (Fig. 13) constituted a foreland basin and forebulge, respectively, generated by collision of Laurentia with an outboard arc during the Mid- to Late Devonian and Mississippian (see Fig. 13 of Chapter 1, which shows an arc contiguous with the “Antler” arc approaching western Canada at this time). The Kaskaskia sequence was terminated by a widespread regression and deep erosion over much of the craton (Henderson, 1989).

PENNSYLVANIAN-PERMIAN (ABSAROKA I AND II SEQUENCES) “Antler overlap basins” developed during the Pennsylvanian and Permian of Nevada (Miller et al., 1992). Little evidence of an ancient continental margin is preserved along the western edge of the craton, from northern Nevada through Idaho. The Oquirrh Basin of northwest Utah contains more than 6 km of deep-to-shallow water carbonates and clastic of Pennsylvanian and Permian age (Rich, 1977; Burchfiel et al., 1992), but the basin appears to be the result of craton-margin subsidence, rather than an embayment in the continental margin. Development of the Oquirrh Basin may be related to intracratonic tectonism caused by the late Paleozoic collision of Gondwana and Laurentia (see Chapters 7 and 8). In Canada, strata of this age are best represented in thrust slices along the Rocky Mountains of British Columbia, and in the subsurface in the northeast part of this province and adjacent areas of west-central Alberta. Tectonic elements that were established during the Mississippian continued with little change through the Pennsylvanian and Permian. A basin extended from northern Yukon to northern British Columbia in much the same position as the earlier Prophet Trough but was named the Ishbel Trough by Henderson (1989). Peace River Embayment persisted as an area of subsidence and clastic sedimentation through the Pennsylvanian. Sediments of Pennsylvanian and Permian age are predominantly siliciclastic, in contrast to the thick and widespread carbonates of Mississippian and older strata. This is probably in part a reflection of the northward drift of Laurentia toward higher latitudes that were less favorable for carbonate biogenesis. Henderson (1989) noted that Permian brachiopod faunas have boreal affinities and are interpreted as temperate in origin. Absaroka-I and Absaroka-II have been divided into five sequences in Canada, separated by regional unconformities. These mostly consist of mixed clastic‑carbonate successions on the shelf but show westward facies transitions into slope facies composed of siltstone, dolomitic siltstone, and shale (e.g., Johnston Canyon and Kindle formations, of Permian age). However, as is the case with the underlying Kaskaskia Sequence, there is no clear evidence of a continental margin, in the sense of deep-water sediments resting on thinned continental crust facing an open ocean. Any remnants of such a margin have been destroyed by Cordilleran orogeny, including thrust faulting and uplift and, during the Cenozoic, major right-lateral strike-slip faulting. A tectonic interpretation of the British Columbia continental margin is discussed in following text.



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TRIASSIC-JURASSIC: TERMINATION OF THE “PASSIVE” CONTINENTAL MARGIN Triassic rocks (Absaroka-III sequence) are thick and well exposed in the thrust belts of the Rocky Mountains, from the US border northwest to the 60th parallel. They thicken westward from an erosional edge in west-central Alberta to a maximum of more than 1200 m in the foothills belt of northeast British Columbia (Gibson, in Gordey et al., 1992). The isopach pattern, paralleling the ancient continental margin, suggests that the thickening is the result of subsidence of the continental margin, but there is no evidence of any distinctively deep-water facies, and as is the case with the underlying Devonian to Permian succession, it would appear that the true continental margin, that is, a deep-water basin developed over thinned continental crust, is now part of the deformed Omineca belt, to the west. In Banff National Park, Triassic rocks comprise the Spray River Group, a mixed succession of fine-grained clastics and sandy and silty carbonates. Some of the clastics are interpreted as thin-bedded turbidites and would appear to constitute the last major suite of siliciclastic sedimentary rocks derived by erosion of the cratonic interior prior to the emergence of the Cordilleran orogen and the reversal of transport directions into the newly formed Western Interior Seaway. Models of Cordilleran orogeny for southern British Columbia suggest that the Middle Cambrian to Permian continental margin of this area had, by late Paleozoic time, evolved into the continental flank of a backarc basin (Fig. 19). During the Jurassic, the west-facing Nicola arc converged against the continental margin, resulting in obduction of continental flakes, delamination of continental crust from its basement roots, and the development of tectonic wedges that thrust eastward. The ancient miogeocline underwent regional metamorphism and deformation, generating what is now the Omineca Belt, comprising an imbricated succession of folded thrust sheets (Gabrielse and Yorath, 1992; Price and Monger, 2003). The first phase of uplift of what became the Omineca belt in the Late Jurassic generated the source for the first westerly derived detritus to be shed from the newly created Cordilleran orogen, and this marked the initiation of the Western Interior Seaway (see Chapter 9).

FIG. 19  Tectonic model for the evolution of the continental margin of southern British Columbia during the Jurassic. Location of cross-section is shown in Fig. 1. Diagram A shows the margin during the Early Jurassic. The miogeocline, as shown in this panel, represents the continental-margin sedimentary rocks of Cambrian to Triassic age that are described in this chapter. Through Triassic and Early Jurassic time the Nicola arc is interpreted to have been an extensional arc, with a broad back-arc basin (Slide Mountain terrane) situated to the east, with the miogeocline functioning as its continental margin. Diagram B (late Early Jurassic) and Diagram C (early Middle Jurassic) show the beginning of the deformation of the arc and its backarc, with delamination of backarc terranes from their roots and the development of a tectonic wedge of arc rocks being thrust eastward. During the Late Cretaceous and early Cenozoic, this tectonic wedge developed into a major anticlinorium, the Selkirk Fan, in the Omineca Belt (Price and Monger, 2003).

264  The Sedimentary Basins of the United States and Canada

The evolution of the continental margin southward through Idaho and into northern Nevada may have followed a similar pattern to that in southern British Columbia, but the evidence is very sparse, owing to intense deformation during the Mesozoic (Burchfiel et al., 1992). In northern British Columbia the continental margin may have been bordered by an approaching east-facing arc by Early Jurassic time, in front of which developed the Whitehorse trough, a forearc basin. This is discussed further by Ricketts (Chapter 10).

CONCLUSIONS Within Canada, the extensional, or “passive,” western continental margin of Laurentia lasted from the late Precambrian, between about 600 and 550 Ma, when oceanic crust is presumed to have appeared separating Laurentia from Rodinia, until the mid-Jurassic, at about 170 Ma. The Antler Orogeny terminated this phase of crustal development considerably earlier in the Great Basin, during the latest Devonian or Early Mississippian (Chapter 11). The duration of the Laurentian margin in Canada is therefore between 380 and 430 million years, which is twice the current age of the Atlantic margin of North America. It is also considerably greater than the duration of the passive-margin phase along any of the other borders of North America. The Arctic margin of the Franklinian Basin was terminated in northern Ellesmere Island by the Early Silurian (Trettin, 1991). The southern Ouachita margin was closed by collision beginning in the mid- to Late Mississippian (Chapter  8); that along the Atlantic margin had an even shorter duration, lasting only until the Middle Ordovician (Chapter 3). Owing to the accretionary tectonism of the Cordilleran orogeny, the original thinned crust of the western Laurentian margin is nowhere exposed. The position of the outer edge of the craton can be established for some periods; for example, the Kicking Horse Rim of British Columbia clearly locates the transition from craton to continental slope during much of the Cambrian and Ordovician. However, the deep continental margin basin and the thinned continental-to-transitional crust that would be expected to be located outboard of the Columbia and Robson basins, the Selwyn Basin, and the Prophet Trough have long since been deformed, metamorphosed, displaced laterally by transcurrent faulting, and buried beneath the terranes accreted since the Jurassic. Interpretation of Lithoprobe data (Cook et al., 1995, 2012) suggests that in southern British Columbia thinned continental crust extends beneath the accreted terranes of the Omineca and Intermontane belts for some 500 km west of the Kicking Horse Rim.

Acknowledgments The author is very grateful to Barny Poole, Elizabeth Miller, and David Morrow for their many valuable comments and suggestions.

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