Sequence stratigraphy and evolution of the Paleoproterozoic intracontinental Baker Lake and Thelon basins, western Churchill Province, Nunavut, Canada

Precambrian Research 125 (2003) 21–53

Sequence stratigraphy and evolution of the Paleoproterozoic intracontinental Baker Lake and Thelon basins, western Churchill Province, Nunavut, Canada夽 R.H. Rainbird a,∗ , T. Hadlari b , L.B. Aspler c , J.A. Donaldson b , A.N. LeCheminant d , T.D. Peterson a b

a Geological Survey of Canada, 601 Booth Street, Ottawa, Ont., Canada K1A 0E8 Department of Earth Sciences, Carleton University, Colonel By Drive, Ottawa, Ont., Canada K1S 5B6 c 23 Newton Street, Ottawa, Ont., Canada K1S 2S6 d 5592 Van Vliet Road, Manotick, Ont., Canada K4M 1J4

Received 24 May 2002; accepted 12 February 2003

Abstract The Baker Lake and Thelon basins lie along the border between the Rae and Hearne domains of the western Churchill Province. Basin fill comprises the Dubawnt Supergroup, a ca. 1.85–1.70 Ga succession of predominantly continental clastic and intercalated volcanic rocks that nonconformably and unconformably overlies granitic and supracrustal rocks, most of late Archean age. The Dubawnt Supergroup comprises three second-order sedimentary sequences that record deposition within a rift basin, a modified rift basin and a thermal sag basin, respectively. The Baker sequence is inferred to represent the principal phase in the development of Baker Lake Basin, a series of generally elongate, northeast-striking, half-graben and fault-bounded troughs filled with continental redbeds and coeval voluminous ultrapotassic volcanic rocks. Faults parallel to the basin margin exhibit evidence for strike-slip ca. 1.83–1.81 Ga. The nature and timing of basin development and associated magmatism allows that it may be a transtensional basin that formed by lateral escape in response to crustal thickening during the Trans-Hudson orogeny, similar to the formation of Mesozoic and Cenozoic strike-slip and associated rift basins on the margins of the Tibetan Plateau. The overlying Whart sequence was deposited in small basins formed by block-faulting and tilting of the Baker sequence. Basin fill comprises eolian and alluvial redbeds with intercalated rhyolite flows and epiclastic rocks. Distribution of facies and stratigraphy suggest that faulting occurred throughout deposition of the Whart sequence, but ceased before deposition of the overlying Barrens sequence (ca. 1.72 Ga). Deformation and basin formation can be linked to a regional episode of granite emplacement and associated thermal metamorphism (ca 1.76–1.75 Ga). The Barrens sequence represents deposition over a broader area, primarily in Thelon Basin. Strata generally are thinner and flatter than underlying sequences, and display lateral continuity, indicating minimal influence from syndepositional faulting. The depositional record reflects progressive upward fining and eventually the first record of marine transgression within the Dubawnt Supergroup. These features suggest that the Barrens sequence was deposited over a broad region of thermal subsidence, likely related to cooling of previously attenuated continental lithosphere. The Barrens sequence may be a remnant of a huge cratonic sand sheet that included the Thelon, Athabasca, Amundsen and Elu basins. A regional subsidence mechanism to account for these basins may be mantle downwelling linked to the late-stage amalgamation of Laurentia. 夽 Contribution

to the Geological Survey of Canada, western Churchill NATMAP. Corresponding author. Tel.: +1-613-943-2212; fax: +1-613-943-5318. E-mail address: [email protected] (R.H. Rainbird). ∗

0301-9268/03/$ – see front matter. Crown Copyright © 2003 Published by Elsevier Science B.V. All rights reserved. doi:10.1016/S0301-9268(03)00076-7

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The tripartite sequence evolution of the Baker Lake and Thelon basins represents a framework for future sequence correlation among late Paleoproterozoic basins in Laurentia, and shares characteristics with the evolution of contemporaneous intracontinental basins from north-central Australia and southeastern Brazil. Crown Copyright © 2003 Published by Elsevier Science B.V. All rights reserved. Keywords: Canada; Paleoproterozoic; Intracontinental basins; Sequence stratigraphy; Western Churchill Province

1. Introduction This paper derives a regional tectonic model for the Baker Lake and Thelon basins employing sequence stratigraphy linked to geochronology and paleoenvironmental analysis (cf. Krapez and Martin, 1999; Jackson et al., 2000; Page et al., 2000). Studies of Precambrian sedimentary successions in the past decade have applied the sequence stratigraphic concept, analogous to studies of Phanerozoic successions in the previous two decades (see review of Sloss, 1988). The basis for sequence stratigraphic analysis is the recognition of unconformities or conformable sequence boundaries that represent time gaps in the sedimentary record and can be correlated over substantial distances. Dating of the sequences is used to provide a chronostratigraphic framework for correlation and mapping of genetically related packages of rocks. The sequences can be assigned orders of hierarchy based on the duration of the sedimentation (or non-depositional) gaps, the amounts of time represented by the strata between bounding surfaces and by the degree of angular discordance between adjacent sequences (Embry, 1993). The order of a sequence is ultimately governed by the duration of its formative process; Krapez (1996, 1997) presented an in-depth discussion of sequence hierarchy, periodicity and causal mechanisms. These overviews provided a guide for analysis of new and previously reported stratigraphic and sedimentological information from the Baker Lake and Thelon basins of southern Nunavut, Canada. Our work aimed at understanding the late Paleoproterozoic reworking of the Archean western Churchill structural province (western Churchill Province) of Laurentia via study of the overlying Baker Lake and Thelon basins. Detailed lithofacies were mapped and stratigraphic sections measured in Baker Lake Basin from 1998 through 2000 and in Thelon Basin in 1998 and 1999. Ultimately we hope that this and related studies will serve as a template for regional tectono-stratigraphic

correlations, including comparison with coeval intercontinental basins on other continents.

2. Regional geological setting 2.1. General geology and previous work The Dubawnt Supergroup is a Paleoproterozoic succession of mainly continental clastic and intercalated volcanic rocks that unconformably overlies granitic and supracrustal rocks of late Archean age and metasedimentary rocks and anorthosite-gabbro of earliest Paleoproterozoic age. Originally defined as the Dubawnt Group, it is exposed over an area of approximately 200,000 km2 in the central part of the western Churchill Province, west of Hudson Bay in Nunavut, Canada (Figs. 1 and 2). The western Churchill Province, comprising the Rae and Hearne structural domains, lies between the Paleoproterozoic Trans-Hudson orogen to the southeast and the Paleoproterozoic Taltson Magmatic Zone–Thelon Tectonic Zone to the northwest (Fig. 1). The lower part of the stratigraphic succession (Baker Lake and Wharton groups) was deposited in a series of generally elongate, northeast-striking, partly fault-bounded basins that are considered erosional remnants of a greater Baker Lake Basin. These extend from Kamilukuak Lake in the southwest to Baker Lake in the northeast and include the informally defined Dubawnt, Angikuni, Wharton, Kamilukuak and Baker Lake sub-basins (Fig. 3). The upper part of the succession (Barrensland Group) is confined mainly to Thelon Basin, a contiguous 400 km long by 200 km wide lunate region with relatively poor exposure (Fig. 2). The Dubawnt Group was defined and its regional distribution outlined by GSC mapping in the early- to mid-1950s (Wright, 1955; Wright, 1957; Wright, 1967). More detailed mapping by Donaldson (Donaldson, 1965; Donaldson, 1966; Donaldson,

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Fig. 1. Geology of northern Laurentia, showing location of Baker Lake and Thelon basins and structural elements discussed in text.

1967), included sedimentological studies and established the initial stratigraphic nomenclature for the Dubawnt Group. The Dubawnt Group was redefined as the Dubawnt Supergroup by Gall et al. (1992), with modification by Rainbird and Hadlari (2000; Fig. 4). Studies of the economic geology of the Dubawnt Supergroup, including comparisons with the Athabasca Group of northern Saskatchewan, were conducted by Miller (Miller, 1980; Miller and LeCheminant, 1985). 2.2. Lithostratigraphy The Dubawnt Supergroup comprises three unconformity-bounded successions: the Baker Lake, Wharton and Barrensland groups (Figs. 3 and 4; Gall et al., 1992; Rainbird and Hadlari, 2000). It has a combined thickness of up to 15 km, but nowhere is the succession complete. The Baker Lake Group is

the most regionally extensive and is exposed in all basins except for Thelon Basin. The base of the succession comprises coarse alluvial redbeds, the South Channel Formation, overlain by finer grained distal equivalents, the Kazan Formation (Donaldson, 1965). Intercalated with, and overlying, these strata is the Christopher Island Formation, a sequence of ultrapotassic lava flows and volcaniclastic deposits. In the Angikuni sub-basin, a separate unit similar to the Kazan Formation but named Angikuni Formation was recognised to unconformably underlie the Christopher Island Formation (Blake, 1980). The Baker Lake Group is capped by the Kunwak Formation, which contains sedimentary rocks similar to the South Channel and Kazan formations, but distinguished by its apparent stratigraphic position above the Christopher Island Formation (Fig. 3). The Baker Lake Group is unconformably overlain by the Wharton

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Fig. 2. Regional geology of Baker Lake and Thelon basins and distribution of the Dubawnt Supergroup (after Miller and LeCheminant, 1985).

Group, which comprises a thick, well indurated sandstone, the Amarook Formation, overlain by porphyritic rhyolite lava flows and pyroclastic and epiclastic sedimentary rocks of the Pitz Formation (Fig. 4). The Wharton Group is exposed in the northwestern part of Baker Lake Basin and in the central-northeastern part of Thelon Basin (Figs. 2 and 3). Wharton Group and Baker Lake Group strata are unconformably overlain by conglomerates and sandstones of the Thelon Formation, lowermost unit of the Barrensland Group (Gall et al., 1992). The Thelon Formation occurs throughout the Thelon Basin and north-central part of Baker Lake Basin. It is overlain by the Kuungmi Formation (Peterson, 1995), a

<10 m thick unit of altered shoshonitic basalt, which, in turn, is overlain by the Lookout Point Formation, a 40 m thick unit of siliceous stromatolitic dolostone with thin interbeds of quartzarenite. The Kuungmi and Lookout Point formations are limited to small isolated exposures in the central part of Thelon Basin (Fig. 2). Possible extensions of Kuungmi Formation are also exposed in the southern part of the Dubawnt sub-basin (Peterson, in press). 2.3. Geochronology of the Dubawnt Supergroup The geochronology of the Dubawnt Supergroup is critical to development of a tectono-stratigraphic

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Fig. 3. Geology of Baker Lake Basin showing outlines of sub-basins described in text as well as locations of other figures and stratigraphic sections (after Hadlari and Rainbird, 2001). Letters A–D show the locations of stratigraphic sections shown on Fig. 13.

model for the Baker Lake and Thelon basins. The Baker Lake Group is difficult to date due to a lack of zircon-bearing volcanic flows or ash layers; therefore other less precise or indirect methods have been employed. Analyses of phlogopite phenocrysts in an ultrapotassic flows and an associated syenite intrusion that intrudes the lower Baker Lake Group yielded preliminary 40 Ar/39 Ar ages of ca 1845 ± 12 Ma and 1810 ± 11 Ma, respectively (Rainbird et al., 2002). Intercalated sandstones contain detrital zircons with diagenetic overgrowths of xenotime (YPO4 ) that provide a mean 207 Pb/206 Pb ion probe age of 1813 ± 37 Ma (Rainbird et al., in preparation). Dating of the Akluilˆak lamprophyre dyke, a probable intrusive equivalent of the volcanic rocks, has provided an age of 1832 ± 28 Ma (Pb–Pb apatite; MacRae et al., 1996). A minimum age for the Baker Lake Group of 1785 ± 3 Ma was derived from laminated carbonate cements interpreted as travertine, in alluvial deposits from the Kunwak Formation in the Kamilukuak

sub-basin (Pb–Pb isochron from calcite; Rainbird et al., 2002). The Wharton Group has proven easier to date because it contains zircon-bearing rhyolite flows (Pitz Formation) that yielded U–Pb ages of 1757.6±3.3 Ma and 1753.7 ± 1.6 Ma (Fig. 4; Rainbird et al., 2001). Diagenetic apatite cement in the unconformably overlying Barrensland Group (Thelon Formation) yielded a Pb–Pb age of 1720 ± 6 Ma (Miller et al., 1989). The above-listed ages are summarised on the stratigraphic column of the Dubawnt Supergroup (Fig. 4).

3. Sedimentology and sequence stratigraphy of the Dubawnt Supergroup As a starting point, we employ the existing lithostratigraphic subdivision of the Dubawnt Supergroup because group boundaries are regionally mapped, major unconformities (Figs. 2 and 3). Each boundary

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Fig. 4. Schematic stratigraphic column showing lithostratigraphy and age of the Dubawnt Supergroup and its relationship to sequence stratigraphy and inferred depositional and tectonic setting (approximately 15 km cumulative thickness is represented). Geochronology sources: Thelon Formation, 1720 ± 6 Ma (Miller et al., 1989); Pitz Formation (Rainbird et al., 2001); Kazan/Christopher Island Formation, 1825 ± 12 Ma (Roddick and Miller, 1994); all others (Rainbird et al., 2002).

is marked by an angular discordance, accompanied by evidence for a significant time gap, including chemical and physical weathering features. The groups therefore are considered to be sequences sensu stricto. To avoid confusion and proliferation of new nomenclature, we will employ abbreviations of group names to identify sequences: Baker Lake Group = Baker sequence; Wharton Group = Whart sequence and Barrensland Group = Barrens sequence. Each sequence best records (may have been deposited over, and therefore represent larger areas) sedimentation within different basins: the Baker sequence is best preserved in the Baker Lake Basin; the Whart sequence is preserved in northwestern Baker Lake

sub-basin and southeastern Thelon Basin (Fig. 2), and the Barrens sequence is confined primarily to the Thelon Basin with an outlier in Baker Lake sub-basin. These regional-scale sequences define individual depositional basins and may therefore be considered as second-order sequences or supersequences (sensu Krapez, 1997). Each supersequence contains examples of smaller cycles, which we interpret to represent third-order depositional sequences or basin-filling rhythms (Krapez, 1996). These sequences and their boundaries vary widely in character, depending on their positions within basins, and their proximity to contemporaneous faults and volcanic centres.

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3.1. Baker sequence The Baker sequence is best preserved in Baker Lake Basin. Strata of closely similar age and lithology, known as the Martin Formation, are exposed north of Lake Athabasca and occupy deep, narrow NNE-trending structural basins that predate the Athabasca Basin (e.g. Donaldson, 1968; Fraser et al., 1970; Macey, 1973; Ramaekers, 1981). For descriptive purposes, the Baker Lake Basin is herein subdivided into sub-basins that are bounded by combinations of faults, basement inliers and younger intrusions. The sub-basins are named, from west to east, the Kamilukuak, Dubawnt, Angikuni, Wharton and Baker Lake (Fig. 3). 3.1.1. Baker Lake sub-basin The Baker sequence is widespread in Baker Lake sub-basin. The sequence is best exposed at the basin’s eastern extremity near the east end of Baker Lake and along its southern margin, particularly at the west end of Thirty Mile Lake (Fig. 3). Recent work describes lithofacies assemblages representing alluvial fan, braided river, ephemeral lake and eolian depositional environments (Hadlari and Rainbird, 2000; Rainbird and Hadlari, 2000; Rainbird et al., 1999). These assemblages form a continuum recording proximal alluvial fan deposits that built basinward from marginal faults and fed transversely into axial braided stream systems. The braided streams fed into floodplains with ephemeral lakes that were inundated by eolian dunes during periods of low runoff. Intercalated volcanic flows and associated pyroclastic deposits record contemporaneous eruption of volcanoes along the axis of the basin. At the eastern end of Baker Lake sub-basin these assemblages form an incomplete, upward-fining succession approximately 1000 m thick. Strata also fine inward, toward the centre of the basin, where they are intercalated with minettes (ultrapotassic volcanic rocks with phlogopite phenocrysts) that comprise partly preserved volcanic edifices exposed on islands in the eastern part of Baker Lake (Rainbird et al., 1999). The succession at the eastern end of the basin is composed of thinner, upward-fining cycles interpreted as alluvial fan flooding events. At the base of the section these cycles are up to 8 m thick beginning with thick, erosive-based, disorganised con-

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glomerate (Fig. 5A), overlain by parallel-stratified conglomerate passing upward to trough-filling, finer grained conglomerate. The tops of some of these cycles are marked by discontinuous lenses of trough cross-stratified to planar-stratified sandstone with erosive upper contacts (Fig. 5B) that are laterally continuous and can be correlated between measured sections several kilometres apart. These strata pass basinward into thinner (10–60 cm) cycles composed of medium-grained, crossbedded to planar-stratified sandstone overlain by parallel-laminated siltstone and mudstone with ubiquitous multigenerational desiccation cracks (Fig. 5C). These strata are interpreted as floodplain deposits that included ephemeral lakes and ergs (Rainbird et al., 1999). The erg deposits are represented by large-scale festoon crossbeds (up to 6 m thick and 150 m wide; Fig. 5D) separated by ∼10 cm interbeds of ripple-laminated sandstone interpreted as interdune ponds (Simpson et al., in press). At the western end of Thirty Mile Lake, on the southern margin of Baker Lake sub-basin, the Baker sequence is up to 2 km thick and consists of up to four, 100–500 m thick, upward-coarsening to upwardfining cycles (Hadlari and Rainbird, 2000, Fig. 6). A complete cycle typically comprises a lower upwardcoarsening section, a middle, non-gradational section and an upper, fining-up section. The lower section commences with a thin unit of fine, parallel-stratified sandstone and siltstone interpreted as floodplain deposits. Some of the sandstones consist of small-scale, low-angle, crossbeds with reverse-graded laminations that are similar to translatent wind-ripple deposits first described by Hunter (1977; Fig. 5E). The floodplain deposits pass upward into crossbedded sandstone and parallel-stratified conglomerate interpreted as alluvial fan and gravel- to sand-rich braided stream deposits (Fig. 5F). The middle section consists of coarse conglomeratic rocks interpreted as proximal alluvial fan deposits that are similar to those described above from eastern Baker Lake sub-basin (cf. Rainbird et al., 1999). The upper section is similar to the lower one except that the succession is reversed. The cycles (many of them incomplete) are stacked in an overall upward-fining pattern. The tripartite, 100–500 m thick, upward-coarsening to upward-fining cycles are interpreted as depositional sequences, composed of progradational, aggradational, and retrogradational parasequence sets (Fig. 6b). Each set is composed of

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parasequences exemplified by fining-up, alluvial fan flooding cycles from eastern Baker Lake and Thirty Mile Lake. These depositional sequences comprise basin-filling accommodation cycles interpreted as recording initial base level drop, subsequent base level rise, increase in gradient as accommodation increased, and decreasing gradient as accommodation space was filled while base level continued to rise. Sequence boundaries are conformable, indicating that creation of accommodation space exceeded sediment flux, and thus the basins were underfilled. This is independantly shown by the overall retrogradational (fining-up) stacking pattern exhibited by the Baker sequence along the southern and eastern margins of the Baker Lake sub-basin. High gradients recorded by the alluvial fan deposits, the linearity of the basin margin, and paleocurrents that are directed northward into the basin (Fig. 7), collectively suggest that the primary subsidence mechanism was movement along growth faults parallel to the preserved southern margin of the basin (Hadlari and Rainbird, 2000). On the northern margin of the sub-basin, southeast of Long Lake (Fig. 3), the Baker sequence is approximately 500 m thick and comprises at least five cycles characterised by continental clastic deposits intercalated with minette flows and volcaniclastic rocks (Fig. 8; Rainbird and Hadlari, 2000). The volcanic rocks occupy the tops of cycles where they are capped by alteration zones with distinctive carbonate cavity infill, which we interpret to be paleoweathering horizons. The underlying clastic deposits are alluvial fan and braided stream deposits, which typically fine upward within individual cycles but exhibit an overall upward-coarsening motif. Paleocurrent data indicate southeastward transport, transverse to the

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inferred basin margin (Fig. 7). These five cycles are interpreted to be third-order sequences hierarchically equivalent to the depositional sequences described from the south side of the sub-basin at Thirty Mile Lake (Fig. 8). They differ in that they are significantly thinner and are capped by volcanic flows with weathered tops. Sequence boundaries on the north side are disconformable, indicating that the combined sedimentary and volcanic flux exceeded accommodation space and overfilled or bypassed the sub-basin in this area. This is also shown by the overall progradational (upward-coarsening) stacking pattern along the north side. Correlation of individual sequences across the sub-basin is tentative (see Fig. 8); if sequences on both sides do correlate, the opposed stacking patterns and basin-filling characteristics of the sequences can be explained by differential subsidence during development of a half-graben. At the southwestern end of Baker Lake sub-basin, a relatively continuous, fault-bounded section of strata mapped as Christopher Island Formation is exposed southeast of the Kunwak River (LeCheminant et al., 1979a). The section is >7 km thick and comprises five, volcanic-dominated cycles comprising pyroclastic and epiclastic rocks overlain by minette flows and an upper cycle of mainly epiclastic rocks (Fig. 9). Flow-tops are sharply overlain by the sedimentary strata, but contacts appear conformable. Pyroclastic rocks in the lower four cycles include welded and non-welded ash-flow tuffs and tuff-breccias formed during vent clearing events, which preceded lava eruption (LeCheminant et al., 1979a; e.g. Fig. 5G). Cycle 5 begins with an upward-fining to upward-coarsening section, about 400 m thick, of sandstones and conglomerates composed of reworked volcanic rocks.

䉳 Fig. 5. Sedimentological features of some selected lithofacies from the Baker and Whart sequences in the Baker Lake sub-basin. (A) Disorganised framework conglomerate of alluvial fan facies, South Channel Formation (Baker sequence). Hammer, for scale, is approximately 40 cm long. (B) Organised-framework conglomerate with lenses of plane-bedded sandstone typical of gravel-bed braided stream facies, South Channel Formation (Baker sequence). (C) Sand-filled dessication cracks from ephemeral lake facies, Kazan Formation (Baker sequence). Knife, for scale, is approximately 10 cm long. (D) Aerial view of giant festoon crossbeds interpreted as eolian dunes, Kazan Formation (Baker sequence). Field of view is about 200 m wide. (E) Oblique view of glacially polished outcrop showing inversely graded foresets in low-angle crossbedding (top is to bottom right). Features are interpreted as subcritically climbing translatent wind-ripple lamination, Kazan Formation (Baker sequence). Coin, for scale, is 2.5 cm in diameter. (F) Crossbedded to planar-bedded sandstone typical of sand-bed braided stream facies, Kazan Formation (Baker sequence). Measuring staff is 1 m long. (G) Crossbedded tuff with syndepositional faults and cognate “dropstone” interpreted as basal penetration sag, Christopher Island Formation (Baker sequence). Coin, for scale, is 2.5 cm in diameter. (H) Altered fragments of rhyolite in matrix of silicified sandstone. Note embayments along top of large clast at bottom of image. Features suggest lava erupted onto wet sediment (“peperite”), Pitz Formation (Whart sequence). Coin, for scale, is 2.5 cm in diameter.

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Fig. 6. (a) Geology of the southern margin of Baker Lake sub-basin, Thirty Mile Lake area (from Hadlari and Rainbird, 2001) showing location of stratigraphic Sections A–C in (b). (b) Sedimentology, sequence stratigraphy and correlation of the Baker sequence, western Thirty Mile Lake (Sections A–C; see Figs. 3 and 6a for section location). Also shown is tentative correlation with a stratigraphic section from the west end of Nutarawit Lake in northern Angikuni sub-basin (from Hadlari and Rainbird, 2001).

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Fig. 7. Paleocurrent rose diagrams summarising crossbedding data in alluvial facies of the Baker sequence, eastern Baker Lake sub-basin. Note axial transport in central parts of the basin and transverse transport at basin margins.

Overlying flows are mainly phlogopite-phyric mafic minette; felsic minette flows mark the top of cycles 2 and 5 (Fig. 9). Cycle 5 is overlain by an upward-fining to upward-coarsening section, about 1100 m thick, of parallel-stratified and graded sandstone, and matrix-supported conglomerate containing boulders of locally derived volcanic rock. These strata have been interpreted mainly as proximal turbidites deposited in shallow water, flanking a volcanic edifice (LeCheminant et al., 1979a). The five volcanic-dominated cycles recognised at the southwestern end of Baker Lake sub-basin (Fig. 9), are also interpreted as depositional (basin-filling) sequences because they are of similar character and frequency to the basin-filling sequences described from sediment-dominated sequences. Mechanisms of subsidence appear to have been similar to those in the central and eastern parts of the sub-basin; differences reflect episodicity of volcanism and proximity to volcanoes. The southwestern depositional sequences are similar to those on the north side of the sub-basin that also are capped by volcanic flows. However, the southwestern cycles are much thicker, recording

greater accommodation, and flow-top unconformities were not observed (LeCheminant et al., 1979a), together suggesting that the basin was underfilled in this region. 3.1.2. Wharton sub-basin Strata of the Baker sequence are exposed along a belt approximately 75 km long and up to 15 km wide that extends from islands in Wharton Lake to southwest of Tebesjuak Lake (Fig. 3). Strata exhibit shallow to moderate north to northeast dips and are truncated against Archean crystalline basement by north- and northwest-trending faults. The faults are interpreted as basin-margin growth faults with significant southwest-side down displacement, based on: (1) facies are distributed asymmetrically (proximal fan to braidplain toward the southwest); (2) bedding attitudes suggest back-rotation. Southern parts of the basin are intruded by 1.75 Ga granites of the Nueltin-suite (Fig. 3; Peterson et al., 2002). In Wharton sub-basin, clastic redbeds interpreted as alluvial fan and fluvial braidplain deposits are stacked in an overall upward-coarsening succession

32 R.H. Rainbird et al. / Precambrian Research 125 (2003) 21–53 Fig. 8. Stratigraphy of the Baker sequence, Aniguq River section, north side of Baker Lake sub-basin (see Fig. 12 for location), showing tentative sequence correlation with strata on the southern margin of the basin (Fig. 6b; Section B). Differences in sequence thickness and character are interpreted as due to relative accommodation potential and composition of basin fill.

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glomerate (LeCheminant et al., 1981). Conglomerate with minor sandstone interbeds characterises an upper cycle, at least 600 m thick, that thickens toward a northeastern basin-margin fault (Fig. 3). Paleocurrents define alluvial fans that built toward the west and southwest, feeding into axial braided fluvial systems (LeCheminant et al., 1981). The two generally upward-coarsening cycles described from the Wharton sub-basin by LeCheminant et al. (1981) are similar to depositional sequences described from elsewhere in the Baker Lake sub-basin. Overall upward-coarsening and upward increase in the content of local basement clasts indicate that sediment flux, controlled by uplift along basin-margin growth faults, outpaced the creation of accommodation space, similar to sequence styles observed along the northern margin of Baker Lake sub-basin. Asymmetric distribution of the basin fill suggests a half-graben geometry, with northeast tilting indicating possible back-rotation toward the basin-bounding faults, which are orthogonal to those in the Baker Lake sub-basin.

Fig. 9. Stratigraphy of the Baker sequence, western Baker Lake sub-basin (Section B-B from LeCheminant et al., 1979b, see Fig. 3 for location). Cycles are interpreted as third-order, basin-filling sequences, similar to those described from Kamilukuak and Dubawnt sub-basins (cf. Fig. 11).

up to 1.5 km thick (LeCheminant et al., 1981). These rocks are similar to redbeds described from the southern and southeastern margin of Baker Lake sub-basin but were mapped as Kunwak Formation because they overlie mafic minette flows of the Christopher Island Formation and lack intercalated volcanic rocks (ibid; see Section 2.2). Above the Christopher Island Formation, a lower cycle (∼900 m thick) comprises mainly parallel-stratified to crossbedded sandstone intercalated with massive to weakly stratified polymictic con-

3.1.3. Dubawnt sub-basin Dubawnt sub-basin underlies the northern third of Dubawnt Lake at the western end of Baker Lake Basin (Fig. 3). Baker sequence rocks are well exposed on several islands and can be subdivided into two sections: (1) a northeast-dipping section on islands near the northwest shore of the lake comprising mainly coarse clastic alluvial redbeds mapped as Kunwak Formation (Peterson et al., 1989); and (2) a north-northwest-dipping section on eastside islands dominated by volcanic rocks and mapped as Christopher Island Formation (Peterson et al., 1989; Rainbird and Peterson, 1990). The western section dips northeast at 30◦ –60◦ and is estimated to be at least 10 km thick (Peterson et al., 1989). The most abundant lithofacies is extremely coarse, massive to crudely bedded, clast-supported conglomerate with disorganised framework. Clasts in the coarsest units average 25–30 cm (up to 1.5 m) and are mainly composed of locally derived granitic rocks and up to 20% minette. Thick, massive conglomerates typically form the tops of 5–20 m thick upwardcoarsening cycles which begin with thin, laminated mudstone and siltstone and pass upward into parallel and cross-stratified sandstone and then crudely stratified conglomerate. The cycles are interpreted as

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Fig. 10. Geology of west-central part of Dubawnt sub-basin summarising paleocurrent data from crossbedding (one station) and conglomerate-clast imbrication (see Fig. 3 for location). The measured stratigraphic section on Lost Boat Island is illustrated in Fig. 11A. Basin-bounding strike-slip faults are inferred as discussed in the text.

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prograding alluvial fans representing pulses of sedimentation triggered by uplift along adjacent basinbounding faults. The 5–20 m thick upward-coarsening cycles are interpreted as parasequences that reflect repeated progradation and abandonment of small allu-

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vial fans over floodplain deposits. Larger scale depositional sequences were not recognised, but only ∼300 m of this >10 km thick section has been measured in detail (Peterson et al., 1989). The island exposures comprising the western section are distributed

Fig. 11. (A) Section of Baker sequence, east side of Dubawnt sub-basin (Lost Boat Island section from Peterson et al., 1989, see Fig. 10 for location). (B) Stratigraphy of the Baker sequence, Kamilukuak sub-basin (section from Tella et al., 1981, see Fig. 3 for location). Cycles are interpreted as volcanic-dominated, basin-filling sequences, similar to those described from Baker Lake sub-basin (cf. Figs. 8 and 9).

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along an east-northeast trend that is truncated northward by Archean granitic rocks (Fig. 10). The contact, which is mainly covered by water, is interpreted as a basin-margin fault (labelled “inferred strike-slip fault”). This interpretation is supported by the observation that the coarsest and most texturally immature conglomerates, including angular monomictic breccias, are exposed immediately adjacent to the inferred fault. Paleocurrents from crossbedding and pebble imbrication measurements reflect predominant south and southeastward transport. Northward transport is observed at two stations suggesting that the basin was very narrow and that alluvial fans also were derived from parallel fault(s?) near the south side of the basin (Fig. 10). The eastern stratigraphic section is completely different, being dominated by volcanic rocks. A basal cycle, which unconformably overlies the Archean crystalline basement, starts with approximately 200 m of stratified, matrix-supported conglomerate that is interbedded and overlain by an upward-fining succession of lithic sandstone and siltstone (Fig. 11). Clasts in the conglomerate are angular, <5 cm diameter and comprise about equal amounts of local volcanic and granitoid rocks. Conformably overlying this is a ∼450 m thick interval of thin (1–10 m) mafic minette flows and intercalated massive breccia. Subaqueous deposition is suggested by quench textures such as hyaloclastite. A second cycle begins above an eroded flow unit with massive clast-supported coarse conglomerate containing rounded clasts derived mainly from the underlying flows. This grades upward to normally graded conglomerate and sandstone and finally into rhythmic sandstone–siltstone beds, 30–100 cm thick, interpreted as lacustrine turbidity current deposits. This lacustrine unit grades upward into ∼350 m of felsic minette flows with thin interflows of siltstone and breccia, interpreted as hyaloclastite (Peterson et al., 1989). The flows are abruptly overlain by intercalated siltstone and crossbedded sandstone, which coarsen upward over 100 m. This upper sedimentary unit possibly represents the base of a third cycle, but is incomplete due to lack of exposure. The two complete cycles are interpreted as third-order sequences; both display an upward-fining (retrogradational) lower parasequence composed of locally derived epiclastic rocks, overlain by minette flows. Contacts between sequences are conformable, suggesting that the basin was underfilled.

Paleocurrents at the east and west ends of the Dubawnt sub-basin are northerly, opposite to those of possibly contemporaneous alluvial fan deposits elsewhere in the basin, suggesting that deposition occurred in a narrow, roughly east-west trending fault-bounded trough. The inferred great thickness (>10 km) and tilting of strata at a high angle to the basin margins together resemble lateral conveyor-belt stacking such has been observed in many strike-slip basins (Crowell, 1974; Steel and Gloppen, 1980). 3.1.4. Angikuni sub-basin Baker sequence rocks in the Angikuni sub-basin are exposed in two segments that extend northeast from Angikuni Lake, as well as in scattered small outliers throughout this region (Fig. 3). Both segments are bounded on their western margins by north-northeast trending faults (e.g. Tulemalu Fault) that display evidence of Neoarchean ductile dextral movement and Paleoproterozoic (ca. 1.85 Ga) brittle reactivation (Aspler et al., 1999). At the southern end of the northern segment, near Angikuni Lake, a thick wedge of alluvial sedimentary rocks lies unconformably between Archean granitoid basement and ultrapotassic volcanic rocks, mapped as Christopher Island Formation (Eade and Blake, 1977). This succession, named Angikuni Formation by Blake (1980), comprises a lower succession of parallel-stratified and crossbedded sandstone with metre-scale interbeds of framework-supported conglomerate (up to 1200 m thick) and a conformably overlying package of red siltstone, mudstone and parallel-stratified fine sandstone containing mud-clast conglomerates and wave ripples (up to 1400 m thick; Aspler et al., 1999; Blake, 1980). The Angikuni Formation is interpreted to represent an alluvial fan-braided stream to lacustrine depositional system. The lower unit likely records high-energy sheetfloods and sand-rich mass flows that spread rapidly across a fluvial plain. The upper unit may record deposition on a low-relief floodplain where fine-grained sediment accumulated in isolated depressions. Shallowly (∼10◦ ) east-dipping minette flows (Christopher Island Formation) lie above steeply (∼45◦ ) east-dipping Angikuni strata and consist of massive to layered, mafic and felsic minette flows, volcanic breccias containing intraformational volcanic clasts, local vesicular bombs and mylonitic basement pebbles, and

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rare beds containing accretionary lapilli. Local units of crossbedded red sandstone and open-framework polymictic conglomerate indicate fluvial sedimentation coeval with subaerial volcanism. Minette dykes, likely feeders to the lava flows, occur throughout the region and display strong northeast trends (Aspler et al., 1999; Peterson et al., 2002). About 25 km north of Angikuni Lake, a 580 m thick section of strata equated with the Angikuni Formation was identified in core drilled by WMC International Ltd. in 1995. Lithofacies include basal breccia, granule–pebble conglomerate, mediumcoarse-grained sandstone, fine-grained sandstone and siltstones, and sandstone to mudstone rhythmites containing a unique evaporite mineral assemblage. The lithofacies are arranged in five, 100 metre-scale cycles interpreted as basin-filling sequences. Sequences 1–4 fine upward, sequence 5 coarsens upward then fines upward. In contrast to the angular unconformity exposed at northern Angikuni Lake, minette flows conformably drape fine-grained sandstones of the Angikuni Formation. Individual flow units are 1–20 m thick and are commonly separated by 0.5–4 m thick zones of carbonate alteration (possible incipient paleosols), as well as by fragmental volcanic and siliciclastic interbeds; field and drill hole data indicate total accumulations of at least 200 m (Cousens, 1999). On the northwest side of Nutarawit Lake, at the northern end of Angikuni sub-basin, lies a ∼1600 m thick, northwest-dipping, upward-fining succession of conglomerate and sandstone, with lithofacies and interpreted depositional settings similar to that of the Angikuni Formation. A stratigraphic section reveals at least three, thick (100–500 m) upward-coarsening to upward-fining alluvial fan-fluvial cycles that we correlate broadly with third-order basin-filling sequences described above and from Thirty Mile Lake on the southern margin of the Baker Lake sub-basin (Fig. 6b; Hadlari and Rainbird, 2000; LeCheminant et al., 1979b). The mainly siliciclastic section is conformably overlain by and intercalated with a thick succession characterised by repeating cycles of volcaniclastic sedimentary rocks overlain by mafic minette flows. These cycles, interpreted as third-order sequences, are similar in character and scale to those described from a section located 20 km to the northwest, near the southwestern end of Baker Lake sub-basin (Fig. 9; LeCheminant et al., 1979b).

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These correlations and the overall upward-fining motif of these strata suggest similar subsidence and basin-filling mechanisms for the Angikuni sub-basin and the Baker Lake sub-basin. Rocks described as Angikuni Formation are equivalent to those mapped elsewhere as South Channel/Kazan formations and together comprise the Baker sequence along the southeast margin of greater Baker Lake Basin. 3.1.5. Kamilukuak sub-basin Kamilukuak sub-basin is situated at the southwestern end of Baker Lake Basin (Fig. 3), where the Baker sequence forms a homocline that dips northeastward at 20◦ –60◦ . The basin is bounded by northeast- and west-northwest-trending oblique-slip faults in the west and by northwest-trending oblique-slip faults to the east. In several areas, the succession is repeated by east-trending normal faults (Peterson, in press; Tella et al., 1981). As elsewhere in Baker Lake Basin, basal Baker sequence strata comprise localised pockets of locally derived breccia overlain by coarse, clast-supported, disorganised framework conglomerate, mapped as South Channel Formation (Tella and Eade, 1980; Tella et al., 1981). Southwest of Kamilukuak Lake, Tella et al. (1981) recognised two unconformity-bounded successions overlying the basal unit (Fig. 11B). The lower succession comprises mainly parallel-bedded, volcanic sandstone–siltstone and granulestone grading upward into massive mafic minette flows. Intercalated with the flows are discontinuous massive and stratified conglomerate and sandstone as well as volcanic sedimentary rocks interpreted as tuff and lapillistone. The overlying succession is similar, but at its base has a higher proportion of proximal sedimentary facies, which are arranged in upward-fining units, 3–50 m thick. Local crossbedding and current ripples indicate transport to the west and northwest. Intercalations of mafic and subordinate feldspar-phyric (felsic) minette, 10–20 m thick, are predominant in the upper part of the upper succession (Fig. 11B). The unconformity-bounded successions mapped by Tella et al. (1981), are similar to third-order depositional sequences elsewhere in the Baker Lake Basin (Fig. 12). The basal succession is interpreted as an alluvial fan sequence and the upper two successions are similar in scale and character to the volcano-sedimentary sequences described from the

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Fig. 12. Geology of the northern margin of Baker Lake sub-basin (modified from Rainbird and Hadlari, 2000). Whart sequence strata are repeated across a basin-parallel normal fault (inferred beneath Thelon Formation). Strata on the south side of the fault dip more gently (<20◦ ), similar to the step-wise flattening observed on the south side of the basin (Fig. 6a). The dip of the normal fault is assumed to be northward, because strata dip to the south with progressive flattening toward the centre of the basin. The Baker and Whart sequences are also segmented along the northern margin of the basin by prominent sets of transfer faults trending 340◦ and 310◦ . Bedding attitude varies across these faults from 20◦ up to 80◦ to the south.

eastern end of Dubawnt sub-basin (cf. Fig. 11A). The lower sequence coarsens upward, and the overlying sequence has a higher proportion of proximal sedimentary facies. In the upper sequence, upward-fining cycles are interpreted as retrogradational parasequences. The predominantly upward-coarsening motif and unconformable sequence boundaries suggest that the basin was overfilled in this region. 3.2. Whart sequence The Whart sequence unconformably overlies the Baker sequence, and is confined to the northwestern part of Baker Lake sub-basin, the south-central part of Thelon Basin and to a small region of the northern Kamilukuak sub-basin (Fig. 3). Lithostratigraphic studies indicate that it comprises a lower succession of clastic continental redbeds (Amarook Formation) overlain by the Pitz Formation, which consists of at least two cycles of felsic volcanic rocks intercalated with coarse alluvial redbeds. 3.2.1. Baker Lake sub-basin Whart sequence rocks are best exposed in a belt along the northern margin of Baker Lake sub-basin (Figs. 3 and 12). Northeast of Pitz Lake, minette flows of the Baker sequence pass upward through a

3–4 m thick regolith into hard, pink, fine-grained, very well-sorted subarkose characterised by 2–8 m thick, simple tabular-trough to wedge-planar crossbeds interpreted as eolian dunes (Amarook Formation; Rainbird and Hadlari, 2000). Paleocurrents indicate strong unimodal transport toward the southwest. Complete sections of the Amarook Formation are lacking, but mapping indicates that, with moderate to steep dips to the SE, it may be up to 500 m thick near the Aniguq River (Rainbird and Hadlari, 2000). West of the northwest fault, which follows the western shore of Long Lake (Fig. 12), the sandstone is thinner (∼200 m) and contains interbeds of parallel-stratified conglomerate and siltstone with wave ripples and pervasive desiccation cracks interpreted as ephemeral stream deposits. Conglomerate is both frameworkand matrix-supported; clasts include well-rounded pebbles and cobbles of vein quartz, minette, pink quartzarenite and basement granitoids. The Amarook Formation appears to be conformably overlain by rhyolite flows and alluvial redbeds referred to collectively as the Pitz Formation (Figs. 12 and 13; Donaldson, 1965). The flows, extrusive equivalents of the 1.76–1.75 Ga Nueltin-suite granites (Peterson et al., 2002), are typically mauve to bright red porphyritic rhyolite containing varying amounts of white kaolinised feldspar phenocrysts up to 3 cm, smaller

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Fig. 13. Regional stratigraphic correlation scheme suggesting that the Whart second-order sequence comprises four, unconformity-bounded cycles, interpreted as third-order depositional sequences, that can be correlated throughout Baker Lake sub-basin and south-central part of Thelon Basin (W-1–4). W-1 is typified by widespread, highly indurated pink eolian sandstone. W-2–4 strata are typified by a lower succession of coarse clastic alluvial braidplain deposits, including eolian and ephemeral lake deposits, which are overlain by red, feldspar-phyric rhyolite flows and associated sedimentary rocks. Sequence thickness and character vary widely with accommodation potential and proximity to centres of volcanism; sequence correlation is complicated by regional syn- to post-depositional block-faulting. Section locations on Fig. 3.

quartz eyes and minor fine-grained mafic clots and grains. Thin and discontinuous layers of rhyolite breccia, lithic arenite and tuffaceous siltstone occur between flows. Some individual flows are tens of metres thick and form weakly defined domes. North of Pitz Lake, the felsic volcanic rocks are erosionally overlain by a second sedimentary cycle, which commences with coarse, matrix-supported conglomerate composed essentially of clasts derived from the underlying volcanic rocks (Fig. 13; Section C). The conglomerate fines upward through mixed conglomeratic sandstone into crossbedded sandstone and siltstone. The sandstones have unimodal east-directed paleocurrents and are interpreted as braided river deposits (Rainbird and Hadlari, 2000). East of Princess Mary Lake (Fig. 12), these sedimentary rocks are overlain by a second cycle of rhyolite flows and interflow sedimentary rocks. Irregular basal contacts and breccia interpreted as peperite point to eruption onto a wet, unconsolidated substrate (Fig. 5H). The upper volcano-sedimentary cycle and underlying flows of the first cycle are cut out across the Long Lake fault

(Fig. 12). Baker and Whart sequence strata dip toward the south at 15◦ –20◦ , but northeast of the fault they are steeper. Whart sequence strata are partly repeated, across an inferred ENE fault, on a parallel ridge to the south (Fig. 12). There, rhyolite flows, similar to those at the top of the second cycle to the north, are erosionally overlain by a third cycle beginning with coarse, clast-supported conglomerate which grades upward into a 20 m thick unit of parallel-stratified pebbly sandstone and siltstone (Fig. 13; Section C). Above a thin covered interval, lies a conglomerate composed of locally derived clasts. Just a few kilometres south, near the southern tip of Pitz Lake, Whart sequence strata are entirely absent, and sandstone of the underlying Baker sequence is unconformably overlain by basal conglomerate of the Thelon Formation (Barrens sequence; Figs. 3 and 13; Section D). Whart sequence rocks also occur along the Kunwak River east and south of Tebesjuak Lake at the western end of Baker Lake sub-basin. (Fig. 3; LeCheminant et al., 1981; LeCheminant et al., 1979a; LeCheminant et al., 1980). The stratigraphic succession is similar to

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that described above, except that sedimentary units are thinner, and the Amarook Formation is absent (Fig. 13; Section B). Conglomerate layers near the base contain clasts of contemporaneous rhyolite, thereby distinguishing them from conglomerates at the top of the Baker sequence mapped as Kunwak Formation. This area contains several isolated exposures of amygdaloidal basalt up to 10 m thick that are considered to be part of the Whart sequence (Pitz Formation), possibly extrusive equivalents of a nearby gabbro intrusion (McRae Lake dyke; LeCheminant et al., 1980). 3.2.2. Kamilukuak sub-basin Rocks assigned to the Whart sequence are exposed in the Kamilukuak sub-basin southeast of Dubawnt Lake (Fig. 3; Peterson, in press; Tella and Eade, 1985; Tella et al., 1981). A 150 m thick succession of interbedded red siltstone, sandstone and volcaniclastic conglomerate unconformably overlies minette flows of the Baker sequence, along the eastern shore of a prominent north-pointing peninsula. The basal unconformity and presence of clasts of red quartz–feldspar porphyry in the conglomerate suggest that the unit is part of the Whart sequence (Tella et al., 1981). Immediately to the south and along the southeast shore of Dubawnt Lake are exposures mapped as “red quartz–feldspar porphyry flows” assigned to the Pitz Formation (Peterson, in press; Tella and Eade, 1985). 3.2.3. Thelon Basin Reconnaissance mapping indicates that the Whart sequence is well exposed southwest of Aberdeen Lake, on the north side of the Dubawnt River, and also on the east side of Marjorie Lake, where a basal sandstone unit was mapped as a lower member of the Pitz Formation (Figs. 2 and 3; LeCheminant et al., 1983). In this region a thin (<2 m) conglomeratic regolith developed on deeply weathered Archean granitoid gneiss is overlain by red, parallel-stratified pebbly feldspathic sandstone (Fig. 13). Clasts in the conglomerate include volcanic rocks of the Baker sequence but also red rhyolite similar to potentially coeval flows from the Baker Lake sub-basin. This thin basal unit is overlain by up to 150 m of medium-grained subarkose with very large (up to 6 m) wedge-planar crossbeds indicating north-northwest paleotransport (Fig. 13; Section A). This sandstone is reminiscent of the Amarook Formation on the north side of Baker

Lake sub-basin (Rainbird and Hadlari, 2000). Several drill cores from the eastern Thelon Basin, north of Aberdeen Lake reveal a distinctive, well-sorted and well-indurated pink sandstone between basement and the overlying Barrens sequence (unpublished data of Cameco Corporation). These observations suggest that the Amarook Formation was deposited throughout the eastern Thelon Basin and northern Baker Lake sub-basin. The eolian sandstone is conformably overlain by ∼20 m of rhyolite flows. Unconformably above the flows is a distinctive red-matrix conglomerate containing mainly clasts of red rhyolite and Amarook sandstone. These rocks, originally distinguished by Donaldson (1966) and described in more detail by Chiarenzelli et al. (in LeCheminant et al., 1983), are identical to those at the base of sequences 3 and 4 from the Pitz Lake North section (Fig. 13; Section C). This unit was considered part of the Thelon Formation (Barrens sequence) in the schematic section from Marjorie Hills of LeCheminant et al. (1983); however, Chiarenzelli et al. (ibid) describe an angular discordance between this unit and overlying, less-well indurated conglomerates that are more typical of Thelon Formation. For these reasons, we include this conglomerate with the Whart sequence (Fig. 13; Section A). The regional stratigraphic correlation scheme proposes that the Whart sequence comprises at least three unconformity-bounded cycles that can be correlated throughout the Baker Lake sub-basin and south-central part of Thelon Basin (Fig. 13; sequences 2–4). These cycles are similar to third-order volcano-sedimentary sequences in the Baker sequence—they begin with generally upward-fining terrestrial redbeds and are capped by volcanic flows and associated volcaniclastic rocks. Each is typified by a lower succession of coarse clastic alluvial braidplain deposits with or without eolian and ephemeral lake deposits, overlain by rhyolite flows and associated sedimentary rocks. Significant variations in sequence thickness and character are interpreted to result from differing accommodation and proximity to centres of volcanism. Stratigraphic correlation is complicated by regional syn- to post-depositional block faults (see Section 4.2). Sequence 1, equivalent to the Amarook Formation, is not preserved in the western Baker Lake Basin, but thickens markedly to the east. The Whart sequence was deposited ca. 1.78–1.72 Ga. Given that

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the younger age was derived from diagenetic apatite in the overlying Barrens sequence, then the duration of Whart sequence deposition is <60 million years. 3.3. Barrens sequence The Barrens sequence is exposed in Thelon Basin and to a lesser extent northern Baker Lake sub-basin (Fig. 2). Lithostratigraphic studies describe a lower sub-horizontal succession of fluvial sandstone, conglomerate and siltstone/mudstone with subordinate eolian and marine sandstones (Thelon Formation) overlain locally by thin units of shoshonitic basalt (Kuungmi Formation) and stromatolitic dolostone (Lookout Point Formation; Gall et al., 1992). The Barrens sequence rests on a regional unconformity; basement rocks are capped by a widely developed paleosol that is locally in excess of 50 m thick (Chiarenzelli, 1983; Donaldson, 1966; Donaldson, 1969; Gall, 1994b). Similarities to paleosols beneath the Athabasca, Amundsen and Elu basins provide strong evidence for interbasinal correlation (Gall, 1992). The presence in these paleosols of distinctive phosphates (Cecile, 1973; Gall, 1994a; Miller, 1983) and local patches of silcrete (Ross and Chiarenzelli, 1985) further strengthens the correlation. 3.3.1. Baker Lake basin The Barrens sequence is confined mainly to the northern margin of Baker Lake sub-basin and is represented there by the Thelon Formation, which crops out sporadically along a corridor between Princess Mary Lake and Baker Lake (Fig. 3). Thickness is estimated from seismic reflection experiments to be approximately 250 m in the area east of Pitz Lake (Overton, 1979). In the eastern Thelon Basin, Hiatt et al. (in press), subdivided the Thelon Formation into three, third-order stratigraphic sequences, but only the basal sequence is preserved in Baker Lake Basin, where it comprises two distinct lithofacies. The basal unit is a very coarse grained, pebbly to conglomeratic sublithic arenite with distinctive white clay-mineral cement. Rocks are pink to mauve, crumbly weathering and composed of moderate to large trough crossbeds with no intervening fines. At the base, the Thelon Formation is dominated by clasts derived locally from the Whart sequence, including rhyolite in the west and hard pink sandstone in the east. Crossbeds yield uni-

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modal, northwest-directed paleocurrents (Donaldson, 1965, Fig. 14). Deposition on an alluvial braidplain is inferred. Estimated thickness of the basal member along the Aniguq River (Fig. 12), is about 100 m. The basal unit on the Aniguq River is overlain conformably by a fine-medium-grained, very well-sorted quartzarenite composed of tabular- and wedge-trough crossbeds up to 10 m thick and >100 m wide. This facies is identical in character and stratigraphic position to sandstones of interpreted eolian origin in the southeastern Thelon Basin (Jackson et al., 1983). The Thelon Formation also is exposed in the Kamilukuak sub-basin, at the north end of the prominent peninsula in southern Dubawnt Lake (Fig. 2). Here the basal sequence comprises the same two lithofacies that are observed in Baker Lake sub-basin. The lower, conglomeratic unit is about 10 m thick; the upper, fine-medium-grained sandstone unit is thicker but of undetermined thickness (Peterson, in press). Conformably overlying the sandstones are aphanitic volcanic flows and associated volcaniclastic rocks that are similar to shoshonites of the Kuungmi Formation in the central Thelon Basin (Fig. 2; Gall et al., 1992; Peterson, 1995). 3.3.2. Thelon Basin In Thelon Basin, the Barrens sequence underlies an area of nearly 80,000 km2 ; the preserved erosional edge defines the basin (Fig. 2). The western side of the basin is abruptly marked by a series of NNE- to ENE-striking faults, including the northeast extension of the McDonald Fault. The eastern side of the basin has been modified by movement along the east-northeast-striking Amer mylonite zone (Tella and Heywood, 1978) and northeast-trending Slave-Chantrey mylonite zone (Tella et al., 1984; Fig. 1). In Thelon Basin, the Barrens sequence comprises the widespread Thelon Formation and the overlying Kuungmi and Lookout Point formations (Fig. 4), which are restricted to a relatively small area in the western part of the basin (Fig. 2). In Thelon Basin, the Thelon Formation comprises four main lithofacies: (1) pebbly to cobbly, trough crossbedded, coarse-grained sublithic arenite and conglomerate interpreted as high-energy braided river deposits; (2) fine-coarse-grained, white clay-cemented, massive to crossbedded subarkose and siltstone interpreted as low-energy braided river

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Fig. 14. Paleocurrents derived from crossbedding in fluvial facies of the Thelon Formation (Barrensland Group = Barrens sequence). Each arrow denotes a minimum of five measurements (number of stations = 157; total number of crossbeds = 1262). Note overall west to northwest trend in the Baker Lake and eastern Thelon Basin and west to southwest trend in the western Thelon Basin.

deposits; (3) medium-coarse-grained, well-sorted quartzarenite with low-angle crossbedding and wave and current ripples interpreted as marine shoreface deposits, and; (4) medium-grained, very well-sorted quartzarenite with large to very large-scale tabularand wedge-trough crossbedding interpreted as eolian deposits (Kyser et al., 2000). Because of near-horizontal bedding attitudes in a region of low relief, the true thickness of the basin fill has not been directly determined, but seismic reflection experiments suggest a stratigraphic thickness exceeding 1000 m in the centre of the basin (Overton, 1979). A regional compilation of paleocurrents for the Thelon Formation, based on crossbedding azimuths from fluvial sandstones, shows a westward to southwest-

ward inclined paleoslope (Fig. 14). This matches general paleocurrent trends in the correlative Athabasca, Amundsen and Elu Basins (see Campbell, 1979; Fraser et al., 1970; Kerans et al., 1981; Ramaekers, 1981). In the western Thelon Basin (Fig. 2), the uppermost part of the Barrens sequence is interrupted by a thin (<10 m) unit of amygdaloidal shoshonite flows and associated volcaniclastic rocks, collectively assigned to the Kuungmi Formation (Gall et al., 1992). The basal flows are rich with detrital sand grains, suggesting that they were erupted onto unconsolidated sand. The uppermost flows are vesicular and strongly oxidised and overlying sandstones contain abundant volcanic detritus.

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The top of the Barrens sequence is marked by the Lookout Point Formation (Donaldson, 1969; Gall et al., 1992), which crops out immediately west of the Kuungmi Formation (Fig. 2). The carbonate-rich section is about 40 m thick and comprises siliceous stromatolitic dolostone with thin interbeds of medium-grained quartzarenite. Quartz sand content decreases upward, giving way to purer dolostones containing tepee structures, oncolites, edgewise breccias and hopper halite casts. The succession suggests an upward-shallowing, arid intertidal environment of deposition. In eastern Thelon Basin, Hiatt et al. (in press) subdivided the Thelon Formation into three stratigraphic sequences based on lithofacies analysis including recognition of intraformational breaks and changes in thickness of fluvial upward-fining cycles. The lower part of each sequence is characterised by conglomerate and coarse-grained sandstone with large trough crossbeds and relatively thin (<1–6 m) upward-fining cycles, representing minimum accommodation potential. The coarse unit is overlain by finer sandstones with thicker (average 9 m), laterally continuous, upward-fining cycles capped by thin siltstone layers interpreted as paleosols (Hiatt et al., in press), representing maximum accommodation potential. Each sequence is capped by laterally continuous, upward-fining sandstones of intermediate thickness, interpreted as marine upper shoreface and eolian deposits. The stratigraphic sequences are interpreted as third-order, depositional sequences (Hiatt et al., in press). The overall fining-upward aspect of the Thelon Formation and its conformable contact with marine dolostones in the western part of the basin suggests a retrogradational pattern of sedimentation with increasing marine influence.

4. Discussion 4.1. Growth-faulting and initiation of Baker Lake Basin The Baker sequence records the initiation of Baker Lake Basin. Structural overprinting, discussed below, including post-Dubawnt Supergroup uplift and erosion, has considerably modified the original geometry of Baker Lake Basin; it has obscured or re-

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activated many structures such as faults, which likely formed during basin initiation. Probable locations for basin-margin growth faults are adjacent to thick packages of coarse, monomictic alluvial fan conglomerates that must have been derived from nearby active fault-scarps. One of the thickest accumulations of such deposits is on the south side of Baker Lake sub-basin near Thirty Mile Lake. A basin-bounding fault has yet to be recognised in outcrop from this area; however, an ENE fault is mapped in the basement, 3–4 km south of Thirty Mile Lake (Fig. 2; LeCheminant et al., 1979a). A half-graben is inferred from lithofacies analysis and sequence correlation (Fig. 6; Rainbird et al., 1999). On the basis of outcrop studies and geophysical maps, Ryan et al. (2000) identified a possibly related fault with significant normal displacement that is parallel to the southeastern margin of Baker Lake sub-basin. Another example of an inferred basin-bounding fault is in the Wharton sub-basin, where a northwestoriented basin-margin fault with normal displacement is consistent with the asymmetric orientation of facies away from the fault and back-rotation of strata toward the basin margin. Extremely thick (>10 km) coarse alluvial fan deposits along the northern margin of Dubawnt sub-basin are inferred to have been deposited in a narrow strike-slip basin, but inferred basin-margin faults are now covered by water. 4.2. Block-faulting of the Baker and Whart sequences The predominant structural feature of Baker Lake sub-basin is inward-dipping successions along its north and south margins, which contrast with sub-horizontal bedding attitudes that prevail in the centre of the basin. This disparity requires the presence of a series of unconformities, folds or faults. Although marked facies variations are apparent, especially across the southern margin of the basin, the Baker sequence appears to be a conformable, genetically related package bounded by high-order unconformities (against Archean basement and the younger Whart sequence). Significant angular unconformities within the Baker sequence have not been recognised, with the exception of an apparent unconformity above sequence BL-0 at Thirty Mile Lake (Fig. 6a), a possible pre-rift depositional phase not recognised elsewhere in Baker Lake Basin (Hadlari and Rainbird, 2000). An alternative explanation for the steeply opposed

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strata in Baker Lake sub-basin is a large syncline, but this is difficult to accommodate given the strong evidence for rotational block-faulting (described below) and lack of folding in the Whart and Barrens sequences. Strata of the Baker sequence dip steeply northward (up to 85◦ ), along the south side of Baker Lake sub-basin (Fig. 6a). Toward the centre of the basin, north of Thirty Mile Lake, conglomerate of the basal Whart sequence crops out above volcanic rocks of the Baker sequence (Fig. 6a; Hadlari and Rainbird, 2000). There, both sequences are tilted to the north about 60◦ . A few kilometres north of this outcrop, an exposure of felsic minette flows of the Baker sequence dips at about 30◦ to the north. Exposure is poor further north, but dips continue to decrease in steps of 10◦ –15◦ such that at the basin centre, the strata are sub-horizontal. Near the southwest tip of Pitz Lake (Fig. 7), flat-lying strata of the uppermost Baker sequence (Kunwak Formation) are disconformably overlain by fluvial conglomerates of the basal Barrens sequence (Thelon Formation), implying that the Whart sequence was either removed or not deposited before deposition of the Barrens sequence. Stratigraphic repetition and incremental decrease in bedding attitude toward the centre of the basin are consistent with a series of subparallel, ENE-striking and south-dipping normal faults that step down southward, toward the margin of the basin. Blocks in the centre of the basin appear to have been uplifted relative to those at or near the margin (see Figs. 6a and 15). Slickensides and foliation along one of these faults, exposed on and north of the shore of “Bunny bay” (Fig. 6a), confirms its south-dipping orientation. The normal faults and the basin margin are separated in an apparent strike-slip sense by a conjugate set of faults oriented at 340◦ and 040◦ . The 340◦ faults show dextral offset; faults trending 040◦ show sinistral offset (Fig. 6a; Hadlari and Rainbird, 2001). A similar pattern of faulting can be seen on the north side of the Baker Lake sub-basin (Fig. 12; Hadlari and Rainbird, 2000) and in the eastern Baker Lake sub-basin on Christopher Island (Hadlari and Rainbird, 2001). At the northern margin of the basin, near Pitz Lake, Whart sequence strata are repeated across an inferred basin-parallel normal fault (Fig. 12). Strata south of the fault dip more gently (<20◦ ). The dip of this normal fault is assumed to be northward

because strata dip to the south with progressive flattening toward the centre of the basin. If the fault geometry on opposing sides of the basin is similar, then the centre of the basin would be a horst, with normal faults stepping out and rotating strata inward (Fig. 15). The Baker Lake and Whart sequences are also segmented along the northern margin of the basin by prominent sets of faults trending 340◦ and 310◦ . These apparent strike-slip faults are similar to transfer faults described by Gibbs (1984), that form along the margins of rift basins to accommodate differences in the slip rate along normal faults in adjacent basin segments. Bedding attitude across these faults ranges from 20◦ to 80◦ toward the south (Fig. 12). Similarly oriented faults, including ENE normal faults, dissect the Angikuni, Dubawnt and Kamilukuak sub-basins (Fig. 3; Aspler et al., 1999; Peterson, 1994; Peterson, in press; Tella and Eade, 1985). Faults post-date the Whart sequence, but do not affect the Barrens sequence, which is sub-horizontal throughout the region. This suggests that block-faulting followed deposition of the Whart sequence, but preceded deposition of the Barrens sequence. However, several lines of evidence lead us to conclude that some faulting was contemporaneous with deposition of the Whart sequence, as follows. In the Baker Lake sub-basin, a prominent 340◦ fault, on the west side of Long Lake, separates blocks of Whart sequence strata with markedly different stratigraphic composition (Fig. 12). On the east side of the fault is a >450 m thick unit of Amarook sandstone (Whart sequence 1) that is overlain unconformably by the Barrens sequence. On the west side, the same sandstone unit is <200 m thick and is overlain by strata of Whart sequences 2 and 3 (see Pitz Lake North composite Section C on Fig. 13). Whart sequence 2 and 3 strata are not preserved on the east side of the fault and thus could have been removed by uplift and erosion before deposition of the Barrens sequence. The greater thickness of the sandstone on the east side could be explained by earlier but opposite vertical movement on the fault that would have occurred before deposition of Whart sequence 2. Other, indirect evidence of syn-Whart sequence faulting is the presence upward-fining depositional sequences characterised by erosional unconformities overlain by coarse, locally derived conglomerates (Fig. 13).

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Fig. 15. Proposed tectonic evolution of the central Baker Lake sub-basin from field mapping and stratigraphic analysis of the Dubawnt Supergroup. Schematic cross-section from Long Lake (N) to western Thirty Mile Lake (S) (Fig. 3). Baker sequence: extension, rifting and development of e.g. Baker Lake sub-basin half-graben; alluvial fan and ephemeral lake sedimentation with coeval minette volcanism. Whart sequence W-1: thermal sag following development of Baker Lake Basin (Baker sequence), represented by deposition of a relatively widespread eolian sandstone. Whart sequence W-2: block-faulting indicated by deposition of coarse conglomerates in restricted basins followed by extension-related felsic magmatism. The cross section reveals a central uplifted block flanked by downward-stepping blocks with progressively inward-steepening bedding attitudes. Whart sequence W-3 and W-4: renewed block-faulting with associated alluvial sedimentation and felsic magmatism. Barrensland sequence B-1: northward shift and broadening of basin depocentre. Thermal subsidence suggested by cessation of faulting and by deposition of regionally correlative succession of fluvial and eolian sandstones.

4.3. Tectonostratigraphic model 4.3.1. Rift phase: Baker sequence The Baker sequence is here interpreted to record the initial and principal phase of development of Baker Lake Basin (Fig. 15). On the southern margin of Baker Lake sub-basin and in northern Angikuni sub-basin, thick, alluvial fan to braided fluvial and lacustrine depositional sequences (third-order) are inferred to represent a response to pulses of uplift,

possibly linked to basin-margin growth faulting. Paleocurrents from these strata indicate mainly transverse drainage of basin-margin uplands (Fig. 7). Finer alluvial deposits in the centre of the basin display axial drainage toward a depocentre in the eastern part of the sub-basin, where thick ephemeral lacustrine and eolian deposits are preserved. Similar thick alluvial deposits are used to infer proximal basin-margin faults in the Wharton and Dubawnt sub-basins. The Baker sequence (second-order) in

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several of these localities is upward fining, indicating increased accommodation as high rates of subsidence outpaced the sediment flux. Elsewhere, such as along the northern margin of Baker Lake sub-basin, third-order sequences are unconformity-bounded and thinner, and show overall upward-coarsening. This indicates that accommodation was less in these areas due to decreasing subsidence. Sediment flux was also diminished on the north side of the basin, but supplemented by episodic volcanic input that caused overfilling and bypassing to areas of greater accommodation. A restored N-S section across-central Baker Lake sub-basin of the Baker sequence indicates a half-graben geometry with principal subsidence and significant normal displacement along the south side of the basin (cf. Leeder and Gawthorpe, 1987; Osmundsen et al., 2000; Figs. 8 and 15). A similar half-graben geometry is interpreted for the Wharton sub-basin and the northern part of Angikuni sub-basin. Along the east side of the northern Angikuni sub-basin, third-order sequences are correlated with those on the south side of the Baker Lake sub-basin at Thirty Mile Lake (Fig. 6b), suggesting that the two sub-basins were contiguous prior to uplift and block-faulting. In the Wharton sub-basin, coarse alluvial fan deposits abut normal faults along the northeastern margin of the basin and taper southwestward into axial fluvial and lacustrine deposits. On the opposite side of the basin, the finer deposits intertongue with and overlie contemporaneous volcanic rocks. The inferred narrow, parallel fault-bounding basin geometry and lateral stacking of extremely thick, coarse clastic fill in the Dubawnt sub-basin are consistent with deposition in a strike-slip basin (Crowell, 1974; Steel and Gloppen, 1980). Its orientation is parallel to northeast-trending faults such as the Tulemalu fault and the Amer mylonite zone, suggesting that the Dubawnt sub-basin may be the expression of a similar major through-going structure. Throughout Baker Lake Basin, sequence stratigraphic analysis indicates that away from basin-margin faults, the Baker sequence is dominated by thick and regionally extensive packages of ultrapotassic volcanic rocks (minettes) including flows and related volcaniclastic rocks. Correlation of volcanic-dominated sequences with characteristically sediment-dominated sequences along basin margins supports a genetic link between volcanism and faulting associated with basin

initiation. The presence of minette clasts in correlative basin-margin alluvial fan facies, also indicates that volcanism and sedimentation were coeval. Petrogenetic studies reveal that minette magmas were derived from an extensive reservoir of enriched upper mantle near the base of the continental lithosphere (Cousens et al., 2001; Peterson et al., 1994, 2002). Magmas likely were transported upward with the aid of faults that penetrated deep into the lithosphere. Such faults are common in continental rift settings where they surface as low-angle crustal-scale detachments (e.g. Wernicke, 1985). The minette magmas of the Baker sequence are coeval with regionally extensive, isotopically similar, granitoid intrusions (1.85–1.80 Ga Hudson suite of Peterson et al., 2002). The intrusions are widely considered to have been emplaced during crustal thickening and associated melting that accompanied northwestward collision of the Superior Province into western Churchill Province (Trans-Hudson orogen, Fig. 1; e.g. Lucas et al., 1999). The Trans-Hudson orogen represents an episode of crustal shortening and was perhaps related to growth of an elevated region, analogous to the Tibetan Plateau, which developed in response to the northward collision into Asia of a series of continental blocks, culminating with India. Resulting growth of the Himalaya and the Tibetan Plateau, beginning in the Eocene (e.g. Molnar and Tapponnier, 1975) was accompanied by north-directed folding and thrusting and by lateral (east and southeastward) escape of a series of strike-slip-fault-bounded blocks. This produced a series of strike-slip and rift basins such as the Qaidam, Junggar, and the Tertiary rift basins of Indochina (Metivier et al., 1998; Morley et al., 2001; Tapponnier and Molnar, 1979). The sub-basins of the Baker Lake Basin are like those of the Tibetan Plateau, both in their proximity and orientation with respect to a major continental collision zone and in their local tectonic settings, stratigraphy and sedimentology. Minettes appear not to be part of the basin fill, but are described from the adjacent foreland (Chung et al., 1998; Miller et al., 1999). Baker Lake Basin formed at the same time as a major phase of tectonothermal reworking of much of the western Churchill Province at ca. 1850–1800 Ma, including NW-directed thrusting and folding and mainly dextral transcurrent faulting (Hanmer et al., in

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preparation). The deformation is considered to be related to terminal collision in the Trans-Hudson orogen. 4.3.2. Extensional modification phase: Whart sequence The Whart sequence represents a stage of basin evolution that is transitional between the initial rifting phase and a later phase of broader, protracted, thermal subsidence. It is the sedimentary record of intrabasinal segmentation generated by a conjugate set of basin-normal, transfer faults and contemporaneous, basin-parallel, normal faults. This block-faulting was accompanied by anorogenic, bimodal magmatism (Nueltin-suite). Associated differential uplift significantly reduced the size of the preserved depositional basin, and shifted northward the locus of sedimentation. A distinctive eolian sandstone at the base of the Whart sequence (W-1) is relatively widespread, suggesting a period of quiescence and broad thermal subsidence following deposition of the Baker sequence (Fig. 15). Overlying sequences (W-2–4) signal a return to fault-influenced sedimentation beginning with coarse but relatively thin alluvial fan conglomerates that typically fine upward into sandy fluvial and eolian-ephemeral lake deposits. Alluvial sedimentation and coeval volcanism were more localised, as indicated by stratigraphic discontinuity and variable paleocurrent patterns between adjacent fault blocks. Most faulting, sedimentation and volcanism were contemporaneous, but the greatest fault offsets and tilting appear to have post-dated deposition of the Whart sequence (i.e. after 1.75 Ga). Faulting ceased before deposition of the overlying Barrens sequence (i.e. by 1.72 Ga). Basin segmentation by block-faulting is common to many continental rifts described from the geological record (for summaries see Leeder, 1995; Sengör, 1995; Sengör and Natal’in, 2001). Analogous and contemporaneous basin evolution and fault geometries are described in studies of the ca. 1.80–1.75 Ga Leichhardt, 1.73–1.69 Ga Calvert and ca. 1.67–1.55 Ga Isa superbasins in northern Australia (Betts et al., 1998; Betts et al., 1999; Jackson et al., 2000; Giles et al., 2002) and in the Espinhaço Basin of Brazil (Martins-Neto, 2000). The block-fault geometry suggests that the Whart sequence developed in response to regional episodes

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of alternating N-S and E-W extension and subsidence. Extension coincided with a thermal episode represented by widespread 1.76–1.75 Ga granite-rhyolite (Nueltin-suite) magmatism in the western Churchill Province (Loveridge et al., 1988; Peterson and van Breemen, 1999; Peterson et al., 2002). Much of the western Churchill Province was uplifted during this episode, but extension seems to have been focused in the region of Baker Lake sub-basin, which was already weakened by earlier extension (see Section 4.3.1). The persistence of terrestrial environments indicates that regional uplift was maintained throughout deposition of Whart sequence. Present levels of exposure indicate that the Baker Lake region has been preserved at higher crustal levels than other parts of Churchill Province, especially to the east and southeast (LeCheminant et al., 1987). The Whart sequence is notably absent throughout most of the southwestern Baker Lake basin (Figs. 2 and 3). In its place are large intrusions of Nueltin-suite granite indicating that this region was uplifted relative to areas in the northeastern part of the basin. Some uplift was before deposition of the Barrens sequence because it directly overlies Nueltin-suite granites along the southeastern margin of Thelon Basin (Figs. 2 and 3). Uplift and faulting ca. 1.76–1.75 Ga were accompanied by injection of basaltic magma into lower to middle crust, resulting in crustal melting and mingled basalt–granite volcanism (Peterson et al., 2002). Triggering of basaltic magmatism requires mantle melting, and hence mantle upwelling. The mechanism must also allow for downdropping and thinning of the lithosphere (possibly by delamination of the lower lithosphere) over the mantle upwelling. Deposition of the Whart sequence was coeval with ∼1.75 Ga thermal events elsewhere in the western Churchill Province (Rb–Sr resetting in titanite and K–Ar from micas; Loveridge et al., 1988 and Pb–Pb/U–Pb ages from rutile; Orrell et al., 1999). Hence the thermal trigger may have been a much broader phenomenon than faulting and volcanism. Near-identical calc-alkaline provinces of this age are common elsewhere in the Ketilidian (Greenland), Baltica, and Amazonia (Peterson et al., 2002). These provinces typically occur within the deformed Archean (less commonly early Paleoproterozoic) hinterlands of major collision zones such as the Trans-Hudson orogen of Laurentia.

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4.3.3. Sag phase: Barrens sequence The Barrens sequence represents deposition over a broader area, primarily in Thelon Basin, and records a significant northward shift in the locus of sedimentation. Strata generally are thinner and gently dipping relative to underlying sequences. Lateral continuity with very little facies variation or asymmetry indicates a lack of influence from syndepositional faulting. Progressive upward fining culminates in the first record of marine transgression anywhere within the Dubawnt Supergroup. Magmatism (Kuungmi Formation) did not play as significant a role in the depositional history of the Barrens sequence, and appears not to be related to faults as in the underlying sequences. Taken together these features suggest the Barrens sequence was deposited over a broad region of thermal subsidence, likely due to cooling of previously thinned continental lithosphere. Deposition closely followed a significant regional crustal melting episode represented by the Nueltin-suite granite and associated volcanic rocks (Peterson et al., 2002). Regional stratigraphic correlation and paleocurrent patterns, which indicate a consistent westerly trend, suggest that the Barrens sequence may be the preserved remnant of a contiguous cratonic sand sheet that included the Athabasca Group of Athabasca Basin to the southwest (Ramaekers, 1981), the Horny Bay Group of Amundsen Basin (Kerans et al., 1981), and the Ellice and Tinny Cove formations of Elu Basin to the northwest (Campbell, 1979; LeCheminant et al., 1996). Distal parts of this huge fluvial and locally eolian system may be preserved in finer grained “basinal” deposits located along the western margin of Laurentia such as the Wernecke Supergroup (Delaney, 1981; Thorkelson et al., 2001) and Muskwa Assemblage (Ross et al., 2001), as originally proposed by Fraser et al. (1970). If these basins were contemporary, then one must appeal to a continent-scale subsidence mechanism such as those that accompany supercontinent assembly and dispersal, and similar to those proposed for the early Phanerozoic intracontinental basins of North America (Burgess et al., 1997; Quinlan, 1987). One model for Thelon Basin holds that it formed by uplift and flexure related to indentation of the Rae Domain by the Slave Province (Fig. 1; Gibb, 1978; Henderson et al., 1990), but this may not apply directly to the potentially correlative basins. It may

apply indirectly in the sense that this collision was part of the late-stage, global-scale amalgamation of Laurentia and subsidence that was accompanied by continent-scale mantle downwelling (Hoffman, 1988; Hoffman and Peterson, 1991). A recent model for similar and contemporaneous intercontinental basins in northeastern Australia proposes that they formed due to the inboard effects of processes such as slab roll-back and subcontinental mantle convection (Giles et al., 2002). These processes were produced along an inferred long-lived subduction zone located up to 1500 km away. This model also draws analogy to development of the Cretaceous-Tertiary basins of eastern China, where initial rift basins are overlain by broad shallow basins filled with fluvial and eventually marine sediments.

5. Conclusions The Dubawnt Supergroup comprises three, secondorder, unconformity-bounded sedimentary sequences that record deposition within an intracontinental rift basin, a modified intracontinental rift basin and a thermal sag basin, respectively. The Baker sequence is inferred to represent the initial and principal phase of the development of Baker Lake Basin, a series of generally elongate, northeast-striking, fault-bounded troughs and half-graben filled with continental redbeds and coeval thick units of ultrapotassic volcanic rocks. The nature and timing of basin development and associated magmatism suggest a transtensional basin that formed by lateral escape in response to crustal thickening during indentation of the Hearne Domain by the Superior craton (Trans-Hudson orogen). Baker Lake Basin is similar to Mesozoic and Cenozoic strike-slip and associated rift basins that developed along the margins of the Tibetan Plateau in response to lateral escape of crust thickened by the collision of India with Asia. Direct evidence of strike-slip deposition is limited to a narrow fault-bounded trough containing very thick alluvial fan deposits (Dubawnt sub-basin). Elsewhere, variably oriented, elongate basins indicate fault-normal extension and half-graben formation. Such extension is normal to the axis of the Baker Lake basin and to the known strike-slip faults and is thus part of and consistent with an overall strike-slip feature. The rarity of exposed basin-bounding faults

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suggests that they were buried during backstepping and subsequent sedimentation. The tectonic escape model is further supported by the proximity and sub-parallel orientation of Baker Lake basin relative to the Trans-Hudson orogenic front. Some structures parallel to, and southwest of, the basin margin, such as the Tyrell and Nowyak shear zones, exhibit evidence for oblique-slip, with a component of northwest-side-down displacement, at the same time as deposition of the Baker sequence (ca. 1.83–1.81 Ga; MacLachlan et al., 2000; ter Meer et al., 2000). Evidence for contemporaneous, northwest-vergent thrusting and folding in the adjacent Rae and Hearne domains is not incompatible with this model as these could be interpreted as frontal thrusts of the Trans-Hudson deformation (Hanmer et al., in preparation). Baker Lake Basin is thus interpreted to have formed adjacent to areas of maximum shortening via lateral escape. A similar relationship between contractional, translational and extensional structures can be observed on tectonic maps of southern Asia (e.g. Sengör, 1995, Fig. 2.18, p. 90). The overlying Whart sequence was deposited in small basins controlled by block-faulting and tilting of Baker sequence strata. It has a more restricted distribution and represents a northward shift in the locus of sedimentation. Basin fill comprises eolian and alluvial to lacustrine redbeds with intercalated rhyolite flows and epiclastic rocks. Mapping and sequence correlation have helped us to recognise block-faulting defined by regional ∼ENE normal faults and contemporaneous conjugate (∼340◦ and 040◦ ) strike-slip faults (Hadlari and Rainbird, 2001). This block-faulting is best documented in the central part of Baker Lake sub-basin where a N-S cross section reveals a central uplifted block flanked by downward-stepping blocks with progressively inward-steepening bedding attitudes (Fig. 15). Distribution of lithofacies within fault-bounded domains suggests that the faults were active throughout deposition, but the greatest offset and tilting occurred after deposition of the Whart sequence (∼1.75 Ga). Major faulting ceased before deposition of the overlying Barrens sequence (Thelon Formation; ca. 1.72 Ga). Fault geometry suggests that the underlying basins developed in response to alternating periods of N-S and E-W extension and subsidence; deformation coincided with therefore must be related to a regional

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episode of granite magmatism and associated regional thermal events (ca. 1.76–1.75 Ga). This faulting and magmatism significantly modified the distribution and preservation of the Baker sequence, thereby obscuring some of the original geometry and stratigraphy of Baker Lake Basin. The Barrensland second-order sequence represents deposition over a much broader area, primarily in Thelon Basin, and represents a northward shift in the locus of sedimentation. Strata generally are thinner and flatter than in underlying sequences and display lateral continuity with little facies variation or asymmetry, indicating a lack of influence from syndepositional faulting. Deposition records progressive upward fining and eventually the first record of marine transgression in Thelon Basin. Magmatism was insignificant compared to that recorded in the underlying sequences. These features suggest the Barrens sequence represents deposition in an intracontinental sag above a broad region of thermal subsidence, likely related to cooling of previously attenuated continental lithosphere. The Barrens sequence may represent a huge cratonic sand sheet, remnants of which are preserved in the Thelon, Athabasca, Amundsen and Elu basins. Distal parts of this huge fluvial and eolian system may be preserved in finer grained “basinal” deposits located along the western margin of Laurentia. If these basins were contemporaneous, then a large-scale subsidence mechanisms, such as those that accompany supercontinent assembly, must be invoked. The Barrens sequence was deposited during the late-stage, global-scale amalgamation of Laurentia, so it is possible that the subsidence resulted from accompanying continent-scale mantle downwelling.

Acknowledgements This study received logistical support from the Polar Continental Shelf Project (Natural Resources Canada) and its operators, Baker Lake Lodge and Comaplex Minerals Ltd. Cameco Corporation provided access to drill core, accommodation and logistical support for our work in Thelon Basin. Numerous discussions with members of the western Churchill NATMAP working group helped us formulate our ideas. Erika Greiner, Sandra Ookout and Derek Smith provided field assistance. Quentin Gall collected the clast imbrication

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