Fluvial, Lacustrine and Volcanic Sedimentation

Precambrian Research 129 (2004) 225–250

Fluvial, lacustrine and volcanic sedimentation in the Angikuni sub-basin, and initiation of ∼1.84–1.79 Ga Baker Lake Basin, western Churchill Province, Nunavut, Canada Lawrence B. Aspler a,∗, Jeffrey R. Chiarenzelli b, Brian L. Cousens c a 23 Newton Street, Ottawa, Ont., Canada K1S 2S6 b Department of Geology, State University of New York at Potsdam, Potsdam, NY 13676, USA c Department of Earth Sciences, Carleton University, Ottawa, Ont., Canada K1S 5B6

Abstract Continental siliciclastic and volcanogenic deposits of the Baker Lake Group accumulated in numerous sub-basins in the interior of the western Churchill Province between 1.84 and 1.79 Ga. In the Angikuni sub-basin, on the southeast flank of greater Baker Lake Basin, Baker Lake Group rocks outcrop in two segments that extend northeast from Angikuni Lake. They are also exposed in scattered outliers throughout the region. At northern Angikuni Lake in the northern segment, conglomerates, pebbly sandstones and mudrocks of the Angikuni Formation unconformably overlie Archean basement, and are unconformably overlain by ultrapotassic volcanic and siliciclastic rocks of the Christopher Island Formation. These rocks record alluvial fan-fluvial and sand flat- playa deposition in a fault-bounded trough formed adjacent to a wedge-shaped basement uplift. Although the Angikuni Formation was tilted before principal Christopher Island Formation volcanism at northern Angikuni Lake, geochemical and Nd isotopic data from mudrocks indicate derivation from earlier or coeval Christopher Island volcanic-like sources. The outliers demonstrate that faulting produced significant changes in the structural level of Archean basement before Christopher Island Formation volcanism. Near “Rack” lake in the southern segment, Angikuni Formation conglomerates, sandstones and mudrocks define 100-m scale upward-fining and upward-coarsening to upward-fining sequences. Relative to the northern segment, these rocks were deposited in a more distal, fine-grained sand flat to semi-perennial fresh-water (±evaporitic) lake setting. The Angikuni Formation at “Rack” lake records deposition between (and likely during) periods of volcanism as indicated by: a conformable Angikuni–Christopher Island contact; abundant volcanic detritus; and local lacustrine chemical sediments that contain magnesite, strontianite, barite and apatite, which reflect the chemistry of the volcanic rocks. Baker Lake Basin may have originated during regional uplift and extension within the western Churchill Province due to terminal collision and post-collision processes in Trans-Hudson orogen to the south, while the western margin of ancestral North America was a free face. © 2003 Elsevier B.V. All rights reserved.

Keywords: Alluvial fan; Fluvial; Sand ﬂat; Lacustrine; Magnesite; Ultrapotassic magmatism; Western Churchill Province

1. Introduction interiors. The western Churchill Province of northern Canada, caught between Taltson-Thelon and Wopmay Orogenic processes on plate margins commonly orogens on the west, and Trans-Hudson orogen on the lead to deformation and sedimentation in continental south (Fig. 1) is a prime example. It was extensively reworked during a series of 2.0–1.7 Ga tectonic events ∗ Corresponding author. historically referred to as the “Hudsonian orogeny”. E-mail address: [email protected] (L.B. Aspler). During and following major Hudsonian events, the

Fig. 1. Regional setting of the Baker Lake Group and tectonic elements of the western Canadian Shield. Interpretation of subsurface domains after Green et al. (1985), Hoffman (1989), Ross et al. (1991), and Hanmer et al. (1995). western Churchill Province was an extensive de- terozoic strata in western North America (Fig. 2) pocentre for predominantly siliciclastic debris. Fraser introduced by Young et al. (1979). The Dubawnt et al. (1970), Donaldson (1973) and Young (1977) Supergroup includes three unconformity-bounded correlated strata that accumulated in different parts sequences, in ascending order (Fig. 3): the Baker of the cratonic interior, and extended these correla- Lake Group (1.84–1.79 Ga); the Wharton Group tions to deposits near the western margin of ancestral (<1.79–>1.75 Ga); and the Barrensland Group North America (Fig. 2). Modern ﬁeld, geochrono- (<1.75–>1.72 Ga). This paper focuses on deposits in logic and geophysical studies (e.g., Ross et al., 2001) the oldest of these sequences, the Baker Lake Group. have largely substantiated and further reﬁned this With the possible exception of the Martin Forma- work. One of the cratonic successions, the Dubawnt tion, unconformably beneath rocks of the Athabasca Supergroup (Gall et al., 1992; Rainbird et al., 2003), Basin in Saskatchewan (Fig. 2; Donaldson, 1968; extends across the north-central part of the western Tremblay, 1972), the Baker Lake Group has no Churchill Province, in Baker Lake and Thelon basins known correlative in northwestern Canada. Hence (Fig. 1). These rocks form part of “Succession A”, basal units in Baker Lake Basin provide a valuable the lowermost of the three (A–B–C) widely accepted record of the magmatic, paleoclimatic and tectonic subdivisions of Paleo-Mesoproterozoic and Neopro- conditions during the middle- to late-stages of the L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250 227

Fig. 2. Syn- to post-Hudsonian cover sequences in northwestern Canada. Modiﬁed after Fraser et al. (1970), Donaldson (1973), Young (1977), Cook and MacLean (1995). Succession A1 to A4 designations after Cook and MacLean (1995) and McLean and Cook (2003). Additional sources: Athabasca Basin (Ramaekers, 1981), Thelon Basin (Rainbird et al., 2003), Elu Basin (Campbell, 1979), Et-Then Basin (Ritts and Grotzinger, 1994), Coppermine Homocline (Baragar and Donaldson, 1973; Kerans et al., 1981; Bowring and Ross, 1985; Ross and Kerans, 1989).

Hudsonian orogeny. Herein we present the results Angikuni sub-basin represent variants of volcanic and of field, petrographic and geochemical work from continental sedimentation in local depocentres during the Angikuni and Christopher Island formations at initiation of greater Baker Lake Basin. Throughout the base of the Baker Lake Group in the Angikuni these initial stages, alluvial fan, fluvial and lacustrine Lake and “Rack lake” (informal name) areas (Fig. 4). (physical and chemical) sedimentation occurred to- These data indicate initial deposition in continental gether with magmatism, albeit in varying proportions, sub-basins that became filled with coarse siliciclastic much like the Ziway–Shala lake basin system of deposits and ultrapotassic volcanic rocks. We inter- the Main Ethiopian Rift (see Le Turdu et al., 1999). pret a relatively wet paleoclimate with brief periods Fault-induced subsidence was likely due to regional of semi-aridity. Together with the South Channel and uplift and extension resulting from 1.84–1.79 Ga colli- Kazan formations in the Baker Lake area, units in the sion and post-collision convergence in Trans-Hudson 228 L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250

Fig. 3. Generalized lithostratigraphy of the Dubawnt Supergroup. Modified after Rainbird et al. (2003). Christopher Island age is from MacRae et al. (1996; Pb–Pb apatite from dyke); Kunwak age is from Rainbird et al. (2000; Pb–Pb calcite, from travertine); Pitz ages are from Rainbird et al. (2003; U–Pb zircon from felsic flows). Thelon age is from Miller et al. (1989; Pb–Pb diagenetic apatite). orogen on the southern flank of the western Churchill near Angikuni and “Rack” lakes as the “Angikuni Province, during which time ancestral North America sub-basin” (Fig. 4). In the Angikuni sub-basin, Baker may have been a free face (Fig. 1). Lake Group rocks outcrop in two segments extending northeast from Angikuni Lake (“northern” and “southern” segments; Fig. 4), as well as in scattered 2. Geologic setting outliers throughout the region. In the Angikuni Lake area, basement to the Baker Continental siliciclastic and volcanogenic deposits Lake Group includes Neoarchean greenschist- to of the Baker Lake Group accumulated in numerous amphibolite-grade supracrustal rocks, upper amphibo- fault-bounded sub-basins in the interior of the west- lite-grade gneisses and gabbroic to granitic plutons ern Churchill Province between ∼1.84 and 1.79 Ga (Aspler et al., 1999b). These rocks define discrete (Figs. 1 and 4). Herein we refer to this collection lithostructural domains that are separated by a net- of sub-basins as “greater Baker Lake Basin”; fol- work of shear zones consisting of mylonitic rocks and lowing Rainbird et al. (2003), we refer to deposits younger greenschist-grade cataclasites (Fig. 4). The L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250 229 . rn segments, Baker Aspler et al. (1999b) . Shear zones at Angikuni Lake after Rainbird et al. (2003) Fig. 4. Regional geology of greater Baker Lake Basin and location of Angikuni sub-basin. In addition to principal exposures in the northern and southe Lake Group rocks outcrop as small outliers throughout the Angikuni Lake area. Modified after 230 L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250 shear zones branch in a pattern similar to the pos- Snowbird zone is a composite entity, in which different itive flower structures described from transpressive segments experienced remarkably different histories. terrains. Two northeast-trending, dextral strike-slip The lower and middle parts of the Baker Lake shear zones are of particular relevance for addressing Group are represented in the Angikuni sub-basin. the style of faulting during deposition of the Baker These include conglomerates, sandstones and mu- Lake Group. The first (“eastern shear zone”, Fig. 4) drocks of the Angikuni Formation, and ultrapotassic continues along the eastern shore of Angikuni Lake, dykes and flows, with interleaved conglomerates extending northward, close to the northwestern mar- and sandstones, of the Christopher Island Formation gin of the southern Angikuni sub-basin segment. (Fig. 3). Geochronologic studies indicate that the This shear zone displays evidence of Neoarchean basin formed between ∼1.84 and 1.79 Ga. Reported (∼2.62–2.61 Ga) amphibolite-grade ductile dextral ages include: (1) 1825 ± 12 Ma from a plutonic body movement, but fault breccias, consisting of myloni- interpreted to be co-magmatic with the Christopher tized tonalitic gneiss fragments set in a quartz stock- Island dykes (40Ar/39Ar hornblende, Roddick and work vein system, and titanite ages of ∼1.85 Ga, Miller, 1994); (2) 1832 ± 28 Ma from a Christopher suggest Paleoproterozoic reactivation (Aspler et al., Island dyke (Pb–Pb isochron, apatite, MacRae et al., 1999b, 2000). The second shear zone (Tulemalu fault 1996); (3) ∼1.84–1.81 Ga from Christopher Island of Tella et al., 1986; Fig. 4) coincides with the north- flows (40Ar/39Ar phlogopite, Rainbird et al., 2002); western margin of the northern Angikuni sub-basin (4) 1813 ± 37 Ma from diagenetic xenotime over- segment. Greenschist grade mylonitic rocks from growths on detrital zircon (U–Pb, Rainbird et al., the Tulemalu fault in western Angikuni Lake have 2002); and (5) 1785 ± 3 Ma from travertine in the yielded complex Paleoproterozoic ages (Aspler et al., Kunwak Formation near the top of Baker Lake Group 2000). As will be discussed further below, although (Pb–Pb calcite, Rainbird et al., 2000). reactivation of both shear zones during sedimentation In the following, we first consider Baker Lake is likely, we lack definitive field evidence of fault Group rocks exposed in the Angikuni Lake area, kinematics during this reactivation. and then examine outcrop and drill-core data from The Angikuni Lake area straddles the Snowbird tec- the southern segment of the Angikuni sub-basin at tonic zone, the geophysical feature defining the bound- “Rack” lake (Fig. 4). ary between the Hearne and Rae domains of the western Churchill Province (Fig. 1). The Snowbird zone at Angikuni Lake is manifested geologically by a broad 3. Angikuni Lake area network of tectonites (Fig. 4; Aspler et al., 1999b), although it is conventionally portrayed as a single In the Angikuni Lake area, Baker Lake Group geological entity, the Tulemalu fault. The Snowbird rocks outcrop in two northeast-trending segments zone was provisionally considered a Paleoproterozoic that extend from northern Angikuni Lake (Fig. 4). suture (Hoffman, 1988), but recent studies in exposed In addition, numerous small (typically <100 m2), but segments of the Snowbird zone west of Hudson Bay significant, outliers drape Archean basement through- indicate that the Rae and Hearne domains share a com- out the region, continuing south and west of Angikuni mon Neoarchean history (Dudás et al., 1991; Schau Lake (Aspler et al., 1998; 1999a,b). and Tella, 1993; Hanmer et al., 1995; Aspler et al., 1999b; Cousens et al., 2001). Hence, although the site 3.1. Angikuni Formation of repeated Paleoproterozoic reactivation, the exposed trace of the Snowbird zone does not appear to consti- The Angikuni Formation (Blake, 1980)isan tute a Paleoproterozoic suture. However, on the basis upward-fining siliciclastic succession. At its type area of drill core and geophysical data from the subsurface in northern Angikuni Lake, it lies unconformably extent of the zone to the southwest, Ross et al. (1991, on Archean basement and is unconformably over- 1995, 2000a,b) maintained that the Rae and Hearne lain by the Christopher Island Formation (Fig. 5). domains did indeed weld along the Snowbird zone It includes a lower unit of conglomerate and arkose between ∼1.85 and 1.80 Ga. If both views hold, the (up to 1200 m thick), and a poorly exposed upper L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250 231

Fig. 5. Baker Lake Group, northern Angikuni Lake, illustrating the angular discordance between steeply dipping Angikuni Formation strata and overlying shallowly dipping Christopher Island Formation flows (after Aspler et al., 1999b). Paleocurrents are from lower Angikuni Formation. unit of fine-grained sandstone, siltstone and mud- (Fig. 5). Shallowly (∼10◦) east-dipping Christopher stone (up to 1400 m thick). The lower unit consists Island mafic and felsic minette flows lie above steeply of carbonate-cemented, parallel and cross-stratified (∼45◦) east-dipping Angikuni strata. The Christopher arkose with metre-scale interbeds of conglomerate Island flows thus form a tongue that drapes the pre- and pebbly sandstone (Fig. 6). Decimetre-scale trough viously tilted contact between the lower and upper cross beds in arkose indicate northwest paleocurrents, parts of the Angikuni Formation. away from a faulted wedge of basement gneisses The Angikuni Formation in the northern Angikuni and granites immediately to the southeast (Fig. 5). Lake area is interpreted to represent an alluvial Clast- and matrix-supported conglomerates contain fan-fluvial to lacustrine depositional system. The angular to sub-rounded gneissic and granitic clasts lower unit likely records high-energy sheetfloods derived from immediately adjacent basement set in and sand-rich mass flows which, unimpeded by land a coarse sand to granule matrix. The upper unit con- vegetation, spread rapidly across a fluvial plain. The sists of red siltstone, mudstone and parallel-stratified, paucity of mudrocks probably reflects efficient by- fine-grained sandstone, and contains abundant mud- passing of muds out of the vegetation-free landscape cracks, mudchip breccia layers, and wave ripple due to flash floods and eolian deflation. The upper marks. An angular discordance between the Angikuni unit likely represents a low-relief sand flat in which and Christopher Island formations is well exposed fine-grained sediment accumulated in isolated ponds near the eastern shore of northern Angikuni Lake filling local depressions. The overall upward-fining 232 L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250

Fig. 6. Lower Angikuni Formation at Angikuni Lake. (A) Pebble conglomerate and pebbly sandstone with well-foliated, basement-derived clasts. (B) Parallel-stratified arkose and pebbly arkose. (C) Parallel-stratified and planar cross-stratified arkose. L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250 233

Fig. 7. Incompatible elements in upper Angikuni Formation mudstones normalized to Primitive Mantle (Sun and McDonough, 1989). Upper Angikuni Formation samples from both Angikuni Lake and “Rack” lake exhibit the same depletions in Nb and Ti and enrichments in Th and Ba as Christopher Island Formation flows and dykes. Christopher Island Formation field is from Cousens et al. (2001). trend comprises a single large retrogradational cycle, an Archean metasomatic event and remained in isola- inferred to represent fault-induced basin subsidence tion until tapped during Paleoproterozoic extension of that was followed by backwasting. the western Churchill interior (Cousens et al., 2001, The angular relationship between the Angikuni 2003). and Christopher Island formations indicates that the In the Angikuni Lake area, the Christopher Is- Angikuni strata were tilted before principal volcan- land Formation directly overlies basement, except ism. The steep eastward dip of the Angikuni Forma- in the north, where the Angikuni Formation is de- tion is consistent with back-rotation of bedding along veloped. The Christopher Island Formation consists west-side-down growth faults bordering on the east. of massive to layered phlogopite-bearing mafic and Although Angikuni Formation sedimentation appears felsic minette flows, volcanic breccias containing to have preceded Christopher Island volcanism, the intraformational volcanic clasts with irregular mar- rare earth element patterns of two Angikuni mudrock gins and alteration rinds, local vesicular bombs and samples are remarkably similar to those displayed by mylonitic basement pebbles, and rare beds of accre- Christopher Island volcanic rocks (Fig. 7; cf. Cousens tionary lapilli (Fig. 8A). Local vent facies consist of et al., 2001). As will be discussed further below, interlayered volcanic flows and beds containing up this supports Donaldson’s (1965) suggestion of early to 70% angular basement clasts with long dimen- Christopher Island-type volcanism, which was based sions up to 50 cm. Local units of cross-stratified red on observations of felsic clasts in lower Baker Lake arkose and open-framework polymictic conglomer- Group conglomerates (South Channel Formation; ate with both basement and volcanic clasts (Fig. 8B Fig. 3) near Baker Lake. and C) indicate fluvial sedimentation between volcanic eruptions. Widespread pristine minette dykes, 3.2. Christopher Island Formation likely feeders to the lava flows, display strong northeast trends (Figs. 9 and 10). Geochemical and Nd and Minette dykes and flows of the Christopher Is- Sr isotopic data from Christopher Island Formation land Formation constitute one of the most extensive dykes and flows in the Angikuni area are presented by (240,000 km2) ultrapotassic suites in the world (Fig. 1; Cousens (1999). Peterson, 1994; Peterson et al., 1994). Geochemical Beyond the limits of the two Angikuni sub-basin and isotopic data indicate derivation from a broad segments, the Christopher Island Formation is exposed lithospheric mantle reservoir that was enriched during in many scattered outliers, which consist of minette 234 L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250

Fig. 8. Christopher Island Formation at Angikuni Lake: coeval ultrapotassic volcanism and fluvial sedimentation. (A) Volcanic breccia; note bomb with concentric zonation defined by vesicles (left of pen). (B) Metre-scale planar cross-stratified redbed arkose. (C) Framework-intact polymictic conglomerate and parallel-stratified arkose with single pebble layer. (D) Near-vertical paleofracture concordant with Archean gneissosity filled with volcanic breccia.

flows and local redbed conglomerate and arkose in- ∼25 km to the northeast, near “Rack” lake in the south- terbeds (Aspler et al., 1998, 1999a,b). Volcanic flows ern segment of the Angikuni sub-basin (Fig. 4), a coat basement surfaces in these outliers, defining an 500-m thick (minimum) section of siliciclastic rocks exhumed paleotopography with a minimum relief of intervenes between Archean basement and flows of 25 m (see also Donaldson, 1965). With the exception the Christopher Island Formation. Albeit with differ- of local sedimentary infilling of paleojoints (Fig. 8D), ences (see below), this section is remarkably similar a paleoregolith is lacking. The outliers are signifi- to the upper part of the Angikuni Formation in its type cant because they demonstrate that the principal con- area, and hence we extend the range of the Angikuni trasts in structural level of Archean basement (see Formation to “Rack” lake. above) were attained before Christopher Island For- mation volcanism, and that volcanic rocks once blan- 4.1. Angikuni Formation keted a much more extensive area. The following is based on field mapping in the vicinity of “Rack” lake and on detailed examination 4. “Rack” lake area of 580 m of core recovered from drill hole “Ang 95-1”. This hole was drilled in 1995 by WMC Inter- Blake (1980) restricted the Angikuni Formation to national Ltd., at 62◦31N and 98◦50W, about 10 km its type area at northern Angikuni Lake. However, northeast of “Rack” lake (Setterfield and Tykajlo, L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250 235

Fig. 9. Undeformed Christopher Island Formation minette dyke cutting mylonitic rocks of the “eastern shear zone” along the eastern shore of Angikuni Lake (see Fig. 4). These mylonites record Neoarchean dextral ductile shear, but the nature of Paleoproterozoic reactivation remains unclear.

1995). Five principal facies are represented (Fig. 11). Angikuni Lake, the continuous vertical profile pro- In order of decreasing grain size these include: basal vided by the Ang 95-1 drill core (Fig. 11), indicates breccias; granule-pebble conglomerates; medium- to that the “Rack” lake facies are arranged in five 100-m coarse-grained arkoses; fine-grained arkoses and silt- scale basin-filling sequences (informally, sequences stones; and arkose to mudstone rhythmites. With the 1–5, Fig. 11). Sequences 1–4 fine upward, sequence exception of the basal breccias, each of these facies 5 coarsens upward and then fines upward. contains a significant component of volcanic-derived material. Whereas outcrop limitations precluded de- 4.1.1. Basal breccias termination of possible cyclic sedimentation patterns Breccias form a 19-m thick unit at the base of the within the overall upward-fining sequence at northern Angikuni Formation in drill hole Ang 95-1. Very angular, irregularly shaped, cobble-sized gabbroic clasts are predominant; these are mixed with rare granitic fragments. Similar to immediately subjacent basement, the gabbroic fragments contain abundant epidote, chlo- rite and secondary biotite due to pre-brecciation hydrothermal alteration. The clasts are self-supporting in a red fine-to-medium arkose matrix; some display a jig-saw fit and are separated by mm-scale sand-filled fractures, suggesting in situ brecciation.

4.1.2. Granule-pebble conglomerates Conglomerates (Fig. 12) form decimetre- to metre- Fig. 10. Trends of Christopher Island Formation minette dykes, scale beds near the base of the Angikuni Formation Angikuni Lake area. in drill hole Ang 95-1, comprising the lower part of 236 L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250

Fig. 11. Measured section from core recovered from drill hole Ang 95-1. For purposes of discussion, large-scale upward-ﬁning sequences are labeled 1–5. L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250 237

Fig. 12. (A) Granule-pebble conglomerate with angular volcanic clasts filling scour cut into parallel-stratified medium- to coarse-grained arkose. (B) Inversely graded coarse-grained sandstone to framework-disrupted granulestone with subrounded volcanic clasts. (C) Photomi- crograph of conglomerate with angular feldspar-phyric clasts in coarse-sandstone matrix with iron oxide cement; note welded tuff fragment (upper left). 238 L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250 sequence 1. They also occur higher in the section, in the middle parts of sequence 5 (Fig. 11). The conglomerates typically form massive beds, but locally display parallel stratification and cross stratification, and normal and inverse grading. Some beds have an intact framework, others are framework disrupted, but in all cases, the matrix is coarse sandstone. The conglomerates also locally constitute the basal parts of decimetre- to metre-scale upward-fining sequences that follow the order: basal scour–conglomerate–coarse-grained sandstone–curled mudstone. The clasts are typically subangular to subrounded and subspherical. They reflect mafic to felsic minette sources exclusively, and many preserve excellent primary volcanic textures (Fig. 12C).

4.1.3. Medium- to coarse-grained arkoses These rocks form metre-scale beds at the base of sequence 2 and in the middle of sequence 5, at the tops of decimetre- to metre-scale conglomerate-based flood units (Fig. 11). Most are parallel stratified, but some are cross-stratified. Although consisting primarily of feldspar and lithic (volcanic) grains, mudchip fragments are common.

4.1.4. Fine-grained arkoses Fine-grained arkoses constitute the predominant lithology of the Angikuni Formation in drill hole Ang 95-1 (Fig. 11). The arkoses form uniform, parallel-stratified, locally low-angle cross-stratified sections many 10’s of metres thick. They are com- Fig. 13. Fine-grained arkose-siltstone upward-fining sequences monly punctuated by centimetre- to decimetre-scale capped by discontinuous mudstone partings and intraclasts. layers of erosionally based, locally graded, volcanic granulestone and medium- to coarse-grained arkose, both of which contain red mudstone clasts. Some fine-grained arkose. These may be pseudomorphs after of the granulestone layers contain tabular felsic vol- gypsum. canic clasts that are distorted, suggesting that they were incompletely consolidated during deposition. 4.1.5. Arkose to mudstone rhythmites Dispersed outsized coarse sand volcanic lithoclasts Rhythmites form metre-scale intervals capping are also common. In addition, many sections contain upward-fining sequences 1–4 in the Ang 95-1 drill red mudstone as discontinuous mm-thick partings, core (Fig. 11). They constitute the lower parts of commonly with upward curled edges, and breccias three metre-scale rhythmite to fine-grained sandstone with cm-scale intraclasts. Red mudstones are also at upward-coarsening sequences at the top of sequence 5 the tops of cm-scale fining upward sequences that (Fig. 11). The rhythmites consist of centimetre-scale, follow the order: scoured base-graded sand–mudstone erosionally based upward-fining sequences compris- (Fig. 13). About 53 m above the base of the sec- ing graded, very fine- to fine-grained arkose and tion, sparse (<1%) prisms of dolomite, up to 3 mm siltstone, to mudstone (Fig. 14). The mudstone-rich long and 1 mm wide, are dispersed within very tops of these sequences commonly contain delicate L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250 239

Fig. 14. Arkose to mudstone rhythmites. mm-scale internal laminae. Near-vertical sandstone magnesite (MgCO3), and local barite (BaSO4), stron- dykes, which taper and wedge-out both upward and tianite (SrCO3) and apatite (Ca5(PO4)3(F, Cl, OH)). downward, are also common. In outcrop, some of the This zone is sharply overlain by a 1-mm thick mud- sandstone layers display straight-crested and interfer- stone layer that grades upward to a second chemogenic ence wave ripple marks. Rare earth element patterns zone that, in turn, grades upward to a layer with from three mudstone samples are similar to those abundant tabular mudchip intraclasts (Fig. 15). At the derived from Christopher Island Formation volcanic 70.5-m level, crystals of magnesite, comprising dis- rocks (Fig. 7). crete euhedra and rosette-like clusters up to 0.5 mm, Evaporitic layers occur in mudstone-rich deposits and masses of dolomite up to 0.3 mm, are dispersed near the top of upward-fining sequence 5, 70.5 and in a 2-cm thick silty mudstone layer. Variation in 86.7 m below the top of Ang 95-1 (Fig. 11). At the the concentration of magnesite and dolomite define 86.7-m level (Fig. 15), a 2-cm thick chemogenic a vague stratification in this layer (Fig. 16). In ad- zone displays crude mm-scale layering defined by dition, apatite occurs as 0.05 mm inclusions within light-toned zones of predominantly dolomite, and magnesite crystals and as discrete 0.03–0.08 mm crys- dark-toned zones that contain abundant crystals of tals (Fig. 17), and strontianite forms local 0.05 mm 240 L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250 masses that contain <0.01 mm inclusions of barite. a cliff face north of “Rack lake”, at 62◦27N and The magnesite-dolomite bearing layer abruptly over- 98◦56W. In contrast to the angular unconformity ex- lies siltstone with micro-cross lamination, and grades posed at northern Angikuni Lake, felsic minette flows upsection to microlaminated mudstone lacking chem- conformably drape underlying Angikuni Formation ical sediment. Above this interval are mm-thick al- sandstones. Load casts and flame structures at the con- ternations of silty mudstone enriched with dolostone tact (Fig. 18) demonstrate that the sandstones were and chemical sediment-free micro-graded siltstone to unlithified during volcanism (cf. Needham, 1978; Hall red mudstone (Fig. 16). and Els, 2002). Similar to the Christopher Island For- mation in the Angikuni Lake area, phlogopite-bearing 4.2. Christopher Island Formation mafic and felsic minette lava flows, are predominant in the vicinity of “Rack” lake. Individual flow units are The contact between the Christopher Island For- 1–20 m thick and are commonly separated by 0.5–4 m mation and the Angikuni Formation is exposed on thick zones of carbonate alteration (possibly incipient

Fig. 15. (A) Alternating chemogenic and fresh-water deposits 86.7 m below the top of drill hole Ang 95-1. (B) Close-up of central zone in panel (A), with magnesite and dolomite mix. L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250 241

Fig. 15. (Continued ). paleosols) and by fragmental volcanic and siliciclas- self-supporting framework; and indications of in situ tic interbeds. Field and drill-hole data indicate total brecciation. accumulations of at least 200 m (Cousens, 1999). The framework-supported granule-pebble conglom- Geochemical and Nd and Sr isotopic data from erates were probably deposited by flash floods, and Christopher Island Formation flows in the “Rack” the matrix-supported conglomerates by debris floods. lake area, including data from two additional drill Erosionally based decimetre-scale upward-fining se- cores, are presented by Cousens (1999). quences capped by curled mudstones likely record single flash-floods. Local inverse grading is prob- 4.3. Interpretation ably the product of high clast-clast interactions in concentrated debris floods. The abundance of suban- In contrast to the relatively high relief setting in- gular to subrounded minette volcanic clasts indicates ferred for the Angikuni Formation at Angikuni Lake, first-cycle erosion of nearby flows. Similarly, the in which alluvial fan–fluvial plain–pond sedimentation parallel-stratified medium- to coarse-grained arkoses is interpreted to have been in response to nearby faults likely record upper flow regime flash floods derived exposing Archean basement, we envisage a more dis- primarily from volcanic sources.s tal, fine-grained sand flat to semi-perennial fresh-water We interpret that the fine-grained arkoses, which to evaporitic lake setting at “Rack” lake. In view of are predominant in the “Rack” lake section, record the conformable contact between the Angikuni and sedimentation on a fine-grained sand flat in which Christopher Island formations, the abundance of vol- turbulent sheet floods spread over a low-relief plain. canic detritus throughout the section, and the unusual Local conglomerate and coarse sandstone layers prob- mineralogy of local chemical sediments in the lake ably represent the farthest reaches that coarse-grained beds, we infer that the Angikuni Formation at “Rack” sediment normally extended from volcanic sources. lake was deposited between, and likely during, peri- Local granulestones with distorted volcanic clasts ods of volcanism. suggest that some flows were reworked immediately The basal breccias are likely talus deposits that following deposition. Some of the outsized vol- draped a relatively low-relief slope on Archean gab- canic clasts within the fine-grained sandstones may broic rock, as indicated by angular clast shapes; have been wind borne. Sparse lenticular dolomite clast compositions that represent derivation almost masses 53 m above the base of the section, possibly entirely from immediately subjacent basement; a pseudomorphs after gypsum, may indicate a brief 242 L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250

Fig. 16. (A) Alternating chemogenic and fresh-water deposits 70.5 m below the top of drill hole Ang 95-1, with magnesite (dark-toned) and dolomite dispersed in central siltstone interval, deﬁning a crude layering. (B) Close-up of magnetite-dolomite interval with magnesite forming single crystals and rosette-like clusters. L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250 243

Fig. 17. Back-scattered electron image of apatite and strontianite from magnesite-bearing zone 70.5 m below top of drill-core Ang 95-1. period of slightly arid conditions shortly after the The intervals with mm- to cm-scale chemical deposits start of sedimentation. Mudstone partings, breccias likely record brief times of raised salinities in what and mudcurls are considered variably reworked and was otherwise a fresh-water system (Figs. 15 and 16). desiccated waning-flood suspension deposits. Local Requiring unusually high Mg/Ca ratios to form in mudstone-rich graded beds probably represent small preference to other carbonate minerals, magnesite fresh-water ponds, which occupied slight depressions. precipitation is relatively uncommon in sedimentary The thick arkose to mudstone rhythmites are inter- environments. It is generally restricted to non-marine preted to record larger, more perennial lakes that were evaporitic settings and/or those fed by surface and capable of trapping the finest material. Individual ground waters that are in contact with high-Mg source sharp-based cm-scale upward-fining sequences prob- rocks (e.g., Pohl, 1989; Renaut, 1993; Spötl and ably represent high-density underflows formed when Burns, 1994). In the present example, we suggest that unchanneled flash floods entered the lakes. The mi- the magnesite, as well as the small amounts of barite, crograded tops of some of these sequences may repre- strontianite and apatite, reflect the unique geochem- sent pulsating underflows or lower density interflows. istry of Christopher Island-like volcanic rocks, and Both debris derived from erosional reworking of vol- record evaporitic concentration of waters that became canic beds, and ash from primary air fall, may have enriched in Mg, Ba, Sr and P because of interac- contributed to the Christopher Island-like geochem- tion with minette flows. Conceivably, juvenile waters ical and Nd isotopic compositions of the mudstones. from volcanic-related hydrothermal springs may have 244 L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250

top sequence 5 are interpreted to represent repeated lake inﬁllings during times of reduced subsidence and/or increased sediment ﬂux.

5. Discussion

5.1. Tectonostratigraphic implications

We propose that the Angikuni Formation is the tectonostratigraphic equivalent of the South Chan- nel and Kazan formations exposed near Baker Lake (Fig. 4). In agreement with Rainbird et al. (2003), alluvial fan–fluvial–lacustrine sedimentation likely occurred together with ultrapotassic magmatism throughout the initial stages of basin development, albeit in varying proportions. The notion of out-of-basin volcanism during deposition of the Angikuni For- mation is supported by recent sequence stratigraphic Fig. 18. Soft sediment deformed contact between Christopher Island felsic minette flow and underlying Angikuni Forma- analysis in the Baker Lake region by Rainbird et al. tion sandstone. Relatively high-density potassium feldspar and (2003). They correlated sequences in a 500-m thick, phlogopite-phyric flow forms lobate load structure; relatively predominantly volcanic succession on the northern low-density very fine grained sandstone forms flame-like injection. side of the basin (near Aniguq River, northeast of Note dark baked zone in sandstone at contact. Pitz Lake; Fig. 3) to sequences in a 2000-m thick sedimentary succession near the southern side of the contributed to elevated lake salinities, such has been basin (near Thirty Mile Lake; Fig. 3). In the north, described from some modern lakes in the East African the relatively thin volcanic units have erosional se- rift system (e.g., Eugster, 1986) and elsewhere (e.g., quence boundaries, whereas in the south, sequence Smoot and Lowenstein, 1991). In addition, ground- boundaries between thick sedimentary units are con- waters charged with volcanic-derived CO2 may have formable (Rainbird et al., 2003). This was attributed contributed to enhanced chemical weathering and flux by Rainbird et al. (2003) to predominantly volcanic of metals from the volcanic rocks, such has been de- accumulation in the north, in an area that was over- scribed from the volcanic aquifer at Mt. Etna (Aiuppa filled relative to the south. Perhaps volcanic sections et al., 2000). similar to the Aniguq River succession formed out- The lower parts of each of the four 100-m scale side of the main “Rack” lake depocentre at the same fining upward sequences are interpreted to record time as the siliciclastic sequences preserved at “Rack” source area uplift events that probably resulted from lake, but were repeatedly uplifted, becoming sources out-of-basin volcanism. We infer that these events led of volcanic debris ultimately shed into (and preserved to rapid spreading of relatively coarse-grained mate- at) sites of greater accommodation. rial across the basin floor, such has been described In summary, the Angikuni and Christopher Island from other volcanic systems (cf. Orton, 1995), and formations record intimate continental sedimentation that they were followed by periods of retrograda- and flood volcanism during initiation of greater Baker tional backwasting during periods of relative tectonic Lake Basin. The significant lateral variation in roughly (and volcanic) quiescence. We consider sequence 5 to coeval volcanism and continental sedimentation ex- represent proximal to distal progradation and aggra- pressed by these units is analogous to, for example, the dation during relatively prolonged source area uplift, modern Ziway–Shala lake basin system of the Main followed by a retogradational, upward-fining phase. Ethiopian Rift (Le Turdu et al., 1999), the Eocene Re- The metre-scale upward-coarsening sequences at the public Basin, Washington State, USA (Gaylord et al., L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250 245

2001), the Cretaceous Etendeka Province, Namibia the Fort Simpson magnetic high (Fig. 1). Docking (Jerram et al., 1999) and the Paleoproterozoic Mt. Isa was consider to have occurred before ∼1.66 Ga, the Inlier, Australia (Eriksson et al., 1993). age volcanic rocks at the top of the Hornby Bay Group, Coppermine Homocline (Fig. 2; Bowring and 5.2. Basin-forming mechanisms Ross, 1985) that cover transcurrent faults (Hoffman and St.-Onge, 1981). Others (e.g., Peterson, 1994; Thick sections of alluvial fan deposits, such as in Peterson et al., 1994; Cousens et al., 2001) consid- the Angikuni Lake area, signify faulting during initial ered that slivers of western Churchill crust escaped deposition of the Baker Lake Group. Furthermore, northeastward because of squeezing between Nahanni field and geochronologic data document 1.84–1.83 Ga docking on the west and Trans-Hudson orogen col- reactivation along the northeast-trending faults that lision on the south. Accordingly, Baker Lake Basin flank the northwestern sides of the Angikuni sub-basin was thought to have formed in zones of transtension segments (Aspler et al., 1999a,b,c; Aspler et al., along northeast-trending strike-slip faults, analogous 2000). Moreover, sedimentologic and paleocurrent to the late Cenozoic basin systems created due to data indicate that a basement wedge in northern lateral escape of Eurasia during late Cenozoic colli- Angikuni Lake was uplifted during deposition of the sion from the south by India (cf. Tapponnier et al., Angikuni Formation. Finally, voluminous minette 1982). magmas derived from the lithospheric mantle imply Recently, however, on the basis of seismic, grav- stretching, thinning, adiabatic melting and creation ity and drill-hole data, the “Nahanni terrane” has been of mechanical pathways for rapid magma release reinterpreted as a sedimentary basin formed by stretch- across the interior of the western Churchill Province ing of pre-Wopmay orogen crust (∼1.84 Ga) (Cook (Cousens et al., 2001, 2003). Nonetheless, the nature et al., 1999). The sedimentary fill of this basin is con- of the continent-scale strain field and the style of sidered equivalent to succession A rocks exposed in basin faulting are open to question. the Cordillera (Fig. 2; Wernecke Supergroup, Muskwa assemblage) bracketed to have been deposited after 5.2.1. Regional boundary conditions? ∼1.78 Ga (age of youngest detrital zircon in Muskwa Hoffman (1980) suggested that regional ultrapotas- assemblage, Ross et al., 2001) and before ∼1.71 Ga sic dyking and initiation of Baker Lake Basin was (age of intrusions cutting the Wernecke Supergroup, due to fracturing of western Churchill lithosphere Thorkelson et al., 2001). Furthermore, the best esti- during brittle eastward indentation by the Slave mates regarding the timing of indentation and tran- Province (Fig. 1). This indentation was manifested scurrent faulting come from the apical region of the by a regional set of conjugate transcurrent structures, Slave indentor, between the McDonald and Bathurst the largest of which includes the northeast-trending, faults. K–Ar data from hornblende suggest a maxi- dextral, McDonald Fault and the northwest-trending, mum age of 1784 Ma (Henderson and van Breemen, sinistral, Bathurst Fault (Figs. 1 and 2; see also Gibb, 1991), and Rb–Sr data from biotite suggest a mini- 1978). Coarse continental successions were deposited mum age of 1735 Ma (Henderson et al., 1990). Al- due to strike slip along both faults, including the though the western edge of the Thelon Formation is Et-Then Group along the McDonald Fault (Ritts and cut by the continuation of the McDonald Fault, the ap- Grotzinger, 1994) and the Tinney Cove Formation parent offset is much less that estimates of strike-slip adjacent to the Bathurst Fault (Fig. 2; Campbell, farther southwest (Henderson et al., 1990). This off- 1979). Hoffman (1989) tentatively ascribed conjugate set probably represents late reactivation along the Mc- faulting to docking of the inferred “Nahanni terrane” Donald Fault, such as in the Coppermine Homocline microcontinent to the western flank of North Amer- (Hoffman and St.-Onge, 1981) and along the Bathurst ica. This docking was thought to have taken place Fault (Campbell, 1979), rather than the Thelon For- after ∼1.84 Ga, the age of the youngest granitic rocks mation having formed during transcurrent faulting (cf. in Wopmay orogen cut by the transcurrent faults Henderson et al., 1990). (Hildebrand et al., 1987), and the age (Villeneuve In summary, the 1.78–1.74 Ga ages are too young to et al., 1991; Ross et al., 2000a) of granitic rocks in relate Baker Lake Basin (∼1.84–1.79 Ga) to regional 246 L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250 shortening and transcurrent faulting near the western while the western margin of ancestral North America flank of the western Churchill Province. However, the was a free face. idea that Baker Lake Basin initiated in response to post-1.83 Ga collisional and post-collisional processes in Trans-Hudson orogen (e.g., Ansdell et al., 1995; 6. Conclusions Chiarenzelli et al., 1998) on the southern flank remains valid. • In the Angikuni Lake area, Angikuni Formation siliciclastic rocks signify alluvial fan-fluvial and 5.2.2. Style of faulting? sand flat-playa deposition in a fault-bounded trough Local cumulative thicknessses of Baker Lake formed adjacent to a wedge-shaped basement up- Group units in excess of 10 km (LeCheminant et al., lift. These rocks define an overall upward-fining 1979; Rainbird et al., 2003) are consistent with lon- trend that is inferred to record retrograda- gitudinal conveyor belt-like stratigraphic stacking tional backwasting following fault-induced subsi- and deposition in local sub-basins created by strike dence. slip. A major difficulty in applying a strike-slip • Although tilted before principal Christopher Island model to the entire basin however, is that synsedi- Formation volcanism, geochemical and Nd iso- mentary strike-slip border faults and fanglomerates, topic data from Angikuni Formation mudrocks at such as found in classical strike-slip basins (e.g., Angikuni Lake indicate earlier or coeval Christo- San Gabriel Fault and Violin Breccia of Ridge Basin pher Island-like sediment sources. California; Crowell, 1974), appear to be lacking. • Volcano-sedimentary outliers scattered across Conceivably, the northeast-trending faults border- lithostructural domains in Archean basement at ing the main segments of the Angikuni sub-basin Angikuni Lake demonstrate that changes in struc- (Fig. 4) could have formed transtensional splays. tural level were attained before Christopher Island However, although these faults were sites of Archean deposition. dextral strike slip, we lack direct field evidence spec- • In the “Rack” lake area, a section of predominantly ifying the sense of motion during Paleoproterozoic arkose, fine-grained arkose and siltstone, and arkose reactivation. Furthermore, even if syndepositional to mudstone rhythmite is interpreted to signify a strike-slip faults were significant, parallelism be- more distal, fine-grained sand flat to semi-perennial tween minette dykes and sub-basin margins implies fresh-water (±evaporitic) depositional system. Pos- a large component of fault-normal extension, such sible gypsum pseudomorphs and local chemogenic as described from the Dead Sea (Ben-Avraham and lake beds suggest brief arid periods in an otherwise Zoback, 1992; Garfunkel and Ben-Avraham, 1996) wet paleoclimate. and the North Agean trough (Mann, 1997). More- • Four 100-m scale upward-fining sequences at over, the scattering of Christopher Island Formation the base of the Angikuni Formation near “Rack” outliers throughout the Angikuni Lake area signifies lake are interpreted to record source area uplift broad regional subsidence that extended well beyond events that were followed by periods of retrogra- the present-day limits of the northern and south- dational backwasting. An upward-coarsening to ern segments and potential strike-slip basin-margin upward-fining sequence at the top of the unit is faults. thought to represent proximal to distal prograda- In summary, regional boundary conditions are no tion and aggradation (during relatively prolonged longer consistent with an India–Eurasia lateral escape source area uplift) followed by retogradation (dur- model, and definitive evidence of strike-slip faulting ing quiescence and backwasting). at the scale of the entire greater Baker Lake Basin • The conformable contact between the Angikuni is lacking. Although we cannot rule out basin forma- and Christopher Island formations at “Rack” lake, tion by strike slip, a more appropriate tectonic model the abundance of volcanic detritus throughout may be regional uplift and extension within the west- the section, and the unusual mineralogy of local ern Churchill Province related to terminal collision chemogenic lake beds (magnesite-, strontianite-, and post-collision processes in Trans-Hudson orogen barite-, apatite-bearing), indicate that the Angikuni L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250 247

Formation was deposited between, and likely dur- References ing, periods of active volcanism. • Conceivably, Archean dextral strike-slip faults that Aiuppa, A., Allard, P., D’Alessandro, W., Michel, A., Parello, border the northeast-trending segments of the Angi- F., Treuil, M., Valenza, M., 2000. Mobility and fluxes of kuni sub-basin could represent releasing (transten- major, minor and trace metals during basalt weathering and groundwater transport at Mt. Etna volcano (Sicily). Geochim. sional) splays. However, although these faults were Cosmochim. Acta 64, 1827–1841. reactivated during the Paleoproterozoic, direct field Ansdell, K., Lucas, S., Connors, K., Stern, R., 1995. Kisseynew evidence of syndepositional strike slip is lacking. metasedimentary gneiss belt, Trans-Hudson orogen (Canada): • The Angikuni Formation is likely a tectonostrati- back-arc origin and collisional inversion. Geology 23, 1039– 1043. graphic equivalent to the South Channel and Kazan Aspler, L.B., Chiarenzelli, J.R., Cousens, B.L., Davis, W.J., formations at the base of the Baker lake Group in MacLachlan, K., 2000. Archean rifted continental margin/back- the Baker Lake region. In agreement with Rainbird arc basin and Archean to Paleoproterozoic transpression near et al. (2003), alluvial fan–fluvial–lacustrine sed- the Snowbird tectonic zone at Angikuni Lake, western Churchill imentation occurred together with ultrapotassic Province, Nunavut. GeoCanada 2000 Conference CD, Calgary. Aspler, L.B., Chiarenzelli, J.R., Cousens, B.L., 1999a. Geology of magmatism throughout the initial stages of basin northern Angikuni Lake. Nunavut. Geol. Surv. Can. Open File development, albeit in varying proportions. 3781. • Although we cannot rule out basin formation by Aspler, L.B., Chiarenzelli, J.R., Cousens, B.L., Valentino, D., strike slip, a more appropriate tectonic model may 1999b. Precambrian geology, northern Angikuni Lake, and a be regional uplift and extension within the west- transect across the Snowbird tectonic zone, western Angikuni Lake, Northwest Territories (Nunavut). Geol. Surv. Can. ern Churchill Province due to terminal collision and Pap. 1999-C, pp. 107–118. post-collision convergence in Trans-Hudson orogen, Aspler, L.B., Chiarenzelli, J.R., Powis, K., Cousens, B., 1998. while the western margin of ancestral North Amer- Geology of southern Angikuni Lake area, District of Keewatin, ica was a free face. Northwest Territories. Geol. Surv. Can. Open File 3608. Baragar, W.R.A., Donaldson, J.A., 1973. Coppermine and Dismal Lakes map-areas. Geol. Surv. Can. Pap. 71-39, 20 pp. Ben-Avraham, Z., Zoback, M.D., 1992. Transform-normal Acknowledgements extension and asymmetric basins: an alternative to pull-apart models. Geology 20, 423–426. Blake, D.H., 1980. 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