Fluvial, Lacustrine and Volcanic Sedimentation
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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 ultra- potassic 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 0301-9268/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2003.10.004 226 L.B. Aspler et al. / Precambrian Research 129 (2004) 225–250 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 extend- ing 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 Fig. 4. Regional geology of greater Baker Lake Basin and location of Angikuni sub-basin. In addition to principal exposures in the northern and southern segments, Baker Lake Group rocks outcrop as small outliers throughout the Angikuni Lake area. Modified after Rainbird et al. (2003). Shear zones at Angikuni Lake after Aspler et al. (1999b). 229 230 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.