Hadlari etal. 2006 baker lake rift basin sedimentology

90 (2006) 47–70www.elsevier.com/locate/sedgeo

Sedimentary Geology 1

Alluvial, eolian and lacustrine sedimentology of a Paleoproterozoichalf-graben, Baker Lake Basin, Nunavut, Canada

Thomas Hadlari a,⁎, Robert H. Rainbird b, J. Allan Donaldson a

a Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada K1S 5B6b Geological Survey of Canada, 601 Booth St., Ottawa, Ont., Canada K1A 0E8

Abstract

The northeast-trending Baker Lake sub-basin was a volcanically active, half-graben during deposition of ca. 1.85–1.76 GaBaker Lake Group. Drainage was oriented along transverse and axial directions with flow to playa lake and deeper perenniallacustrine depocentres. Basin marginal, streamflow-dominated alluvial fans were concentrated along the southern margin, andprovided sediment from Archean crystalline basement rocks. These fed transverse gravel- and sand-bed braided streams. Alluvialdynamics were characterized by channel aggradation and abandonment. Abandoned channel belts were sites of floodplain andeolian deposition. Basin axial braided streams fed northeast and southwest to a depocentre near Christopher Island, where eolian,playa and lacustrine environments were intimately linked. Felsic minette flows were initially erupted from localized centres;contemporaneous sedimentary deposits typically contain minor volcaniclastic components that increase in abundance basinward.Voluminous and widespread younger minette flows prograded outward from volcanic centres contributing significant additionalbasin-infill.© 2006 Elsevier B.V. All rights reserved.

Keywords: Alluvial fan; Braided stream; Floodplain; Eolian; Lacustrine; Half-graben

1. Introduction

This research is part of an integrated study of the BakerLake Group, emphasizing sequence stratigraphy andchronostratigraphy, for the purpose of constructing atectonostratigraphic model for Baker Lake Basin. Utili-zation of non-marine sequence stratigraphic methods toelucidate the relationship between sedimentation andtectonism requires an understanding of the depositionalenvironments throughout the basin. This paper describesthe sedimentology of the ca. 1.83 Ga Baker Lake Groupfromwell-exposed key stratigraphic sections frommargin

⁎ Corresponding author. Fax: +1 613 520 2569.E-mail addresses: [email protected] (T. Hadlari),

[email protected] (R.H. Rainbird).

0037-0738/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.sedgeo.2006.05.005

to inferred depocentre of the Baker Lake sub-basin, as anaid to reconstruction of its paleogeography. In addition,the remarkable preservation and absence of bioturbationfrom these Paleoproterozoic rocks provides sedimento-logical insight into alluvial environments in certaininstances generally not available from the Phanerozoic.Thick alluvial fan deposits are exposed on large, glaciallypolished outcrops. Floodplain deposits are associatedwith braided stream deposits, a relatively undevelopedtopic of study (e.g. Bristow et al., 1999), but observedelsewhere in Precambrian deposits (Sønderholm andTirsgaard, 1998). It has been speculated that eolianitesshould be more prevalent in Precambrian deposits than inthe Phanerozoic due to lack of terrestrial vegetation (e.g.Eriksson and Simpson, 1998). While this generally hasn'tbeen the case, eolian deposits occur throughout the Baker

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http://dx.doi.org/10.1016/j.sedgeo.2006.05.005

48 T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70

Lake Basin, reworking fluvial deposits and forming thinsandsheets to large ergs (Rainbird et al., 2003; Simpsonet al., 2004). This research, representing the first regionallithofacies analysis of the Baker Lake sub-basin,incorporates previously completed fieldwork by theGeological Survey of Canada.

1.1. Regional geology and previous work

Greater Baker Lake Basin extends from Dubawnt Lakenortheast to Baker Lake (Nunavut, Canada) and comprisesa series of northeast-trending intracontinental basins,including the Baker Lake sub-basin (Rainbird et al.,2003; Figs. 1 and 2). Basin fill comprises the faulted butunmetamorphosed, siliciclastic and volcanic rocks of theDubawnt Supergroup (Wright, 1955; Donaldson, 1967;LeCheminant et al., 1979b; Gall et al., 1992; Rainbird andHadlari, 2000; Rainbird et al., 2003Fig. 3). The ca. 1.85–1.70 Ga Baker Lake Basin occupied a unique location in

Fig. 1. Location map indicating the Baker Lake Basin in the

space and time with respect to the evolution of theWesternChurchill Province. Deposition of the ca. 1.85–1.76 GaBaker Lake Group appears to have closely followeddeformation and metamorphism in underlying crystallinebasement rocks, which in some cases were at lower crustallevels at ca. 1.9 Ga (Sanborn-Barrie, 1994). Contempora-neous collisional tectonics were taking place in the ca. 1.9–1.8 Ga Trans-Hudson Orogen, 500 km to the south andsoutheast (e.g. Lucas et al., 1999).

The Dubawnt Supergroup is subdivided into threeunconformity-bounded stratigraphic units that correspondto, from oldest to youngest: the Baker Lake, Wharton andBarrensland Groups (Donaldson, 1967; Gall et al., 1992;Rainbird and Hadlari, 2000); or the Baker, Whart andBarrens second-order sequences (Rainbird et al., 2003;Fig. 3). These groups or corresponding second-ordersequences have been interpreted to represent the tectonicstages of rift, modified rift and thermal sag, respectively(Rainbird et al., 2003).

context of the Western Churchill and Rae Provinces.

Fig. 2. Geology map of the Baker Lake sub-basin and fluvial paleocurrent data.

49T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70

The Baker Lake Group comprises the South Channel,Kazan, Christopher Island (Donaldson, 1965), Kunwak(LeCheminant et al., 1979b) and Angikuni Formations(Blake, 1980), and varies in cumulative thickness fromover 2 km to 500 m. These lithostratigraphic subdivi-sions have provided the framework for regional mappingwithin Baker Lake Basin. Chronostratigraphic control onthe formations is quite poor, but recent studies suggestthat these formations are time-equivalent, reflectinglateral facies boundaries (Rainbird et al., 1999, 2003).

The South Channel Formation comprises boulder tocobble conglomerate interpreted as alluvial fan deposits.It typically overlies crystalline basement rocks at the basinmargin and is composed of locally derived clasts of gra-nite, amphibolite and gneissic lithologies. For this reason,it appears to be the oldest formation, although volcanicrocks of the Christopher Island Fm. also unconformablyoverlie basement (Rainbird and Hadlari, 2000), and occuras clasts within the South Channel Fm. (Hadlari andRainbird, 2001).

The Kazan Formation consists of arkosic sandstone,siltstone and mudstone, representing a variety ofsedimentary environments including eolian, fluvial andplaya lake (Donaldson, 1965; LeCheminant et al.,1979b; Rainbird et al., 2003).

The Christopher Island Formation comprises alkalinevolcanic rocks interbedded with volcaniclastic and sili-

ciclastic sedimentary rocks. Volcanology of the Chris-topher Island Fm. from Baker Lake sub-basin has beendescribed in detail (LeCheminant et al., 1979a,b; Blake,1980). A generalized volcanic stratigraphy for thegreater Baker Lake Basin, from oldest to youngest,consists of: felsic minette flows, minette flows and felsiteflows (Peterson et al., 1989; Hadlari and Rainbird, 2001;Rainbird et al., 2003). The felsic minette flows or equi-valent volcaniclastic deposits are less areally extensivethan younger minette flows, and have been observed tooverlie the basal unconformity of the Baker Lake Group.Mantle-derived minette flows record voluminous extru-sion throughout the entire basin and represent the largestknown ultrapotassic volcanic province (LeCheminantet al., 1987; Peterson et al., 1989, 1994; Cousens et al.,2001). Stratigraphic relations indicate that the flowsoriginated at volcanic centres, which progressivelyexpanded outward to eventually blanket most of thebasin (Hadlari and Rainbird, 2001). The volcanic centreswould have been positive topographic features thatsupplied volcaniclastic sediment, diverted streamflowpossibly altering drainage patterns and replaced sedi-mentary processes as a basin-infilling mechanism.Felsite flows are the youngest and most areally restrictedvolcanic rock. Analyses of phlogopite phenocrysts froma flow and a syenite intrusion that intrudes the lowerBaker Lake Group yield 40Ar/39Ar ages of 1845±12 Ma

Fig. 3. Stratigraphy of the Dubawnt Supergroup (Donaldson, 1967; Gall et al., 1992; Rainbird and Hadlari, 2000; Rainbird et al., 2003).Geochronology sources: Thelon Fm., 1720±6 Ma (Miller et al., 1989); Pitz Fm. (Rainbird et al., 2003); and Baker Lake Group, 1785±3 Ma(Rainbird et al., 2002), 1833±3 Ma (Rainbird et al., 2006).


and 1810±11Ma, respectively (Rainbird et al., 2002; seediscussion Rainbird et al., 2006). A more precise U–Pbzircon age of 1833±3Ma has been obtained from a felsicminette flow from the western end of Baker Lake Basin,providing the best constraint on basin formation(Rainbird et al., 2006).

The Kunwak Formation (LeCheminant et al., 1979b)consists of conglomerate composed primarily of Christo-pher Island Fm. volcanic clasts as opposed to basementrock types in the South Channel Fm. It is differentiatedfrom the Christopher Island Fm. by its stratigraphic posi-tion above volcanic rocks and below the unconformity atthe top of the Baker LakeGroup. This formation primarilyoccurs in the interior of the Baker Lake sub-basin, locatedproximal or downstream from volcanic centres.

The Angikuni Formation (Blake, 1980) is restricted tothe Angikuni sub-basin (Fig. 1). Aspler et al. (2004)consider it to be equivalent to the South Channel andKazan Formations. Incompatible element chemistry ofmudstones suggests derivation from Christopher IslandFormation volcanic rocks, consistent with syn-volcanicsedimentation and probable lateral interstratification ofthe Angikuni and Christopher Island Formations (Aspleret al., 2004).

In the Thirty Mile Lake area of the Baker Lake sub-basin (Fig. 4) steeply inclined, east–northeast-strikingunits of conglomerate, sandstone and volcanic strata ofthe Baker Lake Group unconformably overlie crystallinebasement. Previous mapping in this area identified SouthChannel Fm. conglomerate, Kazan Fm. sandstone andmudstone, and Christopher Island Fm. volcanic rocks(Donaldson, 1965, 1967; LeCheminant et al., 1979b).The Kunwak Formation is exposed to the northwest,along the Kunwak River, where it contains felsite clastsand is unconformably overlain by the Wharton Group(LeCheminant et al., 1979b; Hadlari and Rainbird,2001).

At Christopher Island (Fig. 5), the South ChannelFormation unconformably overlies the Archean Mac-Quoid-Gibson supracrustal belt (Tella et al., 1997;Hanmer et al., 1999) and the 1.9 Ga Kramanituar meta-morphic complex (Sanborn-Barrie, 1994; Sanborn-Barrieet al., 2001). The Kazan Formation comprises eolian,playa and braided stream deposits (Donaldson, 1965,1967; Rainbird et al., 1999). The Christopher Island For-mation locally comprises volcanic flows, pyroclastic andvolcaniclastic deposits. On Christopher Island and sur-rounding islands (not shown on Fig. 5), volcanism was

Fig. 4. Geology map of the Thirty Mile Lake study area. Paleocurrent data are derived from cross-bed measurements.


primarily explosive, as indicated by bomb and accessory-clast sag structures, normal and reverse grading, andcross-stratification within extensive volcaniclastic depos-its (Rainbird et al., 1999). These structures indicatedeposition, in part, by turbulent pyroclastic surges (cf.Fisher and Schmincke, 1984; Cas and Wright, 1987).

2. Lithofacies associations

From the principal study areas at Thirty Mile Lakeand eastern Baker Lake, and other select locations withinBaker Lake sub-basin (Fig. 2), the sedimentary rocks ofthe Baker Lake Group are here subdivided into faciesassociations (FA) more detailed than those presented inprevious formational descriptions. Individual facies are

outlined in Table 1. In general, FA 1 corresponds to theSouth Channel Formation, FA 2 and 3 correspond to theKazan and Kunwak Formations, and FA 3 to 7correspond to the Kazan Formation.

2.1. Facies association 1: alluvial fan

2.1.1. Lithofacies descriptionClast-supported disorganized conglomerate (Gcd)

contains cobble- to boulder-grade angular to subroundedclasts within 1–5 m thick tabular beds with erosionalbasal contacts. Diffuse horizontal stratification gradeslaterally into a more massive framework, which is intactto condensed with slight to no imbrication (Fig. 6d). Thematrix is typically moderately to very poorly sorted, fine

Fig. 5. Location map of the Christopher Island study area with paleocurrent data (St= fluvial, Sw=wave ripple crests, Ste=eolian cross-sets).


to coarse sandstone. Atypically, the matrix exhibits bothhorizontal stratification and small-scale (less than 5 cmthick) cross-stratification adjacent to cobble- to boulder-grade clasts. Randomly distributed within sub-tabularconglomerate beds, mound-shaped accumulations ofgranules and coarse sand overlie certain frameworkclasts (Fig. 6c). Rare examples of reverse grading in thematrix can be seen in some of these beds.

Clast-supported organized conglomerate facies (Gco)contains pebble- to cobble-grade, sub-angular to sub-rounded clasts, within an intact to condensed, imbricatedframework. Thematrix is moderatelywell-sortedmediumto coarse sandstone. Tabular beds, 0.5–2 m thick, gene-rally fine upward, and may form composite conglomeratesheets. A typical occurrence would consist of multiplebeds consisting of 30 cm of cobble to 20 cm of pebbleconglomerate comprising a composite thickness of 2–3 m. Other occurrences include horizontally stratified(Fig. 6b) and less common cross-stratified tabular beds.Horizontal stratification marks the boundaries of rare,thin, lenticular beds of trough cross-stratified sandstone.

Trough cross-stratified conglomerate facies (Gt) ispredominantly pebble-grade with a condensed frame-work and consists of lenticular units up to 2 m thick and10 m wide that fine upward and laterally. The lowersurfaces of these beds are erosional.

Trough cross-stratified sandstone facies (St) consists offine- to pebbly cross-stratified sandstone in sets typicallyranging in thickness from 5 cm to 20 cm. Facies St occursat the top of lenticular conglomerate units or as lenticularunits overlying conglomerate sheets. It may be overlainby parallel-stratified mudstone facies (Fl), consisting oflaminatedmudstone andminor siltstone or fine sandstone,with raremud curls. These layers are overlain by erosionalsurfaces that are laterally continuous for more than 100 m.

2.1.2. Lithofacies interpretationThe clast-supported framework, absence of inverse

grading and weak stratification of the disorganized cong-lomerate facies (Gcd) suggests a streamflow origin asopposed to deposition by a debris flow (e.g. Sohn et al.,1999; Blair, 2000a). A similar facies has been described by

Table 1Lithofacies

Lithofacies Description Interpretation

Ged: framework-supported,disorganized conglomerate

Cobble- to boulder-grade clasts; coarse to fine sandstone matrix; poorly to verypoorly sorted; crude and irregular stratification; tabular geometry;erosional base

Gravel sheets emplaced by highmagnitude flood flows.

Geo: framework-supported,organized conglomerate

Pebble- to cobble-grade clasts; granule to medium sandstone matrix; moderatelysorted; organized framework; erosional base; wedge-shapedand tabular units; predominantly horizontally stratified

Gravel sheets emplaced by bedloadprocesses during flood events.

Gem: framework-supported,massive conglomerate

Pebble- to cobble-grade clasts; coarse to medium sandstone matrix;moderate to well sorted; imbricated; intact framework; tabular geometry

Gravel bars in high-energy braidedstreams.

Gt: trough cross-stratifiedconglomerate

Pebble- to cobble-grade clasts; granule to medium sand grade matrix; fine upward;cross-stratified; lenticular units with erotional base

Filling of channels, scours and channelpools by gravel during flood flows.

Sh: horizontally stratifiedsandstone

Fine- to medium-grained sandstone; well sorted; planar-horizontallamination; ± primary current lineation on bedding planes

Planar bed flow (upper and lower flow regime).

St: trough cross-stratifiedsandstone

Medium to coarse-grained sandstone; trough cross-stratification Migration of three-dimensional dunes.

Ste: trough cross-stratifiedsandstone with pinstripelamination

Fine- to medium-grained sandstone; inversely graded foresets; pinstripelamination; up to 2 m thick

Migration of eolian three-dimensional dunes.

Sr: sandstone withasymmetric ripples

Laminated sandstone with predominantly asymmetric ripples; mudstonedrape; linguoid bedforms common

Bedload deposition/migration ofcurrent ripples.

Sw: sandstone withsymmetrical ripples

Laminated sandstone; symmetrical ripples; ± mud drapes; bifurcating crests;sheet-like geometry

Wave-formed ripples, lacustrineshoreface or delta mouth bar.

SFw: wavy bedded sandstone andsiltstone/mudstone

Inter-stratified sandstone and siltstone/mudstone; asymmetric and/orsymmetric ripples

Overbank, abandoned channel orwaning flood deposits.

Fl: parallel-stratifiedmudstone

Mudstone, siltstone, with parallel-laminated and/or cross-stratifiedsandstone

Prolonged periods of quiet-watersuspension deposition.

Modified from Miall (1977) and Jo et al. (1997).

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Fig. 6. Sedimentological features of the alluvial fan facies association (FA), Thirty Mile Lake area. (a) Three upward-fining bedsets in the alluvial fanFA; top of first beneath rock hammer (centre); two more to left of hammer. (b) Organized framework cobble conglomerate with alternating cobble-pebble tabular layers, similar to conglomeratic “couplets” described by Blair (2000b). Lens cap diameter is 5 cm. (c) Disorganized framework, cobbleconglomerate, with a granular sand accumulation above a framework pebble. Coin is 2.5 cm in diameter. (d) Disorganized framework, cobbleconglomerate from the alluvial fan FA, notebook for scale is 20 cm long.


Jo et al. (1997), in which an erosional lower boundary,weakly developed clast imbrication, crude stratification andlack of inverse grading were considered to be streamflowcharacteristics. The tabular morphology of similar facieshas been attributed to deposition by bedload sheets or low-relief longitudinal bars (e.g. Reid and Frostick, 1987; Todd,1989). Because these conclusions are consistent with ourobservations, this facies (Gcd) is considered to have beenrapidly deposited (intact framework, poor sorting and non-imbrication) by unconfined high-magnitude stream floodflows. Sandstone cross-laminae adjacent to clasts indicatethat deposition of the sand occurred within an intact gravelframework by sediment-laden currents. Where coarse togranular sand mounds occur atop clasts, the sediment isinterpreted to have been transported downward through agravel framework to rest upon upper clast surfaces. Rareinverse grading of the matrix is considered to be formed bya process of sieving, or mechanical sorting through theframework (Hooke, 1967).

The organized conglomerate facies (Gco) is generallyinterpreted as gravel sheets or longitudinal gravel bars (e.g.Reid and Frostick, 1987; Todd, 1989). Weak internalstratification within these deposits probably representswaxing and waning of individual flood flows. Well-stra-tified units are similar to a facies of alternating coarse–fineconglomerate “couplets” described by Blair (2000b) fromthe Hell's Gate alluvial fan in Death Valley. These “coup-lets” were interpreted to have been deposited under upper-flow-regime conditions during the washout stage of thestanding-wave cycle based on similaritieswith documentedfeatures of supercritical sheetflood events (Blair andMcPherson, 1994). Blair (2000b) surmised that the “auto-cyclic growth and destruction of standing waves during asingle sheetflood produces 50–250 cm thick sequences ofmultiple couplets”. These features are identical to some ofour observations, and thus we consider that for the well-stratified conglomerates, deposition occurred by bedloadprocesses during high-magnitude unconfined stream flow

Fig. 7. Stratigraphic section from ThirtyMile Lake study area displayingupward-fining interval in the alluvial fan facies association. Lithofaciesabbreviations from Miall (1977) and Jo et al. (1997), see Table 1.


conditions, possibly due to the washout of standing waves.The development of bedforms differentiates this faciesfrom the clast-supported disorganized conglomerate facies(Gcd), which lacks well-developed stratification.

Lenticular trough cross-stratified conglomerate (Gt) isinterpreted to have in-filled trough-shaped channelsfollowing periods of incision between flood events. Thisfacies is commonly observed within alluvial fan deposits(Jo et al., 1997; Rhee et al., 1998; Blair, 1999, 2000a,b)and is considered to represent secondary, non-catastrophicprocesses that occurred between infrequent sheetfloods.

Cross-stratified sandstone (St) and laminatedmudstone(Fl) indicate that low-energy streamflow conditions pre-vailed over a laterally continuous gravel substrate, infillingpits and gullies with sand and mud. Mud curls recordsubaerial exposure and desiccation between streamflowevents.

Since the coarsest (up to boulder grade) conglomeraticfacies exhibit streamflow indicators, wemust consider thereason for streamflow to prevail over debris flow de-positional processes. Blair (1999) has described adjacentalluvial fans in Death Valley, one streamflow-dominated,the other debris flow-dominated. The debris flow-do-minated alluvial fan was fed by a source region of sedi-mentary rocks; the streamflow-dominated alluvial fan hada source region of crystalline rock. Therefore, the dif-ference was not the gradient of the valley wall nor dis-charge, but simply the type of sediment supplied. Clastlithologies from the alluvial fan deposits at Thirty MileLake and South Channel match the underlying crystallinebasement rocks, consistent with Blair's (1999) theory forstreamflow predominance to be a function of derivationfrom weathered crystalline rock in the source region.

2.1.3. Facies successionsTwo typical bedset end members consist of: (1) 1–3 m

of disorganized cobble to boulder conglomerate (Gcd)overlain by metre-scale channel-fill conglomerate facies(Gt) and/or trough cross-stratified sandstone; or (2) a fewmetres of organized cobble to pebble conglomerate (Gco),incised by channel fill facies (Gt), trough cross-stratifiedsandstone (St), and/or overlain by parallel laminatedmudstone and siltstone (Fl). Coarse, tabular sheetflooddeposits (Gcd, Gco) represent the main accretion units.

Bedsets are commonly arranged in pairs that have acomposite upward-fining character over 5–10 m (Figs. 6aand 7). Although stratal surfaces within the paired bedsetsare discontinuous, the erosional surfaces above ubiquitoussandstone caps that bound the couplets are laterally con-tinuous (N100 m) where viewed transverse to inferredpaleoflow. Whereas bedsets represent sheetflood deposi-tion and subsequent reworking, coupled bedsets have a

higher order of genetic significance. The upward decreasein clast size may be a function of upstream aggradation ofthe alluvial fan, widening of the active lobe or a decrease in



gradient as the toe migrates forward. The inactive lobe ismarked by low-energy streamflow and suspension deposi-tion indicated by stratified sandstone and laminatedmudstone.

Blair (2000b) has described similar stratigraphic unitsfrom Death Valley alluvial fans and noted that generally 2to 8, 50–250 cm sheetflood deposits capped by gully-fillor eolian facies were bound by “progressive tectonic un-conformities”. These are on the scale of our 5–10m pairedbedsets; however, Blair (2000b) was able to observe abedding discordance over the intrafan unconformity, con-cluding that faulting had caused a down-drop of the fan.Therefore, this type of stratigraphic unit may be consi-dered to represent a fault-generated increment of accom-modation, where the succession records the characteristicalluvial response: aggradation of the fan surface.

Alternatively, Mack and Leeder (1999) have de-scribed 3–10 m thick “alluvial fan cyclothems”. Thesewere considered to form primarily due to the combinedeffects of vegetative cover and precipitation (minimumsediment yield would correspond to peak precipitationdue to the binding of sediment by vegetation, and viceversa). This model obviously would not apply to alluvialfan deposits from the Baker Lake Basin, because of anabsence of vegetative cover in the Paleoproterozoic.

Commonality suggests that this nested upward-finingstratal pattern is intrinsic to the alluvial fan depositionalenvironment in fault-bounded basins from the Precam-brian through the Phanerozoic. Since alluvial fans aggradevia lobe accretion and abandonment, this punctuatedprocess superimposed on a gradual fault-induced subsi-dence, though unrealistic, would result in the observedsuccession. Thus, these units do not necessarily indicatespecific fault motions but that subsidencewas sufficient toprovide the grade required for alluvial fan formation.

2.2. Facies association 2: gravel-bed braided stream

2.2.1. Lithofacies descriptionClast-supported massive conglomerate facies (Gcm)

is pebble- to cobble-grade with rare boulders in a coarse

Fig. 8. Sedimentological features from lithofacies associations (FA) 3–7. Lens capbed braided streamFA; knife is 10 cm long. (b) Erosional surfacemarked bygranuinversely graded sandstone laminae, of the sand-bed braided streamFA.The overlyby winnowing of fluvial deposits. (c) Inversely graded lower foresets of a 1.5 mlamination. Faint cross-laminae are visiblewithin these foresets. (d) Floodplain FAunits of horizontally laminatedmudstone–siltstone–sandstone (Fl). Some inclinedof subaerial exposure (inset). To the right side of the photograph, on an oblique exsplay that prograded onto a mud-rich floodplain, was abandoned and subsequenalternating cross-stratified sandstone grading upward into laminated mudstone wsandstone sheet from the lacustrine FA, interpreted as a mouth bar deposit. (g) Emudstone. This is interpreted as a wave-ravinement surface overlying an interdisthinning and upward-fining laminae, containing mudstone clasts: interpreted as d

to granular sandstone matrix. Clasts form a condensed,imbricated framework that is moderately to well-sorted.Tabular beds range in thickness from 10 cm to severalmetres. Sheets of massive conglomerate are continuousfor tens of metres but are discontinuous over hundreds ofmetres perpendicular to the inferred paleoflow direction.

Rare lenticular beds of trough cross-stratified con-glomerate (Gt), 30 cm to 50 cm thick incise into themassive conglomerate, but are less prominent than in thealluvial fan facies association. Angular mudstone clastsare common.

Medium- to thick-bedded, trough cross-stratified sand-stone beds occur as solitary sets or compound sets up to ametre thick, above thick beds of massive conglomerate.Rare laminated mudstone (Fl) occurs at the tops of 10 mthick, upward-fining packages.

2.2.2. Lithofacies interpretationThe gravel-bed braided stream FA is distinguished

from the alluvial fan facies assemblage by a morehomogeneous, better sorted, imbricated and more con-densed framework conglomerate. Clast imbrication infacies Gcm implies bedload transport. The condensedframework and better sorting indicate a more sustainedstreamflow and less rapid aggradation than inferred forconglomeratic facies from the alluvial fan facies assem-blage. Massive texture makes it difficult to differentiategravel-sheet from longitudinal gravel bar deposits, acommon characteristic of gravel-bed braided streamdeposits (Miall, 1977). The lateral discontinuity oflithofacies perpendicular to the inferred paleocurrentdirection indicates that deposition occurred in channelssmaller than a few hundreds of metres in width.

Cross-stratified, channel-fill conglomerate (Gt) prob-ably represents reworking of abandoned-channel gravelsheets prior to deposition of sandstone. Mudstone rip-up clasts within conglomerate sheets attest to intermittentsuspension deposition, although the deposits were subse-quently eroded and transported, between flood events.

Trough-cross stratified sandstone was deposited inabandoned channels. The laminated mudstone (Fl) and

is 5 cm. (a) Linguoid ripples above primary current lineation from the sand-le lag truncatingmedium-grained, cross-stratified sandstone andoverlain bying laminae, interpreted as eolian, indicate that this is a deflation lag formedthick eolian cross-set, interpreted as sub-critically climbing wind ripple

, inwhich an inclined planar-laminated sandstone (Sh) interval lies betweensets are interlaminatedwithmudstone and formmudstone dishes indicativeposure, arrows highlight inclined surfaces. This is interpreted as a crevassetly overlain Fl facies. Rock hammer is 75 cm long. (e) Playa FA, showingith abundant desiccation cracks. (f) Wave-ripple lamination from a rippledrosional surface and lag truncating cross-stratified sandstone overlain bytributary channel in a deltaic environment. (h) Normally graded, upward-elta-front turbidite deposits. Rapidograph pen is 1 cm wide.


trough cross-stratified sandstone that sharply overlyconglomerate probably represent low energy depositionfollowing avulsion and channel abandonment (Miall,1977), similar to gravel-bed abandoned channel depositsfrom the Waimakariri River in New Zealand (Reinfeldsand Nanson, 1993). Alternatively, where non-erosionalcontacts exist between successive beds of conglomerate,sandstone and mudstone, the upward-fining pattern mayreflect deposition from waning flood (Miall, 1977).

2.2.3. Facies successionsUpward-fining packages of the gravel-bed braided

stream facies assemblage are considered to be deposits ofsuperimposed bars (Miall, 1977). Bedsets are arranged in5 to 10 m aggradational to mildly upward-fining suc-cessions of massive conglomerate (Gcm), capped bysandstone (St) or cross-stratified pebble conglomerate(Gt). Such packages are considered to represent verticalaggradation followed by channel belt switching (Miall,1977), represented by sandstone deposition in abandonedchannels.

2.3. Facies association 3: sand-bed braided stream

2.3.1. Lithofacies descriptionMassive, clast-supported conglomerate (Gcm) is a

minor component of the sand-bed braided stream FA,occurring as thin beds at the base of upward-fining bed-sets. Sets of fine-grained to pebbly, trough cross-stratifiedsandstone (St) vary in thickness from 10 cm to 1 m,bedforms of three-dimensional dunes occur on certainoutcrops. These bedsets commonly have a pebble lag andabundant mudstone clasts at the base. Horizontally strati-fied sandstone facies (Sh) consists of planar, horizontallylaminated, well-sorted, fine- to medium-grained sand-stone that commonly displays primary current lineation.Bedding geometry is predominantly tabular, and large-scale lateral accretion surfaces appear to be absent.

Fine- to medium-grained ripple cross-stratified sand-stone (Sr) infrequently occurs at the top of upward-fining bedsets dominated by medium to thick sets oftrough cross-stratified sandstone (St).

Laminated mudstone (Fl) occurs at the top of upward-fining successions. Trough cross-stratified sandstone (Ste)with inversely graded foresets and pinstripe lamination alsooccurs at the top of upward-fining successions. These setsare typically 10 to 50 cm thick, but locally are 1 m thick. Inthin sets (5–10 cm), the foreset angle can be very low, about5°; thick beds commonly overlie symmetrical ripples (Sw).

As a sub-association occurring over tens of metres inthickness, rippled sandstone (Sr) may be exclusivelyinterbedded with horizontally stratified sandstone (Sh)

that displays prominent primary current lineation (Fig.8a). Mudstone drapes are ubiquitous above lenticularripple bedforms.

2.3.2. Lithofacies InterpretationThin conglomerate beds at the base of upward-fining

bedsets indicate that coarse sediment load was depositedat the base of braided channel-fill, perhaps as braid bars.Trough cross-stratified sandstone is a result of three-dimensional dune migration, particularly evident wheredune bedforms are exposed. Horizontally stratified sand-stone with primary current lineation is interpreted tohave been deposited under upper-flow-regime condi-tions (Allen, 1964; Southard and Boguchwal, 1973). Thetabular geometry, lack of lateral accretion surfaces,predominance of trough cross-stratification, occurrenceof upper-flow-regime plane beds are consistent withdeposition by shallow sand-bed braided streams (cf.Miall, 1977).

Rippled sandstone (Sr) capped by mudstone at the topof upward-fining bedsets is inferred to represent waningof flood flow followed by suspension deposition. Whererippled sandstone and upper-flow-regime plane beds arethe dominant lithology in fine-grained sandstone succes-sions, this represents sheetflood deposits for which thegrain size was too small to form dunes (cf. Southard andBoguchwal, 1990), where linguoid ripples transformdirectly into upper-flow-regime plane beds with increas-ing stream velocity (Baas, 1994). Mudstone recordswaning flood or abandoned-channel deposition.

Cross-stratified sandstone with inversely graded fore-sets record wind ripple migration during eolian reworkingof abandoned channel deposits (cf. Hunter, 1977).

2.3.3. Facies successionsUpward-fining bedsets, 0.5–5 m thick, typically

consist of an erosive base with pebble lag, overlain bypredominantly trough cross-stratified sandstone thatpasses gradationally upward into horizontally stratifiedsandstone, current-rippled sandstone and laminatedmudstone (Fig. 9). However, there is a spectrum ofheterolithic to sandstone-dominated deposits. The mostproximal deposits contain conglomerate at the base ofmetre-scale cycles that fine upward to sandstone, depo-sited by a waning flood flow that carried a load of sandand gravel. Medial deposits consist of predominantlycross-stratified sandstone. Less proximal deposits consistof typically less than metre-scale upward-fining cycles ofsandstone with abundant mud drapes, capped by lami-nated mudstone deposited by waning streamflows thatcarried a mixed load of sand and mud. This variation isinterpreted to reflect a spectrum of facies from shallow

Fig. 9. Stratigraphic section from Thirty Mile Lake study area displaying facies successions from braided stream and floodplain facies associations.Lithofacies abbreviations from Miall (1977); see Table 1.



braided stream to mixed-load, ephemeral sheetflood (e.g.Sønderholm and Tirsgaard, 1998). In deposits rich inmudstone, desiccation cracks are common, indicating thatbetween flood events the river bed was subaerially ex-posed. Inversely graded sandstone laminae, characteristicof wind transport, typically occur at the top of upward-fining cycles where mudstone is absent, indicating eolianreworking of dry river beds (Fig. 8b).

While bedsets represent flood events and deposition-abandonment of small braided channels, multiple bed-sets comprise 5–15 m thick composite, upward-finingsuccessions capped by prominent, laterally continuous(up to 50 m at least) mudstone or eolian sandstone (Fig.9). These multiple channels are inferred to comprise alarger channel tract. The upward-fining trend indicatesthat within the channel tract aggradation was accompa-nied by a decrease in stream competency. Aggradationwould result in a reduction of slope, channel switchingand abandonment, to produce upward-fining patterns influvial deposits capped by fine-grained or eolian deposits(cf. Miall, 1977; Hjellbakk, 1997). These thicker up-ward-fining successions therefore likely represent ag-gradation and abandonment of a braided channelcomplex (Fig. 9).

2.4. Facies association 4: floodplain

2.4.1. Lithofacies descriptionThe floodplain FA is typically composed of the

lithofacies Fl, Sr, Sh, St and Ste. Rippled sandstone (Sr)with nearly ubiquitous mudstone drapes is typically inter-stratified with 5 to 20 cm thick laminated mudstone (Fl).Sedimentary structures and bedforms include, ripplelamination and cross-lamination, symmetrical and asym-metrical ripples, and V-shaped polygonal cracks in mud-stone. Thin intervals (generally less than 2 m) of upward-fining, trough cross-stratified sandstone (St) occur withinmudstone-dominated sections. Cross-sets are less than50 cm thick and typically contain up to 5 cm angularmudstone clasts. Cross-stratified sandstone with inverselygraded foresets (Ste; Fig. 8c) occurs at the top of upward-fining intervals, typically as single cross-sets up to 1 m inthickness. Bedding geometries are typically tabular-horizontal; however, low-angle inclined cosets, cumula-tively less than 1 m thick, overlying and overlain by ho-rizontally laminated mudstone occur locally (Fig. 8d).These inclined strata consist of thin beds of parallel-laminated, fine- tomedium-grained sandstone overlain bymudstone drapes, some of which display desiccationfeatures such as mudcracks and mud curls.

As a subdivision within this FA, is the occurrence ofwavy bedded sandstone and mudstone (SFw) with subor-

dinate trough cross-strata (St) and laminated mudstone(Fl). Either symmetrical or asymmetrical ripples maydominate metre-scale intervals of predominantly inter-laminated sandstone and mudstone. The occurrence ofdesiccation structures is variable; an absence of desicca-tion cracks is coincident with a preponderance of symmet-rical ripples.

2.4.2. Lithofacies interpretationThe FA of current ripples, mudstone, and desiccation

cracks suggests periodic overbank flooding followed bysuspension deposition and subaerial exposure within afloodplain setting. Current ripples and planar laminaelacking primary current lineation are indicative of lower-flow-regime deposition, and wave ripples of periodswhere water remained pooled on the floodplain afterfloods. Thin, less than 2 m thick intervals of upward-fining cross-stratified sandstone represent small crevassechannels that traversed the generally mudstone-domi-nated substrate (cf. Rhee et al., 1993). Deposits of eoliansandstone (Ste) indicate subaerial sand dune migrationover the floodplain where flooding was insufficient toinhibit dune formation.

The low-angle inclined sets of parallel-laminatedsandstone contain mudstone laminae, discounting aneolian origin. Desiccation features indicate intermittentsubaerial exposure. The low angle of inclination isinconsistent with formation by dune migration, but toosteep to have been deposited as upper-flow-regime planebeds. The lack of a vertical progression of structures, forexample from dune to ripple-scale cross-sets, suggeststhat this was not a fluvial channel. The horizontal laminaeare therefore considered to have been deposited duringlower-flow-regime conditions on an inclined sand surfacethat migrated over floodplain mud. This is similar tocrevasse splays described from the sand-bed braidedNiobrara River (Bristow et al., 1999), in which ∼1 mthick inclined sets of horizontal lamination and ripplelamination overlie floodplain fines.We therefore interpretinclined sets of laminated sandstone as crevasse splaydeposits that emanated from the thin trough cross-stratified sandstone-dominated (St) crevasse channels.

Intervals dominated by thewavy bedded facies suggestprolonged periods where pools of water might have re-mained on the floodplain, perhaps due to a near-surfacewater table. Such deposits have been described fromrecent braided fluvial floodplain deposits by Bristow et al.(1999) and ephemeral streams by Martin (2000).Floodplain deposits with wave ripples and an apparentabsence of desiccation features have also been describedfrom a Mesoproterozoic braided fluvial system in EastGreenland by Sønderholm and Tirsgaard (1998).


Understanding of the relationship between gradient,sediment grain size and stream type is based mainly onsystems that include sediment-binding vegetative cover. Inthe absence of vegetation, braided streams might exist atlower gradients, lower discharge regimes or finer sedimentgrain sizes. Therefore, heterolithic braided streams andassociated floodplains may be the pre-vegetative equiva-lent to meandering streams and floodplains with respect tothese factors. However, the predominance of braidedstreams, even in the finest deposits and hence lowest gra-dients, could alternatively be due to ephemeral flash-floo-ding that resulted in episodic high-discharge streamflow. Incontrast to recent floodplain deposits, such as along theWaimakariri River which generally contains very few pre-served depositional structures (Reinfelds and Nanson,1993), floodplain deposits from the PaleoproterozoicBaker Lake Basin contain a diverse array of structuresdue to the absence of bioturbation or root growth. Togetherwith other Precambrian deposits, such as the Mesoproter-ozoic braided fluvial system described by Sønderholm andTirsgaard (1998), they provide a perspective on floodplaindeposits generally not available from the Phanerozoic.

2.5. Facies association 5: eolian

2.5.1. Lithofacies descriptionThe eolian FA is typified by up to 10 m thick accumu-

lations of trough cross-stratified sets, 20 cm to 2m thick, offine- tomedium-grained,well-sorted sandstone (Ste). Basalforesets are typically reverse-graded fine- to medium-grained sandstone (Fig. 8c), and most exhibit pinstripelamination (cf. Fryberger and Schenk, 1988). Upper fore-sets are wedge-shaped, tapering downward, and normallygraded, locally coarse- to medium-grained sandstone. Thetops of cross-sets are typically truncated by horizontalsurfaces. These may be associated with granule or pebblelayers, or cross-stratified pebbly sandstone (St). Betweenthese erosional surfaces and the succeeding large-scalecross-set are 10–20 cm thick intervals of wave-rippledsandstone (Sw) and/or interlaminated mudstone (SFw).

2.5.2. Lithofacies interpretationPinstripe lamination and reverse-graded foresets are

interpreted as sub-critically climbing translatent stratifi-cation resulting from the migration of wind-ripples oversubaerial dune slipfaces (cf. Hunter, 1977). Wedge-shaped, normally graded foresets are interpreted as grain-flow deposits. Intervals of wave rippled sandstone (Sw)and/or interlaminated mudstone (SFw) at the base ofeolian cross-sets are considered to be wet-condition inter-dune deposits indicating a near surface water table (Ko-curek and Havholm, 1993). The minor occurrence of

pebbly sandstone indicates that interdune areas were sub-ject to streamflows since the pebbles are too large to havebeen transported by wind, and so these fluvial depositswere reworked resulting in pebble layers interpreted aslags. The large-scale cross-sets are therefore considered tobe formed by the migration of eolian dunes, with wind-ripple lamination preserved on lower slipfaces, and inter-dune areas characterized by standing water with infre-quent streamflow influx from surrounding alluvial plains.This is characteristic of a wet condition eolian system(Kocurek and Havholm, 1993).

Simpson et al. (2004) consider eolian deposits from theBaker Lake Basin to consist of two general occurrences,as thin sandsheets dominated by wind ripple laminationassociated with ephemeral lacustrine and fluvial deposits,and thicker (up to 100m) erg deposits dominated by large-scale cross-sets (up to 6 m thick). The eolian lithofaciesassemblage described herein is primarily based uponobservations from northern Christopher Island, where it isrepresented by up to 10 m thick accumulations of largescale (up to 2 m) cross-sets of sandstone associated withephemeral lacustrine and fluvial deposits. The presence ofinterdune deposits between individual cross-sets indicatesthese were not compound dunes and therefore equivalentto the thin sandsheet subdivision of Simpson et al. (2004).

2.5.3. Facies successionsThere are two types of bedset within the eolian facies

assemblage (Fig. 10). The first is relatively simple andconsists of large-scale trough cross-stratified sandstone(Ste) with wave-rippled sandstone bottom sets (Sw) orcross-stratified sandstone (St), considered to probablyrepresent dune and interdune strata respectively (Kocurek,1981).

The second type of bedset is more complex. A com-plete vertical facies succession consists of: thin (∼10–20 cm) cross-stratified pebbly sandstone (St); pebble orgranule lag; approximately 10–20 cm thick interstratifiedsandstone and mudstone with prominent wave ripples(SFw, Sw); overlain by metre-scale eolian cross-sets. Thepebbly sandstone is rarely preserved, and so boundingsurfaces for multiply stacked bedsets are commonly thehorizontal erosional surfaces. The interpreted successionof depositional events is: fluvial influx of pebbly sand;erosion to produce the lag; intermittent wave currents andsuspension deposition; followed by metre-scale eoliandune migration. These bedsets occur as multiply stackedsets, and so fluvial sandstone overlies eolian cross-sets,representing streamflow flooding of the eolian dune fieldprior to erosion.

Two potential processes for producing the horizontalerosional surfaces are wind deflation and wave-induced


erosion. In the first case, deflation of the eolian dune fieldwould be accompanied by streamflows, accounting forfluvial sandstone overlying the eolian cross-sets. The sur-

Fig. 10. Stratigraphic section from northwest Christopher Island study

face of deflation may have been controlled by the groundwater table as a Stokes surface (Stokes, 1968; Frybergeret al., 1988; Kocurek and Havholm, 1993). Subsequent

area, showing lacustrine, eolian and playa facies assemblages.


erosion following fluvial deposition may have been in-hibited by formation of an armoured pebble lag, as ob-served in periglacial eolian deposits of Iceland (Mountneyand Russell, 2004). This would be followed by shallowsubaqueous conditions with intermittent wave currents andsuspension deposition recorded by the interstratified waverippled sandstone and mudstone laminae. In the absence ofstreamflows an eolian dune field was re-established.

With respect to wave-induced erosion, initial base level(groundwater table) would be steady or low during eoliandune field formation. A rise in base level would be accom-panied initially by an influx of fluvial streamflows, then byshallow standing water as adjacent playa lakes expanded.Wave currents would rework the substrate resulting a ho-rizontal erosion surface, pebble lag and overlying wave-rippled sandstone. This process is analogous to a trans-gressive surface of erosion. Contraction of an adjacentplaya lake was accompanied re-establishment of the eoliandune field.

It is difficult to determinewhether the pebble lags recordwind deflation or transgressive erosion; by association theoverlying SFw/Sw facies are consistent with the latter,however a combination of processes is probable. Sweet(1999) rationalized a rising water table and wind deflationby supposing that as lake expansion occurred sedimentsupply from lake margins was cut off. Winds blowing offthe playa margin were undersaturated with respect to sandand effective at deflating dunes. In the Baker Lake Basin,subsequent to removal of eolian sediment supply and de-flation, shallow lacustrine inundation may have been ac-companied by wave erosion and additional planation.

Both models involve rising base level: If base levelcontrolled the erosion surface, then base level fluctuationscontrol the accommodation increment in eolian systems,consistent with existing theories for preservation of eolianaccumulations (Stokes, 1968; Kocurek and Havholm,1993; Carr-Crabaugh and Kocurek, 1998; Simpson et al.,2004).

This facies association therefore represents environ-ments without significant fluvial sediment flux whereeolian dunes fields were able to develop, which weresubject to episodes of flooding during expansion andcontraction of an adjacent playa lake in response to baselevel fluctuations.

2.6. Facies association 6: playa/mudflat

2.6.1. Lithofacies descriptionThis FA is dominated by 5–20 cm thick layers of

mudstone (Fm) interstratified with 5–20 cm thick troughcross-stratified sandstone (St) and ripple cross-stratifiedsandstone (Sr; Fig. 8e). Within mudstone, V-shaped cracks

up to 5 cm deep filled with sandstone from the overlyingbed are prominent.

2.6.2. Lithofacies interpretation and successionThick mudstone layers indicate sustained periods of

suspension deposition; deep v-shaped desiccation cracksindicate subaerial exposure; thin cross-stratified sandstonebeds overlain bymudstone indicate that bedload depositionpreceded a resumption of suspension deposition. The de-positional environment was characterized by playa lakeexpansion due to episodic flooding, leading to sustainedsuspension deposition to form a mud flat environment (5–20 cm of laminated mudstone), followed by subaerialexposure representing playa lake contraction and desicca-tion of the mudflat.

The basic depositional unit of this association is anupward-fining 10–40 cm cycle of sandstone to mudstone,which represents playa lake expansion followed by con-traction and desiccation, likely recording climatic fluctua-tions. Successions of these cycles are generally less than5 m thick, but can reach thickness' greater than 50 m(Rainbird et al., 1999). Since this facies association istypically intercalated with eolian and lacustrine facies, weinterpret it to have been deposited in a playa lake-mud flatenvironment.

With respect to the association with eolian deposits, aprevalence of playa over eolian environment could be dueto a relatively higher water table that periodically dam-pened the substrate sufficiently to inhibit eolian dunegrowth, or theremay have been a higher proportion of finesediment. Considering the proximity of a vegetation-freesandy braidplain, the playa environmentwasmore likely aproduct of a relatively higher water table.

At southern Christopher Island the playa facies isdominated by mudstone with desiccation cracks and itreaches a maximum thickness of 50 m (Rainbird et al.,1999). This implies a significant source of mud-gradesediment. Macey (1973) identified detrital phlogopite inthe Kazan Formation from southern Christopher Islandand proposed that Christopher Island Formation volcanicand volcaniclastic deposits had supplied volcaniclasticsediment. These chocolate brown, volcaniclastic rockscontain significant amounts of ash-sized particles (Blake,1980; Rainbird et al., 1999), which indicates the avail-ability of a large volume of fine sediment to an ephemerallacustrine environment.

2.7. Facies association 7: lacustrine delta

2.7.1. Lithofacies descriptionThe St facies occurs as 0.2–0.8 m trough cross-sets

that form 1–2 m thick cosets. Mudstone clasts are


common at the base of cosets. Mudstone drapes arecommon at the top of cross-stratified sets and on foresets.These units have lenticular bases and incise into under-lying deposits, which may include facies Fl or St.

Laminated mudstone and siltstone facies (Fl) is inter-stratified with starved symmetrical ripples and beds ofsandstone generally less than 5 cm thick. Pebble lagsoccur at the base of mudstone-dominated intervals thatoverlie pebbly sandstone (Fig. 8g). Desiccation featuresare absent. These intervals reach thicknesses up to50 cm and are commonly incised by the 1–2 m thick Stunits.

Conversely, mudstone drapes are less common in thesymmetrical-rippled sandstone facies (Sw). Ripple typesinclude symmetrical ripples that occur as reworkedcross-set tops, and climbing ripples, locally supercriti-cally climbing (Fig. 8f). Together with thin (5–10 cm)cross-stratified sandstone (St) sets with symmetrical-rippled tops, the rippled sandstone comprises tabularsheets 1–2 m in thickness.

An uncommon facies within this association isnormally graded, horizontally laminated sandstone (Sh;Fig. 8h). It is typified by upward-fining and upward-thinning laminae comprising beds 10–15 cm thick. Basallaminae are coarse-sand grade; upper laminae are fine tovery fine sand. Angular mudstone clasts of mm-scale arecommon in the thicker laminae. Very thinly laminatedsiltstone and mudstone occur at the top of upper gradedsandstone laminae; locally these thin laminae are com-posed almost entirely of horizontal mudstone microclasts.Individual laminae can be traced laterally over a fewmetres, to the extent of outcrop (and lichen) limitations,and beds are continuous for more than 100 m. Troughcross-bedded sandstone with up to 10 cm thick inverse tonormally graded foresets is associated with this facies, aswell as ripple cross-laminated sandstone (Sr).

2.7.2. Lithofacies interpretationUpward-fining bedsets of trough cross-stratified sand-

stone (St) forming sets that incise into underlying depositsare considered to be channel deposits. Mudstone drapes,including those on foresets, indicate suspension deposi-tion within channels between streamflow events. Angularmudstone clasts are interpreted as mudstone rip-ups.Braided stream deposits (FA 3) are characterized as anephemeral, high-discharge fluvial system. The channelswithin this facies association record streamflows alter-nating with standing water suspension deposition, per-haps reflecting seasonal discharge within distributarychannel in a lacustrine environment.

The association of facies Fl with wave ripples and thinbeds of sandstone indicates an environment dominated

by suspension deposition, but subject to wave currentsand occasional bedload deposition of sand. The lack ofdesiccation features, as in the playa deposits (FA 6), andbrief periods of wave currents and bedload sedimenta-tion, would be consistent with deposition in a protectedbay. The pebble layer at the base is interpreted as a lag,and together with wave ripples is suggestive of a pre-ceding phase of wave erosion. Subsequent incisement bychannels that record inter-streamflow slack waterconditions is consistent with deposition in an interdis-tributary bay (cf. Elliott, 1974; Fielding, 1984).

Within the rippled sandstone sheets, the dominance ofsymmetrical ripples indicates the prevalence of oscillat-ing currents and therefore wave processes. In-phaseclimbing ripples indicate high rates of sedimentation.Ubiquitous wave-ripple reworked cross-set bed-topsindicating that unidirectional currents were consistentlyfollowed by wave currents, are suggestive of sand barssubject to shoaling waves. These features and the sheetedgeometry are consistent with deposition at a lake margindelta front mouth bar (cf. Plint and Browne, 1994;Marshall, 2000).

The graded horizontal lamination is identical toBoumadivision Td, which is characterized by fine parallel lami-nation and textural sorting (Bouma, 1962). Oaie (1998)described an Upper Proterozoic occurrence of mudstonemicroclasts from the T3 subdivision (distinctly laminatedsandstone, equivalent to Td; Stow and Shanmugan, 1980),and also noted features such as continuous or discontin-uous parallel lamination due to the orientation of micro-clasts parallel to bedding planes. Ripple cross-laminatedsandstone associated with graded laminae corresponds toBouma division Tc. These packages of Bouma Tc–d divi-sions locally have wave-ripple reworked tops, and closelyoverlie cross-sets that have wave-ripple reworked tops,indicating that they were deposited above storm wavebase. These upward-thinning and upward-fining unitswith ripple cross-stratified sandstone are interpreted to beturbidites, reflecting delta front sediment gravity flowprocesses. In the absence of tidal features, these turbiditesare the best indication of a lacustrine environment. To-gether, the association of turbidites, distributary channels,interdistributary bays and rippled sandsheets comprise aperennial lacustrine deltaic environment.

2.7.3. Facies successionsThere are two types of generally upward-coarsening

successions (Fig. 10). In the first, the base is sharp anderosion is typically indicated by a transgressive lagoverlain by laminatedmudstone (Fl). This is succeeded byan upward-coarsening interval of cross-stratified sand-stone with abundant mudstone clasts, which is truncated

Fig. 11. Schematic block diagram of half-graben and facies tracts from theBaker Sequence during the interval of localized felsic minette volcanism.


and incised by an upward-fining interval of cross-stratified sandstone. Employing the distributary/interdis-tributary bay model presented by Elliott (1974), we in-terpret this as a prograding mouth bar overlain by anupward-fining distributary channel. Such distributarychannel deposits generally display wave-reworked tops,as indicated by wave ripples or a lag. This is likelybecause an abandoned distributary channel will be apositive feature, and subject to wave erosion at the deltaedge until interdistributary bay sedimentation “catchesup” and buries the sand bar.

The second type of upward-coarsening succession issimilar to the first, with a sharp base overlain bymudstoneand upward-coarsening sandstone. Turbidites may occurat the base of these upward-coarsening intervals. Sheets ofwave-rippled sandstone (Sw) and the sub-facies of thincross-stratified sandstone beds with wave-rippled topsoccur at the top, instead of a distributary channel deposit.Similar associations of upward-coarsening successionand thin cross-stratified sets with wave-rippled tops havebeen described by Plint and Browne (1994) from aPhanerozoic strike-slip basin, and interpreted to representthe lake margin bay mouth bar of a lacustrine delta. Wesimilarly interpret this succession as a bay mouthsuccession capped by progradation of a mouth bar at thelake margin. The rippled sandsheet is overlain by eoliandeposits, so continued progradation of the delta systemwas interrupted by relative lake level fall and eolianreworking of the delta top (Fig. 10).

3. Depositional model

Examination of stratigraphic contacts and arealdistribution of the various facies associations enablesreconstruction of the paleobasin through a model oflinked facies tracts. The alluvial fan FA occurs at thepresent day basin margin, primarily at kilometre-scalethickness along the southeastern basin margin. Evidenceof local derivation includes boulder-sized angular clasts,similar to the underlying crystalline basement, eventhough contemporaneous volcanic centres were locallyactive within the basin. Paleocurrent data indicate alluvialtransport was transverse to the basin margin (Fig. 4),which suggests that the present-day basinmargin approxi-mates the paleobasin margin.

The gravel-bed braided stream FA occurs in gradationalstratigraphic contact with the alluvial fan FA. This, in turngrades into the sand-bed braided stream FA. Thegradational transition indicates that these are linked faciesand represent lateral transitions. The change in grain sizeand inferred depositional gradient therefore is representa-tive of a proximal to distal fluvial system; proximal alluvial

fans fed gravel-bed braided streams at their base, which,with decreasing gradient and competence, graded intosand-bed braided streams. The braided streams occurthroughout the basin from the inferred paleomargin (e.g.Thirty Mile Lake) to the depocentre (Christopher Island)(Fig. 11). Paleocurrent data define two drainage patterns forthese braided streams: (1) near the basin margins the trendis transverse to the margin (Fig. 4), and (2) at the centre ofthe elongateBaker Lake sub-basin,where paleocurrent datadefine an axial drainage system (Figs. 2 and 5). Togetherwith asymmetry of stratigraphic thickness of the BakerLake Group from the northwest to the southeast, ∼500 mand N2000 m, respectively (Hadlari and Rainbird, 2000;Rainbird and Hadlari, 2000), the drainage patterns areconsistent with deposition in a half-graben (e.g.. Leeder,1995), the bounding fault of which was adjacent to thesoutheast margin (Fig. 2).

The floodplain FA is associated with the sand-bedbraided stream FA, but also occurs in stratigraphic contactwith eolian, playa and lacustrine FAs. Prominent withinthe floodplain depositional environment are indications ofstanding water, such as abundant wave ripples and localpaucity of desiccation features. Similarly, wet-conditioninterdune deposits characterize the eolian facies, wherethin sandsheet-type eolian deposits are interstratifiedwithin most facies of the Baker Lake Group throughout


the basin. Thicker eolian deposits, dominated by large-scale cross-sets are primarily associated with lacustrine,floodplain, and to a lesser degree braided stream FAs, aremost common near the inferred basin axis along KazanRiver (Simpson et al., 2004) and the main depocentre atChristopher Island (Rainbird et al., 1999; Simpson et al.,2004; Fig. 5). The playa–mudflat FA occurs primarily instratigraphic contact with the eolian FA on ChristopherIsland indicating a close spatial relationship near the maindepocentre of the Baker Lake sub-basin (Rainbird et al.,2003). The lacustrine delta FA has a small areal extent,exposed only at Christopher Island. The stratigraphictransition from fluvial to lacustrine (though rarely com-plete) passes through, eolian, playa and floodplain faciesassemblages, indicating that mud flats and eolian dunesoccupied lake margins adjacent to deltas, depending onlake expansion or infilling, respectively. Paleocurrent datafrom northwestern Christopher Island (Fig. 4) indicatesouthwesterly streamflow. Therefore, northwesternChris-topher Island marked the eastern edge of the basinaldepocentre. Deltas prograded south andwest, into the lakebasin, and were fed by braided streams that originated tothe northeast. The present-day basin margin ends at thenorth shore of Baker Lake and it is likely that thepaleobasin originally extended farther northeast, becausethese paleocurrent data indicate that sand-bed braidedstreams extend to the present margin instead ofconglomerate that would be expected, if the presentnortheast margin coincided with the paleobasin margin.

Paleocurrent data from delta-top eolian cross-sets (Fig.10) indicate southwesterly wind flow, assuming that dunecrests were oriented transverse to the primary wind direc-tion. This appears to be a valid assumption, because thepaleocurrent directions are perpendicular to the trend ofwave ripple crests within the delta complex. Other north-western Christopher Island eolian paleocurrent data thatindicate northwesterly aerial transport (Fig. 5) are simi-larly perpendicular to wave ripple crest trends from inter-dune intervals, suggesting that waves were generated by asimilar prevailing wind direction as the eolian dunes.These paleocurrents are associated with braided streamdeposits with southeast-directed paleocurrents, distinctfrom the aerial paleocurrent direction.

Thus, considering the distribution of alluvial facies,paleocurrent data and stratigraphic thickness (Hadlari andRainbird, 2000; Rainbird and Hadlari, 2000), a model oflinked facies tracts for the Baker Lake sub-basin is setwithin an elongate half-graben basin (Fig. 11). From themargins, a transverse drainage system of alluvial fans tobraided streams, with floodplains and eolian dunes, fed anaxial fluvial system. Axial drainage was primarily directednortheast and less extensively southwest to a depocentre

near Christopher Island (Fig. 2). This pattern indicates thatthe Baker Lake sub-basin was a hydrologically closedsystem: primary drainage was endemic rather than directedto an adjacent basin. At the depocentre, deltas fed into alake that was surrounded by floodplains, mudflats, andeolian dunes with prevailing wind directed northwest andsouthwest (relative to present geography).

4. Discussion: eolian deposits and paleoclimate

Sedimentology of the Baker Lake Group reveals avariety of climatic indicators. Eolian deposits, which are ameasure of aridity, are primarily associated with lacustrine,floodplain, and to a lesser degree braided-stream facies(Fig. 10), and aremost common near the inferred basin axisalong Kazan River (Simpson et al., 2004) and the maindepocentre at Christopher Island (Rainbird et al., 1999; Fig.11). In very thick deposits (30 to 100 m) of eolian sand-stone, some cross-set bounding surfaces indicate dry inter-dune conditions (Rainbird et al., 1999; Simpson et al.,2004). However, thinner sandsheet-type accumulations inassociation with lacustrine and floodplain facies, in parti-cular at the inferred depocentre (Christopher Island), con-tain a greater proportion of interdune deposits composed ofinterlaminated, wave-rippled sandstone and mudstone,indicating flooding of interdune areas (cf. Kocurek andHavholm, 1993). Almost every facies association includesan eolian component: eolian reworking of abandonedchannels, eolian sandstone sheets associated with playa–mudflat environments, and eolian sandstone interstratifiedwith floodplain deposits. However, wave ripples andmudstone laminae within floodplain deposits record pe-riods where standingwater was relatively common. Deltaicdeposits indicate that a perennial lake existed at the maindepocentre. These features confirm that the water table wasclose to the surface, inconsistent with an arid climate.Climatic fluctuations occur at vastly shorter time scalesthan the ∼45 Ma span of time represented by the BakerLake Group. For example, the hyper-arid Rub Al Khalieolian system of the Arabian Peninsula is presently theworld's largest erg; however, lacustrine and paleoground-water deposits such as travertine suggest that the climatewas humid at 35–25 ka and 10–6 ka, coincident withprecessional orbital parameters (Bray and Stokes, 2004).Furthermore, eolian systems are not restricted to hot-climate environments; for example, the cold-climate Askjaregion periglacial sandsheet of Iceland (Mountney andRussell, 2004). These relatively recent, biologically hostileenvironments are analogous to those of the Baker LakeBasin in that they developed on a non-vegetated landscapewith sufficient sediment supply for eolian accumulation. Inthe absence of sediment-binding vegetative cover in the


Paleoproterozoic, it is possible that eolian deposits are notnecessarily an indication of an arid climate, but rather amobile substrate adjacent to a viable sediment source (e.g.active fluvial channel belts). Therefore, theBaker Sequencedeposits broadly suggest a variably semi-arid to semi-humid paleoclimate. With respect to lacustrine deposits,related evaporite minerals and chemogenic lake bedswithin the Angikuni sub-basin, Aspler et al. (2004) hassimilarly suggested a wet paleoclimate with local aridintervals for the Baker Sequence.

5. Conclusion

The alluvial fan FA consists of upward-fining stratalunits 5–10 m thick. These indicate that the alluvial fandeveloped by a succession of lobe accretion and aban-donment events. The main lobe accretion units arerepresented by upward-fining, tabular units of the faciesGcd and Gco, respectively, which record rapid deposi-tion of gravel sheets during high-magnitude streamflowsfollowed by incisement during secondary low-magni-tude streamflows. Inactive lobes were characterized bysand and mud deposition analogous to overbank pro-cesses on alluvial plains. The predominance of stream-flow processes was probably due to weathering anderosion of crystalline rock in the source region, as in-dicated by granitoid and gneissic clast lithologies.

Alluvial fans were primarily located along the south-eastern margin of the basin, and combined with regionalpaleocurrent and stratigraphic thickness variationsindicate that the primary basin-bounding fault of theBaker Lake sub-basin was adjacent to its present south-eastern margin.

The gravel-bed braided stream FA also preservesupward-fining bedsets, 5–10 m thick, which recordaggradation and lateral channel-belt switching. Theseare differentiated from the alluvial fan facies by bettersorting and condensed framework, with imbricated clastsindicating more sustained streamflow and less rapid de-position. Conglomerate facies are discontinuous at scalesover 100 m, indicative of approximate channel widths.The gravel-bed FA is gradational between the alluvial fanand sand-bed braided stream FAs, and distributed fromthe basin margin through the basin axis.

The sand-bed braided stream facies associationcomprises 5–15 m thick upward-fining, stacked bedsetsinterpreted as channel complex successions. Aggrada-tion of mixed-load ephemeral sheetflood and shallow,sand-bed braided streams was followed by upstreamchannel-belt switching. The abandoned channels weresites of suspension deposition of overbank fines andeolian reworking.

The floodplain facies association primarily consists ofinterstratified sandstone and mudstone representing alter-nating bedload and suspension deposition in an overbanksetting. Locally abundant wave ripples record standingwater subsequent to flood events, suggesting a shallowand fluctuating water table. Thin sandstone intervals re-present crevasse channels and inclined sandstone setsrepresent crevasse splays. Eolian dunes indicate subaerialreworking of abandoned fluvial channels.

The eolian facies association includes thin sand-sheets located adjacent to floodplains, playas and deltas.Cross-sets up to 2 m thick record eolian dunes boundedby wet-condition interdune intervals indicative of a nearsurface water table, which controlled accumulation ofeolian deposits. This description is in addition to pre-viously documented erg deposits at the Kunwak River(Simpson et al., 2004), and lesser erg deposits at south-eastern Christopher Island associated with playa de-posits (Rainbird et al., 1999; Simpson et al., 2004). Theoccurrence of eolian deposits within most depositionalenvironments is considered to reflect reworking ofabuandant sand supply on the non-vegetated Precam-brian landscape.

The playa facies association is dominated by lami-nated mudstone with desiccation cracks and subordinatetrough cross-stratified sandstone, representing alternat-ing suspension deposition and desiccation of a lacustrinemudflat, punctuated by bedload flood events. This faciesis associated with the eolian and lacustrine facies assem-blages, and represents a lake margin setting where ex-pansion and contraction due to base level fluctuationsinhibited eolian sandsheet formation.

The lacustrine delta facies association consists ofprodelta turbidites, rippled sandsheets that accumulatedas bay mouth bars, distributary channel sandstone andinterdistributary bay laminated, rippled sandstone–mudstone. The deltaic deposits of northwestern Chris-topher Island record progradation toward a depocentreto the southeast.

In a three-fold subdivision of the volcanic stratigraphy,the lower subdivision comprises felsic minette flows andvolcaniclastics erupted at volcanic centres adjacent tobasin-margin alluvial fans. This was followed by volu-minous minette extrusion, in which flows and volcani-clastic sediments spread from the volcanic centres toblanket most of the basin. Flows are common at ThirtyMile Lake, but are rare at Christopher Island where vol-caniclastics comprise most of the volcanic deposits.Areally restricted felsite domes comprise the upper part ofthe volcanic succession. Where the basin was not entirelyfilled or overfilled due to minette volcanism, gravel-bedbraided streams transported felsite clasts basinward.


The sedimentologyof theBakerLakeGroup indicates atransverse drainage system of alluvial fans to braidedstreams, with floodplains and eolian dunes adjacent toinactive channels. This transverse system fed an axialdrainage system that primarily was directed northeast andless extensively southwest to a depocentre near Christo-pher Island, defining the pattern of a hydrologically closedbasin. At this depocentre, deltas fed into a lake that wassurrounded by floodplains, mudflats and eolian dunesdeposited when the prevailing wind was directed north-west and southwest. Sedimentological features such asephemeral, flash flood-type alluvial deposits, playas, andeolian sandsheets and ergs, indicate a level of ariditymoderated bywet-condition eolian inter-dune deposits andfloodplain deposits that indicative of a near surface watertable, and thus a semi-arid to semi-humid paleoclimate.

Acknowledgements

Extensive logistical support from the GeologicalSurvey of Canada (Natural Resources Canada) is grate-fully acknowledged, accordingly this is GSC contribu-tion #2005407. Financial support was obtained from anNSERC Discovery Grant to Rob Rainbird. Commentsfrom Hazen Russell, Geoff Chiarenzelli, and Guy Plintwere appreciated and improved the manuscript.

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