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Hadlari etal. 2006 baker lake rift basin sedimentology
 

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Alluvial Eolian and Lacustrine sedimentology of a rift basin.

Alluvial Eolian and Lacustrine sedimentology of a rift basin.

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    Hadlari etal. 2006 baker lake rift basin sedimentology Hadlari etal. 2006 baker lake rift basin sedimentology Document Transcript

    • Sedimentary Geology 190 (2006) 47 – 70 www.elsevier.com/locate/sedgeoAlluvial, eolian and lacustrine sedimentology of a Paleoproterozoic half-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 5B6 b Geological Survey of Canada, 601 Booth St., Ottawa, Ont., Canada K1A 0E8Abstract 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-graben1. Introduction to inferred depocentre of the Baker Lake sub-basin, as an aid to reconstruction of its paleogeography. In addition, This research is part of an integrated study of the Baker the remarkable preservation and absence of bioturbationLake Group, emphasizing sequence stratigraphy and from these Paleoproterozoic rocks provides sedimento-chronostratigraphy, for the purpose of constructing a logical insight into alluvial environments in certaintectonostratigraphic model for Baker Lake Basin. Utili- instances generally not available from the Phanerozoic.zation of non-marine sequence stratigraphic methods to Thick alluvial fan deposits are exposed on large, glaciallyelucidate the relationship between sedimentation and polished outcrops. Floodplain deposits are associatedtectonism requires an understanding of the depositional with braided stream deposits, a relatively undevelopedenvironments throughout the basin. This paper describes topic of study (e.g. Bristow et al., 1999), but observedthe sedimentology of the ca. 1.83 Ga Baker Lake Group elsewhere in Precambrian deposits (Sønderholm andfrom well-exposed key stratigraphic sections from margin Tirsgaard, 1998). It has been speculated that eolianites should be more prevalent in Precambrian deposits than in ⁎ Corresponding author. Fax: +1 613 520 2569. the Phanerozoic due to lack of terrestrial vegetation (e.g. E-mail addresses: thadlari@connect.carleton.ca (T. Hadlari), Eriksson and Simpson, 1998). While this generally hasntrrainbir@nrcan.gc.ca (R.H. Rainbird). been the case, eolian deposits occur throughout the Baker0037-0738/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.sedgeo.2006.05.005
    • 48 T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70Lake Basin, reworking fluvial deposits and forming thin space and time with respect to the evolution of the Westernsandsheets to large ergs (Rainbird et al., 2003; Simpson Churchill Province. Deposition of the ca. 1.85–1.76 Gaet al., 2004). This research, representing the first regional Baker Lake Group appears to have closely followedlithofacies analysis of the Baker Lake sub-basin, deformation and metamorphism in underlying crystallineincorporates previously completed fieldwork by the basement rocks, which in some cases were at lower crustalGeological Survey of Canada. levels at ca. 1.9 Ga (Sanborn-Barrie, 1994). Contempora- neous collisional tectonics were taking place in the ca. 1.9–1.1. Regional geology and previous work 1.8 Ga Trans-Hudson Orogen, 500 km to the south and southeast (e.g. Lucas et al., 1999). Greater Baker Lake Basin extends from Dubawnt Lake The Dubawnt Supergroup is subdivided into threenortheast to Baker Lake (Nunavut, Canada) and comprises unconformity-bounded stratigraphic units that corresponda series of northeast-trending intracontinental basins, to, from oldest to youngest: the Baker Lake, Wharton andincluding the Baker Lake sub-basin (Rainbird et al., Barrensland Groups (Donaldson, 1967; Gall et al., 1992;2003; Figs. 1 and 2). Basin fill comprises the faulted but Rainbird and Hadlari, 2000); or the Baker, Whart andunmetamorphosed, siliciclastic and volcanic rocks of the Barrens second-order sequences (Rainbird et al., 2003;Dubawnt Supergroup (Wright, 1955; Donaldson, 1967; Fig. 3). These groups or corresponding second-orderLeCheminant et al., 1979b; Gall et al., 1992; Rainbird and sequences have been interpreted to represent the tectonicHadlari, 2000; Rainbird et al., 2003Fig. 3). The ca. 1.85– stages of rift, modified rift and thermal sag, respectively1.70 Ga Baker Lake Basin occupied a unique location in (Rainbird et al., 2003). Fig. 1. Location map indicating the Baker Lake Basin in the context of the Western Churchill and Rae Provinces.
    • T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70 49 Fig. 2. Geology map of the Baker Lake sub-basin and fluvial paleocurrent data. The Baker Lake Group comprises the South Channel, ciclastic sedimentary rocks. Volcanology of the Chris-Kazan, Christopher Island (Donaldson, 1965), Kunwak topher Island Fm. from Baker Lake sub-basin has been(LeCheminant et al., 1979b) and Angikuni Formations described in detail (LeCheminant et al., 1979a,b; Blake,(Blake, 1980), and varies in cumulative thickness from 1980). A generalized volcanic stratigraphy for theover 2 km to 500 m. These lithostratigraphic subdivi- greater Baker Lake Basin, from oldest to youngest,sions have provided the framework for regional mapping consists of: felsic minette flows, minette flows and felsitewithin Baker Lake Basin. Chronostratigraphic control on flows (Peterson et al., 1989; Hadlari and Rainbird, 2001;the formations is quite poor, but recent studies suggest Rainbird et al., 2003). The felsic minette flows or equi-that these formations are time-equivalent, reflecting valent volcaniclastic deposits are less areally extensivelateral facies boundaries (Rainbird et al., 1999, 2003). than younger minette flows, and have been observed to The South Channel Formation comprises boulder to overlie the basal unconformity of the Baker Lake Group.cobble conglomerate interpreted as alluvial fan deposits. Mantle-derived minette flows record voluminous extru-It typically overlies crystalline basement rocks at the basin sion throughout the entire basin and represent the largestmargin and is composed of locally derived clasts of gra- known ultrapotassic volcanic province (LeCheminantnite, amphibolite and gneissic lithologies. For this reason, et al., 1987; Peterson et al., 1989, 1994; Cousens et al.,it appears to be the oldest formation, although volcanic 2001). Stratigraphic relations indicate that the flowsrocks of the Christopher Island Fm. also unconformably originated at volcanic centres, which progressivelyoverlie basement (Rainbird and Hadlari, 2000), and occur expanded outward to eventually blanket most of theas clasts within the South Channel Fm. (Hadlari and basin (Hadlari and Rainbird, 2001). The volcanic centresRainbird, 2001). would have been positive topographic features that The Kazan Formation consists of arkosic sandstone, supplied volcaniclastic sediment, diverted streamflowsiltstone and mudstone, representing a variety of possibly altering drainage patterns and replaced sedi-sedimentary environments including eolian, fluvial and mentary processes as a basin-infilling mechanism.playa lake (Donaldson, 1965; LeCheminant et al., Felsite flows are the youngest and most areally restricted1979b; Rainbird et al., 2003). volcanic rock. Analyses of phlogopite phenocrysts from The Christopher Island Formation comprises alkaline a flow and a syenite intrusion that intrudes the lowervolcanic rocks interbedded with volcaniclastic and sili- Baker Lake Group yield 40Ar/39Ar ages of 1845 ± 12 Ma
    • 50 T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70Fig. 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 ± 11 Ma, respectively (Rainbird et al., 2002; see In the Thirty Mile Lake area of the Baker Lake sub-discussion Rainbird et al., 2006). A more precise U–Pb basin (Fig. 4) steeply inclined, east–northeast-strikingzircon age of 1833 ± 3 Ma has been obtained from a felsic units of conglomerate, sandstone and volcanic strata ofminette flow from the western end of Baker Lake Basin, the Baker Lake Group unconformably overlie crystallineproviding the best constraint on basin formation basement. Previous mapping in this area identified South(Rainbird et al., 2006). Channel Fm. conglomerate, Kazan Fm. sandstone and The Kunwak Formation (LeCheminant et al., 1979b) mudstone, and Christopher Island Fm. volcanic rocksconsists of conglomerate composed primarily of Christo- (Donaldson, 1965, 1967; LeCheminant et al., 1979b).pher Island Fm. volcanic clasts as opposed to basement The Kunwak Formation is exposed to the northwest,rock types in the South Channel Fm. It is differentiated along the Kunwak River, where it contains felsite clastsfrom the Christopher Island Fm. by its stratigraphic posi- and is unconformably overlain by the Wharton Grouption above volcanic rocks and below the unconformity at (LeCheminant et al., 1979b; Hadlari and Rainbird,the top of the Baker Lake Group. This formation primarily 2001).occurs in the interior of the Baker Lake sub-basin, located At Christopher Island (Fig. 5), the South Channelproximal or downstream from volcanic centres. Formation unconformably overlies the Archean Mac- The Angikuni Formation (Blake, 1980) is restricted to Quoid-Gibson supracrustal belt (Tella et al., 1997;the Angikuni sub-basin (Fig. 1). Aspler et al. (2004) Hanmer et al., 1999) and the 1.9 Ga Kramanituar meta-consider it to be equivalent to the South Channel and morphic complex (Sanborn-Barrie, 1994; Sanborn-BarrieKazan Formations. Incompatible element chemistry of et al., 2001). The Kazan Formation comprises eolian,mudstones suggests derivation from Christopher Island playa and braided stream deposits (Donaldson, 1965,Formation volcanic rocks, consistent with syn-volcanic 1967; Rainbird et al., 1999). The Christopher Island For-sedimentation and probable lateral interstratification of mation locally comprises volcanic flows, pyroclastic andthe Angikuni and Christopher Island Formations (Aspler volcaniclastic deposits. On Christopher Island and sur-et al., 2004). rounding islands (not shown on Fig. 5), volcanism was
    • T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70 51 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- outlined in Table 1. In general, FA 1 corresponds to theclast sag structures, normal and reverse grading, and South Channel Formation, FA 2 and 3 correspond to thecross-stratification within extensive volcaniclastic depos- Kazan and Kunwak Formations, and FA 3 to 7its (Rainbird et al., 1999). These structures indicate correspond to the Kazan Formation.deposition, in part, by turbulent pyroclastic surges (cf.Fisher and Schmincke, 1984; Cas and Wright, 1987). 2.1. Facies association 1: alluvial fan2. Lithofacies associations 2.1.1. Lithofacies description Clast-supported disorganized conglomerate (Gcd) From the principal study areas at Thirty Mile Lake contains cobble- to boulder-grade angular to subroundedand eastern Baker Lake, and other select locations within clasts within 1–5 m thick tabular beds with erosionalBaker Lake sub-basin (Fig. 2), the sedimentary rocks of basal contacts. Diffuse horizontal stratification gradesthe Baker Lake Group are here subdivided into facies laterally into a more massive framework, which is intactassociations (FA) more detailed than those presented in to condensed with slight to no imbrication (Fig. 6d). Theprevious formational descriptions. Individual facies are matrix is typically moderately to very poorly sorted, fine
    • 52 T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70 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 both Trough cross-stratified conglomerate facies (Gt) ishorizontal stratification and small-scale (less than 5 cm predominantly pebble-grade with a condensed frame-thick) cross-stratification adjacent to cobble- to boulder- work and consists of lenticular units up to 2 m thick andgrade clasts. Randomly distributed within sub-tabular 10 m wide that fine upward and laterally. The lowerconglomerate beds, mound-shaped accumulations of surfaces of these beds are erosional.granules and coarse sand overlie certain framework Trough cross-stratified sandstone facies (St) consists ofclasts (Fig. 6c). Rare examples of reverse grading in the fine- to pebbly cross-stratified sandstone in sets typicallymatrix can be seen in some of these beds. ranging in thickness from 5 cm to 20 cm. Facies St occurs Clast-supported organized conglomerate facies (Gco) at the top of lenticular conglomerate units or as lenticularcontains pebble- to cobble-grade, sub-angular to sub- units overlying conglomerate sheets. It may be overlainrounded clasts, within an intact to condensed, imbricated by parallel-stratified mudstone facies (Fl), consisting offramework. The matrix is moderately well-sorted medium laminated mudstone and minor siltstone or fine sandstone,to coarse sandstone. Tabular beds, 0.5–2 m thick, gene- with rare mud curls. These layers are overlain by erosionalrally fine upward, and may form composite conglomerate surfaces that are laterally continuous for more than 100 m.sheets. A typical occurrence would consist of multiplebeds consisting of 30 cm of cobble to 20 cm of pebble 2.1.2. Lithofacies interpretationconglomerate comprising a composite thickness of 2– The clast-supported framework, absence of inverse3 m. Other occurrences include horizontally stratified grading and weak stratification of the disorganized cong-(Fig. 6b) and less common cross-stratified tabular beds. lomerate facies (Gcd) suggests a streamflow origin asHorizontal stratification marks the boundaries of rare, opposed to deposition by a debris flow (e.g. Sohn et al.,thin, lenticular beds of trough cross-stratified sandstone. 1999; Blair, 2000a). A similar facies has been described by
    • Table 1LithofaciesLithofacies Description InterpretationGed: framework-supported, Cobble- to boulder-grade clasts; coarse to fine sandstone matrix; poorly to very Gravel sheets emplaced by high disorganized conglomerate poorly sorted; crude and irregular stratification; tabular geometry; magnitude flood flows. T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70 erosional baseGeo: framework-supported, Pebble- to cobble-grade clasts; granule to medium sandstone matrix; moderately Gravel sheets emplaced by bedload organized conglomerate sorted; organized framework; erosional base; wedge-shaped processes during flood events. and tabular units; predominantly horizontally stratifiedGem: framework-supported, Pebble- to cobble-grade clasts; coarse to medium sandstone matrix; Gravel bars in high-energy braided massive conglomerate moderate to well sorted; imbricated; intact framework; tabular geometry streams.Gt: trough cross-stratified Pebble- to cobble-grade clasts; granule to medium sand grade matrix; fine upward; Filling of channels, scours and channel conglomerate cross-stratified; lenticular units with erotional base pools by gravel during flood flows.Sh: horizontally stratified Fine- to medium-grained sandstone; well sorted; planar-horizontal Planar bed flow (upper and lower flow regime). sandstone lamination; ± primary current lineation on bedding planesSt: trough cross-stratified Medium to coarse-grained sandstone; trough cross-stratification Migration of three-dimensional dunes. sandstoneSte: trough cross-stratified Fine- to medium-grained sandstone; inversely graded foresets; pinstripe Migration of eolian three-dimensional dunes. sandstone with pinstripe lamination; up to 2 m thick laminationSr: sandstone with Laminated sandstone with predominantly asymmetric ripples; mudstone Bedload deposition/migration of asymmetric ripples drape; linguoid bedforms common current ripples.Sw: sandstone with Laminated sandstone; symmetrical ripples; ± mud drapes; bifurcating crests; Wave-formed ripples, lacustrine symmetrical ripples sheet-like geometry shoreface or delta mouth bar.SFw: wavy bedded sandstone and Inter-stratified sandstone and siltstone/mudstone; asymmetric and/or Overbank, abandoned channel or siltstone/mudstone symmetric ripples waning flood deposits.Fl: parallel-stratified Mudstone, siltstone, with parallel-laminated and/or cross-stratified Prolonged periods of quiet-water mudstone sandstone suspension deposition.Modified from Miall (1977) and Jo et al. (1997). 53
    • 54 T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70Fig. 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, The organized conglomerate facies (Gco) is generallyweakly developed clast imbrication, crude stratification and interpreted as gravel sheets or longitudinal gravel bars (e.g.lack of inverse grading were considered to be streamflow Reid and Frostick, 1987; Todd, 1989). Weak internalcharacteristics. The tabular morphology of similar facies stratification within these deposits probably representshas been attributed to deposition by bedload sheets or low- waxing and waning of individual flood flows. Well-stra-relief longitudinal bars (e.g. Reid and Frostick, 1987; Todd, tified units are similar to a facies of alternating coarse–fine1989). Because these conclusions are consistent with our conglomerate “couplets” described by Blair (2000b) fromobservations, this facies (Gcd) is considered to have been the Hells Gate alluvial fan in Death Valley. These “coup-rapidly deposited (intact framework, poor sorting and non- lets” were interpreted to have been deposited under upper-imbrication) by unconfined high-magnitude stream flood flow-regime conditions during the washout stage of theflows. Sandstone cross-laminae adjacent to clasts indicate standing-wave cycle based on similarities with documentedthat deposition of the sand occurred within an intact gravel features of supercritical sheetflood events (Blair andframework by sediment-laden currents. Where coarse to McPherson, 1994). Blair (2000b) surmised that the “auto-granular sand mounds occur atop clasts, the sediment is cyclic growth and destruction of standing waves during ainterpreted to have been transported downward through a single sheetflood produces 50–250 cm thick sequences ofgravel framework to rest upon upper clast surfaces. Rare multiple couplets”. These features are identical to some ofinverse grading of the matrix is considered to be formed by our observations, and thus we consider that for the well-a process of sieving, or mechanical sorting through the stratified conglomerates, deposition occurred by bedloadframework (Hooke, 1967). processes during high-magnitude unconfined stream flow
    • T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70 55conditions, possibly due to the washout of standing waves. higher order of genetic significance. The upward decreaseThe development of bedforms differentiates this facies in clast size may be a function of upstream aggradation offrom the clast-supported disorganized conglomerate facies the alluvial fan, widening of the active lobe or a decrease in(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 laminated mudstone(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, we must 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 Blairs (1999) theory forstreamflow predominance to be a function of derivationfrom weathered crystalline rock in the source region.2.1.3. Facies successions Two typical bedset end members consist of: (1) 1–3 mof 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 inferred Fig. 7. Stratigraphic section from Thirty Mile Lake study area displayingpaleoflow. Whereas bedsets represent sheetflood deposi- upward-fining interval in the alluvial fan facies association. Lithofaciestion and subsequent reworking, coupled bedsets have a abbreviations from Miall (1977) and Jo et al. (1997), see Table 1.
    • 56 T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70
    • T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70 57gradient as the toe migrates forward. The inactive lobe is to granular sandstone matrix. Clasts form a condensed,marked by low-energy streamflow and suspension deposi- imbricated framework that is moderately to well-sorted.tion indicated by stratified sandstone and laminated Tabular beds range in thickness from 10 cm to severalmudstone. metres. Sheets of massive conglomerate are continuous Blair (2000b) has described similar stratigraphic units for tens of metres but are discontinuous over hundreds offrom Death Valley alluvial fans and noted that generally 2 metres perpendicular to the inferred paleoflow direction.to 8, 50–250 cm sheetflood deposits capped by gully-fill Rare lenticular beds of trough cross-stratified con-or eolian facies were bound by “progressive tectonic un- glomerate (Gt), 30 cm to 50 cm thick incise into theconformities”. These are on the scale of our 5–10 m paired massive conglomerate, but are less prominent than in thebedsets; however, Blair (2000b) was able to observe a alluvial fan facies association. Angular mudstone clastsbedding discordance over the intrafan unconformity, con- are common.cluding that faulting had caused a down-drop of the fan. Medium- to thick-bedded, trough cross-stratified sand-Therefore, this type of stratigraphic unit may be consi- stone beds occur as solitary sets or compound sets up to adered to represent a fault-generated increment of accom- metre thick, above thick beds of massive conglomerate.modation, where the succession records the characteristic Rare laminated mudstone (Fl) occurs at the tops of 10 malluvial response: aggradation of the fan surface. thick, upward-fining packages. Alternatively, Mack and Leeder (1999) have de-scribed 3–10 m thick “alluvial fan cyclothems”. These 2.2.2. Lithofacies interpretationwere considered to form primarily due to the combined The gravel-bed braided stream FA is distinguishedeffects of vegetative cover and precipitation (minimum from the alluvial fan facies assemblage by a moresediment yield would correspond to peak precipitation homogeneous, better sorted, imbricated and more con-due to the binding of sediment by vegetation, and vice densed framework conglomerate. Clast imbrication inversa). This model obviously would not apply to alluvial facies Gcm implies bedload transport. The condensedfan deposits from the Baker Lake Basin, because of an framework and better sorting indicate a more sustainedabsence of vegetative cover in the Paleoproterozoic. streamflow and less rapid aggradation than inferred for Commonality suggests that this nested upward-fining conglomeratic facies from the alluvial fan facies assem-stratal pattern is intrinsic to the alluvial fan depositional blage. Massive texture makes it difficult to differentiateenvironment in fault-bounded basins from the Precam- gravel-sheet from longitudinal gravel bar deposits, abrian through the Phanerozoic. Since alluvial fans aggrade common characteristic of gravel-bed braided streamvia lobe accretion and abandonment, this punctuated deposits (Miall, 1977). The lateral discontinuity ofprocess superimposed on a gradual fault-induced subsi- lithofacies perpendicular to the inferred paleocurrentdence, though unrealistic, would result in the observed direction indicates that deposition occurred in channelssuccession. Thus, these units do not necessarily indicate smaller than a few hundreds of metres in width.specific fault motions but that subsidence was sufficient to Cross-stratified, channel-fill conglomerate (Gt) prob-provide the grade required for alluvial fan formation. ably represents reworking of abandoned-channel gravel sheets prior to deposition of sandstone. Mudstone rip-2.2. Facies association 2: gravel-bed braided stream up clasts within conglomerate sheets attest to intermittent suspension deposition, although the deposits were subse-2.2.1. Lithofacies description quently eroded and transported, between flood events. Clast-supported massive conglomerate facies (Gcm) Trough-cross stratified sandstone was deposited inis pebble- to cobble-grade with rare boulders in a coarse abandoned channels. The laminated mudstone (Fl) andFig. 8. Sedimentological features from lithofacies associations (FA) 3–7. Lens cap is 5 cm. (a) Linguoid ripples above primary current lineation from the sand-bed braided stream FA; knife is 10 cm long. (b) Erosional surface marked by granule lag truncating medium-grained, cross-stratified sandstone and overlain byinversely graded sandstone laminae, of the sand-bed braided stream FA. The overlying laminae, interpreted as eolian, indicate that this is a deflation lag formedby winnowing of fluvial deposits. (c) Inversely graded lower foresets of a 1.5 m thick eolian cross-set, interpreted as sub-critically climbing wind ripplelamination. Faint cross-laminae are visible within these foresets. (d) Floodplain FA, in which an inclined planar-laminated sandstone (Sh) interval lies betweenunits of horizontally laminated mudstone–siltstone–sandstone (Fl). Some inclined sets are interlaminated with mudstone and form mudstone dishes indicativeof subaerial exposure (inset). To the right side of the photograph, on an oblique exposure, arrows highlight inclined surfaces. This is interpreted as a crevassesplay that prograded onto a mud-rich floodplain, was abandoned and subsequently overlain Fl facies. Rock hammer is 75 cm long. (e) Playa FA, showingalternating cross-stratified sandstone grading upward into laminated mudstone with abundant desiccation cracks. (f) Wave-ripple lamination from a rippledsandstone sheet from the lacustrine FA, interpreted as a mouth bar deposit. (g) Erosional surface and lag truncating cross-stratified sandstone overlain bymudstone. This is interpreted as a wave-ravinement surface overlying an interdistributary channel in a deltaic environment. (h) Normally graded, upward-thinning and upward-fining laminae, containing mudstone clasts: interpreted as delta-front turbidite deposits. Rapidograph pen is 1 cm wide.
    • 58 T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70trough cross-stratified sandstone that sharply overly that displays prominent primary current lineation (Fig.conglomerate probably represent low energy deposition 8a). Mudstone drapes are ubiquitous above lenticularfollowing avulsion and channel abandonment (Miall, ripple bedforms.1977), similar to gravel-bed abandoned channel depositsfrom the Waimakariri River in New Zealand (Reinfelds 2.3.2. Lithofacies Interpretationand Nanson, 1993). Alternatively, where non-erosional Thin conglomerate beds at the base of upward-finingcontacts exist between successive beds of conglomerate, bedsets indicate that coarse sediment load was depositedsandstone and mudstone, the upward-fining pattern may at the base of braided channel-fill, perhaps as braid bars.reflect deposition from waning flood (Miall, 1977). Trough cross-stratified sandstone is a result of three- dimensional dune migration, particularly evident where2.2.3. Facies successions dune bedforms are exposed. Horizontally stratified sand- Upward-fining packages of the gravel-bed braided stone with primary current lineation is interpreted tostream facies assemblage are considered to be deposits of have been deposited under upper-flow-regime condi-superimposed bars (Miall, 1977). Bedsets are arranged in tions (Allen, 1964; Southard and Boguchwal, 1973). The5 to 10 m aggradational to mildly upward-fining suc- tabular geometry, lack of lateral accretion surfaces,cessions of massive conglomerate (Gcm), capped by predominance of trough cross-stratification, occurrencesandstone (St) or cross-stratified pebble conglomerate of upper-flow-regime plane beds are consistent with(Gt). Such packages are considered to represent vertical deposition by shallow sand-bed braided streams (cf.aggradation followed by channel belt switching (Miall, Miall, 1977).1977), represented by sandstone deposition in abandoned Rippled sandstone (Sr) capped by mudstone at the topchannels. of upward-fining bedsets is inferred to represent waning of flood flow followed by suspension deposition. Where2.3. Facies association 3: sand-bed braided stream rippled sandstone and upper-flow-regime plane beds are the dominant lithology in fine-grained sandstone succes-2.3.1. Lithofacies description sions, this represents sheetflood deposits for which the Massive, clast-supported conglomerate (Gcm) is a grain size was too small to form dunes (cf. Southard andminor component of the sand-bed braided stream FA, Boguchwal, 1990), where linguoid ripples transformoccurring as thin beds at the base of upward-fining bed- directly into upper-flow-regime plane beds with increas-sets. Sets of fine-grained to pebbly, trough cross-stratified ing stream velocity (Baas, 1994). Mudstone recordssandstone (St) vary in thickness from 10 cm to 1 m, waning flood or abandoned-channel deposition.bedforms of three-dimensional dunes occur on certain Cross-stratified sandstone with inversely graded fore-outcrops. These bedsets commonly have a pebble lag and sets record wind ripple migration during eolian reworkingabundant mudstone clasts at the base. Horizontally strati- of abandoned channel deposits (cf. Hunter, 1977).fied sandstone facies (Sh) consists of planar, horizontallylaminated, well-sorted, fine- to medium-grained sand- 2.3.3. Facies successionsstone that commonly displays primary current lineation. Upward-fining bedsets, 0.5–5 m thick, typicallyBedding geometry is predominantly tabular, and large- consist of an erosive base with pebble lag, overlain byscale lateral accretion surfaces appear to be absent. predominantly trough cross-stratified sandstone that Fine- to medium-grained ripple cross-stratified sand- passes gradationally upward into horizontally stratifiedstone (Sr) infrequently occurs at the top of upward- sandstone, current-rippled sandstone and laminatedfining bedsets dominated by medium to thick sets of mudstone (Fig. 9). However, there is a spectrum oftrough cross-stratified sandstone (St). heterolithic to sandstone-dominated deposits. The most Laminated mudstone (Fl) occurs at the top of upward- proximal deposits contain conglomerate at the base offining successions. Trough cross-stratified sandstone (Ste) metre-scale cycles that fine upward to sandstone, depo-with inversely graded foresets and pinstripe lamination also sited by a waning flood flow that carried a load of sandoccurs at the top of upward-fining successions. These sets and gravel. Medial deposits consist of predominantlyare typically 10 to 50 cm thick, but locally are 1 m thick. In cross-stratified sandstone. Less proximal deposits consistthin sets (5–10 cm), the foreset angle can be very low, about of typically less than metre-scale upward-fining cycles of5°; thick beds commonly overlie symmetrical ripples (Sw). sandstone with abundant mud drapes, capped by lami- As a sub-association occurring over tens of metres in nated mudstone deposited by waning streamflows thatthickness, rippled sandstone (Sr) may be exclusively carried a mixed load of sand and mud. This variation isinterbedded with horizontally stratified sandstone (Sh) interpreted to reflect a spectrum of facies from shallow
    • T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70 59Fig. 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.
    • 60 T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70braided stream to mixed-load, ephemeral sheetflood (e.g. dinate trough cross-strata (St) and laminated mudstoneSønderholm and Tirsgaard, 1998). In deposits rich in (Fl). Either symmetrical or asymmetrical ripples maymudstone, desiccation cracks are common, indicating that dominate metre-scale intervals of predominantly inter-between flood events the river bed was subaerially ex- laminated sandstone and mudstone. The occurrence ofposed. Inversely graded sandstone laminae, characteristic desiccation structures is variable; an absence of desicca-of wind transport, typically occur at the top of upward- tion cracks is coincident with a preponderance of symmet-fining cycles where mudstone is absent, indicating eolian rical ripples.reworking of dry river beds (Fig. 8b). While bedsets represent flood events and deposition- 2.4.2. Lithofacies interpretationabandonment of small braided channels, multiple bed- The FA of current ripples, mudstone, and desiccationsets comprise 5–15 m thick composite, upward-fining cracks suggests periodic overbank flooding followed bysuccessions capped by prominent, laterally continuous suspension deposition and subaerial exposure within a(up to 50 m at least) mudstone or eolian sandstone (Fig. floodplain setting. Current ripples and planar laminae9). These multiple channels are inferred to comprise a lacking primary current lineation are indicative of lower-larger channel tract. The upward-fining trend indicates flow-regime deposition, and wave ripples of periodsthat within the channel tract aggradation was accompa- where water remained pooled on the floodplain afternied by a decrease in stream competency. Aggradation floods. Thin, less than 2 m thick intervals of upward-would result in a reduction of slope, channel switching fining cross-stratified sandstone represent small crevasseand abandonment, to produce upward-fining patterns in channels that traversed the generally mudstone-domi-fluvial deposits capped by fine-grained or eolian deposits nated substrate (cf. Rhee et al., 1993). Deposits of eolian(cf. Miall, 1977; Hjellbakk, 1997). These thicker up- sandstone (Ste) indicate subaerial sand dune migrationward-fining successions therefore likely represent ag- over the floodplain where flooding was insufficient togradation and abandonment of a braided channel inhibit dune formation.complex (Fig. 9). The low-angle inclined sets of parallel-laminated sandstone contain mudstone laminae, discounting an2.4. Facies association 4: floodplain eolian origin. Desiccation features indicate intermittent subaerial exposure. The low angle of inclination is2.4.1. Lithofacies description inconsistent with formation by dune migration, but too The floodplain FA is typically composed of the steep to have been deposited as upper-flow-regime planelithofacies Fl, Sr, Sh, St and Ste. Rippled sandstone (Sr) beds. The lack of a vertical progression of structures, forwith nearly ubiquitous mudstone drapes is typically inter- example from dune to ripple-scale cross-sets, suggestsstratified with 5 to 20 cm thick laminated mudstone (Fl). that this was not a fluvial channel. The horizontal laminaeSedimentary structures and bedforms include, ripple are therefore considered to have been deposited duringlamination and cross-lamination, symmetrical and asym- lower-flow-regime conditions on an inclined sand surfacemetrical ripples, and V-shaped polygonal cracks in mud- that migrated over floodplain mud. This is similar tostone. Thin intervals (generally less than 2 m) of upward- crevasse splays described from the sand-bed braidedfining, trough cross-stratified sandstone (St) occur within Niobrara River (Bristow et al., 1999), in which ∼1 mmudstone-dominated sections. Cross-sets are less than thick inclined sets of horizontal lamination and ripple50 cm thick and typically contain up to 5 cm angular lamination overlie floodplain fines. We therefore interpretmudstone clasts. Cross-stratified sandstone with inversely inclined sets of laminated sandstone as crevasse splaygraded foresets (Ste; Fig. 8c) occurs at the top of upward- deposits that emanated from the thin trough cross-fining intervals, typically as single cross-sets up to 1 m in stratified sandstone-dominated (St) crevasse channels.thickness. Bedding geometries are typically tabular- Intervals dominated by the wavy bedded facies suggesthorizontal; however, low-angle inclined cosets, cumula- prolonged periods where pools of water might have re-tively less than 1 m thick, overlying and overlain by ho- mained on the floodplain, perhaps due to a near-surfacerizontally laminated mudstone occur locally (Fig. 8d). water table. Such deposits have been described fromThese inclined strata consist of thin beds of parallel- recent braided fluvial floodplain deposits by Bristow et al.laminated, fine- to medium-grained sandstone overlain by (1999) and ephemeral streams by Martin (2000).mudstone drapes, some of which display desiccation Floodplain deposits with wave ripples and an apparentfeatures such as mudcracks and mud curls. absence of desiccation features have also been described As a subdivision within this FA, is the occurrence of from a Mesoproterozoic braided fluvial system in Eastwavy bedded sandstone and mudstone (SFw) with subor- Greenland by Sønderholm and Tirsgaard (1998).
    • T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70 61 Understanding of the relationship between gradient, pebbly sandstone indicates that interdune areas were sub-sediment grain size and stream type is based mainly on ject to streamflows since the pebbles are too large to havesystems that include sediment-binding vegetative cover. In been transported by wind, and so these fluvial depositsthe absence of vegetation, braided streams might exist at were reworked resulting in pebble layers interpreted aslower gradients, lower discharge regimes or finer sediment lags. The large-scale cross-sets are therefore considered tograin sizes. Therefore, heterolithic braided streams and be formed by the migration of eolian dunes, with wind-associated floodplains may be the pre-vegetative equiva- ripple lamination preserved on lower slipfaces, and inter-lent to meandering streams and floodplains with respect to dune areas characterized by standing water with infre-these factors. However, the predominance of braided quent streamflow influx from surrounding alluvial plains.streams, even in the finest deposits and hence lowest gra- This is characteristic of a wet condition eolian systemdients, could alternatively be due to ephemeral flash-floo- (Kocurek and Havholm, 1993).ding that resulted in episodic high-discharge streamflow. In Simpson et al. (2004) consider eolian deposits from thecontrast to recent floodplain deposits, such as along the Baker Lake Basin to consist of two general occurrences,Waimakariri River which generally contains very few pre- as thin sandsheets dominated by wind ripple laminationserved depositional structures (Reinfelds and Nanson, associated with ephemeral lacustrine and fluvial deposits,1993), floodplain deposits from the Paleoproterozoic and thicker (up to 100 m) erg deposits dominated by large-Baker Lake Basin contain a diverse array of structures scale cross-sets (up to 6 m thick). The eolian lithofaciesdue to the absence of bioturbation or root growth. Together assemblage described herein is primarily based uponwith other Precambrian deposits, such as the Mesoproter- observations from northern Christopher Island, where it isozoic braided fluvial system described by Sønderholm and represented by up to 10 m thick accumulations of largeTirsgaard (1998), they provide a perspective on floodplain scale (up to 2 m) cross-sets of sandstone associated withdeposits generally not available from the Phanerozoic. ephemeral lacustrine and fluvial deposits. The presence of interdune deposits between individual cross-sets indicates2.5. Facies association 5: eolian these were not compound dunes and therefore equivalent to the thin sandsheet subdivision of Simpson et al. (2004).2.5.1. Lithofacies description The eolian FA is typified by up to 10 m thick accumu- 2.5.3. Facies successionslations of trough cross-stratified sets, 20 cm to 2 m thick, of There are two types of bedset within the eolian faciesfine- to medium-grained, well-sorted sandstone (Ste). Basal assemblage (Fig. 10). The first is relatively simple andforesets are typically reverse-graded fine- to medium- consists of large-scale trough cross-stratified sandstonegrained sandstone (Fig. 8c), and most exhibit pinstripe (Ste) with wave-rippled sandstone bottom sets (Sw) orlamination (cf. Fryberger and Schenk, 1988). Upper fore- cross-stratified sandstone (St), considered to probablysets are wedge-shaped, tapering downward, and normally represent dune and interdune strata respectively (Kocurek,graded, locally coarse- to medium-grained sandstone. The 1981).tops of cross-sets are typically truncated by horizontal The second type of bedset is more complex. A com-surfaces. These may be associated with granule or pebble plete vertical facies succession consists of: thin (∼10–layers, or cross-stratified pebbly sandstone (St). Between 20 cm) cross-stratified pebbly sandstone (St); pebble orthese erosional surfaces and the succeeding large-scale granule lag; approximately 10–20 cm thick interstratifiedcross-set are 10–20 cm thick intervals of wave-rippled sandstone and mudstone with prominent wave ripplessandstone (Sw) and/or interlaminated mudstone (SFw). (SFw, Sw); overlain by metre-scale eolian cross-sets. The pebbly sandstone is rarely preserved, and so bounding2.5.2. Lithofacies interpretation surfaces for multiply stacked bedsets are commonly the Pinstripe lamination and reverse-graded foresets are horizontal erosional surfaces. The interpreted successioninterpreted as sub-critically climbing translatent stratifi- of depositional events is: fluvial influx of pebbly sand;cation resulting from the migration of wind-ripples over erosion to produce the lag; intermittent wave currents andsubaerial dune slipfaces (cf. Hunter, 1977). Wedge- suspension deposition; followed by metre-scale eolianshaped, normally graded foresets are interpreted as grain- dune migration. These bedsets occur as multiply stackedflow deposits. Intervals of wave rippled sandstone (Sw) sets, and so fluvial sandstone overlies eolian cross-sets,and/or interlaminated mudstone (SFw) at the base of representing streamflow flooding of the eolian dune fieldeolian cross-sets are considered to be wet-condition inter- prior to erosion.dune deposits indicating a near surface water table (Ko- Two potential processes for producing the horizontalcurek and Havholm, 1993). The minor occurrence of erosional surfaces are wind deflation and wave-induced
    • 62 T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70erosion. In the first case, deflation of the eolian dune field face of deflation may have been controlled by the groundwould be accompanied by streamflows, accounting for water table as a Stokes surface (Stokes, 1968; Frybergerfluvial sandstone overlying the eolian cross-sets. The sur- et al., 1988; Kocurek and Havholm, 1993). Subsequent Fig. 10. Stratigraphic section from northwest Christopher Island study area, showing lacustrine, eolian and playa facies assemblages.
    • T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70 63erosion following fluvial deposition may have been in- up to 5 cm deep filled with sandstone from the overlyinghibited by formation of an armoured pebble lag, as ob- bed are prominent.served in periglacial eolian deposits of Iceland (Mountneyand Russell, 2004). This would be followed by shallow 2.6.2. Lithofacies interpretation and successionsubaqueous conditions with intermittent wave currents and Thick mudstone layers indicate sustained periods ofsuspension deposition recorded by the interstratified wave suspension deposition; deep v-shaped desiccation cracksrippled sandstone and mudstone laminae. In the absence of indicate subaerial exposure; thin cross-stratified sandstonestreamflows an eolian dune field was re-established. beds overlain by mudstone indicate that bedload deposition With respect to wave-induced erosion, initial base level preceded a resumption of suspension deposition. The de-(groundwater table) would be steady or low during eolian positional environment was characterized by playa lakedune field formation. A rise in base level would be accom- expansion due to episodic flooding, leading to sustainedpanied initially by an influx of fluvial streamflows, then by suspension deposition to form a mud flat environment (5–shallow standing water as adjacent playa lakes expanded. 20 cm of laminated mudstone), followed by subaerialWave currents would rework the substrate resulting a ho- exposure representing playa lake contraction and desicca-rizontal erosion surface, pebble lag and overlying wave- tion of the mudflat.rippled sandstone. This process is analogous to a trans- The basic depositional unit of this association is angressive surface of erosion. Contraction of an adjacent upward-fining 10–40 cm cycle of sandstone to mudstone,playa lake was accompanied re-establishment of the eolian which represents playa lake expansion followed by con-dune field. traction and desiccation, likely recording climatic fluctua- It is difficult to determine whether the pebble lags record tions. Successions of these cycles are generally less thanwind deflation or transgressive erosion; by association the 5 m thick, but can reach thickness greater than 50 moverlying SFw/Sw facies are consistent with the latter, (Rainbird et al., 1999). Since this facies association ishowever a combination of processes is probable. Sweet typically intercalated with eolian and lacustrine facies, we(1999) rationalized a rising water table and wind deflation interpret it to have been deposited in a playa lake-mud flatby supposing that as lake expansion occurred sediment environment.supply from lake margins was cut off. Winds blowing off With respect to the association with eolian deposits, athe playa margin were undersaturated with respect to sand prevalence of playa over eolian environment could be dueand effective at deflating dunes. In the Baker Lake Basin, to a relatively higher water table that periodically dam-subsequent to removal of eolian sediment supply and de- pened the substrate sufficiently to inhibit eolian duneflation, shallow lacustrine inundation may have been ac- growth, or there may have been a higher proportion of finecompanied by wave erosion and additional planation. sediment. Considering the proximity of a vegetation-free Both models involve rising base level: If base level sandy braidplain, the playa environment was more likely acontrolled the erosion surface, then base level fluctuations product of a relatively higher water table.control the accommodation increment in eolian systems, At southern Christopher Island the playa facies isconsistent with existing theories for preservation of eolian dominated by mudstone with desiccation cracks and itaccumulations (Stokes, 1968; Kocurek and Havholm, reaches a maximum thickness of 50 m (Rainbird et al.,1993; Carr-Crabaugh and Kocurek, 1998; Simpson et al., 1999). This implies a significant source of mud-grade2004). sediment. Macey (1973) identified detrital phlogopite in This facies association therefore represents environ- the Kazan Formation from southern Christopher Islandments without significant fluvial sediment flux where and proposed that Christopher Island Formation volcaniceolian dunes fields were able to develop, which were and volcaniclastic deposits had supplied volcaniclasticsubject to episodes of flooding during expansion and sediment. These chocolate brown, volcaniclastic rockscontraction of an adjacent playa lake in response to base contain significant amounts of ash-sized particles (Blake,level fluctuations. 1980; Rainbird et al., 1999), which indicates the avail- ability of a large volume of fine sediment to an ephemeral2.6. Facies association 6: playa/mudflat lacustrine environment.2.6.1. Lithofacies description 2.7. Facies association 7: lacustrine delta This FA is dominated by 5–20 cm thick layers ofmudstone (Fm) interstratified with 5–20 cm thick trough 2.7.1. Lithofacies descriptioncross-stratified sandstone (St) and ripple cross-stratified The St facies occurs as 0.2–0.8 m trough cross-setssandstone (Sr; Fig. 8e). Within mudstone, V-shaped cracks that form 1–2 m thick cosets. Mudstone clasts are
    • 64 T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70common at the base of cosets. Mudstone drapes are by suspension deposition, but subject to wave currentscommon at the top of cross-stratified sets and on foresets. and occasional bedload deposition of sand. The lack ofThese units have lenticular bases and incise into under- desiccation features, as in the playa deposits (FA 6), andlying deposits, which may include facies Fl or St. brief periods of wave currents and bedload sedimenta- Laminated mudstone and siltstone facies (Fl) is inter- tion, would be consistent with deposition in a protectedstratified with starved symmetrical ripples and beds of bay. The pebble layer at the base is interpreted as a lag,sandstone generally less than 5 cm thick. Pebble lags and together with wave ripples is suggestive of a pre-occur at the base of mudstone-dominated intervals that ceding phase of wave erosion. Subsequent incisement byoverlie pebbly sandstone (Fig. 8g). Desiccation features channels that record inter-streamflow slack waterare absent. These intervals reach thicknesses up to conditions is consistent with deposition in an interdis-50 cm and are commonly incised by the 1–2 m thick St tributary bay (cf. Elliott, 1974; Fielding, 1984).units. Within the rippled sandstone sheets, the dominance of Conversely, mudstone drapes are less common in the symmetrical ripples indicates the prevalence of oscillat-symmetrical-rippled sandstone facies (Sw). Ripple types ing currents and therefore wave processes. In-phaseinclude symmetrical ripples that occur as reworked climbing ripples indicate high rates of sedimentation.cross-set tops, and climbing ripples, locally supercriti- Ubiquitous wave-ripple reworked cross-set bed-topscally climbing (Fig. 8f). Together with thin (5–10 cm) indicating that unidirectional currents were consistentlycross-stratified sandstone (St) sets with symmetrical- followed by wave currents, are suggestive of sand barsrippled tops, the rippled sandstone comprises tabular subject to shoaling waves. These features and the sheetedsheets 1–2 m in thickness. geometry are consistent with deposition at a lake margin An uncommon facies within this association is delta front mouth bar (cf. Plint and Browne, 1994;normally graded, horizontally laminated sandstone (Sh; Marshall, 2000).Fig. 8h). It is typified by upward-fining and upward- The graded horizontal lamination is identical to Boumathinning laminae comprising beds 10–15 cm thick. Basal division Td, which is characterized by fine parallel lami-laminae are coarse-sand grade; upper laminae are fine to nation and textural sorting (Bouma, 1962). Oaie (1998)very fine sand. Angular mudstone clasts of mm-scale are described an Upper Proterozoic occurrence of mudstonecommon in the thicker laminae. Very thinly laminated microclasts from the T3 subdivision (distinctly laminatedsiltstone and mudstone occur at the top of upper graded sandstone, equivalent to Td; Stow and Shanmugan, 1980),sandstone laminae; locally these thin laminae are com- and also noted features such as continuous or discontin-posed almost entirely of horizontal mudstone microclasts. uous parallel lamination due to the orientation of micro-Individual laminae can be traced laterally over a few clasts parallel to bedding planes. Ripple cross-laminatedmetres, to the extent of outcrop (and lichen) limitations, sandstone associated with graded laminae corresponds toand beds are continuous for more than 100 m. Trough Bouma division Tc. These packages of Bouma Tc–d divi-cross-bedded sandstone with up to 10 cm thick inverse to sions locally have wave-ripple reworked tops, and closelynormally graded foresets is associated with this facies, as overlie cross-sets that have wave-ripple reworked tops,well as ripple cross-laminated sandstone (Sr). indicating that they were deposited above storm wave base. These upward-thinning and upward-fining units2.7.2. Lithofacies interpretation with ripple cross-stratified sandstone are interpreted to be Upward-fining bedsets of trough cross-stratified sand- turbidites, reflecting delta front sediment gravity flowstone (St) forming sets that incise into underlying deposits processes. In the absence of tidal features, these turbiditesare considered to be channel deposits. Mudstone drapes, are the best indication of a lacustrine environment. To-including those on foresets, indicate suspension deposi- gether, the association of turbidites, distributary channels,tion within channels between streamflow events. Angular interdistributary bays and rippled sandsheets comprise amudstone clasts are interpreted as mudstone rip-ups. perennial lacustrine deltaic environment.Braided stream deposits (FA 3) are characterized as anephemeral, high-discharge fluvial system. The channels 2.7.3. Facies successionswithin this facies association record streamflows alter- There are two types of generally upward-coarseningnating with standing water suspension deposition, per- successions (Fig. 10). In the first, the base is sharp andhaps reflecting seasonal discharge within distributary erosion is typically indicated by a transgressive lagchannel in a lacustrine environment. overlain by laminated mudstone (Fl). This is succeeded by The association of facies Fl with wave ripples and thin an upward-coarsening interval of cross-stratified sand-beds of sandstone indicates an environment dominated stone with abundant mudstone clasts, which is truncated
    • T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70 65and incised by an upward-fining interval of cross- fans fed gravel-bed braided streams at their base, which,stratified sandstone. Employing the distributary/interdis- with decreasing gradient and competence, graded intotributary bay model presented by Elliott (1974), we in- sand-bed braided streams. The braided streams occurterpret this as a prograding mouth bar overlain by an throughout the basin from the inferred paleomargin (e.g.upward-fining distributary channel. Such distributary Thirty Mile Lake) to the depocentre (Christopher Island)channel deposits generally display wave-reworked tops, (Fig. 11). Paleocurrent data define two drainage patterns foras indicated by wave ripples or a lag. This is likely these braided streams: (1) near the basin margins the trendbecause an abandoned distributary channel will be a is transverse to the margin (Fig. 4), and (2) at the centre ofpositive feature, and subject to wave erosion at the delta the elongate Baker Lake sub-basin, where paleocurrent dataedge until interdistributary bay sedimentation “catches define an axial drainage system (Figs. 2 and 5). Togetherup” and buries the sand bar. with asymmetry of stratigraphic thickness of the Baker The second type of upward-coarsening succession is Lake Group from the northwest to the southeast, ∼500 msimilar to the first, with a sharp base overlain by mudstone and N 2000 m, respectively (Hadlari and Rainbird, 2000;and upward-coarsening sandstone. Turbidites may occur Rainbird and Hadlari, 2000), the drainage patterns areat the base of these upward-coarsening intervals. Sheets of consistent with deposition in a half-graben (e.g.. Leeder,wave-rippled sandstone (Sw) and the sub-facies of thin 1995), the bounding fault of which was adjacent to thecross-stratified sandstone beds with wave-rippled tops southeast margin (Fig. 2).occur at the top, instead of a distributary channel deposit. The floodplain FA is associated with the sand-bedSimilar associations of upward-coarsening succession braided stream FA, but also occurs in stratigraphic contactand thin cross-stratified sets with wave-rippled tops have with eolian, playa and lacustrine FAs. Prominent withinbeen described by Plint and Browne (1994) from a the floodplain depositional environment are indications ofPhanerozoic strike-slip basin, and interpreted to represent standing water, such as abundant wave ripples and localthe lake margin bay mouth bar of a lacustrine delta. We paucity of desiccation features. Similarly, wet-conditionsimilarly interpret this succession as a bay mouth interdune deposits characterize the eolian facies, wheresuccession capped by progradation of a mouth bar at the thin sandsheet-type eolian deposits are interstratifiedlake margin. The rippled sandsheet is overlain by eolian within most facies of the Baker Lake Group throughoutdeposits, 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 basin margin 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- Fig. 11. Schematic block diagram of half-graben and facies tracts from thetive of a proximal to distal fluvial system; proximal alluvial Baker Sequence during the interval of localized felsic minette volcanism.
    • 66 T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70the basin. Thicker eolian deposits, dominated by large- near Christopher Island (Fig. 2). This pattern indicates thatscale cross-sets are primarily associated with lacustrine, the Baker Lake sub-basin was a hydrologically closedfloodplain, and to a lesser degree braided stream FAs, are system: primary drainage was endemic rather than directedmost common near the inferred basin axis along Kazan to an adjacent basin. At the depocentre, deltas fed into aRiver (Simpson et al., 2004) and the main depocentre at lake that was surrounded by floodplains, mudflats, andChristopher Island (Rainbird et al., 1999; Simpson et al., eolian dunes with prevailing wind directed northwest and2004; Fig. 5). The playa–mudflat FA occurs primarily in southwest (relative to present geography).stratigraphic contact with the eolian FA on ChristopherIsland indicating a close spatial relationship near the main 4. Discussion: eolian deposits and paleoclimatedepocentre of the Baker Lake sub-basin (Rainbird et al.,2003). The lacustrine delta FA has a small areal extent, Sedimentology of the Baker Lake Group reveals aexposed only at Christopher Island. The stratigraphic variety of climatic indicators. Eolian deposits, which are atransition from fluvial to lacustrine (though rarely com- measure of aridity, are primarily associated with lacustrine,plete) passes through, eolian, playa and floodplain facies floodplain, and to a lesser degree braided-stream faciesassemblages, indicating that mud flats and eolian dunes (Fig. 10), and are most common near the inferred basin axisoccupied lake margins adjacent to deltas, depending on along Kazan River (Simpson et al., 2004) and the mainlake expansion or infilling, respectively. Paleocurrent data depocentre at Christopher Island (Rainbird et al., 1999; Fig.from northwestern Christopher Island (Fig. 4) indicate 11). In very thick deposits (30 to 100 m) of eolian sand-southwesterly streamflow. Therefore, northwestern Chris- stone, some cross-set bounding surfaces indicate dry inter-topher Island marked the eastern edge of the basinal dune conditions (Rainbird et al., 1999; Simpson et al.,depocentre. Deltas prograded south and west, into the lake 2004). However, thinner sandsheet-type accumulations inbasin, and were fed by braided streams that originated to association with lacustrine and floodplain facies, in parti-the northeast. The present-day basin margin ends at the cular at the inferred depocentre (Christopher Island), con-north shore of Baker Lake and it is likely that the tain a greater proportion of interdune deposits composed ofpaleobasin originally extended farther northeast, because interlaminated, wave-rippled sandstone and mudstone,these paleocurrent data indicate that sand-bed braided indicating flooding of interdune areas (cf. Kocurek andstreams extend to the present margin instead of Havholm, 1993). Almost every facies association includesconglomerate that would be expected, if the present an eolian component: eolian reworking of abandonednortheast margin coincided with the paleobasin margin. channels, eolian sandstone sheets associated with playa– Paleocurrent data from delta-top eolian cross-sets (Fig. mudflat environments, and eolian sandstone interstratified10) indicate southwesterly wind flow, assuming that dune with floodplain deposits. However, wave ripples andcrests were oriented transverse to the primary wind direc- mudstone laminae within floodplain deposits record pe-tion. This appears to be a valid assumption, because the riods where standing water was relatively common. Deltaicpaleocurrent directions are perpendicular to the trend of deposits indicate that a perennial lake existed at the mainwave ripple crests within the delta complex. Other north- depocentre. These features confirm that the water table waswestern Christopher Island eolian paleocurrent data that close to the surface, inconsistent with an arid climate.indicate northwesterly aerial transport (Fig. 5) are simi- Climatic fluctuations occur at vastly shorter time scaleslarly perpendicular to wave ripple crest trends from inter- than the ∼45 Ma span of time represented by the Bakerdune intervals, suggesting that waves were generated by a Lake Group. For example, the hyper-arid Rub Al Khalisimilar prevailing wind direction as the eolian dunes. eolian system of the Arabian Peninsula is presently theThese paleocurrents are associated with braided stream worlds largest erg; however, lacustrine and paleoground-deposits with southeast-directed paleocurrents, distinct water deposits such as travertine suggest that the climatefrom the aerial paleocurrent direction. was humid at 35–25 ka and 10–6 ka, coincident with Thus, considering the distribution of alluvial facies, precessional orbital parameters (Bray and Stokes, 2004).paleocurrent data and stratigraphic thickness (Hadlari and Furthermore, eolian systems are not restricted to hot-Rainbird, 2000; Rainbird and Hadlari, 2000), a model of climate environments; for example, the cold-climate Askjalinked facies tracts for the Baker Lake sub-basin is set region periglacial sandsheet of Iceland (Mountney andwithin an elongate half-graben basin (Fig. 11). From the Russell, 2004). These relatively recent, biologically hostilemargins, a transverse drainage system of alluvial fans to environments are analogous to those of the Baker Lakebraided streams, with floodplains and eolian dunes, fed an Basin in that they developed on a non-vegetated landscapeaxial fluvial system. Axial drainage was primarily directed with sufficient sediment supply for eolian accumulation. Innortheast and less extensively southwest to a depocentre the absence of sediment-binding vegetative cover in the
    • T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70 67Paleoproterozoic, it is possible that eolian deposits are not The floodplain facies association primarily consists ofnecessarily an indication of an arid climate, but rather a interstratified sandstone and mudstone representing alter-mobile substrate adjacent to a viable sediment source (e.g. nating bedload and suspension deposition in an overbankactive fluvial channel belts). Therefore, the Baker Sequence setting. Locally abundant wave ripples record standingdeposits broadly suggest a variably semi-arid to semi- water subsequent to flood events, suggesting a shallowhumid paleoclimate. With respect to lacustrine deposits, and fluctuating water table. Thin sandstone intervals re-related evaporite minerals and chemogenic lake beds present crevasse channels and inclined sandstone setswithin the Angikuni sub-basin, Aspler et al. (2004) has represent crevasse splays. Eolian dunes indicate subaerialsimilarly suggested a wet paleoclimate with local arid reworking of abandoned fluvial channels.intervals for the Baker Sequence. The eolian facies association includes thin sand- sheets located adjacent to floodplains, playas and deltas.5. Conclusion Cross-sets up to 2 m thick record eolian dunes bounded by wet-condition interdune intervals indicative of a near The alluvial fan FA consists of upward-fining stratal surface water table, which controlled accumulation ofunits 5–10 m thick. These indicate that the alluvial fan eolian deposits. This description is in addition to pre-developed by a succession of lobe accretion and aban- viously documented erg deposits at the Kunwak Riverdonment events. The main lobe accretion units are (Simpson et al., 2004), and lesser erg deposits at south-represented by upward-fining, tabular units of the facies eastern Christopher Island associated with playa de-Gcd and Gco, respectively, which record rapid deposi- posits (Rainbird et al., 1999; Simpson et al., 2004). Thetion of gravel sheets during high-magnitude streamflows occurrence of eolian deposits within most depositionalfollowed by incisement during secondary low-magni- environments is considered to reflect reworking oftude streamflows. Inactive lobes were characterized by abuandant sand supply on the non-vegetated Precam-sand and mud deposition analogous to overbank pro- brian landscape.cesses on alluvial plains. The predominance of stream- The playa facies association is dominated by lami-flow processes was probably due to weathering and nated mudstone with desiccation cracks and subordinateerosion of crystalline rock in the source region, as in- trough cross-stratified sandstone, representing alternat-dicated by granitoid and gneissic clast lithologies. ing suspension deposition and desiccation of a lacustrine Alluvial fans were primarily located along the south- mudflat, punctuated by bedload flood events. This facieseastern margin of the basin, and combined with regional is associated with the eolian and lacustrine facies assem-paleocurrent and stratigraphic thickness variations blages, and represents a lake margin setting where ex-indicate that the primary basin-bounding fault of the pansion and contraction due to base level fluctuationsBaker Lake sub-basin was adjacent to its present south- inhibited eolian sandsheet formation.eastern margin. The lacustrine delta facies association consists of The gravel-bed braided stream FA also preserves prodelta turbidites, rippled sandsheets that accumulatedupward-fining bedsets, 5–10 m thick, which record as bay mouth bars, distributary channel sandstone andaggradation and lateral channel-belt switching. These interdistributary bay laminated, rippled sandstone–are differentiated from the alluvial fan facies by better mudstone. The deltaic deposits of northwestern Chris-sorting and condensed framework, with imbricated clasts topher Island record progradation toward a depocentreindicating more sustained streamflow and less rapid de- to the southeast.position. Conglomerate facies are discontinuous at scales In a three-fold subdivision of the volcanic stratigraphy,over 100 m, indicative of approximate channel widths. the lower subdivision comprises felsic minette flows andThe gravel-bed FA is gradational between the alluvial fan volcaniclastics erupted at volcanic centres adjacent toand sand-bed braided stream FAs, and distributed from basin-margin alluvial fans. This was followed by volu-the basin margin through the basin axis. minous minette extrusion, in which flows and volcani- The sand-bed braided stream facies association clastic sediments spread from the volcanic centres tocomprises 5–15 m thick upward-fining, stacked bedsets blanket most of the basin. Flows are common at Thirtyinterpreted as channel complex successions. Aggrada- Mile Lake, but are rare at Christopher Island where vol-tion of mixed-load ephemeral sheetflood and shallow, caniclastics comprise most of the volcanic deposits.sand-bed braided streams was followed by upstream Areally restricted felsite domes comprise the upper part ofchannel-belt switching. The abandoned channels were the volcanic succession. Where the basin was not entirelysites of suspension deposition of overbank fines and filled or overfilled due to minette volcanism, gravel-bedeolian reworking. braided streams transported felsite clasts basinward.
    • 68 T. Hadlari et al. / Sedimentary Geology 190 (2006) 47–70 The sedimentology of the Baker Lake Group indicates a Bray, H.E., Stokes, S., 2004. Temporal patterns of arid-humidtransverse drainage system of alluvial fans to braided transitions in the south-eastern Arabian Peninsula based on optical dating. Geomorphology 59, 271–280.streams, with floodplains and eolian dunes adjacent to Bristow, C.S., Skelly, R.L., Ethridge, F.G., 1999. Crevasse splays frominactive channels. This transverse system fed an axial the rapidly aggrading, sand-bed, braided Niobrara River,drainage system that primarily was directed northeast and Nebraska: effect of base-level rise. Sedimentology 46, 1029–1047.less extensively southwest to a depocentre near Christo- Carr-Crabaugh, M., Kocurek, G., 1998. Continental sequence stratigra- phy of a wet eolian system: a key to relative sea level change. Relativepher Island, defining the pattern of a hydrologically closed role of eustasy, climate, and tectonism in continental rocks. SEPMbasin. At this depocentre, deltas fed into a lake that was Special Publication 59, 213–228.surrounded by floodplains, mudflats and eolian dunes Cas, R.A.F., Wright, J.V., 1987. Volcanic Successions: Modern anddeposited when the prevailing wind was directed north- Ancient. Allen & Unwin, Boston. 528 pp.west and southwest. Sedimentological features such as Cousens, B.L., Aspler, L.B., Chiarenzelli, J.R., Donaldson, J.A.,ephemeral, flash flood-type alluvial deposits, playas, and Sandeman, H., Peterson, T.D., LeCheminant, A.N., 2001. Enriched Archean lithospheric mantle beneath Western Churchilleolian sandsheets and ergs, indicate a level of aridity Province tapped during Paleoproterozoic orogenesis. Geology 29,moderated by wet-condition eolian inter-dune deposits and 827–830.floodplain deposits that indicative of a near surface water Donaldson, J.A., 1965. The Dubawnt Group, District of Keewatin andtable, and thus a semi-arid to semi-humid paleoclimate. Mackenzie. Geological Survey of Canada, Paper 64-20. 11 pp. Donaldson, J.A., 1967. Study of the Dubawnt Group, Report of Activities, Pt. A. Geological Survey of Canada, Paper 67-1A. 25 pp.Acknowledgements Elliott, T., 1974. Interdistributary bay sequences and their genesis. Sedimentology 21, 611–622. Extensive logistical support from the Geological Eriksson, K.A., Simpson, E.L., 1998. Controls on spatial and temporalSurvey of Canada (Natural Resources Canada) is grate- distribution of Precambrian eolianites. Sedimentary Geology 120,fully acknowledged, accordingly this is GSC contribu- 275–294. Fielding, C.R., 1984. Upper delta plain lacustrine and fluviolacustrinetion #2005407. 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