Architecture and Origin of Fluvial Facies in the Athabasca Group at McClean Lake, Northern Saskatchewan

Darrel Long 1, Cathy Williamson 1, Phillipe Porte/la 3, and Steve Wilson 3

Lon~, D., Williamson. C.,. Portclla, P., and "."ilson. S. (2000): Architecture and origin of lluvia l fac ics in the l\thabasca ( iroup al ~ cClcan Lake. northern Saskatchewan; m Summary of lnvest1gat10ns 2000. Volume 2. Saskatchewan Geological Survey, Sask. Energy Mines. Misc. Rep. 2000-4.2 ·

Abstract

Initial investigations into the character of basal strata of the Athabasca Group in COGEMA Resources Inc. Sue Pit. near the eastern margin of the Athabasca Basin indicate the presence of two distinct form ations deposited under different climatic conditions. Lowermost strata consist predominant(v of fine- l o very coarse-grained sandstones with minor discontinuous cobble and boulder conglomerates and rare mudstones, and rest on a high(v irregular unconformity developed on metamorphosed Hudsonian basement. Well-rounded clasts in the coarser conglomerates are all of local origin. These conglomerates and associated abundant massive and plane laminated sandstones in the basal member appear to have been deposited by sheet-flood and stream-flood processes in an arid to hyper-arid climaric setting. Overly ing strata are predominantly coarse- to very coarse-grained sandstones with abundant cross-stratification and minor granules and small pehb/e conglomerates. These appear to have been deposited on a braidplain f ormed by semi-ephemeral or perennial braided rivers under more humid conditions. 1'he co/1/act between the two members is a stepped erosional surface, which is well represented in the core and in exposures within the pit. This surface represents development of an incised river system associated wirh the inferred climatic change.

l. Introduction

This sub-project of the Atha basca Uranium EXTEC H IV project a ims to complement a regional stratigraphic analysis (Ramaekers, 1990; Yeo et al., this volume) by providing a link between drill core and geophys ical stud ies and open pit/outc rop characteristics of the Athabasca Group.

In order to improve unde rstanding of the stratigraphic framework and architecture of the group, it is essential to document its lithology. sedime ntology, and deta iled facies relationships. The best places to make such observations arc at those deposits that have been, or are being, mined as open pits (e.g. Rabbit, McClean, Key, and Cluff lakes). Areas o f relatively good exposure

(e .g. southeast of Black Lake. C arswell Structure) will also be examined.

2. Three Year Plan

Work will be continued in cooperation with industry. Saskatchewan Energy and Mines. and the Geological Survey of Canada (Yeo et al. , this volume). Work in 2000-01 consisted offacics architecture analysis of the Sue Pit at Mcclean Lake and compilation and synthes is o f relevant DDH core descriptions, and geophysica l information. Investigations at Cluff Lake will coinc ide with Yeo' s work there in 2001-02 . B.Sc. and M.Sc. students will be involved in these projects.

3. New Observations

In order to understand the architecture of the basal strata at the McC lean Lake Sue C Pit. architectural models were established on three accessible faces at the south end of the pit (adjacent to ramps or platforms). O bservations were plo tted directly on a series of overlapping instant photographs. Due to the insta bility of the pit walls, direct observations were limited to the lowe r two metres of exposure. In addition to these observations, twelve cores from closely spaced (10 to 20 m) bore holes were examined in order to fully document lithotypes within the pit and to allow direct comparison with temporary surface expo sures in the pit.

Highly de formed. metamorphosed . and diagene tically alte red basement rocks include coarse recrystallized "quartzite", metasediments including graphitic varieties, and pegmatitic intrusions. The .. quartz ites" may not have had a sedim entary protolith as they consist of an interlocking framework consisting of 70 to 90%, 4 to 8 mm crystals of po lygonized and s trained quartz, and 10 to 30% muscovite or kao lin. They may represent a ·'quartz ite"' or metamorphosed quartz-rich pcgmatite . All the basement rocks show sil!ns o r inte nse diagcnet ic and possibly hydrothcrri:.al a lte ration (c.f. Kyse r. 2000). C lay-filled fractures in the upper few metres of the formati on may re flect development of deep weathering profiles (Fig ure I. left ).

' Department o f Earth Scit:nces, l.aurcntian lJnivcrsity. 935 Ramsey l.ake Road . Sudbury. ON PJ E 2C6: E-mail : dlong,dn k kcl .laun:ntianca ' Dcpartmcnt of Geology. lJnivcr~ity of Regina. R~gina, SK S4S OA2. ' COGEMA Resources Inc .. 8 17 - 825 45th Street West. Sa~katoon. SK S7K J X5.

Summm~v of fni·esl igations 1000. I 'olume 2

Figure I - Leji: Brecciated "(JUart ,ite", with clay-filled seam~. rejlecti11g deep-weathering of the basement. Center· Detail of com act between large brecciated clast of "quartzite" (lower half) in 11 loca/ basal conglomerate anti maHive to plunar-stratijietl small pebble conglomerate am/ 111edi11m-grui11ed .mmlsto11e in the lower member. Night. Detail of thi11 co.~ets of luminMed medium- to courw­grained sandsto11e of possible sheet-flood origin. Note millor co11ce11tration of course material between co.set.f.

The contact between the basal strata (Manitou Falls Formation of Ramaekers, 1990) ofthc Athabasca Group and the basement rocks is an irregular unconformi ty with local slopes of 30° to 45° (Figure 2). Boulder and cobble conglomerates are present locally at the base of the formation and at higher levels within the lower member in proximity to basement highs (Figure 3). Most have very limited lateral continuity, and thin and tine away from basement highs. ( lasts are predominantly "quartzite'' of apparent local origin and are typically well rounded. although some show evidence of in situ brecciation (Figure I , center). Matrix within the conglomerates is of medium to coarse sand grade. and locally sho ws evidence of draped stratification, with rare mudstone laminae.

Strata in the basal member are dominated by pink, moderately well-sorted to poorly sorted , medium- to coarse-grained sandstones, with minor granule and small pebble conglomerates. Most beds appear massive or weakly laminated (Figure I , right; Figure 4) in compo5 itc sets 2 to 4 cm thick, wf1ich may be graded. Planar and trough cross-stratification is rare, as is ripple cross laminat ion. Beds appear to be laterally continuous for more than 20 m. Isolated. well-rounded "quartz ite" clasts, of large pebble and cobble grade

Saskarchewan Geolo!(ical Survey

commonly rest on erosive surfaces at I to 3 m intervals.

The upper member of the sequence exposed in the Sue Pit (Manitou Falls Formation of Ramackers, 1990) rests with an erosional discontinuity on strata within the lower member (Figure 2). Strata are predominantly medium to very coarse sandstones with minor granule

Figure 2 - Basu/ colltact (A) of predominantly flat laminated, medium- to coarse-grai11ed .mndstones of the basal unit of the Athaba.sca Group in the southwest wall <if the Sue Pit, McC/ean Lake. Note that strata above the unco11/ormity are truncated by the ba.~e of the 11pper unit ((If 8) which is ,!ominated by coarse and very coarse cross·­stmtijied .rnndstone. Note local absence of conglomerates at the base of this sequence.

Figure 3 - Annotated fie/ti photograph showing a 1 m wide, 35 cm thick le11s of cobble co11glomerate. This appears to have developed 011 and adjacent to the basement high duri11g depositio,r of the lower member. Note inclined fractures in the "quartzite" that give the rock the appearance of a betided quartzite.

Figure 4 - Thin co:~ets of thick plane-lami11ated, medium­grained santl.~tone in surface exposure of the lower member in the Sue Pit.

14 1

conglomerates and granular very coarse-grained sandstones. Coarser grained conglomerates were not observed in the pit or in cores. Most of the sandstone appears to be massive or is characterized by planar or trough cross-stratification (Figures 5 and 6). Lateral continuity of beds is high (> I 00 m ). No consistent cyclic fining, thinning, or coarsening upwards sequences were observed (Figure 7).

4. Sedimentology

Strata exposed in the Sue Pit are best explained by deposition in ephemeral to semi-permanent braidplain systems in hyper arid to semi-humid conditions. Specific interpretations are outlined below.

a) Basement and Lower Member

The presence of clay-filled fractures and enhanced alteration of deformed Hudsonian basement rocks beneath strata of the Athabasca Group suggest intense weathering during development of a sub-mature topographic surface. The probable local origin ofwe ll­rounded, and in part fractured '' quartzite" clasts in the conglomerates suggests minimal transport distances from basement highs ( inselbergs), with rounding caused by combined hydration and thermal spalling in an arid to hyper-arid climate. Minor stratified matri x in conglomerates at the base of the sequence suggests they were deposited as residual open-work gravel, with

Figure .'i - Planar and trough cross-stratified coarse grained sa11dstone in the upper member of the sequence, adjacent to the ramp at the southeast end of Sue Pit. Field of view i.f approximately 10 cm.

Figure 6 - Plt,nar anti trough cross-stmtijied sand5to11e.5 in the upper member. Note discontinuous hematite staining along strike. Scale divfrions on .fta// are IO cm.

142

voids fi lled by later depositional events (i.e. sieve deposits: Bull, 1972).

The predominance of fla t laminated and massive sandstone in the lower member is suggestive of deposition by superimposed ephemeral sheet-flood and sheet-wash processes in the terminal parts of an arid to hyper-arid braidplain. (c. f. Williams, 1971; Frostick and Reid, 1977; Stear, 1985; Abdullatif. 1989; Tooth, 1999).

Thin sets of flat-laminated sandstones associated with planar and trough cross-stratified sets arc common in modern and ancient fluvial deposits where they are typically interpreted as the rarely preserved products of deposition under upper flow regime conditions (Williams, 1971 ; Frostick and Reid. 1977). The presence of extensive stacked sets of apparently flat laminated sandstones, in the absence of interbedded cross-stratified sands is not typical of modern river deposits and may represent a distinctive pre-vegetat ive fluvial style produced by sheet-floods or stream-floods (c.f. Eriksson et al., 1993; Bhattacharyya and Morad, 1993; Hjellbakk, 1993, 1997; S0nderholm and Tirsgaard, 1998; T irsgaard and 0xnevad, 1998). The presence of rare parting lineations means that this facies cou ld easily be confused with beach-face deposits. The absence of water-escape structures in the associated massive sets, combined with the lack of high-rel ief scour on basal contacts indicates that these were not produced by hyper-concentrated flow mechanisms as has been suggested for massive type sheet sandstones in Phanerozoic braided stream deposits by Martin and Turner ( 1998) and Proterozoic stream deposits by R0e and Hermansen ( 1993).

Thin cosets of massive to laminated strata (Figure 4) are analogous to deposits of Bijou Creek-type rivers (M iall, 1978). The type example of this highly ephemera l river type is characterized by fl ashy d ischarge and, due to source area characteristics. a lack of gravel. Plane lam ination may develop under upper flow regime (sheet-wash) conditions, with a rare cap of mudstone (often removed by aeolian activity) reflecting wan ing flow conditions. Coarser grained laminae between cosets may reflect development of aeol ian deflation lags, a lthough direct evidence of aeolian deposition, such as adhesion ripples. warts, and coarsening upwards laminae (Ross, 1983; Tirsgaard and 0xnevad. 1998) were not seen.

The presence of iso lated large pebbles and cobbles on scour surfaces at l to 3 m intervals th roughout the lower member suggests occasional high energy sheet­flood events followed by periods of fl uvial or aeolian deflation.

Th in intervals with medium-scale cross-beds and ripples suggest that some floods may have become channelized, w ith deposit ion in shallow full y turbu lent ephemeral streams under lower flow-reg ime cond itions .

Summary of lnvesl iga1ions 2()()(), l'olume J

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Figure 7 - Log of typical core from the Sue Pit, showing nature of contacts (First colum11: .w lid line, sharp co11tact; dash, tra11sitim"'I). medi<m grain size (second colum11: deviatio11s to right i11tlicate coarse 1[rai11ed: scale 111 top is in phi divi.5iom for .rnllfl am/ gravel gr11tle material. lsolutet/ ban to right of column imlicate ttUL'l:imum g rui11 size). Nex t five columns indicate metliu11 bed thiclwe.H ill ce11timelres (fJ to 0.2 cm.. thi11 lamillMed; 0.2 lo I cm, thick laminated; I to 5 cm. thin betided; 5 to 60 cm., medium betided; >60 cm, th ick betided). Letters in nex t column i11dic11te colour (Lt, light; C:v, grey ; W, white; Y, yellow; Br, brown; P, pi11k; and R, red). Next eight colum11.v indicate uppurent setlime11t11ry .vtructures ( / , manive or i11di.,·ti11ct; 2, plane beddi11g; 3, wmy ripple lumi11atio11 ; ,t, ripple cross-stratification; 5, trough cross-.vtratijicatio11; 6, pl,mar cross-betiding; 7, sedimentary intr11cla.fts; and 8, desiccation cracks).

Sas katchewan Geological Surve_v I.J3

b) Upper Member

Architectural analysis (in progress) indicates the presence of a stepped eros ion surface at the base of the (0 to 14 m thick) upper member. Th is surface truncates laminae sets in the lower member suggesting burial compaction of the sequence by tens to hundreds of metres of sandstones, prior to development of an incised va lley system.

Medium-scale planar and, to a lesser extent, trough cross-stratified sandstones in the upper member reflect migration of straight and sinuous crested bed forms in shallow semi-ephemeral to perennial sandy braidplain systems (Long, 1978, 2000; Crow ley, 1983 ; R01~ and Hermansen, 1993 ; M iall , 1996). The sheet-like character of the sandstones, combined with the thickness of cross-stratificat ion and apparent a bsence of sequences of bed forms and complex large-scale bed configurations {macrofonns) indicates deposition in streams rarely more than a metre deep, and probably several hundreds to thousands o f metres wide (c.f. Schumm, 1968; Long, 1978, 2000; Fuller, 1985; Martins-Ne to, 1994).

5. Conclusions

Examination of the Athabasca Group exposed in the Sue Pit indicates initial deposition of flash-flood dominated strata in an arid to hyper-arid climatic regime predom inan tly by sheet- flood p'.ocesscs. The characte ristics of modem flash - floods, mcludmg sheet­wash (broad flows, a few cent imetres thick), sheet­floods (several kilometres wide and 4 to 60 cm deep), and stream-floods ( less than 1.5 m deep and up to 4.8 km wide) are poorly understood due to lack of direct measurements (Hogg, 1982; Graf, 1988). In modern dry land river settings, the presence of sparse to dense vegetation tends to break up sheet flows to produce turbulent flows in r ills and small channels (Graf, 1988). Due to the absence of vascu lar plants, th is would not be the case in Proterozoic settings, even in semi-humid to humid env ironments, hence upper flow regime deposits from sheet-floods might be expected to be more common. In terminal fluvial systems, rap id water infi ltration should lead to increased sediment concentrations in the flows, resulting in depos it ion of mass ive beds, or beds with poorly defined lamination ( llje llbakk, 1993).

The absence of identifiable aeo lian depos its in these and other Proterozoic fluvial deposits is enigmatic (Ross, 1983; Eriksson et al. , 1998). Tirsgaard and 0xnevad ( 1998) have sug gested that thi s ma y be a function of preservation due to the pos ition of the groundwater table which wou ld have been too low in ar id and semi-arid c onditions to a llow preservat ion of aeolian deposits. They suggest that optimal condit i<;>ns for preservation occur in semi-epheme ral systems, m more humid se ttings , where a seasonally high water table is combined with preservation space created by subsidence.

1-1-1

Deposition of the lower member was followed by deposition from more substant ia l sem i-ephemera l to perennial stream s in a humid cl imatic setting. The change in depositional sty le was ma rked by significant erosion of t he lower member, wi th development o f an incised va lley system . Preservation of rel ief on this major sequence boundary suggests part ial consolidation, perhaps combined with the deve lopment of early diagenetic cements over a period of several thousa nd years.

The incised valley syste m was quickly buried beneath an extens ive blanket o f sheet -like flu v ial deposits, characterized by broad, shallow braided channe ls dominated by transverse bed form s. Absence of aeo lian bedfom1s in this se lling may be re lated to low preservat ion potential as such deposits would be subject to erosion during fl ood eve nts.

Strata along the eastern margin o f the Athabasca Basin have been corre lated with the Manitou Falls Formation by Ramaekers ( 1990). The presence of a major sequence bounda ry at the contac t of the upper and lower members in the Sue Pit may ind icate previous ly unsuspected loca l preservation of a remnant of the MFa member of the Manitou Fa lls Formation or an equivalent of the stratig raphica lly older Fair Point r: ormation.

The absence of extensive conglome rates in the upper member, combined w ith a lack of abunda nt muds tone intraclasts, suggests that these correlate with the MFb member of the Man itou Falls Formation. which Ramaekers ( 1990) shows to have overstepped the MFa member, and typically forms the oldest strata along the eastern marg in of the basin .

6. Acknowledgments

This sub-project is funded in part by an NSERC g rant to Dr. Darre l Long. Costs were shared by the Geolo0 ica l Survev of Canada. Saskatchewan Energy and Mines, Cameco Corporat ion, and COGEMA -Resources Inc. for students and log istics. Accommodation at McClean Lake was provided by COGEMA Resources Inc .

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facie s seque nces in the epheme ral t erminal river Gash, Kassala. Sudan; Sed. Geol. . v63, p 171 - 1 84.

13hattacharyya , A. and Morad, S. ( 1993) : Proterozoic braide d ephemera l flu via l deposits; an example from the Dhandraul Sandstone Format ion of the Kaimur Group, Son Valley. central India; Scd . Geol., v84, plOl - 11 4.

Bul l. W.B. ( 1972): Recognition of a lluvial-fan deposits in the stratigraphic record ; in Rigby, J.K. and Hamblin. W.K. (eds.) , Recognition of anc ient

Summary c,f" lnvestigalions 2000, 1 "o/ume 2

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Crowley, K.D. (1983): Large-scale bed configurations (macroforms), Platte River Basin, Colorado and Nebraska: Primary structures and formative processes: Geol. Soc. Amer. Bull.. v94, pl 17-133.

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Eriksson, P.G., Schreiber. U.M., van der Neut, M., Labuscha1rne, H .. van der Schvff, W .. and Potgietcr.~G. ( 1993): Alternati've marine and fluvial models for the non-fossiliferous quartzitic sandstones of the Early Proterozoic Daspoot Formation, Transvaal Sequence of southern Africa: J. African Earth Sci., vl6, p355-366.

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Graf, W.L. ( 1988): Fluvia l Processes in Dry land Rivers; Springer-Verlag, Berlin, 346p.

Hjellhakk. A . (1993 ): A ' flash-flood ' dominated braid delta in the upper Proterozoic Nre ringselva Member. Varanger Peninsula, northern Norway; Norsk. Geolgisk Tidsskrift, v73. p63-80.

( 1997): Facies and fluvial architecture of ahigh-energy braided river: The Upper

Proterozoic Seglodden Member. Varanger Penn insular. northern Norwav; Sed . Geol., v I 14. pl J l-1 6 1. ,

Hogg, S.E. ( 1982): Sheetfloods. sheetwash. sheetflow. or ?; Earth Sci. Rev .. vl8. p56-76 .

Kyser. K. (2000): Fluids and basin evolution ; Minera l. Assoc . Can., Short Course Series. No.28. 262 p.

Long, D.G.f. ( 1978): Proterozoic stream deposits: Some problems of recognition and interpretation of ancient sandy lluvial systems; in M ia ll, A.O . (ed.), Fluvial Sedimentology, Can. Soc . Petrol. Geol. , Mem. 5, p313-341.

(2000): Architecture of Proterozoic sand­--ciominated lluvial systems; GeoCanada '.WOO,

Calgary, May 2000, Conference CD, ext. abstr . #342.

Martin , C.A.L. and Turner. R.R. ( 1998): Origins of massive-type sandstones in braided river systems: Earth Sci. Rev .. v44, pl5-38.

Saskatche1ra11 (;eological Survey

Martins-Neto, M.A. (1994): Braidplain sedimentation in a Proterozoic rift basin : The Sao Joao da C hapada Formation, southeastern Braz il ; Sed. Gcol. . v89, p219-23 9.

Mia ll , J\ .D. ( 1978): Lithofacies types and vertical profile models in braided river deposits: A summary; in Miall , A.O. (ed.), Fluvial Sedimentology, Can . Soc. Petrol. Geol. , Mem. 5, p597-604.

- ·· -- -· (1996): The Geology ofFluvia l Deposits: Sedimentary Facies, Basin Analysis, and Petroleum Geology; Spring er-Verlag, Berlin. 582p.

Ramaekers. P. ( 1990): Geology of the Athabasca Group (Helikian) in Northern Saskatchewan; Sask. Energy Min es. Rep. 195, 49p.

R0c. S.L. and Hermansen, M. ( 1993): Processes and products of large, Late Precambrian sandy rivers in northern Norway; Int. Assoc. Sediment., Spec. Publ. No. 17, pl51-l66.

Ross, G.M. (1983): Proterozoic aeolian quartz arenites from the Hornby Bay Group, Northwest Territories, Canada: Implications for Precambrian aeolian processes; Precamb. Resear.. v20. p 149-160.

Schumm, S.A. ( 1968): Speculat ions concerning paleohydrologic controls of terrestrial sedimentat ion; Geol. Soc. Amer. Bull. , v79, p 1573-158 8.

Sonderholm, M. and T irsgaard, H. ( 1998): Proterozoic tluv ia l styles: Response to base-level changes and climate (Rivieradal sandstone, eastern North Greenland); Sed. Geo I. , v 120, p257-274.

Stear. W.O. (1985): Comparison of the bedform distribution and dynamics of modem and ancient sandy ephemeral flood deposits in the southwestern Karoo Region, South Africa; Sed. Gco l. , v45 , p209-230.

Tirsgaard. H. and 0xnevad, I.E.!. ( 1998): Preservation of pre-vegetat iona l mixed fluvio-aeolian deposits in a humid c limatic setting: An example from the Middle Proterozoic Rriksfjord Fonnation , southwest Greenland; Sed. Geol. , vl20, p295-3 17.

Tooth, S. ( 1999): Floodouts in central Australia; in Miller, A.J. and Gupta, A. (eds. ), Varieties of Fluvial Form , John Wiley and Sons Ltd .. Chichester, p2 19-247 .

Williams, G.E. ( 1971 ) : Flood deposits of the sand-bed ephemeral streams of central Australia: Sediment. , vi 7, pl-40.