THE EXTERNIDF.S OF WOPMAY OROGEN, TAKIJUQ LAKE AND KIKERK LAKE SS· MA P AREAS, DISTRICT OF MACKENZIE rroject 810021
1 2 1 F Hoffman, Rein Tirrul, J.P. Grotzinger , S.B. Lucas , and K.A. Eriksson ~~a m br i a n Geology Division H ffm an. P.F., Tirrul, R., Grotzinger, J.P., Lucas, S.B., and Eriksson, K.A., The externides of ~°or>rna y Orogen, Takijuq Lake and Kikerk Lake map areas, District of Mackenzie; ~ Current ~esearc h, Part A, Geological Survey of Canada, Paper 84-1 A, p. 383-395, 1984.
AJJSlr'lC t Re:11me Some r esults of the final summer's fieldwork on this project Le rapport presente certains resultats des derniers travaux re outlined as they pertain to the following topics: sur le terrain effectues dans le cadre du present projet, dans ~l) sedi men tary structures and paleoenvironment of the les domaines suivants: 1) structures sedimentaires et Odjick elastic shelf; (2) paleogeographic zonation of the paleoenvironnement de la plate-forme clastique d'Odjick; Rocknest shelf-edge reef complex; (3) nature and origin of 2) zonation paleogeographique du complexe recifal en bordure Rocknest shelf cycles; (4) correlation of the Rocknest de la plate-forme de Rocknest; 3) nature et origine des cycles dolomi te eastward across Rockinghorse Arch into Kilohigok de la plate-forme de Rocknest ; 4) correlation de la dolomie Basin and i mplications of a revised correlation of formations de Rocknest vers !'est en travers de l'arche de Rockinghorse, in the Epworth and Goulburn groups; (5) regional variation in jusque dans le bassin de Kilohigok, et repercussions d'une 10141 grade metamorphism and its relation to deformation correlation r evisee des formations dans Les groupes d'Epworth events; (6) basement-involved folding without thrusting in the et de Gouldburn; 5) variation regionale du metamorphisme Tree RiYer belt; and (7) development of "shingle" structures faible et lien avec les evenements a l'origine de (crudely hexagonal crustal blocks) by progressive conjugate deformations; 6) plissement sans chevauchement touchant au transcurren t faulting. socle rocheux dans la zone de Tree River; et, 7) evolution des structures imbriquees (blocs grossierement hexagonaux de la crolite) par la formation progressive de decrochements perpendiculaires.
INTRODUCTION deformational wedge (Tirrul, 1983; St-Onge et al., 1984). The externides of Wopmay Orogen (Fig. 52.1) include the The younger event (D3) produced a throughgoing system of foreland t hrust-fold belt and contiguous autochthonous cover conjugate brittle transcurrent faults (Fig. 52.1) indicating fringing the exposed Archean basement of Slave Craton to east-west shortening and north-south extension. the east. The area preserves the landward part of a 1.9 Ga This report, which follows the final summer of continental margin sedimentary prism, the Coronation fieldwork for this project, focuses on the following Supergroup, which is being studied to improve our observations not discussed in previous reports (Tirrul, 1982, understanding of mechanisms of formation and destruction of 1983; Grotzinger, 1982; Hoffman et al., 1983; Grotzinger and a continental margin in Precambrian time. This project is Hoffman, 1983; Grotzinger and Read, in press): closely allied with another studying the seaward part of the marginal prism, which ultimately became the rear part of a 1. Sedimentary structures in the Odjick Formation defor mat ional wedge continuous with the foreland thrust-fold (Fig. 52.2) indicate elastic deposition on a prograding, belt (St- Onge et al., 1984). storm-dominated, open marine shelf. Storm-surge ebb currents are likely responsible for paleocurrents directed In the externides, the prism consists of a lower pre uniformly offshore. An extremely rapid and extensive orogenic sequence (Odjick and Rocknest formations) built on initial transgression of the craton is indicated. the subsided margin of the craton, and an upper syn-orogenic 2. In the Rocknest Formation (Fig. 52.2), a detailed sequence (Recluse Group) deposited diachronously in a paleogeographic zonation of the outer reefal rim and foredeep that migrated eastward in front of the tectonically prograding thrust-fold belt (Fig. 52.2). The principal associated facies has been established. The discovery of deformation of the prism, the 1.89 Ga Calderian Orogeny abundant "neptunian" dykes and sills (synsedimentary fissures filled by submarine cements) in the reef and (~offm an and Bowring, in press), resulted in eastward d1rected thrusting and folding above a basal decollement vadose pisolites in a persistent back-reef shoal complex located 300 m or more above the basement, followed by low add to the list of features shared with some famous Phanerozoic amplitude basement-involved folding of the decollement reef complexes. Systematic changes in (TirruJ, 1983; St- Onge et al., 1984). correlatable cycles across the externides document repeated eastward progradation (enlargement) of the Follow ing hard on the heels of the Calderian Orogeny back-reef shoal complex at the expense of a persistent are tw o unrelated episodes of regional shortening (Hoffman inner-shelf lagoon as the immediate cause of prominent and Bowring, in press). The earlier (D2) produced sporadic Rocknest cyclicity. Incomplete shoaling of cycles in the basem ent-involved folds and related cleavages of variable east prove that shoaling of the entire lagoon was not n~rth e as t trend, strongly developed especially in the Tree required to initiate new cycles. This makes a eustatic Ri_ver Fold Belt in the northeast corner of the externides control on cyclicity more likely and its periodicity, given (F% 52. 1). Transverse D2 arches of regional scale provide the geochronological constraints (Hoffman and Bowring, critical oblique sections through the entire Calderian in press), cannot have exceeded 50 000 years (200 cycles in 10 million years) for each 1-10 m thick cycle. l ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~- 2 Department of Geological Sciences, Virginia Tech, Blacksburg, VA 24061 Department of Geological Sciences, Queen's University, Kingston, Ontario K7L 3N6 383 late transcurrent fault LEGE ND (NE-dextrol, NW-sinistral)
Muskox Intrusion and other post-orogenic gabbros
~ ~ post-orogenic cover Hepburn Intrusives
Great Bear plutonics Foredeep rnolasse (Takiyuak Fm) () IT[].. 0 ::0 Great Bear volcanics Foredeep flysch 0 (McTavish Sgp) 2 (Recluse Gp) )> -; 0 2 Hottah Terrone Passive-margin shelf (/) facies (Epworth Gp) c IJ,,, ::0 G) Passive-margin slope -rise ::0 0 focies (Epworth Gp) c IJ ij\.~ Initial rift sequences (Akaitcho Gp)
0 50 100 r--:-:1 H H H H H ~ km Archean basement
Figure 52.1. Simplified geological map of the northern two-thirds of Wopmay Orogen, showing the locations of measured sections ( A,B,C ,D,S,T) of Figures 52.3-52.5, and location of areas "Y" and "Z" discussed in the te.rt. "KB" indicates the position of the Kangaroo Block. Prominent water bodies are: CG, Coronation Gulf; GBL, Great Bear Lake; TL, Takijuq Lake; H L, H ottah Lake.
3. The eastward extension of the Rocknest Formation has, 5. :\lapping of the Tree River Fold Belt has reaffirmed the for the first time, been traced across Rockinghorse Arch existence of recumbent folds of the basal unconformity into Kilohigok Basin (Goulburn Group), where it correlates without associated thrusts marginal to the basement with a 21 m thick mixed dolomite and elastic unit within uplifts bounding the belt. Metamorphic biotite occurs in the Burnside River Formation. This revised correlation pelite around Eokuk Uplift (Fig. 52.1), perhaps the highest has important regional tectonic implications. It shows grade metamorphism in the externides, but the cause of that a major source of elastic sediment, possibly the this deep-seated thermal anomaly remains problematic. Thelon Front, was being unroofed southeast of Kilohigok Some aspects of D2 deforma tional mechanisms have been Basin concurrent with the rapid opening and closing of the clarified. Coronation margin. 6. Crudley hexagonal crustal blocks ("shingles"), bounded by 4. In an attempt to document variation in low grade D3 faults, were previously observed in north-central metamorphism in the externides, colour change in a Wopmay Orogen and interpreted as east-vergent thrusts specific shale member of the Rocknest Formation was linked by coeval transcurrent faults (Hoffman and systematically recorded, hopefully to be calibrated in the St-Onge, 1981). Discovery of a similar but smaller scale laboratory by determination of illite crystallinity and/or structure in the externides, well exposed and with vi trini te reflectance. The observed colour changes excellent stratigraphic control, led to development of an reflect vertical and lateral varia tion in metamorphic alternative model in which thrusting plays at most a grade during both Calderian and D2 deformations. In both minor role. deformations there is a clear correlation of higher metamorphic grade with development of cleavage, penetrative strain, and degree of basement involvement in folding.
384 pEPOSmONAL HISTORY OF THE ODJICK The sandstones and siltstones of the Lower Member in cLAS11C SHELF- A PRELIMINARY ANALYSIS area "Y'' coarsen and thicken upward into the Middle Member The Odjick Formation (Fig. 52.2) has been subdivided into t~roug~ a stromatoli.tic dolomite unit which is laterally three informal Lower (Eol), Middle (Eo2) and Upper (Eo3) d1sco~tmuous. The Middle Member is mainly arenaceous but contains frequent 5 to 10 cm thick mudstone drapes in the rnem bers (Hoffman et al., 1983). A general alluvial/ coastal lowermost one-third of the member. Hummocky cross rnar ine depositional model was proposed for continental shelf stratification is the predominant sedimentary structure sedi ments of the Odjick Formation which were interpreted to producing a characteristic outcrop pattern of overlapping pass laterally into submarine-fan deposits beyond a shelf fl~t-bottomed lenses. Individual hummocks average 2 m in e FOREDEEP - RECLUSE GROUP +----- Asiok Thrust-Fold Belt ----+- Autochthon minimum eastward tectonic l•00I gabbro sills (Morel Sills) translation of shelf-edge /:: ••• • ••• fluvial lithic-feldspathic arenite .:?:o•o ooo•oo•oo•:•. halokinetic megabreccia basinal limestone-argillite rhythmite calcareous concretionary argillite feld spathic-lithic wa cke turbidite laminated graphitic-sulfidic shale quartz siltite glauconitic dolomite -:·:·:·:·:-:-:·:·:·:-:·:·:·:-:-:-:·:·:·:·:·:·;·. r c c : ...... h·.·.· ...... ·.· .. ·; - · A\ii:m~'"t'•"'•'•'' dNiJ;;i@~}'.'.'..~~'~'..~L~~._.~,,;~;~n<~;;:;::~H::f::~ : :::{:~ : :: :: : ~ '. ·E ': lr-onlono • + - nonstretched Archean crust + + + + + • + ~,....,~;;;.:"!-~ ...... 0 ...... - ...... ·+ - + + + ... -- ... + + + - -- + . - +· ++. +_++++++-+ w -.:;:-;,.__ _,.....,,__..,,;:-:,,,.,,-1~11 0 .. + .... + + - + ... .,.. 1" + ...... + + + + ...... + + + + .... -- + + + + ... - ... + + + ...... + - - + + ...... + + ... - .... PASSIVE MARGIN - EPWORTH GROUP dolomite: s-slope rhythmite, breccia; r-reefal rim, 0 back-reef grainstone; c - lagoanal-peritidal cycles ~ quartz arenite, semipelite: sr ·slope-rise contourite, ~ turbidite; sh - open shelf-shoreline offlap cycles INITIAL RIFTS - AKAITCHO GROUP 3 submarine basalt, tuft, rhyolite, gabbro sills; 2 t submarine elastic sediments, dolomite reefs at top ( IOx vertical exaggeration) + + + ... g~] feldspathic wacke turbidite, semipelite, conglomerate; 20 40 60 80 100 km t submarine volcanics, gabbro, rhyolite porphyry sills PFH Figure ~2.2 . Stratigraphic reconstruction of the Coronation Supergroup, assuming 40% tectonic sho~tenmg ~est of the (rontal thrusts and above the basal decollement. Stratigraphic thicknesses, facies relations CV:d estimated shortening are well controlled by down-plunge projections east of the R~cknest slope (T1rrul, 1983).. To the west, the same shortening (4096) is assumed but stratigraphic thicknesse.s are poorly constramed. The overall construction is artificial for the Recluse Group, which was depos1 ted as deformation progressed from west to east. 385 superimposed on the upper surface of hummocks during the the reefal rim pass laterally into outer-shelf dePos· return to fair-weather conditions followed by settling of consisting of equally interbedded stromatolite mounds its suspended muds during fair-weather sedimentation (e.g. lower ooid/ intraclast grainstone. The outer shelf was subjec ted~ part of Middle Member). Below storm wave base, suspended open-marine conditions involving constant wave and tidaI(?l sediment is deposited from unidirectional, offshore-directed activity as shown by the extreme elongation of stromatolit · currents to produce graded and/or planar-laminated and crossbedding in grains tones. Farther to the east outes sandstones and siltstones. The thickness of these individual shelf deposits pass laterally into a narrow zo~e er storm-generated units decreases outwards on the shelf stromatolite-deficient, crossbedded and planar- laminat of (e.g. Lower Member facies 1 and 2). The planar-laminated ooid/ intraclast grainstones (Fig. 52.4) t hat represeed sandstones and siltstones may be locally scoured and draped shoreface deposits. These facies pass abruptly eastward in~t at storm wave base (e.g. Lower Member facies 3). The a sharply defined, laterally continuous, peritidal sho~ enclosing terrigenous sediments indicate that the complex (Fig. 52.4). This facies belt is dominated stromatolitic dolomite at the top of the Lower Member is of precipitated cryptalgal tufa, locally thrust into tei>eby sublittoral origin and probably developed during a prolonged a ntiform structures, and associated with vadose pisolites ~ period of time when the shelf was starved of terrigenous void-fi~ing silica an~ ~olomite pseudomorphs after botryoidaJ sediment. aragonite that prec1p1tated beneath tepees and in relatec subhorizontal fissures (see Grotzinger and Read, in press). The facies in the Lower and Middle members at area "Z" can be readily interpreted as distal time equivalents of With respect to the diversity and similarity of faci the deposits at area "Y". The Lower Me mber consists mainly and their paleogeographic position immediately behind~ of laminated marlstone, mudstone and carbonate representing reefal rim, there is a striking resemblance between the suspension deposits of a distal shelf environment starved of peritidal shoal complex of the Rocknest Formation, the terrigenous influx. The distal shelf was not necessarily Permian Carlsbad Group of west Texas, and the Triassic deeper than at area "Y" as evidenced by occasional edgewise Latemar Group of the Italian Alps (Assereto and carbonate breccias enclosed within the laminated sediments. Kendall, 1977). In the Rocknest Formation, the width of the The breccias are confined to the uppermost one-third of the Lower Member and are considered to reflect oscillatory reworking of lithified carbonate muds during periodic lowering of wave base. The Middle Member at area "Z" is R.GRP devoid of hummocky cross-stratification and instead comprises several upward-thickening and upward-coarsening sequences of tabular sandstone beds structured by planar 600m lamination with rare wave ripples developed at the tops of beds. Similar sequences in the 1. 7 Ga Ortega Group in New Mexico are interpreted as progradational outer-shelf lobes which accumulated mainly below storm wave base (Eriksson and Soegaard, 1983). The sequences of facies through the Lower and Middle Members of the Odjick Formation at areas "Y" and "Z" indicate a slow progradation following initial rapid 400m transgressive drowning of the craton. A granite-pebble (c) conglomerate and overlying stromatolitic dolomite, both of which are laterally discontinuous and total less than 1.5 m in thickness, represent the only transgressive deposits. Around Carousel Massif, the initial transgression is represented by up to 6 m of white supermature quartz arenite (St- Onge et al., 1982). PALEOGEOGRAPHY, FACIES DISTRIBUTION, AND CYCLIC SEDIMENTATION ON THE ROCK NEST CARBONATE SHELF Ll OOID I INTRA. GRST (d) During the 1983 field season, investigations of the Rocknest (DJ STROMATOLITE MOUNDS Formation by J .P.G. concentrated on detailed mapping and Cd'.) NEPTUNIAN DIKES stratigraphic studies of the shelf-edge reef complex and flanking facies belts, and on changes across depositional W BOTRYOIDAL PSEUDO-FIBROUS CEMENT strike within shoaling-upward cycles at the north end of the 0LAMINAR externides. Paleogeographic reconstruction of the shelf-edge Om 0 FM complex reveals a sharply defined, narrow reefal rim ~RHYTHM ITE composed of strongly elongate stromatolite mounds that (a) interfinger vert ically and laterally along strike with Figure 52.3. (a) Stratigraphic section of Rockn~t subordinate amounts of ooid/intraclast grainstone Formation shelf-edge facies; (b) Crosscutting neptuman (Fig. 52.3a). The reefal rim underwent early cementation and dykes; (c) Details showing truncation of neptunian ~ fracturing as shown by pervasive networks of neptunian dykes followed by deposition of more stromatolitic laminae: and sills, filled with silica and dolomite pseudomorphs after (d) Details showing "fitted' relations of neptunian dyke walls, botryoidal aragonite and other isopachous-fibrous marine and internal cement stratigraphy. Location of section is "S' cements (Fig. 52.3b, d). Early fracture porosity in the reefal in Figure 52.1. rim was furt her enhanced by periods of submarine(?) dissolution which corroded, enlarged, and truncated fractures, and corroded parts of stromatolite mounds (Fig. 52.3c). To the east (landward), stromatolite mounds of 386 peritidal shoal complex varied, but at all times (except during CRYPTALGAL TUFA ~ local drowning and backstepping of th'e reefal rim midway through the evolution of the platform) acted as the R?.1 starting line for numerous short-term progradational cycles ~ PISOLITE I ONCOLITE (1-10 m thick) of the shoal complex over a very broad (at least 150 km) inner-shelf lagoon to the east. In contrast to the outer-shelf, which was wave- and tide(?)-dominated, ~TEPEE STRUCTURE the inner-shelf lagoon was storm-dominated. Storm deposits include abundant rip-up breccias overlain by planar-laminated and hummocky cross-stratified siliciclastic and carbonate ~ BOTRYOIDAL PSE UOO sands. These sequences are commonly capped by wave FIBROUS CEMENT rippled sands and silts, and graded silt-to-mud couplets. The inner-shelf lagoon was the site of intermixing of carbonate sediment derived from the peritidal shoal complex to the west, and siliciclastic sediment derived from a shoreline to the east. Accordingly, the ratio of carbonate to terrigenous sediment decreases from west to east. [] OOID /INT. GRST ; PLANAR LAM. The shoaling-upward cycles in the Rocknest Formation dominate its stratigraphy in the autochthon and all but the westernmost thrust sheets of the foreland thrust-fold belt. ; CROSS-BEDDED They can be correlated for over 200 km parallel to depositional strike (Grotzinger and Hoffman, 1983) and for over 100 km across depositional strike at the north end of the externides (Fig. 52.5). Figure 52.5 shows across-strike correlation of cycles in the lower part of the Lower Shale ~ STROMATOLITE MOUNDS Member (for descriptions of members see Grotzinger and Hoffman, 1983) and the lateral facies changes that occur within them. In any given cycle, the asymmetric vertical 1:-.:.:.11NT GRST.-PK'ff arrangement of facies records gradual shoaling of the shelf followed by rapid submergence represented by the cycle boundary. Note that the eastern part of the shelf was consistently less aggraded than the western part. During Om each cycle, the extent to which the shelf aggraded at any Fif1'Te 52.4. Stratigraphic section through part of Basal location was directly controlled by the proximity of that Member of Rocknest Formation, 10 km (palinspastic) east of location to the back-reef shoal complex; progressive west-to the shelf edge. The section shows a long-term progradational east decrease in the degree of aggradation on the shelf ramp sequence of peritidal shoal-complex f acies overlying immediately before each submergence increment is indicated ooid/intraclast grainstones and stromatolitic boundstones. by the west-to-east transgression of facies boundaries by cycle boundaries. Furthermore, correlation of the Rocknest Location of section is "T" in Figure 52.1. Formation with parts of the Burnside River Formation (see below) far to the east indicates that ( A ) 40km (Bl 20km (Cl 60km (D) the entire Rocknest Formation passes laterally into terrigenous sediments deposited in a shallow-shelf lagoon. These relationships show that progra dation was incomplete during each cycle and that the lagoon was a permanent paleogeographic feature during evolution of the Rocknest shelf. This leads to the inescapable conclusion that complete shoaling of the shelf was not required to induce successive submergence events, and suggests a eustatic control of cyclicity on the shelf. Om EASTWARD EXTENT OF THE ROCKNEST FORMATION D C~YPTALGAL TUFA tOm The apparent eastward disappearance of 8 DCMAL ST ROMATOLITE S - CYCLE BOUNDARY the Rocknest Formation across G EDG E WISE CONGLOMERATE - FACIES BOUNDARY Rockinghorse Arch between the exter 20m D TH IC ~· L AM I NATED DOLOSILTITE nides and Kilohigok Basin (Goulburn • ARGILL ACEOUS DOLOLUT ITE ! OTZ S DOLO SILT/ SAND Group) has always presented a problem 30m for regional stratigraphic correlation D INTRACLAST PACKSTONE (e.g. Fraser and Tremblay, 1969; ~ SCOURED SURFACE Hoffman et al., 1970; Hoffman, 1981; Campbell and Cecile, 1981). The Figure 52. 5. Correlation of four shoaling-upward cycles, lower part of Lower Shale Me mber, Rocknest Formation, at the north end of the externides. Note problem is important because, for eastward transgression of f acies boundaries by cycle boundaries, including pinch example, a disconformity cutting down out of cr yptalgal tufas (supratidal) and domal stromatolites (intertidal) to the east, from the top of the Rocknest Formation ':"ld westward thinning of argillaceous dololutite (sublittoral). Also note lack of could indicate flexural arching during lntraclast packstone (reworked soil) and scoured surface at base of 2, 3 and 4 at the Calderian Orogeny. section ''D", and cycle 4 at section "C", suggesting lack of exposure before ~ub1'.1 erg ence . Cycle boundaries are interpreted to be chronostratigraphic. catton of sections is shown in Figure 52.1. 387 With a view to resolving this problem, a reconnaissance River Formation as previously mapped (Campbell llllc! was made by two of us (P.F.H. and J.P.G.) to Rockinghorse Cecile, 1976) is overlain by an interval of dominantly fin Outlier, the Contwoyto Lake-Peacock Hills area, and the grained elastics with minor intraclastic dolomite transi tion~ area south of Kuuvik Lakes (Fig. 52.6). The Western River upwards into five typical shale-dominated Rocknest cycles and Burnside River formations as mapped by Campbell and each capped by a thin shoal-water stromatolitic dolornit Cecile (1976) in the well-exposed sections at Rockinghorse bed. This interval, 157 m thick with its top not exposed, w: Lake are readily recognized as correlatives of the believe to be correlative with the transitional UpPer Lower (Eol) and Middle (Eo2) members of the Odjick Member (Eo3) of the Odjick Formation and the Rocknest Formation respectively, just slightly coarser grained than in Formation. At section 7, the same dolomitic interval, PQorl the autochthonous externides (e.g. sections 3 and Y of exposed, is overlain in the core of a relatively tight synclin~ Fig. 52 .6). At section 6, the top of the arenaceous Burnside by an intrusive gabbro sill above which is a fine grained ' 0 0 20 30 km + - - -r + Roclunqnorse []IIlE Calderian gabbra _.. _ Contwoyto Loke ~ [:=J Corona t1 on-Goulburn 112• ~ Archean basement 11 3° -r 11a• Coronation Margin Rockinghorse Arch Kiloh igok Basin km .., km u z .., a: 1.5 CD ® @ ::> ::> 0 @) ..,0 a: 1.5] "'.., Ro "'Q. 10 .., 05 Rk .., ::> "'.J ..,0 u a: w 0 ~ G)._.__y:-1:- 0 5 ... 1.0 --~.:~.:.~.~_SQ_~=®:::::::m . --- 0 ,.a::;=;;::;;;;;__~_,.,.,b+.~,~-=--•,.i:.,.+R-lt ::i B -0.5 ,',1 a:::> 0 -10 -10 a: "' ....l: a: -1.5 P-Peacock h ·I 5 0 Ro- As1ok Fm 3: ls L.-Fm_____,~< Q. Rk - K •erk Fm M-Moro Fm (•"BR ') Archean ~ -2.0 Rf-Fonlono Fm B-Burns1de River Fm -20 Rt-Tree River Fm W-Wes1ern River Fm Er- Rock nest Fm ~.p, Boz, 8d - uni ts as 1dent1f1ed ·25 -25 £03-Upper Mb l by Cornpoe ll and Cecile ( 976) Eo2-M ddle Mb . OdJ'Ck Frn -30 Eal -Lowe r Mb 1 -3.0 Figure 52.6. Location of sec tions and proposed correlation of the Rocknest Formation across Rockinghorse Arch into Kilohigok Basin. Section "Y" is located in the Tree River Fold Belt (see Fig. 52. 1) and section 10 is a composite from around Bathurst Inlet. 388 weiJ-indurated, white quartz arenite ("BQZ"). This quartzite There appears to be a relationship bet.ween basement . slightly coarser grained than, but otherwise very similar to, involvement in structures and higher metamorphic grade. ~he Tree River Formation (Hoffman, 1981), which overlies This is observed in both the Tree River Fold Felt and in the the Rocknest Formation. At section 9, an exceptionally well area around Carousel Massif, where shallow dips on the basal ,q>osed river gorge, a 21 m thick interval of mixed elastics unconformity characterize low grade regions, while steep to !nd dolomite ("BD") occurs within the Burnside River overturned attitudes are present in higher grade areas yorrn ation and has been traced over a wide area (Campbell (levels 4 and 5). These observations probably reflect and Cecile, 1976). Although we cannot be certain, based on a temperature control on basement deformation mechanisms. limited reconnaissance, we suggest that this unit is the feather-edge extension of the Rocknest Formation. No NEW DATA ON TRANSVERSE (D2) FOLDING evidence of a significant disconformi ty occurs in this or any of the other sections and it is tentatively concluded that the During 1983, mapping of the Tree River Fold Belt was Burnside River Formation represents, in part, the landward completed. Axial surface traces of megascopic Coronation side of the Rocknest inner-shelf lagoon. Supergroup folds in the belt are shown in Figure 52.8. There are three conspicuous fold trends. North-trending folds (Fl) This correlation, if correct, has important regional restricted to the region west of 113 ° W are associated with tectonic implications, principal among which are: (1) that thrust faults and are of Calderian age. They indicate that a there was no emergent flexural arch associated with decollement below the Rocknest Formation locally extends subduction of the Rocknest shelf, and (2) the Coronation 5 km east of the frontal thrust zone. As previously reported, margi n opened and closed concurrently with unroofing of a the Calderian structures are refolded by Tree River folds (F3) major elastic source area, possibly the Thelon Front(?), of northeast trend which dominate the region shown in southeast of Kilohigok Basin. Figure 52 .8. They are typically open, upright to steeply ' inclined, and polyharmonic, with characteristic wavelengths META MORPHISM IN THE EXTERNIDES of 600-700 m and 60-100 m. The longer wavelength folds usually involve the entire thickness of the Rocknest During the course of mapping in previous field seasons it was Formation; the higher order folds are common at the level of observed that the colour of Coronation Supergroup shale the Domal Stromatolite Member (Grotzinger and shows an areal variation that apparently correlates with Hoffman, 1983), and diminish in amplitude both upward and metamorphic grade, as indicated by mineral assemblages in downward in adjacent members. A less developed Tree River mafic rocks in the Odjick Formation, by the development of Fold set is parallel to the trend of the synclinorium between slaty cleavage, and by the appearance of metamorphic biotite Eokuk and Uyarak uplifts (N30°E). The relative age of these in Tree River Fold Belt. Systematic sampling of the Red folds is not known. Shale Member (Grotzinger and Hoffman, 1983) of the Rocknest Formation was completed in 1983 for a study of The most prominent fabric in the area is penetrative meta morphism in the externides. cleavage (S2) developed at a low angle (about 20°) to bedding in pelitic beds of the Rocknest and Odjick formations. The Red Shale Member samples were graded in the field on cleavage refracts strongly into dolomite and quartzite beds to 5 based on colour and fabric. The lowest a scale of 1 where it is spaced or difficult to detect. The cleavage is apparent grade is represented by level 1 samples, which are folded by the dominant Tree River Fold set. When bedding is moderate red (5R 4/ 4, Goddard et al., 1951), and generally restored, the cleavage dip is exclusively to the north. Its display no tectonic fabric. A complete gradation was development involved a large component of bedding-parallel observed through greyish red (5R 4/ 2) of level 3 to samples shear, as indicated by folded extension veins within pelite which are medium grey (N5, level 5) with a strong slaty that have been rotated through vertical to their present cleavage. Illite crystallinity and muscovite polymorph northerly inclination, analogous to those described by mi nations of the samples are being undertaken in an deter Henderson (1983). effort to calibrate the qualitative grade scale. Poles to bedding and to S2 are shown for one domain (I) The apparent metamorphic grade of pelite within the in Figure 52.8; for the others only bedding/ cleavage Red Shale Member, as reflected by colour and textural intersections are shown. The intersection lineations are variation, is illustrated in Figure 52. 7. If valid, several either indistinguishable from the orientation of younger fold trends, reflecting different factors that control grade, are hinges, apparent. or they show relative clockwise rotation. Similarly, lineations on cleavage surfaces defined by long axes of To some degree, grade is controlled by structural level. reduction spots, mineral alignment and fine corrugations have This is shown by the progressive decrease in apparent grade a mean northerly trend, forming an acute angle with later down-plunge along the west flank of Atanigi Syncline ("A" in fold hinges. Fig. 52. 7), and by a similar decrease down-plunge from both In accord with previous observations (Hoffman, 1973; the north and the south into the structural depression of Hoffman et al., 1983), Archean basement is folded but not White Sandy Syncline. This suggests that syn-Calderian involved in thrusting. Most of the exposed basement/ cover isotherms have been warped by the broad transverse D2 folds. contact has been carefully mapped, and with a very few In the area north of Carousel Massif, a westward minor exceptions, the contact is intact. The basement has increase in apparent Calderian grade is also present. This is been deformed into structures that resemble large-scale thought to be unrelated to structural depth because the mullions, with convex-upward lobes from a few metres to thrust-fold belt has evidently not experienced major post hundreds of metres wide, separated by pinched cusps. The Calderian eastward tilting (Tirrul, 1983). margins of the lobes are commonly overturned, as shown in Figure 52.9. The basement deforms primarily by acquiring an Per haps the most surprising result is that metamorphic inhomogeneously developed retrograde foliation which is grade increases eastward from the thrust-fold belt toward particularly well developed in some cusp zones. It also Eokuk Uplift. This is not an effect of erosional level because deforms by way of discontinuous small-scale conjugate shear the grade in the Tree River Fold Belt is higher than that zones which accomplish northwest shortening with around Takijuq Lake. The reason for localization of deformation and metamorphism in this region is unclear. subvertical extension. 389 CG I N 112° -67° ~ Post - orogenic gabbros t r:---:1 L:___:_J Post - orogenic cover LEGEND .. Late transcurrent fault D . ( NE - dextral , NW - sinistral ) Foredeep molosse (Takiyuak Fm ) Calderian thrust ~ ~ Foredeep flysch (Recluse Gp) b - Appearance of biotite • - 5 Increasing Passive - margin shelf ~ - 4 foci es ( Epworth Gp) @ - 3 grade @ 2 Passive - margin slope - rise 0 - I fac ies (Epworth Gp) Okm 50 100 E3 R E3 R R Archean basement Figure 52.7. Simplified geological map of the externides of ll"opmay Orogen showing apparent metamorphic grade as indicated by the colour and f ab1·ic of shale in the Red Shale .\/ember of the Rocknest Formation. A, Atanigi Syncline: C, Carousel :\Iassif; CC, Coronation Gulf; W, \\'hite Sandy Syncline. 390 67° 45 1 Coronation Gu If / Kik erk T h rust ~~ - / ~ ---- c + .. .,. . Rae Group Coronation Supergroup I+ +I Archean Basement Axial Surface Trace • Mean Fold Hinge c Intersection CS ,.. S ) + Cl eavage Lineation O 2 c Basement Fold Extension (on ) 0 52 Vergence L ineation + = 29 .. = 29 0 km 10 s 1°00' 11 3° + ·----1 112° FifµTe 52.8. Fold trajectory map of the Tree River Fold Belt showing basement/cover contact and selected structural data. Inset along Tree River shows the location of Figure 52.9. Compared with the fold pattern observed in the at "L" to the east, and at "M" to the west. Each is associated ~ o cknest and younger units, the basement surface geometry with one to three generations of cozonal cleavages in the is complex. Although the dominant trend is northeast, the adjacent cover, different in orientation from neighbouring lobes and cusps show a wide variation in orientation and cusps of different trend. From field relations alone the vergence, as shown in Figure 52.8. For exmaple, folds at "J" relative ages of these structures is not clear. Their are overturned to the northwest, at "K" to the southeast, development may have been broadly coeval. 391 .... -- , --- Glacial Deposits A ,I ~ I a ~ 300 I _.... ---' .... -- ~ Gabbro, Diabase --- 200 < Rocknest Formation u ~ 7 " /\ 0 ~ N - Odj ick Member 3 100 "< " < 0 ~ fl: I\ <. " w I- Member 2 Om > " ~ 0 6=J fl: n. ~ Member 0 500m z Monzogroni te 200 ~~r-r~++++ + Eo 2 ----- ~ + + + + + + + + + + + 100 --===;-=;~+::;! t ! t t ! ! ! ! ~ ~ ! ! ! t ! +-+ + + + + + + + + + + + + + + + + + + + + + O m ..L...~~~~~~~~~~~~~~~~~~~~~~~ Figure 52.9. Geological map and cross-sections of a part of the western margin of Uyarak Uplift (see Fig. 52.8 for location) showing the cuspate- lobate geometry typical of the basement/cover contact in the Tree River Fold Belt. Topographic relief here is great enough for excellent structural control. Bot h the shortening of the cover rocks and basement flanking the uplift. The simplest interpretation here is that are interpreted to be due to northwest-oriented compression the cleavage is axial planar to overturned folds which involve throughout the thickness of the lithosphere. The reason for the basement. If gravitationally induced bedding-parallel the wide variation in orientation of basement structures may shear was an important factor in the development of the be due to the inability of an infinite half-space (analogous to cleavage, a linear uplift is required, north of the localities the basement/ cover contact) to buckle. Instead, relief is discussed, and presumably due to tectonic thickening. produced by penetrative strain and shape modification of Southward migration of deformation is required to fold and perturbations into lobate-cuspa te form (Smith, 1979; overprint the north-dipping cleavage at any given locality. Ramberg, 1981, p. 151-156). No doubt, variably oriented The cleavage was not produced by gravitational sliding of weak zones both enhance amplification and refract regional cover off the flanks of a domal uplift since the cleavage and stress trajectories. related lineations are not concentrically and radially disposed A tectonic interpretation of the north- dipping folded about the uplift, respectively. cleavage (S2) is difficult. Despite the fact that subsequent fold hinges do not everywhere lie within S2, they are thought STRUCTURE OF KANGAROO BLOCK to be related to a single progressive tectonic event. Both the Hoffman and St-Onge (1981) proposed that the north-central cleavage and Tree River folds are well developed adjacent to part of Wopmay Orogen is segmented into crudely hexagonal Eokuk Uplift, and poorly developed away from it (e.g. localities "N" and "0", Fig. 52.1). Also, a localities "C" crustal blocks, each bounded by transcurrent faults linking an east-vergent thrust. These overlapping "shingles" were and "E" (Fig. 52.1), t he cleavage is subhorizontal and axial inferred to have developed late during a third major planar to recumbent minor folds on steeply dipping panels 392 mpressional event (03) affecting the orogen and which is points is significant. Aspects of the devE;lopment of the c<> onsible for a transcurrent fault system of regional Kangaroo Block may apply to the larger-scale "shingles" to re~~nt. Since important support for this interpretation was the west. e ed on regional variation in metamorphic pressure, now A simplified map of the Kangaroo Block is shown in ~err ed to outline broad 02 structures that predate 1 Figure 52.lOa. Except for size, its geometrical similarity t o anscurrent faulting (Hoffman et al., 1983; St- Onge, t he blocks of Hoffman and St- Onge (1981) is striking. It is tr press), the question of net vertical displacement along the bounded to the northeast by Kangaroo Fault, with more than 1 ~ock boundaries remains outstanding. In this regard, the 4 km of left-slip. Its curvilinear trace merges with a major b cognition in 1983 of a smaller-scale "shingle", well exposed, boundary fault to the southeast, with approximately 10 km of red offsetting Calderian structures which serve as piercing an Recl use Group ~ Transcurrent mm ~ Fault __.___ Thrust Fault [8 Rocknesf Formation + Syncline [§] Odjick Formation + Anticli ne o km 00 0 0 Er 66° 02 1 Tronscurrent Recl use Group Fa ul t Roc knesf Members Thrust Fau If 5 - 10 Syncline Rocknesf Members 2-4 Anticline Over turned Rocknesf Member Anticline Facing Odji ck Member 3 Direc tion Od jick Member 2 Fif1.Jre 52.10. (a) Simplified geological map of Kangaroo Block and surrounding area. (b) Geological map of the northeast margin of Kangaroo Block with 4.4 km of restored left- slip on Kangaroo Fault. The area is located within the dashed inset of Figure 52.lOa and the general location of the block is shown in Figure 52. 1. 393 right-slip. Both within and adjacent to the block, regional ACKNOWLEDGMENTS structural plunge is to the south. There is no differential We were fortunate in having an outstandingly able alld uplift of the block as a whole. The synclinorium at " A" dedicated group of student assistants. who regular! (Fig. 52.lOa), for example, can be traced across both undertook independent geologica l mapping. It incJud~ boundary faults with no significant change in structural level. \I.E. Grier (Queen's), C.A. Gittins (Toronto). 1\1.D. Dayneka i\lovement on Kangaroo Fault postdates the (\lemorial). Bradford Johnson (Santa Barbara) &lld development of most adjacent structures. Its trace is marked \lark Cunna ne (\'lissoulal_. The .las.t two are particularly by a prominent uninterrupted topographic lineament along commended for their unst in ting efforts without which right-lateral faults are truncated. In Figure 52.lOb, remuneration. For 300 hours of helicopter service without a the geology along part of Kangaroo Fault is shown in more hitch we thank Edgeworth llelicopters of Fort Nelson, B.c. detail, after restoration for 4.4 km of left-slip. With this and pilot -engineer Keith \\'est fa ll. Janien Schwa~ reconstruction, the anticline at .. A" is partially restored, and reaffirmed Napoleon's dictum that an army ma rches on its structural panels at "B" have compatible counterparts on the stomach, while \\'in Bowler and 'llartin Irving provided north side of the fault. logistic support at Yellowknife. We were delighted to host l\I.J. Jackson (Bl\! R, Canberrra. Australia) for half the With the partial reconstruction of Figure 52.lOb, the summer. and briefer visits by D.R. Gray (Virg inia Tech) and right-lateral fault at "C" correlates with a braided syst em of R.A. Price (GSC) provided va luable ins ights. J.P.G. is north-trending faults at ''D'' with right separation and which supported by Na tiona l Science Foundation grant splay westward. The Odjick anticline west of "C" has llEAR-8218618. The manuscript was critically reviewed by apparently been offset from ''E"' along the fault. Other F.H.,\. Campbell. and Simon Hanmer. north-trending faults in Kangaroo Block are also right-la teral faults that have undergone large counterclockwise rotations about vertical axes prior to offset by Kangaroo Fault. REFERENCES Northward from ''F" to ''H", a Calderian syncline cut by a Allen, P.A. swarm of right-lateral faults shows a progressive increase in 1981: \\'ave-generated structures in the Devonian apparent rotation as Kangaroo Fault is approached. Adjacent lacustrine sedi ments of south-east Shetland and to "G", the fold and its cleavage fan have been rotated from ancient wave conditions; Sedimentology, v. 28, an original northerly trend to an azimuth of 100°£. p. 369-379. In addition to large rotations, fault blocks within Assereto, R.L.A.\'I. and Kendall, C.G.St.C. Kangaroo Block exhibit some compressive strain. North to 1977: Nature, origin and classification of peritidal tepee northeasterly trending minor folds with strict chevron structures and related breccias; Sedimentology, profiles and no associated cleavage are common along the v. 24, p. 153-210. eastern margin of the block. They fold Calderian cleavage. Brenchley, P.J. and Newall, G. Evidence for reverse slip along the eastern margin of 1982: Storm- influenced inner-shelf sand lobes in the the block is present but not overwhelming. Where exposed, Caradoc (Ordovician) of Shropshire, England; the faults along the southeast margin of the block have steep Journal of Sedimentary Petrology, v. 52, dips. The fault at "B" (Fig. 52.lOa) is not exposed, but Odjick p. 1257-1269. Middle l\·lember on the west side does not have a match either at "C'' or "D''. A minimum of a few hundred metres of west Campbell, F.H.A. and Cecile, M.P. side- up displacement is required along this margin. 1976: Geology of the Kilohigok Basin, Bathurst Inlet, N.W.T.; Geological Survey of Canada Open In summary, Kangaroo Block was an asymmetric, late File 332, map at 1:500 000 scale. stage development of D3 deformation. It was established after considerable slip and rotation of both right- and left 1981: Evolution of the early Proterozoic Kilohigok lateral transcurrent faults, when a left-lateral fault Basin, Bathurst Inlet - Victoria Island, Northwest propagated into and offset major right-slip faults. Reverse Terri tori es: in Proterozoic Basins of Canada, slip and uplift of the block is relatively minor (less than 10% ed. F.H.A. Campbell; Geological Survey of of strike-slip) and restricted to the leading edge of the block Canada, Paper 81-10, p. 103- 132. unless post-orogenic backsliding (Hoffman and St-Onge, 1981) Eriksson, K.A. and Soegaard, K. has been important. 1983: Storm-deposited outer-shelf facies from Precambrian Ortega Group, New Mexico; FUTURE WORK American Association of Petroleum Geologists, Bulletin, v. 67, p. 456. A 1:250 000 scale geological map of the externides has been prepared by P.F.H. and is available for inspection in Ottawa. Fraser, J.A. and Tremblay, L. -P. It is intended for final publication in colour and will probably 1969: Correlation of Proterozoic strata in the not, because of its extreme complexity, be issued in northwest ern Canadian Shield; Canadian Journal uncoloured form as an Open File map. Geological maps at of Earth Sciences, v. 6, p. 1-9. 1:50 000 scale a re being prepared by R.T. to cover critical Goddard, E.N., Trask, P. D., Deford, R. K., Rove, O.N., parts of the ext ernides, namely the northwest corner, the Singewald, J.T. Jr., and Overdeck, R.\J. area around Carousel :Vlass if, the area of Kangaroo Block, and 1951: Munsell rock colour chart; Geological Society of the area of intersection of frontal Calderian thrusts and the Am erica, New York. Tree River Fold Belt. R.T. will also be preparing journal articles on the three deformations. J.P.G. will likewise be Grotzinger, J.P. preparing articles on the Rocknest Formation, its outer reef 1982: A preliminary account of the internal stratigraphy complex, shelf cycles, and precipitated carbonate cements. of the Rocknest Formation, foreland thrust-fold :\'Iet amorphism of the externides will be the subject of a B.Sc. belt of Wopmay Orogen, Dis trict of Mackenzie;!!! thesis by S.B.L. at Queen's University. C urrent Research, Part A, Geological Survey of Canada, Paper 82-lA, p. 117-118. 394 orotzinger, J.P. and Hoffman, P.F. Hoffman, P.F., Tirrul, R., and Grotzinger, J.P. 1983: Aspects of the Rocknest Formation, Asiak Thrust- 1983: The externides of Wopmay Orogen, Point Lake Fold Belt, Wopmay Orogen, District of and Kikerk Lake map areas, District of Mackenzie; in Current Research, Part B, Mackenzie; in Current Research, Part A, Geological Survey of Canada, Paper 83-lB, Geologial Survey of Canada, Paper 83-lA, p. 83-92. p. 429-435. Grotzinger, J.P. and Read, J.F. Piper, D.J.W. _ Evidence for primary aragonite precipitation, 1972: Turbidite origin of some laminated mudstones; early Proterozoic (1.9 Ga) Rocknest Dolomite, Geological Magazine, v. 109, p. 115-126. Wopmay Orogen, northwest Canada; Geology. (in press) Ramberg, H. 1981: Gravity, Deformation and the Earth's Crust; Hamblin, A.P. and Walker, R.G. Academic Press, London, 452 p. 19 79: Storm-dominated shallow marine deposits: The Smith, R.B. Fernie-Kootenay (Jurassic) transition, southern 1979: The folding of a strongly non-Newtonian layer; Rocky Mountains; Canadian Journal of Earth American Journal of Science, v. 279, p. 272-287. Sciences, v. 16, p. 1673-1690. St-Onge, M.R. Henderson, J.R. Geothermometry and geobarometry in pelitic 1983 : Analysis of structure as a factor controlling gold rocks of north-central Wopmay Orogen (early mineralization in Nova Scotia; in Current Proterozoic), Northwest Territories, Canada; Research, Part B, Geological Survey of Canada, Geological Society of America, Bulletin. (in press) Paper 83-lB, p. 13-21. St-Onge, M.R., King, J .E. , and Lalonde, A.E. Hoffman, P.F. 1982: Geology of the central Wopmay Orogen (Early 1973: Evolution of an early Proterozoic continental Proterozoic), Bear Province, District of margin: the Coronation geosyncline and Mackenzie: Redrock Lake and the eastern portion associated aulacogens of the northwestern Canadian Shield; Royal Society of London, of Calder River map areas; in Current Research, Part A, Geological Survey of Canada, Philosophical Transactions, Series A, v. 273, Paper 82-lA, p. 99-108. p. 547-581. 1984: Deformation and metamorphism of the 1981 : Revision of stratigraphic nomenclature, foreland Coronation Supergroup and its basement in the thrust-fold belt of Wopmay Orogen, District of Hepburn Metamorphic-Plutonic Zone of Wopmay Mackenzie; in Current Research, Part A, Orogen: Redrock Lake and the eastern portion of Geological Survey of Canada, Paper 81-lA, p. 247-250. Calder River map areas, District of Mackenzie; in Current Research, Part A, Geological Survey Of Hoffman, P.F. and Bowring, S.A. Canada, Paper 84-lA, report 25. A short-lived 1.9 Ga continental margin and its Tirrul, R. destruction, Wopmay Orogen, northwest Canada; 1982: Geology. (in press) Frontal thrust zone of Wopmay Orogen, Takijuq Lake map area, District of Mackenzie; in Current Hoffman, P.F. and St-Onge, M.R. Research, Part A; Geological Survey oT Canada, 1981: Contemporaneous thrusting and conjugate Paper 82-lA, p. 119-122. transcurrent faulting during the second collision 1983: Structure cross-sections across Asiak Foreland in Wopmay Orogen; in Current Research, Part A, Geological Survey of Canada, Paper 81-lA, Thrust and Fold Belt, Wopmay Orogen, District of Mackenzie; in Current Research, Part B, p. 251-257. Geological Survey of Canada, Paper 83- lB, Hoffman, P.F., Fraser, J.A., and McGlynn, J.C. p. 253- 260. 1970: The Coronation Geosyncline of Aphebian age; in Symposium on Basins and Geosynclines of the Canadian Shield, ed. A.J. Baer; Geological Survey of Canada, Paper 70-40, p. 200-212. 395