Stratigraphy and Structure of the Southeastern Cape Breton Highlands, Nova Scotia Robert P

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Atlantic Geology

Stratigraphy and Structure of the Southeastern Cape Breton Highlands, Nova Scotia Robert P. Raeside and Sandra M. Barr

Volume 22, Number 3, December 1986 Article abstract The southeastern Cape Breton Highland** are composed of an eastern belt of URI: https://id.erudit.org/iderudit/ageo22_3art04 predominantly granitoid rocks and a western belt of dloritic, granitoid and stratified rocks. The oldest unit is Inferred to be the Chetlcamp Lake gneiss, See table of contents which outcrops in the western belt. This is in faulted contact with the most extensive stratlgraphic unit. the McMillan Flowage Formation, which occurs throughout the western belt. It represents early Paleozoic or older continental Publisher(s) nwrg-fn accumulations of sandy and silty clastic sedimentary, volcanic, volcaniclastic and rare calcareous rocks. These stratified units have been Atlantic Geoscience Society folded and Intruded by late Precambrian(?) to lower Paleozoic granitoid and dloritic plutons. and then have undergone a later strike-slip deforma-tional ISSN event. In the eastern belt, the Baracbois River unit preserves a section of siltstones and greywackes which have been largely metamorphosed to 0843-5561 (print) gneisses, and the Price Point unit includes a small section of volcanic rocks. A 1718-7885 (digital) single pervasive foliation has been recognized in the eastern belt. The two belts appear to represent components of two tectonostratigraphic Explore this journal zones, the western belt being part of the Highlands zone, and the eastern belt part of the Bras d'Qr zone of Cape Breton Island. The western belt displays a pronounced Devonian thermal overprint, whereas the eastern belt has Cite this article preserved late Precambrian or Siluro-Qrdoviclan ages. Neither belt can readily be correlated with the Southeastern zone of Cape Breton Island or the Avalon Raeside, R. P. & Barr, S. M. (1986). Stratigraphy and Structure of the Zone in Newfoundland. Based on lithologic similarities. both In the stratified Southeastern Cape Breton Highlands, Nova Scotia. Atlantic Geology, 22(3), units and in the nature of the plutonic rocks, the western belt (Highlands zone) 264–277. can be correlated with the Gander zone. The eastern belt (Bras d'Qr zone) is a possible manifestation of the Avalon zone at a deeper crustal level.

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Vol. 22 December, 1986 No. 3

Stratigraphy and Structure of the Southeastern Cape Breton Highlands, Nova Scotia

Robert P. Raeslde and Sandra M . Barr Department of Geology, Acadia University Wolfvllle, Nova Scotia, BOP 1X0

The southeastern Cape Breton Highlands are composed of an eastern belt of predomin antly granitoid rocks and a western belt of dioritic, granitoid and stratified rocks. The oldest unit is inferred to be the Cheticamp Lake gneiss, which outcrops in the western belt. This is in faulted contact with the most extensive stratigraphic unit, the McMillan Flowage Formation, which occurs throughout the western belt. It repre sents early Paleozoic or older continental margin accumulations of sandy and silty clastic sedimentary, volcanic, volcaniclastic and rare calcareous rocks. These strat ified units have been folded and intruded by late Precambrian(?) to lower Paleozoic granitoid and dioritic plutons, and then have undergone a later strike-slip deformational event. In the eastern belt, the Barachois River unit preserves a section of siltstones and greywackes which have been largely metamorphosed to gneisses, and the Price Point unit includes a small section of volcanic rocks. A single pervasive foliation has been recognized in the eastern belt.

The two belts appear to represent components of two tectonostratigraphic zones, the western belt being part of the Highlands zone, and the eastern belt part of the Bras d'Qr zone of Cape Breton Island. The western belt displays a pronounced Devonian thermal overprint, whereas the eastern belt has preserved late Precamhrian or Siluro- Qrdovician ages. Neither belt can readily be correlated with the Southeastern zone of Cape Breton Island or the Avalon Zone in Newfoundland. Based on lithologic similarit ies, both in the stratified units and in the nature of the plutonic rocks, the western belt (Highlands zone) can be correlated with the Gander zone. The eastern belt (Bras d'Qr zone) is a possible manifestation of the Avalon zone at a deeper crustal level.

Le sud-est des Hautes-Terres du Cap-Breton se compose d'une chaine orientale forroAe surtout de roches granitoides ainsi que d'une ceinture occidentale montrant des termes dioritiques, granitoides et stratifies. On croit que le gneiss de Cheticamp Lake, qui affleure dans la chaine occidentale, est 1'unite la plus vieille. II est en contact par faille avec 1'unite stratigraphique la plus etendue, f.e. la Formation de McMillan Flowage qui se presente d'un bout A 1'autre de la ceinture occidentale. Cette demiere represente 1'accumulation, au debut du PalAozoique voire plus t3t, de roches terrigenes sableuses et limoneuses, volcanites, volcaniclastites et de rares roches calcaires sur la marge continentale. Ces unites stratifiees ont ete plissees et injecte.es de plutons granitoides et dioritiques tardi-precambrien (?) A Ao- palAozoiques. Elies ont par la suite subi un episode de decrochement. Dans la chaine orientale, l'unite de Barachois River preserve une assise de siltstones et de grauwackes qui ont ete en grande partie metamorphises en gneiss, alors que l'unite de Price Point comprend une petite section de volcanites. On a reconnu une seule foliation penetrante dans la chaine orientale.

Les deux chalnes semblent faire partie de deux zones tectonostratigraphiques; la ceinture occidentale appartiendrait A la zone de Highlands tandis que la ceinture orientale ferait partie de la zone de Bras d'Qr de 1'ile du Cap-Breton. La chaine occidentale accuse d'un episode thermique dAvonien prononce alors que la ceinture orientale a preserve des ages tardi-prAcambriens ou siluro-ordoviciens. La correlation de l'une ou 1'autre de ces chaines avec la zone sud-est de l'ile du Cap- Breton ou la zone d'Avalon de Terre-Neuve est malaisAe. Si l'on en juge par leur similitudes lithologiques, tant en ce qui regarde les unites stratifiAes que la nature

MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 22, 264-277 (1986)

Received April 29, 1986 0711—1150/86/030264-14$3.10/0 Revision Accepted August 4, 1986 265 Raeside and Barf:

des roches plutonlques, on peut corrAler la chalne occldentale avec la zone de Gander. La chalne orientale (zone de Bras d'Or) pourralt reprdsenter la zone d'Avalon A un niveau cortical plus profond. [Tradult par le journal]

INTRODUCTION

The Cape Breton Highlands occupy a significant situation in the Appalachian Orogen, between the larger and more intensely studied areas in Newfoundland and mainland North America. In the southwestern corner of Newfoundland, 150 km distant from the Cape Breton Highlands, four major tectonostratigraphic terranes (Williams and Hatcher 1983) of the Appalachian Orogen converge in an area of poorly defined tectonic relationships (Fig. 1). A fifth terrane, the Avalon, occurs 200 km to the east on the south areas after Williams (1979). Area of study shown ern coast of Newfoundland. in stippled box. The correlation of these tectono stratigraphic terranes identified in by a radial drainage system. Around Newfoundland with Paleozoic and Pre- the perimeter of the plateau, coastal cambrian crystalline rocks in Cape lowlands are underlain by Carboniferous Breton Island has proved to he diffi sedimentary rocks, and areas of exten cult (e.g. Barr and Raeside, this sive outcrop of crystalline rocks are issue, Williams and Hatcher 1983, limited to rivers and streams. There Keppie 1982). This has been in part is good access to the top of the plat due to limited mapping, especially of eau in the southern Highlands using the Cape Breton Highlands, and a lack private road networks of lumber and of radiometric or paleontological age power companies. data, and in part due to differences in Previous maps of the southern Cape the character of the rocks. Most Breton Highlands (MacLaren 1956a, authors have considered that all of 1956b, Kelley 1957, 1960, Murray 1977) Cape Breton Island belongs to the are of a reconnaissance nature and Avalon Terrane, based primarily on the serve to outline the approximate posi similarities of lithologies in the tions of major rock types. No detailed southeastern part of the island to descriptions of stratigraphy, struc those of the Avalon Peninsula of ture, plutonism or metamorphism were Newfoundland (O'Brien et a/. 1983, Rast provided. These maps have been sum and Skehan 1983). However, the largest marized in a regional compilation of block of crystalline basement rocks in the geology of Nova Scotia (Keppie Cape Breton Island, the Cape Breton 1979). Recent geological mapping has Highlands, preserves lithologies dis redefined the lithologic types and tinctly different from those of south their distribution in the southern half eastern Cape Breton Island and has the of the Highlands (Jamieson and Craw potential for providing evidence of the 1983, Jamieson and Doucet 1983, 1984, nature of either the deeper levels of Raeside et a/. 1984, Barr et a7. 1985). the Avalon crust or of a separate The southeastern Highlands (Fig. terr a n e . 2) incorporate two major north-south The Cape Breton Highlands form a trending belts of different lithologi plateau, with an elevation of about 400 cal character. The eastern belt to 500 m, which has been deeply incised includes a variety of granitoid rocks 266 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY

Carboniferous sedimentary rocks

granitoid & dioritic rocks, undifferentiated gneisses

upper clastic division

marble division McMillan middle clastic division Flowage Formatic quartzite division

lower clastic division

Barachois River gneiss

W V W V T v VVVVVVVV VVVVVVVV Price Point unit vvvvvvvv v v v v v v w

Cheticamp Lake gneiss

boundary between eastern 8 western belts

Fig. 2. Simplified geological map of the southeastern Cape Breton Highlands. GFD - Gisborne Flowage diorite, TLD - Timber Lake diorite; CM Bk - West Branch Christopher MacLeod Brook. and subordinate isolated blocks of paper are firstly to define and des gneissic, volcanic and metasedimentary cribe the most extensive stratified units. The western belt includes a unit, the McMillan Flowage Formation, variety of presumed Precambrian diori and to describe more briefly the other tic and granitoid rocks and a contin stratified units of the southeastern uous belt of metasedimentary lith Highlands. Secondly, the paper aims to ologies. The primary purposes of this interpret the tectonic history which 267 Raeside and Barr

these rocks, together with the adja H.W. Krueger, written communication, cent plutonic units, have undergone and 1985). The age of the McMillan Flowage to propose a tectonic model for the Formation is therefore inferred to be development of the southeastern Cape late Precambrian but the radiometric Breton Highlands. controls are equivocal. The formation is subdivided into STRATIGRAPHY five lithologic divisions (Figs. 2 and 3), all of which can be seen in the Four major stratified units are vicinity of McMillan Flowage. Although recognized in the southeastern the rocks have been subjected to exten Highlands, but only one, the McMillan sive deformation, and the original Flowage Formation, has been mapped in contacts between the divisions are sufficient detail to assign a formal largely unknown, the continuity of the name. divisions along strike, and consistent younging directions indicate that 1. McMillan Flowage Formation structural repetition is relatively minor on the small to medium scale The most extensive stratified unit (less than 1 km). Sufficient marker in the southeastern Highlands, the horizons exist to outline the position McMillan Flowage Formation, outcrops in of large scale folds, and these can be a belt extending from Clybum Brook in the north to Baddeck River in the mcmillan flowage formation south, a strike length of 60 km. The rock types are predominantly clastic, but include substantial amounts of upper clastic mafic rocks, at least some of which are division volcanic, and rare calcareous rocks. The age of the McMillan Flowage marble division Formation is not known precisely. In the south it is unconformably overlain E by Carboniferous rocks of the Horton o o Group. It has been intruded by a o V middle clastic variety of dioritic and granitoid division bodies, some of which have been dated a by radiometric methods. In the Gisborne O Flowage area, an intrusive contact was E observed, with xenoliths of semipelite of the middle clastic division of the quartzite formation incorporated in the Gisborne division Flowage diorite which has yielded a KAr date on hornblende of 417 ± 16 Ma lower clastic (H. W. Krueger, written communication, division 1985). The age of the intrusion, how ever, may be substantially older. The McMillan Flowage Formation has also legend been intruded by the Ingonish River petite Tonalite which has yielded ages of 352 semipelite ± 13 M a (Rb-Sr whole rock; J. psammlte, quartzite Blenkinsop, written communication, marble, calc-silicate 1985) and 420 to 485 Ma (K-Ar, whole amphibolite rock; A. Hayatsu, written communi cation, 1984) and by the New Glen Suite Fig. 3. Schematic stratigraphic section of the which has yielded a somewhat dubious McMillan Flowage Formation. The width of the column is a general indication of average grain age of 708 ± 53 Ma (Rb-Sr, whole rock; size. 268 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY confirmed by consistent graded and b. Quartzite division cross bedding observations. Original stratigraphic thicknesses Conformably overlying the lower are difficult to determine because of clastic division is the quartzite divi the extensive folding - thicknesses sion. It underlies a prominent north quoted below are present structural erly trending ridge, 30 km long and up thicknesses. to 1500 m wide, which tapers both to the north and south. As the rocks have a. Lower clastic division a near-vertical dip, the quartzite division is inferred to have a maximum The lower clastic division is thickness of 1500 m. The rocks are recognized only to the south of typically monotonous medium grained McMillan Flowage. It is particularly quartzites, but include arkosic and well exposed in the headwaters of psammitic varieties, which are finely Goose Cove Brook, where an 800 m thick bedded and preserve evidence of cross section has been preserved displaying stratification. Mafic layers within peak metamorphic conditions in the the quartzite can be traced laterally greenschist facies. Small sections of for up to 2 km. In the McMillan Flow- the lower clastic division have also age area, the quartzite division thins been mapped in the central part of the and pinches out, and a conformable area (West Branch Indian Brook and stratigraphic relationship is inferred Barachois River) where metamorphic between the lower clastic division and grade is In the sillimanite zone. the middle clastic division. Else Because of this relatively wide range where, the quartzite division passes of metamorphic conditions, the aspect abruptly, but conformably, up into the of these rocks varies considerably. In lower semipelites of the middle clastic the southern sections, they are a division. series of quartzites, semipelitic phyllites and black slates at the c. Middle clastic division bottom and garnet phyllites interbedded with psammites at the top. The middle clastic division is the Mafic layers occur sporadically thickest division of the McMillan Flow- throughout the unit, but are not age Formation, ranging up to 2000 m laterally persistent. Small outcrops thick in the McMillan Flowage area. of calc-silicate rocks were observed in Like the lower clastic division, the Barachois River in an area previously appearance of the rocks varies consid mapped as marble (Murray, 1977; Keppie, erably depending on metamorphic grade 1979). Marble horizons, up to 1 to 2 which ranges from chlorite in the south m thick, outcrop in the lowermost parts to sillimanite in the vicinity of of the division (Goose Cove Brook to McMillan Flowage. Complete exposure of MacDonald Brook area). On the basis of a lower part of the division is found poorly preserved fining upwards struc in the spillway cut at McMillan Dam, tures and consistently oriented tec and the middle and upper parts of the tonic structures, all sections of the division are moderately well exposed lower clastic division are inferred to along logging roads south of McMillan young from east to west. Nowhere has Flowage. In the south, the most com the base of the division been identi plete sections of the division occur in fied - its eastern margin has been easterly flowing tributaries of West intruded by the granitoid rocks of the Branch Christopher MacLeod Brook. Goose Cove Brook and Cross Mountain In the McMillan Flowage area, the suites, and the Timber Lake diorite. lower part of the division 269 Raeside and Barr

(approximately 500 m thick) is composed d. Marble division of pelitic schists and semipelitic gneisses with thin layers or boudins of The marble division is a thin, but amphibolites and calc-silicate rocks. laterally persistent, horizon which is Many of the clastic layers show mig- an important marker in the upper part matization, and in this area the divi of the McMillan Flowage Formation. It sion has been intruded by at least outcrops in two areas - between three phases of cross-cutting veins and McMillan and Gisborne Flowages, and on granitoid dykes. At lower metamorphic the ridge and brook north of Ingonish grade in Clyburn and Christopher River. In both areas the division is MacLeod Brooks, the lower part of the characterized by a 25 to 35 m thick division occurs as well bedded green- marble and calc-silicate horizon, with grey to black slates, phyllites and 1 to 2 m thick amphibolite layers garnet phyllites with common thin mafic included in it, overlain by a 25 to laye r s . 70 m thick quartzite or feldspathic The middle part of the division quartzite. The most complete exposure (approximately 400 m thick) is composed of the calcareous part of the marble of a series of quartzites and amphibo division occurs in an access ramp to an lites. Individual beds of quartzite adit to an aqueduct between McMillan range from 40 to 150 cm thick, and are and Gisborne Flowages. A detailed very variable in their appearance - description of this outcrop has been blue-white and rusty weathering ortho made by Davis (1983). Correlation with quartzites, micaceous, feldspathic and the marble division in other locali graphitic quartzites, and fine to ties, and detailed examination of the coarse grained varieties of quart structural geology, has indicated that zite are present. Quartzite layers this outcrop preserves a duplicated range from 15 to 60 m thick and are section of the division, folded about inter layered with 25 to 40 m thick a southwesterly plunging minor syncline amphibolite layers. Individual amphi in the core of a large synclinorial bolite layers can be traced for up to 4 structure. km, but do not appear to be correlable throughout the middle clastic division. e. Upper clastic division At lower metamorphic grades quartzites are interlayered with chloritic Small portions of the upper clas schists. tic division are preserved in the core The upper part of the middle clas of a large synclinorial structure in tic division is poorly exposed and the Gisborne Flowage and Ingonish River outcrops on logging roads southwest and areas. The top of the division has east of McMillan Flowage. Its total been removed by the intrusion of the thickness is about 1100 m, assuming no Gisborne Flowage diorite and Ingonish repetition by folding. It is composed River Tonalite, and a maximum thickness of a series of semipelitic schists with of 500 m is preserved. The base of the discontinuous quartzite and amphibolite upper clastic division is gradational layers. A thick amphibolite unit, from the marble division, and consists about 200 m thick, occurs near the of interlayered quartzites, feldspathic middle of the section and can be traced quartzites and semipelites. Further for distances of 8 and 6 km in two up-section, thin, lensoid amphibolites separate areas. The upper part of the occur in a predominantly semipelitic division is not found in the south sequence. The division has everywhere where it has been cut out by the intru undergone metamorphism to the middle sion of the New Glen suite. amphibolite facies. MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 270

Largely on the basis of limited gneisses occurs (Fig. 2). The major reconnaissance mapping and correlation rock type is a monotonous felsic across areas of Carboniferous cover biotite-K-feldspar gneiss, with some much of the area now recognized to be garnet-bearing horizons. The gneiss is underlain by the McMillan Flowage Form monzogranitic in composition, medium ation has been previously assigned to grained and moderately to poorly the George River Group (Murray, 1977; foliated. It contains lenses of peli Milligan, 1970; Keppie, 1979; Wiebe, tic material - some as migmatized bio- 1972). In other parts of Cape Breton tite schists, others as pure biotite Island, however, the George River Group (or biotite and muscovite) rocks. is characterized by thick sequences of Lenses of syenogranitic gneiss also carbonates and quartzites (Milligan, occur in the unit. Possibly infolded 1970), whereas the McMillan Flowage into the predominantly orthogneiss is a Formation is predominantly semipelitic 2 km wide pelitic gneiss belt which in composition with psammitic, quartz- contains garnet, K-feldspar and itic, pelitic and mafic components, and sillimanite-bearing lithologies. Map only very minor carbonates. The George ping has proceeded in this unit only to River Group (carbonate-quartzite the northern edge of the map area in association) outcrops only in central Figure 2 and it is likely that the Cape Breton Island and there are no unit underlies areas to the north. extensive areas of outcrop of pre- Mapping has not been sufficiently Carboniferous carbonate rocks in the detailed to establish a stratigraphic Cape Breton Highlands. Correlation order, and the entire unit is informal between the McMillan Flowage Formation ly termed the Cheticamp Lake gneiss. and the George River Group therefore The relationship between the appears tenuous. If the McMillan Flow- McMillan Flowage Formation and the age Formation is time-equivalent with Cheticamp Lake gneiss is not precisely any portion of the George River Group, known. South of the Clyburn Brook it must represent a deeper water clas Fault, the contact between them is in a tic deposit with a more significant 100 to 500 m wide shear zone charac volcanic component. terized by chloritic felsic myIonites. The correlation of volcanic and North of the fault, the contact is not volcaniclastic layers in the northern exposed, but can be limited to a 150 m part of the area with the volcanic and wide interval of no outcrop across sedimentary Fourchu Group of south which metamorphic grade increases from eastern Cape Breton Island (Wiebe, the sillimanite and muscovite zone to 1972; Keppie, 1979) is yet more ten the sillimanite and K-feldspar zone. uous. There appears to be no separa The relationship between the tion between the volcanic and volcani Cheticamp Lake gneiss and other clastic layers and the clastic sedimen gneissic units in the Cape Breton tary layers in the Clyburn Brook area - Highlands is as yet unknown. the two are largely interbedded with each other throughout the section. 3. Price Point unit Metamorphic grade increases gradually from chlorite grade to sillimanite A sequence of volcanic rocks des grade up the Clyburn valley, and the cribed by Macdonald and Barr (1985) as entire section has been included in the the Price Point unit outcrops in the middle clastic division of the McMillan St. Anns Harbour area (Fig. 2). The Flowage Formation. unit is mostly fault-bounded, although it has been intruded by the Indian 2. Cheticamp Lake gneiss Brook granodiorite which in turn has been intruded by the St. Anns leuco- In the northwestern part of the granite which has yielded a Cambrian study area, a belt of ortho- and para- Rb-Sr age (Cormier 1980), indicating a 271 Raeside and Barr probable Precambrian age for the Price Kellys Mountain block (Barr et a l. Point unit. The Price Point unit is 1982). composed of intermediate crystal and The Barachois River gneiss occupies lithic-crystal tuffs and andesitic- a critical position in the recently dacitic flows of calc-alkalic chemical proposed tectonostratigraphic subdivi affinity. sion of Cape Breton Island (Barr and Based on similarities in geochem Raeside, this issue), lying at or near istry and mineralization, Macdonald and the boundary between the Bras d'Gr and Barr (1985) considered the volcanic Highlands zones. To the west, both rocks of the Price Point unit to be plutonic and stratified rocks preserve equivalent to the Fourchu Group of evidence of Devonian thermal activity southeastern Cape Breton Island. How in the form of extensive development of ever, more recent tectonostratigraphic retrograde muscovite and epidote and analysis (Barr and Raeside, this issue) 40Ar/3,Ar dates on hornblende places the Price Point unit in the Bras (Johnston, 1984) whereas K-Ar dating of d'Gr zone of Cape Breton Island, and rocks to the east has indicated that does not correlate the unit with the there has not been any significant Fourchu Group in the Southeastern zone. thermal activity since the beginning of More radiometric age data are required the Cambrian (Macdonald and Barr, to resolve this problem. 1985). The Barachois River gneiss may therefore represent the remains of 4. Barachois River gneiss and related oceanic or plate margin sedimentary rocks rocks preserved in the predominantly intrusive terrane of this part of the A number of lenses of predominant Highlands. ly gneissic rock occur in the otherwise plutonic eastern half of the map area. STRUCTURAL GEOLOGY Particularly well exposed sections exist in Timber Brook and Barachois The structure of the southeastern River, but similar rocks are also found Highlands is relatively complex, at the confluence of East and West although individual outcrops typically Branches Indian Brook, and south of display simple patterns. Major trends Goose Cove Brook. To the north, near vary from northerly in the south to Ingonish, a block of gneisses may be northeasterly in the McMillan and part of the same unit, and yet further Gisborne Flowages area. In the Clybum north, the Coastal Gneiss of Wiebe Brook-Ingonish River area, trends are (1972) also shows similarities to it. erratic. Structural measurements were In all these sections except the made in the intrusive units as well as Coastal Gneiss, the blocks of gneiss in the stratified units and the two occur as screens between various plu generally show parallel patterns. The tonic bodies. McMillan Flowage Formation is particu The Barachois River gneiss is larly important in the structural typically composed of dark grey, K- analysis as it extends throughout the feldspar augen gneisses, although more study area and contains lithologies psammitic and some amphibolitic varie suitable for the development of a pen ties have been noted. From Goose Cove etrative fabric as well as marker hori Brook metamorphic grade decreases to zons to permit the construction of the south, through zones of sillimanite structural cross-sections and the schist, garnet phyllites and biotite analysis of the large scale structures. siltstones to weakly metamorphosed The map area has been divided into greywackes which are exposed in the Big seven structural domains, depending on Hill inlier near Baddeck. These are the major rock types and dominant fol likely correlative with the greywackes iation trends. Three domains are recog exposed in the southern end of the nized in the McMillan Flowage Forma MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 272 tion (north, central and south), and folds (Fig. 5c). Mesoscopic folds are two in the plutonic and metaraorphic moderately inclined to upright, near- rocks to the east of the McMillan Flow- isoclinal folds which plunge at 30° to age Formation. In addition the 45° to the southwest (Fig. 5d). The Cheticamp Lake gneiss and the plutonic absence of refolded folds implies a rocks west of the McMillan Flowage relatively simple single-stage early Formation are considered as two deformational history. separate domains. 2. Northern Domain McMillan Flowage Formation In the area between Gisborne Flow- 1. Centra1 Domain age and Clyburn Brook, the map pattern of the marker marble division is more In the area of McMillan and complex. Foliation trends are essen Gisborne Flowages, the marble division tially parallel to those in the central of the McMillan Flowage Formation out domain except in the extreme northeast lines a series of near-isoclinal folds. where the structures have an east-west The stratigraphic arrangement indicates orientation (Fig. 5a). Although very that these folds are the core of a 5 km few minor fold hinge lineations were wide synclinorial structure (Fig. 4a). noted, those measured indicate a near The foliation is axial planar to the horizontal structure (Fig. 5b). In

INGONISH RIVER AREA

Fig. 4. Vertical structural cross-sections across the McMillan Flowage Formation illustrating the style of synclinorial folding in the northern and central structural domains. 273 Raeside and Barr

FOLIATION LINEAT/ON

Fig. 5. Summary of structural data for the stratified units of the southeastern Cape Breton Highlands. Figures a to f — McMillan Flowage Formation, g and h - Cheticamp Lake gneiss. Figures a, c, e and g are plots of poles to foliations, figures b, d, f and h are plots of lineations (all minor fold hinge lineations, except figure h, which includes rodding lineations). Contour intervals are indicated below stereographic projections, n - number of data points. vertical section, the marble division to dip gently to the north. The more clearly outlines a series of synclines complex behaviour of these rocks is but with some of the intervening anti likely related to greater competence clines sheared out. A thrust fault contrasts between the layers as deform with a southerly transport direction, ation proceeded at lower metamorphic which outcrops in Power Brook, appears grade conditions. MARITIME SEDIMENTS AND ATLANTIC GEOLOGY 274

3. Southern Domain by almost all the plutonic rocks east and west of the McMillan Flowage Forma In the area south of the prominent tion. Foliations from these are plot flexure in the trend of the map units, ted on Fig. 6, together with the lim south of McMillan Flowage, a single ited number of measurements from the stratigraphic section of the lower Barachois River gneiss. The foliations part of McMillan Flowage Formation is are parallel to those of the adjacent preserved with 30 km of lateral extent. stratified rocks, superficially indi Based on correlation with the central cating a similar deformational history. domain, this is interpreted to be the However, some anomalies exist, for eastern limb of the synclinorial struc ture. Foliations and bedding orienta tions are more northerly (Fig. 5e) but the minor fold hinges cluster around vertical (Fig. 5f). Minor folds are developed on scales ranging from 2 cm wavelength to several metres wave length. Because of the lack of late- recrystallizing micas, finer scale folds tend to be best preserved in quartzites. The origin of the ver tically plunging minor folds is enigma tic. They are predominantly sinistrally asymmetric (S-shaped) when viewed down-plunge, although within individual outcrops, both sinistral and dextral asymmetry can be found. The presence of two orientations of vergence implies that two phases of co-planar deforma tion exist, the first presumably being the one responsible for the large- scale folding, and the second over Fig. 6. Summary of foliation orientations in the plutonic rocks and Barachois River unit. Fig. a, printing the earlier structures with plutonic rocks east of central domain (McMillan the vertical minor fold hinges. The Flowage Formation); b, plutonic rocks (and great lateral extent of consistently Barachois River gneisses) east of southern oriented minor structures and the 50 to domain; c, plutonic rocks west of southern domain. 60° change in orientation of plunge between the central and southern example the Gisborne Flowage diorite. domains indicates that this major This pluton is particularly significant structure formed as a neutral fold as it has been intruded into the core rather than a moderately plunging fold, of the synclinorium, but it has not subsequently re-oriented. A possible been deformed by that folding because means of generating the vertically its outline is clearly cross-cutting plunging fold structures and the 30 km with respect to the internal boundaries amplitude neutral fold is strike-slip in the McMillan Flowage Formation. The movement in a north-south direction. pluton, however, displays a tectonic The western limb of this fold has been foliation which is parallel to the removed by the intrusion of the Kathy foliation in the McMillan Flowage Road diorite and the New Glen suite Formation, indicating a two phase de (Fig. 2). formational history - an earlier event responsible for the large-scale fold Plutonic rocks ing, and a later event which enhanced the axial planar foliation, and im A tectonic foliation is exhibited parted a foliation to the diorite. As 275 Raeside and Barr the two events are coplanar, they are press). This is also manifested by the probably the product of a single oro- extensive development of retrograde, genic event and the intrusion of the but coarse grained, muscovite in alumi diorite was penecontemporaneous with nous rock types and epidote in felsic the second phase of the deformation. lithologies. These effects have not been detected in the eastern part of Cheticamp Lake gneiss the southeastern Highlands, where the rocks have retained late Precambrian or A poorly to moderately developed Siluro-Ordovician ages (Barr et a l. foliation (oriented parallel to the 1985), and lack extensive development foliation in the McMillan Flowage of retrograde metaraorphic minerals. Formation) exists in the Cheticamp Lake gneiss (Fig. 5g). Because of the DISCUSSION Absence of marker horizons, no larger folds have been recognized, although a The various stratified units of broad pelitic belt may be an infolded the southeastern Cape Breton Highlands section. The orientation of minor fold represent a range of depositional sit hinge and rodding lineations is more uations , possibly separated both in complex than in other domains (Fig. time and in space. On the basis of the 5h), suggesting a more complex early preservation of textures which appear history. In addition, some garnet to predate the main northeasterly porphyroblasts from the pelitic section trending foliation of the area, the of tie gneiss preserve a crenulated oldest unit is inferred to be the Chet internal schistosity. These indicate icamp Lake gneiss. However, insuffi that the early deformational history of cient mapping has as yet been done in the Cheticamp Lake gneiss is more com this unit to correlate it with any plex than that of the McMillan Flowage other similar body in the Cape Breton Formation and therefore that the Cheti Highlands or to postulate its origin. camp Lake gneiss may have undergone The preservation of mixed ortho- and deformation before the deposition of paragneisses in the Cheticamp Lake the McMillan Flowage Formation. gneiss implies that its early history involved both intrusive and deposi Younger deformation tional events, before the probable late Precambrian deposition of the McMillan Later deformational effects on the Flowage Formation. rocks of the southeastern Highlands The McMillan Flowage Formation is appear limited to normal faulting and a predominantly clastic unit, incorpor to open folding, which has not signifi ating large amounts of semipelitic and cantly affected the outcrop patterns of psamraitic or quartzitic material, and the crystalline rocks. The surrounding lesser amounts of pelitic, mafic and Carboniferous sedimentary rocks have calcareous lithologies. Only in the undergone moderate deformation, pro extreme northeastern sections (Clybum ducing folded (in places overturned) Brook area) is the metamorphic grade and faulted structures (Kelley, 1957, sufficiently low to observe many of the 1960). This late deformation may also original depositional features, for be responsible for the normal faulting example laminar sedimentary structures, and open folding of the crystalline volcaniclastic breccias and tuffs, and rocks of the Highlands. aphanitic flows. The preponderance of A strong thermal overprint is medium to coarse clastic material responsible for Devonian K-Ar, throughout the formation, its great 4,Ar/**Ar and Rb-Sr ages in the thickness (at least 4000 m), and its granitoid rocks of the western part of considerable lateral continuity imply the area, and elsewhere in the Cape that it was deposited at a relatively Breton Highlands (Jamieson et a l. in stable continental margin. The numer- 276 MARITIME SEDIMENTS AND ATLANTIC GEOLOGY

rous quartzite layers, including the a core of gneisses (for example, the thick quartzite division, may represent Cheticamp Lake gneiss) flanked by lower periodic increased sediment supply, and grade stratified units (for example, the rare calc-silicate and marble hori the McMillan Flowage Formation). These zons may represent periodic shallowing rocks have been intruded by a variety and carbonate deposition. The mafic of granitoid rocks, similar to those of rocks which occur throughout the forma the Gander Terrane of southwestern tion have not yet been studied in de Newfoundland (Barr and Raeside, this tail. Although some are recognized as issue; Chorlton and Dallmeyer, 1986). eruptive in the northeastern part of The Bras d'Or zone, to the southeast, the formation, it is possible that many is characterized by pre-700 Ma gneissic of these rocks may be intrusive. basement (Olszewski et a l. 1981; The Price Point unit and Barachois Gaudette et a l. 1985), overlain by River gneiss have not been studied in carbonate and clastic rocks (for sufficient detail to determine their example, the George River Group) with environments of deposition. The Price minor volcanic rocks. It has been Point unit is volcanic and volcaniclas- assumed to underlie the volcanic rocks tic with calc-alkalic affinities of southeastern Cape Breton Island (for (Macdonald and Barr, 1985) and may example, Rast and Skehan, 1983), represent active orogenic volcanism although this relationship now appears during the late Precambrian. Based on uncertain (Barr and Raeside, this the correlation of the Barachois River issue). This zone cannot be easily gneiss with the lower grade meta- assigned to either the Gander or Avalon siltstones of the Big Hill area, and Terranes. Keen et a l. (1986) reported the correlation of these rocks to those that the Dover-Hermitage Bay Fault on Kellys Mountain, the Barachois which separates these terranes in River unit appears to have originated Newfoundland is a major crustal discon as a thick section of siltstones and tinuity. However, although no fault greywackes, typical of continental mar has yet been recognized in the south gin sedimentation. eastern Highlands, the juxtaposition of The stratigraphic continuity of the eastern and western belts is the McMillan Flowage Formation from at thought to have been effected by least the latitude of Clyburn Brook sinistral strike- slip faulting. south to the southern margin of the Sheared rocks have been observed at two Cape Breton Highlands is particularly localities on the western margin of the useful in the regional analysis of the Barachois River gneiss and the effect role of the southeastern Highlands of the fault is also demonstrated by during the late Precambrian and Paleo the sinistrally vergent minor struc zoic tectonic events. Deformation of tures and neutral folding of the this unit has been effected in two McMillan Flowage Formation. stages - an early folding stage result ing in the extensive synclinorial ACKNOWLEDGEMENTS structure, and later sinistral strike- slip tectonic activity, affecting only We acknowledge with thanks the the southern part of the area. contributions made by other workers in The southeastern Highlands strad the Cape Breton Highlands, in particu dle the boundary between two of the lar Alan Macdonald, who participated in tectonostratigraphic zones identified much of the mapping. We also appre in Cape Breton Island (Barr and Raeside, ciate the co-operation of the Nova this issue). The boundary between Scotia Power Corporation in the use of these zones is placed at the western their private road network and Parks margin of the belt of outcrops of the Canada for access to, and sampling Barachois River gneiss. The Highlands privileges in, the Cape Breton High zone, to the west, is characterized by lands National Park. To R.A. Jamieson 277 Raeside and Barr and G. Pe-Piper we express our appre area of mainly dioritic rocks in the southern Cape Breton Highlands, N.S. Unpublished B.Sc. ciation for suggestions for improve thesis, Dalhousie University, 134 p. ments from a previous version of the KEEN, C.E., KEEN, M.J., NICHOLS, B.. REID, I.. manuscript. This work was funded by ST0CKMAL, G.S.. COLMAN-SADD, S.P., O'BRIEN, Natural Sciences and Engineering S.J., MILLER, H.. QUINLAN. G.. WILLIAMS, H. and Research Council operating grants to WRIGHT, J. 1986. Deep seismic reflection pro file across the northern Appalachians. Geolo both authors. gy. 14, pp. 141-145. KELLEY, D.G. 1957. Baddeck, Nova Scotia. Geo BARR. S.M., JAMIESON, R.A. and RAESIDE, R.P. logical Survey of Canada, Map 14-1956. 1985. Igneous and metamorphic geology of the KELLEY, D.G. 1960. St. Anns, Nova Scotia. Geo Cape Breton Highlands. Geological Association logical Survey of Canada, Map 38-1960. of Canada/ Mineralogical Association of KEPPIE, J.D. 1979. Geological map of the Prov Canada, Joint Annual Meeting, Excursion Guide ince of Nova Scotia. Nova Scotia Department of 10. 48 p. Mines and Energy, scale 1:500,000. BARR. S.M., O ’REILLY, G.A. and O'BEIRNE, A.M. KEPPIE, J.D. 1982. Terrenes in the northern 1982. Geology and geochemistry of selected Appalachians. Nova Scotia Department of Mines granitoid plutons of Cape Breton Island. and Energy, scale 1:5,000,000 Nova Scotia Department of Mines and Energy, MACDONALD, A.S. and BARR. S.M. 1985. Geology and Paper 82-1, 176 p. age of polymetallic mineral occurrences in BARR, S.M. and RAESIDE, R.P. 1986. Tectono- volcanic and granitoid rocks, St. Anns area. stratigraphic subdivision of Cape Breton Cape Breton Island, Nova Scotia. Geological Island. Maritime Sediments and Atlantic Survey of Canada, Paper 85-1B, pp. 117-124. Geology, 22, pp. 252-263. MACLAREN, A.S. 1956a. Ingonish. Geological Sur BARR, S.M., RAESIDE, R.P. and MACDONALD, A.S. vey of Canada, Preliminary map 55-35. 1985. Geological mapping of the southeastern MACLAREN, A.S. 1956b. Cheticamp River. Geologi Cape Breton Highlands, Nova Scotia. In Current cal Survey of Canada, Preliminary map 55-36. Research, Part B, Geological Survey of Canada, MILLIGAN, G.C. 1970. Geology of the George Paper 85-1B, pp. 103-109. River Series, Cape Breton. Nova Scotia Depart CHORLTON, L.B. and DALLMEYER. R.D. 1986. Geo ment of Mines and Energy, Memoir 7, 111 p. chronology of early to middle Paleozoic tec MURRAY, D.L. 1977. The structural relationship tonic development in the southwest Newfoundland between rocks of the George River and Fourchu Gander Zone. Journal of Geology, 94, pp. 67- Groups in the Ingonish River-Clyburn Brook 89. area, Cape Breton Island, Nova Scotia. Unpub CORMIER, R.F. 1980. New rubidium/strontium ages lished M.Sc. thesis. Queen’s University, Kings In Nova Scotia. 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