Stratigraphy and tectonic significance of Cretaceous ...

Stratigraphy and tectonic significance of Cretaceous volcanism in the Queen Elizabeth Islands, Canadian Arctic Archipelago1 ASHTONF. EMBRY AND KIRKG. OSADETZ Institute of Sedimentary and Petroleum Geology, Geological Survey of Canada, 3303-33rd St. NW, Calgary, Alta., Canada T2L 2A7 Received June 9, 1987

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Revision accepted November 5, 1987 Cretaceous volcanic rocks, which consist mainly of basalt flows and pyroclastic rocks, occur on northern Ellesmere Island, Axel Heiberg Island, and northernmost Amund Ringnes Island as part of the Sverdrup Basin succession. Volcanic rocks are associated with each of four regional transgressive -regressive (T -R) cycles that constitute the Cretaceous clastic succession of Sverdrup Basin and are of Valanginian - early Barremian, late Barremian - Aptian, latest Aptian - early Cenomanian, and late Cenomanian - Maastrichtian age; the volcanic component of each increases northward. The centre of volcanism appears to have been north of Ellesmere Island and is interpreted as the site of a mantle plume that was active throughout the Cretaceous. Most of the volcanic activity took place from Hauterivian to early Cenomanian (T-R cycles 1-3) and was accompanied by widespread sill and dyke intrusion. This activity coincided with the main rifting phase of the adjacent oceanic Canada Basin and with minor crustal extension in the Sverdrup Basin. From late Cenomanian to Campanian, volcanism was restricted to the extreme northeast, and trachytes and rhyolites were extruded along with basalts. This volcanic succession is interpreted as being the southern limit of Alpha Ridge, a major volcanic edifice that formed as a hot-spot track across Canada Basin during sea-floor spreading in Late Cretaceous. Les roches volcaniques crCtacCes, formCes principalement de coultes de basalte et de pyroclastites, qui affleurent dans le nord de l'ile Ellesmere, sur l'ile Axel Heiberg et sur l'ile Amund Ringnes ii llextrCme-nord, font partie de la successison lithologique du bassin de Sverdrup. Ces roches volcaniques se retrouvent dans chacun des quatre cycles de transgression-rkgression rkgionale (T-R) que comprend la successiosn clastique du bassin de Sverdrup; leurs Pges correspondent au Valaginien - BarrCmien hbtif, BarrCmien tardif - Aptien, fin de I'Aptien - CCnomanien hPtif et CCnonanien tardif Maastrichtien; dans chaque cycle la proportion en composante volcanique s'accroit en direction nord. Le volcanisme central se situait probablement au nord de I'ile Ellesmere, et il est considCrC comme le lieu d'ascension d'un panache mantellique qui fut actif durant tout le CrCtacC. La pCriode de volcanisme paroxysmal est apparue entre 1'HautCrivien et le CCnomanien precoce (cycles T-R 1-3), et elle fut accompagnCe par la formation de nombreux filons-couches et dykes. Cette activitk coincide avec le stade principal de distension du bassin ocCanique du Canada adjacent et avec autres distensions crustales mineures dans le bassin de Sverdrup. Durant l'intemalle CCnomanien tardif - Campanien, 1'activitC volcanique Ctait confinCe i I'extrCmitC nord-est, et des epanchements de trachyte et de rhyolite accompagnaient les coultes de basalte. Cette succession volcanique est interprCtCe comme dCfinissant la limite sud de la crCte Alpha, un Cdifice volcanique majeur qui reprksente un trace de point chaud qui a traversC le bassin du Canada durant l'expansion ocCanique au Crttack tardif. [Traduit par la revue]

Can. J. Earth Sci. 25, 1209-1219 (1988)

Introduction Volcanic rocks were first recognized in the Cretaceous succession of the Queen Elizabeth Islands during the Geological Survey of Canada's Operation Franklin in 1955 (Fortier et al. 1963). Over the last 30 years, numerous occurrences of Cretaceous volcanic rocks have been described (Fricker 1963; Souther 1963; Tozer 1963; Moore 1981; Balkwill 1983; Ricketts et al. 1985). Recently, these volcanic rocks have been related to the development of Canada Basin, the oceanic basin to the northwest of the Canadian Arctic Islands (Embry and Wall 1985; Sweeney 1985). This paper summarizes the stratigraphy of Cretaceous volcanic rocks in the Queen Elizabeth Islands. The data presented are derived from both published work and our own field studies, which have been carried out over the past 7 years. New data include recently discovered occurrences and more reliable age assignments for some of the major volcanic units. The implications of these data for interpreting the development of Canada Basin are also discussed. 'Geological Survey of Canada Contribution 12387. Printed in Canada / Imprime au Canada

The Cretaceous strata of the Oueen Elizabeth Islands are part of the Sverdrup Basin succession, representing the major depocentre in the Arctic Islands (Fig. 1) from Carboniferous to early Tertiary (Balkwill 1978). Stephenson et al. (1987) recognized four phases of basin development: (1) Carboniferous - Early Permian rifting, (2) Early Permian - earliest Cretaceous thermal subsidence, (3) Early Cretaceous - earliest Late Cretaceous rifting, and (4) Late Cretaceous - Eocene thermal subsidence. The final ~ h a s eis complicated in the eastern part of the basin by uplift and compression from latest cretaceous - Oligocene (Eurekan Orogeny). The tectonic break between phases 2 and 3 was previously recognized by Plauchut (1971) and Balkwill (1978) and coincides with the boundary between the Arctic-wide Ellesmerian and Brookian sequences of Lerand (1973). Mesozoic volcanism took place after this major tectonic realignment. Present data indicate that volcanism occurred between late Hauterivian and Campanian.

Stratigraphy The Valanginian to Maastrichtian stratigraphy of Sverdrup


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FIG.1. Queen Elizabeth Islands, Canadian Arctic Archipelago. Sverdrup Basin and the known extent of Cretaceous volcanic rocks are outlined. Outcrop localities: (1) Northern Amund Ringnes Island, (2) Strand Fiord - Expedition Fiord, (3) North Agate Fiord, (4) Li Fiord, (5) Bals Fiord, (6) Bunde Fiord, (7) Bjarnason Island, (8) Geodetic Hills, (9) Mokka Fiord, (10) Blue Mountains, (11) Emma Fiord, (12) Phillips Inlet, (13) Hansen Point, (14) Lake Hazen, (15) Piper Pass. Basin has been outlined by Tozer (1963, 1970), Balkwill (1978, 1983), and Embry (1985, in press). The succession is characterized by four basin-wide transgressive - regressive cycles (T-R cycles), each of which consists of a shale-siltstone lower portion and a sandstone-dominant upper portion (Fig. 2). The transgressive portion of each cycle is usually very thin, with the bulk of the cycle composed of a regressive succession of prodelta to delta-plain sediments. Sediment supply was mainly from the east and south, with much lesser input from the north (Embry, in press). Figures 2 and 3 illustrate the stratigraphic composition of the four T-R cycles. The first cycle is Valanginian to early Barremian in age and comprises the upper portion of the Deer Bay and Mackenzie King formations and the Paterson Island Member of the Isachsen Formation. The second cycle is of late Barremian to Aptian age and includes the Rondon and Walker Island members of the Isachsen Formation. The third cycle, which is of latest Aptian to early Cenomanian age, comprises the Christopher and Hassel formations over most of the basin. On western Axel Heiberg Island the Bastion Ridge (shalesiltstone) and Strand Fiord (basalt) formations occur at the top of the cycle. The uppermost cycle is of late Cenomanian to Maastrichtian age and, over most of the basin, includes the Kanguk and Expedition formations. On northwestern Ellesmere Island a unit of volcanic strata with clastic interbeds comprises this cycle. These strata are informally referred to as the Hansen Point volcanics (Trettin and Parrish 1987). Most of the significant occurrences of volcanic strata in the Sverdrup Basin are along the line of section illustrated in Fig. 3. Some of the notable stratigraphic features on this section include the following: (i) All four T-R cycles contain volcanic rocks that are mainly basalt flows. (ii) Within each cycle the volcanic component increases northward. (iii) For the first three cycles, the volume and geographic extent of volcanic strata increase for each succeeding cycle. (iv) The fourth

cycle contains the thickest known occurrence of volcanic rocks, but the areal extent of volcanic strata is the least. (v) On northwestern Ellesmere Island, a major unconformity is present beneath the fourth cycle, and the underlying three cycles are absent.

Volcanic rocks Cycle I. Valanginian - early Barremian Volcanic flows within this cycle are known from only two localities: Geodetic Hills on east-central Axel Heiberg Island and Bjarnason Island (Figs. 1,4a). At the Geodetic Hills locality a single basalt flow, 10.5 m thick, is interbedded with coarse-grained, fluvial sandstone of the Paterson Island Member. The flow is about 125 m below the Rondon Member, which is middle to late Barremian in age. Thus the flow is probably late Hauterivian or early Barremian in age. Farther northwest two basalt flows, each about 10 m thick, are present in the Paterson Island Member (Tozer 1963). The stratigraphic position of these flows also suggests a late Hauterivian or early Barremian age for the volcanism in this area. Cycle 2. Late Barremian - Aptian Volcanic rocks are much more widespread in this cycle than in the preceding cycle. They are present in the upper part of the Isachsen Formation (Walker Island Member in most places) in the following areas: (i) northwestern Axel Heiberg Island between Middle Fiord and Bunde Fiord (McMillan 1963; Tozer 1963; Fischer 1985), (ii) central Axel Heiberg Island near the head of Strand Fiord, Geodetic Hills, and the mouth of Mokka Fiord (Fricker 1963; Souther 1963; Tozer 1963; Thorsteinsson 1971a; Ricketts 1985), (iii) northwestern Ellesmere Island in the valley between the Blue and Blackwelder mountains (Thorsteinsson 1971b; Moore 1981 (Figs. 1, 4b). Our studies in the Bunde Fiord region have revealed that the Walker Island Member is about 300 m thick and consists


EMBRY AND OSADETZ

FIG. 2. North-South stratigraphic cross section, Cretaceous strata, southern Axel Heiberg - northwestern Ellesmere Island.

mainly of basalt flows (220 m) with interbedded quartzose sandstone and pyroclastic and epiclastic volcanic sediments. Volcanic flows are approximately 5 -30 m thick, columnar jointed, and amygdaloidal, and they commonly contain abundant petrified wood fragments and stumps of trees having diameters of up to 60 cm. The amygdules are generally composed of agate, carbonate, and lesser zeolite. The flows are fine to medium grained, holocrystalline and aphyric to sparsely porphyritic, and dark greyish green to black. The interbedded fluvial sandstones are thick bedded and commonly trough and planar cross-bedded with basal granule and cobble lags. Volcanic fragments and grains are absent from the thick sandstone beds, although they are present in some thin beds associated with siltstones, coals, and coaly shales that lie on

top of thick sandstone beds. A persistent unit of poorly stratified lahars, thin-bedded volcanic granulestone, and lithic sandstones, approximately 20 m thick, occurs near the top of the formation, commonly below the uppermost volcanic flows. A thin-bedded, fine-grained quartz sandstone, approximately 1 m thick, lies on top of the uppermost flow and is the uppermost unit of the Isachsen Formation. Farther southward at Bals and Li fiords the proportion of agglomerate and lahars is greater, and these units are more widely distributed through the section. At Li Fiord the total thickness of volcanic units is about 125 m. The most southerly occurrence of volcanic strata in the Walker Island Member on western Axel Heiberg Island is at the head of Strand Fiord, where an 80 m thick unit of basalt breccia occurs near the top


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FIG. 3. Time-stratigraphic chart for Cretaceous strata illustrated in Fig. 2. See Fig. 2 for lithology legend.

of the member (Ricketts 1985). At the Geodetic Hills locality on east-central Axel Heiberg Island there are three basalt flows with a combined thickness of 11 m in the Walker Island Member. Farther east at the mouth of Mokka Fiord there is a 28 m thick volcanic unit consisting of three flows. Underlying strata are not exposed, and this volcanic unit is tentatively assigned to the Walker Island Member because it is overlain by shales of the Christopher Formation and has reversed polarities (Wynne et al. 1988). In the Blue Mountains region of northwestern Ellesmere Island, a subaerial basalt unit up to 20 m thick lies near the top of the Isachsen Formation a few metres below the Christopher Formation (Moore 198 1). All of the flows in the Walker Island Member are interbedded with clastic sediments of fluvial origin. The flows appear to have been extruded on subsiding delta plains. The lahars and pyroclastic sediments, which are common in the sections on northwestern Axel Heiberg Island, suggest that poorly consolidated cinder cones may have been constructed on the fluvial-volcanic plain in this area. All of the volcanic units in the Walker Island Member can be dated as late Barremian to Aptian on the basis of the middle to late Barremian age for the underlying Rondon Member and a late Aptian - earliest Albian age for the overlying Christopher Formation (Embry 1985).

Cycle 3. Late Aptian - early Cenomanian Volcanic rocks within this interval outcrop on northern Amund Ringnes Island (Balkwill 1983), west-central Axel Heiberg Island (Fricker 1963; Souther 1963; Tozer 1963; Thorsteinsson 1971c; Ricketts et al. 1985), northwestern Axel

Heiberg Island (Thorsteinsson and Trettin 1972c; Fischer 1985; Ricketts et al. 1985), and northeastern Ellesmere Island (Christie 1964; Osadetz and Moore 1988) (Figs. 1, 4c). Strata of this interval are not preserved in the intervening areas, and thus the full extent of late Aptian - early Cenomanian volcanism is uncertain. However, the ubiquitous Cretaceous dykes and sills in much of northern Axel Heiberg and northern Ellesmere Island suggest volcanism was widespread in the northeastern Sverdrup Basin at this time. The main area of preservation of volcanic units in this interval is western Axel Heiberg Island, where the volcanic rocks form the Strand Fiord Formation, which caps the late Aptian - early Cenomanian T -R cycle (Fig. 2). In the Strand Fiord - Expedition Fiord region of west-central Axel Heiberg Island, the Strand Fiord Formation overlies the Bastion Ridge pelites (Fig. 5) and is abruptly overlain by dark, bituminous shales of the Kanguk Formation. Both of these shale units are of marine origin, but the intervening Strand Fiord Formation, which is up to 250 m thick, consists mainly of subaerial basalt flows. It thins to the east and south and intertongues with the shales of the Bastion Ridge Formation. In these areas, submarine flows, pyroclastic breccias, and epiclastic volcanic strata are common in the Strand Fiord Formation. The various volcanic facies of the Strand Fiord Formation in this area were described in detail by Ricketts et al. (1985). The Strand Fiord Formation is also preserved in the Bunde Fiord - Bals Fiord region of northwestern Axel Heiberg Island where volcanic strata were formerly assigned to the . Christopher Formation (Thorsteinsson and Trettin 1 9 7 2 ~ )At Bunde Fiord the Strand Fiord Formation unconformably overlies marine sandstone of the Hassel Formation, and strata

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'-%

Southern limit of

--, Southern

limit of volcanic r o c k s

volcanic r o c k s

FIG.4. Distribution and thickness of Cretaceous volcanic rocks: (a) Valanginian - early Barremian (cycle 1); (b) late Barremian (cycle 2); (c) latest Aptian - early Cenomanian (cycle 3); and (d) late Cenomanian - Maastrichtian.

younger than the Strand Fiord have not been recognized in the area. The most continuous section through the Strand Fiord Formation is 789 m thick and consists of 28 subaerial basalt flows, most with profusely amygdaloidal flow tops. Southward, at Bals Fiord, the Strand Fiord Formation is 300 m thick and is composed of 17 subaerial basalt flows with rare interbeds of shale and fine-grained sandstone. The other two occurrences of late Aptian - early Cenomanian volcanic rocks consist of thin volcanic units within the upper nonmarine sandstones and shales of the Hassel Formation: two units of volcanic breccia and flows on northern Amund Ringnes Island (Balkwill 1983) and two units of basalt flows, each about 35 m thick, north of Lake Hazen on northeastern Ellesmere Island (Piper Pass locality in Fig. 1) (Osadetz and Moore 1988). A stratigraphically lower occurrence of volcanic-related strata is a volcanic sandstone at the top of the Invincible Point

-

Aptian

Member of the Christopher Formation in the Bunde Fiord region. The sandstone consists mainly of fine to medium grains of basalt and is distinctly different from the quartz-rich sandstones that are common in the upper part of the Invincible Point Member over much of Sverdrup Basin elsewhere (Embry 1985). The quartz-rich sandstones were derived from the south and east, but the volcanic-rich sandstones at Bunde Fiord were derived from the north and northeast. They indicate the presence of a volcanic source area north and northeast of the Sverdrup Basin at this time (mid-Albian). The volcanic strata of the upper Hassel Formation and the Strand Fiord Formation are dated as late Albian to possibly earliest Cenomanian on the basis of palynological studies of underlying, overlying, and laterally equivalent shales (Balkwill 1983; E. H. Davies, personal communication, 1986). The volcanic sandstones in the Christopher Formation at Bunde Fiord are latest early Albian on the basis of associated


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FIG. 5. Albian to lower Cenomanian strata, Dragon Mountain, western Axel Heiberg Island. C, Christopher Formation; H, Hassel Formation; B, Bastion Ridge Formation; SF, Strand Fiord Formation.

ammonites (J. A. Jeletzky, personal communication, 1983). Cycle 4. Late Cenomanian - Maastrichtian With the exception of thin bentonite beds, which are common in the lower Kanguk Formation over most of the Sverdrup Basin, volcanic strata within this interval are confined to northwestern Ellesmere Island. The volcanic rocks are assigned to the Hansen Point volcanics, an informal stratigraphic unit that was recognized at a number of localities (Figs. l , 4 d ) by previous workers (Christie 1957; Thorsteinsson and Trettin 1972a, 1972b; Frisch 1974; Trettin and Frisch 1981; Trettin and Mayr 1981), although it was only recently given an informal name and assigned a Cretaceous age (Trettin and Parrish 1987). Previous age assignments ranged from Silurian to Permian. The Hansen Point volcanics consist mainly of basaltic flows and breccias with minor interbeds of terrigenous clastics. However, in contrast to older Cretaceous volcanic strata, rhyolitic and trachytic flows and breccias are also important constituents in some areas. Our studies of the Hansen Point volcanics have been carried out in the Phillips Inlet area and on the peninsula between Emma Fiord and Audhild Bay (Figs. 1, 4d). In one section near the head of Phillips Inlet the Hansen Point volcanics are > 645 m thick and rest unconformably on the Nansen Formation (Carboniferous - Early Permian). The lower 190 m of the Hansen Point consists of basaltic breccia and tuffaceous agglomerates; the remainder is mainly subaerial basalt flows with minor agglomerate. In a nearby section the lower 105 m of the Hansen Point consists of interbedded chert-pebble conglomerate, glauconitic sandstone, an carbonaceous siltstone and shale. The only volcanic unit within these terrigenous clastics is a 6 m thick basalt flow about 75 m above the base. Overlying the clastics is > 650 m of basalt flows and pyroclastic beds. The Hansen Point volcanics are most lithologically diverse on the unnamed peninsula between Emma Fiord and Audhild Bay. In a section measured near the northeast limit of the exposure approximately 55 m of sediments lies unconformably on Paleozoic Nansen Formation. The base of the section is a chert-cobble and -pebble conglomerate. The clastics are overlain by approximately 205 m of basaltic flows interbedded

with dacitic and rhyolitic pyroclastics and lesser lithic sediments. At least six volcanicflows are present. The intervening pyroclastics are recessive and largely covered. The upper portion of the section consists of 250 m of medium- to thickbedded orthoclase-phyric trachytes. A second section measured near the south shore of the peninsula, adjacent to a normal fault, has a basalt flow at its base. The basalt is overlain by approximately 250 m of lignitic shale and lithic sediments containing abundant wood fragments and purplish-red-weathering tree trunks. The shale is overlain by at least 190 m of poorly exposed basalt flows that are columnar jointed and amygdaloidal. These volcanic and clastic rocks are interpreted as overlying the trachytic unit of the first section, but further structural studies are necessary to confirm this. It is inferred that the composite section on this peninsula is at least 1300 m thick. These outcrops are associated with a swarm of north-northeasterly striking diabase dykes. The Hansen Point volcanics are present near the head of Yelverton Bay (Fig. 4d), where they were studied and briefly described by Trettin and Parrish (1987). These authors noted that "siliceous volcanics are far more abundant than mafic rocks" in the exposures they studied (Trettin and Parrish 1987, p. 261). Most of the volcanic units within the Hansen Point appear to be of subaerial origin, and no indicators of submarine extrusion have been observed. However, some of the interbedded shale units are marine in origin, as evidenced by the presence of dinoflagellates. Thus it is possible that some of the volcanic units are in part marine in origin. The Hansen Point volcanics in the Phillips Inlet - Emma Fiord area are dated as Late Cretaceous on the basis of palynological studies of shales within the unit (E. H. Davies, personal communication, 1986). Dinoflagellate assemblages within a few of the shale units are very similar to those of the Kanguk Formation of Ellesmere and Axel Heiberg islands, which is well dated here as late Cenomanian to early Campanian by pelecypods and foraminifera (Wall 1983). In the Yelverton Bay region the Hansen Point volcanics have a lower intercept UIPb zircon age of 88:: Ma (Trettin and Parrish 1987). A nearby intrusion yielded a 92.0 f 1 Ma (late Cenomanian U/Pb zircon age.

Volcanism and tectonic setting The main tectonic elements of the Canadian Arctic Archipelago during the Cretaceous were Arctic Platform, Sverdrup Basin, Banks Basin, Eglinton Graben, Sverdrup Rim, Canada Basin, and Alpha Ridge (Fig. 6). Sverdrup Basin was the main depocentre, and, as noted by Stephenson et al. (1987), it underwent a rifting phase from Valanginian to early Cenomanian, followed by a phase of thermal subsidence in the Late Cretaceous. Banks Basin and Eglinton Graben were secondary depocentres, and they also experienced elevated rates of subsidence related to rifting in the Early Cretaceous. The Arctic Platform lay to the south and east of the depocentres. Throughout the Cretaceous it alternated between being an area of slow subsidence and deposition and one of gentle uplift and erosion. A positive tectonic element to the northwest of the basins is called Sverdrup Rim (Balkwill 1983; Balkwill et al. 1983). Here, Lower Cretaceous and older strata are progressively truncated to the northwest (Meneley et al. 1975) and are unconformably overlain by strata as old as Santonian (mid -Late Cretaceous). Stratigraphic relationships

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General a r e a of C r e t a c e o u s volcanic r o c k s , Queen E l i z a b e t h Islands


Canada Basin

0

k,"

200

FIG. 6. Cretaceous tectonic setting and distribution of volcanic rocks, Queen Elizabeth Islands.

suggest that most of the uplift on Sverdrup Rim occurred during Early Cretaceous, but it should be noted that the area was one of low subsidence and periodic exposure throughout the late Paleozoic and Mesozoic. The entire Permian to Lower Cretaceous succession is absent in the Phillips Inlet - Emma Fiord region of northwestern Ellesmere Island where the Hansen Point volcanics unconformably overlie the Nansen Formation (Figs. 2, 3). Canada Basin is underlain by oceanic crust and was probably formed primarily during Cretaceous time. A comparison of the features of rifted ocean margins (as described by Falvey 1974) with the stratigraphic and structural relationships along the Canadian margin of Canada Basin indicates that Canada Basin underwent a main rifting phase from late Valanginian to early Cenomanian and a sea-floor spreading phase from Cenomanian to Carnpanian or possibly Maastrichtian (Embry , in press). A break-up unconformity (Falvey 1974, Fig. 9) of early Late Cretaceous age is clearly recognizable on seismic sections along the margin from Tuktoyaktuk Peninsula (Hawkings and Hatlelid 1975, Fig. 10) to Meighen Island (Meneley et al. 1975, Fig. 8). Sverdrup Rim is interpreted by us as a "rift shoulder" that was uplifted during the rift phase and gradually subsided during the spreading phase of Canada Basin. As noted previously, Sverdrup Basin underwent rifting and high rates of subsidence during the rift phase of Canada Basin and lower rates of thermal subsidence in the drift phase. Alpha Ridge forms the northern boundary of Canada Basin. It is a major bathymetric high that extends northwestward from the continental margin north of Ellesmere Island. It rises 1800 m above the floor of Canada Basin, and its crest is at water depths of about 1200 m. Recent seismic refraction studies indicate that the crust of Alpha Ridge is up to 35 km thick and is oceanic in character (Forsyth et al. 1986~).Alpha Ridge has been interpreted as a major volcanic edifice similar

to Iceland that formed over an active hot-spot centre during the opening of Canada Basin (Jackson et al. 1986; Forsyth et al. 19866). The relationship of Cretaceous volcanism within the Queen Elizabeth Islands to the tectonic elements described above was first discussed by Balkwill (1978, 1983). He related the intrusive and extrusive magmatic activity in the Sverdrup Basin to "crustal foundering, accompanied by faulting and tapping of upper mantle fluids," throughout the history of the basin (Balkwill 1978, p. 1021). This interpretation was strongly influenced by the wide range of KIAr dates from intrusive sills (180-90 Ma), the presence of Carboniferous and Permian volcanic flows, and the apparent coincidence of the highest level of sill intrusion and thick volcanic flows with the central portion of Sverdrup Basin. At the time of Balkwill's study little was known about the Cretaceous stratigraphy of northern Axel Heiberg and northern Ellesmere islands. Our more recent data from these areas indicate that the centre of magmatism did not coincide with the centre of Sverdrup Basin but lay to the northeast near the junction of Alpha Ridge with Sverdrup Rim. As shown on Figs. 2 and 4, volcanic units become thicker and more numerous towards the north in the lower three stratigraphic intervals, and the volcanic rocks of the fourth interval are entirely restricted to the extreme northeastern part of the basin. Given the volcanic nature of Alpha Ridge, we presently propose that Cretaceous volcanism of the Queen Elizabeth Islands was due to hot-spot activity, which also gave rise to Alpha Ridge. The interpreted relationship between Cretaceous volcanism in the Queen Elizabeth Islands and the development of Canada Basin is illustrated in Figure 7. Embry (in press) suggests that Canada Basin underwent an early rift phase that lasted from Bajocian to early Valanginian. This coincided with the initiation and early development of Banks Basin and Eglinton


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FIG.7. Relationship of Cretaceous stratigraphy and volcanism, Queen Elizabeth Islands, to development of Canada Basin. Stratigraphy and Canada Basin events after Embry (in press). See Fig. 2 for lithology legend.

Graben, which parallel the margin of Canada Basin. Minor mafic intrusive activity may have occurred at this time within the Sverdrup Basin, as evidenced by the few sills with KIAr dates of 180 to 138 Ma (Balkwill 1978). No volcanic activity is known to have occurred in the Queen Elizabeth Islands during this time. The main rifting phase of Canada Basin occurred from late Valanginian to early Cenomanian. Sverdrup Basin also underwent rifting at this time, although crustal extension was relatively minor. Volcanism in the Queen Elizabeth Islands progressively increased throughout this interval (Figs. 2, 4) and reached its zenith in the late Albian, when basalt flows reached as far southwest as Amund Ringnes Island. Dykes and sills extended farther southwestward to northern Melville Island (Balkwill 1978). Most of the KIAr dates obtained from sills and dykes of the Sverdrup Basin are in the time interval of the "main rifting phase" (Balkwill 1978). Fracture systems along which the dykes were emplaced seem to radiate southwestward from the volcanic centre north of Ellesmere Island (northeast trends of Balkwill and Fox (1982) and north trend of Jollimore (1986) (Fig. 8). To a much lesser degree dyke trends also follow older Sverdrup Basin trends (east -west trend of Jollimore (1986)). Also during the main rifting phase Sverdrup Rim was uplifted and Alpha Ridge began to form over the hot-spot centre. According to the present hypothesis, sea-floor spreading in Canada Basin commenced in Cenomanian time and lasted until Campanian or possibly Maastrichtian. During this time the margins of Canada Basin, as well as Sverdrup Rim and Sverdrup Basin, underwent relatively slow thermal subsidence (Stephenson et al. 1987). Alpha Ridge expanded as a "hotspot track" with the formation of new ocean crust in the vicinity of hot-spot activity (Forsyth et al. 1986b). The southeastern limit of Alpha Ridge volcanism is represented by the

Hansen Point volcanics on northwestern Ellesmere Island. Only minor dyke and sill intrusion occurred farther southwestwards within Sverdrup Basin at this time. Sea-floor spreading in Canada Basin ceased in either late Carnpanian or Maastrichtian. Formation of the Eurasian portion of the Arctic Ocean by rifting and sea-floor spreading followed soon afterwards in Tertiary time (Vogt and Avery 1974). The only other records of Cretaceous volcanism in the areas surrounding Canada Basin - Alpha Ridge are BarremianAptian basalt flows and associated dykes and sills on Kong Karls Land, Svalbard, and on Franz Josef Land (Fig. 9) (Smith et al. 1976; Nalivkin 1973). These flows are stratigraphically equivalent to those of the first and second T-R cycles of the Sverdrup Basin, and like those of the Canadian Arctic, they are associated with quartzose, fluvial sandstones. Younger Cretaceous strata are not preserved on Kong Karls Land and Franz Josef Land. Thus it is not known if volcanism continued throughout much of the Cretaceous in these areas like it did in the Canadian Arctic. Closure of the Eurasian portion of the Arctic Ocean, which formed subsequent to Cretaceous volcanism, places these areas reasonably close to the interpreted hot-spot centre assumed to be responsible for Alpha Ridge and volcanism in the Canadian Arctic (Fig. 9). Thus it would appear that Cretaceous volcanic rocks of Kong Karls Land and Franz Josef Land are further manifestations of Cretaceous hot-spot activity in the region north of Ellesmere Island. Areas more distant from northern Ellesmere Island, such as Prince Patrick and Banks islands, Mackenzie Delta, and northern Alaska, were not subject to Cretaceous volcanism.

Summary Cretaceous volcanic rocks in the Queen Elizabeth Islands consist mainly of basalt flows and pyroclastic rocks. The volcanic rocks are restricted to the northeastern portions of

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General area of Cretaceous volcanic rocks, Queen Elizabeth Islands

FIG.8. Cretaceous dyke trends, Queen Elizabeth Islands, and distribution of volcanic rocks. A Cretaceous hot spot is proposed north of Ellesmere Island.

/'

/ \'-

Continental Cretaceous Volcanics

East Slberlan

Most of the volcanic activity took place from late Hauterivian to early Cenomanian, with late Albian being the zenith of volcanism. This activity coincides with the inferred main rifting phase of the Canada Basin. Basalt flows extend as far southwestward as Amund Ringnes Island, with associated dykes and sills extending farther southwestward to northern Melville Island. Dykes trend mainly north to northeasterly and appear to radiate from the proposed hot-spot site. During the following interval of sea-floor spreading in Canada Basin in the Late Cretaceous, Alpha Ridge developed as a hot-spot track (Forsyth et al. 19863). Volcanic activity in the Queen Elizabeth Islands was much reduced and restricted mainly to northern Ellesmere Island, where thick basaltic, rhyolitic, and trachytic flows were extruded. These volcanic units represent the southeastern limit of Alpha Ridge.

Acknowledgments

FIG. 9. Distribution of Cretaceous volcanic rocks around Canada Basin on pre-Tertiary restoration (Eurasian Basin closed).

Sverdrup Basin and Sverdrup Rim, and volcanic activity increased northwards to an apparent centre north of Ellesmere Island. This centre of volcanism is interpreted as the site of a mantle plume or hot spot that was active throughout the Cretaceous.

The authors would like to thank the Geological Survey of Canada for encouraging this study and allowing its publication. We are also grateful to the Polar Continental Shelf Project for much logistical support during fieldwork. We have benefited from discussions with our colleages, especially Brian Ricketts, Phil Moore, Jane Wynne, and Hugh Balkwill. Billie Chiang typed the manuscript with speed and accuracy, and Elspeth Snow kindly drafted the figures. Hans Trettin, Brian Ricketts, Lorne Ayres, and an anonymous reviewer critically read the manuscript and offered helpful suggestions for its improvement. BALKWILL, H. R. 1978. Evolution of Sverdrup Basin, Arctic Canada.


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