1. Introduction
The Carboniferous–Cretaceous Sverdrup Basin in Arctic Canada preserves strata related to the opening and evolution of the Canada and Amerasian basins (Balkwill, Reference Balkwill1978; Embry & Beauchamp, Reference Embry, Beauchamp and Miall2008; Fig. 1). The litho- and sequence stratigraphic history of the Sverdrup Basin is well characterized (Embry & Beauchamp, Reference Embry, Beauchamp and Miall2008 and references therein); however, the absolute calibration of stratigraphic events within the basin is limited by relatively few high-precision geochronological studies (Tarduno et al. Reference Tarduno, Brinkman, Renne, Cottrell, Scher and Castillo1998; Villeneuve & Williamson, Reference Villeneuve, Williamson, Scott and Thurston2006; Estrada & Henjes-Kunst, Reference Estrada and Henjes-Kunst2013; Evenchick et al. Reference Evenchick, Davis, Bedard, Hayward and Friedman2015; Herrle et al. Reference Herrle, Schröder-Adams, Davis, Pugh, Galloway and Fath2015). The stratigraphic framework of Jurassic and Cretaceous strata of the Sverdrup Basin is based primarily on ammonites and Buchia assemblages and supplemented by other fossil groups preserved mainly in marine strata (Jeletzky, Reference Jeletzky1970; Wall, Reference Wall1983; Galloway et al. Reference Galloway, Sweet, Swindles, Dewing, Hadlari, Embry and Sanei2013, Reference Galloway, Tullius, Evenchick, Swindles, Hadlari and Embry2015; Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014; Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014). However, due to problems associated with long-ranging taxa and provincialism of some fossil groups, absolute age calibration will improve our understanding of high-latitude Mesozoic chronostratigraphy. Much of the available geochronological data is on intrusive phases, such as the widespread dykes and sills of the Queen Elizabeth swarm (Buchan & Ernst, Reference Buchan, Ernst, Hanski, Metanen, Rämo and Vuollo2006) that provide only limited stratigraphic constraints. Volcanic ash beds are key stratigraphic time markers that can be accurately dated using high-precision radiometric techniques to calibrate biostratigraphic and chemostratigraphic events for local and regional correlations within sedimentary basins. Establishing an accurately calibrated stratigraphy provides key temporal constraints to better model the geodynamic development of sedimentary basins.
Shaded relief bathymetry map showing major geographical features of the Arctic region. Areas encompassed by dashed lines identify occurrences of HALIP magmatic rocks (Maher, Reference Maher2001; Buchan and Ernst, Reference Buchan, Ernst, Hanski, Metanen, Rämo and Vuollo2006; Drachev & Saunders, Reference Drachev, Saunders, Scott and Thurston2006; Gaina et al. Reference Gaina, Medvedev, Torsvik, Koulakov and Werner2013; Verzhbitskii et al. Reference Verzhbitskii, Lobkovskii, Byakov and Kononov2013). Solid lines schematically indicate major HALIP-related dyke swarms of the Queen Elizabeth swarm in the Canadian Arctic islands and on Franz Josef Land (after Buchan & Ernst, Reference Buchan, Ernst, Hanski, Metanen, Rämo and Vuollo2006; Døssing et al. Reference Døssing, Jackson, Matzka, Einarsson, Rasmussen, Olesen and Brozena2013). The shaded region on the Alpha Ridge is schematic of a broad magnetic high (Døssing et al. Reference Døssing, Jackson, Matzka, Einarsson, Rasmussen, Olesen and Brozena2013) in the area of the proposed HALIP plume centre. FJL – Franz Josef Land; MB – Makarov Basin; PB – Podvodnikov Basin; Sval – Svalbard. The area within the white rectangle is shown in Figure 2.
Volcanic ash beds, preserved as bentonite layers, are particularly common in the late Cenomanian to Campanian Kanguk Formation of the Sverdrup Basin (Núñez-Betelu, Reference Núñez-Betelu1994; Parsons, Reference Parsons1994), and recent studies of sections on Ellef Ringnes (Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014) and Axel Heiberg islands (Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014) provide important palaeontological and chemostratigraphic datasets for regional correlation. In particular, the organic-rich mudstones of the basal Kanguk Formation preserve chemostratigraphic evidence of the late Cenomanian to early Turonian Oceanic Anoxic Event 2 (OAE2) horizon in the High Arctic (Lenniger et al. Reference Lenniger, Nøhr-Hansen, Hills and Bjerrum2014; Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014; Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014; Herrle et al. Reference Herrle, Schröder-Adams, Davis, Pugh, Galloway and Fath2015). The Kanguk Formation preserves the Arctic response to global Late Cretaceous climate and represents a potential, although thermally immature, regional source rock for hydrocarbons in the Sverdrup Basin. In this paper we report nine new U–Pb zircon ages for bentonite beds from Ellef Ringnes and Axel Heiberg islands that place absolute time constraints on the deposition of the Upper Cretaceous Kanguk Formation. The data help constrain: (a) recently established and developing biostratigraphic frameworks (e.g. Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014; Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014), as well as the age of S. nigricollensis and S. depressus ammonite zones; (b) a minimum age for the carbon isotope excursion correlated with the OAE2 horizon within the Canadian High Arctic (Lenniger et al. Reference Lenniger, Nøhr-Hansen, Hills and Bjerrum2014; Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014; Herrle et al. Reference Herrle, Schröder-Adams, Davis, Pugh, Galloway and Fath2015); and (c) changes in sedimentation rates in different parts of the Sverdrup Basin that will improve our understanding of basin development. In addition, the bentonites provide a record of volcanic events that improves our understanding of the magmatic history of the High Arctic Large Igneous Province (HALIP) during the Late Cretaceous.
2. Geological setting of the Sverdrup Basin
The Sverdrup Basin covers an area of approximately 300000 km2 in the Canadian Arctic Archipelago, extending 1300 km from northern Ellesmere Island to Prince Patrick Island (Fig. 2; Trettin, Reference Trettin, Bally and Palmer1989; Embry & Beauchamp, Reference Embry, Beauchamp and Miall2008). The basin initiated during Early Carboniferous rifting and preserves up to 13 km of marine and non-marine Carboniferous to Eocene strata (Balkwill, Reference Balkwill1978). The basin overlies Neoproterozoic to Siluro-Devonian rocks of the Ellesmerian Orogenic Belt and is bordered to the north by the Amerasia Basin. Evidence of northerly-derived sediment during the Permian through Early Jurassic is interpreted to indicate the presence of a rift shoulder (Sverdrup Rim) and crustal block to the north of the Arctic Islands, termed Crockerland by Embry (Reference Embry, Vorren, Blackadar, Glenister, Greiner, McLaren, McMillan, Norris, Roots, Souther, Thorsteinsson and Tozer1992, Reference Embry2009). Basin rejuvenation in the Early Cretaceous was accompanied by magmatic activity associated with the HALIP event between 130 and 80 Ma (Tarduno et al. Reference Tarduno, Brinkman, Renne, Cottrell, Scher and Castillo1998; Maher, Reference Maher2001; Buchan & Ernst; Reference Buchan, Ernst, Hanski, Metanen, Rämo and Vuollo2006, Villeneuve & Williamson, Reference Villeneuve, Williamson, Scott and Thurston2006). The Amerasia Basin formed through earliest Jurassic to earliest Cretaceous rifting followed by seafloor spreading in the Early Cretaceous (Embry & Dixon, Reference Embry and Dixon1990; Mickey, Byrnes & Haga, Reference Mickey, Byrnes and Haga2002; Grantz, Hart & Childers, Reference Grantz, Hart, Childers, Spencer, Embry, Gautier, Stoupakova and Sorensen2011; Døssing et al. Reference Døssing, Jackson, Matzka, Einarsson, Rasmussen, Olesen and Brozena2013). The eastern parts of the Sverdrup Basin, including Ellesmere and Axel Heiberg islands, were deformed in the late Palaeogene, with tectonic deformation decreasing in intensity to the west (Embry & Beauchamp, Reference Embry, Beauchamp and Miall2008).
Regional map showing the outline of the Sverdrup Basin across the Canadian Arctic Archipelago. EI – Ellesmere Island, AH – Axel Heiberg Island; ER – Ellef Ringnes Island; M – Melville Island; QE – Queen Elizabeth; WIC – Wootten Intrusive Complex; HPVC – Hansen Point Volcanic Complex. The locations of the stratigraphic columns shown in Figure 3 are identified as follows: A – Phillips Inlet, B – Bunde Fiord, C – Strand Fiord, D – Glacier Fiord, E – Ellef Ringnes. The detailed stratigraphic sections of the Kanguk Formation shown in Figure 4 are from locations E and GF.
2.a. Regional stratigraphic framework
Embry & Beauchamp (Reference Embry, Beauchamp and Miall2008) divided the depositional history of the basin into eight phases from the Carboniferous to the Palaeogene, each separated by an angular unconformity. As the main focus of this contribution is the Cretaceous history of the basin, the reader is referred to Embry & Beauchamp (Reference Embry, Beauchamp and Miall2008) for information on the earlier basin evolution. The Cretaceous history includes two phases. First, there was a major rejuvenation of the basin in the Early Cretaceous that led to the development and opening of the Amerasia Basin (Phase 6). This basin rejuvenation is marked by deposition of coarse-grained fluvial and deltaic sandstones and local volcanic rocks of the Valanginian to Aptian-aged Isachsen Formation (Embry & Osadetz, Reference Embry and Osadetz1988; Fig. 3). The basin margin migrated landwards during a significant transgression that drowned the Sverdrup Rim and deposited mud and silt of the late Aptian to late Albian-aged Christopher Formation. This was followed by progradation and deposition of shallow shelf sands of the Hassel Formation during the late Albian to early Cenomanian, depending on location in the basin. The Bastion Ridge Formation overlies the Hassel Formation at Glacier Fiord and consists of mudstone and siltstone that locally interfinger with tholeiitic basalt flows of the Cenomanian-aged Strand Fiord Formation (95±0.2 Ma; Ar–Ar whole rock) on Axel Heiberg Island (Tarduno et al. Reference Tarduno, Brinkman, Renne, Cottrell, Scher and Castillo1998). The volcanic units were deposited towards the centre of the basin and thicken to the north to a maximum thickness of 900 m on northern Axel Heiberg Island (Ricketts, Osadetz & Embry, Reference Ricketts, Osadetz and Embry1985). A major transgression during Phase 7 is marked by a change to slow passive subsidence and deposition of the mudstone- and siltstone-dominated Kanguk Formation, which overlies the Hassel, Bastion Ridge or Strand Fiord formations (Fig. 3).
Regional stratigraphic framework and correlation of units within the Sverdrup Basin (modified from Evenchick et al. Reference Evenchick, Davis, Bedard, Hayward and Friedman2015). Schematic sections from Axel Heiberg and Ellesmere islands are modified from Embry & Osadetz (Reference Embry and Osadetz1988). Note that Herrle et al. (Reference Herrle, Schröder-Adams, Davis, Pugh, Galloway and Fath2015) place the age of the Rondon Member of Isachsen Formation in the early Aptian based on carbon isotope stratigraphy. The stratigraphic framework of Ellef Ringnes Island modified after Evenchick et al. (Reference Evenchick, Davis, Bedard, Hayward and Friedman2015). The sections for Glacier Fiord to Phillips Inlet represent a cross-section showing basin geometry (after Embry & Osadetz, Reference Embry and Osadetz1988); the Ellef part of the figure is not included in this cross-section. Detailed sections of the Kanguk Formation from Ellef Ringnes and Glacier Fiord (GF) shown in Figure 4 are indicated by double-headed arrows.
The sediment input rate increased in the latter part of the Late Cretaceous, and shoreline to shallow marine sand prograded into the basin (Expedition Formation; Fig. 3). Widespread uplift marked the termination of this phase and, in the far northeastern portion of the basin, felsic volcanic rocks of the Hansen Point Formation were erupted (Trettin & Parrish, Reference Trettin and Parrish1987; Estrada et al. Reference Estrada, Piepjohn, Henjes-Kunst and von Gosen2003). These volcanic rocks are interpreted to be related to the initial rifting of the Eurasian portion of the Arctic Ocean and the associated Morris Jesup–Yermak hotspot (Estrada & Henjes-Kunst, Reference Estrada and Henjes-Kunst2004).
2.b. Magmatic history
Detailed knowledge of the timing of magmatic events within the Sverdrup Basin is limited by a paucity of reliable geochronological data. The geochronological dataset suffers from both the small number of analyses and the large number of K–Ar or Ar–Ar analyses that have relatively poor precision, or may be adversely affected by later disturbances or excess Ar (e.g. Corfu et al. Reference Corfu, Polteau, Planke, Faleide, Svensen, Zayoncheck and Stolbov2013). Most of the early magmatism within the basin is manifested as dykes and sills of the Queen Elizabeth swarm that intruded between 130 and 95 Ma (Estrada & Henjes-Kunst, Reference Estrada and Henjes-Kunst2004, Reference Estrada and Henjes-Kunst2013; Buchan & Ernst, Reference Buchan, Ernst, Hanski, Metanen, Rämo and Vuollo2006; Villeneuve & Williamson, Reference Villeneuve, Williamson, Scott and Thurston2006; Evenchick et al. Reference Evenchick, Davis, Bedard, Hayward and Friedman2015). There is limited direct evidence for volcanic or volcaniclastic units within the stratigraphic section during the Early Cretaceous, although bentonites and other volcanic rocks are reported at a number of intervals within the uppermost Valanginian to Aptian Isachsen Formation and upper Aptian to upper Albian Christopher Formation and these may correlate in time with intrusive events (Evenchick et al. Reference Evenchick, Davis, Bedard, Hayward and Friedman2015). Tholeiitic basalt of the Strand Fiord Formation has an Ar–Ar age of 95 Ma (Tarduno et al. Reference Tarduno, Brinkman, Renne, Cottrell, Scher and Castillo1998), and represents the largest volume of volcanic rocks preserved within the basin (Embry & Osadetz, Reference Embry and Osadetz1988). The tholeiitic magmatism was followed by Late Cretaceous alkaline magmatism recorded in a number of localities on Ellesmere Island and northern Greenland. These include the 93–92 Ma Wootton intrusive complex and the Hansen Point volcanics on northern Ellesmere Island (Trettin & Parrish, Reference Trettin and Parrish1987; Estrada & Henjes-Kunst, Reference Estrada and Henjes-Kunst2013). The Hansen Point volcanics have Ar–Ar ages of 83–77 Ma (Estrada & Henjes-Kunst, Reference Estrada and Henjes-Kunst2013), although Bono, Tarduno & Singer, (Reference Bono, Tarduno and Singer2013) indicate that some units in the Hansen Point volcanics have older ages of 92–88 Ma. Evidence for younger magmatic events is documented on northern Greenland and includes the c. 81 Ma Peary dykes (Kontak et al. Reference Kontak, Jensen, Dostal, Archibald and Kyser2001) and the 71–61 Ma Kap Washington volcanics (Tegner et al. Reference Tegner, Storey, Holm, Thorarinsson, Zhao, Lo and Knudsen2011; Thorarinsson, Holm, Tappe et al. Reference Thorarinsson, Holm, Duprat and Tegner2011).
3. The Kanguk Formation
The Kanguk Formation is a marine unit dominated by mudstone with minor interbeds of siltstone/sandstone and bentonite (Núñez-Betelu & Hills, Reference Núñez-Betelu and Hills1994; Parsons, Reference Parsons1994; Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014). The formation is preserved in the Sverdrup Basin and also occurs on Banks and Bylot islands (Miall, Reference Miall1979; Balkwill, Reference Balkwill1983; Ioannides, Reference Ioannides1986). The Kanguk Formation can be correlated with strata in the Mackenzie Delta area (Boundary Creek and Smoking Hills formations) as well as in northern Alaska (Hue, Seabee and Canning formations; Tappan, Reference Tappan1962; Embry & Dixon, Reference Embry and Dixon1990), indicating widespread deposition of marine mudstone at this time.
The Kanguk Formation ranges in age from the late Cenomanian to the late Campanian – early Maastrichtian based on stratigraphic correlation and palaeontological data (Greiner, Reference Greiner1963; Thorsteinsson & Tozer, Reference Thorsteinsson, Tozer and Douglas1970; Balkwill, Reference Balkwill1983; Wall, Reference Wall1983; Núñez-Betelu et al. Reference Núñez-Betelu, MacRae, Hills, Muecke, Thurston and Fujita1994; Tapia & Harwood, Reference Tapia and Harwood2002; Hall, MacRae & Hills, Reference Hall, MacRae and Hills2005; Hills & Strong, Reference Hills and Strong2007; Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014; Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014), as well as chemostratigraphic criteria (Herrle et al. Reference Herrle, Schröder-Adams, Davis, Pugh, Galloway and Fath2015). Locally on Axel Heiberg Island the Kanguk Formation stratigraphically overlies the Bastion Ridge Formation and the time correlative Strand Fiord Formation (Fig. 3), which provides a maximum depositional age of 95.2±0.2 Ma (Tarduno et al. Reference Tarduno, Brinkman, Renne, Cottrell, Scher and Castillo1998). Elsewhere, including on Ellef Ringnes Island, Kanguk Formation overlies the Hassel Formation of Albian to Cenomanian age (Galloway et al. Reference Galloway, Sweet, Pugh, Schröder-Adams, Swindles, Haggart and Embry2012; Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014). Wilson (Reference Wilson1978) proposed a late Cenomanian to Turonian age for the base of the Kanguk based on the occurrence of fossil fish at Banks Island, but preferred an early Turonian age based on correlation to other localities. Greiner (Reference Greiner1963) interpreted a Cenomanian age of the basal Kanguk Formation on Graham Island based on the presence of the marine bivalve Inoceramus cf. I. pictus. Diatoms indicative of the late Cenomanian were recorded at Slidre Fiord on Ellesmere Island (Tapia & Harwood, Reference Tapia and Harwood2002). A latest Cenomanian age is also indicated by other lines of evidence, including pollen evidence from underlying Hassel Formation on Ellef Ringnes Island (Galloway et al. Reference Galloway, Sweet, Pugh, Schröder-Adams, Swindles, Haggart and Embry2012). The occurrence of an organic-rich layer in the lowermost Kanguk Formation was interpreted as the OAE2 event by Schröder-Adams et al. (Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014) at Glacier Fiord. The OAE2 event is interpreted to be a global feature within the Cenomanian/–Turonian boundary interval (93.9 Ma; Meyers et al. Reference Meyers, Siewert, Singer, Sageman, Condon, Obradovich, Jicha and Sawyer2012). The position of the OAE2 interval is also identified by a characteristic carbon isotope variation within the section from Ellef Ringnes Island (Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014) and Axel Heiberg Island (Herrle et al. Reference Herrle, Schröder-Adams, Davis, Pugh, Galloway and Fath2015), placing the Cenomanian/Turonian boundary within the lowermost Kanguk Formation.
Macrofossil biostratigraphic markers include the occurrence of latest Turonian to Coniacian ammonites Scaphites corvensis, S. nigricollensis and S. depressus within the Glacier Fiord section on Axel Heiberg Island (Hills et al. Reference Hills, Braunberger, Núñez-Betelu and Hall1994; Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014; Fig. 4). The S. nigricollensis and S. depressus ammonite zones are accurately calibrated from sections of the Western Interior Seaway in the USA using U–Pb and Ar–Ar dating of volcanic ash beds (Sageman et al. Reference Sageman, Singer, Meyers, Siewert, Walaszczyk, Condon, Jicha, Obradovich and Sawyer2014). The S. nigricollensis Zone contains an ash bed with an Ar–Ar age of 89.87±0.18 Ma (Sageman et al. Reference Sageman, Singer, Meyers, Siewert, Walaszczyk, Condon, Jicha, Obradovich and Sawyer2014) and is of relatively short duration. Ogg, Hinnov & Huang (Reference Ogg, Hinnov, Huang, Gradstein, Schmitz and Ogg2012) propose an interpolated numerical age for the base of this zone at 90.24 Ma, with a duration of 0.26 Ma. The S. depressus Zone contains ash layers in its upper part dated at 87.11±0.08 Ma to 86.52±0.31 Ma (U–Pb zircon and Ar–Ar, respectively; Sageman et al. Reference Sageman, Singer, Meyers, Siewert, Walaszczyk, Condon, Jicha, Obradovich and Sawyer2014). Ogg, Hinnov & Huang (Reference Ogg, Hinnov, Huang, Gradstein, Schmitz and Ogg2012) propose an interpolated numerical age for the base of this zone at 87.86 Ma with a duration of 1.60 Ma.
Detailed stratigraphic columns of measured Kanguk Formation sections from Glacier Fiord, Axel Heiberg Island and Hoodoo Dome on Ellef Ringnes Island, after Schröder-Adams et al. (Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014) and Pugh et al. (Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014), respectively. The two sections are hung from the interpreted stratigraphic level of the OAE2 event defined by carbon isotopic data (Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014; Herrle et al. Reference Herrle, Schröder-Adams, Davis, Pugh, Galloway and Fath2015). Bentonite samples collected as part of this study are identified along with their interpreted U–Pb zircon age and error in Ma. The double-headed arrow connects bentonite horizons ER2010-E55 and AH2011-C56 with comparable ages in the two sections. The width of the covered section between the Bastion Ridge and Kanguk formations at Glacier Fiord is tentative.
In addition to the ammonite occurrences, large inoceramid bivalves identified as Sphenoceramus patootensis occur preserved in situ within the upper part of the Kanguk Formation at Glacier Fiord on Axel Heiberg Island and at Hoodoo Dome, Ellef Ringnes Island (Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014; Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014). This fossil marker is widespread in uppermost Santonian to lowermost Campanian strata of the Arctic (e.g. Jeletzky, Reference Jeletzky1970; Sahagian, Beisel & Zakharov, Reference Sahagian, Beisel and Zakharov1994; Olfer'ev et al. Reference Olfer'ev, Beniamovski, Vishnevskaya, Ivanov, Kopaevich, Ovechkina, Pervushov, Sel'tser, Tesakova, Kharitonov and Shcherbinina2008).
4. Description of sections and samples
Bentonite samples were collected from measured sections at Hoodoo Dome on Ellef Ringnes Island and at Glacier Fiord on Axel Heiberg Island during field visits in the summers of 2010 and 2011, respectively (Fig. 2). Only those bentonites were sampled that were thick enough to yield a large amount of sample and did not appear extremely altered. This resulted in a somewhat random choice of intervals. Detailed descriptions of the sections are provided in Pugh et al. (Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014) for the Ellef Ringnes section and Schröder-Adams et al. (Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014) for the Axel Heiberg Island section. Bentonites occur at numerous stratigraphic levels within both of these sections (Fig. 4). The bentonite layers are easily recognized in the field, being characterized by a distinct yellow-grey colour, and consist of unconsolidated clay with small amounts of phenocrysts including feldspar, apatite and zircon. Bentonites from these sections have not been described in detail; however, Parsons (Reference Parsons1994) described similar bentonites in the Kanguk Formation from the Expedition Fiord area, Axel Heiberg Island. Parsons (Reference Parsons1994) characterized the chemistry of the bentonites as alkaline, consistent with proposed sources related to the HALIP.
The Kanguk Formation exposed within the Hoodoo Dome section on Ellef Ringnes Island consists of two informal members (Balkwill & Hopkins, Reference Balkwill and Hopkins1976; Evenchick & Embry, Reference Evenchick and Embry2012): a lower member (265 m) dominated by alternating beds of black to dark grey platy shale (‘paper shale’) and dark grey to dark brown silty mudstone (Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014), and an upper member (210 m) of dark brown to grey, shaly siltstone and silty mudstone with brown ironstone nodules (Balkwill & Hopkins, Reference Balkwill and Hopkins1976). The top of the section is marked by progradation of the overlying sandstone-dominated Expedition Formation. Several stratigraphic markers were used by Pugh et al. (Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014) to subdivide the lower member. At the base of the member directly above the Hassel Formation, δ13C data reveal the position of the latest Cenomanian OAE2 interval. At the top of the lower member the inoceramid bivalve Sphenoceramus patootensis occurrence indicates the uppermost Santonian / lowermost Campanian boundary interval. Three partial radiolarian range zones were proposed in the lower member (Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014), but their preliminary stratigraphic placements require further study. Eleven bentonite layers were identified within the section: ten in the lower member and one in the upper member (Fig. 4; Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014). Three of them were sampled for age dating at 277.5 m (ER2010-E55), 376 m (ER2010-E82) and 380 m (ER2010-E59) from the lower member (Fig. 4). ER2010-E55 is at the base of the middle Turonian to Santonian Eostichomitra carnegiense partial range zone, and ER2010-E82 and ER2010-E59 are closely spaced within the Coniacian Dictyomitra multicostata partial range zone (Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014).
Data are presented for seven bentonite samples from the Glacier Fiord section described by Schröder-Adams et al. (Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014): one from the Bastion Ridge Formation and six from the Kanguk Formation. Bentonites sampled within a short interval of closely spaced beds tested the relative accuracy of the U–Pb data and their ability to resolve small age difference in stratigraphic sequence. The Bastion Ridge Formation at this section consists of approximately 75 m of silty mudstone that abruptly coarsens upward to a 7 m thick fine-grained sandstone. The bentonite layer is approximately 15 cm thick and occurs in the middle of this succession, 48.5 m above the basal contact with the Hassel Formation. The boundary between the Bastion Ridge Formation and the base of the Kanguk Formation is hidden beneath an 8 m slumped and covered interval. Although both formations are fine-grained they clearly differ in colour where the mudstones of the Bastion Ridge Formation are black to dark grey and those of the Kanguk Formation are light grey to light brown. The overlying Kanguk Formation is approximately 510 m thick, but numerous intervals are covered or obscured by slumped material (Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014). The formation is dominated by mudstone with variable proportions of interbedded siltstone and very fine sandstone (Fig. 4). The basal part of the measured section, above the 8 m covered interval, consists of 55 m of thin platy shale (‘paper’ shale) without microbioturbation interrupted by a more silty bioturbated interval. This section is interpreted as outer shelf facies (Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014). A rapid increase in total organic carbon (TOC) and Hydrogen Index (HI) within this part of the section is correlated with the OAE2 interval, identified on the basis of δ13Corg stratigraphy (Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014; Herrle et al. Reference Herrle, Schröder-Adams, Davis, Pugh, Galloway and Fath2015), suggesting a latest Cenomanian age for the lowermost Kanguk Formation. Overlying the ‘paper’ shale interval is 45 m of mudstone with interbedded siltstone and fine sandstone interpreted as middle shelf deposits (Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014). Bentonites are particularly common within this interval, with six beds mapped, five of which were sampled for geochronological analysis (Fig. 4). The uppermost of these bentonites (C68) occurs approximately 35 m below the position of the Scaphites corvensis and S. nigricollensis ammonite occurrence, indicating a middle to late Turonian age (Sageman et al. Reference Sageman, Singer, Meyers, Siewert, Walaszczyk, Condon, Jicha, Obradovich and Sawyer2014). The upper part of the Kanguk Formation section includes numerous covered intervals. At 1840 m very fine sandstone interbeds occur, followed by a distinctly rusty, siltstone unit at 1872 m. The siltstone interval contains numerous large inoceramid bivalves identified as Sphenoceramus patootensis of latest Santonian to earliest Campanian age and is interpreted to represent a middle shelf to lower offshore facies (Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014). Two bentonite layers occur between 1850 and 1865, with the stratigraphically higher layer sampled for geochronology. This bentonite occurs approximately 15 m below the first occurrence of the inoceramids.
5. U–Pb Geochronology
5.a. Analytical procedures
Bentonite samples consist of unconsolidated, clay-rich material. Heavy mineral concentrates were prepared by decanting fines and then passing the remainder over a Wilfley™ table, followed by density separation in heavy liquids (methylene iodide) and finally sorting by magnetic susceptibility using a Frantz™ isodynamic separator.
All zircon fractions were prepared using the annealing and chemical leaching technique (CA-TIMS) modified from that described by Mattinson (Reference Mattinson2005). Samples were annealed at 1000°C for 48 hours and then leached for 10–20 hours, in HF-HNO3 at 180°C in Savillex 3 mL PFA capsules within a Parr digestion vessel. Isotopic and U–Pb compositional data were determined by isotope dilution thermal ionization mass spectrometry at the Geochronology Laboratory, Geological Survey of Canada. Sample dissolution and chemical methods are slightly modified from Parrish et al. (Reference Parrish, Roddick, Loveridge and Sullivan1987). Individual crystals were selected under a binocular microscope to avoid inclusions and other imperfections, spiked with a mixed 205Pb–233U–235U tracer solution calibrated to ±0.15% against the JMM gravimetric solution and dissolved in high-pressure bombs in HF-HNO3. Data reduction and error propagation follow methods outlined in Roddick (Reference Roddick1987). U and Pb isotopic ratios were measured using a Triton mass spectrometer operated in either static multi-collection mode (U) or using a secondary electron multiplier and ion-counting system (Pb). A Pb mass fractionation correction of 0.1±0.04%/amu was applied as determined by replicate analyses of the NBS981 standard. U fractionation was corrected using the 233U–235U double spike and was typically in the range of 0.12%/amu. Deadtime (20 ns) for the ion-counting system was determined by replicate analyses of the NBS982 solution. The 206Pb/238U ages were corrected for initial 230Th disequilibrium using the formulation of Schärer (Reference Schärer1984), the model Th/U ratio of the zircon determined from the measured 208Pb/206Pb ratio and an assumed Th/U ratio of the magma of 4. This resulted in small increases of the age of approximately 0.09 Ma (Table 1). Age calculations used 238U/235U of 137.88 and decay constants of 1.55125e−10 for 238U and 9.8485e−10 for 235U. Accuracy of the method was monitored by repeated analyses of the Temora2 zircon standard. Thirteen analyses over the course of the analytical session gave a weighted mean age of 417.25±0.25 Ma, relative to the age of 416.8 Ma reported by Black et al. (Reference Black, Kamo, Allen, Davis, Aleinikoff, Valley, Mundil, Campbell, Korsch, Williams and Foudoulis2004).
U–Pb isotopic data
*Weight estimated using image analysis software – concentration uncertainty ~20%.
† Pbr = radiogenic Pb. Pbc = total common Pb in analysis corrected for spike and fractionation.
‡ Atomic ratios corrected for spike, fractionation, blank and initial common Pb, except 206Pb/204Pb ratio corrected for spike and fractionation only. Errors are 1σ absolute. Pb blank <1–1.5 pg; U blank <0.1 pg; Pb blank composition 208:207:206:204 = 0.51966:0.21356:0.25288:0.013895.
‡ Correlation coefficient of errors in isotopic ratios.
‖ 206Pb238U age corrected for initial Th deficit.
6. U–Pb results
All analyses were made on single zircon crystals, and ages were calculated as the weighted mean 206Pb/238U age of the youngest statistical grouping of analyses. This approach assumes that the annealing and leaching process effectively eliminated the potential effects of Pb loss (e.g. Mattinson, Reference Mattinson2005), and that older zircon ages within a bentonite layer reflect inheritance of older crystals, either by recycling older volcanic deposits within calderas during explosive magmatism, or due to long residence in magma chambers prior to eruption (Bachmann et al. Reference Bachmann, Oberli, Dungan, Meier, Mundil and Fischer2007; Folkes et al. Reference Folkes, de Silva, Schmitt and Cas2011; Barboni et al. Reference Barboni, Schoene, Ovtcharova, Schaltegger, Bussy and Gerdes2013). Grains identified as older components in individual bentonite layers yield ages similar to the eruption ages determined for underlying bentonite layers. This suggests that zircons from previous eruption cycles are recycled in younger deposits. The effect of long residence in magma chambers is difficult to evaluate but if it is significant the ages may be biased to older ages by up to several hundred thousand years (e.g. Bachmann et al. Reference Bachmann, Oberli, Dungan, Meier, Mundil and Fischer2007). The summary of interpreted ages is presented in Table 2. Two standard errors of the weighted mean errors are reported without and with decay constant error in Table 2, and separated by a slash in the text and figures. The former error should be used when considering differences in ages within this dataset, whereas the latter value should be considered when comparing this dataset to U–Pb data produced from other laboratories. Decay constant errors should be considered when comparing against age data derived from other isotopic systems. Sample numbers are recorded as follows: locality (AH – Axel Heiberg, ER – Ellef Ringnes), year of collection, number in section (m).
Summary of U–Pb age results
Sample number in parentheses refers to GSC lab number. Characters after hyphen identify samples in Figures 4 and 10.
Age corrected for Th disequilibrium using measured 208Pb/206Pb as described in text.
Error calculated by weighted mean using Isoplot (Ludwig, Reference Ludwig2003).
Error with spike in parentheses includes quadratically added error of spike (0.15%).
Error with spike & decay in parenthesis includes quadratically added error in spike and in 238U decay constant (0.107%).
n = number of analyses included in the age calculation of the total indicated.
MSWD = mean square of the weighted deviates; POF = probability of fit.
6.a. AH2011-B41A Bastion Ridge Formation (Z10765)
Sample AH2011-B41A was collected at 1545 m in the section from approximately within the middle of the Bastion Ridge Formation (Fig. 4). The light-brown bed is somewhat irregular in thickness, ranging from 5 cm to 15 cm, and occurs within silty, slightly rusty-coloured mudstone to siltstone.
A small number of zircons were recovered (Fig. 5a). Four single grain analyses yield rather imprecise results owing to low 206Pb/204Pb ratios. Two of the analyses overlap the concordia curve with a weighted mean age of 98.3±1.8 Ma (mean square weighted deviation (MSWD) = 1.4; probability = 0.24; Fig. 6). The two other analyses are much older, with one concordant analysis with an age of 287 and a very discordant analysis with a 207Pb/206Pb age of 282 Ma. Given the evidence for inheritance in this sample, the 98.3±1.8 Ma is a maximum age for this layer.
Photomicrographs of zircon images from selected bentonite samples prior to thermal annealing. (a) Bastion Ridge Formation, Glacier Fiord section, AH2011-B41A (z10765); (b) Kanguk Formation, Glacier Fiord section, AH2011-C56 (z10770); (c) Kanguk Formation, Glacier Fiord section, AH2011-C59 (z10771); (d) Kanguk Formation, Glacier Fiord section, AH2011-C61 (z10769); (e) Kanguk Formation, Glacier Fiord section, AH2011-C65 (z10768); (f) Kanguk Formation, Glacier Fiord section, AH2011-C68 (z10766); (g) Kanguk Formation, Glacier Fiord section, AH2011-C84 (z10767); (h) Kanguk Formation, Ellef Ringnes Hoodoo Dome section, ER2010-E55 (z11016); (i) Kanguk Formation, Ellef Ringnes Hoodoo Dome section, ER2010-E82 (z11015); (j) Kanguk Formation, Ellef Ringnes Hoodoo Dome section, ER2010-E59 (z11014).
U–Pb concordia diagram for bentonite sample AH2011-B41A from the Bastion Ridge Formation, Glacier Fiord section, Axel Heiberg Island.
6.b. AH2011-C56 Kanguk Formation (Z10770)
Sample AH2011-C56 is from a bentonite at 1641 m in the section, in what is inferred as Turonian-aged strata of the Kanguk Formation on the basis of inference of the occurrence of OAE2 and foramininfera biostratigraphy (Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014; Herrle et al. Reference Herrle, Schröder-Adams, Davis, Pugh, Galloway and Fath2015). This sample is taken from a bentonite that is exposed between two short covered sections where the sediment appears to be dark-grey soft mudstone (Fig. 4). Owing to the poor quality of the section at this interval, its exact position is less accurately determined than for the other bentonites. The orange weathered bentonite is 20 cm thick and appears light yellow underneath the weathered surface.
A number of euhedral zircons with abundant inclusions were recovered (Fig. 5b). Four analyses yield concordant to slightly discordant ages between 93.0±0.2 and 93.9±0.2 Ma, indicating more than one age population (Fig. 7). The three youngest analyses defined a weighted mean age of 93.03±0.21/0.25 (MSWD = 0.57, probability = 0.57). The most precise analysis is slightly discordant and this must represent a minimum age for this layer. The three other analyses do not form a statistical population. The most precise concordant analysis at 93.85±0.24 Ma represents a maximum age for the layer.
U–Pb Concordia diagrams and weighted mean 206Pb/238U age plots for bentonite samples from the Kanguk Formation, Glacier Fiord section, Axel Heiberg Island. Analyses used in the weighted mean calculation are indicated by grey filled bars (open rectangles excluded from age calculation). Length of bar indicates 2σ analytical error of analyses. Two age errors are reported: (1) standard error of the weighted mean (2σ or 95% confidence) calculated using Isoplot (Ludwig, Reference Ludwig2003) and (2) the error that includes the quadratically added spike calibration error to the standard error of the mean. The latter should be used when comparing to other U–Pb ages.
6.c. AH2011-C59 Kanguk formation (Z10771)
Sample AH2011-C59 is from a bentonite at 1650 m, 9 m above sample C56 (Fig. 4). The bed is 30 cm thick and has a similar weathering colour to AH2011-C56. This bentonite occurs on top of a package of coarser-grained rock with concretionized silt beds. Examples of typical zircon morphology are shown in Figure 5c. Most grains have minor inclusions and fractures. Seven analyses yield a range of ages from 91.8±0.2 Ma to 94.7±0.8 Ma (Fig. 7). The three youngest analyses yield a weighted mean age of 91.94±0.18/0.23 Ma (MSWD = 1.5, probability = 0.23), which is interpreted as the best estimate for the age of the bentonite. The four older analyses do not define a single age population, and indicate incorporation of zircon grains 1–3 Ma older than the interpreted eruption age. Two analyses have ages similar to the age of the underlying bentonite, and a third has an age similar to the age of the Strand Fiord volcanic rocks (Tarduno et al. Reference Tarduno, Brinkman, Renne, Cottrell, Scher and Castillo1998), suggesting recycling of earlier volcanic material by younger eruptions.
6.d. AH2011-C61 Kanguk Formation (Z10769)
Sample AH2011-C61 occurs 4.5 m above sample C59 at 1654.5 m. The bentonite is 15 cm thick and has an orange weathered appearance. Its stratigraphic position is within a coarsening upwards cycle with individual siderite concretions. Another 1–5 cm thick bentonite is located 2 m above this sample within the same lithology; however, it was not sampled for geochronology.
Zircons occur as large, euhedral prismatic crystals, with minor inclusions (Fig. 5d). Seven grains record a spread in individual ages from 91.2±0.3 to 92.7±0.7 Ma (Fig. 7). The four youngest analyses yield a weighted mean age of 91.30±0.15/0.20 Ma (MSWD = 1.4, probability = 0.25). The three analyses excluded from the age calculation have ages of c. 92 Ma, within error of the age of the zircon in the underlying bentonite (AH2011-C59), suggesting inheritance of earlier erupted material.
6.e. AH2011-C65 Kanguk Formation (Z10768)
Sample AH2011-C65 was taken at 1665 m within soft grey mudstone underneath an interval of rusty interbedded siltstone. The bed is 10 cm thick. Due to its moisture content it acted as a sliding plane for the surrounding mudstones, resulting in distortion of bedding at this outcrop.
Abundant euhedral, colourless zircons were recovered from this sample. Individual zircon crystals are clear with few inclusions and fractures (Fig. 5e). Eight analyses of zircons yield a small range of ages from 91.0±0.4 Ma to 92.4±0.4 Ma, which do not define a single age population (Fig. 7). The weighted mean age of the five youngest analyses is 91.23±0.22/0.26 (MSWD = 1.8, probability = 0.14), interpreted as the best age estimate of this bentonite layer. The three older analyses have ages of ~92 Ma, comparable to ages from underlying bentonite AH2011-C59.
6.f. AH 2011-C68 Kanguk Formation (Z10766)
Sample C68 is 14 m above sample AH2011-C65 at 1679 m within the Glacier Fiord section. The bentonite has a yellow colour and is 15 cm thick, within grey soft mudstone.
Euhedral, prismatic zircon grains were recovered, with minor inclusions and fractures (Fig. 5f). Six analyses yield a range of ages from 91.0±0.4 to 94.0±0.9 Ma (Fig. 7). The youngest three analyses have a weighted mean age of 91.02±0.30/0.33 Ma (MSWD = 1.08, probability = 0.34).
6.g. AH2011-C84 Kanguk Formation (Z10767)
Sample AH2011-C84 was collected from a bentonite at 1861 m. This position is above what we infer to be the position of the Scaphites depressus occurrence of Hills et al. (1993) and below the first occurrence of Sphenoceramus patootensis in the Glacier Fiord section. The age of this stratigraphic level is inferred to be Santonian, based on these fossil occurrences and foraminifera biostratigraphy (Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014). Sample AH2011-C84 is from the uppermost bentonite layer recognized in the section. Much of the 200 m section between this bentonite and the bentonite-rich lower part of the section is not exposed. The layer is 15 cm thick and occurs underneath rusty very fine-grained sandstone interbedded with siltstone. A second thin bentonite that was not sampled occurs 12 m below sample AH2011-C84.
Euhedral, prismatic zircons were recovered (Fig. 5g). Five of the seven analyses yield a weighted mean age of 83.80±0.21/0.24 Ma (MSWD = 1.09, probability = 0.36; Fig. 7). The excluded analyses include the youngest at 83.2±0.3 Ma and the oldest at 91.9±0.7 Ma. As demonstrated with previous bentonite samples, the older age is interpreted to be inherited from earlier eruptions. The younger age may be due to a small amount of Pb loss that was not eliminated by the chemical leaching process.
6.h. ER2010-E55 (Z11016)
Sample ER2010-E55 is from an approximately 50 cm thick bentonite at 277.5 m within the Hoodoo Dome section, Ellef Ringnes Island. Bed thickness is variable due to cryoturbation. The bentonite is situated between shale, slightly less platy below and platy (‘paper’-like) above the bentonite bed. The bentonite has a light yellow colour when not weathered.
Zircons occur as large, clear prismatic crystals with only minor fractures and few inclusions (Fig. 5h). Six analyses yield a single age population with a weighted mean age of 92.99±0.30/0.33 Ma (MSWD = 1.5, probability = 0.19; Fig. 8).
U–Pb Concordia diagram and weighted mean 206Pb/238U age plot for bentonite samples from the Kanguk Formation, Hoodoo Dome section, Ellef Ringnes Island. Analyses used in the weighted mean calculation are indicated by grey filled bars (open rectangles excluded from age calculation). Length of bar indicates 2σ analytical error of analyses. Two age errors are reported: standard error of the weighted mean (2σ or 95% confidence) calculated using Isoplot (Ludwig, Reference Ludwig2003) and the error that includes the quadratically added spike calibration error.
6.i. ER2010-E82 (Z11015)
Sample ER2010-E82 was collected at 376 m within the Ellef Ringnes section and is 15 cm thick. The bed is embedded within black platy shale. Zircon consists of euhedral elongate prisms often with c-axis elongated melt inclusions (Fig. 5). Four analyses yield a single age population with a weighted mean age of 88.47±0.17/0.22 (MSWD = 0.13, probability = 0.94; Fig. 8), which is interpreted as the age of the bentonite.
6.j. ER2010-E59 (Z11014)
Sample ER2010-E59 was taken 4 m above sample ER2010-E82. The bentonite layer is 40 cm thick and is underlain and overlain by black platy shale. Zircons occur as euhedral, equant to slightly elongate prisms with minor fractures and few inclusions (Fig. 5j). Five analyses yield a weighted mean age of 88.07±0.12/0.18 Ma (MSWD = 1.04, probability = 0.39; Fig. 8). One analysis excluded from the calculation has a slightly older age of 88.5±0.2 Ma, similar to the age of the underlying bentonite layer.
7. Discussion
7.a. Sedimentation rates and position of the OAE2 layer in the lower Kanguk Formation
The Cenomanian–Turonian boundary is placed within the lower Kanguk Formation based on chemostratigraphic and carbon isotopic evidence for the OAE2 event (Fig. 4; Lenniger et al. Reference Lenniger, Nøhr-Hansen, Hills and Bjerrum2014; Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014; Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014; Herrle et al. Reference Herrle, Schröder-Adams, Davis, Pugh, Galloway and Fath2015). In the Glacier Fiord (GF) section the boundary is placed at the 1622 m level (Fig. 4), the point at which the carbon isotope excursion decreases back to more negative values (Herrle et al. Reference Herrle, Schröder-Adams, Davis, Pugh, Galloway and Fath2015). In the Ellef Ringnes section the boundary is placed at the 203 m level based on similar criteria (Fig. 4; Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014). This level marks the top of the OAE2 that corresponds to the Cenomanian–Turonian boundary with an age of 93.9±0.2 Ma (GTS2012; Gradstein & Ogg, Reference Gradstein, Ogg, Gradstein, Ogg, Schmitz and Ogg2012).
The sampled bentonites do not occur within the proposed OAE2 interval and therefore do not provide a direct constraint on the age of the OAE2 layer. In the GF section the lowest bentonite sample occurs 19 m above the excursion and thus provides a minimum age for the excursion at 93.03±0.25 Ma. The approximate position within the section of the 93.9 Ma Cenomanian–Turonian boundary can be estimated by back projection using sedimentation rates calculated from the dated bentonite layers (Fig. 9). Linear regression of the five dated bentonites (AH2011-C56, C59, C61, C65, C68) from the GF section yields an integrated linear sedimentation rate of ~19 m Ma−1, which projects back to the stage boundary age of 93.9 Ma at approximately 1626 m, close to the top of the carbon isotope excursion identified at 1622 m (Fig. 9). There are numerous uncertainties in this approach, including (1) uncertainty in the exact position of the lowermost bentonite layer, as it comes from a part of the section that is largely covered; (2) uncertainties in the measured ages; (3) differential compaction; and (4) variable and non-linear sedimentation rates over the entire interval, as indicated by small changes in silt to mudstone ratios within the interval. These may all contribute to the observed scatter about the linear regression. Although not particularly precise, this analysis is consistent with the interpreted position of OAE2 boundary in the GF section by Schröder-Adams et al. (Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014) and Herrle et al. (Reference Herrle, Schröder-Adams, Davis, Pugh, Galloway and Fath2015) and indicates that it is of comparable age to other global occurrences (Meyers et al. Reference Meyers, Siewert, Singer, Sageman, Condon, Obradovich, Jicha and Sawyer2012; Ma et al. Reference Ma, Meyers, Sageman, Singer and Jicha2014).
Plot of bentonite age against stratigraphic height above the OAE2 horizon. The zero datum is taken at the top of the OAE2 layer based on the negative excursion in carbon isotope composition (Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014; Herrle et al. Reference Herrle, Schröder-Adams, Davis, Pugh, Galloway and Fath2015). The stratigraphic positions of key macrofossil occurrences Scaphites nigricollensis and Scaphites depressus are from Schröder-Adams et al. (Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014) and Hills et al. (Reference Hills, Braunberger, Núñez-Betelu and Hall1994). Chronostratigraphic position of S. nigricollensis and S. depressus ammonite zones based on ages of volcanic ash layers from western USA of Sageman et al. (Reference Sageman, Singer, Meyers, Siewert, Walaszczyk, Condon, Jicha, Obradovich and Sawyer2014) and interpolated numerical ages from Ogg, Hinnov & Huang (Reference Ogg, Hinnov, Huang, Gradstein, Schmitz and Ogg2012). For clarity, sample names are shown abbreviated to their last three characters (e.g. AH2011-c68 is abbreviated to c68). Sedimentation rates are calculated by linear regression where more than two control points are available in the lower part of the Glacier Fiord (GF) section and in the Ellef Ringnes (ER) section. The upper Kanguk Formation sedimentation rate is based on a two-point linear interpolation between bentonite AH2011-C84 and AH2011-C65. The adjusted stratigraphic position of bentonites ER2010-E82 and ER2010-E59, shown as large arrows, is based on correlating the sections at bentonite layers AH2011-C56 and ER2010-E55 and assuming similar sedimentation rates for both sections between 93 and 88 Ma.
The stratigraphic positions of the dated bentonites indicate a substantive change in sedimentation rate within the Kanguk Formation at GF, from relatively low rates within the first 50 m of the section to higher rates up section (Fig. 9). Between 93 and 91Ma the integrated sedimentation rate in the GF section is ~19 m Ma−1. In the upper 200 m of the section the integrated sedimentation rate between 91 and 84 Ma is greater at 26 m Ma−1. This suggests increasing terrigenous input to the basin after 90 Ma, which corresponds to a global regressive cycle during the late Turonian (Gradstein et al. 2012). Generally, the lithology of the Coniacian to Santonian interval at Glacier Fiord is characterized by increased frequency of siltstone to fine sandstone beds interbedded with mudstone, supporting the interpretation of increased allochthonous sediment input (Schröder-Adams et al. Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014; Fig. 4). Increasing terrestrial influence in the upper Kanguk Formation is also indicated by decreasing TOC values and an increase in terrestrial kerogen in both the Glacier Fiord and Ellef Ringnes sections (Núñez-Betelu et al. Reference Núñez-Betelu, MacRae, Hills, Muecke, Thurston and Fujita1994; Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014).
There are fewer data available for the Ellef Ringnes section, but the integrated sedimentation rate over the interval bracketed by bentonite ages between 93 and 88 Ma is calculated at ~21 m Ma−1, slightly lower than the rate calculated for the upper part of the GF section between 91 and 84 Ma (Fig. 9). There is, however, a major discrepancy between the lower parts of the two sections. On Ellef Ringnes, the stratigraphic position of the OAE2 is placed at 203 m (Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014). Assuming an age of 93.9 Ma at this stratigraphic level requires a sedimentation rate of ~72 m Ma−1 to account for the 70 m of section between the OAE2 layer and bentonite layer ER2010-E55 dated at 92.98 Ma. This is in sharp contrast to the GF section, where the sedimentation rate is estimated to be ~19 m Ma−1 over the same time interval.
This difference cannot be explained by an inaccurate age interpretation for bentonite ER2010-E55, as all three samples dated from the Ellef Ringnes section plot at a higher stratigraphic position relative to the proposed OAE2 interval than dated bentonites in the GF section (Fig. 9). The upper parts of the two sections appear to have comparable sedimentation rates. This is demonstrated by pinning both sections at the positions of the ER2010-E55 and AH2011-C56 bentonite layers with ages of ~92.99 Ma, rather than at the interpreted Cenomanian–Turonian boundary as shown on Figure 4 (arrow on Fig. 4). If pinned at the ~92.99 Ma datum, the upper bentonites in the Ellef Ringnes section plot consistently with their predicted stratigraphic position relative to the GF section (Fig. 9). This argues that the upper part of both sections had comparable sedimentation rates, on the order of 20–30 m Ma−1 between ~91 and 88 Ma.
One possible explanation for the discrepancy in the lower part of the section is that the OAE2 horizon occurs higher in the section than interpreted by Pugh et al. (Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014). Assuming comparable sedimentation rates in the two sections, the Cenomanian–Turonian boundary would be predicted to occur at approximately 265 m in the Ellef Ringnes section. However, there is no evidence in the carbon isotope data or other geochemical signals to support this possibility (Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014).
More likely, the difference reflects significant regional variations in sedimentation rate, the cause of which remains speculative. It may relate to the depositional environment immediately prior to the major late Cenomanian transgression. On Ellef Ringnes Island, the Kanguk Formation directly overlies deltaic deposits of the Hassel Formation, indicative of a major fluvially derived sediment source (Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014). During the major transgression represented by the lower Kanguk Formation the area may have continued to receive sediment depositing either as prodelta muds or possibly mass flow deposits. Identification of these types of deposits within sediments affected by cryoturbation is nearly impossible. In contrast, the Kanguk Formation at Glacier Fiord overlies the Bastion Ridge Formation, which represents a restricted basin environment. No evidence for a major fluvial sediment source is noted, suggesting a reduced sediment supply in this area immediately before the major transgression.
7.b. Biostratigraphic constraints
Two relatively well-calibrated ammonite macrofossils are documented in the GF section: S. nigricollensis and S. depressus (Figs 4, 9). Bentonites have not been dated in close proximity to the first occurrence of the ammonites, so constraints on the timing of first appearance in the section rely on estimates based on sedimentation rates. Bentonite AH2011-C65 occurs ~40 m below the first occurrence of S. nigricollensis in the GF section. Based on interpolation using the sedimentation rates described above, the S. nigricollensis occurrence could be as much as 1 Ma older than its predicted stratigraphic position based on the zone age range of 90.24 to 89.98 Ma developed for the Western Interior Seaway (Ogg, Hinnov & Huang, Reference Ogg, Hinnov, Huang, Gradstein, Schmitz and Ogg2012) (Fig. 9). Given the uncertainties in calculation of sedimentation rates, the position of S. nigrocollensis in the section is not precisely dated; however, it occurs within 1 Ma of its age in the Western Interior Seaway. This suggests that the timing of the High Arctic and Western Interior Seaway occurrences is broadly comparable, with an allowable difference estimated to be less than 1 Ma. The report of Scaphites depressus by Hills et al. (Reference Hills, Braunberger, Núñez-Betelu and Hall1994) is inferred to be at ~1770 m in the GF section, in a poorly preserved part of the section without tight chronological control provided by U/Pb geochronology of zircon in bentonites. The basal age range for this ammonite zone is estimated to be 87.86 Ma with a duration of 1.6 Ma (Ogg, Hinnov & Huang, Reference Ogg, Hinnov, Huang, Gradstein, Schmitz and Ogg2012; Sageman et al. Reference Sageman, Singer, Meyers, Siewert, Walaszczyk, Condon, Jicha, Obradovich and Sawyer2014). The lower part of this age range corresponds well to the stratigraphic position estimated by linear projection between bentonite layers C84 and C65 within the GF section, again indicating less than 1 Ma. difference between high- and low-latitude occurrences.
The first occurrence of Sphenoceramus patootensis occurs 15 m above bentonite layer AH2011-C84 within the GF section. Bentonite AH2011-C84 has an age of 83.80±0.21/0.24 Ma, placing these inoceramids within the late Santonian. Accumulation of 15 m of section above the bentonite corresponds to an interval of approximately 0.6 Ma based on calculated sedimentation rates, suggesting that the first occurrence is at c. 83.2 Ma. Sphenoceramus patootensis also occurs within the Ellef Ringnes section; however, the nearest dated bentonite layer occurs at least 70 m before the first occurrence. Calculation based on sedimentation rates predict ages of 83.7–84.7 Ma for the bentonite layer immediately below the S. patootensis occurrence at 470 m, comparable to the 83.80±0.21/0.24 Ma bentonite age determined from below the S. patootensis occurrence in the GF section.
The bentonite ages can also be used to calibrate the partial radiolarian range zones recently established for the Ellef Ringnes section. Pugh et al. (Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014) established the Eostichomitra carnegiense partial range zone and placed its base into the upper middle Turonian. Its first occurrence is above the ER2010-E55 bentonite and thus corresponds to an age of approximately 92 Ma or slightly older, which suggests a middle-middle Turonian age. The last occurrence of E. carnegiense occurs at 433.5 m. Based on the sedimentation-rate estimated age of 83.7-84.7 Ma for the bentonite right below the S. patootensis zone, the last occurrence of this radiolarian taxon falls within middle Santonian. The second radiolarian partial range zone defined by Pugh et al. (Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014) at Ellef Ringnes Island for the Sverdrup Basin is based on the range of Dictyomitra multicostata. Its first occurrence is at 373 m just below bentonite ER2010-E82 with an age of 88.47 Ma, confirming a lower Coniacian age. Its last occurrence is equivalent to the last occurrence of E. carnegiense, discussed above. Determination of the taphonomic bias of these ranges is an area for future work.
The Kanguk Formation at Glacier Fiord yielded agglutinated foraminifera, but no siliceous-walled plankton, which might have been removed by diagenetic processes during deep burial. Schröder-Adams et al. (Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014) describe three benthic foraminiferal assemblage zones for the Kanguk Formation at GF; these are, in ascending order, the Trochammina rutherfordi, Dorothia smokyensis/Evolutinella boundaryensis and Glaphyrammina spirocompressa zones. Trochammina ruthorfordi is a dominant species in the OAE2 interval at GF due to this genus's adaptation to depleted oxygen conditions. Evolutinella boundaryensis has its first appearance between bentonites AH2011-C56 and AH2011-C59 in the GF section, suggesting an age of roughly 92.4 Ma or lower middle Turonian. Glaphyrammina spirocompressa occurs together with latest Santonian S. patootensis.
7.c. Late Cretaceous magmatism in the Sverdrup Basin: regional correlations
The preservation of thick bentonite beds in the Bastion Ridge and Kanguk formations indicates that explosive felsic magmatism occurred episodically over a period of up to 15 Ma, between ~98 Ma and 83 Ma, with particularly frequent deposition of bentonites between 93 and 83 Ma. Geochemical analyses of the bentonite layers within the Kanguk Formation have within-plate, alkaline signatures (Parsons, Reference Parsons1994), consistent with sources associated with younger magmatism in the HALIP (Thorarinsson, Holm, Duprat et al. Reference Thorarinsson, Holm, Duprat and Tegner2011, Reference Thorarinsson, Holm, Duprat and Tegner2012; Estrada & Henjes-Kunst, Reference Estrada and Henjes-Kunst2013; Jowitt, Williamson & Ernst, Reference Jowitt, Williamson and Ernst2014). The volcanic source area for the ash remains conjectural. Distance to volcanic source can be roughly estimated based on bed thickness. Regional changes in volcanic ash bed thickness decrease more or less exponentially with distance from the eruptive centre, and are influenced by a number of additional factors including eruption volume, eruption duration, eruption column height, grain-size distribution and atmospheric conditions (e.g. Mastin, Van Eaton & Lowenstern, Reference Mastin, Van Eaton and Lowenstern2014). These factors cannot be easily quantified without detailed volcanological studies of individual ash deposits; however, some general estimates of distance from eruptive centre can be made based on theoretical models. Mastin, Van Eaton & Lowenstern (Reference Mastin, Van Eaton and Lowenstern2014) developed a numerical model for ash dispersal from large Plinian eruptions typical of the Yellowstone volcanic centre, an intraplate, plume-related volcanic complex in the western United States. Their analysis suggests that for relatively large eruptions (330 km3), ash bed thicknesses of 10–40 cm, typical of those in the Kanguk Formation, occur at maximum distances from source of ~500–1000 km. A radius of 1000 km from the Glacier Fiord and Ellef Ringnes sections extends as far north as the proposed HALIP plume centre on the Alpha Ridge (Fig. 1, approximately 800–1000 km), and to the northeast to northern Greenland in the area of the Kap Washington volcanics (Tegner et al. Reference Tegner, Storey, Holm, Thorarinsson, Zhao, Lo and Knudsen2011; Thorarinsson et al. Reference Thorarinsson, Holm, Duprat and Tegner2011) and Peary dykes (Kontak et al. Reference Kontak, Jensen, Dostal, Archibald and Kyser2001). Included within this radius are known volcanic and intrusive centres of the Wootton Intrusive Complex and the Hansen Point volcanics, which are located on Ellesmere Island within a 400 km radius of the GF and EF sections. Volcanic centres associated with Cretaceous subduction-related magmatism in western North America and Russia are likely too distal to generate decameter thick ash beds in the eastern Arctic. These sources are also not indicated based on the alkaline, within-plate geochemical signature of the ash layers (Parsons, Reference Parsons1994).
The bentonite ages can be correlated with the timing of known magmatic events within the Sverdrup Basin and in Greenland. The bentonites at the base of the GF section and bentonite ER2010-E55 from the Ellef Ringnes section overlap in age with U–Pb zircon ages reported for the Wootton Intrusive Complex (Estrada & Henjes-Kunst, Reference Estrada and Henjes-Kunst2013). The Wootton Intrusive Complex was intruded within a tight time window between 92.7±0.3 Ma and 92.1±0.1 Ma (Estrada & Henjes-Kunst, Reference Estrada and Henjes-Kunst2013), which overlaps in time with the lowermost bentonites from the Ellef Ringnes (92.99±0.30) and Glacier Fiord sections (93.03±0.21 Ma). However, the range of bentonite ages within the Kanguk Formation suggests that volcanic activity was more protracted than indicated by the available ages of the Wootton Intrusive Complex, which span less than 1 Ma. It is possible that the Wootton Intrusive Complex had a more protracted magmatic history than has been currently documented, or that other, as yet unidentified centres were active within the region. The Hansen Point volcanics at Yelverton Bay, northern Ellesmere Island (Trettin & Parrish, Reference Trettin and Parrish1987; Estrada et al. Reference Estrada, Piepjohn, Henjes-Kunst and von Gosen2003) have an imprecise U–Pb age of 88 +20/ −21 Ma (Trettin & Parrish, Reference Trettin and Parrish1987) and 40Ar–39Ar ages of feldspar and whole rock between 83±2 and 77±4 Ma (Estrada & Henjes-Kunst, Reference Estrada and Henjes-Kunst2013). Most of the ages are for basaltic rocks, with a single rhyolitic ignimbrite dated at 79.2±1.9 Ma. The younger Ar–Ar ages reported by Estrada & Henjes-Kunst (Reference Estrada and Henjes-Kunst2013) indicate that the Hansen Point volcanics could only overlap in age with the youngest bentonite layer determined from the GF section. However, Bono, Tarduno & Singer (Reference Bono, Tarduno and Singer2013) report older Ar–Ar ages of 90–88 Ma for samples from the Hansen Point volcanics that correlate with the Coniacian bentonite ages from the Ellef Ringnes section. They also report an older age of ~95 Ma from the Hansen Point volcanics, similar in age to Strand Fiord Formation volcanics. The ages reported by Bono, Tarduno & Singer (Reference Bono, Tarduno and Singer2013) indicate that the volcanic rocks of Late Cretaceous age preserved on northern Ellesmere Island may have a more complex and extended eruption history than previously thought. The similarity of the bentonite ages to ages of magmatic rocks from volcanic centres on Ellesmere Island, together with the distance to source estimated from bed thickness, suggests that the most likely source for the Kanguk Formation ashes are volcanic centres located on northern Ellesmere Island or the Alpha Ridge.
In addition to the dated bentonite horizons, eruption ages can be predicted from the stratigraphic position of undated bentonites in the Ellef Ringnes section. Assuming sedimentation rates of between 27 and 21 m Ma−1 the four bentonites above bentonite E59 have predicted ages of 87.6, 87, 86.4 and 84.3 Ma. This suggests that explosive magmatism occurred frequently through the interval of 93–84 Ma, with a major eruption periodicity on the order of 0.5–2.5 Ma. For comparison, this frequency of eruption is similar to the major eruption frequency of the Yellowstone volcanic centre over the past 2.5 Ma, with a major eruption frequency of ~0.6–0.8 Ma. This argues for either a long-lived volcanic centre on northern Ellesmere Island that generated major Plinian volcanic eruptions over a period of at least 10 Ma, or perhaps several centres periodically active over the same time period.
8. Conclusions
U–Pb dating of bentonite layers within the Kanguk Formation indicates that the age of the OAE2 layer in the Canadian High Arctic (Herrle et al. Reference Herrle, Schröder-Adams, Davis, Pugh, Galloway and Fath2015) is consistent with other global occurrences at 93.89 Ma (e.g. Meyers et al. Reference Meyers, Siewert, Singer, Sageman, Condon, Obradovich, Jicha and Sawyer2012).
Ages for two relatively well-calibrated Scaphites nigricollensis and Scaphites depressus ammonite zones are estimated in the GF section based on interpolation between dated bentonite layers. The ages are comparable to those from lower-latitude sections in the Western Interior Seaway (Sageman et al. Reference Sageman, Singer, Meyers, Siewert, Walaszczyk, Condon, Jicha, Obradovich and Sawyer2014). The bentonite ages also constrain the timing of the partial radiolarian range zones recently established for the Ellef Ringnes section (Pugh et al. Reference Pugh, Schröder-Adams, Carter, Herrle, Galloway, Haggart, Andrews and Hatsukano2014), as well as three benthic foraminiferal assemblage zones defined for the Glacial Fiord section by Schröder-Adams et al. (Reference Schröder-Adams, Herrle, Embry, Haggart, Galloway, Pugh and Harwood2014).
The frequency of bentonite occurrences indicates repeated Plinian eruptions occurred over a 15 Ma interval between 98 and 83 Ma. The bentonites probably have alkaline sources (Parsons, Reference Parsons1994) and, based on numerical models for ash dispersal patterns (Mastin, Van Eaton & Lowenstern, Reference Mastin, Van Eaton and Lowenstern2014), bed thicknesses are consistent with derivation from known igneous centres on the Alpha Ridge, northern Ellesmere Island and Greenland. The frequency of eruption is greater than currently documented for igneous centres on northern Ellesmere Island, such as the Wooten Intrusive Complex, or Hansen Point volcanics, indicating that the alkaline phase of igneous activity within the HALIP region was longer with more frequent eruptions than currently documented.
Acknowledgements
Davis and Galloway were supported by funding from the Geo-mapping for Energy and Minerals (GEM-1) programme of the Geological Survey of Canada, Natural Resources Canada. Financial support to Schröder-Adams was provided by a CRD Grant of the Natural Sciences and Engineering Research Council (NSERC), with the Geo-mapping for Energy and Minerals (GEM) Program and ConocoPhillips Houston and Calgary as partners. Herrle received funding from the Deutsche Forschungsgemeinschaft (HE 3521/6). Julie Peressini, Linda Cataldo and Carole Lafontaine are thanked for their technical support in the U–Pb laboratory at the Geological Survey of Canada. Ray Chung and Ron Christie prepared mineral separates. Kevin Anderson helped check the final reference list. Chris Harrison is thanked for a review of an earlier draft of the paper, and Fernando Corfu and an anonymous reviewer provided comments that improved the paper. Earth Science Sector contribution no. 20150287
