Isachsen Formation Research Articles

Age and Geochemistry of High Arctic Large Igneous Province Tholeiitic Magmatism in NW Axel Heiberg Island, Canada

AbstractThe Cretaceous High Arctic Large Igneous Province (HALIP) in Canada involved extrusion of continental flood basalts (CFBs) at 130–120 Ma and 100‐95 Ma and emplacement of an extensive sill and dike network that intersected the Carboniferous to Paleogene Sverdrup Basin. In this paper, we present new 40Ar/39Ar ages, major and trace elements, and Sr‐Nd‐Pb isotope ratios for HALIP lava, dikes, and sills from Bukken Fiord, NW Axel Heiberg Island, Canadian Arctic Islands. Our best constrained 40Ar/39Ar ages yield a weighted average of 124.1 ± 1 (2σ) Ma, coincident with the first pulse of tholeiitic CFB magmatism in the Arctic‐wide HALIP as exemplified by Isachsen Formation flood basalts on Axel Heiberg Island. The Bukken Fiord samples are plagioclase and clinopyroxene‐phyric tholeiitic basalts, are relatively evolved (3.2–6.5 wt% MgO), and share similar major and trace element compositions to typical HALIP tholeiites. Initial 143Nd/144Nd ranges from 0.51260 to 0.51291 and initial 87Sr/86Sr ranges from 0.70362 to 0.70776, while measured 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb range from 18.614 to 19.199, 15.534 to 15.630, and 38.404 to 39.054, respectively. The most primitive sample in this study has Sr‐Nd‐Pb isotope signatures that suggest an enriched plume‐derived mantle source for HALIP tholeiites. Most samples, however, possess relatively radiogenic isotope signatures that can be explained by moderate degrees of assimilation of Sverdrup Basin sedimentary rocks. Magma‐crust interaction in the HALIP plumbing system was likely widespread and may have increased the environmental impact of the HALIP, particularly if crustal carbon was volatilized.

Sequence stratigraphy and underlying tectonism of the Northern Richardson Mountains and adjacent Mackenzie Delta related to the formation of the Arctic Ocean

Abstract Jurassic-Cretaceous rifting within Arctic North America that eventually resulted in the formation of the Amerasia Basin greatly affected deposition within nearshore terrestrial basins in surrounding areas, such as the Brooks-Mackenzie Basin and the Sverdrup Basin. In this paper, four measured stratigraphic sections and detrital U-Pb zircon data (N = 14; n = 3339) from outcrops within the Northern Richardson Mountains are integrated with well-log and core data from the adjacent Mackenzie Delta to establish a model-independent sequence stratigraphic framework for the Brooks-Mackenzie Basin. This framework is used to elucidate the underlying tectonic forces responsible for the observed stratigraphy, relating patterns of deposition to various phases of rift development. The proposed interpretation classifies the sub-Jurassic unconformity as a first-order sequence boundary, demarcating a change in tectonic setting from pre-rift to rift, meaning it is also classified as the rift onset unconformity for the Amerasia Basin. The overlying Bug Creek Group is largely progradational, representing a first-order lowstand systems tract corresponding to early syn-rift strata. Detrital zircon data from the Bug Creek Group lacks syn-depositional detrital zircon and resembles the underlying Permian, suggesting a recycled source of sediment. A change in the detrital zircon signature is observed within the uppermost Aklavik Formation, representing a new source. This also coincides with a rapid transition from shoreface sandstones of the Aklavik Formation to the lower offshore deposits of the Husky Formation caused by normal faulting in the Husky Lakes Fault Zone which is associated with a dramatic basin expansion. Maximum transgression occurs above the Arenaceous Member of the Husky Formation, likely near the Jurassic-Cretaceous boundary, representing a first-order maximum flooding surface and rift-climax. Above the maximum flooding surface, the overall progradational signature represents a first-order highstand systems tract and early post-rift deposits. The sub-Hauterivian basal Kamik unconformity is observed at the base of the subsequent falling stage systems tract. This first-order sequence boundary is approximately time equivalent to the breakup unconformity for the Arctic Ocean, with the overlying Kamik Formation resembling the Isachsen Formation in the Sverdrup Basin.

Exploring the role of High Arctic Large Igneous Province volcanism on Early Cretaceous Arctic forests

The Hauterivian–Aptian Isachsen Formation at Glacier Fiord, Axel Heiberg Island, in the Sverdrup Basin of the Canadian Arctic Archipelago was deposited contemporaneous with initiation of the High Arctic Large Igneous Province (HALIP). New palynological biostratigraphy and paleoenvironmental reconstruction, in coordination with the emerging geochronology of HALIP igneous rocks, permits exploration of the effects of volcanism on Arctic vegetation during the Early Cretaceous. Four informal terrestrial palynofloral zones are defined and used to reconstruct vegetation change over the Isachsen Formation's ca. 17 million year history and explore the role of the HALIP in these changes. Climate warming during the Hauterivian promoted expansion of a hinterland community dominated by members of the Pinaceae. By the middle Barremian, this community was replaced by mixed heathland and mire, represented by up to 70% fern spores in the uppermost Paterson Island Member, that may be, in part, in response to environmental disturbance associated with volcanic flows of the HALIP. Above the fern spore spike, dinoflagellate cyst assemblages suggest an early Aptian age and a marine setting for mudstones of the Rondon Member in which Ocean Anoxic Event 1a is recorded. An interval of floral instability is recorded in the overlying Walker Island Member, characterized by fluctuations in Pinaceae and Cupressaceae pollen and fern spores, possibly as a result of post-OAE 1a temperature variabilty and landscape disturbance associated with lava flows of the HALIP that were repeatedly extruded onto the subsiding delta plain during deposition of the member.

Carbon isotope and sequence stratigraphy of the upper Isachsen Formation on Axel Heiberg Island (Nunavut, Canada): High Arctic expression of oceanic anoxic event 1a in a deltaic environment

Abstract The Early Cretaceous oceanic anoxic event (OAE) 1a documents a major perturbation of the global carbon cycle with severe consequences for the ocean-climate-biosphere system. While numerous studies over the past decades have provided a relatively detailed picture of the environmental repercussions of OAE 1a at low and mid-latitudes, studies from high latitudes, in particular the High Arctic, are limited. In this study, we present a high-resolution carbon isotopic and sequence stratigraphic framework for the lower to lower upper Aptian interval of the Isachsen Formation of the High Arctic Sverdrup Basin (Canada). These data enable us to precisely locate the stratigraphic position of OAE 1a in a deltaic sedimentary environment. The carbon isotope record allows, for the first time, identification of the different carbon isotope segments (CISs) of OAE 1a in the Sverdrup Basin and thereby correlation of the High Arctic record with sections from lower latitudes. Based on this improved chemostratigraphy, we revise the age of upper Paterson Island, Rondon, and Walker Island Members, important regional lithostratigraphic marker units. Whole-rock geochemical data record two episodes of marine incursion into the Sverdrup Basin during OAE 1a (CISs Ap3 and Ap6), which are interpreted as regional maximum flooding surfaces. This information is used in conjunction with detailed sedimentological logs and geochemical grain-size proxies to refine the sequence stratigraphic framework for the upper Isachsen Formation. We propose that transgressive-regressive cycles in the Sverdrup Basin were controlled mainly by the combined effects of eustatic sea-level changes and regional tectonic uplift, potentially related to the emplacement of Alpha Ridge, which culminated at ca. 122 Ma during CIS Ap9.

A revised stratigraphic framework for Cretaceous sedimentary and igneous rocks at Mokka Fiord, Axel Heiberg Island, Nunavut, with implications for the Cretaceous Normal Superchron

New data and interpretations on geological relationships of igneous rocks at Mokka Fiord, Axel Heiberg Island, Nunavut, provide insight into the timing and nature of magmatism associated with the Sverdrup Basin and High Arctic Large Igneous Province (HALIP). Field relationships indicate that the igneous rocks, previously interpreted to be volcanic flows, are most likely an intrusive unit discordant to regional bedding. An intrusive origin helps resolve chronostratigraphic inconsistencies in previous work. The host rocks are palynologically constrained to be late Barremian to late Aptian in age and are interpreted to be Paterson Island or Walker Island member of the Isachsen Formation. If the igneous body is intrusive, it’s previously reported Ar–Ar age (102.5 ± 2.6 Ma) is no longer in conflict with accepted stratigraphic interpretations and probably reflects the emplacement age of the intrusion. Lingering uncertainties in interpreting the normal and reverse magnetic polarities determined in the previous work remain, and both are considered viable. Although this uncertainty precludes definitive conclusions on the significance of paleomagnetic data at Mokka Fiord, examination of the stratigraphic, paleomagnetic, and geochronologic relationships there highlight potential for the study of excursions, or reversed magnetic polarity subchrons, in the Cretaceous Normal Superchron elsewhere in the HALIP.

U-Pb geochronology of the plumbing system associated with the Late Cretaceous Strand Fiord Formation, Axel Heiberg Island, Canada: part of the 130-90 Ma High Arctic large igneous province

The c. 130-90 Ma High Arctic Large Igneous Province (HALIP) greatly influenced the geological evolution of Canada’s Arctic Islands along with formerly adjacent Svalbard (Norway), Franz Josef Land (Russia) and offshore regions such as the Alpha-Mendeleev Ridge. Three subalkaline tholeiitic pulses constitute the HALIP: (1) an initial two pulses at ca. 120–130 Ma in the Canadian portion are temporally associated with the Isachsen Formation basalts, and a later pulse at ca. 95–100 Ma fed extensive flood basalt lavas of the Strand Fiord Formation. Fieldwork at South Fiord on Axel Heiberg Island revealed a small network of sills and dykes. The U-Pb ID-TIMS baddeleyite age for diabase from a >10 m SSE-trending dyke (sample 13-CK-16) is 95.18 ± 0.35 Ma (2σ) and for a gabbro from a >50 m thick sill ∼7 km to the dyke’s SW (sample 13-WJA-C028-A2) is 95.41 ± 0.12 Ma (2σ), identical within stated uncertainties. The two ages, in tandem with the similar geochemistry of the intrusions and Strand Fiord Formation lavas, provide the first precise age constraints for the feeder system of the Strand Fiord Formation which represents the younger pulse of HALIP. The two samples are distinct from each other in their Sm-Nd isotopic values suggesting they were derived from separate batches of magma that fed separate eruptive episodes.

Mesozoic rift to post-rift tectonostratigraphy of the Sverdrup Basin, Canadian Arctic

Jurassic-Cretaceous rift successions and basin geometries of the Sverdrup Basin are reconstructed from a review and integration of stratigraphy, igneous records, outcrop maps, and subsurface data. The rift onset unconformity is in the Lower Jurassic portion of the Heiberg Group (approximately 200–190 Ma). Facies transgress from early syn-rift sandstones of the King Christian Formation to marine mudstones of the Jameson Bay Formation. The syn-rift succession of marine mudstones in the basin centre, Jameson Bay to Deer Bay formations, ranges from Early Jurassic (Pleinsbachian) to Early Cretaceous (Valanginian). Early post-rift deposits of the lower Isachsen Formation are truncated by the sub-Hauterivian unconformity, which is interpreted as a break up unconformity at approximately 135–130 Ma. Cessation of rift subsidence allowed for late post-rift sandstone deposits of the Isachsen Formation to be distributed across the entire basin. Marine deposition to form mudstone of the Christopher Formation throughout the Canadian Arctic Islands and outside of the rift basin records establishment of a broad marine shelf during post-rift thermal subsidence at the start of a passive margin stage. The onset of the High Arctic Large Igneous Province at approximately 130 Ma appears to coincide with the breakup unconformity, and it is quite typical that magma-poor rifted margins have mainly post-rift igneous rocks. We extend the magma-poor characterization where rifting is driven by lithospheric extension, to speculatively consider that the records from Sverdrup Basin are consistent with tectonic models of retro-arc extension and intra-continental rifting that have previously been proposed for the Amerasia Basin under the Arctic Ocean.

Early Cretaceous vegetation and climate change at high latitude: Palynological evidence from Isachsen Formation, Arctic Canada

Quantitative palynology of the marginal marine and deltaic-fluvial Isachsen Formation of the Sverdrup Basin, Canadian Arctic, provides insight into high latitude climate during much of the Early Cretaceous (Valanginian to early Aptian). Detrended Correspondence Analysis of main pollen and spore taxa is used to derive three ecological groupings influenced by moisture and disturbance based on the botanical affinities of palynomorphs: 1) a mixed coniferous assemblage containing both lowland and upland components; 2) a conifer-filicopsid community that likely grew in dynamic lowland habitats; and, 3) a mature dry lowland community composed of Cheirolepidiaceans. Stratigraphic changes in the relative abundance of pollen and spore taxa reflect climate variability in this polar region during the ∼20 Mya history of the Isachsen Formation. The late Valanginian was relatively cool and moist and promoted lowland conifer-filicopsid communities. Warming in the Hauterivian resulted in the expansion coniferous communities in well-drained or arid hinterlands. A return to relatively cool and moist conditions in the Barremian resulted in the expansion of mixed lowland communities. This work demonstrates the utility of a multivariate statistical approach to palynology to provide insight into the composition and dynamics of ecosystems and climate of high latitude regions during the Early Cretaceous.

Evidence for protracted High Arctic large igneous province magmatism in the central Sverdrup Basin from stratigraphy, geochronology, and paleodepths of saucer-shaped sills

Field evidence, map compilation, geochemistry, geochronology, and potential field data document six intervals of Cretaceous magmatism in the central Sverdrup Basin. These are: (1) Hauterivian (ca. 130 Ma) volcaniclastic deposition in the lower Isachsen Formation; (2) 126.6 ± 1.2 Ma (U-Pb zircon) gabbroic intrusion; (3) 120.8 ± 0.8 Ma (U-Pb baddeleyite) diabasic intrusion; (4) 105.40 ± 0.22 Ma (U-Pb detrital zircon) pyroclastic deposition at the top of the Invincible Point Member, Christopher Formation; (5) upper Albian (ca. 103 Ma) pillow and hydroclastic breccia in the upper Christopher Formation; and (6) uppermost Albian (ca. 101 Ma) volcanic breccia and scoria in the Hassel Formation. Whole-rock geochemical data show that these magmatic rocks are similar to previously documented High Arctic large igneous province tholeiitic basalts, but analyses of fresh glass in tuffs reveal evolved ferroandesite to dacite compositions not recorded in whole-rock data. Approximate ages of saucer-shaped sills inferred from the relationship of sill width to depth of emplacement suggest at least three intervals of sill emplacement between 130 and 120 Ma. The new data show that volcanism in the Sverdrup Basin was of greater spatial extent, and that magmatism occurred more frequently, than was previously recognized. Comparison of the new central Sverdrup Basin data and interpretations with other data sets from the Sverdrup Basin, Svalbard, and Franz Josef Land suggests that High Arctic large igneous province magmatism occurred over a more extended period of time in the central Sverdrup Basin than in other regions.

Arctic plesiosaurs from the Lower Cretaceous of Melville Island, Nunavut, Canada

An expedition to Melville Island in Nunavut, Canada, recovered the fragmentary fossils of several plesiosaurs from non-marine deposits of the Hauterivian–Aptian Isachsen Formation. These plesiosaur fossils are some of the oldest Early Cretaceous records of the group in North America, and they likely predate the formation of a continuous Western Interior Seaway. The plesiosaurs from Melville Island appear to be primarily juveniles, and would have been living in a region that experienced at least seasonally cool temperatures. The presence of these fossils in a fluvial deposit support previous suggestions that juvenile plesiosaurs may have preferentially inhabited shallower waters rather than open marine environments. These fossils also show that polycotylid plesiosaurs were able to rapidly disperse and colonize high latitude coastal regions, as their occurrence in Arctic Canada only slightly postdates the first confirmed appearance of the group in Australia.

Sedimentology and stratigraphy of the Lower Cretaceous Isachsen Formation: Ellef Ringnes Island, Sverdrup Basin, Canadian Arctic Archipelago

The Lower Cretaceous Isachsen Formation is a widespread, siliciclastic succession up to 1.4 km thick that records the northwestward progradation of fluvio-deltaic environments into the Sverdrup Basin in the Canadian Arctic. Although the Isachsen Formation is a prominent unit with proven oil and gas reserves, detailed outcrop descriptions of its sedimentology and stratigraphy, and correlation of these features to subsurface data are rare. We examined outcrop exposures of the Isachsen Formation on Ellef Ringnes Island in the province of Nunavut in the Canadian Arctic in order to document and interpret the sedimentology and stratigraphy of this unit in the central portion of the Sverdrup Basin. Interpretations are based on observations from over 5000 m of measured stratigraphic sections, local mapping of facies contacts, palynology, and sandstone petrography. Deposits of the Isachsen Formation on Ellef Ringnes Island can be divided into six broadly-defined facies associations that record deposition in braided-fluvial, meandering-fluvial, floodplain, tidally-influenced fluvial, shoreface, and offshore environments. Sandstones within the Isachsen Formation are typically quartzose, commonly with compositions of >90% well-rounded, monocrystalline quartz. Although the sedimentary details between outcrop locations vary, a generalized stratigraphic trend is present within the Isachsen Formation. Where exposed, the base of the unit consists of marginal marine sandstones abruptly overlain by braided stream deposits, which grade into mudstone-rich, meandering-fluvial and floodplain deposits. These nonmarine deposits are overlain by a thin marine mudstone (Rondon Member), which is in turn overlain by marginal marine sandstone and fluvial deposits. The uppermost portion of the Isachsen Formation records a transgression and is overlain by marine mudstones of the Christopher Formation. The stratigraphic succession of the Isachsen Formation in outcrops correlates to data from nearby hydrocarbon exploration wells. Stratigraphically, the Isachsen Formation on Ellef Ringnes Island represents a regressive–transgressive sequence, but contains within it two higher-order regressive–transgressive sequences, which is consistent with observations from elsewhere in the Sverdrup Basin. The detailed descriptions of facies within the Isachsen Formation on Ellef Ringnes Island and their correlation to subsurface data can be extrapolated to other parts of the Sverdrup Basin where data are lacking.

Detrital zircon characteristics of the Lower Cretaceous Isachsen Formation, Sverdrup Basin: source constraints from age and Hf isotope data

Detrital zircons from the Lower Cretaceous Isachsen Formation of the Sverdrup Basin, Canadian Arctic Archipelago, have been dated by the U–Pb method and analyzed for Hf isotopes by laser ablation microprobe – inductively coupled plasma – mass spectrometry (LAM–ICP–MS). Five samples from four locations on Ellesmere and Axel Heiberg islands display similar ranges of U–Pb ages, with significant zircon populations at 2.8–2.6, 1.9–1.8, 1.7–1.6, and 1.2–1.0 Ga. Major hiatuses occur between 2.4 and 2.0 Ga and from 0.96 to 0.5 Ga. Low initial εHfvalues indicating recycled crust components are significant in Palaeoproterozoic (1.9–1.8 Ga) and Neoarchaean (2.8–2.6 Ga) zircon populations. Other U–Pb age populations in the studied samples are dominated by zircon with positive εHfvalues, indicating a significant contribution from mantle-derived protoliths. The εHfvalues seen within a given U–Pb age population are generally consistent, with only minor scatter among the different samples. U–Pb and Hf data closely resemble previously published data from Lower Cretaceous rocks in northern Greenland, suggesting they have the same origin. The data are also largely consistent with the East Greenland Caledonides and the Precambrian basement of Greenland and northern Canada as predominant sources of zircon for the studied sandstones. However, based on the level of similarity between data from the Wandel Sea Basin and Sverdrup Basin sediments and on previous Nd isotope work in the Sverdrup Basin, it is likely that the sediments represent redeposited lower and middle Palaeozoic sediments.

Upper Barremian to upper Albian (Lower Cretaceous) dinoflagellate cyst assemblages, Canadian arctic archipelago

The dinoflagellate cyst assemblages from four Lower Cretaceous sections from Melville Island, Axel Heiberg Island and Ellesmere Island in the Northwest Territories, Canada are described. The sections represent the upper part of the Isachsen Formation and the Christopher Formation. The strata studied comprise a composite stratigraphic section of approximately 1400m of shale and siltstone deposits. The study of 33 samples yielded 83 dinoflagellate cyst species, which date the strata as late Barremian to early late Albian. The flora discussed is correlative with assemblages described from other parts of the arctic area. This study provides new and detailed palynological information about the Lower Cretaceous of the Arctic Islands. Comments are included on the morphology and ranges of: Acanthaulax sp. aff. A.? tenuiceras, Apteodinium sp. cf. A. maculatum ssp. grande, Balmula sp. cf. B. tripenta, Batioladinium longicornutum, Batioladinium shaftesburiense, Canningia reticulata, Catastomocystis spinosa, Circulodinium sp. cf. C. attadalicum, Conosphaeridium sp., Cribroperidinium sp. aff. C. conopium, Fromea triquetra, Muderongia pariata, Nyktericysta davisii, Nyktericysta? vitrea, Nyktericysta sp., Pseudoceratium sp. aff. P. toveae, Pseudoceratium sp., Rhombodella paucispina, Senoniasphaera microreticulata, Vesperopsis mayi and Vesperopsis nebulosa.

Paleomagnetism of Cretaceous volcanic rocks of the Sverdrup Basin—magnetostratigraphy, paleolatitudes, and rotations

The principal magnetization of lavas of the Isachsen and Strand Fiord formations on Axel Heiberg Island is shown to predate the Eocene Eurekan Orogeny. Basalt flows of the Strand Fiord Formation, volcanigenic sandstone from the Christopher Formation, and the uppermost flows of the Isachsen Formation are normally magnetized. Reversed magnetizations are found only in the Isachsen Formation, occurring at two horizons, which, we suggest, correspond to M0 and M1 of the M sequence of marine magnetic anomalies (118–123 Ma). It is possible, therefore, that we have located, at least approximately, the base of the Cretaceous normal polarity superchron in these sections. Because inclinations are steep, the analysis of directions of magnetization is not straightforward and has been done by two methods. Method I assumes that no relative rotations have occurred amongst sample localities, and calculations on this basis show a 33 ± 24 °(P = 0.05) counterclockwise rotation with no paleolatitudinal displacement relative to North America. The rotation is in agreement with the rotation of 36 ± 8 °(P = 0.05) determined earlier from the Permian Esayoo Formation on Ellesmere Island. Analysis by method I assumes that the Esayoo and the Isachsen – Strand Fiord sampling localities on Axel Heiberg and Ellesmere Island are contained within what is essentially one large structural domain. The agreement (using method I) of paleolatitude with that of North America is consistent with standard plate reconstructions in which there is a gap of about 300 km between Greenland and Ellesmere Island. However, the dispersion of site-mean directions is greater than that expected for paleosecular variation during the Cretaceous, and therefore some of the dispersion may be attributable to relative motions amongst collecting localities. Therefore, by method II, relative rotations amongst localities are assumed to have occurred, and inclinations and declinations are analysed separately. As with method I, declinations are predominantly counterclockwise from that expected, but by method II the mean inclination (74 ± 2 °standard error) is significantly shallower than that expected (79 ± 1 °standard error). This apparent flattening is consistent with the idea that the Arctic Islands were close to Greenland in the Cretaceous and that there was no gap along Nares Strait. Hence both methods of calculation yield similar counter clockwise rotation, but each gives slightly different paleolatitudes. The latter difference cannot at present be resolved.

Paleocurrent Analysis of Alluvial Sediments: A Discussion of Directional Variance and Vector Magnitude

ABSTRACT A tabulation of recent work on current indicators in modern rivers shows that directional variance increases with decreasing structure scale, in fairly close agreement with the structure hierarchy concept of Allen (1966). Fluvial currents are vectors, definable by direction and magnitude, but most paleocurrent studies ignore magnitude. It is proposed that azimuth readings be weighted according to the cube of current structure thickness, this being a volume measure corresponding to the distance in all three dimensions over which a local flow vector might reasonably be assumed to maintain the same direction. It is also a measure of the quantity of sediment moved by the flow vector. Examples are presented in which the proposed weighting factor is applied to data from the fluvial Isachsen Formation (Cretaceous) and deltaic Eureka Sound Formation (Cretaceous-Tertiary) of Banks Island, Arctic Canada. It is shown that the use of the weighting factor can differentiate flow patterns on the basis of sedimentary structure size, leading to interpretations of channel size, sinuosity, and other parameters of sedimentological importance. The weighting factor also provides an important check on calculations of vector mean.