Keith Dewing

Detrital zircon geochronology and provenance of the Neoproterozoic to Late Devonian Franklinian Basin, Canadian Arctic Islands

More than 1800 detrital zircon uranium-lead (U-Pb) ages collected from Franklinian Basin sedimentary strata of the Canadian Arctic Islands provide important insights into the depositional and tectonic evolution of the northern margin of Laurentia from the late Neoproterozoic to the Late Devonian. The Franklinian Basin succession is composed of strata with three distinctly different U-Pb age provenance signatures, which have implications for the tectonic and paleogeographic evolution of the entire Arctic region. Neoproterozoic and Lower Cambrian formations contain detrital zircon populations of 1750–1950 Ma and 2650–2800 Ma, which are consistent with derivation from Archean to Paleoproterozoic gneisses and granites of the west Greenland–northeast Canadian Shield. The Lower Silurian to Lower Devonian Danish River Formation contains a dominant population of 900–2150 Ma detrital zircons with scattered Archean ages. The 900–2150 Ma zircons were likely transported axially along the foreland basin of the East Greenland Caledonides (Caledonian orogen) and deposited in a deep-water basin between the Pearya terrane and northern Laurentian margin. Middle Devonian to Upper Devonian strata contain detrital zircon populations of 900–2150 Ma, similar to the Danish River Formation, but these units also contain 370–450 Ma and 500–700 Ma detrital zircons. The 900–2150 Ma zircons were likely derived from the East Greenland Caledonian Mountains, the uplifted foreland of the East Greenland Caledonides, and the Pearya terrane. The population of 370–450 Ma detrital zircons potentially comes from uplifting granites in the Caledonian Mountains and Pearya terrane. The 500–700 Ma detrital zircons were likely derived from the continental landmass responsible for the Ellesmerian orogen. The 500–700 Ma age of the zircons suggests that the northern landmass likely had a connection to rocks of the Timanide orogens, located in the Timan Range of northwestern Russia. A dominant population of 365–450 Ma and 500–700 Ma ages in Upper Devonian strata suggests that the Pearya terrane and the northern continental landmass became the dominant source by the end of Franklinian Basin sedimentation. Because detrital zircons are often recycled from older strata into younger deposits, these data provide the basis for understanding the sedimentary provenance of younger units of the Sverdrup Basin and sedimentary wedges along the present Arctic continental margin.

A pericratonic model for the Pearya terrane as an extension of the Franklinian margin of Laurentia, Canadian Arctic

New detrital zircon sensitive high-resolution ion microprobe (SHRIMP) geochronology and published framework geology of Neoproterozoic to Silurian strata are integrated to reexamine tectonic models of the Pearya terrane and the Franklinian margin at Ellesmere Island, Arctic Canada. Compared to Cambrian detrital zircon reference spectra, Neoproterozoic sandstones from the Pearya terrane contain Laurentian detrital zircon ages. In fact, they compare very well with Neoproterozoic strata of Greenland. Ultramafic, tholeiitic, and andesitic basalts of the Maskell Inlet complex, inferred to have an age of ca. 481 Ma, predate the M’Clintock orogeny at ca. 475 Ma. Ordovician granitoid ages within the Pearya terrane span ca. 475–463 Ma. A K-Ar cooling age of 452 Ma records post-tectonic exhumation. Deposited at ca. 450 Ma, new data show that the Cape Discovery Formation contains mainly ca. 500–450 Ma detrital zircon ages, but also older ages of 660–620 Ma. Upper Ordovician sandstones indicate that the Pearya terrane platform continued to receive near-syndepositional zircon. The Pearya terrane platform was submerged at ca. 435 Ma and overlain by Silurian flysch fed by sources similar to detrital zircon within Proterozoic to Ordovician strata of the Pearya terrane. Ties between the Pearya terrane and the Franklinian shelf include: similar Proterozoic-Cambrian stratigraphy; detrital zircon ages from the Lower Cambrian Grantland Formation that appear to be a combination of recycled Marinoan strata of Pearya terrane and Franklinian shelf strata; and profound unconformities on the Franklinian shelf that correlate temporally to the M’Clintock orogeny. A pericratonic model is a straightforward solution to the tectonic history of the Pearya terrane and the Franklinian shelf. We hypothesize that the Pearya terrane was part of the Franklinian margin in the Neoproterozoic and that the intervening deep water basin, or Hazen Trough, originated as a failed rift. The Maskell Inlet complex records ca. 481 Ma volcanism as a response to mantle upwelling beneath a subducting slab, shortly before or as the continent-ocean transition zone of the Franklinian margin was over-ridden. Continued convergence and crustal thickening during arc-continent collision drove the M’Clintock orogeny at ca. 475 Ma and resulted in the unconformities on the Franklinian shelf. After arc-continent collision, magmatism between ca. 475 and 438 Ma was related to subduction on the outboard margin, and the Pearya terrane was overlain by a low-accommodation retroarc foreland basin. A subsequent collisional event led to progressive drowning of the Pearya terrane and Franklinian platforms between 435 and 425 Ma. Silurian flysch then blanketed the Pearya terrane and entered the axis of the foreland basin, which also received sediment from cratonic sources of the flexural basin margin. Limited deformation at northern Axel Heiberg Island was associated with a ca. 390 Ma granitoid within the Pearya terrane. After a Devonian pulse of subsidence, infilling of the basin to form the Devonian clastic wedge overlapped in age with small ca. 368 Ma granitic intrusions within the Pearya terrane and was followed by extensive foreland deformation of the Late Devonian–Mississippian Ellesmerian orogeny.

Fault-mediated melt ascent in a Neoproterozoic continental flood basalt province, the Franklin sills, Victoria Island, Canada

The Neoproterozoic Franklin large igneous province on Victoria Island, Canada, is characterized by continental flood basalts and a sill-dominated feeder system. Field relationships indicate that fault-guided transfer zones allowed magma to jump up-section to form higher-level intrusions. Where sills connect to dikes and magmas moved up-section, roof and wall rocks are characterized by wide and intense contact-metamorphic haloes, consistent with throughflow of magma. The geometric constraints suggest that conduits may have opened episodically and then closed when magma pressure waned. The episodic nature of conduit opening events can explain the pulsed ascent of crystal slurries, and may also play a role in the deposition of Ni-sulfides.

Silurian flysch successions of Ellesmere Island, Arctic Canada, and their significance to northern Caledonian palaeogeography and tectonics

Detrital zircon provenance studies of Silurian flysch units that underlie the Hazen and Clements Markham fold belts of Ellesmere Island, Arctic Canada, were conducted to evaluate models for northern Caledonian palaeogeography and tectonics. Llandovery flysch was deposited along an active plate margin and yields detrital zircons that require northern derivation from the adjacent Pearya terrane. If Pearya originated near Svalbard and NE Greenland, it was transported by strike-slip faults to Ellesmere Island by the Early Silurian. Wenlock to Ludlow turbidites yield Palaeozoic–Archaean detrital zircons with dominant age-groupings c. 650, 970, 1150, 1450 and 1650 Ma. These turbidite systems did not fill a flexural foreland basin in front of the East Greenland Caledonides, but rather an east–west-trending trough that was probably related to sinistral strike-slip faulting along the northern Laurentian margin. The data support provenance connections with the Svalbard Caledonides, especially Baltican-affinity rocks of SW Spitsbergen that were proximal to NE Greenland during the Baltica–Laurentia collision. Pridoli flysch has sources that include Pearya, the East Greenland Caledonides and the Canadian Shield. Devonian–Carboniferous molasse in Arctic Canada has analogous detrital zircon signatures, which implies recycling of Silurian flysch during mid-Palaeozoic (Ellesmerian) collisional tectonism or that some collisional blocks were of similar Baltican–Laurentian crustal affinities. Supplementary material: Detrital zircon U–Pb age results, isotopic data and concordia diagrams of dated samples are available at http://www.geolsoc.org.uk/SUP18797.

Dual provenance signatures of the Triassic northern Laurentian margin from detrital-zircon U-Pb and Hf-isotope analysis of Triassic–Jurassic strata in the Sverdrup Basin

The tectonic setting of northern Laurentia prior to the opening of the Arctic Ocean is the subject of numerous tectonic models. By better understanding the provenance of detrital zircon in the Canadian Arctic prior to rifting, both the prerift tectonic setting and timing of rifting can be better elucidated. In the Sverdrup Basin, two distinct provenance assemblages are identified from new detrital-zircon U-Pb data from Lower Triassic to Lower Jurassic strata in combination with previously published detrital-zircon data. The first assemblage comprises an age spectrum identical to that of the Devonian clastic wedge in the Canadian Arctic and is termed the recycled source. In contrast, the second assemblage is dominated by a broad spectrum of near syndepositional Permian–Triassic ages derived from north of the basin and is termed the active margin source. Triassic strata of Yukon and Arctic Alaska exhibit a similar dual provenance signature, whereas in northeastern Russia, Chukotka contains only the active margin source. Complementary hafnium isotopic data on Permian–Triassic zircon have eHf values that are consistent with the common evolved crustal signature of the Devonian clastic wedge detrital-zircon grains and Neoproterozoic–Paleozoic basement rocks in the Arctic Alaska–Chukotka microcontinent. Furthermore, newly identified volcanic ash beds throughout the Triassic section from the northern part of the Sverdrup Basin, along with abundant Permian–Triassic detrital zircon, suggest a protracted history of magmatism to the north of the basin. We interpret that these zircons were sourced from a magmatically active region to the north of the Sverdrup Basin, and in the context of a rotational model for opening of Amerasia Basin, this was probably part of a convergent margin fringing northern Laurentia from the northern Cordillera along the outboard edge of Arctic Alaska and Chukotka terranes. In Early Jurassic strata, Permian–Triassic zircons decrease substantially, implying the diminution of the active margin as a sediment source as initial rifting isolated the Permian–Triassic source from the Sverdrup Basin.

Escape traces associated with Rafinesquina alternata, an Upper Ordovician strophomenid brachiopod from the Cincinnati Arch region

Abstract Strophomenid brachiopods of the genus Rafinesquina, lying flat, convex-side up on limestone bedding surfaces in the Cincinnati Ordovician, are sometimes associated with moats, which are sediment depressions or gutters ∼5-mm wide surrounding the commissure. Moats are interpreted as trace fossils, excavated by water expelled as the valves snapped shut. Other specimens vary from nearly horizontal to nearly vertical with the hinge line down and commissure up. Meniscate backfill beneath the anterior shell margin traces an arcing path formed as the shell rotated upwards around the posterior hinge line. Rotational tracks are interpreted as trace fossils, recording movement from an initial position buried horizontally to an inclined position as the brachiopod tried to escape burial. The traces form a continuum. Specimens lying flat on the bed surface have moats but no rotational tracks. Inclined shells are associated with deeper burial by obrution events and a greater arc of rotation. The moat shape is inconsistent with differential compaction. The precise association between moats and commissures and the independence of these structures on shell azimuth are inconsistent with current scour. If moats formed by rapid expulsion of water during valve snapping, then rotational tracks may have formed by a similar process. These traces are interpreted as fugichnia formed in response to catastrophic burial, but some moats could be equilibrichnia, formed by adjustment to minor sedimentary events. Rotational traces are similar to type 1 structures of Sowerbyella. If these two genera had similar tracemaking abilities, then other strophomenates probably shared these abilities.

SHELL STRUCTURE AND ITS BEARING ON THE PHYLOGENY OF LATE ORDOVICIAN–EARLY SILURIAN STROPHOMENOID BRACHIOPODS FROM ANTICOSTI ISLAND, QUÉBEC

Abstract Four shell types are recognized from strophomenide brachiopods from Anticosti Island based on their fibrous or laminar character and on the type of taleolae. The shell types consistently co-vary with the types of cardinal process and style of socket plates. Treating shell structure as a conservative, non-reversing character implies that there are four groups of strophomenide brachiopod. The laminar-shell strophomenoids probably originated two separate times; one origination giving rise to two groups within the Strophomenida and one origination producing the Orthotetida. This matches the Treatise classification with only minor variation. The fourth group, that with fibrous shell and trilobed cardinal process and which includes the Plectambonitoidea, does not clearly fit into the Strophomenata and may be more closely related to the Clitambonitoidea. A limited cladistic analysis supports the idea that shell structure should be an important factor in establishing evolutionary kinship and points to ways of optimizing the Treatise classification.

Chapter 38 Thermal maturity of the Sverdrup Basin, Arctic Canada and its bearing on hydrocarbon potential

Abstract Analysis of a large thermal maturity dataset indicates that the Carboniferous to Eocene Sverdrup Basin in the Canadian Arctic had a uniform response to thermal stress with depth for Mesozoic strata. Thermal maturity was established at the level of the widespread Upper Triassic Gore Point Member; a good seismic reflector, occurring in close vertical proximity to the two main oil-prone source rocks in the basin. The Gore Point Member is in the gas window (Ro>1.35%) in the northeastern part of the Sverdrup Basin, whereas in the western Sverdrup Basin its maturity does not exceed 1.2% Ro. This would support the hypothesis that large quantities of gas found at the Drake, Hecla and Whitefish fields have derived from a deeper source, probably in Permian or lower Palaeozoic strata. A normal burial curve is established using boreholes drilled in areas with no structural complexity at time of maximum burial. Low-amplitude structures, including the Drake, Hecla and Whitefish fields, show little or no uplift following maximum burial in the Paleocene, indicating that these structures formed prior to the Eocene folding related to the Eurekan Orogeny. Because they were present at the time of maximum burial, they were available to be charged during hydrocarbon migration. In contrast, high-amplitude structures show evidence of large uplifts following maximum burial. They formed in the Eocene and hence post-date most hydrocarbon migration.

Cenozoic structural evolution on northern Banks Island, N.W.T. Canada

The onshore structural architecture and evolution of large segments of the Arctic continental margin are poorly known because of the generally poor outcrop of pre-Neogene rocks, the remoteness of the area and extent of Neogene cover. Fieldwork on Banks Island during the summer of 2016 has shown that the Devonian, Cretaceous and Paleogene deposits on northern Banks Island are characterized by a number of local, restricted deformation zones, which we interpret to indicate both dextral and subordinate sinistral strike-slip deformation along NNE–SSW striking structures parallel to the continental margin of Banks Island. The presence of Cenozoic strike-slip deformation on Banks Island extends the area of known Cenozoic strike-slip along the continental margin southwestward from where it has been previously documented on northern Ellesmere Island. In addition, field and seismic data indicate that the sedimentary rocks on Banks Island have been affected by extensional movements before and after the strike-slip deformation. The observation of strike-slip motion on Banks Island may imply a component of strike slip over the whole Paleogene North American margin.