Provenance of the Incipient Passive Margin of NW Laurentia (Neoproterozoic): Detrital Zircon from Continental Slope and Basin Floor Deposits of the Windermere Supergroup, Southern Canadian Cordillera

The origin of the passive margin forming the paleo-Pacific western edge of the ancestral North American continent (Laurentia) constrains the breakup of Rodinia and sets the stage for the Phanerozoic evolution of Laurentia. The Windermere Supergroup in the southern Canadian Cordillera records rift-to-drift sedimentation in the form of a prograding continental margin deposited between ~730 and 570 Ma. New U-Pb detrital zircon analysis from samples of the post-rift deposits shows that the ultimate source area was the shield of NW Laurentia and the near uniformity of age spectra are consistent with a stable continental drainage system. No western sediment source area was detected. Detrital zircon from postrift continental slope deposits are a proxy for ca. 676-656 Ma igneous activity in the Windermere basin, likely related to continental breakup, and set a maximum depositional age for slope deposits on the eastern side of the basin at 652±9 Ma. These results are consistent with previous interpretations. The St. Mary-Moyie fault zone near the Canada-U.S. border was most likely a major transform boundary separating a rifted continental margin to the north from intracratonic rift basins to the south, resolving north-south variations along western Laurentia in the late Neoproterozoic at approximately 650-600 Ma. For Rodinia reconstructions, the conjugate margin to the southern Canadian Cordillera would have a record of rifting between ~730 and 650 Ma followed by passive margin sedimentation.

Between the supercontinents: Mesoproterozoic Deer Trail Group, an intermediate age unit between the Mesoproterozoic Belt–Purcell Supergroup and the Neoproterozoic Windermere Supergroup in northeastern Washington, USA

Mesoproterozoic and Neoproterozoic basins in western North America record the evolving position of the Laurentian craton within two supercontinents during their growth and dismemberment: Columbia (Nuna) and Rodinia. The western-most exposures of the Columbia rift-related Belt–Purcell Supergroup are preserved in northeastern Washington, structurally overlain by the Deer Trail Group and depositionally overlying the Neoproterozoic Windermere Supergroup. It has been disputed whether the Deer Trail Group is correlative with the Belt–Purcell Supergroup, or younger. To help resolve the uncertain correlation of these units and their bearing on supercontinent evolution, we characterized the detrital zircon age populations of units from the Deer Trail Group, the Windermere Supergroup, and the Belt–Purcell Supergroup in northeastern Washington. These data show that the western part of the Columbia supercontinent (now located in Australia and eastern Antarctica) remained attached to western Laurentia and continued to supply 1600–1500 Ma detrital zircon grains to the Belt–Purcell Supergroup until after ca. 1391 Ma. The Deer Trail Group is younger than the Belt–Purcell strata, with the basal unit younger than ca. 1362 Ma and a middle unit younger than ca. 1300 Ma. The Deer Trail Group has a pre-Grenville-age provenance from the southwestern USA and possibly east Antarctica. The Buffalo Hump Formation is younger than the Deer Trail Group, with Grenville-age (ca. 1112 Ma) detrital zircon grains and a detrital zircon signature like that of the overlying Neoproterozoic Windermere Supergroup. We interpret the Deer Trail Group to have been deposited during the rift-demise of supercontinent Columbia and before the Grenville-age assembly of the supercontinent Rodinia.

Nailed to the craton: Stratigraphic continuity across the southeastern Canadian Cordillera with tectonic implications for ribbon continent models

Cambrian and Upper Devonian to Mississippian strata can be confidently traced westward, without strike-slip offset, from the autochthonous section above North American basement into the southeastern Canadian Cordillera, and are thus “nailed” to the craton. These strata are in turn stratigraphically pinned to older (Mesoproterozoic Belt-Purcell Supergroup, Neoproterozoic Windermere Supergroup, and Ediacaran), intermediate-aged (Ordovician–Silurian), and younger (Permian to Middle Jurassic) strata found only in the mountains, thus linking them to the adjacent autochthonous craton. The overlapping distribution of linking successions, regionally traceable unique stratigraphic horizons in the Belt-Purcell and Windermere Supergroups, and across-strike stratigraphic features show that the entire Cariboo, northern Selkirk, Purcell, and Rocky Mountains are directly tied to the adjacent North American craton without discernible strike-slip or oblique displacement, or substantial purely convergent plate-scale (>400 km) horizontal displacement. They link the entire width of the Belt-Purcell and Windermere basins in the southeastern Canadian Cordillera to the adjacent craton and show that any proposed Cretaceous ribbon continent suture, with its thousands of kilometers of proposed displacement, cannot run through the southeastern Canadian Cordillera.

The origin and significance of convolute lamination and pseudonodules in an ancient deep-marine turbidite system: From deposition to diagenesis

ABSTRACT Soft-sediment deformation structures, like convolute lamination and pseudonodules, are common in deep-marine turbidites, but details of their origin and timing of formation remain a source of debate. Deep-marine basin-floor deposits of the Neoproterozoic Upper Kaza Group (Windermere Supergroup) crop out superbly in the Castle Creek study area and provide an ideal laboratory to investigate these aspects in convolute-laminated pseudonodules, and also how that deformation influenced later diagenesis. Pseudonodules consist of well-sorted, matrix-poor, upper medium- to coarse-grained, planar-stratified or cross-stratified sandstone that are underlain and overlain by comparatively more poorly sorted, matrix-rich, graded sandstone of similar grain size. Deposition of the stratified pseudonodules is interpreted to have occurred during the same event that deposited the graded sandstone, albeit during a period of general transport bypass, whereby isolated, shallow, seafloor depressions became filled with well-sorted, stratified sand. As stratified sand accumulated the depressions slowly subsided until a critical thickness had built up and exceeded the load-bearing capacity of the substrate composed of graded sand. This destabilized the surface separating the two layers and resulted in the stratified unit foundering, and in some cases becoming completely enveloped by, the upward-displaced lower-density substrate. Surprisingly, despite the deformed macroscopic character of the stratified sediment, primary grain fabric, including intergranular porosity up to 40%, was preserved and influenced early diagenesis, which, owing to dispersed phosphate cement and depleted carbon isotope composition of the pervasive carbonate cement, would have begun very near the sediment–water interface. Importantly also, pseudonodules are common in basin-floor deposits but comparatively rare in continental-slope strata. Expanding flow conditions over the basin floor would have promoted grain settling, and in turn development of a more stably (density) stratified flow structure. Ultimately this resulted in higher local rates of sedimentation on the basin floor and the accumulation of a substrate more prone to later liquidization.

Upper Windermere Supergroup and the transition from rifting to continent-margin sedimentation, Nadaleen River area, northern Canadian Cordillera

Neoproterozoic–Cambrian rocks of the Windermere Supergroup and overlying units record the breakup of Rodinia and formation of the northwestern Laurentian ancestral continental margin. Understanding the nature and timing of this transition has been hampered by difficulty correlating poorly dated sedimentary successions from contrasting depositional settings across Mesozoic structures. Here we present new litho- and chemo-stratigraphic data from a Cryogenian–lower Cambrian succession in east-central Yukon (Canada), establish correlations between proximal and distal parts of the upper Windermere Supergroup and related strata in the northern Canadian Cordillera, and consider implications for the formation of the northwestern Laurentian margin. The newly defined Nadaleen Formation hosts the first appearance of Ediacaran macrofossils, while the overlying Gametrail Formation features a large negative carbon isotope anomaly with δ13Ccarb values as low as –13‰ that correlates with the globally developed Shuram-Wonoka anomaly. We also define the Rackla Group, which includes the youngest (Ediacaran) portions of the Windermere Supergroup in the northern Cordillera. The top of the Windermere Supergroup is marked by an unconformity above the Risky Formation that passes into a correlative conformity in the Nadaleen River area. This surface has been interpreted to mark the top of the rift-related succession, but we draw attention to evidence for tectonic instability through the early-middle Cambrian and argue that the transition from rifting to post-rift thermal subsidence is marked by a widespread unconformity that underlies upper Cambrian carbonate rocks. This is younger than the interpreted age of the rift to post-rift transition elsewhere along the ancestral western Laurentian continental margin.