Carol A. Evenchick

Jurassic-Cretaceous basins along the Canadian Coast Belt: Their bearing on pre-mid-Cretaceous sinistral displacements

The dominantly plutonic Canadian Coast Belt is flanked on the northwest by a Late Jurassic-Early Cretaceous intra- or back-arc basin. In the south it contains strata of the same age deposited in an accretionary fore-arc complex, east of a coeval arc. This distribution is ascribed to pre-mid-Cretaceous sinistral movement of part of the Jurassic-Cretaceous North American plate margin arc to a position west of the accretionary fore-arc complex.

Geometry, evolution, and tectonic framework of the Skeena Fold Belt, north central British Columbia

The Intermontane Belt of the Canadian Cordillera has long been viewed as a passive, relatively rigid block between two metamorphic-plutonic belts, the Coast Plutonic Complex and the Omineca Belt. However, the Skeena Fold Belt, which spans most of the width of the northern Intermontane Belt, exhibits shortening comparable with that in the Rocky Mountain Fold and Thrust Belt. The Skeena Fold Belt has many features of thin-skinned fold and thrust belts, such as thrust faults which sole into a detachment, a wide variety of structural styles which depend on rock type, a foreland basin which was cannibalized by continued deformation, a frontal triangle zone, and a hinterland of metamorphic and plutonic rocks (Coast Plutonic Complex). The Skeena Fold Belt thus is comparable with the Rocky Mountain Fold and Thrust Belt, but rather than deforming a continental terrace wedge, it developed in a terrane (Stikinia) which had accreted to North America in the early Mesozoic, and in Jurassic and Cretaceous clastic successions (Bowser Lake and Sustut groups) which overlie Stikinia. Structural and stratigraphic relationships show that the earliest deformation occurred between Oxfordian and Albian time and that the last folds developed in latest Cretaceous or early Tertiary time. As much as 160 km of northeastward shortening in the Skeena Fold Belt was broadly contemporaneous with crustal thickening in the Coast Plutonic Complex and Omineca Belt, with dextral strike-slip faulting east of the Skeena Fold Belt, and with shortening in the Rocky Mountain Fold and Thrust Belt. Therefore, between latest Jurassic and early Tertiary times, horizontal shortening occured across most of the width of the northern Canadian Cordillera. Concurrent shortening across the Cordillera suggests that a common detachment (or detachments) fed all of these zones as far east as the Rocky Mountain Fold and Thrust Belt.

Bowser basin, northern British Columbia: Constraints on the timing of initial subsidence and Stikinia-North America terrane interactions

Clastic strata composing the northern Bowser basin record the accretion of Stikinia to the composite western edge of the North American plate (Cache Creek-Quesnellia-Slide Mountain-Kootenay North America) in early Middle Jurassic time and the concomitant demise of the intervening Cache Creek ocean. Initial flexural subsidence of the northern Bowser basin, resulting from thrust loading of Cache Creek terrane on Stikinia, is represented by an organic-rich shale of Aalenian age (the Abou Formation of the Spatsizi Group). Coarse-grained sediment first appeared in early Bajocian time following uplift and subaerial exposure of Cache Creek rocks in the upper plate. Thus, the inception of the Bowser basin was Aalenian, rather than Bajocian, as believed by earlier workers. Aalenian southwest-vergent thrusting at the composite western edge of North America is also known from southern British Columbia, a coincidence that implicates collision with Stikinia in the south as a cause of that deformation.

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.

Constraints on the nature and thickness of sedimentary fill and underlying basement rocks in Bowser and Sustut basins, north-central British Columbia from analysis of potential field data

Abstract Qualitative and quantitative interpretations of potential field data, constrained by rock property measurements, are presented for an area in north-central British Columbia encompassing the Bowser and Sustut basins. Rock property measurements demonstrate that the Bowser Lake Group and underlying Hazelton Group have an identical mean density limiting the capacity of gravity to differentiate the two. The finding is important for the design of effective hydrocarbon exploration programs in the area. However, significant contrasts in magnetic susceptibility between basin fill and underlying basement rocks facilitate the use of magnetic data to map first-order variations in the thickness of fill in each of the basins. Results show a relatively uniform fill thickness in Sustut Basin, on the order of 2.5–3 km. In contrast, considerable topography on the basement interface beneath Bowser Basin is identified with variations in the thickness of overlying sedimentary rocks ranging from less than 2 km to more than 6 km. Resolving this topography is an important advance in understanding the hydrocarbon potential of the basin. A large (approximately 50 km × 60 km × 3.5 km) buried intrusion beneath the northeast part of Bowser Basin can account for an observed magnetic anomaly and explain the high coalification gradients and localized high maturation levels of the overlying sedimentary rocks. Neither of the latter can be adequately explained by the estimated burial depths. At least one regionally extensive (>150 km long) fault that cuts basement of Bowser Basin is delineated. The fault may have facilitated migration of hydrocarbons sourced in the basement into Bowser Basin. Correlation of magnetic data with published isotopic age dates indicates that Maitland volcanism spanned five polarity reversals during a 0.8 Ma period, and it facilitates a reduction in the errors associated with published isotopic ages.

The Skeena fold belt: a link between the Coast Plutonic Complex, the Omineca belt and the Rocky Mountain fold and thrust belt

The northern Canadian Cordillera contains a second fold and thrust belt west of the well-known Rocky Mountain fold and thrust belt and the metamorphic/plutonic Omineca belt. The Skeena fold belt occupies one quarter of the width of the Cordillera, and merges to the west with a second plutonic/metamorphic belt, the Coast Plutonic Complex. The Skeena fold belt has many features common to thin-skinned fold and thrust belts, including: low-angle thrust faults which sole into a detachment; a wide variety of fold styles depending on the rock type; a minimum of 44% shortening; a foreland basin that was cannibalized by continued deformation; termination in a frontal triangle zone; and a hinterland of metamorphic and plutonic rocks. The Skeena fold belt is similar in many respects to the Rocky Mountain fold and thrust belt, but rather than involving a continental terrace wedge, it deformed a terrane of Devonian to Lower Jurassic strata (Stikinia) which accreted to North America in the early Mesozoic. Also deformed is the Jurassic to Cretaceous clastic succession which overlies Stikinia, and the Cretaceous clastic succession of the associated foreland basin.