| Summary: | Lithofacies, provenance, and diagenetic studies of more than 3 km of Late Jurassic to mid-Cretaceous silicilastic sediments exposed in the northern Bowser Basin (northern British Columbia) record the tectonic development of the Canadian Cordillera. Strata are divided into the undivided Bowser Lake Group and overlying Currier, McEvoy, and Devils Claw formations Lithofacies include marine mudstone, coarsening upward mudstone, fining upward sandstone, coarsening upward sandstone, chert pebble conglomerate, and coal. Common lithofacies associations are interpreted as a progression (from older to younger) of shallow marine, lower delta plain, upper delta plain, and alluvial braid-plain depositional environments. A subsidence model based on sediment compaction and isostatic load accounts for the necessary accommodation space.
The composition of the sandstone suggests an obducted island arc and oceanic crust asprovenance. Three petrofacies have been identified by modal analysis of framework grains. Petrofacies 1(P1), which occurs in undivided Bowser Lake Group and Currier Formation strata, (QtFL = 34-14-52;QmFLt = 9-14-77) is volcanic lithic rich with subequal to minor chert, minor monocrystalline quartz(generally <10%), and 10-25 % feldspar. Petrofacies 2 (P2) occurs in lower McEvoy Formation, and has higher concentrations of chert reflecting a recycled component in the sandstones, but also retains significant portions of volcanics (QtFL = 62-5-33; QmFLt = 5-5-89). Petrofacies 3 (P3) occurs in the upper McEvoy and Devils Claw formations, and is chert rich like P2, with less volcanic lithics and a small but significant portion of metamorphic lithic fragments (Qt =64 %, F=5%, L=31 %; Qm=7%,F=5%, Lt= 88 %). Paleocurrent directions indicate transport from northeast to southwest. Microprobe analysis of detrital chromian spinel accessory grains demonstrates alpine type peridotite occurs in the provenance. No spinels typical of mid-ocean ridge or Alaskan type complexes were found. The petrofacies and chromian spinel chemistry are consistent with a provenance from island arc and marginal basin lithosphere obducted onto the western margin of North America. Diagenetic history of the sediments provides insight into depositional and post depositional processes in the basin. Seven stages of cement paragenesis are recognized in the sandstones: 1) chlorite; 2) illite; 3) kaolinite; 4) dead oil; 5) quartz; 6) chlorite dissolution; and 7) calcite. Estimated precipitation temperatures begin below 80°C for chlorite, and increase to approximately 100°C to200°C for quartz, and to above roughly 200°C for calcite. Fluid inclusions in quartz cements support such temperature estimates. The succession of cements is interpreted to record replacement of original connate seawater by acid pore waters derived from organic matter maturation that were forced out by compaction of the interbedded muds. Carbon isotopes in carbonate concretions from the mudstones are consistent with formation during a late stage of methanogenesis. Oxygen isotopes from the same concretions suggest pore fluids in the muds at the time of formation were meteoric to brackish waters typical of cool temperate climates. Organic maturation was modelled by using vitrinite reflectance values from interbedded coals and mudstones, and assuming progressive heating as inferred from the sandstone cement paragenesis. Results of the model indicate that a high paleogeothermal gradient, similar to some back arc basins, best explains the diagenetic history of the northern Bowser Basin. Constraints from the lithofacies, provenance, and pore water evolution studies suggest the Bowser Basin began as a deep marine basin which was filled by sediment derived from island arc and marginal basin crust obducted earlier onto the western margin of North America.
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