Bowser Basin Research Articles

An Upper Jurassic ichthyosaur (Ichthyosauria: Ophthalmosauridae) from the Bowser Basin, British Columbia

Although the Jurassic was a period of high diversity in ichthyosaurs, only a small number of specimens have been recorded from Canada to date. We describe here a new occurrence of an ophthalmosaurid ichthyosaur from a shallow marine depositional environment within the Bowser Basin of northern British Columbia. Based on vertebral diameters and the size of the humerus, the ichthyosaur was relatively large compared to other contemporaneous forms, yet possessed teeth that were small for its body size. As well, the height to length ratio of the preserved vertebrae suggests it may have had a more elongate, less regionalized body shape. Although indeterminate at a generic level, the presence of Late Jurassic ichthyosaurs in nearshore waters of northwestern North America further demonstrates their cosmopolitan distribution.

Stratigraphy of the upper Hazelton Group and the Jurassic evolution of the Stikine terrane, British Columbia1ESS Contribution 20120051.

The Lower to Middle Jurassic Hazelton Group represents the final stage of magmatic arc activity in the intraoceanic Stikine terrane, which was followed by accretion within the Cordilleran terrane collage. The Hazelton Group is exposed in the following areas: (i) on the periphery of the Bowser Basin, where arc and back-arc strata are overlain by mainly sedimentary strata of the upper Hazelton Group and then by the clastic basin fill of the Bowser Lake Group; and (ii) within a 300 km long rift system, the Eskay rift, west of the Bowser basin, where a predominantly bimodal volcanic succession contains significant mineral deposits. Examination of representative stratigraphic sections throughout the regional extent of the upper Hazelton Group has suggested significant revisions and clarification of its stratigraphy and include the following: (i) informal division of the Hazelton Group into upper and lower parts and recognition of a diachronous unconformity or unconformities at the boundary between them; (ii) establishment of a type section for the sandstone-dominated Smithers Formation; (iii) establishment of separate Quock and revised Spatsizi formations in the north and extension of the Quock Formation to include all lithostratigraphically equivalent units of blocky, thinly bedded siliceous mudstone and tuff around the periphery of the Bowser basin; and (iv) introduction of the Iskut River Formation for rift-related and volcanic facies in the Eskay rift area. Two independent rifting events occurred during deposition of the Hazelton Group: a Late Sinemurian to Early Pliensbachian phase in the northwest-trending Hazelton trough and a more restricted Aalenian to Bajocian extensional event in the Eskay rift.

Source rock potential of Lower to Middle Jurassic black clastic sequences of the Intermontane belt1This article is one of a series of papers published in this Special Issue on the theme of New insights in Cordilleran Intermontane geoscience: reducing exploration risk in the mountain pine beetle-affected area, British Columbia.

Lower to Middle Jurassic clastic sequences are widespread within the interior of the Canadian Cordillera. These successions cap waning Jurassic volcanism in many Cordilleran arc terranes and are succeeded by clastic sequences of the Intermontane basins. Fine-grained, carbonaceous lithologies, which locally contain elevated levels of organic carbon, characterize these clastic successions. These include sections of the Spatsizi Formation (Abou Member) and Smithers Formation in northern and western Bowser basin, respectively, Ashcroft Formation and equivalent strata in central Quesnellia, and Last Creek Formation and Junction Creek rocks below the Tyaughton–Methow basin. These rocks locally contain metre-thick sections with total organic carbon (TOC) levels >5 wt.% and others have thicknesses approaching 100 m with TOC between 3 and 5 wt.%. Thermal maturation levels are high in many sections, suggesting original organic contents were greater and that these rocks may have been excellent source beds. Associated bitumen in these successions, together with Mesozoic oil in some overlying Intermontane clastic rocks, also suggests these sequences may have been effective oil source rocks. TOC levels, thermal maturity, and thickness of some sections are comparable with shale gas sequences being exploited elsewhere in North America. TOC concentrations within these rocks, together with other elemental abundances, indicate anoxic conditions during deposition. The age of these clastic rocks brackets the global Toarcian anoxic event and that of other organic-rich sequences in North America. Elemental abundances suggest predominantly volcanic-arc complexes as source terranes, although continental signatures are suggested by rocks in western Quesnellia.

Nature and significance of the diachronous contact between the Hazelton and Bowser Lake groups (Jurassic), north-central British Columbia

Abstract In north-central British Columbia, the contact between the Lower to Middle Jurassic Hazelton Group and the Middle Jurassic to Cretaceous Bowser Lake Group is an upward gradational change from fine-grained, commonly tuffaceous sedimentary lithofacies to commonly coarser and more heterogeneous lithofacies assemblages that include abundant chert clasts and generally lack distinct tuff beds. Paleontologic dating reveals that the lowest Bowser Lake Group is generally older (Bajocian) in north-most exposures and younger (Early Oxfordian) to the southwest toward the centre of the Bowser Basin. Interpretation of stratigraphic changes relies upon age data from ammonites, and to a lesser extent marine bivalves; a new bivalve species with biostratigraphic significance, Myophorella richardsi n. sp., is named herein. In many places the contact between the Hazelton and Bowser Lake groups is younger than previously understood. Earlier treatments of this boundary over-emphasized paleontologic content and time correlations rather than lithologic criteria. The diachroneity of the base of the Bowser Lake Group resulted from: southwestward migration of the initial front of coarse, chert-bearing, clastic detritus derived from the Cache Creek terrane, which started in the Bajocian; and subsequent northward migration of coarse facies containing volcanic clasts shed from a volcanic highland near the south margin of Bowser Basin. Although volcanic activity in the immediate vicinity of the Bowser Basin diminished significantly in the Bajocian, volcanism continued to the south of the basin into the latest Jurassic. This southern volcanism was likely the source for tuff beds in the uppermost Hazelton Group in the immediate vicinity of Bowser Basin. Continued southern volcanism and input of ash to the Bowser Lake Group is recorded by U-Pb ages of detrital zircons that are coeval with depositional ages inferred from fossils. The large volume of clastic detritus in the Bowser depositional systems, however, diluted the influx of ash and prevented the accumulation of distinct tuff beds within the Bowser Basin proper.

Sedimentation styles and depositional processes in a Middle to Late Jurassic slope environment, Bowser Basin, northwestern British Columbia, Canada

The Middle to Upper Jurassic Todagin assemblage in northwestern British Columbia, Canada, was deposited in the Bowser Basin above arc-related rocks of the Stikine terrane. Sedimentary structures indicate that a variety of gravity flow processes were involved in transport and deposition in deep-water slope environments. At Mount Dilworth, laterally continuous and channelized turbidites are interbedded with and overlain by mass-transport deposits in which sedimentary clasts are supported in a mudstone matrix. More than 50% of the succession consists of mass-transport deposits, indicating significant slope instability. A 300 m thick mass-transport complex exposed near the top of the succession is interpreted to result from tectonic activity, which triggered a major change in sediment supply from a local source area. At Todagin Mountain, a channel complex displays three successive channel-fills with associated overbank sedimentation units. Mass-transport deposits are rare, and confined to channel axes. Channels 1 and 2 are characterized by 40–50 m thick, ungraded pebble clast-supported conglomerate while the uppermost Channel 3 contains graded beds and occasional traction structures. The gradual change from erosive and amalgamated channel deposits at the base, to more aggradational channels at the top, is related to elevation of the equilibrium profile. Creation of accommodation favored aggradation on the mud-dominated slope succession and construction of well-developed channel-levee systems. The vertical succession exposed at Todagin Mountain is consistent with normal progradation of the slope under high sedimentation rates. In the Mount Dilworth area, extensional faulting associated with development of the restricted Eskay rift in the early Middle Jurassic produced a dissected basement above which the Todagin assemblage was deposited. These structures were inverted during collision of the Stikine and Cache Creek terranes, and likely played a major role in the stratigraphic evolution of the deep-water architectures.

Jurassic subsidence history of the Hazelton Trough-Bowser Basin in the area of Todagin Mountain, north-central British Columbia, Canada

Abstract Backstripping calculations performed on a stratigraphic section near Todagin Mountain, north-central British Columbia, indicate that the northern edge of the Hazelton Trough–Bowser Basin was characterized by two phases of enhanced subsidence. An initial episode of rapid subsidence occurred in the Pliensbachian, during which bimodal volcanic flows and associated sedimentary rocks were deposited in a shallow-marine environment. This episode corresponds to the end of a rifting event in the Stikine arc when most of the accommodation space was generated by extensional faulting of the predominantly igneous basement. Slower subsidence took place in Toarcian to Bajocian time along with deposition of a condensed section in a deep-water basin. This part of the history is consistent with thermal subsidence following the end of volcanism. An increase of subsidence rates in the Bathonian–Callovian interval was accompanied by increased sediment supply, marking the start of deposition of the Bowser Lake Group. This second pulse of rapid subsidence can be explained by sediment loading alone but the subsidence history does not rule out tectonic loading of the basin floor. The Jurassic subsidence history observed at Todagin Mountain is consistent with a simple extensional model defined by a rifting episode followed by thermal contraction and sediment loading. Tectonic loading might also have played a role in the second episode but is not required by the subsidence model.

Evidence of different contractional styles along foredeep margins provided by Gilbert deltas: examples from Bowser Basin, British Columbia, Canada

Abstract The northern margin of the early Middle Jurassic Bowser Basin in British Columbia contains large, solitary shelf-type Gilbert deltas and smaller, multistorey Gilbert deltas stacked against inferred active reverse faults. Analysis of the deltas provides evidence of tectonic structures that can no longer be directly observed. Bowser Basin developed as a foredeep in response to overthrusting of Stikinia by Cache Creek Terrane (oceanic crust) during accretion of Stikinia to the western margin of North America. The terrane boundary is currently represented by the King Salmon and associated thrust faults. The shelf-type Gilbert deltas consist of accretional conglomerate packages up to 100 m thick, that are overlain by and laterally associated with cyclothemic shelf facies. Gilbert deltas responded to low accommodation to sediment supply ratios by prograding to the shelf break of the basin, where they fed large volumes of gravel to slope gullies and the deep basin beyond. At one location, fault-block rotation resulted in oversteepening of foresets and internal discordances. Deposition was dominated by non-cohesive debris flows (50% to 60% of foresets) and intervening muddy debris flows. The coarse-grained sediments are texturally and compositionally mature, consisting predominantly of spherical radiolarian chert clasts derived from the overthrust Cache Creek accretionary prism. A spectacular succession at least 700 m thick near Cartmel Lake records the aggradational stacking of 30 small Gilbert deltas. Like the shelf-type Gilbert deltas, sediment gravity flows predominated. Creation of accommodation space for accumulation of the Gilbert delta stack was repeated many times; the locus of accommodation remained in much the same place over time. This implies a high degree of tectonic control over the geometry of the stacked Gilbert deltas. Accommodation to sediment supply ratios for this kind of Gilbert delta were high. Based on their thickness, geometry, and general association with shelf lithofacies, Bowser Basin shelf-type Gilbert deltas probably required relatively large drainage catchments, such as those associated with thrust ramps, fault transfer zones or blind thrusts. In one example, delta progradation direction was at a low angle (15 –27°) to the main structural trends and in this particular case we favour delta accumulation at a thrust fault transfer zone. In contrast, the stacked Gilbert deltas are inferred to be footwall deltas that accumulated close to an active, reverse fault where the stacking geometry indicates little or no basinward shift in the locus of deposition. Drainage basins were relatively small and were frequently rejuvenated. The Bowser Basin Gilbert deltas accumulated at the margin of a foreland thrust wedge, outboard of a major Early to Middle Jurassic, crustal-scale thrust system that carried oceanic Cache Creek terrane rocks in its hanging wall. Coarse-grained sediment, derived either from northwards erosional back-stepping of King Salmon Fault or from older Bowser Basin sediment, was cannibalized and redeposited in the Gilbert deltas and associated lithofacies. Either case provides a mechanism to account for the reworking necessary to produce the high clast sphericity and the monomict chert composition.

Diagenetic remagnetization in Bowser Basin: implications for timing and tectonics

Abstract Paleomagnetic study of 46 sites in Bowser Basin in central British Columbia, Canada, gives evidence of a pervasive late tectonic hydrothermal event which remagnetized all rock types. Around Oweegee Dome (56.7°N, 129.5°W), samples from Permian through Jurassic strata hold a paleomagnetic component, which postdates most tilting (negative tilt test). The westerly direction (Dec=285.5°, Inc=60.8°, k=40.0, α95=4.2°, N=30 sites) is interpreted to have been acquired during Cretaceous or earliest Cenozoic time after the main stage of Skeena Fold Belt deformation in the western part of the foldbelt. The direction is anomalously shallow and counter-clockwise rotated, due to Late Cretaceous and later lateral displacements, rotations and tilting. The diagenetic event revealed by this study, its timing and its implications on subsequent tectonic motions contribute to the understanding of Bowser Basin development in support of petroleum assessment.

Seismic tomographic cross-sections of the Bowser Basin in northwest British Columbia, Canada

Abstract First-break travel time picks along about 250 km of the Lithoprobe SNORCLE deep reflection profile were inverted using a 2-D crossing-ray tomographic method to estimate bulk P-wave speed within the uppermost 2 km of crust beneath the Bowser Basin in northwest British Columbia. The purpose was to assess the usefulness of resulting wave-speed models in defining local stratigraphy and structure and in petroleum exploration more generally. Two areas were investigated in detail. Near the Oweegee Dome, deeper levels of exposure at the surface and stratigraphic correlations with a nearby drill hole suggest that a marked regional increase in wave speed occurs at the contact between the lowermost Bowser Lake Group strata and lava flows of the underlying Hazelton Group. Near the northwest corner of the Bowser Basin, a triangle zone inferred from surface structure does not show clearly in the seismic wave-speed model but does contain distinctive horizontal layers of higher wave speed that represent thrust duplexes, igneous intrusions, or mineralization.

Seepage of biogenic natural gas in the Intermontane Belt of the Canadian Cordillera

Research Article| December 01, 2007 Seepage of biogenic natural gas in the Intermontane Belt of the Canadian Cordillera K.G. Osadetz; K.G. Osadetz Geological Survey of Canada, 3303 - 33 Street NW, Calgary, AB, T2L 2A7, kosadetz@nrcan.gc.ca Search for other works by this author on: GSW Google Scholar C.A. Evenchick; C.A. Evenchick Geological Survey of Canada, Pacific, 625 Robson Street, Vancouver, BC, V6B 5J3 Search for other works by this author on: GSW Google Scholar F. Ferri; F. Ferri British Columbia Ministry of Energy and Mines and Petroleum Resources, PO Box 9323, 6th Floor, 1810 Blanshard Street, Victoria, BC, V8W 9N3 Search for other works by this author on: GSW Google Scholar B. Mayer; B. Mayer Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB, T2N 1N4 Search for other works by this author on: GSW Google Scholar L.R. Snowdon L.R. Snowdon Snowdon Research Ltd., 4904 Brockington Road NW, Calgary, AB, T2L 1R6 Search for other works by this author on: GSW Google Scholar Author and Article Information K.G. Osadetz Geological Survey of Canada, 3303 - 33 Street NW, Calgary, AB, T2L 2A7, kosadetz@nrcan.gc.ca C.A. Evenchick Geological Survey of Canada, Pacific, 625 Robson Street, Vancouver, BC, V6B 5J3 F. Ferri British Columbia Ministry of Energy and Mines and Petroleum Resources, PO Box 9323, 6th Floor, 1810 Blanshard Street, Victoria, BC, V8W 9N3 B. Mayer Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB, T2N 1N4 L.R. Snowdon Snowdon Research Ltd., 4904 Brockington Road NW, Calgary, AB, T2L 1R6 Publisher: Canadian Society of Petroleum Geologists Received: 10 Oct 2007 Accepted: 23 Oct 2007 First Online: 02 Mar 2017 Online ISSN: 2368-0261 Print ISSN: 0007-4802 © The Society of Canadian Petroleum Geologists Bulletin of Canadian Petroleum Geology (2007) 55 (4): 337–341. https://doi.org/10.2113/gscpgbull.55.4.337 Article history Received: 10 Oct 2007 Accepted: 23 Oct 2007 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation K.G. Osadetz, C.A. Evenchick, F. Ferri, B. Mayer, L.R. Snowdon; Seepage of biogenic natural gas in the Intermontane Belt of the Canadian Cordillera. Bulletin of Canadian Petroleum Geology 2007;; 55 (4): 337–341. doi: https://doi.org/10.2113/gscpgbull.55.4.337 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of Canadian Petroleum Geology Search Advanced Search Abstract The owners of Tatogga Lake Resort informed geologists of persistent flammable natural gas seepage into Tatogga Lake, British Columbia. The gas seep was sampled from anomalously unfrozen openings (UTM zone 9, 440378/6396983, NAD27) when the ice cover was generally 15 to 20 cm thick. Samples of this gas are composed of methane (98.2 %) and carbon dioxide (1.8 %). The stable carbon isotopic composition of both gases is consistent with biogenic generation and possibly slight microbial oxidation. The biogenic origin of this natural petroleum seepage suggests that the gas is not associated with crude oil stains and thermogenic petroleum systems in the Bowser Basin and underlying rocks. The seepage source strata and economic potential remain unidentified and obscured by its underwater occurrence. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Cordilleran Intermontane thermotectonic history and implications for neotectonic structure and petroleum systems, British Columbia, Canada

Heat flow increases northward along Intermontane Belt in the western Canadian Cordillera, as shown by geothermal differences between Bowser and Nechako sedimentary basins, where geothermal gradients and heat flows are ∼30 mK/m and ∼90 mW/m2 compared to ∼32 mK/m and 70 –80 mW/m2, respectively. Sparse temperature profile data from these two sedimenatary basins are consistent with an isostatic model of elevation and crustal parameters, which indicate that Bowser basin heat flow should be ∼20 mW/m2 greater than Nechako basin heat flow. Paleothermometric indicators record a significant northward increasing Eocene or older erosional denudation, up to ∼7 km. None of the heat generation, tectonic reorganization at the plate margin, or erosional denudation produce thermal effects of the type or magnitude that explain the north–south heat flow differences between Nechako and Bowser basins. The more southerly Nechako basin, where heat flow is lower, has lower mean elevation, is less deeply eroded, and lies opposite the active plate margin. In contrast, Bowser basin, where heat flow is higher, has higher mean elevation, is more deeply eroded, and sits opposite a transform margin that succeeded the active margin ∼40 Ma. Differences between Bowser and Nechako basins contrast with the tectonic history and erosion impacts on thermal state. Tectonic history and eroded sedimentary thickness suggest that Bowser basin lithosphere is cooling and contracting relative to Nechako basin lithosphere. This effect has reduced Bowser basin heat flow by ∼10–20 mW/m2 since ∼40 Ma. Neither can heat generation differences explain the northerly increasing Intermontane Belt heat flow. A lack of extensional structures in the Bowser basin precludes basin and range-like extension. Therefore, another, yet an unspecified mechanism perhaps associated with the Northern Cordilleran Volcanic Province, contributes additional heat. Bowser basin’s paleogeothermal gradients were higher, ∼36 mK/m, before the Eocene and this might affect petroleum and metallogenic systems.

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.

Shelfbreak gullies; products of sea-level lowstand and sediment failure; examples from Bowser Basin, northern British Columbia

ABSTRACT The transition from cyclothemic shelf to shale-dominated slope deposits (Callovian to Oxfordian) that accumulated along the northern convergent margin of Bowser Basin is delineated by spectacular, conglomerate-filled channels, or gullies. The gullies, representing the shelf-slope break, formed in two stages: (1) Slumping of upper slope-outer shelf sediment created topography, which determined (2) the course of fluvial, distributary-type channels and focused gravel-dominated sediment transport during relative sea-level lowstands. Gully fill consists predominantly of debris-flow deposits, locally incorporated into small (lowstand) fan deltas that prograded from gully margins. Lowstand fluvial channels overlie shelf cycles, incise the underlying highstand deposits, and are overlain by fossiliferous, transgressive sandstone. However, the shelfbreak gullies deposits are up to 10 times thicker than those of the associated lowstand fluvial channels. Although the lowstand fluvial channels likely acted as bypass conduits for gravel and sand during times of low sea level, the initiation and overdeepening of the shelfbreak gullies by slumping probably took place during both high and low relative sea levels. Deposition within gullies may also have continued during relative highstands, from sediment in storage on the outer shelf. The implications for sequence stratigraphic models are: shelf-break gullies can form at any stage of sea level fluctuation; gully thickness may not be related to the magnitude of lowstand incision on the adjacent shelf; and gullies formed by sediment gravity failure do not provide quantitative information about the magnitude of sea-level fall.

Shelfbreak Gullies; Products of Sea-Level Lowstand and Sediment Failure: Examples from Bowser Basin, Northern British Columbia

Shelfbreak Gullies; Products of Sea-Level Lowstand and Sediment Failure: Examples from Bowser Basin, Northern British Columbia

Pore water evolution in sandstones of the Groundhog Coalfield, northern Bowser Basin, British Columbia

Abstract The succession of sandstone cements in chert and volcanic lithic arenites and wackes from the northern Bowser Basin of British Columbia comprises a record of diagenesis in shallow marine, deltaic, and coastal plain siliciclastic sediments that pass through the oil window and reach temperatures near the onset of metamorphism. The succession of cements is consistent with seawater in the sandstones mixing with acid waters derived from dewatering of interbedded organic rich muds. Sandstone cement paragenesis includes seven discrete cement stages. From earliest to latest the cement stages are: (1) pore-lining chlorite; (2) pore-lining to pore-filling illite; (3) pore-filling kaolinite; (4) oil migration through some of the remaining connected pores; (5) chlorite dissolution; (6) quartz cement; and (7) calcite cement. These seven cement stages are interpreted as a record of the evolution of pore waters circulating through the sandstones after burial. The earliest cement stages, as well as the depositional environments, are compatible with seawater as the initial pore fluid. Seawater composition changed during transport through the sandstones, first by loss of Mg2+ and Fe2+ during chlorite precipitation (stage 1). Dewatering of interbedded organic-rich mudstones probably added Mg2+ and Fe2+ to partially buffer the loss of these cations to chlorite. Acids produced during breakdown of organic matter are presumed to have mixed into sandstone pore fluids due to further compaction of the muds, leading to reduction of initial alkalinity. Reduction in alkalinity, in turn, favours change from chlorite to illite precipitation (stage 2), and finally to kaolinite (stage 3). Pore waters likely reached their peak acidity at the time of oil migration (stage 4). Chlorite dissolution (stage 5) and quartz precipitation (stage 6) occurred when pores were filled by these hydrocarbon-bearing and presumably acidic fluids. Fluid inclusions in fracture-filling quartz cements contain petroleum, high-pressure methane, and methane-rich aqueous solutions. Homogenization temperatures from primary two-phase inclusions are consistent with quartz cementation during progressive heating between approximately 100 and 200°C. Following quartz precipitation, alkaline pore waters were re-established, as evidenced by late-stage calcite cement (stage 7).

Early to middle Cretaceous paleogeography of north-central British Columbia: stratigraphy and basin analysis of the Skeena Group

The Lower–middle Cretaceous Skeena Group records the Early Cretaceous evolution of the southern margin of the Jura-Cretaceous Bowser basin in north-central British Columbia. We formalize Skeena Group nomenclature and present interpretations of three distinct paleogeographic and tectonic phases. During the first phase (Neocomian–Aptian), Skeena deposition was limited to a restricted tidal basin represented by Laventie Formation black-shale deposits, surrounded by coal-swamp deltas of the lower Bulkley Canyon Formation. The lower Skeena Group, correlated to the McEvoy Formation (Bowser Lake Group) in the northern basin, represents final filling of the Bowser foredeep produced by Jurassic accretion of the Intermontane Superterrane to North America. In the second phase (early Albian – Early Cenomanian), marine deposition transgressed eastward and southward accompanied by intrabasinal Rocky Ridge volcanism shedding volcanic detritus into the Kitsuns Creek Member of the Bulkley Canyon Formation. The Rocky Ridge Formation does not correlate northward to other Bowser basin fill but represents intrabasinal volcanism in a transtensional setting along the Omineca continental arc. During the final phase (early–middle Cenomanian), red-bed chert-pebble fluvial deposits of the Rocher Deboule Formation prograded westward, shifting the shoreline to tide-dominated deltas on the far western basin margin. The Rocher Deboule Formation correlates to the Devil's Claw Formation (Bowser Lake Group), the lower member of the Tango Creek Formation (Sustut Group), and, tentatively, to the lower conglomeratic Kasalka Group, all attributed to transpressional Omineca uplift and cannibalization of older Bowser basin fill. Thus the southern basin margin evolved from an Early Cretaceous flexural foredeep to a middle Cretaceous arc setting dominated by oblique convergence, first transtensional then transpressional.

Detrital Chromian Spinel Compositions Used to Reconstruct the Tectonic Setting of Provenance: Implications for Orogeny in the Canadian Cordillera

ABSTRACT Detrital chromian spinel is an important accessory mineral in chert and volcanic lithic sandstones from the northern Bowser Basin. Microprobe analyses show that Bowser Basin detrital chromian spinels have Cr/(Cr + Al) between 0.21 and 0. 89, Mg/(Mg + Fe2+) between 0.24 and 0.70, and Fe3+/(Fe3+ + Al + Cr) values below 0.12. Comparison with spinels from literature on ultramafic rocks indicates that the compositional range of the detrital spinels closely matches that of spinels from Alpine-type peridotites emplaced by obduction of marginal-basin crust and island-arc complexes. Furthermore, Bowser Basin spinels are compositionally distinct from spinels derived from either mid-ocean ridge, stratiform ultramafic complex, or Alaskan-type peridotite. The composition of the detrital spinel demonstrates that the source for Bowser Basin sediments included extensive Alpine-type peridotite, which probably originated as marginal-sea lithosphere. In addition, there is no evidence for material derived from mid-ocean ridge, Alaskan-type peridotites, or stratiform complexes. This provenance interpretation is in agreement with earlier interpretations that related chert pebbles in the Bowser Basin to chert-rich strata of the Cache Creek terrane and is consistent with the detrital modal analysis of sandstones, which calls for a source terrane consisting of obducted oceanic crust and island arc. The chemistry of the detrital spinels further suggests the source area was eroded prior to emplacement of Alaskan-type peridotites or any regional gree schist or higher-grade metamorphic events. Thus, microprobe compositional analysis of detrital spinels adds important detail to the provenance interpretation that is not available through analysis of detrital modes alone. The inference that marginal-basin lithosphere rather than material derived from mid-ocean ridges was the sediment source suggests that the obducted source terranes were closely associated with North America.

Mid‐Cretaceous transtension in the Canadian Cordillera: Evidence from the Rocky Ridge volcanics of the Skeena Group

The age relations, geochemistry, and sedimentology of the Rocky Ridge Formation of the Skeena Group are used to test competing tectonic reconstructions for the mid‐Cretaceous Canadian Cordillera as well as the timing and location of the accretion of the Insular Superterrane. Pollen and macrofossil assemblages indicate that these intrabasinal basalts were erupted along the southern margin of the Bowser basin in the Early Albian to Early Cenomanian. Single‐crystal fusion and step‐heating 40Ar/39Ar dating of hornblendes in one basalt flow from the uppermost part of the formation yielded Middle Cenomanian ages of 94.3 ± 0.4, 95.6 ± 1.6, and 95.0 ± 1.6 Ma. Vesicular basalt flows interbedded with crystal‐rich tuff breccias contain evidence for hot emplacement as pyroclastic flows. Individual eruptive centers are identified by their proximal facies, paleoflow indicators within the lava flows, paleoflow indicators within interbedded volcaniclastic fluvial deposits, geochemical differences, and geographic isolation of volcanic deposits. Major and trace‐element geochemistry from 20 sampled lava flows indicates an alkali basalt composition for the volcanics. The basalts of the northern Rocky Ridge volcanic center show enrichment of light rare earth and large ion lithophile elements with strong negative Nb‐Ta anomalies whereas the basalts of the southern Tahtsa Lake volcanic center show depletion to slight enrichment of light rare earth elements, slight enrichment of large ion lithophile elements with minimal negative Nb‐Ta anomalies. The geochemistry combined with paleogeographic and regional tectonic reconstruction suggests a continental arc setting with intraarc extension. The presence of deeper marine facies to the west and the lack of a western sediment source in the Skeena Group indicate that the technically active Insular Superterrane was not west of the study area during mid‐Cretaceous time. Thus we reconsider the Omineca Belt as the main axis of a mid‐Cretaceous continental arc, placing the Intermontane Superterrane in the intraarc to forearc position with the Rocky Ridge volcanics erupted along the forearc side of the Omineca arc. Coeval regional strike‐slip faulting and reconstructed oblique plate convergence suggest a transtensional setting for Rocky Ridge intraarc extension. An electronic supplement of Tables A1–A2 may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG (LOGIN to AGU's FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95TC03496, Mid‐Cretaceous transtension in the Canadian Cordillera: Evidence from the Rocky Ridge volcanics of the Skeena Group, Kari N. Bassett and Karen L. Kleinspehn). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00. Payment must accompany order.

96/01419 Coal and coalbed methane resource potential of the Bowser Basin, Northern British Columbia

96/01419 Coal and coalbed methane resource potential of the Bowser Basin, Northern British Columbia

Coal and Coalbed Methane Resource Potential of the Bowser Basin, Northern British Columbia

The Bowser Basin covers approximately 50,000 km{sup 2} within the northern part of the Intermontane Belt in northwestern British Columbia. The Groundhog coalfield encompasses approximately 5,000 km{sup 2} in the north-central part of the basin. Coal exploration in the coalfield for the last 100 years and a number of published research projects provide the raw data for an assessment of the potential coal and coalbed methane resource of the Groundhog coalfield. The main coal-bearing sequence in the coalfield is of Late Jurassic to Early Cretaceous age and outcrops on all four sides of the northwest-trending Mount Beirnes synclinorium, which is the most prominent regional structure in the coalfield. The rank of the coal varies from semi-anthracite to meta-anthracite. Coal seams are up to 7 m thick, and cumulative coal thickness in the coal-bearing section ranges up to 53 m. The coalfield contains a potential resource of 37 billion metric tons of coal. The estimated potential coalbed methane resource of the Groundhog coalfield is 228 billion m{sup 3} or 8 trillion cubic feet (tcf). The recoverable reserves will be considerably less. The resource value is large, but the complex structure within the Beirnes synclinorium may make recovery difficult. An adsorption isothermmore » on a drill-core sample of anthracite from the Groundhog coalfield corroborates Kim`s (1977) prediction of the high adsorptive capacity of coal of this rank at low pressure and temperature.« less