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The nature and timing of deformational events and organic and inorganic metamorphism in the Northern Groundhog Coalfield : implications for the tectonic history of the Bowser Basin Moffat, Ian W.


The Groundhog coalfield, located in the northeastern corner of the Bowser Basin of north-central British Columbia, contains approximately a 3700 m thickness of sedimentary rock, ranging in age from Bathonian (Middle Jurassic) to Albian (Early Cretaceous). Four laterally continuous, informal stratigraphic units have been outlined, the Jackson (Bathonian-Oxfordian); Currier (Late Jurassic); McEvoy (Late Jurassic-Cretaceous); and Devils Claw (Albian and younger). An overall regressive sequence, transitional from marine shales in the Jackson through to alluvial fanglomerates in the Devils Claw, documents tectonic uplift of a northern source area in response to convergence of allochthonous terranes with cratonic North America. Vitxinite reflectance studies indicate the basal Currier unit was buried to a maximum depth of approximately 5.5 km prior to deformation. Paleo-geothermal gradients between 30°C/km and 40°C/km indicate a maximum burial paleo-temperature of 180° C-220° C. The observed authigenic mineral assemblage interpreted to be stable at these temperatures, though compatable with observations from other basins, is at odds with experimental studies. Confidence in vitrinite paleogeothermometry leads to the conclusion that pore fluid pressure and chemistry as well as permeability were all important factors in inorganic diagenetic/metamorphic history. Between Albian and early Tertiary times the above stratigraphic units were structurally shortened by approximately 35%. The bulk strain resulted from two or possibly three phases of deformation; the first and third are approximately coaxial and perpendicular to the second phase. Deformational style has been strongly influenced by lithologically controlled, apparent viscosity variations. The predominantly conglomeratic and arenaceous upper stratigraphic levels behaved rigidly whereas the underlying, more argillaceous strata behaved more plastically during deformation. A variably-developed pressure solution cleavage, formed at low buckling strains, is distributed areally as a function of its dependence on temperature, grain size and intergranular stress. The distribution and geometry of syntectonic veins which acted as fluid "sinks" implies that pore fluid overpressuring was an important deformational mechanism, and that differential stresses were generally low. The timing of various deformational events and their predicted paleostress orientations are compatible with the relative timing and geometry of dextral offset along major strike-slip faults in north-central British Columbia. Offsetting relationships indicate a switch from northeasterly directed principal stresses in pre-Albian time to north-northwest in mid-Cretaceous and back to northeasterly in late Cretaceous and/or early Cenozoic. Superposition of strain events is considered a response to accumulation and release of constrictional strain, a necessary consequence of contemporaneous motion along the northerly convergent strike-slip margins bounding the allochthonous terrane which contains Bowser Basin sediments.

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