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CSPG Bulletin

Abstract


Bulletin of Canadian Petroleum Geology
Vol. 41 (1993), No. 1. (March), Pages 1-16

Deformational History of the Groundhog Coalfield, Northeastern Bowser Basin, British Columbia; Styles, Superposition and Tectonic Implications.

I.W. Moffat,, R.M. Bustin

ABSTRACT

The Groundhog Coalfield in the northeastern Bowser Basin of north-central British Columbia contains strata that range in age from Bathonian to Middle Albian. These rocks, which are part of the Bowser Lake Group, lie with structural discordance on Pleinsbachian to Bajocian shales of the Spatsizi Group. Immediately east of the coalfield, strata of the Sustut Group, which ranges in age from Middle Albian to Maastrichtian, lie unconformably on strata of the lower-most Bowser Lake Group.

The entire stratigraphic package has been subjected to two, and possibly three, superposed strains. The earliest deformation (D1) is preserved in northwest-trending, kink style, flexural-slip folds and northeasterly and southwesterly vergent thrust faults that generally have limited displacement. Northeasterly-trending open folds (D2) are superimposed onto the earlier structures, as are flat-lying thrusts that offset first phase structures and whose remnants are preserved as klippen. A third strain event (D3), whose inferred major compressive axes would parallel those of the initial strain, is recorded by gently folded second phase (D2) axial planar cleavage and thrust surfaces.

The history of superposed strains observed in the Groundhog Coalfield appears to be consistent with regional patterns of deformation in the late Mesozoic. The orientation of superposed strains is compatible with the re-orientation of maximum principal compression from northeast-southwest in the pre-Albian to approximately north-south in the middle Cretaceous and to north-northeast-south-southwest in Late Cretaceous and/or early Tertiary -- a history interpreted elsewhere from the offset of major strike-slip faults in the northern Cordillera. The first two superposed strains also roughly correspond to changes suggested by other authors, in relative convergence of the Kula/Farallon and North American plates. This shift may have resulted in a change in shortening direction in the study area from roughly northeast-southwest (120 - 100 Ma) to roughly north-northwest-south-southeast (100 - 43 Ma). The timing of the various superpositions of strain events in the study area are slightly at odds with that documented regionally. Whereas the strain history interpreted from fault offset and from relative plate convergences both suggest an intra-Albian shift from northeast-southwest to approximately north-south directed compression, field relationships in the study area suggest that this shift must have occured after deposition of the Albian Devils Claw Formation.

The apparent "interchange" in orientation of maximum and intermediate compression axes would have been facilitated by low deviatoric stresses, a condition suggested from the orientation of syntectonic quartz/carbonate veins in the study area. Bracketing of the timing of deformation by the aforementioned unconformable relationships suggests that the deformational history of the Groundhog Coalfield was rather short-lived, and spanned the late Middle Albian to Campanian or possibly Santonian.

RESUME

Le bassin houiller Groundhog dans le nord-est du bassin Bowser de la Colombie Britannique septentrionale-centrale contient des strates d'age bathonien a albien moyen. Ces roches, qui font partie du groupe Bowser Lake, sont separees des shales pliensbachiens a bajociens sous-jacents par une discordance tectonique. Directement a l'est du bassin houiller, les strates du groupe Sustut, d'age albien moyen a maastrichtien, recouvrent de facon discordante les couches du groupe Bowser Lake inferieur.

L'entierete de cet intervalle stratigraphique a ete sousmis a deux et peut-etre trois contraintes superposees. La premiere deformation (D1) est preservee par des plis a glissement de flexure du type kink orientes vers le nord-ouest et par des failles de charriages a faible deplacement dont la vergence est orientee vers le nord-est et le sud-ouest. Des plis

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ouverts (D2) orientes vers le nord-est sont superposes a ces premieres structures, ainsi que le sont des failles de charriage qui decalent les structures de la premiere phase, dont les lambeaux de charriage sont preserves en tant que klippes. Un troisieme evenement de contrainte (D3), dont les axes de compression majeurs sont estime avoir ete paralleles a ceux de la premiere phase, est enregistre par le plissement modere du clivage de plan axial et des surfaces de charriage de la seconde phase (D2).

L'histoire des contraintes superposees observee dans le bassin houiller Groundhog apparait en accord avec les tendances regionales de deformation au Mesozoique superieur. L'orientation des contraintes superposees est compatible avec la reorientation de la compression principale maximale du nord-est--sud-ouest au pre-Albien a approximativement nord-sud au Cretace moyen, a nord-nord-est--sud-sud-ouest pendant le Cretace superieur et/ou Tertiaire inferieur, une histoire qui a ete interpretee ailleurs a partir du decalage de failles cisaillantes majeures dans la Cordillere septentrionale. Les deux premieres phases de contrainte correspondent egalement en gros a des changements relatifs dans la convergence des plaques Kula/Farallon et nord americaine suggeres par d'autres auteurs. Ce changement aurait pu causer une modification de la direction de raccourcissement dans la region etudiee depuis approximativement le nord-est--sud-ouest (120-100 Ma) vers approximativement le nord-nord-ouest--sud-sud-est (100-43 Ma). Le moment de la superposition de ces divers evenements de contrainte dans la region etudiee est legerement different de celui documente a l'echelle regionale. Alors que l'histoire des contraintes interpretee a partir des decalages de failles et de la convergence des plaques suggere un changement de direction de compression durant l'Albien depuis le nord-est--sud-ouest approximativement vers le nord-sud, les relations de terrain dans la region etudiee suggerent que ce changement eut lieu apres le depot de la formation Devils Claw albienne.

Cet "echange" apparent entre l'orientation des axes de compression maximum et intermediaire aurait ete facilite par de faibles differences entre les valeurs des stress principaux, une condition qui est suggeree par l'orientation de veines de quartz/carbonate syn-tectoniques dans la region etudiee. L'utilisation des relations de discordance susmentionnees pour preciser l'age de deformation suggere que l'histoire de deformation du bassin houiller Groundhog fut plutot ephemere, couvrant la periode de l'Albien moyen superieur au Campanien ou peut-etre Santonien.

Traduit par Patrice de Caritat


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