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The AAPG/Datapages Combined Publications Database

CSPG Bulletin


Bulletin of Canadian Petroleum Geology
Vol. 46 (1998), No. 4. (December), Pages 564-575

Components of Syn- and Post-Deformational Coalification in the Mountain Park Area, West Central Alberta

C. Willem Langenberg, Wolfgang Kalkreuth, Ken Holmes


The Lower Cretaceous Jewel seam, which is generally about 10 metres thick, is the major economic coal seam of the Mountain Park area. The strata of the area are folded and cut by several thrust faults. Vitrinite reflectance of the Jewel seam (and other coal horizons) was measured from samples collected at outcrops and drill holes from close to one hundred evenly distributed locations. The mean maximum vitrinite reflectance of the Jewel seam ranges from 0.94 to 1.28 percent, with the highest values in the lowest thrust sheet. In addition, there is a slight east to west increase in reflectance in each thrust sheet.

In order to examine relationships between coalification and deformation, vitrinite reflectance anisotropies were determined from oriented coal blocks. Several blocks have biaxial anisotropies, indicating a relation to tectonic stress fields. These biaxial coals display maximum reflectance axes parallel to nearby fold axes, which suggests a relationship between vitrinite anisotropy and local deformation. The biaxial vitrinite reflectance ellipsoids result from superposition of tectonic strains on a primary, sedimentary, burial-related, uniaxial anisotropy. These relationships indicate that coalification resulted largely from pre-deformational sedimentary burial, with components of syn-deformational coalification during the later stages. Post-deformational coalification is indicated by significant reflectance jumps across thrust faults, which cannot be removed by palinspastic restoration. Up to 25 percent of the measured reflectances may have resulted from syn- and post-deformational coalification.


La couche Jewel du Cretace inferieur, en general d'une epaisseur d'environ 10 metres, est la couche de charbon economique majeure de la region de Mountain Park. Les strates de cette region sont plissees et recoupees de plusieurs failles de chevauchement La reflectance de la vitrinite de la couche Jewel (et d'autres horizons de charbon) a ete mesuree a partir d'echantillons provenant d'affleurements et de forages distribues uniformement dans pres d'une centaine de localites. La reflectance moyenne maximum de la vitrinite de la couche Jewel varie de 0,94 a 1,28 pour-cent, ou les plus hautes valeurs se retrouvent dans l'ecaille de chevauchement la plus basse. De plus, on observe un leger accroissement de la reflectance de l'est vers l'ouest de chacune des ecailles de chevauchement.

Pour mieux comprendre les relations entre la houillification et la deformation, les anisotropies de la reflectance de la vitrinite ont ete determinees a partir de blocs de charbon orientes. Plusieurs blocs montrent des anisotropies biaxes, indiquant une relation avec les champs de contrainte tectonique. Ces charbons biaxes montrent des axes de reflectance maximum orientes parallelement aux axes de plis avoisinants, ce qui suggere une relation entre l'anisotropie de la vitrinite et la deformation locale. Les ellipsoides biaxes de la reflectance de la vitrinite sont le resultat de la superposition de deformations tectoniques sur une anisotropie uniaxe reliee a l'enfouissement sedimentaire initial. Ces relations indiquent que la houillification est largement derivee de l'enfouissement sedimentaire pre-deformation, avec des composantes de houillification syn-deformation durant les etapes ulterieures. La houillification post-deformation est indiquee par des sauts significatifs de la reflectance de part et d'autre des failles de chevauchement, qui ne peuvent etre elimines par restauration palinspastique. Jusqu'a 25 pour-cent des reflectances mesurees pourraient etre le resultat d'une houillification syn- et post-deformation.

Traduit par Lynn Gagnon

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