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CSPG Bulletin

Abstract


Bulletin of Canadian Petroleum Geology
Vol. 45 (1997), No. 4. (December), Pages 693-707

Uppermost Permian Conodonts and the Permian-Triassic Boundary in the Western Canada Sedimentary Basin

Charles M. Henderson

ABSTRACT

The Permian-Triassic boundary interval was considered to be absent in many areas of the world, including western Canada, because of the unconformity associated with a global eustatic fall. This boundary is best recognized in strata of the paleo-Tethys Sea and, in particular, southern China, where the boundary will likely be defined. Age assignments discussed in this paper are based upon the assumption that the boundary will be defined by the first appearance of Hindeodus parvus. Biostratigraphic work presented herein indicate that the Permian-Triassic boundary interval is represented in western Canada within the basal black shale of the Sulphur Mountain, Montney and Grayling formations which were previously correlated exclusively with the Triassic. Uppermost Permian faunas have probably not been previously documented in the study area because the significance of considerable condensation within the basal parts of these formations was not recognized. The best section is at Opal Creek in Kananaskis Country where the Permian-Triassic boundary is identified at 1.5 metres above the top of the Ranger Canyon Formation. This suggests that a global eustatic fall, related essentially to the amalgamation of Pangea, occurred during a protracted Late Permian interval and that the subsequent transgression began during the latest Permian and continued into the Triassic. An anoxic depositional site is suggested for these basal shales as they are typically pyritic, lack bioturbation and current structures and have no benthic fossils. This anoxic episode may have been a contributing factor toward the Late Permian extinction event, the largest in geologic history, although extinctions largely occurred earlier in western Canada, since sponge spicules are the only Upper Permian macrofossils in the area. Conodonts, which were minimally affected by this extinction, provide valuable indices for high-resolution sequence biostratigraphic correlation in this interval. The age of the basal shales varies across the region; they are Changhsingian at Opal Creek, Griesbachian at several localities and possibly as young as Dienerian at Meiosin Mountain. This diachroneity can be attributed to the duration of the transgression, as well as paleotopography on the transgressed surface. New exploration insights may result from detailed correlation of Upper Permian and Lower Triassic conodont biozones and sequences/parasequences in the area.

RESUME

L'intervalle de la limite du Permien-Triassique etait considere absent dans plusieurs regions du monde, y compris l'Ouest canadien, a cause de la discordance connexe a la chute eustatique. Cette limite est le mieux identifiee dans des couches de l'ocean Thethys, et en particulier, en Chine du sud, ou la limite est probablement definie. L'etablissement des ages traites dans cet article sont bases sur la suposition que la limite sera definie par la premiere apparition de Hindeodus parvus. Le travail biostratigraphique presente ci-apres indique que l'intervalle de la limite du Permien-Triassique est representee dans l'Ouest canadien dans l'ampelite de fond des formations Sulphur Mountain, Montney et Grayling qui etaient auparavant en correlation exclusivement avec le Triassique. Les faunes du Permien les plus superieurs n'ont probablement pas ete documentees auparavant dans la region etudiee puisque la signification de cette condensation considerable dans les parties de fond de ces formations n'etait pas reconnue. La meilleure section se trouve a Opal Creek, Kananaskis Country, ou la limite du Permien-Triassique est identifiee a 1,5 metres au-dessus de la partie superieure de la Formation Ranger Canyon. Ceci suggere que la chute eustatique, se rapportant essentiellement a l'amalgamation de Pangea, s'est produite pendant l'intervalle etire du Permien tardif et que la transgression subsequente commenca pendant le Permien le plus recent et continua jusqu'au Triassique. Un site de depots anoxiques est suggere pour ces schistes de fond puisqu'ils sont typiquement pyriteux, qu'ils manquent de bioturbation et de structures courantes et ne possedent pas de fossiles benthiques. Cet episode anoxique peut avoir ete un facteur contribuant a l'evenement d'extinction dans le Permien tardif, le plus important de l'histoire geologique; meme si les extictions ont eu lieu plus tot dans l'Ouest canadien, puisque les spicules d'eponges sont les seuls macro-fossiles du Permien

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superieur de la region. Les conodontes, qui ont ete touches au minimum par cette extinction, donnent de precieux indices pour la correlation a haute resolution de la sequence biostratigraphique de cet intervalle. L'age des schistes de fond varie dans la region; ils sont Changhsingien a Opal Creek, Griesbachien a plusieurs emplacements et possiblement aussi jeune que Dienerien a Meiosin Mountain. Cette diachroneite peut etre attribuee a la duree de la transgression, ainsi que la paleotopographie sur la surface en discordance. Les nouvelles perspectives en exploration peuvent etre le resultat de la correlation des biozones de conodontes du Permien superieur et du Triassique inferieur et des sequences/parasequences dans la region.

Traduit par Marie-Louise Tomas


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