ABSTRACT

The Late Paleozoic strata exposed in the Cumberland Basin, part of the larger regional Maritimes Basin of Atlantic Canada, have been considered as classic Carboniferous sections since their original description in the mid-1800s (Joggins Section). Stratigraphic nomenclature applied to the Upper Carboniferous strata, which locally exceed a thickness of 7 km, is problematic due to numerous poorly defined or informal units and a confusing mixture of litho-, bio-, and chrono- stratigraphic terminology. The Cumberland Basin is a key area in the regional nomenclature because it contains the type sections of the Cumberland and Pictou groups, and well exposed sections of strata traditionally assigned to the Riversdale group (abandoned) and Canso group (abandoned and replaced by the Mabou Group).

Mapping and definition of lithostratigraphic units at the formation level has necessitated examination of the criteria for group nomenclature. The constituent formations of the Cumberland and Pictou groups examined by this study include in ascending order: Cumberland Group, 1. Claremont and 2. Boss Point, (both formerly assigned to the Riversdale group, now abandoned); 3. Polly Brook (new); 4. Joggins (redefined); 5. Springhill Mines (new); 6. Ragged Reef (new); 7. Malagash (new); and Pictou Group, 8. Balfron (new); 9. Tatamagouche (new); and 10. Cape John (redefined). The resulting revision of the groups is based upon formal formation subdivisions and utilizes lithostratigraphic criteria such as composition, occurrence of coal and related strata, predominant colour and grain size, vertical and lateral variation, and stratigraphic relationships. This eliminates or minimizes the historical bio- and chrono-stratigraphic bias. The revised Cumberland Group conformably and unconformably overlies strata of the Mabou Group and includes all of the grey (dominated) coal-bearing strata in the basin. It also includes greybeds that were previously assigned to the base of the Pictou Group in the type section. The revised Pictou Group includes all of the redbed dominated, non coal-bearing strata above the Cumberland Group. The proposed lithostratigraphic subdivisions (new and revised) and group assignment criteria will facilitate regional correlation and basin analysis within the Maritimes Basin.

RESUME

Les couches du Paleozoique superieur affleurant dans le Cumberland Basin, qui fait partie de l'entite regionale du Maritimes Basin du Canada Atlantique, ont ete considerees comme des sections Carboniferes classiques depuis leur premiere description dans les annees 1800 (Joggins Section). La nomenclature stratigraphique appliquee aux couches du Carbonifere superieur, dont l'epaisseur excede localement 7 km, est problematique a cause de nombreuses unites mal definies ou officieuses et d'un melange de terminologies litho-, bio-, et chrono-stratigraphiques portant a confusion. Le Cumberland Basin est une region cle pour la nomenclature regionale parce qu'il contient les sections-types des groupes de Cumberland et Pictou, ainsi que des sections bien exposees de strates traditionnellement attribuees au groupe de Riversdale (abandonne) et au groupe de Canso (abandonne et remplace par le Mabou Group).

Le releve de terrain et la definition d'unites lithostratigraphiques au niveau de formation ont necessite l'examen des criteres de nomenclature de groupe. Les formations constituant les groupes de Cumberland et Pictou examines durant la presente etude incluent en ordre ascendant: Cumberland Group, 1. Claremont et 2. Boss Point, (faisant tous deux prealablement partie du groupe de Riversdale, actuellement abandonne) 3. Polly Brook (nouveau); 4. Joggins (redefini); 5. Springhill Mines (nouveau); 6. Ragged Reef (nouveau); 7. Malagash (nouveau); et Pictou Group, 8. Balfron (nouveau); 9. Tatamagouche (nouveau); et 10. Cape John (redefini). La revision des groupes est basee sur des subdivisions formelles en formations et utilise des criteres lithostratigraphiques tels que composition, presence de charbon et strates associees, couleur predominante et granulometrie, variation verticale et laterale, et relations stratigraphiques. Ceci elimine

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ou minimise l'ancienne influence bio- et chrono-stratigraphique. Le Cumberland Group revise recouvre en concordance et en discordance les strates du Mabou Group et comprend toutes les couches carboniferes (principalement) grisatres du bassin. Il comprend egalement les couches grisatres prealablement rattachees a la base du Pictou Group dans la section-type. Le Pictou Group revise inclu toutes les couches a predominance rougeatre et non-carboniferes au-dessus du Cumberland Group. Les subdivisions lithostratigraphiques proposees (nouvelles et revisees) et les criteres de groupes vont faciliter les correlations regionales et l'analyse de bassin dans le Maritimes Basin.

Traduit par Patrice de Caritat