Abstracts
Abstract
Upper Ordovician to lower Devonian rocks in the Antigonish Highlands, Nova Scotia, consist of biinodal volcanics and redbeds at the base overlain by a thick sequence of fossiliferous marine slliciclastic rocks. These rocks belong to the Arisaig Group and correlate well with coeval rocks to the north of the Antigonish Highlands. The definition of the Arisaig Group is revised to Include the uppermost Ordovician-lowermost Silurian Bears Brook Formation which occurs at the base, and the lower Devonian Knoydart Formation which occurs at the top of the succession.
The sequence as a whnle is probably related to a local rifting event. The Bears Brook Formation consists of continental redbeds and biinodal rift-related volcanic rocks. A marine transgression at. the base of the Silurian is indicated by the first appearance of marine fossiliferous rocks and by the overstep relations exhibited by Silurian strata onto Precambrian basement. The marine transgression may have been associated with a world-wide eustatic rise in sea level and with local rifting. The Knoydart Formation is dominated by fluviatile redbeds indicating a return to subaerial conditions. The distribution of the Arisaig Group rocks around the southern and western margins of the Highlands together with the overstep relationships indicate that the Antigonish Highlands became progressively submerged during the Silurian. This was followed by a return to subaerial conditions In the Early Devonian.
RESUME
Dans les Monts Antigonish, les roches de l'Ordovicien superieur au Devonien inferieur comprennent un ensemble inferieur de volcanites bimodales et de llts rouges sur lequel repose une puissante assise de roches fosslliferes marines silicoclastiques. Ces roches appartiennent au Groupe d'Arisaig et peuvent aiseraent etre correlees avec leurs bomologues au nord des Monts Antigonish. On redefinit le Groupe d'Arisaig pour y inclure la Formation de Bears Brook. presente en sa base et s'etageant du sommet de l'Ordovicien au Silurlen basal, ainsi que la Formation de Knoydart, du Devonien inferieur et repartie au sommet de la succession.
Dans son ensemble, la sequence correspond probablement a un episode local de rifting. La Formation de Bears Brook comprend des lits rouges contiuentaux et des volcanites bimodales epparentees au rift. La premiere apparition de roches marines fossillferes et la discordance fondaraentale entre les strates slluriennes et le bati precambrien teraoignent d'une transgression marine a 1'amorce du Sllurien. Cette incursion marine apparalt reliee a une remontee du niveau marin global ainsi qu'a un rifting local. L'importance des llts rouges fluviatlles dans la de Knoydart y atteste d'un retour A des conditions subaerlennes. La distribution des roches du Groupe en bordures meridionale et occidentale des Monts ainsi que leur contact en discordance lndlquent une progressive des Monts Antigonish au cours du Silurlen. L'Eodevonien marque un retour ausc conditions eustatique Formation d'Arisaig submersion subaerlennes
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