ABSTRACT

The Silurian-Devonian sequence in the southern Mackenzie Mountains is part of the miogeoclinal wedge of shelf and slope sediments that borders the entire western margin of the North American Plate. The boundary between shallow-water shelf and offshore slope and basinal deposits fluctuated over a broad region in the southern Mackenzie Mountains during Early Paleozoic time. These relatively rapid changes in the position of the shelf edge in the southern Mackenzie Mountains were the consequence of the evolution of two major, closely related, tectono-sedimentologic entities, the Root Basin and the Prairie Creek Embayment.

Root Basin, an intrashelf basin, developed across the southern part of the Mackenzie Shelf in Ordovician time. Shallow-water dolostones of the Mount Kindle and Root River Formations rimmed Root Basin during Silurian time while argillaceous sediments of the Whittaker and Road River Formations accumulated in the basin centre. By Late Silurian time, marine siltstones of the Cadillac Formation accumulated on a westward-prograding sediment wedge that partly filled Root Basin.

Uplift along the axis of Sombre Salient, the southern extension of Redstone Arch, initiated development of the Prairie Creek Embayment at the south end of Root Basin in earliest Devonian time. The embayment was, in fact, an unfilled remnant of Root Basin, occupying its former southern entrance. The outline of the embayment is marked by the southern depositional limit of the argillaceous and silty carbonates of the Lower Devonian Vera and Camsell Formations. During this period of deposition siltstones of the Cadillac Formation were confined to the region of Prairie Creek Embayment.

In late Early Devonian and early Middle Devonian time the embayment was rimmed by the peritidal carbonates of the Sombre and Arnica Formations. The embayment was filled primarily by the hemipelagic shelf-edge-derived resedimented carbonates of the Sombre detrital member, the Cadillac pink shale member, and the Arnica basinal member. A lack of organic material in the embayment during most of this time period may have permitted almost all the iron in the embayment sediments to remain in the oxidized state. This oxidized iron caused the sediments to be pink-coloured.

Following deposition of the Arnica Formation, large parts of the shelf subsided and Root Basin was re-established as a bathymetric feature in which shales of the Funeral Formation were deposited. After deposition of the Funeral and Landry Formations, the complex paleogeography of earlier times was replaced by a nearly linear, northwest-trending depositional shelf edge, and the Root Basin ceased to influence deposition. Biostromal limestones of the Nahanni Formation were deposited in shallow water east of this boundary, whereas argillaceous and skeletal lime wackestones of the Headless Formation accumulated farther west. Deposition of the Middle and Late Devonian shales of the Horn River and Fort Simpson Formations marked the end of early Paleozoic shelf carbonate deposition in this region.

LA SEDIMENTATION DU BASSIN ROOT ET DE LA BAIE PRAIRIE CREEK (N.W.T.) AU SILURO-DEVONIEN

RESUME

La sequence siluro-devonienne dans le Sud des Montagnes Mackenzie est une partie des sediments miogeoclinaux de plateforme et de pente qui bordent toute la marge ouest de la plaque nord-americaine. Au cours du Paleozoique inferieur la limite entre les depots de plateforme et ceux de bassin oscilla sur une vaste region dans le Sud des Montagnes Mackenzie. Ces changements relativement rapides furent la consequence de l'evolution de deux entites tectono-sedimentologiques etroitement liees, le Bassin Root et la Baie Prairie Creek.

Le Bassin Root s'est developpe dans la partie sud de la plateforme Mackenzie a l'Ordovicien. Les dolostones d'eau peu profonde des formations Kindle et Root River frangeaient le bassin au Silurien tandis que les sediments argileux des formations Whittaker et Road River s'accumulaient dans le centre. Au Silurien superieur, les siltstones marins de la formation Cadillac se deposerent en progradant vers l'Ouest pour remplir en partie le bassin.

Au debut du Devonien, l'uplift de la partie sud de l'Arche Redstone (Sombre Salient) initia le developpement de la Baie Prairie Creek a la terminaison sud du Bassin Root. La forme de la baie est soulignee par la limite depositionnelle sud des carbonates argileux et silteux des formations Vera et Camsell du Devonien inferieur. Pendant cette periode, le depot des siltstones de la formation Cadillac se limita a la region de la Baie Prairie Creek. A la fin du Devonien inferieur et au debut du Devonien moyen, la baie etait frangee par les carbonates de depot peritidal des formations Sombre et Arnica.

La Baie fut remplie essentiellement par les carbonates derives de la plateforme des unites Sombre, detritique, Cadillac, schiste argileux rose, et Arnica, depot de bassin. L'absence de matiere organique dans la baie durant la plupart de cette periode peut avoir permis la conservation a l'etat oxyde du fer des sediments. Cet etat oxyde est a l'origine de la couleur rose des sediments.

Lors de la phase de subsidence qui suivit le depot de la formation Arnica, le Bassin Root fut re-etabli en tant que trait bathymetrique et les schistes argileux de la formation Funeral se deposerent. Apres le depot des formations Funeral et Landry, une bordure de plateforme de direction NW-SE se substitua a la paleogegraphie anterieure, et le bassin cessa d'influencer la sedimentation. Les calcaires biostromaux de la formation Nahanni se deposerent en eau peu profonde a l'Est de cette limite tandis que les wackestones de la formation Headless s'accumulaient plus a l'Ouest. Le depot des schistes argileux des formations Horn River et Fort Simpson du Devonien moyen et superieur marqua la fin de la sedimentation carbonatee de plateforme du Paleozoique inferieur dans cette region.

Traduit par B. Collot

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