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Cambrian-Lower Ordovician rocks outcrop in the northernmost Antigonish Highlands where they unconformably overlie the Precambrian Georgeville Group. They are divided into two groups that are interpreted to be lateral fades equivalents. The Iron Brook Group is predominantly sedimentary and consists of red clastic sedimentary rocks, pink limestones containing late Early Cambrian fossils, quartzites, calcareous tuffs and ironstones containing Late Cambrian-Ordovician inarticulate brachi-opods. The McDonalds Brook Group consists of red clastic sedimentary rocks at the base overlain by bimodal, alkalic, continental rift volcanic rocks with minor interlayered red clastic rocks and late Early Cambrian fossiliferous pink limestones. These rocks are inferred to have been deposited in a pull-apart basin produced in the overlap zone between two dextral NE-trending faults. Connection between the pull-apart basin and a larger ocean is indicated by cosmopolitan, open-ocean. Early Cambrian trilobites.  Cambro-Ordovician rocks in the central part of the basin were subsequently thrusted (?)northeastwards and then back thrusted. Associated NW-SE folds verge away from a central zone. This complex structure dies out towards the NE and S W ends of the basin, where the lowest Cambrian units are relatively mildly deformed and the basal unconformity with Precambrian rocks is preserved. These structures are inferred to have been produced by sinistral motion on the NE-SW bounding faults with compression concentrated locally in the pull-apart basin. This movement expelled Cambro-Ordovician rocks lying on attenuated continental lithosphere from the basin centre onto die basin margins thereby accounting for the limited extent of the deformation. These structures are truncated by die sub-Silurian unconformity, thereby constraining the time of deformation to the Ordovician. Both the Cambro-Ordovician and Siluro-Devonian were deformed during the Devonian by upright NE-SW folds that are preferentially developed adjacent to the major NE-SW faults. The folds are consistently oriented slightly anticlockwise of the faults thereby indicating renewed dextral movements on the NE-SW faults.   RESUME  Des roches du Cambrien a l'Ordovicien inferieur affleurent dans la region la plus au nord des Monts Antigonish et recouvrent en discordance le Groupe precambrien de Georgeville. Elles constituent deux groupes interprets comme des equivalents lateraux de facies. Le Groupe d'Iron Brook est surtout de nature sedimentaire et se compose de roches sedimentaires clastiques rouges, de calcaires roses renfermant des fossiles du debut de l'Eocambrien, des quartzites, des tufs calcaires et des roches ferrugineuses contenant des brachiopodes inarticules tardicambriens a ordoviciens. Le Groupe de McDonalds Brook comprend des roches sedimentaires clastiques rouges en sa base, surmontees par des volcanites bimodales alcalines, de type rift continental et englobant quelquesniveaux a roches clastiques rouges et calcaires roses a fossiles de la finde l'Eocambrien. Ces depots se seraient accumules a l’inferieur d’un basin rhomboedrique sur decrochement installe dans un ilot en distension forme par l’intersection de deux failes dextres orientee NE. La presence de trilobites cosmopolites et franchement marins du debut du Cambrien prouve que ce bassin etait en large communication avec la haute mer.  Dans la partie centrale du bassin, les roches cambro-ordoviciermes furent par la suite chevauchees (?vers le nord-est) puis retrochevauchees. S'y associent des plis NO-SE dont la vergence s'eioigne d'une zone centrale. Cette structure complexe s'estompe vers les extremiles NE et SO du bassin, la ou la deformation des roches cambriennes initiales est relativement modeste et la discordance basale avec les roches precambriennes est preservee. Ces structures seraient le produit d'un jeu senestre des accidents bordiers NE-SO, conjugue a une concentration locale du serage dans le basin de decrochement. Ce movement expulsa des roches cambro-ordoviciennes, sises sur une lithosphere continentale amincie, du centre vers les abords de la cuvette, ce qui explique leur faible deformation. Le recoupement de ces memes structures par la discordance angulaire antesilurienne cantonne la deformation a l’Ordovicien. Tant le Cambro-Ordovicien que le Siluro-Devonien furent deformes lors du Devonien par des plis droits NE-SO developpes de facon preterentielle au voisinage des failles majeures NE-SO. L'orientation de ces plis s'inscrit, conformenient, selon une faible rotation dans le sens inverse des aiguilles d'une montre par rapport aux failles, ce qui suggere un rejeu dextre des failles NE-SO.  [Traduit par le journal]

Anatomy of a telescoped pull-apart basin: an example from the Cambro-Ordovician rocks in the Antigonish Highlands of Nova Scotia