Volume 24, numéro 2, august 1988

EN :

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.

FR :

Des roches du Cambrien a l'Ordovicien infeiieur affleurent dans la region la plus au nord des Monts Antigonish etrecouvrent en discordance le Groupe precambrien de Georgeville. Elles constituent deux groupes interprets comme des equivalents lateiaux de facies. Le Groupe d'Iron Brook est surtout de nature s£dimentaire et se compose de roches se<limentaires 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, surmont£es par des volcanites bimodales alcalines, de type rift continental et englobant quelquesniveaux aroches clastiques rouges et calcaires roses a fossiles de la finde l'Eocambrien. Ces depots se seraient accumul£s armteneurd'unbassmrr»mrx>e^quesurdeCTOchem dextres orienlees NE. La presence de trilobites cosmopolites et franchement marins du debut du Cambrien prouve que ce bassin £tait 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 r£trochevauchees. S'y associent des plis NO-SE dont la vergence s'eioigne d'une zone centrale. Cette structure complexe s'estompe vers les extremil^s 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 pr£serv£e. Ces structures seraient le produit d'un jeu senestre des accidentsbordiers^-SO.conjugueauneconcentrationlocaledusero Cemouvementexpulsa 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 cantoime la deformation a TOrdovicien. Tant le Cambro-Ordovicien que le Siluro-Devonien furent deTormes lors du D£vonien 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, sekm une faible rotation dans le sens inverse des aiguilles d'une montre par rapport aux failles, ce qui suggerc un rejeu dextre des failles NE-SO.

EN :

Previous recordings of Late Ordovician-Early Silurian flysch trace fossils (13 ichnogenera, 15 ichnospecies) from slope deposits of the Matapedia Basin at a single site on the Tobique River, western New Brunswick, are supplemented by the addition of 9 newly discovered ichnogenera (12 ichnospecies). These are:- drculichnis montanus, Cruziana problematica, Cruziana cf. stromnessa, Dipt'tchnites ichnosp., Megagrapton irregulare, Monocraterion ichnosp., Phycosiphon incertum, Rusophycus didymus, Rusophycus ichnosp. types A and H,Scolicia ichnosp, and Tuberculichnus ichnosp. The traces are poorly preserved and occur only in low numbers compared to previously documented forms. Nevertheless, the meticulous sampling procedures adopted at the site emphasize the need for detailed collecting, before drawing conclusions regarding diversity models, stratigraphic distribution, etc., of trace fossils. Of these new forms only Diplichnites ichnosp. has previously been recorded from die Matapedia Basin in the partially coeval Lower Silurian Siegas Formation. Tuberculichnus ichnosp. is recorded for the first time in North America. The combined total of 22 ichnogenera (27 ichnospecies) from the Tobique River represents the most diverse ichnocoenosis yet reported from a single ancient slope deposit. The occurrence of several more typically shallow-water ichnogenera (e.g., Cruziana, Diplichnites, Monocraterion and Rusophycus) suggests that, at least in the Early Palaeozoic, caution must be exercised in their utilization as distinctive facies or environmental indicators.

FR :

9nouveauxichnogenres(12ichnoespeces)s'ajoutentaceuxdejarepertories (13 ichnogenrcs, 15ichnoespcces)enunsite unique localise sur la riviere Tobique, dans 1 'ouest du Nouveau-Brunswick, dans des sediments tardiordoviciens e eosiluriens accumul£s sur le talus continental du Bassin de Matapedia. Ce sont: Circulichnis montanus, Cruziana problematica, Cruziana cf. stromnessa, Diplichnites ichnosp., Megagrapton irregulare, Monocraterion ichnosp., Phycosiphon incertum, Rusophycus didymus, Rusophycus ichnospp, types AetB, Scolicia ichnosp. et Tuberculichnus ichnosp. Cesdemieres sont malconservdes et relativement rares par rapport aux formes deja connues. Ndanmoins, le d£pouillement m&iculeux utilise a ce site d£montre la necessited'unecollecte detailiee avantdetirer des conclusions quant auxmodelesdediversite, aladistributionstratigraphiquc, etc., des traces fossiles. De ces nouvelles formes, seule Diplichinites ichnosp. est deja cormue dans le Bassin de Matapedia, soil dans la Formation sihirienne infeiieure et en partie synchrone de Siegas. On recense Tuberculichnus ichnosp. pour la premiere fois en Amerique du Nord. Le total combine de 22 ichnogenres (27 ichnoespeces) presents sur la riviere Tobique represente l*icrirK>coenoselaplu$divasejamai$repertorieedansunseuldep6tdetalus. Lapresencedcplusieursichnogenrestypiquesd'une faible tranche d'eau (e.g., Cruziana, Diplichnites, Monocraterion et Rusophycus) demontre que la prudence est de misc lors de leur utilisation a litre d'indkateurs de facies on de milieu, tout particulieTement a l'Eopaieozoique.

EN :

Linear furrows and pits were observed from a submersible on the continental slope east of Hudson Strait in water depth ranging from 1004 to 715 m (Fig. 1). The parallel to sub-parallel furrows are typically 15-40cm wide and 20-60 cm deep. These enigmatic features resemble deep-water, current-formed sedimentary furrows but differ in two significant aspects: (1) they consistendy have blocky rims (berms) that appear to have been formed by the ploughing action of a rigid object, and (2) they lack any evidence of formation by bottom currents. We describe the local geological setting and compare the furrows with conventional sedimentary furrows. Our observations lead us to propose three tentative mechanisms for furrow formation: (1) formation by dragging of fins or mandibles of deep diving animals such as whales as they forage on the seafloor, (2) current driven ice which is negatively bouyant because of entrapped glacial sediment, or (3) furrowing by current-driven sponge balls which were observed in the troughs of the furrows.

FR :

Des plongees en submersible sur la pente continentale a Test du detroit d'Hudson ont rev£I£ des sillons lineaires et des depressions par une profondeur de 715 a 1004 m. Ces sillons sont paralleles on presque, typiquement d'une largeur de 15 a 40 cm et d'une profondeur de 20 a 60 cm. Ces structures enigmatiques evoquent les sillons s£dimentaires dus a Taction des courants sur les grands fonds mais en different sur deux points principaux: (1) elles pr£sentent constamment des bordures (replats) morcelles qui semble resulter d'un labourage par un objet rigide et (2) la preuve de leur formation par des courants de fond fait deTaut. On decrit le cadre geologique regional et compare ces sillons aux sillons s&limentaires conventionnels. A la lumiere de nos observations, trois options s'offrent pour expliquer leur formation: (1) tralnage des nageoires ou rostres d'animaux plongeant a de grandes profondeurs, tels les baleines, qui draguent le fond, (2) reclage par des glaces vebiculies par les courants et qui sont plus denses du fait d'un lest de sediment glaciaire ou (3) affouillement par suite de 1' action des courants sur les boules spongiaires qui jalonnent le fond des sillons.

EN :

Cambrian-Devonian sedimentary sequences within southwestern portions of the Meguma Terrane were deformed with attendant developmentof aregionally penetrative cleavage during low-grade regional metamorphismatco. 400-4 lOMa. Contact aureoles developed during subsequent emplacement of the Port Mouton, Shelbume and Barrington Passage plutons at ca. 370-375 Ma. Amphibole within calc-silicate horizons in the contact zones record internally discordant "Ar^'Ar age spectra. However, 3*Ar/*°Ar vs. ,'Ar/*°Ar isotope correlations yield ordinate intercept values which approximate atmospheric values, and abscissa ratios corresponding to ages ranging between 38S.1±2.6 Ma and 366.9±5.6 Ma. These are interpreted to date post-contact metamorphic cooling through temperatures required for iniracrystalline retention of argon. Biotite and muscovite within northern portions of the contact zones record 340-350 Ma *°Ar/"Ar plateau ages which date later post-metamorphic cooling through the lower temperatures required for mica argon closure. Intracrystalline argon systems within biotite and muscovite in southern portions of the contact zone of the Shelbume and Barrington Passage plutons were variably rejuvenated during a later rehearing associated with emplacement of plutons at ca. 315 Ma (Wedgeport Pluton) and 290 Ma (offshore, subsurface pluton). Temperatures attained during these later thermal events were not sufficient to rejuvenate hornblende argon systems.

FR :

Dans les portions sub-ouest de la Laniere de Meguma, les series s6dimentaires cambro-dgvoniennes ont subi, il y a environ 400-410 Ma, une deformation qui s'accompagna du developpement concomitant d'une schistosite penetrative (a l'echelle r^gionale) au cours d'un metamorphisme regional de faible intensity. L'emplacement subsequent des plutons de Port Mouton, Shelbume et Barrington Passage se manifests par 1'implantation d'aureoles de metamorphisme vers 370-375 Ma. L'amphibole provenant de niveaux cake-silicates, au sein des zones de contact, presente des patrons d'age 40Ar/3*Ar marques par une discordance interne. En revanche, les courbes de correlation des isotopes "Ar/*°Ar vs. "Ar/*°Ar montrent des ordonnees a l'origine se rapprochant des valeurs de 1'atmosphere ainsi que des rapports en abcisse correspondant a des ages qui s'etendent de 385,1±2,6 Ma a 366,9±5,6 Ma. Ces ages dateraient, par le biais des temperatures necessaires a la retention intracristalline de 1*argon, un refroidissement post-metamorphisme de contact La biotite et la muscovite provenant des portions septentrionales des zones de contact ont fixe des ages limites 40Ar/s'Ar de 340-350 Maqui datent un refroidissement post-metamorphe ulteneur araidedestempe>aturesplusbasse«regissantlafermeturederargcndanslesmicas. L'emptacementparlasuited'autresplutons aux alentours de 315 Ma (Pluton de Wedgeport) et 290 Ma {pluton de sous-surf ace en of fshore) engendra la rejuvenation a divers degres des systemes d'argon intra-cristallin chez la biotite et la muscovite provenant des secteurs meridionaux de la zone de contact des plutons de Shelbume et Barrington Passage. Les temperatures atteintes lors de ces demiers episodes thermiques resterent en de$a de celles auxquelles fait appel la rejuvenation des systemes d'argon de la hornblende.

EN :

The Hanna Farm pluton is a complex granite - gabbro pluton which appears to be truncated by the Kirkhill fault zone, probably by predominantly dip-slip motion. A leucogranite has been intruded into the Kirkhill fault zone at the southern end of the Hanna Farm pluton. The main Hanna Farm pluton contains secondary biotite and odier evidence of hydrothermal alteration, and is petrographically and geochemically similar to a series of Early Carboniferous (ca. 345 Ma) granites in the western Cobequid Highlands. Secondary biotite within country rock has been radiomelrically dated at 303±11 Ma. The leucogranite within the Kirkhill fault zone lacks evidence of hydro thermal alteration. It shows local ductile deformation, but is not noticeably offset by the Kirkhill fault zone.

The Kirkhill fault zone is one of the major east-west trending fault zones in the western Cobequid Highlands. It cuts and mylonitises late Hadrynian sedimentary and plutonic rocks, and probable Early Carboniferous plutons. The Hanna Farm pluton appears to post-date the last major phase of strike-slip motion on the Kirkhill fault

FR :

Le Pluton de Hanna Farm est un bati complexe de granite et gabbro que semble recouper la zone de failles de Kirkhill, probablement sous I'effet d'un movement a forte composante selon le rejet-pente. Un leucogranite s'est mis en place au sein de la zone de failles de Kirkhill aux confins meridionaux du Pluton de Hanna Farm. Le pluton principal de Hanna Farm contient une biotite secondaire ainsi que d'autres preuves d'une alteration hydrothermale. II ressemble, par sa p£trographie et son geochimisme, a une seYie de granites eocarboniicres (environ 345 Ma) de l'ouest des Monts Cobequid. Une datation radiom£tnquedelabiotitesecondairequerenfermerencaissant,lasituea303±llMa. Onnerencontreaucunetiaced'aMration hydrothermale dans le leucogranite sis au sein de la zone de failles de Kirkhill. Celui-ci pr&ente une deformation ductile locale mais n'est pas decale* de facon notable par la zone de failles de Kirkhill.

Cettederniere est Tune desprincipales zones de failles a tendance Est-Ouest dans 1 'ouest des Monts Cobequid. Elle recoupe et mylonitise des roches plutoniques et s6dimentaires tardihadryniennes ainsi que des plutons probablement eocarboniferes. Le Pluton de Hanna Farm semble postdater le dernier Episode majeur de decrochement sur la faille de Kirkhill.

EN :

The 1988 Colloquium of the Atlantic Geoscience Society was held at the Best Western Claymore Inn and Conference Centre, Antigonish, Nova Scotia on February 5-6, 1988. A workshop organized by the APICS Geology Committee was held on Friday afternoon, February 5, on "Computers in Geology." A special session on "The Age and Style of Faulting in the Canadian and Northern Appalachians" dedicated to the memory of Gao Ruixiang was held on Friday evening. General sessions and poster sessions were also held on Friday evening and on Saturday.

On behalf of the Atlantic Geoscience Society we thank Brendan and Cindy Murphy, other faculty members and students of Saint Francis Xavier University, and all others involved in the organization of the meeting and workshop for organizing and running such a successful and enjoyable Colloquium.

In the following pages we publish the abstracts of talks and poster sessions given at the meeting.