ABSTRACT

The Lower Cambrian Bradore Formation rests unconformably on Precambrian gneiss and was deposited during a time of global sea-level rise. The basal 30-m (Blanc-Sablon Member, new name) consist of pebbly, subarkosic, trough and planar-tabular crossbedded sandstone and pebble conglomerate. The succeeding 20 m (Crow Head Member, new name) are similar in texture, but abundant vertical burrows (Skolithos) obliterate all primary physical structures. The upper 10 m (L'Anse-au-Clair Member, new name) consist of quartzose coarse sandstone with polymodal crossbed orientations (trough and planar-tabular), locally common Skolithos, and scattered Conichnus, Dolopichnus and ?Monocraterion.

The Blanc-Sablon Member is a braided-fluvial deposit with easterly directed trough crossbedding indicating flow away from the centre of the craton. Planar-tabular crossbed sets (up to 1.5 m thick) are commonly oriented at high angles to surrounding troughs (up to 90 degrees) and were probably deposited by migration of transverse or cross-channel bars. Braid channels built directly to the coast, prompting comparison with fan deltas. The river sands were extensively reworked by vertical burrowers (Skolithos, Lingulichnus) in distributary-mouth estuaries (Crow Head Member).

The L'Anse-au-Clair Member is mineralogically more mature than the rest of the formation and rests sharply on the Crow Head Member. It is also in sharp contact with shales and carbonates of the overlying Forteau Formation. The L'Anse-au-Clair Member consists of sands that filled laterally migrating tidal inlets of a barrier coastline. However, the upper parts of the barriers were removed by shoreface erosion during marine transgression.

The Bradore Formation and its equivalents around the North American craton indicate that the craton margin was covered by a blanket of immature alluvium prior to the Early Paleozoic transgression. The alluvium resulted from denudation of the barren, unvegetated North American craton under wet, tropical conditions.

SEDIMENTOLOGIE ET ICHNOLOGIE DE LA FORMATION BRADORE (CAMBRIAN INFERIEU) DE LA COTE DE TERRE NEUVE: SERIE TRANSGRESSIVE FLUVIALE-MARINE PEU PROFONDE

RESUME

La Formation Bradore (Cambrien inferieur), deposee durant une montee globale du niveau marin, est discordante sur les gneiss precambriens. Un gres subarkosique a galets et a stratification entrecroisee planaire et a creux (tabular and trough cross-bedded) forme les trente metres inferieurs (Membre Blanc-Sablon, nom nouveau). Les vingt metres sus-jacents (Membre Crow Head, nom nouveau), comporte des roches a texture semblable, cependant que de nombreux terriers (burrow) verticaux (Skolithos) ont tetruit toute structure physique. Un gres quartzite grossier a stratification entrecroisee a orientation plurimodale (planaire et a creux), avec, localement, de nombreux Skolithos et Conichnus, Dolopichnus eparpilles et ?Monocraterion forment les dix metres superieurs (Membre L'Anse-au-Claire, nom nouveau).

Le Membre Blanc Sablon est un depot de riviere anastomosee, a stratification entrecroisee a creux dont le pendage Est indique un ecoulement s'eloignant du centre du craton. L'orientation des series de stratification entrecroisee planaire (jusqu'a 1.5 m d'epaisseur) est fortement oblique (jusqu'a perpendiculaire) a celle des stratifications entrecroisees a creux; les series planaires furent probablement deposees durant la migration de bancs transversaux. Des chenaux anastomoses furent developpes directement en direction cotiere, suggerant la comparaison a un cone de dejection. Les gres fluviaux furent restructures par des terriers (Skolithos, Lingulichnus) au sein des estuaires effluents (Membre Crow Head).

Le Membre L'Anse-au-Clair comporte des roches mineralogiquement plus murs que le reste de la formation, et presente un contact aigu avec le Membre Crow Head sous-jacent, ainsi qu'avec les argiles et les roches calcaires de la Formation Forteau sus-jacente. Les gres qui remplissent des chenaux de maree qui migrent sur en cordon littoral forment le Membre L'Anse-au-Clair. Les cretes des cordons sableux furent cependant erodees durant la transgression marine qui suivit.

La Formation Bradore et ses equivalents en bordure de craton nord-americain indiquent que la marge cratonique fut couverte avant la transgression du paleozoic inferieur par un manteau d'alluvions peu evolues. Les alluvions furent issus de l'erosion en regime tropical humide du craton nord-americain sterile et sans vegetation.

Traduit par A. Zolnai

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