The North Brook Formation: A temporal bridge spanning contrasting tectonic regimes in the Deer Lake Basin, Western Newfoundland

The North Brook Formation in the Deer Lake Basin of western Newfoundland consists of red to grey, pebble to boulder conglomerates and arkosic sandstones, and less common muds tones and limestones. These rocks represent deposition in alluvial fans, associated downfan braided and meandering (?) systems, and small, carbonate-precipitating lakes. The upper part of the North Brook Formation is believed to intertongue with the mostly lacustrine Rocky Brook Formation of Visean age. Deposition of these nonmarine fades occurred mostly in two lateral basins, which flank elongate flower structures containing strata of the Devonian (?) - Tournaisian Anguille Group. The flower structures were squeezed up as a result of dextral transpression along the Cabot Fault Zone. West of the flower structures there is no evidence that strike-slip faulting directly created the western lateral basin. This is based mainly on the distribution of carbonate clasts in North Brook conglomerates which matches the distribution of carbonate rocks in the adjoining basement. The western lateral basin is interpreted to have formed by a combination of gravity faulting and thermal sagging. One area, however, in a topographically low position in one of the flower structures contains about 7 km of North Brook sediment This stratigraphic thickness is thought to be much greater than the vertical depth to basement, an inference based on gravity measurements, which do not show an anomalously strong gravity low. This discrepancy in thickness vs. depth is interpreted in terms of a pull-apart basin, in which deposited sediment is shunted along the extension direction. The pull-apart area is believed to represent the initial deposition of North Brook sediment, when dextral motion was still in progress along this stretch of the Cabot Fault Zone. Lessening of strike-slip movements, probably in the Visean, was accompanied by more pronounced gravity faulting and North Brook deposition in lateral basins. RESUME La Formation de North Brook (Bassin de Deer Lake, Terre-Neuveoccidenlale) comprend des poudingues a granules el blocs ainsi que des gres arkosiques, gris a rouges, et une plus faible proportion de mudstones et calcaires. Ces roches representent un depot dans des cones de dejection, vers l'aval dans les systemes tresses et a meandres qui leur etaient associes, et dans des petits lacs ou preipitaient des carbonates. On croit que la partie superieure de la Formation de North Brook s'interdigite avec la Formation viseenne a dominante lacustre de Rocky Brook. Le depot de ces facies continentaux prit place surtout dans deux bassins lateraux occupant les flancs de dispositifs a double deversement allonges contenant les strates devoniennes(?)a toumaisiennes du Groupe d'Anguille. La compression vers le haut de ces dispositifs a double deversement decoula d'une transpression dextre le long de la Zone de failles de Cabot. A l'ouest de ces dispositifs, on n’a aucune preuve qu'un decrochement soit directement responsable du bassin lateral occidental. On base ceci surtout sur la distribution des fragments carbonates dans les poudingues de North Brook, distribution qui correspond a celle du socle adjacent. On interprete ce bassin comme la reultante de la formation de failles par gravity combined a un flechissement thermique. Cependant, un depocentre situe en un point bas d'un dispositif a double deversement contient environ 7 km de sediment North Brook. On croit, sur la base de mesures gravimetriques ne montrant aucune forte anomalie negative, que l’epaisseur des strates soit beaucoup plus grande que la profondeur verticale jusqu'au socle. On interprete cette disparity entre l'epaisseur et la profondeur comme l'expression d'un bassin rhomboetlrique sur devrochement, au sein duquel le transit sedimentaire suit la direction de l’extension. La zone rhomboelrique pourrait representer le depot initial des sediments North Brook, lorsque le jeu dextre s'exer^ait encore sur ce jalon de la zone de failles de Cabot. Une diminution des coulissages, probablement au Viseen, s'accompagnait de la formation plus active de failles normales et du depot des sediments North Brook dans les bassins lateraux. [Traduit par le journal]