Structural geology of the northern Liard Range, Franklin Mountains, Northwest Territories

The northern Liard Range is a complex, faulted anticline cored by resistant Early Carboniferous clastic and carbonate rocks, and flanked on both sides by Early Cretaceous clastic rocks. In the study area, the Liard Range defines an eastward-opening arc, striking north-northwest in the south, but swinging toward the northeast, in the north. In the south, the Carboniferous Prophet Formation is thrust over Carboniferous sandstone of the upper Mattson Formation. This east-directed thrust appears to terminate in the apex of the arc. North of the arc, the structure is defined by the southwest-plunging Mattson anticline. In the vicinity of the arc, box folds are common on the east side of the range. The box folds have shallowly east-dipping crestal surfaces and steeply dipping limbs. The folds vary in complexity from ridge to ridge with some folds being harmonic while others display various types of disharmony. 1 Contribution to Central Foreland NATMAP Project Current Research 2002-A5 1 G.F. Hynes et al. Resume : La partie nord du chainon Liard est un anticlinal a geometrie complexe recoupe par des failles, dont le coeur est forme de roches clastiques et carbonatees resistantes du Carbonifere inferieur et les flancs, de part et d’autre, se composent de roches clastiques du Cretace inferieur. Dans la region d’etude, le chainon Liard decrit un arc qui s’ouvre vers l’est; d’une direction nord–nord-ouest au sud il s’incurve vers le nord-est dans sa partie nord. Au sud, la Formation de Prophet du Carbonifere chevauche les gres carboniferes de la partie superieure de la Formation de Mattson. Ce chevauchement a vergence est semble se terminer a la pointe de l’arc. Au nord de l’arc, le style structural est defini par la presence de l’anticlinal de Mattson a plongement sud-ouest. Aux environs de l’arc, les plis coffres sont communs du cote oriental du chainon. Les plis coffres presentent des surfaces sommitales faiblement inclinees vers l’est et des flancs abrupts. La complexite des plis varie d’une crete a l’autre, certains etant harmoniques et d’autres disharmoniques a des degres divers. INTRODUCTION The Liard Range lies within the southern portion of the Franklin Mountains in the Northwest Territories (Fig. 1). The area, which is part of the Canadian Cordillera foreland fold and thrust belt, is underlain by Paleozoic and Mesozoic strata. The topography is structurally controlled and the area is host to important gas reserves, with production from fields such as the Beaver River, Kotaneelee, Pointed Mountain, and La Biche fields. The Liard Range hosts some of the most prolific gas-producing wells in Canada. The Fort Liard area encompasses a fundamental change in the character of the Cordilleran foreland. This is manifest as a change from a thin, narrow, Paleozoic carbonate shelf in the south to a broad, thick platform in the north (Cecile and Norford, 1993); and as an abrupt 200 km eastward swing in the Cordilleran deformation front (Wheeler and McFeely, 1991). The craton beneath western Canada can be divided into five distinct segments bounded by major northeast-trending features defined by surface or subsurface expression. One of these segment boundaries is the Liard line, which has been interpreted as a crustal-scale transfer fault active during the Late Proterozoic rifting that created the paleo-Pacific continental margin. This crustal feature has been interpreted as the essential control that produced the profound character changes noted above (Cecile et al., 1997b). Approximately 100 km north of the Liard line, a second, less prominent feature is suggested by the sigmoidal geometry of the Kotaneelee and Liard ranges, as well as by other surface features. Morrow and Miles (2000) have used these surface expressions, based on regional reconnaissance mapping (Douglas, 1976; Douglas and Norris, 1976), combined with potential field signatures and physiography to postulate the existence of a second subsurface feature, the Beaver River structure, which comprises a secondary element of the Liard transfer fault system (Cecile et al., 2000). Current Research 2002-A5 2 G.F. Hynes et al. ALBERTA N.W.T. Pacific Ocean 55°N 5°N Edmonton YU KO N TE R R IT O R Y AL AS KA O M IN E C A B E LT FO R E LA N D B E LT IN TE R M O N TA N E B E LT IN S U L A R B E LT