Diagenetic Evolution of the Middle Devonian Stone and Dunedin Formations of the Liard Basin, Northeast British Columbia

Abstract

The Middle Devonian Stone and Dunedin formations are correlative to the Chinchaga and Keg River formations in Northwest Alberta, as well as the Headless and Nahanni formations in Southern Yukon. Core study has identified twenty carbonate sedimentary facies in the Stone and Dunedin formations. The lower to middle Stone Formation comprises of a subtle vertical facies transition from intertidal mudstones to lagoonal wackestones and amphiporid floatstone. The upper Stone Formation in c-10-E/94-N-7 comprises of dominantly clast-rich silty dolomudstone to silty dolomudstone, which is interpreted to have deposited in peritidal settings and represent the middle Chinchaga depositional break caused by the middle Eifelian regression. The Stone Formation as a whole is interpreted to be shallowing upward sequence. The Dunedin Formation comprises of a vertical facies transition from mudstones of restricted shallow marine origin, to reefal floatstones and boundstones, and to clast-bearing crystalline rock of slope facies. These are interpreted to be a deepening upward sequence. Integrated geochemical and geophysical analysis have identified two sets of fractures and three stages of local structural deformations, all of which postdate the precipitation of the coarsely crystalline saddle dolomite and calcite cements in the Stone and Dunedin formations. Systematic sampling and geochemical analysis including stable oxygen and carbon isotopes, radiogenic strontium isotopic ratio, and fluid inclusion microthermometry on various diagenetic phases were conducted. The most significant finding is that several samples of the coarsely crystalline calcite cement are highly depleted in δ13C stable carbon isotope values and as low as -11.23 ‰ V-PDB. Pseudo-secondary to secondary carbonic (vapor only?) inclusions were also found in the coarsely crystalline calcite cement. These can be correlated to a major phase of organic maturation. These new structural and geochemical findings led to a reinterpretation that the coarsely crystalline saddle dolomite and calcite in the Middle Devonian Stone and Dunedin formations of the Liard Basin in Northeast British Columbia were precipitated during the Carboniferous and Permian.