Regional coalification pattern of Lower Cretaceous coal-bearing strata, rocky mountain foothills and foreland, Canada — implications for future exploration

Abstract

The regional coalification pattern of Lower Cretaceous strata in parts of the Canadian Foothills Belt and adjacent foreland has been determined by vitrinite reflectance measurements. The present study focusses on two coal-bearing sequences, Bluesky-Gething and Gates Formation, both of which contain coal resources of economic interest. The vitrinite reflectances ( R max) in the Bluesky-Gething Formation range from 0.76% (high-volatile A bituminous) to 2.55% (semianthracite). Rank changes from top to the base of Bluesky-Gething Formation follow 1st-order regression lines. Factors influencing the rate of increase of R max per depth interval (coalification gradient) include paleogeothermal gradients, the rank level under investigation, conductivities of host rocks, and thicknesses of coal seams. Time-depth (burial) curves for the Lower Cretaceous Bluesky-Gething Formation suggest that the regional coalification pattern for the top of the formation results largerly from variations in the depth and/or duration of burial beneath Maastrichtian-Tertiary foredeep deposits. Coalification largerly predates deformation. The coalification pattern is more complicated for the base of the Bluesky-Gething Formation because thickness changes in the Lower Cretaceous Bluesky-Gething interval locally have a greater effect on the rank than does the regional change in the thickness of Maastrichtian-Tertiary sedimentary wedge. Detailed rank studies on laterally continuous coal seams of the Gates Formation showed that coalification levels were largerly established before folding and thrusting started. Isoreflectance lines of the beds run parallel to the bedding of the folded strata. Seams collected from various thrust sheets exposed at surface do not, in general, show significant changes in vitrinite reflectances from one thrust sheet to another. Many of the coals of the Gates Formation were found to have biaxial negative reflectance in dicatrices. Orientation of R max is in all cases parallel of subparallel to the mactroscopic fold axes indicating preferential orientation of the aromatic lamellae in the coals in the direction of minimum compressive stress. It is suggested that the occurrence of biaxial negative coals is related to a tectonic stress field that existed during the later stages of burial and subsequent deformation.