Regional coalification of lower cretaceous coal-bearing strata, Rocky Mountain Foothills and Foreland, British Columbia and adjacent parts of Alberta, Canada

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, i.e., Bluesky-Gething and Gates Formations, both of which contain coal resources of economic interest. Bluesky-Gething Formation. The vitrinite reflectances ( R max) range from 0.76% (high-volatile A bituminous coal) to 2.55% (semianthracite). ASTM rank maps for the study area indicate high-volatile A bituminous coal to low-volatile bituminous coal ranks for localities in the Inner Foothills, where seams of sufficient thickness occur at or near surface. These ranks place many of the Gething coals in a rank range suitable for the production of metallurgical coal. Vertical rank changes within the Bluesky-Gething Formation follow first-order regression lines. Factors influencing the rate of increase of R max per depth interval (coalification gradient) include present and paleogeothermal gradients, the rank level under investigation, conductivities of host rocks, thickness of coal seams and possibly locally increased heat flows by groundwater movements. Calibrated time-depth (burial) curves for the Lower Cretaceous Bluesky-Gething Formation suggest that the regional coalification pattern for the top of the formation results largely from variations in the depth and/or duration of burial beneath Maastrichtian-Tertiary foredeep deposits. Local variations in paleogeothermal gradients modified the overall pattern. Coalification increases from west to east across the Foothills occurred largely because of diachronous Laramide deformation that resulted in an eastward increase in duration and depth of burial. Maximum coalification occurs near the eastern limit of deformation. Coal rank decreases farther east as a consequence of eastward thinning of the Late Cretaceous-Tertiary sedimentary wedge. Rank also decreases significantly along strike because of northward thinning of the Maastrichtian-Tertiary section. Locally, northward thinning is enhanced by the north-side-up reactivation of old basement structures associated with the Peace River Arch/Embayment. The coalification pattern is more complicated from 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 thickness of Maastrichtian-Tertiary sedimentary wedge. Gates Formation. Detailed rank studies on laterally continuous coal beds of the Gates Formation showed that coalification levels were 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 do not in general show significant changes in vitrinite reflectances from one thrust sheet to the other. Many of the coals of the Gates Formation were found to have biaxial negative reflectance indicatrices. Orientation of R max is in all cases parallel or subparallel to the macroscopic fold axes which indicates preferential orientation of the aromatic lamellae of 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.