Preliminary Map 42 presents the geology of the southern part of the Crowsnest Coalfield in southeastern British Columbia. The Preliminary Map consists of 7 sheets that wrap around the south end of the coalfield. Each is a 1:10 000 scale geological map with legend, explanatory notes and sections through the coal measures showing coal seams.
The maps show the distribution of coal seams of the Jurassic-Cretaceous-age Kootenay Group in the vicinity of Morrissey Creek and Flathead Ridge, across the western part of the South Dominion Coal Block (Parcel 82). At the south end of Morrissey Ridge (section G-H) the thickness of the coal measures is 560 metres, of which 10 per cent is coal; the structurally thickened section exposed on Flathead Ridge (K- J) is by contrast 770 metres thick yet did not reach the overlying Elk Formation. Of this succession, 11 per cent is coal.
The rank of coal has been determined by measuring in oil the mean maximum reflectance (Ro) of the coal maceral vitrinite. Values >1.51 per cent indicate low volatile bituminous coal; values <1.51 per cent indicate medium volatile bituminous coal. All coals exposed in Morrissey Creek valley are of low volatile rank; all coals exposed beneath the Flathead Ridge fault are low volatile in rank. Medium volatile coals are restricted to the structural plate riding on the Flathead Ridge fault, though they are also present on the eastern margin of the map sheet close to the ridge crest.
The rank of coal changes down-dip of seams. For example, the seam at 400 metres elevation above the basal sandstone with Ro maximum of 1.38 per cent shown in section G-H has increased in reflectance in Morrissey Creek valley to 1.65 per cent Ro. This is because much of the coalification in this area of the coalfield occurred subsequent to folding and tilting.
Flathead Ridge fault, which cuts across the hinge zones of a syncline-anticline couple at the south margin of the sheet, is a post-coalification structure that separates coal of markedly different rank. Overturned, constrictional structures at the southern extremity of the fault may indicate a thrust component prior to normal movement of about 1 000 metres (vertical).
Moving south along the strike of the measures on Morrissey Ridge, there is an increase in rank, so that at the south end of the map sheet the coals exposed in the lower half of the succession are of low volatile rank. The Ridge exposes a striker section of coal measures with the thickest development of the overlying Elk Formation in the Crowsnest Coalfield. However, correlation of individual seams is difficult due to rapid changes in lithology.