Preliminary Map 68 describes the geology of the north half of the Elk Valley coalfield in southeastern British Columbia. The Preliminary Map consists of 8 sheets with geology presented at 1:10 000 scale on orthophoto bases. The Preliminary Map includes legends on each sheet, explanatory notes, plots of structural measurements and columnar sections through the coal measures.
Preliminary Map 68 is a contribution to the Canada/British Columbia Mineral Development Agreement 1985-1990. This MDA funded project extends and completes the 1:10 000-scale geological mapping carried out by the Geological Survey Branch in the East Kootenay coalfields of southeastern British Columbia. It covers an area immediately north of Preliminary Map 51 (Greenhills Range) and Preliminary Map 63 (south half of the Elk Valley coalfield).
Mapping, based on outcrops, roadcut exposures and trenches, defines the locations of formational contacts, coal seams, sandstone units and major structural features. Stratigraphic sections were measured at six locations. Vitrinite reflectance data and extensive descriptive notes are included.
The study area extends from Henrietta and Britt creeks in the south to the British Columbia - Alberta boundary in the north. Jurassic-Cretaceous Kootenay Group exposures throughout were mapped.
Most of the area is Crown land and includes all or part of three separate coal properties. The most southerly of these comprises the north end of the Fording Coal Ltd. Fording River property. The northern boundary of the Fording River property is in the vicinity of U.T.M. gridline 5 580 000 metres north, which is just north of Aldridge Creek.
Adjacent to the north is the Elk River property in which Fording Coal held a 50 per cent interest at the time of writing (June 1989). The northern boundary of the Elk River property is in the vicinity of U.T.M. gridline 5 590 000 metres north, close to Cadorna Creek. Coal rights in the area north of Cadorna Creek, formerly known as the Vincent option, were reserved to the Crown at the time of writing.
Data presented are based on examination of surface outcrops, roadcuts and trenches, and are supplemented by air-photograph interpretation. In many cases poor exposure has limited the amount of data and has restricted the ability to correlate seams. Seams less than 1 metre thick have generally not been plotted. Exposure on Map Sheet 2 is quite variable, with Mount Tuxford on the east side of the Elk River valley having good exposure, while the west side is virtually devoid of outcrop. Coal seam traces have been interpreted only on Mount Tuxford.
Stratigraphic sections were measured on Mount Veits and Tuxford, Henretta Ridge, Weary Ridge and at Coal Creek", a tributary of Bleasdell Creek. Sections were measured using either pogo stick or compass and chain. They have been generalized for publication. Note that coal seams less than 1 metre thick are not indicated, nor are partings within seams that are less than 1 metre in thickness.
The basal unit of the Kootenay Group is the Morrissey Formation. The Morrissey is a resistant sandstone unit consisting of two members and ranging in thickness from 20 to 80 metres (Gibson, 1985). The upper member, the Moose Mountain member, is a distinctive marker unit consisting of medium-grained sandstone, which is utilized in all surface and subsurface studies of the Kootenay to demarcate the base of coal occurrences. Within the study area the Moose Mountain member was observed to be more variable than normal. For example, it includes one or more carbonaceous partings in the area between Weary Ridge and Mount Tuxford, and at two localities south of Aldridge Creek it was observed to include an unusual light grey to white weathering, quartzose facies. The trace of the Morrissey Formation on the east limb of the Alexander Creek syncline is plotted with moderate confidence on Sheet 2.
The Mist Mountain Formation contains essentially all the coals of economic interest in southeastern British Columbia. Its thickness in the study area ranges between 450 and 550 metres. Coal forms between 8 and 12 per cent of the total thickness of the formation at most locations. Individual seams in the study area range from less than 1 to greater than 10 metres in thickness. In the area covered by Map Sheet 2, a section 550.0 metres thick comprising the complete Mist Mountain Formation was measured along the north ridge of Mount Tuxford. It hosts 39.5 metres of coal, representing only 7.2 per cent of the total thickness of the formation. However, some 50.6 metres of the section are covered, a portion of which could contain coal.
The precise identification of the contact of the Mist Mountain Formation with the overlying Elk Formation in southeastern British Columbia is generally difficult. The Elk is a coarser grained facies than the Mist Mountain and it generally lacks thick coal seams and contains unusual sapropelic coals known as "needle coals". The thickness of the Elk Formation in the study area is inferred to range between 350 and 450 metres. The Elk - Mist Mountain contact on Sheet 2 is located just below the peak of Mount Tuxford. It overlies a 2-metre-thick coal seam and is at the base of a thick sandstone sequence. This position also corresponds with a change in colour; the Elk Formation sandstones weather to a distinctive yellow or orange-brown colour compared with the predominantly grey weathering colour of the Mist Mountain Formation sandstones. A section of Elk Formation comprising the lowest 178 metres was measured. The lower two-thirds of the section consists of several stacked channel - sandstone units separated by thin, recessive, finer grained intervals, including a 1.0- metre thick coal seam. The upper third is a recessive interval including coal seams 1.0 and 1.6 metres thick. The section is overlain by a prominent cliff-forming sandstone that marks the base of another sequence of stacked sandstone units.
The Kootenay Group is unconformably (?) overlain by the Lower Cretaceous Blairmore Group. The basal unit of the Blairmore Group is the Cadomin Formation, a distinctive cliff-forming conglomerate unit.
The study area lies in the Front Ranges of the Rocky Mountains and is part of the Lewis thrust sheet. The major structure in the coalfield is the Alexander Creek syncline, which extends throughout its entire 100-kilometre length. The northern extension of the Greenhills syncline, which is west of the Alexander Creek syncline, also influences the extreme southern end of the study area.
The Alexander Creek syncline is overturned in the central part of the study area, and open elsewhere. It generally plunges to the north.
The Bourgeau thrust fault, which marks the western boundary of the surface exposures of the Lewis thrust sheet, forms the western boundary of the Mist Mountain Formation the Elk valley coalfield in the study area. The relative footwall stratigraphic position of the fault varies along its trace from the Fernie Formation to uppermost Kootenay. At the north end of the study area it is offset 2 kilometres to the northeast by a transverse fault.
Minor structures associated with the Bourgeau thrust are not generally observable because of poor exposure. The area known as "Coal Creek" on the west limb of the Alexander Creek syncline is one exception. See Sheet 3 for more detail. The complex folding in the northern part of Map Sheet 2 is interpreted to be caused by the Bourgeau thrust ramping to the east at this point.
Small-scale folds affect the Mist Mountain Formation strata in three general areas: the extreme south end of the study area (see Sheet 1), the lower west-facing slopes of Mount Tuxford (see Sheet 2), and the small eastward-flowing creeks north of Cadorna Creek (see Sheets 6 and 7). These are mainly interpreted as minor folds associated with the Alexander Creek syncline, and, with few exceptions, are not attributed directly to thrusting.
In contrast with the south half of the coalfield (see Grieve and Fraser, 1985), thrust faults within the Kootenay Group are not a major feature of the study area. On Henrietta Ridge at the extreme south end of the study area, however, two thrust faults are observed (see Sheet 1). One causes a repeat of the Morrissey Formation and lowermost part of the Mist Mountain Formation; the other occurs higher in the Mist Mountain Formation and is exposed on the south-facing slope of the ridge. A third thrust fault is inferred to occur in the east limb through part of the north end of the study area, based on sporadic occurrences of deformed strata (see Sheet 6 for example).
Late-stage, crosscutting normal faults are present in the Mount Tuxford and Mount Veits areas. These appear to follow prominent joint orientations and have resulted in mass wasting of the Morrissey Formation, forming both topographic steps and landslide blocks. A large landslide block of Morrissey Formation occurs on Weary Ridge (see Sheet 4). A set of two normal faults is shown at the north end of Mount Tuxford on Map Sheet 2. A combined 16 metres of normal displacement is estimated. A system of three faults, striking to the north and dipping steeply east, occurs approximately 400 metres due east of the peak of Mount Tuxford. The faults show a combined 8 metres of reverse displacement.
The rank of the coals has been determined petrographically by measuring the mean maximum vitrinite reflectance on grab samples. These values are plotted on the map. To convert reflectance values to ASTM rank equivalents use 1.12 per cent as the boundary between high-volatile and medium-volatile coals and 1.51 per cent as the boundary between medium-volatile and low-volatile rank. Coals of the Mist Mountain Formation vary in rank from low-volatile to high- volatile bituminous. The highest rank Mist Mountain Formation coals are found in the lower part of the formation on the east limb of the Alexander Creek syncline near Weary Creek. The lowest rank coals are exposed on the west limb of the Alexander Creek syncline near Bleasdell Creek, and on both limbs at the north end of the study area. Elk Formation coals are all of high-volatile bituminous rank in the study area. On Map Sheet 2 reflectance values in the Mist Mountain Formation vary from 1.57 to 1.02 per cent.