Summary:
The Willow Creek property is located within the Peace River Coalfield (PRC) and forms part of the Rocky Mountain Foothills structural belt, which lies to the east of the Canadian Rocky Mountain Trend.
The Willow Creek deposit is characterized by tight folding and faulting, which aligns closely with the definition for Complex geology.
Due to the location of major coal seams near the surface and the complexity of the geologic structure in the area, the Willow Creek Mine is considered a surface mining deposit.
Local Geology
The coal seams occurring on the property are contained within the non-marine Gething Formation. There are nine major coal zones occurring in the Willow Creek area. The major seams, however, are frequently split by non-coal partings, with 24 separate sub-seams of interest identified in the current geological model. Seam nomenclature is grouped by major seams, Seam 1 through Seam 8, in descending order, while Seam A is positioned between Seam 4 and Seam 5 at the contact between the Upper and Middle Gething units, and the Bird seam is positioned at the top of the coal-bearing sequence.
The coal-bearing strata within the Willow Creek area are exposed in a series of northwesterly trending folds that are cut by thrust faults. Deformation was characterized by high lateral compressive stresses that had a near-horizontal orientation. Depth of burial was not excessive, resulting in brittle to semi-brittle styles of deformation.
Coal Occurrences
The coal seams occurring on the property are contained within the non-marine Gething Formation. the Gething Formation consists of dark grey mudstone, siltstone, very-fine to coarse-grained sandstone, carbonaceous mudstone, silty and sandy mudstone, coaly plant debris, minor bentonite, black shale, occasional minor tuffs in the upper part, minor conglomerates and coal. The sandstone in the upper portion of the formation contains pebbles and coal stringers. It is overlain by the Bluesky Formation, a very distinctive glauconitic and conglomeratic sandstone with a green-grey colour.
There are nine major coal zones occurring in the Willow Creek area. The major seams, however, are frequently split by non-coal partings, with 24 separate sub-seams of interest identified in the current geological model. Seam nomenclature is grouped by major seams, Seam 1 through Seam 8, in descending order, while Seam A is positioned between Seam 4 and Seam 5 at the contact between the Upper and Middle Gething units, and the Bird seam is positioned at the top of the coal-bearing sequence. The seams and seam splits have been assigned a hierarchical number within a three-digit or four-digit seam code (e.g. the major Seam 4 is named as Seam 400, then the seam splits above the Seam 400 are named as Seams 401 and 410. If the seam splits are below the Seam 400, the seam splits are named Seam 440, 480). While the geometry of seams splitting and coalescing is fairly complicated, the individual major seams mostly occur as discrete units.
Structural Geology
The regional trend in the Foothills region, for both fold axes and thrust faulting, is northwest to southeast, with fault planes dipping to the southwest. The regional folding in the Foothills is generally broad and gentle, with major fold set axes spaced on the order of 2 km to 4 km apart and dips of less than 20 ^ 6 Smaller scale folds and undulations modify these larger structures. Faulting tends to be of the thrust variety and occurs with varying severity throughout the Foothills; while bedding inclinations can be locally steepened by drag folding associated with these structures.
The coal-bearing strata within the Willow Creek area are exposed in a series of northwesterly trending folds that are cut by thrust faults. Deformation during early Campanian to late Eocene time was characterized by high lateral compressive stresses that had a near-horizontal orientation. Depth of burial was not excessive, resulting in brittle to semi-brittle styles of deformation.
Large sheets of strata became detached during deformation and were displaced to the southwest or northeast along thrust faults. These thrusts generally have a staircase-like geometry, with wide flats almost parallel to bedding, connected by narrow ramps oblique to bedding, that cut upsection toward the northeast. The tendency is for the faults to be subparallel to bedding in less competent rocks such as coal, mudstone and shale, and to be more oblique to bedding in competent lithologies such as sandstone.
Deformation tended to migrate northeastward across the region, and as a rule, the lower or more external of any two thrusts is the younger. Comparatively little folding preceded faulting; consequently, thrusts tended to develop in essentially horizontal strata. As movement occurred within a thrust, its staircase geometry caused folding in the overlying strata and older thrust faults. Because of this folding, an overlying older thrust dips either southwest or northeast, although the overall movement of the plate of strata remains northeastward.
Concentric folds, angular folds and box folds are typical fold geometries. The dip of fold limbs can vary from nearly horizontal to overturned, but is usually in the range of 20° to 50°. Fold axes trend northwesterly. Plunge oscillates between northerly and southerly over the length of a large-scale fold and is usually shallow, but can steepen locally to as much as 35°. The major fold axes persist for large distances, in several cases more than 50 km, and usually have an en echelon alignment.
Concentric folds are roughly U-shaped, and in concentric folding both the competent and incompetent layers can maintain a constant thickness throughout the structure.
Chevron folds are V-shaped, with relatively short hinge areas and straight limbs. Strain in the hinge zone is usually accommodated by limb faults, the development of bulbous hinge zones, or boudinage of incompetent layers. The latter can produce localized tectonic thickening and thinning of coal seams. Examples of all of these features have been found associated with chevron folds in the property, but have been simplified for portrayal on the cross-sections.
Box folds are characterized by broad, nearly flat crests, flanked by steeply dipping limbs. The plates of strata that form the top and limbs tend to be largely undeformed, but the abrupt hinge zones often exhibit some of the small-scale structures that are associated with the hinge zones of chevron folds.
The Willow Creek Mine area has been subdivided into four structural blocks: Willow North, Willow Central, Willow South and Willow West. Of these, the Willow North and Willow Central Blocks (the subject of this study), are located along the ridge on the northeast margin of the property, immediately south of the Pine River. All of the blocks occur on either limb of the Pine River Anticline, a broad, regional structure that exhibits localized parasitic folding of the concentric and chevron type. Thrust faulting in the Willow Creek area is both northeastward- and southwestwarddirected, with southwestward-verging thrusts predominating within the mined area.