EFFECTS OF PRAIRIE FORMATION HALITE DISSOLUTION ON OVERLYING BEAVERHILL LAKE GROUP STRATA IN NORTHEASTERN ALBERTA

In Devonian rocks of northeastern Alberta, dissolution of Prairie Formation halite caused deformation and fracturing of the overlying Beaverhill Lake Group. A combination of subsurface stratigraphy, core analyses, field work, and historical literature review was used to reconstruct various scales of Beaverhill Lake Group structure caused by halite dissolution and collapse.

At the regional scale, the Prairie Formation decreases in thickness eastward through a salt dissolution zone into an area of complete halite loss and collapse of non-halite beds. As a result, strata overlying the zone of Prairie halite dissolution are folded into dome-and-saddle structures, which can span several kilometers in areal extent and which have been truncated by pre-Cretaceous erosion.

In outcrop, folds within the Beaverhill Lake Group have wavelengths of 50 to 500 m, often with second-order parasitic folds occurring on the limbs of first-order folds. Most outcrops also contain faults with vertical offsets of up to 0.5 m. In the subsurface, localized collapse breccias of undifferentiated Prairie and post-Prairie rock occur in areas of complete halite loss.

In core, collapse-related fractures are often sub-vertical, calcite and/or pyrite-filled, and cross-cut bedding. A small proportion of these fractures (<5%) contain millimeter-scale vertical offsets.

Dissolution of the Prairie halite and its effects on overlying strata has the potential to significantly impact industrial activities in northeastern Alberta. With the loss of the thick Prairie Formation aquitard, underlying saline aquifers and overlying freshwater aquifers could be contiguous. Fractures in collapsed Waterways Formation limestone may be conduits for fluid movement and prime environments for karst processes.