McIntyre Lake Salt Deposit Stratigraphy and Structure

TGI: Targeted Geoscience Initiative - South-central Cape Breton Island

The upper portion of the McIntyre Lake composite section was used to construct the regional composite section, shown below, and is therefore identical. The data represent drillhole CS-5A in its entirety. Below the level of the D1 limestone, the regional composite was derived from Malagawatch salt deposit data. Thicknesses shown for sub-D1 correlative rocks at McIntyre Lake, corrected for tectonic tilt, show some contrast with the regional composite.

Below the level of the "Triplet Marker" in the Middle Windsor Group, the stratigraphy of the McIntyre Lake deposit is uncertain. A relatively thick carbonate member intersected twice in the lower portion of drillhole CS-7A may represent lowest Middle Windsor Group strata, and lower Windsor Group halite is no doubt represented in the highly deformed rocks at McIntyre Lake. Lowest Windsor Group rocks were not intersected in drilling at McIntyre Lake.

A reconstructed section joining all but one of the deep holes drilled at McIntyre Lake, shown on the right, is a challenge. There is, of course, a "correct" reconstruction. Recognition of the latter is problematic, when multiple interpretations are not only possible but equally valid as working hypotheses. The interpretation shown is one of several possible reconstructions, and no claim is made here that it is necessarily correct.

Several important observations, however, remain valid.

1. Deformation at the McIntyre Lake deposit is intense; halite is commonly schistose, and faults cut the succession, unusual in saline strata which flow under stress
2. Folds are well documented at a variety of scales; fold limbs are often attenuated by shear, and faults may be in some cases broken fold axes.
3. The succession is relatively complete, except for lowermost Windsor Group strata, and saline strata extend upwards above the Windsor Group.
4. Potash salts occur at comparable levels within the middle and lower Windsor Group at most Cape Breton salt deposits - the McIntyre Lake deposit is no exception in this regard.

Click on well names to view detail of the individual wells.

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Almost every well drilled at McIntyre Lake collared in basal grey mudrocks of the Hastings Formation (basal Mabou Group). Holes 5 and 2 penetrated a normal contact with uppermost Windsor Group strata, which are notably saline in character. Each of the upper Windsor Group carbonate marker horizons in these wells is part of a small scale cycle of sedimentation recorded by an ascending sequence of carbonate to anhydrite to halite to non-marine siltstone. Hole 5 bottomed in beds of the upper Windsor Group. Hole 2 extended to greater depths, and passed through a faulted contact between upper Windsor Group strata and highly deformed middle Windsor Group rocks. Hole 3 collared in upper Windsor Group beds at approximately the level of the D3 limestone marker. Hole 3 recorded several fold and fault-repeated sections, mainly in beds of the upper Windsor Group, and in addition drilled a small interval of the middle Windsor Group. Hole 7 drilled Mabou Group grey mudrocks in its upper part, then passed into highly deformed salt of the lower or possibly middle Windsor Group. In the lower part of that deep well, one distinctive fossiliferous marine carbonate interval with associated anhydrite and salmon-coloured siltstone was drilled twice without change in facing, indicating fault repetition. The assignment of that carbonate marker is uncertain since it was observed only in one well where it could not be linked to any associated carbonate markers. Salt that overlies and underlies this carbonate marker contains minor associated potash, suggesting that the carbonate represents one of the numerous markers of the middle Windsor Group. Potash has not been documented in upper Windsor Group saline successions in Atlantic Canada.

Each well at McIntyre Lake can be interpreted in terms of its structural configuration, and the stratigraphic relationships for the most part can be compared with less deformed saline successions of the Windsor Group. Correlation between the seven wells is in many cases tenuous at best, attesting to the degree of structural complexity in this deposit, perhaps more severe than in any other salt deposit in southern and central Cape Breton Island. Nevertheless, the deposit is situated relatively close to the Strait of Canso, and is currently of interest for possible development of underground storage caverns for hydrocarbons.