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The Cleveland salt deposit is defined by a single deep well drilled in a search for potash resources. Located in the east-central portion of NTS sheet 11F/11, the well was cored from surface. The stratigraphic setting of the deep sub-surface revealed dramatic complexity, quite unexpected when compared with gently dipping strata at surface.
Cleveland 227-2 collared in strata of the Mabou Group. On the far left, these rocks are compared with an intersection of comparable strata in Corkan "Port Malcolm", drilled in the south-central part of NTS Sheet 11F/11. This correlation suggests that a significant thickness of the lower Hastings Formation is missing at the faulted contact with the Windsor Group in the Cleveland 227-2 well.
Highest strata in the well are assigned to the Pomquet Formation of the Mabou Group and comprise mainly fine-grained red siltstone and sandstone. At intermediate depths, the hole penetrated grey mudrocks with minor red mudrocks and rare limestone of the Hastings Formation (lower Mabou Group). All Mabou Group strata are essentially flat-lying, as seen in surface exposures near the well head. Beneath the Mabou Group, the well intersected significant thicknesses of salt-bearing Windsor Group. The uppermost Windsor strata represent the main Lower Windsor Group salt, impure with interbedded siltstone and overturned.
The well passed downwards and up-section into Middle Windsor Group beds represented by intercalated anhydrite and fossiliferous marine carbonate rocks, still facing downwards in the hole, and then passed through a fold axis and drilled the same beds facing normally. At this depth, the hole passed through a fault and cored coral-bearing limestone of the Herbert River limestone (basal Upper Windsor Group), also facing normally. Near the bottom of the hole, the Herbert River limestone was intersected for a second time, still facing normally, and interpreted as a fault repeated section. The hole bottomed in uppermost beds of the Middle Windsor Group a short distance below the base of the Herbert River limestone. The total thickness of Windsor Group strata intersected in this well represents only a small portion of the total Windsor Group known in other salt deposits in southern Cape Breton Island, such as Malagawatch and McIntyre Lake.
Click the heavy black lines to view cored strata at the Cleveland deposit.
Click the heavy black lines to view cored strata at the Cleveland deposit.
The contact of the Mabou strata with the underlying Windsor Group is a fault, the nature of which is problematic. In southern Cape Breton Island, Mabou strata in some areas are juxtaposed on the basal limestone of the Windsor Group across an interpreted extensional detachment. The Mabou-Windsor contact in Well 227-2 may represent this same detachment. However, the occurrence beneath this fault surface of Middle Windsor Group above Upper Windsor Group strata, all facing normally, across a second faulted contact, suggests that reverse faults must be invoked in the deeper subsurface. The data provided do not allow a full assessment of the potential of the deposit to host significant salt and/or potash, and indeed do not provide any firm indication of depth to the true base of the Windsor Group. The degree of stratigraphic and structural complexity revealed by this well suggests a need for caution in constructing any cross-sections to depths below those penetrated by deep salt wells in South-central Cape Breton Island.
Stratigraphic relationships in the lower portion of Noranda 227-2 clearly suggest that reverse faults (either high-angle or low-angle thrusts) must be invoked. The contact between lower Mabou grey shale and Lower Windsor Group impure salt with interbedded siltstones, overturned, is shown as a low-angle structural surface along which the Mabou Group is detached structurally from the underlying Windsor Group.
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Overall, the structural setting is one of marked contrast between the Mabou and Windsor groups, with the latter providing a strain focus through its evaporitic character. This strain is expressed by significant structural shortening by large-scale folding and by reverse faulting. |
Experience in other salt deposits, supported by surface mapping of Windsor Group strata, suggests that this shortening differentially affects the middle and upper parts of the Windsor Group, and that a major structural discontinuity exists within or at the base of the main Lower Windsor Group salt. The depth to this discontinuity is very often unknown, and has a bearing on the economic potential of deep salt and potash deposits in areas where highly deformed Windsor Group strata have been documented. Beds of the Mabou Group are flat-lying at surface, and cores show similar geometry to depths greater than 2000 feet. The age of these rocks has been confirmed by palynomorphs recovered from samples taken throughout the Mabou interval. The orientation of major faults and fold axial planes is unknown. The sketch above is schematic only, intended to illustrate contrasts in structural configuration in the lower portion of 227-1.
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