Structure and petrology of the Gypsumville gypsum deposit
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The Gypsumville sulphate deposit is approximately 130 feet thick and overlies a sequence of red and grey shales. The upper 40 or 50 feet is composed mainly of well-stratified gypsum, which is underlain by anhydrite. Fusuline foraminifera indicate a Permian or younger age for the deposit. The gypsum has been deformed. Two sets of ridge-forming anticlinal folds are recognized and are interpreted as having resulted from ice-dragging during Pleistocene glaciation. This interpretation is based on a correlation between fold geometry and inferred directions of ice-movement. Minor intrastratal convolutions also occur and are regarded as the product of slumping which occurred not long after deposition. Petrographic observations indicate that the gypsum of the outcrop is a product of the hydration of anhydrite and that no increase in volume accompanied this replacement. The development of gypsum crystals from anhydrite occurred in three stages: a) the growth of large, coarse gypsum crystals at the expense of the anhydrite (G-1 stage); b) recrystallization of coarse gypsum crystals and formation of fine grained gypsum (G-2 stage); and c) growth of euhedral or lath-shaped crystals at the expense of fine grained gypsum ( G--3 stage). It is likely that the anhydrite formed after an earlier generation of gypsum but there is no petrographic evidence to substantiate this conclusion.