Structure and petrology of the Gypsumville gypsum deposit
Abstract
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.