452
9 AUTOCHTHONOUS SEDIMENTS
Windsorian (Carboniferous) of Nova Scotia, the nodular anhydrite units are overlain
by red beds. In these cases it appears that the regressions were followed by the establishment of continental conditions.
Carbonate-anhydrite sabkha cycles are present in the Purbeck evaporites of southern England (West, 1964; Shearman, 1966), and thick developments have been encountered in borehole cores in the Lower Carboniferous of parts of the east Midlands
of England (Llewellyn et al., 1969).
It is appropriate to refer briefly to the chemical process that appears to operate in the
formation of sabkha-type evaporites. The essential feature is that the evaporite minerals grow interstitially within earlier formed sediment. Dolomite and gypsum develop in
the sediments of the present-day high intertidal and low supratidal environment, and
they are also present in the older buried intertidal sediments under the sabkha plain.
Anhydrite characterizes the supratidal concentration of the interstitial brines inward
through the intertidal into the supratidal zone. It appears that concentration is an important factor in determining which of the two calcium sulfate minerals will be formed.
Gypsum forms during the early stages of concentration but anhydrite is not generated
until high concentrations are achieved. Although much of the anhydrite appears to have
formed directly as anhydrite (Plate 5D), there are occurrences where the anhydrite
pseudomorphs gypsum. In the latter cases it would appear that gypsum, formed during
the early stages of concentration of the groundwaters, became made over into anhydrite as the concentration increased. The dehydration of gypsum to anhydrite is a reversible reaction:
CaSO4"2H20 ~ CaSO 4 4- 2H20
Increasing temperature and salinity
Dehydration of gypsum occurs with increasing salinity and temperature; hydration of
anhydrite occurs with decreasing salinity and temperature (MacDonald, 1953). Thus
anhydrite can only form at the surface of the earth in arid hypersaline environments.
Gypsum may form in cooler, less saline environments, but dehydrates to anhydrite on
burial. Murray (1964) showed that, for an average salinity and geothermal gradient, the
dehydration of gypsum takes place at about 1000 m below the surface of the earth. Anhydrite hydrates to gypsum at a similar depth.
The dolomite in the sabkha forms by reaction between the brines and the host carbonate sediment, and the reaction releases calcium ions. Normal seawater carries more
sulfate ions than are required to satisfy the calcium in seawater, so that as seawater is
concentrated by evaporation, the calcium will be precipitated as calcium sulfate, but the
excess sulfate ions remain in the brine. This excess sulfate is available to combine with
the calcium ions released by dolomitization of the carbonate sediments, and a further
crop of calcium sulfate is generated. Thus by virtue of dolomitization, the sabkha mechanism of evaporite genesis may lead to production of almost twice the amount of calcium sulfate minerals as would be formed by simple evaporation of the same volume of
seawater.
Halite deposits are uncommon in carbonate-anhydrite evaporite sequences. They do
occur occasionally, however, as for example in the Middle Devonian Stettler Formation
of western Canada (Fig. 9.34). Such a large lens of halite could be accounted for if local
subsidence developed at the back of an otherwise emergent sabkha plain. The ground-
9 AUTOCHTHONOUS SEDIMENTS
Windsorian (Carboniferous) of Nova Scotia, the nodular anhydrite units are overlain
by red beds. In these cases it appears that the regressions were followed by the establishment of continental conditions.
Carbonate-anhydrite sabkha cycles are present in the Purbeck evaporites of southern England (West, 1964; Shearman, 1966), and thick developments have been encountered in borehole cores in the Lower Carboniferous of parts of the east Midlands
of England (Llewellyn et al., 1969).
It is appropriate to refer briefly to the chemical process that appears to operate in the
formation of sabkha-type evaporites. The essential feature is that the evaporite minerals grow interstitially within earlier formed sediment. Dolomite and gypsum develop in
the sediments of the present-day high intertidal and low supratidal environment, and
they are also present in the older buried intertidal sediments under the sabkha plain.
Anhydrite characterizes the supratidal concentration of the interstitial brines inward
through the intertidal into the supratidal zone. It appears that concentration is an important factor in determining which of the two calcium sulfate minerals will be formed.
Gypsum forms during the early stages of concentration but anhydrite is not generated
until high concentrations are achieved. Although much of the anhydrite appears to have
formed directly as anhydrite (Plate 5D), there are occurrences where the anhydrite
pseudomorphs gypsum. In the latter cases it would appear that gypsum, formed during
the early stages of concentration of the groundwaters, became made over into anhydrite as the concentration increased. The dehydration of gypsum to anhydrite is a reversible reaction:
CaSO4"2H20 ~ CaSO 4 4- 2H20
Increasing temperature and salinity
Dehydration of gypsum occurs with increasing salinity and temperature; hydration of
anhydrite occurs with decreasing salinity and temperature (MacDonald, 1953). Thus
anhydrite can only form at the surface of the earth in arid hypersaline environments.
Gypsum may form in cooler, less saline environments, but dehydrates to anhydrite on
burial. Murray (1964) showed that, for an average salinity and geothermal gradient, the
dehydration of gypsum takes place at about 1000 m below the surface of the earth. Anhydrite hydrates to gypsum at a similar depth.
The dolomite in the sabkha forms by reaction between the brines and the host carbonate sediment, and the reaction releases calcium ions. Normal seawater carries more
sulfate ions than are required to satisfy the calcium in seawater, so that as seawater is
concentrated by evaporation, the calcium will be precipitated as calcium sulfate, but the
excess sulfate ions remain in the brine. This excess sulfate is available to combine with
the calcium ions released by dolomitization of the carbonate sediments, and a further
crop of calcium sulfate is generated. Thus by virtue of dolomitization, the sabkha mechanism of evaporite genesis may lead to production of almost twice the amount of calcium sulfate minerals as would be formed by simple evaporation of the same volume of
seawater.
Halite deposits are uncommon in carbonate-anhydrite evaporite sequences. They do
occur occasionally, however, as for example in the Middle Devonian Stettler Formation
of western Canada (Fig. 9.34). Such a large lens of halite could be accounted for if local
subsidence developed at the back of an otherwise emergent sabkha plain. The ground-