9.6 EVAPORITES
453
waters, already concentrated sodium chloride brines, would break surface and form a
brine pool from which halite would be precipitated.
9.6.4 Halite-Potash Evaporite Successions
The halite-potash evaporite successions differ from the cyclic carbonate-anhydrite sequences in a number of important respects. Although many of them contain significant
proportions of carbonate and anhydrite rocks, they are characterized by a substantial
thickness of halite. Generally they comprise extensive basin-shaped accumulations that
appear to have formed in large, partially enclosed embayments that had only restricted
access to the open sea. They are exemplified by the Permian Zechstein evaporites of
northwest Europe, by the Middle Devonian evaporite complex of the Elk Point basin
of western Canada, the Silurian evaporites of the Michigan basin, the Pennsylvanian
evaporites of the Paradox basin, Utah, and the Cambrian evaporites of Siberia.
In a general way the succession of mineral salts tends to be cyclic, in the sense that
they pass up from carbonate-anhydrite rocks into thick piles of halite and in some instances terminate with potassium salts. Four such cycles are developed in the Zechstein
evaporites of Germany (refer back to Fig. 9.30). It is the thicknesses of halite that are
the remarkable feature of these deposits. The Prairie Halite of the Elk Point basin in
Saskatchewan is approximately 200 m thick, and some 500 m of halite are present in the
Zechstein of Germany. Vast quantities of seawater had to be processed to form these
thick accumulations and space had to be provided to accommodate them. The latter
consideration has led to long controversy as to whether the halite was formed in deep
brine-filled basins or in shallow brine pools against a background of subsidence.
A common rock type in the thick halite successions is the so-called "layered halite
rock." This consists of repeated alternations of layers of halite, 2-10 cm in thickness,
separated by I-ram-thick laminae of anhydrite, or anhydrite and dolomite sometimes
with organic matter. Some geologists have interpreted these cyclic alternations of anhydrite and halite as recording annual evaporation cycles. Such cycles are termed $ahresringe by German geologists. On this basis it has been argued that the 200 m thickness
of salt of the Prairie Halite of Saskatchewan was deposited in 4000 years, and that the
500 m of halite in the Zechstein accumulated in 10,000 years. The Jahresringe concept
argues against a shallow-water origin, because to accommodate the observed thicknesses would require a background of subsidence of approximately 5 cm a year. Such a
rate of subsidence is greatly in excess of what could reasonably be expected in any tectonic or diastropic setting, and it becomes necessary, therefore, to postulate an initially
deep basin. Keep in mind, however, that the deep brine hypothesis depends largely on
the validity of the Jahresringe concept. The petrology of the Prairie Halite has been described by Wardlaw and Schwerdtner (1966). Each halite layer is an admixture of two
types of halite crystals. Some of the crystals carry abundant tiny brine inclusions; these
are arrayed in planes parallel to the cube faces, and give the crystals a zoned appearance.
In thin sections, under the microscope, the zoned crystals are elongated upward and the
zones appear as chevrons with their apices directed upward. This type of fabric is indicative of competitive growth of crystals upward from a substrate, and the abundant
brine inclusions suggest intermittent episodes of rapid growth. The other type of halite
crystal is clear and free of inclusions, and the mutual relationships of the two types of
crystals suggest that the clear halite replaced halite with inclusions.
453
waters, already concentrated sodium chloride brines, would break surface and form a
brine pool from which halite would be precipitated.
9.6.4 Halite-Potash Evaporite Successions
The halite-potash evaporite successions differ from the cyclic carbonate-anhydrite sequences in a number of important respects. Although many of them contain significant
proportions of carbonate and anhydrite rocks, they are characterized by a substantial
thickness of halite. Generally they comprise extensive basin-shaped accumulations that
appear to have formed in large, partially enclosed embayments that had only restricted
access to the open sea. They are exemplified by the Permian Zechstein evaporites of
northwest Europe, by the Middle Devonian evaporite complex of the Elk Point basin
of western Canada, the Silurian evaporites of the Michigan basin, the Pennsylvanian
evaporites of the Paradox basin, Utah, and the Cambrian evaporites of Siberia.
In a general way the succession of mineral salts tends to be cyclic, in the sense that
they pass up from carbonate-anhydrite rocks into thick piles of halite and in some instances terminate with potassium salts. Four such cycles are developed in the Zechstein
evaporites of Germany (refer back to Fig. 9.30). It is the thicknesses of halite that are
the remarkable feature of these deposits. The Prairie Halite of the Elk Point basin in
Saskatchewan is approximately 200 m thick, and some 500 m of halite are present in the
Zechstein of Germany. Vast quantities of seawater had to be processed to form these
thick accumulations and space had to be provided to accommodate them. The latter
consideration has led to long controversy as to whether the halite was formed in deep
brine-filled basins or in shallow brine pools against a background of subsidence.
A common rock type in the thick halite successions is the so-called "layered halite
rock." This consists of repeated alternations of layers of halite, 2-10 cm in thickness,
separated by I-ram-thick laminae of anhydrite, or anhydrite and dolomite sometimes
with organic matter. Some geologists have interpreted these cyclic alternations of anhydrite and halite as recording annual evaporation cycles. Such cycles are termed $ahresringe by German geologists. On this basis it has been argued that the 200 m thickness
of salt of the Prairie Halite of Saskatchewan was deposited in 4000 years, and that the
500 m of halite in the Zechstein accumulated in 10,000 years. The Jahresringe concept
argues against a shallow-water origin, because to accommodate the observed thicknesses would require a background of subsidence of approximately 5 cm a year. Such a
rate of subsidence is greatly in excess of what could reasonably be expected in any tectonic or diastropic setting, and it becomes necessary, therefore, to postulate an initially
deep basin. Keep in mind, however, that the deep brine hypothesis depends largely on
the validity of the Jahresringe concept. The petrology of the Prairie Halite has been described by Wardlaw and Schwerdtner (1966). Each halite layer is an admixture of two
types of halite crystals. Some of the crystals carry abundant tiny brine inclusions; these
are arrayed in planes parallel to the cube faces, and give the crystals a zoned appearance.
In thin sections, under the microscope, the zoned crystals are elongated upward and the
zones appear as chevrons with their apices directed upward. This type of fabric is indicative of competitive growth of crystals upward from a substrate, and the abundant
brine inclusions suggest intermittent episodes of rapid growth. The other type of halite
crystal is clear and free of inclusions, and the mutual relationships of the two types of
crystals suggest that the clear halite replaced halite with inclusions.