454
9 AUTOCHTHONOUS SEDIMENTS
Layered halite rocks comprising layers of halite 2-10 cm in thickness separated by thin
laminae of gypsum are forming at the present day, or formed in the Recent geological
past, in brine pools on coastal salt fiats at the head of the Gulf of California, Mexico.
The deposit is rarely more than 25 cm thick. The halite layers are built of zoned crystals
that have the same arrangement as those in the layers of the Prairie Halite, but the rock
is riddled with small dissolution hollows. The salt flats dry out frequently, and when
they are periodically flooded by the sea, this incoming water pipes its way down into the
halite rock. The way in which the dissolution pipes corrode the crystals of zoned halite
is very similar to the manner in which the clear halite appears to replace the zoned halite in the layered Prairie Halite. Indeed, if the dissolution hollows in the present-day
occurrence could be filled with clear halite, the two rocks would be identical. If the apparent replacement of zoned halite by clear halite in the layers of the Prairie Halite is
the record of piping of the rock by dissolution and subsequent filling by a later generation of clear halite, then this would argue against a deep-water origin for the salt, because
it is difficult to conceive how piping could take place beneath a deep standing body of
brine. Another feature evident in the Recent occurrence in Mexico is that the layers are
not annual but may have taken long periods of time to form.
Thus interpretations of the environment of deposition of salt deposits, like the 2-m
thickness of the Prairie Halite, range from the extremes of, on the one hand, a brinefilled basin approximately 200 m deep with rapid deposition, to slow accumulation in
very shallow water against a background of gentle subsidence on the other.
Where potash salts are present in the major halite evaporite sequences, as for example
in the Middle Devonian of the Elk Point basin of Canada or the Permian Zechstein of
northwest Europe, the potash salts usually occur in the upper part of the succession or,
in the case of the Zechstein, in the upper part of the major evaporite cycles. It would
seem at first sight that they were precipitated residual lakes of brine left after the deposition of halite. However, the deposits pose problems. One of these is that their overall chemical composition is not that that which would be expected by simple evaporation of seawater. In the theoretical direct evaporation of seawater, precipitation of
halite should be followed by, first, precipitation of MgSO4"nH20 (epsomite), then by
KC1 (sylvite), and finally by MgC12"6H20 (bischofite). In most occurrences, the epsomite, or its mineralogical equivalent, is only weakly developed or is absent, and the
bischofite is also characteristically absent. Although it is reasonable to argue that bischorite is so soluble that it is unlikely to survive even if its precipitation was achieved, the
depletion with respect to magnesium and sulfate demands explanation. The Middle Devonian potash deposits of Saskatchewan provide an interesting example, because they
consist solely of sylvite (KC1) and carnallite (KMgC13"nH20). Not only is sulfate absent,
but the proportion of magnesium is much lower than would be predicted. Evidently
the brines had been conditioned and their chemistry modified earlier in their history.
The absence of sulfate ions could be accounted for by the activities of sulfate-reducing
bacteria, but such a process cannot explain the combined deficiency in both sulfate and
magnesium.
It is of interest at this stage to refer back to the reactions that are taking place in the
formation of the Recent evaporites of the UAE sabkhas. Recall that dolomitization of
the carbonate sediments releases calcium ions, and these promote precipitation of more
calcium sulfate than would occur with simple evaporation of seawater. The resultant
sabkha brines are, in consequence, stripped of their sulfate and depleted with respect
9 AUTOCHTHONOUS SEDIMENTS
Layered halite rocks comprising layers of halite 2-10 cm in thickness separated by thin
laminae of gypsum are forming at the present day, or formed in the Recent geological
past, in brine pools on coastal salt fiats at the head of the Gulf of California, Mexico.
The deposit is rarely more than 25 cm thick. The halite layers are built of zoned crystals
that have the same arrangement as those in the layers of the Prairie Halite, but the rock
is riddled with small dissolution hollows. The salt flats dry out frequently, and when
they are periodically flooded by the sea, this incoming water pipes its way down into the
halite rock. The way in which the dissolution pipes corrode the crystals of zoned halite
is very similar to the manner in which the clear halite appears to replace the zoned halite in the layered Prairie Halite. Indeed, if the dissolution hollows in the present-day
occurrence could be filled with clear halite, the two rocks would be identical. If the apparent replacement of zoned halite by clear halite in the layers of the Prairie Halite is
the record of piping of the rock by dissolution and subsequent filling by a later generation of clear halite, then this would argue against a deep-water origin for the salt, because
it is difficult to conceive how piping could take place beneath a deep standing body of
brine. Another feature evident in the Recent occurrence in Mexico is that the layers are
not annual but may have taken long periods of time to form.
Thus interpretations of the environment of deposition of salt deposits, like the 2-m
thickness of the Prairie Halite, range from the extremes of, on the one hand, a brinefilled basin approximately 200 m deep with rapid deposition, to slow accumulation in
very shallow water against a background of gentle subsidence on the other.
Where potash salts are present in the major halite evaporite sequences, as for example
in the Middle Devonian of the Elk Point basin of Canada or the Permian Zechstein of
northwest Europe, the potash salts usually occur in the upper part of the succession or,
in the case of the Zechstein, in the upper part of the major evaporite cycles. It would
seem at first sight that they were precipitated residual lakes of brine left after the deposition of halite. However, the deposits pose problems. One of these is that their overall chemical composition is not that that which would be expected by simple evaporation of seawater. In the theoretical direct evaporation of seawater, precipitation of
halite should be followed by, first, precipitation of MgSO4"nH20 (epsomite), then by
KC1 (sylvite), and finally by MgC12"6H20 (bischofite). In most occurrences, the epsomite, or its mineralogical equivalent, is only weakly developed or is absent, and the
bischofite is also characteristically absent. Although it is reasonable to argue that bischorite is so soluble that it is unlikely to survive even if its precipitation was achieved, the
depletion with respect to magnesium and sulfate demands explanation. The Middle Devonian potash deposits of Saskatchewan provide an interesting example, because they
consist solely of sylvite (KC1) and carnallite (KMgC13"nH20). Not only is sulfate absent,
but the proportion of magnesium is much lower than would be predicted. Evidently
the brines had been conditioned and their chemistry modified earlier in their history.
The absence of sulfate ions could be accounted for by the activities of sulfate-reducing
bacteria, but such a process cannot explain the combined deficiency in both sulfate and
magnesium.
It is of interest at this stage to refer back to the reactions that are taking place in the
formation of the Recent evaporites of the UAE sabkhas. Recall that dolomitization of
the carbonate sediments releases calcium ions, and these promote precipitation of more
calcium sulfate than would occur with simple evaporation of seawater. The resultant
sabkha brines are, in consequence, stripped of their sulfate and depleted with respect