Historical Geology/Saline giants

Saline giants are vast deposits of soluble minerals. What do we mean by "vast"? Well, to take one example, the Louann Salt covers 800,000 square kilometers and is four kilometers deep, amounting to some seven quadrillion tonnes of salt. Even a small saline giant, such as the one found in the Michigan Basin, covers an area of 100,000 square kilometers and has a depth of 250-350 meters.

In this article we shall review what is known, and what may plausibly be conjectured, about the formation of saline giants.

Difficulties
Although precipitation of salts from seawater can be observed today in an ordinary bucket, the formation of saline giants cannot be observed anywhere. This is not really surprising: the geological record shows that the formation of saline giants has only happened at certain times and in certain places, and it is not unexpected that we should happen to be living in one of the times when no saline giants are forming.

While it is not unexpected, it is annoying. Our understanding of sedimentary rocks is in other cases greatly enhanced by the fact that we can see the sediments being deposited in the present day. In the case of saline giants, we lack this information and must do the best we can.

Evaporation of seawater
Seawater contains a variety of dissolved ions, such as (in descending order of abundance by mass) Cl-, Na+, SO42-, Mg2+, Ca2+, K+, HCO3- and Br-; these eight ions alone make up more than 99% of the dissolved ions in seawater, and other ions can be neglected for the purposes of this article.

When seawater evaporates, these precipitate out as minerals such as halite (NaCl) and gypsum (CaSO4·2H2O). The evaporation of 1 liter of seawater will produce around 35 grams of evaporites.

The degree of evaporation required for precipitation varies from mineral to mineral: so gypsum will begin to precipitate out when seawater has been reduced to about 30% of its original volume, but it needs to be reduced to 10% of its volume for the precipitation of halite.

The relevant facts are summarized in the table below. The first column specifies the mineral, the second gives its abundance as a percentage of all the minerals precipitated, and the table as a whole has been ordered roughly according to the ease with which the various minerals precipitate out, from those that precipitate out most readily down to the most soluble. Minerals which occur only in the tiniest traces have been omitted.

The figures given here are based on the pioneering work of Usiglio, still thought to be reasonably accurate: more information will be found here.

The upshot of all this is that as sea water evaporates, a small quantity of calcite will be deposited first. As evaporation continues, gypsum will be deposited: as there is much more gypsum than calcite, and as most of the calcite will have been deposited already, this means that the gypsum will swamp the calcite being deposited, and so the resulting rock will be almost exclusively gypsum. Similarly, when the halite starts being deposited, more halite will be deposited than gypsum or calcite, and the result will essentially be halite. A really intensely concentrated brine, reduced to a few percent of its original volume, will precipitate other salts, but the greater abundance of halite will ensure that it predominates.

We should note also that further dehydration of gypsum (CaSO4·2H2O) will remove the water molecules associated with the calcium sulphate, converting it to anhydrite (CaSO4).

It follows that saline giants will, broadly speaking, consist either of halite, gypsum, or anhydrite.

Models for the formation of saline giants
Saline giants are invariably covered over by a blanket of more conventional sediment, otherwise they would long ago have been washed away by rain (if on land), or dissolved in the sea (if underwater). This observation leads us to a question which is initially perplexing: how in the world can they have formed in the first place? The sea dissolves soluble minerals, and is nowhere near the saturation point at which they must start precipitating out. How, then, is it even possible for these saline giants to form within a marine basin?

Your first guess might be that they are the result of a bit of sea becoming closed off from the main body of the sea and simply drying up. However, there is too much salt in saline giants to be accounted for by a single such event, since the drying up of a kilometer's depth of seawater would result in the deposition of only 14 meters of salt. What we need is some model in which the basin keeps being filled, either continuously or periodically, with new supplies of salt water.

We shall describe four such models. Note that although they cannot all apply to the same saline giant at the same time, it is perfectly possible for them to apply to different saline giants, or, conceivably, to the same saline giant at different times. In this sense, it is possible for all the models described to be correct.


 * Model 1: A barrier with a small gap in it.

Our first model is this: suppose we have a sedimentary basin which is connected to the sea only by a very narrow channel. Combine this with an arid climate and little input of fresh water into the basin from rivers and streams or from rain. So long as the rate of evaporation is greater than the input of fresh water, the physical necessity that the surface of the sea inside the basin must always be at the same level as the surface of the sea outside the basin ensures that salt water will always be flowing into the basin; and as water will be continually evaporating from it, leaving the dissolved minerals behind, this will increase the salinity of the water in the basin until it reaches the saturation point and precipitation occurs.

Something of the sort can be seen today: it was first calculated by Edmond Halley (of Halley's Comet fame) that the Mediterranean loses more water to evaporation than the input of fresh water; this, he realized, explained why there is always a current flowing into the Mediterranean through the Straits of Gibraltar.

We may note that the Mediterranean is indeed somewhat more salty than the Atlantic. However, there is no saline giant forming on the floor of the Mediterranean today. The reason is (so it has been calculated) that in order for this proposed mechanism to work, the cross-sectional area of the channel must be many orders of magnitude smaller than the surface area of the water in the sedimentary basin; otherwise the tendency of water to mix will prevent the water in the basin from ever becoming saturated enough for precipitation to take place. The Straits of Gibraltar today are simply not narrow enough for the formation of saline giants.

Now a channel of just the right size would be unstable: we might expect it within a short space of time to get blocked up or, alternatively, broadened out, either of which would spoil the proposed mechanism. The deposition of the saline giants under the Mediterranean took 300,000 years — a blink of an eye by the usual standards of geologists, but a long time for such a narrow channel to stay just the right width. For this reason, although we must regard this proposed mechanism as possible in theory, we should require very definite evidence to endorse it in any particular case.


 * Model 2: A barrier overtopped at high tide.

Another proposal is of a barrier (a "sill") between the main body of the sea and a sedimentary basin such that water will surmount it only at high tide. Again, this is possible, but as with the mechanism previously described, it needs to be just right, and, to stay just right for hundreds of thousands of years. This is possible but implausible. A further problem with such a model is this: the influx of water at high tide needs to just balance out the water lost by evaporation. For if the water input was less than the water lost, then the water in the basin would be reduced to an intermittent puddle which would not account for basin-wide sedimentation; and if the water input was greater than the water lost, then eventually the basin would fill up until, at high tide, it was at the same level as the main body of the sea, allowing mixing to take place. It is hard to see what effects could keep the situation in equilibrium, so that the basin is always reasonably full but never quite fills up.


 * Model 3: A barrier overtopped by rises in sea level.

A third, similar model again requires a barrier completely blocking off the sedimentary basin, which is periodically overtopped, not as a result of the tide, but as a result of an increase in the global sea level caused by changes in the Earth's climate. Such a model would predict that layers of evaporites should alternate with layers of more conventional marine sediments deposited during periods when the basin is full. It seems that this is what we see in the case of the Mediterranean deposits (see here for further information) but it is by no means true of all such evaporites.

This requires a less precise set of circumstances than the previous model, in that fluctuations in global sea level caused by climate change might be expected to be greater in magnitude than local fluctuations caused by the tide.

This model might be combined with the first or second models: variations in sea level might alternately allow and prevent the mechanisms described in model one or two: again, we should then expect to see an alternation of evaporites with more conventional sediments.


 * Model 4: A permeable barrier.

A fourth model is as follows: the basin is completely cut off from the main body of the sea, but by porous sediment or sedimentary rock, so that sea water can seep through the barrier. As with our other models, we require that the output of fresh water through evaporation should be greater than the input: however in this case, unlike the "narrow channel model", the water in the basin is free to drop below sea level when it evaporates: this produces a pressure differential between the two sides of the barrier and ensures that water flows in just one direction, from the main body of the sea into the basin. The nearest analog to this model in the modern world would be the deposition of salt in lagoons.

You will notice that this model does not require anything to be just right: neither the height nor the width of the barrier are crucial: so long as the barrier is above sea level, the system described will work.

Such a model is immune to the problem of equilibrium that we raised with respect to the second model. For the lower the water level sinks in the basin by evaporation, the greater the pressure differential on the two sides of the barrier, and the greater the influx of water; and conversely, the higher the level in the basin, the less water will seep through the barrier. So we might well expect such a system to be in equilibrium, with the basin never either drying out completely or filling up so as to overflow the barrier.

Saline giants: what do we know and how do we know it?
All the models described above require two things: that the basin in which the saline giant is deposited should be nearly or totally isolated from the main body of the sea; and that the climate should be such that more fresh water is lost through evaporation than is input by rivers and rain. We can test whether these conditions were in place, and show that these are the conditions under which saline giants form. For example, the Mediterranean is in the present day nearly cut off from the sea, and it does not strain the imagination to suppose that 5.9 million years ago it was more isolated still. To take another example, it would be strange to see evaporites forming in the Gulf of Mexico today; but conditions were just right at the time when they formed, when it was almost, or entirely, blocked off from the main body of the sea by what is now West Africa. To take a third example, the Castile formation in the Delaware Basin (which, despite its name, is located in Texas and New Mexico) was ringed around by a reef complex during the time of its formation. Independent evidence from paleoclimatology also confirms that the climatic conditions were right for evaporite formation in each case.

So although in many cases there is controversy over which of the models we have described best explains the existence of a particular saline giant (a controversy which perhaps in some cases will never be fully resolved) this is really a dispute about details: geologists are agreed, and can confirm, that what is required is a basin which is almost, or entirely, cut off from the main body of the sea, plus an arid climate.

This explains why such deposits are rare in the geological record. There is no particular reason why this set of circumstances should be common: they occur by happenstance and not by any sort of geological inevitability.