Popular Science Monthly/Volume 82/February 1913/Immense Salt Concretions
|IMMENSE SALT CONCRETIONS|
By Professor G. D. HARRIS
Crystalline salt masses may be a mile in diameter! Where are they? How were they formed? Who said so? Interrogations like these are sure to be forthcoming from layman, chemist and geologist alike whenever such startling assertions are made.
Salt is a common substance. Its occurrence in the waters of the ocean, as well as those of land-locked, mouthless seas is a matter of common knowledge. Interesting articles too, have been written regarding the immense layers of rock salt within the earth's crust. They have told of the hundreds of years required in excavating the great chambers and galleries in the Austro-Hungarian mines at Hallstadt, Ischl and Weiliczka. Such mines have been the chose-à-voir for travelers in this monarchy for the past two or three centuries. The Stassfurt mines have become known throughout the world for the richness of their potassium deposits. The Salt Mountain of Cordova, Spain, and the Salt Cliff at Bahadur Khel, in the Trand Indus region of India, are among the notable rock-salt occurrences.
All these salt accumulations have been explained (and perhaps properly) by supposing that they represent the residue of evaporated saline waters, waters that occurred in cut-off bays or sounds, receiving but occasionally supplies from the neighboring ocean, scarcely equaling the vapors lost by evaporation.
Of late an entirely new method of accumulation or growth of rock salt masses has been discovered. Here the salt no longer occurs in thin but wide-extended sheets, layers or strata, but in huge lumps, concretions we may say, with vertical and horizontal diameters approximately equal. These are the masses we wish here to bring to the attention of the reader. We do not have to go to Spain or India to see these marvels. They are, so to speak, right at home. They occur encysted in the sands and clays of the later geological formations along our gulf coast, from east Texas to south Alabama inclusive. Not all are immediately along the gulf border, to be sure, but the majority are but a few score miles from this line. All have doubtless a general conception of the low, grassy marsh-lands of southern Louisiana with its intricate system of tidal bayous beset here and there with dark green live oaks giving the appearance of old-time great apple trees in a great meadow, when viewed from a distant vantage ground. Doming up here and there in these monotonous marshlands are great swells of terra-firma, one hundred or more feet in height and a mile or more in diameter. They are seen from a great distance, and strike one at once as being something out of the ordinary, surely formed by no common method of uplift, less yet by circumdenudation. Of these coastal mounds the so-called "Five Islands," lying to the east of Vermilion and Atchafalaya bays are splendid examples. Belle Isle is just to the west of the Atchafalaya River, between Morgan City and the Gulf, Côte Blanche, Grande Côte, Petite Anse and Côte Carline follow to the northwestward in the order named. The first, or Belle Isle, is famous as the fabled residence of Lafitte, the great Gulf pirate, Grande Côte and Petite Anse for their salt mines and Côte Carline for the southern home of Joe Jefferson, the actor. The drill has demonstrated the fact that these rounding hills are the surface indices of salt masses below. Down one, two or three thousand feet drills have penetrated with but little variation of matter and structure, making, as already observed, the salt masses perhaps as deep or deeper (thicker) than they are in horizontal diameter. Just off the mound one may drill two thousand feet and encounter nothing but soft clays and sand of Quaternary or "Recent" age. Below are similar materials belonging to the Miocene Tertiary; there is no salt, sometimes not even salt water. Such strangely local salt lumps naturally have troubled the philosophical geologist not a little. Some have said they must have been formed in the crater of some dying volcano, sea-waters having oozed in and evaporating deposited salt for years and years in a streaming caldron. But alas for this explanation, these salt masses are not simply the residue of evaporated sea water, they are 99 per cent, chloride of sodium and without the admixture of crater debris. They are pure and solid. Again, though careful magnetic surveys have been made about them, they fail to show any of those erratic local variations sure to occur in volcanic regions. Finally there is proof positive they were never deposited in a hole or depression, but on the contrary have even moved upward bodily through hundreds of feet of surrounding deposits! This seems at first absolutely impossible and as certainly absurd. Nevertheless, we can demonstrate the point beyond doubt. Note that we have said that certain of these salt lumps occur some distance from the Gulf coast, up country, so to speak, where the terranes are of Tertiary and Cretaceous age and are more or less consolidated. For example, in north central Louisiana salt comes near the surface of the soil in circular areas. Surrounding these areas are rings of highly tilted Cretaceous deposits, still outside are the lower Tertiaries, 1,000 or 1,200 feet thick. Clearly then these salt punches, so to speak, have pushed themselves from amongst Cretaceous rocks right through the Lower Tertiaries, bending these strata up on all sides of the mass, to a height of 1,000 or 1,200 feet. The case then seems clear that the salt masses have come from below and have moved upwards. This is as clearly demonstrated as the fact that the battleship Maine was wrecked from a force without "because the plates were bent inwards." Were the Tertiary and Quaternary beds removed from the flanks of these salt masses we should see a cylinder or perhaps more accurately a truncated cone of salt standing upon mid-Cretaceous rocks towering upwards half a mile or perhaps a mile, though the upper end of the cone might not be over 1 mile across. Some one will say that is certainly similar to the church-spire spur that was lifted out of the crater of Mt. Pelée after its recent destructive eruption. Others will be reminded of Bogoslof Island in Alaskan waters. But here again, in endeavoring to explain the phenomenon there is no need of invoking vulcanicity. For the past ten years we have had exceptional chances to study all these interesting salt masses and are prepared to confidently affirm that the origin of both salt masses and their movements has nothing to do with volcanic action.
The true explanation of the origin, growth and movement of these salt masses seems simple when once we have a clear understanding of certain structural features of the lower Mississippi region. Observe on any geological map that Quaternary and older rocks back to the medieval or Cretaceous beds all slope Gulf-wards at a much greater angle than the surface of the ground makes with the horizontal. In other words, if water should enter a pervious Cretaceous or older bed in Arkansas and follow the same to the latitude of the Gulf border it would find itself several thousand feet below the Gulf level. Such waters would naturally become very warm as compared with water at or near the surface. They would take soluble substances in solution. If a break or point of weakness occurred in the superincumbent beds such hot waters would ascend after the manner of water in an artesian well. If the waters were saturated with salt at a high temperature they would be obliged to part with some of their saline burden as they approached the upper, cooler strata. The amount of salt held in solution by water at various temperatures, it is true, increases not greatly with increased heat; nevertheless, it is appreciable, and in the end the giving up of salt by lowering temperature would produce notable results. Again, though salt masses might tend to accumulate as just outlined at a certain place in the crust of the earth, would not pressure prevent such a growth, and even if growth takes place what would tend to push the salt up bodily say 1,000 feet or more? Here again we need none of Vulcan's aid, for we all know that when once crystallization commences each little crystal will have its growth in spite of almost any resistance. Witness the growth of ice crystals in our water pipes in zero weather. In other words, the force exerted by growing crystals is known to be at least of the same order of magnitude as the crushing strength of the grown crystal. Therefore we are sure growing crystals of salt can lift a column of Gulf coast deposits at least 3,500 feet thick. If brine is supplied to a salt mass from below, crystallization will take place mainly on the bottom of the mass. Therefore the mass will grow from bottom up. The top will be thrust through the superincumbent beds, bending and tilting them up at high angles. Some growth would doubtless take place on the sides of the mass till it attained considerable dimensions; afterwards it would be confined to the base, for the column of rock salt would be a better conductor of heat than the surrounding clays and sands, hence the marked change of heat, hence the salt deposition would take place at the base of the salt column. The mass would therefore be of comparatively small diameter, though its depth might be great.
We see from the above considerations how salt masses might be formed and how they would by receiving their growth increments from the bottom seem, to move upwards and pierce the superficial layers of the earth's crust and there be truncated by atmospheric agencies if they actually reached the surface, or how they might produce great weales on the surface in case they did not quite pierce through. Now we wish to give a few facts indicating that the process outlined above is truly that by which these salt masses were formed and pushed up. Referring again to Gulf-coast structural features, noting the location of all the salt masses known to date, we have little difficulty in satisfying ourselves that such masses are located in a rectilinear manner, row after row as we approach the Gulf border. These lines are parallel in a general way to fault lines farther up country in Arkansas and Texas. A movement along a fault line, similar to these, most readers will remember caused considerable trouble in the region of San Francisco but a few years ago. Where such lines cross (for in Louisiana there are two sets) points of weakness occur permitting the upflow of artesian waters. In several of the " mounds " these waters are saline and " hot."
Finally the source of the salt itself has been a subject of much speculation. However, it is a matter of no serious concern for us here. We know that artesian conditions occur in the general region we are discussing, we know that there are breaks or fault lines and points of weakness through which artesian flows may take place, we know that deep artesian waters are always regarded as " hot." We know that cooling saturated solutions of salt in water must be continually giving up salt; and as this crystallizes it forces aside and upwards superficial rock strata even to depths of several thousand feet. Not only do we know it has strength to do this, but, best of all, there in the Gulf region are the salt masses and there are the bent-up and folded-back rocks. Still we may be permitted perhaps to speculate regarding the source of the brines that have fed these growing crystalline masses. It is well understood that the thick coal-bearing rocks of west central Arkansas derived their material from the south. The Carboniferous continent extended Gulf-wards doubtless as far as the southern limits of Louisiana and perhaps considerably beyond. These old lands were eroded and swept northward into the Carboniferous seas of west Arkansas already referred to. In Permian or slightly later times this continental area was base-leveled, standing on a par with west Kansas and north Texas, receiving deposits of salt and gypsum in shallow sun-baked seas. After considerable accumulation of these saline materials the Gulf region was depressed at the south and covered by later and later deposits and the Gulf invaded the Mississippi valley to Cairo, Illinois. This was in late medieval geological times (late Cretaceous). Since then the central part of the continent has gradually raised, the Gulf border has sunk so that, through the Tertiaries and recent ages, the formations have been tilted more and more to the south till the salt-bearing Permian beds are doubtless 5,000 to 8,000 feet beneath these younger deposits. Hence in all probability the source of the artesian flow of brines that produce the salt masses under discussion.