CHAPTER VI. Sedimentation and Geological Time

SEDIMENTATION AND GEOLOGICAL TIME

The maximum thickness of the sedimentary rocks—Rate of deposit—Uniformitarian basis of the method as usually applied—Arguments against the validity of this assumption—Present a period of land extension and continental elevation—Present geological rates not true standards—Ideal sedimentation curve and rates of deposit—Difficulties in the application of the data—Summary of time estimates based on this method—The hour-glass method applied to the accumulation of sediments and of calcium carbonate.

The most familiar method of estimating geological time is based upon the total observed thickness of stratified rocks and the rate at which they may have been deposited. Our knowledge of the so-called maximum thickness of each of the stratigraphical systems has been carefully summarised by Sollas, to whom we owe the following table.

Maximum Thickness of the Geological Systems.

FEET Recent and Pleistocene 4,000 Pliocene 13,000 Miocene 14,000 Oligocene 12,000 Eocene 20,000 ------ 63,000 feet. Cretaceous 44,000 Jurassic 8,000 Triassic 17,000 ------ 69,000 ” Permian 12,000 Carboniferous 29,000 Devonian 22,000 ------ 63,000 ” Silurian 15,000 Ordivician 17,000 Cambrian 26,000 ------ 58,000 ” Algonkian 82,000 82,000 ” Archean ? ? ------------- Total 335,000 feet. -------------

The rate of deposit of sediment on the ocean floor is a factor over which there has been much dispute. The rate varies between very wide limits, according to distance from the shore and from the mouths of great and active rivers. What is generally sought is to give an estimate which will correspond to the maximum thickness of accumulated material, i.e. an estimate of the average maximum rate of deposit. The same rate is then applied in turn to the whole of the geological column in the hope that no serious error will be introduced by the assumption of uniformity. This attitude was taken up in defence against the attacks of Kelvin and his followers. The tendency to invoke more active geological agencies in the past, greater floods and tidal waves, a more stupendous upheaval of mountains and more violent volcanic eruptions, did not commend itself to most geologists. Geikie stated the case very clearly in his eloquent address of 1892, when he affirmed that “the geological record furnishes a mass of evidence which no arguments drawn from other departments of Nature can explain away, and which, it seems to me, cannot be satisfactorily interpreted save with an allowance of time much beyond the narrow limits which recent physical speculation would concede.” But while the conception of greater activity in the past met with little favour, the assertion of uniformity was as far as geologists dared to go. No one suggested that we might be living in an age of more than average activity. Yet there are many reasons which favour this hypothesis in preference to the alternative views.

That the average land area of the past was less than that of to-day has already been stated. According to the palæo-geographical researches of Mr. C. Schuchert, the mean area of North America since Cambrian times has been four-fifths of its present area. In the case of the other continents a smaller fraction would probably be more representative. Of very much greater importance is the fact that the average height of the land areas above sea-level has often been less than it is, so that the present average is excessive when viewed from the broader standpoint of geological time. The chief defect in the time estimates based on the rate of sedimentation lies, according to Chamberlin, in the too full dependence on standards derived from the geological processes now in action. It is tacitly assumed that current rates are representative, or that the departure from the mean rate is not such as to involve any grave error. Joly, for example, in discussing the divergent evidence of geological processes and radioactive minerals, points out that to bring the different methods into agreement we must assume “that the rivers are now bearing to the sea about 14 times the average percentage of the past—not less than 9 times.” Then he says, “It seems quite impossible to find any explanation of such an increase.”

In the present high relief of the earth’s surface at least a partial explanation may be found. Prof. Chamberlin writes in a private communication to the author: “Because of the relatively high gradients, the wash of clastic material from the slopes and its deposition in the basins, as well as the transfer of salts to the sea, are to-day more rapid than in average times. We seem to be at, or near, one of the great extremes of intensification of the processes of solution and degradation. And so, whether conclusions are based upon degradation and clastic deposition, or upon solvent action and the accumulation of solutes in the sea, the present rates are high rates, and if these are made the basis of time estimates, the estimates are minimum ones. There are abundant evidences that periods of base-levelling have occupied a notable part of geological time. There is cogent evidence that the Archean and Proterozoic (Algonkian) terranes were reduced well towards base-level in pre-Cambrian times, and that subsequently extensive base-levelling clearly seems to have intervened at repeated intervals. To me the evidence seems to support the existence of a dozen or a score of stages of peneplanation, some of which appear to have made a notable advance toward complete base-levelling.”

During those intervals, when the average continental height was low, denudation and deposition would proceed very slowly. How slowly, we have no adequate means for determining. A very careful study of drainage basins with reference to their mean elevation would be a step towards a sounder method than it has yet been possible to apply. That the departure from present rates, in past time, may have been very considerable, is indicated crudely by a very simple calculation. It has been found experimentally that the carrying power of water varies as the sixth power of its velocity. Roughly, we may say it varies as the sixth power of the mean square of the heights of the drainage basin from which it finds its way to the sea. If this were strictly true, then, if the present day contours of the land were reduced to half their value, the power of removing material would be reduced to less than one four-thousandth. Of course, other factors would begin to operate which would prevent the attainment of any variation as extreme as this. Nevertheless, our faith in the value of present standards as applied to geological time cannot but be seriously shaken. Chamberlin feels warranted in thinking “that the substitution of mean velocities of denudation, deposition, and saline accumulation, if it could be made to approach the realities of the case, would have the effect of multiplying by a considerable figure the best estimates that have been made on the basis of current velocities.” Other factors tending to increase the age estimate are numerous, but comparatively insignificant beside those already briefly discussed.

In the last chapter we concluded with an estimate of the quantity of different types of sediment which are now annually deposited.

Shales 6300 million tons. Sandstones 1440 ” ” Limestones 1260 ” ” ------------------ 9000 million tons.

If it be assumed that all the arenaceous sediments form littoral, deltaic and estuarine types, i.e. that they are concentrated over an area of say 100,000 square miles, the rate of accumulation would be one foot in 150 years, or dealing only with fresh unconsolidated sediment, of one foot in about 100 years. But these figures are obviously too high, for a great deal of sandstone comes under shallow water marine types. Fortunately, in the deltaic deposits of two important rivers human remains of recognised age are found buried; and from measurements of the thickness in each case it is known that the Nile has deposited loose sediment at one foot in 320 years, and the Po at one foot in 174 years.

We have here, as in all deltaic deposits, a mode of growth analogous to that of the glacial clays studied by De Geer. It by no means follows that these rivers will continue to raise their beds at the same rates, for the main growth of deposit is not upwards, but seawards.

To get a rough idea of the rate of sedimentation, the ideal section opposite has been constructed. The continental shelves are assumed on the average to be representable as a band of 100 miles in width fringing a coastline of 100,000 miles. The average thickness of a deposit laid down according to this plan would be about 0·4 of the greatest thickness, the latter, on the scale of the diagram, having reached 400 feet. In the particular case illustrated the land is slowly sinking, and as the sea encroaches upon it the deposits gradually overlap.

If deposited over one square mile, as consolidated sediments of density 2·5, our 9000 million tons of material would form a rectangular mass 4570 feet in thickness. Being deposited over the ten million square miles of the continental shelf, the average thickness of the layer is 0·000457 feet, corresponding to a rate of deposit of one foot in 2200 years. The maximum rate is therefore one foot in 880 years. If this figure be applied to the total thickness of the geological column, then, disregarding its imperfection, the time implied would be about 300 million years.

The distribution of conglomerate, sandstone and shale is sufficiently indicated in the diagram. It is difficult to know how to distribute the argillaceous and calcareous types. They are mutually exclusive, for limestone cannot form by organic agencies in places where mechanical detritus is being deposited. They are therefore taken together, on the basis that where one is not being formed the other is.

Section illustrating the formation of Sediments on the Continental Shelf while the latter is being slowly depressed.]

The following table gives the rates of deposit at various distances from the shore, for the case illustrated in Fig. 11:

-------------------------+-------------------------------- | YEARS FOR DEPOSIT OF ONE FOOT. DISTANCE FROM +----------------+--------------- SHORE IN MILES. | FRESH | CONSOLIDATED | DENSITY = 1·8. | DENSITY = 2·5. -------------------------+----------------+--------------- { 0 | | { 10 | 2,780 | 2,000 Sandstones { 20 } | 1,670 | 1,200 { 30 } | 1,120 | 880 { 40 } | 1,430 | 1,030 50 } | 2,280 | 1,660 60 } Shales | 5,210 | 3,750 70 } | 10,420 | 7,500 80 } | 13,900 | 10,000 90 } | 20,850 | 15,000 100 } | 41,700 | 30,000 -------------------------+----------------+---------------

These figures, however, have but little value, for there is no single law of deposition. The effects of ocean currents, of earth movement, and of the presence or absence of great rivers should all be considered, and they provide a problem so complex that as yet it is hopelessly beyond a general solution. Difficulties are encountered at every stage; not only are the estimated rates of doubtful value, but their application is discredited by our ignorance as to what constitutes the real maximum thickness of sediments.

As already indicated, the latter difficulty is due to the tendency for successive beds to overlap while the cycle of deposition is running its course. As was admirably stated by Prof. Watts in his Presidential address to the Geological Society in 1911, deltaic deposits gradually extending seawards are more characteristic during periods when the land is being elevated relative to sea-level. On the contrary, during periods of depression estuarine conditions prevail and the beds grow landwards.

The same problem has been attacked by Sederholm from a rather different point of view. He says, “As the layers successively formed cover each other like scales or roof-tiles, no vertical section contains them all. If we mean by maximum thickness the sum of the maxima of the layers formed in successive years, it certainly measures millions of feet.” In this case the rates of deposit as ordinarily found would not, of course, be applicable.

Before proceeding farther, it may be well to review the various estimates of time which have been founded upon the geological method. The earlier geologists believed that the sediments were deposited widespread over the ocean floor and the rate of deposition was therefore taken as even less than that of denudation. Then came the Challenger expedition in 1872-5, and it became certain that the formation of all except the less important deep-sea deposits takes place almost entirely on the submarine continental shelves. A considerable modification of the estimated rates of deposition was then made necessary and the time periods were correspondingly shortened.

------+--------------+------------+-----------+---------- | | | RATE OF | | | MAXIMUM | DEPOSIT | TIME IN DATE. | AUTHOR. | THICKNESS | YEARS FOR | MILLIONS | | IN FEET. | ONE FOOT. | OF YEARS. ------+--------------+------------+-----------+---------- 1860 | Phillips | 72,000 | 1332 | 96 1869 | Huxley | 100,000 | 1000 | 100 1871 | Haughton | 177,200 | 8616 | 1526 1878 | Haughton | 177,200 | ? | 200 1883 | Winchell | — | — | 3 1889 | Croll | 12,000 | 6000 | 72 1890 | de Lapparent | 150,000 | 600 | 90 1892 | Wallace | 177,200 | 158 | 28 1892 | Geikie | 100,000 | 730-6800 | 73-680 1893 | McGee | 264,000 | 6000 | 1584 1893 | Upham | 264,000 | 316 | 100 1893 | Walcott | — | — | 45-70 1893 | Reade | 31,680 | 3000| 95 1895 | Sollas | 164,000 | 100 | 17 1897 | Sederholm | — | — | 35-40 1899 | Geikie | — | — | 100 1900 | Sollas | 265,000 | 100 | 26·5 1908 | Joly | 265,000 | 300 | 80 1909 | Sollas | 335,800 | 100 | 80 ------+--------------+------------+-----------+----------

Most of these estimates are little more than rough guesses. We do not know how much of the story is lost to us, or how much is hidden away. The time which has usually been regarded as expressing the geological requirements most adequately is 100 million years. The fanciful figures arrived at by Winchell, and McGee (who suggested a probable age of 6000 million years) are merely illustrations of how the data could be twisted to produce impossibly extreme results. The latest estimate, due to Sollas, includes an allowance of 25·4 million years for the duration of pre-Cambrian time—the same period as that which has apparently elapsed since. A further allowance is made for unconformities, those gaps in the sequence which are unrepresented by sediment. Taking the great unconformities as probably numbering six, each being equivalent to 40,000 feet of sediment, 24 million years are added. For minor unconformities and interruptions in the record other 5 millions are granted and the total is thus brought up to 80 million years.

In an attempt to free ourselves from the difficulties with which this method is beset, we may adopt a mode of procedure similar to that followed in the last chapter. It was there assumed that the total volume of the sediments which have ever existed amounts to some 210 million cubic miles. The present annual supply of sediment when ultimately compressed and consolidated would occupy 0·83 of a cubic mile. If the present rate of accumulation were reliable, geological time would then be of the order 250 million years. A still nearer approach to the truth may be made by calculating on a uniformitarian basis how long the existing sediments, which we placed at 70 cubic miles, have taken to form. From the 8,000,000 square miles of igneous and metamorphic rocks, one cubic mile would be denuded away in 4·54 years, which implies that one cubic mile of consolidated sedimentaries would be formed in about five years. The age then works out at 350 million years. Making a further correction for the slower rate of denudation of igneous rocks, this figure may perhaps be doubled. Finally, there is the correction for average rate in place of present rate, and to what extent this would increase the estimate it is impossible to say.

We may revert to the maximum thickness of the sedimentary rocks to support the estimate of the total volume of sediments which has ever existed. On the basis of the sedimentation curve, the sediments have been deposited on an area of 10 million square miles, and if laid down everywhere at their average maximum thickness, 60 miles, they would cover about 0·4 of that area. The total volume which can ever have existed, leaving unconformities out of the question, is therefore, 60 × 10,000,000 × 0·4 cubic miles, or 240 million cubic miles.

Finally, a crude estimate may be based on the amount of calcium carbonate which has accumulated in geological time. Several estimates of the volume of limestones in existence have been made, e.g.:

Dana 18·40 million cubic miles. Reade 10·00 ” ” ” Van Hise 6·25 ” ” ”

The limestones now forming make up 14% of the total sediments which collect on the continental shelves. On the land the proportion must be lower than this, because limestone is denuded at a rate well above the average. If we take limestone formations at 10%, the volume would be about 7 million cubic miles, a figure not far from that of Van Hise. The calcium carbonate may now be estimated. Limestones contain on an average 75% and shales and sandstones together about 7%. The total volume, calculated at density 2·6, would therefore be in round figures 10 million cubic miles. Igneous rock contains 3·43% of calcium, and if in the process of denudation all of this is dissolved and removed, the present rate of production of calcium would be equivalent to 1 cubic mile of calcium carbonate in 32 years, on the same basis as in previous calculations. The time estimate at this rate would be 320 million years.

We may now sum up our various results as follows:

1. Accumulation of Sodium. MILLION YEARS. (a) Uncorrected quotient Naₒ/Naᵣ 80·8 (b) Partially corrected Naₒ/Naᵣ 90 (c) Unchloridised sodium alone 180 (d) Primary sodium alone 210-340

2. Accumulation of Sediments. (a) Maximum thickness 300 (b) Total volume which has ever existed 250 (c) Total volume now existing 350

3. Accumulation of CaCo₃. (a) Total volume now existing 320

Not one of these estimates is to be regarded as final; the uncertainties are too many and too great. The whole trend of this chapter has been to show that whatever may be the true reading of the hour-glass of denudation and deposition, it ought probably to be very much higher than has been generally assumed.