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.
[Illustration: FIG. 11.
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[3]
70 } | 10,420 | 7,500[4]
80 } | 13,900 | 10,000[5]
90 } | 20,850 | 15,000[6]
100 } | 41,700 | 30,000
-------------------------+----------------+---------------
[Footnote 3: Cf. Rates of deposit of the Cretaceous sediments of
Colorado, p. 47.]
[Footnote 4: Cf. Rates of deposit of the Cretaceous sediments of
Colorado, p. 47.]
[Footnote 5: Cf. Rates of deposit of the Cretaceous sediments of
Colorado, p. 47.]
[Footnote 6: Cf. Rates of deposit of the Cretaceous sediments of
Colorado, p. 47.]
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[7] | 6000[8] | 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[9] | 3000[10]| 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
------+--------------+------------+-----------+----------
[Footnote 7: Spread evenly over the land areas.]
[Footnote 8: Rate of denudation.]
[Footnote 9: Spread evenly over the land areas.]
[Footnote 10: Rate of denudation.]
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.[11] 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.
[Footnote 11: Leith finds 12·5-22 million cubic miles.]
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.