Tuatara: Volume 10, Issue 1, April 1962
Dating Recent Mountain Growth By Fossil Pollen
Dating Recent Mountain Growth By Fossil Pollen
In The Last Few Decades proofs have been discovered of the recency (previously unsuspected) of mountain uplift in various parts of the world. In New Zealand the dating within the last few years by microfloral evidence, largely by the study of fossil pollen grains (palynology), of deposits laid down in early Pleistocene times has made it evident that at least some of our mountains have arisen in that period — and quite probably within the last few hundred thousand years. In California also, the Sierra Nevada is a conspicuous example of a great mountain range that was upheaved at an equally late date, as has been shown by examination of pollen contained in lake beds that accumulated at low levels across the site of the present-day mountains.
Geomorphologists long ago exposed the fallacy of ‘the everlasting hills’. Many of them have, however, clung tenaciously to the doctrine of survival for many millions of years of mountainous relief forms or at least of considerable relics of ancient landscapes on the upraised blocks and arches from which mountains are being carved by erosion. It is true that the tempo of erosion, notably that of dissection of the land surface by streams of water, varies considerably from region to region, depending apparently on differences in distribution and intensity of rainfall that seem at first sight negligible. C. H. Crickmay, a whole-hearted believer in the importance of the role of lateral corrasion and the sideward swinging of rivers in shaping the landscape, goes so far as to claim that there may be ‘differences of intensity of superficial geologic activity of at least ten million to one’ (Crickmay, 1959, p. 50); but this takes into account both local differences of ground slope (Crickmay describes the erosive processes on level ground as ‘stagnant’) and the important fact of local exposure to or immunity from active corrasion by wave or river action. Allowance being made, however, for appreciable regional differences in the tempo of erosion, such as that evidenced by a postglacial development of fine-textured erosional relief in New Zealand but not in north-western Europe, it is obvious that the rapidity of mountain dissection and the kaleidoscopic changes of form which some evolving landscapes undergo have very often been overlooked. The oversight may be attributed in general to the acceptance of exaggerated estimates of the length of time that has elapsed since the upheavals took place that produced the initial forms of mountain ranges.page 6
There is evidence of large-scale upheaval of mountains in Pleistocene time in Italy, in New Zealand, in the Himalayas and the ranges of central Asia, in Scandinavia, and in Iceland, as well as in western North and South America. Although information bearing on such recent mountain growth has been accumulating for several decades, little precise dating has been attempted, and so there has been a lingering tendency to minimise the actual recency of the events by referring to them in such terms as ‘Miocene and Pliocene movements some of which persisted strongly into the Pleistocene’ (Flint, 1947, p. 514).
Recent Mountain Growth in New Zealand
Figure 1. The Paparoa Range (west) and Victoria Range (east), carved by erosion from arches upraised in the Pleistocene period and separated by a gravel-filled syncline. Data from Suggate (1957).
The apparent absence of any glaciations except those of very late Pleistocene date in peninsular Italy and in the Central Range of New Guinea (Verstappen, reviewed by Cotton, 1961) may indicate simply that in early and even in middle Pleistocene times the mountains of these regions had not yet grown high enough to be glaciated. In New Zealand non-discovery of glacial morainic debris of early Pleistocene date in the mountains might, though it is only negative evidence, be acceptable as an indication that high mountains were not in existence in the early glacial ages; but this inference is supported by the discovery at low levels of some undoubtedly glacial deposits which must have originated in the early Pleistocene as they are conformable with stratified beds of that age. The Ross Glaciation, to which Gage (1961) and others refer these, may be correlated tentatively with the ‘first’ or ‘second’ of the world glacial ages (Günz, Mindel, Riss, Würm), perhaps with the Günz, seeing that the glacial deposits are associated with stratified deposits of New Zealand's early Pleistocene stage, the Lower Nukumaruan. (The paleontology of the Upper Nukumaruan, in the North Island, affords evidence of another Pleistocene cooling of ocean waters that may be tentatively correlated with the Mindel refrigeration.)* Neither the Günz nor the Mindel age is as yet precisely dated in years, but a date as late as about 320,000 B.P. (before present) has been suggested for the Günz and about 235,000 B.P. for the Mindel (Fairbridge, 1961, fig. 10, p. 133). Though it is quite possible that these estimates of age are too low it is perhaps a safe guess that the Ross Glaciation occurred not more than half a million years ago.
The deposits laid down during this early New Zealand glaciation are themselves part of a sequence of strata which, though their age is fixed by microfloral evidence, are in part marine. These beds, and with them the glacial deposits, have been folded by movements contemporaneous with strong upheaval of the adjacent Mount Greenland, west of the Southern Alps — and very probably of the main range of the Southern Alps itself. The main range had begun to rise rapidly in earliest Pleistocene times, its erosion supplying a vast flood of coarse debris before the Paparoa and Victoria ranges were arched up (Suggate, 1957, p. 24), but much of the upheaval has certainly taken place since the Ross Glaciation, which Gage (1961, p. 631) suggests was due to ‘ice-cap glacierisation’ and which may, therefore, have occurred on comparatively low ground.page 8
While admitting that ‘we cannot picture either the details of the early Pleistocene landscape or the character of the Ross Glaciation’, Gage envisages the landscape as ‘an early stage in the mountain-building, when dissection was incomplete, the relief less severe, and the scenery less strongly alpine than in the late Quarternary [and the present day], so that there may have been extensive plateaus upon which ice-caps or plateau glaciers could have been generated’. In other words, though not anterior to the whole of the uprise of the Southern Alps this was a time during — and perhaps midway through — the growth of the range and so near to its first up-bulging that incision of valleys had reached only a young stage and the landscape was as yet very far from the stage of maturity and intricate dissection it has since attained.
Microfloral Dating of Upheaval
In the last few years the precise methods of pollen study have been employed in dating a great mountain upheaval, and this has proved to be of startlingly recent date as compared with earlier estimates and guesses. In 1957 Axelrod established by study of fossil floras that uniform climates prevailed in Tertiary times from the coast of California eastward into the interior of North America —the region being then of small relief and low-lying. The basins and ranges, that is to say, which now cause rain-shadow effects and diversification of climate and flora were not yet in existence. This applies in particular to the high Sierra Nevada range which separates desert interior basins from the relatively humid coastal region. In two studies dated 1960 and 1961 Axelrod and Ting have applied the method of dating by pollen content to lake deposits now at high levels on the Sierra Nevada and at lower levels east of the range.
On the Sierra Nevada extensive remnants are preserved of two more or less level erosion surfaces (Fig. 2). The older of these was developed as a lowland by erosion before upheaval of the range began; the younger surface was cut by the widening of valleys which had been excavated below the level of the older after it had been raised several thousand feet by the first spasm of upheaval. Since the widening of valleys took place, with the formation of the lower recognisable erosion surface, there has been even stronger upheaval, which has raised both the erosion surfaces to altitudes of 10,000 feet and more on the highest parts of the range. Relics of the surfaces that have survived the more recent ravages of erosion are now isolated plateaus between deep, newly-cut canyons. Until very recently the problem of dating the origin of the two main erosion surfaces baffled investigators, who all regarded them as ancient but who failed to agree on their ages. F. E. Matthes, the leading authority on the geomorphology of the Sierra Nevada, described the upper surface as of Eocene age and the lower as Miocene, dates which implied that the first upheaval was very ancient (post-Eocene but pre-Miocene) and that the second also could have occurred many millions of years ago.
Both the upper surface (to which the name Boreal, formerly of local application, is now given throughout) and the lower (now referred to as the Chagoopa or Broad Valley surface) are overlain in places by sediments that accumulated in lakes existing when the surfaces were low-lying. From these sediments abundant pollen has been obtained at a number of places, study of which has revealed the date of accumulation of the sediments as well as affording much information regarding the climates prevailing when the lakes were in existence. The deposits spread on the Boreal surface are of Late Pliocene age (Axelrod and Ting, 1961, p. 121).
At some localities east of the Sierra Nevada beds containing the same pollen flora as these are associated with others containing mammalian fossils of Late Pliocene or possibly Early Pleistocene age; but Axelrod and Ting (1960, p. 27) are satisfied that their age is definitely very late Pliocene, being convinced of this by the floristic composition of the pollen assemblage, which they have compared with that in coastal Californian Iocalities. This whole region was one of changing climates and changing plant distributions in the Pliocene period, so that a Late Pliocene floristic composition can scarcely be misinterpreted. From the uniformity of the fossil pollen flora on the high plateau of the Sierra and at many localities in rain-shadow deserts to the east and its similarity to that found at places near the coast of California it is certain that the Sierra Nevada range was not in existence in the very late Pliocene — as is obvious also from the near-senile character of the relief on the Boreal surface of the high Sierra, the date of that surface being now established. To quote from Axelrod and Ting (1960, p. 1): ‘Analysis of Late Pliocene vegetation leeward [i.e. east] of the page 10 present Sierra Nevada, and comparison with contemporaneous floras in western California, shows that the Sierra had only low to moderate relief [i.e. had not yet begun to rise as a mountain range] and that drainage was westward across [the site of] the range. Although the Late Pliocene floras represent only one major life zone (transition yellow-pine forest), the fossil localities now occur in life zones ranging from lower desert to upper subalpine forest, which have evolved in response to the 8,000-9,000 feet of relief that has developed more recently.
In 1960 Axelrod and Ting were of the opinion that some of the deposits with the Late Pliocene flora lay on the Broad Valley or Chagoopa surface, besides some on the Boreal surface, so that these two surfaces appeared to be of approximately the same age notwithstanding that several thousand feet of upheaval followed by incision of valleys and then by the opening out of these into ‘broad valleys’, the last especially being a time-consuming process, had occurred between the two periods in which lakes existed and sediments accumulated. This apparent inconsistency between the floristic and geomorphic histories of the region has been removed by the correction of an error. In their later report these authors (1961, p. 12 footnote, p. 145) have withdrawn the statement that the Late Pliocene flora occurs on the Broad Valley surface, investigation having shown that the beds containing this flora (at altitudes of 9,000-10,000 feet on the high Sierra) lie only on the higher (Boreal) surface, whereas floras of Early Pleistocene age are found at localities situated on the plateaus of the Broad Valley (or Chagoopa) surface, these being now also at altitudes of over 8,000 feet.
On the Kern Plateau of the Sierra Nevada a similar Early Pleistocene flora (a ‘pine-fir ecotone’) is found not only at very high altitudes but also at localities through a vertical range of 2,500 feet in a distance of thirty miles, which is taken to indicate tilting of the surface (now a plateau) at some time after fossilisation took place (A. and T., 1961, pp. 142-3). Moreover, they are present also at much lower levels (at altitudes of less than 3,500 feet) east of the Sierra Nevada in Owens Valley, which has obviously suffered very much less upheaval since their accumulation. This serves to establish a recent date for the long-known great fault, or zone of step faulting, between the large Sierra Nevada and Owens Valley earth blocks (Fig. 2).
As regards more or less precise dating of the period at which lakes existed on the then low-lying Broad Valley-Chagoopa surface, ‘cool, moist climates like those of the Chagoopa phase appear to be contemporaneous with glacial conditions’ (A. and T., 1961, p. 140). A decision to correlate it with the ‘second’ (Kansan or Mindel) rather than with an earlier glaciation has been influenced to some extent by the fact that the earliest glaciation recorded in the Sierra Nevada region, the McGee Glaciation, has been referred to page 11 the Kansan age, and McGee glacial till is known to rest on the Chagoopa surface (A. and T., 1961, p. 141). Needless to say, the glaciers that laid down the deposits attributed to the McGee Glaciation. like those of the Ross Glaciation of New Zealand, cannot be traced to an origin in valleys in any way resembling those of the present day. They existed, that is to say, before the upgrowth of the mountain forms of the present-day landscape. This is only one more example of upheaval of mountains during, instead of anterior to, the Glacial Period — and within the last half million or perhaps even the last quarter of a million years.
Another consideration that must be given due weight in deciding whether to correlate the Chagoopa episode with the ‘first’ or the ‘second ’ glacial age is the lapse of time that must be allowed for the events between the two floral stages, ‘Late’ Pliocene and Early Pleistocene. These comprise (1) uplift of the Sierra Nevada range amounting to 2,500-3,000 feet, which, however rapid it may have been, was followed by (2) a long period of erosion during which not only were valleys incised but — a process demanding much more time — the floors of these valleys were opened out to a great width, with development of the Broad Valley-Chagoopa surface. The Pliocene period as a whole having been at least ten times as long as the Pleistocene, it seems at first sight reasonable to suggest that much of the time that must have elapsed between the development of the Boreal and of the Broad Valley-Chagoopa surface was within the Late Pliocene. According to Axelrod, however, this cannot have been the case, for the ‘Late ’ Pliocene flora on the Boreal surface flourished really at the very end of the Pliocene. He has described the climate that conditioned it as ‘the beginning of a glacial stage’ (A. and T., 1961, p. 140), which can only mean a time transitional to the Pleistocene glacial period. In accordance with this, the first upheaval of the Boreal surface, i.e. of the Sierra Nevada as a whole, must have occurred ‘in the Plio-Pleistocene transition and/or in basal Pleistocene time’, and acceptance of this necessitates placing the (much later) second, or major, upheaval of the range as a ‘mid-Pleistocene event’ (A. and T., 1961, p. 141) rather than early Pleistocene. Thus the major upheaval of the Sierra Nevada range may have taken place as late as the Yarmouth, or Mindel-Riss, interglacial age, which began, according to some recently published estimates, only about 235,000 years ago (cf. Fairbridge, 1961, fig. 10).
Axelrod, D. I., 1957. Late Tertiary Floras and the Sierra Nevadan Uplift. Geol. Soc. America Bull., 68: 19-46.
Axelrod, D. I., and Ting, W. S., 1960. Late Pliocene Floras East of the Sierra Nevada. Univ. Calif. Publ. Geol. Sci., 39(1) : 1-118.page 12
— 1961. Early Pleistocene Floras from the Chagoopa Surface, Southern Sierra Nevada. Univ. Calif. Publ. Geol. Sci., 39(2) : 119-94.
Cotton, C. A., 1961. Growing Mountains and Infantile Islands on the Western Pacific Rim. Geographical Journal, 127 : 209-11.
Crickmay, C. H., 1959. A Preliminary Inquiry into the Formulation and Applicability of the Geological Principle of Uniformity. Calgary: Evelyn de Mille.
Fairbridge, R. W., 1961. Eustatic Changes in Sea Level. Chapter 3 of Physics and Chemistry of the Earth, vol. 4 : 99-185. Pergamon Press. Flint. R. F., 1947. Glacial Geology and the Pleistocene Epoch. New York: Wiley.
Gage. M., 1961. On the Deformation. Date, and Character of the Ross Glaciation. Trans. Roy. Soc. N.Z., 88 : 631-8.
Suggate. R. P., 1957. The Geology of the Reefton Subdivision. N.Z. Geol. Surv. Bull. 56.
Vella. P., 1961 (December). The Pattern of Eustatic Sea-level Fluctuation During the Quaternary Period. Geol. Soc. N.Z. Newsletter. 11 : 16-17.
Woldstedt, 1961 (November). Die Vergletscherung Neuseelands and die Frage ihrer Gleichzeitigkeit mit den europaischen Vereisungen. Eiszeitalter u. Gegenwart, 12 : 18-24.
* Since this was written Woldstedt (1961), following a suggestion by Suggate, has proposed to correlate the Nukumaruan-age glaciations with pre-Günz glaciations in Europe; while Vella (1961), on the other hand, has suggested dating them as of Mindel age and younger.