C.—Pre-Glacial Geological History.
The main features of the Rakaia Valley appear to be structural in origin. This idea was first of all enunciated by Edward Dobson (1865, p. 50 and map), who pointed out that the valleys of the
Canterbury rivers were oriented on lines, probably fault lines, diverging from a point about 40 miles due west of the town of Hokitika. This hypothesis was adopted with some reserve by Haast (1879, pp. 175–6). There appears to be some coincidence of the heads of the valleys with the lines drawn by Dobson, but too many divergencies occur, if the hypothesis is to be regarded as sound. There is also no direct evidence of the presence of these faults. Hutton (1884, pp. 91–2) considered the valleys to be of ancient date, and formed even before the Cretaceous and Tertiary sediments were deposited. He was of the opinion that the sea invaded the mountain portions, and that the limestones, etc., were laid down in them when they were arms of the sea, and he uses as a part of his main line of evidence the fact that a limestone mass in the bed of the Rakaia near Redcliffe Gully showed by its presence that erosion had taken place before its deposition. This mass of limestone turns out to be grey-wacke, so that the evidence and the conclusion deduced therefrom must be set aside.
The present author (1916, pp. 142–3) drew attention to certain features of the Rakaia region, notably to the series of parallel valleys which lie east of Lake Coleridge, and suggested that they were of structural origin. This is apparently the status of the question at present. It will be best, before considering this in detail, to give a brief statement of the geological history of the area, as far as it is known, up to the end of the Tertiary era. This is as follows:—Grey-wackes, slaty shales, argillites, with minor conglomerates of probable Triassic to Lower Jurassic age were folded at the end of the Jurassic or the beginning of the Cretaceous into a mountain range of modified Alpine type. The folding is characterised by overthrusts, faulting, and crushing, but this has not been of an intensity comparable with related movements in the European Alps, and it produced little meta-morphism of the beds. The movement was evidently of a shallow depth, and its results are fracture and breaking rather than a re-crystallisation of the materials under the stresses involved. This mountain range was reduced to a peneplain towards the end of the Cretaceous, a levelling probably finished off by marine planation, and on the surface thus produced a veneer of Senonian and Tertiary sediments was deposited.
1. The Tertiary Veneer:
The basal beds of this are Senonian in age, consisting of coal measures, but they pass up into Tertiary greensands, limestones, and shelly beds. Whether this veneer was continuous over the whole area is not known, but the presence of marine beds wherever the veneer is encountered suggests that the sea had a wide incidence, and the elevations above the surface of the peneplain, and standing above sea-level, were of small extent, even if they existed at all, although land certainly occurred in close proximity.
The remnants of this veneer are found in the valley of the Rakaia in the following places at the indicated heights:—
Near Lake Heron, in the valley of the Cameron and in the Smite, at heights of approximately 2500 ft. Also on
the high country west of Lake Heron, between the Cameron and the Rakaia, at a height of 4000 ft.
In Redcliffe Gully, opposite the Power Station, to a height of 3060 ft.
In the valley of the Harper River, just above the road crossing, and towards the head of the river, at heights in the latter exceeding 3000 ft. The former of these two occurrences consist of sands, with calcareous concretionary layers, striking E.-W. and dipping S. at low angles. The occurrence is obscured by wind-blown and glacial material, and can only be seen for a few chains on the left bank of the river. No fossils were seen.
In the valley of the Acheron and Coleridge Creek, at heights of just over 3000 ft. There is besides the outlier in the lower Acheron, but this is only at a height of 1300 ft. to 1500 ft.
The widespread occurrence of these beds and their height above the floor of the valley can hardly be explained on Hutton's hypothesis that the valleys were eroded by Cretaceous times. There is also a general though not a precise similarity in the sequence in various places.
In addition to the localities just cited, Senonian beds occur in the gorge itself at heights of 900 ft. to 1000 ft., and Tertiaries at the Curiosity Shop, some three miles below the gorge, at a height of 650 feet above sea-level, but these occurrences are extra-montane, and have no apparent bearing on the origin of the mountain valley of the river.
The two areas in the vicinity of the Rakaia, where the Tertiaries are well developed and which are fairly well known, are the Trelissick Basin and Mount Somers. The general sequence in both places is similar, in that coal-measures occur at the base, and these pass up through sands, greensands, and marls into limestones and shelly beds with interstratified volcanics at various levels, but the age of the two series is not identical. In the Trelissick the lower coal-measures are definitely Senonian, and in the Mount Somers area the lowest marine beds lying immediately above the coal-measures are definitely Tertiary (Bortonian). This implies either a wider extension of the same beds as time progressed, that is, a diachronism existed, so that coal-measures of one part of the area may be synchronous with limestones in another part of the area, as maintained by Marshall (1911, p. 23), or that the same succession took place in separate diastrophic areas as suggested by Thomson (1917, pp. 399–401).
The succession of beds as developed in Coleridge Creek is apparently analogous to that in the Trelissick basin, from which it is now separated by the southern end of the Craigieburn Range, but the thickness is much less, suggesting the approximation of a shore line. It cannot be said definitely in the absence of fossil evidence that the relationship is proved, and it may, after all, be advisable to assign this case to the southern diastrophic province. The relation of the
coal-measures in the Acheron River to this occurrence is, however, doubtful. The succession in Redcliffe Gully (1913, pp. 337–8) seems to belong to the Mount Somers province, so that its boundary appears to extend to just beyond the Rakaia. The occurrences of Tertiaries in the higher parts of the river basin are more definitely littoral in character, and do not have any development of the purer limestone facies. They seem more in accord with the higher beds of the two areas suggested for comparison, indicating a wider spread of the shallow water conditions, no doubt simultaneously, in Awamoan times, that being their probable age, and also that land existed in close proximity to the area where the main divide is now situated. It may be true, therefore, that this part of the area was not invaded by the sea when it extended over the outer fringe of what is now the mountain area. The fact that this facies of the Tertiaries is widespread over the deeper water deposits also indicates an actual shallowing of the sea, so that it seems to imply that the movements which culminated in the elevation of the mountain area in its present form were initiated when these beds were being deposited, that is, they date from Awamoan or Miocene times.
It is reasonable to think that the covering veneer persisted at a low level, probably submerged by the sea, till towards the close of the Tertiary era, when the shallowing of the Awamoan sea was accentuated, the area was raised, and land of some decided relief was formed. The evidence for this rise is furnished both by the Trelissick basin and by the Mount Somers area, for in both of these districts, and elsewhere in North and South Canterbury, the uppermost Tertiary beds consist of gravels which have experienced some warping, thus showing that distortional movements had not ceased when they were being laid down. These gravels definitely indicate changes in level, and also the proximity of a land of fairly high relief.