Late Cretaceous and Tertiary Geosynclines in Westland, New Zealand
[Read before the Wellington Branch, August 8, 1946; received by the Editor, October 13, 1947; issued separately, May, 1949.]
The distribution of thicknesses of the upper groups of the Greymouth coal-measures and the lower groups of the overlying Tertiary strata, backed by consideration of facies distribution, indicates that these sediments accumulated in a narrow tectonic furrow that continually deepened from late Cretaceous to mid-Oligocene, when the region experienced a period of diastrophic calm. Marine transgression occurred in mid-Eocene.
Between late Oligocene and early Miocene a ridge began to form on the site of the furrow, the contents of which were partly eroded and acquired an anticlinal structure. Although marginally or completely submerged during phases of the subsequent renewed marine transgression, the ridge thereafter was repeatedly raised. A new furrow formed in early Miocene, parallel with and immediately east of the first, and has continued to subside virtually until the present time. The orogenic climax at the close of the Tertiary gave the ridge its present form as the Paparoa Range, and the abundant waste eroded from this and other simultaneously rising areas expelled the sea from the younger furrow in the upper Pliocene.
The furrows, described as miniature geosynclines within the “western geosyncline” recognized by Macpherson, illustrate on a small scale the successive development of adjacent and subparallel geosynclines in a region of considerable crustal mobility. They show most of the characteristics of larger geosynclines, but are too narrow to be interpreted in terms of isostatic response to sedimentary loading.
The Greymouth bituminous coalfield, on the west coast of the South Island, has recently been examined in detail in the course of a general survey of the coal resources of New Zealand. A by-product of the economic survey was a well-documented chronicle of geologic events with some bearing on diastrophic theory, and with special interest in view of Macpherson's recently published outline of diastrophism in New Zealand since the Late Cretaceous 1
This paper, however, deals with a small part of the story, inferred from a study of systematic variations in the thicknesses of certain formations, and from the general pattern of their deformation.
The coal-measures at Greymouth, resting unconformably on a basement of indurated sandstone and argillite of probably early Palaeozoic age, are inferred, from fairly satisfactory indirect evidence, to be Late Cretaceous. The following table summarizes the content and classification of the stratigraphic succession at Greymouth, with emphasis upon the rock groups with which this paper is chiefly concerned.
[Footnote] 1 Macpherson, E. O., “An Outline of Late Cretaceous and Tertiary Diastrophism in New Zealand.” N.Z. Dept. Sc. Ind. Res. Geol. Mem. 6, 1946.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Age. Miocene-Upper Pliocene||N.Z. Stage.2 Altonian-Waitotaran||Group. Lower Marine Tertiary||Formation. (Not subdivided)||Lithology and Thickness. Terrestrial conglomerate, sand, clay, lignite; marine sands, conglom., silts, limestone.0–6,000 ft.|
|Lower-mid-Oligocene||Whaingaroan-Waitakian||Cobden limestone||Glauconitic, arenaceous, and argillaceous lime stone. 100–700 ft.|
|Runangan||Port Elizabeth mudstone||Calcareous mudstone. 200–1,000 ft.|
|Middle-upper Eocene||Kaiatan||Lower Marine Tertiary||Omotumotu formation||Marine breccia, conglom erate. sandstone, mudstone; intra-formational slumping. 0–3,000 ft.|
|Kaiata siltstone||Micaceous siltstone, often carbonaceous. 1,500–5,000 ft.|
|Bortonian||Island sandstone||Calcareous sandstone. 100–600 ft.|
|Brunner||Brunner coal-measures||Predominantly quartz conglom. and sands, carbonaceous shale, coal. 50–400 ft.|
|Upper Cretaceous||?-pre Pilipauan||Upper Paparoa||Dunollie coal-mesasures||Conglomeiate, quartz mica sand, carbonaceous mudstone, coal. 0–1,000 ft.|
|Goldlight mudstone||Lacustrine brown mud-stone. 0–600 ft.|
|Middle And Lower Paparoa||(5 formations)||Breccia, conglomerates, sands, carbonaceous mudstone, lake muds, coal; volcanics. 100–1,600 ft.|
|Palacozoic||–||–||Greenland||Indurated sandstone, argillite.|
Pleistocene and Recent drift formations unconformably succeed the Upper Tertiary Group. The above classification, replacing that of Morgan,3 will be dealt with fully in Bulletin 45 of the New Zealand Geological Survey (“The Greymouth Coalfield”; now in press).
It is proposed to discuss mainly the Upper Paparoa, Brunner, and Lower Marine Tertiary groups, and in less detail, the Upper Tertiary sediments. Isopach maps (Text Figs. 1–5) illustrate the range and pattern of the thickness-distribution of all the formations from Goldlight to Kaiata inclusive, and were compiled from drill logs and surface sections. These are supplemented by a structure diagram
[Footnote] 2 Finlay, H. J., and Marwick, J. “The Divisions of the Upper Cretaceous and Tertiary of New Zealand.” Trans. Roy. Soc. N.Z., vol. 70, pt. 1, pp. 77–135, 1940; “New Divisions of the New Zealand Upper Cretaceous and Tertiary.” N.Z. Journ. So. and Tech., vol. 28, no. 4 (Sec. B), pp. 228–36, 1947.
[Footnote] 3 Morgan, P. G. “The Geology of the Greymouth Subdivision, North Westland.” N.Z. Geol. Surv. Bull. 13, 1911.
and hypothetical cross-sections (Text Figs. 6–8) which are an attempt to reconstruct the conditions at the close of Kaiata time, and compare them with the present.
It will be observed first that the isopachs exhibit a general similarity of pattern, and, second, that the maximum thicknesses of all formations illustrated, totalling nearly 8,000 ft., lie within a north-north-east belt from 0 5 to 1·5 miles wide. The Dunollie formation has two distinct maxima, and the Island sandstone shows signs of a second line of maxima, a little offset from and parallel with the first. Text Figs. 7 and 8 emphasize that before the covering strata 1 were deformed into their present anticlinal form, these formations occupied a narrow crustal downwarp, for it should be noted that the vertical and horizontal scales are the same.
The isopachs of the Middle and Lower Paparoa groups, not reproduced here, do not show this distinctive pattern. Those of the two lowest formations, although based on less complete data, suggest that the sediments accumulated in narrow, pre-existing depressions that may have been either erosional or deformational, and that were oblique to the elongation of the later isopach patterns. The remaining members of the lower non-marine groups show no consistent arrangement. Isopachs were not prepared for formations above the Kaiata siltstone, for they outcrop only around the margin of the area studied. The Omotumotu formation is a wedge of incompletely sorted delta beds that does not conform to the earlier picture. When traced westwards, it becomes thinner, finer and better sorted, and interdigitates with the upper part of the Kaiata formation. On the basis of cross-sections to the south of the coalfield, it is inferred that the Port Elizabeth and Cobden formations, which undoubtedly once extended over the whole region, were thickest within the same belt as the pre-Omotumotu beds, but that the range of variation was less.
Unconformity above the Cobden limestone is now established 5 in the field and reinforced by recent palaeontological advances. It is now known that all three stages of the lately re-established Pareora Series.6 representing the upper half of the Oligocene, are missing at Greymouth, where the sediments succeeding the Cobden are of the Altonian Stage (lowest Miocene). These beds are known from exposures along the eastern and south-western fringes of the coalfield. All but one of the stages absent at Greymouth are present in South-West Nelson, according to recent stratigraphic mapping by Wellman. 7
[Footnote] 1 Cotton, C. A. “The Later Geological History of New Zealand.” Geol. Mag., dec. 6, vol. 3, 1916, p. 243 (footnote).
[Footnote] 5 Compare Morgan, P. G. “Records of Unconformities in New Zealand.” Trans. N.Z. Inst., vol. 48, 1916, p. 12.
[Footnote] 6 Finlay, H. J., and Marwick, J., loc. cit., 1947, p. 220.
[Footnote] 7 Wellman, H. W. “Geology of the Fox River Headwaters, Brighton Survey District, South-West Nelson” Trans. Roy. Soc. N.Z., vol. 76, pt. 2, 1946, pp. 221–3 “Geology of the Pike River Coalfield,” in MS. (quoted with permission) The Upper Waitakian Stage mentioned in the first paper is now called the Otaian Stage (see earlier reference to Finlay and Marwick. 1947).
The Late Tertiary group of sediments attains a thickness of over 6,000 ft. in the area immediately east of the coalfield, the youngest marine member being Lower Pliocene (Waitotaran Stage). At the top of the younger succession, marine and littoral sediments give place to terrestrial conglomerates, sands and lignite, probably representing parts of the middle and upper Pliocene. Finally, Pleistocene and Recent alluvial gravels rest indiscriminately upon various older rocks.
The covering strata have been deformed to a degree varying in different parts of the area. Faulting and folding are most intense at the north end and along the east side of the coalfield, where Pliocene beds are tilted nearly vertically, and even Pleistocene terrace gravels are dislocated by renewed faulting along old lines. The axis of the asymmetrical Paparoa Anticline roughly follows the line of maximum thicknesses of the Upper Paparoa, Brunner, and Lower Marine Tertiary groups of sediments, and the flanks of the primary anticline are modified by subsidiary folds and faults. The thick belt of younger sediments east of the coalfield is broadly synclinal (Text Fig. 8).
The main topographic axis approximately coincides with the chief axis of elevation of the covering strata. The pattern of the main streams is consequent on this elevation, and is little influenced by the secondary structural elements.
in a well-defined belt. This strongly suggests a persistent, narrow, crustal downfold that progressively deepened from Late Cretaceous to Middle Oligocene. At first it received only lake and fluvial sediments, but the differential movements were superimposed upon slow regional subsidence that brought about a transgression by the sea in middle Eocene times (Bortonian Stage) and initiated the lower Tertiary marine phase of deposition in this area. When the thickness variations are considered in conjunction with the constitution and variations of grain-size of the formations, the history of the area during this period emerges as an interplay of minor marginal uplifts and accelerated sedimentation, rests and planation of marginal ridges, and continual deepening of the trough. These events are recorded in the trough sediments by alternations of deltaic or fluvial sands and gravel with lake or marine silts and muds. Times of transition from lake to delta sedimentation were particularly favourable to the accumulation of coal.
Differential depression of the trough continued throughout the period of slow, regional submergence; but the movements later became less strongly differential, and by the middle Oligocene were very slow and almost uniform. Then, as observed by Macpherson 8 most of the New Zealand area was beneath clear, shallow seas in which fine, lightcoloured, calcareous sediment was slowly accumulating.
The persistent trough that has been described conforms with Schuchert's concept of a geosyncline in all respects except that it is much narrower in relation to depth than the classic American examples. No limitations of magnitude that would exclude this trough from being classed as a geosyncline are known to the writer. It is, therefore, proposed to refer to it as the Paparoa Geosyncline, and to regard it as a secondary element within Macpherson's “western geosyncline.” 9
The total thickness of sediment in the deepest part at the stage of maximum differential depression amounted to 8,000 ft., and the width of the geosyncline, as judged from its present width with some allowance for subsequent crustal shortening, was about eleven miles. How far it extended to the south is unknown, for the equivalent formations in this direction are mainly subsurface, and the evidence is therefore fragmentary. The northwards extent was similarly uncertain, but Wellman 10 has demonstrated a similar feature sixteen miles north of the northern boundary of the coalfield that may be the northwards extension of the Paparoa Geosyncline. In that area, however, the lower marine Tertiary sedimentation continued until the Hutchinsonian Stage.
The whole sequence of events so far corresponds, in the general diastrophic history of the region, to the “initial geosynclinal phase” recognised by Macpherson. The mid-Eocene marine transgression he interprets as a flooding of the western geosyncline from the north. The recent studies in Westland and South-West Nelson, however, do not lend support to the view that Macpherson's “median land”
[Footnote] 8 Op. cit., pp. 13, 18.
[Footnote] 9 Op. cit., p. 14.
[Footnote] 10 Op. cit., 1947.
separating the castern and western geosynclines formed a land ridge at this time. Similar fine-grained and (after Kaiatan times) lightcoloured sediments represent the upper Eocene and Oligocene on both sides of the South Island, and so near to the alpine axis as to suggest continuity of the post-Bortonian beds and over-all submergence, or at least broad sea-ways connecting the areas by way of saddles in the median ridge. The median land is recognised as an axis of recurrent rising tendency. It is known to have been elevated repeatedly as a land ridge from mid-Tertiary onwards, and the divergent trend of the lower Paparoa rocks at Greymouth is not necessarily evidence against the existence at that carlier time of a north-east median ridge. Nevertheless, the writer at present prefers the simpler hypothesis of progressive westward migration of the shoreline from Canterbury to Westland between late Cretaceous and middle Eocene.
As Macpherson pointed out, merely mild orogenic impulses interrupt sedimentation from late Cretaceous to late Oligocene, such as those producing wedges of coarse clastics in otherwise mainly fine scdiments during middle Paparoa and Omotumotu times, and deepening of the troughs was the dominant process.
Basal conglomerates of the younger Tertiary succession around the borders of the coalfield include pebbles of the lower groups. It is, therefore, inferred that between the close of the Lower Marine Tertiary phase and the Altonian transgression, the Paparoa Geosyncline
began to be raised and its contents eroded, but there is no certainty that the axis of elevation formed a land ridge at this stage, and the erosion may have been entirely submarine. Elsewhere in Westland, however, there is evidence that the main alpine axis of the South Island, thirty miles farther east, was then beginning to appear as land. 11 Both the Paparoa and the main Alpine axes thereafter showed persistently rising tendencies.
The Altonian transgression was not a return to the earlier condition of regional sinking, for the subsequent history, as indicated mainly by Wellman's work in other parts of Westland and Nelson, tells of repeated marginal flooding of the rising orogenic belts, with interludes of erosion.
The transgression marked the initiation of a new trough, to the east of the partly inverted Paparoa Geosyncline, that subsided differentially, but with minor interruptions marked by faunal breaks, until it held more than 6,000ft. of sediments. The title of Grey Valley Geosyncline is proposed for this late Tertiary depositional trough which extended south beyond Kumara. Another area of thick late Tertiary beds may exist, west of the southern extension of the Paparoa axis.
Early in the Pliocene, there began a crescendo of mountainbuilding culminating in the Pleistocene as the Kaikoura Orogeny. 12 The Sothern Alps, the Paparoa Range, and the other tectonic mountains then attained the greater part of their present elevation, and erosion stripped them of most of what remained of the Cretaceous and lower Tertiary covering strata. The Grey River Geosyncline filled rapidly with abundant coarse debris from rising “highs” on either side, with the result that the sea was expelled from it after the Waitotaran Stage. The uppermost few hundred feet of sediment in this trough are littoral gravels and sands followed by terrestrial sand, gravel, and lignite, and even these have been substantially deformed during the crisis of the orogeny. Diminishing posthumous movements continued through the glacial epoch and until Recent times.
The anticlinal character of the Paparoa Range and the close coincidence of its structural and topographic axes are due to the lateness of the major elevation. Erosion denuded the comparatively soft younger sediments before appreciably eroding the more highly indurated early Tertiary and Cretaceous beds. As noted earlier, the present drainage pattern at the south end of the range is consequent upon the culminating uplift. The larger streams established their courses before the soft cover was removed, and are now superimposed upon the lower beds.
[Footnote] 11 The oldest Tertiary conglomerate in Westland known to contain pebbles of the distinctive Alpine schists is one of Altonian age at Koiterangi. Gage, M., and Wellman, H. W., “The Geology of Koiterangi Hill, Westland,” Trans, Roy. Soc. N.Z., vol. 73, 1944, pp. 359, 360.
[Footnote] 12 Cotton, C. A., loc. cit., 1916; “Some Peneplanations in Otago, Canterbury, and the North Island of New Zealand.” N.Z. Journ. Sci. and Tech., vol. 20, 1938, no. 1, pp. IB-8B.
In the deformation of the covering strata of the Paparoa fold range, both tensional and compressional features may be observed (Text Fig. 7). The compressional faults along the east side of the range have certainly been active recently, for they involve late Pliocene strata; but the age of the tensional faults on the broad westdipping flanks of the secondary folds is not so well known. These may have originated during the geosynclinal phase, for a number of normal faults interrupting coal-measures appear to die out when followed southwards, that is, in the direction of pitch of the Paparoa Anticline, and do not affect the higher part of the Kaiata siltstone or higher formations. These faults may, therefore, have originated in response to crustal tension during the phase of most rapid and most intensely differential subsidence during the Kaiatan Stage. Against this, when the deformation suffered by one particular bed is examined in different parts of the area, the intensity of deformation is seen to decrease southwards. Finally, the normal faults could all be late, the thick Kaiatan sediments having plastically absorbed the deformation, allowing the faults to die out upwards. The evidence is thus inconclusive. Although some normal faults may have moved first during the orogenic phase, others probably date back to the initial geosynclinal phase, for the formation of the geosyncline involved a certain amount of crustal lengthening, of which normal faulting would be the most probable mechanism.
In the later orogenic phases, uplift of the Paparoa Anticline as a whole proceeded concurrently with deepening of the Grey Valley Geosyncline, but the precise timing of events is unknown. Epicycles of alternating compression and tension may have been superimposed upon the dominating compressional trends of late Tertiary times that culminated in the compressional paroxysm of the Kaikoura Orogeny.
Raised beaches of late Pliocene or Pleistocene age at first sight give a false impression of uniformity of later uplifts, probably because the coastline is roughly parallel with the orogenic axis. Further study of them, and of their relation with inland geologic features is needed; but the earlier explanations of these features on a basis of uniform uplifts are likely to give way to more complicated mechanisms, as suggested already by Cotton's recent observations in South Westland. 13
The evolution of the Paparoa Geosyncline and its conversion to an anticline reflect various stages in the later diastrophic history of New Zealand, as outlined by Macpherson, who worked out his scheme with a detailed knowledge of the critical North Island east coast areas and a broad comprehension of New Zealand geology, but without access to the data presented in this paper. It is pleasing, therefore, to find a good general accordance when his hypothesis is checked against information yielded by recent studies in the equally important South Island west coast Cretaceous and Tertiary sequences. The Paparoa tectonic element is one of the secondary folds that Macpherson recognizes on the concave side of the South Island segment of the growing New Zealand arc. While not inconsistent
[Footnote] 13 Cotton, C. A. “The Alpine Fault of the South Island of New Zealand from the Air.” Trans. Roy. Soc. N.Z., vol. 75, 1947, p. 372.
with his belief that the present-day ridges of basement originated in the late Cretaceous, persisted thereafter as continually upwardtrending potential highs, and were most vigorously elevated between early Miocene. and late Pliocene times, the Greymouth studies do not support repeated appearance of the median ridge as land throughout this long period. Macpherson recognizes at least one period. between late Eocene and late Oligocene times, when the sea probably extended over the whole country. The main axis may have experienced earlier elevations, but no trace of them has so far been recognized on the Westland side, and the distribution of the coarse clastics in the lowest post-Palaeozoic strata is better explained on the basis of a pre-Paparoa north-west trend.
Even when regarded as a secondary element superimposed upon Macpherson's western geosyncline, the narrowness of the Paparoa folded belt in relation to its thickness implies a higher degree of crustal mobility than is usually allowed, and it is difficult to reconcile this narrow tectonic unit with the idea of geosynclinal collapse following prolonged isostatic subsidence under load. 14 The Paparoa Geosyncline was only eleven miles wide, compared with the accepted minimum width of fifty miles for elements capable of isostatic adjustment, but it seems inadvisable to apply isostatic concepts to such a small tectonic feature.
Bucher 15 discussed the zonal migration of geosynclines within the large heterogeneous mobile belts. The development of the Grey Valley Geosyncline, alongside and parallel with the earlier Paparoa Geosyncline, illustrates that the same process can involve much smaller units.
Before concluding, the writer wishes to thank the Director of the New Zealand Geological Survey for permission to use in this papermatter abstracted from the manuscript of a forthcoming Survey publication, and also Mr. A. W. Hampton, Chief Draughtsman of the Geological Survey for arranging to have the text-figures re-drawn for publication.
[Footnote] 14 E.g., Lawson, A. C. “The Isostasy of Large Deltas.” Bull. Geol Soc. Ain., vol. 49, 1938, pp. 401–416.
[Footnote] 15 Bucher, W. H. “The Deformation of the Earth's Crust,” p. 373, Princeton, 1933.