Evolution of the Stream Pattern
Four distinct elements are recognizable in the present drainage pattern. They are:
The trunk river system of the Grey.
Superposed consequents. Secondary streams and parts of streams descending the flanks of the Paparoa anticlinorium in directions essentially normal to its general contours.
Subsequent elements. Two groups of secondary streams adjusted to the structure of the covering strata.
Insequents. Minor tributaries uninfluenced by structure.
1. The Trunk River System
The history of the trunk river system begins with emergence of the Paparoa axis as a permanent ridge of land, and expulsion of the sea from the geosyncline to the east. Deltas of streams extending westwards from the rising Alps would eventually have united with those advancing eastwards from the Paparoa Range. Their combined aggradational products filled the depression, which had not yet ceased to subside. The birth of the river system in the depression thus dates from the integration of drainage from both sides into trunk streams running parallel with its axis. We are not concerned here with the portion now occupied by the Inangahua Valley, but it should be noted that the divide between Grey and Buller drainage within the depression has not always been where it is now, that is, at the Reefton Saddle.
The earliest date for the inauguration of the Grey system is fixed by lower Pliocene (Waitotaran) fossils in the highest marine sediments of the depression. On the other hand, the overlying terrestrial gravels, though deformed by the most recent strongly differential earth movements, need not be older than early Pleistocene, for there is evidence elsewhere in Westland* showing that these movements were still going on after the first phase of glacier advance.
[Footnote] * Gage, M., “The Tertiary and Quaternary History of Ross, Westland,” Trans. Roy. Soc. N.Z., vol. 75, pt. 2, pp. 138–159, 1945.
As vigorous differential movements declined, the supply of waste would have diminished until the rivers could commence the dissection of the constructional surface over which they flowed, but we have not the information to decide whether the cycle of erosion thus introduced led directly to the planation of the lowlands, or whether other cycles intervened. Nor have we much chance of tracing the shifting courses of the main rivers during the planation period. But since the beginning of the phase of progressive uniform emergence, the Grey seems to have remained fixed in position, and to have become superposed upon the hard core of the Paparoa anticlinorium at the Brunner Gorge, where there is a structural saddle and a group of large faults transverse to the axis of the range. The position of the coastline would have fluctuated to some extent during the intermittent emergence, yet it can at no stage have been more than a very few miles west of its present position, for, during at least two of the intervening halts, marine erosion was successful in cutting back to a position east of the present shore.
To sum up, the trunk river system was (a) consequent upon the final constructional surface in the structural depression; (b) antecedent to the later deformational movements involved in the uprise of the Paparoa Range; (c) superposed upon folded structures that had been impressed upon the covering strata during earlier phases of the uplift.
2. Superposed Consequent Elements
Certain elements still discernible in the present stream pattern (Fig. 2) seem to reflect an earlier system of streams flowing directly down the general slope of the range, disregarding prominent structural features transverse to their courses. These are interpreted as descendants of a system of consequents initiated during the rapid culmination of uplift of the range, and they present a problem because the subsidiary structures of the anticlinorium that they disregard had probably been in existence from an early stage in its history.* In explanation, it is suggested that the influence of structural detail upon the pattern of the developing streams was unusually delayed because the anticlinorium was at first covered by a thick mantle of uniform lower Tertiary marine sediments. The Paparoa upfold, it will be recalled, coincides with the position of the earlier geosynclinal trough, which contained more than 5,000 ft. of Kaiata Siltstone in its deepest part. This formation is now folded and faulted in conformity with the lower members of the covering strata, but because its variations in hardness and texture occur gradually through thousands of feet of thickness, the secondary fold structures could have had no expression in the scenery so long as the Kaiata mantle survived. Thus an initial simple consequent pattern became well established while accelerating differential uplift was converting a surface of marine planation into a rising land ridge.
[Footnote] * The evidence for the age of these structures is presented elsewhere—“The Geology of the Greymouth Coalfield.” N.Z. Geol. Surv. Bull. 45 (in press), by M. Gage.
As the ridge gained in altitude and degree ot” relief, the consequents would have become deeply incised and their courses firmly stabilized. Insequent tributaries would have been dissecting the interfluves, but without threat to the main consequent pattern. It may be inferred from the strong survival of this pattern that when vertical corrasion by the consequents at length exposed harder and more variable lower rocks where they cut across transverse “highs” of the secondary folds, they became superposed upon them. In this way the consequent pattern became heavily inscribed upon the lower covering strata and basement rocks. There is resemblance with the development of a radial drainage pattern in the English Lake District, as described by J. E. Marr in these words:
“The district may be compared to a…tablet covered with wax, through which lines were impressed by a style, and after the removal of the wax a fresh set of lines has been engraved on the actual surface of tablet, which however has not completely obliterated the earlier set.”*
3. Subsequent Elements
With the climax of differential uplift past, and the rate of differential uplift diminishing, downcutting by the streams would have given place to valley-widening processes, widely exposing the harder cores of anticlines. The extensive dip-slopes and structural terraces of Island Sandstone and Brunner beds remaining to-day attest to the erosion-resisting qualities of these formations compared with the strata above them. The anticlinal cores of lower, harder members of the covering strata thus emerged as ridges, resisting dissection, as the general level of the land was lowered. Stream development thenceforwards would have been influenced by the north-south trend of the secondary folds and their associated fault-zones, and thus arose the prominent subsequent elements of which Spring Creek, parts of Seven Mile Stream, Coal Creek and other streams are the modern descendants (Fig. 3), following synclinal belts that at an earlier stage were occupied by strips of softer rocks and belts of rock weakened by faulting and shearing.
It is part of the doctrine of the geomorphic cycle that in maturity, adjustment of the stream pattern to accord with the underlying structure should take place by way of captures, with derangement of the original consequent pattern. In the southern Paparoa Range, there are a number of separate but noticeably co-linear elements now forming parts of different streams, and sharp bends having the appearance of elbows of capture, involving both consequent and subsequent streams. But the surprising feature is that so much of the original consequent pattern remains, considering the magnitude of the secondary fold system and the marked variation of hardness of the rocks now exposed at the surface. To explain this, it seems possible to suggest only the influence of the thick Kaiata beds during the engraving process described above.
[Footnote] * “The Influence of the Geological Structure of the English Lakeland upon its Present Features,” Quart, Journ. Geol. Soc., vol. 62, p. lxvii, 1906.
The time of appearance of the subsequent pattern on the range itself is uncertain, but the second set of subsequents at lower levels, including Kaiata and Omotumotu streams and the lower reaches of Coal Creek and Seven Mile Stream, are probably more recent. They have been excavated beneath the extensive planation surface during the final phase of regional emergence, and are guided by weaker members of the covering strata (Kaiata Siltstone, Port Elizabeth Mudstone), and separated by strike-ridges supported by harder formations (Cobden Limestone, Omotumotu beds). This seems the only tenable explanation of the conspicuous strike valley east of the Twelve Apostles Ridge. The curvature of these subsequent elements in the present landscape demonstrates the southward continuation and plunge of the Paparoa anticline.
Adjustment to structure is less pronounced on the eastern slopes of the range, despite several major faults and belts of intensely crushed rock. The only notable subsequent streams are: part of Ford Creek, parallel with a major fault; Blackball Creek, following the lower contact of the covering strata for some distance; Blackwater and Bray creeks, influenced for a distance by alternating hard and soft bands in the Omotumotu beds; and a branch of Soldier Creek, following a mudstone band. Otherwise, all but the smallest and youngest tributary rills disregard structural features, and there is only one suggestion of an important capture.
These differences between east and west slopes of the range are ascribed to the fact that the severest deformation, with overturning and intense shearing, and the most recent posthumous movements, occurred along the eastern flank of the anticlinorium. Downstream tilting has affected the east-flowing streams more recently and more noticeably, and being thus continually rejuvenated, they have failed to attain maturity or to become adjusted to structure. It is convenient to note here that between the main streams on this side, sloping flattopped interfluves are underlain by coarse poorly sorted and poorly rounded angular debris resting unconformably on strongly deformed Tertiary and older strata forming the east flank of the anticlinorium. They appear to be remnants of a once continuous piedmont alluvial apron, beneath which the modern streams are deeply entrenched and superposed upon the underlying rocks. The origin of the coalesced fans is probably connected with the latest pulse of differential elevation, which through accompanying seismic activity and accelerated erosion would have led to aggradation and fan-building at lower levels. Alternatively, the accelerated erosion may have followed the disappearance of an earlier forest cover during severely cold conditions in the Pleistocene. The present streams must be regarded as descendants of consequents at one time flowing down the constructional surface of the piedmont apron.
The upper reaches of Blackball and Ten Mile creeks and the large tributary of the latter, Otto Creek, all cut across the north-westerly trend of the Palaeozoic rocks in which their valleys are now carved, but. run parallel with the range. These streams are interpreted as superposed subsequents, having long ago adjusted their courses to the structure of covering strata now entirely removed.
Fig. 1–Stream map of the Greymouth district. 1, Otto Creek; 2, Ten Mile Stream; 3, Seven Mile Stream; 4, Coal Creek; 5, Grey River; 6, Sawyer Creek; 7, Omotumotu Creek; 8, Kaiata Creek; 9, Card Creek; 10, Stillwater Creek; 11, Arnold River; 12, Bray Creek; 13, Blackwater Creek; 14, Ford Creek 15, Blackball Creek; A, crest of Paparoa Range; B, Twelve Apostles or Rapahoe Range; C, Kaiata Range; D, unnamed strike-ridge of Omotumotu beds.
Fig. 4–View from the upper slopes of the Paparoa Range into the upper valley of Coal Creek. The prominent valley towards the right is the Seven Mile Stream. The observer is thus looking down two valleys inheriting their direction from streams consequent on the initial upheaval of the range. Transverse subsequent stream elements and anticlinal ridges are also visible. In the distance, note the wide strike-valley, the even crest of the Twelve Apostles Range, and the present outlet of the Grey at upper left.