Geological Reconnaissance of District Between Franz Josef Glacier and Copland Valley
[Read before Geology Section. Royal Society Congress, Auckland. May, 1954; received by Editor, July 12, 1954.]
A map shows the approximate boundaries of metamorphine zones, ranging from Chlorite 1 to Garnet, which are aligned approximately parallel to the principal chain of the New Zealand Alps between Mt. Sefton and Mt. de la Bêche. Metamorphism increases from east to west. The isograds appear to be generally coincident with planes of stratification and the latter can be identified in the highest metamorphic grades. Microfolds become conspicuous in rocks of Chlorite 4 and higher grades. Petrographic detail is not included in this paper.
Although several writers have referred to the geology of the central portion of the Southern Alps and quite a number of papers discuss the glaciers and ancient glaciation, descriptions of the “solid” geology are few. Haast (1871) wrote a short description of the most common lithologic types and the dominant structures, and drew a section (on a very small scale) which passes through Mt. Sefton, Sealy Peak and Lake Tekapo. A report by Cox (1877) describes briefly the geology of the region lying between Reefton and Bruce Bay and extending as far east as the western flanks of Mt. Cook. That report is accompanied by a map and some rough sections including one from Mt. Cook to the West. Coast.
The district north of the Central Alps has been more thoroughly described in reports which are informative concerning the whole Alpine region because of its regular structural trends. McKay's reports and maps of 1893 and 1894, both very similar, were followed by the Hokitika bulletin of Bell and Fraser (1906) and by the Mikonui bulletin of Morgan (1908). The latter writers draw detailed sections illustrating within the schist terrains folds with very steep, and in places overturned, limbs. Park, in his “Geology of New Zealand” (1910), also draws several sections showing overturned anticlines and synclines in the massif of Mt. Cook and other ranges east of it, but he gives no clear description of these folds. The sequence of the Harper's Pass region has recently been described by Wellman, Grindley and Munden (1952) whose paper, although concerning a district some hundred miles to the north, is of special interest because it introduces more stratigraphical and structural precision than the work of earlier writers. The line of the Alpine Fault has been mapped in several papers by Wellman and Willett (1942, 1951) and from their data it has been sketched on our map.
The present paper summarizes the results of a brief visit in January, 1954, whose chief object was to examine and collect metamorphic rocks. Examination of the geology was largely confined to the lines of three traverses and the geology of adjoining tracts of country was usually only observed at a distance. These three traverses were from the Hooker Glacier over the Copland Pass to the road bridge over the Karangarua River, up the Fox Glacier to Newton Pass, and from there down the Franz Josef Glacier to the terminal face. In that portion of the
Copland Valley below Scott Creek clear exposures were lacking, but specimens were collected from very large blocks and boulders of rock regarded as more or less in situ and as representative of the immediately surrounding country rock. Elsewhere specimens were taken from outcrops.
The Alpine Fault is parallel to the general strike of the strata which, as noted by earlier writers, lies between 20° and 40° east of north. Similarly we confirm that that the beds which generally show steep dips between 60° and 90° are of highest metamorphic grade to the west and become of progressively lower grade to the east. Cox, for the region under discussion, divides the metamorphic rocks into a lover and gneissic portion exposed at the foot of the Fox Glacier, a middle portion dominantly of mica schist and an upper and eastern portion dominantly of chlorite schist. McKay, who also shows this group of strata dipping eastwards, and therefore in ascending vertical succession, as well as in order of decreasing metamorphism eastwards, divides these strata more definitely on his map of the region between the Mikonui River and Arthur's Pass Bell and Fraser and Morgan confirm this metamorphic sequence. The maps and sections by Well-man, Grindley and Munden employ a subdivision along similar lines. The latter writers, as well as McKay, note the passage towards the east, and ascending the stratigraphic succession, of the chlorite schists into less metamorphosed strata— the Carboniferous (Maitai) of McKay and the subschists of Wellman et al, who have found Triassic fossils in the upper part of these beds. Cox also had mapped Mt. Cook and parts of its western flanks as composed of Maitai rocks, generally
Fig. 2.—Microfold in chlorite schist, south wall of Copland Vally south of jungle Creek Ptygmate tolds formed from sandstone bands and quartz vens cuttinh across bedding
Fig. :3—Microfold in quartz brothe [ unclear: ] t, west wall of Franz Joset [ unclear: ] same locates as Fig. 4 Anal plane cleavage (on S2) coincident with [ unclear: ] bedding surprises on sleeper [ unclear: ]
Fig. 4—Quartz-brotite schist, west wall of Franz Joset Glacier, hall a mile north west of Defrance Hut.
Fig. 5.—Crest of microfold in quartz-biotite schist. Thin section from rock shown in Fig. 4. Marked crystallization of biotite along axial-plane cleavages (S2) oblique to relict sedimentary banding (S1) All terms of passage visible between [ unclear: ] cleavage and true cleavage.
blue silky slates and sandstones, and Haast, in discussing the indurated sandstones and shales of the Mount Cook region and the schistose rocks west of it, had earlier noted the increase of metamorphic grade to the west of the Moorehouse Range.*
Both Cox (loc. cit., p. 77, Section H-H) and von Haast (loc. cit., p. 22, 23) suggest, although they describe no clear contacts, that within this field there is unconformity between the metamorphic and the less altered rocks. To the writers the evidence of unconformity is inadequate, and the idea of unbroken passage seems preferable All the other writers who have worked in adjoining regions have been unable to trace any break in the sequence.
On the western edge of the field a steep regional dip to the east is confirmed by Cox, McKay, and by Haast in the western portion of his section between the West Coast and Sefton Peak it is doubtless on this disposition of the strata that Haast (loc. cit., p. 23) says “the whole appears as the eastern wing of a huge anticlinal arrangement”. This anticline must have had the most metamorphosed rocks in its core and, if it did exist, it must have had its wing broken off by the Alpine Fault. Haast's text (p. 23) is a little obscure on this point, especially as lie stresses the westward dip of the rocks on the Moorehouse Range and yet notes that “going westerly, and in descending order” the rocks are of increasingly metamorphosed nature. However, his section indicates a synclinal axis passing through Mt. Sefton—a synclinal axis which would appear to reconcile his different verbal observations and to place the most metamorphosed rocks to the west near the core of the inferred anticline.
Our map shows the distribution of the metamorphic zones, each of which grades insensibly and perpendicular to the strike into its neighbour. The rocks range in grade from unmetamorphosed sediments on the east flanks of the Main Divide to garnetiferous biotite schists and gneisses towards the west. As most of the rocks outcropping west of the first appearance of garnet are garnetiferous they may represent a distinct metamorphic zone although observations in other parts of Westland (Turner, 1933; Mason and Taylor, in press) show that garnet does not always appear in rocks of this grade. The few detailed observations which follow show why we consider the schistosity planes to coincide nearly everywhere with bedding planes with only local and small departure from this coincidence.
The slopes of the Main Divide over-looking the Hooker Glacier consist dominantly of indurated mudstones, siltstones and fine sandstones with coarser bands subordinate in volume. The siltstones are more siliceous and of lighter colour than the mudstones, and both alternate in beds of varying thickness. In places bands of fine quartzose siltstone form regular laminae only a few millimetres thick within the mudstones giving a very finely-ribboned appearance to the strata. These beds break with a flaggy fracture parallel to the very flat bedding surfaces and show little signs of faulting or shearing. Quartz veins are inconspicuous, and though the rocks can hardly be called metamorphic they may perhaps be compared to those of the Chlorite 1 zone of Otago. These strata show a dip at 60° to 70° on a bearing of 320° and evidently strike in the direction of Mt. Cook where distinct banding, inferred to be stratification, is visible at a distance. Park (p. 63) draws a view of the west wall of Mt. Cook which shows the strata there
[Footnote] * The Moorehouse Range is an old name for that part of the Main Divide lying between La Perouse and Mt. Sefton.
cut by a fault. A distant view from the Hooker Hut suggests that a sharp syncline with both limbs dipping steeply west underlies the summit of Mt. Cook.
Because of the consistency of strike it seems likely that all the Moorehouse Range south and south-east of the Copland Pass is composed of similar sedimentary rocks and most unlikely that any kyanite schist occurs there. (An excellent specimen of kyanite schist in the Canterbury Museum bears an identification label giving the Moorehouse Range as the locality of collection, and this specimen is referred to by Morgan (1927) in a list of minerals collected in New-Zealand.)
A cursory examination of the strata on the ascent from the hut to the Copland Pass indicates that these westward dipping strata are not inverted but unfortunately sufficient observations have not been made over the whole field to confirm the direction of “younging”. Proceeding westwards from the Copland Pass fissility becomes gradually more marked. Finely banded phyllites, containing numerous quartz veins and stringers, are well exposed about two miles east of the Douglas Rock Hut, indicating that the Chlorite 2 zone has been entered. On the path three-quarters of a mile north-west of this hut and immediately south of Jungle Creek the outcropping quartz-chlorite schists, in places containing abundant crystals of magnetite, are ascribed to the Chlorite 4 zone. They are markedly foliated (as defined by Harker, “Metamorphism”, 2nd edition, p. 203), with strong lineations and are lustrous with abundant chlorite. They still show a finely-ribboned appearance parallel to schistosity planes and the most siliceous laminae now appear as thin quartz layers. Quartz veins are now very abundant, particularly aligned along schistosity planes. Although the latter planes are still generally coincident with bedding planes, dipping steeply west-north-west, some of the fine laminae are locally much crumpled to form very sharp up folds and downfolds, of an amplitude of some two to three feet and wave length of about a foot, which themselves bear even smaller drag-folds. Concentrated near the crests and troughs of these folds comparatively wide and contorted lensoid quartz layers of concertina-like outline (ptygmatic folds) mark the recrystallised relicts of sandstone beds whose material had moved towards the crests and troughs. The thin and flatter quartz veins aligned along the axial planes appear to have been largely fed from such crestal segregations.
On closer examination of sections showing such folds two sets of schistosity surfaces ire visible, one parallel to the contorted bedding laminae and the other parallel to axial planes, the latter in places giving abundant evidence of minute displacements (false cleavage as defined by Harker). At such places the “split” is markedly more hackly and less perfect than on those flanks which show coincidence of both schistosity surfaces :the rock may break more readily along planes parallel to the axial planes at one place; in another it breaks more readily along planes determined by the bedding schistosity. Ignoring the variable strikes and dips of laminae on the flanks of these small folds, the general strike of the strata and axial planes at this locality is approximately south-south-west with a westward dip of 75° to 80°. The lineation formed by the intersection of the two surfaces is particularly marked and plunges nearly south-south-west at an inclination of 10°. This detailed description covers other contorted beds examined elsewhere in the region. Wellman and others (p. 221) found similar rocks, but here we would emphasize that the most sinuous quartz segregations appear to mark relict disrupted beds.
Photographs from the summits of Mt. Copland (Dr. J. Rattenbury) confirm that the west-north-west dip continues as a regional element for some distance north of the Copland Valley.
At Splintery Creek, opposite the mouth of the Ruera River, quartz-biotite schists occur in loose blocks, the biotite tending to porphyroblastic development: the steep regional dip as seen in distant view appears to be still towards the west, but rocks in situ were not examined to confirm this. From Welcome Flat Hut westwards no rocks were collected in place. The abundant blocks in the bed of Shields Creek, one mile west of this hut, consists of finely banded quartz-biotitc schist and coarser gneisses of similar mineral composition, both containing many crystals of garnet up to 2 mm. in diameter. Interbanded with these are lighter coloured very quartzose hornblende-garnet gneisses, the hornblende occurring as conspicuous needles. Despite the higher grade of metamorphism the minute laminae similar to those of the unmetamorphosed sediments are still conspicuous in the schists and are parallel to the schistosity planes. Also abundant in the material of separate blocks are fine-grained silky and dark green rocks composed almost entirely of amphibole and epidote.
Beyond this point the most interesting locality is Architect Creek, in which occur abundant blocks of garnetiferous biotite schist and gneiss. The garnet porphyroblasts commonly attain a diameter of ¾ in. and show evidences of rotation and fracture. Some of the garnets have displaced the overlying and underlying quartz layers whilst other crystals are sliced by thin quartz bands and the portions laterally displaced along the shear planes which are themselves parallel to relict stratification planes. Beyond this point no clear structure was observed although a view up the Karangarua Valley suggested that the dip was to the south-south-west.
On the walls of the Fox and Franz Josef Glaciers exposures are better and the rocks may be described more briefly by comparison with those in the Copland Valley. At the foot of the Fox Glacier biotite-garnet schists dip south-east at an inclination between 80° and vertical. Further up the Fox Glacier, on the south wall one mile east of Boyd Creek, quartz-biotite schists accompanied by epidote-hornblende-quartz schists show corrugations and minute folds with the Same deformational features observed in detail in the Copland Valley. Again, despite the obliquity of certain axial plane cleavages or schistosity surfaces and the relict stratification in crestal sections of small folds, most of the exposures show coincidence of the two planes on a larger scale. The structure here is obscure, and it is not clear whether an apparent complexity is real or is due to creep on the steep slopes. On the north wall above the outlet of the Victoria Glacier the biotite schists still occur, striking regularly on a 33° bearing and dipping south-east at 70°; but beyond this point chlorite schists follow them with a dip nearly vertical. Between the east end of the Chancellor Ridge and Newton Pass all the exposed strata are flaggy argillites and sandstones which in metamorphic grade cannot exceed Chlorite 2 and are mostly of Chlorite 1 grade. The dip of the strata east of Chancellor Ridge is dominantly westward and appears to be consistently so over a considerable distance to the south-south-east and north-north-west.
At the Franz Josef Glacier biotite schists only appear west of the Carrel Glacier and the boundaries of the metamorphic zones follow the regional strike from the Fox Glacier, the bedding still dipping steeply to the south-east Small folds, in general very similar to those described in the Copland Valley, appear
in the biotite schists which outcrop on the south wall of the Franz Josef about half-a-mile north-west of the Defiance Hut. The axial planes dip at 60° on a bearing of 140°, and the axial plunge is about 45° on a bearing of 192°, and parallel to a lineation formed by the intersection of axial-plane and bedding schistosity. These strata show extremely fine bedding-laminae and also bands and boudins of coarser quartz. The general strike of the uncontorted strata coincides with that of the axial planes and going towards the end of the glacier the dip steepens and approaches the vertical. Garnets first appear in outcrops along the Waiho River and are abundant at the bridge over the Callery gorge.
It would seem from observations at both glaciers that the stretch of country along which biotite schists are replaced by less metamorphosed rocks is approximately coincident with a change of steep regional dip from east to west; there is no sign of any major displacement but rather an appearance of passage. Observations of direction of “younging” which have not. been made in this hurried reconnaissance might answer some of the queries which the field relations raise: if the small scale corrugations are part of a more complicated regional fold pattern such observations may be necessary at very close intervals. On the other hand, the small folds may well be only evidences of local incompetence in strata folded on a fairly simple regional pattern : it would seem that in rocks that have attained a certain degree of metamorphism microfolds do become more prevalent.
On this latter working hypothesis and assuming furthermore that increasing metamorphism is likely to reflect an originally greater stratigraphic depth a major syncline may intervene between the east-dipping schists and the lower grade schists of the Main Divide. To locate the position of the hypothetical axial plane on the map would evidently require more detailed studies, for parts of the west flank of this syncline could well consist of overturned strata. Probably considerations of this sort led Haast to draw a syncline through Mt. Sefton.
Bell, J. M. and Fraser, C., 1906. The Geology of the IIokitika Sheet. North Westland Quadrangle. N. Z.G.S. Bull, No. 1.
Cox, S. H., 1877. Report on Westland District. Rep Geol. Explor. during 1874–6. No. 9. pp. 63–93.
Haast, Julius, Von, 1871. Notes on the Geology of the Central Portion of the Southern Alps. including Mount Cook. Rep. Geol. Explor. during 1870–71. No. 6. p. 19–25.
Mckay, Alexander, 1893. Geological Explorations of the Northern Part of Westland pp. 132–186. Mines Statement. N.Z. Dept. of Mines.
— 1894. On the Geology of the Northern Part of Westland, and the Gold-bearing Drifts between the Teremakau and Mikonui Rivers. Rep. Oral. Explor. N.Z. Geol. Surv. during 1892–93. No. 22. pp. 11–50.
Morgan, P. G., 1908. The Geology of the Mikonui Subdivision, North Westland N.Z. G. S. Bull, No. 32
— 1927. Minerals and Mineral Substances of New Zealand. N.Z. G. S. Bull. No 6.
Park, James, 1910. The Geology of New Zealand. Whitcombe & Tombs Ltd, Christchurch.
Turner, F. J., 1933. The Metamorphic and Intrusive Rocks of Southern Westland. Part II. Trans. Roy. Soc. N.Z., Vol. 63, Pt 3. pp. 237–284.
Wellman, H., 1951. The Geology of Bruce Bay—Haast River. South Westland. N.Z.G.S. Bull., No. 48.
— and Willett, R. W. The Geology of the West Coast horn Abut Head to Milford Sound. Part 1. (1942). Trans. Roy. Soc. N. Z., Vol. 72, Part 4. pp. 286–306.
— 1942. Ibid. Part 2—Glaciation. Trans. Roy. Soc. N. Z., Vol. 72. Part 3. pp. 199–219.
— Grindley, G. W. and Munden, F. W., 1952. The Alpine Schists and the Upper Triassic of Harper's Pass (Sheet S 52), South Island, New Zealand, Trans, Roy Soc, N.Z., Vol. 80, Part 2, pp. 213–227.