A Geological Traverse from the Waitaki River to Dunstan Peak, Otago
[Read before the Otago Institute November, 1934; received by the Editor, July 5, 1935; issued separately, September, 1936.]
Commencing at a point about five miles upstream from the township of Kurow, on the Waitaki River, the traverse described in the following pages extends across Northern into Central Otago, and terminates on the north-eastern end of the Dunstan range, viz., at Dunstan Peak. It crosses five mountain-ranges which, in order, from the north-eastern end are the St. Mary Range (6000 ft.), two branches of the Hawkdun Range (5500 ft.), the St. Bathan Range (6000 ft.), and the Dunstan Mts. (5000 ft.).
The tops of the ranges (above 4500 ft.) are littered with broken rock, while the lower slopes are protected by the ubiquitous tussockgrass which forms practically the only vegetation. With the exception of the main river valley (Waitaki and Manuherikia) the country studied is very steep and rugged, devoid of habitation, and populated only by a few shepherds in the sheep-mustering season.
Although the total length of the traverse (as the crow flies) is no more than forty miles, some seventy miles in all were followed in order to collect sufficient data for a connected account of the whole traverse route. The field-work occupied about six weeks in all, and was carried out at intervals during 1934.
Excluding reports on the Tertiary rocks of the Waitaki Valley, every little work on the older folded strata to the south-west appears to have been done. Hutton and Ulrich (1875) in their book Report on the Geology and Goldfields of Otago give a brief account of the regional geology, with but few details. In 1881 McKay, reporting on the Vincent and Lake Counties for the Geological Survey, described a section in the same general direction as that illustrated in this paper, but lying about fifteen to thirty miles further north. He was unable to carry out any detailed petrographical work, and his conclusions were based on field-observations only. The section described herein is more complicated than that illustrated by McKay, and the writer makes no attempt to correlate (as does McKay) the rocks considered with those of other districts.
Professor James Park (1903) published a paper in which he described the section from the Waitaki River to the summit of Mt. St. Mary; he assigned a Permo-Carboniferous age to the upper members of this series on the evidence of certain included fossils which, however, were later shown by Trechman (1917) to be typical of the New Zealand Trias. The only other geological investigation
was in connection with a soil-survey carried out by the late Dr H. T. Ferrar, the results of which were published in 1929. This covered the St. Bathans and part of the Turnagain Survey Districts near the south-west end of the section.
Data Obtained and Method of Treatment.
Wherever possible along the line of traverse, dip and strike measures were taken, not only of bedding and schistosity planes, but also of the joints and veins which traverse the rocks. A study of the relative positions of the bedding-planes (which, in almost every case, coincide with the planes of schistosity where the latter are developed), supplemented to some extent by microscopical investigation, reveals the structure and extent of folding. The joints and veins fall into a number of well-defined systems whose relations will be discussed later.
For convenience of description the traverse has been divided into three parts, the first comprising the region of green greywackeschists, phyllites, conglomerates, etc., of the north-eastern flank of Mt. St. Mary; the second the long sequence of contorted, faulted, and sometimes reconstituted greywackes, slates, etc., lying between the Mt. St. Mary Range and the St. Bathans Range; and the third the rocks between the St. Bathans Range and the Dunstan Mts.
Any petrographic correlation between the various members is rendered difficult by variation in the amount of reconstitution induced by folding and faulting at different points. A rock-type may thus have its original characters almost entirely obscured within a short distance, while the general uniformity of the unaltered types themselves also increases the difficulty of correlation.
The number of specimens collected averages about three per mile. All these have been studied microscopically.
A.—Waitaki River to Summit Mt. St. Mary Range.
This portion of the traverse was taken from the most part in the bed of the Awahokomo Creek, which flows into the Waitaki River some five miles upstream from Kurow Township. The older rocks near the main river are obscured by Recent stream-deposits and by Tertiary beds of coal-measures, sandy limestone, etc., but are exposed in the Awahokomo creek-bed, not far from the McAuley homestead, which is about three and a-quarter miles upstream from the junction.
In order to obtain specimens from as near the Waitaki as possible, the hill between the Awahokomo and Wharekuri Creeks was examined, but, in the absence of outcrops, some chips had to be taken from angular surface-boulders which could hardly have moved any great distance from the original position of outcrop. The two sections (2166, 2167) cut from these are partly reconstituted greywackes, which are only moderately sheared. They are made up mostly of angular to subangular quartz and feldspar fragments with interstitial much-finer-grained recrystallised material consisting of quartz, feldspar (probably albite), sericite, and clinozoisite.
Twisted muscovite flakes, chlorite arising from the breaking-up of biotite, actinolite prisms (probably from original hornblende), brown stilpnomelane and epidote are present; accessories include clastic granular sphene, ilmenite and some carbonaceous matter.
The first rocks in situ which appear along our traverse are in the creek-bed not far from the McAuley homestead, and are green greywacke-schists dipping steeply (85°) to the south-west. These greywacke-schists, with interlaminated green and purple phyllites, continue right to the summit of the range, where there are several bands of conglomerate which are associated with the fossiliferous Triassic rocks. The strike is fairly uniform (315°), but the dip is variable, becoming flat (27°) at the junction of the first large tributary, about one and a-quarter miles above the homestead. Between this point and about a quarter of a miles above the homestead there are marked variations in the dip and strike, and contortions are to be seen in several places. A thrust-plane (discussed later) is believed by the writer to cross the creek between these two points. Dips of 40° to 50° are common in the upper reaches of the Awahokomo, but they become steeper (60°) on the summit. Several of the more coarsely grained strata in this series are massive rather than schistose, but on the whole the rocks have a semi-schistose to slabby macroscopic structure.
Section 2185 may be taken as typical of the green greywackeschists (called by McKay the “Kurow Schist”). These rocks are all very similar in character and mineral-content, and differ in grainsize only. About 20% of the rock is made up of rounded strained quartz porphyroclasts up to ⅓ mm. in diameter, together with one or two almost totally recrystallised feldspar fragments. These are enveloped in a reconstituted sheared groundmass of quartz and albite with plentiful chlorite and epidote and a little sphene. No. 2185, however, differs from the other rocks of this series in the presence of a number of small pleochroic blue prisms of tourmaline, which appear to have grown in place. There is no definite evidence to show whether the mineral is due to the action of magmatic emanations, to the presence of boron in the parent sediment (cf. Goldschmidt and Peters), or to recrystallisation of clastic tourmaline.
The Triassic conglomerate occurring at the summit of the range does not seem to contain any igneous rocks. Flattened pebbles, consisting of a variety of types of sedimentary rocks–sheared quartzo-feldspathic sandstones, greywackes of varying grainsize and different degrees of reconstitution, and phyllites–are set in a matrix of subangular strained quartzes and altered feldspars, with plentiful stilpnomelane (pleochroic from golden to golden-brown to deep reddish-brown) in fibrous masses and clusters of slender needles. Other minerals are epidote, sericite, sphene, ilmenite, carbonaceous matter, chlorited biotite, and perhaps zoisite. The conglomerate, as a whole, breaks into flat slabs with a definite fissility parallel to the bedding-plane. The fossils found hereabouts are frequently so distorted as to be unrecognisable.
B.—Summit St. Mary Range to Summit St. Bathans Range.
Between these two localities seventy-six specimens were taken and thin sections thereof examined. A full description of each of these is unnecessary since they are all very similar in mineralogical constitution and structure. The descriptions are therefore presented in tabular form with an accompanying summary of (1) the minerals present, and (2) the classes of rock represented (with regard to the amount of reconstitution).
Characteristics of Minerals Present.
Quartz.—This is perhaps the commonest mineral seen in these rocks. It occurs in two forms: (a) angular to subangular or rounded clastic fragments, usually quite clear, but sometimes exhibiting small rows of inclusions; strain-shadows are almost always observable between crossed nicols, and locally granulation and recrystallisation may have taken place; (b) as a fine-grained recrystallised constituent of the “groundmass” in which the larger minerals are set.
Feldspar.—Any variety of members of this group have been noted, but in general the more basic plagioclases (i.e., those with a greater anorthite content than An30) cannot be determined on account of their replacement by fine-grained masses of sericite, epidote, and occasionally calcite. However, several porphyroclastic fragments as basic as Ab50An50 were noted. The more acid members of the series are not so susceptible to alteration, with the result that many clear, twinned, unaltered elastic fragments ranging from albite (Ab95An5) to oligoclase-andesine (Ab70An30) were observed. Orthoclase is not nearly so plentiful nor widely distributed as the plagioclases. It yields to alteration more easily than the acid plagioclases, and is thus frequently recognised by its somewhat cloudy aspect, combined with characteristic lack of twinning and low refractive index. Microcline is occasionally present. The above feldspars all occur as clastic or porphyroclastic fragments; in the so-called “groundmass” feldspar, in the form of clear small grains of albite, is associated with the recrystallised quartz, sericite, etc. The alteration-products arising from the feldspars are sericite, epidote, occasionally calcite and kaolin dust.
Biotite.—The usual type is a brown, strongly pleochroic variety altering in most cases to green chlorite, and sometimes shows straineffects, such as contortion of cleavage-laminae. The mineral is sometimes bleached light brown, though still retaining its initial high birefringence. Occasionally (e.g., Section No. 2327) pleochroic haloes surround small zircons enclosed in biotite.
Hornblende.—Both green and brown varieties were noted, occurring as clastic fragments, often broken, and almost universally showing the first stages of replacement by actinolite.
Stilpnomelane.—This mineral is an abundant constituent of many of the rocks, and appears to have formed in the very early stages of metamorphism. It occurs in several different ways: (a) as bunches or clusters of fine prisms, sometimes with a tendency toward radiating habit (Section No. 2213); (b) as felted, fibrous
masses (No. 2215); (c) as isolated needles or cluster of needles scattered throughout a granular aggregate of clear quartz or sometimes albite. Often the needles are arranged in two series intersecting at angles which vary in different cases (Nos. 2215 and 2214). In No. 2213 stilpnomelane is associated with chlorite. The pleochroism is in all cases from light golden yellow to dark brown* except No. 2110.
Epidote.—Both clastic and secondary varieties may be present, the former occurring as rounded or broken grain almost always pleochroic from very pale yellow to light yellowish green. The secondary variety is very widely distributed, and occurs as fine, almost colourless or very pale green granular aggregates which appear as dark clusters under low-power magnification or as coarse granular aggregates replacing feldspar. The tenor of Fe2O3 is very variable, as indicated by a wide range of interference tints. The variety clinozoisite occurs in many of the sections throughout the “groundmass” as slender prisms with low double refraction.
Sphene.—This mineral is a universal accessory. As clastic grains it normally appears in the form of rounded or broken grains, although in some cases the almost perfect lozenge-shape has been retained (No. 2238). Under high power the mineral nearly always appears to be slightly pleochroic from very pale pink to pale pinkishbrown. In No. 2238 lamellar twinning and prismatic cleavage are both well developed.
Chlorite.—This is a light green fibrous variety showing anomalous interference tints; it is always secondary in origin.
Iron-ore.—In most cases this seems to be ilmenite, since very frequently the black grains are surrounded by a whitish dusty decomposition product which is probably leucoxene.
Prehnite.—This mineral occurs in a narrow veinlet of quartz in one rock (No. 2276).
Other Minerals.—The remaining minerals—actinolite, zircon, zoisite, sericite, rutile, and calcite–are usually present in relatively small amount, and show no unusual features.
Classes of Rock Represented.
All the rocks in this section are altered to some extent. According to the amount of reconstitution suffered, the rocks have been classified as:—
Slightly reconstituted greywacke.
Partly reconstituted greywacke.
Largely reconstituted greywacke.
The above classification embraces a complete graduation from slightly to totally reconstituted rocks. The divisions are of course
[Footnote] * cf. Turner and Hutton, 1935, p. 2.
arbitrary, but give some indication of the relative intensity of metamorphism to which the rocks concerned have been subjected. It must be remembered that the amount of reconstitution depends also to some extent on the grainsize of the rock, e.g., a fine-grained rock is more susceptible to alteration under shearing-stress than a coarse-grained one.
A type section for each of the above classes will now be described.
Slightly Reconstituted Greywacke (No. 2232).—The rock is made up of subangular quartz and feldspar fragments, the latter being slightly more plentiful than the quartz, together with a little clastic muscovite and decomposing biotite in flakes and wisps. The less-altered feldspar comprise albite, oligoclase, and orthoclase, but the remainder, presumably more calcic, are obscured by sericite, granular epidote, and in one or two pieces, calcite. A few of the quartz fragments show a little recrystallisation around the margins, while strain-effects are universal, being seen in the feldspar also. The biotite is altering sometimes to chlorite and sometimes to a golden-yellow compound which may be a form of stilpnomelane. Between the larger grains is a recrystallised aggregate of quartz, albite, sericite, epidote, and a little chlorite. Accessories include sphene and ilmenite. A certain amount of rotation of the quartz and feldspar fragments resulting in a very roughly parallel structure has taken place.
Partly Reconstituted Greywacke (No. 2213).—The rock is of a dark-grey, medium-grained, somewhat sheared, slabby nature, exhibiting rough fissility. The section reveals subangular to rounded quartz and altered feldspar fragments up to ¾ mm. diameter, with hornblende, partially chloritised biotite, muscovite, stilpnomelane, composite and carbonaceous fragments, and grains of sphene, ilmenite, epidote, and a little zircon, set in a recrystallised “groundmass” of quartz and albite with epidote, clinozoisite, and sericite. Markedly parallel structure is evident. The feldspar include albite, oligoclase, acid andesine, some microcline and orthoclase, as well as more basic varieties altered to quartzo-feldspathic aggregates enclosing sericite and slender prisms of clinozoisite. All the quartz fragments show undulose extinction, some having been almost completely reconstituted to a mosaic of interlocking grains, while in others recrystallisation is confined to the borders. Green and brown hornblende (10%) are both represented, the general alteration being to actinolite, which usually appears as clusters of fine prisms bordering the original mineral. Stilpnomelane (10%) occurs both as bunches of radiating needles and in clusters.
Largely Reconstituted Greywacke (No. 2254).—A pale greenish-white, medium to fine-grained, non-fissile, crushed rock. About 30% of the section consists of porphyroclasts of quartz and feldspar (mainly albite-oligoclase) up to 1 mm. in diameter, the former being almost entirely recrystallised on the borders, and often
throughout, to a mosaic of sutured grains, while the latter are usually only affected on the margins. There is a great abundance of greenish, fibrous, dark clusters, which, under high power (80 diams.), resolve themselves into a member of the epidote-clinozoisite series, occurring as small prisms in felted masses. The pleochroism is from colourless or very pale apple-green to pale apple-green; refractive index fairly high; double refraction about 0.012; elongation negative. Between this material and the porphyroclasts is a mosaic of crushed quartz with some albite, a little stilpnomelane (1–2%), limonite (4%), and a few grains of sphene. Streakiness and some elongation of the porphyroclasts is well demonstrated in parts of the section, while the twin-lamellae of the feldspar frequently are broken across or bent. A few quartz grains and limonite-filled cracks traverse the rock.
Greywacke-schists (No. 2247).—A blue-grey, sheared, fairly coarse-grained, schistose rock. The section shows strained, rounded, more or less clear quartz, and clouded relict-grains of largely recrystallised feldspar up to 1 mm. in diameter, with a few almost completely decomposed biotites, a trace of chlorite, granular epidote, a good deal of dirty carbonaceous and iron-stained material, and grains of sphene (often broken), set in a sheared recrystallised finegrained base of quartz, albite sericite and epidotic material. The original feldspar has recrystallised usually to albite, quartz, much sericite, and a good deal of clinozoisite. The quartz relicts are often surrounded by a clear ring of newly-formed quartz, albite, and sericite.
Slates (No. 2205).—In addition to the above types there are the slates, of which No. 2205, a dark-grey highly fissile rock, may be taken as representative. By far the greater part of the rock consists of a very fine-grained recrystallised streaked aggregate of quartz, feldspar (probably albite), and sericite, with much carbonaceous and chloritic material. A few rounded grains of clastic quartz stand out clearly by reason of their coarseness, while grains of clastic epidote, phene, and ilmenite occur here and there.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Femic Minerals Present.|
|Serial Number.||Distance NE of Parson's Rk. Ck.||Type of Rock.||Class of reconstn.||Fissility.||Max. gr. size of fragments.||Dip.||Strike.||Original (Clastic)||New-formed.|
|2217||2 mls. 2 chs.||Gwcke||2||Poor||⅜ mm.||40° SW||323°||Acces.*||Acces.||Acces.||Acces.|
|2216||1 " 74 "||Gwcke||2||Poor||1 "||Acces.||Acces.||Acces.||Acces.|
|2215||1 " 66 "||Gwcke||2||Poor||1¼ "||Acces.||15%|
|2214||1 " 55 "||Gwcke||3||Poor||⅝ "||Acces.|
|2213||1 " 44 "||Gwcke||2||Fair||¾ "||73°SW||325°||Acces.||5–10%||Acces.||Acces.||10%|
|2212||1 " 35 "||Gwcke||2||Fair||⅙ "||62° SW||317°||Acces.||5–10%|
|2211||1 " 24 "||Gwcke||2||Fair||⅓ "||Acces.||Acces.||20%|
|2210||1 " 16 "||Slate||3||Good||Fine||85°SW||327°||5–10%||(green)|
|2209||1 " 8 "||Slate||3||Good||Fine||70°SW||312°||Acces.|
|2206||55 "||Slate||3||Good||Fine||48° NE||305°||Acces.|
|2204||31 "||Gwcke||2||Poor||1 ¼ mm.||84°NE||320°||Acces.||Acces.|
|2203||20 "||Gwcke||2||None||1 "||83° NE||330°||Acces.||Acces.||Acces.|
|2202||10 "||Giwcke||2||None||1 ¼ "||75° NE||325°||20%||Acces.||5–10%||Acces.|
|Distance SW of Parson's Rk. Ck.|
|2230||23 chs.||Gwcke||1||None||2 "||70° SW||337°||30%||Acces.||Acces.|
|2232||62 "||Gwcke||1||None||1 ¾ "||45° SW||347°||15–20%||Acces.|
|2332a||1 ml. 12 "||Conglom|
|2233||1 " 27 "||Gwcke||1||None||1 "||69° SW||332°||15–20%||Acces.|
|2234||1 " 34 "||Gwcke||1||None||1 "||Acces.||20%||Acces.|
|2235||1 " 41 "||Gwcke||1||None||¾ "||Acces.||20–30%||Acces.||Acces.|
|2236||1 " 49 "||Gwcke||1||Poor||½ "||90°||345°||15–20%||10%||Acces.||Acces.|
|2237||1 " 60 "||Gwcke||3||None||3/20||77° NE||327°||Acces.||10%|
[Footnote] *Percentages less than 5 are represented as “Acces.”
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|2238||1 ml. 54 chs.||Gwcke||1||None||1 ¼ mm.||Acces.||Acces.||Acces.||Acces.||Trace|
|2239||1 " 42 "||Slate||2||Semi||Fine||10%||Acces.|
|2240||1 " 25 "||Gwcke||1||None||1 "||Acces.||5%||5%||Acces.||Trace|
|2241||1 " 14 "||Gwcke||2||None||1 ¼ "||10%||Acces.||Acces.||Acces.||Acces.|
|2242||48 "||Gwcke||1||None||1 ¼ "||52° SW||307°||10%||10%||Acces.|
|2244||8 "||Slate||3||Good||Fine||30° NE||302°||Acces.|
|2245||0 "||Gwcke-Schist||4||Fair||1 "||75° NE||347°|
|Distance SW of Otematata River.|
|2246||0 mls. 13 chs.||Gwcke-Schist||4||Good||1 ¼ "||80° SW||337°|
|2247||29 "||Gwcke-Schist||4||Good||1 "||68° SW||342°||Acces.|
|2249||45 "||Gwcke||3||Fair||½ "||78° SW||340°||Acces.||Acces.||Acces.|
|2250||53 "||Gwcke||3||Fair||¼ "||83° SW||344°||Acces.||Acces.|
|2251||1 " 4 "||Gwcke||3||Fair||½ "||57° NE||354°||Acces.||Acces.|
|2252||1 " 18 "||Gwcke||2||Fair||1 ¼ "||75° SW||322°||Acces.||5%||10–15%|
|2253||1 " 32 "||Gwcke||3||Poor||½ "||85° SW||327°||Acces.||Acces.||10%|
|2254||1 " 49 "||Gwcke||3||None||1 "||75° SW||302°||Acces.|
|2255||1 " 70 "||Gwcke||2||None||1 ½ "||73° SW||337°||Acces.||Acces.||10%|
|2256||2 " 14 "||Gwcke||2||None||1 ¼ "||75° SW||325°||Acces.||Acces.||Acces.||5–10%|
|2260||2 " 41 "||Gwcke||2||Poor||1 "||75° SW||327°||Acces.||5–10%||Acces.||15%|
|2261||2 " 72 "||Gwcke||1||None||1 ¼ "||10–15%||Acces.|
|2262||4 " 0 "||Gwcke||1||None||1 ¼ "||15–20%||Acces.||5%|
|2263||4 " 38 "||Gwcke||1||None||1 ½ "||15%||5%||Acces.|
|2264||5 " 24 "||Gwcke||1||None||½ "||90°||302°||5%||10%||Acces.||Acces.||Acces.|
|2265||5 " 62 "||Gwcke||1||None||1 "||90°||352°||15%||Acces.||Acces.||Acces.|
|2266||6 " 73 "||Gwcke||2||None||1 "||75° NE||302°||10%||5%||Acces.||Acces.|
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|2324||5 mls. 8 chs.||Gwcke||1||None||1 mm.||5%||5%||Acces.|
|2325||4 " 57 "||Gwcke||2||None||1 "||75° SW||319°||5–10%||10–15%||Acces.||Acces.|
|2326||4 " 10 "||Gwcke||1||None||1 ¾ "||10–15%||10–15%||Acces.|
|2327||3 " 33 "||Gwcke||1||None||1 "||70° SE||37°||5–10%||Acces.||Acces.|
|2328||2 " 77 "||Gwcke||1||None||1 ¼ "||60° SE||37°||10–15%||Acces.||Acces.|
|2329||2 " 38 "||Gwcke||1||None||¾ "||85° SE||7°||10%||Acces.|
|2330||1 " 61 "||Gwcke||1||None||¾ "||5–10%||Acces.||Acces.|
|2331||63 "||Gwcke||1||None||1 "||85° SW||355°||10%||Acces.|
|2332||25 "||Gwcke||2||None||1 "||5–10%||Acces.|
|2333||1 "||Slate||3||Good||Fine||75° NW||32°||Acces.|
|Distance NE of Manuherikia River.|
|2335||3 mls. 40 chs.||Gwcke||1||None||1 "||15–20%||Acces.||Acces.||Acces.|
|2336||3 " 18 "||Slate||3||Good||Fine||45° NE||352°||Acces.|
|2337||2 " 68 "||Slate||3||Good||Fine||Acces.|
|2338||2 " 11 "||Gwcke||2||Very poor||1 mm.||15–20%||Acces.||Acces.||Acces.||Acces.|
|2339||1 " 47 "||Gwcke||1||None||1 "||50° NE||357°||10–15%||Acces.||Acces.||Acces.|
|Distance SW of Manuherikia River.|
|2340||2 mls. 65 chs.||Gwcke||2||None||1 "||80° SW||350°||Acces.||10%||Acces.||Acces.||Acces.|
|2341||3 " 33 "||Gwcke||2||Poor||¾ "||Acces.||Acces.||Acces.||10%||Acces.|
|2342||3 " 77 "||Gwcke||2||Very poor||1 "||70° NE||345°||10%||5%||Acces.||5%|
|2343||4 " 41 "||Gwcke||2||None||1 "||5%||Acces.||5%||5–10%||Acces.|
|2344||5 " 42 "||Gwcke||1||Very poor||1 "||5%||Acces.||Acces.||Acces.||Acces.|
|2345||5 " 63 "||Gwcke||1||Poor||¾ "||Acces.||Acces.||Acces.||Acces.|
|2346||6 " 12 "||Gwcke||3||None||1 "||15%||Acces.||5–10%||5%|
|2347||6 " 46 "||Gwcke||3||None||1 ¼ "||80° NE||337°||Acces.||Acces.||15%|
|2348||Summit St. Bathans Range.||Gwcke||3||Poor||1 ¼ "||55° W||2°||Acces.||5–10%||15–20%|
Comment on Tables.
Taking into consideration the grainsize, a glance at the column dealing with the amount of reconstitution reveals an almost constant degree of alteration resulting from stress between the upper south-western slopes of Mt. St. Mary and Parson's Rock Creek. Proceeding south-westwards along the section-line from the latter point, however, the intensity of reconstitution becomes less across the Parson's Rock Spurs; but when the Otematata River is neared increase again becomes apparent, until at the river itself, and for short distance on its south-western side, greywacke-schists are encountered, indicating an area of maximum shearing located along the general direction of the main channel.
The intensity of metamorphism begins to wane again southwestwards from the Otematata River, and with two or three exceptions remains fairly constant at the “slightly reconstituted” level until the Hawkdun Fault-block is reached. From here to the summit of the St. Bathans Range a slight general increase is noted.
C. Between St. Bathans Range and Dunstan Peak.
The localities to which reference is made are shown in Textfigure 1. Selected rocks typical of this part of the section are described below:—
No. 2350.—Sheared, largely reconstituted greywacke, in handspecimen a greyish, medium to coarse-grained, poorly fissile rock. The section indicates considerable shearing, the quartz crystals frequently showing recrystallisation and granulation. Acid plagioclase, with some orthoclase, forms rounded crystals up to 11/4 mm. diameter, while the more basic feldspars have altered to dark, finely granular epidotic matter which has now become interstitial to the more resistant quartz and feldspar, and may possibly have the composition of clinozoisite. Green hornblende is present up to about 8%, but much of it has been replaced by actinolite, which is quite abundant (10%). There is also about 10% of stilpnomelane appearing both in the rock itself and in the quartz veins traversing it. Fine material consisting of minute granules of quartz and feldspar (albite) with numerous little sericite flakes forms the matrix in which the larger crystals are enclosed. Small clastic grains of sphene, ilmenite, and epidote, together with decomposing biotite and carbonaceous matter, constitute the accessories.
No. 2351.—Totally recrystallised rather fissile sheared grey-wacke. In section this is a thoroughly recrystallised sheared rock consisting of a fine mosaic of quartz and albite containing a few more or less rounded quartz and plagioclase relicts, as well as streaks and cluster of dark, granular epidotic matter. Some serieite and a little actinolite are present.
No. 2354 is a dark-grey fissile slate consisting of fine quartz, feldspar, sericite, chlorite, epidote and dark carbonaceous matter, enclosing occasional porphyroclasts of quartz or acid feldspar. Accessories are iron-ore, sphene and zircon. The carbonaceous matter is concentrated in separate layers, the others being relatively light coloured. Signs of strain-slip are evident.
No. 2355—A sheared, totally reconstituted greywacke (grey-wacke-schist) with distinct macroscopic foliation. The microsection shows a reconstituted rock with a few granulated relicts of quartz and clouded feldspar. The foliae are of two kinds: (a) greyish sheared quartz-albite aggregates with epidote sericite and some sphene enclosing a few almost totally recrystallised quartz and feldspar relicts; (b) clear quartz-albite aggregates containing irregular black patches of iron ore.
Nos. 2357–2360 are foliated quartz-albite-epidote-sericite-schists resembling those of Central Otago, but rather finer in grain. Nos. 2357 and 2358 are corrugated on a small scale. In No. 2357 the laminae of which the rock is composed are of two types. One consists of allotriomorphic quartz and albite grains (often with sutured margins) with a few granules of epidote and pieces of muscovite (often stained yellowish by iron oxide) and rare acicular crystals of stilpnomelane. In the other variety quartz and albite are relatively unimportant, the abundant minerals being epidote and muscovite. The epidote occurs here also as granules, while the muscovite (often stained greenish and yellowish by iron solutions) forms little elongated flakes. Chlorite is fairly plentiful in some parts. The parallelism and folding of the laminae is frequently emphasised by the presence of streaks of carbonaceous matter. In
addition, two or three slender prisms of tourmaline were observed cutting across the general direction of the foliation, while sphene is an unimportant local accessory.
It is probable that a major fault separates the schists of the Dunstan Range from the less metamorphosed rocks to the east in the valley of Dunstan Creek. The evidence for the existence of such a fault is somewhat indefinite, and is based upon the present topography, the contorted state of the rocks in this neighbourhood, and the rapid change in metamorphic grade between Dunstan Peak and Dunstan Creek.
Throughout the area between Dunstan Creek and the Waitaki River the faults fell into an older compressional series and a younger tensional series (block faults).
(a) The Compressional Series.
The most obvious member of this series is the Waitaki Thrust (T) which crosses the Awahokomo Valley about four miles above its junction with the Waitaki. The movement probably took place at the same time as the folding of the greywackes, under the influence of pressure from the north-east in late Jurassic or-early Cretaceous times. It may be considered as a result of shearing along an overturned aniclinal fold, probably not confined to any single plane, but rather the result of movements distributed through a zone. The greywackes lying immediately along the south-west of this shear-zone, and to some extent also those to the north-east, were so strongly affected by the movements that they developed a schistosity which almost obliterated their original clastic structure (“Kurow Schists” of earlier writers). The intensity of this purely local dynamic metamorphism decreases rapidly in a south-westerly direction.
A somewhat similar though less powerful movement probably took place along the line now marked by the Otematata River (above its junction with the Otamatakau River). Here, again, the rocks locally becomes schistose, the amount of reconstitution increasing suddenly as the fault-line is approached from the north-east, and dying away on the south-westerly side of the river. A band of conglomerate in which the pebbles have been sheared to lensoid shapes outcrops on the north-east side of the river, but is absent on the other side, where, in the absence of a fault, it would be expected to appear.
A notable feature in the Otematata Valley when viewed from the Parson's Rock Spur is the large bend which occurs in the river gorge just above the Chimney Creek confluence (Text-fig. 2). From such evidence as fairly intense corrugation, jointing, and shattering of the surrounding rocks, and topographic features, it appears probable that a fault crosses the major Otematata Fault here. Its continuations are traceable both where it crosses Parson's Rock Creek and in the north-west branch of Chimney Creek (which follows
its general direction). Other movements in a roughly parallel direction may account for the courses taken by portions of the Otamatakau River, Long Gully, an other creeks. From the somewhat complex nature of the structure near the large bend in the Otamatakau River east of Turnagain, it is possible that this spot marks the intersection of three faults: (i) The Otamatakau Fault—i.e., that determining the lower reaches of this river, and having an approximate bearing of 40°; (ii) a fracture-zone bearing about 110°–120°, and determining the course of the Otamatakau River south of Turnagain (a parallel movement is indicated by the topography to the north of Turnagain; (iii) a minor block-fault running almost north and south, belonging to the younger fault-series. The rocks near this point have suffered some reconstitution, while the direction of strike has been locally disturbed and is highly variable.
Text-Fig. 2.—The fault running from A to B cuts across the narrow saddle directly above C, on the ridge between the Otematata River (seen in the middle foreground) and Chimney Creek.
The Otematata Fault is probably one of the same age as the Waitaki Thrust. The other fractures, with the exception of the minor block-fault, may also be of this age, but the evidence in support of the correlation is not so striking.
(b) The Tensional Series.
The block-faults (probably a tensional series) whose general direction in this district appears to be almost north and south may be summarised thus:—
A possible renewal of movement long the Otematata Fault.
A minor fault about two miles to the east of Turnagain.
A major fault forming the western scarp of the Hawkdun Mts. as described by Dr C. A. Cotton.
A major fault forming the western scarp of the St. Bathans Mts.
All these block-faults belong to a later (Tertiary) tectonic movement, and hence have had a considerable effect upon the evolution of the modern topography. The amounts of the movements involved can be gauged roughly by the break in the old peneplain level across the lines of the faults.
Attention is drawn to the fact that the three main directions of joints and veins (see below) coincide approximately with the three principal fault directions, indicating some connection (see circular figure, Plate 18).
Joints and Veins.
Comparison of over one hundred dip and strike observations on joint-surfaces and over thirty on vein-systems revealed the following data:—
The strikes of over 85% of the joint planes can be classified into four groups whose average bearings taken clockwise round the compass are: 27°, 76°, 124°, 164°.
The strikes of the vein-systems fall into three classes, which correspond in direction with three of the above joint classes, viz., 27°, 124°, 164°. Only one vein-system was observed whose strike was about 76°.
Taking the above four directions in rotation, the following generalisations hold:—
Strike 27° : Joints and veins all dip south-easterly at angles varying from 45°–90°.
Strike 76°: Joints are divided into two sets, one of which dips north-westerly at 15°–90°, mostly over 45°, while the other dips south-easterly at 30°–82°.
Strike 124°: Joints and veins are divided into two sets, one of which - dips north-easterly at 20°–75°, while the other dips south-westerly at 10°–50°.
Strike 164° : Joints and veins are divided into two groups, one of which dips westerly at 36°–90°, while the other dips easterly at 50°–60° (mostly 20°–30°).
The section put forward by McKay (1881) was taken further north than that described in this paper, although traversing the same formations. His conception appears to have been a wide and fairly shallow syncline in Maital and Te Anau beds flanked unconformably on the west by the “higher part of the lower schist” and also
uncomformably on the east by the “Kurow Schist.” In the centre of this syncline, but lying unconformably on the Maitai and Te Anau beds, he shows a contorted series of Wairoa and Kaihiku rocks.
Hector and Ulrich (1875) represent the Kurow Mts. as belonging to their Kaikoura Series (Carboniferous), which appears to lie unconformably in a syncline of Kakanui rocks (Upper Silurian), of which the eastern wing flanks the Waitaki River on the west and the western wing makes up the Hawkdun Mts., the St. Bathans Range, and the eastern end of the Dunstan Mts.
Trechmann (1917) assigns a Kaihiku age (Upper Middle Triassic) to the somewhat sheared fossiliferous rocks discovered on Mt. St. Mary in 1903 by Professor Park (see also Park, 1925, and Wilckens, 1927). The “Kurow Schists” which conformably underlie these Kaihiku rocks are therefore older and may thus be of Lower Middle or even Lower Triassic age. Amongst the earlier writers these “Kurow Schists” were figured as a separate block, older than all the surrounding rocks. The hypothesis favoured by the present writer is against these views, which is local and of a low grade, to the stress set up by a thrusting movement from the north-east.
Between Mt. St. Mary and the lower eastern flank of Dunstan Peak is a long series of folded and faulted greywackes, slates, grits, and occasional conglomerates whose age most probably lies wholly within the Triassic and Jusassic periods. No fossils were found in these rocks, but it is quite possible that a more intensive search may give a definite indication of their age. The Trias-Jura rocks in other parts of New Zealand are noteworthy for their great thickness (e.g., see Henderson and Grange, 1926; Mackie, 1935), and, in view of the many folds between Mt. St. Mary and Dunstan Creek, the actual thickness of rocks involved need not necessarily exceed that found in other areas.
At the western end of the section the contact between the (?) Trias-Jura greywackes and the schist of Dunstan Peak (comparable with Central Otago Schist) is obscured by a great series of contortions, both large and minute. The greywackes near the contact become schistose, and, except for the presence of the contortions, might conceivably have graded imperceptibly into the schists. The presence of the contortions, however, coupled with the comparative flatness of the dip of the schist-laminae of the Dunstan Mts., casts doubt on any theory of metamorphic gradation between the two-rock series, but would suggest rather that the margin of an older resistant block of schist had been crumpled by the thrusting against it of the greywacke block, whose own margin had also suffered similarly to some extent, although not showing the same amount of contortion as the schist itself.
The hypothesis tentatively put forward by the present writer, then, is that a large thickness of Trias-Jura sediments has been pressed against an older resistant schist-block on the south-west, as a result of the operation of a powerful north-easterly force, with concomitant folding and some faulting of the sediments, together with overthrusting on the north-east, and crumbling of the schists on the south-west.
This suggestion is put forward with some diffidence, for the data recorded during the present study are admittedly capable of other interpretations. They include nothing definitely contradictory to the orthodox theory that the Triassic rocks were involved in the same orogeny as was responsible for the metamorphism of the Otago Schists, and that the two groups of rocks differ in grade of metamorphism only. At the same time it has now been established that the gradual transition hitherto supposed to exist between the Triassic fossiliferous rocks and the completely reconstituted schists many miles to the west is lacking. The change is relatively abrupt and takes place near the western end of the section examined. This is readily explained on the writer's hypothesis. Further, the contrast between the steeply folded structure of the Triassic greywackes throughout the entire length of the section and the subhorizontal or gently dipping disposition of the schists of the Dunstan Range (Park, 1908) appears to favour the writer's view. Nowhere in the Kurow—St. Bathans section has any evidence been found that is inconsistent with the view that the rocks of Central Otago attained their schistose condition before the commencement of Middle Triassic sedimentation.
The writer is greatly indebted to Dr F. J. Turner, of Otago University, for kind assistance and much helpful criticism given him during the compilation of this paper.
Henderson, J., and Grange, L. I., 1926. The Geology of the Huntly-Kawhia Subdivision, Bull. Geol. Surv. N.Z., no. 28 (n.s.).
Hutton, F. W., and Ulrich, G. H. F., 1875. Report on Geology and Goldfields of Otago, Mills Dick & Co., Dunedin.
Mackie, J. B., 1935. The Geology of the Glenomaru Survey District, Trans. Roy. Soc. N.Z., vol. 64, pp. 275–302.
McKay, A., 1881. Reps. Geol. Expl. N.Z. Geol. Surv., pp. 56–92.
Park, J., 1903. Discovery of Permo-Carboniferous Rocks at Mt. St. Mary, Trans. N.Z. Inst., vol. xxxvi, pp. 447–453.
—— 1908. The Geology of the Cromwell Subdivision, Bull. Geol. Surv. N.Z., no. 5 (n.s.).
—— 1925. A Text Book of Geology, Charles Griffin & Co., London.
Trechman, C. T., 1917. The Trias. of New Zealand, Q.J.G.S., vol. lxxiii, pp. 165–246.
Turner, F. J., and Hutton, C. O., 1935. Stilpnomelane and Related Minerals as Constituents of Schists from Western Otago, New Zealand, Geol. Mag., vol. lxxii, pp. 1–8.
Wilckens, O., 1927. Contributions to the Palaeontology of the New Zealand Trias, Pal. Bull. Geol. Surv. N.Z., no. 12.