The Bob's Cove Tertiary Beds and the Moonlight Thrust-Fault.
Petrologist, New Zealand Geological Survey.
[Read before the Royal Society, May 29, 1935; received by the Editor, December 15, 1938; issued separately, June, 1939.)]
Occurrence Of The Tertiary Rocks.
The Sediments Between Bob's Cove and Twelve Mile Creek.
The Rocks In The Zone Of The Moonlight Thrust-Fault.
Interpretation Of The Movement Involving the Tertiary Strata.
The, Age, of, the, Bob's, Cove, Beds.
The, Date, of, the, Involvement,.
Since 1864, when T. R. Hacket (1864) announced the discovery of limestone near Bob's Cove, on the shores of Lake Wakatipu, western Otago, much attention has been paid to this formation of Tertiary rocks, the Bob's Cove Beds. Hector (1870), Hutton (1872, 1875), Blair (1876), Cox (1879), McKay (1881, 1894), Park (1909) and Benson (1934, 1935) have all either examined them in the field or discussed their relations with the metamorphic rocks or with other Tertiary formations. Both McKay and Park made careful field examinations, but the latter writer alone appears to have realised that profound involvement of these beds within the schists had occurred. Park (loc. cit., p. 66) established the following sequence for the Bob's Cove Beds:
(a) Sandstone and conglomerates, passing downwards into bands of impure limestone.
(b) Pure limestone.
(c) Sandstone, marly and clayey.
(d) Marly clays.
(e) Calcareous breccia-conglomerates.
Occurrence Of The Tertiary Rocks.
The Tertiary deposits occur on a low promentary along the shores of Lake Wakatipu, between Bob's Cove and Twelve Mile Creek (Few Creek), forming a small ice-shorn outlier about 250–300 acres in extent. Extending from the edge of Lake Wakatipu, for approximately 22 miles in a direction slightly east of north, is a narrow strip of the Tertiary sediments, never more than 150 feet thick, which have been caught in and thrust under the schists, by, it is believed, an easterly directed overfold (figs. 1 and 2).
The Sediments Between Bob's Cove and Twelve Mile Creek.
The Bob's Cove Beds dip steeply and have been very strongly warped so that the outcrop is S-shaped (fig. 3). The beds abut against the north shore of Lake Wakatipu and for a distance of about 15 chains in a northerly direction they strike at approximately 17–20° east of north and dip west at 55°. The beds then swing round to strike at 255–260° and dip to the south at 50°, until a point just south of Trig, station E is reached, when they again swing round to strike at 11–12° west of north and dip west at 50–55°. About six or seven chains north of the Trig. station, the beds end abruptly in a steep escarpment some 300 feet high. The writer believes with Park that these beds have been involved and overturned in an easterly thrust overfold, the interpretation of which will be considered later; therefore the apparently uppermost beds are really the oldest. The Tertiary sequence in order of decreasing age has been determined as follows:
Sandstone with bars of heavy conglomerate: about 300 feet thick (apparently highest, but actually the inverted basal bed of the Tertiary sequence).
Pebbly Limestone with numerous small fragments of schist; about 30 feet thick.
Limestone; about 65–70 feet thick.
Very Gritty Limestone; about 15–25 feet thick
Sandstone; about 300 feet.
Silty Sandstone; about 200 feet.
Siltstone (apparently lowest, but actually the youngest bed of the Tertiary sequence); at least 530 feet remain after over-folding and erosion.
The basal member1 of the Tertiary Series, consisting of yellowish sandstone with bars of both heavy and fine conglomerates, adjoins the underlying schists with strong angular unconformity. The sand stone is sometimes very calcareous and may grade into a gritty limestone with increase of lime or into a fine conglomerate with increase in the amount of quartz or schist pebbles. A thin section of an average specimen contains much quartz, an important amount of calcite, with rare feldspar, clinozoisite, iron-ores, chlorite, muscovite, glauconite and foraminifera; in some sections fine fragments of schist were noted. Bars of conglomerate, 3–4 feet in thickness, consist of well cemented pebbles of various types of schists, up to 6 inches in diameter; fragments of a large Ostrea are sometimes abundant. From this bed McKay (1881) obtained species which he identified as Crassatella ampla Zittel and Natica solida Sowerby.
With apparent conformity these beds pass into the Pebbly Limestone, which is estimated to be at least 30 feet thick; it forms a steep dip slope facing west and south. This sediment is a fine compact greyish rock containing numerous pebbles of green schist and frequently large grains of pyrite, now in part replaced by limonite-pseudomorphs. Narrow pebbly and shelly bands are common, while
[Footnote] 1 The basal member of the Series is now the topmost bed owing to overturning.
silty bands were also noted. Microscopically the rock (No. 3487)1 is composed of plentiful calcite and numerous organic remains including algae and foraminifera; glauconite, quartz, feldspar and schist fragments are also present. From this horizon the following fossils were collected2:
Indeterminate coral remains.
Venericardia aff. ponderosa Suter. (From silty bands)
Cirsotrema sp. (From silty bands)
Balanus sp. (From silty bands)
The Pebbly Limestone is followed conformably by the next bed, the Limestone. The outcrop, about 65–70 feet thick, forming the ridge of the crest of the outlier, is much shattered and broken, but exhibits very fine fluting and solution channels. Where it abuts against the lake it has been worn smooth by wave action and just at the water's edge contains many well-developed potholes, usually filled with numerous rounded pebbles. It is a fine grey, close-textured rock (No. 3450) containing in addition to predominant calcite, fragments of schist, quartz, feldspar, chlorite, muscovite, pyrite, glauconite and abundant organic remains. An analysis of this rock made by the writer is as follows:—
|SiO2 and insol||1.40|
|Organic and H2O||0.66|
Fossils occur very sparingly, but unfortunately none could be collected.
At the edge of the lake the Limestone appears to pass imperceptibly into the next bed, the Very Gritty Limestone, or the “Greensandstone” of McKay, which is here 20 feet thick. On being traced northwards, however, this bed appears to become somewhat thinner and more gritty, when the passage from the limestone to this bed is much more sharply defined. North of the Trig, station, however, the bed thickens once more to about 25 feet. In a specimen taken at the shore of the lake, calcite is dominant, though glauconite and quartz are abundant; in the more gritty parts, however, at a point 15 chains north of the lake shore, fragments of schist and
[Footnote] 1 Rock section numbers refer to the collection at the University of Otago.
[Footnote] 2. The writer is very much indebted to Dr. C. R. Laws, of Auckland, for determining the Mollusca.
grains of calcite are present in about equal amounts. On weathering this rock gives rise to a yellowish-brown sand, the colour being due to the alteration of the chlorite in the component pebbles of green schist. Near the lake-shore joint-planes are numerous, one of which, striking at 140° with a south-westerly dip of 50°, is particularly well marked. The bed may be quite fossiliferous, sparsely so, however, in the more calcareous parts. The following types have been collected by the writer:
Odontocyathus aff. japonicus Yabe and Eguchi.
? Eupatagus sp.
? Waiparia sp.
Lentipecten sp. [probably L. hochstetteri (Zittel)]
? Kuia sp.
Xenophora prognata (Finlay).
In the University of Otago geological museum there is a Hemipatagus sp. which appears to have come from this horizon, but the collector is unknown. In-addition to these fossils, Park (1918) collected the following “from the sandstone below the limestone,” a horizon presumably near to this bed; the modern nomenclature is given in brackets:
Picten huttoni Park. [Lentipecten hochstetteri (Zittel).]
Cucullaea alta Sowerby. [This species has recently been dropped from New Zealand records, as it is a South American species.]
Limopsis zitteli Iher.
Cardium huttoni Iher. [Probably Trachycardium sp.]
Venericardia purpurata (Desh.). [See below.]
Ostrea wuellerstorfi Zittel. [Gigantostrea wuellerstorfi (Zittel).]
Polinices ovatus (Hutton).
Ancilla hebera Hutton. [Baryspira sp.]
Dentalium mantelli Zittel. [Probably Fissidentalium sp.]
Note: Venericardia awamoaensis Harris may have been mistaken for Venericardia purpurata (Desh.), for the latter is a typically Recent species.
The coral Odontocyathus aff. japonicus Yabe and Eguchi is a sufficiently interesting type to warrant further notice, and was first described by Yabe and Eguchi (1932) from the Neogene of Japan. In the Otago University geological museum there is an unnamed, well-preserved coral from North Otago, and it appears to be identical
with the Bob's Cove specimen. In both specimens the septa are regular and 96 in number, the septa of the first cycle being most prominent. The North Otago type has the following dimensions:—
|Diameter of calyx measured along septa of the first cycle||27||(32)|
|Diameter of calyx measured along septa of younger cycle||26||(26)|
|Greatest height to summit of exert primary septa.||10||(22)|
|Least height in interval between two primary septa||9||(15)|
|Length of radial processes at base||?||(?)|
|Length of radial processes at base of Bob's Cove specimen||10|
The figures in brackets are the dimensions of the type specimen described by Yabe and Eguchi from Japan. The ratio, diameter of calyx to least height in the New Zealand specimen is 3:1, while that in the Japanese type is 2:1. Hence the New Zealand specimen is a slightly flatter type, while the primary costae in contrast to the Japanese type are not nearly so pronounced. But its other features, such as number and arrangement of the septa and the presence of peculiar radiating spines or spiniform processes around the margin of the base clearly indicates its close relation to Odontocyathus japonicus.
The Very Gritty Limestone is followed conformably by the Sandstone, a yellow somewhat calcareous bed, about 300 feet thick, which in places may be slightly silty and sometimes rather glauconitic. The outcrop of this bed forms a steep cliff face from which thick slabs loosened by exfoliation have accumulated as talus. It contains many concretions, averaging about one foot in diameter, which, when split open, are seen to contain abundant fossil remains, unfortunately all unrecognisable. In a concretion from this bed McKay (1881, p. 145) discovered what he believed to be part of a skeleton of a Plesiosaurus-like reptile, but later (1894, p. 13) he considered that the vertebrate remains might be cetacean or reptilian. Professor W. B. Benham has recently examined the vertebrate material collected by McKay and has most kindly allowed the present writer to make use of his notes in this connection. Professor Benham states “that one tooth in longitudinal section is 2 cm. in length by 4 mm. in diameter with a narrow pulp cavity surrounded by black or dark brown dentine enclosed in thick enamel. The tooth is a long narrow cone, somewhat curved; there is no ‘root.’ The block also shows 3 or 4 other isolated fragments of similar teeth. One is a transverse section in which a pulp cavity is not evident; evidently near apex. Another is a longitudinal section with very narrow pulp-cavity near the base. The teeth probably belong to some ‘modern Odontocete’ (as opposed to ancient Odontocetes such as Squalodon, etc) such as a Dolphin or Porpoise, but are too large for either of the existing species, but approach more nearly to Cephalorhynchus hectori, the ‘Porpoise.’ They are too slender and too short for Globicephalus (‘Black Fish’), too long and too slender for Tursiops. They are much too big for any Teleost and lack the characteristic basal thickening of a shark's tooth.”
Lying conformably on the Sandstone is the Silty Sandstone, a bed approximately 200 feet thick, with the junction between the two formations, except at the edge of the lake, obscured by the talus mentioned above. This bed appears to be the “Blue Sandy Marl” of McKay. As with the underlying sandstone, the bed usually forms a steep cliff, the present base of which is obscured by a talus of fragments loosened by exfoliation. The Sandstone is again calcareous, but more glauconitic and silty and contains more abundant fine fragments of schist than the underlying sandstone. Fossils are not abundant, but the following types have been collected by the writer:
Lentipecten sp. [Probably L. hochstetteri (Zittel.]
Trachycardium sp. [Cf. T. waitakiensis Suter).]
Fissidentalium solidum (Hutton).
? Kuia sp.
This bed passes up into the uppermost bed, the Siltstone or “Marl” of Park. It is at least 530 feet thick and is inturned against a highly irregular surface of brecciated schist, but at the lake-edge it is in contact with the breccia of the zone of the thrust-fault. It is a fine-grained, homogeneous sediment, sometimes rather gritty due to admixture with fragments of schist. Park (1909, p. 100) gives an analysis of this sediment:
|CaO (otherwise combined)||2.39|
|SiO2 and insol||51.60|
|Alkalis and loss||2.51|
Analyst: G. M. Thomson, 1909.
This Siltstone possibly had a much greater thickness than is seen now, for much of the sediment has been sheared out by tremendous thrusting, and, further, it must have been somewhat reduced by erosion before thrusting took place. Fossils are not abundant and are confined almost entirely to the lowermost horizon, just above the Silty Sandstone. The following types have been collected by the writer:
Indeterminate coral remains.
Cucullaea worthingtoni Hutton.
? Austrofusus sp.
In 1875 Hutton also collected fossils from the Bob's Cove Beds, but unfortunately did not state from what horizon they were obtained. The species reported by that worker are listed as follows:
Dentalium giganteum Sowerby. [D. solidum Hutton.]
Voluta corrugata Hutton.
Panopaea plicata Hutton.
Panopaea worthingtoni Hutton.
Cucullaea ponderosa Hutton.
Cucullaea worthingtoni Hutton.
Cucullaea alta Sowerby.
Cucullaea attenuata Hutton.
Waldhemia lenticularis Desh.
With regard to V. corrugata Hutton, Marwick (1926, p. 299) remarks that “until some topotypes have been secured, however, the best course is to ignore this species,” a view later upheld by Finlay (1927, p. 513). Finlay (1926, p. 473) points out that the correct spelling of Panopaea is Panope and further he does not appear to recognise P. plicata Hutton as a type. Nevertheless Finlay and Marwick (1937, p. 36) point out that according to Sherborn Panopea appears to be the correct spelling. Finlay (1926, p. 448) notes that Cucullaea worthingtoni Hutton is identical with and has priority over C. attenuata Hutton. In the list of fossils above it is possible that Hutton's Waldhemia lenticularis Desh. is Waiparia sp.
Brief reference might be made here to the investigation of the heavy residues in some of the Bob's Cove sediments (Hutton and Turner, 1936). This work has shown that except for the interesting but rare occurrence of monazite, all the minerals identified are known to be present in the local metamorphic rocks.
The Rocks In The Zone Of The Thrust-Fault.
The thrust-zone with its involved Tertiary sediments has an average strike of about 15° east of north and dips to the west at angles between 50–70°, while the character and the amount of the sediments varies considerably as they are traced northwards.
At the edge of Lake Wakatipu, the thrust zone is approximately 60 feet thick, with a strike about 3° west of north and a westerly dip of 54°. Here the involved material is a calcareous breccia composed of angular fragments of schist, abundant grains of quartz, feldspar and some foraminifera, cemented by calcium carbonate. On weathering, this rock has the appearance of broken concrete, for solution of the corbonate has left the schist fragments protruding from the matrix. Immediately to the west of this zone is the Siltstone described earlier, but 50 yards north from the lake shore this Siltstone gives way to a mass of brecciated schist (see fig. 3); to the east of this zone of thrusting for a distance of about 75 yards the schist is intensely brecciated and shattered. Continuing northwards the thrust-zone can be traced for a distance of about 600 yards, when it disappears under a thick covering of alluvium.
Just less than a mile from the edge of the lake, the zone reappears from beneath the alluvial covering, outcropping close to the right bank of Twelve Mile (Few) Creek; the zone of involvement runs parallel with it for some 400 yards, then crossing the creek ascends a steep gully to a low saddle, 1,875 feet high, situated between Lake Dispute and Twelve Mile Creek. The rocks exposed on the bank of this creek strike at 14–15° east of north and the dip is to the west at 55°; the rock walls are very shattered, particularly those to the east. The sequence determined here is as follows:
A narrow black band of pulverised schist which is followed by a coarse siltstone (about six feet thick) containing grains of glauconite and remains of dicotyledonous leaves; this bed passes up into a fine conglomerate (about eight feet thick) containing fragments of a large Ostrea.
At the saddle the sequence has changed, and here a green gritty limestone containing foraminifera passes up into a breccia with fragments of a large Ostrea. Continuing north, the zone clings to the top of the ridge and passes over a spur of Mount Hanley at an altitude of approximately 3,800 feet, at which point it intersects the Twelve Mile Thrust. It then rapidly descends a precipitous slope almost to the fans south of the west arm of Moke Lake and continues north across the Left Hand Branch of Moke Creek. After passing Mount Hanley the zone is narrow and consists chiefly of brecciated schist, together with a small amount of red siltstone at the outcrop in the gorge of the Left Hand Branch. From this creek the zone continues north over ridge and valley to Slip Creek, which it crosses at an altitude of about 2,100 feet. The zone is best observed on the north slope of a ravine at 2,800 feet, where the outcrop is about 75 feet wide, the strike being 15° east of north and the dip 72° to the west. The schist wall to the west is crumpled, but that to the east is intensely brecciated. In the nearby schists an easterly dip may be seen, but this reversal of dip is only local, for 100 yards to the east of the zone of involvement the normal westerly dip is resumed. The sequence at this point begins with:
A poorly feldspathic sandstone, interbedded with a very thin band of carbonaceous material. This is followed by a series of green and brown coloured breccias, the latter interstratified with a band of reddish siltstone containing finely divided haematite; the latter is probably the outcrop referred to by Cox (1882) as a haematite lode.
In the feldspathic sandstone the grains of quartz and feldspar show strong undulose extinction and granulation (Nos. 3430, 3436). The green breccia (No. 3429) is composed of angular fragments of green quartz-albite-chlorite-schist, up to 60 cms. in diameter, while the brown breccia (No. 3419), though similar, contains much calcite and is stained by the red siltstone. The composite fragments are usually deeply slickensided.
The zone continues north to cross Gill's Creek, on the north side of which the sequence is well exposed at an altitude of 3,000 feet. Here crumbling and contorted schist on the west is followed by a
zone of pulverised schist, three inches wide, 40 feet of fine calcareous grit (No. 3424) and 30 feet of fine breccia. This latter bed abuts against an eastern wall of strongly brecciated schist. The zone now crosses the ridge to descend to a tributary of Gill's Creek and after ascending a steep ridge to 3,650 feet, descends to cross Dead Horse Creek at 2,200 feet. At this locality a strike of 15–16° east of north and a dip of 65° to the west were recorded and in the zone itself occur 55 feet of gritty limestone divided in the middle by a band of sandstone, 2 feet wide. The limestone contains traces of dicotyledonous leaves, while a concretion was found which, when broken open, emitted a strong odour of hydrocarbons and contained indistinct carbonaceous remains. Rare fossils occurred in the sandstone, but only fragments of a large Ostrea were determinable.
The outcrop of the Tertiary rocks now ascends to 3,000 feet, crosses the upper reaches of Butcher's Creek and ascends the slopes of Craigallachie to an altitude of 3,600 feet, dropping rapidly to 1,800 feet to cross the Moonlight Creek. Here the outcrop, approximately 150 feet in width, is well exposed on the south face of the Moonlight Gorge, where it is crossed by the Lake Luna-Moke Creek pack-track; the strike is 15° east of north and the dip to the west, at 70–75°. The zone rock is a feldspathic sandstone containing a minor amount of chlorite, muscovite and epidote; the quartz and albite show strong undulose extinction and granulation. The sandstone contains abundant indeterminable casts, fucoids and rarely a Glycymeris sp. Towards the footwall the bed appears to become more indurated.
The zone crosses the river and ascends the slopes of Mount Gilbert to pass east of the summit and then descends to Stony Creek, which it crosses at 1,800 feet. Here the Tertiary rocks consist of rather feldspathic sandstone and some conglomerate, about 20 feet thick in all; once again the eastern wall is intensely shattered and in places slickensided. From the sandstone Park (1909, p. 63) has reported the following fossils: Venus sp., Pinna sp., Ostrea sp. (very large and thick like O. wuellerstorfi Zittel), Turritella sp., Natica sp., Dentalium sp. (like D. mantelli Zittel), undetermined coral remains and Fucoid stems.
From Stony Creek the involved beds rise to a point almost at the summit of the Silverhorn (Trig. N, Shotover S.D.) and at this point reach an altitude of 5300 feet, which according to Park (1909, p. 63) is the greatest elevation at which Tertiary rocks have been found in New Zealand. From this point the zone continues north, clinging to the ridge west of Skipper's Creek and then ascending towards Mount Aurum. At this point the zone consists of some sandstone and breccia, but the outcrop is not very distinct. Later deep snow prevented any further work north of Mount Aurum, but the writer hopes to be able to continue this investigation soon.
Interpretation Of The Movement Involving The Tertiary Strata.
Any hypothesis advanced to explain the mechanism of the movement that caused the involvement of the Tertiary strata must explain the following points:
(1) Only a narrow band of Tertiary strata has been involved from Twelve Mile Creek to Mount Aurum, a distance of about 23 miles, while a thick sequence has been wedged in along the shores of Lake Wakatipu between Bob's Cove and Twelve Mile Creek.
(2) The same movement that involved the thin strip must likewise have involved the whole series at Bob's Cove.
(3) The Siltstone, the youngest bed of the Bob's Cove Series, lies adjacent to the breccia of the thrust-zone at the very edge of the lake, but 50 yards north from this point the sediment abuts against a mass of finely brecciated schist.
(4) The strike of the thrust-plane from the edge of the lake to the point where it disappears under the alluvial covering is approximately 3° west of north, but the average strike from Twelve Mile-Creek to Mount Aurum is 15° east of north. Therefore there is a marked displacement between the outcrop of the thrust-zone south, of the alluvium and that north of this covering.
(5) The schist wall east of the thrust-fault is always very much more shattered than the western wall. Also breccias, when they occur, almost always form the footwall, and the pebbles composing the breccias are most commonly Chl. 2 and Chl. 3 sub-zone schists.
In considering the Tertiary beds along the lake shore it was shown that they had suffered a peculiar warping in the process of which, it is believed, the whole series has been overturned. The evidence for overturning is twofold: (a) The succession on stratigraphic grounds appears to be inverted, and (b) the position of the sole of movement east of the strip of Tertiary rocks indicates overturning.
Now consider point (a) above; it has been shown that the Bob's Cove Beds grade down1 from sandstone with bars of heavy conglomerates through limestone, sandstone and silty sandstone to silt-stone. Again the siltstone, where it abuts against the schist, does contain a small amount of fine fragments of schist, but there is no sign of a basal conglomerate that so frequently occurs at the bottom of such a sequence. Therefore if the sequence were merely tilted, it would appear that sedimentation must have commenced in deep water and continued throughout a long period with the sea gradually becoming more shallow until finally subaqueous sediments gave place to littoral or beach deposits. Hence the writer would suggest that the beds are not tilted, but, instead, that they are completely overturned, so that the present uppermost beds, the coarse sandstone and conglomerates, are really the basal beds. The suggestion that basal conglomerates may have existed between the siltstone and the thrust-zone but have been torn away by the thrusting may be raised, but it will be shown that though some destruction of the sediments must have occurred, overturning alone will explain all the facts. Should overturning not have taken place the actual surface of thrusting, that is the sole of the movement, should not be below the siltstone but between the bed of conglomerate and sandstone and the adjacent schists.
[Footnote] 1. “Down” is not used in the stratigraphic sense.
It has been pointed out in statement (5) above that the schist wall to the east is always intensely shattered or brecciated, while the rocks to the west are barely disturbed or at the most are crushed or pulverised along a narrow zone only. Now if the sequence at Bob's Cove is an inverted one, the same must apply to this narrow strip. The deformation would seem to have been brought about through regional compression by the overturning of the Tertiary covering beneath an easterly thrust overfold (fig. 4, diagram A-C). A continuation of pressure from the west would probably result in thrusting along a plane between the Tertiary strata and the lower schist wall, so that the wedge of Tertiary rocks would be driven upwards along this inclined plane with consequent tearing, brecciating and slickensiding of the metamorphic rocks adjacent to this plane (fig. 4, diagram D). If a movement such as that figured by Park (1909, p. 64, fig. 15) took place, by which the Tertiary covering was merely folded in and embraced by the schists, then some evidence of a double sequence would be expected, but this is not the case. Two suggestions may be put forward to account for the absence of a double sequence:
(a) It is possible that at least half the involved formation has been torn away and destroyed by thrusting.
(b) After a period of compression, culminating in intense deformation by folding and thrust-faulting has occurred, relaxation of horizontal pressure may have brought about gravitational sinking of the upthrust block down the thrust-plane (fig. 4, diagram E). This would cause a deep involvement of a wedge of the Tertiary sediments which would not exhibit a double sequence. It is not stretching the mechanics of crustal movement too far to assume such a reversal of movement, for Bucher (1933, p. 2) points out in law 2 that “in the process of crustal deformation, the direction of radial displacement is reversible.” Subsequent peneplanation (fig. 4, diagram E, plane a-b) of this region would then remove all sign of the Tertiary covering on both sides of the outcrop of the thrust-plane. The writer favours the latter hypothesis, for there are numerous instances where the former occurrences of both upward and downward movement along a fault-plane has been demonstrated such as along the south-west boundary of the Kakanui and Horse Ranges where greywacke makes up the high-standing block while the lowlands are composed of schist (Cotton, 1917, p. 273); further Waghorn (1928, p. 26) believes that the Ruahine block in the North Island is now subsiding, the movement taking place along the ancient fault-lines.
Comparable examples of involvement with overturning along thrust-faults of covering beds by underlying formations are not uncommon, for Bucher (1933, pp. 162–169) mentions several cases from the Southern Rocky Mountain region, while Heritsch (1929) gives many instances from the homogeneous mobile belt of the Swiss Alps. Examples of involvement without overturning are common in New Zealand; in the Murchison district of New Zealand, Fyfe (1929) has described the occurrence of a strip of Tertiary rocks lying between two broadly arcuate faults among granites, but although a
cross-section shows the sediments to be lying vertically, Henderson (1937, p. 87) indicated that the Tertiary rocks dip steeply to the eastwards and suggested that the eastern block of granite has overridden these sediments. Further, Turner (1935, p. 330) has described a zone of mid-Tertiary sediments involved among sheared greywackes in the Eglinton Valley, but there is no evidence to show whether overturning has taken place.
The greater thickness of the strata at Bob's Cove than that in the zone of the thrust-fault requires consideration. With overfolding and thrusting such as has been described, the cross-section of the involved strip is most likely to be wedge-shaped, the Tertiary rocks becoming sheared out at depth. Now if a block of the crust enclosing this wedge is faulted down below the base-level between faults at right angles to the zone of the thrust-fault, it will be preserved from intensive erosion; but as the base-level is lowered and the more elevated region is worn away, the outcrop of the Tertiary beds in that block will become narrower. Eventually the down-faulted block will reveal a wide infolded mass in contrast to the narrow strip in the block on the upthrown side (see fig. 4, diagrams G. H and I).
This reasoning may be applied to the Bob's Cove area. The Bob's Cove Tertiary beds occur on a promentary separated from the main landmass by thick alluvium and the writer believes that beneath this alluvial covering there is concealed a fault (as was suggested by Benson, 1934, 1935), the Bob's Cove fault, running approximately east and west, from near the mouth of Twelve Mile Creek to Bob's Cove itself; the Bob's Cove beds are on the down-thrown side. Therefore these Tertiary beds represent the thick end of the wedge while the strip from Twelve Mile Creek to Mt. Aurum corresponds to the thin end of the wedge enclosed within the much-eroded and peneplained block. A fault such as is postulated above must necessarily have brought about a displacement of the outcrops of the thrust-plane to the north and to the south, and this is precisely what is observed when the outcrop of the thrust-fault is plotted on an arbitrary plane surface (fig. 4, diagram J). As the thrust-plane dips at angles between 55–80° to the west, eastward displacement of the outcrop in the downthrown block should occur; and since the throw of the cross-fault (Bob's Cove fault) must have been very great, if it is to account for the observed variation in thickness of the involved beds, the eastward displacement of the beds at Bob's Cove must be considerable. Actually, however, the displacement, though obvious, is in the opposite direction, that is, the block of the Bob's Cove beds shows a relative displacement to the west. Therefore it would appear that the gravitative sinking was accompanied by considerable lateral movement of the down-thrown block in an east to west direction thus more than compensating the west to east displacement resulting from the vertical component of the movement (fig. 4, diagrams I and K).
Further evidence in support of the presence of the suggested Bob's Cove fault is seen in the fact that it is parallel to the main structure lines blocking out the Wakatipu graben, these lines being,
according to Benson (1935, p. 15), really revivals of ancient shatter-belts. In this case the Bob's Cove fault is probably a post-peneplain revival of an ancient pre-peneplain line of dislocation.
Two rather striking points remain to be explained:
(a) The remarkable warping of the Bob's Cove beds.
(b) The overlap of the siltstones from a mass of brecciated schist on to the breccia in the thrust-zone itself.
In regard to the first point, it is probable that the involved sediments caught up by the advancing fold would not move eastwards with a perfectly straight front, but there would be a certain amount of buckling here and there. Consider the limestone band advancing eastwards in such a way that one point in the band advances eastwards more than another point 20–25 chains to the north; this would cause the shape of the outcrop of the involved wedge to conform more or less with that of the beds at Bob's Cove. The cause of such buckling may possibly be due to variations in thickness of the semi-plastic silty sediments which might have been squeezed laterally to some extent, thus presenting an uneven resistance to the schists moving from the west. As a result of this, sharp deflections of the competent beds of limestone, sandstone, conglomerate, etc., would probably take place. A continuation or inequality of the thrusting from the west might then shear through the wedge producing flaw-faulting along such planes as the Bob's Cove fault (see fig. 4, diagram F). The section betwen c and d in fig. 4, diagram F, therefore represents the Bob's Cove beds.
Now consider the second point; the block of schist lying between the siltstone and the zone of thrust-faulting near the shore of the lake may be regarded as a “horse” of the basement rock caught up and involved in the thrust.
Finally it must be admitted that comparison of the involved Tertiary beds at different vertical heights (e.g. on the Silverhorn at 5,300 feet and at the level of Stony Creek at 1,800 feet) fails to show any very great variation in thickness such as might be expected on the assumption that the involved strip is wedge-shaped. But it must be remembered that the depth of involvement must be very great, therefore, the wedge-shape need not be a markedly noticeable feature in a vertical section of about 3,500 feet. Further, the width of the outcrop widens northwards from the West Branch of Moke Creek and this is interpreted as due to (a) decreasing depth of involvement northwards or (b) the greater shearing out of the wedge towards the south.
The Age Of The Bob'S Cove Beds.
Unfortunately the fossils have been very poorly preserved and are commonly so distorted by shearing that specific determinations were in most cases impossible. Dr. C. R. Laws suggests that certain of the mollusca, especially Xenophora prognata (Finlay), Cucullaea worthingtoni Hutton, Trachycardium sp., Venericardia ponderosa Suter and Solecurtus (if it be allied to S. bensoni), indicate Ototaran
age. In the Otago University Geological Museum there is a single specimen of a well-preserved Rhizothyris fortis Thomson, labelled Skipper's, and this is a type confined solely to the Hutchinsonian Stage of the New Zealand Tertiary. Undoubtedly this fossil must have been carried down from the Tertiary strata either in Stony Creek or a tributary of Skipper's Creek. It must be pointed out that there is a marked absence in the Bob's Cove beds, excluding the doubtfully identified Venericardia awamoaensis Harris, of typical Awamoan forms such as: Lima colorata Hutton, Spissatella trailli (Hutton), Neilo awamoana Finlay, Tawera marshalli Marwick and Limopsis zelandica Hutton. Therefore it would seem best to fix the upper limit of these beds as Hutchinsonian and the lower as Ototaran.
The Date Of The Involvement Of The Tertiary Strata.
It has been shown that the Tertiary beds involved in the movement are at the latest Hutchinsonian (late Oligocene) in age and hence the thrust-faulting must be post-Hutchinsonian. It would not be possible to place an upper limit upon the date of the thrust-fault were it not for the fact that there is evidence that the district has since undergone a profound peneplanation affecting both Tertiary and older formations alike. The evidence is to be seen in the accordance of summit levels in the district through which the thrust-fault runs. Marshall (1918), Benson (1934, 1935) and Service (1934) have noted the occurrence of a late Tertiary peneplain in eastern coastal districts of Otago and also faulting movements occurring after the time of deposition of the Tertiary beds but before the peneplanation. It has been made clear by Benson that the peneplain recognised in the Wakatipu area is coeval with that in eastern Otago and further that writer (1934, 1935) considers that the peneplain in the Wakatipu area is merely an extension of the very extensive Fiordland peneplain. From evidence in eastern Otago it is only possible, at least at present, to restrict the age of the peneplain to somewhere between the Hutchinsonian and the Pliocene, so that the age of the Moonlight involvement, though it must antedate the peneplanation, can be placed only between the same limits.
The date of the involvement of the Tertiary rocks has been considered, but there is evidence pointing to the fact that an ancient zone of dislocation may have guided the course of the late Tertiary movement. It seems clear that at the time the Tertiary rocks were laid down pre-Tertiary erosion had laid bare a range of metamorphic rocks such as are now exposed, for the Tertiary rocks rest on poorly metamorphosed Te Anau rocks in the Eglinton Valley and on more metamorphosed schists in the Lake Wakatipu region. Dr. Benson has pointed out to the writer that the apposition of the Chl. 2 and Chl. 3 sub-zone rocks on the east side of the Late Tertiary Moonlight thrust-fault to the Chl. 4 rocks on the western side suggests that such formations were in close proximity prior to the deposition of the Cainozoic sediments; and further that the great overthrusting movements which occurred here in Late Tertiary time were in large measure a revival of dislocation along a plane of faulting that had been active in Pre-Tertiary times.
In conclusion, the writer wishes to thank sincerely Professor W. N. Benson and Dr. F. J. Turner for the assistance they have given him with the development of this paper. To Professor W. B. Benham acknowledgment is due for having allowed the writer to make use of his notes on the Bob's Cove “porpoise”.
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