Art. XXIII.—Tertiary Geology of the Area between Wharekuri and the Otiake River, North Otago.

By G. H. Uttley, M.A., M.Sc., F.G.S., Scots College, Wellington.

the greensands below the limestone in that locality with the Hutchinsonian-Awamoan horizon at Oamaru, instead of correlating the fossiliferous beds above the limestone at Wharekuri with that horizon, as McKay had done (1882B, p. 103). A similar error was made by Hutton in correlating the greensands below the limestone at Waihoa with the Pareora (Awamoan) horizon. Park's correlation lent support to his view that the Ototara limestone and the Waitaki Valley limestone were at different horizons. The latter in a later work (1918, p. 110) still maintained that the Wharekuri greensands are Hutchinsonian in age, but the writer is in agreement with Marshall (1915, p. 386) that the greensands are pre-Ototaran.

The geological evidence clearly shows that the Waitaki Valley is a tectonic depression of post-Awamoan age, as affirmed by Cotton on geomorphological evidence, in opposition to the view of Park (1905, p. 523) that the depression was in existence before the deposition of the Tertiary sediments.

II. General Description of the Area

From the vicinity of the Wharekuri-Otekaike fault, which bounds the present area on the southwest to within half a mile of the Waitaki River, prominent ridges and hills, possibly composed almost entirely of heavy gravels and silts, form prominent features of the landscape between the lower middle course of the Awahokomo Creek and the Kurow River. These gravel uplands are in many places 800 ft. above the Waitaki valley-plain. From the Kurow River to the Otiake River a gravel-covered tableland sloping gently towards the Waitaki River forms a rather strong contrast to the pyramidal hills and ridges just mentioned. Several prominent salients stand above the general surface of this evenly sloping plain, and, where their flanks have been cut into, fossiliferous beds are exposed.

The southwestern portion of the area is bounded by the steeply rising foothills of the Kurow Mountains. Within a short distance of the fault-line the country rises rapidly to a height of 3,000 ft., or over 2,000 ft. above the valley-plain of the Waitaki River. To the northeast the area is flanked by the block mountains of South Canterbury, reaching to heights of 3,000 ft. not far from the Waitaki River, which now flows close to the almost undissected front of these mountains.

The average height of the Waitaki valley-plain is 650 ft. above sea-level. Kurow Hill (1,947 ft.), composed of Maitai sediments (greywacke), rises abruptly from the general level of the plain, and farther to the northwest the hill on which Trig. Station G is situated reaches a height of nearly 1,300 ft., these two prominences being separated by the depression known as the Little Awakino Valley.

The district is thus a relatively depressed area lying between the block mountains of North Otago and South Canterbury, drained by the Waitaki River, which is fed, from the Otago side, by a number of small streams, flowing in a northeasterly direction from the Kurow Mountains.

III. Geology of the Area.

The beds dip 70° in a direction N. 10° E. Beds (a), (c), and (e) are gravel deposits, composed of pebbles up to the size of a cricket-ball, intermingled with fine sands. The pebbles consists of greywacke, sandstone, and quartz, all well water-worn. Bed (c) contained a piece of lignitized wood. Beds (b) and (f) are light-coloured greenish-grey unctuous clays. Bed (d) is a fine micaceous sandy bed. On the opposite side of Wharekuri Creek, not 20 yards away, a steep face of the Maitai rocks rises abruptly from the bed of the creek. Slickensided surfaces were noted, and much crush-breccia; and extensive faulting is everywhere indicated. The steeply dipping beds just described show no contact with the other Tertiary rocks in the area.

McKay's section (1882B, p. 101) certainly indicates the order in which the various rocks crop out, as the Wharekuri Creek is followed from its junction with the Waitaki River to the point past the coal-mine, where the tilted beds occur, except that the limestone does not occur in the section exposed in the creek. The tilted beds just described were said by McKay to be of Upper Pareora (Awamoan) age, and to contain, the coal deposits that are worked at Wharekuri. He observed no junction of these tilted beds, and the quartz, sandstone, and clays in which the coal-seam occurs are separated from them by slope deposits and heavy river-gravels. The composition of the beds is also quite different from the beds associated with the coal at Wharekuri Coal-mine. Park (1905, p. 524) stated that there was no evidence to show that the coal-rocks lie at the top of the sequence. The tilted beds, however, may lie conformably at the top of the Tertiary series, although this cannot be definitely affirmed, as no junction was observed. They have certainly been involved along with the Tertiaries in the tectonic movements of the district, the evidence for which is seen at many points in the Waitaki Valley. These tilted gravels appear to be widespread in the Upper Waitaki Valley and in the Waihao district, for McKay described another section in the former locality as follows: “In this section [Quail Burn] the lowest beds seen are soft sandstones, divided into thick bands by beds of greenish greasy clay. These beds dip northwest at an angle of 45°…. At two or three places along the southeast slopes of these hills pieces of lignite have been found and … having, as I consider, proved that the Wharekuri coal-seam farther down the Waitaki occurs in beds of this age, there is more than a possibility of coal being found near the mouth of the Quail Burn.” No other geologist would, agree with McKay in the inference drawn in the last part of this quotation, but the description of these gravel-beds indicates that they are similar to the tilted beds at Wharekuri.

In the Waihao district the gravels are also often tilted, and Hector refers to these in the following words: “With the Waitaki Valley as it now is these beds have no direct connection, since they abundantly show that movements of the lands involving a considerable, alteration of its surface configuration have taken place since their deposition; the beds being frequently tilted at high angles, especially in districts distant from the coastline” (1882, p. xxv), These tilted gravels in the Wharekuri locality are overlain unconformably by the high-level terrace - gravels (McKay 1882B, p. 102).

(2.) Awahokomo Basin

McKay (1882A, p. 66) states that the Otekaike limestone at Wharekuri “is traceable as a continuous line for three miles.” The exposure, however, has nothing like the extent ascribed to it by McKay; it crops out

the Maitai rocks, which, at this point rise very steeply. From the green sands the following forms were obtained:—

• Corbula canaliculata Hutt.

• Cucullaea sp.

• *Limopsis aurita (Brocchi)

• Panope orbita Hutt.

• Polinices gibbosus (Hutt.)

• Ostrea sp.

• Turritella cavershamensis Harris

Corals similar to the genera collected on the south, bank of the Waitaki River also occur here

Marshall (1915, p. 381) obtained several species from these greensands at an horizon lying 20.ft. above the quartzgrits. This writer recognized the fault in this locality, and traced the fault-breccia towards Wharekuri, and there can be no doubt that this fault is a continuation of that described above at Wharekuri.

(3.) Awakino Basin.

Traces of the quartzgrits are seen in many places in the basins of the Awahokomo and Little Awakino Streams. In the basin of the latter, about a mile and a half from the main road, these rocks are exposed on the right and left banks, and to the southwest the eroded surface of the Maitai rocks rises from beneath them, and slopes gently upwards towards the crest of Kurow Hill. The quartzgrits evidently formerly covered the whole of this “fossil plain,” which has now been partially stripped of its former cover. The plain forms the back slope of the tilted block figured by Cotton (1917B, p. 432). This block will be referred to later as the Awakino tilted block. Quartzgrits and greensands occur as mapped in several places in a southwesterly direction towards the Big Awakino River, and in many places close to the line of the Wharekuri-Otekaike fault, where the Maitai rocks rise very abruptly. A mile west of the trigonometrical station on Kurow Hill there is a small coal-mine, which supplies an inferior type of coal. It is being worked at present close up to the face of a steeply rising greywacke scarp. The coal-rocks dip away from the scarp at 45° in a direction S. 28° W. The scarp, which is almost undissected in this locality, extends in a northwesterly direction, gradually diminishing in height, and reaching the road-level about half a mile northwest of the point where the west branch of the Little Awakino crosses the road on the southwest side of Kurow Hill. The scarp is evidently a fault-scarp, determined by a fault of diminishing throw, which trends N. 60° W. to meet the main Wharekuri-Otekaike fault. The fault bounds the Awakino tilted block on the southwest. In the angle defined by these two faults the gravel deposits are of great thickness, but the basal quartzgrits and overlying greensands (McKay, 1882B, p. 102) crop out occasionally. The Tertiary rocks have evidently been extensively eroded, and their remnants are buried by the gravel deposits, which now form hills 2,000 ft. in height (loc. cit., p. 99). Cuttings show that the gravels are of at least two types—heavy greywacke and sandstone boulder deposits, and deposits composed of well-rounded sandstone pebbles and sands. At one place on the road between the basin of the Little Awakino and the Big-Awakino the soft unctuous bluish clay similar to that described at Wharekuri was observed in a cutting. McKay states that the latter (the so-called “Pareora gravels”) are highly tilted in the present locality, and he refers the coal deposits of the neighbourhood to the horizon of these rocks (1882B, p. 102). The gravels extend to the Kurow River, forming even-topped elongated ridges and pyramidal hills, which have been blocked out by the action of the numerous intermittent streams that drain the area. Across

this gravel-mass the Big Awakino flows, entrenched below the general surface of its former valley-plain, which in this portion of its course, over the area lying between the two faults referred to above, is about a quarter of a mile wide. At the coal-mine the stream enters a narrow gorge, which it has cut through the Awakino tilted block on its way to join the Waitaki River. The Big Awakino pursues a remarkably straight course from its source, near the crest-line of the Kurow Mountains, to its mouth, and this course is evidently consequent on the initial deformation; but, as the movements were probably not simultaneous over the whole of northern Otago, the uplift of the Awakino block may have commenced later. The course of the Big Awakino across the Awakino block in a narrow gorge-like channel must be considered as antecedent to the uplift of this block. Its lower course is therefore what Cotton (1917A, p. 253) has termed “anteconsequent.”

(4.) Kurow River to Otiake River.

On the left bank of the Kurow River, four miles above its junction with the Waitaki River, an outcrop of fossiliferous Tertiary rocks occurs. There is only a small exposure, extending along the bank for about 30 yards. The rocks dip at an angle of 46° towards the south-west, and the Maitai rocks rise steeply a short distance from these beds, The junction is again obscured by the gravel deposits, but it is undoubtedly a faulted one. The Tertiary exposure here consists of a hardened calcareous greensand containing Foraminifera, mainly in the form of glauconitic casts. Minute subangular grains of quartz also occur. The larger fossils are poorly preserved, and specific identifications could not be made. The following genera were found: Cucullaea, Malletia, Panope, Pecten. This outcrop is on the line of the Wharekuri-Otekaike fault.

The outcrops of Tertiary rocks are few and scattered between the Kurow River and the Otiake River. About three-quarters of a mile to the south-east of the last locality quartz-grits occur in places, and farther on there is a small exposure of a glauconitic calcareous rock. The only forms obtained were Dentalium solidum Hutt. and Limopsis aurita (Brocchi). Near the source of the most southerly tributary of Malcolm's Creek the quartz-grits again crop out, flanked to the south-west by the steeply rising foothills of the ranges. Traces of greyish-green glauconitic sandstones were found near Trig. Station N. In a cutting on the road that leads from Malcolm's Creek to the basin of the Otiake River there is a small outcrop of calcareous rock, from which the following fossils were obtained:—

• Corbula canaliculata Hutt.

• Cytherea chariessa Sut.

• Dentalium solidum Hutt.

• *Limopsis aurita (Brocchi)

• *Malletia australis (Q. & G.)

• Nucula saggitata Sut.

• Panope sp.

• Pecten huttoni (Park)

Traces of brachiopods were also found.

A mile from the railway-line the following section (fig. 3) is exposed on the right bank of the Otiake River:—

Fig. 3.—Section, right bank of Otiake River. (a), Limestone (Otekaike limestone); (b), glauconitic calcareous shell-bed; (c), calcareous sandy mudstone; (d), hardened calcareous glauconitic bed; (e), calcareous sandy mudstone.

IV. General Succession and Palaeontological Notes.

The Tertiary rocks in the Waitaki Valley form a conformable sequence. The general succession is similar to that in the Waihao district of South Canterbury. Quartzgrits, often containing coal, are followed by micaceous quartzose, greensands with interbedded concretionary bands, usually fossiliferous; these are followed by calcareous glauconitic greensands (containing a little microscopic quartz and mica), which are often fossiliferous. In the basin of the Maruwenua River and at Black Point a few fossils have been determined, and these undoubtedly represent an horizon near the base of the greensands—Park's Bortonian. The looser glauconitic greensands lying above the Bortonian have not proved fossiliferous in the

done by Park in his latest work (1918, p. 26). Many of the species contained in these lower greensands occur also in the Awamoan, but the latter horizon contains a much greater variety of species, and a large number of these appear to be restricted to this horizon. In the Oamaru district the brachiopods have proved serviceable in differentiating several horizons, but some of these brachiopods are apparently restricted to this area, and detailed correlation with beds in the Waitaki Valley is not yet possible. This brachiopod fauna has been discussed in another paper in this volume (pp. 152–53). Corals, Echinoderms, and Foraminifera occur abundantly in the Oamaruian of North Otago, but our knowledge of them is very incomplete, and useless for detailed stratigraphical work. A revision of all these groups is urgently needed.

Of the ninety-four species of Mollusca determined from the beds above the limestone in the Waitaki Valley (Otiake beds), seventy-three species occur in the typical Awamoan at Oamaru, four occur in the Hutchinsonian (of Thomson), twelve forms have apparently not been recorded elsewhere in North Otago, whilst only five species have not been previously recorded from post-Ototaran beds. Five of these not recorded elsewhere are new species, seventy-nine species have now been listed from the greensands below the limestone in the Waitaki Valley, and fifty-six of these occur in the Awamoan beds at Oamaru. Nearly three hundred species have been recorded from the Awamoan beds at Oamaru; but in the case of the beds of the Waitaki Valley the collections are few, and previously unrecorded species are continually turning up. The figures quoted above, in the absence of fairly exhaustive collections, do not give much ground for definite conclusions, but they do show that the Otiake beds contain a molluscan fauna of which 78 per cent. occurs in the typical Awamoan, beds of Oamaru. The fauna of the greensands below the limestone contains 70 per cent, of the fossils recorded from the Awamoan at Oamaru, showing that in the Waitaki Valley the fauna above and below the limestone has a strong resemblance to the typical Awamoan fauna (Hutton's Pareora).

V. Physiography of the Area.

VI. The Gravels (excluding Recent Deposits).

The terraces and gravels of the Waitaki Valley are well worthy of a detailed study, but good topographical maps are a prime necessity. Some remarks, however, should be made on the gravel deposits. McKay clearly recognized that the gravels were not all of the same origin. His descriptions are somewhat difficult to follow. He distinguishes three types of these deposits—(1) angular gravels, (2) well-rounded coarse gravels, (3) gravels and sands with lignite deposits. He notes that (1) and (2) contain fossiliferous Triassic and Permian boulders, and that (3) are often highly tilted. These three types can undoubtedly be recognized, but it will be

a difficult matter to trace their boundaries, and this has not been attempted in the present paper. The angular gravels are ascribed by McKay to the action of glaciers—that is, they are glacier-deposits that owe their origin to former glaciers from the neighbouring Kurow and Hakataramea Mountains. Not having discovered these Triassic and Permian fossiliferous rocks in the Kurow Mountains, McKay concluded that glaciers brought them from the Canterbury mountains which were known to contain these older fossiliferous rocks. There is, however, a total absence of the characteristics of glacial deposits, glacial striations being quite lacking. It is true that large masses of rock and fine silts and clays are mixed confusedly together in the deposits near Wharekuri. Marshall explained these angular deposits differently, and (1915, p. 381) stated that the Maitai rocks in the neighbourhood of the fault (Wharekuri-Otekaike fault) have been much shattered; that weathering has developed their shattered nature, and they break down into “a clayey material which still contains angular fragments of rock.” Park (1904, p. 448) traced the fossiliferous boulders to their source near the summit of Mount Mary. The rocks were found in situ at a height of 5,160 ft., at a point distant about three miles and a half from the Wharekuri-Otekaike fault-line. These deposits which occur close to this line were probably derived from the dissection of the fault-scarp as it rose, for deep aggradation would take place as the deformation proceeded. The sloping surface of the tilted block, to the west of the fault-line, would also undergo degradation by the numerous consequent streams, and the waste would be spread out on the floor of the depression, forming an alluvial gravel-plain. This plain appears to have been built up after the reduction of the valley lowland to somewhat low relief. The extensive aggradation would therefore imply a great increase in the supply of waste due to the increasing differential elevation. As the streams that deposited these gravels are now well entrenched, regional uplift has probably been the most recent movement. McKay believed that the tilted sandstone gravels at Wharekuri lay conformably above the higher fossiliferous beds. They may do so, but the writer was unable to satisfy himself on that point. There is no doubt, however, that they have been involved in the differential movements, and may possibly represent the period of emergence of the land.

VII. Summary and Conclusion.

(1.) The Maruwenua limestone is overlain directly by the Otiake beds (Hutchinsonian-Awamoan).

(2.) McKay's unconformity between the lower and upper parts of the limestone is non-existent, and all other observers agree that the rock is a unit.

(3.) McKay correlated the base of the limestone with the Ototaran limestone; the upper part (his Otekaike limestone) must, therefore be Upper Ototaran, -and the overlying beds (McKay's Hutchinson's Quarry beds) are the equivalent of the Hutchinsonian-Awamoan of the coastal district.

(4.) The upper part of the limestone at Landon Creek contains a number of brachiopods that are similar to the brachiopods from the lower portions of the Maruwenua limestone (so-called Waitaki stone). Some of these brachiopods do not rise above the Ototaran of the typical Oamaru district. The evidence points to the limestone of the Waitaki Valley being Ototaran.