The Geology of Mount Grey District, North Canterbury.
The area under consideration consists of 70 square miles of hilly country, lying about 10 miles north and west of the town of Rangiora. It forms a rectangular block seven miles from north to south, and ten miles from east to west, sparsely settled, and practically roadless, on account of the poor quality of the land. Excellent geological sections are visible along the larger streams—the Okuku, Grey, and Kowhai Rivers—but elsewhere exposures are poor, on account of a thick mantle of waste and dense vegetation.
The writer is indebted to Dr. R. S. Allan and Dr. G. J. Jobberns, of Canterbury University College, and to Dr. H. J. Finlay and Dr. J. Marwick, of the New Zealand Geological Survey, for much useful criticism and advice during the course of this work. The stratigraphical correlations are largely based on the work of Dr. Finlay on the Foraminifera, of Dr. Marwick on the Mollusca, and of Dr. Allan on the Brachiopoda collected during the survey. The writer is glad to express his thanks for this help, which has contributed materially to the results obtained. He is also indebted to Mr. A. Hampton, New Zealand Geological Survey, for drawing the map and geological columns.
Work of Previous Investigators.
The Mount Grey district has received little attention from geologists. As early as 1849 Forbes (1855), assistant surgeon on the Admiralty survey ship Acheron, journeyed up the Kowhai River to the foot of Mount Grey, and returned down the Karetu River, a tributary of the Okuku. His account is of historical interest only, as being among the earliest writings on the geology of New Zealand. The area he traversed included much of that described in this paper; he mentioned the tilted gravels, since assigned to the Kowhai series, and a stratum of hard blue clay, dipping south-east, and containing an immense number of marine shells, the genera of which were specified. The blue clay was overlain by sandstone and underlain by dense hard limestone. These beds are obviously Tertiary, although Forbes expressed no opinion on this point.
Hutton (1877) referred to the district in a general account of the geology of the north-east part of the South Island. He recorded his Pareora formation in the undulating country around the base of Mount Grey.
McKay (1877B) spent a day examining the limestone and associated beds in the Karetu River and in the west branch of the Grey River. His account, which was illustrated by a section, is remarkably clear and comprehensive; he particularly noted the absence of the Amuri limestone in the west branch of the Grey River.
Haast (1879), in his geological map of Canterbury and Westland, recorded his Mount Torlesse formation (Upper Palaeozoic),
Waipara formation (Cretaceo-Tertiary), and Oamaru and Pareora formations (Middle Tertiary), in this area. He does not appear to have visited the area personally, but he correlated the tilted gravels around the base of Mount Grey with the beds forming the Mairaki Downs on the south side of the Ashley River, and suggested a Lower Pliocene age for them.
These tilted gravels around the base of Mount Grey have been mentioned by a number of writers in general accounts of the geology of New Zealand. Hutton (1885) referred them to the Wanganui system, but remarked that they were difficult to distinguish from the upper gravels of the Pareora system. Park (1910) considered them to be older fluvio-glacial drifts. Thomson (1917) referred to them more fully, but was extremely doubtful whether they should be assigned to his Notocene or Notopleistocene deposits.
Speight (1919) described the Tertiary sequence in this area in a paper on the older gravels of North Canterbury. He was mainly concerned with the older gravels and associated beds, which he named the “Kowai” series and assigned to the Pleistocene period, but he also described sections through the Tertiary beds in the two branches of the Grey River and in the Kowhai River.
Morgan (1919) gave a very brief summary of the geology of Ashley and Kowhai Counties, of which this area forms part, from the viewpoint of limestone resources. He quoted analyses of limestone from the area, and listed previous reports on the geology.
Allan (1938) described the Weka Pass stone in this area, and cited palaeontological evidence to show that it correlated with his Duntroonian stage of the Oamaru system. He discussed Speight's account (1919) of the Tertiary sequence in the Grey River in the light of more recent work, and corrected some errors of correlation.
In addition to the above brief references, much of what has been writen about the Middle Waipara-Weka Pass district, which adjoins this area to the north-east, is of interest, as the stratigraphical sequence is similar. The most comprehensive account of the geology of the Middle Waipara-Weka Pass district is that of Thomson (1920).
The history of this area begins in the Lower Mesozoic with the deposition of a thick series of coarse sediments, which now make up the conglomerates and sandstones of the Hokonui series. These sediments were derived from a nearby high land of sedimentary rocks. Little is known of the time of these events, for the rocks are unfossiliferous, and correlation rests on lithological similarity with known Triassic rocks in other parts of Canterbury.
In the Lower Cretaceous, orogenic movements—the post-Hokonui orogeny of Thomson (1917)—caused intense folding and compression of these sediments, and probably uplifted them to form a considerable land mass. Erosion followed, and by late Cretaceous times this land mass had been reduced to a surface of low relief which was then flooded by the sea. On the truncated surface of the older rocks a considerable thickness of marine sandstones and clays were deposited. Then, from the Upper Cretaceous to the Middle Eocene, no sediments were deposited, or if any were deposited they were eroded away be-
fore the greensands, clays, and sandstones of Bortonian age were laid down. At the end of this period the supply of terrigenous sediment decreased sharply, and for a considerable period more or less calcareous sediments were deposited. This period was one of similar conditions over most of North Canterbury, and was marked by the deposition of the Amuri Limestone, the Weka Pass Stone, the Grey Marl, and the Whiterock (Hutchinsonian) Limestone. After the Whiterock Limestone was deposited, conditions changed somewhat, and the limestone is overlain by marly clays which grade upwards into sandstones—the Tokama series (Upper Miocene).
The following table summarizes the geological history of the area, as interpreted from the stratigraphy, structure, and physiography.
|Diastrophic Events.||Corresponding Geological Events.|
|Orogenic movements—late Palaeozoic?||Production of an elevated land surface outside the area.|
|Period of relative crustal inactivity.||Rapid erosion with the deposition of the Hokonui series in a geosynclinal trough.|
|Post-Hokonui orogenic movements—Lower Cretaceous.||Folding and uplift of the Hokonui series, with the production of a high land, and resulting erosion.|
|Stage of Upper Cretaceous sea-advance.||Continued erosion of the high land with the production of a surface of low relief, on which was deposited Upper Cretaceous sediments.|
|Interruption of deposition.||Disconformity separating Upper Cretaceous and Middle Eocene strata.|
|Stage of Lower Tertiary sea-advance.||Deposition of the Ashley series, Karetu series and Amuri Limestone. Decrease in amount of terrigenous sediment deposited.|
|Stage of oscillatory movements.||Deposition of the Weka Pass Stone, Grey Marl and Whiterock Limestone.|
|Stage of Middle Tertiary sea-retreat.||Deposition of the Tokama series on a rising sea floor.|
|Crust movements—late Miocene or early Pliocene. Commencement of the Kaikoura deformation.||Uplift of the earlier Tertiary rocks with local folding and faulting, followed by erosion. Uplift of the Southern Alps to the west. Deposition of the Kowhai series by erosion of these mountains, with the development of an outwash gravel plain in front of them.|
|Continuation of orogenic movements.||Uplift of the Mount Grey block; erosion of the earlier formed outwash gravel plain with the development of a new plain at a lower level.|
|Climatic changes, coastal recession, or regional uplift.||Rejuvenation.|
The increasingly coarse nature of the latter beds presaged an important change in physical conditions at the end of this period, in late Miocene time. Elevation of the area took place. Emergence as dry land appears to have been local, as in the western part of the area the Kowhai series rests unconformably on the eroded surface of the Whiterock Limestone, the Tokama series having been removed by erosion, while in the eastern part the Kowhai series rests disconformably on the Tokama series, the upper surface of which is channelled and bored by marine organisms.
Profound alterations in the physical geography of North Canterbury took place at this time. On the planed surface of the earlier
Tertiary rocks the Lower Pliocene Kowhai series was deposited. The change in the sediments from the limestones and clays of the Miocene to the coarse sandstones and greywacke gravels of the Lower Pliocene indicates the rise of a high land of pre-Cretaceous rocks. This high land was probably the Southern Alps to the west. The movements which uplifted the Southern Alps were part of the “Kaikoura deformation” (Cotton, 1916). It has become increasingly apparent that the orogenic movements of the Kaikoura deformation did not begin contemporaneously all over New Zealand. In this part of the South Island these movements began as early as the beginning of the Pliocene; but the Kaikoura Mountains and the Tararua-Ruahine-Kaimanawa Ranges were probably not uplifted till late in the Pliocene (cf. Marwick, 1933, p. 959).
After the deposition of the Kowhai series, block-faulting movements resulted in the elevation of the Mount Grey-Mount Karetu Range in the northern part of the area, and in the dislocation of the previously continuous sheet of Tertiary rocks. Since that time erosion has removed the covering beds from the uplifted Mount Grey-Mount Karetu Range, and the basement of Hokonui greywackes is exposed on the summit of the range as a fossil plain sloping to the north-west. The streams flowing from Mount Grey and Mount Karetu bevelled the relatively soft Cretaceous and Tertiary rocks lying in front of the hard rock highland, and spread a veneer of loose sandy gravels on the planed surface.
Since that time a general lowering of base-level has resulted in the entrenchment of the streams draining the area, and in the development of extensive terraces at varying levels.
The structure of that part of North Canterbury within which this district lies consists of elevated blocks of pre-Cretaceous rocks with a general north-east trend, separating lowland areas which in the interior are intermontane depressions. The elevated blocks are often bounded by faults on one or more sides. In the lowland areas Cretaceous and Tertiary rocks are found, remnants of a once wide-spread unit, which has been dislocated by faulting and eroded off the higher country.
The Mount Grey district can be divided into three structural and topographic units:
The Mount Grey-Mount Karetu Range, a highland of hard rocks of pre-Cretaceous age, with a general elevation of about 3000 feet.
The undulating country around the base of this highland, formed of Cretaceous and Tertiary rocks. Its elevation varies from about 1400 feet at the base of Mount Grey to 500–600 feet where it merges into the next unit.
The level country bordering the Ashley River and its tributaries. This level country extends in places into the southern part of the area, where the Tertiary beds of zone 2 everywhere dip below the post-Pliocene gravels forming the plain. This zone is part of the-Canterbury Plains, from which it is nearly cut off by the Mairaki Downs, low hills of tilted Pliocene gravels on the south bank of the Ashley River.
The part of the Mount Grey-Mount Karetu Range within the area is defined by three faults: the fault on the north-east side of Mount Grey (F1), the fault on the south side of Mount Grey (F2), and the fault up the Karetu Valley (F3). Faults F1 and F2 are prominent physiographically as well-defined scarps which have been somewhat dissected by streams flowing from Mount Grey. The uplifting of Mount Grey along faults F1 and F2 took place after Lower Pliocene times, since the Kowhai series, together with the older Tertiary and Cretaceous rocks, are cut by these faults. The steepness of the scarps and the well-marked faceted spurs suggest a comparatively recent date for the faulting. It is probable, however, that these features are the result of differential erosion of the soft Tertiary rocks brought against the hard greywacke highland by the faulting, the scarps being wholly or in part fault-line scarps. This conclusion is supported by the variation in the height of the scarps; the maximum height is developed where the hard pre-Cretaceous rocks are faulted against the soft younger rocks. Where both sides of the faults are greywacke, the scarps tend to disappear; and where the faults run out into the little-consolidated gravels of the Kowhai series the scarps have been entirely obliterated by erosion. The physiographic history of the area suggests that the faulting took place not later than the Upper Pliocene. The displacement along faults F1 and F2 is not known definitely, as Tertiary rocks do not occur on the upthrow side, having been stripped off by erosion. It is probably several hundred feet.
The fault along the Karetu Valley, F3, is not marked topographically by a prominent scarp, but is indicated by the geological structure. Cliffs of Middle Tertiary (Hutchinsonian) limestone form the east side of the valley, while the west side consists of pre-Cretaceous rocks. This fault probably continues to the Okuku River, where, in the south-west corner of the area, there is an isolated exposure of Hutchinsonian limestone 10 chains long, overlain by Kowhai beds. Immediately to the north-west of this exposure of limestone the section is obscured by slumping, but the Kowhai beds exposed in the high banks of the river show signs of faulting. The fault probably dates from Miocene times, as the Kowhai series rests on pre-Cretaceous rocks on the west side of the fault and on Middle Tertiary rocks on the east side. This suggests that the area to the west was upthrown by faulting after the deposition of the Middle Tertiary rocks and before the deposition of the Kowhai series in the Pliocene. Erosion stripped the soft Tertiary and Cretaceous rocks off the pre-Cretaceous basement of the upthrown area, leaving an approximately plane surface on which the Kowhai series was deposited. Erosion and minor renewed movement in post-Tertiary times has determined the present topographic features of this fault-line.
The Cretaceous and Tertiary rocks of this area occur along the downthrow side of faults F1, F2, and F3. They form a structural and physiographic unit, and dip at about 30° in directions varying between south-east and south. The part lying north-east of fault F1 is continuous with the well-known sequence of the Middle Waipara-Weka Pass district. Between faults F2 and F3 the relationship of
the Cretaceous rocks to the undermass is probably a normal unconformity, although no section was seen to confirm this, the boundary between the two being invariably obscured by slumping.
The evolution of the present topography of the Mount Grey district began with the deposition of the Kowhai series in the Lower Pliocene. This series grades from coarse marine sandstones at the base to estuarine or fluviatile gravels in the upper part. At the time of deposition, the Kowhai series probably formed a widespread out-wash gravel plain in front of the elevated land which gave rise to it. This plain, analogous to the present Canterbury Plain, may have extended over much of Canterbury, as ancient gravels, lithologically similar to the gravels of the Kowhai series, occur in many places where conditions have been favourable for their preservation from erosion. Some of these occurrences are: the Mairaki Downs, on the south bank of the Ashley River; in the Malvern Hills, between the Waimakariri and the Rakaia Rivers (Speight, 1928); in the Mount Somers district (Speight, 1938); at many places in South Canterbury, along the eastern edge of the uplands; and in the Waitaki Valley, where these ancient gravels have been named the Kurow series (Marwick, 1935B).
This gravel plain, which probably covered the Mount Grey district, was dislocated in Upper Pliocene or later times by the upfaulting of the Mount Grey-Mount Karetu Range. The streams flowing from the newly-uplifted block rapidly planed out a broad frontal apron to the greywacke highland. A large part of the Kowhai series was removed, the soft sediments of the Cretaceous and Tertiary sequence were neatly bevelled, and a cover of loose sandy gravels spread over the planed surface (Fig. 1A).
This planed surface was similar in nature and origin to the pediments which have been described from the dry country of western North America. It extended beyond the boundaries of the area south and east, perhaps as far as the present coast. This was suggested by Dr. G. Jobberns (1937), in a paper on the lower gorge of the Waipara River, when he wrote (p. 131): … “The possibility presents itself that some of these upper terraces (around the south-eastern base of Mount Grey) of undoubted fluvial origin, may be correlated with our Teviotdale terrace, all forming now discontinuous remnants of an old pediment planed out on the relatively soft beds lying in front of the hard rock upland of Mount Grey …”
The present diversified topography of that part of the Mount Grey district covered with Cretaceous and Tertiary rocks has been derived from the old pediment by sub-aerial erosion (Fig. 1B).
After the development of the pediment, a change in conditions took place, and the previously-established streams began to degrade. Whether this change, which affected the whole of North Canterbury, was due to uplift as suggested by McKay (1877A), to cutting back of the coast (Jobberns, 1937), or to changes in climate and the supply of water and waste (Speight, 1907), is not clear; it was probably due to a combination of these causes. As a result of this change to degradation the streams are now flowing at 400 to 600 feet below the
surface of the old pediment (A—A, Fig. 1B). The gradual lowering of base-level resulted in the formation of terraces at different heights along the principal streams.
A.—Section illustrating the topography of the Mount Grey district about the end of the Pliocene period.
B.—Section illustrating the topography of the Mount Grey district at the present day.
Figure 1.—Generalized sections across the southern part of the Mount Grey district, illustrating the evolution of the present topography.
Weka Pass Stone and Grey Marl.
These streams are at present engaged in planing out a new surface, and, given a sufficiently long continuance of present conditions, this will result in the removal of all trace of the old pediment, and the development of a new one at the level represented by A—A, Fig. 1B. The smaller streams have not progressed very far in this new planation, but the larger rivers—the Ashley, the Okuku, the Kowhai, and the Waipara have removed large portions of the old pediment with the formation of a new plain at a much lower level.
The present topography forms two contrasting types; the first, the steep and rugged topography of the pre-Cretaceous rocks, rising to a height of 3055 feet in Mount Grey and 3184 feet in Mount Karetu; the second, the undulating topography which has developed on the Cretaceous and Tertiary rocks underlying the old pediment surface, which sloped gently downward from a height of about 1400 feet at the base of the Mount Grey massif. The topography of the Cretaceous and Tertiary rocks has been governed by the presence of the hard band of Hutchinsonian limestone, which forms a prominent homoclinal ridge, extending in a general south-westerly direction across the area. The soft Cretaceous and Tertiary sandstones and clays with which this limestone is interbedded have been eroded away to give subsequent depressions between the homoclinal ridge and the hard massif of the pre-Cretaceous uplands, and between the homoclinal ridge and the moderately resistant gravels of the Kowhai series.
The principal streams of the area are probably consequent in origin, but where they flow across the strip of Cretaceous and Tertiary rocks they show considerable adjustment to structure. The streams draining the east side of Mount Grey flow along the strike of the Kowhai series, and the lower course of the east branch of the Grey River is also guided by the strike of these beds. Where the streams run in the direction of the dip they have cut deep, narrow channels, with high, precipitous banks, but where they flow along the strike their valleys are considerably more open and easy to traverse. The Karetu River is an example of a stream that has probably been guided by the fault-line along which its valley is eroded.
The dissection pattern which has developed on that part of the area underlain by the beds of the Kowhai series is interesting. These beds are very porous, and the undulating country underlain by them does not support permanent streams. The gravels forming the greater part of the series are very uniform in their resistance to erosion. The result is that the occasional heavy rains have cut numerous closely-spaced insequent gullies, which are especially well developed in the steep banks bordering the larger streams. In some places this gullying affords striking examples on a small scale of bad-land topography: e.g., the Horseshoe Cliffs, in the east branch of the Grey River.
The following table is a summary of the exposed rock formations in the Mount Grey district.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Age.||New Zealand Stage.||Local Series Name.||Description of strata.||Thick ness (feet).|
|Upper Pliocene-Recent||Terrace gravels||Gravels|
|Lower Pliocene||Waitotaran||Kowhai||Gravel, sand, clay||0–1600|
|Miocene||Upper Hutchinsonian-Taranakian||Tokama||Sandy mustone and sandstone||350–550|
|Hutchinsonian||Whiterock Limestone||Fossiliferous limestone||0–100|
|Waitakian (?). Hutchinsonian||Grey Marl||Sandy marl||50|
|Duntroonian||Weka Pass Stone||Arenaceous glauconitic limestone||40|
|Oligocene||Ototaran||Amuri Limestone||Hard fine-grained limestone||30|
|Upper Bortonian (also Tahuian?)||Karetu||Argillaceous sandstone||400|
|Upper Cretaceous||Loburn||Black clay||250|
|Sulphur Sands||Quartz sands||400|
|Triassic (?)||Hokonui||Conglomerate, grey-wacke, argillite||?|
The rocks forming the elevated mass of the Mount Grey-Mount Karetu Range in the northern part of the district are grouped in the Hokonui series. The series name covers the sedimentary rocks of probable Triassic and Jurassic age, which make up much of the mountain country of New Zealand. In this area the rocks of the Hokonui series are conglomerates, coarse grey felspathic sandstones, and dark-grey argillites. These different rock types would require exhaustive field-work to map separately, on account of the great amount of shearing, shattering, and crushing they have undergone at many places.
The most common rock type is a coarse, grey, massive, felspathic sandstone, commonly known as greywacke, with which is interbedded dark-grey slaty argillite and rare bands of red jasperoid material. Conglomerates are exposed on the Okuku River and in the east branch of the Grey River. The conglomerates on the Okuku River are similar to those that have been described from other parts of Canterbury; they are made up of well-rounded pebbles of brown and grey sandstone, varying from one to three inches in diameter, set in a sandy matrix. The conglomerates in the east branch of the Grey River are of an unusual type; they are coarse and unsorted, containing poorly-rounded rock fragments up to one foot in greatest length. These rock fragments consist mainly of grey sandstone, but there is also a considerable amount of light-grey finely crystalline limestone. The place
of origin of these limestone fragments is unknown, and their age-could not be determined, as no fossils were found in them.
Owing to the massive nature of these rocks, and the extensive-shattering and jointing they have undergone, it is usually difficult or impossible to determine dip or strike. In places where the strike was measured, it was found to vary between north-west and north-north-west; the dip is uniformly high, from 60° to 90°. The thickness of this formation is unknown, as the structure has not been worked out, but the shallow-water nature of the rocks, and their wide distribution, suggest that the thickness is many thousands of feet.
The rocks in this area are unfossiliferous, so their age cannot be directly determined. Lithologically they are similar to rocks which in other parts of Canterbury have yielded fossils of Triassic and Jurassic age, and they may be grouped with these rocks in the Hokonui series. Fossils are known from the Okuku Range, the continuation of the Mount Grey-Mount Karetu Range to the north-west, that are of Upper Triassic (Noric) age (Trechmann, 1917, p. 175 p Marwick, 1935B, p. 300).
The beds immediately overlying the pre-Cretaceous rocks crop out in the two branches of the Grey River. The contact with the underlying formation is nowhere exposed, but in the east branch it is a fault plane, while in the west branch it is probably a normal unconformity.
The Sulphur Sands consist of blue or grey little-cemented quartz sands, with a small proportion of argillaceous material, and some glauconite. The term “Sulphur Sands” arises from the yellow efflorescence on these rocks, which has been ascribed to the presence of sulphur compounds (Thomson, 1920, p. 343). This efflorescence is combined with a distinct smell of sulphurous gases, especially where these rocks are cut through in narrow gorges. It is presumably due to the oxidation of iron sulphide, present mostly in a state of fine division in the rocks, but partly in small nodular concretions. This iron sulphide also gives rise to the iron-bearing colloidal matter in water seeping from these beds, which forms a rusty-brown incrustation on the rocks in many places.
These beds were probably deposited in shallow water; the general texture is coarse, and in the west branch of the Grey River is exposed a one-inch band of conglomerate of small greywacke pebbles set in a sandy matrix. The Sulphur Sands are approximately 400 feet thick; the bedding is poor, so the thickness cannot be accurately estimated.
The “Sulphur Sands” may be correlated with the “Sulphur Sands” and “Saurian Beds” of the Middle Waipara-Weka Pass district. Their stratigraphical position is the same, they are lithologically similar and they contain large concretions like those which in the Middle Waipara-Weka Pass district have yielded numerous reptilian bones. In the Mount Grey district these concretions are very large, generally cylindrical in shape, and up to 20 feet in length
a few molluscan remains were found in them, but no reptilian bones. These beds belong to the Piripauan stage of Thomson (1920), equivalent to the Senonian of Europe.
The beds for which the name Loburn series is proposed consist of soft black or dark-grey clays which contain considerable quantities of finely divided mica. These clays contain also occasional small concretions of marcasite, and show in places a yellow efflorescence, similar to, but not so well developed as, the efflorescence on the underlying Sulphur Sands. Their thickness is about 250 feet. When followed downwards the Loburn series passes without any perceptible break into the underlying Sulphur Sands; the clay becomes sandy, and lighter in colour, and glauconite appears. There is probably no great time break between the two series, the Loburn series having been laid down on the Sulphur Sands without any long pause in deposition.
No macro-fossils were found in the Loburn series. A sample of clay from the Loburn series in the east branch of the Grey River (5329*) was submitted to Dr. H. J. Finlay for examination. He found that the foraminiferal fauna correlated the Loburn series with the lower Rakauroa formation of the Poverty Bay district, the age being Upper Cretaceous (Finlay, 1939B, p. 535).
In a more recent letter to the writer Dr. Finlay remarks “The fauna of your Loburn series sample (5329) compared with the Middle Waipara section is definitely like one I have from the top of the Saurian Beds, and unlike the higher Piripauan Waipara Greensands, so these latter seem to be missing in the Grey district, unless represented by the overlying greensand” (included in the Ashley series). This strengthens the evidence for the close relationship between the Loburn series and the underlying Sulphur Sands, assuming that the Sulphur Sands represent part of the Saurian Beds of the Middle Waipara-Weka Pass district.
The Loburn series is overlain by a band of greensand, dark-green, almost black, in colour, consisting of coarse-grained glauconite with a little quartz sand. It is well exposed in the two branches of the Grey River, where it varies from four to six feet thick. There is a clear-cut lithological separation from the beds above and below, with angular conformity and without any sign of an erosion surface.
No fossils were found in this greensand, and a sample sent to Dr. H. J. Finlay was barren of Foraminifera. Thus its age cannot be determined from internal evidence. The age of the overlying mudstone is lower Bortonian (Middle Eocene); that of the underlying Loburn series is Upper Cretaceous. Lithologically this greensand belongs with the overlying mudstone rather than with the underlying
[Footnote] * Numbers of samples sent to Dr. Finlay are his locality numbers—see Finlay, 1939A, p. 505.
black clay. It has therefore been included with the mudstone in this description, the name Ashley series being proposed to cover the two lithological units.
The boundary between the Cretaceous and Tertiary in this area is therefore placed at the base of the Ashley series. In the field there is no sign of an erosion break covering this interval, in spite of the considerable time break involved. The boundary probably represents a disconformity of some magnitude.
Overlying the greensand is about 100 feet of soft grey mudstone, which contains large grains and nests of glauconite. This mudstone is best exposed in the west branch of the Grey River, but was also seen in the east branch, where it is largely obscured by slip debris. The contact with the overlying Karetu series is sharply defined, the boundary between them being marked by a thin layer of limonitic material, probably deposited by ground water moving along the contact between pervious and impervious rocks.
No macroscopic fossils were found in this mudstone. A sample of the mudstone (5328, from the east branch of the Grey River) yielded a good foraminiferal fauna, which showed its age to be lower Bortonian (Finlay, 1939C, p. 111).
The name Karetu series is proposed for the beds lying between the Ashley series and the Amuri Limestone. Lithologically these beds consist of soft grey and blue-grey argillaceous sandstones. Glauconite occurs throughout the formation, but is more abundant near the base, where clayey matter is rare, the beds approximating to a quartz-glauconite sand. This series contains bands of concretions of grey sandstone cemented with calcium carbonate. The sandstones are generally massive and unbedded, and it is therefore difficult to estimate the thickness of the series; it is probably of the order of 400 feet.
The Karetu series is best exposed in the gorges of the two branches of the Grey River; isolated outcrops were also seen in the tributaries of the Karetu River. Like other Cretaceous and Tertiary sandstones and mudstones, these beds have little resistance to erosion and weather readily into soil, so that exposures are non-existent except in stream sections.
There is a sharp lithological separation between the Karetu series and the Ashley series. The relationship between the Karetu series and the overlying Amuri Limestone is not clear. There is no sign of a physical break between the two formations, the Amuri Limestone at its base passing rapidly but imperceptibly into a glauconitic limestone and then into a glauconitic argillaceous sandstone, characteristic of the Karetu series. A sample taken from the Karetu series in the east branch of the Grey River, about 150 feet above its base (5449), contained Foraminifera indicating an upper Bortonian age (Finlay, 1939C, pp. 93, 111). The Amuri limestone is Ototaran. If there is no break between the Karetu series and the Amuri Limestone, then the Karetu series contains both upper Bortonian and Tahuian horizons. If there is a break between the two formations (for which
there is no field evidence), then the Tahuian stage may have been a period of non-deposition, or, less likely, a period of uplift and erosion. The writer is of the opinion that the two series are conformable, but decisive evidence might be obtained if samples from the upper 250 feet of the Karetu series were examined for Foraminifera.
The exposures of the Amuri Limestone in this area are the most southerly ones that can definitely be said to be part of this classic formation, which is so widespread throughout North Canterbury and Marlborough.
In the east branch of the Grey River the full thickness of the Amuri Limestone is well exposed, from the top of the Karetu series to the base of the Weka Pass Stone. The greater part of the limestone is a very hard, white, fine-grained rock, closely jointed at the surface into small cuboidal blocks. At the base it passes rapidly, though by imperceptible stages, into a glauconitic limestone and then into a glauconitic argillaceous sandstone characteristic of the Karetu series. The glauconite in this part is in large grains, and appears to be locally concentrated in worm borings. The upper part of the Amuri Limestone also shows a transition, in the space of two or three feet, into the massive sandy limestone of the Weka Pass Stone. Similar contacts between the Amuri Limestone and the Weka Pass Stone were seen on the north-east side of Mount Grey.
The Amuri Limestone is about 30 feet thick in the east branch of the Grey River; on the north-east side of Mount Grey the thickness is about the same, but when followed north across the Waipara River into the Weka Pass district it thickens considerably. The Amuri Limestone does not occur in the west branch of the Grey River; when followed south-west from the fault on the south side of Mount Grey it thins out, and finally disappears near the saddle between the two branches of the Grey River.
The Amuri Limestone is probably the most discussed formation of the Tertiary sequence in New Zealand, and its age, together with its relationship to the overlying formation, the Weka Pass Stone, has been the subject of controversy for many years. During the survey of this area, new evidence bearing on these problems has been obtained. Previously the age of the two formations had been the subject of speculation, fossil evidence being practically lacking—save for a faulty determination by Chapman (1926, pp. 10, 11) of a Danian age for Foraminifera from the Amuri Limestone. On the evidence of brachiopods from the Weka Pass Stone in the Grey River this formation has been placed in the Duntroonian stage (Allan, 1938, p. 91). The Amuri Limestone has been shown to be Ototaran (Finlay, 1939C, pp. 107–8). If there is a disconformity between the two formations the time break separating them is much shorter than previously thought. In this area the contact between the Amuri Limestone and the Weka Pass Stone is quite regular, and shows no sign of a break in deposition. There is no evidence that the lensing-out of the Amuri Limestone is due to erosion prior to the deposition of the Weka Pass Stone. But the question of conformity or otherwise between the two formations cannot be decided on the evidence from
this area alone. When the relationship of the two formations over the whole of North Canterbury and Marlborough is considered, it appears probable that, in certain areas at least, some kind of a break does exist between them. The general change in physical character means some important change in conditions of sedimentation throughout the whole area in which the two formations are found. Such a change in conditions need not have amounted to disconformity. Elevation may have been small, sufficient, perhaps, to bring the upper surface of the Amuri Limestone into the zone of current erosion prior to the deposition of the Weka Pass Stone. The extent of the elevation probably differed from place to place, this being manifested in the variable nature of the contact between the two formations.
No macro-fossils were found in the Amuri Limestone in this area. Two samples were sent to Dr. Finlay for foraminiferal examination. One (5575) came from an old quarry on the west bank of the east branch of the Grey River, about 250 feet above the stream. Neither the top nor the base of the formation was visible in the quarry face, but from the thickness of the limestone exposed this sample probably came from near the middle of the formation. The other sample (5448) came from the glauconitic base of the limestone exposed in the bed of the east branch of the Grey River. The sample 5575 contained Siphogenerina striatissima, Notorotalia stachei, and Rotaliatina sulcigera, an association indicating a Whaingaroan age; the sample 5448 contained a poorly preserved fauna of Kaiatan age. Thus the Amuri Limestone in this district appears to be a compressed deposit filling the whole Ototaran sequence (Finlay, 1939C, p. 107).
Weka Pass Stone.
The Weka Pass Stone is the name originally given to the building stone of the Weka Pass district. It is a formation which extends over a belt of country in Marlborough and North Canterbury nearly 100 miles long, with a maximum width of 15 miles. It is not known in its typical form south of the Mount Grey district.
The Weka Pass Stone in this area varies from an arenaceous glauconitic limestone to a calcareous greensand. The glauconite increases as the formation is traced from north-east to south-west across the area, but is not abundant enough to characterise the rock except in the Karetu Valley and on the Okuku River. Locally the basal part of the Weka Pass Stone contains large numbers of fucoids. The Weka Pass Stone is coarse in texture, much more so than the Amuri Limestone, and unlike the latter formation is thick-bedded and unjointed, this feature giving its outcrops a much more massive appearance.
In the two branches of the Grey River, where both the upper surface and the base of the Weka Pass stone are visible, its thickness is 35–40 feet. It thickens considerably between the west branch of the Grey River and the Karetu River, standing out prominently as white cliffs below a scarp formed by the Whiterock Limestone.
The Weka Pass Stone in this area is sparsely fossiliferous, the best locality for collecting being the cliffs overlooking the west branch of the Grey River. Brachiopods are most abundant, but molluses, echinoderm fragments, bryozoa, and fish teeth are also found.
Enough fossils have been obtained from the Weka Pass Stone to correlate it with the Duntroonian stage (Allan, 1938, p. 91), this correlation replacing the older ones of Ototaran (Henderson, 1929), or Waiarekan (Thomson, 1920).
The fauna of the Weka Pass Stone is as follows:
Cirsostrema lyratum (Zittel). Not uncommon as fragments.
Lentipecten hochstetteri (Zittel). Rare.
These mollusca are not very helpful in the matter of correlation, both being long-ranging forms.
Aetheia gualteri (Morris). Not uncommon.
Liothyrella neglecta (Hutton). Very rare.
Pachymagas sp. Very rare.
Waiparia cf. elliptica (Thomson). Not uncommon.
Aetheia gualteri is usually very abundant in strata of Duntroonian age. Waiparia elliptica is the most characteristic species of the Duntroonian stage. Liothyrella neglecta, while not occurring in the type locality for the Duntroonian stage, is found in strata of the same age at the Curiosity Shop, on the Rakaia River.
Between the Weka Pass Stone and the Whiterock Limestone is some 50 feet of calcareous mudstone, which is the equivalent in this area of the well-known Grey Marl of the Middle Waipara-Weka Pass district. Good sections of the Grey Marl are visible in the two branches of the Grey River; elsewhere its occurrence is indicated by a grassy slope separating the outcrops of the much harder Whiterock Limestone and Weka Pass Stone.
As in most other districts, it is impossible to draw a sharp division between the Weka Pass Stone and the Grey Marl, the top of the Weka Pass Stone passing insensibly after a few feet into glauconitic sandy marls. The glauconitic sandy marl at the base of the Grey Marl becomes finer and much less glauconitic in its upper part. The Grey Marl is overlain by the Whiterock Limestone. In the east branch of the Grey River, where the Whiterock Limestone is absent, the Grey Marl passes up without any perceptible break into the Tokama series.
Macroscopic fossils are very rare in the Grey Marl; a few specimens of Lentipecten hochstetteri (Zittel), some fragmentary brachiopods, and an irregular echinoid were all that were found, and these were of no value for determining the age of the formation.
Two samples from the Grey Marl in this area were sent to Dr. Finlay for examination for Foraminifera. One (5648), from the passage zone between the Weka Pass Stone and the Grey Marl in the east branch of the Grey River, gave a fauna which occurs in this zone in other parts of North Canterbury (Finlay, 1939C, p. 93; the other (5263), from the top of the Grey Marl in the west branch of the Grey River, yielded characteristic Hutchinsonian species.
From studies on the foraminiferal fauna of the Grey Marl Dr. Finlay has recently (1939A, p. 516) placed this formation in the Hutchinsonian stage. In the same paper (p. 530) he expresses the view that the Waitakian stage, lying between the Duntroonian and the Hutchinsonian, cannot be satisfactorily separated from the Duntroonian.
The placing of the Grey Marl in the standard sequence rests on the relationship of the formation to the overlying beds, and on the adequacy of the foraminiferal evidence. A survey of the Grey Marl over a wider area than the Mount Grey district is necessary before a satisfactory conclusion can be reached.
The Whiterock Limestone is the local name given to the moderately hard cream-coloured Hutchinsonian limestone of this area. In the early days of settlement it was extensively quarried for building stone, and is still much used as a source of agricultural lime. It forms the crest of a prominent homoclinal ridge stretching north-east from the Karetu River to the east branch of the Grey River. The limestone is also exposed in the banks of the Okuku River, in the south-west corner of the area, where it is bent up into an anticline and overlain unconformably by the Kowhai series. The limestone varies considerably in thickness; in the Okuku River it is some 40 feet thick, having been reduced by erosion prior to the deposition of the Kowhai series; in the Karetu Valley it reaches a maximum of 100 feet; eastward, it thins gradually till in the west branch of the Grey River it is only three feet thick; farther east it thickens slightly, but later thins again, and is not found north-east of Trig. T, not appearing in the east branch of the Grey River. To the north-east of Mount Grey the typical Whiterock Limestone does not occur, but it is probably represented by part of the Mount Brown series.
The Whiterock Limestone is made up mainly of comminuted fossils; it is a shallow-water deposit, and is distinctly current-bedded in places. Analyses show that it varies considerably in composition, containing from 75 to 96 per cent of calcium carbonate, the impurity being quartz sand and a little clayey matter. The lateral variation in thickness of the limestone and the way it lenses out along the strike are probably due to its shallow-water origin; the upper surface of the limestone shows no sign of erosion, except on the Okuku River. In this locality it is overlain unconformably by the Kowhai series, and not by the Tokama series, as is the case further east.
The only exposure showing the contact between the Whiterock Limestone and the underlying formation, the Grey Marl, is in the gorge of the west branch of the Grey River. The two series are stratigraphically conformable, the agreement in strike and dip being absolute. The actual contact shows some peculiarity; the upper surface of the Grey Marl is slightly irregular, and the irregularities are filled with pockets of shelly material belonging to the overlying limestone. The irregularity of the contact is probably due to current action during the deposition of the limestone. Samples taken from just above (5262) and just below (5263) the Whiterock Limestone in the west branch of the Grey River contained characteristic
Hutchinsonian Foraminifera (Finlay, 1939C, p. 123). This would indicate that the Whiterock Limestone is an intra-Hutchinsonian horizon. (Cf. samples from East Grey, 5654, 5653, 5652, 5651, 5650, 5649, 5648.) In the east branch of the Grey River, where the Whiterock Limestone does not occur, the Grey Marl passes up without a break into the Tokama series.
Fossils are locally abundant in the Whiterock Limestone; the hard matrix renders them difficult to extract, and they are often crushed or otherwise distorted. The most favourable locality for collecting is the site of the old lime kilns on the Karetu River, where brachiopods are common. Mollusca are rare. Echinoderm and bryozoan fragments make up a large part of the limestone. Ophiuroid ossicles occur in the limestone on the Okuku River, and fragments of cetacean bone have been found in the quarry on the Karetu River.
The identified fossils from the Whiterock Limestone are as follows:—
Lentipecten hochstetteri (Zittel). Locally common.
Placopecten hectori (Hutton). Rare.
(The following note on the species was kindly supplied by Dr. R. S. Allan.)
Pachymagas aff. bartrumi Thomson.
Pachymagas aff. cottoni Thomson.
Pachymagas cf. coxi Thomson.
Pachymagas haasti Thomson.
Pachymagas cf. marshalli (Andrew).
Pachymagas speighti Thomson.
Rhizothyris pirum Thomson.
The “Pachymagas” n.sp. is an interesting specimen, but too crushed to name. It has the beak characteristics of Pachymagas (?) uttleyi Allan and “Neothyris” anceps Thomson, two species for which a new genus is necessary. The Whiterock specimen also shows incipient anterior multiplication.
Large brachiopods belonging to Pachymagas and Rhizothyris are common in this formation. The specimens are normally crushed and/or distorted, this accounting for the lack of precise naming in the above list. For the same reason, it is not possible to record the relative abundance of the species in the list.
Nevertheless, the assemblage is clearly of Hutchinsonian age, and the Whiterock Limestone may be compared with:
The Main Mount Brown Limestone (D) of Thomson in the Weka Pass and Middle Waipara district. (Thomson, 1920, pp. 361–2.)
Upper limestone, Bland's Bluff, Ashburton River. (Speight, 1938, pp. 62–3.)
Forest Hill Limestone—Middle limestone. (Allan, 1937, pp. 140–1.)
Clifden Limestone, Clifden, Southland. (Park, 1921, p. 52.)
On comparison with the nearby Hutchinsonian assemblages in the Weka Pass area, one notes the absence at Whiterock of Magadina, Neobouchardia, and Waiparia, all of which are abundant further north and are associated with comparable species of Pachymagas and Rhizothyris.
Overlying the Whiterock Limestone is a thick series of blue-grey sandy mudstones, sometimes calcareous, argillaceous at the base, but becoming gradually sandier towards the top of the series, this feature perhaps presaging a change to terrestrial conditions. Numerous bands of hard calcareous concretions occur throughout, and at the top of the series is a shell-bed crowded with large mollusca. The name Tokama series is proposed for this formation.
The Tokama series is best exposed in the two branches of the Grey River; it is also to be seen in the first tributary of the Kowhai River across the divide from the Grey River, but here only the upper 200 feet of the formation is visible, faulted against the pre-Cretaceous rocks of Mount Grey. In the east branch of the Grey River the series is 550 feet thick; it thins when followed south-west, and in the west branch is only 350 feet thick. Between the west branch of the Grey River and the Karetu River the series is not exposed, but can be traced as a grassy subsequent valley eroded against the much harder Whiterock Limestone. It does not occur on the Karetu River or the Okuku River, because here the Tokama series and part of the underlying Whiterock Limestone were removed before the deposition of the Kowhai series, which rests on the eroded surface of the Whiterock Limestone.
North-east of Mount Grey, the typical blue-grey sandy clays of the Tokama series do not occur. In this locality the interval between the Grey Marl and the Kowhai series is represented by two bands of limestone separated and overlain by sandy beds—the Mount Brown series of Thomson (1920). From the brachiopod fauna the Whiterock Limestone correlates best with the D limestone of the Mount Brown series. The Tokama series is probably equivalent, in part at least, with the beds of the Mount Brown series overlying the D limestone, but palaeontological evidence to establish a good correlation is lacking.
With the exception of the shell-bed at the top of the Tokama series macro-fossils are rare in this formation. Such as do occur are deformed and difficult to collect. Rare brachiopods occur near the base of the series. Molluscs occur sparingly throughout. Fragments of a coral tentatively referred to the genus Flabellum were noted in a number of places.
The shell-bed at the top of the Tokama series is a close-packed mass of large Mollusca. There is no great variety of forms, large species of Polinices and Austrodosinia making up nearly the whole bed. This shell-bed was seen in the two branches of the Grey River, but the exposure in the west branch is only visible after heavy floods have shifted the river gravel which usually covers it. All the species in the following list are recorded from the east branch of the Grey River.
The fauna of this shell-bed is as follows:
Austrodosinia cf. magna (Hutton). Common.
Bassina cf. speighti (Suter). Rare.
Diplodonta (Zemysia) zelandica (Gray). Common.
Eucrassatella cf. ampla (Zittel). Fragment.
Eumarcia plana Marwick. Lunule more concave than typical
Gari n.sp. Fragment. Very narrow anterior end.
Glycymeris (Manaia) hurupiensis Marwick. Juvenile.
Lima aff. colorata Hutton.
Maoricrypta wilckensi Finlay.
Mauia n.sp. aff. huttoni (Suter). Juvenile.
Polinices huttoni von Ihering. Common.
Polinices intracrassus Finlay. Common.
Zelandiella fatua Finlay. Rare.
Zelandiella aff. subnodosa (Hutton). Rare.
The identifications and the notes on the species are the work of Dr. J. Marwick.
This assemblage of Mollusca is difficult to match with that of any of the type localities in New Zealand stratigraphy. The only comparable fauna occurs in a horizon in the lower gorge of the Waipara River. Most of the genera recorded are of long-ranging types. Austrodosinia magna, Polinices huttoni, and the Zelandiellas are typical Miocene forms. A close relative of Z. fatua,* if not the same form, occurs in the Mohakatino beds of the North Island. The position of the Mohakatino beds is not certainly known, but it is either Awamoan or lower Taranakian. Similar specimens to the Z. aff. subnodosa of the Mount Grey district occur in the basal beds of the Urenui series. From the evidence of the molluscan fauna the age of this shell-bed appears to be probably lower Taranakian, i.e. Tongaporutuan.
This correlation is supported by the Foraminifera contained in the shell-bed. A sample (5184), from the shell-bed in the east branch of the Grey River, gave a fauna which, while containing no characteristic Taranakian forms, was extremely like that from a marl occurring at Balmoral, in the Culverden district. The fauna from this marl is definitely Taranakian, as indicated by the presence of the genus Bolivinita. It is not uncommon to find the key genus Bolivinita missing from samples from near the base of the Taranakian (Dr. H. J. Finlay, private communication).
A sample (5262), from the base of the Tokama series in the west branch of the Grey River, has yielded characteristic Hutchinsonian Foraminifera, which correlate it approximately with 5651—100 feet above the Weka Pass Stone in the east branch, i.e., just above the Grey Marl (Finlay, 1939C, p. 123). Samples from higher in the Tokama series did not yield decisive Foraminifera.
[Footnote] * Since this was written, Z. fatua has been collected at several localities probably of Taranakian age.—J. M.
The palaeontological evidence indicates that the base of the Tokama series is Hutchinsonian and the shell-bed at the top is probably lower Taranakian. The 350–550 feet of beds included in the Tokama series thus appear to cover the time interval between the Hutchinsonian stage and the lower part of the Taranakian stage—that is, the Awamoan stage, together with any hiatus above or below it.
The Tokama series may be compared in stratigraphical position to the following formations:
Bands 4–8 at Clifden, Southland, which overlie the Clifden Limestone, the equivalent in that district of the Whiterock Limestone.
The higher beds of the Mount Brown series of the Middle Waipara-Weka Pass district, overlying the main Mount Brown Limestone (D).
The Stethothyris epsilon Limestone exposed at Balmoral, in the Culverden district, which underlies sandy marl containing a foraminiferal fauna similar to that from the shell-bed at the top of the Tokama series.
The three above-mentioned formations are all characterised by a zone containing the brachiopod Stethothyris epsilon Allan. The Stethothyris epsilon zone is probably an Awamoan horizon (Finlay, 1939A, p. 530), but may perhaps be slightly older. The absence of Stethothyris epsilon from the Tokama series might be argued as evidence for a concealed time break therein, but more probably the conditions under which this Tokama series was laid down were not favourable for the existence of this brachiopod.
The Kowhai series of this area is a variable series of littoral beds, consisting largely of gravels, sands, and clays, with shell-beds near the base, and occasional thin lignite seams and bands of carbonaceous shale. The best sections are exposed in the two branches of the Grey River, but incomplete sections are also to be seen in the Okuku River, and in the various branches of the Kowhai River. Away from the streams, except where exposed by slips and washouts, these beds form rolling downs covered with scrub and gorse, and outcrops are poor.
The material making up these beds was derived almost entirely from the greywackes and argillites of the pre-Cretaceous rocks which form the mountains of North Canterbury. Occasional pebbles of the lower Tertiary rocks were noted, further evidence for an erosion break at the base of the Kowhai series. The subangular nature of the pebbles show that the greywacke land must have been near to the area of deposition. The absence of large pebbles suggests that this land was of moderate relief, although it was probably the outlying part of a more elevated tract. The gravels show no trace of a glacial or fluvio-glacial origin, for the pebbles are uniform in size, large boulders being entirely absent; the gravels are quite similar to those that are being deposited near the mouths of the North Canterbury rivers at the present time. The beds of the Kowhai series are generally little-compacted and are easily eroded, but some of the gravel
layers are locally cemented into hard conglomerate by a deposit of limonite, and these bands stand out topographically as small cuestas. The lower beds of the Kowhai series are undoubtedly marine, but the upper beds were laid under estuarine conditions or actually on a land surface.
Throughout the series the beds show intraformational unconformities, especially in the case of gravels overlying sands and clays. The upper surface of the finer beds nearly always suffered some erosion prior to the deposition of the gravels, but in no case is this great enough, or sufficiently widespread, to amount to regional unconformity.
The maximum thickness of the Kowhai series is unknown. When, traced upwards, the formation is always truncated by an erosion surface, on which rests more recent gravels deposited when the base-level was higher than it is to-day. In accordance with the deltaic type of deposition, the series varies considerably in thickness from place to place. In the west branch of the Grey River at least 1650 feet of beds are exposed, and the total thickness in this area locality may well be considerably greater. Of the 1650 feet measured, gravels make up about 1000 feet, sands 400 feet, and clays 250 feet.
The change in the Tertiary sediments of this area from the limestones and clays of the earlier Tertiary formations to the greywacke gravels of the Kowhai series shows that, in late Miocene times, profound alterations took place in the physical geography of North Canterbury, probably the rise of high land to the west. The Kowhai series overlies the earlier Tertiary rocks with marked unconformity in the Okuku and Karetu Rivers, and further west appears to overlap the younger rocks and lie directly on the surface of the pre-Cretaceous rocks. In the valley of the Grey River the Kowhai series lies disconformably on the Tokama series, the upper surface of the Tokama series showing borings by marine animals. North-east of Mount Grey the Kowhai series overlies the upper beds of the Mount Brown series, the equivalent in that area of the Tokama series.
The age of the Kowhai series is given by the fossil Mollusca associated with it, which are moderately abundant in the lower part. The fossils are poorly preserved and difficult to collect on account of the coarseness of the matrix. The specimens from which the following list was compiled were collected from outcrops of the Kowhai series in the valley of the Grey River. The list could possibly be considerably enlarged if collections were made from the shell-beds exposed in the north branch of the Kowhai River, just below the bridge on the Onepunga Road.
The fossils generally occur in thin shell-beds with a sandy or pebbly matrix. They are all of a distinctly shallow-water environment, as is shown by the massive nature of the shells and the coarseness of the matrix. The pelecypods occur as single valves, with the exception of an occasional double-valved oyster, and the rock-borer, Anchomasa. Anchomasa occurs around the New Zealand coast at the present time, where it is found boring into any soft rock around tidal limits. Its environment was quite similar at the time when the Kowhai series was being deposited; a band of clay in the west branch
of the Grey River contains numerous specimens of this fossil, all oriented perpendicularly to the plane of stratification, with the gaping end just below the upper surface of the bed.
The identified fossils from the Kowhai series in this area are as follows:—
Acominia cf. hendersoni (Marwick). Fragments.
Anchomasa n.sp. aff. similis (Gray). Locally common. K.N.G.
Baryspira cf. mucronata (Sowerby). Fragment.
Barytellina aff. crassidens Marwick. Rare.
Bassina speighti (Suter). Juvenile.
Chione (Austrovenus) cf. stutchburyi (Gray). Common. N.G.
Cominella (Cominista [?]) n.sp. Fragment.
Dosinia n.sp. aff. kaawaensis Marwick. Locally common. K.W.
Eucrassatella cf. ampla (Zittel). Common in one shell-bed.
Eumarcia, probably two species. Fragments.
Lutraria solida Hutton. Common. W.N.G.
Mactra n.sp. aff. discors (Gray). Rare. K.R.
Nucula cf. nitidula Adams. Common. K.N.
Ostrea cf. ingens Zittel. Common in one shell-bed. W.N.G.
Ostrea (?) n.sp. Common in one shell-bed.
Ostrea sinuata Lamarck. Common. N.
Paphirus (?) sp.
Struthiolaria cf. incrassata Powell. Rare. W.
Venericardia purpurata (Deshayes). Common. N.G.
Verconella cf. adusta (Phillipi). Rare. N.G.
Zeacumantus n.sp. Rare.
Zeacuminia orcyta (Suter). Rare. K.W.
Zemysia cf. zelandica (Gray). Locally common. K.
The identifications are the work of Dr. J. Marwick.
In the above list identical or closely related species are indicated thus: K—recorded from the Kaawa beds (Laws, 1936, p. 44); W—recorded from the type Waitotaran (Powell, 1931, pp. 87–89); N—recorded from the type Nukumaruan (Marshall and Murdoch, 1920, p. 123; Allan, 1933, p. 103); G—recorded from the Greta beds (Thomson, 1920, pp. 364–366).
Analysis of the fauna reveals some interesting features. The first is the occurrence of long-range stragglers from Miocene times, such as Eucrassatella and Zeacuminia, which persist until the Waitotaran stage and die out therein. Another is the entire absence of such typical Waitotaran fossils as the large pelecypods Phialopecten triphooki, Pallium crawfordi, and Cardium spatiosum. The characteristic Waitotaran form Ostrea ingens, however, is common in one shell-bed in the east branch of the Grey River.
Comparison of the fauna with those of other localities reveals that the Kowhai series has most in common with the Kaawa beds, the
type Waitotaran, and the type Nukumaruan. The Kaawa beds are regarded as being Waitotaran or slightly older (Laws, 1936, p. 42). They differ from the type Waitotaran in station of deposition; the Kaawa beds were deposited near a shoreline in water not deeper than about 15 fathoms (Laws, 1936, p. 41); the beds of the type Waitotaran at Hawera, on the other hand, were laid down at a distance from a shoreline, and at depth ranging from 25 to 30 fathoms (Powell, 1931, p. 90).
The fossils therefore indicate a Waitotaran or Nukumaruan age for the Kowhai series. On the evidence of the Miocene element in the fauna, together with the occurrence of Ostrea ingens, the Kowhai series is assigned to the Waitotaran stage. The fauna of the Kowhai series has more in common with that of the Kaawa beds than with that of the type Waitotaran. This feature is due to the deposition of the Kowhai series and the Kaawa beds in much shallower water than the beds of the type Waitotaran. The Kowhai series represents a shallower water facies than the Kaawa beds; it was laid down very close to a shoreline where the sediments were coarse and the currents strong—as witness the gritty or conglomeratic nature of the shell-beds, and the worn and rubbed surfaces of the fossils. The shallow water origin of the Kowhai series probably explains the resemblance of the fossils to those of the type Nukumaruan, which was laid down under very similar conditions.
The determination of the Kowhai series as Waitotaran introduces difficulties when comparison is made with the account of the geology of the Middle Waipara-Weka Pass district by Thomson (1920). He distinguishes two series, lithologically similar to the Kowhai series, separated locally by an unconformity. The lower series, his Greta or Motunau beds, which he placed in the Waitotaran, contains a molluscan assemblage similar to that of the Kowhai series, as is shown by the list above. The upper series, which was unfossiliferous, he correlated with the Kowhai series of this area, at that time thought to be Pleistocene. The present investigation has shown that the Kowhai series as defined by Professor Speight is Waitotaran. From the faunal relationships it is reasonable to correlate the Kowhai series with the Greta Beds. Either Thomson's Kowhai series is a higher formation, in which case a new name for it is necessary, or the unconformity he described is intraformational and his Greta Beds and Kowhai series should be grouped together. The writer has not seen the locality where Thomson described the unconformity between his Greta Beds and Kowhai series, but evidence from other places favours the second explanation.
The gravels lying on the truncated surface of the Tertiary and Cretaceous rocks were deposited by the rivers traversing the area at various stages in their history. These river gravels are similar in lithology to the gravels of the Kowhai series, but the greywacke pebbles are not so well rounded and vary more in size than do the pebbles of the Kowhai series. Where contacts can be seen the terrace gravels can be easily differentiated from the Lower Pliocene gravels, but elsewhere, especially on the lower slopes of the downs around
the base of Mount Grey, they are difficult to separate from the upper members of the Kowhai series, which were also laid down on a land surface.
The oldest and most extensive of these terrace gravels is the series which forms the dissected surface of the old pediment in front of the hard-rock upland of Mount Grey. This surface is some 500–600 feet above the present level of the rivers draining the area. These gravels must be at least as old as Pleistocene, and probably date from the Upper Pliocene. Between the level of the old pediment and the present river level are a number of less extensive terraces capped with more recent gravels.
Allan, R. S., 1933. On the System and Stage Names applied to Subdivisions of the Tertiary Strata in New Zealand, Trans. N.Z. Inst., vol. 63, pp. 81–108.
— 1937. Tertiary Brachiopoda from the Forest Hill Limestone (Hutchinsonian) of Southland, New Zealand, Rec. Cant. Mus., vol. 4, no. 3, pp. 139–153.
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