Art. V—The Flint-beds associated with the Amuri Limestone of Marlborough.
[Read before the Wellington Philosophical Society, 27th October 1915]
|I. The Amuri limestone||48|
|II. Distribution of the flint-beds||52|
|III. Character of the flints||53|
|IV. Origin of the flint-beds||55|
|List of papers referred to||58|
I The Amuri Limestone
The Amuri limestone, which takes its name from Amuri Bluft, seventeen miles south of Kaikoura, is a great limestone formation composed of a variety of rocks, of which the most characteristic and the most abundant is a much-jointed, thin-bedded limestone of chalky appearance, but considerably indurated and much harder than chalk. Typical soft chalk is known from the formation only at Oxford, in Canterbury, near the southern-most part of its range. In North Canterbury the lower beds of the limestone are generally softer and more argillaceous, with a coarser and more conchoidal fracture, and pass down gradually in the Waipara district into a dark-coloured mudstone. In East Marlborough, where the formation is very much thicker, there are several coarse alternations of hard chalky limestone with more argillaceous bands, and in places there are fine alternations of hard white limestone with greenish argillaceous limestone, in all cases fine-grained. The base of the formation in this district is formed by the beds of flint which form the subject of this paper. By its fine gram and chalky appearance, together with the manner in which it breaks into small cuboidal blocks, the Amuri limestone is easily distinguished from all other limestones in New Zealand,* and it has evidently had a different mode of origin from the Oamaruian (Ototaran) limestones of Otago and South Canterbury, which are largely composed of fragments of Bryozoa. It contains a fair proportion of unbroken tests of Foraminifera, especially Globigerina, but the greater part of its mass is made up of an exceedingly fine-grained calcareous mud, with little terrigenous matter. Analyses, however, reveal a considerable amount of silica, which must also be present in a state of minute subdivision, while there are occasionally grains of glauconite.
Although the Amuri limestone is now distributed in an irregular and discontinuous manner throughout the east part of North Canterbury and Marlborough, there can be little doubt from its lithological characters that it once spread over the greater part of these districts, a conclusion previously
reached by Hector (1890), McKay (1891), and Cotton (1913). In thickness, however, it is very variable, being less than 300 ft. in the Waipara district, and, together with the flint-beds, over 2,500 ft. in the Middle Clarence Valley. At Amuri Bluff, roughly half-way between these places, the thickness is estimated by Hector (1877) at 780 ft, and it probably increases gradually between this point and the Middle Clarence Valley. Near Kekerangu the limestone appears to be divided into two or three parts by mudstone intercalations, but it is possible that this appearance is deceptive, and is due to faulting or sharp folding, since a mudstone overlies the limestone. A somewhat similar phenomenon occurs in the hills north of the Lower Ure River. The thickness of the formation decreases again in the Cape Campbell district, but it is still considerable. Apparently the typical Amuri limestone reappears on the north side of Cook Strait in the Cape Palliser district, but it has not been personally investigated in that area by the writer.
The age of the Amuri limestone is approximately fixed by the fact that it always overlies beds containing Cretaceous fossils, and is always followed by beds containing a Tertiary (Oamaruian) fauna. In all localities where a continuous section can be traced the limestone follows the Cretaceous beds with complete conformity of bedding; and in localities south of Amuri Bluff there is also a lithological gradation between these two sets of rocks, proving that the apparent conformity is there a real one. The Amuri limestone is thus much older at its base, even where it is thinnest, than any beds in the Oamaru district; and the diastrophic correlation of the Amuri and Ototara limestones by Marshall, Speight, and Cotton (1911) is in conflict with the palaeontological evidence. The Cretaceo-tertiary problem in New Zealand owes much of its complexity to an earlier false correlation of these rocks, which, it may be noted, was never accepted by Hutton. Within the central part of New Zealand, where the Amuri limestone is developed, Cretaceous beds always underlie the limestone conformably, while it is followed by physically conformable beds containing an Oamaruian fauna. Outside this area—at any rate, to the south of it—the sequence generally commences with Oamaruian coal-beds, and, in the few areas where Cretaceous rocks are developed, there is probably unconformity.
In the Waipara district the Amuri limestone is followed by a calcareous sandstone, the Weka Pass stone, and this in turn is followed by more or less calcareous mudstones, the “grey marls.” In places the Weka Pass stone is glauconitic at the base, and in such places there is the peculiar and often-described junction which gives the appearance of an erosion of the limestone prior to the deposition of the glauconitic bed. Whether or not this is a correct explanation of this peculiar contact, it is certain that no such contact is present at places within the same district where the base of the Weka Pass stone is not glauconitic, but that there is a passage so gradual between the two rocks that one cannot say within a foot of rock where the one ends and the other begins. It is, therefore, evident that even if there has been some erosion of the Amuri limestone prior to the deposition of the Weka Pass stone, it is a purely local phenomenon, and not indicative of a non-sequence of any extent. The argument that an unconformity between the two limestones may well be present from the analogy of similar well-proved unconformities in other parts of the world entirely overlooks the facts that in these other places the apparent conformity of the two limestones in question is local, and that in New
Zealand the parallelism in dip and strike of the Amuri limestone and the succeeding Tertiary rocks is found throughout some hundreds of miles of outcrop.
In the Middle Clarence area the uppermost band of the Amuri limestone is argillaceous, and passes quite gradually into fossiliferous Oamaruian mudstones similar to the grey marls. The use of the term “grey marls” by Marshall, Speight, and Cotton (1911) for mudstones following the Ototara limestone, and, in particular, for the Wanganuian mudstones of the Wanganui River, has robbed this term of any geological significance, and made it practically synonymous with Tertiary mudstone of any age or position. Some of McKay's uses of the term in South Canterbury and elsewhere are also equally unfortunate; but as originally used by him within the area occupied by the Amuri limestone it has a perfectly definite significance—viz., for mudstones following the Amuri limestone—and if confined to this usage it may continue as a most useful geological term. It does not follow that by terming certain rocks the grey marls of the Clarence Valley one necessarily considers them the correlatives of the grey marls of the Waipara district, for the greater thickness of the Amuri limestone of the Clarence Valley may be due not only to an earlier beginning, but also to a later cessation of deposition than in the case of the limestone at Amuri Bluff and the Waipara district. All that the term implies is that they are mudstones following the Amuri limestone.
On the theory of conformity between the Weka Pass stone and the Amuri limestone, the age of the top beds of the latter can be fixed if the age of the former is known; but, although a considerable number of fossils are known from the Weka Pass stone and the grey marls, the range of these species within the Oamaruian is not yet well enough ascertained to allow it to be stated whether these beds are Middle or Lower Oamaruian. The Weka Pass stone certainly does not correlate with the Hutchinson Quarry beds, as supposed by Marshall, since the main band of the Mount Brown beds, containing Rhizothyris rhizoida and Pachymagas parki, occupies this horizon in the Waipara district. The probabilities are that it will be found that the Weka Pass stone correlates with the Waiarekan.
It has apparently been assumed by the opponents of a conformable Cretaceo-tertiary succession in any part of New Zealand that the whole of the Amuri limestone is Cretaceous in age. Actually, however, the only determmable fossils obtained from the limestone, excluding fish-teeth and crinoid stems, are of Tertiary aspect. McKay has recorded Pecten zittei Hutt. and Rhynconella squamosa Hutt from the limestone of Amuri Bluff. The, former species I have also collected from fallen blocks of the limestone in the same locality, but the specimen identified as Rhynconella squamosa is, in my opinion, not specifically determinable. In addition, Mr. H Suter has identified the following forms from the collections made by Mr. A. McKay and myself from Amuri Bluff.—
Pecten zelandiae Gray.
Pecten delicatulus Hutt (?)
Pecten sp. nov. cf P chathamensis
Pecten sp nov.
Pecten zelandiae is a Recent species, and its occurrence strongly suggests that the rock from which it comes is of Tertiary age. Unfortunately, the exact horizon within the limestone of any of the above forms is unknown.
In the green argillaceous limestone of the Ure River, a little above the junction of the Isolated Hill Creek, Teredo heaphyi occurs. Again the exact horizon within the limestone cannot be stated, owing to the complex faulting in the vicinity.
In the Coleridge Creek section of the Trelissick Basin, Mr. R. Speight and I recently discovered a band of fossiliferous tuff, about 25 ft. thick, and interbedded 10 ft. below the top of the Amuri limestone. This tuff yielded twenty-one species of Mollusca, determined by Mr. Suter as follows:—
Calliostoma aucklandicum E. A. Smith.
Seila huttoni Suter.
Siphonum planatum Suter.
Polinices huttoni Ihering.
Polinices ovatus (Hutton).
Ampullina miocoenica Suter.
Ampullina suturalis (Hutton).
Epitonium zelebori (Dunker) var.
Epitonium rugulosum lyratum (Zittel).
Fusinus bicarinatus Suter.
Hemifusus goniodes Suter.
Siphonalia turrita Suter.
Cominella intermedia Suter (?)
Admete trailli (Hutton).
Ancilla papillata (Tate).
Ancilla subgradata (Tate).
Marginella harrisi Cossman.
Surcula seminuda Suter.
Terebra costata Hutton.
Limopsis catenata, Suter.
Chione chiloensis truncata Suter.
Of the above, two species are new, four (or 19 per cent.) are Recent, while the others are all well-known Tertiary, and mostly Oamaruian, species.
It seems clear from these facts that the top at least of the Amuri limestone is of Tertiary age, and that the Amuri limestone is in itself a Cretaceotertiary rock, Cretaceous at the base and Tertiary at the top. In view of this, the conformity or unconformity of the Weka Pass stone sinks to a question of purely local Tertiary geology without significance for the relationship of the Cretaceous and Tertiary in New Zealand.
The Cretaceous fossils of the rocks underlying the Amuri limestone have recently been studied by Mr. H Woods, of Cambridge, and, as stated by Morgan (1915), he has come to the conclusion that two distinct faunas occur. “The older of these, found at Coverham, is considered to correspond to the Lower Utatúr (approximately Upper Greensand and Gault) fauna. The younger, of approximately Senonian age, occurs at Amuri Bluff and other points to the south.”
It would appear at first sight from this statement that the Amuri limestone of the Clarence Valley (Coverham) must be separated from the underlying Cenomanian beds by an unconformity. All the natural sections at Coverham and in the neighbourhood, however, show a perfectly conformable junction between the flint-beds at the base of the limestone and the underlying Cenomanian mudstones. One would expect, in any case, that the base of the limestone in this locality, where it is over 2,500 ft. thick (including the flint-beds), would be lower in horizon than the base of the
limestone at Amuri Bluff, where it is less than 800 ft. thick, in view of the lithological similarity of the limestones in the two localities. It appears that depression and deposition commenced in the Middle Clarence area at an earlier date than farther south, and continued in this area while it gradually spread farther south (and probably north). A confirmation of this view will ensue if the Amuri limestone in the Puhipuhi Mountains, between Kaikoura and Clarence Mouth, is found to be intermediate in thickness between that at Amuri Bluff and that in the Middle Clarence area, and if fossils from the underlying Cretaceous beds, none of which have been available to send to Mr Woods, prove to be intermediate in age between Senonian and Cenomanian—i.e, Turonian.
II. Distribution of The Flints
Small rounded flints similar in manner of occurrence to those of the English Chalk occur fairly abundantly embedded in the limestone at Amuri Bluff*. They are generally of a light-brown or flesh colour, but are sometimes opaque white and hardly distinguishable from the limestone in which they occur. Unlike the English flints, they are never hollow, thus destroying any hopes of discovering sponge-spicules or other fossils in their interiors, nor does the surrounding limestone exhibit the hollow casts of sponge-spicules which are often so well displayed in the Chalk. In the Middle Clarence and the Ure River area similar flints are found in the upper part of the limestone, but in the lower part an entirely different mode of occurrence prevails. The base of the limestone is entirely replaced by beds which are composed of large lenticules of black flint in the centre, with a variable amount of grey external matter, very often composed of euhedral crystals of a carbonate set in a flint matrix.
These flint-beds were first described by McKay (1877) from the Hapuka River, a little to the north of Kaikouia, where he found “20 ft. of a peculiar rock consisting of flinty nodules in a matrix of dark shale” between sandy micaceous (Cretaceous) clays and the base of the limestone. This appears to be the southernmost point of their range. At Waipapa boat-harbour, farther to the north, they are 50 ft. or more in thickness, and are described by McKay (1886) as “formed by black flint in layers averaging about 6 m in thickness, which are parted by a lesser thickness of white decomposed flint, or fine-grained sandstone”. They have not been studied in detail between Clarence Mouth and Kekerangu, but are described by McKay (1886) as of “very considerable thickness” in the Benmore Stream, and here he notices the light-coloured exterior to the flints in their upper part. From this point the Amuri limestone swings round in a great curve through Benmore, the Isolated Hill, and Brian Boru, leaving its former course approximately parallel to the coast to enter the Middle Clarence Valley (cf. Cotton, 1913, Thomson, 1915). In the Isolated Hill Creek, a tributary of the Ure River which separates Benmore from the Isolated Hill, a very good section of the flint-beds is displayed. They are about 400 ft thick, and are perfectly conformable to the underlying mudstones, into which they pass almost imperceptibly within a thickness of 2 ft of rock. Forty feet below the main mass of flints there is a thin intercalation of flint and flint-carbonate rock within the Cretaceous mudstones. At Coverham, in the Middle Clarence Valley, a few miles to the south-west,
[Footnote] * Cf. Hutton (1877), Hector (1887), Liversidge (1877). I have not seen any flint in the Amuri limestone of the Waipara district.
the flint-beds are estimated by McKay (1886) as over 500 ft. in thickness. In Sawpit Gully, a tributary of the Nidd Stream, at Coverham, the junction between the flint-beds and the underlying sandy mudstones is sharp, and not gradual. The mudstones contain numerous pyritic concretions in their topmost 20 ft., and at the junction itself there is a strong yellow efflorescence. The lowest flint lenticules are light-coloured on the exterior, but do not show the carbonate rhombohedra that are so characteristic of the lenticular flint-beds in general. In the valley of the Nidd Stream, above the junction of Sawpit Gully there is a thick band of dark, but not black, flint or chert within the Cretaceous mudstones, over 1,000 ft. below the base of the main flint-beds. This band of flint is not lenticular in character, and does not exhibit the flint-carbonate rock associated with the flints at the base of the limestone series.
Between Coverham and the Dee River, still farther to the south-west, the flint-beds at the base of the limestone, as well as the limestone itself, attain their maximum thickness. In the Mead Gorge there are massive beds of flint at the base of the limestone series, and then alternations of limestone and flint-beds for some distance higher up. By measuring the dip and pacing the width of the outcrops I made the following rough estimate of the thickness at this point, neglecting some small faults which reduce the apparent thickness.—
|Top||Grey marls (fossiliferous Oamaruian mudstone).||Ft.|
|Hard argillaceous limestone (Weka Pass stone of McKay)||150|
|Hard chalky limestone||280|
|Hard chalky limestone||90|
|Flint-beds with limestone intercalations||1,410|
To the south-west of the Dee River the flint-beds again decrease in thickness, and, according to McKay (1886), they disappear altogether before the limestone reaches the Dart River. In the Upper Awatere Valley the Amuri limestone is much thinner than in the Clarence Valley, and the flint-beds are mentioned by McKay (1890) as occurring at its base. Finally, a considerable development of these beds with the Amuri limestone is reported by McKay (1877) near the mouth of the Flaxbourne River, in the Cape Campbell district.
III. Character of the Flints.
What have been termed “flint-beds” in the above account are layers composed either wholly of black flint or of large lenticules of flint surrounded by a semi-crystalline material. The layers are distinct from one another above and below, and are in close contact along surfaces resembling bedding-planes. Where the layers are composed of flint they are generally about 6 m to 8 m. in thickness, and the surfaces of contact are approximately parallel. Where the layers consist of dark flint only in certain lenticules, the bounding surfaces are more irregular, as the layers exhibit numerous swellings following in a reduced degree the shape of the lenticules, and reaching in some cases a thickness of 18 m. or more. Plates II and III will give a better idea of the phenomena than any lengthy description. It seems reasonable to suppose that the surfaces of separation of the layers
are bedding-planes, possibly in some cases distorted by the chemical rearrangement that has taken place.
The flint-beds are in most outcrops much shattered, and it is difficult to collect hand specimens free from flaws, although more compact specimens may be obtained from the gravels of the streams cutting through the series. In general the boundary between the light-coloured exterior and the inner dark flint is sharp, but there is no surface of easy separation between the two, and flints of the Chalk type preserving their original surface cannot be obtained either naturally or artificially. The amount of the light-coloured exterior varies considerably from place to place, and there is also considerable variation in its composition. It may consist of dark flint containing more and more rhombohedra of carbonate as the exterior is approached, until finally a rock is obtained which consists only of carbonate, and resembles marble. Such a gradation from dark flint has been observed in boulders obtained from the Ure River, and in this case there is no sharp boundary between the inner and outer layers. In the usual case there is a sudden change from a flint practically without carbonate crystals to one containing them abundantly, the outermost layer again consisting almost entirely of carbonate crystals without any flint matrix. Occasionally thin lenticules of flint occur within beds of limestone, and there may or may not be an intermediate rock with crystals of carbonate set in either a flint
Fig. 1.—Flint with relatively few euhedral crystals of dolomite, Ure River gravels Magnified 15 diameters
Fig 2.—Dolomite rock with a little flint in the interstices of the crystals, Mead Gorge Magnified 15 diameters.
or a partly calcareous matrix Text Fig 1 shows the appearance in microscopic section of a flint with relatively few crystals of the carbonate, which are euhedral towards the flint, but anhedral towards one another. Text Fig. 2 shows the carbonate rock with only a little flint in the interstices of the crystals. Where larger crystals of carbonate occur in a fine-grained calcareous matrix they are less regularly euhedral than when they occur in a flint matrix.
The size of the carbonate crystals is approximately constant in any one specimen, but varies considerably in different specimens, a common size being a little over ½ mm in the greatest diameter. One exceptional specimen, obtained from the gravels of the Isolated Hill Creek, a tributary of the Ure River, contains rhombs with a largest diameter of as much as 11 mm.
There is reason to suspect that this specimen came from the band of flint-carbonate rock intercalated near the top of the mudstones in this section, for this band exhibits much larger rhombs than usual.
A qualitative chemical test, carried out by Mr. B. C. Aston, showed that the carbonate of the coarsely crystalline specimen from the Isolated Hill Creek was strongly magnesian. A specimen from the Ure River gravels of the normal type of flint-carbonate rock, containing little flint matrix, was then submitted to the Dominion Analyst, Dr. Maclaurin, for analysis, with the following result:—
|Al2O3 + Fe2O3||3.12||CaCO3||41.60|
This analysis reveals two interesting facts. In the first place, it shows that the carbonate crystals have approximately the composition of dolomite. It seems most probable that they are pure dolomite, in view of the known power of crystallization of that mineral, and that the excess of calcium carbonate over the dolomite molecule is contained in the matrix. In the second place, it shows that the silica of the flint is little hydrated, and is not opal. In microscopic section the flint matrix is nearly dark between crossed nicols, but often shows numerous very small laths with moderate birefringence, straight extinction, and positive elongation, probably one of the orthorhombic forms of silica. They are altogether too minute for study in convergent light with the apparatus available.
The flint-beds at the base of the flint series in Sawpit Gully, Coverham, differ from those higher up in the series in that they contain a large amount of clastic quartz and other minerals in a matrix similar to that of the more normal flint-beds. The quartz is the most plentiful of the included minerals, and is in small angular grains. Muscovite in thin flakes appears in considerable amount, while there are scattered grains of glauconite and numerous minute crystals of rutile and pyrite with some iron - hydroxide staining. There are small rectangular areas of finely crystalline silica, often containing sericite, which possibly represent a replacement of clastic grains of feldspar. The light-coloured exteriors of these flints have a similar composition, except that there is less of the flint matrix and a greater proportion of subparallel plates of muscovite. There seems little doubt that in this case the flint-beds are replacing an impure fine-grained sandstone containing a little glauconite.
The flinty band intercalated in the Cenomanian mudstones of the Nidd. Stream, above Sawpit Gully, is similar in composition and structure to the last described, with the difference that the minerals enclosed in the flint matrix are about half as large, and include very little glauconite.
IV. Origin of the Flint-Beds.
Chert-beds not occurring in association with limestones or dolomites are usually the result of the molecular rearrangement in situ of silica derived from skeletons of such organisms as Radiolaria, sponges, or diatoms. The
absence of such skeletons in any of the numerous microscopic sections examined removes any ground for accepting such an explanation for the origin of the silica in the present case.
Chert-beds associated with dolomites are of frequent occurrence in the pre-Cambrian and Palaeozoic, and in these cases it is generally held that the silica has replaced limestone or dolomite, whether its origin lies within the mass of the carbonate rock or not. The Chalk flints are believed to result from the replacement of the chalk by silica, the latter being dissolved from sponge-spicules throughout the mass of the chalk and collected into concretionary bodies by some process not well understood. The absence of the casts of sponge-spicules in the Amuri limestone militates against such an explanation for the origin of the silica in the present case, although it is not a fatal objection, since the limestone has practically everywhere been subjected to such pressure as might be competent to close up any spaces of dissolution so left. The unbroken state of the tests of Foraminifera, however, suggests that little internal movement of the limestone has taken place. The fact that the flint-beds are thickest where the whole series is thickest is in accord with the derivation of the silica of the flints from the overlying limestone series, but again the total absence of the flint-beds at Amuri Bluff and farther south is, on this assumption, difficult of explanation. It seems most probable, therefore, that the silica was chemically deposited along with the dolomite or that it is of extraneous origin. If the flint-beds were of strictly local occurrence, an extraneous origin would be not improbable, though it would be difficult to explain the silicification of only the lowest beds of the limestone series, but in view of the widespread occurrence of the flint-beds, apparently at a definite horizon, the theory of original deposition seems most acceptable. The chemical deposition of silica along with dolomite is believed to have taken place in other parts of the world. Writing of the magnesian limestone of Durham, England, Trechmann (1914, p 258) states, “At certain periods, it seems, a deposit of siliceous material took place Silica, in the form of compact or friable nodules of chert, occurs in several beds, but chiefly in the middle bedded rocks on the eastern side of the reef. In many cases the nodules merge with every gradation into the suirounding dolomite.”
The conditions that permit the precipitation of silica are intimately related to those that permit also the precipitation of dolomite, calcium carbonate, and sulphides (Daly, 1907). They are found best displayed at the present day in the Black Sea (Andrussow, 1897), and are believed to have been operative also in the closed sea of Permian times in western Europe, in which were deposited the Kupferschiefer and the Zechstem dolomite (Schuchert, 1915). The fundamental cause of these conditions is the absence of bottom-scavengers, the lack of which allows the sea-bottom to become foul with accumulated remains of the swimming animals of the upper levels of the sea. The absence of bottom-dwelling life is caused by the lack of oxygen, a condition which can exist only in a more or less enclosed or sheltered sea not accessible to the creep of heavier and colder polar waters which oxygenate the bottoms of the open oceans. In the foul bottoms sulphur bacteria thrive, and decompose the organic matter, precipitating sulphides and liberating ammonium carbonate. The latter acts on the calcium and magnesian salts in solution in the sea, precipitating calcium and magnesium carbonates, and probably also on dissolved colloidal silica, which is precipitated as flint or chert.
There is no direct evidence that such conditions caused the formation of the flint-dolomite rocks at the base of the Amuri limestone, but it is
difficult to formulate any other hypothesis for their origin, while there are other groups of rocks the formation of which can also most easily be explained by the adoption of such an hypothesis. The Amuri limestone itself appears to be in large part a chemical deposit, and its silica-content and its poverty in fossils becomes then easily explicable. The Senonian sulphur sands and mudstones of Amuri Bluff and the Waipara district are certainly the kind of deposits to be expected not far from the margins of a sea with a foul bottom.
It is quite possible that the sea in which the Amuri limestone and the underlying rocks were deposited was more or less enclosed, for although Marshall, Speight, and Cotton (1911) considered that after the close of the post-Jurassic folding a shore-line ran in a north-east direction from New Zealand to New Caledonia, recent work by Cotton and by the officers of the Geological Survey on the West Coast has abundantly demonstrated that the present trend of the mountains of the north part of the South Island is due not to original folding in the directions of their present trends, mainly north-east and south-west, but to post-Oamaruian block-faulting. Older rocks similar to those forming these ranges outcrop in the Chatham Islands, and the presence of Oamaruian sediments in these islands suggests that there was a land surface close at hand during Oamaruian times to furnish material for the sediments. Upper Cretaceous rocks have not been recorded from the Chatham Islands, and, should their absence under the Oamaruian rocks be proved, it will establish a considerable degree of probability that there was a land surface there during the Upper Cretaceous. Should this be established, east and west coasts to the Amuri sea will be known to exist. The sediments that mark the northern and southern coasts, if any, are unfortunately below the waters of the ocean. It is not necessary, however, to have an entirely closed sea for the existence of sulphur bacteria in quantity, for these are known in foul bottoms in sheltered fiords in Norway and in the Bay of Kiel (Schuchert, 1915). It is, indeed, quite possible that foul bottoms exist in deeps of the open ocean that are cut off from the currents of oxygenating cold polar waters by submarine ridges, and on such bottoms, if they exist, rocks resembling the Amuri limestone must be in course of formation.
According to the hypothesis here put forward, the sequence of events was somewhat as follows. The Amuri sea was at first restricted in extent so far as the present New Zealand area is concerned, and on its margins it received terrigenous deposits (conglomerates, sandstones, and mudstones) in Cenomanian times. One point of the margin at this epoch is fixed by the coal-beds and fresh-water fossils of Quail Flat, at the upper end of the Middle Clarence Valley. As depression extended the margins, the area of original deposition passed beyond the range of the terrigenous deposits, and the precipitation of flint and dolomite, together with the shower of tests of Foraminifera, built up the lower layers of the Amuri limestone in the area of which Coverham is the centre. During this period—possibly the Turoman—the terrigenous sediments presumably continued on the sea-margins, and should be found, as indicated above, in the Puhipuhi Mountains. With further depression the area covered by the limestone increased, but for some reason the deposition of flint, and probably also of dolomite, ceased. During the Senonian the area south of Kaikoura Peninsula apparently came under similar conditions, during which the peculiar sulphur sands and mudstones were deposited. Still further depression removed this area also beyond the range of terrigenous deposits, and the limestone between Kaikoura and Oxford was formed between the Senonian and some
stage of the Oamaruian, together with the upper part of the limestone of the northern area. The marginal terrigenous deposits of this period are unknown unless they are represented in the Malvern Hills beds, though the presence of Conchothyra parasitica in these renders this unlikely. The area occupied by the Amuri sea now appears to have been elevated, or at least to have remained stationary, allowing it to shoal with sediments, while at the same time its margins became very widely extended north and south, forming the eastern Oamaruian sea, which was probably only a part of the Pacific Ocean. It will be noticed that the above account postulates that the Amuri sea was not entirely closed, for an extension of its margins with depression is dependent on a connection with the open ocean.
Much of the above is speculative, but will serve a useful purpose if it calls attention to the peculiar nature of the Amuri limestone, the flint-beds, and the sulphur sands, and provokes an alternative explanation. During most of the field-work on which this paper is based I was accompanied by Dr. C. A. Cotton, to whose kindly criticism I desire to express my indebtedness.
List of Papers Referred To.
Andrussow, N. 1897. “La Mer Noire,” Guides des Excursions, VIIe Cong. GéAol. Internat., St. PéAtersbourg, 1897, art. xxix.
Cotton, C. A. 1913. “The Physiography of the Middle Clarence Valley, New Zealand” Geogr. Journal, vol. 42, pp. 225–45.
Daly, R. A. 1907. “The Limeless Ocean of Pre-Cambrian Times” Am Jour Sci. vol. 23, pp. 93–115.
Hector, Sir J. 1877. “Marlborough and Amuri Districts.” Rep. Geol Explor. during 1873–74, pp. ix–xiii.
— 1890. “Marlborough District.” Rep Geol Explor during 1888–89, No. 20, pp. xxxvi–xxxviii.
Hutton, F. W. 1877. “Report on the Geology of the North-east Portion of the South Island, from Cook Strait to the Rakaia” Rep Geol. Explor. during 1873–74, pp. 27–58.
Liversidge, A. 1877. “Notes on some of the New Zealand Minerals belonging to the Otago Museum.” Trans N Z Inst., vol 10, pp. 490–505.
Marshall, P.; Speight, R; and Cotton, C. A. 1911. “The Younger Rock-series of New Zealand.” Trans. N Z. Inst., vol 43, pp. 378–407.
McKay, A. 1877. “Report on Kaikoura Peninsula and Amuri Bluff” Rep. Geol Explor. during 1874–76, pp. 172–184.
— 1886. “On the Geology of the Eastern Part of Marlborough Provincial District” Rep Geol Explor. during 1885, No. 17, pp. 27–136.
— 1887. “Report on Cape Campbell District.” Ibid., pp 185–91.
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— 1891. “On the Geology of Marlborough and South-east Nelson: Part II” Rep. Geol Explor. during 1890–91, No. 21, pp. 1–28.
Schuchert, C. 1915. “The Conditions of Black Shale Deposition as illustrated by the Kupferschiefer and Lias of Germany.” Proc. Am Phil. Soc., vol. 44, pp. 260–69.
Thomson, J. A. 1915. “Oil-indications in the Benmore District, East Marlborough.” 9th Ann. Rep. (n.s.) N Z Geol. Surv., Parl Paper C.–2, pp. 100–1.
Trechmann, C. T. 1914. “On the Lithology and Composition of Durham Magnesian Limestone.” Quart Jour. Geol Soc, vol. 70, pp. 232–65.