Art. XXXIII.—Concretions in the Recent Sediments of the Auckland Harbour, New Zealand.
[Read before the Auckland Institute, 13th December, 1916; received by Editors, 30th December, 1916; issued separately, 30th November, 1917.]
During dredging operations recently carried out in St. George's Bay, Auckland Harbour, a considerable number of concretions were brought up, along with fine sands and numerous molluscan shells, from a depth of about 28 ft. to 35 ft. below mean high-water level.
It is no new discovery that concretions can form contemporaneously with the beds in which they are found: deep-sea manganese oolitic grains and nodules are classical examples. It is, however, seldom that one finds definite mention in literature of contemporaneous concretions of the type to which these at Auckland belong.
Description of the Concretions.
The majority of the concretions are irregular nodular masses varying in size from ½ in. up to 6 in or more in diameter. (See Plate XXIX, fig. 1.) Their material is very hard and compact limestone (about 70 per cent. calcium carbonate) enclosing numerous angular grains of quartz, a few of glauconite, numerous specks of carbonaceous matter, small fragments of partially carbonized wood, and, as a rule, numbers of Recent fossils—diatoms, a few foraminifera, small crabs, and molluscs. In some concretions there are comminuted fragments of the more delicate shells, but the stronger shells are intact. Small crabs are exceedingly common nuclei of small rounded concretions, such as the smallest of Plate XXIX, fig. 1, and the mollusc Atrina zealandica frequently the nucleus of the larger ones. (See Plate XXIX, fig. 2.). It shows its peculiar tooth-like ornamentation unimpaired, and the prisms of its outer calcareous layer are still uncemented. Few of the other molluscs found in the concretions can be regarded as nuclei, as the photograph (Plate XXIX, fig 3) shows well.
No attempt has been made to list the shells found in the nodules, but all belong, so far as the writer is able to determine them, to Recent species. Many, such as Cominella maculosa and Fulgoraria gracilis (depicted in Plate XXIX, fig. 3), show unbleached natural colours. Amphibola crenata (Plate XXIX, fig 3), various species of Turritella (Plate XXIX, fig. 1), and small Pecten valves are frequent.
In a few of the larger concretions some very irregular drusy cavities up to ½ in. in diameter are present. They are arranged without any relation to the boundaries of the concretion, or its nucleus. Their irregularity prohibits their being considered hollow casts of some since-removed organism; nor do they show the characters of shrinkage cracks, although this last explanation seems to be the only feasible one. They cannot be regarded as solution cavities, because there is ample evidence that the harbour deposits containing the concretions have not been subjected to any change of conditions such as would be necessary to permit percolation and solution to be effective.
Examination with the microscope shows that the calcareous material of the concretions was deposited as a microgranular aggregate, which is so fine in grain that it often has infilled minute diatom capsules which are even yet arranged in their characteristic bead-like strings. The included quartz-grains are somewhat sparsely but uniformly scattered throughout the calcareous matrix, and the only direct evidence of their having been outcrowded during growth of the concretion is that the somewhat incoherent surface zones are crowded with sand-grains, which, however, can equally well be accounted for by mechanical intermixture during pumping operations. Concentric structures and bedding-planes are absent.
It is evident from the facts already stated—the largely uncarbonized nature of many included wood fragments and the preservation of the natural colours of the shells in particular—that these concretions not only are contemporaneous with the beds in which they exist, but, further, have been formed at no very distant date.
Origin of the Concretions.
It is agreed by practically all geologists that the manganese nodules of deep-sea deposits, the phosphatic concretions near shore, and the minute pisolites and oolites of many limestones, with many other similar chemically precipitated accretions, are contemporaneous in origin with the beds in which they he. With respect to numerous accretionary nodules of other types there is less agreement, and the criteria of primary origin set forth by one writer may be construed by another as criteria of secondary origin. For example, the passage of bedding-planes through a concretion is considered by Geikie* to be evidence of contemporaneous origin, but by Chamberlin and Salisbury† to indicate secondary origin. The last-named authorities indicate the general lack of decision on this point in the following statement: “Some concretions probably form during the accumulation of the beds in which they lie”‡
Because such indecision is shown by many authors in dealing with concretions of this type the writer feels justified in emphasizing the indubitably contemporaneous origin of those described herein. There is no possibility that they have come by virtue of wave-action from the soft mid-Tertiary sandstones forming the prominent cliffs of the shores of Auckland Harbour, for these beds contain very few molluscan fossils, and, further, the concretions themselves have not been subjected to any erosive action.
The large quantity of calcium carbonate present in the matrix of the nodules cannot have been derived from the inferior amount constituting the included shells. Percolation cannot be effective under the conditions obtaining in this instance, so that the calcareous material must have been precipitated directly from sea-water by much the same process as that explaining the formation of the “coal-balls”§ of the English Coal Measures and similar accretionary bodies
This long-established principle of direct precipitation has been suggested by one or two writers in explanation of the concretions in many magnesian
[Footnote] * Sir A Geikie, Text-book of Geology, vol. 1, 4th ed, 1903, p. 647.
[Footnote] † T. C Chamberlin and R D. Salisbury, Geology, vol. 1, 2nd ed., 1909, p. 496.
[Footnote] ‡ Loc. cit, p 439
[Footnote] § M. C. Stopes and D M Watson, Phil Trans Roy Soc. Lond, ser. B, vol. 200, 1997, pp 167–218
limestones,* and without doubt it has led to the oft-made statements, such as that quoted above from Chamberlin and Salisbury, to the effect that many concretions are contemporaneous in origin with the beds enclosing them.
The writer has been unable to find any description of concretions mentioned by Chapman as occurring in muds at Melbourne,† but imagines that they are comparable with those of the Auckland Harbour.
Cause of the Precipitation of the Calcium Carbonate.
The cause of the precipitation of the calcium carbonate in the Auckland Harbour concretions is often uncertain, for many of them lack definite nuclei. Some pencil-like ones probably formed around a decaying twig or some such organic remnant, when the organic compounds liberated induced precipitation of the carbonate. The commonest nuclei are shells of the mollusc Atrina zealandica and of a small crab; the skeletal parts are almost invariably filled by the material of the concretion, and this fact is perhaps evidence that the soft parts had disappeared before the shells came to rest. Probably precipitation was initiated by the decomposition of the organic matter in the epidermis of molluscs such as Atrina and the hard parts of the crabs. This seems the more probable since molluscs lacking a horny epidermis are very abundant in the harbour sands and yet seldom, if ever, form the true nuclei of the nodules described.
Summary and Conclusion.
The writer has described certain calcareous concretions forming in present-day sands and other silts of the shallow, sheltered waters of Auckland Harbour (N.Z.), and contemporaneous with these sediments. He considers that they result largely from direct precipitation of calcium carbonate from sea-water. He wishes to bring forward this example of such nodules in order to supplement the somewhat vague available information about concretions which have formed as primary component parts of a stratum.
In conclusion, the writer wishes to thank Mr. M. Ongley, New Zealand Geological Survey, for considerable help in looking up literature.
Through the courtesy of the members of the Geological Section of the Wellington Philosophical Society, the above paper was discussed at the June meeting (20th June, 1917) of the section, and several very relevant criticisms and suggestions were put forward.‡
It was suggested, inter alia, that (i) the concretions may have been dredged from the underlying mid-Tertiary Waitemata standstones; (ii) the evidence for the Recent age of the concretion-bearing sands and silts set forth in the paper was inconclusive.
Mr. Hamer, Engineer to the Auckland Harbour Board, has very kindly allowed the writer to see the mapped records of many scores of bores, and
[Footnote] * Cf. A. J. Jukes Browne, Concretions in Magnesian Limestones, Geol. Mag.(n.s), dec m, vol. 8, 1891, p. 528.
[Footnote] † F Chapman, On Concretionary Nodules with Plant-remains in the Old Bed of the Yarra at South Melbourne, Geol. Mag. (n s), dec. v, vol. 3, 1906, pp. 553–56.
[Footnote] ‡ Communicated to the writer by Dr. C. A. Cotton, Victoria University College.
of the soundings taken before and during dredging in the area of the harbour concerned.
Dealing with the suggestion that portions of the Waitemata beds have been dislodged and brought up, it need only be mentioned that the dredge employed was of the suction type and quite incapable of disrupting the coherent Waitemata sandstones disclosed beneath the silts by boring. The possibility of the concretions being a rewash of the Waitematas has already been considered.
The Harbour Board bores and the more recent physiographic history of the Auckland area prove, in the writer's opinion, that the concretions belong to beds now forming, which have had an uninterrupted course of deposition since their inception
The pertinent facts of the recent physiographic history of Auckland are that the Auckland Harbour represents a former stream-valley drowned by invasion of the sea within fairly recent geological times, and that, subsequent to the drowning, the only recognizable movement of elevation (evidenced by wave-cut platforms) is one involving an uplift of not more than 5 ft. or 6 ft.
The bores demonstrate that the concretion-bearing sediments occur in a steep-walled narrow gut, with its bed approximately 30 ft. below mean low-water level (the Harbour Board datum), flanked by a level platform of firm sandstone (Waitemata) approximately 3 ft. or 4 ft. below low-water level. This gut is a continuation of the well-marked gulch spanned by Grafton Bridge. Gut and platform alike, prior to the commencement of dredging, were masked by sands and muds which raised the harbour-bottom to within about 2½ ft of low-water level. The filling of the gut was every-where unconsolidated.
Auckland, 24th August, 1917.