Red and Green Argillites.
In the reports of the old Geological Survey the term “slates” has been used for these rocks as also for those now called “argillites.” The red and green slates do not differ, except in colour, from the common argillites; they show parting parallel to the bedding-planes, and, save where weathering has been active, slaty cleavage is no more developed than in the argillites (9, p 44; 18, p. 43). Their chemical similarity to a typical argillite is seen by comparing analyses Nos. 3, 4, and 5. The name “argillite” is here used for these rocks. The exposures of red argillites in the Wellington Peninsula are indicated on the map (Fig. 1). The outcrop at Red Rock Point, illustrated by the section (Fig. 2), is the clearest. Only in the case of this outcrop is the development of green argillite in connection with red argillite plain. Outcrops on hillsides are conspicuous by reason
of their red colour, but the green rock is hardly to be distinguished from the common darker green or black argillite.
The material of the green argillites shows, under the microscope, quartz, magnetite and pyrite, and sericite. The green colouring-matter is indeterminable; in the case of green argillites examined elsewhere this colouring has been considered to be epidote (9, p 47) or amphibole (12, p. 99) Haematite is the chief recognizable mineral of the red argillites.
The so-called red and green slates are green argillites partly reddened Sometimes a gradual change of tint from deep red through light red to green is observable. In other cases the colour changes abruptly from red to green. Sometimes the red argillite shows veins of deeper red (Plate VI, Fig 2). The appearance under the microscope with reflected light is of a roughly equidimensional mass of grains, each coated with red, while in places red colouring is gathered in veins which show a deeper tone.
The description of the red clays and shales of Nova Scotia as given by J. W. Dawson (19, p 26) applies equally to the microscopic appearance of these argillites. “[the colouring-matter] having indeed the aspect of a chemical precipitate rather than of a substance triturated mechanically. In addition to oxide of iron distributed through the beds, there is, in fissures traversing them, a considerable quantity of the same substance in the state of brown haematite and red ochre, as if the colouring-matter had been subperabundant and had been in part removed and accumulated in these veins.”
Where the red argillites have been subjected to weathering, cleavage is more pronounced; and often the red colour has been leached out, leaving a light-grey to white product. A similar result of weathering of argillite has been noted by C Fraser (18, p 47)
Writing in “The Geological History of New Zealand” (20, p 164), F. W Hutton says, when speaking of the Maitai system, “In several localities in both Islands red-jasperoid slates occur, sometimes associated with manganese oxide, and this, together with the paucity of fossils and the general absence of plant-remains, points perhaps to a deep-sea origin” The manganese oxide which occurs in the rocks of the Wellington Peninsula is found in small nests or stringers in the softer strata. In most cases it forms no more than an incrustation sufficient for blowpipe testing a sample from Duck Creek, Porirua, yielded 5.4 per cent of MnO2. It is never found as concentric shells around a nucleus, nor exhibiting mammillated structure, nor yet impregnating a mass of palagonite or forming layers alternate with any such substance—these being the more general modes of occurrence of manganese oxide in deep-sea deposits (21 22) Here its occurrence seems to have no more significance than that of iron oxides, and in habit it appears to parallel closely the latter oxides.
Further, the Maitai rocks of Wellington Peninsula, as mentioned before, yield plant-remains, these having been collected on both sides of the argillites at Sinclair Head and elsewhere, while the black colouring-matter of the argillites is presumably carbonaceous. Paucity of fossils is no criterion of deep-sea deposits. The absence of radiolarian cherts and glauconitic sands anywhere in the rocks of the Maitai series, as well as the nearness in the series of conglomerate bands both above and below the red and green slates, are evidence that the Maitai rocks are not of deep-sea origin.
Analyses Nos 4 and 5 of the accompanying list are of green and red argillite respectively. Save for the proportions of ferrous and ferric
iron, the two analyses are almost identical. The total content of iron does not differ by more than 0–4 per cent., but, while in the green argillite the proportions of ferrous and ferric oxide are 3 : 1, in the red argillite the proportions are equal. This shows that the difference in colour is due to the presence of more ferric iron in the red argillite, and indicates that the change of colour has been brought about by the production of ferric iron from the ferrous iron present in the green argillite.
In discussing the formation of red sediments it seems a common premise that the iron-content in the sediment was in the higher state of oxidation at the time of its deposition, either as a hydrated sesquioxide, as supposed by Joseph Barrell (23, p 286), or already in the form of anhydrous red haematite, in which state I. C. Russell concludes all red sediments were deposited (24, p. 56).
J. D. Dana has attributed the red colour of certain shales to the oxidation of their iron-content by the action of heat resulting from orogenic movements (25). How the oxidation has been brought about is not stated. Some such hypothesis as Dana's seems best suited to the case of the red argillites of Wellington Peninsula.
The connection between the red argillites and the strike-faults of the Maitai rocks is indicated by the map. That the areas of red argillites have suffered from faulting-effects is shown by the fact that a quartz lode is developed in connection with each band. As stated above, these lodes are silicified fault or shear zones (26, p. 135). Genetically, however, they are segregated veins, as distinct from true fissure-veins, and as such have been described by J Park (27, p 64). Siliceous solutions, circulating mostly in a downward or lateral direction, may be considered efficient agents in supplying the oxygen necessary to convert the ferrous iron to the ferric state.
In the field the appearance of the green argillites is consistent with the idea of leaching; the argillites are of a dull greyish-green colour, and, although quite compact as distinct from weather-rotted, they are without the sheen commonly noticed in light-coloured argillites (11, p. 47; 29, p. 50).
The effects of vein solutions on country rock composed of “clay slates, greywacke slate, and similar rocks” have been investigated by A. von Groddeck (28). He finds that the result of such action on “variegated slates” will be a leaching-out of iron and magnesia, a loss in sericite, and a gain in quartz. The final product, however, is a slate composed of “quartz and sericite with a little rutile and considerable specular iron.”
On comparing the analysis of a typical dark-coloured argillite with that of the red argillite it will be seen that the small differences in the analyses vary in accordance with von Groddeck's results; while the percentage of silica shows a slight increase in the red argillite, the amounts of alumina and potash are slightly less. The total loss on ignition is high in the case of the dark-coloured argillite, due presumably to the carbonaceous matter present. In the case of the red argillite, water-content is probably responsible for the ignition-loss of 3–42 per cent.
The leached-out products of iron and magnesia are not necessarily lost to the rock; in the case of the slates investigated by A. von Groddeck the resulting product has “considerable specular iron.” Probably the leached iron, as in the case of the Wellington argillites, has been oxidized and redeposited in the rock.
Discussing the origin of red formations, Joseph Barrell (23, p. 290) concludes that the chief factors operating in the production of red shales from
those of lighter colour are—(1n) Dehydration of iron oxides under great pressure and moderate temperature; (2) diffusion operating under conditions of warmth and moisture. Oxidation of the ferrous iron might be accompanied by hydration: if so, the conditions postulated by Barrell for its dehydration would obtain in the neighbourhood of a fault-zone. That some diffusion of the dehydrated iron oxide has taken place is evident from the appearance of the reddened argillite, and is also shown by the fact that in most cases the silica of the associated quartz veins is also slightly reddened. The necessary conditions for this diffusion would also be found as an accompaniment of the faulting.
The changes mentioned above have no doubt been induced as the result of more than one movement of faulting. The folded quartz vein at Sinclair Head points to movement after its formation. In this connection it is interesting to recall the position of the Tertiary conglomerate at 150 ft. above sea-level. That there has been revival of faulting along old fault-lines has been pointed out by C. A Cotton (13, p 295).