a. Le Petit Carenage (Brough's Bay) and Tikao Bay.
(See Plate 12, Photo. 5).
Immediately behind the shore-line the land rises steeply to a shelf with a general height of approximately 350 to 400 feet above sea-level, and accordant with the similar but less extensive shelf between Barry's Bay and French Farm (see Photo. 1). Between Le Petit Carenage (Brough's Bay) and Tikao Bay it increases in width since the spur from which it has presumably been cut is broader and stretches further out into the harbour. Exposures of rock in situ are rare except along the sea-front, where there is an interesting development having an important bearing on the question at issue.
Along the southern shore of Le Petit Carenage a trachyte of somewhat peculiar character is exposed. At sea-level it dips approximately N.W. at moderate angles, and it weathers whitish, suggesting that it is soft, but the fresh rock is very hard, with glistening surfaces when recently fractured, and dark grey in colour with a greenish tint when seen in mass on the shore platform. In this section it shows occasional micro-phenocrysts of sanidine and rare plagioclase feldspars and also probable anorthoclase; numerous greenish, pleochroic aegirine-augite; in a base composed essentially of twinned and untwinned feldspar, the
former irregular and flaky in shape, and the latter in lath-like form, both with index of refraction lower than that of balsam, and the twinned forms with the extinction angle of albite-oligoclase; many of the laths have the denticulate margin and extinction of anorthoclase. Included in the base are numerous grains of augite coloured like the phenocrysts, and grains of titaniferous magnetite Analysis No. 6, p. 71, which shows it to be a trachyte decidedly Analysis No. 6, p., which shows it to be a trachyte decidedly sodic in character. I did not recognise any carbonate as occurring in the section. Rock of this type occurs over a fairly wide area of the Carenage-Tikao shelf, for it outcrops round the head of the tributary valley on the south leading down to the Carenage; it is developed along a considerable length of the shore-platform round the south-eastern corner of the shelf, where it shows some variation; it occurrs in the typical facies near the top of the shelf overlooking the shore-line on the east, and also at the base of the slope facing south near the beach at the head of Tikao Bay.
Between the two occurrences on the eastern shore another trachytoid rock appears. At its northern end it has a somewhat flaky appearance, and it dips N.W. at moderate angles. This facies continues for some 12 chains, when it changes in colour from a whitish to a pink or red, and where this is washed by the sea it has all the external appearance of a syenite; this facies continues for about 6 chains, and forms the north-western corner of the promontory between the two bays. Its relations to the aegirine-augite trachyte just mentioned are not clear, but I think it underlies this rock, an opinion given with much reserve.
In micro-section the two facies of the rock present little variation except in coarseness of grain. The phenocrysts appear to be sanidine up to 2 mm. in length, with probable anorthoclase, and subordinate plagioclase; quartz also occurs occasionally. The matrix is of trachytic texture, in some cases moderately fine in grain, but still coarse for a normal trachyte, while in other cases it is very coarse with quadrate and stumpy laths up to 5 mm. in length, many of the laths showing the characters of anorthoclase. While the base shows little signs of decay, there are many aggregations and flecks of iron-ore, some of which is derived from small and irregular laths of aegirine-augite. A few grains of titaniferous magnetite or ilmenite also appear, and there are accumulations of leucoxene derived therefrom; some of the iron-ore may be derived from the former. Aggregations and masses of calcite up to 1.5 mm. in diameter also appear, and the rock reacts strongly to acids in places, and specially so with the coarser types, the finer giving little or no response; the source of this calcite was not discovered, but it may have been derived from the plagioclase feldspars. The analyst notes in calculating the norm that in addition to the calcite a considerable quantity of carbonate is present as MgCO3 and FeCO3, so these minerals may be present as well. The composition of the rock as a whole is given by Analysis No. 7, p. 71.
The question arises as to the name to be given to this rock, and it seems to me that it might be called a trachyte-prophyry or quartz-bostonite as defined by Johannsen in his “Descriptive Petrography of Ingneous Rocks” (1937, vol. 3, pp. 26–7). The rock is too coarse for a typical trachyte, but the rocks called by the name suggested are dykes, and the occurrence under consideration is apparently not one, but either a massive flow or the margin of a plutonic body. It is visible only near the shore-line, so that its field relations are difficult to determine, but it has evidently a close genetic connection with the aegirine-augite trachyte just referred to. The presence of so much carbonate in the prophyry as well as the lower percentage of silica, differentiates them, the latter feature being somewhat remarkable since the porphyry contains some free quartz.
In addition to these major occurrences trachytes outcrop in at least three places on the summit of, the shelf, the first near its highest point, the second on the south-east corner, and the third near its western margin. The former two of these outcrops are certainly not dykes since they are exposed over too wide an area, but there may be some doubt concerning the last, a reference to which will be made later. Trachyte rocks also fringe the northern shore, and also the western half of the southern shore of Tikao Bay, but I have made only a cursory examination of parts of the former. The rocks I have seen consist of flows, tuffs, and coarse fragmentaries, the last containing blocks of trachyte as well as basic material. The trachytes, including the coarse fragmentaries, are exposed on the southern side of the bay, and extend as far as small beach; the rocks at the other end of the beach are basalt or basic andesite, and such probably extend towards Wainui.
Crossing the head of Tikao Bay is a massive trachyte, over 30 feet thick, striking approximately north–south, and dipping west at an angle of about 35°. At first I considered this occurrence to be one of the later trachyte dykes, and this may be correct after all, but I am inclined to think that it may be a flow with a dip in agreement with that of the other trachytes exposed along the margin of the bay. It is oriented in such a direction that it might coincide with the mass referred to earlier as being exposed towards the western border of the summit of the shelf, and a similar rock is exposed on the south side of the head of a small valley leading down to the Carenage. These facts fit the conclusion that it is either a titled flow or a dyke. All the trachytoid rocks of the shelf are penetrated by a later series of trachyte dykes, and by occasional basalts, all oriented towards Onawe, and it is possible that this major occurrence as well as some of the minor outcrops belong to these later intrusions.
The wide spacing of the major outcrops on the summit of the shelf and the continuity of the trachytoid rocks round the shore-line from the Carenage to the head of Tikao Bay and a little beyond indicate that the main mass of the shelf above sea-level is constructed of these rocks, and that it constitutes the greatest development of such rocks within the Akaroa area. Their presence materially
strengthens the contention that the cone of the Akaroa volcano has been built on a basement which includes volcanics of an earlier date, and of a different petrological facies. Certainly some doubt may be expressed in this particular in connection with the Carenage-Tikao trachytes since basic rocks have not been observed to overlie them definitely, but these occur at a higher level on the interior slopes of the caldera in the immediate background of the shelf, and it would not be reasonable to conclude that the trachytes represent a separate and subsequent development on the floor of an already formed caldera, especially since related rocks are clearly overlain by basic flows at French Farm, Onawe, and Lushington Bay. These are older than the Akaroa basic rocks, and it is therefore almost certain that the Carenage-Tikao trachytes are also older. Trachyte dykes penetrate both series, but I do not think that any close relationship can be inferred as existing between the later trachyte dykes and the older trachyte flows and clastics. It only means that a reversal to an earlier condition of alkalinity has occurred either accompanying the extrusion of basic material or more probably marking an end-phase of these extrusions. These remarks as to the relationship of the trachyte dykes to the older trachytes also apply to the occurrences elsewhere round the harbour. Also it must be noted that in no locality is there any clear-cut exposure of the basement on which these older trachytes lie.