Stratigraphical Position of the Charteris Bay Sandstone.
[Read before the Philosophical Institute of Canterbury, 5th November, 1924; received by Editor, 31st December, 1924; issued separately, 31st March. 1926]
In the author's account of the Geology of Banks Peninsula (Trans. N.Z. Inst., vol. 51, 1919, p. 373) mention was made of a sandstone occurring in places round the head of Lyttelton Harbour, notably at Governor's Bay and at Charteris Bay, which was stated to underlie the rhyolite of that area. When the paper was written no fossils had been found in the sandstone by which its precise age could be determined. However, towards the end of the year 1923 Mr. Orton Bradley, of Charteris Bay, brought to my notice a find of fossils close to the road leading to the wharf. These I considered from a casual inspection of a limited number to show a Senonian facies, and gave this as a tentative opinion in the Records of the Canterbury Museum (vol. 2, pt. 3, p. 150). Subsequently a more extensive collection was obtained, and this was submitted to Dr. J. Marwick, the Government Palaeontologist, and he concluded that they were of mid-Tertiary age. This conclusion necessitated a reconsideration of the question of the stratigraphical relationship of the beds in which the fossils occur, so that I made a re-examination of the locality with a view to determining their exact position, and also the age of the associated volcanics.
The fossils submitted to Dr. Marwick came from a large boulder of ferruginous sandstone or grit in the bank close alongside the road, and traces of similar fossils were found in position in a bed about 2 chains away from the boulder and striking in its direction, so that the boulder is not likely to have rolled down the hill for any distance, and may be treated as being practically in situ. Although, by the kindness of Mr. Bradley, a good deal of the surface covering of soil and slip material was cleared away, we could not see the fossils in the solid. The number of individuals is large, but they are, unfortunately, represented only by casts and moulds—no shells were seen. The matrix of the fossils also contained numerous small quartz pebbles, so that it might be called a ferruginous quartz-grit.
There are exposures of the same sandstone all along the shore, much intersected by dykes, so that their precise stratigraphical relationship is not easy to make out. The beds appear to have a northerly strike and an easterly dip at moderate angles. However, on the south side of a small bend in the road south of the fossils the dip is reversed and there is a small syncline. At this point, too, there are beds of white sandstone, occasionally well bedded, with layers of harder material interstratified with softer rock. A little distance up the hill towards the south-east a white sandstone has been quarried for building purposes, but owing to the extensive jointing it was found impossible to obtain reasonably-sized blocks, and so the quarry was abandoned. Near it there are also bands of small pebbles in the rock striking towards the spot where the fossils are, so that the white sandstone is in all probability closely associated with the fossil-bearing beds, but the covering of soil which masks the whole surface does not allow this point to be definitely decided.
There is a close lithological resemblance between these beds and those occurring on the saddle between Charteris Bay and the Head of the Bay where there is a small quarry for road-metal, and also at the various places along Potts Peninsula, specially on the east side, on the end, and on Little Quail Island, which is adjacent thereto. I have examined afresh all these occurrences, and there seems to be little doubt that the sandstone in these places does really antedate the rhyolite. Both the occurrence on the end of Potts Peninsula and that on the west side furnish very strong evidence that this is the case. In the former the sandstone is exposed close down to the sea along the shore-platform, and the rhyolite outcrops just above it on the steep hillside face, but no actual contacts can be seen. This occurrence and that on the adjacent Little Quail Island apparently constitute the denuded core of an anticline, since the rhyolites dip south on the end of Potts Peninsula, and both flows and agglomerate beds dip north on the south side of Quail Island.
On the western side of the peninsula the sandstone underlies the rhyolite, and the same is probably true of the quarry on the saddle at its proximal end. In the former two cases the possibility of faulting is not entirely eliminated, but it seems extremely unlikely. Further, in no case has a remnant of sandstone been observed overlying the rhyolite, and the absence of such a remnant is hard to explain if it be granted that the sandstone is younger than the rhyolite, since there is a very high probability that such would be preserved underlying the andesite somewhere or other round the head of the harbour. The general conclusion is, therefore, that a sandstone of similar lithological character to that containing the fossils does underlie the rhyolite. All the same, on the eastern side of Charteris Bay near the fossils there is no certainty as regards the relationship of the sedimentaries and the rhyolite—one set may as well be underneath as the other, considering the evidence at one's disposal.
The only precise determination of its age is that derived from the identification of the fossils. In a personal communication Dr. Marwick says: “I have had a good look over the Charteris Bay material and have come to the conclusion that it is probably of Middle or Upper Oamaruian age, somewhere from Waiarekan to Awamoan. The most important fossil is the Venerid, of which a large number of casts are present, showing the hinge and the interior of the shell. At first sight it resembles a large Callista thomsom Woods, but the clear casts of the hinge show there was no anterior lateral tooth. This with the smooth margin and disposition of the pallial line proves that the shell is a Marcia. This genus as far as I know does not go back to the Cretaceous. In fact, the loss of the anteriorlateral tooth in the Veneridae appears to have been a post-Cretaceous development. Suter generally classed these shells as Paphia—e.g., Paphia curta Hutton. The first known appearance in New Zealand is just below the Aotea limestone in the Huntly-Kawhia district, and the last in the Nukumaruian. I would not like to say whether the Charteris Bay shells belong to a new species or not, as the exterior is not well shown. There are several other genera present, but only the exterior of the shells is preserved, so their determination is uncertain. The case with strong radial ribs is like Cardium spatiosum. I know of no other New Zealand shell with which it could be compared. The Venerid of which casts were made showing concentric ridges may have been a Marcia, but I do not think the hinges spoken of above belong to it. It could also have been an Antigona or a Paphia. The marginal fragment of a large flat shell may belong to a Miltha: the sculpture was apparently very weak, and the
inflation not sufficient for a Dosinia. There is also a doubtful Glycymeris. The rounded closely-placed radial ribs and strong concentric growth-lines remind one of some members of this genus. Apart from the Marcia, the general resemblances, though vague, thus favour a Middle Tertiary age.”
The conclusions arrived at by Dr. Marwick as to the age of these fossils necessitates the consideration of two alternatives—viz., either our ideas as to the age of the rhyolites must be modified, or there are two sandstones in the district of similar lithological character.
The age of the rhyolites has usually been given as Cretaceous, since rhyolites of similar lithological character occur at Malvern and Rakaia Gorge whose age is definitely pre-Senonian, and there is no reason for considering the Gebbies Pass rhyolites of different age, seeing that their stratigraphical position as well is apparently similar to that of the rhyolites in other localities. Therefore we are driven to consider the possibility of two sandstones existing in the area, one of pre-Tertiary date and the other of mid-Tertiary date. Now, if both occur, both are penetrated by trachyte dykes belonging to the Lyttelton volcanic system, and, as these trachytes are certainly contemporaneous with the building-up of the Lyttelton volcano, it follows that the volcano dates from a time later than the mid-Tertiary, so that it may, after all, be much later than was first supposed. Further, since the volcano was much denuded before the lavas of Mount Herbert were poured out, it makes the date of the Mount Herbert volcano a great deal later than has usually been believed—that is, it is late Tertiary, or even Pleistocene—and this serves to confirm the conclusions drawn from the form of the stream-valleys eroded on the land-surface reaching down from the summit towards Purau and Diamond Harbour.