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Volume 51, 1919
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Art. XIX.—Further Notes on the Geology of the Trelissick or Castle Hill Basin.

[Read before the Philosophical Institute of Canterbury, 18th September, 1918; received by Editor, 30th December, 1918; issued separately, 26th May, 1919.]

The following additional notes with regard to the Trelissick Basin are made as the result of further examination since my former paper* was published. The first matter concerns the existence of another detached block of limestone on the western side of the area at the base of the Craigieburn Range. This was hidden by bush during my former examination of the locality, but fire has swept over it and exposed the rock clearly to view. I should have observed it, however, had I ascended the Hog's Back Creek about a quarter of a mile above the turning it makes on striking the Hog's Back Ridge, as the bed crosses the creek and is exposed on its banks under the covering of shingle. The limestone here forms a heavy band, striking east-north-east, pointing towards the saddle between the Hog's Back Creek and Waterfall Creek in one direction, and in the other running towards one of the spurs of the Craigieburn Range. In the creek the bed is 100ft. thick, dipping generally north-north-west at an angle of 85°, but is much contorted in places. It is well jointed, slightly crystalline, and distinctly coralline. When traced towards the south-west through the burnt bush the dip flattens out and has a general direction of south-south-east at an angle of 45°. The relations to the greywackes at the point where it abuts on the spur of the Craigieburn Range are obscured by bush and a covering of loose debris, but the general circumstances suggest a fault contact, and the relations to the limestone of the Hog's Back Ridge are also difficult to make out, but some strong deformation either of faulting or acute folding, certainly occurs between them.

Another detached block of limestone occurs on the north side of the Long Spur, which divides the basin of the Hog's Back Creek from that of the Thomas River. This dips west at an angle of 45°, but at its southern end the strike turns round so that it becomes north-east. The rock is much brecciated. It is possible that this forms a syncline with the block of rock just referred to, the uppermost members of the Tertiary sequence being developed in a basin shape between them.

Near the southern end of the Hog's Back a fossil locality was discovered in the bed of the creek. The bed is 3 ft. in thickness where exposed, and consists of sand and shell layers, striking east, and dipping south at an angle of 75†—that is, parallel to the limestone of the Hog's Back in its vicinity. The following fossils were obtained: Cucullaea ponderosa Hutt., Fulgoraria arabica elongata (Swains.), Paphia curta (Hutt.)

My attention has been drawn by Dr. W. P. Evans and Mr. A. E. Flower, who were recently camping in the district, to another important fossil horizon in the Thomas River. This occurs in the bed and on the north bank of the stream just opposite the junction of the little creek that flows past the farm and joins the Thomas in the vicinity of the old wool-scouring works. The strata consist here of sands, sandy clays, and dark sandy shales with numerous specimens of Ostrea ingens. Underneath the oyster-bed are

[Footnote] * R. Speight, The Stratigraphy of the Tertiary Beds of the Trelissick or Castle Hill Basin, Trans. N.Z. Inst., vol. 49, p. 321, 1917.

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dark sandy shales, light-coloured and greenish-grey sands, sometimes glau-conitic, in the upper part of which are very fragmentary fossil shells. The dark sands weather brown, and occasionally have lenses of crystalline calcite running through them. Farther down stream there is the same alternation of sands and shales with lignite, and in one of the beds of sand, about 2 chains above a fence crossing the creek, is a layer of shell-fragments which include Struthiolaria tuberculata and Plejona huttoni pseudorarispina. These beds pass down into the sands with numerous shell-fragments, to be referred to later.

The beds in the vicinity of the wool-scour strike north-west and dip south-west at angles of about 20°, and they continue to do so for a little distance up-stream, where lignite again occurs associated with greenish sands, but above this the strike must swing sharply round till it becomes east-north-east—that is, along the creek. As the beds are followed upstream, the dip becomes steeper, as much as 60°, in agreement with the strike and dip of the limestone, south of the road, forming the north-eastern corner of Castle Hill. The beds here consist of greenish and yellowish white sands and sandy shales, the latter occasionally carbonaceous. I was fortunate on this occasion to be able to locate exactly the bed of Ostrea ingens, first recorded by Park,* for the heavy floods during the last season had swept the face free from surface debris and exposed the fossil-bed cleaily. I had repeatedly searched for the bed without success, and am glad to give confirmatory evidence of his interesting discovery. It was quite by oversight that I made no reference to this point in my original paper, and I regret the omission.

At the point where this bed is exposed the following series is shown on the face of the cliff, the sequence being in ascending order and the total thickness about 25 ft.: (1) impure lignite; (2) sandy shales; (3) greyish sands; (4) sandy shales; (5) greyish argillaceous sands; (6) oyster-bed, 12 in. to 18 in. thick, in dark argillaceous matrix; (7) greyish sandy shales; (8) greyish sands, weathering brown; (9) white sands.

Park is quite correct in his inference that this oyster-bed (6) is considerably higher than the beds developed just above the limestone gorge of the Porter, since this bed is probably at a higher level than that occurring opposite the wool-scour, which is undoubtedly at a higher level strati-graphically than that containing numerous Struthiolaria, which in turn overlies the limestone. However, the exact correlation of the two oyster-beds is a matter of doubt owing to the absence of continuous exposure and the slipped character of the country, especially the high banks of the Thomas, where they are overlain by gravels. Although shell-fragments were observed higher up the stream still, above the road-crossing, no other bed of oysters was located, although it may easily occur buried up by surface debris The occurrence of the Ostrea ingens indicates clearly that the beds associated with it are of Mio-Pliocene age.

Still another fossil horizon was located in the Porter River, about a mile above the limestone gorge, and just below the junction of the river with a rocky creek coming from Mount Torlesse. In the bed of the stream is a small exposure of greensands containing numerous specimens of the black oyster (Ostrea angasi). The beds strike north-east, and dip north-west 40†. Associated with this is another bed containing fragmentary shells in poor

[Footnote] * J. Park, Marine Tertiaries of Otago and Canterbury, Trans. N.Z. Inst., vol. 37, p. 535, 1905.

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state of preservation. A fragment of a gasteropod showed part of a suture-line with a well-defined band below it, suggesting the Cretaceous genus Nerinaea, which has recently been obtained in Southland by Marshall.

A special object of my recent visits to the locality was to collect fossils from the loose sands and concretionary bands above the limestones, beds which have been classed as “Pareora” by all who have examined the Castle Hill Basin. A full description of these beds, specially those occurring between the two gorges of the Porter River, was given in my previous paper, and so it is unnecessary to mention their lithological features any further; but the beds are also exposed in the bed of Broken River below the road-crossing, in the Thomas, in Home Creek near its junction with the Porter, and in a tributary coming in on the north side of Whitewater Creek, and elsewhere. Collections were made in four of these localities. It is unfortunate, however, that the great majority of the fossils are in a poor state of preservation. Between the two gorges of the Porter there are two distinct horizons yielding shells. In the lower of these the following were obtained:—

  • Cardium ? sp.

  • Cerithium hectori Harris.

  • Comminella inflata (Hutt.).

  • *Macrocallista multistriata (Sow.).

  • *Mactra discors Gray.

  • *Mesodesma australe (Gmel.).

  • Paphia curta (Hutt.).

  • Polinices gibbosus (Hutt.).

  • Polinices ovatus (Hutt.).

  • *Tellina eugonia Sut.

In the lower horizon at Whitewater Creek, in the same stratigraphical position as the above, the following were obtained: *Calyptraea maculata infiata (Hutt.), Cerithium hectori Harris, Crassatellites attenuatus (Hutt.).

In both these localities there is a well-defined upper horizon characterized by numerous individuals of Struthiolaria, which for that reason may be called the Struthiolaria bed. Between the Porter gorges the following were collected:—

  • Cerithium hectori Harris.

  • Crepidula striata (Hutt.).

  • Fastigiella australis Sut.

  • Glycymeris globosa (Hutt.).

  • Glycymeris laticostata (Q. & G.).

  • Paphia curta (Hutt.).

  • Plejona (Athleta) huttoni pseudorarispina Sut.

  • Polinices gibbosus (Hutt.).

  • Struthiolaria tuberculata Hutt.

In beds in similar stratigraphical position in the tributary of the Whitewater the following were obtained:—

  • Cerithium hectori Harris.

  • Crassatellites attenuatus (Hutt.).

  • Crepidula striata (Hutt.).

  • Fastigiella australis Sut.

  • *Mactra discors Gray.

  • Polinices gibbosus (Hutt.).

  • Paphia curta (Hutt.).

  • Struthiolaria tuberculata Hutt.

[Footnote] * Recent.

[Footnote] † New species described in this volume (p. 68).

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In Broken River the following were obtained:—

  • *Calyptraea maculata inflata (Hutt.).

  • Cerithium hectori Harris.

  • Crassatellites attenuatus (Hutt.).

  • Crepidula striata (Hutt.).

  • *Diplodonta zelandica (Gray).

  • *Dosinia greyi Zitt.

  • Hemiconus ornatus (Hutt.).

  • Struthiolaria tuberculata Hutt.

From Home Creek the following were obtained:—

  • Ancilla papillata (Tate).

  • *Calyptraea maculata (Q. and G.).

  • *Calyptraea maculata inflata (Hutt.).

  • Cardium spatiosum Hutt.

  • Cerithium hectori Harris.

  • Chione speighti Sut.

  • Crassatellites attenuatus (Hutt.).

  • *Crepidula monoxyla (Less.).

  • Cytherea enysi Hutt.

  • *Diplodonta zelandica (Gray).

  • *Divaricella cumingi (Ad. & Ang.).

  • Lima crassa (Hutt.).

  • Lima huttoni Sut.

  • Lima jeffreysiana Tate.

  • Mactra attenuata Hutt.

  • Modiolaria elongata (Hutt.).

  • Paphia curta (Hutt.).

  • Pecten aff. chathamensis Hutt.

  • Plejona (Athleta) huttoni pseudorarispina Sut.

  • Polinices gibbosus (Hutt.).

  • Solariella cf. sulcatina Sut.

  • Struthiolaria tuberculata Hutt.

  • Turritella ? sp.

An examination of these lists does not disclose any noteworthy difference between the fossil content of the lower set of beds in the Porter and Whitewater Rivers and that of the upper set, but the number of specimens collected is no doubt insufficient to detect with certainty any difference that may exist.

Of the thirty-five species enumerated in the combined lists only twelve are Recent—that is, 32 per cent.—a percentage which does not differ appreciably from that of Recent forms in the “shell-bed” of the Porter River or the tuff-beds interstratified in the limestone as recorded in my paper on the Trelissick Basin. In this paper it is maintained on stratigraphical grounds that there is no unconformity between the limestones and the overlying beds yielding the fossils enumerated above, and this contention is now supported by the palaeontological evidence.

In concluding these notes I have to express my indebtedness to the late Mr. H. Suter for kindly helping me with the identification of the majority of these specimens, and for describing the new species of Fastigiella included in the list

[Footnote] * Recent.