Art. 3.—On the Discovery of the Liothyrella boehmi Greensand Band at Flume Greek, Waitaki Valley.
[Read before the Otago Institute, 8th November, 1921; received by Editor, 12th November, 1921; issued separately, 1st February, 1923.]
At the lower end of Flume Gully, about a mile from Papakaio, the yellow-and-black speckled mineral tuffs which occupy the floor and lower sides of the valley are overlain conformably by a bed of compact yellowish-brown calcareous sandstone about 40 ft. thick. The lower part of this sandstone is highly glauconitic, and the upper part is intercalated with bands and lenses of hard impure limestone.
At the junction of the tuffs and overlying glauconitic layer there is an irregular streak of gritty, pebbly conglomerate, ranging from almost nothing to about 2 in. thick. The material in this grit-band is mainly flinty quartz and black minerals. In size it ranges from sand-grains to well-rounded pebbles ½ in. in diameter.
The dip of the tuffs and associated calcareous sandstone is towards the north at a low angle. The course of Flume Gully is almost at right angles to the strike, which coincides approximately with the trend of the Waitaki Valley.
On the west side of the gully, at a point about 100 yards above the big flume and 55 ft. above the floor of the valley, I discovered in the soft glauconitic band overlying the, tuffs and in the pebbly band a rich marine fauna, containing prominently, among many other shells, a profusion of well-preserved examples of the large and beautiful Lioihyrella boehmi (Thomson).
The molluscs collected here included—
Epitonium lyratum (Zitt.)
Siphonalia conoidea (Zitt.)
Turbo marshalli Thomson
Crepidula costata (Sow.)
Pecten hochstetteri Zitt.
Pecten delicatulus Hutt.
Pecten huttoni (Park)
Ostrea wuellerstorfi. Zitt.
From the same bed I collected the brachiopods—
Pachymagas ellipticus Thomson
Neothyris tapirina (Hutt.)
Rhizothyris rhizoida (Hutt.)
Hemithyris sp. cf. squamosa (Hutt.)
Liothyrella boehmi (Thomson)
Liothyrella oamarutica Boehm
Terebratulitia suessi Hutt.
Aetheia gualteri (Morris)
Of these, Liothyrella boehmi, Aetheia gualteri, and the species of Hemithyris are the most common.
Associated with the molluscs and brachiopods occur many Polyzoa, represented typically by a large cup-shaped Celepora, several corals, including Isis dactyla T.—Woods (some of the joints of which were found over 1 in. in diameter and 2 in. long), Cidaris spines, Graphularia sp., and cetacean-bones, the latter fairly abundant. Some 30 yards nearer the flume, in the uppermost layer of the tuffs, I found a good example of Liothyrella boehmi and one example of Aetheia gualteri.
My success at Flume Gully led me to an examination of the lower glauconitic horizon further afield. I discovered almost the same assemblage of brachiopods in the glauconitic band underlying the Maruwhenua limestone at the old gold-workings; also in the same band below the Ngapara stone at Weka Creek, Ngapara, at Pukekarara Creek, a mile and a half due east of Tokarahi Railway-station.
The brachiopods seem to occur only where the band is moderately glauconitic. Where it is intensely glauconitic there is usually an absence of both molluscs and brachiopods.
The assemblage of brachiopods found at Flume Gully is practically the same as that occurring in bed c, fig. 25, overlying the Oamaru stone in Landon Creek;* in the calcareous glauconitic tufts overlying the Oamaru stone near Sebastopol;† in the calcareous glauconitic tuffs overlying the Oamaru stone east of Rocky Peak ‡ and in the calcareous tuffs underlying the Kakanui (Deborah) limestone and overlying the Oamaru stone at Kakanui and neighbourhood.§
But this brachiopod assemblage also occurs in the glauconitic sandstone band underlying, or forming, the lowermost part of the Waitaki stone near Duntroon∥ Maruwhenua, and Ngapara. And from these facts I conclude (a) that the Waitaki stone overlies the Oamaru stone: (b) that the Kakanui, Flat Top Hill, and Deborah limestones, as well as the upper bands of limestone or calcareous glauconitic sandstone at Sebastopol, Landon Creek, and Flume Gully, are the equivalent of the Waitaki stone; and (c) that the Liothyrella boehmi brachiopod horizon below the Kakanui limestone is the equivalent of the glauconitic sandstone. band immediately below the Waitaki stone.
The new evidence strengthens my contention (a) that in the Oamaru area there are two limestone horizons—viz., the Oamaru limestone and
[Footnote] * J. Park, The Geology of the Oamaru District, N.Z. Geol. Surv. Bull. No. 20 (n.s.), p. 63, 1918.
[Footnote] † J. Park, l.c., p. 74.
[Footnote] ‡J. Park, l.c., p. 72.
[Footnote] § J. Park, l.c., pp. 58, 69, 71.
[Footnote] ∥ J. Park, l.c., p. 83. See also G. H. Uttley, Trans. N.Z. Inst., vol. 5O, p. 124,1918.
the Kakanui limestone (or Waitaki stone); and (b) that in the Maruwhenua-Ngapara area the Oamaru limestone is represented by clayey and sandy beds underlying the Liothyrella boehmi zone.
At Kakanui the Kakanui limestone is separated from the Oamaru stone by a great thickness of mineral tuffs, and the outcrops of the two limestones lie far apart. To the west the tuffs diminish rapidly in thickness, and eventually disappear near Weston and Totara. As a consequence of this the limestone outcrops converge to the westward, and at Sebastopol and in the area lying to the west of that hill the two rocks rest on one another, forming one escarpment; but the glauconitic band is always present between them, though not everywhere fossiliferous.
Towards the Waitaki area the Oamaru stone thins out and then disappears, but the glauconitic Liothyrella boehmi band and the overlying Waitaki stone are still present.
It is noteworthy that the discovery of this useful reference-horizon was due to the assiduous and careful collecting of Mr. Uttley, supplemented by the painstaking identification of species by Dr. Thomson.
The only modification introduced into my grouping (1918) of the Oamaruian by the discovery of the Laothyrella boehmi zone, omitting the conglomerate band, which is often absent, is the correlating of the Kakanui limestone with the Waitaki stone. Thus we get—
Awamoan . . Clays and shelly sands.
(a) Upper-Limestone (Waitaki, Kakanui) (Pachymagas parci horizon).
(b) Lower—Glauconitic sandstone band (Liothyrella boehmi horizon).
(a) (Mineral tuffs) To west and north, clayey and sandy beds.
(b) Oamaru stone
Waiarekan. . Volcanic tuffs. To west and north, clayey and sandy beds
Ngaparan. . Quartz sands, grits, and conglomerates, with lignite.