Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations
Sequence and Character of Rocks, and Position of Strata
Sequence and Character of Rocks, and Position of Strata.
The principal beds of this formation consist mostly of bluish or greenish argillaceous sands, with harder calcareous, mostly fossiliferous beds interstratified with them. One of the best and most extensive sections is situated on the left, or southern banks of the Waipara, where the strata which immediately repose upon the Mount Brown series consist mostly of arenaceous and argillaceous beds, clays, sometimes marly or loose marine sands. The clays often enclose concretions of sandy limestones, and also harder beds of the same rock, impregnated with lime and alternating many times with each other. In them we meet at the foot of that mountain, and often in a perfect state of preservation, with a great variety of fossils, as, for instance, Voluta pacifica, Natica solida, Struthiolaria (several species), Lutraria solida, Cytheriz Enysi, Dosinia (two species), Venericardia intermedia, Pectunculus laticostatus, Lima crassa, and many others. These beds are either of littoral origin or shallow water deposits. They are overlaid by beds of conglomerate, mostly formed of small river shingle, deposited in a shallow estuary, and consisting of the debris of the palæozoic ranges near the upper course of the Waipara; but the destruction of some of the older tertiary rocks has also furnished material for their formation. These beds are together of a thickness of about 300 feet. Some contain two species of oysters, of which one is Ostrea Nelsoniana, which sometimes forms page 318regular beds. There occur, also, some casts of gasteropoda, mostly filled in the interior with crystals of calcareous spar. The beds are capped by loose marine sands, sometimes very calcareous, full of fragments of shells, and of a light-yellowish or greenish colour. Here and there harder bands of calcareous sandstones stand out from them as protuberances. The same Ostrea, Pectunculus laticostatus, and Pecten secta occur in them. Between them beds of conglomerate are interstratified, and the interstices between the pebbles mostly filled up with calcareous spar. The beds along the banks of the Motanau river and of Motanau island consist of similar strata, also resting upon the Oamaru formation, apparently conformably. The lowest beds consist of bluish sandy clays, with rolled pieces of shells, sometimes forming regular layers; they are divided in numerous banks by beds of calcareous sandstone six inches to two feet thick, standing out as protuberances. Sometimes the latter is separated in lenticularshaped masses, lying side by side in the sands. The uppermost deposit consists of a thick layer of hard fossiliferous calcareous sandstone, sometimes forming cliffs along the sea coast for a considerable distance, and in which most of the shells appear only as casts. This bed dips near the mouth of the Motanau river 10 deg. to the south-east.
There is a great similarity in the rocks belonging to this formation, owing, without doubt, to the same physical conditions prevailing during their deposition. The occurrence of small beds of lignite in them is only an exception. The principal localities where such beds have been met with are situated in the Moeraki Downs, at the mouth of the Waipara, in the Broken River basin, and in White Rock creek, a source branch of the Pareora river. The strata belonging to this series lie either conformably upon the Oamaru formation, or, what is still more usual, unconformably upon it. In many localities, as, for instance, in the Pareora and Waihao rivers, the calcareous greensands have become greatly denuded before the Pareora beds were deposited above them, or which is more commonly the case, along them, when the former stood as islands in the tertiary sea. From the sections attached to this chapter the relations of the beds to each other can easily be made out, so that I need not enter here into a more minute description.
The Kanieri group at the West Coast resembles in many respects the Pareora beds on the East Coast. The lowest bed visible consists of bluish sandy marls, glauconitic at their base. In them calcareous page 319nodules are of frequent occurrence, containing generally fossil shells, or cetacean remains. Gradually they alter to ferruginous sands, with layers of fossiliferous sandstone having a calcareous matrix, interstratified with them. Upon them reposes a thick bedded conglomerate, apparently the highest bed in the series; occasionally rolled pieces of shells and cetacean bones are enclosed in it.
In the list of fossils found in this formation, I have indicated those which are only found in the Westland series, the beds of which generally dip so slightly that they sometimes appear in a horizontal position.