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Geology of the Provinces of Canterbury and Westland, New Zealand : a report comprising the results of official explorations

Oamaru Formation

Oamaru Formation.

As previously stated, there are a number of coal seams of some economic value in the Oamaru formation, stretching from the northern page 457Ashburton to the Waitaki; however they are generally confined to localities of less extent than those of the preceding formation. As far as I am aware, no seams of any value are found at the base of this formation north of the Kowhai river, the first seam of any thickness (about 4 feet) being found near the southern foot of Mount Grey. It doubtless lies at the base of the Oamaru formation. It is a lignite, the woody structure of most of its component parts being still clearly discernible. After crossing the Rakaia, small outliers with brown coal have been preserved in many localities, consisting generally of one seam only, but of considerable thickness. Thus, for instance, the seam on the right bank of Taylor's Creek two miles above its entrance into the - Canterbury plains, is at least 28 feet thick. It consists of a lignite of fair quality, but stands at a very high angle (70 degs.). A similar seam exists on the western slopes of the Clent Hills, in the Upper Ashburton plains, of which section No. 9 on plate 3 gives the details.

In ascending the hillside close to the Clent Hill station, and after passing through 10 to 12 feet of shingle deposit, the lowest beds belonging to this outlier are reached, consisting of—

  • 42 feet of loose ferruginous sands dipping E.S.E. 76 degs., and containing pockets and concretions of ferruginous clays. Upon, them reposes—
  • A band of clay marls 12 inches thick, full of casts of a Gypris and of a bivalve, allied to Cyrene, thus proving the fresh water origin of the beds under review. They are overlaid by—
  • 14 feet of arenaceous sands gradually becoming sulphurous, separated in many layers by small bands of clays, through which the whole obtains a well stratified appearance; they are succeeded by a bed of under-clays, several inches thick, upon, which reposes a large seam of—
  • Brown coal, dipping E.S.E. 63 degs., 28 feet 6 inches thick, separated by several small bands of shales into different banks.

This seam of brown coal, of fair quality, is covered by loose quartzose sands, first white and afterwards assuming yellowish tints, which, 80 feet above the coal seam, abut against the palaeozoic rocks, here forming steep cliffs and being greatly decomposed, the whole overlaid, as before stated, by post-pliocene alluvium.

There are some other coal seams near the Canterbury plains, either in small outliers, or skirting the ranges; for instance, in the bight page 458formed by the junction of the Cox range with Mount Somers, a series of seams of lignite occurs having an average dip of 14 deg. towards N.N.W., and reposing on quartziferous porphyry. This series begins with beds of porphyry tufa, and fire-clays, covered by a succession of shales, alternating with seams of coal, two to five feet thick—the coal consisting of distinct layers of earthy brown coal and lignite, the latter exhibiting quite clearly the woody structure. Another series of coal seams, situated on a spur on the Cox hill ranges, about five to six hundred feet above the foot of these hills where they rise from the Canterbury plains, seem to skirt the ranges. They consist of deposits similar in character to those previously described, also reposing on porphyry tufa. There is first a lower seam of 2 feet 6 inches, separated from an upper seam 2 feet 4 inches by 4 inches of shale covered by fire-clay. These strata dip 35 deg. to the N.W. by W., and consequently appear to dip below the quartziferous porphyries of which the ranges consist. However, it is apparent, from the ridges rising in front of the series, that a small lagoon was at one time here formed by the sea, and filled up in couse of time by the above described beds.

Another basin, already of practical value, is situated in the neighbourhood of the junction of the River Stour with the Ashburton. It occurs in a depression amongst the quartziferous porphyries, which, before the main river had cut through their eastern boundary, formed here, doubtless, a ridge of considerable dimensions, behind which the sedimentary beds of lacustrine or littoral origin could be accumulated. This basin, somewhat triangular in shape, is about two miles in breadth and length. It appears that the porphyries had already undergone considerable denudation before the newer beds were formed, they having been deposited on the sides of steep escarpments and cliffs. The lowest beds consist of porphyry tufas, lying generally at a high angle, and following the outlines of the spurs. They are of great variety in colour and texture, often with a fine ribboned appearance, white and yellowish colours being predominant. They are well exposed in Coal Creek; gradually they become darker, and are succeeded in that locality, by shales upon which a seam of brown coal of good quality reposes, 14 feet in thickness, of which however only the lower portion of 8 feet was extracted, when I visited the locality in 1871. Since then another method has been adopted, the coal seam being stripped and its whole thickness worked. This coal has a dip of 8 deg. towards the S.S.E.

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The seam is capped by shales, with smaller seams of brown coal interstratified, and sandy clay marls, overlaid unconformably by post-pliocene alluvium. Section No. 11, on plate 3, gives the details of this locality.

On the opposite side, and on the southern banks of the River Ashburton, I discovered another portion of the same basin, consisting of porphyry tufas, shales, and two seams of brown coal, about 4 and 5 feet thick, separated by a few feet of inferior coal, or shale. However, as the outcrop was very much decomposed, the two might possibly belong to one seam; this can easily be ascertained by opening up the ground, which is here also covered by post-pliocene alluvium. The deposits in Alexander Creek, a tributary of the River Stour, are doubtless only a portion of this larger Ashburton-Stour basin; the coal seams are, however, too small and too irregular to be of any practical value.

In the southern portion of the Province in several localities, brown coal of fair quality has been discovered and is partly worked, but the seams are always irregular and do not extend over a large area without either thinning out or becoming of very inferior quality. I have alluded already to the occurrence of a workable seam at Elephant Hill, on page 310 (section 4, plate No. 5). Similar coal seams are also found at the base of the Oamaru formation in the middle course of the different Waihao branches, but they are as previously pointed out, sometimes of indifferent quality, and often unworkable or difficult of access.

Section No. 7, on plate 9, makes us acquainted with the geological features of the Oamaru formation in the middle course of the Opuha, where it leaves the small palæozoic ranges, forming the eastern boundary of the upper Opuha plains. The sequence of the beds is as follows:—

On the palæozoic rocks repose—

  • No. 1. White quartzose sands with some harder layers, containing impressions of dicotyledonous leaves, and some pieces of driftwood changed into lignite.
  • No. 2. Seam of brown coal 8 feet thick. It consists of a good dull brown coal, containing a number of layers of glance coal.
  • No. 3. Quartzose sands gradually becoming ferruginous.
  • No. 4. Ferruginous sand with harder calcareous beds (fossiliferous).
  • No. 5. Greensands, gradually becoming marly, the upper bed—
  • No. 6. Consisting of the characteristic calcareous sandstone, known as the Oamaru or Weka Pass building stone.
page 460

And finally, I wish to allude to another occurrence of coal in the southern portion of Canterbury, which in years to come may be of considerable importance to the district. The coal seams in question are situated in the Otaio district, and exposed in the river-bed not far from where it leaves the palæozoic ranges. For about 100 yards post-pliocene alluvium forms the banks of the river, after which a series of deposits of shale and brown coal is exposed, but not sufficiently to take any reliable measurements.

Then follow in ascending order:—

Shales, thickness about 4 ft. 6 in.
Brown coal 3 ft. 0 in.
Dark sandy shale 2 ft. 0 in.
Brown coal 1 ft. 4 in. dip 13 deg. to N.E.
Shale, blackish sandy 1 ft. 2 in. dip 13 deg. to N.E.
Brown coal 6 ft. 0 in. dip 13 deg. to N.E.

For about six chains the continuation of these beds is covered by alluvium, after which greensands capped by calcareous sandstone appear, the latter forming everywhere the uppermost beds of the series in the district. The principal coal seam consists of a fair brown coal, with layers of glance coal interstratified.

A number of outliers belonging to the Oamaru formation occuring far in the interior, contain sometimes thick coal seams, but they are generally difficult of access, and of limited extent.