The Geology of the Malvern Hills, with Map and Sections, Panoramic
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NEW ZEALAND. Jle:paxtmenf of ~cienfific anb 6lnbuztriar ~ezearcf?. GEOLOGIOAT.--J MEMOIRS. ME M 0 I R No. 1. THE GEOlOGY. OF THE MAlVERN HillS. By R. SPEIGHT, M.Sc., F.G.S., F.Am.G.S., Curator of the Canterbury Museum. With a Section on the Physical and Chemical Properties of the Clays and Sands, by S. PAGE, B.Sc. WITH MAP AND SECTIONS, PANORAMIC SKETCHES, AND PHOTOGRAPHS. WELLINGTON BY AUTHORITY: W. A. G. SKINNER, GOVERNMENT PRINTER. 1928 NoTE.--We have been enabled to do the work contained in this report by means of a grant made by the New Zealand Institute for that purpose. CONTENTS. PAGE A. Introductory 1 B. General Physiography of the Area 1 C. Stratigraphy 3 I. Pre-Senonian 4 (a) Triassic Sedimentaries and Volcanics, and Lower Jurassic Sedimentarias 4 (b) Upper Jurassic or Lower Cretaceous Volcanics .. 12 (i) Rhyolites 14 (ii) Andesites 16 II. Senonian 17 (a) Coal-measures 17 (i) Hororata-Glentunnel-Sheffield Area 17 (ii) Outliers 34 Cordy's Flat, Hart's, St. Helens, Phillips Saddle, Rockwood, Hawkins Valley, Kowai Valley, High Peak, Acheron, Rakaia Gorge. (b) Age of the Beds 44 III. Post-Senonian 44 (a) Igneous Rocks-Effusives and Intrusives 46 (b) Post-Tertiary-Gravels, &e ... 52 D. Economic Geology 53 (a) Coals 53 (b) Clays and Sands 56 (c) Building-stone 67 ( rl) Metalliferous Minerals 68 E. Bibliography 69 Index 7l THE GEOLOGY OF THE MALVERN HILLS. A. INTRODUCTORY. IN the early days of geological work in New Zealand the Malvern Hills district attracted considerable attention, chiefly, perhaps, since it promised to provide a considerable amount of coal for a region deficient in fuel, but also because it furnished points of purely scientific interest. The latter were fully recognized by Dr. von Haast, who was then Provincial Geologist for Canterbury, and he spent much time and trouble in attempting to elucidate its geological features and to determine the precise economic possibilities of the area. Although this work was done fifty years ago, it is surprising how well it has stood the test of time. The records of his work are to be found in his reports to the Provincial Government, and also in the earlier reports of the New Zealand Geological Survey. In addition to Haast's pioneer work, special mention must be made of that done by Captain Hutton when he was on the Survey and later, and also of that by S. Herbert Cox. The accounts given by these early geologists still furnish the basis of subsequent work, and I must personally express my indebted ness to them for suggestions so numerous that it is impossible to mention them further. Quite recently most important palooontological work has been carried out by Woods and Trechmann, and many points connected with the chemical aspects of the coal problem have been thoroughly dealt with by Evans. 'Ihe fact that the latest accoun~, of the area was given by S. H. Cox in 1884-that is, forty years ago has prompted me to redescribe it in the light of more modern knowledge, and hence the following account. B. GENERAL PHYSIOGRAPHY. FoR the purpose of this paper the area called the Malvern Hills will be considered as that lying to the south-east of Big Ben Range, and forming the basin of the Upper Selwyn and its tributaries, the Hawkins, Waianianiwha, Wairere, and the Hororata. Since the main matter for consideration is concerned with the nature and stratigraphy of the Cretaceous coal-measures occurring in the district, there are three other areas which must naturally be considered with it-viz., the outliers at Benmore, the Acheron, and Rakaia Gorge. · As has been explained in a previous paper (Speight, 1924), the main surface features of the area arise from fault-movements which have affected a peneplain developed out of Trias-Jura rocks covered with a thin veneer of weak Cretaceous beds. These movements have resulted in the formation of a series of subparallel more or less continuous ridges, with intervening strips, whose orientation has determined initially the directions of the main streams. Commencing from the north-west, these faulted strips are as follows :- 1. The Benmore Area, with a possible continuation on the line of the Acheron Valley. South-east of this lies the mass of the Big Ben Range, a long even-topped ridge, rising to 5,436 ft. in Benmore, with lower foothills farther south-east. Then follows 2. The High Peale Area, occupying a portion of the Upper Selwyn basin and reaching north- east across the Upper Hawkins. South-east of this extends a well-defined ridge, of which the following are the chief features, commencing from the south-west end of the range: Round Top (2,917 ft.), Snowy Peak (2,983 ft.), High Peak (3,176 ft.), Four Point Range (2,414 ft.), Flagpole Hill (2,939 ft.), and the Black Hills'(2,264 ft.). The Selwyn has cut its gorge between the last two of these, (See panorama.) 1-Geol. Mem. No. l. 2 3. The Kowai Area, bounded on the south by the Russell Range (3,086 ft.), which forms a kind of splinter connecting the foothills of Big Ben with the north-east end of the Round Top-Flagpole- Black Hills ridge. 4. The Rockwoocl A1·ea, occupying the upper valley of the Hororata and lying between Snowy Peak, High Peak, and a ridge extending south-west from Pullwool Peak (2,404 ft.). 5. 001·dy' s Flat Area, which lies between the Four Point Range- Flagpole Hill- Black Hills ridge and the broken ridge of which Mount Misery (1,910 ft.) and the Cairn Range (1,634 ft.) are segments. This area perhaps extends across Pig Saddle (Knight's Saddle) to the west of Abner's Head and on to the north-east under Russell's Flat, where it is masked by recent river-gravels, except at the northern corner, close to the Township of Springfield. To the south-east of the Pullwool Peak-Mount Misery- Cairn Range- Abner's Head ridge lies the main area of coal-measures, stretching from the Hororata River to the Hawkins, near Sheffield, a belt about 13 miles in length, with a maximum width of 4 miles, the latter corresponding practically with the line of railway between Coalgate and Whitecli:ffs. The following streams are associated with these strips of country :- (1) Macfarlane's Stream, which runs north-east to the Upper Kawai, behind Big Ben, and probably the Acheron, which discharges south-west to the Rakaia. (2) The Upper Selwyn, which runs north-east and receives the greater part of its water from the south-east slopes of Big Ben. (3) The Kawai, in its middle and lower course, running north-east to the Waimakariri. (4) The Upper Hororata, which runs almost south past the Rockwood Station. (5) The Glendore Stream, which runs,north-east past Mount Misery to join the Selwyn near Whitecli:ffs. While attributing the major outstanding surface features of the area to faulting, it should be noted that the deformational movements did not in all cases result in rupture, but rather warping. In the D;Orth-east, especially in the vicinity of Abner's Head and the Black Hills, also on the northern flank of the Cairn Range, this is most marked. Stripped surfaces occur here in such a perfect condition that it is reasonable to infer not only the former presence of covering beds, but also their dip and strike from the slope of these surfaces. This may seem somewhat doubtfU;l evidence, but when one sees remnants of the covering beds associated with such surfaces, and then again the surfaces without the covering beds, the conclusions drawn therefrom are hardly as far-fetched as persons unacquainted with the locality might imagine. As a result of observations on these I have concluded that the faulting movements lost somewhat of their intensity and graded into folds in the north-east part of the area, although there is certainly evidence that some faulting existed there. The association of faults with folds is a characteristic feature of many parts of the submontane area of New Zealand, and it may have been characteristic of the montane area, but we cannot say, since the covering beds, from whose position inferences may most readily be drawn, have been stripped away almost completely. The only streams that have been powerful enough to break across the ridges which bound them on the south-east are the Selwyn and Hawkins, the former aided, no doubt, by the interference of the glaciers coming from the direction of the Rakaia Valley ; otherwise the drainage has been in a north-easterly or a south-westerly direction. In the area occupied definitely by the coal- measures the control of structure on the directions of the subordinate streams is very marked. There is decided evidence of glaciation in the High Peak Valley and on the western part of the area under consideration. Not only did the Rakaia Glacier cover the plains in the vicinity of the point where it issued on to them with morainic and fluvio-morainic material, but there is undoubted proof that the ice-stream crossed the Rockwood Range at two points-viz., at Middle Saddle, which lies between High Peak and Snowy Peak, and also between Snowy Peak and Round Top. This conclusion is based not only on the smoothed landscape features, but on the fact that massive blocks of greywacke up to 20 tons in weight occur in many parts of the range, resting on a surface consisting of andesite or of rhyolite.