Submitted by admin on April 22, 2009 - 21:23
Canterbury Land District
The Canterbury district comprises four regions: the high main divide of the Southern Alps forming the western margin, composed of greywackes and argillites; the range-and-basin terrain of the foothills of similar rocks interspersed with later sediments and some volcanics; the extensive piedmont Canterbury Plains veneered with Quaternary alluvium; and to the east the volcanic complex of Banks Peninsula.
The rugged mountain ranges of the Southern Alps were uplifted during the Kaikoura Orogeny over much of their length to over 8,000 ft, culminating in peaks such as those of Mount Tasman, (11,475 ft) and Mount Cook (12,349 ft). They are formed from complexly folded and faulted greywackes and argillites, thought to be mainly of Triassic age. Large advancing glaciers and the effects of a periglacial climate have subjected the rocks to typical ice and freeze-thaw sculpturing; this has already been seen in Southland.
The second region comprises the foothills of the Alps, the basement greywackes and argillites of which apparently become progressively younger in an easterly direction. The oldest rocks are thought to occur in some of the ranges marginal to the Alps in South Canterbury, for instance, in the Kirkliston Range and Hunters Hills, which are greywackes and argillites, locally schistose, probably of Permian and Lower Triassic age. The greywackes and argillites of the foothills north-east from the Hunters Hills (for example, Torlesse and Puketeraki Ranges) are of Triassic age. These rocks continue eastwards to floor the Canterbury Plains. Where outcrops occur, as in the Cheviot area, they contain Upper Jurassic fossils. This eastward younging is shown in section D of diagram 6.
The present range-and-basin topography of this region results from diastrophism accompanied and followed by sedimentation. During the mid-Mesozoic Rangitata Orogeny, the basement rocks were uplifted and deformed. There was also a brief outburst of volcanic activity in the Mount Somers and Rangitata River area when andesites and rhyolites were erupted, together with some ash, to form beds several thousands of feet in thickness. A period of peneplanation and deep weathering produced clays. In late Cretaceous times slow marine transgression led to deposition of freshwater coal measures and marine greensands (containing marine reptiles at Waipara) followed by early Tertiary limestones, the best known being the Amuri limestone exposed at Weka Pass and widespread elsewhere in North Canterbury and Marlborough.
With the onset of the Kaikoura Orogeny the sea retreated and, during the uplift of the Southern Alps, the region was fractured by numerous faults. Extensive fault-bounded blocks were raised to varying heights and tilted to angles commonly of 10–20°. The tilted fault blocks so formed comprise the foothills of the Alps, the intermontane “lows” corresponding to depressed or little raised blocks, for example, the Mackenzie Plains and Hanmer Basin. (Cross-section D shows this tectonic structure with the more recent glacially scooped Lakes Pukaki and Tekapo lying on the Mackenzie Plains.) As the uplifted blocks rose, the relatively weak cover of Cretaceous and Tertiary strata was rapidly stripped from the Mesozoic basement rocks so that the younger rocks are preserved only in some marginal areas, as at Waipara and Weka Pass. The tectonic basins filled with Quaternary debris.
The material derived from the stripping of the fault blocks during the late Tertiary was deposited in the sea immediately to the east over the submerged Mesozoic basement rocks. These constitute the deeper strata of the third region of the land district, the piedmont plains. During Pleistocene glacial periods this aggradation was accelerated. Ice erosion attacked both the soft cover and hard basement rocks of the uplifted blocks, and vast quantities of coarse debris were deposited by glaciers as terminal moraine and thence washed down to the plains by huge rivers, such as the Waimakariri, Rangitata, and Rakaia. Enormous depths of gravels accumulated in the intermontane basins and built up the present surface of the Canterbury Plains. Loess deposited under periglacial conditions mantles the gravels and is sometimes up to 60 ft thick.
The fourth region, comprising the two basaltic and andestic cones of Banks Peninsula, was built up in the early and middle Pleistocene, and volcanic activity also occurred at this time near Timaru and Geraldine in South Canterbury. Both the Banks Peninsula cones have been cliffed by the sea and radial drainage has eroded numerous valleys, now drowned in their lower parts by the post-glacial rise in sea level to form bays as at Port Levy and Pigeon Bay. The sea has breached both cones, gaining access to the craters and forming Lyttelton and Akaroa Harbours.
Earthquakes occur frequently in Canterbury, especially the northern part, indicating that the earth movements responsible for the rise of the Southern Alps and other ranges are still proceeding. The Hope Fault, which branches off the Alpine Fault and extends along the southern side of the Hanmer Basin and into the Seaward Kaikouras of Marlborough seediagram 8), is particularly active and movement on it in 1888 (Glynnwye earthquake) produced horizontal displacements. An earthquake in 1929, locally known as the Arthur's Pass earthquake, had its epicentre on this fault. The Cheviot earthquake of 1901 produced some superficial slumping but no known scarp, as any movement in the underlying Mesozoic rocks was probably cushioned by the cover of soft Tertiary strata.
Canterbury is not well endowed with economic mineral deposits, but it does have abundant supplies of limestone in the Tertiary rocks, small areas of marble in the greywackes and argillites, and plentiful good quality gravel deposits. The Canterbury Plains form an important ground water system, providing water for domestic and industrial use and for irrigation.