Geology of the Western Taylor and Mount Somers ranges, mid-Canterbury.

Beggs, John McIntyre, 1953-

The western Taylor and Mount Somers Ranges are-formed of Torlesse rocks which are characteristically thick-bedded massive coarse-grained sandstones, with minor siltstone and thin-bedded flysch units. These rocks are assigned to the Fingers Formation. They are overlain in the northwest with sharp angu1ar unconformity by a sequence of Tertiary sediments which are subdivided into four new formations. The Tertiary sequence and the underlying Torlesse rocks are truncated along the northwest margin of the area by a post-early Miocene normal fault, named the Clent Hills Fault.
Three phases of pre-Cenozoic deformation are recognised:
[1] An original sequence probab1y a few kilometres thick was imbricated by tectonic sliding and tilted to produce a steeply-dipping, mainly north-younging sequence.
[2] The rocks were gently folded about steeply-p1unging axes. The major structure is a north-facing syncline, of which the eastern and western limbs have subsequently been deformed to different extents, probably because of the contrast in orientation of bedding (considered an anisotropy). The two limbs of that syncline are recognised as distinctive structural blocks.
[3] Thrusting of the rocks of this area over the Jurassic Clent Hills Group to the south. Evidence for this phase mainly lies outside the area.
In the Cenozoic Kaikoura Orogeny the earliest structures have been reactivated during uplift of the area.
Post-early Miocene offset on the Clent Hills Fault is at least 2000 m.
Sandstones of the Fingers Formation are feldspathic (50-60%) and micaceous (about 5%). Study of the diverse suite of lithic clasts in these sandstones, supported by the mineralogy of monocrystalline grains, suggests a source area including acid plutonic, acid volcanic, and metamorphic rocks, probably in that order of importance. Metamorphic clast types show a great variety of textural and mineralogical reconstitution, the lovrest-grade members being quartz--sericite greywackes. The chemistry of some detrital muscovites is consistent with a metamorphic terrane ranging from lower greenschist to upper amphibolite facies. The data may indicate a distinctive compositional characteristic of the metamorphic source-rocks, of lower Na content. The petrographic evidence favours ultimate derivation of these rocks from the early Paleozoic. (Tuhua Sequence of Carter et al. 1974) rocks of New Zealand or their correlatives on the Campbell Plateau or in West Antarctica.
The rocks of the area have been metamorphosed to the zeolite facies. Sedimentary features of the thick-bedded, coarse grained sandstones point to deposition by mass-flow processes, and they are interpreted as submarine channel deposits. Intervening thin-bedded flysch and siltstone sequences have the characteristics of interchannel deposits. It is suggested that the Fingers Formation was-deposited in a base-of-slope, inner and/or middle submarine fan environment.
Fossils from one locality within the area indicate a Kaihikuan (middle Triassic) age for at least part of the Fingers Formation. Lithological considerations suggest that the age range of the formation is unlikely to extend outside the Triassic.
Considering the sedimentary environment of this area during the Triassic and that of other Torlesse rocks known to be of the same age, it is suggested that an eastward directed paleoslope existed at that time in mid-Canterbury. The sediments may have been derived from an archipelago made up of older Torlesse rocks which themselves were derived from Western Province rocks, bypassing the Hokonui volcanic arc.
The 200 m Tertiary sequence represents nearly continuous deposition from mid-Eocene to early Miocene. The lower part of the section is transgressive and the upper part, regressive. The Oligocene part of this sequence differs from that of other sections in the Canterbury basin in that terrigenous sediments are dominant over carbonates. This is cited as evidence for the continuous existence of an emergent land area in the central South Island, throughout the Cenozoic.

104 leaves : illus., maps (in pocket) ; 30 cm.

1978Beggs

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