No abstract

Pre-Cretaceous basement in the Mt Somers area consists of Torlesse Supergroup rocks ofprehnite-pumpellyite facies which are here subdivided into the Mt Taylor Group (Middle-Late Triassic; new group), the Clent Hills Group (Middle-Late Jurassic; amended group), and undifferentiated Torlesse Supergroup. Mt Taylor Group is further subdivided into three new formations: (a) Fingers Formation (Kaihikuan-?Oretian (Ladinian-?Carnian) of late Middle to perhaps early Late Triassic age) of medium-coarse-grained sandstone and minor interbedded alternating sandstone-siltstone sequences containing the key fossils Daonella sp., Praegonia sp., and Spiriferina cf. abichi Oppel. Part of this formation consists of distinctive massive, white, micaceous, coarse-grained feldsarenite with dark calcareous spheroidal concretions. (b) Pudding Hill Formation (Middleearly Late Triassic) a flysch-like sequence of alternating dark grey, fine sandstone and siltstone turbidites, and contains the tube fossil Torlessia mackayi Bather. This formation appears to interdigitate with the Fingers Formation. (c) Balmacaan Formation (Kaihikuan (Ladinian), late Middle Triassic), a fine-to mediumgrained sandstone and siltstone sequence containing conglomerate beds. The key fossil of this formation is Agonisca corbiensis Fleming. Balmacaan Formation appears to be a near-shore (proximal) deposit, Fingers Formation an intermediate reworked deposit, and Pudding Hill Formation a distal turbidite deposit of a continental shelf submarine fan. A west to east palaeoslope is suggested. Undifferentiated Torlesse rocks, of probable Triassic age, occur in the area. These unfossiliferous rocks are lithologically similar to the Mt Taylor Group and in particular to the Balmacaan Formation but in places grade into a phyllite or cataclasite, apparently related to faulting. The Clent Hills Group contains both non-marine and marine s~quences with probable gradations between the two. A late Jurassic (Puaroan) age is indicated by the presence of Inoceramus cf. gracilis Holdhaus and Buchia sp. indet., but other marine fossil assemblages indicate a possible range of Middle-Late Jurassic (Temaikan-Puaroan). Conglomerate channel deposits containing tree trunks, and siltstone beds containing well preserved plant remains are a conspicuous feature of the formation. The Clent Hills Group is interpreted as a deltaic sequence.

The Mount Somers Volcanics of mid Canterbury in eastern South Island are a high-K calcalkaline andesite-dacite-rhyolite suite erupted subaerially approximately 95 Ma ago during the long Late Cretaceous episode of normal geomagnetic field. Thermoremanent magnetic directions at 46 sites, corrected for post-eruption tectonic tilt, are stable and group well (a95 = 3'8°, K = 31'7) with a very steep mean inclination (I = -S5°, D = 354°). The resulting paleomagnetic South

Potassium-argon total rock, sanidine, and plagioclase ages are presented for 24 rhyolite, dacite, andesite, and basalt rocks of the Upper Cretaceous, Mt Somers Volcanics of mid Canterbury. Ages are grouped as follows: (1) total-rock ages of andesites and dacites: 45-94 Ma, but with a significant concentration at 82-91 Ma; (2) plagioclase ages of andesites and dacites: 87-93 Ma; (3) sanidine ages of pitchstone flows and rhyolitic intrusives: 88-99 Ma. The wide range of andesite total-rock ages, which do not correlate with co-existing plagioclase ages, indicate that substantial argon loss (up to 50%) may, in a few cases, have occurred from a second potassic phase, most probably volcanic glass. The majority of total-rock/plagioclase pairs show concordant ages, which in turn are similar to sanidine ages of closely associated rhyolitic rocks. Detailed stratigraphic evidence and potassium variations in the feldspars show that argon loss (about 5%) has occurred in most plagioclases (K> 0-8%) and some sanidines (K < 7%), causing younger ages than anticipated. In addition, despite favourable potassium concentrations (7% < K < 0-9%) of their feldspars, andesites known to predate rhyolites none the less yield younger ages. To some extent this pattern is explained by slight differences in the andesite-rhyolite sequences at the three main eruptive centres at Mt Somers, Malvern Hills, and Rangitata Gorge. Both andesite and rhyolite volcanism at Mt Somers (94-98 Ma) may predate rhyolites in the Malvern Hills area, 91-94 Ma. Mt Somers Volcanics extend from about 98 Ma (upper Albian) to at least 92 Ma (Turonian). Owing to the slight argon loss problem, the younger age limit for volcanism cannot be assessed. A basalt sample from Malvern Hills, dated as 37 Ma, is probably related to widespread tholeiitic Oligocene volcanism in North Canterbury, rather than to Mt Somers Volcanics. Mt Somers Volcanics show age similarities with more basic alkaline volcanics of inland Marlborough (and may form part of an Upper Cretaceous volcanic arc in the South Island). More distant age correlations can be made with rhyolites on Lord Howe Rise (Tasman Sea) and in central Marie Byrd Land, West Antarctica.

The first synthetic aperture radar image ofNew Zealand covered an area of mountain ranges in the North Island. These ranges comprise Mesozoic greywacke and argillite and are flanked by Tertiary marine sediments and mantled with Holocene tephras. Variations in radar image texture delineated areas of different lithology and induration with some success. However, reasons for many of the textural variations are unknown but might relate to vegetation. The image also clearly depicts major faults where these have a significant effect on the topography, but no new fault lines were identified.