The stratigraphy of the Torlesse rocks of the Mt Somers area (S81) mid-Canterbury

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.

Upper crustal structure beneath the eastern Southern Alps and the Mackenzie Basin, New Zealand, derived from seismic reflection data

“…They comprise compositionally monotonous sandstone, interlayered with siltstone and argillite in submarine-fan-type sequences, with rare red and black argillites and conglomeratic units. Bedding is typically <1-50 m scale, with large-scale variations in lithological proportions at c. 1 km and c. 3 km scales that in places are pronounced enough to define lithological associations or units (e.g., Hicks 1981;Oliver et al 1982).…”

Tectonic significance of Cretaceous bivergent extensional shear zones in the Torlesse accretionary wedge, central Otago Schist, New Zealand

“…In the Clent Hills Group north of the Otago Schist, weakly indurated Late Jurassic deltaic deposits rest on highly indurated Late Triassic Torlesse greywacke (Fig. 1 A) (Oliver et al 1982). These sediments are little studied in terms of their provenance and structure but are probably trench-slope rather than extensional basins.…”

Lower Tertiary transgressive sediments of the Broken River Formation, Mt Somers area, Canterbury, New Zealand

“…1) are slightly metamorphosed (Zeolite Facies) sandstone, mudstone, and conglomerate of the Triassie-Jurassie Torlesse Supergroup (Oliver 1977(Oliver , 1979Oliver et al 1982). These rocks were deformed and uplifted in the Early Cretaceous during the Rangitata Orogeny (Oliver 1979), and eroded 10 a peneplain, before the subaerial extrusion of the Cenomanian calcalkalic rocks of the Mt Somers Volcanics Group (Oliver 1979).…”