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

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Abstract
Pre-Cretaceous basement in the Mt Somers area consists of Torlesse Supergroup rocks of prehnite-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 sequences 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.

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