Stratigraphy of stephens subgroup (maitai group) in the countess range — mararoa river area, northwestern southland, New Zealand

Abstract

Stephens Subgroup (upper Maitai Group) is well exposed (except for its top) for over 30 km in the Countess Range, Snowdon Peak and Bare Peak-Mararoa River areas. The sequence consists of 2750 m of sedimentary rocks which include volcaniclastic sandstone, conglomerate, and siltstones, conspicuous tuffs and occasional limestone lenses. The subgroup is subdivided into five formations; four of them are new. Kiwi Burn Tuffs (new) is characterised by tuff. Acheron Lakes Sandstone (new) is a thick sequence of volcaniclastic sandstones. Cerberus Formation (new) is dominated by red coloured siltstone and sandstone. Eldon Formation (new) is another sequence of volcaniclastic sandstones, and Snowdon Formation is of variable lithology including volcaniclastic sandstone, conglomerate, siltstone and tuff. Fossils are rare in the Stephens Subgroup. However, Permophorus obovatus is found in the Retford Conglomerate Member, which has now been traced to the Mararoa River. This, along with Mellarium and radiometric ages from middle Snowdon tuff, clearly indicate an Early-Middle Triassic age for the Snowdon Formation. Basal Stephens Subgroup formations may also be of Early Triassic or even of Permian-Triassic boundary age. © Journal of the Royal Society of New Zealand.