Middle Triassic coastal outwash plain deposits in Tank Gully, Canterbury, New Zealand

Abstract

A homoclinal sequence of Middle Triassic rocks crops out in the southern flanks of Mt Potts between Tank and Rocky Gullies, Canterbury. The basal conglomerates and sandstones of this sequence were deposited by braided streams filling an early Triassic valley, cut into deformed older sandstones. The basal conglomerates and sandstones are conformably overlain by coal measures containing diverse megafossil plant assemblages typical of Late Anisian to Ladinian rocks of Gondwanaland. These coal measures are in turn conformably overlain by beach sandstones and shallow-marine siltstones containing Ladinian (Kaihikuan local stage) marine fossils. Two new formations are described, namely Bench Sandstone and Tank Gully Coal Measures, and a new fault, the Rocky Peak Fault, is located. The whole Triassic sequence was deposited on a narrow coastal outwash plain and shallow marine shelf adjacent to alpine fold mountains in a cool temperate to subantarctic climate and latitude. This interpretation of the Triassic palaeoenvironment supports the view that the Torlesse rocks of New Zealand were derived from land to the east. © Journal of the Royal Society of New Zealand 1979.