Otapirian and Aratauran sequences (latest Triassic and earliest Jurassic) along the northern Marokopa coast (SW Auckland, New Zealand) and observations on the Triassic/Jurassic boundary in New Zealand

Abstract

Data on the changes in lithology and fauna across the Otapirian/Aratauran (Triassic/Jurassic) boundary are provided by 4 representative sections along the northern Marokopa coast. The latest Otapirian is characterised by a thick bed (40+ m) of indurated sandstone, extensively bioturbated. Aratauran sediments consist of mudstone, siltstone and sandstone, with some intercalated tuffaceous beds. In the absence of ammonites, the transition from the bivalve Otapiria dissimilis (Otapirian) to O. marshalli (Aratauran) is used as faunal control points across the boundary. The first appearance of Otapiria marshalli also serves as a proxy for the Triassic/Jurassic boundary. At Marokopa, ammonite assemblages first appear some 15–20 m above the boundary and the occurrence of Metophioceras is indicative of the base of the Conybeari Subzone (earliest Sinemurian). The hiatus in the ammonite record in New Zealand is a reflection of the worldwide biotic crisis across the Triassic/Jurassic boundary.