Permian‐Jurassic strata at Productus Creek, Southland, New Zealand: Implications for terrane dynamics of the eastern Gondwanaland margin

Abstract

An area in the Wairaki Hills lying to the east of the Takitimu Mountains and extending from the vicinity of the Wairaki River south to near Ohai is described. The area straddles the faulted contact between the Brook Street Terrane and the Murihiku Terrane. Sparsely fossiliferous volcaniclastic strata of the 15 km thick Takitimu Group (Early Permian) comprise the oldest rocks of the region. Youngest Takitimu rocks, the newly recognised Caravan Formation, contain ankaramitic dikes and pyroxene‐rich marine volcaniclastics. These are conformably overlain by richly fossiliferous sandstone and impure limestone of Mangarewa Formation (120 m) and atomodesmatinid prism limestones of Glendale Limestone (400 m), which together constitute the newly restricted Productus Creek Group of latest Early to early Late Permian age. Takitimu and Productus Creek Group rocks are intruded by Triassic to Early Jurassic igneous rocks ranging from andesite to diorite and olivine monzonite. Barretts Formation, a newly recognised Middle Jurassic conglomerate‐sandstone unit (200 m), unconformably overlies the Takitimu Group, Productus Creek Group, and the intrusive bodies. All of these rocks are included in the Brook Street Terrane. Triassic strata of the Murihiku Terrane comprising a fore‐arc or back‐arc basin assemblage have been thrust westward (Letham Ridge Thrust) over Barretts Formation and underlying rocks of the Brook Street Terrane. Slices of melanged olistostrome (Hawtel‐Coral Bluff Melange) are developed along this thrust. Rocks incorporated within the melange include the entire Wairaki Breccia, Trig Z, and Nemo Formations and parts of units originally defined as Hawtel, Letham, Mangarewa, and Wether Hill Formations. All these rocks have previously been mapped as components of the Productus Creek Group but are here excluded from that unit. Petrographically, Barretts Formation resembles Murihiku sediments of Jurassic age exposed in southeastern Otago. In contrast, Early and Middle Triassic Murihiku sandstones show distinctly different framework compositions. On the basis of sandstone petrography, paleomagnetic data, regional structure, and the unconformable contact of the Jurassic Barretts Formation on Middle Permian Caravan Formation, we conclude that some degree of spatial proximity between the Brook Street Terrane and the Murihiku arc was achieved during the Jurassic. There is no evidence for close proximity before that time. Accretion of the New Zealand suite of oceanic terranes (i.e., Eastern Province) onto the Gondwanaland margin was probably accomplished during latest Jurassic to middle Cretaceous time.