The geology of an area surrounding Hyde, Central Otago

Abstract

Geological mapping conducted in an area surrounding Hyde, Central Otago, indicated that chlorite subzone schist underlay a succession of Tertiary rocks consisting of marine and freshwater sediments of the Stranraer Group, and alkali olivine basalts of the Waipiata Formation.

The Haast schists consisted of massive and layered rock; the major mappable schist structure was a planar schistosity, and minor locally developed mesoscopic folding was observed.

The Stranraer Group was subdivided into the Horse Burn and Hyde Formations, and the Hyde Formation was subdivided into the Fullarton and White Cliffs Members. Marine, glauconitic, sparsely fossiliferous, Bortonian (mid Eocene) age Horse Burn sands were deposited as a thin prism only in the north-east corner of the mapped area. During fresh-water Fullarton deposition gravels and sands poured into two localised, structurally controlled depocentres, and a thin sheet of Fullarton sediment was deposited across part of the remainder of the area. Fullarton sediments overlay Horse Burn sands in the Horse Burn type section. The White Cliffs depositional event was in part marine, at times hypersaline. White Cliffs sediments overlay Fullarton sediments in the west and south, and lay directly on schist in the centre of the examined area.

A thin flow of Waipiata basalt overlay the Stranraer Group, and formed resistent caps on flat-topped hills near Hyde, and the extrusive centre, a large and structurally differentiated lava pond intruded by a sub-parallel dike swarm and numerous plugs, was exposed near Tiroiti.

Correlation of Stranraer rocks with sections to the south and north-east indicated that in the Tertiary the Hyde area was situated near the shallow southern edge of a transgressive sea that passed inland to the north and west, and back to the present day coast, with only minor influence on the local depositional history.