Basement Rock facies—Northern North Island

Abstract

Geographic names are proposed for three geosynclinal facies of the northern North Island in preference to those in current use. Rocks of the Oparau Facies (synonyms—Hokonui; Shelf; Western; and Marginal, in part) are fossiliferous and tuffaceous, are simple structurally, and show increasing low-grade metamorphism with age. They form part of the zeolite mineral facies. Rocks of the Morrinsville Facies (synonyms—Alpine, in part; Marginal, in part; and Eastern) are thick monotonous sparsely fossiliferous greywackes, chipwackes, and siltwackes that include neither tuff beds nor lavas. They lie in the quartz-prehnite zone of the prehnite-pumpellyite metagreywacke facies. Rocks of the Hunua Facies (synonyms—Alpine, in part; Axial, in part; and Marginal, in part) are very thick, commonly sheared, siltwackes and greywackes, with intercalated spilitic lavas, cherts, and limestone. They lie in the quartz-prehnite zone of the prehnite-pumpellyite metagreywacke facies, probably somewhat higher grade than rocks of the Morrinsville Facies. The depositional environments of the facies, determined by inference only, may well have been: Oparau Facies, on the continental shelf; Morrinsville Facies, at the foot of the continental slope; and Hunua Facies, in the deeper area beyond the continental slope. These facies terms could be used, where applicable, throughout New Zealand. The essential features are those that can be recognised in the field, and thus “Hunua Facies” may be applied in (e.g.) the Wellington and many other regions in place of “Alpine”, while “Oparau” is generally applicable in place of “Hokonui”.