The geology of the Upper Takaka - Riwaka district, North-West Nelson

Abstract

A map is presented showing the geology of the Upper Takaka - Riwaka district. Lower Paleozoic, mid-Tertiary, and Pleistocene rocks are present. Lower Paleozoic strata comprise four distinct units. The oldest, Upper Ordovician, consists mainly of quartz-biotite-garnet schist, (Pikikiruna Schist). The second unit, Uppermost Ordovician, consists entirely of black limestone and marble and contains diagnostic corals (Takaka Marble). The third unit, Uppermost Ordovician? to Silurian contains two isolated formations. Low-rank, brachiopod-bearing quartzites (Hailes Knob Quartzites) overlie Takaka Marble at Hailes Knob; high-rank gneisses with brachiopod-bearing quartzites (Riwaka Gneiss) overlie marble in the Riwaka Valley. Greywackes lying in the west of the district are tentatively included in this unit. The fourth unit, Silurian to ?Devonian, consists of massive amphibolites (Riwaka Amphibolite) which overlies Riwaka Gneiss in the Riwaka Valley. The Paleozoic strata are strongly folded along north-south axes, and increase rapidly in metamorphic grade towards the east, where they pass into rocks of the Separation Point Granite formation. A major clockwise transcurrent fault in the south of the area, striking 060°T offsets Paleozoic folds by 3 miles. Mid-Tertiary sediments representing a marine transgression, unconformably overlie Paleozoic rocks in a late Tertiary syncline marked by the Takaka Valley. Waitakian to Hutchinsonian limestone (Takaka Limestone) is conformably overlain by Hutchinsonian to Altonian mudstone (Tarakohe Mudstone) which in turn is unconformably overlain by Lillburnian to Tongaporutuan sandstone and conglomerate (Hope Creek Sandstone). Pleistocene deposits underlie at least three distinct levels of terraces in the Takaka Valley, and are mapped separately. Descriptions and illustrations of the Uppermost Ordovician corals from the Takaka Marble, together with a list of Lower Paleozoic fossil localities of the Takaka Valley and Pikikiruna Range are given in Appendix 1. The Upper Tertiary syncline (Takaka Syncline) and the adjacent, prominent, topographic scarp (Pikikiruna Scarp), are discussed in detail in Appendix 2.