Ordovician Biostratigraphy of North-West Nelson

Abstract

Eleven graptolite zones are recognized in New Zealand and are considered to represent most of the Ordovician Period. The most complete fossiliferous section, at Aorangi Mine in Northwest Nelson, is described in detail. New fossiliferous Ordovician sections at Wangapeka Valley and Hailes Knob, in Northwest Nelson, are described and the previously Known sections at Cobb Valley (Northwest Nelson) and the Cape Providence - Preservation Inlet region (Southwest Otago) are discussed The New Zealand zones are correlated with those of Victoria, Australia, and the Victorian stage nomenclature is adopted for New Zealand. In upward sequence the stages and zones are: Lancefieldian (zones 1, Adelograptus, and 2, T.approximatus), Bendigonian (zone 3, T.fruticosus), Chewtonian (zone 4, D.protobifidus), Castlemainian (zones 5, I.c.lunata, and 6, I.c.maximodivergens), Yapeenian (zone 7,Oncograptus), Darriwilian (zones 8, P.etheridgei, and 9, D.decoratus), Gisbornian (zone 10, N.gracilis), Eastonian (zone 11, Orthograptus - lower part) and Bolindian (zone 11, Orthograptus - upper part?). Fossils of lowermost Lancefieldian and uppermost Bolindian age are not yet known in New Zealand. Diagnostic Silurian fossils are also unknown and the next youngest New Zealand fossils are Lower Devonian (Gedinnian?). Previous Work in the Aorangi Mine area is outlined, the geology illustrated by a map, and the stratigraphic column shown to include both older, and younger beds than were previously known. About 8500ft thick, the column comprises, in upward sequence, Webb Formation, Aorangi Mine Formation (with four informal members), Formation A (new and informal, with three members), and Formation B (new and informal). Graptolites are confined to the three younger formations and represent zones 1 to 10, ranging in age from Lancefieldian to Gisbornian. The structure is outlined and illustrated by cross-sections. In Wangapeka Valley, argillites of the Wangapeka Formation contain zone 11 graptolites of Eastonian and lower Bolindian age, and are overlain by 1500ft of quartzite of the Ellis Formation, which has yielded no diagnostic fossils but which is inferred to represent at least part of the Silurian Period. Lower Devonian brachiopods, bivalves, corals and trilobites are known from the uppermost beds of the Ellis Formation, about 1550ft above its base. At Hailes knob, Mount Arthur Marble with fossils of uppermost Ordovician age is overlain by Hailes Knob Quartzite of probable Silurian age. From the Aorangi Mine and Wangapeka areas, one hundred and thirteen graptolite species and subspecies, representing 35 geners, are described. They include one named new species and 49 new records for New Zealand. Stratigraphic and geographic distribution elsewhere in the world is noted. The geners Isograptus Moberg and Paraglossograptus Mu are reviewed, and their New Zealand members described and discussed in detail. The important zonal species and subspecies are also described in detail; the remainder are briefly discussed. One hundred and six taxa are figured. Fossil localities in the areas described are listed with their fossils in appendix I, and details of measured sections are given in appendix II. A summary of Lower Paleozoic fossil localities of Northwest Nelson is given in a supporting paper.