Stratigraphy and paleontology of the oligocene Te Kuiti group, Waitomo County, South Auckland, New Zealand

Abstract

Abstract The Te Kuiti Group includes all those sedimentary lithologies in the South Auckland area lying above the regional unconformity developed on rocks of the Murihiku Supergroup and Manaia Hill Group, loosely named the Mesozoic basement rocks, and below the Mahoenui Group or its more northerly counterpart, the Waitemata Group. The sediments record a major marine transgression in the Oligocene and consist mainly of calcareous mudstones, calcareous sandstones, and sandy and pure skeletal limestones with subordinate, but locally abundant, coal measures and conglomerates. Mudstones and muddy sandstones are predominantly massive and bioturbated. Mudpoor sandstones and limestones are characteristically thinly bedded (3–10 em thick) in subhorizontal wavy- and lenticularly stratified units and, less commonly, in tabular cross-stratified sets. The typical flaggy appearance of these latter lithologies in outcrop reflects this thinly bedded stratification. The diverse and confusing lithostratigraphic nomenclature applied to the group in the past is largely unnecessary and strata are readily described in terms of the formations originally proposed by Kear & Schofield. In Waitomo County the group is divided formally into two subgroups (Lower and Upper Te Kuiti), six formations (Waikato Coal Measures, Whaingaroa Siltstone, Aotea Sandstone, Orahiri Limestone, Waitomo Sandstone and Otorohanga Limestone), and seven limestone members (Awamarino, Waitetuna, Mangaotaki, Te Anga, Pakeho, Waitanguru and Piopio); six of the members are new. Rapid lateral and vertical lithologic variation in formations and members is accommodated by establishing 30 infarmally named beds in the group. Formations and members are commonly bounded by unconformities, mainly disconformities, which are typically sharp, slightly irregular, and often bored, pebbly, and glauconitic. The contact with mudstones of the overlying Mahoenui Group is generally conformable in Waitomo County. The diverse fauna of the group consists mainly of bryozoans, echinoids, and benthonic foraminifers, with less abundant molluscs, planktonic foraminifers, coralline algae, and brachipods, and occasional solitary corals, serpulids, arthropods, and vertebrates. However, because most of the skeletal material in the group is highly fragmented, the comprehensive macropalecntologic lists presented for each formation do not reflect this faunal diversity, and are consistently dominated by robust bivalves. Zone fossils indicate the Waikato Coal Measures and Whaingaroa Siltstone are mainly of Lower Whaingaroan age, the Aotea Sandstone ranges in age from Middle Whaingaroan to Duntroonian, the Orahiri Limestone and Waitomo Sandstone are Duntroonian and locally Waitakian in age, and the Otorohanga Limestone is of Waitakian age. Regionally, parts of different formations are laterally equivalent.