OSTRACOD BIOSTRATIGRAPHY AND PALEOECOLOGY OF THE KOKOAMU GREENSAND AND OTEKAIKE LIMESTONE (LATE OLIGOCENE TO EARLY MIOCENE), NORTH OTAGO AND SOUTH CANTERBURY, NEW-ZEALAND

An improved Mid Tertiary ostracod stratigraphy has been obtained from detailed sampling of four outcrop sections of Kokoamu Greensand and overlying Otekaike Limestone in the South Island, New Zealand. The sections include the stratotypes and reference sections for the New Zealand Duntroonian and Waitakian Stages (Late Oligocene to Early Miocene). Three key evolutionary events provide the main basis for correlation of the sections, and allow recognition of four zones. These evens are, from oldest to youngest: the first appearance of Cytheralison amiesi, at about the Whaingaroan/Duntroonian Stage boundary; the last appearance of Agrenocythere? semivera, together with a major faunal replacement, in the lower Duntroonian; and the first appearance of Trachyleberis jilletti sp. nov. at the Duntroonian/Waitakian Stage boundary. The lower boundary of the lowermost zone and upper boundary of the uppermost zone are not defined since they occur beyond the limits of the sequence. This correlation is consistent with that provided by foraminifera and enhances stratigraphical resolution particularly through the Duntroonian Stage. Changes in relative abundance and species composition throughout the sequence are similar in the sections studied and are considered to be controlled by facies and water depth. An outer shelf fauna in the lower part of the Kokoamu Greensand is replaced in the upper Kokoamu Greensand by a relatively shallow mid shelf fauna which persists into the Otekaike Limestone. In the upper Waitakian part of the Otekaike Limestone an inner shelf environment is indicated. Five new species are proposed: Cytheralison spinosa sp. nov., Cytheralison parafava sp. nov., Rugocythereis reticulata sp. nov., Trachyleberis jilletti sp. nov. and Vitjasiella dulplicispina sp. nov.