Macrofossils and paleoenvironment of the Tapui formation

Zehnder, Allyce.

Eight samples from a relatively small area (c. 7 km2 ) were analysed from a single formation (Tapui Formation) to identify faunal and paleoecological changes within an inner- to mid-shelf setting during the New Zealand Bortonian stage (37-43 my ±2 (Cooper, 2004); Bartonian, Middle Eocene, equivalent). 48 taxa were identified from 786 individuals. The Bortonian Stage was proposed by James Park (1918) based on molluscs from what Gage later termed the Tapui Formation, North Otago. The Tapui Formation comprises marine sandstone, silty in places and gravelly in others, spanning shoreface to inner and perhaps middle shelf settings; as inferred from patchy exposures. Past studies clearly identify the Bortonian as a time of global warmth (primarily through invertebrate paleontology and &180). Here we report on laboratory studies of new assemblages of Tapui macrofossils. Taxonomic structure analysis and substrate niche analysis were used to assess depth as an alternative method separate from taxonomic uniformitarinism. The Simpson Index indicates diversity in each sample. Cluster Analysis and the Simpson Coefficient of Similarity elucidate possible links between faunal similarity, paleoenvironment and geographical location. Raman spectroscopy, LA-ICP-MS and stable isotope analysis (& 180, &13C) were used to assess diagenetic alteration of molluscs, and the possible presence of primary temperature signals. Taxonomic uniformitarianism and taxonomic structure analysis both indicate deposition at shelf depths. Depth correlations using substrate niche was proven to be difficult, possibly due to preservational bias. There appear to be broad correlations between assemblage composition and sediment texture (sandy vs silty, bedded vs massive - active vs quiet), but sampling is too patchy to identify firm trends. The Simpson Diversity Index varies between samples (e.g. 0.07-0.8), but results from the Simpson Coefficient of Similarity are ambiguous. Cluster analyses show links to geographical location, current energy (thus depth of deposition) and diversity for both new samples and published faunal lists. Geochemical analyses indicate that some samples are diagenetically altered so that no primary temperature signal is retained. One sample is sufficiently pristine to produce a &180 paleotemperature of20.6°C.

78, [10] leaves, 1 leaf of plates (folded) : ill. (some col.), maps ; 30 cm.

2009Zehnder