Ichnocoenoses of the Mount Messenger Formation, a Miocene submarine fan system, Taranaki Basin, New Zealand

Abstract

The late Miocene Mount Messenger Formation along the North Taranaki coast comprises a fining‐upwards succession of interbedded sandstones and mudstones previously interpreted as deposits of a series of submarine fans in bathyal to upper bathyal waters. The sequence was divided into three lithofacies associations: thick‐bedded basin floor fan (BFFTK); thin‐bedded basin floor fan (BFFTN); and proximal to distal channel levee complex (CLC). This study recognises a fourth unit, the thick‐bedded Ophiomorpha sandstone lithofacies association (TBOS), as another depositional environment. Ichnology is an important tool in distinguishing the lithofacies assemblages and interpreting original depositional conditions. Each of the lithofacies associations shows a distinctive relationship between lithologic parameters and ichnofauna and ichnofabric. Four ichnocoenoses are recognised, corresponding well with the four lithofacies assemblages. Ichnocoenose I, dominated by fodinichnia, coincides with BFFTK and falls into the Cruziana ichnofacies. Ichnocoenose II, dominated by fodinichnia, coincides with BFFTN and falls into the Zoophycos ichnofacies. Ichnocoenose HI, dominated by fugichnia, coincides with CLC and falls into the Cruziana ichnofacies. Ichnocoenose IV, dominated by domichnia, coincides with TBOS and falls into the Skolithos ichnofacies. Although ichnocoenoses I and III are classed as Cruziana ichnofacies, they are discrete in trace fossil diversity, density, and ethology. Ichnocoenose IV has a unique, monospecific Ophiomorpha assemblage; together with the well‐sorted sand texture, absence of mud, and thick massive bedding, the TBOS lithofacies appears to reflect a different environmental setting to the Mount Messenger Formation with which it is associated.