Paleoenvironmental analysis of the Hautawa Shellbed, Whanganui Basin

Abstract

The Hautawa Shellbed, Whanganui Basin is described in detail to uncover lateral variations in depositional paleoenvironment. This was achieved through the in situ documentation of the macrofaunal assemblage and its taphonomic attributes at three localities. The sites from west to east are: Ridge Road, Old Hautawa Road, and the type section on West Road. They are all exposures on farm tracks and cover a 20-km range across the central Whanganui Basin. The descriptions were collected at 15-cm intervals and analysed using k-means clustering and Principal Component Analysis (PCA) to uncover trends within the data set. Combining the assemblage data with the taphonomic has allowed six major biofacies to be recognised. In turn, the arrangement of the biofacies in the sections suggest three subunits: A, B, and C. Subunits A and C are laterally continuous between all of the sections and always relate to the lowermost and upper-most portions of the Hautawa Shellbed. In contrast, subunit B is only observed to occur at West Road overlying subunit A. These subunits have can also be equated to sequence stratigraphic terminology. Subunits A and B form an onlap shellbed and subunit C a backlap shellbed. Hence, the Hautawa Shellbed represents deposition during the transgressive systems tract of a single cyclothem. This study is unique compared to other Whanganui Basin stratigraphic research in its statistically robust approach for comparing data gathered at various sites along outcrop strike to better understand the preserved paleoenvironment. To support the macro-faunal investigation, census counts of foraminifera were conducted for samples collected from the fine-grained sediments encompassing the Hautawa Shellbed at each of the three sites. Together, the macrofaunal and foraminiferal studies reveal temporal and spatial paleoenvironmental changes within the Hautawa Shellbed. The presence of biostratigraphically important fauna within the Hautawa Shellbed has been used to link the unit to other similar formations in both the Whanganui and East Coast Basins. This key assemblage which highlights the Nukumaruan-Mangapanian Stage boundary at 2.40 Ma includes: Zygochlamys delicatula, Crassostrea ingens, Phialopecten thomsoni, Phialopecten triphooki, and Mesopeplum convexum. The paleoenvironmental variations observed and presented here for the Hautawa Shellbed have been combined with published work on other parallel formations to produce a paleogeographic map of the Whanganui Basin for 2.40 Ma.