Palynology of predominantly Last Glaciation sediments from the Mangaroa Drillhole, Hutt Valley, New Zealand

Abstract

Spores and pollen recovered from about 40 m of sediment in a drillhole in the Mangaroa Valley, a tributary of the Hutt Valley, Wellington, indicates that the sediments are predominantly of Last Glaciation age topped by about 4 m of Holocene and modern fill. Two glacial episodes, the Last Glaciation and a much older glaciation, separated by a disconformity, are represented in the drillhole. The pollen samples are in close proximity to two tephras—the Kawakawa Tephra at a depth of 9.2 m, and the Rangitawa Tephra (locally called the Mangaroa Ash) at 38.4 m. These tephras indicate an age of c. 350 000 years for the base, and c. 22 600 years for about the top 8 m. The sediment above the Rangitawa Tephra has been zoned according to the predominant pollen types derived from woody vegetation. The zonation indicates that changes occurred from Nothofagus fusca type above the disconformity, through Phyllocladus (?alpinus) to Nothofagus menziesii, back to Phyllocladus, and finally to Dacrydium cupressinum at the base of the Holocene. Apart from the mid to basal Holocene sample, all samples are dominated by pollen from herbaceous plants, especially grasses, sedges, and occasional rushes. This sequence is similar to sequences identified in Last Glaciation sediments from previous drillholes in the Wellington area and suggests that all productive sediments below the Holocene at Mangaroa represent the Last Glaciation. Although there is the possibility that the boundaries between the pollen zones may be small disconformities, it is likely that only two major breaks occur, both within a metre or so of the top of the two tephras at the upper limits of the Nothofagus fusca and the upper Phyllocladus zones, respectively. This suggests that the sediments represent deposition from only the mid to late Holocene, the Last Glaciation, and the Nemona Glaciation, or an earlier unnamed glaciation. Pollen samples from the latter do not contain palynomorphs.