Late quaternary glacio-geological and vegetation history of the Cobb Valley, North-west Nelson, New Zealand

Abstract

This study presents a Quaternary glacial and vegetation history of part of the Cobb Valley, North-west Nelson, focussing on Late Pleistocene and Holocene records. The glaciogeological evidence in the field area revealed a complex history of multiple glaciations. Though dating control is poor, three periods of glaciation were identified. The youngest of these is probably of Late Otiran age (Kumara-22, 22,300-18,000 yr BP), which resulted in suites of small terminal moraines on the valley floor upvalley from Trilobite Hut. An earlier larger glacial event is recorded by large roche moutonnée fields and may date from Early Otiran (Kumara-21, Oxygen Isotope Stage 4) to Waimaungan (Oxygen Isotope Stage 8) times. From morphological evidence a third, older glacial event was inferred. This glaciation was of a significantly larger extent, and occurred at a higher base level in an altered topographic setting. This event may be a correlative of the so-called 'Porika' glaciation in the upper Buller area. The palynological analyses revealed a record from soon after the Last Glacial Maximum (18,000 yr BP) to modern times. At 17,000 yr BP a herbfeld-Tussock grassland existed in the Cobb Valley. Between c. 14,000-13,000 yr BP this vegetation was abruptly replaced by montane podocarp-forest/ shrubland, marking climatic amelioration and deglaciation. Significant beech (Fuscaspora types) forest expansion began at c. 7,000 yr BP followed by N. menziesii invasion in the valley floor at c. 3,500 yr BP. In the youngest part of the record (c. 1875 AD) increasing grassland and decreasing N. menziesii stands, is interpreted as the consequence of anthropogenic alterations in the 19th Century. A detailed record through the Younger Dryas-event (11,000-10,000 yr BP), showed no significant vegetation or climate fluctuations during this interval, and suggests that the Younger Dryas was not marked by significant temperature changes in New Zealand.