Jane M. Soons

The glacial sequence in part of the Rakaia Valley, Canterbury, New Zealand

Abstract From examination of the glacial deposits and landforms of the Rakaia Valley, four major ice advances have been recognised. In chronological order, these are the Woodlands, Tui Creek, Bayfield, and Acheron Advances. The Bayfield Advance was double, and the Acheron triple. All are regarded as of late Pleistocene age, the Waimaunga Glaciation being represented by the Woodlands Advance, and the Otira Glaciation by the three later advances. Differences in the height of moraines and outwash surfaces allow the first three advances to be clearly differentiated; the Woodlands Advance is further distinguished by the degree of weathering of its deposits. The Acheron Advance occurred after the initiation of the present Rakaia Gorge, and outwash surfaces comparable to those of the earlier advances are lacking in the main valley, although present elsewhere. Morainic deposits of this advance show that two distinct glaciers derived from different sources were present in the area, one occupying the main Rakaia Va...

Environmental and sea-level changes on Banks Peninsula (Canterbury, New Zealand) through three glaciation–interglaciation cycles

Abstract A greater than 200 ka record of marine transgressions and regressions is recorded from a 75 m core from Banks Peninsula, Canterbury, New Zealand. This record comprises thick suites of muddy sediments attributed to back barrier, lake and lagoonal environments alternating with thin soil and loess complexes. These deposits have been dated using radiocarbon and thermoluminescence (TL) techniques supported by proxy data (diatoms, phytoliths, pollen and sedimentology). The aqueous deposits are attributed to three interglacials and an interstadial (Marine Isotope Stages 1, 5a, 5c, 6, and 7). The loesses and paleosols date to the intervening stadials (Isotope Stages 2, 5d (or 6?) and probably 8). On the basis of transgressive beach facies, back barrier swamps and barrier-blocked lake deposits, a partial sea-level curve including data from Isotope Stage 5 is presented. Our data indicate that Banks Peninsula has been tectonically stable over that period and we provide sea-level points that support the existing isotope curve during Stages 5 and 6. Detailed diatom records are limited to Isotope Stage 1 and the latter part of Stage 5. Diatom histories recorded from these stages are remarkably consistent. Both indicate a progressive floral change from marine types through freshwater colonising species to freshwater planktonic assemblages. These reflect parallel histories of coastal evolution during the two interglacials. In both cases, marine transgression in the early part of the isotope phase was followed by lagoon development implying that a gravel spit extended across the embayment from the west. This was succeeded by lake development when the lagoon was cut off by the juncture of the spit with Banks Peninsula. This lake deepened as the coast rotated into swash alignment and the spit was converted into a gravel barrier. The vegetation history of the site indicates that mixed podocarp broadleaf forests, similar to the pre-European flora of Banks Peninsula, occupied the region during Isotope Stages 1 and 7. This contrasts with the palynological interpretation of a marine record (DSDP Site 594) from off the Canterbury coast which suggested that Isotope Stage 7 was markedly cooler than the Holocene. During glacial periods, forest was eliminated and replaced by a tall shrubland of mixed montane and coastal affinities.

Dates for Otiran deposits, including plant microfossils and macrofossils, from Rakaia Valley

Abstract Dates of 22800 ½ 800 yr B.P. (new T½), 22200 ½ 750yrB.p. (old T½) (NZ 3940) and 19750 ½ 600 yr B.P. (new T½), 19200 ½ 550yrB.p. (old T½) (NZ 4298) have been obtained for plant remains in lake deposits in the Lyndon-Acheron Valley, which links the Rakaia and Waimakariri drainage systems. The deposits occur in a channel cut into outwash gravel of Bayfield 2 age, and preceding a period of aggradation related to a third and previously unrecognised Bayfield advance. The dates thus provide a base for establishing a firmer correlation between deposits on the east and west sides of the Southern Alps than has been available previously. The plant remains indicate the absence of woody vegetation and the presence of a sparse herbaceous vegetation. However, there are indications that the climate was less cold than might have been suspected from other evidence.

Glacial advances in the Rakaia Valley, New Zealand

Abstract Re-alignment of the road near the Rakaia Gorge bridge (S82) has clearly exposed deposits of the Woodlands and younger glacial advances, and their erosional or faulted relationships. Renewed geomorphological mapping in the area indicates that the Tui Creek Advance was in three phases, not one as previously recognised, so that it may have the status of a full glaciation. New details of the deposits and geo-morphology related to the three phases of the last (Acheron) advance of the Otira Glaciation indicate greater aggradation than previously supposed, and the formation of a complex of lakes during fluctuations of the glacier.

The Holocene evolution of a well nourished gravelly barrier and lagoon complex, Kaitorete “Spit”, Canterbury, New Zealand

Abstract Morphological, stratigraphical, micropaleontological and radiocarbon dating investigations of the gravelly Kaitorete “Spit” system, Canterbury, New Zealand show that a barrier/spit system has existed at this site for the last 8000 yr. Kaitorete “Spit” developed as a true spit extending north from near the Rakaia river mouth, during sea level rise in the Late Pleistocene and Early Holocene. This produced an estuarine coastline along the southwestern flank of Banks Peninsula. The spit extended to Banks Peninsula sometime in the early mid-Holocene creating a barrier lake complex behind the spit. Since the mid-Holocene, there have been at least three fluctuations between lacustrine and estuarine/lagoonal conditions in the barrier blocked system. The most recent fluctuation, which resulted in the present barrier lake complex, occurred within the last 200–500 yr. These fluctuations are associated with the avulsion of a large braided river, the Waimakariri, to and from the Lake Ellesmere basin. When the river flows through the basin, a permanent channel is maintained to the ocean with estuarine conditions present. When the river flows elsewhere, the barrier closes and a freshwater lake system forms behind it. This study emphasises the importance of fluvial control on the evolution of gravelly coastlines where sediment supply is not the limiting factor. Both the sediments and the water in the back barrier area are predominantly derived from the terrestrial catchment. We highlight the absence of true marine influence in the back barrier area, except in the vicinity of the tidal channel, even when such a channel is present. Marine processes have been limited to the gradual re-orientation of the barrier system from drift to swash alignment.

Quaternary vegetation and climate changes on Banks Peninsula, South Island, New Zealand

Late Quaternary terrestrial and marine pollen records from the Canterbury Plains and Banks Peninsula suggest that climates during the peak of Marine Isotope Stage 7 (MIS 7) were similar to those prevailing during stage 5e and the Holocene. Podocarp forest (notably Prumnopitys taxifolia-matai) characterises each interglaciation. In contrast, marine records from DSDP 594 cores, off the cast coast of Canterbury, indicate that stage 7 was dominated by montane forest (Libocedrus sp. and Phyllocladus). This suggests temperatures as much as 3 degreesC colder than indicated by the Banks Peninsula assemblage. Age control from both sites appears to be robust. Some of the differences may be related to the taphonomy of the pollen at both sites. DSDP 594 may reflect a more southerly catchment of fluvially and aeolian-derived pollen than does the Banks Peninsula site. Banks Peninsula was alternately separated from, and joined to, the mainland as Quaternary sea levels fell and rose. Assuming modem ocean current patterns, during interglacials the south-north Southland Current would have swept through the seaway separating the island from the mainland, diverting the flow of rivers embouching on the Canterbury coast, and moving sediments and fluvially transported pollen northwards. Little of this material would have reached DSDP 594, nor, if wind patterns were similar to those of today, would wind-borne pollen from Banks Peninsula have reached the site. It is probable that vegetation on the Peninsula was consistently distinct from that recorded at DSDP 594, which has a more southerly derivation. In contrast to the high mountain areas of the South Island, the low levels of grass pollen in the available record suggest that the Peninsula retained a woody vegetation over much of its area during glacial periods. This was favoured by the physiography of the area, with a variety of micro-climates, and by the extensive areas available for colonisation at times of low sea level. The podocarp forest of MIS 7 was replaced by an open shrubby vegetation in which Leptospermum and Kunzea (Leptospermum-type pollen) was locally dominant, and in which Plagianthus, Phyllocladus, Coprosma and Myrsine were prominent. Charcoal is associated with this change. Most of the recorded taxa, with the exception of Phyllocladus, are present on the Peninsula today. A gap in the pollen record coincides with the Last Interglacial and Last Glaciation, but a return of forest vegetation is documented in the later Holocene. The reconstructions do not exclude the possibility of a cooler stage 7. They do highlight the importance of excluding local/regional non-climatic effects before interpreting climate change from data sets, and reinforce the necessity of testing marine records against compatible terrestrial ones

Changes in geomorphic environments in Canterbury during the Aranuian

Abstract Changes in landforms in Canterbury consequent on the change from glacial to non-glacial conditions during the Aranuian are examined. Land-form evidence may supplement that provided in the vegetation record and suggest that the montane environments of Canterbury were more sensitive to climate change than the lower, including coastal, environments. On the whole, however, it appears that landforms record only the broad change from glacial environments to those of the present. Alternations of erosion and deposition in any given valley may reflect a variety of causes, including seismic events, fire, and intense local storms, as well as longer term and regional climate change. It is concluded that the vegetation record will provide a more sensitive and detailed record of climatic variation than will the geomorphological record.

Evolution of the New River drainage system, Westland

Abstract The small catchment of the New (Paroa) River, on the West Coast of the South Island, displays a markedly asymmetrical drainage pattern which reflects its tectonic and Quaternary glacial history. Outwash from ice in the Taramakau valley and Lake Brunner basin during the Nemona Glaciation created a new surface on which the forerunner of the modern drainage pattern developed. This pattern was modified through successive glaciations as valleys were deepened and streams diverted in a series of captures. Continuing deformation where the streams crossed the nose of the Brunner Anticline also played a part. A chronological framework of the capture sequence is provided by the input of sediments during the Waimea and Otira Glaciations.