Glacigenic systems in the Upper Clutha valley in Central Otago, New Zealand

Abstract

This report describes the glacigenic stratigraphy from the Upper Clutha Valley in Central Otago. The deposits from seven glacial advances are described and modelled on a wet based glacier. Stratigraphic classification of these deposits is based on the concept of allostratigraphy. Deposits of separate glacial advances are distinguished from each other by surfaces of discontinuity rather than on lithic characteristics.

Deposits of each glacial advance are given formational status. The name of the formation is different from the name of the glacial advance which produced the deposit. Where adequate preservation exists and distinction can be made from moraine and outwash, formations are subdivided into a Diamict Member and Gravelly Member respectively.

Deposits from each advance are described in ascending stratigraphic order, according to the normal rules of procedure. Deposits from the oldest glacial advance by nature of their preservation are at the highest elevation.

Deposits from the Criffel Advance occur at a single locality known as the Criffel Diggings, situated on a weathered bedrock spur at an elevation of 1220 m a.s.l. on the eastern flanks of the Criffel Range. Diamict in character, this deposit is described under the Luggate Creek Formation (new). Tentative correlation is made with fluvial remnants at an elevation of 525 m in the vicinity of Lowburn.

Deposits from the Lowburn Advance are described from a northwesterly facing ice-cut bench on the northern flanks of the Pisa Range at an elevation of 762 m a.s.l. A stratigraphic succession of diamicts, gravels, sands and clays is described under the Pisa Formation (new). This succession is interpreted as an ice-marginal sequence produced during an advance/retreat cycle. Similar lithofacies are described from Bendigo to Northburn Station.

Deposits from the Lindis Advance are the most diverse and widespread in the Upper Clutha Valley and are described under the Bendigo Formation (Wood, 1962). Remnant lateral morainic debris is described from the western flanks of the Lindis Peaks at an elevation of 550 m and from the eastern flanks of the Pisa Range at an elevation of 610 m. Cluden Hill is recognized as a major push moraine and is composed of three rampart loops. The outer main rampart, designated Ro, spans the width of the lower Lindis Valley.

Proglacial sedimentary sequences are described from Bendigo, Pisa Station and as far south as the westerly facing flanks of Mount Oho. These 'ice contact' deposits are laid down in the glaciolacustrine environment, probably as the Lindis Glacier debouched into a standing body of water.

Recession of the Lindis Glacier produced an extensive outwash plain. The ‘Tarras terrace block’, which extends from Tarras to Bendigo, a distance of 8 km, is a preserved remnant of this feature. The Clutha Formation (new) comprises a suite of interglacial (post Lindis Advance) lacustrine sediments which outcrop sporadically along both banks of the Clutha River from Maori Point, northwards to Luggate. Drillcore investigations by the Ministry of Works into Upper Clutha damsites have enabled a lithostratigraphic subdivision to be proposed. The Clutha Formation is subdivided into two Members; the Sandy Point Member, Luggate (maximum thickness to date, +109 m) and the Maori Point Member, Queensberry (maximum thickness to date, 75.8 m). The Sandy Point Member is further subdivided into the Upper, Middle and Lower Sandy Point Submembers. The Maori Point Member is further subdivided into Submembers A-F. Lithostratigraphic correlation between the Luggate and Queensberry areas is uncertain however.

Selective granulometric and geochemical analysis of Clutha Formation sediments indicate irregular, but cyclic couplets of normally graded rhythmic laminations composed of a darker layer of clay and a lighter silty-clay grade layer, generally with less than 30% clay sized material. There is no fractionation of the allogenic mineral fraction within the lighter and darker layers. Vivianite nodules are described from these sediments.

The sequence at Luggate is interpreted as a deltaic progradation in which bedding thickness trends reflect an upward increase in rhythmic couplet thickness from a mean value x̅ = 2 mm in the Lower Sandy Point Submernber, up to x̅ = 30 mm in the Middle Sandy The Upper Sandy Point Submernber is composed of massive and horizontally bedded sands and draped laminations, sedimentary characteristics interpreted to result in the delta slope environment.

The sequence at Queensberry reflects a more complex depositional history than the sequence at Luggate. Massive and ripple laminated sands are overlain by massive and rhythmically laminated silts and clays. Montmorillonite is a conspicuous component of these clays. Biologic homogenization of these sediments is common. Lebensspuren traces are common, cf. Chironomid midge larvae. Subaqueous mass flow is an important depositional process at Queensberry.

The Clutha Formation rests unconformably on a massive pale grey diamict at Luggate (Borehole L77). Granulometric and S.E.M. studies indicate a supraglacial or perhaps englacial rather than a subglacial origin for this diamict.

A S.A.S. graphics plot models the basement profile in the Upper Clutha Valley. This result is based upon D.S.I.R. gravity surveys and shows sharp basement profile depth increments at the confluence of the Clutha and Lindis Valleys. This feature is referred to as the Queensberry basin.

Generally remnant morainic debris deposited by the Luggate Advance is scarce. Minor diamict lithofacies described under the Queensberry Formation (new) occur northwest of Luggate at an elevation of 425 m. Remnant bouldery lithofacies veneer an ice-carved spur west of Luggate at an elevation of 426 m.

Deposits of the Albert Town Advance are described under the Mount Barker Formation (new). Two well preserved terminal moraines of the Mount Barker Diamict Member are located mid-valley Hawea/Wanaka Flat. A hummocky diffuse loop of end moraine extends from the base of Mount Barker into the lower Cardrona Valley in the southwest to the Cardrona/Clutha River confluence in the northeast at a maximum elevation of 381 m a.s.l. A second well defined arcuate loop, composed of 6 - 10 transverse ridges lies between 7 – 9 km south of Lake Hawea, maximum elevation 403 m a.s.l. Deposits of the Mount Barker Gravelly Member are tentatively traced as far south as the Cromwell Gorge. An end morainic loop wraps around the base of Mount Iron, maximum elevation 330 m a.s.l., 2 km east of Wanaka township. This loop is deposited by the Mount Iron Advance and is described under the Cardrona Formation (new).

Arcuate end morainic loops wrap around the southern outlets of Lakes Wanaka and Hawea at maximum elevations 405 m and 379 m respectively. These define the terminal locations of separate ice loops near synchronous in each subcatchment. These loops are deposited by the Hawea Advance and are described under the Wanaka Formation (new). The terminal moraine at Hawea, the Wanaka Diamict Member is deposited as a sequence of flowed diamicts consistent with deposition under a relatively high fluid/sediment ratio. The moraine at Wanaka, Wanaka Diamict Member, is composed of a more massive diamict consistent with deposition under a relatively low fluid/sediment ratio. Upper and mid outwash fan regions are described under the Wanaka Gravelly Memb er. Proximal fluvial deposits are characterized by poorly imbricated, horizontally bedded clast supported cobbly gravel. Mid fan regions are dominated by trough and planar trough cross stratified gravels.

Chapter nine presents an empirical approach to paleohydraulics based on computer modelling of terrace gradients and contemporary gravel bed studies.