QA_Vol 24_No 1_July 2007 - Australasian Quaternary Association

volume of ice would have accumulated before outflowto lower altitudes would have occurred. Outflow towarmer, lower altitudes is the primary mechanism bywhich glaciers achieve neutral mass balance. Thus MtOwen may have accumulated an unusual thickness ofice occupying the central plateau.Williams (1992) has inferred the presence of a numberof radiating glaciers from the Sentinel Hill core. Initiallythese may have been numerous, lapping over every lowpoint in the encircling divide. A number of outlet colsconveyed ice from Reverse Basin into Blue Creek (figure3). Competition between outlets will be regulated bytheir ice discharge, velocity and erosion rate. The deep,steep flanked cut through Granity Pass to the GranityCreek appears to have eroded down. The Granity Passroute was finally captured by Blue Creek. This issomewhat surprising as the overall terrain gradient isnot steep. This indicates significant ice accumulationsin the Sanctuary Basin. Culliford Hill ice would haveoverflowed into an occupied Blue creek, forcing SentinelHill ice out into Grantity creek. Under late glacial conditions,ice no longer flowed directly into Blue Creekfrom Culliford Hill and the Granity lobe reverted downBlue Creek. Sanctuary Basin was the last point of conspicuousice on the northern part of the plateau as isindicated by the low lying ice contact deposits.There are significant morphological incompatibilitiesbetween the land surface and presumed ice flow: deeplyincised valleys such as Blue Creek and closed depressionssuch as Poverty Basin and Castle Basin (Figure 3,6 and online appendix). Ice flow is primarily controlledby surface gradient, so basal slope means that icecannot readily flow through closed depressions.However, sediment fills adjacent to these depressionsindicate that there were occupied by sedimentsubsequently swallowed by the underlying karst.This sediment fill concept is readily transferred toradial valleys where incision slots can be occupied bysediment during ice advances, and re-excavated onretreat. The Railway ridge is inferred to be a filloccupying the head of Blue Creek when ice flow wasfocused though Granity Pass. Off the plateau, postglacialerosion of steep slopes and dense forest covermake detailed mapping of outlet valleys difficult. Theradial valleys were occupied by glaciers with very largegross mass balances (massive accumulation of1–5m.m 2 . a -1 and mean annual temperature of +5°-+10°C). Such ice is unlikely to have well defined stabletermini and would be dominated by sandurs (in valleys)and alluvial fans (at outlets). Most of the glacialevidence in the engorged valleys was probably destroyedby fluvial processes (floods) during the deglaciation.The style of glaciation inferred here diverges from that inother parts of north-west Nelson. In Cobb Valley, a fullsuite of valley glacier phenomena are recorded includinga large sequence of terminal moraines and numerouslateral moraines and roche mountonee fields (Shulmeisteret al., 2001). On the Owen Massif the evidence ismuch more fragmentary, largely because karst processeshave modified or destroyed much of the evidence.Provenance analysis is quite revealing. The Wangapekaclasts found in the Railway ridge crop out near SentinelHill while the clasts of andesite should come from theonly known outcrop which is 3–4 kilometres west ofGranity Pass. The Wangapeka clasts imply that ice flowinto Granity Pass was sourced from the major peaks tothe south of the Pass and, rather more surprisingly, theandesite requires eastward flowing ice derived from thevery western most limits of the massif. This not feasibleif ice flows were minor and implies either a) that otherunmapped outcrops of the Andesite occur on the massifor b) that reversals of ice flow occurred at times ofsubstantial ice cover. We cannot discriminate betweenthese hypotheses based on our data. We conclude thatice flows involved at least two stages. During the earlystages of advances ice flowed into the basins on themassif. As ice built up in the basins, divides were overtopped,flows locally reversed and a small ice cap developedover the massif, directing flow off the massif in aroughly radial pattern.The four main rivers (Granity, Nuggety, Fyfe and Owen)that drain the massif flow through at least one deepgorge within a few kilometres after leaving the massif(Figure 2 & 3). There are inset V-shape profiles in U-shape valleys. These are represented by terraces aboveor on top of gorges. This V in U topography has beennoted elsewhere in northwest Nelson (Henderson, 1931and Shulmeister et al., 2005) and was inferred byHenderson to indicate an earlier, much larger glacialphase. In order for a V-shaped profile to develop withinthe U there are two possibilities. The first, as perHenderson, is that the U-shaped valleys have not beenextensively occupied by ice during recent glaciationswhich suggests that the scale of glaciation is diminishingin NW Nelson. Since this area was more tectonicallyactive during the early Quaternary, such an inferenceis reasonable. However, an alternative hypothesisthat the V’s are either filled with ice, or more likelyaggradation gravels during glacial advances, and thatactive ice may over-run an occupied V-shaped valleywithout eroding the V out. This model has beenproposed on the European Alps (De Graaf, 1996).ConclusionsThe weight of evidence suggests that the whole of theOwen Massif was an ice accumulation zone at timesduring previous glaciations. We infer that accumulationwas focused on pre-existing karst closed drainages andaccumulated as a central ice body until ice overflowed17 | Quaternary AUSTRALASIA 24 (2)