Kol-Uchkol & Gurumdy area (Southern Alichur Range)

The oldest glacial deposit in the southern Alichur Range area, isolated erratic boulders on a shoulder in above 4400 m a.s.l. in the Kol-Uchkol Valley (UK1), yielded exposure ages of 66-86, and 93-136 ka (Tab. 5.1, Fig. 5.9A). If the highest age is considered to be the one closest to the deposition age of these boulders, they were left by an MIS 5 or MIS 6 advance. If, as Kuhle (1997) believes, they have been left by a plateau glacier covering the whole plateau with a height of more than 1000 m, this glaciation must have ended within the MIS 5. If, as we think is more likely, these boulders are remnants of a moraine the matrix of which has been completely washed away, the moraine most probably was left by a middle Pleistocene or even earlier advance. The same deposit in the Gurumdy Valley (GU1, Tab. 5.1, Fig. 5.9B) has yielded exposure ages only below 70 ka, which show that the deposit has been degrading all through the late Pleistocene. Phases of accelerated moraine degradation may be recognized at 40-50, around 25, and around 20 cal. ka B.P., close to dated or inferred glacial advances in the region (see below). Moraine degradation may have occurred in association with solifluction activity, which has been high in this area all over the Pleistocene (Gorbunov & Seversky, 1999). Even if we cannot decide,

whether the deposit represented by UK1 and GU1 is of middle Pleistocene or earlier age, it is certainly older than the last Pleistocene maximum as mapped by Zabirov (1955).

The maximum glaciation of Zabirov (1955) in the Kol-Uchkol-Gurumdy area is repre-sented by the deposits UK2 and GU2 (Tab. 5.1, Fig. 5.9B) with an ELA depression of 380 m. The oldest boulders of UK2 yield exposure ages of 57-75 ka, covering the MIS 4. These ages most likely represent the end of the respective glacial advance, as boulder UK28 is situated several 100 m behind the end moraine wall. They show that the typical "chukur"

moraine of this maximum advance was deposited early in the last glacial cycle and does not belong to the middle Pleistocene, as assumed by some Russian researchers (Pakhomov

& Nikonov, 1977, Dodonov, 2002). The correlative lateral moraine in the Gurumdy catch-ment GU2 yields exposure ages below 60 ka only (Tab. 5.1, Fig. 5.9B). As they parallel the ages from the older GU1 deposit, these ages are obviously all degradation ages.

Kol-Uchkol

exposure age [ka]

0 20 40 60 80 100 120 140 160

Gurumdy

exposure age [ka]

0 20 40 60 80 100 120 140 160

UK1 UK2 UK3 UK4 UK5 6 GU1 GU2 GU3 GU4

?

?

?

MIS 5-4 MIS 3? MIS 2 MIS 6? MIS 5-4? MIS 2

?

A B

Fig. 5.9. Interpretation of exposure ages from the A) Kol-Uchkol and B) Gurumdy catchments.

Minimum ages are depicted by white dots, conservative maximum ages by black dots. Samples interpreted to be affected by inheritance in boxes cross-hatched in grey, phases of glacier advance in boxes cross-hatched in black, phases of moraine degradation in white boxes. See text for expla-nation.

Tab. 5.1. Results of ¹⁰Be surface exposure dating in Central Asia.

Tab. 5.1 continued.

Tab. 5.1 continued.

1 correction factor for topographic shielding of fast neutrons, corrected for the influence of surface inclination

2 correction factor for topographic shielding of muons, corrected for the influence of surface inclination

3 maximum slope angle of the sampled surface

4 thickness of the sample

5 ratio of ages calculated using the scaling systems of Lal (1991) as used in this study, and the scaling system of Dunai (2001) as modified by Schaller et al. (2002), for details see section 2.

The recessional deposit UK3 clearly yields exposure ages that are too old in comparison with the ages of the stratigraphically lower UK2 moraine (Tab. 5.1, Fig. 5.9A). Most like-ly, UK3 contains some preexposed material carried down from above-slope by the adjacent kolluvial fan. The ditch separating both deposits may have been formed at a later time.

Processes allowing for such coverage of parts of lateral moraines have been described as common from the neighbouring Karakoram (Iturrizaga, 2003). No unambiguous deposition age can therefore be inferred from the UK3 data. Most likely, the moraine has been laid down between 40 and 60 cal. ka B.P., in possible analogy to the Yashilkul area.

The deposits UK4, GU3 and GU4 yield similar exposure ages between 13 and 28 ka, if one boulder (GU32) is excluded as obviously preexposed (Tab. 5.1, Fig. 5.9). Therefore, UK4 can be correlated to GU3 or GU4, or even both, as its horizontal dimensions exceed those

of the latter, and its ELA depression lies in between. As GU3 is stratigraphically lower than GU4, it probably dates to around 27 cal. ka B.P., while GU4 probably dates to around 22 cal. ka B.P. UK4 most likely has been deposited 27-20 cal. ka B.P.

The recessional stages represented by UK5 and UK6 have occurred in the lateglacial period, around ~15.5 cal. ka B.P., and ~14.3 cal. ka B.P. It may be noteworthy that calcu-lation applying the scaling system of Dunai (2001) results in ages of UK5 and UK6 that correspond with the beginning and end of the Younger Dryas period (13.0 ka, 11.5 ka, respectively). However, the two moraines are too different in their dimensions and too far separated from each other to be an example of a typical double Younger Dryas advance (Easterbrook, 2003), hence this implication is rather doubtful at present.

All these dating results show, that the up to nine moraine walls constituting the Alichur complex (Dodonov, 2002) have been deposited during the second half of the late Pleisto-cene, while the most prominent advance among them has been synchronous with or slight-ly earlier than the global LGM, 21-25 cal. ka B.P., but has been much more constrained than the maximum late Pleistocene glaciation, which in this region occurred early in the last glacial cycle.

5.3.6 Comparison with neighbouring regions