3.1 Fan morphology and valley fill

The four fans studied here have a common, coalesced aggradation surface (∼ 1380 m a.s.l.) that once must have extended all the way across the valley to the southern slope. Although the Pulanera fan is located at the mouth of Val Mila, the fan emanates from the small erosional-collapse catchment named Val Pulanera (Fig. 2a). At the toe of the Pulanera fan, incision by the Vorderrhein created a sequence of nine terraces. Riser height of the oldest terrace is 20 m. The most recent terrace is 1 m above today's active channel. Just to the west at Val Giuv numerous lateral and frontal moraines mark past glacier positions (Fig. 2b), but there is very little outwash downstream of the moraines and no fan, and the Giuv stream enters a bedrock canyon before joining the Vorderrhein. The lower part of the canyon is filled with and draped by sediment of the neighbouring Pulanera fan. The L'Ondadusa fan is built up of material from the small catchment L'Ondadusa, which lies between Val Mila and Val Strem. Both the Pulanera and L'Ondadusa fans are largely inactive with little material being delivered to the fan today. Only tiny streams, which lack incised channels, traverse the two fans. The erosion escarpment along the Vorderrhein is about 50 m at L'Ondadusa and 70 m at the toe of the Drun fan. The Strem torrent, which separates the L'Ondadusa and Drun fans (Fig. 2a), has incised about 30 m into the topographically contiguous fan surface.

The Drun fan, which lies at the mouth of Val Strem, consists of a lower main fan and an upper fan to the northeast. Surface morphology shows that the fan itself does not emanate from Val Strem, but instead the upper fan sediment is sourced in the Drun catchment (Fig. 2c). The fan is a ski area and both the lower and upper fans were cleaned of large boulders. The upper Drun fan bears scattered boulders (< 0.5 m in height) and faint levees of abandoned channels. Outcrops of the upper fan deposit along the incised Drun channel (∼ 35 m) near Valtgeva reveal several-metre-thick, matrix-supported block layers with infrequent interbedded laminated sands (Fig. 3a). The former are dominated by angular to subangular schist and gneiss blocks in a matrix of silty sand. The Drun catchment lies at the foot of the Cuolm da Vi, the site of active gravitational collapse and toppling with reported movement rates of some tens of centimetres per year (Amann et al., 2006). Several-metre-thick debris-flow deposits within the channel itself attest to recent deposition by the Drun torrent. Left lateral moraines present in Val Strem at Valtgeva are partially buried by the debris-flow sediments of the upper Drun fan (Fig. 2c). The most upvalley moraine was buried in part by the 14 March 2016 Val Strem debris avalanche (Fig. 2a). During this event the western flank of Cuolm da Vi collapsed and ∼ 200 000 m³ of rock debris mixed with snow detached and ran up tens of metres on the opposite valley slope to eventually form a ∼ 1 km debris tongue (https://www.nzz.ch/panorama/ungluecksfaelle-und-verbrechen/felssturz-in-sedrun-geroell-verschuettet-wasser–und-stromversorgung-1.18712845, last access: 17 January 2018).

Table 1Sample information, measured ¹⁰Be concentrations and calculated surface exposure ages; all errors are 1σ.

^a Analyses were normalized to the ETH AMS (accelerator mass spectrometry) standard S2007N (Christl et al., 2013). ^b Calculated with the online calculator of Balco et al. (2008) using the northeast North America calibration data set, a rock density of 2.65 g cm⁻³, and no erosion. Errors include the statistical uncertainties of the AMS measurement and the error on the subtracted blank (3.5 ± 2.5 × 10⁻¹⁵).

Download Print Version | Download XLSX

Two of the NEAT cores (Schneider, 1992) were taken close to the middle of the valley (locations shown in Fig. 2a). Examination of the published core log data allowed us a glimpse into the valley fill beneath the L'Ondadusa and Drun fans. Core SBA at 1342 m a.s.l. (2701381/1170326) lies in the Vorderrhein channel, while core SBB at 1358 m a.s.l. (2701314/1170165) lies in the Strem channel ∼ 240 m north of the Vorderrhein. Both cores traversed bedrock (Schneider, 1992), in SBA at 55 m depth (1288 m a.s.l.) and in SBB at 89 m depth (1269 m a.s.l.). This reach of the Vorderrhein Valley is overdeepened as shown by the top bedrock surface at 1269 m a.s.l. (core SBB), which is tens of metres lower than the bedrock threshold in the canyon near Bugnei at 1340 m a.s.l. The first 5–10 m of sediment on top of bedrock was interpreted by Schneider (1992) as subglacial till (overconsolidated diamicton). This is overlain by glaciofluvial gravels, rare interbedded glacigenic sediments (“tills”) and lake sediments. The glaciofluvial deposits contain a broad spectrum of lithologies attesting to both local and Vorderrhein contributions. Starting at an elevation of about 1300 m a.s.l., Vorderrhein gravels with well-rounded clasts begin to dominate both cores. Schneider (1992) interpreted this as signalling the onset of the Holocene. No ¹⁴C dates are reported for any of the NEAT cores in the valley. In core SBB, the presence of clean, well-sorted and at times cross-bedded sands between elevations of 1330 and 1340 m a.s.l. was interpreted to indicate the presence of a lake (Schneider, 1992). Starting at elevation 1350 m a.s.l. and continuing to the top of core SBB poorly sorted, locally sourced debris-flow sediments with subangular gneiss blocks ranging up to 1 m³ in diameter were logged. The elevation of the (abandoned) Drun fan surface just adjacent to core SBB is at 1385–1390 m a.s.l., allowing 35–40 m of debris-flow sediments, which comprise the fan topography visible today.

Figure 4Reconstructed paleoglaciers in the three tributary valleys. Pink lines show the AAR ELAs calculated with the ArcGIS tool GlaRe (Pellitero et al., 2015, 2016). Yellow dots indicate the locations of ¹⁰Be-dated boulders. The corresponding ¹⁰Be ages and their uncertainties are given. The coherence of the ELAs and ΔELAs with respect to the Little Ice Age ELAs (see text) in concert with the ¹⁰Be ages suggest that these glaciers advanced during the Alpine Lateglacial Egesen stadial.

Download

Topographically, the abandoned Drun fan surface continues to the east to coalesce with the Bugnei fan, where the Vorderrhein has incised more than 120 m at the fan toe. Fan outcrops along the northern canyon wall indicate the presence of a pre-existing canyon that was filled by Bugnei debris. The canyon is contiguous with the Cavorgia bedrock canyon that separates the Sedrun fan surface from the Disentis fan surface (Fig. 2a). The rather uniform 7^∘ radial angle of the Bugnei fan surface on both the west and east side underscores the idea that during fan aggradation the (former) Cavorgia canyon must have been already completely filled up.

3.2 Moraines and paleoglacier reconstruction

Results of ¹⁰Be exposure dating of moraine boulders are given in Table 1 and shown in Fig. 4. In Val Giuv, the prominent left-lateral moraine (Fig. 2b) is an accretionary moraine with the outermost left-lateral moraine (age 12 070 ± 450 years, Giuv3) having been partially buried by the larger more prominent inner left-lateral moraine (age 10 880 ± 430 years, Giuv1, Fig. 3b). The apparently too old age of Giuv2 19 610 ± 840 years (also on outermost moraine) reflects cosmogenic nuclide inheritance and is not discussed further. Sub-parallel latero-frontal moraines at the mouth of Val Mila indicate that, 13 430 ± 670 years ago (Mila2), the local glacier had just reached the main valley. The upvalley left-lateral moraine records stabilization of the glacier at a recessional position 10 120 ± 440 years ago (Mila1). From a boulder on the left flank of the Vorderrhein Valley an age of 13 600 ± 580 years (Mila3) was obtained. This gives a minimum age for the timing of an ice-free main valley. Pollen data indicate ice-free conditions around Sedrun by the beginning of the Bølling interstadial (ca. 14.6 kyr ago; Burga, 1987).

Comparison of the ELAs of the paleoglaciers provides an independent means of assessing contemporaneity of moraine formation in the three tributary valleys. Determined ELAs for the reconstructed paleoglaciers are 2300, 2230, and 2240 m a.s.l. (accumulation area ratio, AAR) for Val Giuv, Mila and Strem, respectively (Fig. 4). All three valleys hosted glaciers during the Little Ice Age. ELA depressions with respect to the Little Ice Age ELAs, which were 2700, 2570 and 2650 m a.s.l. (Maisch, 1992), are 400, 340 and 410 m, respectively. ELA depression during the Egesen stadial across the Alps varies between 250 and 350 m (Ivy-Ochs, 2015, and references therein). Both the ¹⁰Be ages and ELA depressions support the hypothesis that the studied moraines record advance of Val Giuv, Mila, and Strem glaciers during the Egesen stadial, which was between about 13 and 11.5 kyr ago (Ivy-Ochs, 2015).