Catastrophic rock avalanches ina glaciated valley of the High Atlas, Morocco: 10Be exposure ages reveal a 4.5kaseismicevent
Philip D. Hughes1, David Fink2,William J. Fletcher1, George Hannah1,3
1School of Environment, Education and Development, The University of Manchester, Manchester M13 9PL, UK.
2Australian Nuclear Science and Technology Organisation, PMB1, Menai, NSW 2234, Australia.
3Cambridge Quaternary, Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ.
ABSTRACT
Surface modification processes leading to large debris accumulations in high relief mountain areas are important for understanding landscape evolution, especially in some of the Earth’s most active orogens. The Arroumd rock avalanche at the foot of the NW face of Mt Aksoual (3912 ma.s.l.) in the Jebel Toubkal area of the High Atlas, Morocco, represents one of the largest mass movement landforms in North Africa.The age and origin of this extensive feature has been contested for over a century. Late Pleistocene moraines are also present in the same valley, adjacent to the avalanche debris. The mean of six10Be cosmogenic exposure ages show thata series of catastrophic rock slope failures occurredat 4.5 ± 0.5 ka, whilst a set of 8 exposure ages from two of the three mapped moraineshave a far larger spread from 1.5 to 7.5 ka. This suggests that the avalanche eventswere effective agents in modifying the true surface exposure age of the Pleistocene moraines in the Arroumd valley.This has resulted in similar mean 10Be apparent exposure ages forthe pre-existing Late Pleistocene morainesurfacesand Holocene catastrophic rock slope failures. Similar rock avalanche deposits are present in other glaciated valleys in the High Atlas. We conclude that the trigger for collapse wasseismic activity related toproximity of the major Tizin’Test fault.The findings have important implications for interpreting and dating glacial landforms in tectonically-active settings.
Introduction
Tectonically-active orogens produce high-relief topography that is subject to intense erosion by glacial, fluvial and slope processes. The interplay of each of these processes is important in shaping and controlling landscape development. The importance of glacial processes in limiting mountain growth (Egholm et al., 2009) and control of fluvial incision are intertwinedwith respect to mountain erosion (e.g. Tomkin and Braun, 2002). The role of slope processes in response to topographic pre-conditioning by glaciation is also well established (Ballantyne,2002). Cosmogenic exposure dating can now help test ideas of landscape evolution and establish the sequence of events in areas that exhibit evidence of significant glacial, fluvial and slope processes.This paper examines the relationship between all of these processesin the High Atlas, an active orogen(Babault et al., 2012) comprising the highest mountains in North Africa.
The highest massif in the Atlas Mountains, Jebel Toubkal (4167 m a.s.l.), has received the attention of numerous geomorphologists over the past 135 years. In particular, one site has received special attention: Arroumd (alternative spellings: Aremd; Aroumd; Armed) to the north of Jbel Toubkal. In 1878, Hooker and Ball (accompanied by Maw) interpreted the extensive accumulations of rock debris at Arroumd as glacial moraine features, formed by a large former valley glacier (Hooker and Ball, 1878, p. 199). De Martonne, (1924) sketched moraine ridges in the Arroumd valley and suggested that the large debris field, amassed~50 metres lower in elevation than these moraines and adjacent to the village of Arroumd, resembled a rock glacier like those in Colorado and the Alps (p. 300). This interpretation of the Arroumd landform as a rock glacier was reiterated by later workers (Dresch, 1941; Mensching, 1953; Chardon and Riser, 1981). However, questions remain as to the origin, age and processes which led to the assemblage of the various landforms in this valley. Recently, Hughes et al. (2011a) suggested that the boulder debris resembled a rock avalanche deposit that could have been depositedduring or shortly after glacier retreat – although this was based on only preliminary geomorphological observations.
These different landformsof the Assifn’Imserdane valley at Arroumd are revisted in this paper. New detailed field mappingand stratigraphical observationsare employed to re-evaluate the evolution of the rock deposits in this valley. The geomorphological and stratigraphical data provide the basis for interpretations of the processesresponsible for these deposits and establishing the sequence of events. Cosmogenic 10Be radionuclide (CRN) exposure dating is then applied toestablisha robust geochronological frameworkfor the sequence of events. This new data will provide a major step forward in understating landscape evolution in theHigh Atlas and test ideas relating to a series of enigmatic landforms that have puzzled geologists for more than 135 years.
Study Area
Arroumd is situated in the High Atlas, ~ 5 km north east of Jebel Toubkal (4167 m a.s.l.), the highest peak in North Africa (Fig. 1). Toubkal and neighbouring mountains are formed in Precambrian extrusive lavas, including basalts, andesite and rhyolite (Dresch, 1941; Pouclet et al., 2007; Delcaillau et al., 2010).Arroumd village is situated at the entrance to the Assifn’Imserdane valley, which is northwest-facing and bounded to the south-east by one of the largestrockwalls in the Atlas, >1500 m in height, culminating at the western ridge (Azroun’Tamadoute) ofAksoual (3912 m a.s.l.). This rockwall is fault-controlled and is closely associated with the nearby Tizin’Test fault(Delcaillau et al. 2010, 2011) (Fig. 1). As with other areas of the Atlas Mountains, the cirquesand valleys of the Jebel Toubkal contain clear evidence of glaciation (see reviews in Messerli and Winiger 1992; Hughes et al. 2004, 2011a; Mark and Osmaston, 2008).
Methods
The geomorphology of the Assifn’Imserdane valley was mapped in the fieldonto topographic base maps over the period 2007-2013 as part of a wider programme investigating the glacial history of the High Atlas (see Fig. 2).
Landform units were subdivided on the basis of morphological and lithological criteria (morpholithostratigraphy, see review by Hughes, 2010).Landform profiles were determined using an abney level, ranging poles and 30 m measuring tape. Surface clast properties were measured on each geomorphological unit, including clast shape (a, b, c axes; and C40 index, after Benn and Ballantyne, 1994), roundness (very angular – VA; angular – A; subangular – SA; subrounded – SR; rounded – R; well rounded – WR), lithology and texture (presence or absence of striae). Clast properties were noted and measured on a sample of between 50 and 100 surface clasts. The size of the largest clasts were characterised by taking the mean of the largest 10 clasts where sample size was 50 or the mean of the largest 20 clasts where the sample size was 100.The clast density of boulder units was estimated at both surfaces and section exposures using a visual clast density chart (e.g. Tucker, 2003, his Figure 3.3).The relative degree of weathering of landform surfaces was also determined using the Harden profile development index to quantify the degree of surface soil development (cf. Harden, 1982; Birkeland 1999).
Site Geomorphology
Geomorphological description
The Assifn’Imserdane valley contains extensive accumulations of rock debrisand thick sand and gravel sediment sequences. Two distinct morphological assemblages can be identified: a series of boulder-supportedsediment ridges and mounds (Units-1, 2 & 3 in Fig. 2 & 3) and two regions covered with denser masses of larger boulders(Units-A and B in Fig. 2 & 3).In addition to these boulder landforms, accumulations of bedded sands and gravels are also present in the valley and these are exposed in section by the modern river channel (Fig. 3).
On the north side of the valley, between 2100 and 2200 m a.s.l., a linear boulder ridge, Unit-1, extends for about 1 km along the northern valley flank and is 3-4 metres in height at its apex. On the southern valley side about 0.5 km south of Arroumd village, a boulder ridge at similar elevation, and with similar geomorphological and lithological characteristics, is also evident.
The cross-section profile of the boulder ridge on the northern valley side is asymmetrical with a steeper south-facing slope (towards the valley centre) compared with the north-facing slope (towards the valley side-wall). Section exposures of Unit-1 reveal that it is composed of boulders set within a silty matrix and the clast density is c. 40-50%. The mean size of the 10 largest boulders is 4.9 m.The boulders are largely subangular and subrounded (VA: 2%; A: 16%; SA: 30%; SR: 44%; WR: 8%) and block-shaped (C40 = 20%) with 38% of clasts displaying striae. However, on the exposed surface of the ridge, clast shape is laterally variable and there is increased clast roundness with increased distance down-valley. Boulder lithologies are variable with many different grades of basalt, andesite and rhyolite represented.Near its more down-valley position, thisboulder ridge of Unit-1bounds a small sediment infilled basin. A series of boreholes has revealed that the basin is filled with about 2 m of fine silts.
Two sets of boulder mounds and ridges higher up-valley are assigned to Unit-2 and Unit-3 and these have similar sedimentological characteristics and contain striated boulders (Fig. 4). Based on morphostratigraphy,Unit-1 is the oldest deposit and Unit-3 the youngest. The soils on these landform units are thin (285 to 325 mm depths to un-weathered parent material) and yielded increasing profile development index (PDI) values downvalley (Unit-3: 3.46; Unit-2: 4.57; Unit-1: 5.57). Clay content also increases down-valley (Unit-3: 6.7%; Unit-2: 8.5%; Unit-1: 16.5%).
Units A and B are piles of larger and more densely-packed boulders situated to the south of Units 1 and 2 described above. Unit-A, is the largest accumulation of boulders and extends c. 2 km from the base of the northeastern cliffs of Azroun’Tamadout to the Kasbah near Imlil, between elevations of c. 2350 to 1700 m. Higher in the Assifn’Imserdane valley, a smaller cone of boulder debris is also present between 2200 and 2400 m a.s.l. (Unit-B).
Unit-A is characterised by denseaccumulations of angularand sub angular(A: 53%; SA: 47%) and predominantly block-shaped boulders (C40: 40%, indicating that only this percentage are not approximately cubic; cf. Benn and Ballantyne, 1993). Unit-A commences from the steep cliffs on the southern side of the valley, extends over a large central sector of the modern valleyfloor and continuesdownvalley past Arroumd village towards Imlil (Fig. 3).The surface clast density ranges from 60-80%. The boulders are very large with the 20 largest clastshaving a mean a-axis of 12.6 mfar larger than the mean dimensions of Units-1,2 and 3 .These densely-packedand very largeboulder accumulations extend for 2 km with a width of 1.25 km between altitudes of 2400 and 1700 m covering an area of 1.32 km2. Based on sediment exposures of over 40m in depth cut by the modern river channel through the debris, the volume of the boulder accumulation is estimated to be potentially >60million m3.In these exposed sections the boulders are predominantly clast-supportedwith a clast density of60-80%, similar to the distribution of surface boulders. In several places the surface of the boulder debris is covered in a layer (10-20 cm) offine silts.However, apart from this there is no clear soil development. The boulder accumulations are cut by two deep channels with a northerly channel hosting the current stream, whilst the southern channel at the base of the steep southern cliffs has been abandoned (Fig. 3). We found no striated surface boulders in Unit-A. However, in section exposures revealed by the current stream channel, striated clasts are present at depths of >10 m below the surface. This suggests that Unit-A overlies Unit-1 which does contain striated clasts – see below.
Unit-B is characterised by a very denseaccumulation of angular and sub angular (A: 62%; SA: 38%) and predominantly block-shaped (C40: 50%) boulders. It is located higher in elevationand has characteristics similar to Unit-A. The boulders are very large with the 10 largest boulders having a mean a-axis size of 10.8 m. The boulder pile forms a cone with strong grading in boulder sizes with the largest found at the base. Above the apex of this boulder cone a rock scar is visible in the cliff face (Fig. 5). Soil is absent on this feature, although a silt layer is present as appears in Unit-A.
The densely-packed boulder Units-A and -B cross-cut the less densely-packed ridges and mounds of Units-1 and -2, respectively. On the south side of the valley, boulders of Unit-A have overrun and cut through the boulder ridge which was located on this side of the valley. A remnant of this ridge isstill preserved at its western tip(see Fig. 2). However, Unit-A appears not to have extended up to, nor cross cut, the northern Unit-1 boulder ridges (Fig. 3).Unit-B has also cross-cut Unit-2, which lies up-valley of Unit-1. The uppermost deposits, Unit-3, are not cross-cut by any other unit but lie up-valley of all the other units described above.
Just above the northern bank of the active modern river channel and slightly below the lateral boulder ridge of Unit-1 on the north side of the valley,a terrace surface (P1 in Fig 6B) with a low slope angle of 5° is preserved (see also Fig. 3B). Channel cuttings reveal >30 m of bedded sands and gravels resting ontop of boulders of both Unit-A and Unit-1. Above the southern bank of the prominent active channel, boulder accumulations are separated by a gently sloping plateau surface between 2050 and 2100 m a.s.l. (P2 in Fig. 3B and 6B). This surface is composed of gravels and very few boulders and is at a similar elevation and has an identical slope angle (5°) to the sand and gravel terrace surface of P1 to the north. Below the central plateau of P2, the dense boulders reappear and form a lobe upon which the village of Arroumd is built (Fig. 6B).
Geomorphological Interpretation
The geomorphology of the Assifn’Imserdane valley is mapped and interpreted in Fig. 2 & 3 and shown in annotated photographs in Fig. 6A & 6B.
The boulder ridge on the north side of the valley(Unit-1) along with ridges to the north (Unit-2 and -3) are interpreted as moraines in accordance with De Martonne (1924) and Hughes et al. (2011a). Importantly, the Unit-1 linear ridge trends gradually up-slope along the valley side from an altitude of 1900 m and ends at 2300 m (see Figs. 2 and 3) and is set apart from Unit-A rock avalanche depositswhose edge is situated 300-400 m to the south.This is consistent with interpretation as a lateral moraine that formed below an equilibrium line altitude at 2300 m (cf. Benn and Lehmkuhl, 1998). The asymmetry of the moraine profile in cross-section, with a steeper southern slope, is also consistent with formation of a lateral moraine ridge with a steeper proximal ice-contact slope and gentler distal slope (Glasser and Hambrey, 2002). The presence of striated boulders (Fig. 7) is anotherkey criterion for the interpretation of this feature as a moraine. The boulders of the moraines are much smaller than boulders of the rock avalanche deposits and can be explained byclastcomminution at the sole of a glacier. In addition, the fact that surface boulders on the moraine ridge also become rounder and less-angular down-valley is consistent with greater clast abrasion with distance travelled in the down-valley (Benn and Ballantyne, 1994). The silt in-filled basin on the northern (distal) side of the linear moraine ridge is interpreted as a former lake basin bounded by a lateral moraine. Units -2 and -3further up valley have an arcuate form and this again is consistent with interpretation as moraines, in these cases as end moraines.
The soil PDI data supports the morphostratigraphy in that the lowest glacial Unit-1 is more weathered than the upper glacial Units-2 and -3. The magnitudes and differences in the PDIs are small compared with other areas (cf. Hughes et al. 2010), although this is largely caused by the fact that the soils were thin (285-325 mm, total depth from soil surface to parent material). The difference in soil weathering indicated by the increasing PDIs is illustrated by the progressive increased clay content, a product of enhanced weathering, between Unit-3, Unit-2 and Unit-1. In neighbouring valleys, there are also three moraine units and preliminary cosmogenic exposure ages indicate that these surfaces are separated by thousands of years corresponding with successively older glaciations at c. 10-12, 15-24 and 30-88ka, respectively(Hughes et al., 2011; Fink et al., 2012).
The dense boulder accumulations in the Assifn’Imserdane valley (Units A and B) are interpreted as representing at least two generations of rock avalanche deposits following rock slope failure of the NW face of Aksoual (and subsidiary peak Azroun’Tamadot). The very large boulder sizes, significant quantities of sub angular boulders, and the presence of fine silts on the surface of the boulder debris is consistent with the settling out of dust clouds following major rock avalanches (Hewitt 2009). In the case of Unit-B, the cone morphology below a clear rock scar also supports this interpretation (Fig. 5). Striated clasts are present at depth (> 10 m) below the rock avalanche deposits (unit-A) described above and it appears that the rock avalanche has overran pre-existing moraines in the centre of the valley.
The level area of bedded sands and gravels, P1, to the north of the rock avalanche deposit of Unit-A and which separates the modern channel from Unit-1 moraine, is interpreted as a fluvial terrace. Similarly, thelevel sand and gravel surfaces, P2, superimposed onto the rock avalanche deposit Unit-A are also interpreted as abandoned alluvial surfaces.The deep channel that incises the northern part of Unit-A hosts the modern river course. However, the southern channel (Fig. 6A & 6B) is no longer occupied by the main riverand represents a former water course.