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Abstract

Rock avalanches have been widespread in the Karakoram Himalaya. More than 100 events have been identified in recent surveys and they include the largest catastrophic landslides known in the region. Some occurred in the present-day glacierized zone and during the past decade. Most have been identified and reconstructed from more or less ancient deposits at lower elevations. The incidences of these deposits and features that place them in the general class of rock-avalanche fragmentites are described. However, the main focus is on certain morphological and sedimentary phenomena that reflect interactions of rock avalanches with rugged terrain. In most cases, relations between runout path and geometry of opposing valley walls and interfluves have had major effects on the overall shape, surface morphology, and internal structures of the deposits. In some cases there are complex interactions with erodible substrates. Further complications arise from later erosion and burial of rockavalanche deposits, and the large changes they bring about in other geomorphic conditions. Styles of rock avalanche are proposed based upon the configuration of depositional complexes and their relations to surrounding topography, especially to local valley systems. The analysis and terminology for such topography-constrained forms are developed, in particular, from the work of Albert Heim. An understanding of these forms provides a useful guide to field identification and reconstruction of past rock avalanches in rugged mountain valleys, and necessary background for assessing future risks from catastrophic landslides.

Introduction

Karakoram and catastrophic mass movements

The Karakoram Himalaya is part of the South-Central Asian mountain system. This extensive mountain complex includes the northwest part of the Greater Himalaya, which is to the south of the Karakoram, the Hindu Kush to the west, the Pamirs to the northwest, and Kun Lun to the north. The Karakoram contains the highest peaks of these ranges and is the most heavily glacierized, with some 16 000 km2 of perennial ice in the higher, more rugged areas (Young and Hewitt, 1993). The entire region reflects the enormous orogenic forces initiated by the collision of the Asian and Indian plates ca. 50 Ma, with continuing large-scale crustal shortening, subduction, faulting, and high rates of surface uplift (Coward et al., 1986).

The Karakoram Mountains are carved mainly in a microplate that was attached to the southern rim of the Asian plate (Fig. 1). Surviving sedimentary and metasedimentary formations of the Karakoram plate range from Carboniferous to Cretaceous in age (Searle, 1991). The rocks of the Karakoram batholith and Metamorphic Complex of Searle are believed to reflect igneous and metamorphic events of an Andean-type convergence zone prior to collision with the Indian continental plate, as well as major tectonic deformation, metamorphism, and intrusive episodes after collision (Le Fort, 1988; Searle, 1991). On the southern flank of the range are systems of plutons and metasedimentary rocks attributed to an island-arc phase of collision, the Kohistan-Ladakh terrane. Punched through this and into the south-central Karakoram is a zone of active faulting and northward-directed thrusting in the Nanga Parbat-Haramosh massif (Madin et al., 1989). The highest rates of denudation and incision of rivers in bedrock have been found here, and fission-track dating suggests that uplift has been as much as 8 mm/yr since the late Pliocene (Zeitler et al., 1989; Burbank et al., 1996). However, the Quaternary has involved rapid uplift and large climatic changes throughout the region; most, if not all, of the Karakoram has been glaciated several times (Haserodt, 1989; Shroder, 1993b). Geomorphic activity is extreme by global standards, especially the work of snow avalanches, glaciers, rockfalls, rockslides, debris flows, and flood-prone rivers (Hewitt, 1968, 1993; Goudie et al, 1984). The conditions are also those commonly associated with catastrophic landsliding (Voight and Pariseau, 1978). Reports of sudden, large mass movements began with the earliest modern accounts of the region. Large debris flows have been the most frequently recorded and there are classic descriptions of them in Godwin-Austen (1864), Conway (1894), and de Filippi (1912). They are the catastrophic mass movements most commonly associated with disasters (Hughes and Nash, 1986; Shroder, 1989; Kreutzmann, 1994). First-hand accounts also testify to the great scale and devastation of landslides triggered by earthquakes (Mason, 1914; Workman and Workman, 1917; Cockerill, 1902; Hewitt, 1976). A widely discussed example was that of 1841 from the slopes of Nanga Parbat, just south of the Karakoram. The landslide dammed the Indus for six months before draining catastrophically to cause the largest recorded flood on this river (Hewitt, 1968; Shroder, 1993a). The second-largest flood also came from the outburst of a landslide dam that blocked the Hunza River for seven months in 1858 (Becher, 1859; Todd, 1930).

Figure 1.

Location and regional geology of Karakoram Himalaya. Geological divisions, structures, and nomenclature are after Searle (1991).

Figure 1.

Location and regional geology of Karakoram Himalaya. Geological divisions, structures, and nomenclature are after Searle (1991).

However, until the late 1980s no rock avalanches were reported from the Karakoram and, if they were mentioned, no example was identified or differentiated from other large landslides (Hewitt, 1968; Jones et al., 1983; Goudie et al., 1984; Owen, 1991). No catastrophic mass movements from this mountain range were referred to in several major overviews of large landslides (Indian Society of Engineering Geology, 1975; Coates, 1977; Voight, 1978; Rouse, 1984; Eisbacher and Clague, 1984; Hutchinson, 1988). Upon reflection, this seems surprising given that the Karakoram environment resembles others where rock avalanches are well known, including the New Zealand, Japanese, and European Alps, and the American Cordilleras (Luckman, 1976; Coates, 1977; Whitehouse and Griffith, 1983; Cruden, 1985; Evans and Clague, 1988). Moreover, local relief, ruggedness, and degree of dissection in the Karakoram exceed that in most of these other ranges. There are tens of thousands of square kilometers of rock wall steeper than 45° and with elevation ranges of 500 m or more which, along with the conditions already described, seem likely to promote rockslide and rockfall events of great size.

The first rock avalanches actually reported from the Karakoram occurred at Bualtar Glacier in 1986 (49 in Fig. 2; Hewitt, 1988). Surveys by Hewitt, directed at this problem since 1987, have identified deposits of more than 100 rock avalanches at 97 locations (Fig. 2). Because recognition of the events depended mainly on ground-based surveys in a logistically difficult environment, the geographical distribution of events reflects the routes chosen and accessible parts of the valleys visited. Nevertheless, it is clear that rock avalanches have occurred in all the main valleys of the Karakoram, at all elevations, and in each of the main geological terranes and climate-geomorphic zones. They appear to be characteristic and influential mass-movement processes throughout the region, and include the largest of catastrophic landslides yet found in the Karakoram. Because barely 5% of an ∼70 000 km2 area was examined, far more examples must remain to be discovered. Meanwhile, several of the large landslides of Nanga Parbat, including that of 1841, have been recognized as rock avalanches (Owen, 1991; Shroder, 1993a).

Figure 2.

Locations of rockslide-rock avalanches identified in Karakoram Himalaya. Numbers identify sites of rock-avalanche deposits and shaded areas show valleys along which surveys were carried out. In some cases site is location of more than one rock avalanche. Total of 107 events had been identified by 1995. Structures and nomenclature are after Searle (1991).

Figure 2.

Locations of rockslide-rock avalanches identified in Karakoram Himalaya. Numbers identify sites of rock-avalanche deposits and shaded areas show valleys along which surveys were carried out. In some cases site is location of more than one rock avalanche. Total of 107 events had been identified by 1995. Structures and nomenclature are after Searle (1991).

A major reason for the absence of rock avalanches from previous studies turned out to be misinterpretation of their deposits. More than 40 examples, including the most accessible ones, had been mapped as moraines and tills. They were incorporated as such into the more influential reconstructions of Quaternary events (Dainelli, 1922; Norin, 1925; Derbyshire, 1996). In many other mountain areas, rock-avalanche deposits have been mistaken for glacial deposits or for other types of mass movement, sometimes for disturbed bedrock or volcanic and meteorite-impact features (e.g., Goguel and Pachoud, 1972; Pflaker, 1978; Porter and Orombelli, 1980; Whitehouse and Griffiths, 1983; Heuberger et al., 1984). Difficulties can arise because superficial features of rock-avalanche deposits may resemble glacial or other forms of coarse, rubbly deposits (Heim, 1932; Mollard, 1977; Laznicka, 1988). However, an outstanding factor in the Karakoram, and perhaps in other high mountains, is the nature and legacy of rock-avalanche behavior in exceptionally rugged terrain.

Of the rock-avalanche deposits in the Karakoram, 90% record substantial and complex modifications due to interactions with rugged topography. The remaining 10% spread far across relatively gentle intermontane basin areas or over glaciers. Their deposits record complex interactions with erodible substrates, large-scale entraining of material from the latter, or eventual soaking of the moving rock-avalanche debris by moisture from lakes, river flats, or melted snow and ice. These interactions of rock avalanches with terrain and substrate are dominant and pervasive features of Karakoram examples. If they have caused problems of identification, they also provide essential clues for recognizing and reconstructing rock-avalanche events in high mountains. Before discussing this, the criteria used to place the Karakoram deposits in the general class of rock avalanches, and some measures of the distribution and dimensions of the events, are reviewed.

Diagnostic features of rock-avalanche deposits

This study developed from an attempt to establish the incidence and significance of catastrophic rockslides in the Karakoram, and depended mainly upon recognition and interpretation of more or less ancient deposits. It also involved distinguishing rock-avalanche deposits from other coarse fragmentites (Laznicka, 1988), i.e., diamictons when unconsolidated and diamictites or breccias when cemented or lithified. Appendix 1 provides a detailed breakdown of depositional features encountered in rock avalanches and the terms applied to them. Some of the more important diagnostic features used in the Karakoram surveys are briefly discussed in the following.

The undisturbed surfaces of rock-avalanche deposits usually consist of openwork covers of angular boulders with megaclasts (Fig. 3). The largest surface clasts in a given local area may show preferred orientation parallel or transverse to rock-avalanche movement. Local zones of imbrication, jostling, and impact fracturing record conditions in the moving debris sheet. In 42 of the Karakoram examples, this bouldery rubble can be traced to a clear detachment scar on nearby mountain walls, and the rockslide-rock-avalanche events were readily reconstructed. In others, this was not possible with the time and resources available, and properties of the deposits were used to confirm their origin. Where exposed, sedimentary features at depth were an important part of the diagnosis.

Figure 3.

Examples of bouldery, openwork surface of rock avalanches with megaclasts. A: 1986 Bualtar Glacier event (50 in Fig. 2), showing an area of deposits ∼500 m from distal rim. Material in this region is broken and crushed marble bedrock. Some crude imbrication of large clasts is apparent. Largest boulder, with person on top, has volume of ∼5200 m3, and a longest axis of 32 m oriented in direction of movement from left to right of photograph. Rock avalanche had traveled 3.5 km horizontally and 1400 m vertically at this point (see Hewitt, 1988). B: View over main accumulation and distal region of Bhurt, Ishkoman Valley rock avalanche, northwest of Gilgit (13 in Fig. 2). View is from east flank of Ishkoman Valley looking upstream. Terminal moraines of Karambar Glacier are in distance (K). Debris came from left of photograph and detached from rock wall in intrusive body, exposure of Karakoram Batholith, on west side of Ishkoman gorge (cf. Fig. 1). Central portion of rock avalanche crossed and (formerly) dammed Ishkoman River at Bhurt village. Debris still forms barrier across mouth of Bhurt Glacier valley to right of photograph whose stream crosses deposit in foreground. Largest of blocks (A) has volume of almost 27 000 m3 and longest axis of 44 m and had been transported ∼4.5 km horizontally and 1000 m vertically.

Figure 3.

Examples of bouldery, openwork surface of rock avalanches with megaclasts. A: 1986 Bualtar Glacier event (50 in Fig. 2), showing an area of deposits ∼500 m from distal rim. Material in this region is broken and crushed marble bedrock. Some crude imbrication of large clasts is apparent. Largest boulder, with person on top, has volume of ∼5200 m3, and a longest axis of 32 m oriented in direction of movement from left to right of photograph. Rock avalanche had traveled 3.5 km horizontally and 1400 m vertically at this point (see Hewitt, 1988). B: View over main accumulation and distal region of Bhurt, Ishkoman Valley rock avalanche, northwest of Gilgit (13 in Fig. 2). View is from east flank of Ishkoman Valley looking upstream. Terminal moraines of Karambar Glacier are in distance (K). Debris came from left of photograph and detached from rock wall in intrusive body, exposure of Karakoram Batholith, on west side of Ishkoman gorge (cf. Fig. 1). Central portion of rock avalanche crossed and (formerly) dammed Ishkoman River at Bhurt village. Debris still forms barrier across mouth of Bhurt Glacier valley to right of photograph whose stream crosses deposit in foreground. Largest of blocks (A) has volume of almost 27 000 m3 and longest axis of 44 m and had been transported ∼4.5 km horizontally and 1000 m vertically.

The coarse surface rubble overlies and may hide the underlying, densely compacted diamicton into which the surface boulders pass at depth (Fig. 4). This composes the main body of any rock-avalanche deposit and may consist, predominantly, of coarse clasts. However, there are also substantial amounts of matrix material of sand grades and finer, ranging from as little as 5% (matrix poor), to 25% or more (matrix rich). Heim (1932, p. 106–107 and 112, respectively) emphasized “… the enormous grinding of bedrock to dry rock flour (‘Steinmehl’) or dust, with which the entire breccia is interspersed …” and that it “… is found only in the great rockslides (‘Bergsturzen’).” Matrix-supported sections occur, but clast-supported material generally predominates. Unlike the texture of most tills, matrix content rarely reaches or exceeds 50%. It is quite variable locally, but commonly increases with depth. Variations in the amounts of matrix material between examples seem directly proportional to total rockslide volume and inversely proportional to rock hardness.

Figure 4.

A: Vertical cross section in medial right flank region of Bualtar Glacier rock avalanches (50 in Fig. 2; cf. Fig. 3), exposed within 1 yr of event by breakup of glacier. Main body is matrix-rich (∼33% mass), but mostly clast-supported diamicton of crushed and pulverized marble. Note person for scale. B: Erosional remnant from upper 10 m of Gol-Ghoro rock-avalanche deposit (64 in Fig. 2). Angular coarse clasts and clast-supported, matrix-rich (∼20% coarse sand and finer material) rubbly diamicton are shown. Location is 5 km from source in lobe that moved upstream along Indus gorge (cf. Fig. 18). Coarse surface rubble and surrounding material have been removed by river. Note 1 m staff for scale.

Figure 4.

A: Vertical cross section in medial right flank region of Bualtar Glacier rock avalanches (50 in Fig. 2; cf. Fig. 3), exposed within 1 yr of event by breakup of glacier. Main body is matrix-rich (∼33% mass), but mostly clast-supported diamicton of crushed and pulverized marble. Note person for scale. B: Erosional remnant from upper 10 m of Gol-Ghoro rock-avalanche deposit (64 in Fig. 2). Angular coarse clasts and clast-supported, matrix-rich (∼20% coarse sand and finer material) rubbly diamicton are shown. Location is 5 km from source in lobe that moved upstream along Indus gorge (cf. Fig. 18). Coarse surface rubble and surrounding material have been removed by river. Note 1 m staff for scale.

Except where large quantities of sediment and water were incorporated from the substrate, all materials reflect production by fracturing, crushing, and grinding of locally derived bedrock. That, in turn, depends solely upon gravity- and friction-driven fragmentation of the rock due to interactions occurring within the moving debris. No transport medium intervenes in these comminution processes or in the emplacement of deposits from a rock avalanche. The clasts, in all size ranges and throughout the main body of the rock avalanche, are angular or very angular and have been called sharpstones and chinkstones (Long-well, 1951; Laznicka, 1988). Unless disturbed or weathered, the matrix materials have tight packing. They are held in place by the framework of coarser clasts and intergrain friction. Where exposed by undercutting, the material may stand in vertical cliffs. However, the removal of a larger clast, which usually takes some force, liberates the dust and sand that simply flows out or blows away.

A number of rock-avalanche deposits from carbonate source rocks include cemented, concrete-like breccias (34, 35, 42, and 47 in Fig 2). Crackle, mosaic, or jigsaw breccias were also found in many of the Karakoram examples (Fig. 5): these refer, respectively, to entrained bedrock units that are severely fractured but not separated, or have slight separation of fragments but in the same relative positions, or with very slightly changed positions (Laznicka, 1988). These breccias are typical of the largest rock avalanches and thicker exposures.

Figure 5.

A: Individual boulders with crackle and jigsaw breccia. This matrix-poor (<12% sand grades and finer), clast-supported diamicton has been exposed by erosion in medial region of Ghoro Choh 1 rock avalanche (79 in Fig. 2). Material is 100% greenish tonalite and is ∼8 m below rock-avalanche surface and 2 km from its source on west wall of Shigar Valley. Depth of deposit is not known. Brunton compass for scale. B: Large crackle-brecciated unit of transported granitic bedrock is in medial zone of Haldi deposit, where Saltoro River has cut through (57 in Fig. 2). Entire exposure of unit is not shown, but it is ∼150 m wide and 45 m deep and is more than 100 m above base of rock-avalanche deposit. Unit is surrounded, above and beneath, by thoroughly broken, crushed, and pulverized diamicton material. It was carried ∼2.5 km. Person at left for scale. (Another perspective on exposure is in Figure 21B, above C, and to right side of B).

Figure 5.

A: Individual boulders with crackle and jigsaw breccia. This matrix-poor (<12% sand grades and finer), clast-supported diamicton has been exposed by erosion in medial region of Ghoro Choh 1 rock avalanche (79 in Fig. 2). Material is 100% greenish tonalite and is ∼8 m below rock-avalanche surface and 2 km from its source on west wall of Shigar Valley. Depth of deposit is not known. Brunton compass for scale. B: Large crackle-brecciated unit of transported granitic bedrock is in medial zone of Haldi deposit, where Saltoro River has cut through (57 in Fig. 2). Entire exposure of unit is not shown, but it is ∼150 m wide and 45 m deep and is more than 100 m above base of rock-avalanche deposit. Unit is surrounded, above and beneath, by thoroughly broken, crushed, and pulverized diamicton material. It was carried ∼2.5 km. Person at left for scale. (Another perspective on exposure is in Figure 21B, above C, and to right side of B).

A crucial diagnostic indicator in the Karakoram, as elsewhere, is homogeneous lithology from boulders to dust in the main body of these deposits. At a certain grain size, matrix materials of the polycrystalline rock types change from mainly lithic to single mineral fragments. This is seen in a speckled appearance of the separated finer fractions, the grade depending on the original texture. The fines generally have mineral and element compositions identical to those of the coarse clasts. This serves to confirm that the material derives from a single bedrock unit.

Where the failed bedrock mass comprised two or more distinct lithologies, these are preserved in the deposits as lithologically “… uniform bands (‘Strichweise Einförmigkeit’) of materials” (Heim, 1932, p. 108), another unique feature of rock avalanches (Fig. 6). Rock units from the lowest part of the detachment zone tend to form the distal deposits, whereas the highest are generally found in the proximal region (Eisbacher, 1979). However, departures from this sequence were discovered in several Karakoram examples. They reflect how higher and later arriving parts of the mobile debris rode over or swept around earlier or lower parts that were stalled against topographic obstacles (see following and Mollard, 1977, p. 31). Where individual bedrock units included minor intrusions or distinct, minor stratification, and schistosity, these are generally preserved, but often as severely crushed and attenuated material. In Karakoram rock avalanches that are tens of meters thick, other distinctive structures occur in the crushed rock, but they appear related to topographical constraints and are dealt with later.

Figure 6.

Lithological banding in rock-avalanche debris. Outer lobe of Bualtar Glacier event (50 in Fig. 2) looking across and down glacier shows bands of distinctive lithology, arranged concentrically at right angles to movement (cf. Fig. 3A). Each band represents unit in original bedrock, fractured, crushed, and attenuated by catastrophic descent, but preserved as remnant stratigraphy. Distal rim is composed of light gray marble. Inner areas are bands of other carbonates, dark metamorphic lithologies, or minor intrusives. Rock avalanche traveled from left to right down valley, but has also been disturbed by surge of glacier (cf. Hewitt. 1988). This phenomenon is seen in many Karakoram rock-avalanche deposits, but scale of bands, erosion, or less contrasted colors, usually makes it difficult to show in photograph.

Figure 6.

Lithological banding in rock-avalanche debris. Outer lobe of Bualtar Glacier event (50 in Fig. 2) looking across and down glacier shows bands of distinctive lithology, arranged concentrically at right angles to movement (cf. Fig. 3A). Each band represents unit in original bedrock, fractured, crushed, and attenuated by catastrophic descent, but preserved as remnant stratigraphy. Distal rim is composed of light gray marble. Inner areas are bands of other carbonates, dark metamorphic lithologies, or minor intrusives. Rock avalanche traveled from left to right down valley, but has also been disturbed by surge of glacier (cf. Hewitt. 1988). This phenomenon is seen in many Karakoram rock-avalanche deposits, but scale of bands, erosion, or less contrasted colors, usually makes it difficult to show in photograph.

The fines may contain minor amounts of material not derived from the same rock as surrounding coarse clasts (Hewitt, 1988). Presumably this occurs because the finer dust and powder migrates through the dilated moving mass of the rock avalanche, whereas coarser debris cannot (Hsii, 1975). Other complications arise in the basal zone and margins of the deposits (Yarnold and Lombard, 1989). Here, entrained substrate material may be mixed in (Fig. 7). Lines of shear and stringers of material scraped from the bed penetrate the rock-avalanche material (Johnson, 1978). Some Karakoram rock avalanches that entered flood plain or lacustrine areas became saturated. Their outer parts were transformed into wet debris flows before final deposition (73 and 79 in Fig. 2). A number of those in the intermontane Skardu and Shigar Basins of the Central Karakoram mobilized millions of cubic meters of valley floor, flood plain, lacustrine, and coarse fluvial sediments from the runout path (Fig. 8). Broken up, the complexly contorted and faulted stratified alluvium is mixed with rock-avalanche debris and forms large parts of the exposed deposits (77–85 and 66–70, Fig. 2). For more than a century, these had been misinterpreted as glaciotectonic features, supporting the assumption that surrounding bouldery rock-avalanche debris represents morainic complexes (Norin, 1925; Owen, 1988). However, while dependent on substrate conditions in runout zones, such features are also located in distinctive, usually distal and basal, zones of rock-avalanche facies (Krieger, 1977; Yarnold, 1993). They are organized by the large-scale unity of the singular, catastrophic movement and depositional event. Once recognized, they also help in field identification and reconstruction of rock-slide-rock-avalanche events in complicated terrain.

Figure 7.

Complex basal zone facies of Gol-Ghoro event (64 in Fig. 2) exposed by undercutting at present-day river level (cf. Fig. 23). Shown are remnant units of original bedrock as crushed, attenuated, and contorted rock-avalanche debris, and dikes of alluvium (d) intruded from substrate and carrying river-rounded clasts (arrow) (cf. Yarnold, 1993, p. 355). Direction of rock-avalanche movement was left to right. Note 1 m staff for scale.

Figure 7.

Complex basal zone facies of Gol-Ghoro event (64 in Fig. 2) exposed by undercutting at present-day river level (cf. Fig. 23). Shown are remnant units of original bedrock as crushed, attenuated, and contorted rock-avalanche debris, and dikes of alluvium (d) intruded from substrate and carrying river-rounded clasts (arrow) (cf. Yarnold, 1993, p. 355). Direction of rock-avalanche movement was left to right. Note 1 m staff for scale.

Figure 8.

Yarbah Tsoh rock avalanche (73, Fig. 2) swept into and across Shigar Valley, partly slicing through, partly transporting lacustrine and alluvial material on valley floor. Rock avalanche boulders and diamicton units are seen beneath (a) and within (b) large volumes of alluvium, while an imbricated stack of upended boulders occurs on top (c). Debris moved from left to right of photograph. Alluvial material shows severe contorting (d), shearing (e), and detachment of individual blocks of sediment, resulting from transport across valley. Person for scale (vertical arrow).

Figure 8.

Yarbah Tsoh rock avalanche (73, Fig. 2) swept into and across Shigar Valley, partly slicing through, partly transporting lacustrine and alluvial material on valley floor. Rock avalanche boulders and diamicton units are seen beneath (a) and within (b) large volumes of alluvium, while an imbricated stack of upended boulders occurs on top (c). Debris moved from left to right of photograph. Alluvial material shows severe contorting (d), shearing (e), and detachment of individual blocks of sediment, resulting from transport across valley. Person for scale (vertical arrow).

Distribution and general dimensions of Karakoram rock avalanches

Rock avalanches were identified in each of the main geological zones of the Karakoram, and represent all major rock types with roughly equal numbers from plutonic, metamorphic, and sedimentary or metasedimentary bedrock sources (Table 1). Their deposits were found at elevations to ∼5300 m, roughly at the perennial snowlines or around firn limits on glaciers. The head of detachment zones, where identifiable, was generally ∼1000 m higher, but sometimes more than 2000 m. The highest yet found has an elevation of more than 7000 m (47 in Fig. 2). The only examples known to have occurred in the twentieth century have been confined to glacierized areas between 3500 and 6000 m (10, 49 in Fig. 2). However, most of the rock avalanches recognized originated from glacially oversteepened rock walls below 4500 m and left deposits in valleys that have been ice free at least since the beginning of the Holocene.

Table 1.

Geological Settings and Lithologies of Karakoram Rock Avalanche Deposits

No. of events
Geological Terranes* (99 events)
   Indian Plate
     High Himalaya (margins)—Haramosh4
   Kohistan-Ladakh Arc
     Ladakh terrrane and batholith (Baltistan)34
     Kohistan terrane and batholith (Gilgit)25
   Karakoram or “Northern” Plate
     Karakoram metamorphic complex19
     Karakoram batholith3
     N. Karakoram terrane (central region)9
     N.W. Karakoram and Hindu Raj9
   Predominant Lithology (82 events)
     Plutonic21
     Metamorphic26
     Sedimentary or metasedimentary§20
     Mixed#15
No. of events
Geological Terranes* (99 events)
   Indian Plate
     High Himalaya (margins)—Haramosh4
   Kohistan-Ladakh Arc
     Ladakh terrrane and batholith (Baltistan)34
     Kohistan terrane and batholith (Gilgit)25
   Karakoram or “Northern” Plate
     Karakoram metamorphic complex19
     Karakoram batholith3
     N. Karakoram terrane (central region)9
     N.W. Karakoram and Hindu Raj9
   Predominant Lithology (82 events)
     Plutonic21
     Metamorphic26
     Sedimentary or metasedimentary§20
     Mixed#15

* Following Searle (1991), cf Figures 1 and 2.

The Darkot and Yarkhun Formations of Buchroithner and Gamerith(1978).

§ There are 17 events in which carbonates predominate, including sedimentary or metasedimentary and metamorphic varieties.

# Substantial bands of two or more lithologies found in deposit (cf Fig.6).

Figure 9 identifies elements and dimensions of rock avalanches commonly measured to compare events. Substantial erosion or burial of deposits, inaccessible source zones, and poor topographical maps make estimates of these dimensions difficult or impossible for many of the Karakoram events. Where orders of magnitude could be determined for volume, areal extent, height, and horizontal distance traveled, these may be compared with dimensions of known rock avalanches elsewhere. The original areas covered by the deposits range from ∼2 km2 to almost 40 km2. Original volumes of material all exceeded 1.5 × 106 m3 and most exceeded 20 × 106 m3 (Fig. 10). Broadly, the data relating runout to volume and height of fall (Fig. 11) accord with well-established relations among these parameters (e.g., Scheidegger, 1973; Hsü, 1975; Nicoletti and Sorriso-Valvo, 1991). However, the correlations are rather weak. Likewise, the distribution of deposit area is noticeably different from that of their volumes. It is suggested that these results reflect, above all, interference in the runout and emplacement of deposits by topography and substrate. Table 2 provides comparative data for examples used to illustrate the latter in the rest of the paper.

Figure 9.

Schematic longitudinal cross section of rockslide-rock avalanche in mountain valley, to illustrate features referred to in text and certain measures and terminology used to describe them. H is vertical distance between highest point of detachment zone and lowest reach of debris; Hr is runup height to raised distal rim; and L is maximum horizontal travel. Fahrböschung, or travel slope angle, is angle from highest point in detachment zone to furthest travel distance, tangent of which gives equivalent coefficient of friction (Hsii, 1975).

Figure 9.

Schematic longitudinal cross section of rockslide-rock avalanche in mountain valley, to illustrate features referred to in text and certain measures and terminology used to describe them. H is vertical distance between highest point of detachment zone and lowest reach of debris; Hr is runup height to raised distal rim; and L is maximum horizontal travel. Fahrböschung, or travel slope angle, is angle from highest point in detachment zone to furthest travel distance, tangent of which gives equivalent coefficient of friction (Hsii, 1975).

Figure 10.

Area and volume dimensions of Karakoram rock-avalanche deposits. A: Estimated original areas of deposits for 51 events. B: Estimated original volume of deposits for 56 events.

Figure 10.

Area and volume dimensions of Karakoram rock-avalanche deposits. A: Estimated original areas of deposits for 51 events. B: Estimated original volume of deposits for 56 events.

Figure 11.

Relations of mobility for selected Karakoram rock avalanches, where dimensions could be estimated. A: Ratio of maximum horizontal travel to estimated volume for 20 events. B: Ratio between maximum vertical descent (H) and maximum horizontal travel (L) for 20 events (cf. Fig. 9).

Figure 11.

Relations of mobility for selected Karakoram rock avalanches, where dimensions could be estimated. A: Ratio of maximum horizontal travel to estimated volume for 20 events. B: Ratio between maximum vertical descent (H) and maximum horizontal travel (L) for 20 events (cf. Fig. 9).

Table 2.

Location, Dimensions and Lithologies of Selected Rock Avalanche Events Discussed in the Text

Survey #13Survey #47Survey #57Survey #68Survey #79Survey #86
36°34′N:74°05′N36°04′N:7455′N35°15′N:76°26′N35°37′N:35°18′N35°40′N:75°28′N35°22′N:75°25′N
BhurtGannish ChisshHaldiSatpara*Ghoro Choh IKatzarah
Area of deposit (km2):
    (i) exposed now3.51.22.541012
    (ii) estimated original7.517.025.0221420
Volume of deposits (×106m−3):
    (i) estimated now2504.830030060120
    (ii) estimated original500+200600+400120200
Rock-wall Source:
    Maximum elevation (m asl)44006800420040003800500
    ExposureENNWSSWWNWNESSW
    Rock type(s)PlutonicCarbonate (metamorphic)Plutonic and (minor) metamorphicPlutonic and metamorphicPlutonic (tonalite)Plutonic and metamorphic
Run out of debris:
    Lowest elevation239042002600220025002100
    Maximum drop, (m)109026001600170013002400
    Maximum travel, (m)50001050070009000700011000
    Farböschung angle (N)§12°13°13°11°<12°
    ACoefficient of friction (H/L)#0.220.230.230.160.190.22
    Highest run up, hr(m)**180250500250150730
Survey #13Survey #47Survey #57Survey #68Survey #79Survey #86
36°34′N:74°05′N36°04′N:7455′N35°15′N:76°26′N35°37′N:35°18′N35°40′N:75°28′N35°22′N:75°25′N
BhurtGannish ChisshHaldiSatpara*Ghoro Choh IKatzarah
Area of deposit (km2):
    (i) exposed now3.51.22.541012
    (ii) estimated original7.517.025.0221420
Volume of deposits (×106m−3):
    (i) estimated now2504.830030060120
    (ii) estimated original500+200600+400120200
Rock-wall Source:
    Maximum elevation (m asl)44006800420040003800500
    ExposureENNWSSWWNWNESSW
    Rock type(s)PlutonicCarbonate (metamorphic)Plutonic and (minor) metamorphicPlutonic and metamorphicPlutonic (tonalite)Plutonic and metamorphic
Run out of debris:
    Lowest elevation239042002600220025002100
    Maximum drop, (m)109026001600170013002400
    Maximum travel, (m)50001050070009000700011000
    Farböschung angle (N)§12°13°13°11°<12°
    ACoefficient of friction (H/L)#0.220.230.230.160.190.22
    Highest run up, hr(m)**180250500250150730

Note: cf Figure 1; asl = above sea level.

* Deposits may represent two events.

Estimated from existing maps; probably not more accurate than ±100 m.

§ See Heim (1932) and Figure 4.

# See Scheidegger (1973).

** Estimated in field.

TOPOGRAPHIC CONSTRAINTS ON THE MORPHOLOGY AND STRUCTURE OF ROCK-AVALANCHE DEPOSITS

Mode of emplacement and overall forms of rock-avalanche deposits

It is generally accepted that the character and mobility of rock avalanches varies with rock type and geological conditions, the mode of bedrock failure and triggering mechanism, the volume of the failed mass, and the height and steepness of descent (e.g., Evans, 1984; Corominas, 1996). The distance traveled by the debris is further constrained by the terrain over which it moves. However, while these factors control overall dimensions, the material deposited mainly reflects the unique composition and mechanics of the “sturzstrom,” or streaming flow of dry rubble and powder (Heim, 1932).

The most widely described rock-avalanche deposit is sheet like and thins away from the source slope. Its plan form is tongue like, recording much greater momentum transfer and transport of debris directly out from the source, but limited lateral spreading (Fig. 12). These features only occur, however, if certain conditions are satisfied. The debris must be introduced onto a relatively gentle, continuous, and fairly uniform slope or valley floor. The debris should not be very thick or confined at the base of the source slope. In these circumstances the mechanics of a dry, cohesionless flow of rubbly debris dominate the emplacement and form of the deposit (Rouse, 1984; Hutchinson, 1988). Details of surface morphology, especially at the distal rim, and of facies in the basal zone almost always reflect some interaction with topography and substrate, but they are minor features (Cruden, 1985).

Figure 12.

Plan forms of rock-avalanche deposits in which there was minimal interference by topography or mobile substrates (A) after Johnson, 1978; (B) Cruden, 1982; (C) after Cruden and Beaty (no date) (Fig. 1); (D) after Eisbacher and Clague, 1984; (E) this paper (Fig. 13).

Figure 12.

Plan forms of rock-avalanche deposits in which there was minimal interference by topography or mobile substrates (A) after Johnson, 1978; (B) Cruden, 1982; (C) after Cruden and Beaty (no date) (Fig. 1); (D) after Eisbacher and Clague, 1984; (E) this paper (Fig. 13).

There are subsidiary lobes of most Karakoram rock avalanches that display the classic, narrow, lobate, sheet-like form. However, their overall and local morphologies depart substantially from it. The only exception was a relatively small rock avalanche that moved directly down Chillinji Glacier in 1991 (Fig. 13; 10 in Fig. 2). Most others reflect strong topographic interference that impeded and blocked the mobile debris. Topographic features in the runout zones funneled or separated parts of the rock avalanches. Many were split into two or more independent streams or lobes, moving around obstacles, upvalley or downvalley, and into other valleys at junctions. Each lobe could have a very different history and resulting morphology, although connected as arms or distributaries of the same event. Unless following entirely different paths, debris streams might reconverge farther on, to interact once more. These modifications affected the relative concentration or dispersal of the depositional mass. They created a range of distinctive plan forms, surface morphologies, and internal or facies characteristics.

Figure 13.

Chillinji Glacier rock avalanche of 1991 (10 in Fig. 2), seen 1 yr later. It shows typical tongue-like, thin sheet of debris, raised distal rims, and minor lobes. Rock-wall source is to left of photograph. ∼500–700 m above ice surface.

Figure 13.

Chillinji Glacier rock avalanche of 1991 (10 in Fig. 2), seen 1 yr later. It shows typical tongue-like, thin sheet of debris, raised distal rims, and minor lobes. Rock-wall source is to left of photograph. ∼500–700 m above ice surface.

Topography is widely recognized as an important influence upon the behavior of rock avalanches, and on emplacement of their deposits (e.g., Mudge, 1965; Shreve, 1968; Fahnestock, 1978; Whitehouse and Griffiths, 1983; Evans, 1989; Evans et al., 1994; Hungr, 1995). In a study of Martian landslides, some of which strongly resemble Earth's rock avalanches, Lucchitta (1979) also emphasized the importance of degree of confinement on trough floors in controlling the forms and emplacement of debris.

Nicoletti and Sorriso-Valvo (1991) investigated topographic relations of rock avalanches in terms of runout distance and energy dissipation. They recognized three types of overall deposit morphology. All of the Karakoram examples except the Chillinji Glacier event would fall in their high-energy-dissipative class, in which topographic interference causes a high rate of dissipation of mechanical energy in the runout path. This they identify with a “deformed T shape” seen in rock avalanches that cross narrow valleys and impact an opposing slope with “subsequent upstream and downstream surging” (Nicoletti and Sorriso-Valvo, 1991, p. 1370). They briefly identified a number of other features attributed to geomorphic controls, described in the following. However, it may be more accurate to place the Karakoram examples in an implied fourth class, in which both terrain and rock-avalanche shape are morphologically complex. As Nicoletti and Sorriso-Valvo (1991) pointed out, it is difficult to apply their methods to such examples.

Corominas (1996) addressed and extended the analysis of landslide mobility, including rock avalanches, in terms of the relationship to the fahrböschung, or angle of reach. He stressed the role of obstructions or topographical constraints and included some effects of confinement, deflection, and runup over moderate inclines or minor obstructions. However, examples with complex or poorly defined topographic constraints and with complex movements and splitting of the debris were not considered.

However, it is these complications, and features that result from them, that predominate in the depositional record in the Karakoram. Moreover, what topography-constrained forms look like and where they are encountered in the field seem the most relevant issues for the state of our knowledge of rock avalanches in the Himalaya. In this regard, the pioneering work of the Swiss geologist Albert Heim remains appropriate. He defined a range of distinctive rock-avalanche morphologies and related features due to topographic interference observed in the European Alps (Heim, 1932). His insights are built upon here, to help interpret Karakoram rock-avalanche deposits.

Opposing slope effects: The main accumulation

Where steep slopes and salients oppose or block the runout path, they lead to substantial and complex effects in the rock avalanche. These vary according to the geometric relations of debris-stream movement and opposing slope (Heim, 1932). Their relations to the overall shape of the depositional complex are shown schematically as types 2–5 in Figure 14.

Figure 14.

Schematic long and cross sections, and plan forms of cross-valley rock-avalanche deposits. Emphasis is placed on geometric relations of debris emplacement to opposing slopes. Terms are discussed in text. In types 1, 2, and 4, subtypes are indicated where debris, initially confined in tributary chutes or canyons, is emplaced in main valley.

Figure 14.

Schematic long and cross sections, and plan forms of cross-valley rock-avalanche deposits. Emphasis is placed on geometric relations of debris emplacement to opposing slopes. Terms are discussed in text. In types 1, 2, and 4, subtypes are indicated where debris, initially confined in tributary chutes or canyons, is emplaced in main valley.

In the distal regions of a rock avalanche, especially, three main situations can be discerned, usually in combination.

1. The first is climbing up and stalling against an opposing slope that was orthogonal to the direction of debris movement, the swash features of Whitehouse and Griffiths (1983).

2. The second is deflecting of debris moving obliquely across opposing slopes. This may pass into the phenomenon of deflective rise and fall, or caroming, along slopes and reentrants (Heim, 1932; Fahnestock, 1978).

3. The third is splitting into independent debris streams around obstructions or at an opposing wall (Heim, 1932; Butler et al., 1986; Nicoletti and Sorriso-Valvo, 1991).

These situations have different consequences for the outermost and lowest layers of material, the parts of the rock avalanche that arrive at the obstruction first, compared to later arriving or higher parts of the moving debris. In the latter we find a fourth development; i.e., forms of interaction within the debris stream that are indirectly related to topographic interference. These include pressure ridges and mounds built against the debris ahead that is slowed or stalled by obstructions (Mollard, 1977), or deflection and separation of flow around it. In extreme cases, later arriving debris may override or climb far up slower moving and stalled material.

The resulting depositional features and their degree of development also depend upon the relations of rock-avalanche size to valley geometry. Where a large fraction of the rock-avalanche mass travels directly across a mountain valley and is blocked by the opposing slope, the deposit has an asymmetrical long profile. The thicker, higher, and often larger volume rests against the impact slope, forming the main accumulation (“Hauptanlage” of Heim, 1932). It is most marked in narrow, steep-walled valleys where a deep, cross-valley ramp of debris may form (types 3, 4, and 5 in Fig. 14). There may also be a conspicuous raised area, distinguished as the crown of the deposit here. A well-known example is the Tauferberg hill of the Köfels event in Ötztal, Austria, a main accumulation on the opposite side of the Maurach gorge from the detachment zone, the remarkable thickness and height of which had deceived many geologists as to its origin, prior to identification as a rock-avalanche deposit (Heuberger et al., 1984). Comparable features are found in many of the Karakoram rock avalanches where they descended into deep gorges (13, 17–29, 36–46, 53–56, 61–64, 71, and 87 in Fig. 2).

In the larger ones the main accumulation or crown against the opposing slope may be several hundred meters higher and thicker than the material closer to the source slope (Fig. 15).

Figure 15.

Satpara Lake rock avalanche, Skardu Basin, Baltistan (#69 in Fig. 2; cf. Table 2). A: Aerial view looking southward up Satpara Valley from above Skardu Basin. The rock avalanche descended from a rock wall to left of photograph, and surged up opposing slope to emplace well-defined main deposit (M) with higher crown (C). Satpara Lake (S) was dammed and rose at least 50 m higher in early phase of impoundment. Satpara stream, lower left, has breached the barrier and cuts down ∼110 m through the proximal part of the rock avalanche deposit. Arrow is station for photo B. B. View over the main accumulation from the crown toward the detachment zone (B), the highest part of which is ∼950 m above Satpara stream and 1000 m below the peak, which is 4798 m above sea level. The surface rubble in the foreground consists entirely of intrusive igneous rock debris of quartz diorite or tonalite composition. Boulders and underlying diamictons in the low, proximal region (E), and deflected up and down valley, consist of a metasedi-mentary graywacke. Debris deflected around the main accumulation (right side of photo = left flank of rock avalanche) traveled several kilometers up Satpara Valley. Most of the debris is drowned by the lake (D), except for a small, boulder-covered island. Substantial debris streams were also deflected to the left part of photo and down the fan into Skardu Basin, where they reached the present course of the Indus River, 9 km from the source (A). Beside the detachment zone is a large block (C). apparently part of the original rockslide, that became wedged in the bottom of the gorge without breaking up.

Figure 15.

Satpara Lake rock avalanche, Skardu Basin, Baltistan (#69 in Fig. 2; cf. Table 2). A: Aerial view looking southward up Satpara Valley from above Skardu Basin. The rock avalanche descended from a rock wall to left of photograph, and surged up opposing slope to emplace well-defined main deposit (M) with higher crown (C). Satpara Lake (S) was dammed and rose at least 50 m higher in early phase of impoundment. Satpara stream, lower left, has breached the barrier and cuts down ∼110 m through the proximal part of the rock avalanche deposit. Arrow is station for photo B. B. View over the main accumulation from the crown toward the detachment zone (B), the highest part of which is ∼950 m above Satpara stream and 1000 m below the peak, which is 4798 m above sea level. The surface rubble in the foreground consists entirely of intrusive igneous rock debris of quartz diorite or tonalite composition. Boulders and underlying diamictons in the low, proximal region (E), and deflected up and down valley, consist of a metasedi-mentary graywacke. Debris deflected around the main accumulation (right side of photo = left flank of rock avalanche) traveled several kilometers up Satpara Valley. Most of the debris is drowned by the lake (D), except for a small, boulder-covered island. Substantial debris streams were also deflected to the left part of photo and down the fan into Skardu Basin, where they reached the present course of the Indus River, 9 km from the source (A). Beside the detachment zone is a large block (C). apparently part of the original rockslide, that became wedged in the bottom of the gorge without breaking up.

In the proximal area of such deposits, below the source slope, a marked depression may occur and the thinner part of the cross-valley ramp (Mollard, 1977; Butler et al., 1986). Because rock-avalanche deposits can form strong and enduring cross-valley barriers, water dammed behind them usually overflows and erodes a breach across this lower, proximal part (Heim. 1932). More than 40 Karakoram rock avalanches impounded large lakes. Those breached across the proximal area include 13, 17, 28, 39–42, 45, 54, 64, 69, 77, and 86 in Figure 2. However, several rock avalanches from tributary valleys that formed high barriers (>50 m thick) across a main valley have been breached along the distal rim, between the raised main accumulation and the valley wall (e.g., 1, 53, and 71 in Fig. 2). In either case, the new channel is rarely where the original stream flowed. Later, the river may be superimposed on a buried rock spur to begin cutting the so-called epigenetic gorges of the Karakoram (Dainelli, 1922; Hewitt, 1968). Epigenetic gorges occur at rock-avalanche barriers (1, 22, 41, 45, 57, 61, and 91 in Fig. 2).

Opposing slope effects: The Brandung phenomena

The circumstances described here also lead to a feature at the distal rim that Heim (1932) identified and termed the “Brandung.” It is above, sometimes tens of meters higher than, the asymmetrical main accumulation and crown deposit. English equivalents such as surge, swash, surf, or wave seem likely to cause confusion, so Heim's term brandung (lower case), is adopted here.

The brandung represents the culmination of the rock avalanche's upward climb of an opposing slope and consists of a distal ridge with steep front, sometimes an equally steep slope facing into the rock-avalanche deposit. The form and height varies across the impact slope in a given case, and between examples as a function of the steepness of the slope, height of climb, and thickness of debris reaching the upper limit. Heim interpreted the plan form (his Brandungswelle or brandung wave, by analogy with a wave breaking on a shore) as roughly parabolic around the center line of flow, assuming a constant strike of the impact slope (Fig. 16). The nearer the trajectory is orthogonal to the impact slope, the more pronounced is the brandung. Then, the leading mass of debris will be stalled against the slope. The angle of the impact slope is also important. If it is fairly gentle, the debris is likely to be more dispersed; if steep, the debris stream will be more concentrated, but to a limiting angle at which it will fall back. Well-developed brandung ridges usually occur against opposing slopes between 30° and 50°. Heim first demonstrated the high rate of energy dissipation where the 1881 Elm event climbed an opposing slope, and that the fahrböschung to a brandung may be considerably steeper than where the rock avalanche is free to move downvalley (cf. Hsü, 1978). Nevertheless, the height of climb represented by some of the Karakoram brandlings is remarkable evidence for the mobility of rock-avalanche debris (Table 3).

Figure 16.

Rock-avalanche deposit with well-developed brandling: Katzarah event, Baltistan (86 in Fig. 2), which created cross-valley deposit impounding Indus River where it leaves Skardu Basin. A: Overview from distal rim toward source slope in rock walls of Brogardo hanging valley (A*). Detachment zone may be as much as 2000 m above Indus and 1200 m above photo station. It is 7 km away. Indus cuts across low, proximal zone of rock avalanche (C). Barrier is only partially breached so that sediment and high summer flows are backed up behind it for more than 30 km across basin of Skardu (to right of photograph). After its initial descent, debris surged up slope in foreground to highest point (730 m+ above Indus; see B and Table 3). Bulk of deposit is on this opposing side of valley. Ridges and depressions in debris have a relief of 50 m+ and debris is > 100 m thick. Katzarah Lake (B*) is impounded in deep, closed trough between pressure ridges in rock avalanche. Surface rubble and underlying diamicton materials in foreground consist of fine-grained schistose rock; debris in proximal zone and in remnants up far transport slope (e.g., at g) consists of coarse-grained, amphibole-bearing quartz monzonite. Division between these lithologies forms narrow, well-defined zone, traceable across deposit at right angles to direction of movement, and is example of remnant stratigraphy (cf. Fig. 6). B: Brandung of Katzarah rock avalanche (86 in Fig. 2). View is from near the same photo station as 16A, looking to the right (eastward) and upvalley along brandung where it curves up to highest point reached (cf. Fig. 17). Almost parabolic curve closely resembles that predicted by Heim (see Fig. 19). Note people for scale (arrow).

Figure 16.

Rock-avalanche deposit with well-developed brandling: Katzarah event, Baltistan (86 in Fig. 2), which created cross-valley deposit impounding Indus River where it leaves Skardu Basin. A: Overview from distal rim toward source slope in rock walls of Brogardo hanging valley (A*). Detachment zone may be as much as 2000 m above Indus and 1200 m above photo station. It is 7 km away. Indus cuts across low, proximal zone of rock avalanche (C). Barrier is only partially breached so that sediment and high summer flows are backed up behind it for more than 30 km across basin of Skardu (to right of photograph). After its initial descent, debris surged up slope in foreground to highest point (730 m+ above Indus; see B and Table 3). Bulk of deposit is on this opposing side of valley. Ridges and depressions in debris have a relief of 50 m+ and debris is > 100 m thick. Katzarah Lake (B*) is impounded in deep, closed trough between pressure ridges in rock avalanche. Surface rubble and underlying diamicton materials in foreground consist of fine-grained schistose rock; debris in proximal zone and in remnants up far transport slope (e.g., at g) consists of coarse-grained, amphibole-bearing quartz monzonite. Division between these lithologies forms narrow, well-defined zone, traceable across deposit at right angles to direction of movement, and is example of remnant stratigraphy (cf. Fig. 6). B: Brandung of Katzarah rock avalanche (86 in Fig. 2). View is from near the same photo station as 16A, looking to the right (eastward) and upvalley along brandung where it curves up to highest point reached (cf. Fig. 17). Almost parabolic curve closely resembles that predicted by Heim (see Fig. 19). Note people for scale (arrow).

Table 3.

Runup Height and Related Features of Selected Karakoram Rock Avalanches

EventLocationRunupCommentVertical
*(hr)Descent
(m)(H)
(m)
Katzarah#86735To highest part of brandung (Fig. 16)>1200
Gol-Ghoro#64>740To highest part of brandung (Fig. 17)∼1600
Haldi#57>500To highest crossing of interfluve (Fig. 23)1300
Rondu A#92>1000To brandung above Shoat?(>1400)
Rondu B#92650To brandung above Mendi∼1400
Gupis#17410To crown above village1100
EventLocationRunupCommentVertical
*(hr)Descent
(m)(H)
(m)
Katzarah#86735To highest part of brandung (Fig. 16)>1200
Gol-Ghoro#64>740To highest part of brandung (Fig. 17)∼1600
Haldi#57>500To highest crossing of interfluve (Fig. 23)1300
Rondu A#92>1000To brandung above Shoat?(>1400)
Rondu B#92650To brandung above Mendi∼1400
Gupis#17410To crown above village1100

Note: cf Evans et al. 1994. p. 766. Heights are measured from present river level to the highest identifiable exposure of rock avalanche debris on the impact slope, along a straight line flow path connecting the exposure, river level and central part of the detachment zone. In every case, the river still flows in debris of the rock avalanche barrier and the ancient valley floor is not exposed. It may be tens of meters beneath the river, perhaps more than 100 m in some cases. Hence, the runup values only approximate H, in Figure 9. Assuming the debris at the distal rim represents the leading edge of the rock avalanche material that had descended first to the ancient valley bottom, the actual heights climbed may be significantly greater. The greater height climbed by the Rondu, Gol-Ghoro, and Katzarah events than other examples reported to date (Evans et al. 1994), seems to reflect a combination of greater mass, local relief and the geometric relations of steep descent and steep opposing valley walls.

*See Figure 2 for numbered locations.

No specific detachment zone identified on rockwalls and canyons from which rock avalanche debris came.

Where the brandung is well developed, it is associated with a pronounced depression between the depositional ridge and the valley wall. Heim called this the “little brandung trough” (Brandungstälchen). Karakoram examples occasionally exceed 10 m in depth and extend for hundreds of meters, sometimes, if intermittently, for several kilometers. Slope deposits accumulate in the trough. Local drainage is channeled along it and is sometimes held in ponds or temporary small lakes. A distinctive depositional complex develops that may be hundreds of meters above the main valley floor (Fig. 17).

Figure 17.

View along little brandling trough of Katzarah rock avalanche (86 in Fig. 2). between rim of rock avalanche and opposing slope. View looks westward toward exit of Skardu Basin along crest. Rock avalanche moved up and along valley slope from right side of photograph (cf. Figs. 15 and 16).

Figure 17.

View along little brandling trough of Katzarah rock avalanche (86 in Fig. 2). between rim of rock avalanche and opposing slope. View looks westward toward exit of Skardu Basin along crest. Rock avalanche moved up and along valley slope from right side of photograph (cf. Figs. 15 and 16).

The brandung of the Gol-Ghoro event (64), in the Indus gorge 20 km northeast of Skardu, is remarkable in several respects (Fig. 18). The crest of the brandung and ∼20% of its mass, some 2 × 104 m3, consist of boulders and larger units of bedrock stripped from the rock wall immediately below by the rock avalanche. Much of this material was only slightly broken up and rotated in transport, and includes crackle and jigsaw breccia masses easily mistaken for in situ weathered rock. However, it is separated from the bedrock by a rubbly diamicton tens of meters thick that has the same composition as the main body of the rock-avalanche deposit and forms the bulk of the brandung. The rock avalanche had traveled ∼5 km from the detachment zone at this point, and the highest part of the brandung is ∼740 m above the present level of the Indus. The full height of the runup must have been even more, because the river has not cut down to the ancient valley floor, and still flows entirely in a gorge through the rock-avalanche material. That the leading edge of the rock avalanche could still entrain such a mass of bedrock and lift it almost vertically as much as 120 m is an indication of the power of these events.

Figure 18.

Brandung of Gol-Ghoro rock avalanche (64 in Fig. 2) in Indus Valley 15 km upstream of Skardu Basin. A: Well-developed part of brandling (B), showing its inward-facing, eroded slope. View looks eastward and upstream from crown deposit (C) and main accumulation (A) across highest part of impact slope. Height of brandling, from head of rock wall below it to crest, is ∼55 m. Feature consists of brandung crest with boulders and large units of broken igneous rock displaying crackle and jigsaw breccia (a), thick wedge of rock-avalanche debris (b) sandwiched between a and rock wall (c), from which material forming brandling crest was stripped. B: View along strike of brandung in southerly upvalley direction, showing crest of large, imbricated boulders of rock stripped from rock wall (shown in A). Most of distal, outward-facing slope to left is buried in wind-blown dust. Several meters of debris washed from mountain wall above fill brandung trough (BT). Irrigated field and terraces of Gol (G) are 4 km away and 600 m lower on terrace above Indus.

Figure 18.

Brandung of Gol-Ghoro rock avalanche (64 in Fig. 2) in Indus Valley 15 km upstream of Skardu Basin. A: Well-developed part of brandling (B), showing its inward-facing, eroded slope. View looks eastward and upstream from crown deposit (C) and main accumulation (A) across highest part of impact slope. Height of brandling, from head of rock wall below it to crest, is ∼55 m. Feature consists of brandung crest with boulders and large units of broken igneous rock displaying crackle and jigsaw breccia (a), thick wedge of rock-avalanche debris (b) sandwiched between a and rock wall (c), from which material forming brandling crest was stripped. B: View along strike of brandung in southerly upvalley direction, showing crest of large, imbricated boulders of rock stripped from rock wall (shown in A). Most of distal, outward-facing slope to left is buried in wind-blown dust. Several meters of debris washed from mountain wall above fill brandung trough (BT). Irrigated field and terraces of Gol (G) are 4 km away and 600 m lower on terrace above Indus.

A related observation concerns the well-rounded cobbles and boulders mixed with rock-avalanche material in the outer face of this Gol-Ghoro brandung. Similar fluvially rounded stones are scattered over the surface of some mounds and ridges of its crown deposit. They obviously come from a large river but are not fluvial deposits. They include lithologies not found in the local bedrock or in the main body of the rock avalanche, but that are common in river gravels of the Indus, Shyok, and Saltoro Rivers upstream of the Gol-Ghoro rock-avalanche barrier. Presumably they represent Indus gravels that the rock avalanche entrained as it crossed the ancient valley floor and carried to its outer rim (cf. Evans et al., 1994). Moreover, rounded stones of the same lithologies are found in dikes of substrate alluvium incorporated into the basal zone of the rock-avalanche deposit and exposed in a few places at river level (Fig. 7). However, clasts of these lithologies are found only rarely in the present river channel. Well-rounded clasts of any type are rare. For ∼11 km of rapids through the barrier, and for 10 km below it to the intermontane basin of Skardu, the river flows over angular or poorly rounded coarse debris from the rock avalanche.

The development of a brandung also depends upon the relation of rock-avalanche trajectory to the direction of opposing slope. The more oblique the angle at which the debris strikes the slope, the more spread out is the deposit (Fig. 19). Inside a low, stalled brandung ridge or raised rim, the debris may be guided across the slope, even swinging away from its highest reach. A sequence of forms can be recognized showing the progressive reduction in height of the brandung and depth of accumulation, and an increase in streaming along the slope, as the impact becomes more oblique. Because a rock-avalanche extends across a wide front and is usually spreading laterally before it reaches an opposite valley wall, the brandung form and strike will reflect the changing geometry of impact. It will be further complicated by irregularities in the orientation and steepness of the valley side. These features can be found in the dolomite debris across the distal rim of the well-known Madison Canyon, Montana, slide of 1959 (Hadley, 1964). They are also amply demonstrated by the Gol-Ghoro, Satpara, Katzarah, and Rondu-Mendi events.

Figure 19.

Simplified geometric relationships of rock-avalanche movement and opposing valley walls as they influence form and degree of development of brandung and deflection of flowing debris across slope (after Heim, 1932, p. 87–89).

Figure 19.

Simplified geometric relationships of rock-avalanche movement and opposing valley walls as they influence form and degree of development of brandung and deflection of flowing debris across slope (after Heim, 1932, p. 87–89).

It is appropriate here to note that these particular deposits are some of those consistently mistaken for late-glacial morainic complexes and, as such, have played a large role in landform interpretations in the Karakoram. The main deposit of the Katzarah event, for example, has long been mapped as a terminal or hummocky moraine complex, and its brandung described as a lateral moraine (Dainelli, 1922; Owen, 1988). Augusto Gansser, who recognized that the deposits near the Indus are from a rock avalanche, thought the brandung recorded a glacier that had worked over an earlier landslide (Burgisser et al., 1982). At first, his view seemed reasonable to me. The highest parts of the brandung, and the mix of deposits in the well-developed brandung trough, do resemble a lateral moraine-kame terrace complex (Fig. 17). They are remarkably similar to the ablation valleys found along Karakoram and Nanga Parbat glaciers (Hewitt, 1993). However, the brandung debris shows no evidence of a glacial origin. It has the same lithology and sedimentary and facies characteristics as the main rock-avalanche deposits that fill the valley below. The detailed geometry and topographical relations of the brandung show that ice could not have formed it. Although large parts of its steep, inner face have fallen back, some parts of the boulder-covered surface and all of the deeper layers form a continuous debris sheet with the rest of the rock avalanche across the valley floor. Nevertheless, in this and other examples, it is seen how topography-related hummocks, pressure ridges, and other forms, especially when combined with the extraordinary runup of rock avalanches on distant impact slopes, have encouraged confusion with glacial deposits. Albeit Heim also confronted this problem: his criteria for distinguishing glacial from rockslide-rock-avalanche materials (Heim, 1932, p. 107) remain valid and helpful aids to preliminary identification in the Karakoram situation.

Transport slope or tributary valley confinement and lateral dispersal

The effects of topographic interference described so far apply to the rock avalanche in the runout zone (Fig. 9). However, topographic constraints operating as debris moves down and outward from the transport slope also affect the emplacement configuration of rock-avalanche deposits (Fig. 14, types 1b, 2b, and 4b).

Initial descent of the rockslide mass and developing rock avalanche may be confined between spurs or in a chute or steep canyon. More than one-half of the Karakoram examples in Figure 2 moved first into and along narrow tributary valleys, before runout and deposition in a main valley, or on the floor of an intermontane basin. The rock avalanche has enormous kinetic energy but lacks cohesion and tensile strength, so that it assumes the form of a confining valley (Heim, 1932, p. 94). When this debris moves over a wide, flat surface, it will disperse radially. Most of the mass, and the fastest rate of flow, will still be projected directly outward from the mouth of the confining canyon, resembling the hose effect. Nicoletti and Sorriso-Valvo (1991, type A), showed that, if otherwise unobstructed, these rock avalanches can have the greatest runout and elongation in proportion to their mass and height of descent. However, in most Karakoram examples, the tributary canyons are still relatively large valleys rather than narrow chutes below the breakout zone. Few of the lobes entering the main valley spread unobstructed, and most encountered an opposing valley wall so that the central debris stream was stalled to form a thick cross-valley deposit. However, rapidly spreading flanks traveled much farther, the stalled central part deflecting later arriving material into separate lobes moving up and down valley (see Figs. 13, 2b and 4b, and 1, 3, 20, 51, 53, 71, and 92 in Fig. 2). Spread that suggests lateral stretching of the rock avalanche occurs where the debris sheet has descended over a convex surface. The situation is seen especially on the widening convexity over preexisting sediment fans. It is difficult to separate from the role of confinement in a tributary canyon as the chutes and narrow tributary canyons of the Karakoram nearly always debouch onto debris cones or sediment fans (13, 31, 51, and 71 in Fig. 2). However, the point to emphasize is that the various styles of rock-avalanche deposit reflecting topographical constraints are not only the result of obstructions in the runout zone. Many are topographically confined on the transport slope as well, or by tributary valleys, and this affects the later runout and emplacement of deposits.

Valley side caroming flow

Even where the rock avalanche moves more or less parallel to the valley sides, the mobile debris interacts with spurs and reentrants along the way. This can leave an irregular trim line of marginal deposits, but its rise and fall along the valley sides has a different profile from lateral deposits of a glacier occupying the same valley (Fahnestock, 1978; Porter and Orombelli, 1980).

The prehistoric Gannissh Chissh (7027 m) event that descended from near the summit of the peak onto the upper Barpu Glacier provides an example (47 in Fig. 2). The rock-avalanche deposits that survive blanket the lateral moraines of the upper Sumaiyar Bar tributary. Opposite the source slope are remnants of a brandung plastered against the valley wall as much as 250 m above the glacier (Fig. 20). However, farther down the valley are discontinuous bouldery remnants as much as 50 m above the main lateral moraine complex. These record a sort of bobsled path, as the debris swung across the contours, from side to side of the twisting valley, and shot up into reentrants.

Figure 20.

Gannish Chissh prehistoric rock avalanche (47 in Fig. 2). A: View to northeast across avalanche-nourished Gannish Chissh Glacier (f), tributary of Barpu Glacier. Rockslide-rock avalanche descended as much as 2000 m to glacier surface from face of mountain (Gannish Chissh, Golden Peak, or Spantik, 7027 m above sea level) above and to right of photograph (a). Most of original deposit has been removed by glacier. Remnants exist of brandung (b) opposite source slope, and raised rim blanketing lateral moraines (c). Rock-avalanche debris is partly covered by later, lateral moraine (d). Remnants of debris that surged up into reentrants are also found (e). Debris had been deflected through almost 180° to travel toward photo station. Behind and to left of this photo station, it was again deflected sharply across valley, as shown in B. Down glacier is to left. B: View in opposite direction from point C (in A), toward photo station in A (S). Rock-avalanche debris was deflected to left by spur at right of photo down steep slope of lateral moraines, which it blankets in foreground (R) and crossed glacier again (yaks for scale). Rock-avalanche debris apparently did not spread over lateral moraines (T) and was completely removed from middle ground by glacier and by avalanching from far valley wall. However, lower in background, debris was deflected back to right side of valley. Its deposits reappear below U, on lateral moraines at exit of this tributary, and for ∼ 1 km down main valley.

Figure 20.

Gannish Chissh prehistoric rock avalanche (47 in Fig. 2). A: View to northeast across avalanche-nourished Gannish Chissh Glacier (f), tributary of Barpu Glacier. Rockslide-rock avalanche descended as much as 2000 m to glacier surface from face of mountain (Gannish Chissh, Golden Peak, or Spantik, 7027 m above sea level) above and to right of photograph (a). Most of original deposit has been removed by glacier. Remnants exist of brandung (b) opposite source slope, and raised rim blanketing lateral moraines (c). Rock-avalanche debris is partly covered by later, lateral moraine (d). Remnants of debris that surged up into reentrants are also found (e). Debris had been deflected through almost 180° to travel toward photo station. Behind and to left of this photo station, it was again deflected sharply across valley, as shown in B. Down glacier is to left. B: View in opposite direction from point C (in A), toward photo station in A (S). Rock-avalanche debris was deflected to left by spur at right of photo down steep slope of lateral moraines, which it blankets in foreground (R) and crossed glacier again (yaks for scale). Rock-avalanche debris apparently did not spread over lateral moraines (T) and was completely removed from middle ground by glacier and by avalanching from far valley wall. However, lower in background, debris was deflected back to right side of valley. Its deposits reappear below U, on lateral moraines at exit of this tributary, and for ∼ 1 km down main valley.

In the upper Yasin Valley, the Dulung Bar event descended the narrow gorge of that name, the mobile offshoot of a much larger rockslide (1 in Fig. 2). Above the floor of the lower Dulung Bar is a trim line of ridges and eroded remnants of rock-avalanche debris, 100–150 m higher than the main deposit, and dipping across the south flank shoulder into the main valley, which the rock avalanche dammed. The Shikarjerab event (35 in Fig. 2) left complex features of this kind where the rock avalanche spread out rapidly after being confined in the narrow Shikarjerab canyon, and before descending to cover the Hunza Valley at Sost. The features are well preserved, the carbonate debris being cemented in place.

Overtopping of interfluves

Another demonstration of the remarkable mobility of rock avalanches involves several Karakoram events that climbed up and over rugged watersheds to enter adjacent valleys. The resulting deposits straddle intervening ridges (Fig. 14, type 5). Of course, whether this happens and to what extent also depends upon the height and steepness of interfluves in the path of the rock avalanche.

The southward-moving debris of the Haldi event, for example, descended first into the lower Saltoro Valley (57 in Fig. 2 and Fig. 21). While the bulk of the material was trapped in the gorge and formed a thick barrier across the river, some of it surged up the opposite wall, overtopping parts of the crest to a height of more than 500 m above the valley floor. Much of this debris continued on down the far slope, but a 5–15-m-thick ridge remains hung up on the interfluve. The origin is confirmed by lithology. The bouldery diamicton and megaclasts draping the interfluve consist of pale gray, often porphyritic, plutonic rocks identical to those forming the main body of the rock avalanche on the north side of the Saltoro River. That is identical with the rock-wall source in the Kande Plutonic Unit of Searle (1991, p. 162). The bedrock of the interfluve, however, consists of steeply dipping (070°) metamorphic and dark intrusive lithologies of the Shyok Melange of Brookfield (1980). A thick tongue of the rock avalanche also moved westward out of the mouth of the Saltoro to cross and block the Hushe Valley. Its distal remnants now survive, separated from the rest of the deposit by the Hushe River.

Figure 21.

Haldi prehistoric rock avalanche event (57 in Fig. 2; cf. Table 2). A: Map based on LANDSAT-satellite imagery and field observations. (Available 1:250 000 topographic map is extremely crude in this area.) B: View northeast toward detachment zone (A) and deposits in proximal zone, including ridges covered by megaclasts (B) and underlain by large, crackle-brecciated units (cf. Fig. 5B). Photo station is on interfluve between Saltoro (C) and Shyok Rivers. Debris (D) in foreground on interfluve is part of rock avalanche and, in places, 500 m+ above river. C: View south down Haldi stream valley toward interfluve that rock avalanche crossed, on opposing south side of Saltoro Valley. Arrows identify ridge of rock-avalanche debris deposited on interfluve above Saltoro River. Arrow at b indicates photo station for A. Haldi Valley is cut through rock-avalanche debris (a-a), where it was moving westward toward Hushe Valley.

Figure 21.

Haldi prehistoric rock avalanche event (57 in Fig. 2; cf. Table 2). A: Map based on LANDSAT-satellite imagery and field observations. (Available 1:250 000 topographic map is extremely crude in this area.) B: View northeast toward detachment zone (A) and deposits in proximal zone, including ridges covered by megaclasts (B) and underlain by large, crackle-brecciated units (cf. Fig. 5B). Photo station is on interfluve between Saltoro (C) and Shyok Rivers. Debris (D) in foreground on interfluve is part of rock avalanche and, in places, 500 m+ above river. C: View south down Haldi stream valley toward interfluve that rock avalanche crossed, on opposing south side of Saltoro Valley. Arrows identify ridge of rock-avalanche debris deposited on interfluve above Saltoro River. Arrow at b indicates photo station for A. Haldi Valley is cut through rock-avalanche debris (a-a), where it was moving westward toward Hushe Valley.

Flow separation and shear zones at depth

Closely associated with an asymmetrical main accumulation, brandung, overtopped interfluves and splitting of debris streams at the surface, are flow separations and shear zones at depth (Fig. 22). These record how, in deeper rock avalanches, the faster moving upper or later arriving materials become detached from earlier or lower material that is slowing or stalled against obstructions. There may be several shear zones separating distinct, thick, superimposed diamicton units. This can be seen exposed on a grand scale in high cliffs cut through the Ganesh-Saukien (45 in Fig. 2), Haldi (57), Gol-Ghoro (64), Satpara Lake (69), Rondu-Mendi (92), and Tsok (71) depositional complexes. They may be compared with better known cases such as the Flims, Switzerland, and Köfels, Austria, events (Heim, 1932; Heuberger et al., 1984).

Figure 22.

Major shear zone at depth in Haldi deposit (57 in Fig. 2). Above and below are units of structureless, coarse, matrix-rich diamicton of crushed porphyritic igneous rock, 15–20 m thick. Irregular shear zone consists of gouge and laminae of rock crushed to fine sand and silt grades. Much of material in shear zone has different color from main body of deposit as well as distinctive texture and structure. Whether that partly reflects debris of another lithology in shear zone, or involves physical, possibly chemical alteration of same bedrock material by extreme frictional heating (cf. Heuberger et al., 1984; Hewitt, 1988), is not yet determined. Note horizontal pen for scale.

Figure 22.

Major shear zone at depth in Haldi deposit (57 in Fig. 2). Above and below are units of structureless, coarse, matrix-rich diamicton of crushed porphyritic igneous rock, 15–20 m thick. Irregular shear zone consists of gouge and laminae of rock crushed to fine sand and silt grades. Much of material in shear zone has different color from main body of deposit as well as distinctive texture and structure. Whether that partly reflects debris of another lithology in shear zone, or involves physical, possibly chemical alteration of same bedrock material by extreme frictional heating (cf. Heuberger et al., 1984; Hewitt, 1988), is not yet determined. Note horizontal pen for scale.

The deepest, most confined layers in these very thick Karakoram rock avalanches, as with the Flims event, display complex shearing, crushing, and jumbled blocks distributed through the mass. There is smearing out of quartz veins and other distinct subunits, multiple microshears, and crackle or jigsaw breccia in units of bedrock. However, close inspection shows that, however severely contorted, sheared, crushed, and attenuated, material of distinct lithologies does not mix (Fig. 23). These products of intense crushing and tectonism deep in rock avalanches may have been mistaken for glaciotectonic features as well, because the exposures in question have been mapped as tills (Norin, 1925; Derbyshire et al., 1984; Searle, 1991).

Figure 23.

View of Gol-Ghoro rock avalanche showing debris exposed at depth, where it encountered steep opposing valley wall (right, foreground). At this point debris was climbing steeply upslope and to right of photograph. While there are occasional intact megaclasts and crackle or jigsaw brecciated bedrock units, >70% of material is crushed to sand- and silt-sized particles. Two main lithologies are present, pale granites with green intrusions. Although thoroughly crushed, contorted, and attenuated, lithologies do not mix (see inset at arrow). Their original units are preserved as movement distorted, remnant stratigraphy. Exposure is in cliff undercut by Indus River (I), 110 m below. In left background, detachment zone (S) is 2.5–3.5 km away and its highest part is ∼1200 m above river level. Highest climb of impact slope was 550 m above and to right of this exposure (see Fig. 18). Exposure of similar but less severely crushed materials, shown at river level in Figure 4B, is 2.5 km upriver and to left of this view. Inset: Detail of crushed bedrock, crackle, and jigsaw breccia at depth in Gol-Ghoro rock-avalanche deposit (see arrow in main figure). Note sharp dividing line between two lithologies present, immediately left of Brunton compass and climbing from left to right of picture. Degree of crushing partly reflects lithology.

Figure 23.

View of Gol-Ghoro rock avalanche showing debris exposed at depth, where it encountered steep opposing valley wall (right, foreground). At this point debris was climbing steeply upslope and to right of photograph. While there are occasional intact megaclasts and crackle or jigsaw brecciated bedrock units, >70% of material is crushed to sand- and silt-sized particles. Two main lithologies are present, pale granites with green intrusions. Although thoroughly crushed, contorted, and attenuated, lithologies do not mix (see inset at arrow). Their original units are preserved as movement distorted, remnant stratigraphy. Exposure is in cliff undercut by Indus River (I), 110 m below. In left background, detachment zone (S) is 2.5–3.5 km away and its highest part is ∼1200 m above river level. Highest climb of impact slope was 550 m above and to right of this exposure (see Fig. 18). Exposure of similar but less severely crushed materials, shown at river level in Figure 4B, is 2.5 km upriver and to left of this view. Inset: Detail of crushed bedrock, crackle, and jigsaw breccia at depth in Gol-Ghoro rock-avalanche deposit (see arrow in main figure). Note sharp dividing line between two lithologies present, immediately left of Brunton compass and climbing from left to right of picture. Degree of crushing partly reflects lithology.

VALLEY SYSTEM RELATIONS AND THE RECONSTRUCTION OF ERODED REMNANTS

Topography not only affects the local behavior of moving debris and the configuration of thickened and separated parts of the debris sheet. If these effects are substantial, then the entire depositional body will have a shape reflecting the movement and emplacement of the rock avalanche in rugged terrain. Its geometry and the organization of morphological and sedimentary features described herein will form a depositional complex shaped by the local valley system. A diverse range of deposit types and their degrees of complexity in the Karakoram reflect these relations between each rock-avalanche event and valley layout. Relations to terrain shown in Figure 14 must be extended to encompass local valley systems (Fig. 24).

Figure 24.

Types of valley system or valley junction relations of rock-avalanche deposits. Main Valley T-shapes (cf. Nicoletti and Sorriso-Valvo, 1991; numbers refer to Fig. 2): I, Main valley source and cross-valley deposit with upvalley and down-valley lobes (e.g., Ghoro Choh I, 79); II, Tributary valley source, main valley T-deposit (Dulung Bar, 1; Tsok, 71). Triple lobe types: III, Main valley source with upvalley and downvalley lobes, plus lobe entering and plugging tributary valley (Jullah, 70; Katzarah, 86); IV, Tributary valley source and upvalley lobe, plus main valley T shape (cf. Satpara, 69; Fig. 15A). Multiple valley junction and/or lobe type example: V, Tributary valley source, deposit at junction of three other valleys (Gannish-Saukien, 45; Bordon Tir-Sost, 34). Multiple lobes with overtopping of interfluve example: VI, Tributary valley source with upvalley and downvalley lobes and main valley lobe from overtopping (e.g., Hum Bluk, 51; cf. Haldi, Fig. 21).

Figure 24.

Types of valley system or valley junction relations of rock-avalanche deposits. Main Valley T-shapes (cf. Nicoletti and Sorriso-Valvo, 1991; numbers refer to Fig. 2): I, Main valley source and cross-valley deposit with upvalley and down-valley lobes (e.g., Ghoro Choh I, 79); II, Tributary valley source, main valley T-deposit (Dulung Bar, 1; Tsok, 71). Triple lobe types: III, Main valley source with upvalley and downvalley lobes, plus lobe entering and plugging tributary valley (Jullah, 70; Katzarah, 86); IV, Tributary valley source and upvalley lobe, plus main valley T shape (cf. Satpara, 69; Fig. 15A). Multiple valley junction and/or lobe type example: V, Tributary valley source, deposit at junction of three other valleys (Gannish-Saukien, 45; Bordon Tir-Sost, 34). Multiple lobes with overtopping of interfluve example: VI, Tributary valley source with upvalley and downvalley lobes and main valley lobe from overtopping (e.g., Hum Bluk, 51; cf. Haldi, Fig. 21).

Valley Junction phenomena

Nearly all the Karakoram events in Figure 2 split into debris lobes moving upvalley and downvalley (Fig. 24, type I). Many were not confined to a single valley. They moved into one or more valleys, other than that from whose walls they were detached. At least 44 that originated in a tributary valley, often a hanging valley, emplaced the larger part of the deposits across a main valley (types II and IV in Fig. 24). Here most of them split into upvalley and downvalley lobes, often for the second time. Conversely, 18 events that derived from the walls of a main valley sealed or traveled up into one or more tributary valleys (type III). At least 14, whether derived from the walls of a tributary or main valley, were emplaced in and around the junctions of three or more valleys (type V). Where debris crossed an interfluve, further complications arose (type VI).

Individual cases involve various permutations and combinations of these emplacement relations. The schematic diagrams hardly represent the actual variety of splayed plan forms or the many minor lobes and variable patterns of ridges and depressions that occur. They only hint at the variable thickness of the debris sheets, which is partly due to infilling of preexisting topographic depressions and partly to thickening in pressure ridges or raised rims (cf. Mudge, 1965). Such complexities apply to all examples that resemble the deformed T shape of Nicoletti and Sorisso-Valvo (1991).

Nevertheless, these are complications that occur in sedimentary bodies that have a large-scale unity of emplacement. Each depositional complex records a single catastrophic event that cannot have lasted more than two to three minutes, and a composition due to a singular and unitary process, the rock avalanche. While deposit morphology and facies are modified by interactions with terrain and substrate, these occur as predictable responses of the mobile debris. This assumes even greater significance when we consider the actual state in which the deposits are found.

Remnant forms and their identification

In the Karakoram, few of the rock-avalanche deposits are extensively preserved, which may help explain why they went undetected for so long. Most survive as scattered remnants, isolated on opposite sides of valleys, in different valleys, or protruding from later deposits. The difficulties arising from this situation are further complicated because deposits were often emplaced many kilometers from, and out of sight of, the rock-wall source. Topography-related features then assume unique importance in the record of events. They are most likely to survive erosion and burial, or to be most evident in the landscape amid what remains. Boulder-covered mounds and ridge forms, areas where the deposits were thickened by substrate conditions or against opposing valley walls, most often provide the first clues to a possible rock-avalanche deposit. Initial recognition of such features and how they might relate to a rockslide-rock-avalanche event, is essential to the reconstruction of events and interpretation of depositional complexes.

Equally relevant are relations of the emplaced deposits to their landscape setting and subsequent geomorphic developments at the rock-avalanche sites. Deposits in the proximal region are less likely to survive or to be exposed because detachment zones and steep transport slopes, to judge from more recent examples, remain sites of frequent rock falls and debris flows for decades, perhaps centuries after initial failure. The detachment scar of the rock avalanche often is among an array of steep rock walls from which rockfalls and rockslides repeatedly descend in summer and snow avalanches in winter and spring. Rock-avalanche barriers are most often breached and removed first in the proximal zone. Enduring remnants are more likely to be found along opposing valley flanks and in the distal regions.

Where rock avalanches spread over valley floors, sedimentladen rivers caused early burial of the deposits when impounded, and their removal with later breaching. Among the Karakoram rock avalanches identified thus far, 89 straddle valley floors and interfere with the Indus streams. More than 40 have impounded large lakes in which deep, multilayered lacustrine sediments accumulated against the barriers and for tens of kilometers upvalley. A large number of these landslide dams remain only partially breached today, and hold back extensive river flats. As a result, much of the upvalley rock-avalanche deposits, below the height of overtopping of the barrier, have been buried beneath lake and flood-plain sediments. At most, a few bouldery islands and ridges stick out to hint at the former extent of the rock avalanche (Fig. 16A). They also tend to be features that resulted from topographic or substrate interference. Meanwhile, the abundant sand and dust blowing from the river flats and arid slopes has buried large areas of the rock-avalanche deposits in dunes and blankets of dust. Pressure ridges and mounds, or megaclasts that cap them, rise out of the eolian deposits to signal the presence of the rock-avalanche deposit (e.g. 22, 27, 45, 68–70, 77, and 85 in Fig. 2).

Once the rivers breach the barriers, there is segmenting and trenching of the rock-avalanche deposits, and of the slack-water alluvium and sediment fans built up behind the barrier. The impact-thickened parts of the rock avalanche tend to resist erosion longer. As pressure-induced forms, they are not only thicker, but probably the more densely compacted and toughest diamictons.

When rock avalanches fall upon glaciers, the survival of their deposits and forms of depositional remnants become even more problematic. The surging of the Bualtar Glacier following the rock avalanches of 1986 (49 in Fig. 2) led to rapid breakup and reworking of the on-ice debris (Gardner and Hewitt, 1991). Ten years later, the remains of the rock-avalanche debris, carried 4 km farther down the glacier, can still be discerned, but only if one knows where to look. The remnants might also be taken for part of the heavy supraglacial debris covers common to Karakoram glaciers. Reworking of the debris associated with often high but very variable rates of ablation, abundant melt-water, and irregular, heavily crevassed ice, has ensured that the debris retains few of the distinctive sedimentary features of rock-avalanche deposits. Soon, only opposing slope and valley-side depositional remnants above the ice will survive from the rock avalanche, much as in the Gannissh Chissh event (Fig. 20). However, once the possibility and nature of topography-related forms and erosional remnants is recognized, then other criteria can be employed for identifying their materials (Appendix 1). Then, even the considerably destroyed depositional complex may be reconstructed, much as with examples from the more distant geological past (e.g., Krieger, 1977; Yarnold, 1993).

CONCLUSION

Rugged terrain is a large factor in the forms and structures of rock-avalanche deposits throughout the world's high mountains. Steering and splitting of the dry rock streams by irregular topography leave complicated plan forms and systems of pressure ridges. Steep, opposing slopes affect the final configuration of the depositional mass, creating asymmetrically thickened and ridged forms. At a more local level, distinctive surface morphologies and internal tectonic and facies characteristics are associated with topographic interference.

In the Karakoram, such phenomena are seen in almost all examples and appear characteristic rather than incidental. However, they do complicate the interpretation or, at least, initial recognition, of prehistoric events. The problem is aggravated by relatively rapid, often substantial erosion and burial, when features associated with topographic and substrate interference tend to survive the longest, or to be more readily found. Rather than the well-known, boulder-covered, sheet-like deposit exhibiting lobate forms with minor longitudinal, transverse, and distal ridges, one first encounters huge, isolated ridges or locally thickened, hummocky remnants and high vertical cliffs of rock-avalanche debris exposed by erosion. These usually are cut off from, or have no immediately obvious relation to, their distant rock-wall source. The main body of the rock-avalanche deposit may be in one or more valleys, other than the one where the event originated. It may be severed from the latter by subsequent erosion and burial of intervening sections.

Other complex forms of deposit are observed in certain Karakoram examples where parts of the substrate and obstructions were deformed, eroded, and entrained by the rock avalanche. Although not dealt with herein, they involve striking and distinctive deposits, and are found in some of the more densely inhabited and frequently described valleys, notably the Skardu-Shigar Basins of Baltistan (65–70, 79–85 in Fig. 2). In general, awareness of these various forms relating to topography and substrate is essential for initial field identification, and to differentiate rock-avalanche deposits from the many other and varied coarse fragmentites of the region.

As in other mountain lands subject to rapid economic development and social changes, the ability to locate and estimate the incidence of rock avalanches is a matter of some practical urgency (Shroder, 1989; Evans and Savigny, 1994). A majority of the Karakoram rock-avalanche deposits described herein have permanent settlements located on or near them. The river flats, terraces, and sediment fans backed up behind the barriers they formed, are the principal areas of agricultural land and sites of airports, tourist destinations, military bases, and communications routes in much of the region. Dozens of villages and some major centers, such as Khapalu, Skardu, Gupis, Sost, and Karimabad-Ganesh, are sited amid the rubble of ancient rock avalanches (58–68, 17, 34–35, 45 in Fig. 2).

In these ways, perhaps, the catastrophic landslides of the Karakoram have been a great benefit to human settlement. They have created depositional features favorable to human use in an environment of extreme rates of erosion and steep slopes. The possibility that rock avalanches are not just a thing of the past, or of today's high glacier valleys, needs to be addressed. The hazards from frequent mass movements, especially along the road system, have received detailed study (Jones et al., 1983; Kreutzmann, 1994). However, few of the rare, catastrophic rockslides of the past had been recognized. The fact that so many of the rock avalanches have been confused with other processes aggravates this problem. Moreover, most of the rock-avalanche deposits identified so far were emplaced across ice-free valley floors, so that they relate to postglacial and, most likely, to Holocene rather than to Pleistocene conditions. There are few reliable dates for the glacial or rock-avalanche deposits. Thus, questions of chronology and the actual timing and frequency of rock avalanches remain unanswered. This adds to a certain sense of urgency about the need for an assessment of catastrophic landslide hazards, at least in the inhabited areas of the Karakoram. An important conclusion here is that an understanding of topography-constrained styles of rock avalanche and field features associated with the deposits is essential for estimating future risks as well as in recognizing past occurrences.

APPENDIX 1. SEDIMENTARY AND RELATED CHARACTERISTICS OF ROCK AVALANCHE DEPOSITS

Following is a checklist of characteristics that have proved useful in field identification and in differentiating rock avalanche deposits from other diamictons or “coarse fragmentites.” Established but specialized terms applied to these sediments are underlined. Terms from the literature that are not well established or of vernacular type are in double quotes; others, specifically coined or adopted here, are in single quotes. The general organization by sedimentary properties is based upon Pettijohn (1975) and modified, where more appropriate to these coarse “fragmentites” or “rudrocks,” after Laznicka (1986). The numbered citations are: (1) Heim, 1932; (2) Laznicka, 1988; (3) Krieger, 1977; (4) Mudge, 1965; (5) Rouse, 1984; (6) Hutchinson, 1988; (7) Yarnold and Lombard, 1989; (8) Hsü, 1978; (9) Norton, 1917; (10) Longwell, 1951; (11) Johnson, 1978; and (12) Kerr, 1984. Features relating to topographical controls are not included, but are the main subject of the text.

GENERAL
Originrock avalanche (= sturzstrom) event (1)
Provenancelocal bedrock (rarely <1 or >20 km radius from deposit rim)
Emplacementtransported; sudden, frictional “freezing” (1,8), usually in a dry state; large-scale ‘event-emplacement’ unity
Rock classsedimentary “rudaggregates” or “rudrocks” (2)
Type of materialfragmental, coarse clastic cataclastic, “crush” breccia/rubble consisting of fracture- (compressional), shatter- (impact), and abrasion-generated clasts (9)
Type of sedimentcoarse fragmentite (2)
Unconsolidatedrubble or diamicton
Consolidatedbreccia or diamictite commonlv megabreccias or ‘megadiamicts’ (?) (i.e., larger clasts >1 m diam.)
Constituents
Interfragmental fillingmatrix of sand-sized grades and smaller (2), sometimes cemented (= breccia or diamictite)
Matrix proportionfrom (rarely) <5% to (rarely) >50%
SEDIMENTARY BODYcompact, continuous, unified
Geometry (3, 4, 7)
overalllobate or tongue-like
thin compared to areal extent
local sectionstabular or lens-like (pressure and impact thickening may occur; see text)
Magnitude (4, 5, 6)
Volumeminimum 1.5 × 106 m3; may exceed 1000 × 106 m3
areal extentrarely <2 km2, may exceed 40 km2
thicknessmostly 2–10 m, (may exceed 150 m, usually due to topographic or substrate interference; see text)
Contactsgenerally sharp with surroundings and substrate. May incorporate erodable substrate material in basal zone and distal regions (see below)
TEXTUREcoarse (dominant mode)
Grain sizefrom (usually minor) clay-size to megaclasts (>1m diam.); “sandy-silty-rubble or breccia” (2)
Largest clasts(usually) >1 m, (often) >10m, (sometimes) >60m diam.
Fragment shape
roundnessangular or very angular, (sharpstone, chinkstone) usually some lozenge-shaped (2, 10)
sphericityhighly variable (seems lithology-dependent, but compact forms tend to dominate except in surface rubble)
Fragment textureroughened, no polishing or striations, (rarely) crude scratches, percussion scars, may be “snubbing” of edges in softer lithologies
Sortingvery poorly sorted, very “immature”*
Porosity
main bodylow, impermeable
surface rubblehighly porous, ‘openwork’
LITHOLOGYallogenic, detrital materials
Provenancelocal bedrock (rarely <1 or >20 km radius from rim of deposit), exact or very close identity with parent rock
Fragmentsmostly lithic, mineral grains may form finer matrix
Composition
At a sample sitesingle rock type (= monolithologic, monomictic, homolithologic)
may be traces of exotic materials in fines
Between sitesmay involve several lithologies arranged in distinct bands transverse to movement/emplacement
Matrix/fragments relationexact correspondence (i.e., same composition and generally same colour)
FACIES/ARCHITECTUREmassive, structureless (2)
Frameworkinterlocking coarse clasts, tightly interlocking matrix grains, clast-supported, sometimes matrix-supported
Packing
main bodytight (overconsolidated)
surfaceopenwork rubble
Facies subsequence (7)
(i) mixed basal layerwith material entrained from substrate in “stringers” and dikes injected into rock avalanche or carried upward along shear planes or in load structures (11)
(ii) main bodystructureless, homogeneous, perhaps crude upward coarsening
(iii) surfaceopenwork rubble with (some) distinctive fabric/architecture (see below)
(iv) horizontallittle or no variation, except in relative
organizationthickness or development of subsequence elements, unless there is strong topographic or substrate interference (see text)
(v) distal rimoften raised, thickened, often increased shearing and deformation of substrate (3, 12)
(vi) substratemay be eroded, deformed, or scraped up and carried as transported basal layer or distal unit (7, 11)
Lithology relationsuniform, or “in bands” representing remnant stratigraphy from bedrock source (1)
Fabric (in surface rubble)(i) largest clasts oriented with or transverse to movement Direction; (ii) appositional or imbricated large clasts: and (iii) “jostling” (2) and ‘final moment’ torque, imbrication, and impact fracturing of megaclasts (i.e., during the rapid “accelerated deceleration” and sudden “frictional freezing” of the main body, (1, 8)
Special featurescrackle, jigsaw, and mosaic breccia (2, 7)
GENERAL
Originrock avalanche (= sturzstrom) event (1)
Provenancelocal bedrock (rarely <1 or >20 km radius from deposit rim)
Emplacementtransported; sudden, frictional “freezing” (1,8), usually in a dry state; large-scale ‘event-emplacement’ unity
Rock classsedimentary “rudaggregates” or “rudrocks” (2)
Type of materialfragmental, coarse clastic cataclastic, “crush” breccia/rubble consisting of fracture- (compressional), shatter- (impact), and abrasion-generated clasts (9)
Type of sedimentcoarse fragmentite (2)
Unconsolidatedrubble or diamicton
Consolidatedbreccia or diamictite commonlv megabreccias or ‘megadiamicts’ (?) (i.e., larger clasts >1 m diam.)
Constituents
Interfragmental fillingmatrix of sand-sized grades and smaller (2), sometimes cemented (= breccia or diamictite)
Matrix proportionfrom (rarely) <5% to (rarely) >50%
SEDIMENTARY BODYcompact, continuous, unified
Geometry (3, 4, 7)
overalllobate or tongue-like
thin compared to areal extent
local sectionstabular or lens-like (pressure and impact thickening may occur; see text)
Magnitude (4, 5, 6)
Volumeminimum 1.5 × 106 m3; may exceed 1000 × 106 m3
areal extentrarely <2 km2, may exceed 40 km2
thicknessmostly 2–10 m, (may exceed 150 m, usually due to topographic or substrate interference; see text)
Contactsgenerally sharp with surroundings and substrate. May incorporate erodable substrate material in basal zone and distal regions (see below)
TEXTUREcoarse (dominant mode)
Grain sizefrom (usually minor) clay-size to megaclasts (>1m diam.); “sandy-silty-rubble or breccia” (2)
Largest clasts(usually) >1 m, (often) >10m, (sometimes) >60m diam.
Fragment shape
roundnessangular or very angular, (sharpstone, chinkstone) usually some lozenge-shaped (2, 10)
sphericityhighly variable (seems lithology-dependent, but compact forms tend to dominate except in surface rubble)
Fragment textureroughened, no polishing or striations, (rarely) crude scratches, percussion scars, may be “snubbing” of edges in softer lithologies
Sortingvery poorly sorted, very “immature”*
Porosity
main bodylow, impermeable
surface rubblehighly porous, ‘openwork’
LITHOLOGYallogenic, detrital materials
Provenancelocal bedrock (rarely <1 or >20 km radius from rim of deposit), exact or very close identity with parent rock
Fragmentsmostly lithic, mineral grains may form finer matrix
Composition
At a sample sitesingle rock type (= monolithologic, monomictic, homolithologic)
may be traces of exotic materials in fines
Between sitesmay involve several lithologies arranged in distinct bands transverse to movement/emplacement
Matrix/fragments relationexact correspondence (i.e., same composition and generally same colour)
FACIES/ARCHITECTUREmassive, structureless (2)
Frameworkinterlocking coarse clasts, tightly interlocking matrix grains, clast-supported, sometimes matrix-supported
Packing
main bodytight (overconsolidated)
surfaceopenwork rubble
Facies subsequence (7)
(i) mixed basal layerwith material entrained from substrate in “stringers” and dikes injected into rock avalanche or carried upward along shear planes or in load structures (11)
(ii) main bodystructureless, homogeneous, perhaps crude upward coarsening
(iii) surfaceopenwork rubble with (some) distinctive fabric/architecture (see below)
(iv) horizontallittle or no variation, except in relative
organizationthickness or development of subsequence elements, unless there is strong topographic or substrate interference (see text)
(v) distal rimoften raised, thickened, often increased shearing and deformation of substrate (3, 12)
(vi) substratemay be eroded, deformed, or scraped up and carried as transported basal layer or distal unit (7, 11)
Lithology relationsuniform, or “in bands” representing remnant stratigraphy from bedrock source (1)
Fabric (in surface rubble)(i) largest clasts oriented with or transverse to movement Direction; (ii) appositional or imbricated large clasts: and (iii) “jostling” (2) and ‘final moment’ torque, imbrication, and impact fracturing of megaclasts (i.e., during the rapid “accelerated deceleration” and sudden “frictional freezing” of the main body, (1, 8)
Special featurescrackle, jigsaw, and mosaic breccia (2, 7)

*The notion of ‘maturity’ hardly has meaning or relevance in this process and type of sediment.

But in an opposite sense to ‘supermature’ fluvial and beach gravels—i.e., single rock type not of residual minerals, but ‘whole-rock’ and mineral fragments of parent rock.

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Acknowledgments

Parts of the research on which this paper is based were funded by the International Development Research Centre, Ottawa, and the Office of Research, Wilfrid Laurier University, Waterloo. I thank officers of the Water and Power Development Authority, Pakistan, and local guides from the Northern Areas for assistance in the field, Barbara W. Murck, University of Toronto, for petrographic analysis of rock types mentioned in the text, Pam Schaus for preparing the figures, Sally Gray for preparing the text, and the volume referees and editors for many helpful suggestions.

Figures & Tables

Figure 1.

Location and regional geology of Karakoram Himalaya. Geological divisions, structures, and nomenclature are after Searle (1991).

Figure 1.

Location and regional geology of Karakoram Himalaya. Geological divisions, structures, and nomenclature are after Searle (1991).

Figure 2.

Locations of rockslide-rock avalanches identified in Karakoram Himalaya. Numbers identify sites of rock-avalanche deposits and shaded areas show valleys along which surveys were carried out. In some cases site is location of more than one rock avalanche. Total of 107 events had been identified by 1995. Structures and nomenclature are after Searle (1991).

Figure 2.

Locations of rockslide-rock avalanches identified in Karakoram Himalaya. Numbers identify sites of rock-avalanche deposits and shaded areas show valleys along which surveys were carried out. In some cases site is location of more than one rock avalanche. Total of 107 events had been identified by 1995. Structures and nomenclature are after Searle (1991).

Figure 3.

Examples of bouldery, openwork surface of rock avalanches with megaclasts. A: 1986 Bualtar Glacier event (50 in Fig. 2), showing an area of deposits ∼500 m from distal rim. Material in this region is broken and crushed marble bedrock. Some crude imbrication of large clasts is apparent. Largest boulder, with person on top, has volume of ∼5200 m3, and a longest axis of 32 m oriented in direction of movement from left to right of photograph. Rock avalanche had traveled 3.5 km horizontally and 1400 m vertically at this point (see Hewitt, 1988). B: View over main accumulation and distal region of Bhurt, Ishkoman Valley rock avalanche, northwest of Gilgit (13 in Fig. 2). View is from east flank of Ishkoman Valley looking upstream. Terminal moraines of Karambar Glacier are in distance (K). Debris came from left of photograph and detached from rock wall in intrusive body, exposure of Karakoram Batholith, on west side of Ishkoman gorge (cf. Fig. 1). Central portion of rock avalanche crossed and (formerly) dammed Ishkoman River at Bhurt village. Debris still forms barrier across mouth of Bhurt Glacier valley to right of photograph whose stream crosses deposit in foreground. Largest of blocks (A) has volume of almost 27 000 m3 and longest axis of 44 m and had been transported ∼4.5 km horizontally and 1000 m vertically.

Figure 3.

Examples of bouldery, openwork surface of rock avalanches with megaclasts. A: 1986 Bualtar Glacier event (50 in Fig. 2), showing an area of deposits ∼500 m from distal rim. Material in this region is broken and crushed marble bedrock. Some crude imbrication of large clasts is apparent. Largest boulder, with person on top, has volume of ∼5200 m3, and a longest axis of 32 m oriented in direction of movement from left to right of photograph. Rock avalanche had traveled 3.5 km horizontally and 1400 m vertically at this point (see Hewitt, 1988). B: View over main accumulation and distal region of Bhurt, Ishkoman Valley rock avalanche, northwest of Gilgit (13 in Fig. 2). View is from east flank of Ishkoman Valley looking upstream. Terminal moraines of Karambar Glacier are in distance (K). Debris came from left of photograph and detached from rock wall in intrusive body, exposure of Karakoram Batholith, on west side of Ishkoman gorge (cf. Fig. 1). Central portion of rock avalanche crossed and (formerly) dammed Ishkoman River at Bhurt village. Debris still forms barrier across mouth of Bhurt Glacier valley to right of photograph whose stream crosses deposit in foreground. Largest of blocks (A) has volume of almost 27 000 m3 and longest axis of 44 m and had been transported ∼4.5 km horizontally and 1000 m vertically.

Figure 4.

A: Vertical cross section in medial right flank region of Bualtar Glacier rock avalanches (50 in Fig. 2; cf. Fig. 3), exposed within 1 yr of event by breakup of glacier. Main body is matrix-rich (∼33% mass), but mostly clast-supported diamicton of crushed and pulverized marble. Note person for scale. B: Erosional remnant from upper 10 m of Gol-Ghoro rock-avalanche deposit (64 in Fig. 2). Angular coarse clasts and clast-supported, matrix-rich (∼20% coarse sand and finer material) rubbly diamicton are shown. Location is 5 km from source in lobe that moved upstream along Indus gorge (cf. Fig. 18). Coarse surface rubble and surrounding material have been removed by river. Note 1 m staff for scale.

Figure 4.

A: Vertical cross section in medial right flank region of Bualtar Glacier rock avalanches (50 in Fig. 2; cf. Fig. 3), exposed within 1 yr of event by breakup of glacier. Main body is matrix-rich (∼33% mass), but mostly clast-supported diamicton of crushed and pulverized marble. Note person for scale. B: Erosional remnant from upper 10 m of Gol-Ghoro rock-avalanche deposit (64 in Fig. 2). Angular coarse clasts and clast-supported, matrix-rich (∼20% coarse sand and finer material) rubbly diamicton are shown. Location is 5 km from source in lobe that moved upstream along Indus gorge (cf. Fig. 18). Coarse surface rubble and surrounding material have been removed by river. Note 1 m staff for scale.

Figure 5.

A: Individual boulders with crackle and jigsaw breccia. This matrix-poor (<12% sand grades and finer), clast-supported diamicton has been exposed by erosion in medial region of Ghoro Choh 1 rock avalanche (79 in Fig. 2). Material is 100% greenish tonalite and is ∼8 m below rock-avalanche surface and 2 km from its source on west wall of Shigar Valley. Depth of deposit is not known. Brunton compass for scale. B: Large crackle-brecciated unit of transported granitic bedrock is in medial zone of Haldi deposit, where Saltoro River has cut through (57 in Fig. 2). Entire exposure of unit is not shown, but it is ∼150 m wide and 45 m deep and is more than 100 m above base of rock-avalanche deposit. Unit is surrounded, above and beneath, by thoroughly broken, crushed, and pulverized diamicton material. It was carried ∼2.5 km. Person at left for scale. (Another perspective on exposure is in Figure 21B, above C, and to right side of B).

Figure 5.

A: Individual boulders with crackle and jigsaw breccia. This matrix-poor (<12% sand grades and finer), clast-supported diamicton has been exposed by erosion in medial region of Ghoro Choh 1 rock avalanche (79 in Fig. 2). Material is 100% greenish tonalite and is ∼8 m below rock-avalanche surface and 2 km from its source on west wall of Shigar Valley. Depth of deposit is not known. Brunton compass for scale. B: Large crackle-brecciated unit of transported granitic bedrock is in medial zone of Haldi deposit, where Saltoro River has cut through (57 in Fig. 2). Entire exposure of unit is not shown, but it is ∼150 m wide and 45 m deep and is more than 100 m above base of rock-avalanche deposit. Unit is surrounded, above and beneath, by thoroughly broken, crushed, and pulverized diamicton material. It was carried ∼2.5 km. Person at left for scale. (Another perspective on exposure is in Figure 21B, above C, and to right side of B).

Figure 6.

Lithological banding in rock-avalanche debris. Outer lobe of Bualtar Glacier event (50 in Fig. 2) looking across and down glacier shows bands of distinctive lithology, arranged concentrically at right angles to movement (cf. Fig. 3A). Each band represents unit in original bedrock, fractured, crushed, and attenuated by catastrophic descent, but preserved as remnant stratigraphy. Distal rim is composed of light gray marble. Inner areas are bands of other carbonates, dark metamorphic lithologies, or minor intrusives. Rock avalanche traveled from left to right down valley, but has also been disturbed by surge of glacier (cf. Hewitt. 1988). This phenomenon is seen in many Karakoram rock-avalanche deposits, but scale of bands, erosion, or less contrasted colors, usually makes it difficult to show in photograph.

Figure 6.

Lithological banding in rock-avalanche debris. Outer lobe of Bualtar Glacier event (50 in Fig. 2) looking across and down glacier shows bands of distinctive lithology, arranged concentrically at right angles to movement (cf. Fig. 3A). Each band represents unit in original bedrock, fractured, crushed, and attenuated by catastrophic descent, but preserved as remnant stratigraphy. Distal rim is composed of light gray marble. Inner areas are bands of other carbonates, dark metamorphic lithologies, or minor intrusives. Rock avalanche traveled from left to right down valley, but has also been disturbed by surge of glacier (cf. Hewitt. 1988). This phenomenon is seen in many Karakoram rock-avalanche deposits, but scale of bands, erosion, or less contrasted colors, usually makes it difficult to show in photograph.

Figure 7.

Complex basal zone facies of Gol-Ghoro event (64 in Fig. 2) exposed by undercutting at present-day river level (cf. Fig. 23). Shown are remnant units of original bedrock as crushed, attenuated, and contorted rock-avalanche debris, and dikes of alluvium (d) intruded from substrate and carrying river-rounded clasts (arrow) (cf. Yarnold, 1993, p. 355). Direction of rock-avalanche movement was left to right. Note 1 m staff for scale.

Figure 7.

Complex basal zone facies of Gol-Ghoro event (64 in Fig. 2) exposed by undercutting at present-day river level (cf. Fig. 23). Shown are remnant units of original bedrock as crushed, attenuated, and contorted rock-avalanche debris, and dikes of alluvium (d) intruded from substrate and carrying river-rounded clasts (arrow) (cf. Yarnold, 1993, p. 355). Direction of rock-avalanche movement was left to right. Note 1 m staff for scale.

Figure 8.

Yarbah Tsoh rock avalanche (73, Fig. 2) swept into and across Shigar Valley, partly slicing through, partly transporting lacustrine and alluvial material on valley floor. Rock avalanche boulders and diamicton units are seen beneath (a) and within (b) large volumes of alluvium, while an imbricated stack of upended boulders occurs on top (c). Debris moved from left to right of photograph. Alluvial material shows severe contorting (d), shearing (e), and detachment of individual blocks of sediment, resulting from transport across valley. Person for scale (vertical arrow).

Figure 8.

Yarbah Tsoh rock avalanche (73, Fig. 2) swept into and across Shigar Valley, partly slicing through, partly transporting lacustrine and alluvial material on valley floor. Rock avalanche boulders and diamicton units are seen beneath (a) and within (b) large volumes of alluvium, while an imbricated stack of upended boulders occurs on top (c). Debris moved from left to right of photograph. Alluvial material shows severe contorting (d), shearing (e), and detachment of individual blocks of sediment, resulting from transport across valley. Person for scale (vertical arrow).

Figure 9.

Schematic longitudinal cross section of rockslide-rock avalanche in mountain valley, to illustrate features referred to in text and certain measures and terminology used to describe them. H is vertical distance between highest point of detachment zone and lowest reach of debris; Hr is runup height to raised distal rim; and L is maximum horizontal travel. Fahrböschung, or travel slope angle, is angle from highest point in detachment zone to furthest travel distance, tangent of which gives equivalent coefficient of friction (Hsii, 1975).

Figure 9.

Schematic longitudinal cross section of rockslide-rock avalanche in mountain valley, to illustrate features referred to in text and certain measures and terminology used to describe them. H is vertical distance between highest point of detachment zone and lowest reach of debris; Hr is runup height to raised distal rim; and L is maximum horizontal travel. Fahrböschung, or travel slope angle, is angle from highest point in detachment zone to furthest travel distance, tangent of which gives equivalent coefficient of friction (Hsii, 1975).

Figure 10.

Area and volume dimensions of Karakoram rock-avalanche deposits. A: Estimated original areas of deposits for 51 events. B: Estimated original volume of deposits for 56 events.

Figure 10.

Area and volume dimensions of Karakoram rock-avalanche deposits. A: Estimated original areas of deposits for 51 events. B: Estimated original volume of deposits for 56 events.

Figure 11.

Relations of mobility for selected Karakoram rock avalanches, where dimensions could be estimated. A: Ratio of maximum horizontal travel to estimated volume for 20 events. B: Ratio between maximum vertical descent (H) and maximum horizontal travel (L) for 20 events (cf. Fig. 9).

Figure 11.

Relations of mobility for selected Karakoram rock avalanches, where dimensions could be estimated. A: Ratio of maximum horizontal travel to estimated volume for 20 events. B: Ratio between maximum vertical descent (H) and maximum horizontal travel (L) for 20 events (cf. Fig. 9).

Figure 12.

Plan forms of rock-avalanche deposits in which there was minimal interference by topography or mobile substrates (A) after Johnson, 1978; (B) Cruden, 1982; (C) after Cruden and Beaty (no date) (Fig. 1); (D) after Eisbacher and Clague, 1984; (E) this paper (Fig. 13).

Figure 12.

Plan forms of rock-avalanche deposits in which there was minimal interference by topography or mobile substrates (A) after Johnson, 1978; (B) Cruden, 1982; (C) after Cruden and Beaty (no date) (Fig. 1); (D) after Eisbacher and Clague, 1984; (E) this paper (Fig. 13).

Figure 13.

Chillinji Glacier rock avalanche of 1991 (10 in Fig. 2), seen 1 yr later. It shows typical tongue-like, thin sheet of debris, raised distal rims, and minor lobes. Rock-wall source is to left of photograph. ∼500–700 m above ice surface.

Figure 13.

Chillinji Glacier rock avalanche of 1991 (10 in Fig. 2), seen 1 yr later. It shows typical tongue-like, thin sheet of debris, raised distal rims, and minor lobes. Rock-wall source is to left of photograph. ∼500–700 m above ice surface.

Figure 14.

Schematic long and cross sections, and plan forms of cross-valley rock-avalanche deposits. Emphasis is placed on geometric relations of debris emplacement to opposing slopes. Terms are discussed in text. In types 1, 2, and 4, subtypes are indicated where debris, initially confined in tributary chutes or canyons, is emplaced in main valley.

Figure 14.

Schematic long and cross sections, and plan forms of cross-valley rock-avalanche deposits. Emphasis is placed on geometric relations of debris emplacement to opposing slopes. Terms are discussed in text. In types 1, 2, and 4, subtypes are indicated where debris, initially confined in tributary chutes or canyons, is emplaced in main valley.

Figure 15.

Satpara Lake rock avalanche, Skardu Basin, Baltistan (#69 in Fig. 2; cf. Table 2). A: Aerial view looking southward up Satpara Valley from above Skardu Basin. The rock avalanche descended from a rock wall to left of photograph, and surged up opposing slope to emplace well-defined main deposit (M) with higher crown (C). Satpara Lake (S) was dammed and rose at least 50 m higher in early phase of impoundment. Satpara stream, lower left, has breached the barrier and cuts down ∼110 m through the proximal part of the rock avalanche deposit. Arrow is station for photo B. B. View over the main accumulation from the crown toward the detachment zone (B), the highest part of which is ∼950 m above Satpara stream and 1000 m below the peak, which is 4798 m above sea level. The surface rubble in the foreground consists entirely of intrusive igneous rock debris of quartz diorite or tonalite composition. Boulders and underlying diamictons in the low, proximal region (E), and deflected up and down valley, consist of a metasedi-mentary graywacke. Debris deflected around the main accumulation (right side of photo = left flank of rock avalanche) traveled several kilometers up Satpara Valley. Most of the debris is drowned by the lake (D), except for a small, boulder-covered island. Substantial debris streams were also deflected to the left part of photo and down the fan into Skardu Basin, where they reached the present course of the Indus River, 9 km from the source (A). Beside the detachment zone is a large block (C). apparently part of the original rockslide, that became wedged in the bottom of the gorge without breaking up.

Figure 15.

Satpara Lake rock avalanche, Skardu Basin, Baltistan (#69 in Fig. 2; cf. Table 2). A: Aerial view looking southward up Satpara Valley from above Skardu Basin. The rock avalanche descended from a rock wall to left of photograph, and surged up opposing slope to emplace well-defined main deposit (M) with higher crown (C). Satpara Lake (S) was dammed and rose at least 50 m higher in early phase of impoundment. Satpara stream, lower left, has breached the barrier and cuts down ∼110 m through the proximal part of the rock avalanche deposit. Arrow is station for photo B. B. View over the main accumulation from the crown toward the detachment zone (B), the highest part of which is ∼950 m above Satpara stream and 1000 m below the peak, which is 4798 m above sea level. The surface rubble in the foreground consists entirely of intrusive igneous rock debris of quartz diorite or tonalite composition. Boulders and underlying diamictons in the low, proximal region (E), and deflected up and down valley, consist of a metasedi-mentary graywacke. Debris deflected around the main accumulation (right side of photo = left flank of rock avalanche) traveled several kilometers up Satpara Valley. Most of the debris is drowned by the lake (D), except for a small, boulder-covered island. Substantial debris streams were also deflected to the left part of photo and down the fan into Skardu Basin, where they reached the present course of the Indus River, 9 km from the source (A). Beside the detachment zone is a large block (C). apparently part of the original rockslide, that became wedged in the bottom of the gorge without breaking up.

Figure 16.

Rock-avalanche deposit with well-developed brandling: Katzarah event, Baltistan (86 in Fig. 2), which created cross-valley deposit impounding Indus River where it leaves Skardu Basin. A: Overview from distal rim toward source slope in rock walls of Brogardo hanging valley (A*). Detachment zone may be as much as 2000 m above Indus and 1200 m above photo station. It is 7 km away. Indus cuts across low, proximal zone of rock avalanche (C). Barrier is only partially breached so that sediment and high summer flows are backed up behind it for more than 30 km across basin of Skardu (to right of photograph). After its initial descent, debris surged up slope in foreground to highest point (730 m+ above Indus; see B and Table 3). Bulk of deposit is on this opposing side of valley. Ridges and depressions in debris have a relief of 50 m+ and debris is > 100 m thick. Katzarah Lake (B*) is impounded in deep, closed trough between pressure ridges in rock avalanche. Surface rubble and underlying diamicton materials in foreground consist of fine-grained schistose rock; debris in proximal zone and in remnants up far transport slope (e.g., at g) consists of coarse-grained, amphibole-bearing quartz monzonite. Division between these lithologies forms narrow, well-defined zone, traceable across deposit at right angles to direction of movement, and is example of remnant stratigraphy (cf. Fig. 6). B: Brandung of Katzarah rock avalanche (86 in Fig. 2). View is from near the same photo station as 16A, looking to the right (eastward) and upvalley along brandung where it curves up to highest point reached (cf. Fig. 17). Almost parabolic curve closely resembles that predicted by Heim (see Fig. 19). Note people for scale (arrow).

Figure 16.

Rock-avalanche deposit with well-developed brandling: Katzarah event, Baltistan (86 in Fig. 2), which created cross-valley deposit impounding Indus River where it leaves Skardu Basin. A: Overview from distal rim toward source slope in rock walls of Brogardo hanging valley (A*). Detachment zone may be as much as 2000 m above Indus and 1200 m above photo station. It is 7 km away. Indus cuts across low, proximal zone of rock avalanche (C). Barrier is only partially breached so that sediment and high summer flows are backed up behind it for more than 30 km across basin of Skardu (to right of photograph). After its initial descent, debris surged up slope in foreground to highest point (730 m+ above Indus; see B and Table 3). Bulk of deposit is on this opposing side of valley. Ridges and depressions in debris have a relief of 50 m+ and debris is > 100 m thick. Katzarah Lake (B*) is impounded in deep, closed trough between pressure ridges in rock avalanche. Surface rubble and underlying diamicton materials in foreground consist of fine-grained schistose rock; debris in proximal zone and in remnants up far transport slope (e.g., at g) consists of coarse-grained, amphibole-bearing quartz monzonite. Division between these lithologies forms narrow, well-defined zone, traceable across deposit at right angles to direction of movement, and is example of remnant stratigraphy (cf. Fig. 6). B: Brandung of Katzarah rock avalanche (86 in Fig. 2). View is from near the same photo station as 16A, looking to the right (eastward) and upvalley along brandung where it curves up to highest point reached (cf. Fig. 17). Almost parabolic curve closely resembles that predicted by Heim (see Fig. 19). Note people for scale (arrow).

Figure 17.

View along little brandling trough of Katzarah rock avalanche (86 in Fig. 2). between rim of rock avalanche and opposing slope. View looks westward toward exit of Skardu Basin along crest. Rock avalanche moved up and along valley slope from right side of photograph (cf. Figs. 15 and 16).

Figure 17.

View along little brandling trough of Katzarah rock avalanche (86 in Fig. 2). between rim of rock avalanche and opposing slope. View looks westward toward exit of Skardu Basin along crest. Rock avalanche moved up and along valley slope from right side of photograph (cf. Figs. 15 and 16).

Figure 18.

Brandung of Gol-Ghoro rock avalanche (64 in Fig. 2) in Indus Valley 15 km upstream of Skardu Basin. A: Well-developed part of brandling (B), showing its inward-facing, eroded slope. View looks eastward and upstream from crown deposit (C) and main accumulation (A) across highest part of impact slope. Height of brandling, from head of rock wall below it to crest, is ∼55 m. Feature consists of brandung crest with boulders and large units of broken igneous rock displaying crackle and jigsaw breccia (a), thick wedge of rock-avalanche debris (b) sandwiched between a and rock wall (c), from which material forming brandling crest was stripped. B: View along strike of brandung in southerly upvalley direction, showing crest of large, imbricated boulders of rock stripped from rock wall (shown in A). Most of distal, outward-facing slope to left is buried in wind-blown dust. Several meters of debris washed from mountain wall above fill brandung trough (BT). Irrigated field and terraces of Gol (G) are 4 km away and 600 m lower on terrace above Indus.

Figure 18.

Brandung of Gol-Ghoro rock avalanche (64 in Fig. 2) in Indus Valley 15 km upstream of Skardu Basin. A: Well-developed part of brandling (B), showing its inward-facing, eroded slope. View looks eastward and upstream from crown deposit (C) and main accumulation (A) across highest part of impact slope. Height of brandling, from head of rock wall below it to crest, is ∼55 m. Feature consists of brandung crest with boulders and large units of broken igneous rock displaying crackle and jigsaw breccia (a), thick wedge of rock-avalanche debris (b) sandwiched between a and rock wall (c), from which material forming brandling crest was stripped. B: View along strike of brandung in southerly upvalley direction, showing crest of large, imbricated boulders of rock stripped from rock wall (shown in A). Most of distal, outward-facing slope to left is buried in wind-blown dust. Several meters of debris washed from mountain wall above fill brandung trough (BT). Irrigated field and terraces of Gol (G) are 4 km away and 600 m lower on terrace above Indus.

Figure 19.

Simplified geometric relationships of rock-avalanche movement and opposing valley walls as they influence form and degree of development of brandung and deflection of flowing debris across slope (after Heim, 1932, p. 87–89).

Figure 19.

Simplified geometric relationships of rock-avalanche movement and opposing valley walls as they influence form and degree of development of brandung and deflection of flowing debris across slope (after Heim, 1932, p. 87–89).

Figure 20.

Gannish Chissh prehistoric rock avalanche (47 in Fig. 2). A: View to northeast across avalanche-nourished Gannish Chissh Glacier (f), tributary of Barpu Glacier. Rockslide-rock avalanche descended as much as 2000 m to glacier surface from face of mountain (Gannish Chissh, Golden Peak, or Spantik, 7027 m above sea level) above and to right of photograph (a). Most of original deposit has been removed by glacier. Remnants exist of brandung (b) opposite source slope, and raised rim blanketing lateral moraines (c). Rock-avalanche debris is partly covered by later, lateral moraine (d). Remnants of debris that surged up into reentrants are also found (e). Debris had been deflected through almost 180° to travel toward photo station. Behind and to left of this photo station, it was again deflected sharply across valley, as shown in B. Down glacier is to left. B: View in opposite direction from point C (in A), toward photo station in A (S). Rock-avalanche debris was deflected to left by spur at right of photo down steep slope of lateral moraines, which it blankets in foreground (R) and crossed glacier again (yaks for scale). Rock-avalanche debris apparently did not spread over lateral moraines (T) and was completely removed from middle ground by glacier and by avalanching from far valley wall. However, lower in background, debris was deflected back to right side of valley. Its deposits reappear below U, on lateral moraines at exit of this tributary, and for ∼ 1 km down main valley.

Figure 20.

Gannish Chissh prehistoric rock avalanche (47 in Fig. 2). A: View to northeast across avalanche-nourished Gannish Chissh Glacier (f), tributary of Barpu Glacier. Rockslide-rock avalanche descended as much as 2000 m to glacier surface from face of mountain (Gannish Chissh, Golden Peak, or Spantik, 7027 m above sea level) above and to right of photograph (a). Most of original deposit has been removed by glacier. Remnants exist of brandung (b) opposite source slope, and raised rim blanketing lateral moraines (c). Rock-avalanche debris is partly covered by later, lateral moraine (d). Remnants of debris that surged up into reentrants are also found (e). Debris had been deflected through almost 180° to travel toward photo station. Behind and to left of this photo station, it was again deflected sharply across valley, as shown in B. Down glacier is to left. B: View in opposite direction from point C (in A), toward photo station in A (S). Rock-avalanche debris was deflected to left by spur at right of photo down steep slope of lateral moraines, which it blankets in foreground (R) and crossed glacier again (yaks for scale). Rock-avalanche debris apparently did not spread over lateral moraines (T) and was completely removed from middle ground by glacier and by avalanching from far valley wall. However, lower in background, debris was deflected back to right side of valley. Its deposits reappear below U, on lateral moraines at exit of this tributary, and for ∼ 1 km down main valley.

Figure 21.

Haldi prehistoric rock avalanche event (57 in Fig. 2; cf. Table 2). A: Map based on LANDSAT-satellite imagery and field observations. (Available 1:250 000 topographic map is extremely crude in this area.) B: View northeast toward detachment zone (A) and deposits in proximal zone, including ridges covered by megaclasts (B) and underlain by large, crackle-brecciated units (cf. Fig. 5B). Photo station is on interfluve between Saltoro (C) and Shyok Rivers. Debris (D) in foreground on interfluve is part of rock avalanche and, in places, 500 m+ above river. C: View south down Haldi stream valley toward interfluve that rock avalanche crossed, on opposing south side of Saltoro Valley. Arrows identify ridge of rock-avalanche debris deposited on interfluve above Saltoro River. Arrow at b indicates photo station for A. Haldi Valley is cut through rock-avalanche debris (a-a), where it was moving westward toward Hushe Valley.

Figure 21.

Haldi prehistoric rock avalanche event (57 in Fig. 2; cf. Table 2). A: Map based on LANDSAT-satellite imagery and field observations. (Available 1:250 000 topographic map is extremely crude in this area.) B: View northeast toward detachment zone (A) and deposits in proximal zone, including ridges covered by megaclasts (B) and underlain by large, crackle-brecciated units (cf. Fig. 5B). Photo station is on interfluve between Saltoro (C) and Shyok Rivers. Debris (D) in foreground on interfluve is part of rock avalanche and, in places, 500 m+ above river. C: View south down Haldi stream valley toward interfluve that rock avalanche crossed, on opposing south side of Saltoro Valley. Arrows identify ridge of rock-avalanche debris deposited on interfluve above Saltoro River. Arrow at b indicates photo station for A. Haldi Valley is cut through rock-avalanche debris (a-a), where it was moving westward toward Hushe Valley.

Figure 22.

Major shear zone at depth in Haldi deposit (57 in Fig. 2). Above and below are units of structureless, coarse, matrix-rich diamicton of crushed porphyritic igneous rock, 15–20 m thick. Irregular shear zone consists of gouge and laminae of rock crushed to fine sand and silt grades. Much of material in shear zone has different color from main body of deposit as well as distinctive texture and structure. Whether that partly reflects debris of another lithology in shear zone, or involves physical, possibly chemical alteration of same bedrock material by extreme frictional heating (cf. Heuberger et al., 1984; Hewitt, 1988), is not yet determined. Note horizontal pen for scale.

Figure 22.

Major shear zone at depth in Haldi deposit (57 in Fig. 2). Above and below are units of structureless, coarse, matrix-rich diamicton of crushed porphyritic igneous rock, 15–20 m thick. Irregular shear zone consists of gouge and laminae of rock crushed to fine sand and silt grades. Much of material in shear zone has different color from main body of deposit as well as distinctive texture and structure. Whether that partly reflects debris of another lithology in shear zone, or involves physical, possibly chemical alteration of same bedrock material by extreme frictional heating (cf. Heuberger et al., 1984; Hewitt, 1988), is not yet determined. Note horizontal pen for scale.

Figure 23.

View of Gol-Ghoro rock avalanche showing debris exposed at depth, where it encountered steep opposing valley wall (right, foreground). At this point debris was climbing steeply upslope and to right of photograph. While there are occasional intact megaclasts and crackle or jigsaw brecciated bedrock units, >70% of material is crushed to sand- and silt-sized particles. Two main lithologies are present, pale granites with green intrusions. Although thoroughly crushed, contorted, and attenuated, lithologies do not mix (see inset at arrow). Their original units are preserved as movement distorted, remnant stratigraphy. Exposure is in cliff undercut by Indus River (I), 110 m below. In left background, detachment zone (S) is 2.5–3.5 km away and its highest part is ∼1200 m above river level. Highest climb of impact slope was 550 m above and to right of this exposure (see Fig. 18). Exposure of similar but less severely crushed materials, shown at river level in Figure 4B, is 2.5 km upriver and to left of this view. Inset: Detail of crushed bedrock, crackle, and jigsaw breccia at depth in Gol-Ghoro rock-avalanche deposit (see arrow in main figure). Note sharp dividing line between two lithologies present, immediately left of Brunton compass and climbing from left to right of picture. Degree of crushing partly reflects lithology.

Figure 23.

View of Gol-Ghoro rock avalanche showing debris exposed at depth, where it encountered steep opposing valley wall (right, foreground). At this point debris was climbing steeply upslope and to right of photograph. While there are occasional intact megaclasts and crackle or jigsaw brecciated bedrock units, >70% of material is crushed to sand- and silt-sized particles. Two main lithologies are present, pale granites with green intrusions. Although thoroughly crushed, contorted, and attenuated, lithologies do not mix (see inset at arrow). Their original units are preserved as movement distorted, remnant stratigraphy. Exposure is in cliff undercut by Indus River (I), 110 m below. In left background, detachment zone (S) is 2.5–3.5 km away and its highest part is ∼1200 m above river level. Highest climb of impact slope was 550 m above and to right of this exposure (see Fig. 18). Exposure of similar but less severely crushed materials, shown at river level in Figure 4B, is 2.5 km upriver and to left of this view. Inset: Detail of crushed bedrock, crackle, and jigsaw breccia at depth in Gol-Ghoro rock-avalanche deposit (see arrow in main figure). Note sharp dividing line between two lithologies present, immediately left of Brunton compass and climbing from left to right of picture. Degree of crushing partly reflects lithology.

Figure 24.

Types of valley system or valley junction relations of rock-avalanche deposits. Main Valley T-shapes (cf. Nicoletti and Sorriso-Valvo, 1991; numbers refer to Fig. 2): I, Main valley source and cross-valley deposit with upvalley and down-valley lobes (e.g., Ghoro Choh I, 79); II, Tributary valley source, main valley T-deposit (Dulung Bar, 1; Tsok, 71). Triple lobe types: III, Main valley source with upvalley and downvalley lobes, plus lobe entering and plugging tributary valley (Jullah, 70; Katzarah, 86); IV, Tributary valley source and upvalley lobe, plus main valley T shape (cf. Satpara, 69; Fig. 15A). Multiple valley junction and/or lobe type example: V, Tributary valley source, deposit at junction of three other valleys (Gannish-Saukien, 45; Bordon Tir-Sost, 34). Multiple lobes with overtopping of interfluve example: VI, Tributary valley source with upvalley and downvalley lobes and main valley lobe from overtopping (e.g., Hum Bluk, 51; cf. Haldi, Fig. 21).

Figure 24.

Types of valley system or valley junction relations of rock-avalanche deposits. Main Valley T-shapes (cf. Nicoletti and Sorriso-Valvo, 1991; numbers refer to Fig. 2): I, Main valley source and cross-valley deposit with upvalley and down-valley lobes (e.g., Ghoro Choh I, 79); II, Tributary valley source, main valley T-deposit (Dulung Bar, 1; Tsok, 71). Triple lobe types: III, Main valley source with upvalley and downvalley lobes, plus lobe entering and plugging tributary valley (Jullah, 70; Katzarah, 86); IV, Tributary valley source and upvalley lobe, plus main valley T shape (cf. Satpara, 69; Fig. 15A). Multiple valley junction and/or lobe type example: V, Tributary valley source, deposit at junction of three other valleys (Gannish-Saukien, 45; Bordon Tir-Sost, 34). Multiple lobes with overtopping of interfluve example: VI, Tributary valley source with upvalley and downvalley lobes and main valley lobe from overtopping (e.g., Hum Bluk, 51; cf. Haldi, Fig. 21).

Table 1.

Geological Settings and Lithologies of Karakoram Rock Avalanche Deposits

No. of events
Geological Terranes* (99 events)
   Indian Plate
     High Himalaya (margins)—Haramosh4
   Kohistan-Ladakh Arc
     Ladakh terrrane and batholith (Baltistan)34
     Kohistan terrane and batholith (Gilgit)25
   Karakoram or “Northern” Plate
     Karakoram metamorphic complex19
     Karakoram batholith3
     N. Karakoram terrane (central region)9
     N.W. Karakoram and Hindu Raj9
   Predominant Lithology (82 events)
     Plutonic21
     Metamorphic26
     Sedimentary or metasedimentary§20
     Mixed#15
No. of events
Geological Terranes* (99 events)
   Indian Plate
     High Himalaya (margins)—Haramosh4
   Kohistan-Ladakh Arc
     Ladakh terrrane and batholith (Baltistan)34
     Kohistan terrane and batholith (Gilgit)25
   Karakoram or “Northern” Plate
     Karakoram metamorphic complex19
     Karakoram batholith3
     N. Karakoram terrane (central region)9
     N.W. Karakoram and Hindu Raj9
   Predominant Lithology (82 events)
     Plutonic21
     Metamorphic26
     Sedimentary or metasedimentary§20
     Mixed#15

* Following Searle (1991), cf Figures 1 and 2.

The Darkot and Yarkhun Formations of Buchroithner and Gamerith(1978).

§ There are 17 events in which carbonates predominate, including sedimentary or metasedimentary and metamorphic varieties.

# Substantial bands of two or more lithologies found in deposit (cf Fig.6).

Table 2.

Location, Dimensions and Lithologies of Selected Rock Avalanche Events Discussed in the Text

Survey #13Survey #47Survey #57Survey #68Survey #79Survey #86
36°34′N:74°05′N36°04′N:7455′N35°15′N:76°26′N35°37′N:35°18′N35°40′N:75°28′N35°22′N:75°25′N
BhurtGannish ChisshHaldiSatpara*Ghoro Choh IKatzarah
Area of deposit (km2):
    (i) exposed now3.51.22.541012
    (ii) estimated original7.517.025.0221420
Volume of deposits (×106m−3):
    (i) estimated now2504.830030060120
    (ii) estimated original500+200600+400120200
Rock-wall Source:
    Maximum elevation (m asl)44006800420040003800500
    ExposureENNWSSWWNWNESSW
    Rock type(s)PlutonicCarbonate (metamorphic)Plutonic and (minor) metamorphicPlutonic and metamorphicPlutonic (tonalite)Plutonic and metamorphic
Run out of debris:
    Lowest elevation239042002600220025002100
    Maximum drop, (m)109026001600170013002400
    Maximum travel, (m)50001050070009000700011000
    Farböschung angle (N)§12°13°13°11°<12°
    ACoefficient of friction (H/L)#0.220.230.230.160.190.22
    Highest run up, hr(m)**180250500250150730
Survey #13Survey #47Survey #57Survey #68Survey #79Survey #86
36°34′N:74°05′N36°04′N:7455′N35°15′N:76°26′N35°37′N:35°18′N35°40′N:75°28′N35°22′N:75°25′N
BhurtGannish ChisshHaldiSatpara*Ghoro Choh IKatzarah
Area of deposit (km2):
    (i) exposed now3.51.22.541012
    (ii) estimated original7.517.025.0221420
Volume of deposits (×106m−3):
    (i) estimated now2504.830030060120
    (ii) estimated original500+200600+400120200
Rock-wall Source:
    Maximum elevation (m asl)44006800420040003800500
    ExposureENNWSSWWNWNESSW
    Rock type(s)PlutonicCarbonate (metamorphic)Plutonic and (minor) metamorphicPlutonic and metamorphicPlutonic (tonalite)Plutonic and metamorphic
Run out of debris:
    Lowest elevation239042002600220025002100
    Maximum drop, (m)109026001600170013002400
    Maximum travel, (m)50001050070009000700011000
    Farböschung angle (N)§12°13°13°11°<12°
    ACoefficient of friction (H/L)#0.220.230.230.160.190.22
    Highest run up, hr(m)**180250500250150730

Note: cf Figure 1; asl = above sea level.

* Deposits may represent two events.

Estimated from existing maps; probably not more accurate than ±100 m.

§ See Heim (1932) and Figure 4.

# See Scheidegger (1973).

** Estimated in field.

Table 3.

Runup Height and Related Features of Selected Karakoram Rock Avalanches

EventLocationRunupCommentVertical
*(hr)Descent
(m)(H)
(m)
Katzarah#86735To highest part of brandung (Fig. 16)>1200
Gol-Ghoro#64>740To highest part of brandung (Fig. 17)∼1600
Haldi#57>500To highest crossing of interfluve (Fig. 23)1300
Rondu A#92>1000To brandung above Shoat?(>1400)
Rondu B#92650To brandung above Mendi∼1400
Gupis#17410To crown above village1100
EventLocationRunupCommentVertical
*(hr)Descent
(m)(H)
(m)
Katzarah#86735To highest part of brandung (Fig. 16)>1200
Gol-Ghoro#64>740To highest part of brandung (Fig. 17)∼1600
Haldi#57>500To highest crossing of interfluve (Fig. 23)1300
Rondu A#92>1000To brandung above Shoat?(>1400)
Rondu B#92650To brandung above Mendi∼1400
Gupis#17410To crown above village1100

Note: cf Evans et al. 1994. p. 766. Heights are measured from present river level to the highest identifiable exposure of rock avalanche debris on the impact slope, along a straight line flow path connecting the exposure, river level and central part of the detachment zone. In every case, the river still flows in debris of the rock avalanche barrier and the ancient valley floor is not exposed. It may be tens of meters beneath the river, perhaps more than 100 m in some cases. Hence, the runup values only approximate H, in Figure 9. Assuming the debris at the distal rim represents the leading edge of the rock avalanche material that had descended first to the ancient valley bottom, the actual heights climbed may be significantly greater. The greater height climbed by the Rondu, Gol-Ghoro, and Katzarah events than other examples reported to date (Evans et al. 1994), seems to reflect a combination of greater mass, local relief and the geometric relations of steep descent and steep opposing valley walls.

*See Figure 2 for numbered locations.

No specific detachment zone identified on rockwalls and canyons from which rock avalanche debris came.

GENERAL
Originrock avalanche (= sturzstrom) event (1)
Provenancelocal bedrock (rarely <1 or >20 km radius from deposit rim)
Emplacementtransported; sudden, frictional “freezing” (1,8), usually in a dry state; large-scale ‘event-emplacement’ unity
Rock classsedimentary “rudaggregates” or “rudrocks” (2)
Type of materialfragmental, coarse clastic cataclastic, “crush” breccia/rubble consisting of fracture- (compressional), shatter- (impact), and abrasion-generated clasts (9)
Type of sedimentcoarse fragmentite (2)
Unconsolidatedrubble or diamicton
Consolidatedbreccia or diamictite commonlv megabreccias or ‘megadiamicts’ (?) (i.e., larger clasts >1 m diam.)
Constituents
Interfragmental fillingmatrix of sand-sized grades and smaller (2), sometimes cemented (= breccia or diamictite)
Matrix proportionfrom (rarely) <5% to (rarely) >50%
SEDIMENTARY BODYcompact, continuous, unified
Geometry (3, 4, 7)
overalllobate or tongue-like
thin compared to areal extent
local sectionstabular or lens-like (pressure and impact thickening may occur; see text)
Magnitude (4, 5, 6)
Volumeminimum 1.5 × 106 m3; may exceed 1000 × 106 m3
areal extentrarely <2 km2, may exceed 40 km2
thicknessmostly 2–10 m, (may exceed 150 m, usually due to topographic or substrate interference; see text)
Contactsgenerally sharp with surroundings and substrate. May incorporate erodable substrate material in basal zone and distal regions (see below)
TEXTUREcoarse (dominant mode)
Grain sizefrom (usually minor) clay-size to megaclasts (>1m diam.); “sandy-silty-rubble or breccia” (2)
Largest clasts(usually) >1 m, (often) >10m, (sometimes) >60m diam.
Fragment shape
roundnessangular or very angular, (sharpstone, chinkstone) usually some lozenge-shaped (2, 10)
sphericityhighly variable (seems lithology-dependent, but compact forms tend to dominate except in surface rubble)
Fragment textureroughened, no polishing or striations, (rarely) crude scratches, percussion scars, may be “snubbing” of edges in softer lithologies
Sortingvery poorly sorted, very “immature”*
Porosity
main bodylow, impermeable
surface rubblehighly porous, ‘openwork’
LITHOLOGYallogenic, detrital materials
Provenancelocal bedrock (rarely <1 or >20 km radius from rim of deposit), exact or very close identity with parent rock
Fragmentsmostly lithic, mineral grains may form finer matrix
Composition
At a sample sitesingle rock type (= monolithologic, monomictic, homolithologic)
may be traces of exotic materials in fines
Between sitesmay involve several lithologies arranged in distinct bands transverse to movement/emplacement
Matrix/fragments relationexact correspondence (i.e., same composition and generally same colour)
FACIES/ARCHITECTUREmassive, structureless (2)
Frameworkinterlocking coarse clasts, tightly interlocking matrix grains, clast-supported, sometimes matrix-supported
Packing
main bodytight (overconsolidated)
surfaceopenwork rubble
Facies subsequence (7)
(i) mixed basal layerwith material entrained from substrate in “stringers” and dikes injected into rock avalanche or carried upward along shear planes or in load structures (11)
(ii) main bodystructureless, homogeneous, perhaps crude upward coarsening
(iii) surfaceopenwork rubble with (some) distinctive fabric/architecture (see below)
(iv) horizontallittle or no variation, except in relative
organizationthickness or development of subsequence elements, unless there is strong topographic or substrate interference (see text)
(v) distal rimoften raised, thickened, often increased shearing and deformation of substrate (3, 12)
(vi) substratemay be eroded, deformed, or scraped up and carried as transported basal layer or distal unit (7, 11)
Lithology relationsuniform, or “in bands” representing remnant stratigraphy from bedrock source (1)
Fabric (in surface rubble)(i) largest clasts oriented with or transverse to movement Direction; (ii) appositional or imbricated large clasts: and (iii) “jostling” (2) and ‘final moment’ torque, imbrication, and impact fracturing of megaclasts (i.e., during the rapid “accelerated deceleration” and sudden “frictional freezing” of the main body, (1, 8)
Special featurescrackle, jigsaw, and mosaic breccia (2, 7)
GENERAL
Originrock avalanche (= sturzstrom) event (1)
Provenancelocal bedrock (rarely <1 or >20 km radius from deposit rim)
Emplacementtransported; sudden, frictional “freezing” (1,8), usually in a dry state; large-scale ‘event-emplacement’ unity
Rock classsedimentary “rudaggregates” or “rudrocks” (2)
Type of materialfragmental, coarse clastic cataclastic, “crush” breccia/rubble consisting of fracture- (compressional), shatter- (impact), and abrasion-generated clasts (9)
Type of sedimentcoarse fragmentite (2)
Unconsolidatedrubble or diamicton
Consolidatedbreccia or diamictite commonlv megabreccias or ‘megadiamicts’ (?) (i.e., larger clasts >1 m diam.)
Constituents
Interfragmental fillingmatrix of sand-sized grades and smaller (2), sometimes cemented (= breccia or diamictite)
Matrix proportionfrom (rarely) <5% to (rarely) >50%
SEDIMENTARY BODYcompact, continuous, unified
Geometry (3, 4, 7)
overalllobate or tongue-like
thin compared to areal extent
local sectionstabular or lens-like (pressure and impact thickening may occur; see text)
Magnitude (4, 5, 6)
Volumeminimum 1.5 × 106 m3; may exceed 1000 × 106 m3
areal extentrarely <2 km2, may exceed 40 km2
thicknessmostly 2–10 m, (may exceed 150 m, usually due to topographic or substrate interference; see text)
Contactsgenerally sharp with surroundings and substrate. May incorporate erodable substrate material in basal zone and distal regions (see below)
TEXTUREcoarse (dominant mode)
Grain sizefrom (usually minor) clay-size to megaclasts (>1m diam.); “sandy-silty-rubble or breccia” (2)
Largest clasts(usually) >1 m, (often) >10m, (sometimes) >60m diam.
Fragment shape
roundnessangular or very angular, (sharpstone, chinkstone) usually some lozenge-shaped (2, 10)
sphericityhighly variable (seems lithology-dependent, but compact forms tend to dominate except in surface rubble)
Fragment textureroughened, no polishing or striations, (rarely) crude scratches, percussion scars, may be “snubbing” of edges in softer lithologies
Sortingvery poorly sorted, very “immature”*
Porosity
main bodylow, impermeable
surface rubblehighly porous, ‘openwork’
LITHOLOGYallogenic, detrital materials
Provenancelocal bedrock (rarely <1 or >20 km radius from rim of deposit), exact or very close identity with parent rock
Fragmentsmostly lithic, mineral grains may form finer matrix
Composition
At a sample sitesingle rock type (= monolithologic, monomictic, homolithologic)
may be traces of exotic materials in fines
Between sitesmay involve several lithologies arranged in distinct bands transverse to movement/emplacement
Matrix/fragments relationexact correspondence (i.e., same composition and generally same colour)
FACIES/ARCHITECTUREmassive, structureless (2)
Frameworkinterlocking coarse clasts, tightly interlocking matrix grains, clast-supported, sometimes matrix-supported
Packing
main bodytight (overconsolidated)
surfaceopenwork rubble
Facies subsequence (7)
(i) mixed basal layerwith material entrained from substrate in “stringers” and dikes injected into rock avalanche or carried upward along shear planes or in load structures (11)
(ii) main bodystructureless, homogeneous, perhaps crude upward coarsening
(iii) surfaceopenwork rubble with (some) distinctive fabric/architecture (see below)
(iv) horizontallittle or no variation, except in relative
organizationthickness or development of subsequence elements, unless there is strong topographic or substrate interference (see text)
(v) distal rimoften raised, thickened, often increased shearing and deformation of substrate (3, 12)
(vi) substratemay be eroded, deformed, or scraped up and carried as transported basal layer or distal unit (7, 11)
Lithology relationsuniform, or “in bands” representing remnant stratigraphy from bedrock source (1)
Fabric (in surface rubble)(i) largest clasts oriented with or transverse to movement Direction; (ii) appositional or imbricated large clasts: and (iii) “jostling” (2) and ‘final moment’ torque, imbrication, and impact fracturing of megaclasts (i.e., during the rapid “accelerated deceleration” and sudden “frictional freezing” of the main body, (1, 8)
Special featurescrackle, jigsaw, and mosaic breccia (2, 7)

*The notion of ‘maturity’ hardly has meaning or relevance in this process and type of sediment.

But in an opposite sense to ‘supermature’ fluvial and beach gravels—i.e., single rock type not of residual minerals, but ‘whole-rock’ and mineral fragments of parent rock.

Contents

References

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