Rheologic survey of mass transport events from the geologic record of an Andean Precordilleran slope

1. Introduction

Gravitational mass movements are the main mechanisms of sediment transport in orogenic belts. Besides that, fluvial and glacial processes also contribute to the sediment erosion, transport, and deposition. Alluvial fans placed along the mountain front in the Andes, Alps, and Himalaya, as in other young orogens, have been widely studied to understand how climate and tectonics controlled their evolution (Sah and Srivastava, 1992; Srivastava et al., 2009; Fontana et al., 2014; Cesta and Ward, 2016; Terrizzano et al., 2017). Since alluvial fans are products of various processes, analysis of sedimentary deposits can provide data to characterize the regime and the rheology of the mass transport processes that produced the deposit (Costa, 1984, 1988; Coussot and Meunier, 1996). Mass transport events along mountain fronts are triggered by intense precipitation, meltwater, seismic events, or a combination of these factors. Besides controlling the trigger, climate also regulates the low chemical weathering rates in arid and semi-arid regions and therefore the low production of silt and clay sediments. Alluvial fans in these areas are characterized by a dominance of sandy and gravelly deposits generated by non-cohesive flow processes, whereas humid conditions favor the abundance of soil and cohesive flows processes (Blair and McPherson, 1994; Coussot and Meunier, 1996). The Andes Mountain is a potential area for alluvial fans dominated by non-cohesive flow. Thus, this study aims to classify and characterize the mass transport rheology in a small alluvial fan located in the Andean Precordillera.

2. Study area

The study area is situated in the Andean Precordillera, in the Northern Patagonian Andes, approximately 60 km from the municipality of Aluminé, Nequén Province (Fig. 1A). This Andean sub-region, known as the Neuquén Cordillera, is characterized by a sequence of NE-SW oriented mountain belts, separated from each other by U-shaped valleys (Fig. 1B–C). The U-shaped form is evidence of previous glacial erosion (Giardino and Vitek, 1988; Brenning, 2005), whichmight have occurred during the last glaciation in Patagonia (maximum pick between 27 and 25 ky BP) (Hulton et al., 2002; Hein et al., 2010; Moreno et al., 2015). The outcropping bedrock in the study area comprises granitoids of the Granodiorita Paso de Icalma Formation, which display a clear to bluish-gray color,mediumto coarse grain size, holocrystalline structure, and porphyritic texture (Cingolani et al., 2011). The granitoids comprise plagioclase (50%), quartz (20%), bluish-green hornblende, yellowishpink biotite (10%), opaques (5%), apatite and chlorite (2%), and sericite (1%) (Danieli et al., 2011). The soil in the high-altitude areas is very slightly developed or absent, with common exposure of bare rock or a thin saprolite layer, and little to no edaphic development.

According to Köppen's classification (Koppen and Geiger, 1928), the climate is cold-temperate (Dfb), varying from cold and humid from the Patagonian Cordillera to Tierra del Fuego, to cold sub-humid and snowy at the Fueguina Magallanica Cordillera (Secretaria de Ambiente y Desarrollo Sustentable, 2003). The Andean mountain chains works as a natural barrier to the humid winds originating in the Pacific Ocean, causing high rainfall to the west and low rainfall to the east of the Cordilleras. The 12-year (2003–2015) historical series of precipitation recorded by the meteorological observatory of Aluminé-AR shows an average annual precipitation of 720.5 mm, with minimum and maximum annual precipitation of 216 and 1048 mm, respectively, thus characterizing a steppe climate (Koppen and Geiger, 1928). The rainy season occurs between April and September, with mean precipitation in the historical series ranging between 42.4 and 149.2 mm/month; in the dry season, precipitation ranges between 10.1 and 31.3 mm/month. The large thermal amplitude favors the frost shattering processes that produce an abundance of small rock fragments. Larger fragments are likely related to the occurrence of discontinuities in the rock mass. Because the higher mountains are snow-covered for a considerable portion of the year (Fig. 1B–C), snow melting is likely the trigger of mass movements in the study area, although intense seasonal rainfall and seismic activity may also be triggers.

3. Methodology

The study was carried out by: a) acquiring and processing satellite images and constructing a digital elevation model (DEM); b) conducting a geological survey and collecting samples; and c) interpreting the sedimentary processes and rheological aspects ofmass transportphenomena. Two satellite images were used: one acquired by the Pleiades (CNES/ ASTRIUM; April 1, 2014) with a 2 m resolution (Google Earth, 2014) and the other by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER; October 17, 2011) with a 30 m resolution (ASTER, 2011). The Pleiades image was georeferenced using ArcGIS 10.3 software in the UTM reference system (datum Sirgas 2000, 29 zone) with control points extracted from Google Earth Pro. The ASTER (2011) image was projected to the same coordinate system as the Pleiades image (Google Earth, 2014). The ASTER (2011) provided contour datawith a 5mresolution, the basis for the DEM which allowed the estimation of physical parameters, such as horizontal and vertical distances, elevation, slope angle, thickness, and volume of layers. The estimation of the uncertainty related to the DEM could not be reported due to the absence of topographic maps of the study area. The volume estimate was obtained by subtracting the current DEM from the reconstructed pre-event DEM. The reconstruction of the pre-event DEM in the alluvial fan area was done manually by adjusting the contour data to obtain a U-shaped bottom valley as found in adjacent areas, corresponding to the time immediately after the last glaciation. In the drainage basin, the contour data adjustment was done considering the topography and mean slope in the surrounding area. In the field, geological and geotechnical description was carried along four transverse sections and one longitudinal section. In each section, landslide head surfaces and the associated sediment depositswere described, and bedrock fractures measured (dip direction and dip angle). Based on section measurements, we estimated the channelform index (CI) by the ratio between the width (w) and depth (d), which expresses the flow efficiency and gives relative channel shape information (Schumm, 1960). Sedimentary deposits in each sectionwere characterized in terms of geometry and sedimentary texture, such as size and grain selection, gradation (normal or inverse), morphology (sphericity and roundness), preferential orientation of clasts, imbrication, and grain/matrix relation (clast- ormatrix-supported framework) (Tucker, 2011). A total of eight significant sediments were sampled to perform physical characterization analyses (grain size, specific weight, and plastic and liquid limits).

4. Results

The study area was segmented into three geomorphological zones according to predominant mechanisms and features: rupture zone, erosion/transport zone, and deposition zone (Fig. 2). At the rupture zone, the dominant process is sediment production;

the supply is transported to the erosion/transport zone, which in turn is a bypass zone where the sediments are transferred to the depositional zone. Temporary deposits occur locally in the rupture and erosion/transport zones. The erosion/transport zone was subdivided into two subzones, the erosion/transport and the outfall, due to changes in valley shape. 4.1. Structural analysis and geomorphology The rocky massif displays intense fracturingwith no preferential dip direction, although it is possible to recognize three families of fractures with different dip directions (dipping southeast, southwest, and northeast, Fig. 2). Step slope (Fig. 3D) and fractures dipping southeast, southwest, and northeast intensify gravitational movements in the left margin of the drainage basin, while also producing a more stable right margin. The channel-form index (CI) through the longitudinal section in Fig. 4 shows distinct behaviors: i) a continuous increase from the rupture zone to the beginning of the erosion/transport subzone, from 0.29 to 1.47; ii) a slight decrease of the CI between the erosion/transport subzone and the beginning of the outfall subzone, from 0.67 to 0.47; and iii) highest CI levels in the final outfall subzone (8.50). The low CI value in the rupture zone corresponds to degraded and/or deeper channels, while the high CI values in the erosion/transport and outfall subzones are related to aggrading or shallow channels (Schumm, 1960). Values close to 1.0, as occur in the beginning of the erosion/transport subzone, correspond to ideal and semicircular channels with the best conveyance characteristics. Thus, in the rupture zone, there is a high potential for fluvial incision caused by turbulent flows; in the beginning of the erosion/ transport subzone the efficiency of the flow provides for downward sediment transportation; at the end of the erosion/transport and the beginning of the outfall subzone, fluvial incision and turbulent flow take place once again; and at the end of the outfall subzone sediment deposition is predominant. The analysis of sediment volume revealed a discrepancy between the volumes mobilized in the mountain and those deposited in the alluvial fan. The estimated volume of rock removed from the rupture and erosion/transport zones is 11 × 106 m3, whereas that in the alluvial fan is 4 × 106 m3. The difference in volume (7 × 106 m3) suggests that all sediments mobilized from the rupture and erosion/transport zones are not deposited in the alluvial fan. One likely explanation is the continuous loss of fine-grained sediments from the fan fringe to the Remeco River or the previous occurrence of more fluid flows reaching the river channel. 4.2. Drainage basin and alluvial fan zoning 4.2.1. Rupture zone The rupture zone exhibits a deep V-shaped valley (Fig. 5A), with slope angles between 24° and 46° (Fig. 3D). At the left margin, the bedrock crops out (Granodiorita Paso de Icalma Formation), whereas at the rightmargin it is covered by saprolite. In this zone, at least 30 landslide head scarps were identified from the field survey and satellite image data (Google Earth, 2014), as well as two different sedimentary deposits. The drainage system in this zone is underground, emerging at the surface 250 m downslope of the drainage basin head (Fig. 2). Gravel- and cobble-sized sediments, with a sparse sandy matrix, cover the drainage bottom. The leftmargin exhibits intense fracturing and planar landslide head scarps with a slightly semi-circular geometry in plan view (Fig. 2) and attached lobe-shaped deposits that reach up to 65 m in length and 23 m in width (Fig. 6A). These deposits are poorly selected, composed mainly of gravel, cobble, and boulder size particles, with clastsupported texture and the absence of matrix, gradation, and/or organic remains. The clasts display low sphericity and roundness, varying from sub-angular to very angular, without any preferential alignment. In the right margin, the granitoid is covered by saprolite, and is also affected by planar landslide scarps. Attached to the landslide head scarps are elongated bars migrating downslope (Fig. 2), which reach up 150 m in length and 1–3 m in width. The elongated bar sediments vary from very coarse-grained sand to gravel, with poor selection and gravel-sized sediment dispersed in a coarse-sand matrix (matrix supported framework). From base to top, the bars display an inverse grading (with sand sediments at the base that grade upward into progressively coarser ones) and a progressive decrease in matrix content, with an absence of matrix in the topmost layers. Bars are roughly triangular in shape with steeper margins and a flat bottom with gravel and cobble between two crests. Both roundness and sphericity values closely resemble values present in sediments from the left margin. Locally, it is possible to observe imbrication where the long axes of the clasts are oriented perpendicular to the flow direction. Based on the above characteristics, different mass movement mechanismswith distinct rheologies occur in eachmargin. Intense fracturing, with plane fractures parallel and dipping into the left margin face, and physical weathering controlled the occurrence of planar landslide head scarps, steep slopes, and rock detachment processes (Savalli and Engelder, 2005; Molnar et al., 2007; Gomes, 2009; Wyllie, 2017). Predominance of angular gravel, clast-supported arrangement, absence of clast orientation, and lack of matrix in the sediment deposits attached to these scarps indicate falling, bouncing, and short distance rolling rock fragments (Drew, 1873). These rock fragment deposits are the result of frequent rockfall events in the rock wall (Cruden and Varnes, 1996), which has a brittle rheology. The proximity between scarps and deposits characterizes small distance transport, and thus a talus slope or talus deposit (Bertran et al., 1997; Blikra and Nemec, 1998; Bertran and Jomelli, 2000; Suguio, 2003). Talus slope deposits rest at angles between 31° and 33° (Fig. 3D), the typical residual frictional angle of granitoids according to Winter et al. (2005). Planar landslides in the rightmargin alsomobilize soil since the bedrock is overlaid by a saprolite horizon. The plane fracture dipping into the right margin face produces more stable bedrock in this margin than in the leftmargin and consequently enough timefor soil formation. The poor selection of grains (from sand to gravel size) in the sediment deposits is explained by the source area characteristics. The sparse content of clay (S-01, Fig. 9), the clast-rich, matrix-supported arrangement, and the inverse grading are indicative of non-cohesive high-density sediment-gravity flow (Blair and McPherson, 1994). These elongated bars are the marginal deposits of debris flow, the lateral levees that delimit the flow channel produced by debris flow (Collinson, 1986; Giraud, 2005). Specific characteristics identified in themarginal levees give detailed information about the rheology of this debris flow. The existence of larger clasts on top of the deposit suggests the action of intergranular collision, thus creating dispersive pressure that results in the kinetic sieving of thematerial (Costa, 1984, 1988; Collinson, 1986). The absence of matrix on top of the layermay reflect the continuation of water flow, even aftermass sediment freezing. The scarce occurrence of imbrication is the result of the rolling clasts downstream in a temporary and local low viscosity flow (Collinson, 1986). The distance traveled by the flow is directly related to its viscosity and is intensified by the slope steepness. Therefore, the debris flow process is characterized by dominance of the laminar flow regime (non-Newtonian fluid) with plastic rheologic properties (Collinson, 1986). 4.2.2. Erosion/transport subzone The erosion/transport subzone is a V-shaped valley (Fig. 5B) with slope angles between 6° and 46° (Fig. 3D). The left margin comprises a thin soil layer completely covered by vegetation,whereas the rightmargin comprises saprolite covered by grass. Landslide head scarps were observed only in the right margin, accompanied by sedimentary deposits. Through the drainage system, the sediments produced in the rupture zone and in the erosion/transport subzone are transported downstream. At the right margin, N10 planar landslideswere identifiedwith associated elongated bar deposits. These deposits (Fig. 6C–D) display the same sedimentary characteristic as the marginal levees (Fig. 6E) described in the rupture zone, thus sharing the same interpretation: a product of non-cohesive debris flows (Collinson, 1986; Giraud, 2005). The width is similar to that of the bars in the rupture zone, but lengths reach up 280 m. 4.2.3. Outfall subzone The outfall subzone is narrower than the upstream sector, which causes strangulation of the flow (Fig. 2). Inside this subzone, the valley changes froma narrowto awider V-shape (Fig. 5C–D). The slope angles range from 14° to 33° (Fig. 3D). A soil horizon occurs in the left margin and saprolite in the right margin, the same as observed in the previous subzone. Planar landslide head scarps are only observed in the right margin, with talus deposits attached, as occurs in the LM of the rupture zone, although these deposits extend downward, reaching the drainage system. In themain drainage system, longitudinal bars comprise cobble and boulder-sized sediments, with low sphericity, moderate selection, absence of matrix, and a clast-supported framework (Fig. 7). Imbrication of clasts was observed, with the long axes oriented perpendicular to the flow direction. Marginal levees were also seen in this subzone (Fig. 7A). Abandoned bars show the intergranular space filled with sand and plant residues (Fig. 7B). The long clast axes oriented perpendicular to the flowin the longitudinal bars is evidence of bedload transport. The clast-supported framework suggests water as the sediment transport agent. Boulders in these deposits are typical of catastrophic flooding: overflow events related to higher energy discharge and steep slopes (Collinson, 1986; Giraud, 2005), probably triggered bymeltwater or intense precipitation. Thus, these longitudinal bars were deposited by stream flow (Costa, 1984, 1988; Giraud, 2005) under a turbulent regime and with Newtonian rheological characteristics. 4.2.4. Depositional zone The depositional zone is fan-shaped, with a maximum length of approximately of 500mfromthe apex to the distal fan (or 640mincluding the fan fringe) and awidth of 900m(Fig. 8A). The slope angle varies between 4° and 18°,with amean value of 9° (Fig. 3D). The sediment grain size ranges from fine-grained sand to boulders, and based on the grain size distribution along the depositional zone, the fan was segmented into four distinct subzones: proximal, medial, distal, and fringe. The proximal fan subzone comprises fine-grained sand to boulders, the medial subzone comprises fine-grained sand to cobble, the distal subzone comprises sand, and the fan fringe very fine-grained sand. The grain-size distribution of sediments in Fig. 9 (S-05 to S-08) shows very low concentrations of silt and mud (b10%). The liquid and plastic limit tests performance resulted in non-plastic and non-liquid for all samples collected, which confirms the low concentration of silt and mud. The proximal fan exhibits marginal levees and terminal lobes. The marginal levees (Fig. 10A–C) have lengths between 11 and 131 m, widths between 2 and 14 m, and the thickness is usually b0.6 m. These deposits display the same sedimentary characteristics observed in the previous zones. Bars located in the southern sector of this subzone are abandoned and thus contain sandy sediments and plant remains filling the intergranular porosity, characteristics that are not present in the deposits northward. Terminal lobes are over 50 m in length, 11 m in width, and 3.9 m in height (Fig. 10D–E). They comprise very fine-grained gravel to boulders, with a clast-supported framework, poorly selected, with normal gradation (Fig. 10D–E) and the absence of matrix. Clasts are characterized by low roundness and sphericity, with imbrication paleocurrent measurements showing sediment transport toward the northeast. At the frontal part of the deposit, an accumulation of cobble and boulders forming a rocky front was observed (Fig. 10E). In front of the rocky front, dispersed clasts of smaller grain sizes are present. Toward the top of the deposit and to the back of the rocky front there is a decrease in grain size. A rocky front, selection of grains, and decreasing grain size toward the top of the deposit are the same characteristics described for the head of the debris flow terminal lobe, also called the frontal lobe or detrital wave (Takahashi et al., 2001; Iverson, 2014). These deposits constitute the material that passed between the levees and flowed farther downslope. Frontal lobes also have a body and tail, although they were not identified in the field. Once the movement of rocky front ceases, the fine-grained debris in the wetter part of the flow (body and tail) might still have enough kinetic energy to override or pass through the rocky front and flow downslope (Collinson, 1986). Debris flow terminal lobes associated with intense ice melting are described by Reineck and Singh (1973), Costa (1984, 1988), and Giraud (2005). There is a rheological difference in the behaviors observed in the head and tail of terminal lobes. In the head of the flow, clasts move as a cohesive mass due to kinetic energy. As such, the margins of the slope are eroded and sediments are incorporated into the body and tail by preferential transport (Iverson, 1997). The head is characterized as a non-liquefied mass under a laminar flow regime, whereas at the body and tail there is a transition from laminar to turbulent flow. As such, the event was interpreted as a non-cohesive debris flow under a laminar flow regime with plastic rheologic behavior. Satellite image interpretation of the depositional zone shows a radial distribution of the abandoned channels and allowed the identification of four depositional lobes (Fig. 11). The active lobe is located inthenorthern part of the fan, which means that the main channelmight be shifting laterally from the south toward the north. The density and radial pattern of the distributary channels indicate streamflow as the dominant process in the fan sediment transport. Stream flows are Newtonian flows characterized by moderately high water/sediment ratios, with up to 60% of the mass volume comprising water (Costa, 1984, 1988). The fan grain size distribution, with coarse-grained deposits in the proximal area and fine-grained sediment more distally, represents the rapid loss of carrying capacity of the flow. Physical weathering and very weak chemical weathering, or even the bypass of the fine sediments and their deposition in the fluvial flood plain, likely explain the fan absence of silt andmud in the fan. The sediment-gravity mechanism is predominant in the proximal fan (marginal levees and terminal lobe), and fluid-gravity processes are predominant in themedia and distal fan.

5. Discussion

Morphologic features identified in the study area, including the drainage basin (rupture zone and erosion/transport subzones), distributary channels, and depositional lobes (depositional zone), are diagnostic of the alluvial fan. Besides that, the alluvial fan primary processes were also recognized and include rock fall and debris flow occurring in the drainage basin, as well as debris flow and stream flow in the fan area. Thus, the alluvial fan was produced by sediment-gravity and fluid-gravity processes. All depositional features are clast-rich deposits, with very low concentrations of fine sediments (b10% of silt and mud), representing non-cohesive sediment transport processes. The triggering mechanisms are not the same for all processes. Intense bedrock fracturing, physical weathering, and steep slopes together control the lowering of the internal friction and shear strength, thus generating rockfall that is predominant in the left margin of the drainage basin. Events of rapid and intense snow melting or precipitation associated with a steep slope triggered the debris flow processes. Catastrophic flooding deposits in the outfall subzone also attest to the runoff of an enormous water flow, controlled in this case by the flow strangulation caused by the abrupt narrowing of the valley transverse section. The depositional arrangement and composition give evidence of a laminar flow regime for the non-cohesive debris flow and a turbulent regime for stream flows. The ratio of water to sediment controls the flow regime during the sediment transport process. In the debris flow, the high density and the strength of the flow support particles and produce laminar flowthat is normally non-erosive.When the flowceases, it generates matrix-supported deposits. The sandy matrix of the debris flow deposited in the study area is controlled by the intense physical weathering rate in comparison to the chemical one. Although sandy

matrix debris flow deposits are widely described in the literature (Jeletzky, 1975; Lewis, 1976, 1980; Galloway and Hobday, 1983; Shultz, 1984; Larsen and Steel, 1978; Capra et al., 2002; Wagreich and Strauss, 2005), some authors consider only silt and mud as the matrix in debris flow processes (Varnes, 1978; Guidicini and Nieble, 1983; Hungr et al., 2001). Stream flow deposits in the outfall subzone and depositional zones are clast-supported, and the higher proportion ofwater compared to sediment produces turbulent flow, with sediment being transported as a bedload or suspended load. The mass flow in debris flow events had no cohesion and still produced deposits with inverse gradation. This results from the dispersive pressure and kinetic sieving that take place during the transport event, concentrating coarser sediments at the top of the deposit, as occurs in cohesive debris flow (Reineck and Singh, 1973; Blair and McPherson, 1994; Ledder, 1999). Normal gradation toward the top and the back of the rocky front of the terminal lobes is explained by the particle segregation in granular flows (Pierson, 1986; Sohn et al., 1999; Kokelaar et al., 2014). Debris flow is also characterized by plastic rheologic behavior and thus low viscosity. The plastic behavior is interpreted from deposit characteristics located near the source area, such as massive or inverse gradations and the absence of traction structures, which are evidence of frictional freezing (Costa, 1984; Costa, 1988; Collinson, 1986). The flows that were able to reach the thalweg were conditionedmainly by the slope steepness but also by their higher viscosity. All processes recognized in the study area are recurrent through time, although temporarily a single type of flow dominates (Nilsen, 1982; Costa, 1984, 1988; Giraud, 2005). Such a situation was verified in the study area where stream flow processes currently dominate, as recorded in the reworking sediments of the alluvial fan. Therefore, the main process transporting sediment from the drainage basin to the depositional zone is likely the non-cohesive debris flow, whose event recurrence period is relatively long according to Costa (1988).

6. Conclusion

Based on the distinct depositional and erosional features recognized in the small alluvial fan located in the Andean Precordillera, we conclude that the study area remains in constant activity. The planar landslides, controlled by intense bedrock fracturing, generate rockfall process, with a brittle rheologic characteristic and talus deposit. Planar rupture also triggered non-cohesive debris flows, which have a laminar regime and plastic rheology. Rockfall occurs in the rupture zone and erosion/transport subzone, and non-cohesive debris flows occur in all zones of the study area. Besides marginal levees, debris flow terminal lobes are also preserved in the proximal alluvial fan. Clast-supported elongated bar deposits documented in the thalweg of the outfall subzone were produced by stream flow. However, the presence of boulders attests to overflooding caused by catastrophic events. Streamflowis also recorded in the alluvial fan by the occurrence of lobes with radial drainage line patterns. The current drainage system located northward explains the migration of the alluvial fan lobes from the south toward the north. The alluvial fanwas likely built by recurring episodic events of debris flow and by sediment reworking during stream flow. With regard to the rheologic behavior of themass flow that gave rise to the depositional features observed, it is possible to distinguish clearly between fluid flows under the turbulent regime (stream flow) and plastic flows under the laminar regime (debris flow). Such conclusions were fundamentally based on the sedimentary characteristics of deposits, mainly size and grain selection, absence or existence of gradation, preferential orientation of clasts, grain/matrix relationship, and depositional geometry. Themain triggers for the gravitational mass movements were meltwater and/or intense rainfall. In regard to the sediment transportmechanisms, debris flow type movement originates from gravitational transport, whereas stream flows rely on water flows, generally intensified by meltwater or precipitation. All evidence indicates that deposition took place in a mixed gravity flow and stream flow dominated alluvial fan, which was seasonally influenced by melting snow.

Acknowledgements

This study is part of the first author's Ph.D. thesis at the Geology Graduate Program of the Universidade do Vale do Rio dos Sinos (Unisinos), supported by PROSUP/CAPES (process number 1202942). The authors wish to thank Francisco M.W. Tognoli, the Advanced Visualization Laboratory (VISLAB-Unisinos), the Viamão Project, and LASERCA-Unisinos for the technical support provided. The authors also thank the editor and the anonymous reviewers for the valuable contributions.

References

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(Eds.), Landslides: Analysis and Control, Transportation Research Board, Washington, D.C., Transportation Research Board. National Academy Press,Washington, pp. 11–33 Special Report 176. Wagreich,M., Strauss, P.E., 2005. Source area and tectonic control on alluvial-fan development in the Miocene Fohnsdorf intramontane basin, Austria. Geol. Soc. Lond., Spec. Publ. 251:207–216. https://doi.org/10.1144/GSL.SP.2005.251.01.14. Winter, M.G., Shackman, L., MacGregor, F., Nettleton, I.M., 2005. Background to Scottish landslides and debris flows. In: Winter, M.G., MacGregor, F., Shackman, L. (Eds.), Scottish Road Network Landslides Study. Scottish Executive, Edinburg, pp. 12–24. Wyllie, D.C., 2017. Rock Fall Engineering. CRC Press, London.

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Review of the classification of landslides of the flow type. Environ. Eng. Geosci. 7:221–238. https://doi.org/10.2113/ gseegeosci.7.3.221. Iverson, R.M., 1997. The physics of debris flows. Rev. Geophys. 35 (3):245–296. https:// doi.org/10.1029/97RG00426. Iverson, R.M., 2014. Debris flows: behaviour and hazard assessment. Geol. Today 30 (1): 15–20. https://doi.org/10.1111/gto.12037. Jeletzky, J.A., 1975. Hesquiat Formation (New): a Neritic Channel and Interchannel Deposit of Oligocene Age,Western Vancouver Island, British Columbia (92 E). Energy, Mines and Resources, Canada, Ottawa (54 pp.). Kokelaar, B.P., Graham, R.L., Gray, J.M.N.T., Vallance, J.W., 2014. Fine-grained linings of leveed channels facilitate runout of granular flows. Earth Planet. Sci. Lett. 385:172–180. https://doi.org/10.1016/j.epsl.2013.10.043. Koppen, W., Geiger, R., 1928. Klimate der Erde. Verlag Justus Perthe, Gotha. Larsen, V., Steel, R.J., 1978. The sedimentary history of a debris-flow dominated, Devonian alluvial fan–a study of textural inversion. Sedimentology 25 (1):37–59. https://doi. org/10.1111/j.1365-3091.1978.tb00300.x. Ledder, M.R., 1999. Sedimentology and sedimentary basins: from turbulence to tectonics. 2 ed. Blackwell Science Ltd., Oxford. Lewis, D.W., 1976. Subaqueous debris flows of early Pleistocene age at Motunau, North Canterbury, New Zealand. N. Z. J. Geol. Geophys. 19 (5):535–567. https://doi.org/ 10.1080/00288306.1976.10426308. Lewis, D.W., 1980. Storm-generated graded beds and debris flow deposits with Ophiomorpha in a shallow offshore Oligocene sequence at Nelson, South Island, New Zealand. N. Z. J. Geol. Geophys. 23 (3):353–369. https://doi.org/10.1080/ 00288306.1980.10424145. Molnar, P., Anderson, R.S., Anderson, S.P., 2007. Tectonics, fracturing of rock, and erosion. J. Geophys. Res. Earth Surf. 112:F03014. https://doi.org/10.1029/2005JF000433. Moreno, P.I., Denton,G.H., Moreno,H., Lowell, T.V., Putnam, A.E., Kaplan, M.R., 2015. Radiocarbon chronology of the last glacialmaximum andits termination innorthwesternPatagonia. Quat. Sci. Rev. 122:233–249. https://doi.org/10.1016/j.quascirev.2015.05.027. Nilsen, T.H., 1982. Alluvial fan deposits. In: Scholle, P.A., Spearing, D. (Eds.), Sandstone Depositional Environments, American Association of Petroleum Geologists, Memoir. 603, pp. 49–86. Pierson, T.C., 1986. Flow behavior of channelized debris flows, Mount St. Helens, Washington. In: Abrahms, A.D. (Ed.), Hillslope Processes. Allen & Unwin, Boston, pp. 269–296. Reineck, H.-E., Singh, I.B., 1973. Glacial environment. In: Reineck, H.-E., Singh, I.B. (Eds.), Depositional Sedimentary Environments. Springer, Berlin, Heidelberg, pp. 164–182. Sah, M.P., Srivastava, R.A.K., 1992. Morphology and facies of the alluvial-fan sedimentation in the Kangra Valley, Himachal Himalaya. Sediment. Geol. 76 (1–2):23–42. https://doi.org/10.1016/0037-0738(92)90137-G. Savalli, L., Engelder, T., 2005. Mechanisms controlling rupture shape during subcritical growth of joints in layered rocks. GSA Bull. 117 (3–4):436–449. https://doi.org/ 10.1130/B25368.1. Schumm, S.A., 1960. The shape of alluvial channels in relation to sediment type. US Geol. Surv. Prof. Pap. 352-B, 17–30. Secretaria de Ambiente y Desarrollo Sustentable, S, 2003. Atlas de los bosques nativos argentinos. Dirección de Bosques. Secretaría de Ambiente y Desarrollo Sustentable. Shultz, A.W., 1984. Subaerial debris-flow deposition in the upper Paleozoic Cutler Formation, western Colorado. J. Sediment. Res. 54 (3):759–772. https://doi.org/10.1306/ 212F84EF-2B24-11D7-8648000102C1865D. Sohn, Y.K., Rhee, C.W., Kim, B.C., 1999. Debris flow and hyperconcentrated flood-flow deposits in an alluvial fan, northwestern part of the Cretaceous Yongdong Basin, Central Korea. J. Geol. 107 (1):111–132. https://doi.org/10.1086/314334. Srivastava, P., Rajak, M.K., Singh, L.P., 2009. Late Quaternary alluvial fans and paleosols of the Kangra basin, NW Himalaya: tectonic and paleoclimatic implications. Catena 76: 135–154. https://doi.org/10.1016/j.catena.2008.10.004. Suguio, K., 2003. Geologia Sedimentar. Blucher, São Paulo. Takahashi, T., Nakagawa, H., Satofuka, Y., Kawaike, K., 2001. Flood and sediment disasters triggered by 1999 rainfall in Venezuela; a river restoration plan for an alluvial fan. J. Nat. Disaster Sci. 23 (2), 65–82. Terrizzano, C.M., García Morabito, E., Christl, M., Likerman, J., Tobal, J., Yamin, M., Zech, R., 2017. Climatic and tectonic forcing on alluvial fans in the Southern Central Andes. Quat. Sci. Rev. 172:131–141. https://doi.org/10.1016/j.quascirev.2017.08.002. Tucker, M.E., 2011. Sedimentary Rocks in the Field: A Practical Guide. Wiley-Blackwell. Varnes, D.J., 1978. Slope movement types and processes. In: Schuster, R.L., Krizek, R.J. (Eds.), Landslides: Analysis and Control, Transportation Research Board, Washington, D.C., Transportation Research Board. National Academy Press,Washington, pp. 11–33 Special Report 176. Wagreich,M., Strauss, P.E., 2005. Source area and tectonic control on alluvial-fan development in the Miocene Fohnsdorf intramontane basin, Austria. Geol. Soc. Lond., Spec. Publ. 251:207–216. https://doi.org/10.1144/GSL.SP.2005.251.01.14. Winter, M.G., Shackman, L., MacGregor, F., Nettleton, I.M., 2005. Background to Scottish landslides and debris flows. In: Winter, M.G., MacGregor, F., Shackman, L. (Eds.), Scottish Road Network Landslides Study. Scottish Executive, Edinburg, pp. 12–24. Wyllie, D.C., 2017. Rock Fall Engineering. CRC Press, London.