Hydrologic remobilisation of tephra‐fall deposits: A sedimentological analysis throughout fluvio‐lacustrine systems of North‐West Patagonia

Abstract

This paper explores the response to Holocene ash inundation in different sub‐environments of two fluvio‐lacustrine systems, Las Piedritas and Totoral, focussing on the processes of volcaniclastic remobilisation. Sediment cores and outcrops were used to recognise three deposit types, noting how some differ from lahars, through sedimentological and stratigraphic analysis: (1) thick, laterally discontinuous, pumice‐dominated deposits occur overlying alluvial plain and lower terrace surfaces across both watersheds; (2) chaotic, extremely poorly sorted and matrix‐supported beds composed of both epiclastic and pyroclastic material occur as two conspicuous deposits at Las Piedritas outcrops, in a low‐gradient confined channel setting; and (3) layers with an exclusively pyroclastic composition and no flow sedimentary structures or fragment roundness were found draping each of the two chaotic intervals and intercalating with the background sediment in Las Piedritas prodelta. Thick pumice packages capping the floodplains resulted from the overflow of streams carrying a floating pumice load. Due to its positive buoyancy, the pumice is transported as a slowly moving overlying mantle without mixing with the streamflow. The disorganised pattern and poor sorting of the matrix‐rich polymictic beds are consistent with laminar, gravity‐driven, high‐concentration flows. They constitute secondary lahars, originating upstream from large run‐off processes that reworked a pyroclastic substrate. The lack of reworking features in the vitric‐rich units identified in the lake core retrieved from Las Piedritas prodelta, as well as the continuous and widespread occurrences of those interbedded in upstream outcrops, indicate an airfall origin. No reworked tephra unit is recognised in the prodelta sediment record, although various deposits from the hydrologic remobilisation of airfall tephra are found along the subaerial portion of both watersheds. Apparently, large amounts of flotation‐remobilised tephra were retained on the delta plain, while another considerable fraction would have been carried away from the creek mouth under the influence of dominant regional winds.

INTRODUCTION

Northern Patagonia is frequently affected by explosive activity from the Southern Volcanic Zone (SVZ; Stern, 2004), characterised by large-volume eruptions capable of generating about 0.5 to 1.0 km³ of tephra (i.e., 2008 Chaiten, 2011 Puyehue-Cordón Caulle, PCC and 2015 Calbuco eruptions; Alfano et al., 2011; Alloway et al., 2015; Van Eaton et al., 2016). The ash clouds are mostly carried east of the Andes as a consequence of prevailing westerly winds in this region (Figure 1), causing abundant tephra fallout across the Argentine provinces of Río Negro, Neuquén and Chubut (Villarosa et al., 2006, Durant et al., 2012; Collini et al., 2013; Reckziegel et al., 2016; Romero et al., 2016). For example, in the aftermath of the 2011 PCC eruption, Las Piedritas and Totoral (P–T) watersheds were respectively covered by an airfall pumice layer reaching a maximum thickness of 17 and 30 cm (Wilson et al., 2013; Alloway et al., 2015; Figure 2). Episodes of intense volcanism like this may lead to different volcano-hydrologic events triggered by the remobilisation of pyroclastic material throughout Andean watersheds (Smith & Lowe, 1991; Wilson et al., 2013; Córdoba et al., 2015; Elissondo et al., 2016; Baumann et al., 2018, 2020). The main scope of this paper is to characterise the depositional mechanisms involved in these phenomena from sedimentological–stratigraphic observations along P–T fluvio-lacustrine basins. This work aimed to provide an approach to identify such disturbance processes and assess potential impacts in other similar settings.

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Study site

The studied fluvio-lacustrine systems are located on the eastern (Argentine) side of the Andes (Figure 2), a prominent mountain chain formed in response to subduction-related processes operating continuously along the western margin of South America since the late Early Jurassic Period (Charrier et al., 2007 and references therein).

In particular, the mid-latitude to high-latitude Andean environments have been heavily influenced by both volcanic and glacial phenomena. Therefore, U-shaped valleys, ice-carved lake basins and associated moraines are dominant features of the glaciated landscape enclosing the study region (40–41°S) that also comprises pyroclastic deposit sequences associated with the eruptive history of the central SVZ (Alloway et al., 2022).

Such a glacio-volcanic setting provides an excellent Holocene sedimentary archive, containing deposits related to explosive volcanism of primary (airfall) and secondary (reworked) origin. Hence, the coring of lakes and wet meadows, together with the scrutiny of subaerial records, is extremely useful in assessing volcanic disturbance events (Villarosa et al., 2006; Chapron et al., 2006, 2007; Bertrand et al., 2014; Sabatier et al., 2022).

Lahar flows, related processes and their implications

Lahars have been defined by Smith and Lowe (1991) as volcaniclastic sediment-laden flows that possess characteristics of debris flows or hyperconcentrated flows. Such rapidly flowing mixtures of volcaniclastic material and water tend to follow pre-existing channels, capable of travelling hundreds of kilometres from the source (Griswold & Iverson, 2008; Procter et al., 2010; Vallance & Iverson, 2015).

Laharic events are dubbed primary when they occur on the flanks of ice-capped volcanoes during syn-eruptive activity, as the incandescent material mixes with and melts the snow and ice cap (Vallance, 2005). On the other hand, since tephra deposits can extend into a huge area downwind of an erupting volcano, secondary lahars may occur on mountains other than the volcano, triggered by heavy rains, snow melting, dam failure or a combination of these events (Massey et al., 2010; Mothes & Vallance, 2015; Córdoba et al., 2015). These events can take place even during post-eruptive periods. Moreover, the possibility of lahar formation in this region presumably increases during the austral winter, which is the rainy season in the northern Patagonian Andes (average rainfall between 4000 and 2500 mm year⁻¹; Barros et al., 1983).

Although there is some literature on the occurrence of secondary lahar flows and their deposits in the region following the 2011-PCC eruption (Córdoba et al., 2015; Baumann et al., 2018, 2020), it remains interesting to further explore these and any other related phenomena that might result from tephra-fall accumulation in proximal to mid-volcaniclastic environments. Thus, a sedimentological analysis is presented here to broaden insights into the alluvial response to intense ashfalls in this setting.

High bulk densities and mobility make lahars highly destructive along their paths (Neall, 1976; Iverson et al., 1998). Understanding these and other potentially hazardous processes involved in the hydrologic remobilisation of airfall tephra deposits is relevant for developing effective mitigation strategies and protecting vulnerable populations.

Previous works

Despite numerous studies documenting the Holocene tephrostratigraphy of the northern Patagonian Andes (Villarosa et al., 2006; Moreno et al., 2015; Naranjo et al., 2017; Alloway et al., 2017, 2022; Cottet, 2020, among others), only a few cases have addressed tephra remobilisation in the region's watersheds. Beigt et al. (2019) investigated the 2011-PCC tephra resedimentation at Pireco-Totoral delta and its implications for deltaic morphology and subaqueous mass-wasting processes. At the same time, Amat et al. (2022) have been studying the hydrogeomorphic impacts of the 2011-PCC eruption along the Totoral watershed (Figure 2).

Chapron et al. (2006, 2007) reported that ca 7 × 10⁶ m³ of pyroclastic material fell over the drainage basin of the Lago Puyehue during the earthquake-induced 1960-PCC eruption (Lara et al., 2004; Watt et al., 2009). This ash inundation was followed by heavy rainfall and snowmelt, and a mixture of volcaniclastic and regolith-derived sediments from the catchment were transported over ca 10 km throughout the Río Golgol. They recognised this flow deposit not only in terrestrial records but also in the lacustrine archive (through seismic profiling and short sediment coring). Van Daele et al. (2014) also identified Volcán Villarrica eruption-induced event deposits in sediment cores from lakes Villarrica and Calafquén, including detrital fining-upward units interpreted as lahar deposits.

After the 2008 Volcán Chaitén eruption, Ulloa et al. (2016) revealed an extraordinary input of pyroclastic sediments in adjacent river basins and subsequent hyperconcentrated flows were described by Pierson et al. (2013), Umazano et al. (2014) and Major et al. (2016). At the same time, Mella et al. (2015) documented lahar occurrence throughout Pescado, Tepú, Blanco Sur and Blanco Este rivers following the 2015 eruption of Volcán Calbuco. However, the scope of these studies was restricted to sediment-laden flows originating on or near the volcanoes.

Córdoba et al. (2015) drew attention to possible lahar generation not only in volcanic proximal areas but also in broad regions of Andean Patagonia under the influence of explosive volcanic activity. Although some sedimentological implications were discussed, it was not the main focus of that work which presents an evaluation of a computer-modelling tool to study and forecast the secondary lahar hazard in the town of Villa la Angostura (Argentina). Likewise, Baumann et al. (2018, 2020) used physical models for assessing rainfall-induced lahars in the area following the 2011-PCC eruption.

Finally, other brief mentions about destructive mass wasting events involving tephra deposits, triggered by intense precipitation in the region, are found in the literature gathering impacts of the 2011-PCC eruption (Wilson et al., 2013; Elissondo et al., 2016).

METHODS

Since the recurrence period of major eruptive events is often longer than the time covered by historical data, an integration of the sedimentary record is necessary to successfully reveal the occurrence of the studied remobilisation processes over an extended time frame. Accordingly, this study attempts to trace remobilised tephra deposits along P–T watersheds through terrestrial and limnogeological studies (Figure 3).

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On-site work

To document the subaerial path of such phenomena, extensive riverside sedimentological surveys have been carried out across both watersheds, including the examination, description and sampling of different deposits. Additionally, a sediment core was retrieved from Mallín Grande, a wet meadow located in Las Piedritas delta plain, using a 5 cm diameter modified Livingstone piston corer (Figure 3B).

For the purposes of tracking these events into the lacustrine environment, a 46 cm long core was taken at Las Piedritas outlet into Bahía Craft (Lago Nahuel Huapi; Figure 3B) with a modified UWITEC corer. To select a suitable coring site, a bathymetric survey covering the subaqueous portion of the delta was conducted using a dual-frequency echosounder.

Laboratory analyses

Once in the laboratory, samples collected during terrestrial surveys were prepared for petrographic examination under a stereo microscope. Preparation procedures consisted of washing a reduced amount of sample through a 63 μm sieve, cleaning the >63 μm fractions in a warm ultrasonic bath with distilled water (to remove silt, clay, organic matter and weathering products) and drying them in an oven at 38°C (Steen-McIntyre, 1977).

The lake core was X-rayed to document the stratigraphy before it was split and described visually. Magnetic susceptibility (MS) of the core was measured at 0.5 cm increments using a Bartington MS2E loop sensor. Afterwards, the core was systematically subsampled at 1 cm increments to quantify the organic matter content by loss on ignition at 550°C (Bengtsson & Enell, 1986; Heiri et al., 2001). Water content was calculated by weighing subsamples before and after drying at 105°C overnight. Those interbeds identified as tephra layers were also subsampled and prepared for optical microscopic examination.

According to their petrographic features (including glass fragment morphology), tephra deposits described from both subaqueous and soil-forming environments were associated with different eruptive styles (Heiken, 1972), allowing a preliminary attribution to central SVZ eruptive sources as well. Comparison with well-identified tephra samples sourced from the region's eruptive centres, stored at Laboratorio de Tefrocronología y Limnogeología (IPATEC, S.C. de Bariloche), enabled a better constraint on such correlation and, furthermore, contributed to matching some of the tephra units documented here to previously reported historical eruptions.

RESULTS

A large part of the study area remains covered by the 2011-PCC tephra, constituting a significant feature of the current landscape. As observed in the Totoral watershed (40°42′12.8″S—71°46′49.30″W/40°43′10.3″S—71°47′46.2″W), thick packages of this remobilised pumiceous material overlie horizontal surfaces of the alluvial plain and lower terraces (Figure 4A,B,C). These units can reach 130 cm thick and are almost entirely composed of tephra. An equivalent deposit is found along Las Piedritas floodplain (Figure 4D,E) and its remarkable thickness was corroborated by taking the sediment core at Mallín Grande (MG-0223; 40°46′20.1″S—71°38′04.2″W).

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Chaotic, extremely poorly sorted and matrix-supported beds are found at Las Piedritas riverside outcrops (exposed along ca 250 m and reaching up to 2.5 m thick; 40°45′49.5″S—71°38′20.4″W; Figure 5A). They occur as two distinct deposits composed of coarse lapilli and epiclastic blocks dispersed within a finer cineritic–epiclastic matrix. Debris comprising a pumice lapilli layer that rapidly grades into a dark scoria is found among the block size fraction of the basal unit (Figure 5C).

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By examining the matrix components under a stereo microscope (Figure 5D), besides the vitric and epiclastic populations, a pyroclastic mineral fraction is identified by the occurrence of crystals partly covered with glass. The latter comprises (i) colourless, tabular and euhedral to sub-euhedral plagioclase, (ii) greenish prismatic pyroxene and (iii) prismatic crystals of black hornblende (Figure 5E). Among the vitric fragments, both scoriaceous and pumiceous constituents are observed. Scoria particles are black with low vesicularity, whereas pumice fragments are brownish and moderately vesicular, with mostly spherical vesicles and mineral inclusions. Most of the white pumiceous components are generally concentrated in the coarse fraction, some of which show highly elongated vesicles. These conspicuous volcaniclastic beds are intercalated with two decimetre-thick sheet-like layers, which show an exclusively pyroclastic composition and no flow sedimentary structures or fragment roundness.

A continuous 22 cm thick scoria-rich tephra bed stands out in the middle part of these sections. This tephra bed begins with a lapilli lower layer and rapidly grades into an ash-sized tephra. This is a distinctive and regionally pervasive feature within the interval. It is not only composed predominantly of glass shards (94%) but also includes mineral grains (5%) and lithic fragments (1%). Within the glassy population, two particle types can be distinguished, one scoriaceous and the other pumiceous. Scoria is the most common vitric component in the unit (Figure 5F). These black vitric particles show angular shapes and low vesicularity. On the other hand, pumiceous fragments are medium-vesicular and, despite exhibiting parallel pipe-shaped vesicles in some cases, they are mostly spherical. Inclusions of microphenocrysts are generally common in this glass type. The mineral fraction consists of plagioclase, pyroxene and olivine crystals, partly covered with a thin layer of glass. Plagioclase is colourless, tabular and euhedral to sub-euhedral. Pyroxene occurs as greenish prismatic crystals. Olivine exhibits a yellowish hue and may include opaque minerals (Figure 5G).

The second tephra layer is identified at the top, preserved within soil material. The vitric components are dominant (96%) in this thinner (ca 10 cm thick), yellow-brown fine ash, whereas a minor portion of crystals (4%) is recognised. In addition, four subdivisions can be made within the glassy fraction. The main sub-population consists of colourless and light brown pumiceous fragments with high vesicularity, including tubular vesicles. Additionally, light brown to greenish vitric particles, showing coalescing bubbles and smooth curved vesicle walls, are found (Figure 5H). Greyish-black scoriaceous particles with well-developed vesicularity, as well as black, brownish and colourless non-vesicular glass grains with conchoidal shapes (obsidian; Figure 5I), are present in minor proportions. The mineral fraction comprises plagioclase (colourless, tabular and euhedral) and pyroxene (as green prismatic crystals).

Seven discrete event layers (T1–T7), analogous to the continuous vitric-rich units interbedded in the outcrops, can also be distinguished interrupting the fine organic matter-rich background sedimentation in the 46 cm long lake core retrieved from Las Piedritas prodelta in April 2022 (PIE1-10422; 40°47′07.8″S—71°37′47.2″W). These tephra units correlate positively with MS fluctuations and negatively with water and organic matter content (Figure 6). As a result, they can be traced by peaks in the MS profile and troughs in the other parameters. In addition, tephra layers are generally visible to the naked eye due to their coarser nature and changes in colour compared to background sediments.

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The presence of pyroclastic materials stands out at the base of the core (T0), where greyish-black scoriaceous fragments with low to very low vesicularity are found. Green-coloured broken droplets and long threads of glass (Pele's hair) are also identified. Isolated occurrences of brownish and colourless pumiceous particles, as well as lithic grains, are observed.

A very fine ash-sized, grey, dominant pumiceous layer (T1) is documented at 41 cm depth. Their pumice fragments are colourless and light brown, with angular shapes, thin walls and high vesicularity. Vesicles are spherical, oval-shaped, elongated and parallel tube-like. Likewise, a significant presence of brownish-black obsidian is recognised. Plagioclase occurs as anhedral crystals. Another pumiceous layer (T2) with the aforementioned petrographic features is found at 39 cm depth.

A very fine to medium ash-sized, dark grey tephra at 19 cm depth (T3), clearly identified by a prominent spike in MS, shows vitric (78%), crystal (17%) and lithic constituents (5%). Within the vitric particles three sub-populations occur: light to dark brown pumiceous fragments, greyish scoriaceous particles and brownish/green-coloured broken glass droplets. Pumiceous shards show angular shapes, thick walls and coalescing bubbles. Their vesicles are spherical, oval-shaped or elongated. Scoria fragments, however, are angular to subangular, with mid to high vesicularity. In this case, the vesicles are spherical or irregularly shaped. A significant portion of the crystal population consists of felsic minerals (quartz and plagioclase), whereas mafic minerals (mostly pyroxene) constitute a minor fraction. Opaque mineral inclusions are common. Another ash layer (T4) with petrographic features similar to the previously described unit is recognised in the interval from 16 to 17 cm.

A very fine lapilli-sized, grey tephra unit (T5) is visible at a depth of 13 to 15 cm. Glassy particles can be grouped into three sub-populations: abundant pumiceous fragments (91%), small quantities of scoria (5%) and obsidian (2%). Mineral and lithic grains (1% each) are also found. Pumiceous particles are mostly colourless and, to a lesser extent, light brown-coloured. They look angular, with thin walls and high vesicularity. Spherical, oval-shaped, elongated and parallel pipe vesicles are observed in them. These fragments may also show pyroxene and opaque mineral inclusions. In contrast, scoria grains are black, angular to subangular, with mid to low vesicularity. Their vesicles are spherical or irregularly shaped. Finally, obsidian can be either black, brownish, colourless or banded. Both felsic and mafic minerals are found in equal proportions. The felsic fraction is composed of colourless, tabular and euhedral to sub-euhedral plagioclase, showing inclusions of pyroxene and opaque minerals; whereas the mafic portion mostly comprises greenish prismatic pyroxene crystals. Minerals might be partly covered by a thin layer of glass.

A vitric-rich, very coarse-to-fine ash-sized, light grey tephra (T6) is observed between 6.5 and 10 cm depth, comprising 97% pumiceous fragments. Scarce amounts of scoria, obsidian, mineral grains and lithic particles can be recognised. Although light brown-coloured fragments are found within the pumiceous population, it is dominantly white. They are angular, with thin walls and high vesicularity. Their vesicles exhibit spherical, oval, elongate and even parallel-pipe shapes. Pyroxene crystals and opaque minerals are commonly included within the pumiceous fragments. Obsidian grains are black, brownish, slightly coloured or banded and can also include opaque minerals. Greenish prismatic pyroxene, containing opaque mineral inclusions, is observed. Black pyroxene grains are present in minor proportions and plagioclase occurs as colourless, tabular and euhedral crystals.

An upper, fine ash-sized, brownish tephra layer (T7) is documented by another prominent spike in the MS profile at 3 cm depth. It is primarily composed of vitric fragments (80%), most of which are pumiceous. However, the mineral components make up a substantial portion of this tephra (20%). The pumiceous fraction is mostly light to dark brown in colour, with minor amounts of colourless particles. These fragments are angular, with thin walls and coalescing bubbles. Vesicles are spherical, oval-shaped or elongated. The scoria fragments are black, angular to subangular, with mid to high-vesicularity consisting of spherical or irregularly shaped vesicles. The mineral constituents are both felsic and mafic. The felsic sub-population comprises colourless, tabular and euhedral to sub-euhedral plagioclase crystals; whereas the mafic cluster is represented by greenish prismatic pyroxene. Crystals are frequently covered with a thin layer of glass and may also include other opaque minerals.

DISCUSSION

Numerous ash and lapilli airfall beds are commonly recognised throughout the region and are distinctive enough to provide stratigraphic context for other intervening disturbance event deposits. Therefore, following an analysis of the sedimentary record, the regional tephrostratigraphic framework previously established by different authors (Villarosa et al., 2006; Naranjo et al., 2017; Alloway et al., 2022) can serve as a guiding reference in the discussion of the tephra remobilisation events discussed.

Naranjo et al. (2017) presented a stratigraphic synthesis of Holocene fall deposits derived from the PCC and Antillanca Volcanic complexes. They assigned Playas Blanca-Negra (PB-N) to a conspicuous and widespread hornblende-bearing airfall tephra, characterised by a white pumiceous layer with a black scoriaceous cap (see also Alloway et al., 2022). It is an important late Holocene chrono-marker bed since it can be easily identified due to its colour, grain size, thickness and extent. PB-N had previously been reported in the vicinity of the study area by Villarosa et al. (2006; then mapped as Nahuel Huapi Tephra), who estimated a probable age interval of 2050 to 2350 cal yr BP for this tephra. Subsequently, Naranjo et al. (2017) obtained ages similar to that chronology and correlated the unit to the Antillanca Volcanic Complex, confirming a tephra dispersion to the east based on isopach mapping. At the same time, Naranjo et al. (2017) assigned Nahuel Huapi to a ca 1.9 cal ka BP scoriaceous tephra bed overlying the PB-N unit. They also established an eastward dispersion from the Antillanca Volcanic Complex for this basaltic eruption deposit, and it was found near the study area (ca 40 km downwind from the eruptive centre). The same airfall layer can be correlated to the tephra unit that Laya (1977) identified as Member Lago Espejo in the Nahuel Huapi region.

Mil Hojas tephra (MH; Naranjo et al., 2017), the latest large-scale explosive eruptive deposit sourced from Volcán Puyehue (ca 1.1 cal ka BP), invariably overlies the Nahuel Huapi layer. This yellow-brown/pale brown/white pumiceous ash and lapilli deposit is traceable up to 90 km from the source, showing an east to south-east dispersion. Medial occurrences of this late Holocene tephra marker bed were reported in the study area by Alloway et al. (2022). In this setting, the stratigraphy overlying MH comprises the pumiceous products of the most recent Cordón Caulle eruptions, reported in 1921–22, 1960 and 2011 (Lara et al., 2006; Chapron et al., 2006; Alloway et al., 2015; Seropian et al., 2021). Centimetre-thick 1921–22-PCC and 1960-PCC tephra layers occur within the A-horizon formed at the present-day ground surface, which is in turn mantled by pyroclastic material of the 2011-PCC eruption (Alloway et al., 2022).

Examining Las Piedritas riverside outcrops, the continuous and widespread occurrences of the interbedded vitric-rich units, with no remobilisation evidence, indicate an airfall origin. According to the observed petrographic and bedding features, the dark scoriaceous bed found in the middle part of the sections corresponds to Nahuel Huapi tephra (after Naranjo et al., 2017). On the other hand, based on the glass shard morphology, stratigraphic position and thickness, the yellow-brown ashfall layer recognised at the top corresponds to MH tephra (Naranjo et al., 2017). These surveyed sections do not cover a time frame spanning the PB-N tephra-fall deposit. However, the presence of amphibole and block size debris exhibiting a rapid transition from pumice to scoria, as observed in the basal unit (Figure 5C,E), reveal stratigraphic proximity to this tephra.

Such components suggest that the basal deposit resulted from the scouring and reworking of the PB-N tephra, and the appreciable mixing with epiclastic components indicates entrainment of upper basin regolith-derived debris as well. The disorganised pattern and extremely poor sorting observed in this deposit, as in the other matrix-rich polymictic bed that completes the sections, are consistent with the laminar nature of gravity-driven, high-concentration flows (lahars sensu Smith & Lowe, 1991). They originated from large run-off processes and, due to erosion and sediment incorporation, evolved as secondary lahars capable of suppressing turbulence, preventing better organisation and internal segregation of their deposits (e.g. 1961 Calbuco lahars, Castruccio et al., 2010; Fisher & Schminke, 1994).

In an attempt to assess the runout of the aforementioned flow events, the subsurface core sampling conducted downstream from Las Piedritas fluvio-lacustrine system was hindered by the presence of a prominent pumice lapilli deposit capping the floodplain, comparable to remobilised 2011-PCC tephra packages found at the Totoral watershed, in the same depositional environment. The fabric and compositional differences noticed between these and the laharic deposits documented upstream (Table 1) reveal a hydrologic tephra remobilisation process that is rheologically different from a secondary lahar. Such thick and laterally discontinuous tephra-dominated beds result from the overflow of streams carrying a floating pumice load. This phenomenon, clearly observed during the 2011-PCC eruption in the proximal to mid-volcaniclastic environments studied here (Figure 7), occurs during eruptive events when large amounts of this high-porosity and lightweight material reach the active fluvial channels. Due to its positive buoyancy, the pumice load does not mix with the streamflow below and is transported as a slowly moving overlying mantle.

TABLE 1 Criteria for differentiation and interpretation of volcano-sedimentary deposits recognised in this work.

	Tephra-fall deposits	Laharic deposits	Remobilised pumice flood deposits
Composition	Exclusively pyroclastic composition (mostly juvenile glass fragments)	Polymictic. Mixtures of pyroclastic and epiclastic populations	Almost entirely composed of tephra (mostly pumice)
Sorting	Well sorted	Extremely poorly sorted. Various-sized block fragments are scattered within large amounts of fines	Poorly sorted. Grain size ranging from ash to coarse lapilli
Fabric	Grain-supported, open framework	Matrix-supported (high proportion of matrix)	Dominantly grain-supported
Bedding features	Continuous, widespread, sheet-like layers, which can reach decimetre thickness. Contacts are non-erosive	Thick deposits (metre-scale beds), showing lateral thickness variations and sharp gently undulating non-erosive contacts	Thick deposits (can exceed a metre in thickness), laterally discontinuous, with sharp non-erosive lower contacts and wavy tops. Stratification can be imparted by the alternation of coarse and fine-grained layers
Internal organisation	Commonly massive or showing fining-upward trends	Disorganised and extremely poorly segregated	Internally homogenous bodies
Depositional environment	Widespread areal distributions, covering both subaqueous and soil-forming environments	Confined channel setting (following abrupt changes in the channel direction or slope)	Floodplains
Interpretation	The exclusively pyroclastic composition, together with the continuous and widespread occurrences, indicates an airfall (primary) origin	The chaotic pattern and very poor sorting are consistent with laminar, gravity-driven, high-concentration flows (lahars)	Overflow of streams carrying a floating pumice load. Due to its positive buoyancy, it is transported as a slowly moving overlying mantle without appreciable mixing with the streamflow

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Naturally, an analogous expression of those remobilised pumice flood deposits was expected to be found in the core sampled from the subaqueous portion of Las Piedritas delta. However, the lack of reworking features indicates a primary origin for all the tephra deposits identified within this short sediment core (T0–T7). Obtaining a longer core was not possible due to the presence of non-coherent coarse-grained sediments underlying the retrieved lacustrine stratigraphic record. There might be a discrete tephra bed comprising the pyroclastic material at the base of the core (T0), which can be tentatively associated with the eruptive activity from both Osorno and Calbuco volcanoes based on the abundance of greyish-black scoriaceous fragments and vitric particles with fluidal shapes (cf. López-Escobar et al., 1992; Romero et al., 2021). Likewise, according to the petrography and morphology of the glass, a Calbuco source is interpreted for three ashfall units (T3-T4-T7), assigning the upper one (T7) to the 2015 eruption (see Romero et al., 2016, 2021). Several pumice and obsidian-bearing tephra layers are attributed to PCC eruptions (T1-T2-T5-T6), including the 1960 (T5) and 2011 (T6) events (broadly described by Lara et al., 2006; Daga et al., 2006; Chapron et al., 2006; Collini et al., 2013; Naranjo et al., 2017; Seropian et al., 2021).

The >1 m thick reworked tephra deposits observed at the wet meadow in Las Piedritas delta plain indicate that this flat open environment is a sediment sink. This could explain the relatively minor delta growth following the 2011-PCC eruption compared to that documented by Beigt et al. (2019) in the Totoral and Pireco fluvio-lacustrine systems.

Moreover, the prevailing wind directions may generate a current capable of driving a floating pumice mass out of the bay. This would account for both the lack of secondary tephra deposits and the relatively low primary thickness of the 2011-PCC products.

However, the presence of remobilised tephra within the lake sediment record cannot be ruled out. Pyroclastic material could have reached the lacustrine environment by floating processes, and eventually sinking once the pores became saturated with water. As a result, they would look as though they had not been remobilised, since floating fragments can be transported for long distances and finally deposited without appreciable changes in size, roundness or shape (Elzouki & Elfigih, 2020).

CONCLUSIONS

Many aspects of the effects of large ashfalls in proximal to mid-volcanic settings still remain under-explored. Recent explosive eruptions (e.g. 2011-PCC) afford clear opportunities for recording responses, analysing depositional mechanisms and evaluating potential impacts following ash inundation in fluvio-lacustrine systems. This work entailed sedimentological studies to recognise volcano-hydrologic events along P–T watersheds. This presents a significant advancement by describing and distinguishing the downstream tephra transportation by floating processes from the secondary lahars.

Run-off processes originating upstream from heavy rains or snow melting may involve erosion and entrainment of pyroclastic materials and, consequently, lead to lahar formation. Thus, 2011-PCC products in the region's headwaters still could contribute to lahar formation today, as run-off is enhanced because of reduced infiltration into slopes mantled by tephra and the lack of vegetation cover (Figure 8). The occurrence of past analogous events in Las Piedritas watershed is documented in a low-gradient confined-channel setting; whereas remobilised pumice flood deposits, resulting from the syn-eruptive overflow of streams carrying a floating tephra load, fill the flatter low-lying environments (P–T floodplains).

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Although different deposits resulting from the airfall tephra hydrologic remobilisation are found along the subaerial portion of both watersheds, no reworked tephra unit is recognised in the sediment core obtained from Las Piedritas prodelta. Large amounts of flotation-remobilised tephra were retained in the delta plain, while another considerable load would have been carried away from the creek mouth under the influence of dominant regional winds. Even so, the possibility of secondary tephra sedimentation at this subaqueous environment is not discarded. These findings highlight the need for developing methods that allow the differentiation of flotation-remobilised tephra deposits from airfall layers within the lacustrine record, which would be a natural continuation of this line of research. The characterisation conducted on the well-documented processes in the study area provides helpful insights on identifying them at other sites. Likewise, the field observations presented here about these past volcanic disturbance events are useful for calibrating simulations of future flooding scenarios. Work is underway to map secondary lahars from models run based on the geological record.

Global models project a precipitation decrease ranging from −10 to −30% in western Patagonia. Several authors have suggested that this consistent signal may be associated with a southward shift of the Pacific Ocean storm track caused by the poleward expansion of the Pacific subtropical high in a global warming context (Boisier et al., 2016 and references therein). Since sediments accumulated in catchments are commonly removed by rain or melt water, the aridification of climate can cause increased sediment availability and subsequently enhanced susceptibility to the formation of debris flows. However, even though decreasing winter precipitation is predicted, an increase in the intensity of extreme rainfall is also projected (Barros et al., 2015; IPCC, 2021). It is feasible that these concentrated episodes of heavy rain may trigger more frequent debris flows (Moreiras et al., 2021). Thus, the magnitude and frequency of laharic events under a climate change scenario are worthy of further research.

Finally, the volcano-hydrologic events reported here constitute hazardous processes as they can damage infrastructure and disrupt road traffic, leading to significant economic losses in the region. An amplified impact is expected as well due to rising exposure linked to urban expansion towards more susceptible areas. Therefore, a comprehensive analysis of these phenomena is fundamental for developing accurate hazard assessments associated with the explosive volcanism of the central SVZ. This paper serves as a useful framework to guide and inform such assessments.

ACKNOWLEDGEMENTS

This study was funded by the Agencia Nacional de Promoción de la Investigación, el Desarrollo Tecnológico y la Innovación (PICT-2020-SERIEA-02245), the Consejo Nacional de Investigaciones Científicas y Técnicas (PUE 229201801 00052 CO, PIP 11220200102980CO) and the Fundación de la Universidad Nacional del Comahue para el Desarrollo Regional (PIN I 04/B226). Lucía I. Dominguez, Lautaro de Luca and Pablo A. Salgado are all thanked for their assistance in the field. We acknowledge the constructive feedback from reviewers and editors, which significantly contributed to enhancing this work.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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