Glacial geomorphology of the central and southern Chilotan Archipelago (42.2°S–43.5°S), northwestern Patagonia

ABSTRACT

We present a geomorphic map of the glacial landforms associated with the Golfo Corcovado ice lobe in northwestern Patagonia. Built upon prior studies, our map elaborates on the central and southern sectors of Isla Grande de Chiloé and neighboring islands. Through a combination of remote sensing techniques and exhaustive fieldwork, we identified a suite of ice-marginal, subglacial, and glaciofluvial features created by the Golfo Corcovado ice lobe during four maxima within the last glacial cycle, in none of which the ice-front reached the Pacific coast of Isla Grande de Chiloé. Our mapping builds a foundation and provides insights for future interdisciplinary research on the Late Quaternary sequence of glacial and paleoclimatic events in this key sector of northwestern Patagonia.

KEYWORDS:

• Glacial geomorphology
• Chilean Lake District
• Isla Grande de Chiloé
• Northwestern Patagonia
• Last Glacial Maximum
• Last Glacial Termination

1. Introduction

Isla Grande de Chiloé (IGC) constitutes the largest island of the Chilotan Archipelago, which is composed by ∼40 islands located in the Chilotan Inner Sea, just south of the Chilean Lake District (CLD) and west of Chiloé Continental, in northwestern Patagonia (40°S–44°S; Figure 1). This region has been central in the study of former glacial fluctuations and paleoclimatic conditions in the middle latitudes of the southern hemisphere (e.g. Denton et al., 1999; Heusser, 1990; Lowell et al., 1995; Moreno et al., 2015; Villagrán, 1985; Villagrán, 1988).

Figure 1. (a) Location of the study area in southern South America. Former limit of the Patagonian Ice Sheet according to Davies et al. (2020) is outlined as a white polygon. Present Patagonian Ice Fields are depicted in grey. (b) Overview of the Chilean Lake District and the Chilotan archipelago showing the coverage of used aerial photographs (dark blue dots: centroids of GEOTEC images) and satellite imagery (light green boxes: Sentinel 1 imagery; blue light boxes: ALOS PALSAR digital elevation model). Field-checking areas (solid red line: inspected on the ground; dashed red line: helicopter inspection). (c) Digital elevation model of Isla Grande de Chiloé region highlighting the main sites of the study area.

Three-boxes figure. Upper-left panel shows the southern tip of South America corresponding to Chile and Argentina with the inferred extent of the Patagonian Ice Sheet during the last glaciation, which broadly coincides with the Cordillera de los Andes. Upper-right panel show a DEM of the Chilean Lake District and the Chilotan Archipelago with the coverage of the different imagery used to compile the main geomorphic map. Lower panel presents a DEM of the central-southern Isla Grande de Chiloé divided in four sectors. From north to south, these sectors are Dalcahue-Castro, Lago Cucao-Isla Talcán, Lago Tepuhueico-Estero Compu and Tantauco. Red squares in Castro, Lago Quilque, Estero Compu and Tantauco mark the area covered by images presented in the paper.

During the last ice age, locally known as the Llanquihue glaciation (Heusser, 1974), the region was covered by multiple piedmont glacier lobes that extended from Cordillera de los Andes (Figure 1; Davies et al., 2020) which produced a suite of well-preserved glacial landforms and sedimentary deposits. Previous studies have reconstructed the glacial history of CLD during the Last Glacial Maximum (LGM: 35–18 ka, ka = 1000 calibrated years before present) by mapping the extent of glacial landforms, particularly between Lago Ranco and Seno Reloncaví (e.g. Denton et al., 1999, 1976; Laugénie, 1982; Lowell et al., 1995; Mercer, 1972; Porter, 1981; Figure 2). Mapping efforts in the Chilotan archipelago have focused on the central-northern half of IGC, with fragmentary work on its southern third and some islands to the east (Andersen et al., 1999; García, 2012; Figure 2).

Figure 2. ALOS PALSAR digital elevation model presenting the approximate extent of the major glacial limits in the Chilean Lake District and Isla Grande de Chiloé according to Andersen et al. (1999) and García (2012). The GEBCO 2019 bathymetry of the Chilotan Inner Sea is also shown. Most constraining radiocarbon ages (Denton et al., 1999; García, 2012; Moreno et al., 2015) and recent Bayesian-modelled age based on composite ¹⁴C, ¹⁰Be and infrared stimulated luminescence (García et al., 2021) of the Golfo Corovado ice lobe activity. Green dots indicate close maximum-limiting ¹⁴C ages for glacial advances. Yellow dots represent close minimum-limiting ¹⁴C for glacial advances. Dark blue dot points the deposition age of the upper Lago Cucao outwash terrace. Radiocarbon ages have been recalculated using the calibration curve from Hogg et al. (2020). From north to south, black dashed arrows indicate the inferred ice flow of the Lago Llanquihue, Seno Reloncaví, Golfo Ancud and Golfo Corcovado ice lobes.

ALOS PALSAR digital elevation model of the Chilean Lake District and Isla Grande Chiloe showing the extent of the major glacial drifts in the region from Lago Llanquihue to Lago Tepuhueico, named as Colegual, Casma and Llanquihue, from the westernmost (outer) to easternmost (inner). The Chilotan Inner Sea is represented with the GEBCO 2019 bathymetry map. White boxes present the most constraining ages of the Golfo Corcovado ice lobe fluctuations from numerous sites across Isla Grande de Chiloé. Purple, black, and brown squares show the extent of previous maps in the region compiled by Heusser and Flint (1977), Andersen et al. (1999) and García (2012), respectively. Dashed blue square show the area mapped in this study including the minor islands of the Chilotan Archipelago.

Our study seeks to fill the gap in glacial geomorphic and stratigraphic studies in the central and southern sectors of IGC and adjoining archipelago. This contribution is intended as an initial effort to expand our knowledge into a virtually unmapped sector, build a geomorphic framework integrating new sectors with previous site- or sector-specific studies, and laying the foundations to unravel the structure and timing of local glacial fluctuations during the last glacial cycle.

2. Previous work

The earliest sketch of former glacier limits in IGC was published by Caldenius (1932). Subsequently, Heusser and Flint (1977) mapped the landforms-sedimentary associations in glacial drift packages along the northern and central sectors of IGC (Figure 2). They distinguished three glacial limits based on surface morphology and weathering criteria, which they named San Antonio, Intermediate, and Llanquihue drifts, from outermost to innermost. These glacial limits were correlated to the Colegual, Casma, and Llanquihue drifts (Table 1), previously identified by Mercer (1972, 1976) in the CLD area. Within the latter, and several years later, Porter (1981) defined and mapped four glacial drifts in the Lago Llanquihue area, he termed Caracol, Río Llico, Santa María, and Llanquihue, from outermost to innermost (Table 1).

Table 1. Stratigraphic names of the glacial drift hosted in the Chilean Lake District and Isla Grande de Chiloé according to Mercer (1972, 1976), Heusser and Flint (1977), Porter (1981) and Denton et al. (1999) Golfo Corcovado ice lobe drift sequence labeled by Mercer (1972, 1976) and Denton et al. (1999).

Mercer (1976)	Heusser and Flint (1977)	Porter (1981)	Andersen et al. (1999)	Morphostratigraphic position	Age
Llanquihue	Llanquihue	Llanquihue	Llanquihue	Inner Outer	Younger Older
Casma	Intermediate	Santa María	Casma
Colegual	San Antonio	Río Llico	Colegual
Río Frío	?	Caracol	Pre-Casma/Colegual

Andersen et al. (1995, 1999) published the first comprehensive geomorphic map of northern IGC (Figure 2), developed in the context of an interdisciplinary effort to reconstruct the glacial, vegetation, and climate history of CLD during the LGM (Denton et al., 1999; Lowell et al., 1995). They identified at least two glacial drifts, composed by nested moraine ridges they termed Casma/Colegual and Llanquihue drifts, from outermost to innermost (Table 1). More recently, García (2012) mapped the glacial landforms of central IGC (Figure 2), which he tentatively correlated with the Santa María and Llanquihue drifts (Table 1).

The initial glacial chronologies from the CLD and IGC relied on relative dating techniques, including morphostratigraphy and weathering rinds (e.g. Heusser & Flint, 1977), followed by a generation of studies that benefited from the advent of the radiocarbon dating technique (e.g. Heusser, 1990; Mercer, 1972; Porter, 1981). Subsequently, Lowell et al. (1995) and Denton et al. (1999) developed a glacial chronology based on an extensive radiocarbon-dating effort based on multiple minimum and maximum bracketing dates obtained from numerous stratigraphic sections and sediment cores from lakes and bogs associated to the Llanquihue drift. They concluded that ice lobes in the CLD achieved maxima during the last glacial cycle at ∼33.9, ∼30.9, ∼27.0, ∼26.2 and ∼18 ka (Moreno et al., 2015). In IGC they show that the Golfo Corcovado ice lobe advanced to the outermost Llanquihue moraine prior to ∼53.0 ka (Figure 2; Denton et al., 1999; Heusser et al., 1999), predating the LGM. More recently, García et al. (2021) used a combination of radiocarbon, infrared stimulated luminescence and ¹⁰Be depth profiles to conclude that the outermost Llanquihue glacial limit of the Golfo Corcovado ice lobe was formed at ∼57.8 ka, in agreement with previous findings. Subsequent ice advances occurred prior to ∼34.5 ka, after ∼26.7 ka, and shortly after ∼18.1 ka (Figure 2; Denton et al., 1999; Heusser, 1990). They suggested that the final advance of the Golfo Corcovado ice lobe was the most extensive event of the LGM, contrasting with the behavior of ice lobes in the CLD (Denton et al., 1999). The westernmost extent of the Golfo Corcovado ice lobe in the southern third of IGC, however, was unaddressed by these studies. The demise of the Golfo Corcovado ice lobe is constrained by basal radiocarbon dates from multiple lakes and bogs inboard or atop the innermost Llanquihue moraine yielding close minimum-limiting ages of ∼17.8 ka. Radiocarbon dates for the onset of ice-free conditions in Chiloé continental of ∼16.7 ka (Figure 2; Moreno et al., 2015) indicate sustained glacier recession across ∼100 km eastward through the Chilotan Inner Sea within ∼1000 years or less.

3. Setting

The Chilotan archipelago spans from ∼41.8° to ∼43.5°S in the Pacific shore of southern Chile. The western sector of IGC is dominated by Cordillera de la Costa (CC) running N-SE. Two topographic gaps corresponding with the Canal de Cachao and Lago Huillinco-Cucao basin divide the range into Cordillera Piuchén (∼900 m a.s.l.) in northern IGC, and Cordillera Pirulil in central-southern IGC. The interior-eastern sector of IGC is characterized by a flat to smooth hilly topography extending eastwards to the inner sea (Figure 1(c)).

The CC consists of Paleozoic metamorphic and Miocene sedimentary rocks, which limit with Quaternary glacial, glaciolacustrine, and glaciofluvial deposits toward the interior and eastern sectors of IGC (SERNAGEOMIN, 2003). The tectonic setting is dominated by the subduction of the Nazca Plate under the South American Plate and the Liquiñe-Ofqui Fault Zone in the continent, accounting for several stratovolcanoes located ≥70 km east of IGC. These have produced numerous tephra layers interbedded within local soil sequences in the Chilotan archipelago, Chiloé continental, and east of the Andean divide (Alloway et al., 2017a, 2017b, 2018) and seismic activity, such as the recent (AD 2016) Mw 7.6 earthquake that occurred offshore south of Tantauco (Ruiz & Madariaga, 2018; Figure 3).

Figure 3. Schematic map showing main oceanographic and tectonic features in the study area. Offshore, the South Pacific current splits into the Humboldt current to the north and the Cape Horn current to the south broadly at the latitude of the Chilotan Archipelago (Strub et al., 2019). In the continent, the location of some of the volcanoes and larger caldera systems in the Southern Volcanic Zone (∼33–46°S) of South America (modified from Stern et al., 2007). The grey shaded line indicates the eastern limit of the Cenozoic arc volcanism, while the dashed north-south line labeled LOFZ is the Liquiñe-Ofqui Fault Zone (Cembrano et al., 1996, 2000). Hypocenters of main earthquakes since 1950, and their spatial classification with respect to the Chile-Perú Trench, are also indicated (Ruiz & Madariaga, 2018).

Schematic map of southern South America showing the Chile-Perú Trench running from N to S and the main surficial oceanic currents of Humboldt and Cape Horn heading to the north and south, respectively, close to the Chilean coast in the Pacific Ocean. In the continent, numerous stratovolcanoes and Calderas are associated to the Liquiñe-Ofquí Fault Zone in the Cordillera de los Andes. The hypocenters of main earthquakes since 1950. An Interplate earthquake (Mw 7.6) occurred in 2016 very close to the southern coast of Isla Grande de Chiloé.

The climate of northwestern Patagonia is controlled by the annual cycle of the southern westerly winds, which promotes maximum precipitation during winter and minimum during summer (Garreaud et al., 2013). From west to east, meteorological stations distributed across IGC show annual precipitation values that range from ∼2500 to ∼1600 mm and a mean air temperature fluctuating between ∼14°C in summer and ∼7°C in winter (data retrieved from www.explorador.cr2.cl). Satellite data indicate offshore sea surface temperatures ranging between ∼15°C in summer and ∼11°C in winter, whereas the Chilotan Inner Sea follows a similar pattern but slightly colder during the entire year (Saldías et al., 2021).

4. Map production

We present a glacial geomorphic map of central and southern IGC (∼42.2°∼43.5°S) that encompasses areas within and beyond previous mapping efforts (Figure 2; Andersen et al., 1999; García, 2012; Heusser & Flint, 1977). We developed descriptions and interpretations from raw observations, extending our analysis into neighboring islands and sea floor whenever pertinent.

We conducted an initial identification of major glacial landforms (Figure 1(b)) using aerial photographs (1:70,000) and satellite imagery from Google Earth (2016 Cnes/SPOT, ∼15 m spatial resolution), SENTINEL-2 (∼10 m spatial resolution), and ESRI Imagery (2017, TerraColor, ∼15 m spatial resolution and SPOT, ∼2.5 m spatial resolution). We analyzed minor topographic features using images from the Advanced Land Observation Satellite (ALOS PALSAR, ∼12 m spatial resolution). Submarine landforms were examined with the General Bathymetric Chart of Oceans 2019 (GEBCO 2019, ∼0.6 km/pixel spatial resolution). We ground-truthed the preliminary interpretations based on glacial geomorphologic mapping through field campaigns between 2016 and 2019 (Figure 1(b)). While the more accessible locations in central IGC and adjacent islands were covered on the ground, the southern sector, corresponding to the Tantauco area, was scouted from a helicopter.

5. Results

We summarized the glacial landforms identified throughout central and southern IGC in Table 2, indicating their corresponding: (i) morphological description, (ii) identification criteria, (iii) potential uncertainties, (iv) interpretation, and (v) previous mapping efforts of the individual landforms recognized in IGC.

Table 2. Summary of glacial landforms, identification criteria, uncertainties, and previous mapping of the geomorphology of the Golfo Corcovado ice lobe (adapted from Bendle et al., 2017; Darvill et al., 2014; Leger et al., 2020; Lovell et al., 2012; Soteres et al., 2020).

	Landform	Morphology	Identification criteria	Uncertainties	Interpretation	Previous mapping
Ice-marginal landforms	Moraine ridge	Medium to large positive relief with plan morphology ranging between conic to linear and composed of glacial sediments with large boulders of Andean lithologies (granites, tonalites, metasedimentary) periodically protruding.	Sharp and continuous feature elevated above surrounding topography. Often aligned in the same direction. Limited by marked slopes on DEM.	Difficult to distinguish from moraine terrain. It can be confused with bedrock promontory.	Generally, it marks the former position of the glacier margin. In some cases, it could also reflect minor stand-stills positions of the ice-front.	Andersen et al. (1999) García (2012)
	Moraine terrain	Irregular topography composed by rounded hills made of glacial sediments. Sometimes, it is possible to identify large boulders (erratics) on the surface of these landforms	Diffuse pattern without significant slope breaks on DEM.	When subdued, difficult to differentiate from outwash plains. It can also be hard to distinguish from bedrock promontories, especially in the Tantauco sector.	Indicative of areas previously covered by ice.	Andersen et al. (1999) García (2012) Both studies designated this unit as hummocky moraine.
	Ice-contact slope	Prominent linear to arcuate slopes with homogeneous inclinations extending for tens to hundreds of meters. The top of this landform often exhibits moraine ridges and/or the headwater of an outwash plains. Locally, the slope is heavily incised by fluvial channels.	Extensive and sharp breaks on the slopes on DEM.	Difficult to differentiate when located within mountain areas and/or steep sloped coastlines.	Marks the former position of the glacier margin.	Andersen et al. (1999) García (2012)
Subglacial landforms	Subglacial channel	Linear or sinuous long depression reaching several tens of km with smooth slopes and flat bottoms. Usually ends abruptly in lithological boundaries. Frequently occupied by lakes or wetlands. Punctually delimited by moraine ridges.	Extensive linear-to-sinuous features delimited by smooth slope breaks and sub-horizontal valley floors forming U-shaped valleys on DEM.	Potential misidentification with meltwater channels.	Indicative of former ice coverage and major subglacial drainage pathways.	García (2012) but labeled as glacial trough.
	Ice-sculpted bedrock	Major bedrock outcrops with rounded plan form and smooth surface. Minor moraine ridges and ice-contactslopes are often attached to bedrock flanks.	Extensive rounded features of contrasting color within surrounding landscape in satellite imagery. Delimited by marked slope breaks in DEM.	When covered by vegetation, difficult to distinguish from moraine ridges or moraine terrain.	Indicative of former ice coverage.	Unmapped
	Rock lineation	Minor bedrock outcrops with a linear plan form reaching ∼1 km in length and usually nearby the Cordillera de la Costa.	Linear features with contrasting colors with adjacent landscape on satellite imagery. Parallel and subdued slope-breaks on DEM.	Potential misidentification with moraine ridges.	Indicative of former ice-flow direction.	Andersen et al. (1999) García (2012)
Glaciofluvial landforms	Outwash plains	Extensive gently inclined surfaces (0–5°) grading from ice-marginal features. Frequently composed by sequences of distinctive terraces and dissected by channels and composed by glaciofluvial sediments. Presence of sharp scarp marking different terrace levels. Often dissected by meltwater channel.	Areas of homogeneous color imprinted by linear to undulating features in aerial and satellite imagery. Homogeneous fine-grained texture interrupted by clear breaks on the predominant slope on DEM.	Potential misclassification when appear in contact with subdued moraine terrain. Surface gradient only apparent on DEM.	Indicative of former meltwater drainage pathways. Headwaters might mark a former position of the ice-front.	Andersen et al. (1999) García (2012)
	Outwash scarp	Linear-to arcuate slope breaks imprinted on the outwash plains. Often, they delimit different outwash terrace levels or meltwater channels.	Feature marked by shadowing in aerial and satellite imagery. Sharp slope breaks on the outwash plains in DEM.	Potential misidentification with meltwater channels scarps.	Indicative of several stages of outwash construction associated to different glacial events.	Andersen et al. (1999)
	Meltwater channel	Deeply incised channel with gently inclined floor (0–5°) generally delimited by step scarps, often imprinted over outwash plains. Straight to sinuous pattern. Headwaters are associated to ice marginal features. Usually, oversized related to modern drainage. Punctually occupied by lacustrine basin.	Linear features with margins defined by shadowing and different colors in aerial and satellite imagery. Fine graded texture delimitated by sharp slope breaks in DEM.	Difficult to distinguish from fluvial channels when constrained in mountainous areas. Potential misidentification with subglacial channels.	Indicative of former meltwater pathways. When associated to moraine ridges, delimitates the approximate position of ice lateral margins.	Andersen et al. (1999) García (2012)
Glaciolacustrine landforms	Paleodeltas	Minor horizontal surface (0–5°) with fan-like plan form, commonly associated to outwash plains or fluvial channels ending in depressions.	Small flat surface with fan-like limits and homogeneous fine-grained texture in DEM.	Minor features, difficult to identify in aerial and satellite imagery. Potential misinterpretation with outwash plains. Fieldwork and stratigraphic analysis is required for confirmation.	Marks the maximum height of former glacial lake levels.	Unmapped
	Glaciolacustrine irregular terrain	Irregular topography, heavily incised by fluvial channel. Associated to the proximal portion of ice-contact slopes located at the eastern coastline of the main island.	Distinctive fluvial erosion pattern in aerial and satellite imagery. Irregular pattern with no significant slope breaks in DEM.	Potential misidentification with moraine terrain. Fieldwork and stratigraphic analysis are needed for confirmation.	Delimits the extent of former glacial lakes.	Heusser and Flint (1977)
Fluvial landforms	Fluvial channel	Linear and deeply incised channel ranging from 1 to 10 km in length and V-shape profile.	Elongated and deep valleys in aerial and satellite imagery. Sharp and steep breaks on the slope, marked V-shaped profile in Dem.	Difficult to differentiate from meltwater channel when located at mountainous areas.	Marks modern drainage pathways.	García (2012) identified this feature as Holocene riverbed.
Coastal landforms	Sea cliff	Linear and continuous scarps of steep slopes and tens of m in high along the coastline.	Sharp and linear brakes in the slopes of the coastline on DEM.	Potential misidentification with ice-contact slopes.	Indicates areas dominated by marine processes.	Unmapped
	Coastal features	Nearly horizontal surfaces (0–3°) located at sea level on the coastline. Often composed by sandy beaches delimited by sea cliffs.	Areas of homogeneous light colors close to the coastline. Sandy bars are often visible in aerial and satellite imagery. Fine grained texture with flat topography on DEM.	Difficult to delimitate when associated with outwash plains.	Marks areas dominated by marine processes (sandy beaches) or the between fluvial to marine processes (estuaries).	García (2012) mapped sandy to gravel beaches mostly in the western shore of the main island.
Subaqueous landforms	Subaqueous moraine	Linear-to-arcuate positive relief in the sea floor associated with terrestrial ice marginal features.	Continuous linear-to-arcuate positive relief delimitated by contrasting breaks on the slope of the sea floor in bathymetry DEM.	Hard to differentiate from morainal banks or grounding zone wedges. Difficult to field check.	Potentially marks the former position of the glacier margin.	Unmapped
	Subaqueous channel	Linear-to-sinuous depressions delimited by marked subaqueous scarps reaching several tens of km.	Continuous depression with linear-to-sinuous plan form marked by sharp brakes on the slope of the sea floor in bathymetry DEM.	Difficult to field check.	Potentially indicative of former meltwater drainage.	Rodrigo (2006)
	Subaqueous lineation	Long and linear positive relief reaching ∼10 km in the bottom of the sea floor.	Long linear feature with 10 km in length and delimitated by sharp brakes on the slope of the sea floor in bathymetry DEM.	Difficult to field check.	Indicative of former ice flow direction.	Rodrigo (2006)

We divided the study area in four sectors (Figure 1(c)) to facilitate the descriptions, which we present from north to south.

5.1. Castro-Dalcahue sector

This sector extends between Río Butalcura to the mouth of Estero Castro (Figure 1(c)). To the west, the CC reaches its maximum elevation in IGC (∼900 m.a.s.l.). Plastered against the eastern flank of the range, we recognize a sequence of diffuse ice-contact slopes (Table 2) accompanied by poorly-defined till deposits running at elevations of ∼370 m a.s.l. Immediately to the east, we identify at least three outwash terraces (Table 2), extending from CC to the Pindapulli community, that descend in elevation toward the NW from the Dalcahue-Castro coastline through the Mocopulli-Pupetra sector into the Río Butalcura channel. The lower outwash plain extends for ∼10 km ranging from ∼180 m a.s.l. at its headwaters to ∼100 m a.s.l. at its distal portion, the upper subsidiary outwash terraces reach ∼190 and ∼200 m a.s.l. at their headwaters, respectively.

To the east and south of the outwash plains, the landscape is characterized by irregular morainic topography (Figure 4; Table 2) extending to the eastern coast of IGC (Andersen et al., 1999; García, 2012). In this area, we distinguish remnants of nested, and relatively sharp moraine ridges (Table 2) oriented from NE to SW between Dalcahue and Río Butalcura. These ice-marginal features are partially buried by the outwash plain that mantles the Mocopulli and Pupetra area (Main map).

Figure 4. Oblique aerial photographs of the morainal topography and moraine ridge near Castro (Figure 1(c)).

Four-boxes figure. Upper panoramic view showing a landscape composed by irregular topography interpreted as morainic terrain covered by meadows and disperse forest. This landform ends abruptly on a steep sea cliff facing a channel of the Chilotan Inner Sea. Lower left panel shows a conical hill covered by open forest and meadow which is interpreted as part of a moraine ridge. Lower central and right panels present irregular topography inferred to be morainal terrain.

Another distinctive feature in the east coast of IGC corresponds to a continuous and steep ice-contact slope (Table 2) that runs between Dalcahue and Castro. At its proximal side, the slope presents an irregular topography with smooth hills accentuated by minor creeks. Numerous stratigraphic exposures in this area reveal a sequence of laminated sands with discrete lenses of silts and clays and occasional dropstones, which we interpret as glaciolacustrine beds. These deposits were first described by Heusser and Flint (1977), who inferred the presence of a proglacial lake during the last deglaciation we name, informally, ice-dammed lake Castro. At the top of this ice-contact slope, we identify moraine ridges and/or the headwaters of the outwash plain terraces discussed above, as well as a mayor channel that dissects the lower outwash (Table 2; Main map). This channel starts at ∼100 m a.s.l. east of Dalcahue and presents a sinuous plan flowing northward into the Mocopulli-Pupetra outwash plain and ending in Río Butalcura. This channel, which is currently inactive, likely evacuated meltwater from its respective ice-front or functioned as the spillway of lake Castro during the final phases of the last glaciation (Heusser et al., 1995).

To the southeast of the Castro-Dalcahue ice-contact slope, we distinguish a suite of continuous and sharp moraine ridges running N-S in Península Rilán. The interior of the peninsula features an outwash plain that descends northward from an elevation of ∼120 m a.s.l. at its headwater (Main map). Its distal portion shows a tilted sequence of sand and gravel layers capped by horizontal layers of sands and gravels we interpret as the topsets and foresets, respectively, of a Gilbert-type delta (Table 2). The paleodelta reaches ∼100 m a.s.l., marking a maximum level for the glacial lake that deposited the glaciolacustrine sediments discussed in the previous paragraph, which most likely discharged through the channel located east of Dalcahue.

We recognize several ice-marginal features further east on Isla Quinchao, including groups of moraine ridges running NW-SE in the island interior, and ice-contact slopes forming its northern shore, where glaciolacustrine beds in natural outcrops are ubiquitous (Main map).

The small islands to the east exhibit irregular morainal topography lacking sharp and laterally continuous ridges that would indicate former ice-front stabilization in the archipelago following withdrawal from the LGM margins.

5.2. Lago Cucao – Isla Talcán sector

From north to south, this sector spans from the southern limit of Península Rilán to Lago Tepuhueico. The most conspicuous topographic feature is a wide valley that crosses CC from east to west, where Lago L. Cucao and Lago L. Huillinco are located (Figure 1(c)).

We observe two levels of outwash plains sloping westward, north and south of L. Cucao (Figure 5). The upper outwash plain ranges between ∼150 and ∼60 m a.s.l. from its headwaters to its distal portion, and the lower between ∼120 and ∼30 m a.s.l., respectively. We recognize moraine remnants partially buried by the upper outwash south of L. Cucao (Main map). A well-defined ice-contact slope immediately west of Lago Quilque (Figure 5) separates both outwash plains. Further evidence supporting our interpretation of this landform comes from a road cut near its culmination revealing a stratigraphy composed by massive sands with gravel lenses in discordant contact with a sequence of horizontal layers of sand and gravels we interpret as a glaciofluvial deposit (Figure 5(b); Turbek & Lowell, 1999).

Figure 5. (a) Overview of the sequence of outwash plains nearby Lago Cucao and ice-contact slope of Lago Quilque (Figure 1(c)). (b) Detail of a stratigraphic exposure at the culmination of the Lago Quilque ice-contact slope.

Two-boxes figure. Upper box shows a panoramic view of the Lago Cucao area highlighting a lower outwash plain connected through an ice-contact slope to an upper outwash plain. The same geomorphic elements are observable on the opposite shore of the lake. Lower box present a stratigraphic exposure at the culmination of the ice-contact slope composed by two main unit. The base consists on massive sands with gravel lens in erosional contact with the upper unit composed by subhorizontal beds of sands and gravels.

Further east of L. Cucao, along its northern shore, we identify a sequence of ice-contact slopes between ∼200 and ∼300 m a.s.l. resting on the eastern slope of CC (Main map). These features can be traced southward to a narrowing located between L. Cucao and L. Huillinco, where we find minor moraine ridges and ice-moulded bedrock outcrops (Table 2).

Eastwards, the area between L. Huillinco and the eastern coast of IGC features an irregular morainal topography with discontinuous moraine belts. The most distinctive moraine ridges occur on the eastern shore of L. Huillinco. These ice-marginal features appear to correlate with some isolated moraine fragments located north of the meltwater channels that originate from the Quinched area (Main map). Inboard from these moraines, we recognize numerous parabolic-shaped valleys reaching ∼10 km in length and ∼120 m in depth, occasionally occupied by lakes, and abruptly ending when changes in bedrock lithology occur, we interpret as subglacial channels (Figure 6; Table 2). These landforms were previously identified by García (2012), who interpreted as glacial troughs formed in a subglacial environment and reworked during subsequent glacial incursions.

Figure 6. (a) ALOS PALSAR digital elevation model of the subglacial channels located in the eastern shore of Isla Grande de Chiloé between Queilen and Quellón (Figure 1(c)). (b) Geomorphic map of the area illustrating the extent of major subglacial channels. (c) Oblique aerial overview of a subglacial channel. The location and direction are indicated by a red circle with lines in 6b.

Three-boxes figure. Upper left panel show an elevation model of Isla Grande de Chiloé east coast between Queilen and Quellón, including Estero Compu. High elevations are shown in yellow/orange color, whereas low elevations appear in blue. A marked channel with U-shaped profile clearly runs from Quellón area towards the north and then abruptly turn westwards near Estero Compu. Upper right panel show a hillshade of the same area with the channels mapped as pink polygons forming and intricate east-to-west trending network. Lower panel shows an oblique aerial overview of the mayor channel of the area with gentle slopes culminated on a nearly flat surface. A road runs along the wide valley floor towards the west.

To the east of IGC, we identify an ice-contact slope and moraine ridges running N-S along the eastern coast of Isla Lemuy. To the north, this ice-contact slope can be correlated with the moraine ridges and ice-contact slopes of Península Rilán and Isla Quinchao; to the south, this landform is aligned with submerged geomorphic ridges that cross Bahía Yal toward IGC, landforms we interpret as subaqueous moraines (Table 2; Batchelor & Dowdeswell, 2015). The continuity of this feature matches the ice-contact slope that establishes the eastern coast of IGC between L. Tarahuín and Queilen (Main map).

The rest of the Chilotan islands to the east of Isla Lemuy consist of irregular morainal topography and outwash deposits lacking clear ice-marginal features, suggesting that the Golfo Corcovado ice lobe did not stabilize during deglacial retreat in the interior archipelago.

5.3. Lago Tepuhueico – Estero Compu sector

This sector extents from L. Tepuhueico in the north to L. Yaldad in the south (Figure 1(c)). From west to east, the first distinctive glacial landforms correspond to a group of outwash plains occupying east-to-west trending valleys that dissect the core of CC and descend toward the western coast of IGC. The most extensive of these originates west of L. Tepuhueico and ranges in elevation from 150 to 50 m a.s.l. We also detect multiple subtle ice-contact slopes between ∼350 and ∼250 m a.s.l. on the eastern flank of CC, adjacent to L. Tepuhueico. Immediately east of that lake, we recognize a group of small outwash plains that descend to the west and follow a N-S direction (Main map).

The interior of IGC is dominated by several subglacial channels imprinted over irregular morainal topography with occasional moraine ridges (Figure 6). These channels flow westwards reaching ∼50 km in length, and originate from ice-contact slopes or former U-shaped valleys currently flooded by the sea, such as Estero Compu, along the eastern coast of IGC.

A prominent ice-contact slope culminating ∼120 m a.s.l. rims the east coast of IGC between Bahía Yal and Queilen, where it curves westward forming the slopes of Estero Compu. Closely inboard, intermittent moraine ridges occur near Queilen following a N-S direction. These moraines can be traced to submarine ridges, we interpret as subaqueous moraines, towards Isla Tranqui. South of Isla Tranqui, we find NW-SE trending ice-contact slopes running along the east coast of IGC that match the direction of subaqueous moraines (Main map).

5.4. Tantauco sector

This sector corresponds to the southern tip of IGC (Figure 1(c)) and has a landscape dominated by the CC, which, at this latitude, curves towards the east occupying most of the island. To the west, close to the Pacific shore, we observe extensive and westward-dipping outwash plains (Figure 7(a)) enclosed in the valleys excavated on the Miocene sedimentary rocks of CC (SERNAGEOMIN, 2003). Northwest of L. Chaiguaco, imprinted over the outwash plains, we detect a well-preserved ice-contact slope topped by sharp moraine ridges in the Río Medina valley (Figure 1(c) and Figure 8). The next valley to the south exhibits remnants of moraine ridges located at similar morphostratigraphic positions.

Figure 7. (a) Oblique aerial overview of the outwash constrained by the Cordillera de la Costa topography in the Tantauco area (Figure 1(c)). (b) Oblique aerial overview of the southern coastline of Tantauco (Figure 1(c)).

Two-boxes figure. Left panel oblique aerial view of the Tantauco sector showing an extensive outwash plan with a channel running between low relief mountain of Cordillera de la Costa. Right panel oblique aerial view of the south coast of Isla Grande de Chiloé in Tantauco area. The image presents a sequence of minor, linear and parallel hills interpreted as beach ridges, possibly affected by post-glacial uplift.

Figure 8. (a) Slope map derived from the ALOS PALSAR digital elevation model of the ice-contact slope located in the Río Medina valley of the Tantauco sector (Figure 1(c)). (b) Geomorphic map illustrating the distribution of main ice marginal features of a portion of the Río Medina valley and considered as the outermost position of the Golfo Corcovado ice lobe in the Tantauco sector. (c) Oblique aerial overview of the ice-contact slope located in the Río Media basin in the Tantauco sector.

Three-boxes figure. Upper left panel shows a slope model of the Río Medina valley in Tantauco sector. Upper right panel presents our mapping of the area composed by a sinuous channel running from south to north and cross cutting an ice-contact slope with moraine ridges on its culminations. Lower panel shows an oblique aerial image of the ice-contact slope and associated moraine ridges and outwash plains.

East of L. Chaiguaco, the landscape exhibits a smooth topography mainly composed by ice-moulded knobs of the Paleozoic metamorphic rocks from CC. We identify a subtle ice-contact slope running N-S at an elevation of ∼300 m a.s.l. south of L. Chaiguaco and west of L. Chaiguata (Main map).

The east coast of IGC exhibits a well-defined ice-contact slope culminating at ∼130 m a.s.l. from Estero Yaldad, near Quellón, to the north of Isla San Pedro (Figure 1(c)). In the latter, massive bedrock outcrops present a rounded ice-sculped plan form, occasionally resulting in parallel rocky lineations of hundreds to thousands of meters in length following a SE-NW direction (Main map), indicative of former ice flow.

The south coast of IGC is characterized by a well-preserved ice-contact slope running E-W at ∼150 m a.s.l. (Main map). Its culmination is associated to moraine terrain accompanied by scattered moraine ridges, which turn towards the Pacific Ocean south of L. Huillín.

In general, the distribution of ice-marginal landforms in the Tantauco sector suggests that glacier flow occurred along a SE to NW axis along Canal Moraleda (Figure 1(c)), where Rodrigo (2006) mapped large-scale subaqueous mega-lineations reaching ∼15 km in length.

6. Discussion

6.1. Glacial limits of the Golfo Corcovado ice lobe

The predominantly discontinuous and poorly preserved nature of ice-marginal landforms in central and southern IGC allow us to address a first-order interpretation of the behavior of the Golfo Corcovado ice lobe. The distribution and morphostratigraphic relations of the most distinctive moraine ridges, ice-contact slopes, and outwash plains, suggest at least four potential glacial limits associated to the Llanquihue glaciation. We named these positions as Corcovado (COR) 1–4, from outermost to innermost (Figure 9).

Figure 9. Extent of the Golfo Corcovado ice lobe inferred from our geomorphic map. Solid lines represent ice-front position reflected by the geomorphic record. Dashed lines represent inferred ice-front positions due to the fragmentary nature of the geomorphic record. Semitransparent dark blue polygon indicates the inferred extent of the proglacial Lake Castro and black dashed arrow the potential drainage path during its early stages.

Read the detailed description of this figure

Hillshade model of Isla Grande de Chiloé showing the extent of four positions of the Golfo Corcovado ice-front, we named COR1 to COR4 from the outermost (west) to the innermost (east). COR1 remains mostly butted against the eastern flank of Cordillera de la Costa except for the Cucao-Huillinco lakes. COR2 is mostly displayed closely inboard of COR1 but it is recognizable between Dalcahue and L. Tepuhueico. COR3 runs across the interior of Isla Grande de Chiloé eastwards of L. Huillinco, L. Tepuhueico. To the south of the latter, COR3 is not clearly represented until L. Chaiguaco y L. Huillin in the Tantauco sector. Finally, COR4 runs along the east coast of Isla Grande de Chiloé, punctually crossing marine channels in the Dalcahue-Castro area, Estero Compu and Estero Yaldad.

The outermost glacial limit, COR 1 (Figure 9), is defined by the Butalcura moraines which were first mapped by Andersen et al. (1999), and subsequently recognized by García (2012). These ice-marginal features are morphostratigraphically associated with the sequence of diffuse ice-contact slopes running at the eastern flank of CC distal to L. Pastahué. To the south, in the L. Cucao area, COR 1 appears as the ice-contact slope of L. Quilque, which continues southward along the eastern slope of CC in the L. Tepuhueico sector. In the Tantauco sector, we correlate this ice margin with a series of ice-contact slopes and moraine ridges associated to the outwash plains hosted in valleys that dissect CC. Except for the L. Cucao area, the distribution of deposits and landforms suggests that the Golfo Corcovado ice lobe remained buttressed against CC without reaching the west coast of IGC, at least, during the Llanquihue glaciation. This conclusion validates previous hypotheses based on the southward extrapolation of ice marginal features from the CLD and central IGC (García, 2012).

COR 2 (Figure 9) is represented by a diffuse ice-contact slope along the eastern flank of CC distal to Laguna Pastahué, closely inboard of COR 1. To the south, this ice marginal feature can be tentatively traced to the narrowing connecting L. Cucao and L. Huillinco, further south these landforms can be associated with an ice-contact slope plastered along the eastern slope of CC in the L. Tepuhueico sector, at broadly the same elevation. We could not identify ice marginal features attributable to COR 2 in the Tantauco sector of IGC, possibly due to fluvioglacial erosion on the narrow sectors of the valleys traversing the CC, outwash burial, or, alternatively, to the deposition of moraines on the western slope of CC. Hence, the extent of this glacial limit remains elusive in the Tantauco sector of IGC.

The next glacial limit is COR 3 (Figure 9), which is represented by a group of subtle moraine ridges inboard the Butalcura moraines recognized by Andersen et al. (1999) and García (2012). To the south, near Quinched, COR 3 follows a group of moraine ridge remnants heading west toward the eastern shore of L. Huillinco. Further south, in the L. Tepuhueico area, we did not recognize any distinctive ice-marginal features. We hypothesize that the Golfo Corcovado ice lobe could have reached the headwaters of the outwash plains located east of CC between L. Tepuhueico and L. Yaldad. In the Tantauco sector, COR 3 is represented by an ice-contact slope and minor moraine ridges running N-S between the west shore of L. Chaiguata and south of L. Huillín, where they originate outwash plains that drain toward the Pacific Ocean.

The innermost glacial limit, COR 4 (Figure 9), comprises prominent ice-contact slope and moraine ridges extending along the eastern and southern coast of IGC, from Punta Tenaún in the north to the Tantauco shore in the south. This ice marginal feature appears to be associated through subaqueous moraine ridges with ice-contact slopes and moraine ridges hosted in Isla Quinchao, I. Lemuy and I. Tranqui. This ice-front position could account for the formation of ice-dammed lake Castro in the Castro-Dalcahue area (García, 2012; Heusser et al., 1995; Heusser & Flint, 1977) and suggests that the Golfo Corcovado ice lobe experienced a stepwise retreat from the east coast of IGC following withdrawal from COR 3, as opposed to a single irreversible pulse.

Overall, the northern and central-southern sectors of IGC present different glacial landform types. To the north of the Castro-Dalcahue area, distinctive moraine ridges associated to extensive outwash plains clearly delineate former ice lobe boundaries (Andersen et al., 1999), whereas to the south of Lago Cucao-Huillinco, ice-marginal and glaciofluvial features are diffuse and the landscape is mostly dominated by ice-moulded CC hills furrowed by subglacial channels (Main map). This finding suggests that (i) glaciers in northern IGC advanced over a soft sedimentary bed, whereas, in southern IGC the Golfo Corcovado ice lobe mostly advanced over bedrock outcrops; and (ii) the differences in CC topography between the northern (absence of CC) and central-southern (high and low relief, respectively) sectors of IGC established topographic barriers that affected local glacial dynamics and hydrology. Based on our mapping, which covers for the first time the southern half of IGC, we infer that the Golfo Corcovado ice lobe did not surpass the CC toward the west coast in central and southern IGC, at least, during the last glaciation.

7. Concluding remarks

We present a new map of the landform-sediment associations sculpted and deposited by the Golfo Corcovado ice lobe in the central and southern thirds of IGC and the adjoining archipelago, including the seafloor of the Chilotan Inner Sea.

We distinguish four marginal positions of the Golfo Corcovado ice lobe in IGC (Figure 9), defined by successive glacial and glaciofluvial landforms along a west-to-east axis. Our results suggest that (i) the Golfo Corcovado ice lobe was mostly butted against the CC during the last glaciation without reaching the Pacific coast of IGC, and (ii) that disappearance of the Golfo Corcovado ice lobe proceeded in a stepwise manner during the initial portion of the Last Glacial Termination (∼17.8–16.8 ka) forming the ice-dammed lake Castro. Our map, thus, provides constraints for further empirical and modeling developments in a key area of the middle latitudes of the Southern Hemisphere.

Software

Spatial information analysis was carried out using ArcGIS 10.4 software. World image in the inset map was composed in MATLAB 2017. Final map production was completed with Adobe Illustrator CC 2020.

Open Scholarship

This article has earned the Center for Open Science badge for Open Data. The data are openly accessible at 10.17632/hzg2tvvxk8.1

Acknowledgements

The authors are grateful to Gaspar González, Pablo Ugalde, Joaquín Rivera and Ignacio Aguirre for field assistance. The authors also thank to Dr. Bethan Davies, Dr. Jorge Rabassa and Mr. Chris Orton for contributing to improve the original manuscript and map with their reviews.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

ESRI shapefiles (*.shp) for visualizing the map produced in this study are freely available, as long as original publication is properly cited. A folder with twenty-four shapefiles can be downloaded from a Mendeley Data online repository using the following doi:10.17632/hzg2tvvxk8.1.

Additional information

Funding

This research was supported by Chilean Ph.D. Fellowship of the Comisión Nacional de Investigación Científica y Tecnológica - CONICYT [grant number #21161417], the Chilean Fondo Nacional de Desarrollo Científico y Tecnológico - FONDECYT [grant numbers #1160488, #1191435], and the Chilean Agencia Nacional de Investigación y Desarrollo - ANID/Millennium Science Initiative/Millennium Nucleus Paleoclimate [grant number #NCN17_079].