ABSTRACT
We map the glacial geomorphology of the former Patagonian Ice Sheet between 44°S and 46°S. Building on previous work, our map covers a ∼50,000 km2 region of west-central Patagonia. The study area includes the eastward-flowing Río Pico, Río Caceres, Río Cisnes, Lago Plata-Fontana, El Toqui, Lago Coyt/Río Ñirehuao, Simpson/Paso Coyhaique, and Balmaceda palaeo-outlet glaciers, adjacent valleys, and the Andean Cordillera. The inventory contains >70,000 individual landforms mapped from remotely-sensed imagery and field surveys. Mapping was classified into ice-marginal (e.g. moraine ridges, trimlines), subglacial (e.g. glacial lineations, flutes), glaciolacustrine (e.g. palaeolake shorelines, perched deltas), glaciofluvial (e.g. proglacial outwash plains, meltwater channels), and non-glacial (e.g. palaeochannels, landslides or slumps) landform groups. The new map will inform future interpretations of regional glacier dynamics, and the development of robust geochronological datasets that test the timing of glaciation and deglaciation.
1. Introduction
Patagonia, in southernmost South America, has undergone repeated glaciations since ∼6 to 5 Ma (CitationMercer & Sutter, Citation1982; CitationRabassa et al., Citation2005, Citation2011; CitationRabassa, Citation2008). The former Patagonian Ice Sheet (PIS; (a)), a temperate ice sheet with active and fast-flowing outlet glaciers (CitationDarvill et al., Citation2017; CitationGlasser & Jansson, Citation2005), covered most of Chile and western Argentina between 38° and 56° S (CitationCaldenius, Citation1932; CitationDavies et al., Citation2020; CitationGlasser & Jansson, 2008). The Andean Cordillera is aligned north–south through western South America, intersecting the Southern Hemisphere Westerly Wind Belt and generating a marked west–east precipitation gradient (CitationAravena & Luckman, Citation2009; CitationGarreaud et al., Citation2013). Arid conditions east of the Andean Cordillera have therefore preserved a unique, well-defined, record of terrestrial Quaternary glacier advances (CitationDavies et al., Citation2020; CitationGlasser et al., Citation2008). This makes Patagonia an ideal location to investigate the role of Southern Hemisphere mid-latitude palaeoclimate on local palaeoglacier dynamics.
Until recently, glacial geomorphological mapping of the PIS focused on the largest and most distinctive landforms (e.g. CitationCaldenius, Citation1932; CitationGlasser & Jansson, 2008; CitationMercer, Citation1965), providing a broad-scale synthesis of ice extent and dynamics. Since ∼2015, freely available high-resolution satellite imagery has offered new opportunities to refine our understanding of palaeoglaciological processes, but disparities in mapping extent and detail remain prevalent (cf. CitationDavies et al., Citation2020). Most data are focused around the Northern Patagonian Icefield (Campo de Hielo Norte, 46–47.5 °S), Southern Patagonian Icefield (Campo de Hielo Sur, 48–51 °S), and Cordillera Darwin Icefield (Campo de Hielo de la Cordillera Darwin, 54–55 °S). To date, deglaciated areas north of the Northern Patagonian Icefield have received comparatively little attention. By generating comprehensive inventories of Patagonian glacial geomorphology (CitationBendle et al., Citation2017b; CitationDarvill et al., Citation2014; CitationGarcía et al., Citation2014; CitationLeger et al., Citation2020; CitationLovell et al., Citation2011; CitationMartin et al., Citation2019; CitationSoteres et al., Citation2020), we can begin to assess spatiotemporal variations in palaeoglaciological processes.
We provide detailed mapping across two degrees of latitude (44–46° S, (b)). The main aim of this study is to identify and classify key glacial landform assemblages in west-central Patagonia. Our objectives are to: (1) produce a high-resolution map of glacial geomorphology using satellite imagery and field surveys and (2) group and characterise the key landform assemblages for this region of the former PIS. Mapping will provide the basis for subsequent reconstructions of glacier extent and dynamics based on morphostratigraphy (cf. CitationLukas, Citation2006), glacial landsystems (cf. CitationEvans, Citation2003), and the generation of robust geochronological datasets (cf. CitationGarcía et al., Citation2020; CitationMendelová et al., Citation2020; CitationPeltier et al., Citation2021; CitationSagredo et al., Citation2018; CitationStrelin et al., Citation2014).
2. Study area
The mapping area is located at ∼44–46° S and ∼70.5–72.5° W. The area included at least eight eastward-draining outlet glaciers during the last glacial cycle (). These were the Río Pico (∼44.2° S), Río Caceres (∼44.5° S), Río Cisnes (∼44.6° S), Lago Plata-Fontana (∼44.8° S), El Toqui (∼45° S), Lago Coyt/Río Ñirehuao (∼45.3° S), Simpson/Paso Coyhaique (∼45.5° S), and Balmaceda (∼46° S) glaciers.
Figure 1. Continental and regional setting of valleys mapped in this study. (b) Regional setting of mapped valleys, including the location of the main outlet glaciers: P: Río Pico, Ca: Río Caceres, C: Río Cisnes, PF: Lago Plata-Fontana, T: El Toqui, Ñ: Lago Coyt/Río Ñirehuao, SP: Simpson/Paso Coyhaique, and B: Balmaceda. LGC/BA: Lago General Carrera-Buenos Aires. Arrows represent main ice-flow directions. (a) Southern South America, including the mapped area. NPI: Northern Patagonian Icefield; SPI: Southern Patagonian Icefield; CDI: Cordillera Darwin Icefield. Ice extent at 35 ka is also presented and was extracted from PATICE (CitationDavies et al., Citation2020). Modern ice extent from RGI v. 6.0, region 17 (CitationArendt et al., Citation2017).
![Figure 1. Continental and regional setting of valleys mapped in this study. (b) Regional setting of mapped valleys, including the location of the main outlet glaciers: P: Río Pico, Ca: Río Caceres, C: Río Cisnes, PF: Lago Plata-Fontana, T: El Toqui, Ñ: Lago Coyt/Río Ñirehuao, SP: Simpson/Paso Coyhaique, and B: Balmaceda. LGC/BA: Lago General Carrera-Buenos Aires. Arrows represent main ice-flow directions. (a) Southern South America, including the mapped area. NPI: Northern Patagonian Icefield; SPI: Southern Patagonian Icefield; CDI: Cordillera Darwin Icefield. Ice extent at 35 ka is also presented and was extracted from PATICE (CitationDavies et al., Citation2020). Modern ice extent from RGI v. 6.0, region 17 (CitationArendt et al., Citation2017).](/cms/asset/03ca4d1f-7ee3-4a52-9ee2-547965b90eb4/tjom_a_1986158_f0001_oc.jpg)
The Andean Cordillera dominates the landscape in the west (), with local summit elevations of up to ∼2600 m a.s.l., before an eastward transition through the Patagonian foothills, which grade into the Argentine steppe. Ice originated from the Chilean highlands, advancing west towards the Pacific and east into present-day Argentina. During deglaciation, Atlantic-bound meltwater was stored in large proglacial lakes formed in overdeepened glacial troughs east of the ice divide (cf. CitationDavies et al., Citation2020; CitationGarcía et al., Citation2019). Further ice recession then led to the opening of palaeolake drainage routes through the Andean Cordillera (cf. CitationGarcía et al., Citation2019). Today, ≥1482 glaciers remain in the region (CitationArendt et al., Citation2017); most of these are found adjacent to the Queulat ice cap (44.4° S, ∼2000 m a.s.l.), and Mt. Hudson volcano (45.9° S, ∼1900 m a.s.l.).
3. Previous mapping
CitationCaldenius (Citation1932) first mapped glacial landforms and glaciolacustrine sediments in the study area. Since this time, a number of early field surveys (e.g. CitationBeltramone, Citation1991; CitationDal Molin, Citation1998; CitationLapido, Citation2000; CitationPloszkiewicz, Citation1987; CitationRamos, Citation1981) highlighted the largest and most extensive landforms, including moraine complexes or deposits, and proglacial outwash plains ( and ).
Figure 2. The location of published mapping between 44° and 46°S. Mapping type has been included: field or remotely sensed, as well as mapping extent. Where detailed maps or georeferenced locations were not available (), mapping extent has been estimated according to the data that has been described in each publication (legend: previous mapping). Areas that were field verified in this study have also been included (grey boxes). Modern ice extent from RGI v. 6.0, region 17 (CitationArendt et al., Citation2017). LGC/BA: Lago General Carrera-Buenos Aires.
![Figure 2. The location of published mapping between 44° and 46°S. Mapping type has been included: field or remotely sensed, as well as mapping extent. Where detailed maps or georeferenced locations were not available (Table 1), mapping extent has been estimated according to the data that has been described in each publication (legend: previous mapping). Areas that were field verified in this study have also been included (grey boxes). Modern ice extent from RGI v. 6.0, region 17 (CitationArendt et al., Citation2017). LGC/BA: Lago General Carrera-Buenos Aires.](/cms/asset/ad6db2a6-51cf-46f1-99c7-d10c35e0d645/tjom_a_1986158_f0002_oc.jpg)
Table 1. Relevant glacial geomorphological landforms and glacier inventories presented in studies from , described using the terminology presented in this paper.
In the Río Pico valley, five moraine complexes were mapped in association with an eastward-draining outlet glacier (CitationBeraza & Vilas, Citation1989 and CitationLapido, Citation2000 In: CitationRabassa et al., Citation2011). Likewise, in the Río Cisnes valley, CitationGarcía et al. (Citation2019) mapped five main moraine complexes (Named: CIS 1, 2, 3, 4, and 5) and identified the limits of a proglacial palaeolake. Distinct palaeolake shorelines at 950-920 m a.s.l. and 860-850 m a.s.l. can be traced from the upper-to-middle Río Cisnes valley (CitationDavies et al., Citation2020; CitationGarcía et al., Citation2019). A col located at ∼920 m a.s.l. drained the palaeolake towards the Atlantic Ocean at this level (CitationDavies et al., Citation2020), whilst the prominent Winchester Delta (), south of the modern Río Cisnes, clearly marks the ∼860-850 m a.s.l. lake level (CitationGarcía et al., Citation2019).
Figure 3. Broad geomorphological context of mapped valleys, including key moraine complexes. It is likely that the stratigraphically oldest moraine complexes span multiple glacial advances. Moraine complexes were mostly identified from remote mapping and should be field verified. Outlet glaciers: P: Río Pico, Ca: Río Caceres, C: Río Cisnes, PF: Lago Plata-Fontana, T: El Toqui, Ñ: Lago Coyt/Río Ñirehuao, SP: Simpson/Paso Coyhaique, and B: Balmaceda. Location and extent of maps presented in have also been included (black boxes) and photos of glaciolacustrine sediments (red dots). Modern ice extent from RGI v. 6.0, region 17 (CitationArendt et al., Citation2017).
![Figure 3. Broad geomorphological context of mapped valleys, including key moraine complexes. It is likely that the stratigraphically oldest moraine complexes span multiple glacial advances. Moraine complexes were mostly identified from remote mapping and should be field verified. Outlet glaciers: P: Río Pico, Ca: Río Caceres, C: Río Cisnes, PF: Lago Plata-Fontana, T: El Toqui, Ñ: Lago Coyt/Río Ñirehuao, SP: Simpson/Paso Coyhaique, and B: Balmaceda. Location and extent of maps presented in Figures 4–6 have also been included (black boxes) and photos of glaciolacustrine sediments (red dots). Modern ice extent from RGI v. 6.0, region 17 (CitationArendt et al., Citation2017).](/cms/asset/fb7253b0-54b1-48cd-a93e-d5190e75f4dc/tjom_a_1986158_f0003_oc.jpg)
Figure 4. Ice-marginal landforms within the main outlet valleys and the Andean Cordillera. Raw satellite images (left) and mapped geomorphology (right). (a) Satellite image of central-northern sector of the Río Pico valley, including the location of (g). (b) Mapped landforms, including the PIC 6 moraine and an associated proglacial outwash plain. (c) Satellite image of a morainal bank, which is the lateral margin of the PIC 6 moraine, and a small isolated palaeolake basin in the northern Río Pico valley. (d) Mapped landforms surrounding the PIC 6 morainal bank, which is adjacent to ice-contact fans, and multiple, stepped, palaeolake shorelines. (e) Satellite image of the area southeast of the Queulat ice cap. (f) Mapped moraines and trimlines associated with the stratigraphically youngest glacier advances in the Andean Cordillera. (g) Ice-proximal view of the PIC 6 moraine in the field overlooking older moraines and terminating into a proglacial outwash plain. Satellite images are from ESRI™ DigitalGlobe World Imagery via ArcGIS Pro. Figure panel locations are shown in .
![Figure 4. Ice-marginal landforms within the main outlet valleys and the Andean Cordillera. Raw satellite images (left) and mapped geomorphology (right). (a) Satellite image of central-northern sector of the Río Pico valley, including the location of (g). (b) Mapped landforms, including the PIC 6 moraine and an associated proglacial outwash plain. (c) Satellite image of a morainal bank, which is the lateral margin of the PIC 6 moraine, and a small isolated palaeolake basin in the northern Río Pico valley. (d) Mapped landforms surrounding the PIC 6 morainal bank, which is adjacent to ice-contact fans, and multiple, stepped, palaeolake shorelines. (e) Satellite image of the area southeast of the Queulat ice cap. (f) Mapped moraines and trimlines associated with the stratigraphically youngest glacier advances in the Andean Cordillera. (g) Ice-proximal view of the PIC 6 moraine in the field overlooking older moraines and terminating into a proglacial outwash plain. Satellite images are from ESRI™ DigitalGlobe World Imagery via ArcGIS Pro. Figure panel locations are shown in Figure 3.](/cms/asset/fcfe4b7d-1911-4eea-a809-ba3cb817f3ab/tjom_a_1986158_f0004_oc.jpg)
Figure 5. Subglacial and glaciolacustrine landforms and sediments. (a) Satellite image of glacial lineations in the Simpson/Paso Coyhaique valley. (b) Mapped glacial lineations in the Simpson/Paso Coyhaique valley, dissected by proglacial outwash plains. Glacial lineations are aligned in a west-east and southwest-northeast direction, and adjacent to moraine complexes or deposits. (c) Satellite image of recently deglaciated ice cap south of Volcán Macá. (d) Flutes mapped within the Andean Cordillera which demonstrate multiple, diverging, local ice-flow directions. (e) Satellite image of the CIS 4 moraine, Río Cisnes valley. (f) Short-lived, narrow, palaeolake shorelines embedded on the CIS 4 moraine. (g) Laminated glaciolacustrine sediments located in the centre of the Río Pico palaeolake basin. (h) Glaciolacustrine sediments dissected by a tephra layer (cf. CitationStern et al., 2015) in the Río Cisnes valley.
![Figure 5. Subglacial and glaciolacustrine landforms and sediments. (a) Satellite image of glacial lineations in the Simpson/Paso Coyhaique valley. (b) Mapped glacial lineations in the Simpson/Paso Coyhaique valley, dissected by proglacial outwash plains. Glacial lineations are aligned in a west-east and southwest-northeast direction, and adjacent to moraine complexes or deposits. (c) Satellite image of recently deglaciated ice cap south of Volcán Macá. (d) Flutes mapped within the Andean Cordillera which demonstrate multiple, diverging, local ice-flow directions. (e) Satellite image of the CIS 4 moraine, Río Cisnes valley. (f) Short-lived, narrow, palaeolake shorelines embedded on the CIS 4 moraine. (g) Laminated glaciolacustrine sediments located in the centre of the Río Pico palaeolake basin. (h) Glaciolacustrine sediments dissected by a tephra layer (cf. CitationStern et al., 2015) in the Río Cisnes valley.](/cms/asset/ea82ff78-b46b-4056-bc1d-a7580fbaaa04/tjom_a_1986158_f0005_oc.jpg)
Figure 6. Examples of glaciofluvial landforms in the study area. (a) Location of catastrophic flood landforms and direction of final palaeolake drainage in the Río Cisnes valley. (b) Mapped catastrophic flood landforms along the banks of the modern Río Cisnes, adjacent to the CIS 5 moraine. Includes the location of (c). (c) Catastrophic flood landform in the foreground, adjacent to the CIS 5 moraine. (d) Location and ice-flow direction at Lago La Paloma, the site of a tributary glacier of the main Balmaceda valley. (e) Mapping of the La Paloma glacier, including moraine ridges and proglacial outwash terraces. Proglacial outwash terraces indicate that glaciofluvial drainage from the La Paloma glacier was directed towards the main Balmaceda valley.
![Figure 6. Examples of glaciofluvial landforms in the study area. (a) Location of catastrophic flood landforms and direction of final palaeolake drainage in the Río Cisnes valley. (b) Mapped catastrophic flood landforms along the banks of the modern Río Cisnes, adjacent to the CIS 5 moraine. Includes the location of (c). (c) Catastrophic flood landform in the foreground, adjacent to the CIS 5 moraine. (d) Location and ice-flow direction at Lago La Paloma, the site of a tributary glacier of the main Balmaceda valley. (e) Mapping of the La Paloma glacier, including moraine ridges and proglacial outwash terraces. Proglacial outwash terraces indicate that glaciofluvial drainage from the La Paloma glacier was directed towards the main Balmaceda valley.](/cms/asset/5e7408f4-682a-45ca-aa87-2198c94f87cd/tjom_a_1986158_f0006_oc.jpg)
CitationBeltramone (Citation1991) identified multiple moraine complexes or deposits and moraines associated with the Simpson/Paso Coyhaique glacier (). CitationDal Molin (Citation1998) and CitationDal Molin and González Díaz (Citation2002) further mapped broad moraine complexes and proglacial outwash plains from the Lago Coyt/Río Ñirehuao to Balmaceda valleys, east of the international boundary. CitationGlasser and Jansson (Citation2005) then used satellite remote sensing data to map in greater detail and presented the first indication of fast ice-flow in the region, evidenced by glacial lineations. Subsequently, CitationTaylor (Citation2019) provided thorough mapping of the Balmaceda outlet glacier (). This included the identification of previously unmapped moraine complexes and ridges associated with several glacial advances, proglacial outwash plains, and multiple palaeolake levels.
Within the Andean Cordillera, CitationMardones et al. (Citation2011) mapped moraines deposited in tributary valleys of the main Balmaceda outlet glacier ( and ). CitationDavies and Glasser (Citation2012) then identified late-Holocene moraines and trimlines, allowing an assessment of recent ice extent. Multiple studies have now provided mapping within the Andean Cordillera (CitationGlasser & Jansson, Citation2005, Citation2008; CitationMardones et al., Citation2011; CitationMoreno et al., Citation2021), but at variable spatial scales.
The PATICE database compiled and generated new mapping () for the entire PIS (CitationDavies et al., Citation2020). In the area located between 44° and 46° S, detailed mapping has been limited to the Río Cisnes valley (e.g. CitationGarcía et al., Citation2019), and a regional, high-resolution, inventory is needed to assess palaeoglaciological processes.
4. Methods
Mapping of the eastern outlet glaciers was completed at 1:10,000–1:40,000 spatial scales, and a 1:1000–1:10,000 scale was used inside the Andean Cordillera. Landform identifications were undertaken according to descriptions outlined by CitationBendle et al. (Citation2017b, pp. 660–661), CitationMartin et al. (Citation2019, pp. 114–115), and CitationLeger et al. (Citation2020, pp. 654–656), following protocols outlined in CitationChandler et al. (Citation2018). Identification criteria were then modified for the study area (). Where possible, published landforms were re-mapped and incorporated into the new inventory (). River and proglacial outwash plain shapefiles were adapted from the PATICE database (CitationDavies et al., Citation2020), and modern glaciers were extracted from the Randolph Glacier Inventory (RGI v. 6.0, region 17; CitationArendt et al., Citation2017).
Table 2 . Rationale, uncertainties, and identification criteria for ice-marginal, subglacial, glaciolacustrine, glaciofluvial, and non-glacial landforms mapped between 44 – 46°S. Total mapped landform numbers and/or areas are provided where applicable. Landform occurrence (✘) within the study region: P: Río Pico outlet glacier, Ca: Río Caceres outlet glacier, C: Río Cisnes outlet glacier, PF: Lago Plata-Fontana outlet glacier, T: El Toqui outlet glacier Ñ: Lago Coyt/Río Ñirehuao outlet glacier, SP: Simpson/Paso Coyhaique outlet glacier, B: Balmaceda outlet glacier, and A: landforms mapped within the Andean Cordillera. Identification criteria were modelled after CitationBendle et al. (Citation2017b), CitationMartin et al. (Citation2019), and CitationLeger et al. (Citation2020): criteria were then modified and updated for the study area.
Mapping was completed with ArcGIS Pro using the Southern Hemisphere WGS-1984-UTM Zone 18S Projected Coordinate System. A ∼50,000 km2 area was mapped using an ASTER (v.3) 30 m resolution Digital Elevation Model (ASTER DEM). Composite ESRI™ DigitalGlobe World Imagery (1–2 m resolution) and Sentinel-2 (10 m resolution) imagery were used to identify landforms. Hillshade and Slope Digital Terrain Models were generated using the Spatial Analyst function and overlain onto the ASTER DEM to highlight topographic relief and elevation (CitationOtto & Smith, Citation2013). Field surveys validated geomorphological interpretations at multiple locations ().
5. Mapped landforms
Mapping was classified into ice-marginal, subglacial, glaciolacustrine, glaciofluvial, and non-glacial landform categories (). Within these groups, a total of 27 landform types were digitised, which resulted in a database of >70,000 mapped features. We describe landforms mapped in the eight outlet valleys and the Andean Cordillera.
5.1. Ice-marginal landforms
5.1.1. Moraine complexes or deposits
Arcuate moraine complexes were deposited by the main outlet glaciers, and extend over tens of kms across the mouths of the individual valleys. Moraine complexes or deposits were mapped in multiple previous publications and have been re-mapped in greater detail. Herein, moraine complexes have been named to follow a standard nomenclature ().
Moraines can be grouped into sets (moraine complexes) using morphostratigraphy to determine the relative ice-margin age (cf. CitationLukas, Citation2006) prior to developing geochronological datasets (CitationLüthgens & Böse, Citation2012). Moraine complexes or deposits were defined from satellite imagery based on geographic position, as well as moraine morphology and proximity. Where available, proglacial outwash plains were also used to identify the lateral and terminal extent of moraine complexes. However, the easternmost moraine complexes commonly lack distinct moraine ridges and are instead comprised of mounds or flat-topped and irregular terrain. In particular, the limits of older or proximal moraine complexes or deposits (e.g. : SPC 1) can become difficult to define and should be field verified.
5.1.2. Moraine ridges
Moraine ridges mark the extent of past glacial advances, readvances, or stillstands (e.g. CitationDavies et al., Citation2020; CitationGlasser & Jansson, 2008) and define the geometry of the glacier terminus (CitationEly et al., Citation2021). Moraine ridges were mapped in all outlet valleys and within the Andean Cordillera.
Strings of latero-frontal moraines cap the eastern margins of the former outlet glaciers. Moraine morphology varies between straight, arcuate, or meandering ridges ((a,b)) or mounds, ranging from <0.01 km to ∼50 km in length (). In the field, the moraine crest/apex usually possesses an undulating morphology ((f)) and are commonly overlain with sub-rounded, faceted boulders. The highest relief (ca 250 m) and most distinctive of these possess a sharp apex, steep ice-distal slope, and crosscut the Río Cisnes valley (CIS 3, 4 & 5; CitationGarcía et al., Citation2019). The longest moraine in the region is found in the Río Pico valley, a laterally continuous moraine (∼50 km in length), with a sinuous morphology ( and (a,b)).
Within the Andean Cordillera, stratigraphically younger moraines are usually low-relief (<10 m), continuous ridges that possess an arcuate form and point down-slope of empty and occupied mountaintop cirques (e.g. ). Straight or winding, discontinuous ridges with close spacing are found nested up-slope of larger latero-frontal moraines. The best preserved of these are found surrounding the Mt. Hudson volcano.
5.1.3. Morainal banks
A rounded wedge of sediment, possessing a steep ice-distal face, and gently tapering ice-proximal slope (44°03′32.8″ S, 71°34′22.9″ W) has been mapped adjacent to palaeolake shorelines and ice-contact fans in the northern Río Pico valley. This landform has been interpreted as a morainal bank (cf. CitationFitzsimons & Howarth, Citation2018; CitationRovey & Borucki, Citation1995), indicating a glaciolacustrine terminus with subaqueous deposition (cf. CitationDavies et al., Citation2018; CitationMartin et al., Citation2019). The morainal bank marks the northern lateral edge of the PIC 6 moraine ((c,d)), aligned in a south-north direction, ∼5 km in length, and terminating into a small (∼12 km2), isolated, palaeolake basin (44°01′56.7″ S, 71°32′57.0″ W).
5.1.4. Trimlines
Trimlines were mapped within the Andean Cordillera (e.g. (e,f)), and are concentrated around the remaining ice caps (e.g. ). Trimlines are found in two forms: the first is lobate landforms, which possess a sharp or gradational break between glacially scoured bedrock, sediment, or shrubs to the developed treeline, and point down-valley from active glaciers or abandoned cirques. The second possesses a sharp transition between underlying sediment/bedrock to low-lying shrubs and run parallel with the valley sides. According to criteria outlined by CitationRotes and Clark (2020), these are classified as glacial trimlines based on a clear contrast between surface age (e.g. developed vegetation, glacially scoured bedrock), proximity to moraines, or position down-slope of glaciers.
5.2. Subglacial landforms
5.2.1. Cirques
Landforms identified on mountain tops in the Andean Cordillera and marked by arcuate headwalls which taper into sub-rounded erosional hollows are mapped as cirques ((e,f)). Many cirques in Patagonia still contain glaciers (e.g. CitationAraos et al., Citation2018; CitationMartin et al., Citation2019), though here we map abundant deglacierised cirques. Cirques can be identified in the surrounding landscape based on their position up-slope of ice-marginal (e.g. latero-frontal moraines) and subglacial (e.g. flutes, glacially scoured bedrock) landforms. The cirque bed will often appear overdeepened from subglacial erosion (CitationHooke, Citation1991), and sometimes contains a lake.
5.2.2. Crag-and-Tails
Crag-and-tails can be differentiated from other subglacial landforms by a resistant bedrock core on their up-ice (stoss) end, with a gently tapering lee slope composed of softer glacial sediments (CitationBenn & Evans, Citation2014; CitationStroeven et al., Citation2013). Elongate ridges (individually ≤2 km in length, ≤0.4 km in width), only mapped in the Río Cisnes valley, are interpreted as crag-and-tails, aligned southwest-northeast and streamlined in the direction of local ice-flow.
5.2.3. Glacially scoured bedrock
Glacially scoured bedrock is distinguishable from the surrounding landscape by a smoothed bedrock surface often interspersed with vegetation, till, or found overlain with perched boulders. These glacial erosional landforms may indicate points of greater ice thickness, where basal ice has reached its pressure-melting point (CitationGlasser & Bennett, Citation2004; CitationShaw, Citation1994). Similarly to the more northerly Río Corcovado, Río Huemul, and Lago Palena-General Vintter glaciers (CitationLeger et al., Citation2020, ∼43° S), the Río Pico valley possesses extensive areas of glacially scoured bedrock (∼84 km2), and displays a broad-scale west–east streamlining in the direction of local ice-flow.
5.2.4. Glacial lineations
Glacial lineations are defined by an ovate or linear, elongate morphology, valley-floor position, and orientation in the direction of ice-flow. These landforms can be utilised as flow lines and may indicate areas of thick or warm-based ice that scoured and deposited sediment into linear ridges (CitationGlasser & Jansson, Citation2005; CitationStroeven et al., Citation2013). Regionally, the most well-defined, elongate (≤2.5 km in length) and narrow (≤0.1 km in width) ridges occur in the Simpson/Paso Coyhaique valley (e.g. (a,b)), aligned west–east and southwest–northeast reflecting local ice-flow directions.
5.2.5. Flutes
Narrow, elongate, and streamlined ridges (≤1.5 km in length) are a product of subglacial deformation (cf. CitationVan der Meer, Citation1997), and were mapped as sedimentary flutes. Flutes are only preserved within recently deglacierised areas, and are useful in determining the direction(s) of local ice-flow (CitationGordon et al., 1992). We mapped 2521 flutes on mountaintops within the Andean Cordillera. The best example of these can be found south of Volcán Macá ((c,d)), up-valley of glacial trimlines, and demonstrating multiple, diverging, ice-flow paths.
5.3. Glaciolacustrine landforms
5.3.1. Palaeolake shorelines
In Patagonia, during glacial maxima, erosion of sediment/bedrock beneath outlet glaciers formed glacial overdeepenings (cf. CitationCook & Swift, Citation2012), which were sometimes exposed subaerially during deglaciation. These overdeepenings commonly developed into proglacial lakes during deglaciation (e.g. CitationGarcía et al., Citation2019; CitationLeger et al., Citation2020). In the study area, most of these lakes have now drained, leaving well-preserved palaeolake shorelines to mark their former extent (CitationDavies et al., Citation2020; CitationGarcía et al., Citation2019).
We mapped palaeolake shorelines in two common forms. The first is continuous, flat-topped terraces calved into valley sides, moraines, or associated with perched deltas. These shorelines represent the most long-lived lake levels in the region. The largest and most distinctive of these shorelines are found in the Río Cisnes valley at 950–920 m a.s.l., and 860–850 m a.s.l. (CitationDavies et al., Citation2020; CitationGarcía et al., Citation2019), and the Río Pico valley at ∼870 m a.s.l.
The second type is sets of multiple, discontinuous, shorelines found on moraines or eroded into valley-side deposits ((e,f)). These are more difficult to discern in the field due to narrow terracing and closely spaced steps, and represent a punctuated lake level lowering through time (e.g. CitationBell, Citation2008; CitationThorndycraft et al., Citation2019a). These sets are found in the Río Pico (1010–910 m a.s.l.), Río Cisnes (790–650 m a.s.l.), Lago Coyt/Río Ñirehuao (730–600 m a.s.l.) and Balmaceda (∼600–610 m a.s.l.) valleys.
5.3.2. Perched deltas
Perched deltas formed when rivers met proglacial lakes and remained raised above the (palaeo-)lake basin as lake levels fell (e.g. CitationDulfer & Margold, Citation2021; CitationStroeven et al., Citation2016). Perched deltas are flat-topped or possess a gentle slope oriented towards, but elevated above, a palaeolake basin. Mapping the distribution and elevation of perched deltas, and comparing them with palaeolake shorelines, informs on the extent of former palaeolake levels (cf. CitationBell, Citation2008; CitationGlasser et al., Citation2016; CitationTurner et al., Citation2005). We mapped 91 perched deltas in the study area, the largest of these is the Winchester Delta (44°36′30.9″ S, 71°27′08.4″ W) in the Río Cisnes valley (CitationGarcía et al., Citation2019), marking a local palaeolake level of 860–850 m a.s.l. Large deltas are also found in the Río Pico valley (44°04′54.3″ S, 71°35′34.1″ W; 44°05′12.8″ S, 71°33′07.0″ W), associated with a ∼850 m a.s.l. delta-top area.
5.3.3. Ice-contact fans
Ice-contact fans morphologically resemble perched deltas but can be distinguished based on their proximity to moraines. These landforms are composed of a build-up of glaciofluvial outwash on the ice-distal side of moraines during a prolonged period of glacier stability (CitationDowdeswell et al., Citation2015), and indicate the former ice-contact lake level at the time of moraine formation. The largest ice-contact fan (∼15 km2) is found adjacent to the southern lateral margin of the PIC 6 moraine (44°24′48.4″ S 71°33′38.1″ W), gently sloping into the Río Caceres palaeolake basin, and marking a ∼900 m a.s.l. lake level.
5.3.4. Glaciolacustrine sediments
CitationCaldenius (Citation1932) first described glaciolacustrine sediments in the study region, but precise locations were previously unknown. Fine grained, well-sorted, and laminated glaciolacustrine sediments have accumulated in association with proglacial palaeolake formation during deglaciation ((g,h)). At the macro-scale, these deposits resemble varved sediments studied at Lago Buenos Aires (CitationBendle et al., Citation2017a). Laminated glaciolacustrine sediments were field verified in the Río Pico, Río Cisnes, Simpson/Paso Coyhaique, and Balmaceda valleys.
5.3.5. Palaeolake spillways (cols)
We updated previous mapping of palaeolake spillways (cols) identified in the Río Cisnes, Lago Coyt/Río Ñirehuao, and Balmaceda valleys (CitationDavies et al., Citation2020; CitationGarcía et al., Citation2019), which are important for studying the switch between Atlantic-bound and Pacific-bound drainage routes during deglaciation (CitationThorndycraft et al., Citation2019a; Citation2019b). We mapped two additional cols in the Río Pico valley at ∼810 m a.s.l. and ∼770 m a.s.l., by analysing contour elevations of known palaeolake shorelines in ArcMap. These data suggest that the Pico palaeolake drained east, towards the Atlantic, until the palaeolake level dropped below ∼770 m a.s.l.
5.4. Glaciofluvial landforms
5.4.1. Catastrophic flood landforms
We identified boulder bars in the middle Río Cisnes valley (∼470–560 m a.s.l.), above the banks of the modern Río Cisnes, and in association with the CIS 5 moraine ((a–b)). The boulder bars are characterised by steep scarp slopes with a flat-topped surface, interspersed with sub-rounded boulders (≤3 m in height) and cobbles. In the field, we interpreted one boulder bar (44°40′08.0″ S, 71°49′06.6″ W, 545 m a.s.l.) as an expansion bar with a fossa channel located on the southern edge of the deposit ((c)). These landforms are similar to those identified by CitationBenito and Thorndycraft (Citation2020) further south in the Baker valley, which were interpreted as forming as the result of catastrophic floods (>105 m3/s) during palaeolake drainage.
5.4.2. Proglacial outwash plains
Proglacial outwash plains are characterised by broad, flat-topped expanses of glaciofluvial sediment, forming on the ice-distal side of moraines, or found nested between moraine complexes (e.g. CitationBendle et al., Citation2017b; CitationDarvill et al., Citation2014). Similarly to moraine complexes, proglacial outwash plains may provide approximate ice-marginal positions but additionally indicate the direction(s) of glaciofluvial drainage. Mountainous areas to the east of the main outlet glaciers constrained the area of former glaciofluvial activity; beyond this, the landscape transitioned into the Argentine steppe, where proglacial outwash plains formed continental drainage networks (e.g. CitationClapperton, Citation1993).
5.4.3. Proglacial outwash terraces
Raised, flat-topped terraces of glaciofluvial sediment, located within the margins of proglacial outwash plains, are interpreted to form as a result of isostatic uplift (CitationMaizels, Citation2002) or (glacio-)fluvial incision (CitationEilertsen et al., Citation2015). Stratigraphically younger terraces usually display well-preserved meltwater channels, whilst the surfaces of outwash terraces associated with the most extensive glacial advances appear homogenous. Based on geographic position, some of the stratigraphically youngest outwash terraces, oriented in a southwest–northeast direction, are found in a tributary valley of the Balmaceda glacier ((d,e)).
5.4.4. Meltwater channels
Meltwater channels were mapped in the main outlet valleys, and in two main forms. The first are proglacial meltwater channels, which form extensive networks and are found on the ice-distal side of moraines, preserved on proglacial outwash plains (e.g. CitationBendle et al., Citation2017b; CitationLeger et al., Citation2020). These are generally sinuous, discontinuous, channels with narrow spacing. The second are ice-marginal and subglacial meltwater channels that dissect moraine ridges (e.g. (a,b)) or the valley floor. These channels are sinuous, vary in width from less than 50 m to over 150 m, and can only be traced for short distances. Where moraine complexes or ridges are heavily dissected, ice-marginal meltwater channels provide additional evidence for ice extent (CitationGreenwood et al., Citation2007).
5.4.5. Kettle holes
Kettle holes were mapped inside proglacial outwash plains or nested within/between moraine complexes. Kettle holes are enclosed depressions with an ovate or sub-rounded planform shape and can be dry or contain small lakes. These landforms indicate regions where dead ice has been buried, melted, and left a residual pit (CitationEvans & Orton, Citation2015). The largest kettle holes (∼2 km2) are located between the ÑIR 1 and ÑIR 2 moraine complexes, in the Lago Coyt/Río Ñirehuao valley.
5.5. Non-glacial landforms
Some non-glacial features were mapped in addition to glacial geomorphology. Holocene rivers, fans and deltas, palaeochannels, wetlands, and major floodplains determine the direction of modern or recent drainage routes and point to areas of possible post-depositional re-working. River terraces provide an indication of the relative age of drainage routes, and therefore the timing of continental drainage re-organisation. Mapping residual lakes can also be useful, particularly in determining the structure of palaeolake evolution in the main valleys.
Landslides or slumps were common following deglaciation, and particularly in the southern region (below 45.5° S) of the study area (cf. CitationPánek et al., Citation2021). Mapping landslides or slumps is important, as gravitational instabilities can be mistaken for moraines (e.g. CitationSoteres et al., Citation2020). In particular, younger landslides or slumps appear to have overlaid segments of the Simpson/Paso Coyhaique and Balmaceda lateral moraine complexes.
6. Concluding points
We present a glacial geomorphological map covering a ∼50,000 km2 area of west-central Patagonia (44–46° S), building on existing mapping in the region (). The map provides a detailed, georeferenced, inventory of glacial landforms mapped within eight outlet valleys and the Andean Cordillera. The inventory contains >70,000 features and 27 landform groups which were classified as ice-marginal, subglacial, glaciolacustrine, glaciofluvial, or non-glacial landforms. Our mapping reveals: (1) Multiple, previously unmapped, moraine complexes or deposits and ridges () spanning from the most extensive glacial advances to the stratigraphically youngest; (2) the widespread occurrence of palaeolake shorelines, perched deltas, ice-contact fans, and glaciolacustrine sediments in association with the main outlet glaciers informing on the existence and geolocation of multiple, large palaeolakes; and (3) the distribution of proglacial outwash plains, proglacial outwash terraces, and meltwater channels, which provide evidence for the extent, longevity, and direction of regional glaciofluvial drainage networks. Data will underpin future assessments of palaeo-glacier dynamics, the generation of geochronological datasets through direct-dating methods, and the basis for refining reconstructions of the Quaternary Patagonian Ice Sheet.
Software
Mapping was originally undertaken in ArcMap (v. 10.3) and completed in ArcGIS Pro. Full layout design for all maps was completed using ArcGIS Pro.
Supplemental Material
Download Zip (103.4 MB)TJOM_1986158_supplemental material
Download PDF (49.4 MB)Acknowledgements
The authors would like to acknowledge Joshua Pike for his support in the field, Stephanie Bouckaert for granting generous access to the Cisnes Estanica, and both Amalia Nuevo Delaunay and César Méndez for local guidance, field support, and off-road transport. Finally, the authors would also like to thank Tancrède Leger, Harold Lovell, Heike Apps, and Jasper Knight for providing detailed reviews or comments which helped significantly in refining the finished manuscript.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Data Availability Statement
All shapefiles are available in the supplementary materials of this paper.
Additional information
Funding
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