This article summarises a recent publication by the AntarcticGlaciers.org author Bethan Davies and colleagues on glaciolacustrine sediment-landform assemblages in Chilean Patagonia:

Davies, B. J., Thorndycraft, V. R., Fabel, D. & Martin, J. R. V. Asynchronous glacier dynamics during the Antarctic Cold Reversal in central Patagonia. Quaternary Science Reviews 200, 287-312, (2018).

Ice-dammed lakes in Patagonia | Glacial geomorphology | Glaciolacustrine landforms | Glacial landforms | Glaciolacustrine landsystems | Further reading | References | Comments |

Ice-dammed lakes in Patagonia

Following the Last Glacial Maximum in Patagonia, outlet glaciers from the Patagonian Ice Sheet (see figure below) receded1, resulting in the formation of large ice-dammed lakes that drained across Argentina to the Atlantic Ocean2,3.

These ice-dammed lakes grew as the glaciers receded, and this may have affected ice-sheet dynamics by influencing calving. These “glaciolacustrine” environments have a number of distinctive landforms, including morainal banks, deltas4, shorelines5 and push moraines.

We conducted a detailed study of the glaciolacustrine landforms near Cochrane in Chile (see Davies et al., 2018)6, using both glacial geomorphological mapping and cosmogenic nuclide dating to analyse glacial landforms and establish the manner and timing of their formation.

You can inspect the study area yourself, using Google Maps:

Glacial geomorphology

The Río Baker valley preserves a number of detailed glacial and glaciolacustrine landforms relating to outlet glaciers from the North Patagonian Icefield and Monte San Lorenzo.

During the last glaciation, these outlet lobes coalesced to form part of the Lago Cochrane outlet lobe at the Last Glacial Maximum. These landforms were formed later, in association with a large ice-dammed lake that occupied the Baker Valley. This ice-dammed lake had several different levels that related to different drainage pathways as glaciers receded and new cols opened up.

Our glacial geomorphological map shows the detailed landform assemblage in the area around Cochrane.

Glaciolacustrine landforms

Glacier lake shorelines

The level that had the highest geomorphic imprint in the Cochrane area is the 350 m lake. This ice-dammed lake can be traced throughout the Baker Valley, around Lago Cochrane/Pueyrredón, and into the Lago General Carrera/Buenos Aires valley. It probably drained through a 350 m col above Río Bayo, draining around the northern margin of the Northern Patagonian Icefield and into the Pacific Ocean7. We call this lake “Lago Chalenko”.

This lake resulted in the formation of shorelines, which are eroded into glacial sediments on valley sides at a consistent elevation; 460 m north of Cochrane, and 340-350 m around Valle Grande, around the Juncal Massif and around Lago Esmeralda. The shorelines are gently valley-dipping platforms, with a flat long-profile and often with exotic and local boulders.

Sediments associated with the glacier lake include well sorted laminated silts and clays, with dropstones indicating iceberg rafting.

Deltas

There are a series of perched flat-topped delta terraces (350 – 460 m) at the foot of Río Estero Elva, which currently drains into Lago Cochrane/Pueyrredón. Gilbert-type deltas such as these form when streams enter lakes 4,8-10, and form in stepped sequences upstream of modern lake deltas.

Morainal banks

Adjacent to the Esmeralda Moraines, is an asymmetric moraine closely associated with the 350 m shoreline. The moraines have a wedge shape, with a shallow ice-proximal slope and a steep ice-distal slope. The moraines reach 330 m in height, and are below the 350 m shoreline.

The Salto Moraines are at the edge of a hanging valley, perched above Valle Grande. The Salto Moraines reach heights of 350 m above sea level, and are closely associated with the 350 m shoreline, which is cut into the top of the moraines around the Juncal Massif.

The Salto Moraines have a gentle ice-proximal slope, which is interrupted with numerous small recessional moraines, and a steep ice-distal slope. The steep ice-distal slope of the Salto Moraines is imprinted with shorelines and deltas, indicating lower lakes persisted in Valle Grande after Lago Chalenko drained.

These asymmetrical moraines are interpreted as ‘Morainal Banks’ formed at the grounding line of a valley glacier that terminated in Lago Chalenko.

You can view these moraines yourself, and compare the morainal bank with the Esmeralda push moraines, in Google Maps above.

Ice-contact fans

On the eastern flank of the “Juncal Massif” there is an accumulation of sediments that dips towards the valley floor. This is interpreted as an ice-contact fan, which formed subaqueously at the grounding line of the glacier that terminated in Lago Chalenko as it receded.

Glacial landforms

Push moraines

The Esmeralda Moraines (see geomorphological map above) are classical push moraines, formed above the level of the lake. They are arcuate, sharp-crested, cross-valley ridges with a ridge crest at 360-366 m. The narrow ridge crest is only 3 m wide, and the moraine ridges are 60 m high, with symmetrical, steeply sloping sides. Inside the main moraine crest are numerous smaller moraines, interpreted as recessional moraines.

These moraines represent an advanced position of Calluqueo Glacier from Monte San Lorenzo. South of the Esmeralda Moraines are a set of recessional moraines, called the “Moraine Mounds”11, which represent a stillstand during the recession of Calluqueo Glacier.

A shoreline is cut into glacial sediments around Lago Esmeralda below the height of the Esmeralda Moraines, suggesting that the 350 m glacial lake (Lago Chalenko) flooded the valley upon recession of Calluequeo Glacier from this position.

Lateral moraines

Numerous lateral moraines are perched on the hillsides of the Salto Valley. One moraine, field-checked at 510 m, is contiguous with the terminal Esmeralda Moraines. These moraines are sharp-crested, sloping, and wrap around the hillsides. The moraines photographed below are all above Lago Esmeralda.

Kame terraces

Directly opposite the Juncal Fan, against the other side of the Salto Valley, is an accumulation of sediments with a flat topped surface. This is interpreted as a kame terrace that formed in the gap between the glacier and the valley side, with meltwater streams washing in sediments. The flat flop surface is equivalent to the ice surface.

Ice-scoured bedrock

Patches of ice-scoured bedrock crop out in the valley floor and on the valley sides regionally. They form well-developed bedrock lineations that are aligned along the valley long-axis. These lineations are interpreted as roche moutonnées. They form by a process of glacial abrasion and polishing as the glacier ice flows over the bedrock.

Glaciolacustrine landsystems

The Cochrane region bears a distinctive assemblage of landforms, with valley glaciers terminating in a glaciolacustrine environment, forming sub-aqueous, asymmetric morainal banks and sharp-crested, terminal push moraines.

Glaciolacustrine landforms in this landsystem include deltas, palaeoshorelines and subaqueous fans. The morainal banks formed before large drops in the altitude of the valley floor, indicating that calving and lake depth were some of the main controls on glacier terminus position.

Sediments were deposited on the lake floor by turbidity currents and underflows emanating from the glacier margin in front of the morainal bank. Glaciolacustrine deposition is likely to be strongly influenced by underflows into a sediment-stratified proglacial lake. Freshwater buoyant plumes emanating from the ice margin are usually more important in glaciomarine environments.

The confined environment has led to a great diversity in sediments and landforms, with asymmetric morainal banks forming during periods of stability and ice-contact fans forming along the valley sides during periods of recession. These landforms are indicative of abundant meltwater and a high sediment flux.

Further Reading

There are several pages on this website on the Patagonian Ice Sheet:

See also our webpages on Glacial Lakes.