Rapid terrace incision and Quaternary landscape evolution in central Patagonia

The debate over isostatic uplift versus discharge as drivers of Quaternary river incision is explored here through geospatial analysis of a ~250‐m‐relief terrace sequence from the Río Pinturas (Argentine Patagonia). The geomorphic setting of the Cañadon Caracoles reach allows evaluation of discharge as a driver of terrace incision because advancing ice during Pleistocene glaciations blocked Pacific drainage and routed meltwater from an expanded ~104‐km2 catchment to the Atlantic through the dryland steppe of the study area. Twenty‐two terrace levels, some assigned to previously dated ice margins [Marine Isotope Stage (MIS) 32–36, MIS 16, MIS 8 and MIS 2], were identified. Average net incision since 800 ka (~0.34 m ka−1) exceeded regional uplift rates. The MIS 2 terraces, with ante quem and post quem age constraint on the timing of terrace formation, show that terrace incision was episodic and faster still during a transitional warming climate. Glacier recession and proglacial lake formation at ~18 ka led to rapid incision of ~11.7 m ka−1 over a few millennia. River capture and negligible flow from ~15.0 ka caused fan‐dammed lake formation on the valley floor and vertical stability during MIS 1. The Pinturas terraces demonstrate rapid incision can be driven by discharge and sediment dynamics.


Introduction
Fluvial palaeohydrological tools are applied to evaluate allogenic and autogenic drivers of change in river catchments over geologically instantaneous to 10 6 -year timescales (Schumm, 1977;Daniels, 2008;Thorndycraft, 2013;Baker et al., 2022).In Patagonia (Fig. 1a), a region shaped by marked climatic gradients (Garreaud et al., 2013), neotectonics (Bourgois et al., 2021) and Plio-Quaternary glaciations (Rabassa et al., 2005;Rabassa and Coronato, 2009;Rabassa et al., 2011) there is great potential for fluvial palaeohydrological research to: (i) provide baseline data on fluvial landscape processes in this large, diverse, data-sparse region; and (ii) elucidate fluvial drivers over a range of timescales (Baker et al., 2022).Despite this, modern studies on Quaternary palaeohydrology and river development are geographically isolated across the region.Tobal et al. (2021) investigated the long-term Quaternary terrace sequence of the Atlantic draining Deseado valley (46.3°S,Fig. 1b and c).Here they dated terrace levels from 1167 to 447 ka, each terrace probably associated with glacial phases.Skirrow et al. (2021) dated terrace incision and planform change in the upper Chubut River (42°S, 70°W) identifying a time transgressive change from braided to meandering across the late Pleistocene to early Holocene transition.This planform change was hypothesized to have been driven by a latitudinal shift in the austral westerlies that resulted in drier conditions in the catchment (Skirrow et al., 2021).In the Pacific-draining Baker catchment research has focused on reconstructing the magnitude and timing of glacial lake outburst floods (GLOFs) through landforms (Benito and Thorndycraft, 2020), slackwater flood sediments in bedrock gorge reaches (Benito et al., 2021) and alluvial palaeochannel fills (Vandekerkhove et al., 2020).
In all the examples above, climate is an important driver of change whether it be through fluvial response to the shifting austral westerlies (Skirrow et al., 2021) or its role in modulating the timing and spatial extent of glaciations, and the subsequent patterns of deglaciation and proglacial lake development (Benito and Thorndycraft, 2020;Benito et al., 2021).However, there are a range of other geophysical drivers in the region that also need careful evaluation, for example Miocene to Quaternary neotectonics (Vogt et al., 2010;Guillaume et al., 2013;Bourgois et al., 2021), volcanic activity (Coronato et al., 2013) and glacio-isostatic rebound (Troch et al., 2022).Fluvial palaeohydrology research in Patagonia, therefore, has the potential to quantify magnitudes and rates of landscape change over a range of timescales, from ungauged modern systems (Thorndycraft, 2022) to long-term Quaternary timescales (Coronato et al., 2013;Tobal et al., 2021).This contribution investigates the long-term Quaternary evolution of the upper Río Pinturas catchment (Argentina), from the Mid-Pleistocene to Holocene, to evaluate the role of Quaternary glaciations in the landscape evolution of Atlantic-draining Patagonian rivers.As such the work provides an exemplar of the type of inferences that can be read from Quaternary riverine landscapes in Patagonia.
The aims of the present study are to: (i) constrain the rates of fluvial terrace incision over the middle Pleistocene to Holocene; (ii) qualitatively evaluate the influence of long-term drivers of landscape evolution; and (iii) reconstruct a model of landscape evolution over glacial/interglacial cycles.The main approaches applied are remotely sensed geomorphological mapping and geospatial analysis of fluvial landforms, in particular river terraces, interpreted within the context of the glacial chronology of the Puerreydón ice-lobe (Hein et al., 2010(Hein et al., , 2011) ) of the former Patagonian Ice Sheet (Davies et al., 2020).

Study area
The main study area is the Cañadon Caracoles river valley (Figs 1 and 2a), a 1568-km 2 catchment (BasinATLAS, 2023;Linke et al., 2019) in Argentine Patagonia (47°S, 71°W, Fig. 1a).Cañadon Caracoles is a right bank tributary of the Río Pinturas (6454 km 2 ), which in turn feeds the Atlantic draining Río Deseado (30054 km 2 ) (Fig. 1c).For much of its length upstream from the Deseado confluence the Pinturas is incised in a bedrock gorge.The walls and caves in the gorge at Cueva de las Manos, now a UNESCO World Heritage archaeological site (https://whc.unesco.org/en/list/936/),were used by hunter gatherers for rock art, the most famous depictions being negative-relief hand stencils.Cañadon Caracoles joins the Pinturas upstream of the Cueva de las Manos archaeological site (Fig. 1d).At this point, the Pinturas drains an area of 1807 km 2 principally through the Río Ecker tributary.Herein, the Pinturas catchment upstream of its Cañadon Caracoles confluence (HYBAS_ID 6070963930, BasinATLAS, 2023;Linke et al., 2019) is referred to as the Ecker.
In terms of the modern drainage network, Cañadon Caracoles is an ephemeral river with a catchment elevation range of 435-1405 m asl.The Pinturas and its principal tributary, the Río Ecker, drain the basalt tableland of the Meseta del Lago Buenos Aires (Fig. 1b).The Ecker rises at ~2500 m a.s.l. and flows off the eastern escarpment of the meseta.Here the catchment comprises volcanic cones, lava flows, crater lakes and, in the escarpment sector, volcanic wet meadows (Mazzoni, Rabassa, 2013, 2018;Melendi et al., 2021) (Mercer, 1976;Hein et al., 2010;Thorndycraft et al., 2019a).The Cañadon Caracoles watershed here marks the present-day continental drainage divide.Note the landslide scar (Pánek et al., 2020)  to precipitation regime (Aravena and Luckman, 2009).The Argentine steppe of central Patagonia, to the east of the Andean Cordillera, is in the marked rain shadow of the austral westerlies (Garreaud et al., 2013).For the period 1931-2020 at Cañadon Caracoles, according to the CRU TS 4.05 half degree grid box climate data for 47.25°S, 70.75°W (Harris et al., 2020), mean annual temperature was 7.6 °C with a mean diurnal temperature range of 10.7 °C, mean annual precipitation of 89.7 mm, and mean annual vapour pressure of 5.1 HPa.According to the BasinATLAS database the potential natural vegetation of the study area is grassland/steppe (Ramankutty and Foley, 1999).
Cañadon Caracoles is an underfit valley that, during glacial periods, conveyed glacial meltwater from the Puerreydón ice-lobe of the former Patagonian Ice Sheet to the Atlantic (Caldenius, 1932;Hein et al., 2010).Following the last glaciation this Atlantic drainage pathway was probably abandoned by lake level fall ~15.5 ka (Mercer, 1976;Hein et al., 2010).This timing of lake level fall was corroborated by a weighted mean optically stimulated luminescence (OSL) age of 14. 8 ± 1.1 ka from a prograding delta forming in the lowered lake level (Vásquez et al., 2022).The cause of the drainage reversal was ice-sheet thinning and recession that opened a Pacific drainage route via the lower Baker valley (Hein et al., 2010;Thorndycraft et al., 2019a).Bourgois et al. (2021) hypothesized that the Río Baker (Fig. 1c) has probably drained through the Patagonian Cordillera to the Pacific Ocean since 14-18 Ma.Initially the Baker valley drained the interior presently occupied by Lago Puerreydón (Fig. 1b), but following uplift of the Andean Cordillera beneath the Northern Patagonian Icefield, the Baker captured drainage of the Lago Buenos Aires basin at ~3-4 Ma (Bourgois et al., 2021).Christeleit et al. (2017) argued, based on low-temperature thermochronometry, that early (pre-5 Ma) Patagonian glaciations incised valley relief in the region.The implication of these studies is that the glacial/interglacial, Atlantic/Pacific drainage reversals documented for the last glacial/interglacial cycle have occurred throughout the Quaternary period when expansion of the former Patagonian Ice Sheet was great enough to block Baker valley drainage.
A consequence of these Mid-to Late Pleistocene ice advances is that a large area of the Cañadon Caracoles tributary is underlain by Quaternary deposits (Fig. 1d), primarily icemarginal glacigenic sediments and glaciofluvial outwash cobbles and gravels (Hein et al., 2011).Outcrops of Jurassic bedrock occur beyond the MIS 32-36 ice limit and exposed along the incised Cañadon Caracoles gorge.The former outwash plains have been incised since the Late Pleistocene forming a terrace staircase (Fig. 3a-c) >250 m in relief.These terraces are the principal focus of this paper.
The tectonic setting of central Patagonia is dominated by the Chile Triple Junction.Here, the Nazca, South American and Antarctic plates converge off the Pacific coast at 45.5°S.Geological and geomorphic evidence suggests the opening of a slab window that resulted in the dynamic topography in the region (Guillaume et al., 2009;2013) leading to anomalous uplift rates in the study area, probably playing a role in shaping retrograde slopes in the region (Thorndycraft et al., 2019b).Troch et al. (2022) quantified the amount of glacial isostatic rebound beneath the present North Patagonian Icefield based on the record from an isolation basin to the west of the Andean Cordillera.They reported 96 m of glacial isostatic uplift between 16.5 and 9.1 cal ka BP in the centre of the former Patagonian Ice Sheet, with a mean uplift rate of 13 m ka −1 during this timeframe.

Methods
Geomorphological mapping was carried out in the study area through on-screen vectorization (Chandler et al., 2018) using a combination of cloud-free Sentinel (15-m resolution) satellite imagery, and ESRI™ World Imagery, which includes DigitalGlobe imagery of ~1-2-m resolution (GeoEye-1, IKONOS).Bendle et al. (2017a) used this methodology to map over 35 000 landforms across 46-48°S, including the Cañadon Caracoles study area, and shapefiles from that study were used as the starting point for mapping herein.Some landforms from Bendle et al. (2017a) were modified based on new interpretations and field observations.Mapping was carried out in ArcGIS Pro 2.6 and the WGS-1984 UTM-Zone19S coordinate system was used.The Digital Elevation Model (DEM) used for mapping and evaluation of terrace altitudes was the 30-m spatial resolution ALOS Global Digital Surface Model (DSM) (tiles ALPSMLC30_S048W071_DSM and ALPSMLC30_S048W072_DSM). Studies have shown this DSM has one of the best vertical accuracies of global open-source elevation data, especially in open non-forested landscapes (e.g.Ferreira, Cabral, 2021;Chai et al., 2022).
The terrace landforms in the study area were digitized at 1:5000 scale.Prior to geospatial analysis of terrace elevation, the preserved terrace landforms were mapped as one feature class, creating a layer of individual terrace fragments (Fig. 1d).These were then divided into separate aged terraces based on an iterative geospatial analysis.This involved the creation of valley long-profiles and crosssections, followed by cross-referencing these to the planform relationships between terraces, ice margins and the previously published geochronology from Hein et al. (2009Hein et al. ( , 2010Hein et al. ( , 2011)).
Once terraces had been assigned (from the oldest T1 terrace), elevation point cloud data were generated in ArcGIS Pro 2.6 for both ice-marginal landforms from each dated icelimit, and the terraces.This was seen as an approach to first test the assigned terrace numbers, and second as a means of data visualization.To create the elevation point cloud data, a grid of transect lines was produced in ArcGIS Pro.A baseline was drawn, west of the Cañadon Caracoles catchment, perpendicular to the dominant WSW-ENE drainage alignment of the valley.The Generate Transects Along Lines tool was used to draw lines, perpendicular to the valley axis, with line intervals set at 500 m.The Generate Points Along Lines tool was then used to add vector points to each transect line at 100-m intervals.These two tasks created a grid of points spaced at 500 × 100 m.Elevation from the ALOS-PALSAR DSM was added to these points using the Add Surface Information tool.The x-y coordinates for each point were also added.The gridded data layer was then clipped to each of the ice-marginal and terrace (T1, T2, etc.) layers to extract the point elevation data.To create an elevation point cloud graph, elevation data for each layer were plotted against the x (longitude) coordinate based on the general NE flow direction along the Cañadon Caracoles valley.The data, therefore, do not reflect true long profile distance down-valley but this was considered a robust approach to visualize the data, given the variability of down-valley flow orientations followed by meltwater and river flow over the million-year timeframe of the terraces, and the individual nature of ice-limit/outwash spatial relationships during different glacial periods.In addition, down-terrace long-profiles were extracted from the DEM, the pathway of the long-profiles drawn by following palaeochannels on satellite imagery.

Results
Geomorphic mapping revealed 83 terrace fragments covering 427.4 km 2 in the study area (Table 1).These were divided into 22 distinct terrace levels, where the precision of the DEM allowed (Table 1, Fig. 4).Terraces were assigned based on: (i) their spatial relationship (Fig. 4) to the east-west transect of older to younger morainic deposits (Figs 1d and 4), and (ii) their relative elevations to older and younger terraces (Figs 5 and 6).The oldest, T1, terrace surface is identified beyond the ice limit of the Great Patagonian Glaciation (GPG) and lies outside the Cañadon Caracoles catchment (Fig. 4).Downstream of the Caracoles and Pinturas tributary junction, terrace deposits outcrop atop the Pinturas gorge above the Cueva de las Manos site and can be traced to the GPG ice margin.The constraining age for the GPG ice margin and T1 comes from a cap of fluvioglacial gravels, dated by Hein et al. (2011), that lie stratigraphically above Plio-Quaternary sediments at a locality associated with the right lateral margin of the GPG glacier (see the black circle labelled T1 on Fig. 4).At Cueva de las Manos, the T1 terraces are situated ~225 m above the present Pinturas valley floor, constraining a rate of incision averaged over the last 1100 ka to 0.20 m ka −1 at this reach.
The oldest terrace (T2) of the Cañadon Caracoles catchment (Fig. 3a) features an ice-contact fan that ties these braided, outwash fluvioglacial deposits to the Outermost Caracoles ice limit of Hein et al. (2011), which probably constrains this glaciation to 780-900 ka, based on reversed polarity (Sylwan et al., 1991) and the age of the stratigraphically older GPG ice limit (Hein et al., 2009).Terrace T2 is The continental drainage divide marked by the windgap, the former outflow of the proglacial lake formed by ice-lobe recession in the Puerreydón basin (Fig. 1b).Note the landslide post-dating lake drainage (Pánek et al., 2020).[Color figure can be viewed at wileyonlinelibrary.com]Table 1.Spatial and timing information for terraces T2-T22 located within Cañadon Caracoles and the Great Patagonian Glaciation ice limit.
Terrace Area (km 2 therefore assigned to MIS 20-22 and an age of 800 ka is used for the incision rate calculation (Table 1).T2 is ~275 m above the valley floor (Fig. 6b) in this reach, constraining a net incision rate of 0.34 m ka −1 .Terrace T3, in the stratigraphy presented herein, was sampled for cosmogenic nuclide exposure dating by Hein et al. (2011) and was assigned to MIS 16.Based on planform relationships between moraines and glaciofluvial deposits, terraces T3-T5 are attributed to a package of glaciofluvial deposits associated with the MIS 16 ice margin (Hein et al., 2009(Hein et al., , 2011)).Terraces T3 and T5 are separated altitudinally by ~30 m of incision (Fig. 6b), a similar amount to the better dated MIS 2 terraces discussed below.The younger T5 includes an ice-contact fan (location indicated on Fig. 3c), fed by an ice-margin, which is inferred to be an MIS 16 moraine formed during a stillstand or re-advance following recession from the MIS 16 glacial maximum.The T3-T5 terraces are situated 205-225 m above the valley floor, constraining a net incision since 650 ka (Table 1) of 0.32-0.35m ka −1 .
The next sequence of terraces (T6-T11) cannot be pinned to any ice-marginal sediments.Spatially, they are smaller in areal extent, ranging from 0.06-5.5 km 2 (Table 1), and terraces T6 and T8-T10 have insufficient data to be plotted on Fig. 5.All these terraces lie within Cañadon Caracoles to the east of the MIS 16 ice limit, and the larger terraces T7 and T11 are shown on Fig. 3a-c.T7 sits atop bedrock at the entrance to a deeper section of the Cañadon Caracoles bedrock gorge (Fig. 3a-c).Altitudinally these terraces are positioned between the MIS 16 and MIS 8 (T12-T13) terraces (see T7 and T11 on Fig. 5), with elevations ranging from 190 m (T6) to 130 m (T11) above the valley floor (Table 1).It is therefore likely that T6-T11 date to MIS 10-14 in the stratigraphic sequence and provide further evidence from Patagonia for the potential of outwash deposits to record ice advances not preserved in the moraine record, as has also been found from the proglacial landscape of the Buenos Aires glacier lobe (Smedley et al., 2016).
The next pinning points in the chronology are terraces T12 and T13, which cover a large spatial area (26.8 and 83.8 km 2 , respectively, Table 1) to the west of the MIS 16 ice margin (Fig. 4).Both these terraces were outwash plains fed by the MIS 8 ice margin (Hein et al., 2011).Downcutting of ~120 m since 250 ka constrains a rate of net incision of 0.48 m ka −1 (Table 1).Following recession of ice from the MIS 8 moraine, a proglacial lake formed, as evidenced by palaeoshorelines imprinted on the ice-proximal side of the moraines (Fig. 3).The palaeoshoreline elevations range from ~530 to 565 m a.s.l., compared to ~590 m a.s.l. for the T13 outwash plain close to the ice-marginal deposits.This suggests ~60 m of incision associated with a lake outflow post-dating terrace  T13.Terraces T14 and T15 are located ~30 and ~45 m lower than T13, respectively (Table 1), so possibly correlate to a period of glacier recession post-dating the MIS 8 ice limit.
The preservation of the T15 terrace extends both up-and down-valley of the MIS16 ice margin (Fig. 4) and is the youngest terrace outcropping on top of bedrock within Cañadon Caracoles (Fig. 3a-c).At the gorge reach, T15 lies ~80 m above the valley floor, constraining a lower bound net incision rate of 0.32 m ka −1 over the last 250 ka.It is possible, however, that T15 corresponds to an ice advance during MIS 6, for which is there is evidence from further north in Patagonia in the Ñirehuao valley (Peltier et al., 2023), but in the absence of further dating control this interpretation is speculative and not considered further.
The youngest terrace package in the study area (T16-T22) is located furthest to the west (Fig. 4) and dates to MIS 2 and the local Last Glacial Maximum (lLGM) Río Blanco, and younger, moraines (Hein et al., 2010).T16 was an outwash plain (167.2 km 2 , Table 1) fed by the lLGM moraine (~25 ka) and presently located ~40 m above the valley floor.Located ~5 km to the east, T17 (Fig. 3d) was an outwash plain (44.5 km 2 ), ~35 m above the current valley floor, associated with an ice margin at 18 ka.Ice-lobe recession from the 18-ka moraine formed proto Lago Puerreydón and terraces T18-T22 are tied to the palaeolake outflow.Channel planform shifted from braided (as evidenced from satellite imagery for all terraces T2-T17) to a wandering gravel bed river through terraces T18-T21, then meandering (Fig. 2b) by abandonment of the T22 level.T22 is the youngest mapped terrace (+3 m) and is therefore closest in timing to the river capture event, dated to ~15 ka (Mercer, 1976;Turner et al., 2005;Hein et al., 2010;Thorndycraft et al., 2019a;Vásquez et al., 2022), that created the wind gap (Fig. 3d).
The down-valley gradients of selected Mid-to Late Pleistocene terraces are illustrated in Fig. 7, which shows dimensionless slope plotted against distance down-valley.The steepest slopes are those of terraces T2 (0.0095) and T13 (0.0052).Terraces deposited east of the MIS 16 icemarginal deposits, and confined by the bedrock gorge (T4, T7 and T15), show a cluster of slope values between 0.0033 and 0.0037.Where there are two terraces shown for one glacial period the younger terrace shows a lower slope; for example, the slopes of T16 and T17 are 0.0041 and 0.0027 respectively.
The final stage of landscape evolution (post-dating 15.5 ka) in Cañadon Caracoles is evidenced by the valley floor geomorphology (Fig. 8).Geomorphic mapping identified three key MIS 1 valley floor landform types: (i) mass movements, such as rock falls (Fig. 8a); (ii) alluvial fans forming at the mouths of tributary gullies eroded into Quaternary moraine and glaciofluvial terraces (Fig. 9); and (iii) palaeolake shorelines and lacustrine sediment fills (Fig. 8b and c).The valley floor long profile (Fig. 9) shows positive relief, associated with alluvial fan localities, punctuating the overall down-valley decrease in elevation.A further notable feature of the long profile is the reverse bed gradient over the lower ~4 km of Cañadon Caracoles to the Pinturas confluence.Main valley sedimentation blocking or impeding the drainage of tributary valleys has been documented (An et al.,2022), including in dryland environments (Grenfell et al., 2008).Alternatively, it is noted from remote sensing that steep tributary gullies enter Cañadon Caracoles along this reach also with the potential to deliver coarse sediment to the valley floor.The largest of these tributaries, located 0.9 km upstream of the confluence, drains ~4 km 2 of dissected unconsolidated Quaternary sediments, probably >100 m thick.However, this

Discussion
The discussion starts with a spatio-temporal synthesis of terrace incision rates along Cañadon Caracoles to review the data in the context of reach-specific geomorphology, such as the transition from alluvial to bedrock reaches and spatiotemporal distribution of the published geochronology.There then follows an evaluation of drivers of terrace incision before a model of landscape evolution over glacial/interglacial cycles from the Mid-Pleistocene (MIS 32-36) to MIS 1 is presented.

Spatio-temporal synthesis of terrace incision rates
The older part of the terrace sequence within the Cañadon Caracoles catchment, shown in cross-sections XS2 and XS3 (Fig. 6b), occurs between the MIS 32-36 and MIS 16 ice limits.In this reach terraces T2-T15 are preserved, while up-valley (westwards) from the MIS 16 ice-marginal deposits, the terrace sequence is younger (XS4 and XS5, Fig. 6c), spanning T12 (MIS 8) to T22 (MIS 2).The most downstream limits of terraces T2, T4, T7, T11 and T15 all sit atop bedrock (Fig. 3b), with the elevation of the bedrock gorge lip declining up-valley (westwards), as seen in the field photo in Fig. 3a.These terrace levels evidence cycles of alluvial aggradation and incision cutting down into the Jurassic bedrock (Fig. 3a-c).At this reach T15 marks a transition from alluvial/strath terraces to bedrock incision with no terrace preservation (Fig. 3c).The calculated rate of average net incision varies depending on the location due to terrace gradient (see Fig. 9 for dimensionless slope values) and the substrate, with higher rates of incision calculated for steeper terraces cut into unconsolidated sediments, for example the T13 outwash plain with a slope of 0.0052 (Fig. 7) and incision rate of 0.48 m ka −1 (Table 1).Incision rates are also sensitive to the elevation differences between terraces associated with the same glaciation and assigned ages.For example, using the elevations of T3-T5 to constrain incision from MIS 16 (650 ka) to the present provides an envelope of 0.32-0.37m ka −1 .The MIS 8 terraces, T13 (alluvial) and T15 (bedrock), provide a range of 0.32-0.48m ka −1 for average net incision from 250 ka to the present.Figure 10 provides a summary of incision rates for the Cañadon Caracoles terraces with age control, the presented values based on incision from T2 to T3, T5 to T13, T15 to T16 and T17 to the river channel.
The most confident chronological tie points in the oldest part of the terrace sequence are terraces T3 and T5 (both MIS 16), the latter an ice-contact fan formed by the MIS 16 icemargin.Terrace T2 also features an ice-contact fan at its upstream end, this linked to the Outer Caracoles glacier, probably MIS 20-22 based on a magnetic reversal (Sylwan et al., 1991;Hein et al., 2009).An average net incision of 85 m (Fig. 6b), spanning at least MIS 20-22 (T2, 800 ka) to MIS 16 (T3-T5, 650 ka) therefore constrains an upper rate of net incision through these alluvial terraces of 0.33 m ka −1 , comparable to the average net incision rate of 0.34 m ka −1 for the complete record from T2 to the valley floor (Fig. 10).The net incision rate from MIS 16 (T3-T5, 650 ka) to MIS 8 (T13-T15, 250 ka) slowed to 0.26 m ka −1 , probably due to bedrock control.Average incision from T15 to the valley floor then increased to 0.32 m ka −1 possibly because flow since MIS 8 was concentrated through a narrower bedrock gorge, with no alluvial terrace preservation after T15 in the lower reach.
Greater precision on incision rates can be calculated for the youngest terraces mapped in the study area.Terraces T16-T22 date to the lLGM (25 ka) and onset of deglaciation (from 18 ka) in this region of Patagonia (Hein et al., 2010;Bendle et al., 2017b), and the available chronology provides ante quem and post quem ages for the terrace sequence T17-T22.Crucially, the geomorphic evidence for abandonment of the Caracoles outflow provides a minimum timespan of ~10 ka for formation (aggradation and incision) of terraces T17-T22.Incision of T17 post-dates the 18-ka moraine constraining an incision rate of 11.7 m ka −1 (Table 1), with terraces T18-T22 incised over a timespan of ~3 ka until the geologically instantaneous river capture event at ~15.0 ka (Mercer, 1976;Turner et al., 20005;Hein et al., 2010;Vázquez et al., 2022).The evidence from T17-T22, therefore, where terrace formation is constrained by ante quem and post quem ages and narrower dating uncertainties, demonstrates a more precise and rapid (11.7 m ka −1 ) rate of terrace incision compared to the average net incision rates calculated for the rest of the terrace sequence.

Evaluating drivers of terrace formation and rapid incision
What are the dominant drivers of MIS 2 terrace formation and rapid incision?The geomorphological evidence from the MIS 2 moraine and terrace landforms point to meltwater and sediment discharge as key controls.The cycles of aggradation and incision recorded by terraces T17-T22 must have occurred between the lLGM (25 ka) and 15 ka, when drainage switched from its Atlantic course via the Río Deseado to flow to the Pacific via the Baker catchment (see Fig. 1c and Thorndycraft et al., 2019a for a review).Aggradation of the T17 outwash plain started forming in accommodation space created once ice had receded westwards from the lLGM moraine, the river abandoning the T16 terrace.The T17 braided outwash plain was extant from the end of the lLGM until the onset of deglaciation from the 18-ka ice-marginal moraine position, providing a timespan of ~7 ka for aggradation  of these deposits.The accumulation of this large outwash plain (Fig. 4) was probably driven by high sediment and meltwater discharge from the ice-lobe.Following ice recession and formation of proto Lago Puerreydón, the now lakefed river began to incise due to reduced sediment discharge and a probably enhanced meltwater discharge caused by the increasingly negative mass balance of the ice-sheet (Hubbard et al., 2005).The outwash plain (terrace T17) was abandoned and the inset terraces T18-T22 were formed, the river shifting first to a wandering gravel bed planform, then meandering (Fig. 2b).Evidence from annually laminated lake sediments at Lago Buenos Aires to the north (Fig. 1b) shows an increasing rate of glacier recession from 17.322 ± 115 cal ka BP, following formation of the proto lake at 18.086 ± 214 cal ka BP (Bendle et al., 2017b).Rapid glacier recession, therefore, probably enhanced discharge from the Lago Puerreydón outflow.This, combined with the lake acting as a sediment trap, promoted rapid downcutting.Other potential geophysical drivers of incision in the region, however, require evaluation for their potential influence on landscape change.These include: (i) neotectonic uplift; (ii) glacial isostatic uplift; (iii) erosional isostasy; (iv) base level; and (v) combinations of (i) to (iv).Evidence for uplift in the Deseado catchment comes from marine terraces of the Atlantic seaboard (Pedoja et al., 2010).Nine marine terraces spanning the last 900 ka, with displacement up to ~140 m at a net uplift rate of 0.09-0.16m ka −1 (the higher rate illustrated on Fig. 10), were identified in the coastal sector north and south of the Deseado river mouth.The uplift rate in this region of the Patagonian Atlantic seaboard was found to be twice that of the rest of South America, the enhanced uplift attributed to dynamic uplift caused by subduction of the Chile ridge (Pedoja et al., 2011).Semi-analytical modelling of dynamic uplift quantified ~400 m of uplift over the last 8 Ma in the Cañadon Caracoles region (see fig. 6 in Guillaume et al., 2013), at an average rate of 0.05 m ka −1 .These modelling data are too coarse for the last 800 ka, but the 0.05 m ka −1 rate is shown on Fig. 10 for illustrative purposes.
These uplift data can be compared with incision rates calculated for the Deseado and Cañadon Caracoles terrace sequences.For the Deseado terraces, Tobal et al. (2021) calculated an average rate of incision of 0.135 m ka −1 (Fig. 10) since 1200 ka, within the range of uplift rate calculated by Pedoja et al. (2010).However, the data suggest an initially higher rate of incision (0.387 m ka −1 ) from 1200 to 900 ka (prior to the Deseado marine terrace record), falling to 0.007 m ka −1 during ~750-450 ka (Tobal et al., 2021).These data suggest lower average net incision rates compared to the last 800 ka in Figure 10.Plot of age against elevation above the channel for the Cañadon Carcoles terraces with age control, as well as the average rate of incision curves for the Cañadon Caracoles (this study) and Deseado terrace sequences (Tobal et al., 2021).The maximum calculated uplift curve (metres above sea level) is shown for marine terraces at Puerto Deseado (Pedoja et al., 2011).The uplift curve from regional modelled dynamic uplift for the last 8 Ma (Guillaume et al., 2013) is also shown as an illustration of long-term uplift rates in the region.[Color figure can be viewed at wileyonlinelibrary.com]Cañadon Caracoles (0.34 m ka −1 ).Furthermore, the evidence from MIS 2 demonstrates a much higher rate still (11.7 m ka −1 ), greatly exceeding neotectonic uplift rates, for the episodic incision forming terraces T17-T22 (Fig. 10).The spatio-temporal data on uplift and incision rates from the wider Deseado catchment suggests, therefore, that uplift cannot explain the magnitude of incision within the upper Pinturas catchment.
There is evidence for glacial isostatic warping of the Cañadon Caracoles terraces.For example, the southeast, right valley margin MIS 6 (T13) and MIS 2 (T17) terraces (Fig. 6) show altitude sloping down from the incised valley and along the expected axis of uplift.Thorndycraft et al. (2019a) plotted palaeoshoreline elevations across 45-46°S to demonstrate uplift of ~40-50 m close to the Andean Cordillera, providing minimum uplift rates of ~3 m ka −1 constrained by the palaeolake drainage chronology.Troch et al. (2022), based on sediments from an isolation basin to the west of the Andean Cordillera, report a magnitude of 96 m of glacial isostatic uplift between 16.5 and 9.1 cal ka BP in the centre of the former Patagonian Ice Sheet, with a mean uplift rate of 13 m ka −1 during this period.These values are comparable to the rate of net incision of 11.7 m ka −1 for terraces T17-T22, but Cañadon Caracoles incision occurred earlier in time (~18-15 ka), over a shorter timespan, and also in a zone of lower magnitude glacial isostatic uplift according to the palaeoshoreline evidence (Thorndycraft et al., 2019a;Vásquez et al., 2022).Given competent flows ceased by ~15 ka at Cañadon Caracoles it is unlikely the high rate of terrace incision can be explained by glacial isostatic uplift.In summary, therefore, at Cañadon Caracoles the geomorphic evidence suggests that discharge and sediment supply were the dominant drivers of terrace incision during MIS 2.
Evaluating the full terrace sequence at Cañadon Caracoles and regional geological evidence (e.g.Christeleit et al., 2017;Bourgois et al., 2021), it is hypothesized the same climatedriven (glacial/interglacial) meltwater and sediment discharge controls also occurred during older terrace formation events.For example, the MIS 8 terraces (T12-T15) record a comparable spatial pattern and magnitude of incision to those of MIS 2, and similarly there is palaeoshoreline evidence for the existence of a proglacial lake during MIS 8 (Fig. 4).Both the MIS 8 and MIS 2 ice margins and outwash terraces occur in a sedimentary basin, with large accommodation space.There is no geomorphological evidence for proglacial lakes preserved down-valley (eastwards) from the MIS 16 ice margin, where incision has cut the Cañadon Caracoles bedrock.However, current understanding of regional Miocene to Quaternary landscape evolution (Christeleit et al., 2017;Bourgois et al., 2021) suggests proglacial lakes, and continental-scale drainage reversals, probably occurred during earlier glacial/ interglacial cycles because glacial ice expansion and recession would have blocked and opened, respectively, the Pacific drainage pathway.Atlantic/Pacific drainage reversals were, therefore, probably a feature of glacial/interglacial cycles throughout the Quaternary in this sector of Patagonia and should be considered in models of landscape evolution that explain the Mid-to Late Pleistocene terraces of Cañadon Caracoles (see next section).
The Cañadon Caracoles terrace sequence fits the global model for enhanced fluvial incision since the Mid-Pleistocene Transition at 1200-800 ka (e.g.Bridgeland and Westaway, 2007;Gibbard and Lewin, 2009;Westaway et al., 2009) when an intensification and lengthening (from 41 to 100 ka) of glacial cycles occurred (Clark et al., 2006;Chalk et al., 2017).Westaway et al. (2009) argued that increased rates of isostatic uplift driven by this intensification and lengthening of glacial cycles caused a shift in morphology with rivers forming narrower, entrenched valleys within broader palaeovalleys.Gibbard and Lewin (2009), however, urged caution in interpreting interterrace incision matching contemporaneous uplift.They argued incision processes were largely episodic (in cold climate phases) with their own process timescales.The long-term natural experiment from Cañadon Caracoles supports Gibbard and Lewin's view of episodic incision, but the data from MIS 2 demonstrate rapid incision during transitional warming at the onset of deglaciation.The intensification and lengthening of glacial cycles in the upper Pinturas probably increased glacial sediment budgets and enhanced the hydrological cycle creating large outwash plains of unconsolidated sediments, while melting of these larger ice-sheets contributed increased meltwater discharge once deglaciation started.
Other examples of rapid episodic incision have been documented elsewhere in Patagonia.In the Baker valley (Fig. 1c) within the Andean Cordillera, a geologically instantaneous increase in discharge of ~0.1 × 10 6 m 3 s −1 caused by catastrophic glacial lake outburst flooding explained ~30 m of rapid bedrock gorge incision (Benito and Thorndycraft, 2020).Rapid terrace formation has been documented using satellite remote sensing for the modern ice-dammed Laguna del Viedma valley of the Southern Patagonia Icefield (49.5°S).Here there is evidence for terrace incision >1.33 m a −1 triggered by base level fall in response to net lake level fall (Thorndycraft, 2022) during the recession and thinning of the Viedma glacier (Foresta et al., 2018;Minowa et al., 2021).The implications of these (presently) geographically sparse studies, covering a wide span of timescales, and contrasting in their geomorphic processes, are that drivers of landscape change can be diverse, highlighting the role palaeohydrological data can play in elucidating the magnitude and rates of landscape processes in Patagonian river catchments.

A landscape model of terrace formation at Cañadon Caracoles
The following model of terrace formation in Cañadon Caracoles (Fig. 11) is proposed for glacial/interglacial cycles since the Mid-Pleistocene.During glacial cycles an expanding and thickening Patagonian Ice Sheet blocked the Pacific drainage route of the Río Baker initially forming a proglacial lake that drained to the Atlantic (Fig. 11, Stage 1).Once Pacific drainage was blocked the watershed draining to the Pinturas through Cañadon Caracoles increased by ~10 4 km 2 , therefore enhancing the hydrological cycle through ice-sheet runoff (Marshall and Clarke, 1999;Wickert, 2016).Ice continued to advance up a retrograde slope until, by the lLGM, ice reached Cañadon Caracoles forming a terminal moraine and a glaciofluvial outwash plain (Fig. 11, Stage 2).At this time, based on the relationship between area and volume from icesheet modelling of the Northern Patagonia Icefield (Hubbard et al., 2005), it is likely the ice-shed reaching Cañadon Caracoles was ~10 4 km 3 in volume.
Decoupling of ice from the local glacial maximum moraine created accommodation space for the creation of a new outwash plain (Fig. 11, Stage 3) with both net vertical and westwards accretion as ice started to recede westwards towards the Andes.This recession was punctuated by still-stands or readvances, leading to further moraine and outwash plain formation (Fig. 11, Stage 4).Once persistent deglaciation was underway, further glacier recession led to the formation of a proglacial lake reducing sediment supply and promoting net incision (Fig. 11, Stage 5).During this phase of low sediment discharge, meltwater flows would have probably been high.During deglacation of the Laurentide Ice Sheet, for example, Marshall and Clarke (1999) modelled peak runoff (~1.0 × 10 6 m 3 s −1 ) at the onset of deglaciation (14.5 ka in North America), with an average flow of 0.675 × 10 6 m 3 s −1 sustained over the period 15.0-14.0ka.
Incision continued within the lake-fed Cañadon Caracoles river until recession and thinning opened the Baker valley and a Pacific drainage route was captured (Thorndycraft et al., 2019a), causing lake level to fall below the outflow (Fig. 11, Stage 6).This triggered an instantaneous reduction in drainage area of ~10 4 km 2 , and the end of perennial discharge based on the Late Pleistocene/Holocene geomorphology, such as the lacustrine landforms at the bed of the Cañadon Caracoles gorge (Fig. 8b and c).The counter, therefore, to an enhanced hydrological cycle during glacials, and high meltwater flows during glacial/interglacial transitions, was low competent discharge associated with ephemeral flow in an underfit, dryland, valley during interglacials (Fig. 11, Stage 6).Water sources for fauna and early humans in this dryland environment may have included springs at the base of alluvial terraces (Fig. 8d).

Conclusions
In this study a geospatial analysis of remotely sensed mapping of river terraces from the upper Pinturas catchment, combined with previously dated ice-margins (Hein et al., 2009(Hein et al., , 2010(Hein et al., , 2011)), revealed 22 terrace levels spanning the last ~1100 ka.The key findings from the Cañadon Caracoles terrace sequence are the following: • Terraces were identified associated with five previously dated ice margins at MIS 32-36, MIS 20-22, MIS 16, MIS 8 and MIS 2. Additionally, intermediary terraces were identified, these probably dating to other glacial phases most probably between MIS 10 and MIS14.• Average net terrace incision, including phases of aggradation, occurred at a rate of 0.34 m ka −1 over the last ~800 ka, probably faster than regional rates of long-term uplift.• The MIS 2 terrace sequence demonstrates episodic and rapid incision during transitional warming at the onset of deglaciation.Aggradation of the T17 outwash plain occurred during 25-18 ka before incision of ~35 m occurred over ~18-15 ka, at a rate of 11.7 m ka −1 .• The MIS 1 landform record at Cañadon Caracoles is characterized by low-competence, ephemeral flow in the main valley.Tributary gullies eroded into glacigenic and terrace deposits, bringing coarse sediment into the valley floor.These alluvial fans, along with rock falls and other mass movements, blocked Cañadon Caracoles drainage and formed a landscape mosaic of shallow valley floor lakes.• The Mid-to Late Pleistocene terrace sequence at Cañadon Caracoles can be explained by the lengthening and intensification of glacial cycles from the Mid-Pleistocene Transition causing expansion of the Patagonian Ice Sheet, blocking Pacific drainage over longer timescales than older, shorter glaciations.This enabled the aggradation of large outwash plains in the upper reaches of the Pinturas catchment.Rapid terrace incision occurred during transitional warming phases at the onset of deglaciation followed by interglacial vertical stability once drainage had re-opened to the Pacific, leaving an ephemeral, underfit river in Cañadon Caracoles.
Figure 2. Examples of satellite imagery of the study area.(a) Sentinel image of the Cañadon Caracoles river catchment indicated by the blue polygon (see Fig. 1c and d for location).(b) ESRI basemap imagery of the former outflow of proglacial palaeolake Puerreydón that was abandoned by river capture ~15.5 ka(Mercer, 1976;Hein et al., 2010;Thorndycraft et al., 2019a).The Cañadon Caracoles watershed here marks the present-day continental drainage divide.Note the landslide scar(Pánek et al., 2020) and the meandering planform of the incised river channel.(c) ESRI basemap imagery showing the deepest section of the Caracoles gorge and gullied alluvial terraces.(d) ESRI basemap imagery showing multiple terrace levels, active gullying, valley floor alluvial fans and a shallow lake.[Color figure can be viewed at wileyonlinelibrary.com] Figure 2. Examples of satellite imagery of the study area.(a) Sentinel image of the Cañadon Caracoles river catchment indicated by the blue polygon (see Fig. 1c and d for location).(b) ESRI basemap imagery of the former outflow of proglacial palaeolake Puerreydón that was abandoned by river capture ~15.5 ka(Mercer, 1976;Hein et al., 2010;Thorndycraft et al., 2019a).The Cañadon Caracoles watershed here marks the present-day continental drainage divide.Note the landslide scar(Pánek et al., 2020) and the meandering planform of the incised river channel.(c) ESRI basemap imagery showing the deepest section of the Caracoles gorge and gullied alluvial terraces.(d) ESRI basemap imagery showing multiple terrace levels, active gullying, valley floor alluvial fans and a shallow lake.[Color figure can be viewed at wileyonlinelibrary.com]

Figure 3 .
Figure 3. Photomontage of the Cañadon Caracoles terraces.(a) View from the right valley margin looking to the northwest across Cañadon Caracoles towards the Meseta del Lago Buenos Aires (Fig.1b).The deepest section of the Caracoles gorge is visible to the right with terraces (T2, T7 and T11) visible up-valley.(b) View looking downstream from terrace T15 on the left margin of Cañadon Caracoles.The photograph shows relationships between the terraces T4, T7 and T11 and strath bedrock terraces in the gorge reach.(c) View upstream from the entrance to the Caracoles gorge (left margin).In the background the T5 icecontact fan is visible, associated with the Inner Caracoles moraines (cf.Hein et al., 2011) on the horizon.(d) The continental drainage divide marked by the windgap, the former outflow of the proglacial lake formed by ice-lobe recession in the Puerreydón basin (Fig.1b).Note the landslide post-dating lake drainage(Pánek et al., 2020).[Color figure can be viewed at wileyonlinelibrary.com]

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outwash plain fed by ice contact fan pinned to MIS 20-22 ice margin(Hein et al., 2011Sequence of small terrace fragments (T6-T11) located within a range of ~90 m elevation difference from MIS 16 to MIS 8. Possible evidence for glacial advances during MIS 10Elevation above channel and incision rate not calculated because T12 is located 6-7 km from valley floor.T12 is differentiated from T13 by a 5-10-m Outwash plain pinned to the MIS 8 ice margin(Hein et al.Two terrace levels located with a range of 80-m elevation difference from MIS 8 to MIS 2. T14 and T15 could be recessional terraces associated with ice receding from the MIS 8 maximum, or one or both may date to MIS

Figure 4 .
Figure 4. Geomorphology map showing the alluvial terraces of the Cañadon Caracoles tributary of the Rio Pinturas.Some key terraces underpinning the terrace chronology are labelled.[Color figure can be viewed at wileyonlinelibrary.com]

Figure 5 .
Figure 5. Point cloud elevation data from clipped rasters plotted against x coordinates (WGS1984 UTM Zone 19).The rasters were clipped to terrace and ice-marginal shapefiles, the latter divided by age based on data from Hein et al. (2011).Also shown is the location of the two palaeolake outflows (POF) dated to MIS 8 (POF1) and MIS 2 (POF2) (+) and the position of the gorge rim at the entrance to the deepest section of Cañadon Caracoles (x).Note the T1 data are from outside the Cañadon Caracoles catchment (see the T1 label on Fig. 4) and were not used for incision rate calculations.Terraces T6 and T8-T10 have insufficient data to be plotted.[Color figure can be viewed at wileyonlinelibrary.com]

Figure 6 .
Figure 6.Valley cross-sections showing the relative elevations and inferred ages of terraces.(a) Map showing the location of the cross-sections.Note: cross-sections are drawn from the left to right valley margin looking downstream.(b) Bedrock channel reaches of the Río Pinturas (XS1) and Cañadon Caracoles (XS2 and XS3).(c) Sedimentary basin reaches (XS4 and XS5).XS2 is unlikely to be perpendicular to palaeoflow direction so the right bank T2 terrace level appears lower than the left.[Color figure can be viewed at wileyonlinelibrary.com]

Figure 7 .
Figure 7.Long profiles for selected terraces of Cañadon Caracoles, with elevations taken from the ALOS DSM.The starting point for each terrace is organized qualitatively for clarity due to the contrasting terrace orientations through time.Dimensionless slope is shown for each terrace.

Figure 8 .
Figure 8. Field photos showing valley floor geomorphology and features of the modern-day hydrology of Cañadon Caracoles.(a) Rock falls from both sides of the gorge providing evidence for low-competence flow.(b) View upstream from the same photo location as shown in Fig. 8(a) showing an almost dry lake bed upstream of the rockfalls.(c) Dry lake bed with palaeoshorelines showing evidence for a shallow lake on the Cañadon Caracoles valley floor.(d) A spring at the base of terrace T7 overlying Jurassic bedrock.[Color figure can be viewed at wileyonlinelibrary.com]

Figure 9 .
Figure 9.Long profile of Cañadon Caracoles based on minimum elevations extracted from the ALOS-PALSAR DSM from cross-sections spaced at 50-m intervals.The palaeolake outflow marks the modern continental drainage divide between Pacific (left) and Atlantic (right) drainage.Positions of shallow lakes, based on remotely sensed mapping, are indicated (L), but those visited in the field were seasonal dry (see photos in Fig. 8).Locations of alluvial fans are shown.

Figure 11 .
Figure 11.Six-stage schematic model through a typical glacial/interglacial cycle at Cañadon Caracoles, where Q and Q s are discharge and sediment load respectively, and '+' and '-' signs denote qualitative increases/decreases in each.Moraine (M) and terrace (T) formation occur at time Z 1 and Z 2 .[Color figure can be viewed at wileyonlinelibrary.com]