Ar t i c l e i n f o
Article history:
Received 5 September 2008
Received in revised form
3 December 2008
Accepted 9 December 2008
Available online xxx
A b s t r a c t
Relict sand wedges are ubiquitous in southern Patagonia. At six sites we conducted detailed investigations of stratigraphy, soils, and wedge frequency and characteristics. Some sections contain four or more buried horizons with casts. The cryogenic features are dominantly relict sand wedges with an average depth, maximum apparent width, minimum apparent width, and H/W of 78, 39, 3.8, and 2.9 cm, respectively. The host materials are fine-textured (silt loam, silty clay loam, clay loam) till and the infillings are aeolian sand. The soils are primarily Calciargidic Argixerolls that bear a legacy of climate change. Whereas the sand wedges formed during very cold (4 to 8 C or colder) and dry (ca. 100 mm precipitation/yr) glacial periods, petrocalcic horizons from calcium carbonate contributed by dustfall formed during warmer (7 C or warmer) and moister (250 mm/yr) interglacial periods. The paleo-argillic (Bt) horizons reflect unusually moist interglacial events where the mean annual precipitation may have been 400 mm/yr. Permafrost was nearly continuous in southern Patagonia during the Illinoian glacial stage (ca. 200 ka), the early to mid-Pleistocene (ca. 800–500 ka), and on two occasions during the early Pleistocene (ca. 1.0–1.1 Ma). 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Relict sand wedges and ice-wedge pseudomorphs have played an important role in reconstructing periglacial environments in many parts of the world (Washburn, 1980; French, 1996). These features are manifested on the undisturbed ground surface by patterned ground and reflect the former existence of permafrost (Black, 1976; Washburn, 1980; Pe´we´ , 1983). Ice-wedges generally form today only in areas with permafrost and where the mean annual air temperature (MAAT) is 3.5 C or colder (Pe´we´ et al., 1969; Black, 1976; Washburn, 1980; Hamilton et al., 1983; Burn, 1990). Murton and Kolstrup (2003) provided a detailed discussion
of the limitations to using ice-wedge pseudomorphs as indicators of paleo-temperatures. Active sand wedges are of limited distribution on the earth’s surface and occur primarily in Antarctica
(Pe´we´ , 1959; Berg and Black, 1966; Black, 1973). These features appear to be limited to areas where the MAAT is 4 to 8 C or colder and the mean annual precipitation is <100>
Southern Patagonia has a long history of research on cryogenic structures, beginning with the pioneering work of Corte (1967). In Patagonia ice-wedge pseudomorphs have been used to reconstruct the distribution of permafrost during major glaciations (Beltramone, 1993; Vogt and del Valle, 1994; Trombotto, 1996, 2008) and to show changes in mean annual air temperature during glacial–interglacial cycles (Corte,1967; Galloway,1985; Corte,1991; Meglioli, 1992). Further south, on the island of Tierra del Fuego, relict wedges have been recognized and interpreted as belonging to the Last Glacial Maximum. (Coronato et al., 2004a) Whereas the
previously cited studies suggest that the cryogenic features are icewedge pseudomorphs, our research will show that most of these features in southern Patagonia are relict sand wedges, which has important ramifications for paleo-environmental reconstruction.
The objectives of this study are (1) to describe the physical and chemical properties of relict sand wedges and ice-wedge pseudomorphs and their host materials and (2) to interpret the stratigraphic and paleo-environmental significance of these features.
* Corresponding author. Tel.: þ1 608 263 5903; fax: þ1 608 265 2595.
E-mail address: bockheim@wisc.edu (J. Bockheim).
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Quaternary Science Reviews
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2. Regional setting
The study was conducted near Rı´o Gallegos in Santa Cruz province, Argentina, southern Patagonia, at 52S, 68 540–70 380W (Fig. 1).
Detailed investigations were made at six sites, including Estancia Tres de Enero, Chimen Aike, Monte Aymont, Punta Loyola, Estancia Bella Vista, and Rio Chico (Table 1). The study area is part of the southern Patagonia Steppe eco-region (Soriano, 1983), and the dominant plant species is ‘‘coiro´ n’’ bunchgrass (Stipa humilis). The current mean annual temperature is 7 C, and the mean annual precipitation is 250 mm. The soils are classified dominantly as
Paleargids (INTA, 1985). Relict sand wedges usually are developed on flat surfaces in Patagonia. To establish the setting in which the wedges are formed, we will give a detailed account of the glacial chronology of the region. The regional landscape includes ancient moraines, glaciofluvial
plains, periglacial fans, river valleys, volcanic tablelands and basins, and ancient and modern marine beaches.
The glacial landscape on which the sand wedges formed is composed of downwasted ground moraine and glaciofluvial plains of different ages.
Fig. 2 shows the stratigraphy proposed by Meglioli (1992) and modified by Coronato et al. (2004b,c).
The study area was affected by the Estrecho de Magallanes and Rı´o Gallegos glacial lobes (Meglioli, 1992), which expanded on several occasions during the early-middle Pleistocene from the then much larger Cordillera Darwin (54 300S, 70 460–71 350W) and Southern Patagonian Icecap (51S, 73W) mountain sheets, respectively.
Fig. 1. Location of the study sites and glacial limits during the middle-upper Pleistocene in southern Patagonia (after Meglioli, 1992).
White areas represent the hypothetical Patagonian ice mountain sheet location (after Caldenius, 1932).
A) Great Patagonian Glaciation (MIS 32-30) limits covering the continental area and the northern part of Tierra del Fuego Island.
B) Cabo Vı´rgenes Glaciation (MIS 20-18?) limits and location of the sand-wedge casts, BV: Bella Vista site, TE: Tres de Enero site, ChA: Chimen Aike site, RCh: Rı´o Chicosite.
C) Punta Delgada Glaciation (MIS 12-10?) limits and location of Monte Aymond (MA) site.
D) Primera Angostura Glaciation (MIS 6) limit and location of Punta Loyola (PL) site.
E) Segunda Angostura Glaciation (MIS 2) limits including Tierra del Fuego Island and wedge-cast sites mentioned in the text, CP: Cabo Pen˜as and RE: Rı´o Ewan. Although the
relationship between ice limits age and sand-wedge development could not be clearly established, the age of the drift units offer a maximum age for the development of the
cryogenic features.
Table 1
Characteristics of study sites, southern Patagonia.
Site No. Location Drift unit Approx age
(Ma)
Soil classification
1 Monte Aymont Cabo Virgenes 0.6–0.8 Pachic Argixerolls
2 Est. 3 de Enero Sierra de los
Frailes
1.0–1.2 Calciargidic
Argixerolls
3 Chimen Aike Sierra de los
Frailes
1.0–1.2 Calciargidic
Argixerolls
4 Punta Loyola (alluvial fan) w0.2 Calciargidic
Argixerolls
5 Est. Bella Vista Bella Vista 1.0–1.2 Typic Argixerolls
6 Rio Gallegos city Rio Chico w1.0 Calciargidic
Argixerolls
2 J. Bockheim et al. / Quaternary Science Reviews xxx (2009) 1–12
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Please cite this article in press as: Bockheim, J., et al., Relict sand wedges in southern Patagonia and their stratigraphic and paleo-environmental
significance, Quaternary Science Reviews (2009), doi:10.1016/j.quascirev.2008.12.011
Fig. 2.
Chronostratigraphical chart of glacial drifts recognized along the Magellan Straits, including ice-wedge casts developed on them. Whereas the Great Patagonian Glaciation
(GPG) and Pre-GPG glaciations were interpreted as of the piedmont type, the later ones are defined as ice-lobe glaciations. Nested, frontal moraine arcs bound the external limits of each advance. The older the age of the drift on which the casts are generated, the more difficult it is to relate them with a certain cold-glacial period. The data for benthonic forams originates from Opdyke (1995) and Shackleton (1995).
The Rıo Gallegos glacial lobe deposited its oldest and easternmost moraine sequence at 70 400W close to the present Estancia Bella Vista, where the Bella Vista till forms a hummocky topography with huge scattered boulders. Eastwards, a very extensive glaciofluvial outwash plain is developed following an ancient valley carved down by the Rı´o Gallegos. This drift unit has been interpreted as belonging to the Great Patagonian Glaciation, which is equivalent to the Sierra de los Frailes drift along the Magellan straits (Meglioli, 1992). The Bella Vista basal till is host to multiple horizons of sand-wedge relicts. As the till sediment is overlaid by aeolian sandy soils of unknown age, it is not possible to establish yet in which cold period or glaciation the cryogenic structures were actually formed.
The Estrecho de Magallanes lobe reached several positions along the straits and represents the easternmost the glacial advance named as ‘‘Initioglacial’’ by Caldenius (1932). According to Meglioli (1992), this advance deposited the Sierra de los Frailes drift and reached the present Atlantic Ocean submarine platform, south of the Rıo Gallegos mouth (Fig. 2). The landscape is a very extensive till plain on which several volcanic cones have grown that are aligned forming a sinuous range known as the Pali Aike Volcanic Field. Flat glaciofluvial plains surround the basal till plain along its northern and eastern sides. Caldenius (1932) and Meglioli (1992) interpreted
this drift unit as belonging to the Greatest Patagonian Glaciation (GPG). The GPG was defined by Mercer (1976) as the glaciation that extended from the Andes Mountains to the present Atlantic Ocean platform. The age of the GPG has been radiometrically constrained between 1.1 and 1.0 Ma at Lago Buenos Aires, in northern Santa Cruz province, and in Estancia Bella Vista based on 40Ar/39Ar dating of stratigraphically related lavas (Meglioli, 1992; Ton-that et al.,
1999; Singer et al., 2004). At the Bella Vista, Chimen Aike and Estancia Tres de Enero sites, relict sand wedges penetrate this old till unit. At the latter site, it is also possible to define at least two till units (see discussion below). Meglioli (1992) identified four frontal moraine arcs representing the Middle–Late Pleistocene glaciations along the Magellan Straits depression. Two of them have preserved evidences of ice-wedge pseudomorphs. The Cabo Vırgenes glaciation followed the GPG (Fig. 2). Its morphology is represented by large, flat topped and well preserved moraines which wrap around both sides of the Magellan Straits depression at around 100 ma.s.l. (above sea level). They were severely dissected by outwash streams flowing northeastwards to the Atlantic Ocean. These relict channels are still well preserved on the landscape. The age of this drift unit was assigned to the ‘‘Daniglacial’’ stage by Caldenius (1932) and later dated at >1.07 Ma and <0.36>
3. Methods
Because of problems with accessibility, exposures were examined along road cuts and in gravel pits. Stratigraphic sections were described and measured with a level and stadia rod. The most
detailed stratigraphic sections at the Estancia Tres de Enero and Chimen Aike sites were fully described and are presented below.
Ground soils and buried soils were described according to criteria in the Soil Survey Manual (Soil Survey Division Staff, 1993) and classified using the Keys to Soil Taxonomy (Soil Survey Staff, 2006).
We developed a scheme and spreadsheet for describing relict sand wedges and ice-wedge pseudomorphs that is based partially on Murton et al. (2000). Three types of cryogenic features were recognized, including sand wedge casts, ice-wedges pseudomorphs, and composite wedge-pseudomorphs using criteria of Jahn (1978) and Washburn (1979). Previous investigations in
southern Patagonia have recognized ice-wedge pseudomoprhs but not necessarily relict sand wedges. We delineated sand-wedge relicts from criteria derived from Pe´we´ (1959) and Black (1976) for active sand wedges in Antarctica and criteria for relict sand wedges by Carter (1983), Murton (1996), French et al. (2003), Harris et al. (2005), and Murton and Bateman (2007). These criteria include:
(i) Wedge or V shape with apex pointing downward;
(ii) Dimensions: top of wedges 0.5–3.5 min width and height 0.1–10 m when measured at right angles to the axial plane of thewedge; diameter of polygon 7–20 m when measured in plan
view;
(iii) Distinct foliated fabric of near-vertical to vertical laminations in the wedge centers grading outward to inclinations that parallel the wedge edges;
(iv) The laminations vary from 1 to 5 mm in width and often show cross-cutting relationships;
(v) Bedding in the host material is upturned at wedge edges;
(vi) No evidence of slumping in the upper top of the wedge, which is typical of ice-wedge relicts from melting of the ice-wedge. It should be emphasized that not all of these features are present
in relict sand wedges (Murton, 1996), and some of the features (i, ii, and v) are also characteristic of ice-wedge pseudomorphs (Black, 1976; Carter, 1983; Karte, 1983).
Although we recognize four shapes of the cryogenic features, wedge, funnel, vein, and composite (Murton et al., 2000), we refer to them collectively as ‘‘wedges’’. Although three forms of deformation of the host material are commonly recognized, upturned, downturned, and none (Murton et al., 2000), we only observed upturning. However, this form was not always evident because of the fine texture of the host material and pedogenesis within the casts. The origin, texture, and color of the host material and infilling were noted.
Transects were established across the top of the relict wedges, and each relict wedge along each transect was characterized according to shape, depth, maximum apparent width, minimum
apparent width, reaction to HCl, carbonate stage following Birkeland (1999), color, and field texture.
The frequency of relict sand wedges and ice-wedge pseudomorphs was determined from
transects and reported in number of relicts per 100 m. Because the true width of relict wedges in a randomly oriented section can only be determined from measurements taken on the ground surface, we applied the correction factor to apparent widths using the equation of Mackay (1977): Ww0:64a (1) where W¼true width and a ¼ apparent width.
We observed two horizons of sand-wedge formation at two of the sites, Estancia Tres de Enero and Chimen Aike, where we conducted detailed stratigraphic studies.
Grain-size analysis was performed in 11 samples from C and Bt horizons of the host materials and on selected wedge infillings. Fine-sediment analysis was done by in the Sedimentology Laboratory at the Universidad Nacional de La Pampa (La Pampa University) at Santa Rosa city, Argentina. The grain-size distribution was measured in wet samples using a Malvren Mastersizer 2000 laser counter. Low-angle light dispersion methods were used following Fraunhofer and Mie theories. Samples were pre-treated with acetic acid (5%); oxygenated water (100 vol.), sodium hexametaphosphate (1%), and ultrasound. Grain-size analyses from phi -3 (pebbles) to 4 (clays) in 3 de Enero site till units were performed by Centro de Investigaciones Geológicas (CIG) from Universidad de La Plata (La Plata city, Argentina). The techniques included quartering, mechanical shaking, disaggregation, drying, sieving, and
weighing the material.
4. Results
4.1. Stratigraphy and sedimentology
4.1.1. Estancia Tres de Enero
At Estancia Tres de Enero, the lower portion of the sedimentary sequence contains a till layer, that is here named Till 1, with a mean thickness of 3 m, and a range in thickness of 2–6 m
(Fig. 3A). This till has two sub-layers, including
(1) a lower one that is 2 m thick and dark brown in color with minimal fissility, and (2) an upper one that is 1 m thick and light brown in color and has well-developed fissility and a small percentage of large (>8 cm) clasts.
Till 1 is a basal till and has a highly compact, silt–loam matrix. Clasts from the Andean crystalline basement may be up to 17 cm in diameter. The mean size of the larger clasts is 8 cm in diameter. The clasts are sub-rounded and irregular in shape. The largest ones are distributed chaotically in all directions, with some having their longest axes in a vertical position, which may reflect frost heaving. The matrix bears many small pebbles of 1–3 cm in diameter, including clasts of quartz, quartzite, granite, and metamorphic and volcanic rocks, most of them coming from Cordillera Darwin, in the Chilean side of the island of Tierra del Fuego.
A second till bed unconformably overlies Till 1 (Till 2a); this till bed cuts the upper portions of the sand wedges appearing within Till 1. Till 2a has a thickness of 50 cm and has similar characteristics to the lower bed in terms of matrix and larger clasts grain size.
However, this unit lacks fissility and exhibits sand-wedge casts that cross its entire thickness, even penetrating a few cm into Till 1.
The top of this unit appears to have been eroded by anthropogenic activities related to road building.
A ‘‘U’’ shaped paleo-channel infilled by glaciofluvial deposits has eroded and cut Tills 1 and 2a, and it is covered by a third till layer, which is herein named the Till 2b. This channel has a minimum visible width at its base of 9 m and a width of 15 m at its uppermost portion. The visible thickness of the paleo-channel is 3 m. The lower part, 2 m thick, is composed by gravel layers conforming approximately to the channel shape with an upward concavity, and the channel thins out laterally on the sides.
The gravel size varies between cobbles and pebbles, with the largest clasts up to 25 cm in diameter, and is supported in a silt loam matrix. These beds alternate with clast-supported, horizontally stratified gravel lenses.
The upper 100 cm of the paleo-channel contains horizontally bedded, sandy gravels grading into well-stratified sand lenses, with clearly visible layers of heavy-mineral concentration and strong
lamination. The laminations are composed of very fine sands, silts and clays and each is less than 5 mm thick. Some energy diminution can be interpreted toward the upper portion of the channel, which changes into materials containing cross-bedding and coarse sands inter-bedded with open-work gravel layers.
This sequence of fine-grained beds and stratified layers continue toward the top of the glaciofluvial unit. Till 2b, only 1 m thick, is separated by an erosional unconformity from the underlying sands; it is eroded at the top by deflational and human activities and is overlies the
glaciofluvial sequence. The two till units have a similar grain-size distribution (Fig. 4).
Grains ranging between pebble-size and very-fine-sand size comprise 65.3% of the upper till and 54.9% of the lower till; the remainder contains silt and clay.
Fig. 3. Stratigraphic sections description of multiple set of ice-wedge casts developed on till at (A) Estancia Tres de Enero site and (B) Chimen Aike
4.1.2. Chimen Aike
At Chimen Aike site, the stratigraphy is well exposed in a gravel pit. Glaciofluvial sediments with a greyish to bluishgrey color and some distinctive light grey layers, named Unit 1, occur at the base of the sequence with a total thickness of 9 m (Fig. 3B). They correspond to coarse sands with trough crossbedding structures to fine gravels with cross-bedding stratification.
In most cases these structures indicate a west to southwest transport direction. The thickness of individual beds ranges between 10 and 20 cm. Several beds of very fine sands and light-colored silts appear to be inter-bedded with the gravels. A few isolated cobbles up to 8 cm in diameter were observed.
A lens-like stratified body composed of glaciofluvial sands and gravels up to 4 m in thickness, named Unit 2, lies uncomformably over the previous unit (Fig. 3b). The color varies from grey to very dark grey. This overlying unit starts with a fine sand layer and silt beds with planar stratification and deformational structures which penetrate the lower unit. This first layer is 10 cm thick and it is overlain by 20 cm of a coarse lag conglomerate with pebbles and cobbles of a bullet shape up to 20 cm in diameter, which is typical of glacial deposits (Boulton, 1978). This
unit, unlike the previous one, shows continuous, well-developed planar bedding, with individual strata ranging from 10 to 50 cm in thickness.
Overlying this unit is Unit 3, which has a thickness of 2 m and is composed of coarse sand in a silty matrix and with scattered lenses of conglomerates and strongly deformed bedding. It contains many deformation features due to overloading, some of which are intruding the underlying unit. There are important, well-defined protrusion structures, possibly related to fluid emission due to overloading or even perhaps to paleo-seismic activity. The load casts are of a ‘‘ball-and-pillow’’ structure. The lens-like layers are composed of fine gravels, which appear to
have been severely deformed, in some cases intruding the overlying sandy sediments. Clastic dykes are common in this unit, some of which are 30–40 cm in length and 3–5 cm in thickness. A few reddish layers are found which are probably related to hydromorphic activity, most likely chemical deposition from underground water movement.
Overlying this latter unit is a well-defined diamicton that is approximately 1 mthick and is named herein Unit 4. In some layers, cross-bedded gravels occur, which indicate a transport from a western provenance. This unit contains pebbles and cobbles up to 10 cm in diameter, including some layers of open-work gravels. CaCO3 deposition occurs as sub-vertical veins, with a thickness up to 50 cm. In some areas, the carbonate deposition appears horizontally
continuous and homogeneous, whereas in other areas it is discontinuous or in the shape of crossing bars. This unit is host to very frequent sand wedges and cryogenic features, including platy structures, deformed beds, reorganization of pebble and cobble clasts in vertical or subvertical position, and upward deformation of the sandy-silty beds around the sand wedges. In some cases these features penetrate down to Unit 3. The sequence culminates with the presence of colluvial sediments up to 50 cm in thickness, which shows the development of a soil or a sequence of superposed soils of postglacial age. This paleosol has prismatic structure and a welldefined clay-enriched horizon that is cross-cut by a younger generation of sand wedges.
4.2. Soils
Most of the ground-soils (as opposed to buried soils) are derived from aeolian sand over a thin (19–30 cm) layer of alluvium or outwash, which overlies the dominant material of the section that includes a combination of till, outwash, and alluvium. The soils of the study sites have a thick (34–105 cm) mollic epipedon and, therefore, are classified in the Mollisol order (Table 1). Because they have an aridic soil moisture regime, they are classified in the Xeroll suborder. All of the ground-soils have an argillic (Bt) horizon (25– 54 cm thick) and, therefore, are classified in the Argixeroll great group. Soils with abundant (stages III–IV) secondary carbonates are classified as Calciargidic Argixerolls; the soil at Monte Aymont has a thick (105 cm) mollic epipedon and is a Pachic Argixeroll; and the soil at Estancia Bella Vista lacks carbonates and is a Typic Argixeroll
4.3. Wedge properties
We investigated a total of 67 wedges at the six sites. Two of the sites (Estancia Tres de Enero and Chimen Aike) have multiple horizons of wedges (Fig. 5A and B). About 55% of the cryogenic features had a wedge shape, followed by a funnel shape (23%), indistinct (14%), and veins (8%) (Table 2). The wedge frequency ranged from 14 to 52 and averaged 46 wedges per 100 m(excluding the Punta Loyola site), or a spacing between wedges that are 2.2 m. Height of the wedges ranged from 40 to 124 cm and averaged 78 cm. The apparent maximum width ranged from 24 to 55 cm and averaged 40 cm; the apparent minimum width ranged from 2.0 to 5.4 cm and averaged 3.6. The height/apparent width (average of maximum of minimum) ratio ranged from 1.6 to 4.1 and averaged 2.8. The wedges normally lacked visible carbonates, despite an abundance of carbonates in the host materials at specific localities, notably at Chimen Aike and Punta Loyola (Tables 1 and 2). At several locations we observed a 2.6 2.6 m polygonal form with 0.5 m wide relict sand wedges in smoothed ditches alongside roads (Fig. 5C). The frequency of these wedges was 16 per 100 m, which is less than the corrected value (26 per 100 m) that we recorded along exposures.
Grains in wedge infillings have a different size than their host materials or the soils horizons in which they penetrate (Fig. 4). In most cases the wedge infillings are composed of particles larger than fine sand, which are not present in the host materials. The wedge infillings of 3 de Enero site samples are the only ones with some clasts larger than pebbles (phi -1).
5. Discussion5.1. Wedge interpretation
All six of the sites have what appear to be relict sand wedges of primary infilling. The shape and dimensions of the relict sand wedges are comparable to contemporary sand wedges in Antarctica (Pe´we´ , 1959) and relict sand wedges in northern Alaska (Carter, 1983) and northern Canada (Murton, 1996; Murton and Bateman, 2007). They are also similar to what were described as ice-wedge pseudomorphs in southern Patagonia (Galloway, 1985; Trombotto, 2008). In particular many of the wedges have distinct foliated fabric of near-vertical to vertical laminations in the wedge centers grading outward to inclinations that parallel the wedge edges.
Fig. 6 compares a relict sand wedge at Estancia Tres de Enero with an active sand wedge in Beacon Valley, Antarctica. In both cases, the laminations vary from 1 to 5 mm in width and often show crosscutting characteristic of contemporary sand wedges (Pe´we´ , 1959).
We did not observe slumping of the features suggestive of icewedges, but the upper horizons may have been truncated.
5.2. Paleo-environmental reconstruction
The geologic setting and morphology of the relict wedges suggest a cold dry climate when wind-driven sand moving across southern Patagonia dropped into open cracks from a combination
of thermal contraction and desiccation. The vegetation must have been even more sparse than today. The relict sand wedges and polygons in southern Patagonia imply that permafrost existed in the area at various stages during the Pleistocene. Our data suggest that in southern Patagonia there were at least four cold intervals during the past 1.1 Ma, including two that are post-1.0 Ma, one that is post-0.5–0.8 Ma, and one that is post-0.3 Ma. We found no relict sand wedges in sediments assigned to the Last Glacial Maximum in this region; however, they likely exist further west where LGM moraines occur and are common 200 km to the south in Tierra del Fuego (Coronato et al., 2004a; Perez-Alberti et al., 2008).
Although we recognize the problems in interpreting paleoenvironment from cryogenic features (e.g., Murton and Kolstrup, 2003), we will attempt to provide rough estimates of the change in mean annual air temperature and moisture during the Patagonian cold events. Sand wedges form today in regions with a mean annual air temperature of 4 to 8 C or colder (Pe´we´ ,1959; Mackay,1974; Karte, 1983). Since the city of Rı´o Gallegos has a present mean annual air temperature of around 7 C, our data suggest that the mean annual air temperature during the mid-Pleistocene cold intervals was at least 11–15 C colder than today. CLIMAP (1976)indicates a mean annual sea-surface temperature along the southern Chilean coast of 2 C for glacial times. Because these interpretations do not directly apply to terrestrial environments, the study of sand-wedge development is important to the reconstruction of paleo-environments in southern Patagonia. At least six glacial advances have been occurred in the past 1 Ma (Coronato et al., 2004b,c) and 16 since the late Miocene (Rabassa et al., 2005) in southern Patagonia. The stable isotope record of planktonic and benthic foraminifers compared with ice-rafted debris (IRD) from Site ODP 861 along the Chilean coast (45 510S–75 41.50W) record that glacial conditions occurred at this latitude close to 18.3 ka B.P.
(MIS 1a); 23.5 ka B.P. (Marine Isotope Stage (MIS 2.2)), 65 ka B.P.
(MIS 4.2), 135 ka B.P. (MIS 8.2) (Scho¨ nfeld et al., 1995) and 340 ka
(MIS 10.2) (Spiegler et al., 1995). The two first cold stages mentioned correlate with the Fenix and Moreno I and II moraines, which are dated by radiocarbon and cosmogenic isotopes at 23–
16 ka and 150–140 ka, respectively at Lago Buenos Aires (46 300S) along the eastern slope of the Andes (Kaplan et al., 2005).
Sand wedges in Antarctica often have gravimetric soil moisture contents ranging from 3 to 7% (Campbell and Claridge, 2006). Based on 20 months of data that we collected at two sites in southern Patagonia (Ercolano and Bockheim, unpublished), the soil moisture content at a depth of 25 cm averaged 10%. The meanannual precipitation of areas with contemporary sand wedges ranges from 50 to 150 mm (Pe´we´ , 1959; Karte, 1983). Therefore, it is likely that the mean annual precipitation during the cold intervals was less than 250 mm/yr and possibly as low as 100 mm/yr. Numerous investigators have challenged the position that relict ice-wedge pseudomorph networks could be used to infer climate trends at lower latitudes (Washburn, 1979; Burn, 1990; Plug and Werner, 2002; Murton and Kolstrup, 2003). Wedge spacing and
width in ice-wedge networks mainly reflect infrequent episodes of rapidly falling ground temperatures rather than mean conditions.
Fig. 5. Photographs of selected cryogenic features including (A) multiple features at Estancia Tres de Enero, (B) multiple features at Chimen Aike, and (C) former sand-wedge polygons
exposed along Route 3 15 km southwest of Rio Gallegos. See Fig. 3 for stratigraphic location of photograph 5A. There are lag deposits above each horizon of sand-wedge casts.
Narrow relict wedges, such as those recorded in our analysis (mean maximum width¼ 0.40 m), are consistent with a cold, inherently variable climate, or with a moderate climate marginally suitable for ice-wedge development. The characteristics of ice-wedge networks reflect interplay among fracturing, ice-wedge growth, and network development, which are all nonlinear processes with differing intrinsic timescales.
We have observed numerous instances whereby sand-wedge relicts cut through petrocalcic horizons, particularly on drifts of mid-Pleistocene age. In Fig. 7, a sand-wedge relic cuts through stage IV carbonates of Sierras de los Frailes drift. These data imply that petrocalcic horizons likely formed during warm interglacials and sand wedges developed during cold-glacial periods. Vogt and del Valle (1994), Vogt and Corte (1996), and Trombotto (1996, 2008) reported similar findings elsewhere in Patagonia.
All of the ground soils that we examined in southern Patagonia had argillic horizons, as did many of the truncated buried soils.
Many pedologists from other parts of the world believe that the argillic horizon in Aridisols and related soils was formed during Holocene or Pleistocene events when the climate was moister than today (Nettleton et al., 1975; Southard and Southard, 1985; Khademi and Mermut, 2003). In some areas of the world, such as the western United States, argillic horizons formed during the glacial periods (pluvials) when precipitation was greater (Reheis, 1987).
However, in Patagonia the glacial periods were extremely cold and dry and the interglacial periods had greater moisture and higher temperatures. Previous studies indicate that argillic horizons require a minimum of 400 mm/yr of precipitation to form (Reheis,
1987; Birkeland, 1999).
5.3. Landform deflation rates and wedge infilling
Cold arid conditions, promoted by a decrease of air temperature and the domain of the southern westerlies, accompanied by a limited vegetation cover would have promoted wind erosion in
a cold rain-shadow environment. Wind-blown sand is the likely source of the material filling the sand-wedge casts, although pebbles also infill one of the wedge sets.We calculated the mass of
sand infilling per unit area from the equation:
M ¼ D½ðH W LÞ=2Db (2)
where M¼ mass of infilling (Mg/ha); D¼ density of contraction fissures (number/ha); H ¼height of contraction fissure (m); W¼adjusted width of contraction fissure (m); L¼ length of
contraction fissure segment (m); and Db ¼ bulk density (Mg/m3).
Fig. 6. Photographs of a relict sand wedge at (A) Tres de Enero and (B) a contemporary sand wedge in Beacon Valley, Antarctica (photo by J. Bockheim). The stones of the surface of
the active sand wedge from Antarctica have fallen from the adjacent high-center polygon and do not represent a lag pavement.
According to these estimates, sand wedge relicts contain an average of 48 Mg/ha of infilled sand. This corresponds to an overall regional deflation of 27 cm and supports the conjecture that
landscape deflationwas more rapid during cold and wet climates in southern Patagonia and Tierra del Fuego than in northern regions of Patagonia, as it had been postulated by Kaplan et al. (2007). Further research is needed to certainly know about the source of sands which infill the wedge and to establish comparisons among them. Hypothetically, the sands may have originated from Pliocene-Early Pleistocene glacial and glaciofluvial deposits overlaying volcanic and sedimentary tablelands located to the north and west of the study sites. Also, the wide and dry downstream valley floor of the Gallegos river system during glacial periods could have been the
source of sand.
5.4. Evidence for multiple advances during the Sierra de los Frailes
Glaciation
Two sets of sand-wedge relicts are developed at Estancia Tres de Enero along a new cut in Route 3 (Figs. 1 and 3A). This site is located on the oldest moraine system of the region based on
mapping by Caldenius (1932) and Meglioli (1992). Since this new road cut allows the observation of the two sets of cryogenic features developed over two different till units, it is possible to interpret that this region was affected by extreme cold climate conditions at least during four distinctive periods. The lower set of sand wedge casts intrude an old basal till (Till 1) whose original surface has been eroded, thus destroying its initial topography; paleo-channel sediments show also the effect of younger episodes of meltwater erosion on it (Fig. 3A). The upper set of sand wedges intrudes the second till unit (Till 2b) and also the glaciofluvial
sediments.
The presence of Till 1 indicates that ice covered this site sometime during the GPG maximum (Meglioli, 1992) or before. During the following climate reversal, periglacial conditions would have prevailed, no glacial ice would have reached the site, and sand wedges would have developed. A third cold climate period allowed the glacial ice to cover this area again, as in the first glacial event, and its basal deposits covered the older till as well as the sand
wedge developed on it. Their excellent state of preservation could perhaps be explained by rapid freezing of the sedimentary deposits before the ice advanced over them again. A fourth cold episode allowed the formation of the upper horizon of sand wedges, developed on the younger glacial deposits.
Two of the glacial periods described above would have been humid enough to allow huge glaciers to extend more than 250 km eastwards from the Andes, but the other two would have been
extremely cold and dry, promoting the formation of a cold desert and polygonal soil features instead of large glacial lobes.
The chronology of these periglacial events is not known yet but it could be compared to the regional glacial chronology. Three possibilities can be postulated for interpretation: 1) If the lower set of sand-wedge casts is related to the time of the maximum glacial advance recognized in the area, (Great Patagonian Glaciation or Sierra de los Frailes Glaciation) around 1 Ma, it is possible to consider the lower till as a glacial expansion earlier than the GPG. The existence of extensive pre-GPG piedmont glaciations could be interpreted from inter-bedded till deposits between lava flows in several tablelands of Patagonia (Coronato et al., 2004c; Rabassa et al., 2005). Mercer (1976) reported a till bed overlying lava flows dated ca. 2.8 Ma (K–Ar age determination) at Estancia Cóndor Cliff (50 110S, 70 510W). This author also reported the
presence of five till beds inter-bedded with lava flows at Cerro del Fraile (50 330S, 72 400W), with K–Ar ages of 2 Ma for the lower lava flow which overlies a till bed and 1.03 Ma for the upper lava flow which overlies the upper till bed. More recently, Singer et al. (2004) redefined this sequence and reported seven till beds and ten lava flows. While the lower till (‘‘Till 1’’ at Cerro del Fraile) is overlain by a lava flow dated at 2.18 Ma by the 40Ar/39Ar incremental
heating technique, the upper till (‘‘Till 7’’ at Cerro del Fraile) overlies a lava flow dated at 1.42 Ma. Isolated glacial erratics occur on the uppermost lava. These ages are very close to
those of Fleck et al. (1972) in the same place. Considering that Till 7 may correspond to the GPG and that it would be equivalent to the upper till unit (Till 2) of the Estancia Tres de Enero site, Till 6, dated between 1.5 Ma (Fleck et al., 1972) and 1.4 Ma (Singer et al., 2004), at Cerro del Fraile could be them comparable with the lower till unit of Estancia Tres de Enero site (Till 1), thus being a pre-GPG glaciation.
As a second possibility, the lower till in Estancia Tres de Enero site (Till 1) could be interpreted as belonging to the GPG or Sierra de los Frailes Glaciation. In this case, the upper till (Till 2) would indicate that a younger post-GPG glaciation was large enough to reach the boundaries of the 1 Ma glacial advance near the Atlantic coast.
A third possible interpretation is that the GPG or Sierra de los Frailes Glaciation had several clearly defined advance stages characterized by extremely cold and dry climatic conditions and separated by recessional events. Further dating studies should be performed on these till units to constrain a more accurate local chronology. In that they are not covered by other glacial deposits, the second sand-wedge episode could have happened anytime from the early to the late Pleistocene, and it could have been formed as recently as the Last Glacial Maximum.
Fig. 7. Example of sand-wedge relics formed during glacial intervals cutting through
petrocalcic horizons formed during interglacials in Sierra de los Frailes drift at Estancia
Tres de Enero
6. Conclusions
Relict sand wedges and ice-wedge pesudomorphs are common
in southern Patagonia and are important as markers for delineating
multiple glaciations and for reconstructing paleo-environments.
The presence of two sets of sand-wedge relicts at Estancia Tres de
Enero provides evidence for multiple advances close to the Atlantic
coast during the Sierra de los Frailes glaciation around 1 Ma ago, or
even before.
Based on stratigraphic analysis, four sets of sand-wedge relicts
suggest that permafrost accompanied by patterned ground affected
glacial deposits from the early Pleistocene (ca. 1.0–1.1 Ma or even
earlier) at Bella Vista, Tres de Enero and Chimen Aike sites, from the
the early to mid-Pleistocene (ca. 800–850 ka) at Monte Aymond
site and periglacial deposits from the middle Pleistocene (ca.
300 ka) at Punta Loyola site. In that most of the ice-wedges are not
overlain by other glacial deposits, it is not clear in which glacial
period they were formed.
The geologic setting and morphology of the relict wedges
suggest a cold dry climate when strong winds moving across the
region dropped sand and occasionally pushed pebbles into the
open thermal-contraction cracks. The vegetation must have been
even more sparse than today. Based on the distribution of active
sand wedges, the climate during sand-wedge formation must have
been at least 11–15 C colder than today, and the mean annual
precipitation may have been less than half of what it is today.
The soils of the region are primarily Calciargidic Argixerolls that
bear a legacy of climate change. Whereas sand wedges formed
during very cold (4 to 8 C or colder) and dry (ca. 100 mm
precipitation/yr) glacial periods, petrocalcic horizons from calcium
carbonate contributed by dustfall formed during warm (7 C or
warmer) and moister (250 mm/yr) interglacial periods. The
paleo-argillic (Bt) horizons reflect unusually moist interglacial
events where the mean annual precipitation may have been
400 mm/yr.
Finally, ice and sand wedge casts are a powerful cryo-sedimentary
tool available in the lowland landscapes of southern
Patagonia. They provide useful paleo-environmental information,
which should encourage further research in this region.
Acknowledgments
This projectwas supported by the Argentinian Science Secretary
(FONCyT-ANPCyT; PICTR 67-02) to Jorge Rabassa and the Universidad
Nacional de la Patagonia Austral at Rı´o Gallegos city, Santa
Cruz province.We are grateful to two reviewers for their thoughtful
suggestions.
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Please cite this article in press as: Bockheim, J., et al., Relict sand wedges in southern Patagonia and their stratigraphic and paleo-environmental
significance, Quaternary Science Reviews (2009), doi:10.1016/j.quascirev.2008.12.011
En tiempos en los que el cambio climático está en boca de
