the cuaternary glacial record of the colombian andes

Quaternary Glaciations - Extent and Chronology, Part III Editors J. Ehlers and P.L. Gibbard �9 2004 Elsevier B.V. All fights reserved

The Quaternary glacial record of the Colombian Andes

Karin F. Helmens

Arctic Centre, University o f Lapland, P.O. Box 122, 96101 Rovaniem4 Finland,"

Abstract

The onset of glaciations in the Colombian Andes as recorded by the start of glaciofluvial sedimentation in the inter-montane Bogotfi basin is dated near the Gauss/ Matuyama polarity reversal at 2.6 Ma; episodes of increased glacial activity occurred since ca. 0.8 Ma. Moraines and till beds in the higher parts of the Andes record a series of glacier fluctuations of Late Quaternary age. Radiocarbon dates of organic-rich sediments and palaeosols found associated with glacial landforms/deposits in mountain ranges exceeding 3600 m altitude in the Eastern Cordillera, in combination with evidence provided by the radiocarbon-dated palaeosol sequence in the region, place glacial events between probably 43 ka and 38 ka, 36 to 31 ka, 23.5 to 19.5 ka, 18.0 to 15.5 ka, 13.5 to 12.5 ka, and most probably 11 to 10 ka (radiocarbon years BP). Independent chronologies for the glacial and palynological records of the Eastem Cordillera suggest a close match between stadials characterised by low upper Andean forest limits and glacier advances in the surrounding high- mountain ranges. Major glacier advances during the Middle Weichselian seem to have responded to cool and humid climatic conditions. The Late Weichselian Glacial Maximum (LGM) is recorded as a two-fold glaciation maximum just before 19.5 and 15.5 ka, with glaciers advancing some 1200 to 1100 m below their present limits; during the cooling events, the forest limit was depressed by 1100 to 900 m, implying a drop in mean annual temperature of ca. 8 to 6 ~ respectively. Interstadial conditions prevailed around 18 ka, when temperatures rose considerabb to values up to 4 ~ higher than during the preceding and following stadial periods. Mountain ranges below 4000 m altitude were deglaciated at ca. 12.5 ka following a Late-glacial advance of cirque glaciers. Younger Dryas cooling is well-registered in the palynological record; glaciers in the highest parts of the Andes seem to have responded to the cooling by extending to elevations some 700 m lower than at present. The glacial record registers a high climatic variability in the northern Tropical Andes during the Late Quaternary period.

Introduction

The Andes in Colombia, in the northernmost part of South America, extends between latitudes of c. 1-11 ~ N. The Andes consists of three N-S to NE-SW orientated mountain

chains named the Cordilleras Occidental, Central and Oriental, or the Western, Central and Eastern Cordilleras, respectively. The Cordilleras are separated from each other by the deep inter-Andean valleys of the Cauca and Magdalena Rivers (Fig. 1). The position of the Equilibrium Line Altitude (ELA) in the northern Andes lies at the high elevation of c. 5000 m (Kuhn, 1981). This, and the fact that little terrain extends above this height, means that only a small portion of the Colombian Andes is currently glaciated. The total area of glacier ice in Colombia, measured from Landsat images from the early 1970's, has been determined at 104 km 2 (Hoyos-Patifio, 1998). The Central Cordillera is the only volcanic mountain chain and encompasses a series of glacier-capped stratovolcanoes including the Nevado del Huila and the Ruiz-Tolima volcanic massif (Fig. 1). The Eastern Cordillera only rises high enough to support glaciers on its northern portion, where the Sierra Nevada del Cocuy and adjacent ranges are ice-covered over a total length of c. 35 km. Glaciers are also found on the highest peaks and ridges of the Sierra Nevada de Santa Marta, which, as a tetrahedral-shaped mountain massif, stands isolated along the Caribbean coast.

The total area of glaciated terrain was substantially enlarged during the Pleistocene. Estimates by Thouret et al. (1996) indicate that c. 26,000 km 2 of the Colombian Andes would bear evidence of Pleistocene glaciation, amounting to c. 7.5% of its total surface area. Only a minor portion of the formerly-glaciated mountains, however, has been studied. Raasveldt (1957) notes the limited economic use of natural resources in the higher mountains of Colombia, arising from the sparse habitation and poor industrial development, as an important factor that explains why glacial deposits have not been mapped at a large-scale as in N-America and NW Europe. The study by Raasveldt (1957) on the glaciations of the Sierra Nevada de Santa Marta, like more recent studies in the Eastern and Central Cordilleras, has been of a purely scientific nature: to study the timing and extent of fluctuations of mountain glaciers located in the tropical zone. Despite the fact that only a small part of the Colombian Andes has been studied, the investigations of the Eastern and Central Cordilleran morainic sequences (van der Hammen et al., 1980/81; Helmens, 1988; Helmens et al., 1997b; Thouret et al., 1996) are characterised by their great detail, and provided one of the best-dated Late Pleistocene glacial records for the Andes (Clapperton, 2000). Chronologies are based on the radiocarbon dating of organic-rich sediments and palaeosols, the accumulation of organic matter in depressions and the soil cover being

115

116 Karin F. He lmens

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Fig. 1. Andean Colombia showing the Western (1), Central (2) and Eastern Cordilleras (3). Sites mentioned in the text are indicated, �9 those underlined are currently glaciated.

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favoured by the cool and humid climate of the northem high Andes. In addition, the study of glaciofluvial sediment accumulated in the inter-montane basin of Bogota (Eastern Cordillera; Fig.l) has provided evidence on the timing and intensity of older glaciations in the region (Helmens et al., 1997a).

Early and Middle Pleistocene glaciations recorded by glaciofluvial sediment in the Bogota basin

The first part of this paper deals with the outer valleys of the tectonic basin of Bogot~i in which thick sequences of glaciofluvial sediment have accumulated. Fission-track dating of intercalated tephras, combined with magnetic

polarity dating of the sediment sequences, have provided a chronology for Early and Middle Pleistocene glaciations in the region. The central part of the Bogot~i basin is infilled by up to 600 m of mostly lake sediments of Late Pliocene- Pleistocene age (Hooghiemstra & Ran, 1994). The fiat surface of the sedimentary infill at an elevation of c. 2600 m forms the high plain of Bogota in the central part of the Eastern Cordillera (Fig. 1).

Where the inter-montane basin of Bogota is bordered by high mountains, such as at the foot of the western slopes of the P~iramo de Palacio (Fig. 2), a distinct lithological transition is observed from morainic deposits at high elevations towards the glaciofluvial sediment in the basin (Helmens, 1990; Helmens & van der Hammen, 1994). The coarse and angular boulders of which the moraines are

Colombia 117

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composed (Rio Chisac~i Formation in Fig. 2) pass into rounded boulders and gravel in a series of large coalescing outwash fans directly at the foot of the formerly-glaciated mountain slopes (Rio Siecha Formation). The latter deposits grade, within the Bogot~i basin, into thick sequences of well-rounded gravel (Rio Tunjuelito Formation), which in their turn grade into a series of sand and gravel units away from the main rivers that enter the basin. The sand and gravel units alternate with, and in

places truncate, more fine-grained sediment beds of partly organic silt and clay of a mostly lacustrine origin. The resulting sediment sequence has a total thickness of c. 30 m and has been defined as the Subachoque Formation (van der Hammen et al., 1973). Palynological evidence from the fine-grained lacustrine intercalations in this formation, from thin organic beds in the Rio Tunjuelito Formation, and from sequences of up to 150 m thick of clays interbedded with peaty and sandy sediment in the deeper parts of the central

118 Karin F. Helmens

basin, that have been correlated with the type Subachoque, reflect the changing climatic conditions of the Pleistocene (van der Hammen et al., 1973; Kuhry & Helmens, 1990; Hooghiemstra & Ran, 1994). These pollen assemblages indicate that during deposition of the Subachoque and Rio Tunjuelito Formations the slopes surrounding the Bogotfi basin were alternately covered by Andean forest vegetation, representing interglacial (or interstadial) conditions, and treeless, tropic-alpine vegetation, called paramo in the northern Andes, that indicate glacial (stadial) conditions. The sand and gravel interbeds in the Subachoque Formation, and the gravels of the Tunjuelito Formation, are interpreted to represent the coldest intervals of the Pleistocene when the surrounding mountains were glaciated. The glaciers caused the outer valleys of the Bogot~i basin to be infilled by glaciofluvial sediment, restricting the Bogot~ lake to the central part of the basin. Radiocarbon dates from the Tunjuelito Formation and from organic-rich sediments and palaeosols found associated with the moraines of the Rio Chisac~i Formation suggest synchrony between the deposition of gravels in the Bogotfi basin and glacial events in the mountains for the Late Pleistocene (van der Hammen et al., 1980/81; van der Hammen, 1986).

A detailed geochronology for the sediment infill of the outer parts of the Bogot~ basin is presented by Helmens et al. (1997a). This study focussed on two major outcrops in the Subachoque Formation and the sediments of the underlying Guasca Member of the Upper Tilatfi Formation (Guasca and Subachoque sections in Fig. 2). The clays and silts of the Guasca Member represent the oldest sediments deposited in the Bogot~ basin (Helmens, 1990). Palynological evidence from organic-rich intercalations in the lower and middle part of the member indicates warmer conditions than those prevailing in the Bogotfi area today. The warmer conditions have been ascribed to a lower altitude of the Bogotfi area, since the major tectonic uplift of this part of the Eastern Cordillera had not yet been completed, and/or to a warmer Pliocene climate (Wijninga & Kuhry, 1993). The two outcrops that expose a total thickness of c. 75 m were sampled at c. 10 cm intervals for palaeomagnetic polarity studies (Helmens et al., 1997a). Interbedded tephras, derived from the Ruiz-Tolima volcanic massif in the Central Cordillera (Fig. 1), were sampled for fission-track dating. The polarity records obtained from the Guasca and Subachoque sections were not expected to mimic the geomagnetic polarity reference timescale (Cande & Kent, 1995), considering that the alternating glaciofluvial and lacustrine/paludal sediments of the Subachoque and Upper Tilat~i Formations must have accumulated at different rates, and periods of sedimentation alternated with periods of erosion. The palaeomagnetic correlation presented in Helmens et al. (1997a) takes into account the discontinuity of sedimentation, as well as the error limits of the fission-track dates. The sediments of the Guasca Member yielded fission-track dates of c. 3 and 2.9 Ma, and their polarities are assigned to the upper part of the Gauss Chron. The Subachoque Formarion yielded fission-track

dates of 2.5, 1.7 and 1 Ma and its magnetisation indicates that it ranges from the Matuyama to the Brunhes Chron. Magnetic susceptibility (MS) measurements were also undertaken, and are used as a proxy for periglacial and glacial erosion in the mountains. The environmental record was further enhanced by detailed lithological descriptions of the sediments. The results obtained by Helmens et al. (1997a) are summarised in Figure 3.

Onset o f glaciations near the boundary (2.6 Ma)

Pliocene-Pleistocene

Magnetic polarity and fission-track dating places the first glaciation in the Bogot~i mountains near the Gauss/Matuyama magnetic reversal at 2.6 Ma. The onset of glaciation is represented by a sudden influx of glaciofluvial sand and gravel into the Bogot~i basin, recorded at the base of the Subachoque Formation. The Gauss/Matuyama boundary is often used by terrestrial stratigraphers as the Pliocene-Pleistocene boundary (e.g., Zagwijn, 1992).

Warm conditions during deposition of the Guasca Member in the Gauss Chron, recorded by palynological evidence (Wijninga & Kuhry, 1993), also seem to be indicated by relatively low MS values (Fig. 3). Higher MS values are interpreted by Helmens et al. (1997a) as possibly reflecting minor climatic oscillations, with a larger amount of magnetite-rich sediment being transported towards the Bogot~i basin during colder intervals. Peaks with anomalously high MS in tephra-rich samples from the lowermost part of the member are possibly related to a high admixture of volcanic magnetite. A distinct increase in the amplitude of the MS signal is recorded at the base of the Subachoque Formation. MS peaks in the sediments of the Subachoque Formation are interpreted as reflecting periods of enhanced influx of magnetite into the basin resulting from repeated glaciation. The continuous Funza pollen record obtained from long boreholes in the central part of the Bogot~ basin (Fig. 2) records a considerable lowering of the regional forest limit at the base the Subachoque Formation, which indicates rapid cooling (Hooghiemstra & Ran, 1994). As the Funza pollen sequence indicates that tectonic uplift had most probably ceased, the first glaciation is thus related to significant climate cooling. The age of c. 2.6 Ma for the onset of glaciations in the Bogoth mountains as obtained by Helmens et al. (1997a) confirms an earlier fission-track age of slightly younger than 2.7 + 0.6 Ma for the initiation of deposition of the Subachoque sediments in the Funza boreholes (Andriessen et al., 1993). An Early Matuyama age for the first glaciation in the Bogot~i area in northern South America corresponds closely to that obtained for the Bolivian Andes in central South- America, as established by magnetostratigraphic 'dating' of sediments in the La Paz Basin (Thouveny & Servant, 1989). Evidence for large-scale glaciation, based on ice-rafted debris records, indicates that glaciation in the North Atlantic region also began around 2.6 Ma (Shackleton et al., 1984; 1990).

Colombia 119

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Fig. 3. The Late Pliocene-Quaternary environmental record of the Guasca Member of the Upper Tilat6 Formation and the Subachoque Formation in the marginal valleys of the Bogot6 basin, based on magnetostratigraphy, fission-track chronology, lithology and magnetic susceptibility of the Guasca and Subachoque sections (Fig. 2). The geomagnetic reversal chronology is based on Cande & Kent (1995). The vertical scale of lithological columns is linear The sand and gravel interbeds in the Subachoque Formation represent glaciofluvial sediment &,rived from glaciers in the higher mountain ranges surrounding the Begot6 basin (Helmens et al., 1997a).

120 Karin F. He lmens

Increased glacial activity since c. O. 8 Ma

A lithological change in the Subachoque Formation marked by an influx of coarser-grained glaciofluvial deposits and a possible increase in amplitude of the magnetic susceptibility signal occur near the Matuyama/Brunhes boundary at 0.8 Ma, indicating a shift towards higher magnitude climate oscillations.

The detailed lithological description of the Subachoque Formation by Helmens et al. (1997a) has revealed that the sand and gravel interbeds become distinctly more coarse- grained in the upper part of the formation (Fig. 3). Since their sampling was initially directed towards studies of palaeomagnetic remanence, only fine-grained lithologies were collected, and as a result there are gaps in the MS record corresponding to the coarsest units. Nevertheless, the data obtained seems to indicate a further increase in MS amplitude in the upper part of the Subachoque Formation. The change in lithology and MS signal is dated to occur near the Matuyama/Brunhes magnetic reversal at 0.8 Ma. Apparently, episodes with conditions colder than those prevailing during the upper part of the Matuyama Chron resulted in more extensive glaciations (and a larger influx of magnetite-rich sediment into the Bogotfi basin), with rapidly aggrading floodplains leaving a distinct series of coarse-grained sand and gravel beds. Additionally, episodes with conditions warmer than during the Matuyama Chron, and higher evaporation and evapotranspiration rates (Hooghiemstra, 1984; Kuhry, 1991), resulted in lower lake levels and, in the orographically dry Guasca valley, in periods of soil formation (Fig. 3).

A strong increase in climatic variability represented in the upper part of the Funza pollen sequence has been also placed at c. 0.8 Ma, based on fission-track dating and land- sea correlation (Andriessen et al., 1993; Hooghiemstra et al., 1993; Hooghiemstra & Ran, 1994). The change in climate is recorded by pollen in the clays of the Sabana Formation (Fig. 2). The change from low-amplitude to high-amplitude climatic oscillations registered near the Matuyama/Brunhes boundary in the oxygen isotope record, obtained from deep-sea foraminifera, is associated with a distinct change in the frequency of oscillations from 41 to 100 ka climate cycles (Ruddiman et al., 1986, 1989). The 100 ka periodicity determined in the Funza arboreal pollen record also seems to be restricted to the last c. 0.8 Ma (Hooghiemstra et al., 1993).

have resulted in a high-resolution record of glacier fluctuations for the last c. 45 ka. Modern glaciers in the Sierra Nevada del Cocuy on average lie above 4700 m a.s.1., with ice tongues locally descending to 4500 m. The highest peak is the Ritacuba Blanco at :5490 m. The mountains near Bogotfi reach to almost 4000 m altitude. Although currently ice-free, a distinct glacial landscape of cirques, U-shaped valleys and glacially-scoured, relatively flat areas dotted with glacial lakes, combined with numerous morainic ridges, characterise the mountain ranges, the higher parts of which exceed 3600 m a.s.1. (Helmens, 1988).

Maximum glaciation in the Colombian Eastern Cordillera pre-dated the Late Weichselian glacial maximum. One (possibly two) major glacial advance(s) of Middle Weichselian age; a two-fold glaciation maximum for the Late Weichselian (referred to here as an early and late stadial of the LGM); and a minor advance of early Late-Glacial age, are presented, based on the Bogotfi record. Evidence from the Sierra Nevada del Cocuy is discussed which additionally indicates a glacial event of probable Younger Dryas (YD) age, and a series of glacier limits for the Holocene. The glacial limits marked by the Middle Weichselian, the LGM and the YD moraines are indicated on the digital map. The small scale of the digital map inhibits the reproduction of the morainic ridges. However, several maps are included in this paper, which illustrate in detail the morainic sequence in both the Bogotfi and Cocuy mountains, as well as the situation of sites that have provided radiocarbon dating control for the glacial events recognised.

The glacial sequence discussed below is compared with the palynological record from the Eastern Cordillera. Numerous palynological studies have been carried out in this latter region. Pollen records have been obtained from sediment and peat accumulations in deep tectonic basins, weathering depressions in sandstone and, in the high altitude areas, in glacial basins. Syntheses of palynological evidence for the radiocarbon-dated interval 25 ka to modem day have been published by van der Hammen et al. (1980), Kuhry (1988) and Helmens & Kuhry (1995).

Finally, the Late Quaternary glacial record of the Eastern Cordillera is compared with results obtained from a detailed study of moraines on the Ruiz-Tolima volcanic massif in the Central Cordillera (Thouret et al., 1996).

Late Pleistocene and Holocene glaciations recorded by moraines in the Eastern Cordillera

The second part of this paper focuses on the mountains surrounding the high plain of Bogot~ (c. lat. 5 ~ N) and on the Sierra Nevada del Cocuy (c.lat. 6 ~ N), both situated in the Eastern Cordillera (Fig. 1). A detailed mapping of moraines and till beds in these mountain ranges, together with the radiocarbon dating of organic-rich sediments and palaeosols associated with the glacial deposits/landforms,

Mountain ranges near Bogot~i

Glacial landforms in the high mountain ranges of the P~iramo de Guerrero, the P/lramo de Pefia Negra, the westernmost part of the Pfiramo de Palacio and the northern part of the Pfiramo de Sumapaz (Figs 1 and 2) have been mapped in detail by Helmens (1988). The mapping focussed on the mountain slopes that drain towards the high plain of Bogotfi, because it formed part of a project on the Pliocene-Quaternary stratigraphy, palaeoenvironments and landscape evolution of the Bogot/l basin and surrounding

Colombia 121

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Fig. 4. The Late Quaternary glacial sequence in the Bogot6 mountains (Cuchilla Boca Grande, Phramo de Sumapaz; Fig. 2). The older glacial deposits pre-date 38 ka and possibly post-date 43 ka (radiocarbon years BP)," morainic complexes 1-4 have been dated to the time interval c. 36-12.5 ka (Helmens et al., 1997b). The location of exposures/boreholes mentioned in the text are indicated.


mountains (Helmens, 1990). The main morainic lobes are shown in Figure 2. For comparison studies, Helmens (1988) also mapped glacial landforms in one of the valleys of the Pfiramo de Palacio along the outer, eastern slope of the Cordillera, and in the northern valleys of the Pfiramo de Sumapaz that are drained towards the inter-Andean Magdalena River (Fig. 1).

Based on general morphology and degree of modification by erosion and denudation, the maps by Helmens (1988) differentiate four morainic complexes, defined from oldest to youngest as morainic complexes 1 to 4. All four morainic complexes are well-represented on the slopes below the Cuchilla de Boca Grande in the Pfiramo de Sumapaz (Figs 2 and 4). In addition, the latter area includes a site with 'older glacial deposits'. These deposits, which no longer display a morainic topography, have been encountered beyond the sequence of moraines (van der Hammen et al., 1980; Helmens, 1990).

Absolute chronological control for the glacial sequence in the Bogotfi mountains is provided by a series of radiocarbon dates (uncalibrated) from peat, lake sediments and palaeosols from borehole cores and exposures in the formerly-glaciated areas (van der Hammen et al., 1980; Helmens, 1988, 1990; Helmens & Kuhry, 1995; Helmens et al., 1997b). The palaeosols are formed in volcanic ash or ash-dominated slope deposits. Aluminium-humus complexes have protected organic matte, against microbial degradation, and the organic material accumulated in considerable quantities in the soil profile. This stable humus has been successfully used to radiocarbon date palaeosol sequences in the area (e.g. F61ster & Hetsch, 1978; Guillet et al., 1988). Combined dating results on palaeosols and peaty or lacustrine sediments further illustrate the reliability of the palaeosol dates (Helmens & Kuhry, 1995). A histogram of dating results on soil humus from the volcanic ash-palaeosols in the Bogotfi area (Fig. 5) indicates that intervals of soil formation occurred throughout the region over the last 50 ka (van der Hammen, 1981; Helmens & Kuhry, 1995). Because of their widespread occurrence, palaeosols have been tentatively used as stratigraphical markers for the glacial sequence (Helmens & Kuhry, 1995).

Middle Weichselian glacial advance(s)

The glacial record of the Bogotg area provides evidence for major glaciation(s) during the Middle Weichselian Substage. A major glacial advance occurred here before 31 ka, and an even more extensive advance took place before 38 ka. The glacial events are represented by morainic complex 1 and the older glacial deposits, respectively. The Middle Weichselian glacial limit shown on the digital map corresponds to the ice limit marked by the moraines of complex 1.

Morainic complex 1 consists of the oldest morainic landforms identified. These moraines are strongly subdued and only preserved locally. They occur at elevations some 200 m below the moraines of complex 2 which represent an

I I I I z ' 5 0 . . . . .

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AGE (xlO 3 yr BP)

Fig. 5. Histogram of radiocarbon dates of soil humus from volcanic ash-palaeosols in the Bogotgt area (Helmens & Kuhry, 1995).

early stadial of the LGM. A palaeosol directly overlying morainic complex 1 yielded a radiocarbon date of 30,930 • 420 yr BP (Chisacfi 8 in Fig. 2; Helmens, 1990). A similar date of 31,350 • 500 yr BP has been obtained from a palaeosol developed on top of sediments interpreted as reworked sediment of morainic complex 1. The latter sediments compose the upper part of a terrace sequence found exposed at the foot of the Cuchilla Boca Grande (section 190 in Fig. 4; Helmens, 1990; Helmens & Kuhry, 1995). Remnants of moraines of complex 1 were encountered in the apical zone of a similar terrace situated some distance to the north. The reworked sediment of morainic complex 1 in section 190 is separated from an underlying series of older glacial deposits by a volcanic ash bed overlain by organic-rich clays that have yielded a radiocarbon date of 38,100 +2,500/-1,900 yr BP. The older glacial deposits in the lower part of the terrace sequence include a thick layer of basal till. These rather compact clays scattered with rock fragments are underlain by rounded gravels and boulders most probably of glaciofluvial origin, and overlain by solifluction deposits that probably represent reworked supraglacial till. Slightly reworked supraglacial till has also been found in another valley several kilometres down-valley from morainic complex 1 (Subachoque 4 in Fig. 2; van der Hammen et al., 1980). The surface here, at an elevation of 2850 m a.s.1., is characterised by concentrations of large erratic boulders. A palaeosol developed on the till has been dated to 35,800 +1,100/-900 yr BP. Moreover, a possible maximum age of 43 ka for the older glacial deposits is indicated by a palaeosol developed in the Bogotfi area during this time interval (Fig. 5), that is represented in neither of the above- mentioned sequences of the older glacial deposits (Helmens & Kuhry, 1995).

An early timing for maximum glaciation during the Last Glacial stage in the Colombian Andes was first indicated by van der Hammen et al. (1980/81) and van der Hammen (1986). Their interpretation was based on limited absolute dating control for the morainic sequence of the Sierra Nevada del Cocuy, combined with evidence on glacial deposits, glaciofluvial gravels, palaeosols and pollen from the Bogotfi area. Van der Hammen et al. (1980/81) correlated the older glacial events with stadial periods of Middle Weichselian age, characterised in the palynological and lake-level records from the area by cold and distinctly humid conditions. More limited ice advances during the even colder Late Weichselian were ascribed to prevailing drier climatic conditions in the Colombian Andes.

Colombia 123

La Laguna GRIP V23-81

(Tropical Andes) (Greenland) (Noah Atlantic)

n'

1 . _ >, 15-

o

l o r- t~

O 2 0 - t"

. m

I ~ _ <

1 0 - _ ' ~ " - ~ - " - - - , ~ ~-,

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r - - - - I

La Laguna ~

Int, e ~ ~ m) ~ ~ l S 2 . . . . '_ __ ~ __ " '

-8 -6 -4 -2 0 -42 -40 -38 -36 100 80 60 40 20 ( ,(as deviations from present)

Temperature (*C) r (%.)in ice Percent N. pachyderma (s.)

Fig. 6. The climate record obtained from the La Laguna pollen record (Fig. 2) compared with the GRIP ice core from Greenland (Dansgaard et al., 1993) and the North Atlantic Ocean V23-81 core (Bond et aL, 1993). The correlation between the Greenland and North Atlantic cores is according to Bond et al. (1993); the timescale follows the radiocarbon chronology of the marine record (from Helmens et al., 1996). Intervals of glacier expansion recorded in the Bogotd mountains (the average lowering in glacier-front positions given in brackets) have been added directly to the right of the La Laguna temperature curve (Helmens et al., 1997b).

Additional glacial geomorphological/geological, palaeoecological and absolute chronological evidence later available for the Bogotfi area (Helmens & Kuhry, 1986; Helmens, 1988, 1990) supported this interpretation, and placed the most extensive glaciations recognised before 38 ka and probably after 43 ka, and between 36 and 31 ka (Helmens & Kuhry, 1995).

An early timing for the last maximum glacial advance has been reported from different parts of the Andes (Laugenie & Mercer, 1973; Wright, 1983; Espizua, 1993), including from the Ecuadorian Andes where radiocarbon dates from peat interbedded between tills yield ages in the range of 33 to > 43 ka (Clapperton, 1987). The maximum glacial advance in these areas, as in the Colombian Andes, pre-dated the maximum global ice volume for the Weichselian at c. 18 ka indicated by the deep-sea record (CLIMAP, 1981). Evidence for major glacial events predating the last global ice volume maximum in mountain ranges around the world is summarised and discussed in Gillespie & Molnar (1995).

Late Weichselian Glaciation Maxima

In the Bogot~i mountains, the Late Weichselian Glacial Maximum (LGM) consisted of two glacial maxima

separated by an interval of glacial retreat. The glacial advances are marked by morainic complexes 2 and 3. The glaciers had retreated from complex 2 just before 19.5 ka, but readvanced shortly afterwards close to this limit and deposited morainic complex 3, from where they sub- sequently receded just before 15.5 ka. These glacial limits are indicated on the digital map as representing an early and late stadial of the LGM. The LGM limit indicated on the digital map for the central and southern parts of the Pfiramo de Sumapaz (Fig. 2) is based on Brunnschweiler (1981). Although the glacier extension shown in Brunnschweiler (1981) seems to represent mostly the late LGM glacial advance, marked by the well-preserved morainic complex 3 of Helmens (1988), it is possible that in some places the extension includes the early LGM advance as indicated by the closely-related moraines of complex 2.

The youngest of the two LGM morainic complexes, complex 3, shows the most impressive morainic morphology of the different morainic complexes recognised in the Bogotfi mountains. The arcuate, multiple ridge system rises tens of metres above the valley floors, and the related maximum ice extent can be continuously traced throughout the mountain ranges studied. Its distribution shows that limits of former glacier systems were strongly influenced by local climatological conditions (Helmens, 1988). It


appears that glaciers reached farther down-valley in areas with high orographic precipitation, most significantly on the wet, eastern slopes of the Cordillera, where morainic complex 3 is found at the low elevation of 3100 m. In the rain-shadow created by large topographic barriers, complex 3 occurs at elevations as high as 3750 m. A rain-shadow effect is to some extent demonstrated in the area of the Cuchilla Boca Grande (Fig. 4). Northwards, a decrease in elevation in combination with a narrowing of the water divide gives rise to an increasing influence of moisture- bearing winds from the eastern slopes of the Cordillera. Moraines of complex 3 reach down to elevations of c. 3400 m in the northern part of the Cuchilla Boca Grande area, but only to about 3600 m in the more southerly parts. The moraines of complex 2 partially enclose the moraines of complex 3 but reach c. 100-150 m farther down-valley. Morainic complex 2, however, has been distinctly more affected by erosional and denudational processes, is generally incomplete, and displays more subdued ridges than the sharp crests of morainic complex 3.

Two radiocarbon dates are available for the retreat of glaciers from morainic complex 3, i.e. 15,510 + 190 yr BP (Colorado 5 in Fig. 2; Helmens et al., 1997b) and 14,660 + 280 yr BP (Boca Grande 3 in Fig. 4; Helmens, 1988) obtained from basal lake sediments enclosed by the moraines. Minimum ages for complex 2 of 19,190 + 120 yr BP and 18,130 + 170 yr BP, obtained from organic-rich sediments found overlying glaciofluvial gravel directly behind the moraines of complex 2 (Pefia Negra 6 in Fig. 2; Helmens, 1988), are in accordance with a radiocarbon date of 19,370 + 230 yr BP for a palaeosol found on top of the complex (section 9 in Fig. 2; Khobzi, oral commun. , in Helmens, 1988). The maximum age for morainic complex 2 is most probably not older than 23.5 ka (Helmens & Kuhry, 1995). This age corresponds to a period of widespread formation of organic-rich soils in the Bogot/l area (Fig. 5); the period is not represented in the dated palaeosol sequence developed on the morainic complex, nor is it expressed in the sediment sequence behind the moraines.

The time interval between 21 and 14 ka was originally defined in the palynological record as the Ffiquene Stadial (van Geel & van der Hammen, 1973). Pollen data from the inter-montane Ftiquene lake, located at c. 100 km NE of Bogotfi, reflects a lowering in temperature in the order of 8 ~ C compared to present conditions for this interval. The Ffiquene Stadial was followed by the Susacfi Interstadial (14-13 ka) when temperatures rose to values c. 4 ~ C lower than at present (van der Hammen & Vogel, 1966; Kuhry, 1988). Recently, a pollen record obtained from a series of lake sediments, collected from an ecologically highly sensitive situation on the westem slopes of the Eastem Cordillera (La Laguna in Fig. 2) has revealed that cold glacial conditions did not persist throughout the Ffiquene Stadial, but instead were interrupted by a distinct interval of climate warming around c. 18 ka (Helmens et al., 1996). High pollen frequencies during the Holocene for Andean forest elements reflect the present position of the La Laguna site within the Andean forest belt. During the Late

Weichselian, the pollen record shows that the Andean forest was replaced by open, treeless, paramo vegetation, indicating that under the influence of considerable climate cooling, the forest limit had fallen to well below the elevation of the La Laguna site. A more detailed inspection of the Late Weichselian, however, reveals several intervals, including around 18 ka, with a clear increase in tree pollen. The application of well-studied quantitative relationships of modem vegetation cover and pollen rain (Grabandt, 1980, 1985) translates into an important upward shift in the upper Andean forest limit for the 18 ka interval.

Figure 6 shows the climate record obtained from the La Laguna site for the last 30 ka. The upper Andean forest limit throughout the Eastern Cordillera corresponds to the 9 ~ C annual isotherm, both in wet and relatively dry areas (Kuhry et al., 1993). Altitudinal shifts in the forest limit are translated into temperature values using a thermal lapse rates with elevation, which at present are consistently in the order of 0.66 ~ C/100 m (van der Hammen & Gonzalez, 1963; Kuhry, 1988), and which were similar throughout the last 25 ka (Bakker, 1990; Kuhry et al., 1993). The chronology of the La Laguna core is based on ten radiocarbon dates from macrofossils and bulk peaty samples. The La Laguna pollen record indicates a lowering of the forest limit by 1100 to 900 m beneath its present elevation at c. 3300 m for the stadial intervals directly preceding and following the 18 ka interval, implying a drop in mean annual temperatures of 8 to 6 ~ C. Glaciers advanced down-valley during the stadial periods to elevations of c. 3350 (morainic complex 2) respectively 3500 m (complex 3), reflecting a lowering of the ice-front by c. 1200 to 1100 m compared than present. Between 19.5 and 17 ka, the ice front retreated, extensive soil formation took place (Fig. 5), and the upper Andean forest limit shifted to elevations similar to that of the La Laguna site (2900 m). At the same time temperatures rose considerably to values only 3 to 4 ~ C lower than those in the present interglacial period. The interval between 19.5 and 17 ka has been defined as the La Laguna Interstadial (Helmens et al., 1996). Following Kuhry (1988), the preceding stadial (21 to 19.5 ka) and the subsequent stadial (17 to 14 ka) are termed the Early Ffiquene Stadial and the Late Ffiquene Stadial, respectively.

In Helmens et al. (1996; Fig. 6), the temperature record of the La Laguna core is compared with the high-resolution climate records obtained from the Greenland ice cores (Dansgaard et al., 1993) and North Atlantic Ocean sediments (Bond et al., 1993). The ice-core and marine records show several abrupt temperature shift to markedly warm interstadial periods for the Last Glacial period including during the LGM around 20 ka. Helmens et al. (1996), however, note that it is still premature to correlate precisely the three climate records in time, considering the various sources of error in radiocarbon dating of lake sediments, palaeosols and marine sediments, the uncertainties in the ice flow model of the Greenland record, and the uncertainties of converting the radiocarbon chronology of the marine and terrestrial records to the calender

Colombia 125

chronology of the ice-core record. Nevertheless, the paper concludes that Late Weichselian climate underwent large and rapid changes not only at high northern latitudes but also in the montane tropics.

In different parts of the world, the LGM has been recorded as a two-fold glaciation maximum at c. 19 ka and 15 ka, as indicated by a review of ice-marginal fluctuations of the Laurentide and Cordilleran Ice Sheets on the North American continent, and of mountain glaciers as far apart as the Southern Andes and the Southern Alps of New Zealand (Broecker & Denton, 1990). Rapid and widespread deglaciation is recorded immediately after the second maximum, but little evidence is available for the amount of glacier retreat following the first maximum (Broecker, 1994). Climate warming during the La Laguna Interstadial suggests that glacier retreat during the interval separating the two LGM glaciation maxima was substantial in the northern Andes.

Glacial advance o f Weichselian early Late-Glacial age

The Bogot/l mountains were deglaciated at c. 12.5 ka. It is at this time that glaciers had retreated from the youngest morainic complex 4. This complex 4 includes a distinct system of winding morainic ridges that stand a few metres high. The moraines are only found on high mountain ridges with high orographic precipitation, where at elevations of c. (3300) 3500-3700 m they generally enclose well-defined cirques.

Basal lake sediments in a cirque basin inside morainic complex 4 have yielded a radiocarbon date of 12,760 • 160 yr BP (Boca Grande 1 in Fig. 4; Helmens, 1988). A date of 14,460 + 170 yr BP, obtained on lake sediments below till, directly behind the moraines of complex 4 (Boca Grande 2 in Fig. 4; Helmens, 1988), appears to correspond to the time when ice associated with morainic complex 3 was ablating at this site, having began to disappear from a nearby site by 14,660 + 280 yr BP (Boca Grande 3 in Fig. 4; Helmens, 1988). The pre-existing lacustrine sediments apparently were not eroded during the subsequent re-advance, when glacier ice just overtopped the rock threshold of the adjacent cirque, depositing morainic com-plex 4 close to the riegel's base. A rough maximum age for complex 4 is provided by a radiocarbon date of 13,710 • 80 yr BP obtained from a peat horizon in lake sediments that were found underlying glaciofluvial sands and gravels in front of the complex (Boca Grande 4 in Fig. 4; Helmens, 1988).

The time interval during which morainic complex 4 was formed most probably correlates with the La Ciega Stadial defined in ttle palynological record. This stadial represents a short but intense cold phase, between 13 and 12.5 ka, with temperature depletion estimated at about 6 ~ C compared to present conditions. The stadial was followed by the relatively warm Guantiva Interstadial (12.5-11 ka) when temperatures rose considerably to values only 1 to 2 ~ C lower than present (van der Hammen & Gonz~ilez, 1965; van Geel & van der Hammen, 1973; Kuhry, 1988).

Following the widespread deglaciation that began at c. 14 ka, mountain glaciers on different continents, as well as segments of the large continental ice sheets, are recorded to have readvanced during the early part of the Late-Glacial period. While moraines of this period are seldom well- dated, they seem to have formed during the time interval c. 13-12 ka (Clapperton, 1995, 2000).

Sierra Nevada del Cocuy

The currently glaciated mountain range of the Sierra Nevada del Cocuy is c. 300 km NE of Bogot/l (Fig. 1). A map of the region covering some 1250 km 2 of glaciated and formerly glaciated terrain, which shows the ice cover in the late 1950's and a sequence of six drift bodies with accompanying moraines, was constructed by van der Hammen et al. (1980/81). This map was based on earlier reconnaissance work by Gonzalez et al. (1966). Figure 7 gives a simplified version of part of the map by van der Hammen et al. (1980/81), which covers the extensively glaciated western slopes of the Cocuy range, and which includes the type areas for the drifts that they numbered 3 to 6 (old to young). The type area for the still older drift 2 is situated along the steep and sparsely glaciated eastern slopes of the Sierra Nevada del Cocuy mountains (Fig. 8). The glacial origin of landforms and deposits of drift 1 is uncertain. Recently, Helmens et al. (1997b) have presented a detailed map of the morainic sequence along the Rios San Pablin and Crncavo (Figs. 7 and 9), and have defmed the still unnamed moraines in the upper parts of drifts 3 and 5. Here, new detailed maps are presented of the morainic sequence of the youngest drifts 5 and 6, in their type areas along the Rio Bocatoma and in the vicinity of the Laguna de la Sierra (Figs. 7 and 10-11). They allow a further subdivision of the moraines in the lower part of drift 5. The maps in Figures 9-11, which are based on aerial photographs taken in the early 1980's, additionally distinguish an extensive zone of glacially-scoured bedrock adjacent to the retreating ice front. Only limited dating control is available for the moraines of the Sierra Nevada del Cocuy. Extrapolation of a radiocarbon date from sediments cored in Laguna Ciega (Laguna Ciega III in Fig. 7; van der Hammen et al., 1980/81) yields a possible age of 24.5-27.0 ka for the beginning of lake sedimentation on the lower part of drift 3 following glacier retreat from the Crncavo moraines, re- named the Lower Crncavo moraines by Helmens et al. (1997b). No dates are available for the still older Rio Negro moraines of drift 2, which like the Lower Crncavo moraines also only found very locally. Although van der Hammen et al. (1980/81) use this data combined with data from the Bogot/l area, including palynological evidence, to suggest extensive glaciation during the Middle Weich- selian, additional dating control is necessary to correlate the older moraines in the Cocuy mountains with the Middle Weichselian moraines and older glacial deposits in the mountains near Bogoti


I

/ / \ J /

Rio Cardenillo

-~ Laguna Ciega

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/ 7 G * Laguna Ciega III uicdn

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~ . ~ . ~ main waterd iv ide

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" ' - - " limit of drift 5

)'rr-r~ limit of drift 4

~ , " " limit of drift 3

/ ~ morainic ridges

�9 exposure/coring

Fig. 7. Drifts 3-6 and moraines on the western slopes of the Sierra Nevada del Cocuy. The initiation of lake sedimentation on the lower part of drift 3 and on the upperpart of drift 4 have been dated to c. 24.5-27 ka and 12.5 ka, respectively (based on van der Hammen et al., 1980/81). The positions of exposures~ boreholes mentioned in the text indicated.

Colombia 127

\

g ',~i/" F N r 0 ] km

| |

I -~ '~ main waterdivide

.,.,~,~bq~//~/~ " - " " limit of drift 5

Prr~ limit of drift 4

Laguna Pozo Negro

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t~,~( ,~,.,r, limit of drift 3

~ ~ ) / morainic ridges

Y . Laguna - P o z o Verde

R(o Negro moraines ," ~ (drift 2) \~ib (2600 m)

" Lagunas I Las Risaguidas \ ~ (2400 m)

Fig. 8: Drifts 2-4 and moraines on the eastern slopes of the Sierra Nevada del Cocuy (van der Hammen et al., 1980/81).

Glacial retreat from the Upper Lagunillas moraines in the upper part of drift 4 took place shortly before c. 12.5 ka. The radiocarbon date of 12,320 + 100 yr BP was obtained from a thin peat horizon overlying basal, laminated minerogenic lake sediments at a site directly behind the moraines (Lagunillas V in Figs. 7 and 10; Gonzalez et al., 1966). The dates of 24.5-27.0 ka and 12.5 ka obtained at the Laguna Ciega III and Lagunillas V sites seem to place both the Lower Lagunillas moraines in the lower part of drift 4 (van der Hammen et al., 1980/81) and the Upper C6ncavo moraines in the upper part of drift 3 (Helmens et al., 1997b) at the time of the LGM. Helmens et al. (1997a) have noted the striking similarity in morphology displayed by the moraines in the Cocuy and Bogota mountains. The Lower Lagunillas moraines, which rise up to 100 m above the valley floors (Fig. 9), and the late-LGM morainic complex 3 of the Bogota mountains show a most impressive morainic morphology. The arcuate, multiple ridge systems are enclosed by the Upper C6ncavo moraines and the early- LGM morainic complex 2, which are distinctly more subdued and are incomplete. Like the Late-Glacial morainic complex 4. the Upper Lagunillas moraines include winding ridges of much lesser height. Along the Rio San Pablin, on the western slop,es of the Sierra Nevada del Cocuy,

the Upper and Lower Lagunillas moraines reach down- valley to c. 3600 m and c. 3400 m a.s.l.; and the Upper and Lower C6ncavo moraines to c. 3300 m and c. 2900 m, respectively. The Rio Negro moraines on the eastern slopes of the Cocuy range occur at elevations of c. (2200) 2600- 2800 m.

Younger Dryas glacial advance

A glacial event corresponding to the Younger Dryas (YD) climatic oscillation of the northem North Atlantic region between c. 11 and 10 ka has been proposed by Gonzalez et al. (1966) and van der Hammen et al. (1980/81) based on the Cocuy record. The event is marked by the Bocatoma moraines in the lower part of drift 5, re-named the Lower Bocatoma moraines by Helmens et al. (1997b). The massive, lobate morainic complex displays ridges which are distinctly sharper and fresher looking than the morainic crests of drift 4. The moraines reach down on the westem slopes of the Cocuy range to elevations of c. 3800-4000 m. They indicate a maximum lowering of the glacier fronts by some 700 m compared to present conditions.

Detailed mapping of the Lower Bocatoma moraines suggests that they represent two glacial advances, in which the moraines of the younger advance in part have buried the moraines formed by the older advance (indicated as respectively outer to outermost moraines in Figs 9-11), and a minor readvance or distinct still-stand that left an inner moraine or lobe of moraines. As Gonzalez et al. (1966) mentioned, the individual morainic ridges that make up the Lower Bocatoma moraines do not show a noticeable difference in intensity of weathering, suggesting only a small age difference between their periods of formation. A YD age for the Lower Bocatoma moraines is suggested by a radiocarbon-dated sediment sequence deposited in front of the moraines and a pollen record obtained from basal lake sediments accumulated directly behind the (outer) moraines (Lagunillas V respectively XI in Fig. 10; Gonzalez et al., 1966). The Lagunillas V section is exposed in a major body of valley-floor sediments enclosed by the Upper Lagunillas moraines in the upper part of drift 4. Ice should have retreated from the Upper Lagunillas moraines and sandy, laminated lake sedi-mentation had started at the section-site shortly before 12,320 + 100 yr BP. Around the level of that date, the sediments in the section become more clayey with intercalations of peat. After c. 11,900 yr BP peat is deposited. Sandy intercalations in this peat layer are dated between 11,350 + 140 yr BP and 10,030 + 90 yr BP and might indicate that a glacier occurred nearby, behind the Lower Bocatoma moraines. Palynological evidence fromthe Lagunillas XI section suggests glacier retreat from the outer morainic ridges of the Lower Bocatoma moraines and the start here of sandy/clayey, laminated lake sedimentation near the transition from the Late-Glacial to the Holocene.

An attempt to provide the Lower Bocatoma moraines with additional chronological control has resulted in a


Ritacuba Blanca

(5490 m)

A _c#)C,,j,,(///A'--~__..~"

~ - " " " ~ & / / I

Corra l i tos mora ines ~00 43O0

Upper Bocatoma moraines

Lower Bocatoma m o r a i n e s

(inner moraines) ~ '

(outer moraines) ~ -

(outermost moraines) -"

" ~ " ~ " ~ 4200

4100

Laguna 3an Pablfn_ ,

~

Cz 0 1 km I �9 _ I

Upper Laguni l las mora ines

Lower Lagunil las moraines

Upper C6ncavo moraines

Lower C6ncavo moraines

._~ main waterdivide

lower limit of present-day icecap ~', glacially scoured bedrock k'~ crest of morainic ridges

-.~ moraines overriden by later ice advance

: outwash deposits �9 .

lake and valley-bottom deposits

slumping

,~ro~--" contour-line (m) G u i c f i n J ) i / "~ %

t300

"-- 3200

%

Fig. 9. The Late Quaternary morainic sequence along the Rios San Pablin and C6ncavo, Sierra Nevada del Cocuy (Helmens et al., 1997b), modified from van der Hammen et al. (1980/81" Fig. 7). The subdivision of the Lower Bocatoma moraines Onto outermost, outer and inner

moraines) is based on the present paper.

Colombia 129

Fig. 10. The Late-Glacial-Holocene morainic sequence along the Rio Bocatoma, Sierra Nevada del Cocuy (modified from van der Hammen et al., 1980/81; Fig. 7). The Lower Bocatoma moraines are probably of Younger Dryas age (11-10 ka), whereas the Corralitos moraines were formed during the Little Ice Age. The location of exposures/boreholes mentioned in the text are indicated.

Fig. 11. The Late-Glacial-Holocene morainic sequence in the vicinity of the Laguna de la Sierra, Sierra' Nevada del Cocuy (modified from van der Hammen et al., 1980/81; Fig. 7). The Lower Bocatoma moraines are probably of Younger D~as age (11-10 ka), whereas the Corralitos moraines were formed during the Little Ice Age. The location of exposures /boreholes mentioned in the text are indicated. For legend, see Fig. 10.


Table 1. Radiocarbon date of macrofossils from peat in the Sierra Nevada del Cocuy (K.F. Helmens & K. van der Borg, unpublished data).

AMS 14 C age Laboratory nr. Section name Elevation Depth Sample description (yr BP) (m) (m)

8540 + 260 UTC-4632 C6ncavo II 3860 2.26-2.28 Cyperaceae macrofos.

rough minimum age for the corresponding glacial event. The base of a peat deposit found overlying sand directly behind the inner ridges of the Lower Bocatoma moraines on the slopes below the Laguna de la Sierra yielded a radiocarbon date of 8540 + 260 yr BP (C6ncavo II in Fig. 11 and Table 1; K.F. Helmens & K. van der Borg, unpublished data). The sample dates the start of formation of the slope bog at the site (today dominated by a Espeletia- Carex-Sphagnum vege-tation). Since it usually takes some time for a slope bog to develop on deglaciated terrain, the date of c. 8.5 ka does not necessary provide a precise timing for the glacial retreat from the moraines. The date, however, suggests that ice retreated from the Lower Bocatoma moraines shortly after 10 ka, because during the time interval c. 10 to 8.5 ka mostly peat accumulated at the Lagunillas V site. Basal sandy lake sediments are exposed at several sites behind the outer and inner ridges of the Lower Bocatoma moraines. However, their organic con- tents are too low to provide reliable radiocarbon dates.

A cooling event of YD age, the E1 Abra Stadial (van Geel & van der Hammen, 1973), is distinguished in the palynological record. The E1 Abra Stadial has been recognised in many pollen records of the Eastern Cordillera and has been dated to the time interval c. 11 to 10-9.5 ka (Kuhry et al., 1993). New evidence obtained by van't Veer et al. (2000) also indicates the lowest temperatures between c. 11 and 10.5 ka; cool and dry conditions are inferred after c. 10.5 ka, extending into the earliest Holocene. The maximum temperature decline during the E1 Abra Stadial, compared to Late Holocene conditions, is estimated in the order of 3-4 ~ C (Kuhry et al., 1993; van't Veer et al., 2000).

YD cooling was the last significant change prior to the global warming which heralded the onset of the Holocene. The cooling event is well-represented in many records from the northern North Atlantic region. Discussion has arised whether the YD was a global cooling event or a regional, North Atlantic phenomenon (e.g., Alley et al., 1993; Peteet, 1995). Although the YD age assignment for the Lower Bocatoma moraines requires supporting radiocarbon dates, the evidence available on the glacial sequence of the Cocuy mountains, in combination with that provided by the palynological record of the Eastern Cordillera, strongly suggests that the moraines were formed at the time of the YD cooling event. Strong evidence for glacier expansion in the northern Andes during the YD S tadial has also recently come from the Ecuadorian Andes. Here radiocarbon dates from macrofossils, peat and gyttja above and below till indicate that a c. 140 k m 2 ice cap developed between c. 11 and 10 ka on ground with a mean elevation of 4200 m where none exist now (Clapperton et al., 1997). The

weighted mean radiocarbon age of 10,035 yr BP for the minimum limiting dates (which include three radiocarbon dates of about 10,100 yr BP from peat) excludes an Early Holocene age for the glacial advance, as proposed by Heine & Heine (1996) and Heine (2000) based on earlier work in the same region. A major glacier readvance in Peru in the southern Tropical Andes has been dated at c. 11 ka (Rodbell & Seltzer, 2000). Despite sustained cool temperatures during the YD Chron, indicated by the oxygen isotope record from an ice core in the region (Thompson et al., 1998), rapid glacial retreat is recorded by c. 10.9 ka, apparently in response to reduced precipitation.

Holocene glacier fluctuations

Numerous morainic ridges in the Cocuy mountains indicate that glaciers were more extensive during earlier parts of the Holocene than today. The Corralitos moraines, which comprise the youngest drift 6, most probably formed during the later part of the last millenium. The lower limit of drift 6 is found on the western, ice-covered slopes of the Cocuy range between c. 4200 m and 4500 m altitude. During the maximum advance, glaciers extended some 200-400 m lower than at present.

The Corralitos moraines are very conspicuous and complete, and are vegetation free. Radiocarbon dates obtained for similar moraines in other parts of the Colombian Andes (Herd, 1982; van der Hammen, 1984) place their time of formation within the last c. 500 years during the Little Ice Age (van der Hammen et al., 1980/81). Glaciers may still have been near the outer ridges of the Corralitos moraines as late as c. 1850 A.D. At this time, Ancizar on a pilgrimage through the region reported 4150 m as the lowest elevation for a glacier terminus in the Cocuy mountains (cited in Hoyos-Patifio, 1998). Glacier retreat during the latter part of the last century has been significant. For example, a photograph taken by E. Kraus in 1938 shows glacier-ice in contact with Laguna de la Sierra in the uppermost part of drift 6 (van der Hammen et al., 1980/81). Aerial photographs taken in 1981 (Fig. 11) indicate that the ice had disappeared from the lake and that glacier margins have retreated considerably to elevations of c. 100 to 150 m above the lake. During this recent retreat, glaciers in the Cocuy range have exposed an extensive zone of glacially scoured bedrock which is covered by only very little till.

On the upper part of drift 5, aerial photographs very clearly depict still another series of moraines, which have b~en defined the Upper Bocatoma moraines by Helmens et

Colombia 131

al. (1997a). The moraines are similar in appearance, but much smaller in size than the Late Glacial Lower Bocatoma moraines in the lower part of drift 5. No radiocarbon dates are available to constrain the timing of formation of the Upper Bocatoma moraines. They might have formed in the Early Holocene during short still-stands in the glacial retreat following the YD-cooling event.

The palyn01ogical record of the Eastern Cordillera shows slightly elevated temperatures between c. 7 and 3 ka (Kuhry, 1988). No distinct short episodes of cooling are represented.

Comparison with the glacial record of the Central Cordillera

A detailed study of the morainic sequence on the Ruiz- Tolima volcanic complex in the Central Cordillera (Fig. 1; c. lat. 5 ~ N; highest peak, Nevado del Ruiz at 5400 m) has been made by Thouret et al. (1996). An absolute chronology for a series of morainic belts is provided by radiocarbon dating and inter-site correlation using tephra/palaeosol/peat stratigraphies.

Although the late Rio Recfo moraines have not been adequately dated, stratigraphical evidence appears to indicate that the moraines here were formed before 28 ka and after 42 ka. This information suggests, that glaciers were more extensive during the Middle rather than during the Late Weichselian as in the Eastern Cordillera. Peat directly overlying the still older early Rio Recio moraines has yielded radiocarbon dates of c. 49 to > 53 ka. Since no evidence of an interglacial soil was found in the locally more than 10 m thick sediment-soil sequence on the early Rio Recio moraines, it is assumed that the relatively flesh moraines were formed during the early part of the Last Glacial period. The Rio Recio moraines occur at c. (2900) 3200-3300 m a.s.1.

The Murillo moraines, which represent two phases of glacier expansion down to c. 3300-3400 m and to c. 3400- 3600 m a.s.l., have been dated as older than 16 ka and probably younger than 28 ka. Using palynological evidence, the glacial advances are correlated with stadial intervals between c. 28 and 21 ka (early Murillo moraines) and c. 21 and 14 ka (late Murillo moraines). The glacial position marked by the late Murillo moraines is indicated on the digital map as the LGM limit. An independent radiocarbon chronology is required for the early Murillo moraines in order to support the correlation made by Thouret et al. (1996), or, as an alternative, propose a correlation ,of the early and late Murillo moraines with the Early (21-19.5 ka) respectively Late Ffiquene Stadials (17- 14 ka) of the Eastern Cordillera (Kuhry, 1988; Helmens et al., 1996). The early Murillo moraines form the most voluminous glacial deposits in the Ruiz-Tolima massif.

Peat near the base of the up to 4 m thick sediment-soil sequence overlying the late Otfin moraines (at c. 3800-4000 m altitude) has yielded a radiocarbon date of c. 8.8 ka. The overlying stratigraphical sequence lacks a widespread

tephra, dated between c. 11.5 and 10.8 ka, which occurs in the sequence on the early Otfin moraines (3600-3800 m) of early Late-Glacial age. The late Otfin moraines are correlated by Thouret et al. (1996) with the Younger Dryas Stadial of the northern North Atlantic region.

An Early Holocene age for the Santa Isabel moraines (4150-4300 m) is interpreted on the basis of a radiocarbon date of c. 7.5 ka obtained from a palaeosol which forms the oldest stratigraphic unit overlying the moraines. The youngest moraines on the Ruiz-Tolima volcanic complex occur at c. 4300-4600 m altitude; they only bear a few centimetres thick entisol (inner Ruiz moraines), with a thin tephra bed (outer Ruiz moraines) at the base. Peat found at the base of the outermost ridge of these unweathered and vegetation-free moraines has yielded a date of c. 500 radiocarbon years (Herd, 1982).

Acknowledgements

Research by the author on the glaciations of the Eastern Cordillera of Colombia has been supported by grants of the Dutch Foundation for the Advancement of Tropical Research (WOTRO; doss. nr. W77-103), the Natural Sciences and Engineering Research Council of Canada ('Canada International Fellowship') and the National Geographic Society, U.S.A. (grant 4871-92). Thanks are due to the following persons (in alphabetical order) who have contributed in an important way to the research over the years: Dr. P.A.M. Andriessen (Free University, Amsterdam, The Netherlands), Dr. R.W. Barendregt (University of Lethbridge, Canada), Dr. R.J. Enkin (Geological Survey of Canada, Pacific Geoscience Centre Subdivision), Dr. P. Kuhry (University of Lapland, Rovaniemi, Finland), Dr. W.G. Mook (University of Groningen, The Netherlands), Dr. N.W. Rutter (University of Alberta, Edmonton, Canada), Dr. T. van der Hammen (Tropenbos, Bogotfi, Colombia) and Dr. K. van der Borg (University of Utrecht, The Netherlands).

References

Alley, R.B., Bond, G., Chappelaz, J, Clapperton, C.M., Del Genio, A., Keigwin, L. & Peteet, D.H. (1993). Global Younger Dryas? Eos, 74, 587-589.

Andriessen, P.A.M., Helmens, K.F., Hooghiemstra, H., Riezebos, P.A. & van der Hammen, T. (1993). Absolute chronology of the Pliocene-Quaternary sediment sequence of the Bogot~t area, Colombia. Quaternary Science Reviews, 12,483-501.

Bakker, J.G.M. (1990). Tectonic and climatic controls on Late Quaternary sedimentary processes in a neotectonic intramontane basin (The Pitalito basin, South America). Unpublished Ph.D dissertation, Agricultural University of Wageningen, 160 pp.

Bond, G., Broecker, W.S., Johnsen, S.J., McManus, J., Labeyrie, L., Jouzel,. J. & Bonani, G. (1993). Correla-


tions between climate records from North Atlantic sediments and Greenland ice. Nature, 365, 143-147.

Broecker, W.S. (1994). Massive iceberg discharges as triggers for global climate change. Nature, 372, 421-424.

Broecker, W.S. & Denton, G.H. (1990). The role of ocean- atmosphere reorganizations in glacial cycles. Quaternary Science Reviews, 9, 305-341.

Brunnschweiler, D. (1981). Glacial and periglacial form systems of the Colombian Quaternary. Revista CIAF, 6(1- 3), 53-56.

Cande, S.C. & Kent, D.V. (1995). Revised calibration of the geomagnetic polarity timescale for the Late Cretaceous and Cenozoic. Journal of Geophysical Research, 100, 6093-6095.

Clapperton, C.M. (1987). Maximal extent of Late Wisconsin glaciation in the Ecuadorian Andes. Quaternary of South America and Antarctic Peninsula, 5, 165-179.

Clapperton, C.M. (1995). Fluctuations of local glaciers at the termination of the Pleistocene: 18-8 ka BP. Quaternary International, 28, 41-50.

Clapperton, C.M. (2000) Interhemispheric synchroneity of Marine Oxygen Isotope Stage 2 glacier fluctuations along the American cordilleras transect. Journal of Quaternary Science, 15, 435-468.

Clapperton, C.M., Hall, M., Mothes, P., Hole, M.J., Helmens, K.F., Kuhry, P. & Gemmell, A.M.D. (1997). A Younger Dryas Icecap in the Equatorial Andes. Quater- nary Research, 47, 13-28.

CLIMAP Project Members (1981). Seasonal reconstruc- tions of the Earth's surface at the Last Glacial Maximum. Geological Society of America Map and Chart Series, vol. MC-36.

Dansgaard, W., Johnsen, S.J., Clausen, H.B., Dahl-Jensen, D., Gundestrup, N.S., Hammer, C.U., Hvidberg, C.S., Steffensen, J.P., Sveinbj6msdottir, A.E., Jouzel, J. & Bond, G. (1993). Evidence for general instability of past climate from a 250-kyr ice-core record. Nature, 364, 218- 220.

Espizua, L.E. (1993). Quaternary glaciations in the Rio Mendoza Valley, Argentine Andes. Quaternary Research, 40, 150-162.

F61ster, H. & Hetsch, W. (1978). Palaeosol sequences in the Eastern Cordillera of Colombia. Quaternary Research, 9, 238-248.

Gillespie, A. & Molnar, P. (1995). Asynchronuous maximum advances of mountain and continental glaciers. Reviews of Geophysics, 33 (3), 311-364.

Gonzalez, E., van der Hammen, T. & Flint, R.F. (1966). Late Quaternary glacial and vegetational sequence in Valle de Lagunillas, Sierra Nevada del Cocuy, Colombia. Leidse Geologische Mededelingen, 32, 157-182.

Grabandt, R.A.J. (1980). Pollen rain in relation to arboreal vegetation in the Colombian Cordillera Oriental. Review of Palaeobotany and Palynology, 29, 65-147.

Grabandt, R.A.J. (1985). Pollen rain in relation to vegetation in the Colombian Cordillera Oriental. Unpublished Ph.D dissertation, University of Amsterdam.

Guillet, B., Faivre, P., Mariotti, A. & Khobzi, J. (1988). The ~4C dates and 13C/12C ratios of soil organic matter as a means of studying the past vegetation in intertropical regions: examples from Colombia (South America). Palaeogeography, Palaeoclimatology, Palaeoecology, 65, 51-58.

Heine, K. (2000). Tropical South America during the Last Glacial Maximum: evidence from glacial, periglacial and fluvial records. Quaternary International, 72, 7-21.

Heine, K. & Heine, J.T. (1996). Late-Glacial climatic fluctuations in Ecuador: glacier retreat during Younger Dryas time. Arctic and Alpine Research, 28, 496-501.

Helmens, K.F. (1988). Late Pleistocene glacial sequence in the area of the high plain of Bogotfi (Eastern Cordille- ra, Colombia). Palaeogeography, Palaeoclimatology, Pa- laeoecology, 67, 263-283.

Helmens, K.F. (1990). Neogene-Quatemary Geology of the High Plain of Bogotfi, Eastem Cordillera, Colombia (Stratigraphy, Palaeoenvironments and Landscape Evolution). Dissertationes Botanicae, 163. J. Cramer, Berlin-Stuttgart, 202 pp.

Helmens, K.F. & Kuhry, P. (1986). Middle and Late Quaternary vegetational and climatic history of the P/lramo de Agua Blanca (Eastern Cordillera, Colombia). Palaeogeography, Palaeoclimatology, Palaeoecology, 56, 291-335.

Helmens, K.F. & Kuhry, P. (1995). Glacier fluctuations and vegetation change associated with Late Quaternary climatic oscillations in the Andes near Bogot~, Colombia. Quaternary of South America and Antarctic Peninsula, 9, 117-140.

Helmens, K.F. & van der Hammen, T. (1994). The Pliocene and Quaternary of the high plain of Bogot/l (Colombia): a history of tectonic uplift, basin development and climatic change. Quaternary International, 21, 41-61.

Helmens, K.F., Kuhry, P., Rutter, N.W., Van der Borg, K. & De Jong, A.F.M. (1996). Warming at 18,000 yr B.P. in the Tropical Andes. Quaternary Research, 45, 289-299.

Helmens, K.F., Barendregt, R.W., Enkin, R.J., Bakker, J. & Andriessen, P.A.M. (1997a). Magnetic polarity and fission-track chronology of a Late Pliocene-Pleistocene palaeoclimatic proxy record in the Tropical Andes. Quaternary Research, 48, 15-28.

Helmens, K.F., Rutter, N.W. & Kuhry, P. (1997b). Glacier fluctuations in the Eastern Andes of Colombia (South- America) during the last 45,000 radiocarbon years. Quaternary International, 38139, 39-48.

Herd, D.G. (1982). Glacial and volcanic geology of the Ruiz-Tolima volcanic complex Cordillera Central, Colombia. Publicaciones geol6gicas especiales del 1NGEOMINAS (Bogot~, Colombia), 8, 1-48.

Hooghiemstra, H. (1984). Vegetational and climatic history of the high plain of Bogot/l, Colombia: a continuous record of the last 3.5 million years. Dissertationes Botanicae, 79, 368 pp. J. Cramer, Vaduz.

Hooghiemstra, H. & Ran, E.T.H. (1994). Late Pliocene- Pleistocene high resolution pollen sequence of Colombia:

Colombia 133

an overview of climatic change. Quaternary Inter- national, 21, 63-80.

Hooghiemstra, H., Melice, J.L.,Berger, A. & Shackleton, N.J (1993). Frequency spectra and palaeoclimatic variability of the high-resplution 30-1450 ka Funza I pollen record (Eastem Cordillera, Colombia). Quaternary Science Reviews, 12, 141-156.

Hoyos-Patifio, F. (1998). Glaciers of South America- Glaciers of Colombia. In." Williams, R.S., Jr. & Ferrigno, J.G. (eds), Satellite Image Atlas of Glaciers of the WorM, U.S. Geological Survey Professional Paper, 1386-I, 11- 30.

Kuhn, M. (1981). Vergletscherung, Nullgradgrenze, und Niederschlag in den Anden. Jahresbericht des Sonnblick- Vereines j~r die Jahre 1978-1980, pp. 3-13. Springer Verlag, Wien.

Kuhry, P. (1988). Palaeobotanical-palaeoecological studies of tropical high Andean peatbog sections (Cordillera Oriental, Colombia). Dissertationes Botanicae, 116. J. Cramer, Berlin-Stuttgart, 241 pp.

Kuhry, P. (1991). Comparative palaeohydrology in the Andes of ,Colombia. Abstracts of the XIII INQUA Congress, Beijing, China, p. 179.

Kuhry, P. & Helmens, K.F. (1990). Neogene-Quatemary biostratigraphy and palaeoenvironments. In." Helmens, K.F. (ed.), Neogene-Quaternary Geology of the High Plain of Bogot6, Eastern Cordillera, Colombia (Stratigraphy, Palaeoenvironments and Landscape Evolution), Dissertationes Botanicae, 163, 89-132. J. Cramer, Berlin-Stuttgart.

Kuhry, P., Hooghiemstra, H., Van Geel, B. & van der Hammen, T. (1993). The E1 Abra stadial in the Eastem Cordillera of Colombia (South America). Quaternary Science Reviews, 12, 333-343.

Laugenie, J.C. & Mercer, J.H. (1973). Glacier in Chile ended a major readvance 36,000 years ago: some global comparisons. Science, 183, 1017-1019.

Peteet, D.H. (1995). Global Younger Dryas? Quaternary International, 28, 93-104.

Raasveldt, H.C. (1957). Las glaciaciones de la Sierra Nevada de Santa Marta. Revista de la Academia Colombiana de Ciencias Exactas, Fisicas y Naturales, 9 (38), 469-482.

Rodbell, D.T. & Seltzer, G.O. (2000). Rapid ice margin fluctuations during the Younger Dryas in the Tropical Andes. Quaternary Research, 54, 328-338.

Ruddiman, W.F., Raymo, M.E. & McIntyre, A. (1986). Matuyama 41,000-year cycles: North Atlantic Ocean and Northern Hemisphere ice sheets. Earth and Planetary Science Letters, 80, 117-129.

Ruddiman, W.F., Raymo, M.E., Martinson, D.G., Clement, B.M. & Backman, J. (1989). Pleistocene evolution of Northern Hemisphere climate. Palaeoceanography, 4, 353-412.

Shackleton, N.J., Backman, J., Zimmerman, H., Kent, D.V., Hall, M.A., Roberts, D.G., Schnitker, D., Baldauf, J.G., Desprairies, A., Homrighausen, R., Huddlestun, P., Keene, J.B., Kaltenback, A.J., Krumsiek, K.A.O.,

Morton, A.C., Murray, J.W. & Westberg-Smith, J. (1984). Oxygen isotope calibration of the onset of ice- rafting and history of glaciation in the North Atlantic region. Nature, 307, 620-623.

Shackleton, N.J., Berger, A. & Peltier, W.R. (1990). An alternative astronomical calibration of the lower Pleistocene timescale based on ODP Site 677. Transactions of the Royal Society of Edinburgh." Earth Sciences, 81, 251-261.

Thompson, L.G., Davis, M.E., Mosley-Thompson, E., Sowers, T.A., Henderson, K.A., Zagorodnv, V.S., Lin, P.-N., Mikhalenko, V.N., Campen, R.K., Bolzan, J.F., Cole-Dai, J. & Francou, B. (1998). A 25,000-year Tropical climate history from Bolivian ice cores. Science, 282, 1858-1864.

Thouret, J-C., van der Hammen, T. & Salomons, B. (1996). Palaeoenvironmental changes and stades of the last 50,000 years in the Cordillera Central, Colombia. Quaternary Research, 46, 1-18.

Thouveny, N. & Servant, M. (1989). Palaeomagnetic stratigraphy of Pliocene continental deposits of the Bolivian Altiplano. Palaeogeography, Palaeoclimatolo- gy, Palaeoecology, 70, 331-334.

van der Hammen, T. (1981). Glaciales and glaciaciones en el Cuatemario de Colombia: palaeoecologia y estratigrafia. Revista CIAF, 6, 635-638.

van der Hammen, T. (1984). Datos sobre la historia de clima, vegetaci6n y glaciaci6n de la Sierra Nevada de Santa Marta. In. van der Hammen, T. & Ruiz, P.M. (eds), Studies on Tropical Andean Ecosystems, 2, 561-580. J. Cramer, Vaduz.

van der Hammen, T. (1986). La Sabana de Bogotfi y su lago en el Pleniglacial Medio. Caldasia, 15, 249-262.

van der Hammen, T. & Gonzalez, E. (1963). Historia de clima y vegetaci6n del Pleistoceno Superior y del Holoceno de la Sabana de Bogotfi. Boletin Geol6gico (Ingeominas, Bogot~), 11(1-3), 189-266.

van der Hammen, T. & Gonzalez, E. (1965). A Late-Glacial and Holocene pollen diagram from Ci6naga del Visitador (Dept. Boyacg, Colombia). Leidse Geologische Medede- lingen, 32, 193-201.

van der Hammen, T & Vogel, J.C. (1966). The Susac~ Interstadial and the subdivision of the Late-Glacial. Geologic en Mijnbouw, 45 (2), 33-35.

van der Hammen, T., Werner, J.H. & Van Dommelen, H. (1973). Palynological record of the upheaval of the Northem Andes: a study of the Pliocene and Lower Quaternary of the Colombian Eastem Cordillera and the early evolution of its High-Andean biota. Review of Palaeobotany and Palynology, 16, 1-122.

van der Hammen, T., Duefias, H. & Thouret, J.C. (1980). Guia de excursi6n - Sabana de Bogot~i. Primer seminario sobre el Cuatemario de Colombia, Bogot~-Colombia, 25 al 29 de Agosto,1980, 8 pp. Bogot~.

van der Hammen, T., Barelds, J., De Jong, H. & De Veer, A.A. (1980/81). Glacial sequence and environmental history in the Sierra Nevada del Cocuy (Colombia).


Palaeogeography, Palaeoclimatology, Palaeoecology, 32, 247-340.

Van Geel, B. & van der Hammen, T. (1973). Upper Quaternary vegetational and climatic sequence of the Ft~quene area (Eastern Cordillera, Colombia). Palaeo- geography, Palaeoclimatology, Palaeoecology, 14, 9-92.

Van't Veer, R., Islebe, G.A. & Hooghiemstra, H. (2000). Climatic change during the Younger Dryas chron in northern South America: a test of the evidence. Quaternary Science Reviews, 19, 1821-1835.

Wijninga, V.M. & Kuhry, P. (1993). Late Pliocene palaeoecology of the Guasca Valley (Cordillera Oriental, Colombia). Review of Palaeobotany and Palynology, 78, 69-127.

Wright, Jr., H.E. (1983). Late-Pleistocene glaciation and climate around the Junin plain, central Peruvian Highlands. Geografiska Annaler, 65 A, 35-43.

Zagwijn, W.H. (1992). The beginning of the Ice Age in Europe and its major subdivisions. Quaternary Science Reviews, 11,583-591.

the cuaternary glacial record of the colombian andes

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