- Email: [email protected]
LOESS RECORDS | South America
South America M A Za´rate, Universidad Nacional de la Pampa, La Pampa, Argentina ã 2013 Elsevier B.V. All rights reserved. This article is reproduced from the previous edition, volume 2, pp. 1466–1479, ã 2007, Elsevier B.V.
Introduction The loess record of southern South America, the largest of the Southern Hemisphere, was originally reported in the nineteenth century under different names, among others by Charles Darwin in 1844. Its resemblance with the loess deposits of the Rhine valley was first noticed in 1866. Since then the term ‘loess’ has been loosely used in the literature to refer to the vast mantle of late Tertiary and Quaternary sediments of the Chaco-Pampean plain. Loess-like deposits, loessoid deposits, reworked loess, loessic deposits, and secondary loess have been used to designate fine-grained, light-colored and massive sediments with the appearance of loess. Generally, loess-like and loessoid deposits are the terms most commonly found in the literature. Neotropical loess has been recently used to designate primary and reworked sediments of loessic character, distributed between latitudes 20 and 30 S. Also, the term tropical loess has been proposed to account for fine-grained surface deposits of massive appearance in northeastern Argentina, areas of southern Brazil, and northern Uruguay. These sediments, traditionally interpreted as in situ weathered byproducts from the underlying late Jurassic basaltic rocks, have been recently interpreted as a wide and extensive loess mantle that accumulated during the last glacial cycle. This hypothesis generated controversy and is presently under debate. Ongoing geochemical, mineralogical, and pedological analysis reinforce the in situ origin of the surface deposits. During most of the twentieth century, research on South American loess was fragmentary and discontinuous. With few exceptions, a great number of contributions were focused on the stratigraphy and the fossil vertebrate content. A renewed interest began in the late 1980s specially driven by the necessity of testing global paleoclimatic models. Numerous studies were conducted particularly across the Pampean plain. The more detailed mapping, magnetostratigraphy, dating, paleopedological studies, and compositional analysis have substantially increased the understanding of the record. In recent years, detailed analysis substantially increased the knowledge of the loess–paleosol record of the mountain valley loess of Tucuma´n in northwestern Argentina. The Chaco loess is less known than the other two loess records, although regional aspects of its composition and geographical distribution have been reported.
Geographical Distribution Loess distribution has always been associated with the extensive and heterogeneous Chaco-Pampean plain of southern South America, a region over 1 million km2, covering most of northern central Argentina, western Uruguay, southern Brazil, western Paraguay, and southeastern Bolivia (Figure 1). This
generalization probably comes from the geographic distribution of loessoid deposits in Argentina and was later adopted in other studies, including not only loess but also sand dunes, sand mantles, and fine-grained deposits of noneolian origin. Subsequently, more detailed mapping across the ChacoPampean plain (Table 1) indicated a more restricted geographic distribution of loess; extensive areas of central Argentina are covered by eolian sands, while alluvial deposits are common in the northeast.
The Pampean Loess and Loess-Like Record General Characteristics The exposures of Pampean sediments are scarce and limited to quarries, road cuts, and river banks. The exceptions are the Chapadmalal sea cliffs near Mar del Plata and the Parana´ River banks, 20–30 m thick and laterally continuous for several kilometers. Both of these unusually well-exposed sections have been the subject of numerous studies (Figure 2). The Pampean sequences do not represent a continuous sedimentary record; stratigraphic discontinuities are indicated by several erosional unconformities while the rate and characteristics of the sediment supply varied through time. Loess is a secondary and minor component of the sedimentary sequences, being prevalent only during the last glacial cycle. Instead, loess-like deposits generated by the reworking of primary eolian sediments are dominant. Running water, masswasting processes on hill slopes, and deposition in swampy and lacustrine environments are thought to have been involved in the retransportation and resedimentation of primary loess. The reworking by aqueous transport agents resulted in several loess-like deposits including siltstones, clay siltstones, and sandy siltstones. They usually exhibit a general massive appearance or have poorly defined sedimentary structures frequently masked by pedogenesis and diagenesis.
Paleosols The occurrence of pedogenic features is ubiquitous throughout the stratigraphic sequence of loess and loess-like deposits (Figure 3). The recognition and classification of paleosol soil horizons present several difficulties in field exposures. Paleosols were originally interpreted as truncated profiles with A horizons eroded by eolian and/or fluvial processes. Their identification has been mostly based on field morphological properties. Detailed micromorphological analysis carried out at some stratigraphic intervals of the northern and southern Pampas show that pedogenesis was active throughout the sedimentation process resulting in accretionary soil profiles. Welding of pedological features on preexisting soil surfaces gives way to pedocomplexes. In addition, paleosols are not always discrete
629
630
LOESS RECORDS | South America
68⬚
Bolivia
60⬚
52⬚
Paraguay 24⬚
Br
az
il
24⬚
1 2 Tucuman
28⬚
32⬚
Uruguay
e
a
n
Buenos Aires
O
c
36⬚
Rio Cuarto
Ocean
32⬚
28⬚
a
Mar del Plata
i
Legend
Last glacial loess after Iriondo (1990–1997) Loess + loessic sediments after Panario and Gutiérrez (1999)
t
Tucumán mountain loess after Sayago (1995) Frenguelli (1995)
a
A
tl
a
n
ti
o
c
n
Loessic distribution after Sayago (1995)
Polanski (1963)
P
a
g
44⬚
Pacific
40⬚
Tropical loess(?)after Iriondo and Kröhling (1997)
48⬚
Eolian sands
52⬚
0
500 km
Figure 1 Loess and loessoid distribution following several authors. Adapted from Frenguelli J (1955) Loess y Limos pampeanos. Ministerio de Educacio´n de la Nacio´n. Serie Te´cnica y Dida´ctica. La Plata, No. 7; Iriondo M (1990) Map of the South American plains – Its present state. Quaternary of South America and Antarctic Peninsula 6: 297–308; Panario D and Gutie´rrez O (1999) The continental Uruguayan Cenozoic: An overview. Quaternary International 62: 75–84; Polanski J (1963) Estratigrafı´a, Neotecto´nica y Geomorfologı´a del Pleistoceno pedemontano entre los rı´os Diamante y Mendoza. Asociacio´n Geolo´gica Argentina Revista XVII: 127–349; Za´rate M (2003) The loess record of Southern South America. Quaternary Science Reviews 22: 1987–2006; Za´rate M and Blasi A (1993) late Pleistocene-Holocene eolian deposits of the southern Buenos Aires Province, Argentina: A preliminary model. Quaternary International 17: 15–20.
entities, as their boundaries are sometimes obliterated by bioturbation, aqueous reworking, and grain size variation. Micromorphology also suggests that the balance between pedogenesis and sedimentation varied cyclically over time, with alternating phases of pedogenesis and intervals of much higher sedimentation rate. This pattern resulted in the identification of pedosedimentary cycles, thought to be ultimately driven by climatic changes.
Carbonate Accumulations Carbonate accumulations, known locally as ‘tosca,’ constitute a very common feature of the loess and loess-like deposits of
the southern and northern Pampas. Their origin has been attributed to both pedogenesis and groundwater processes. Carbonate accumulations exhibit very diverse morphologies including carbonate-cemented sedimentary layers, nodules, and concretions ranging from millimeters to half a meter in size, as well as veins (Figure 4). Carbonate rhizoconcretions are also present, associated with paleosol horizons. Calcrete crusts are another common and characteristic feature of the southern Pampas sequences. In addition, crusts are developed on the surfaces of late Miocene and Pleistocene loess-like deposits. They exhibit complex morphologies, resulting from multiple episodes of brecciation, dissolution, and reprecipitation. The thickness varies from a few centimeters
LOESS RECORDS | South America
Table 1
631
Summarized characteristics of the loess record in the Pampas, Chaco and the mountain valleys of Tucuma´n
Area
General knowledge
Sedimentary record
Stratigraphic record
Pampean loess record
Numerous studies on stratigraphy, vertebrate fossil remains, paleopedology, magnetostratigraphy, and geochronology: several numerical dates by 14C, TL, OSL, Ar/Ar
Discontinuous sedimentary record
late Miocene (10 Ma) to Holocene record
Chaco loess record Northwest Argentina, Tucuma´n Mountain valley loess record
Not known in detail. Information available on areal distribution and partially on composition, very few dates (14C) Studies performed since the 1980s; several recent papers on pedostratigraphy, micromorphology, 14C, and OSL dates, geochemistry, magnetostratigraphy
(a)
(b)
(c)
(d)
Loessoid deposits dominant component Loess prevalent during last glaciation forming a continuous apron Loess covering several areas of the plain Continuous stratigraphic loess–paleosol record. High resolution
Last Glacial–Holocene loess Mid-Pleistocene to Holocene loess– paleosol record
Figure 2 Exposures of the Pampean loess and loess-like record. (a) Chapadmalal sea cliffs south of Mar del Plata. (b) late Miocene and Pliocene loess deposits at a river bankful in Ventania range. (c) Gorina quarry in La Plata. (d) Outcrop along the left margin of the Parana´ River in Baradero, northern Pampas.
to more than 2 m, depending on the relative age of the geomorphic surfaces where they are present. No numerical ages have been obtained.
Vertebrate Fossil Content The abundant vertebrate fossil remains recovered from the late Cenozoic loess and loess-like deposits have been the subject
of numerous studies, dealing particularly with their taxonomy and biostratigraphic implications. Fossil assemblages consist of autochthonous endemic groups (including edentates, litopterns, notoungulates, and marsupials) and Holarctic immigrants (including carnivores, cervids, camelids, and horses) which gradually migrated to South America at different stages. A major faunal turnover, whose origin is still debated, is recorded in the late Pliocene loess-like deposits of Chapadmalal.
632
LOESS RECORDS | South America
Impact Glasses A remarkable feature of the loess and loess-like deposits of South America is the occurrence of vesicular and glassy fragments, ranging in size from a few millimeters up to clusters 1–2 m in diameter. These fragments, known as ‘escorias’ are found at localities of the southern Pampas and in Rio Cuarto (northern Pampas; Figure 6). In stratigraphic sections, they form horizons characterized by a relatively high concentration of fragments of different sizes and can be traced laterally for several hundreds of meters or even kilometers at some localities. Reworked escorias fragments from these main horizons are commonly embedded in younger fluvial deposits; some secondary reworking is due to vertebrate bioturbation. Their genesis, debated for decades, had been previously attributed to natural fires, volcanic and igneous activity, and even in situ diagenetic processes. However, recent studies of the geochemical and petrographic character of escorias show clear signatures of an impact origin. High-resolution argon-40/argon-39 (40Ar/39Ar) dating of the fresher-appearing glass indicates the occurrence of five impact events in the southern Pampas recorded at sections cropping out in Chapadmalal, Centinela del Mar (two events), Bahı´a Blanca, and Chasico´ (Table 3). In Rı´o Cuarto, impact glasses have been found scattered on exhumed surfaces with relative age, fission track, and 40Ar/39Ar dating indicating the presence of at least two additional Holocene and Pleistocene impact events.
(a)
Magnetostratigraphy
(b)
Figure 3 Field exposures of paleosols. (a) Truncated paleosol of the mid-Pleistocene developed on loess deposits, Gorina quarry, near La Plata. (b) late Pliocene loess–paleosol sequence at Chapadmalal, Bt horizons more eroded and with darker colors.
Fossil assemblages have been grouped into different land mammal ages or stages–ages based on their relative degree of evolution (Table 2). All the type localities of the late Miocene to Pleistocene stage–ages (except for the Huayquerian, situated in the Andean piedmont), are located in the southern and northern Pampas of Buenos Aires province. In addition to fossil bones, loess and loess-like deposits exhibit striking mammal bioturbation features represented by burrows and caves of variable sizes (Figure 5). The smaller burrows, with diameters of less than 20 cm, were formed by fossil rodents. Large burrows, with diameters between 0.80 and 1.80 m, are also common. Their morphological characteristics and the occurrence of claw marks on the walls and roofs, along with biomechanical analysis of fossil limbs, suggest that the mylodontid sloths Scelidotherium and Glossotherium were the probable builders of these burrows. Prior to the initiation of magnetostratigraphic studies, fossil assemblages were the only tools available to assign relative ages and correlate isolated and distant loess sections across the Pampean plain. This resulted in a regional chronostratigraphic scheme of the loess and loess-like record.
Magnetostratigraphic analysis has been performed on several sections of loess and loess-like deposits of the northern Pampas in the vicinity of Buenos Aires and the Chapadmalal sea cliffs in the southern Pampas (Table 4). When no numerical ages are available for chronology the magnetostratigraphic interpretations, however, are constrained by the stratigraphic discontinuity and the common lateral facial changes. As a result, several alternative interpretations have been proposed at some sections. The magnetostratigraphy of Chapadmalal suggests an early Gauss-to-Gilbert magnetic age (>3.15 Ma) for the lowermost levels supported by later studies indicating ages close to 4 Ma. The sequence is completed by a discontinuous record of loess-like deposits accumulated during the Gauss–Matuyama and Brunhes magnetic ages. In the northern Pampas, the paleomagnetic analysis suggests a Matuyama age younger than 2 Ma for the oldest levels cropping out at the bottom of several quarries. The Brunhes– Matuyama boundary is identified at several sections at a variable depth of 5–10 m from the surface. The onset of the Brunhes chron is thought to be characterized by a new pulse of loessic sedimentation in coincidence with an increase in the Andean volcanic activity and a change to drier and colder climatic conditions.
Magnetic Susceptibility Near the city of Buenos Aires in the northern Pampas, low susceptibility is recorded in paleosols and higher susceptibility in loess layers. This contrasts with the loess–paleosol record of
LOESS RECORDS | South America
(a)
(b)
(c)
(d)
633
Figure 4 Morphologies of carbonate accumulations. (a) Calcrete crusts in late Pliocene and Pleistocene loess-like deposits at a Chapadmalal quarry. (b) Calcrete crust on top of Pliocene loess deposits in Tandilia. (c) Veins, nodular and massive calcium carbonate layers eroded by paleochannels, early Pleistocene loess-like deposits at Chapadmalal. (d) Rhizoconcretions and reticulate pattern of carbonate stringers associated with a paleosol level, Chapadmalal.
Table 2
Stages–ages (land mammal ages) and type localities in the Pampean region (adapted from Cione and Tonni (2001), and references therein)
Area
Type locality
Stages–ages (land mammal ages)
Sedimentary deposits
Northern Pampas of Buenos Aires province
Stratigraphic sections located in the surroundings of Buenos Aires city and La Plata
Platan
Lacustrine and fluvial facies – loess
Lujanian Bonaerian Ensenadan
Fluvial deposits – loess Loess–loessoid deposits Loessoid deposits of paludal and fluvial environments Loessoid deposits of fluvial environments
Southern Pampas of Buenos Aires province
Chapadmalal sea cliffs
Marplatan
Chapadmalal sea cliffs
Chapadmalan
Monte Hermoso Arroyo Chasico´ in southern Buenos Aires
Montehermosan Chasicoan
China, but is similar to what has been reported for Alaska. Magnetic susceptibility changes have been interpreted as evidence of alternating periods of wet and dry conditions. Low susceptibility values are reported in calcrete layers and reduced sediments and high values in the B horizons. The magnetic
Loessoid deposits of hill slopes and of fluvial environments Loessoid deposits of fluvial environments Very fine sandstones and loessoid deposits siltstones of fluvial and paludal environments
mineralogy is dominated by multidomain magnetite, behavior attributable to fine silt-sized particles most likely of detrital origin. Diagenetic processes and past climatic conditions have been considered as possible controlling factors of magnetic susceptibility. The magnetic signals of late Pleistocene and
634
LOESS RECORDS | South America
Table 3
(a)
Numerical ages of impact glasses
Localities
Host sediments
Glass age
Rio Cuarto northern Pampas
Surface material
Centinela del Mar southern Pampas
Loessoid deposits of fluvial environment
4–10 ka 2.3 1.6 ka 6 2 ka 570 100 ka 114 26 ka 230 30 ka
Chapadmalal sea cliffs (Mar Loess and loessoid del Plata) southern Pampas deposits Bahia Blanca southern Loessoid deposits Pampas Chasico´ southern Pampas Loessoid deposits and fine sandstones
445 21 ka 3.27 0.08 Ma 5.33 0.05 Ma 9.23 0.09 Ma
Source: Adapted from Schultz P, Za´rate M, Hames B, et al. (2004) The Quaternary impact record from the Pampas, Argentina. Earth and Planetary Science Letters 219: 221–238.
The Mountain Valley Loess of Tucuma´n
(b)
Figure 5 Bioturbation structures. (a) Rodent burrow in Pliocene loess at the Chapadmalal sea cliffs. (b) Cave excavated in loess-like deposits north of Mar del Plata.
Figure 6 Fragment of impact glass (escoria) found in late Pliocene loessoid deposits of the southern Pampas.
Holocene alluvial and eolian deposits at two Pampean fluvial environments of Buenos Aires suggest that the genesis of a high-coercive fraction in loessic sediments is tentatively associated with dissolution. Pedogenesis is considered the cause of magnetic signal variations in paleosols showing extensive dissolution of detrital ferrimagnetic minerals. High-coercitivity minerals are thought to indicate climatic conditions characterized by a dry season.
Loess–paleosol sections over 40 m thick have been reported at various sites in a mountain valley of Sierras Pampeanas in Tucuma´n province (Figure 7). The sections, located at altitudes between 2300 and 2500 m above sea level, are exposed along gully and river walls of the Tafi del Valle intramountain basin, a closed basin that acted as a sediment trap to eolian dust throughout the Quaternary. Twenty-eight discrete paleosols consisting of Bw, Bt, and C horizons were recognized at the La Mesada section on the basis of morphological properties (color, structure, texture, and clay coating). Detailed micromorphological analysis carried out at two stratigraphic intervals of La Mesada permitted the identification of a series of three main pedosedimentary stages (Table 5), interpretation probably applicable to the rest of the sequence. The pedosedimentary stages include an interval of accretionary pedogenesis, followed by a relatively stable surface characterized by the dominance of pedogenic process and a return to accretionary pedogenesis. Also, a cyclical pedosedimentary history is inferred at the 45-m-thick loess– paleosol sequence of El Lambedero on the basis of field properties, magnetic susceptibility, particle size, calcium carbonate content, and soil micromorphology. Magnetostratigraphy, geochemistry, and micromorphology were carried out at the 50-m-thick Las Carreras loess–paleosol sequence. The succession comprises 32 paleosols classified as Luvisols and (Luvic) Kastanozems. Carbonate leaching and reprecipitation, soil aggregation, and clay translocation are, among others, the major pedogenic processes involved in paleosol development. It is considered a quasicontinuous and complete continental record of climatic changes and environmental conditions in subtropical South America. The paleomagnetic analysis indicates a Matuyama magnetic age for the lower 15 m with the Brunhes–Matuyama boundary found at a depth of 26.7 m. A minimum age of 1.15 Ma is assigned to the onset of loess sedimentation. The relative maxima of magnetic susceptibility correspond mainly with loess and relative
LOESS RECORDS | South America
635
Table 4 Location of the main magnetostratigraphic sections studied in the Pampean loess sequences Area
Localities
Magnetostratigraphy
Mountain valleys of Tucuma´n
Las Carreras section
Brunhes
Northern Pampas
Buenos Aires city: three magnetostratigraphic sections (building excavations)
Baradero: two magnetostratigraphic sections La Plata: three main magnetostratigraphic sections in building excavations and quarries
Southern Pampas
Chapadmalal sea cliffs, Mar del Plata: four magnetostratigraphic sections
Matuyama (<1.15 Ma) Brunhes
Matuyama (upper part), Jaramillo subchron recorded. Exposed lower level >1.5 Ma Brunhes
(a)
Matuyama Brunhes
Matuyama (upper part), subchron Jaramillo recorded. Exposed lower levels >1.5 Ma Brunhes
Matuyama Gauss Gilbert (lower levels exposed >4 Ma)
minima with paleosols following a pattern similar to the Pampean loess–paleosol sequences. High susceptibility values were found associated with the presence of organic matter in both fossil and the modern Ah soil horizons. The paleoenvironmental or sedimentological significance of magnetic susceptibility signals, however, remains unclear and more studies are needed to explore the relation of the magnetic properties and lithology.
The Last Glacial Loess Record Geographic Distribution The eolian sediments of the last glacial cycle form a very extensive and continuous apron that blankets the landscape of the Pampean plain as well as extensive areas of the Chaco plain. The deposits constitute the parent material of the present cultivated soils (Figure 8). Small patches of loess are also found along the proximal Andean piedmont and intramountain valleys of Sierras Pampeanas (Figure 1).
(b)
Figure 7 Mountain loess from Tafi del Valle, Tucuma´n. (a) Panoramic view of the upper part of the loess–paleosol sequence near La Mesada. (b) General view of El Lambedero section. Photos courtesy of Rob Kemp.
Loess is surrounded by a large sand dune field west and south-southwestward which extends along the distal eastern Andean piedmont, covering most of the wide valley of the Bermejo–Desaguadero–Salado fluvial system, as well as the floodplains and fan environments of the San Juan, Mendoza, Tunuya´n, Diamante, and Atuel rivers. The sand dune fields and sand mantles extend eastward to around 150 km west of Buenos Aires, including large-scale dunes trending northnortheastward–south-southwestward. Southward, the sandy eolian facies continue along the Colorado and Negro fluvial valleys surrounding the southern Pampas where sandy deposits grade into loessial sands and sandy loess facies (Figure 9). Local topographic factors promote the deposition of loess on the surface of structural plains of La Pampa province (Figure 9) and along the proximal eastern piedmont and intramountain basins of Sierras Pampeanas of Co´rdoba and San Luis. In the Chaco plain, the loess cover is more discontinuous, alternating with alluvial deposits. Sand dunes composed of fine sands of dominantly quartz composition are reported in the Bolivia–Paraguay low plain.
Mineralogical Composition Grain Size Zonation Across the Pampean plain eolian deposits exhibit a regional west-southwest to east-northeast grain size zonation.
In the southern Pampas, sandy loess is mostly volcaniclastic consisting of andesitic plagioclase, hornblende, pyroxene, volcanic glass shards, and quartz grains. Volcanic rock fragments
636
Table 5
LOESS RECORDS | South America
Main characteristics of Tucuma´n mountain valley paleosols
Sections and thickness (m)
Number of paleosols
Paleosol characteristics
La Mesada (1, 2) (42 m)
28
(1) Bw and Bt horizons interbedded with loess (C horizons)
El Lambedero (2) (45 m)
One in the uppermost 6 m profile. Two in the lowermost 4.5 m profile
Las Carreras (3) (50 m)
32
(2) Micromorphology suggests three pedosedimentary stages: accretionary pedogenesis, relative stable surface, and return to accretionary pedogenesis Bt, BC, and C horizons identified pedocomplex in the upper section; three pedosedimentary stages cyclical development with four pedosedimentary stages Bt, BC horizons, A horizons mostly present but difficult to identify Main pedogenic processes: carbonate leaching and reprecipitation, pedogenic formation and translocation of clay and organic matter
Source: Adapted from Kemp R, Toms P, Sayago JM, et al. (2003) Micromorphology and OSL dating of the basal part of the loess-paleosol sequence at La Mesada in Tucuma´n Province, Northwest Argentina. Quaternary International 106–107: 111–117; Kemp R, King M, Toms P, et al. (2004) Pedosedimentary development of part of a Late Quaternary loess-paleosol sequence in northwest Argentina. Journal of Quaternary Science 19: 1–10; Schellenberger A, Heller F, Veit H (2003) Magnetostratigraphy and magnetic susceptibility of the Las Carreras loess-paleosol sequence in Valle de Tafı´, Tucuma´n, NW-Argentina. Quaternary International 106/107: 159–167; Zinck JA and Sayago JM (1999) Loess-paleosol sequence of La Mesada in Tucuma´n province, northwest Argentina. Characterization and paleoenvironmental interpretation. Journal of South American Earth Sciences 12: 293–310; Zinck JA and Sayago JM (2001) Climatic periodicity during the late Pleistocene from a loess-paleosol sequence in northwest Argentina. Quaternary International 78: 11–16.
(a)
(b)
(c)
(d)
Figure 8 Last glacial loess deposits. (a) Late Glacial sandy loess with present soil profile (argiudoll) near Mar del Plata, southern Pampas. (b) Last glacial loess in Co´rdoba, northern Pampas, gully generated by agricultural practices. (c) Last glacial loess at the Gorina quarry, northern Pampas. (d) Detailed view of the present soil developed on last glacial loess at the Gorina quarry.
LOESS RECORDS | South America
68⬚
66⬚
64⬚
62⬚
60⬚
Sala
Buenos Aires La Plata
do
r
Chasico
Co
Ne
gro
Loess
l
ora ago do rive r nia
Pat
aR
ia
Ra
Rio de la Plata terrace
ng
e Mar Del Plata
ang
e
Monte Hermoso
38⬚
Chapadmalal Centinela Del Mar Necochea
Late glacial shoreline
Bahia Blanca
n
n
A
ani
dil
ea
n
d
e
e ro
s
Desag u d
ther
rive
36⬚ Ta n
Ven t
Nor
56⬚
58⬚
riv
er
Colorado-Negro delta
40⬚
At la
nti
c
Sandunes and sand mantles Transport direction Volcanoes
Oc
70⬚
637
0
100
200 km
42⬚
Figure 9 Loess and eolian facies of the southern Pampas. Source areas and inferred wind transport direction. Adapted from Za´rate M and Blasi A (1993) late Pleistocene–Holocene eolian deposits of the southern Buenos Aires Province, Argentina: A preliminary model. Quaternary International 17: 15–20. Areal distribution of the Colorado and Negro rivers delta adapted from Parker G, Violante RA, Paterlini MC (1996) Fisiografı´a de la plataforma continental. In: Ramos VA and Turic MA (eds.) XIII Congreso Geolo´gico Argentino y III Congreso de exploracio´n de hidrocarburos, Buenos Aires. Geolog´ıa y Recursos Naturales de la Plataforma Continental Argentina, Relatorio, vol. 1, pp. 1–16.
are very common. Particles are very fresh and usually well rounded (volcanic rock fragments, magnetite, pyroxenes) or subrounded to subangular (clinopyroxenes, orthoclase, plagioclase). Illite, poor in potassium, is the dominant clay mineral in Pleistocene–Holocene loess sequences, while smectite and kaolinite are secondary components (Table 6). In the northern Pampas of Buenos Aires, the mineralogical composition of loess varies throughout the sections. It is predominantly composed of volcaniclastic material (glass shards and plagioclases); the sand fraction includes metamorphic rock fragments (gneisses, migmatites, etc.). Illite and smectite are the predominant clay minerals (Table 6). In southern Santa Fe´, the last glacial loess is composed of volcaniclastic material as well as a secondary percentage of metamorphic and igneous rock. Quartz is a dominant component of Chaco loess.
Geochemistry and Isotopic Composition The isotopic ratios (strontium-87/strontium-86 (87Sr/86Sr), neodymium-143/neodymium-144 (143Nd/144Nd)) of loess samples covering the time interval of ca. 10–150 ka from four localities of the Pampas (Hipodromo, Gorina, Baradero, and Lozada) and one locality from the mountain valleys of Tucuma´n (Lambedero) are characterized by a broad range of 87Sr/86Sr values and restricted 143Nd/144Nd ratios (0.5123–0.5126). A significant geographic variability exists and individual sites show less isotopic variability. Chondrite-normalized rare-Earth element (REE) patterns are generally consistent with loess from other regions and characterized by light REE enriched patterns, flat heavy REE trends, and negative europium anomalies.
Geochronology Some radiocarbon ages together with a few thermoluminescence (TL) ages were first used to determine the chronology of
loess deposition in South America. A correlation with the marine isotopic stage stratigraphy was proposed for the northern Pampas of Santa Fe´. New optically stimulated luminescence (OSL) analyses have provided numerical ages of some representative Pampean loess–paleosol sections and La Mesada and El Lambedero sections in the Tucuma´n mountain valleys. The intervals of dominant soil formation and periods of increasing depositional rates identified on the basis of detailed micromorphological analysis were chronologically bracketed and tentatively correlated with the marine isotope stratigraphy. Among other results, the OSL chronology suggests different deposition rates across the region with loess accumulating until the midHolocene in the western part of the northern Pampas. Discrepancies have arisen with OSL ages that suggest older ages than previously indicated by both radiocarbon and TL chronologies. At La Mesada, OSL ages ranging from 150 to 195 ka BP are much older than those indicated by the radiocarbon chronology (<30 ka BP) Consistent with this, the magnetostratigraphy in another nearby section (Las Carreras) suggests much older ages (Table 7).
Provenance of Loess Deposits The mineralogy and geochemistry of loess in South America indicates that the Andes Cordillera, Sierras Pampeanas of Co´rdoba, and San Luis, and the Parana´ River basin have been the most important source areas, with their relative inputs varying across the region (Figure 10). The isotopic data are consistent with multiple loess sources with a majority of particles being derived from the Andes. Discrete tephra layers, along high percentages of fresh volcanic shards within the loess suggest the direct input of Andean volcanic eruptions coming from the Central Volcanic Zone and the Southern Volcanic Zone. These two volcanic districts seem to have had a major
638
LOESS RECORDS | South America
Table 6 Geographic distribution, mineralogical composition, source areas, and inferred wind systems of last glacial loess (adapted from Za´rate, 2003 and references therein). Source areas of mountain valleys in Tucuma´n according to Smith et al. (2003) and Schellenberger and Veit (2006) Area
Eolian deposit
Mineralogical composition
Source area
Inferred transportwind systems
Western Chaco
Loess
Bolivian Andes
Northerly winds
Mountain valleys of Tucuma´n
Loess
Dominant quartz composition Volcaniclastic composition dominant
Western Andean source (Chilean altiplano) Local sources. Tephras from CVZ Andes Cordillera, Sierras Pampeanas, Parana´ basin, Uruguayan shield. Tephras from CVZ and the SVZ
Northerly winds
Northern Pampas (Pampa Ondulada of Buenos Aires, southern Santa Fe´, Co´rdoba)
Northern Pampas (Pampa Deprimida)
Loess
Volcaniclastic composition dominant
Clayey loess
Secondary amount of metamorphic and igneous rocks
Sandy loess (WSW) grading to typical loess (ENE)
Volcaniclastic composition dominant ENE area: not determined
Northern Patagonia (late Tertiary volcaniclastic sediments) Andes Cordillera (34–38 S)
Southern Pampas
Sand mantles
Volcaniclastic (
Sierras Pampeanas (?). Tephras from SVZ and CVZ (?) Northern Patagonia (late Tertiary volcaniclastic sediments) Andes (34–38 S). Tandilia and Ventania. Tephras from SVZ
Loessial sands. Sandy loess. Clayey loess (<)
Westerly winds Southsouthwesterly winds (?)North/ northeasterly winds (?)Tropospheric winds from the Puna/Altiplano Southsouthwesterly winds Holocene: westerly winds
Southsouthwesterly winds Holocene: westerly winds
Table 7 Chronology proposed for the loess–paleosol sections studied in the mountain valleys of Tucuma´n according to Zinck and Sayago (1999), Zinck and Sayago (2001), Kemp et al. (2003, 2004), and Schellenberger et al. (2003) Sections
14
La Mesada El Lambedero
Four dates between 5–42 m of depth: age range from ca. 17.5 to 27.5 ka –
Las Carreras
–
C dates
OSL dates
Magnetostratigraphy
Three dates between 35 and 42 m of depth: ages >152 ka Upper 6.1 m: 33–96 ka and lowermost 4.7 m: >165 ka –
–
influence on the different compositions of loess in different regions of South America. The volcanoes of the Puna plateau probably were the main source of loess in the Chaco region and some of the northern Pampas tephras. The volcanic eruptions of the Southern Volcanic Zone are more significant as a loess source material in the southern Pampas. The loess and sand deposits of the southern Pampas are mostly derived from Andean pyroclastic and volcanic rocks. Only a minor and localized contribution from the mountain ranges of southern Buenos Aires is present. In the northern Pampas, Andean volcaniclastic material is dominant in Santa Fe´ and northern Buenos Aires province. The Sierras Pampeanas of Co´rdoba and San Luis constitute a secondary source area, providing particles of metamorphic and igneous rocks and traces of kaolinite. In addition, the
– Brunhes–Matuyama including Jaramillo subchron and Kamikatsura event
Parana´ River basin contributed as a secondary source. The metamorphic outcrops located on the Uruguayan margin of the Rı´o de la Plata might be another potential secondary source area. The isotopic signatures suggest that the Chilean Altiplano was the source area of the mountain valley loess of Tucuma´n. Presumably direct ash falls and local dust sources were secondary contributors.
Paleoclimatic Significance of the Loess Record Wind Systems Two regional-scale sedimentological models have been proposed to explain loess origins in South America. The ‘Chaco
LOESS RECORDS | South America
Bolivia
cvz
68⬚
52⬚
Paraguay 24⬚
H
O
C
Late glacial shoreline Rio de la Plata terrace
40⬚
Deltaic system Colorado Negro rivers
Hudson volcano
Atla
S
44⬚
ntic
V
Z
Pacific
32⬚
ce
an
Uruguay
lado
36⬚
Ocean
I
32⬚
28⬚
Brazilian shield
Motamorphic+Igneous
L
Desaguadero - Sa
28⬚
rocks
E
B
ra
zi
l
24⬚
60⬚
639
Legend Late Tertiary volcaniclastic bedrocks Volcanic rocks
48⬚
Uruguayan basement rocks Patagonian inner shelf (deltaic deposits)
52⬚
Alluvial fans Volcanoes Glaciation Surface wind direction Tropospheric wind direction
0
500 km
Drainage Mountain range
Figure 10 Source areas of loess and eolian sands. Adapted from Za´rate M (2003) The Loess record of Southern South America. Quaternary Science Reviews 22: 1987–2006.
model’ and the ‘Pampean model’ were formulated by Iriondo (1997) to illustrate source areas, transport pathways, and depositional areas for loess of the northern and southern regions, respectively. The Chaco model suggests that during last glacial period, northerly wind systems deflated the floodplains of the Parapetı´, Pilcomayo, and Bermejo rivers, depositing the Chaco loess. The loess of the mountain valleys of Tucuma´n is now considered to be part of the Chaco model. The Pampean model explains the loess record further south. Particles were deflated from the floodplains of the Colorado and Negro rivers, and the Desaguadero fluvial system, a tributary of the Colorado (Figure 9). The westerly and southwesterly transport directions inferred from geomorphological,
sedimentological, and geochemical evidences agree with westerly paleowind simulations derived from climate models.
South American Source of Antarctic Dust A South American origin has been attributed to dust found in Antarctic ice cores. In general, the isotopic ratios of the eolian dust records from the Epica-Dome C and Vostok ice cores suggest that the Pampas, Patagonia, and possibly the exposed continental shelf of Argentina were the dominant sources of Antarctic dust. Detailed studies are needed to find out which of these regions were the main contributors of Antarctic dust.
640
LOESS RECORDS | South America
Conclusions Information is still lacking from several areas across the region, particularly the Chaco plain and the Andean mountain valleys north and south of Tucuma´n whereas recently proposed chronologies need to be tested in order to understand fully the paleoclimatic significance of the loess record. Many of the studies have been carried out in the Pampean plain. Here, the late Miocene to Holocene record is a complex stratigraphic succession mostly composed of loess-like deposits, numerous welded paleosols, and several erosional unconformities. The paleoenvironmental and paleoclimatic interpretations derived from the record should be examined carefully considering local and regional factors of controls. At a regional scale, loess deposits become a dominant component of the sequences during the last glacial period. Differences in sedimentation rates, sediment supplies, and soil development reflect heterogeneous environmental conditions through time across the vast Chaco-Pampean plain. To date, the apparently continuous and complete mountain valley loess–paleosol sequence of Tucuma´n, in north-western Argentina, dating back to around 1.15 Ma, is the most promising continental proxy record of paleoenvironmental and paleoclimatic conditions of southern South America.
See also: Loess Deposits: Origins and Properties. Beetle Records: Late Pleistocene of South America. Glacial Climates: Effects of Atmospheric Dust. Glaciations: Late Pleistocene in South America; Middle Pleistocene Glaciations in the Southern Hemisphere. Ice Core Records: South America. Lake Level Studies: Latin America. Loess Records: China; Europe; North America. Luminescence Dating: Optical Dating. Paleosols and Wind-Blown Sediments: Nature of Paleosols; Soil Micromorphology. Pollen Records, Late Pleistocene: South America. Pollen Records, Postglacial: South America. Vertebrate Records: Late Pleistocene of South America.
References Bidegain JC (1998) New evidence of the Brunhes/Matuyama polarity boundary in the Herna´ndez-Gorina quarries, northwest of the city of La Plata, Buenos Aires Province, Argentina. Quaternary of South America and Antarctic Peninsula 11: 207–228. Cione AL and Tonni EP (2001) Correlation of Pliocene to Holocene southern South American and European vertebrate-bearing units. Bolletino della Societa´ Paleontologica Italiana 40: 167–173. Clapperton CM (1993) Quaternary Geology and Geomorphology of South America, p. 779. Amsterdam: Elsevier. Delmonte B, Basile Doelsch I, Petit JR, et al. (2004) Comparing the Epica and Vostok dust records during the last 220,000 years: Stratigraphical correlation and provenance in glacial periods. Earth Science Reviews 66: 63–87. Frenguelli J (1955) Loess y Limos pampeanos. Ministerio de Educacio´n de la Nacio´n. Serie Te´cnica y Dida´ctica. La Plata, no. 7. Imbellone PA and Teruggi ME (1993) Paleosols in loess deposits of the Argentine Pampas. Quaternary International 17: 49–55. Iriondo M (1990) Map of the South American plains – Its present state. Quaternary of South America and Antarctic Peninsula 6: 297–308. Iriondo MH (1997) Models of deposition of loess and loessoids in the Upper Quaternary of South America. Journal of South American Earth Sciences 10: 71–79.
Iriondo MH and Kro¨hling D (1997) The tropical loess. In: Zhisheng A and Weijian Z (eds.) Quaternary Geology, Proceedings of the 30th International Geological Congress 21: 61–77. Kemp R, King M, Toms P, et al. (2004) Pedosedimentary development of part of a Late Quaternary loess–paleosol sequence in northwest Argentina. Journal of Quaternary Science 19: 1–10. Kemp R, Toms P, Sayago JM, et al. (2003) Micromorphology and OSL dating of the basal part of the loess–paleosol sequence at La Mesada in Tucuma´n Province, Northwest Argentina. Quaternary International 106–107: 111–117. Kemp R, Za´rate M, Toms M, et al. (2006) Late Quaternary paleosols, stratigraphy and landscape evolution in the Northern Pampas, Argentina. Quaternary Research 66: 119–132. Kro¨hling DM (1999) Upper Quaternary geology of the lower Carcaran˜a´ Basin, North Pampa, Argentina. Quaternary International 57/58: 135–148. Morra´s H (1999) Geochemical differentiation of Quaternary sediments from the Pampean region based on soil phosphorus contents as detected in the early 20th century. Quaternary International 62: 57–67. Morra´s H, Moretti L, Piccolo G, and Zech W (2005) New hypothesis and results on the origin of stonelines and subsurface structure in ferralitic soils of Misiones, Argentina. Geophysical Research Abstract 7: 05522. Muhs D and Za´rate M (2001) Late Quaternary eolian records of the Americas and their paleoclimatic significance. In: Markgraf V (ed.) Interhemispheric Climate Linkages, pp. 183–216. San Diego: Academic Press. Nabel P, Cione A, and Tonni E (2000) Environmental changes in the Pampean area of Argentina at the Matuyama–Brunhes (C1r–C1n) Chrons boundary. Palaeogeography, Palaeoclimatology, Palaeoecology 162: 403–412. Nabel PE, Morra´s HJM, Petersen N, and Zech W (1999) Correlation of magnetic and lithologic features of soils and Quaternary sediments from the Undulating Pampa, Argentina. Journal of South American Earth Sciences 12: 311–323. Orgeira MJ (1990) Paleomagnetism of late cenozoic fossiliferous sediments from Barranca de los Lobos (Buenos Aires province, Argentina). The magnetic age of the South American land-mammal ages. Physics of the Earth and Planetary Interiors 64: 121–132. Orgeira MJ, Walther AM, Tofalo RO, et al. (2003) Environmental magnetism in fluvial and loessic Holocene sediments and paleosols from the Chacopampean plain (Argentina). Journal of South American Earth Sciences 16: 259–264. Orgeira MJ, Walther AM, Vasquez CA, et al. (1998) Mineral magnetic record of paleoclimatic variation in loess and paleosol from the Buenos Aires formation (Buenos Aires, Argentina). Journal of South American Earth Sciences 11: 561–570. Panario D and Gutie´rrez O (1999) The continental Uruguayan Cenozoic: An overview. Quaternary International 62: 75–84. Parker G, Violante RA, and Paterlini MC (1996) Fisiografı´a de la plataforma continental. In: Ramos VA and Turic MA (eds.) XIII Congreso Geolo´gico Argentino y III Congreso de exploracio´n de hidrocarburos, Buenos Aires. Geolog´ıa y Recursos Naturales de la Plataforma Continental Argentina, Relatorio, vol. 1, pp. 1–16. Polanski J (1963) Estratigrafı´a, Neotecto´nica y Geomorfologı´a del Pleistoceno pedemontano entre los rı´os Diamante y Mendoza. Asociacio´n Geolo´gica Argentina Revista XVII: 127–349. Sayago JM (1995) The Argentinian neotropical loess: An overview. Quaternary Science Reviews 14: 755–766. Schellenberger A, Heller F, and Veit H (2003) Magnetostratigraphy and magnetic susceptibility of the Las Carreras loess–paleosol sequence in Valle de Tafı´, Tucuma´n, NW-Argentina. Quaternary International 106/107: 159–167. Schellenberger A and Veit H (2006) Pedostratigraphic and pedological and geochemical characterization of Las Carreras loess–paleosols sequence, Valle de Tafı´, NW-Argentina. Quaternary Science Reviews 25: 811–831. Schultz P, Za´rate M, Hames B, et al. (2004) The Quaternary impact record from the Pampas, Argentina. Earth and Planetary Science Letters 219: 221–238. Smith J, Vance D, Kemp R, et al. (2003) Isotopic constraints on the source of Argentinian loess – With implications for atmospheric circulation and the provenance of Antarctic dust during recent glacial maxima. Earth and Planetary Science Letters 6682: 1–16. Teruggi ME (1957) The nature and origin of Argentine loess. Journal of Sedimentary Petrology 27: 322–332. Tonni EP, Nabel P, Cione AL, et al. (1999) The Ensenada and Buenos Aires formations (Pleistocene) in a quarry near La Plata, Argentina. Journal of South American Earth Sciences 12: 273–291. Vizcaino S, Farin˜a RA, Za´rate MA, Bargo MS, and Schultz P (2004) Palaeocological implications of the mid-Pliocene faunal turnover in the Pampean region (Argentina). Palaeogeography, Palaeoclimatology, Palaeoecology 213: 101–113.
LOESS RECORDS | South America
Vizcaino SF, Za´rate MA, Bargo MS, and Dondas A (2001) Pleistocene burrows in the Mar del Plata area (Buenos Aires province, Argentina) and their probable builders. Acta Paleontologica Polonica 46: 289–301. Za´rate M (2003) The loess record of southern South America. Quaternary Science Reviews 22: 1987–2006. Za´rate M and Blasi A (1993) Late Pleistocene–Holocene eolian deposits of the southern Buenos Aires Province, Argentina: A preliminary model. Quaternary International 17: 15–20.
641
Zinck JA and Sayago JM (1999) Loess–paleosol sequence of La Mesada in Tucuma´n province, northwest Argentina. Characterization and palaeoenvironmental interpretation. Journal of South American Earth Sciences 12: 293–310. Zinck JA and Sayago JM (2001) Climatic periodicity during the late Pleistocene from a loess–paleosol sequence in northwest Argentina. Quaternary International 78: 11–16.
Introduction The loess record of southern South America, the largest of the Southern Hemisphere, was originally reported in the nineteenth century under different names, among others by Charles Darwin in 1844. Its resemblance with the loess deposits of the Rhine valley was first noticed in 1866. Since then the term ‘loess’ has been loosely used in the literature to refer to the vast mantle of late Tertiary and Quaternary sediments of the Chaco-Pampean plain. Loess-like deposits, loessoid deposits, reworked loess, loessic deposits, and secondary loess have been used to designate fine-grained, light-colored and massive sediments with the appearance of loess. Generally, loess-like and loessoid deposits are the terms most commonly found in the literature. Neotropical loess has been recently used to designate primary and reworked sediments of loessic character, distributed between latitudes 20 and 30 S. Also, the term tropical loess has been proposed to account for fine-grained surface deposits of massive appearance in northeastern Argentina, areas of southern Brazil, and northern Uruguay. These sediments, traditionally interpreted as in situ weathered byproducts from the underlying late Jurassic basaltic rocks, have been recently interpreted as a wide and extensive loess mantle that accumulated during the last glacial cycle. This hypothesis generated controversy and is presently under debate. Ongoing geochemical, mineralogical, and pedological analysis reinforce the in situ origin of the surface deposits. During most of the twentieth century, research on South American loess was fragmentary and discontinuous. With few exceptions, a great number of contributions were focused on the stratigraphy and the fossil vertebrate content. A renewed interest began in the late 1980s specially driven by the necessity of testing global paleoclimatic models. Numerous studies were conducted particularly across the Pampean plain. The more detailed mapping, magnetostratigraphy, dating, paleopedological studies, and compositional analysis have substantially increased the understanding of the record. In recent years, detailed analysis substantially increased the knowledge of the loess–paleosol record of the mountain valley loess of Tucuma´n in northwestern Argentina. The Chaco loess is less known than the other two loess records, although regional aspects of its composition and geographical distribution have been reported.
Geographical Distribution Loess distribution has always been associated with the extensive and heterogeneous Chaco-Pampean plain of southern South America, a region over 1 million km2, covering most of northern central Argentina, western Uruguay, southern Brazil, western Paraguay, and southeastern Bolivia (Figure 1). This
generalization probably comes from the geographic distribution of loessoid deposits in Argentina and was later adopted in other studies, including not only loess but also sand dunes, sand mantles, and fine-grained deposits of noneolian origin. Subsequently, more detailed mapping across the ChacoPampean plain (Table 1) indicated a more restricted geographic distribution of loess; extensive areas of central Argentina are covered by eolian sands, while alluvial deposits are common in the northeast.
The Pampean Loess and Loess-Like Record General Characteristics The exposures of Pampean sediments are scarce and limited to quarries, road cuts, and river banks. The exceptions are the Chapadmalal sea cliffs near Mar del Plata and the Parana´ River banks, 20–30 m thick and laterally continuous for several kilometers. Both of these unusually well-exposed sections have been the subject of numerous studies (Figure 2). The Pampean sequences do not represent a continuous sedimentary record; stratigraphic discontinuities are indicated by several erosional unconformities while the rate and characteristics of the sediment supply varied through time. Loess is a secondary and minor component of the sedimentary sequences, being prevalent only during the last glacial cycle. Instead, loess-like deposits generated by the reworking of primary eolian sediments are dominant. Running water, masswasting processes on hill slopes, and deposition in swampy and lacustrine environments are thought to have been involved in the retransportation and resedimentation of primary loess. The reworking by aqueous transport agents resulted in several loess-like deposits including siltstones, clay siltstones, and sandy siltstones. They usually exhibit a general massive appearance or have poorly defined sedimentary structures frequently masked by pedogenesis and diagenesis.
Paleosols The occurrence of pedogenic features is ubiquitous throughout the stratigraphic sequence of loess and loess-like deposits (Figure 3). The recognition and classification of paleosol soil horizons present several difficulties in field exposures. Paleosols were originally interpreted as truncated profiles with A horizons eroded by eolian and/or fluvial processes. Their identification has been mostly based on field morphological properties. Detailed micromorphological analysis carried out at some stratigraphic intervals of the northern and southern Pampas show that pedogenesis was active throughout the sedimentation process resulting in accretionary soil profiles. Welding of pedological features on preexisting soil surfaces gives way to pedocomplexes. In addition, paleosols are not always discrete
629
630
LOESS RECORDS | South America
68⬚
Bolivia
60⬚
52⬚
Paraguay 24⬚
Br
az
il
24⬚
1 2 Tucuman
28⬚
32⬚
Uruguay
e
a
n
Buenos Aires
O
c
36⬚
Rio Cuarto
Ocean
32⬚
28⬚
a
Mar del Plata
i
Legend
Last glacial loess after Iriondo (1990–1997) Loess + loessic sediments after Panario and Gutiérrez (1999)
t
Tucumán mountain loess after Sayago (1995) Frenguelli (1995)
a
A
tl
a
n
ti
o
c
n
Loessic distribution after Sayago (1995)
Polanski (1963)
P
a
g
44⬚
Pacific
40⬚
Tropical loess(?)after Iriondo and Kröhling (1997)
48⬚
Eolian sands
52⬚
0
500 km
Figure 1 Loess and loessoid distribution following several authors. Adapted from Frenguelli J (1955) Loess y Limos pampeanos. Ministerio de Educacio´n de la Nacio´n. Serie Te´cnica y Dida´ctica. La Plata, No. 7; Iriondo M (1990) Map of the South American plains – Its present state. Quaternary of South America and Antarctic Peninsula 6: 297–308; Panario D and Gutie´rrez O (1999) The continental Uruguayan Cenozoic: An overview. Quaternary International 62: 75–84; Polanski J (1963) Estratigrafı´a, Neotecto´nica y Geomorfologı´a del Pleistoceno pedemontano entre los rı´os Diamante y Mendoza. Asociacio´n Geolo´gica Argentina Revista XVII: 127–349; Za´rate M (2003) The loess record of Southern South America. Quaternary Science Reviews 22: 1987–2006; Za´rate M and Blasi A (1993) late Pleistocene-Holocene eolian deposits of the southern Buenos Aires Province, Argentina: A preliminary model. Quaternary International 17: 15–20.
entities, as their boundaries are sometimes obliterated by bioturbation, aqueous reworking, and grain size variation. Micromorphology also suggests that the balance between pedogenesis and sedimentation varied cyclically over time, with alternating phases of pedogenesis and intervals of much higher sedimentation rate. This pattern resulted in the identification of pedosedimentary cycles, thought to be ultimately driven by climatic changes.
Carbonate Accumulations Carbonate accumulations, known locally as ‘tosca,’ constitute a very common feature of the loess and loess-like deposits of
the southern and northern Pampas. Their origin has been attributed to both pedogenesis and groundwater processes. Carbonate accumulations exhibit very diverse morphologies including carbonate-cemented sedimentary layers, nodules, and concretions ranging from millimeters to half a meter in size, as well as veins (Figure 4). Carbonate rhizoconcretions are also present, associated with paleosol horizons. Calcrete crusts are another common and characteristic feature of the southern Pampas sequences. In addition, crusts are developed on the surfaces of late Miocene and Pleistocene loess-like deposits. They exhibit complex morphologies, resulting from multiple episodes of brecciation, dissolution, and reprecipitation. The thickness varies from a few centimeters
LOESS RECORDS | South America
Table 1
631
Summarized characteristics of the loess record in the Pampas, Chaco and the mountain valleys of Tucuma´n
Area
General knowledge
Sedimentary record
Stratigraphic record
Pampean loess record
Numerous studies on stratigraphy, vertebrate fossil remains, paleopedology, magnetostratigraphy, and geochronology: several numerical dates by 14C, TL, OSL, Ar/Ar
Discontinuous sedimentary record
late Miocene (10 Ma) to Holocene record
Chaco loess record Northwest Argentina, Tucuma´n Mountain valley loess record
Not known in detail. Information available on areal distribution and partially on composition, very few dates (14C) Studies performed since the 1980s; several recent papers on pedostratigraphy, micromorphology, 14C, and OSL dates, geochemistry, magnetostratigraphy
(a)
(b)
(c)
(d)
Loessoid deposits dominant component Loess prevalent during last glaciation forming a continuous apron Loess covering several areas of the plain Continuous stratigraphic loess–paleosol record. High resolution
Last Glacial–Holocene loess Mid-Pleistocene to Holocene loess– paleosol record
Figure 2 Exposures of the Pampean loess and loess-like record. (a) Chapadmalal sea cliffs south of Mar del Plata. (b) late Miocene and Pliocene loess deposits at a river bankful in Ventania range. (c) Gorina quarry in La Plata. (d) Outcrop along the left margin of the Parana´ River in Baradero, northern Pampas.
to more than 2 m, depending on the relative age of the geomorphic surfaces where they are present. No numerical ages have been obtained.
Vertebrate Fossil Content The abundant vertebrate fossil remains recovered from the late Cenozoic loess and loess-like deposits have been the subject
of numerous studies, dealing particularly with their taxonomy and biostratigraphic implications. Fossil assemblages consist of autochthonous endemic groups (including edentates, litopterns, notoungulates, and marsupials) and Holarctic immigrants (including carnivores, cervids, camelids, and horses) which gradually migrated to South America at different stages. A major faunal turnover, whose origin is still debated, is recorded in the late Pliocene loess-like deposits of Chapadmalal.
632
LOESS RECORDS | South America
Impact Glasses A remarkable feature of the loess and loess-like deposits of South America is the occurrence of vesicular and glassy fragments, ranging in size from a few millimeters up to clusters 1–2 m in diameter. These fragments, known as ‘escorias’ are found at localities of the southern Pampas and in Rio Cuarto (northern Pampas; Figure 6). In stratigraphic sections, they form horizons characterized by a relatively high concentration of fragments of different sizes and can be traced laterally for several hundreds of meters or even kilometers at some localities. Reworked escorias fragments from these main horizons are commonly embedded in younger fluvial deposits; some secondary reworking is due to vertebrate bioturbation. Their genesis, debated for decades, had been previously attributed to natural fires, volcanic and igneous activity, and even in situ diagenetic processes. However, recent studies of the geochemical and petrographic character of escorias show clear signatures of an impact origin. High-resolution argon-40/argon-39 (40Ar/39Ar) dating of the fresher-appearing glass indicates the occurrence of five impact events in the southern Pampas recorded at sections cropping out in Chapadmalal, Centinela del Mar (two events), Bahı´a Blanca, and Chasico´ (Table 3). In Rı´o Cuarto, impact glasses have been found scattered on exhumed surfaces with relative age, fission track, and 40Ar/39Ar dating indicating the presence of at least two additional Holocene and Pleistocene impact events.
(a)
Magnetostratigraphy
(b)
Figure 3 Field exposures of paleosols. (a) Truncated paleosol of the mid-Pleistocene developed on loess deposits, Gorina quarry, near La Plata. (b) late Pliocene loess–paleosol sequence at Chapadmalal, Bt horizons more eroded and with darker colors.
Fossil assemblages have been grouped into different land mammal ages or stages–ages based on their relative degree of evolution (Table 2). All the type localities of the late Miocene to Pleistocene stage–ages (except for the Huayquerian, situated in the Andean piedmont), are located in the southern and northern Pampas of Buenos Aires province. In addition to fossil bones, loess and loess-like deposits exhibit striking mammal bioturbation features represented by burrows and caves of variable sizes (Figure 5). The smaller burrows, with diameters of less than 20 cm, were formed by fossil rodents. Large burrows, with diameters between 0.80 and 1.80 m, are also common. Their morphological characteristics and the occurrence of claw marks on the walls and roofs, along with biomechanical analysis of fossil limbs, suggest that the mylodontid sloths Scelidotherium and Glossotherium were the probable builders of these burrows. Prior to the initiation of magnetostratigraphic studies, fossil assemblages were the only tools available to assign relative ages and correlate isolated and distant loess sections across the Pampean plain. This resulted in a regional chronostratigraphic scheme of the loess and loess-like record.
Magnetostratigraphic analysis has been performed on several sections of loess and loess-like deposits of the northern Pampas in the vicinity of Buenos Aires and the Chapadmalal sea cliffs in the southern Pampas (Table 4). When no numerical ages are available for chronology the magnetostratigraphic interpretations, however, are constrained by the stratigraphic discontinuity and the common lateral facial changes. As a result, several alternative interpretations have been proposed at some sections. The magnetostratigraphy of Chapadmalal suggests an early Gauss-to-Gilbert magnetic age (>3.15 Ma) for the lowermost levels supported by later studies indicating ages close to 4 Ma. The sequence is completed by a discontinuous record of loess-like deposits accumulated during the Gauss–Matuyama and Brunhes magnetic ages. In the northern Pampas, the paleomagnetic analysis suggests a Matuyama age younger than 2 Ma for the oldest levels cropping out at the bottom of several quarries. The Brunhes– Matuyama boundary is identified at several sections at a variable depth of 5–10 m from the surface. The onset of the Brunhes chron is thought to be characterized by a new pulse of loessic sedimentation in coincidence with an increase in the Andean volcanic activity and a change to drier and colder climatic conditions.
Magnetic Susceptibility Near the city of Buenos Aires in the northern Pampas, low susceptibility is recorded in paleosols and higher susceptibility in loess layers. This contrasts with the loess–paleosol record of
LOESS RECORDS | South America
(a)
(b)
(c)
(d)
633
Figure 4 Morphologies of carbonate accumulations. (a) Calcrete crusts in late Pliocene and Pleistocene loess-like deposits at a Chapadmalal quarry. (b) Calcrete crust on top of Pliocene loess deposits in Tandilia. (c) Veins, nodular and massive calcium carbonate layers eroded by paleochannels, early Pleistocene loess-like deposits at Chapadmalal. (d) Rhizoconcretions and reticulate pattern of carbonate stringers associated with a paleosol level, Chapadmalal.
Table 2
Stages–ages (land mammal ages) and type localities in the Pampean region (adapted from Cione and Tonni (2001), and references therein)
Area
Type locality
Stages–ages (land mammal ages)
Sedimentary deposits
Northern Pampas of Buenos Aires province
Stratigraphic sections located in the surroundings of Buenos Aires city and La Plata
Platan
Lacustrine and fluvial facies – loess
Lujanian Bonaerian Ensenadan
Fluvial deposits – loess Loess–loessoid deposits Loessoid deposits of paludal and fluvial environments Loessoid deposits of fluvial environments
Southern Pampas of Buenos Aires province
Chapadmalal sea cliffs
Marplatan
Chapadmalal sea cliffs
Chapadmalan
Monte Hermoso Arroyo Chasico´ in southern Buenos Aires
Montehermosan Chasicoan
China, but is similar to what has been reported for Alaska. Magnetic susceptibility changes have been interpreted as evidence of alternating periods of wet and dry conditions. Low susceptibility values are reported in calcrete layers and reduced sediments and high values in the B horizons. The magnetic
Loessoid deposits of hill slopes and of fluvial environments Loessoid deposits of fluvial environments Very fine sandstones and loessoid deposits siltstones of fluvial and paludal environments
mineralogy is dominated by multidomain magnetite, behavior attributable to fine silt-sized particles most likely of detrital origin. Diagenetic processes and past climatic conditions have been considered as possible controlling factors of magnetic susceptibility. The magnetic signals of late Pleistocene and
634
LOESS RECORDS | South America
Table 3
(a)
Numerical ages of impact glasses
Localities
Host sediments
Glass age
Rio Cuarto northern Pampas
Surface material
Centinela del Mar southern Pampas
Loessoid deposits of fluvial environment
4–10 ka 2.3 1.6 ka 6 2 ka 570 100 ka 114 26 ka 230 30 ka
Chapadmalal sea cliffs (Mar Loess and loessoid del Plata) southern Pampas deposits Bahia Blanca southern Loessoid deposits Pampas Chasico´ southern Pampas Loessoid deposits and fine sandstones
445 21 ka 3.27 0.08 Ma 5.33 0.05 Ma 9.23 0.09 Ma
Source: Adapted from Schultz P, Za´rate M, Hames B, et al. (2004) The Quaternary impact record from the Pampas, Argentina. Earth and Planetary Science Letters 219: 221–238.
The Mountain Valley Loess of Tucuma´n
(b)
Figure 5 Bioturbation structures. (a) Rodent burrow in Pliocene loess at the Chapadmalal sea cliffs. (b) Cave excavated in loess-like deposits north of Mar del Plata.
Figure 6 Fragment of impact glass (escoria) found in late Pliocene loessoid deposits of the southern Pampas.
Holocene alluvial and eolian deposits at two Pampean fluvial environments of Buenos Aires suggest that the genesis of a high-coercive fraction in loessic sediments is tentatively associated with dissolution. Pedogenesis is considered the cause of magnetic signal variations in paleosols showing extensive dissolution of detrital ferrimagnetic minerals. High-coercitivity minerals are thought to indicate climatic conditions characterized by a dry season.
Loess–paleosol sections over 40 m thick have been reported at various sites in a mountain valley of Sierras Pampeanas in Tucuma´n province (Figure 7). The sections, located at altitudes between 2300 and 2500 m above sea level, are exposed along gully and river walls of the Tafi del Valle intramountain basin, a closed basin that acted as a sediment trap to eolian dust throughout the Quaternary. Twenty-eight discrete paleosols consisting of Bw, Bt, and C horizons were recognized at the La Mesada section on the basis of morphological properties (color, structure, texture, and clay coating). Detailed micromorphological analysis carried out at two stratigraphic intervals of La Mesada permitted the identification of a series of three main pedosedimentary stages (Table 5), interpretation probably applicable to the rest of the sequence. The pedosedimentary stages include an interval of accretionary pedogenesis, followed by a relatively stable surface characterized by the dominance of pedogenic process and a return to accretionary pedogenesis. Also, a cyclical pedosedimentary history is inferred at the 45-m-thick loess– paleosol sequence of El Lambedero on the basis of field properties, magnetic susceptibility, particle size, calcium carbonate content, and soil micromorphology. Magnetostratigraphy, geochemistry, and micromorphology were carried out at the 50-m-thick Las Carreras loess–paleosol sequence. The succession comprises 32 paleosols classified as Luvisols and (Luvic) Kastanozems. Carbonate leaching and reprecipitation, soil aggregation, and clay translocation are, among others, the major pedogenic processes involved in paleosol development. It is considered a quasicontinuous and complete continental record of climatic changes and environmental conditions in subtropical South America. The paleomagnetic analysis indicates a Matuyama magnetic age for the lower 15 m with the Brunhes–Matuyama boundary found at a depth of 26.7 m. A minimum age of 1.15 Ma is assigned to the onset of loess sedimentation. The relative maxima of magnetic susceptibility correspond mainly with loess and relative
LOESS RECORDS | South America
635
Table 4 Location of the main magnetostratigraphic sections studied in the Pampean loess sequences Area
Localities
Magnetostratigraphy
Mountain valleys of Tucuma´n
Las Carreras section
Brunhes
Northern Pampas
Buenos Aires city: three magnetostratigraphic sections (building excavations)
Baradero: two magnetostratigraphic sections La Plata: three main magnetostratigraphic sections in building excavations and quarries
Southern Pampas
Chapadmalal sea cliffs, Mar del Plata: four magnetostratigraphic sections
Matuyama (<1.15 Ma) Brunhes
Matuyama (upper part), Jaramillo subchron recorded. Exposed lower level >1.5 Ma Brunhes
(a)
Matuyama Brunhes
Matuyama (upper part), subchron Jaramillo recorded. Exposed lower levels >1.5 Ma Brunhes
Matuyama Gauss Gilbert (lower levels exposed >4 Ma)
minima with paleosols following a pattern similar to the Pampean loess–paleosol sequences. High susceptibility values were found associated with the presence of organic matter in both fossil and the modern Ah soil horizons. The paleoenvironmental or sedimentological significance of magnetic susceptibility signals, however, remains unclear and more studies are needed to explore the relation of the magnetic properties and lithology.
The Last Glacial Loess Record Geographic Distribution The eolian sediments of the last glacial cycle form a very extensive and continuous apron that blankets the landscape of the Pampean plain as well as extensive areas of the Chaco plain. The deposits constitute the parent material of the present cultivated soils (Figure 8). Small patches of loess are also found along the proximal Andean piedmont and intramountain valleys of Sierras Pampeanas (Figure 1).
(b)
Figure 7 Mountain loess from Tafi del Valle, Tucuma´n. (a) Panoramic view of the upper part of the loess–paleosol sequence near La Mesada. (b) General view of El Lambedero section. Photos courtesy of Rob Kemp.
Loess is surrounded by a large sand dune field west and south-southwestward which extends along the distal eastern Andean piedmont, covering most of the wide valley of the Bermejo–Desaguadero–Salado fluvial system, as well as the floodplains and fan environments of the San Juan, Mendoza, Tunuya´n, Diamante, and Atuel rivers. The sand dune fields and sand mantles extend eastward to around 150 km west of Buenos Aires, including large-scale dunes trending northnortheastward–south-southwestward. Southward, the sandy eolian facies continue along the Colorado and Negro fluvial valleys surrounding the southern Pampas where sandy deposits grade into loessial sands and sandy loess facies (Figure 9). Local topographic factors promote the deposition of loess on the surface of structural plains of La Pampa province (Figure 9) and along the proximal eastern piedmont and intramountain basins of Sierras Pampeanas of Co´rdoba and San Luis. In the Chaco plain, the loess cover is more discontinuous, alternating with alluvial deposits. Sand dunes composed of fine sands of dominantly quartz composition are reported in the Bolivia–Paraguay low plain.
Mineralogical Composition Grain Size Zonation Across the Pampean plain eolian deposits exhibit a regional west-southwest to east-northeast grain size zonation.
In the southern Pampas, sandy loess is mostly volcaniclastic consisting of andesitic plagioclase, hornblende, pyroxene, volcanic glass shards, and quartz grains. Volcanic rock fragments
636
Table 5
LOESS RECORDS | South America
Main characteristics of Tucuma´n mountain valley paleosols
Sections and thickness (m)
Number of paleosols
Paleosol characteristics
La Mesada (1, 2) (42 m)
28
(1) Bw and Bt horizons interbedded with loess (C horizons)
El Lambedero (2) (45 m)
One in the uppermost 6 m profile. Two in the lowermost 4.5 m profile
Las Carreras (3) (50 m)
32
(2) Micromorphology suggests three pedosedimentary stages: accretionary pedogenesis, relative stable surface, and return to accretionary pedogenesis Bt, BC, and C horizons identified pedocomplex in the upper section; three pedosedimentary stages cyclical development with four pedosedimentary stages Bt, BC horizons, A horizons mostly present but difficult to identify Main pedogenic processes: carbonate leaching and reprecipitation, pedogenic formation and translocation of clay and organic matter
Source: Adapted from Kemp R, Toms P, Sayago JM, et al. (2003) Micromorphology and OSL dating of the basal part of the loess-paleosol sequence at La Mesada in Tucuma´n Province, Northwest Argentina. Quaternary International 106–107: 111–117; Kemp R, King M, Toms P, et al. (2004) Pedosedimentary development of part of a Late Quaternary loess-paleosol sequence in northwest Argentina. Journal of Quaternary Science 19: 1–10; Schellenberger A, Heller F, Veit H (2003) Magnetostratigraphy and magnetic susceptibility of the Las Carreras loess-paleosol sequence in Valle de Tafı´, Tucuma´n, NW-Argentina. Quaternary International 106/107: 159–167; Zinck JA and Sayago JM (1999) Loess-paleosol sequence of La Mesada in Tucuma´n province, northwest Argentina. Characterization and paleoenvironmental interpretation. Journal of South American Earth Sciences 12: 293–310; Zinck JA and Sayago JM (2001) Climatic periodicity during the late Pleistocene from a loess-paleosol sequence in northwest Argentina. Quaternary International 78: 11–16.
(a)
(b)
(c)
(d)
Figure 8 Last glacial loess deposits. (a) Late Glacial sandy loess with present soil profile (argiudoll) near Mar del Plata, southern Pampas. (b) Last glacial loess in Co´rdoba, northern Pampas, gully generated by agricultural practices. (c) Last glacial loess at the Gorina quarry, northern Pampas. (d) Detailed view of the present soil developed on last glacial loess at the Gorina quarry.
LOESS RECORDS | South America
68⬚
66⬚
64⬚
62⬚
60⬚
Sala
Buenos Aires La Plata
do
r
Chasico
Co
Ne
gro
Loess
l
ora ago do rive r nia
Pat
aR
ia
Ra
Rio de la Plata terrace
ng
e Mar Del Plata
ang
e
Monte Hermoso
38⬚
Chapadmalal Centinela Del Mar Necochea
Late glacial shoreline
Bahia Blanca
n
n
A
ani
dil
ea
n
d
e
e ro
s
Desag u d
ther
rive
36⬚ Ta n
Ven t
Nor
56⬚
58⬚
riv
er
Colorado-Negro delta
40⬚
At la
nti
c
Sandunes and sand mantles Transport direction Volcanoes
Oc
70⬚
637
0
100
200 km
42⬚
Figure 9 Loess and eolian facies of the southern Pampas. Source areas and inferred wind transport direction. Adapted from Za´rate M and Blasi A (1993) late Pleistocene–Holocene eolian deposits of the southern Buenos Aires Province, Argentina: A preliminary model. Quaternary International 17: 15–20. Areal distribution of the Colorado and Negro rivers delta adapted from Parker G, Violante RA, Paterlini MC (1996) Fisiografı´a de la plataforma continental. In: Ramos VA and Turic MA (eds.) XIII Congreso Geolo´gico Argentino y III Congreso de exploracio´n de hidrocarburos, Buenos Aires. Geolog´ıa y Recursos Naturales de la Plataforma Continental Argentina, Relatorio, vol. 1, pp. 1–16.
are very common. Particles are very fresh and usually well rounded (volcanic rock fragments, magnetite, pyroxenes) or subrounded to subangular (clinopyroxenes, orthoclase, plagioclase). Illite, poor in potassium, is the dominant clay mineral in Pleistocene–Holocene loess sequences, while smectite and kaolinite are secondary components (Table 6). In the northern Pampas of Buenos Aires, the mineralogical composition of loess varies throughout the sections. It is predominantly composed of volcaniclastic material (glass shards and plagioclases); the sand fraction includes metamorphic rock fragments (gneisses, migmatites, etc.). Illite and smectite are the predominant clay minerals (Table 6). In southern Santa Fe´, the last glacial loess is composed of volcaniclastic material as well as a secondary percentage of metamorphic and igneous rock. Quartz is a dominant component of Chaco loess.
Geochemistry and Isotopic Composition The isotopic ratios (strontium-87/strontium-86 (87Sr/86Sr), neodymium-143/neodymium-144 (143Nd/144Nd)) of loess samples covering the time interval of ca. 10–150 ka from four localities of the Pampas (Hipodromo, Gorina, Baradero, and Lozada) and one locality from the mountain valleys of Tucuma´n (Lambedero) are characterized by a broad range of 87Sr/86Sr values and restricted 143Nd/144Nd ratios (0.5123–0.5126). A significant geographic variability exists and individual sites show less isotopic variability. Chondrite-normalized rare-Earth element (REE) patterns are generally consistent with loess from other regions and characterized by light REE enriched patterns, flat heavy REE trends, and negative europium anomalies.
Geochronology Some radiocarbon ages together with a few thermoluminescence (TL) ages were first used to determine the chronology of
loess deposition in South America. A correlation with the marine isotopic stage stratigraphy was proposed for the northern Pampas of Santa Fe´. New optically stimulated luminescence (OSL) analyses have provided numerical ages of some representative Pampean loess–paleosol sections and La Mesada and El Lambedero sections in the Tucuma´n mountain valleys. The intervals of dominant soil formation and periods of increasing depositional rates identified on the basis of detailed micromorphological analysis were chronologically bracketed and tentatively correlated with the marine isotope stratigraphy. Among other results, the OSL chronology suggests different deposition rates across the region with loess accumulating until the midHolocene in the western part of the northern Pampas. Discrepancies have arisen with OSL ages that suggest older ages than previously indicated by both radiocarbon and TL chronologies. At La Mesada, OSL ages ranging from 150 to 195 ka BP are much older than those indicated by the radiocarbon chronology (<30 ka BP) Consistent with this, the magnetostratigraphy in another nearby section (Las Carreras) suggests much older ages (Table 7).
Provenance of Loess Deposits The mineralogy and geochemistry of loess in South America indicates that the Andes Cordillera, Sierras Pampeanas of Co´rdoba, and San Luis, and the Parana´ River basin have been the most important source areas, with their relative inputs varying across the region (Figure 10). The isotopic data are consistent with multiple loess sources with a majority of particles being derived from the Andes. Discrete tephra layers, along high percentages of fresh volcanic shards within the loess suggest the direct input of Andean volcanic eruptions coming from the Central Volcanic Zone and the Southern Volcanic Zone. These two volcanic districts seem to have had a major
638
LOESS RECORDS | South America
Table 6 Geographic distribution, mineralogical composition, source areas, and inferred wind systems of last glacial loess (adapted from Za´rate, 2003 and references therein). Source areas of mountain valleys in Tucuma´n according to Smith et al. (2003) and Schellenberger and Veit (2006) Area
Eolian deposit
Mineralogical composition
Source area
Inferred transportwind systems
Western Chaco
Loess
Bolivian Andes
Northerly winds
Mountain valleys of Tucuma´n
Loess
Dominant quartz composition Volcaniclastic composition dominant
Western Andean source (Chilean altiplano) Local sources. Tephras from CVZ Andes Cordillera, Sierras Pampeanas, Parana´ basin, Uruguayan shield. Tephras from CVZ and the SVZ
Northerly winds
Northern Pampas (Pampa Ondulada of Buenos Aires, southern Santa Fe´, Co´rdoba)
Northern Pampas (Pampa Deprimida)
Loess
Volcaniclastic composition dominant
Clayey loess
Secondary amount of metamorphic and igneous rocks
Sandy loess (WSW) grading to typical loess (ENE)
Volcaniclastic composition dominant ENE area: not determined
Northern Patagonia (late Tertiary volcaniclastic sediments) Andes Cordillera (34–38 S)
Southern Pampas
Sand mantles
Volcaniclastic (
Sierras Pampeanas (?). Tephras from SVZ and CVZ (?) Northern Patagonia (late Tertiary volcaniclastic sediments) Andes (34–38 S). Tandilia and Ventania. Tephras from SVZ
Loessial sands. Sandy loess. Clayey loess (<)
Westerly winds Southsouthwesterly winds (?)North/ northeasterly winds (?)Tropospheric winds from the Puna/Altiplano Southsouthwesterly winds Holocene: westerly winds
Southsouthwesterly winds Holocene: westerly winds
Table 7 Chronology proposed for the loess–paleosol sections studied in the mountain valleys of Tucuma´n according to Zinck and Sayago (1999), Zinck and Sayago (2001), Kemp et al. (2003, 2004), and Schellenberger et al. (2003) Sections
14
La Mesada El Lambedero
Four dates between 5–42 m of depth: age range from ca. 17.5 to 27.5 ka –
Las Carreras
–
C dates
OSL dates
Magnetostratigraphy
Three dates between 35 and 42 m of depth: ages >152 ka Upper 6.1 m: 33–96 ka and lowermost 4.7 m: >165 ka –
–
influence on the different compositions of loess in different regions of South America. The volcanoes of the Puna plateau probably were the main source of loess in the Chaco region and some of the northern Pampas tephras. The volcanic eruptions of the Southern Volcanic Zone are more significant as a loess source material in the southern Pampas. The loess and sand deposits of the southern Pampas are mostly derived from Andean pyroclastic and volcanic rocks. Only a minor and localized contribution from the mountain ranges of southern Buenos Aires is present. In the northern Pampas, Andean volcaniclastic material is dominant in Santa Fe´ and northern Buenos Aires province. The Sierras Pampeanas of Co´rdoba and San Luis constitute a secondary source area, providing particles of metamorphic and igneous rocks and traces of kaolinite. In addition, the
– Brunhes–Matuyama including Jaramillo subchron and Kamikatsura event
Parana´ River basin contributed as a secondary source. The metamorphic outcrops located on the Uruguayan margin of the Rı´o de la Plata might be another potential secondary source area. The isotopic signatures suggest that the Chilean Altiplano was the source area of the mountain valley loess of Tucuma´n. Presumably direct ash falls and local dust sources were secondary contributors.
Paleoclimatic Significance of the Loess Record Wind Systems Two regional-scale sedimentological models have been proposed to explain loess origins in South America. The ‘Chaco
LOESS RECORDS | South America
Bolivia
cvz
68⬚
52⬚
Paraguay 24⬚
H
O
C
Late glacial shoreline Rio de la Plata terrace
40⬚
Deltaic system Colorado Negro rivers
Hudson volcano
Atla
S
44⬚
ntic
V
Z
Pacific
32⬚
ce
an
Uruguay
lado
36⬚
Ocean
I
32⬚
28⬚
Brazilian shield
Motamorphic+Igneous
L
Desaguadero - Sa
28⬚
rocks
E
B
ra
zi
l
24⬚
60⬚
639
Legend Late Tertiary volcaniclastic bedrocks Volcanic rocks
48⬚
Uruguayan basement rocks Patagonian inner shelf (deltaic deposits)
52⬚
Alluvial fans Volcanoes Glaciation Surface wind direction Tropospheric wind direction
0
500 km
Drainage Mountain range
Figure 10 Source areas of loess and eolian sands. Adapted from Za´rate M (2003) The Loess record of Southern South America. Quaternary Science Reviews 22: 1987–2006.
model’ and the ‘Pampean model’ were formulated by Iriondo (1997) to illustrate source areas, transport pathways, and depositional areas for loess of the northern and southern regions, respectively. The Chaco model suggests that during last glacial period, northerly wind systems deflated the floodplains of the Parapetı´, Pilcomayo, and Bermejo rivers, depositing the Chaco loess. The loess of the mountain valleys of Tucuma´n is now considered to be part of the Chaco model. The Pampean model explains the loess record further south. Particles were deflated from the floodplains of the Colorado and Negro rivers, and the Desaguadero fluvial system, a tributary of the Colorado (Figure 9). The westerly and southwesterly transport directions inferred from geomorphological,
sedimentological, and geochemical evidences agree with westerly paleowind simulations derived from climate models.
South American Source of Antarctic Dust A South American origin has been attributed to dust found in Antarctic ice cores. In general, the isotopic ratios of the eolian dust records from the Epica-Dome C and Vostok ice cores suggest that the Pampas, Patagonia, and possibly the exposed continental shelf of Argentina were the dominant sources of Antarctic dust. Detailed studies are needed to find out which of these regions were the main contributors of Antarctic dust.
640
LOESS RECORDS | South America
Conclusions Information is still lacking from several areas across the region, particularly the Chaco plain and the Andean mountain valleys north and south of Tucuma´n whereas recently proposed chronologies need to be tested in order to understand fully the paleoclimatic significance of the loess record. Many of the studies have been carried out in the Pampean plain. Here, the late Miocene to Holocene record is a complex stratigraphic succession mostly composed of loess-like deposits, numerous welded paleosols, and several erosional unconformities. The paleoenvironmental and paleoclimatic interpretations derived from the record should be examined carefully considering local and regional factors of controls. At a regional scale, loess deposits become a dominant component of the sequences during the last glacial period. Differences in sedimentation rates, sediment supplies, and soil development reflect heterogeneous environmental conditions through time across the vast Chaco-Pampean plain. To date, the apparently continuous and complete mountain valley loess–paleosol sequence of Tucuma´n, in north-western Argentina, dating back to around 1.15 Ma, is the most promising continental proxy record of paleoenvironmental and paleoclimatic conditions of southern South America.
See also: Loess Deposits: Origins and Properties. Beetle Records: Late Pleistocene of South America. Glacial Climates: Effects of Atmospheric Dust. Glaciations: Late Pleistocene in South America; Middle Pleistocene Glaciations in the Southern Hemisphere. Ice Core Records: South America. Lake Level Studies: Latin America. Loess Records: China; Europe; North America. Luminescence Dating: Optical Dating. Paleosols and Wind-Blown Sediments: Nature of Paleosols; Soil Micromorphology. Pollen Records, Late Pleistocene: South America. Pollen Records, Postglacial: South America. Vertebrate Records: Late Pleistocene of South America.
References Bidegain JC (1998) New evidence of the Brunhes/Matuyama polarity boundary in the Herna´ndez-Gorina quarries, northwest of the city of La Plata, Buenos Aires Province, Argentina. Quaternary of South America and Antarctic Peninsula 11: 207–228. Cione AL and Tonni EP (2001) Correlation of Pliocene to Holocene southern South American and European vertebrate-bearing units. Bolletino della Societa´ Paleontologica Italiana 40: 167–173. Clapperton CM (1993) Quaternary Geology and Geomorphology of South America, p. 779. Amsterdam: Elsevier. Delmonte B, Basile Doelsch I, Petit JR, et al. (2004) Comparing the Epica and Vostok dust records during the last 220,000 years: Stratigraphical correlation and provenance in glacial periods. Earth Science Reviews 66: 63–87. Frenguelli J (1955) Loess y Limos pampeanos. Ministerio de Educacio´n de la Nacio´n. Serie Te´cnica y Dida´ctica. La Plata, no. 7. Imbellone PA and Teruggi ME (1993) Paleosols in loess deposits of the Argentine Pampas. Quaternary International 17: 49–55. Iriondo M (1990) Map of the South American plains – Its present state. Quaternary of South America and Antarctic Peninsula 6: 297–308. Iriondo MH (1997) Models of deposition of loess and loessoids in the Upper Quaternary of South America. Journal of South American Earth Sciences 10: 71–79.
Iriondo MH and Kro¨hling D (1997) The tropical loess. In: Zhisheng A and Weijian Z (eds.) Quaternary Geology, Proceedings of the 30th International Geological Congress 21: 61–77. Kemp R, King M, Toms P, et al. (2004) Pedosedimentary development of part of a Late Quaternary loess–paleosol sequence in northwest Argentina. Journal of Quaternary Science 19: 1–10. Kemp R, Toms P, Sayago JM, et al. (2003) Micromorphology and OSL dating of the basal part of the loess–paleosol sequence at La Mesada in Tucuma´n Province, Northwest Argentina. Quaternary International 106–107: 111–117. Kemp R, Za´rate M, Toms M, et al. (2006) Late Quaternary paleosols, stratigraphy and landscape evolution in the Northern Pampas, Argentina. Quaternary Research 66: 119–132. Kro¨hling DM (1999) Upper Quaternary geology of the lower Carcaran˜a´ Basin, North Pampa, Argentina. Quaternary International 57/58: 135–148. Morra´s H (1999) Geochemical differentiation of Quaternary sediments from the Pampean region based on soil phosphorus contents as detected in the early 20th century. Quaternary International 62: 57–67. Morra´s H, Moretti L, Piccolo G, and Zech W (2005) New hypothesis and results on the origin of stonelines and subsurface structure in ferralitic soils of Misiones, Argentina. Geophysical Research Abstract 7: 05522. Muhs D and Za´rate M (2001) Late Quaternary eolian records of the Americas and their paleoclimatic significance. In: Markgraf V (ed.) Interhemispheric Climate Linkages, pp. 183–216. San Diego: Academic Press. Nabel P, Cione A, and Tonni E (2000) Environmental changes in the Pampean area of Argentina at the Matuyama–Brunhes (C1r–C1n) Chrons boundary. Palaeogeography, Palaeoclimatology, Palaeoecology 162: 403–412. Nabel PE, Morra´s HJM, Petersen N, and Zech W (1999) Correlation of magnetic and lithologic features of soils and Quaternary sediments from the Undulating Pampa, Argentina. Journal of South American Earth Sciences 12: 311–323. Orgeira MJ (1990) Paleomagnetism of late cenozoic fossiliferous sediments from Barranca de los Lobos (Buenos Aires province, Argentina). The magnetic age of the South American land-mammal ages. Physics of the Earth and Planetary Interiors 64: 121–132. Orgeira MJ, Walther AM, Tofalo RO, et al. (2003) Environmental magnetism in fluvial and loessic Holocene sediments and paleosols from the Chacopampean plain (Argentina). Journal of South American Earth Sciences 16: 259–264. Orgeira MJ, Walther AM, Vasquez CA, et al. (1998) Mineral magnetic record of paleoclimatic variation in loess and paleosol from the Buenos Aires formation (Buenos Aires, Argentina). Journal of South American Earth Sciences 11: 561–570. Panario D and Gutie´rrez O (1999) The continental Uruguayan Cenozoic: An overview. Quaternary International 62: 75–84. Parker G, Violante RA, and Paterlini MC (1996) Fisiografı´a de la plataforma continental. In: Ramos VA and Turic MA (eds.) XIII Congreso Geolo´gico Argentino y III Congreso de exploracio´n de hidrocarburos, Buenos Aires. Geolog´ıa y Recursos Naturales de la Plataforma Continental Argentina, Relatorio, vol. 1, pp. 1–16. Polanski J (1963) Estratigrafı´a, Neotecto´nica y Geomorfologı´a del Pleistoceno pedemontano entre los rı´os Diamante y Mendoza. Asociacio´n Geolo´gica Argentina Revista XVII: 127–349. Sayago JM (1995) The Argentinian neotropical loess: An overview. Quaternary Science Reviews 14: 755–766. Schellenberger A, Heller F, and Veit H (2003) Magnetostratigraphy and magnetic susceptibility of the Las Carreras loess–paleosol sequence in Valle de Tafı´, Tucuma´n, NW-Argentina. Quaternary International 106/107: 159–167. Schellenberger A and Veit H (2006) Pedostratigraphic and pedological and geochemical characterization of Las Carreras loess–paleosols sequence, Valle de Tafı´, NW-Argentina. Quaternary Science Reviews 25: 811–831. Schultz P, Za´rate M, Hames B, et al. (2004) The Quaternary impact record from the Pampas, Argentina. Earth and Planetary Science Letters 219: 221–238. Smith J, Vance D, Kemp R, et al. (2003) Isotopic constraints on the source of Argentinian loess – With implications for atmospheric circulation and the provenance of Antarctic dust during recent glacial maxima. Earth and Planetary Science Letters 6682: 1–16. Teruggi ME (1957) The nature and origin of Argentine loess. Journal of Sedimentary Petrology 27: 322–332. Tonni EP, Nabel P, Cione AL, et al. (1999) The Ensenada and Buenos Aires formations (Pleistocene) in a quarry near La Plata, Argentina. Journal of South American Earth Sciences 12: 273–291. Vizcaino S, Farin˜a RA, Za´rate MA, Bargo MS, and Schultz P (2004) Palaeocological implications of the mid-Pliocene faunal turnover in the Pampean region (Argentina). Palaeogeography, Palaeoclimatology, Palaeoecology 213: 101–113.
LOESS RECORDS | South America
Vizcaino SF, Za´rate MA, Bargo MS, and Dondas A (2001) Pleistocene burrows in the Mar del Plata area (Buenos Aires province, Argentina) and their probable builders. Acta Paleontologica Polonica 46: 289–301. Za´rate M (2003) The loess record of southern South America. Quaternary Science Reviews 22: 1987–2006. Za´rate M and Blasi A (1993) Late Pleistocene–Holocene eolian deposits of the southern Buenos Aires Province, Argentina: A preliminary model. Quaternary International 17: 15–20.
641
Zinck JA and Sayago JM (1999) Loess–paleosol sequence of La Mesada in Tucuma´n province, northwest Argentina. Characterization and palaeoenvironmental interpretation. Journal of South American Earth Sciences 12: 293–310. Zinck JA and Sayago JM (2001) Climatic periodicity during the late Pleistocene from a loess–paleosol sequence in northwest Argentina. Quaternary International 78: 11–16.