The Plio-Pleistocene record of the central eastern Pampas, Buenos Aires province, Argentina: The Chapadmalal case study

Palaeogeography, Palaeoclimatology, Palaeoecology, 72 (1989): 27-52

27

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

THE PLIO-PLEISTOCENE RECORD OF THE CENTRAL EASTERN PAMPAS, BUENOS AIRES PROVINCE, ARGENTINA: THE CHAPADMALAL CASE STUDY M A R C E L O A. Z A R A T E a n d J O R G E L. F A S A N O Centro de Geologia de Costas y del Cuaternario Universidad Nacional de Mar del Plata and C O N I C E T cc 722, correo central 7600 Mar del Plata (Argentina)

(Received November 1, 1987; revised and accepted September 2, 1988)

Abstract Zarate, M. A. and Fasano, J. L., 1989. The Plio-Pleistocene record of the central eastern Pampas, Buenos Aires province, Argentina: the Chapadmalal case study. Palaeogeogr., Palaeoclimatol., Palaeoecol., 72: 27-52. The Plio-Pleistocene record of central eastern Pampas, Argentina, is preserved in loess-like sediments exposed in the sea cliffs south of Mar del Plata. The uplift of the Andes during Miocene triggered the deposition of wind-blown material. The source areas are located about 1000 km away, in western Argentina. Volcanic ashfalls play a significant role in the formation of eolian sediments, which were reworked and redeposited by aqueous transport in a flat, low relief landscape under varying climatic conditions. Vertebrate fossil assemblages document warm and wetter Pliocene and Early Pleistocene climates during Montehermosan and Uquian land-mammal ages. More arid and cooler conditions alternating with humid and warmer intervals prevailed during the Middle to Late Pleistocene in Ensenadan and Lujanian land-mammal ages. The Chapadmalal locality is the best exposed and most complete section. Almost all the units have been modified by pedogenesis. Truncated paleosols as well as pedogenic and non-pedogenic calcretes are very frequent throughout the stratigraphic succession. Several paleosurfaces with prominent paleosols and calcretes suggest that sedimentation was an episodic process with numerous depositional gaps, recording relatively long lasting intervals of landscape stability. The depositional events represent alternations of fluvial and eolian sedimentation. Low frequency catastrophic episodes played an important role in the sedimentation process.

Introduction T h e p u r p o s e of t h i s p a p e r is to p r e s e n t a g e n e r a l r e v i e w of t h e m a i n c h a r a c t e r i s t i c s of t h e P l i o - P l e i s t o c e n e r e c o r d in t h e c e n t r a l e a s t e r n P a m p a s of B u e n o s A i r e s p r o v i n c e , A r g e n t i n a ( F i g . l ) . I t a l s o i n t e n d s to s t r e s s t h e n e e d of i n c o r p o r a t i n g p a l e o s o l s a n d c a l c r e t e s and their relations within lithofacies into the i n t e r p r e t a t i o n of t h e s e d i m e n t a r y s e q u e n c e s , Present knowledge about the geological e v o l u t i o n of t h e P a m p a s is r a t h e r m e a g e r . Vertebrate paleontology has a long tradition in t h e c o u n t r y a n d is by f a r t h e m o s t k n o w n and studied discipline. In a way, it triggered 0031-0182/89/$03.50

t h e i n i t i a l i n t e r e s t in L a t e C e n o z o i c i n v e s t i g a t i o n s , p a r t i c u l a r l y b e c a u s e of t h e a b u n d a n c e of fossil v e r t e b r a t e s i n t h e p a m p e a n s e d i m e n t s . A l t h o u g h t h e first Q u a t e r n a r y s t u d i e s d a t e b a c k as e a r l y as t h e m i d - n i n e t e e n t h c e n t u r y , the researches have been discontinuous. D ' O r b i g n y in 1847 w a s t h e first w h o a t t e m p t e d a g e o l o g i c a l c l a s s i f i c a t i o n of L a t e C e n o z o i c d e p o s i t s of A r g e n t i n a , t h e n n a m e d " a r g i l e p a m p ~ e n n e " ( P a m p e a n clay). S i n c e t h e n on, t h e sediments received different names. Darwin (1846) i n c l u d e d t h e s e d e p o s i t s i n a u n i t t h a t he called Pampean Mud or Pampean Formation. B o t h s c i e n t i s t s a t t r i b u t e d t h e a c c u m u l a t i o n of t h e s e d i m e n t s in t h e P a m p a s to a m a r i n e -

© 1989 Elsevier Science Publishers B.V.

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29 related catastrophic event. The term loess pampeano (pampean loess)was originally used by Heusser and Claraz (1863). Bravard (1857) who emphasized the role played by the eolian activity in the genesis of these sediments, coined the term Formacion Pampa (Pampa Formation). Burmeister (1864) considered intense rains and catastrophic floods as mainly involved in the origin of these deposits. The pioneer work carried out by Ameghino (1876, 1889, 1908, 1909) almost a century ago established the basic framework of the Quaternary geology of the Pampas. Since then no detailed studies were done except for some isolated but outstanding contributions such as those by Frenguelli (1928, 1950, 1957). In fact most of the original ideas and concepts, are still used and accepted as generally valid although presently under revision, An increased number of contributions during the last fifteen years obtained additional evidence on the Late Cenozoic evolution of the region. Fidalgo and coworkers (1973, 1 9 7 5 ,

1979) provided a great deal of information especially with reference to the Late Pleistocene-Holocene interval. More recently paleomagnetic, palynological and archaeological investigations were also initiated. G e o g r a p h i c and s t r u c t u r a l setting Buenos Aires province located in the eastcentral part of the Pampas, is an extensive flat grassland whose general monotony is interrupted by two northwest-trending mountain ranges, Tandilia and Ventania (Fig.l, 2a). Extremely low gradients characterize the basin of Salado river, the major autochthonous stream which crosses the region. In the northeast, the province is drained by several minor tributaries of the Rio Paran~ and Rio de la Plata, in a gently rolling landscape. Southwards, the area called Pampa interserrana, between the Tandilia and Ventania ranges, is predominantly flat with some hilly sectors. Numerous creeks and minor rivers flow across the area. The western

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30 part of the province corresponds to the socalled Pampa arenosa (sandy Pampa). It is characterized by complex dune systems, presently inactive and cultivated. Mean annual temperatures range from 13 to 17°C. Average annual precipitations decrease progressively westward and southward (e.g. 1000 mm in Buenos Aires; 600 mm in Bahia Blanca and 740 mm in Trenque Lauquen). This climatic characteristic gave rise to the inforreal subdivision of the area into Pampa hfimeda (humid Pampa) and Pampa seca (dry Pampa). The limit between both climatic zones is a transitional fringe which has shifted west during the last fifteen years because of remarkable increase of rainfall. Presently the rain averages 200-300 mm higher than the historical one; a wide area is now affected by catastrophic fioodings. Yrigoyen (1975) divided the province into positive and negative structural elements (Fig.2b). Tandilia and Ventania are considered as a single and major structural unit, named Positivo bonaerense. This structure separates the Colorado and Salado tectonic basins which represent negative structural elements (Fig.2b). Tandilia is a fault-mountain system (Teruggi and Kilmurray, 1975)consisting of discontinuous groups of low hills with absolute maximum elevation of 524 m above sea level. It is cornposed of metamorphic and igneous bedrock of Precambrian age, 2200 m.y. old, the oldest of Argentina, overlain by a Paleozoic sedimentary cover mostly composed of quartzite rocks. The Ventania range is a fold-mountain system (Harrington, 1947), with a maximum height of 1247 m above sea-level. It is mainly composed of Paleozoic quartzites while some igneous rocks of Late Precambrian and Paleozoic age are also present. Part of the Paleozoic cover comprises Carboniferous and Permian rocks which closely relate to similar stratigraphic units of the Gondwana supercontinent, The Colorado and Salado basins are bounded by steeply inclined normal faults with a significant vertical displacement. The sedimentary filling is composed of marine and conti-

nental deposits of Cretaceous and Cenozoic age. The bedrock is found at a depth of about 6000 m. The Colorado and Salado basins are presently undergoing subsidence (Introcaso and Gerster, 1985). Although some authors (Tapia, 1937; Kraglievich, 1953) pointed out the existence of tectonically disrupted Cenozoic stratigraphic units, no neotectonic movements have yet been confirmed.

Stratigraphy Fidalgo et al. (1975) and more recently Marshall et al. (1984) carried out an up-to-date revision of the stratigraphic and paleontological nomenclature of the Pampean region. Since the first stratigraphic scheme formulated by Ameghino, many others have proposed different schemes throughout the years. The result was a proliferation of local stratigraphies with the general system. Many new terms as well as the classic ones were misapplied to name same or different units. Intraregional correlation is very difficult because of similar lithologic characteristics of sediments of different age. On the other hand, the great variability of the depositional environments have resulted in different lithologic and morphologic properties of a layer of single age. Therefore, the recognition of a given unit is difficult and sometimes impossible when tracing laterally along discontinuous profiles. Among the numerous contributions dealing with the stratigraphy of the Pampean sediments, those by Ameghino (1889, 1908, 1909), Frenguelli (1928, 1950, 1957) and Kraglievich (1952, 1953, 1959) are judged to be the milestone achievements. In 1876, Ameghino divided the Pampean sediments into a lower part of Pliocene age (Formacion Pampeana) and an upper part that he called Postpampeano and which he assigned to Quaternary. Wind, tectonism and particularly water were supposed to be the main factors involved in the accumulation of the Pampean sediments. Later, in 1889 Ameghino published his most outstanding

31 contribution, in which, based on faunal assemblages, he established a stratigraphic and chronologic scheme dividing the Pampean sequence into 8 pisos (or horizontes). These are from the oldest to the youngest the Ensenadense, Belgranense, Bonaerense, and Lujanense (comprising the Pampeana Formation); Querandino and Platense (which correspond to the Postpampeano); and the topmost horizons, Aimara and Ariano (which represent the recent times). Ameghino also recognized a thin intervening marine layer (Interensenadense) within the Ensenadense that he used to subdivide the latter unit into a lower part (Ensenadense basal) and an upper part (Ensenadense superior). In 1908, Ameghino defined the Chapadmalense that he assigned a Miocene age. Finally in 1909, Ameghino identified the Preensenadense or Ensenadense inferior. Frenguelli (1957) refined Ameghino's framework. He suggested that the Chapadmalense, Ensenadense and Bonaerense were of the Pleistocene age, whereas the Lujanense marked the Pleistocene-Holocene boundary. He also introduced some modifications of the stratigraphy and chronology of the Holocene. Kraglievich carried out detailed stratigraphic and paleontologic work in the sequence exposed along the sea cliffs between Mar del Plata and Miramar. In 1952 he published his first results. He recognized lithostratigraphic units from the oldest to the youngest: Chapadmalal Formation (Pliocene): Barranca de los Lobos and Vorohue Formations (Early Pleistocene); San Andres and Miramar Formations (Middle Pleistocene) and Arroyo Seco and Loberia Formations (Upper Pleistocene). The ages assigned were based on the fossil contents. In 1953, Kraglievich separated the upper part of Arroyo Seco Formation into a new unit that he called Santa Isabel Formation. Finally, in 1959 he included the San Andres in the upper part of the Vorohue Formation. The lithologic variability of the sediments makes the recognition of these lithostratigraphic units very difficult. Their differentiation is only possible based from fossil content. The lithologic characteristics of the sediments

would support the existence of only one f o r m a t i o n ( T e r u g g i e t a l . , 1957). Although local stratigraphic columns are still in use, the PlioPleistocene sediments are now again grouped under the collective name of Pampeano or Pampeano Formation by some authors (e.g. Fidalgo et al., 1973; Fidalgo, 1979). Presently, the stratigraphy of the Pampeano is under revision. Taking into account its general character, allostratigraphic units seem to be the most suitable for stratigraphic subdivision. The units known as Interensenadense, Belgranense, Querandinese and Platense represent marine entities. They consist of thin mollusk bearing silts restricted to a narrow fringe which parallels the present coastline. The Interensenadense, found at about 7 m below the current level of Rio de la Plata (Ameghino, 1889) was considered by Frenguelli (1957) to represent a local inland penetration of the estuarine waters of Rio de la Plata. According to Frenguelli, the Belgranense was a lateral facies of the Bonaerense, whereas the Querandinense and Platense deposits represented the last transgressive and regressive hemicycles respectively. Recent investigations in the Salado basin by Fidalgo et al. (1973) and Fidalgo (1979) resulted in the recognition of three marine-related units: Pascual, Destacamento Rio Salado and Las Escobas Formations, which may be correlated to the Belgranense, Querandinense and Platense. Fidalgo concluded that Pascua and Las Escobas Formations correspond to two marine transgressions whereas Destacamento Rio Salado Formation are typical deposits of coastal lagoons and marshes. Mollusk shells from the Belgranense or equivalent units have yielded radiocarbon ages over 30,000 yr B.P. (Cortelezzi and Lerman, 1971; Cortelezzi, 1977). Samples of Glycymeris longior(Sowerby) taken from an outcrop in the Mar Chiquita area, 35 km northward of Mar del Plata, of what is thought to be an equivalent unit, was analyzed for amino acids by Dr. N. Rutter from the University of Alberta, Canada. According to the D/L ratio of aspartic acid, the deposit could be of Sanga-

32 mon Interglacial age or older (N. Rutter, pers. comm., 1988). In the same area, Schnack et al. (1982b) recognized an Holocene transgression with marine and estuarine facies formed during the regressive phase. The latter yielded radiocarbon ages between 3859 and 7340 yr B.P. These results are consistent with other radiocarbon dates for other regions within Buenos Aires province (e.g. Gonzalez and Weiler, 1983; Isla et al., 1986; Fasano et al., 1987). In Mar Chiquita area Holocene sea is thought to have reached a maximum of about 2-2.5 m above the present level (Schnack et al., 1982a and b). Both Pleistocene (Belgranense and Pascua) and Holocene beach ridges show similar heights in the Salado basin and in Mar Chiquita region, Lithology Pampean sediments which are mostly windblown are composed of a dominant silt fraction with minor amounts of fine sand and clay. The two latter increase in channels and floodplains. Psefitic fractions found in the channel lithofacies, consist of calcrete clasts and bone fragments. They include clasts of Precambrian and Paleozoic rocks in the piedmont areas of Tandilia and Ventania. The mineralogical analyses carried out by Teruggi et al. (1957) in the Chapadmalal sequence did not show differences in the composition of lithostratigraphic units recognized by Kraglievich (1952, 1953). The light minerals are mainly composed of plagioclase, not very abundant quartz, volcanic glass shards and volcanic lithoclasts (Fig.lib). Opaque iron minerals, amphiboles and pyroxenes are the most common heavy minerals. Gonzalez Bonorino (1965)found similar mineralogical assemblages in the city and suburbs of Buenos Aires. However, he recognized two different mineralogical zones with different montmorillonite/quartz ratio. The lower one has low illite/montmorillonite and kaolinite and plagioclase/quartz ratios. In the upper zone these ratios are higher,

The source area of the deposits, judged from their mineralogy, is located about 1000 km away, in the western and southwestern piedmont of the Andes (Teruggi, 1957). In the area of Buenos Aires city, the composition of the lower mineralogical zone reveals an important contribution from the Brazilian Shield. The upper mineralogical zone shows a source area similar to that of the Chapadmalal sequence. A secondary participation of the bedrock of the Pampean ranges, in the central part of Argentina is also recorded (Gonzalez Bonorino, 1965). Late Cenozoic volcanism in the Andes range played a remarkable role in the formation of the Pampean sediments. Ashfalls were very frequent throughout the Tertiary and the Quaternary, continuing until present times. The last major ashfall which affected all the eastern Pampas, took place in 1932. It came from the eruption of the Quiza Pu volcano, at the border of Argentina and Chile. In the northwestern part of Buenos Aires province, the ash layer buried by eolian sands is still observed (Camilion and Imbellone, 1984); while in the rest of the region it was mixed up by pedogenesis. Some thick ash layers of late Pleistocene age were also found in the surroundings of Mar del Plata (Fasano et al., 1984) and near Bahia Blanca. Although the sediments are generally labeled as loess deposits, primary loess is not very frequent. Most of the originally eolian material has been reworked and redeposited by either water or in some cases slope processes. Hence the term ~sedimentos loessoides or depSsitos loessoides". Besides, the pedogenesis and diagenesis have modified to a different degree the original characteristics of the sediments, especially those of Pliocene and Early Pleistocene age. The term "weathered loess" (Pye, 1987) may be used :to describe them. Late Pleistocene and possibly Holocene eolian deposits are p r o b a b l y the only ones which indisputably represent a true primary loess. Chronological indicators The dating of Late Cenozoic sequences is based on fossil vertebrate assemblages. Only

33

recently was it complemented by paleomagnetic results. Absolute dating methods were still not fully applied.

Epoch

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Vertebrate paleontology

South American Land M a m m a l Ages Lujanian

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South America was an island continent throughout most of the Tertiary period. The fauna which evolved in isolation was dominated by autochthonous endemic groups (Patterson and Pascual, 1972; Pascual et al., 1985). During the late Miocene a limited interchange of faunas between North and South America began. Later the emergence of the Panamanian land bridge during Pliocene brought about an extensive exchange of terrestrial faunas (Marshall and Pascual, 1978; Marshall et al., 1983, 1984). Pascual et al. (1965, 1966) defined separate land-mammal ages for the Cenozoic era based on the faunal sequence originally proposed by Ameghino. The three main criteria considered to formally define those ages are: (1) the stage of evolution of the taxa, (2) the change of faunal associations through time, and (3) the first and/or last appearance of the taxa in the fossil record (Pascual, 1984). The Plio-Pleistocene interval is represented by four landmammal ages. From the oldest to the youngest they are Montehermosan, Uquian, Ensenadan and Lujanian. A tentative chronology of these land-mammal ages has been proposed assuming periods of synchronization of Late Cenozoic interchange between North and South America (Marshall and Pascual, 1978) and also on the basis of some absolute dating methods (Marshall et al., 1979, 1982) (Fig.3). The authors carried out radioisotopic and magnetostratigraphic determinations of Late Tertiary sediments (Montehermosan and Early Uquian Ages) in the northwestern part of Argentina. No absolute data is available for Buenos Aires province. The Montehermosan Age which belongs to the Eopampean faunistic cycle includes among other, cariamid birds, mirmecophagid anteaters, rodents of the present eremic Ctenomydae, the water-loving Myocastortidae and

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Hydrochoeridae, the cursorial Dinomidae and varied Caviidae both represented by eremic forms (Dolichotinae) (Pascual, 1984, p. 21). The type-locality is Montehermoso on the Atlantic coast, near Bahia Blanca. A Chapadmalalan Age, upper part of the Montehermosan, has been tentatively proposed. This age is defined on the basis of the first known appearance in South America of the North American immigrants (e.g. Mustelidae) and the presence of autochthonous guide fossils (e.g. Dolicavia, Paraglyptodon, Scelidotheridium). This mammal age is not accepted by some authors (e.g. Marshall et al., 1983). It is represented by the Chapadmalal Formation, exposed along the sea-cliffs between Mar del Plata and Miramar. The Uquian Age is characterized by the first

34 major contingent of North American immigrants marking the faunal interchange between North and South Americas (Marshall et al., 1983). It also marks the last appearance of some marsupials, such as Thyiophorops, Thylatheridium, Sparassocynidae, and Argyrolagidae, the notoungulates Hegetotheriidae, the rodents Dankomys, Eucoelophorus, Pithanotorays and Eumisops. First appearance of Tayassuidae (Pascual, 1984, p. 22) is noted in the strata. The type locality is at Esquina Blanca, Jujuy province, northwest of Argentina (Fig.l) (Marshall et al., 1982). In Buenos Aires province this fauna is represented in Barranca de los Lobos, Vorohue and San Andres Formations, which overlie Chapadmalal Formation along the sea-cliffs between Mar del Plata and Miramar. The Ensenadan Age corresponds to the climax of the Pampean faunistic cycle. Some taxa exhibit a rather general trend towards the differentiation of giant forms (Pascual et al., 1966). The name derives from the locality of Ensenada, near La Plata. Guide fossils include Doedicuroides, Neuryurus, Theriodictis a n d Brachynasua (Marshall et al., 1984). The Lujanian Age is characterized by the last record of large numbers of Uquian and Ensenadan taxa (i.e. Proboscidea, Notoungulata, Glyptodontoidea, Megatherioidea, Litop-

terna, Equidae, Smilodon) with the appearance of many new forms which persist to the present as conspicuous representatives of the Neotropical region (i.e. Cavia, Holochilus) (Pascual et al., 1965; Marshall et al., 1984). This age derives its name from a locality near Buenos Aires city. A paleontologic criterion has been almost the only one used so far to define both the PlioPleistocene and the Pleistocene-Holocene boundaries. The Plio-Pleistocene boundary was supposed to lie within the upper part of the Uquian (Marshall et al., 1983). The appearance of a large number of Neoarctic mammals in South America has been considered as the indicator of the boundary. However, since the invasion of the Neoarctic mammals was gradual, this criterion can only be used as a

complementary one (Marshall et al., 1983). The Pleistocene-Holocene boundary is marked by the extinction of almost all large mammalian herbivores and their specialized predators such as ground sloths, glyptodons, proboscidean horses, notoungulates, litopterns, sabretooth cats (Marshall et al., 1984). Presently, Late Cenozoic land-mammal ages are under intensive revision. New fossil species are being collected with detailed stratigraphic control in different type-localities including the Chapadmalal sequence. Great attention is being paid to microvertebrates (J. Prado, pers. comm.).

Paleomagnetism Valencio and coworkers carried out paleomagnetic investigations in the Chapadmalal sea-cliffs as well as in Buenos Aires and La Plata cities. In Chapadmalal sea cliffs, Valencio and Orgeira (1984) studied the section known as Escalera de Barranca de los Lobos. According to Kraglievich's stratigraphic scheme (1952), the Chapadmalal, Barranca de los Lobos and Vorohue Formations are here exposed. The authors suggested the Gilbert to Early Gauss magnetic age for the mostly reversed Chapadmalal Formation (Late Montehermosan landmammal age). An early to middle Gauss magnetic age is assigned to the mostly normally magnetized Barranca de los Lobos and Vorohue Formations, both pertaining to the Uquian land-mammal age (Fig.4a). More recently Ruocco made known the results obtained from a profile located 7 km southwest of Barranca de los Lobos. All the units recognized by Kraglievich occur, except Chapadmalal Formation, comprising the Uquian, Ensenadan and Lujanian faunal Ages. Ruocco found these units mostly reversely magnetized and assigned them to the Matuyama epoch (Ruocco, 1987, and this issue). The magnetostratigraphy studied in the subsoil of la Plata and Buenos Aires cities, 60 km away from each other, comprise the Ensenadense and Bonaerense (conventionally taken as Mid to Late Pleisto-

35

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cene). B o b b i o et al. (1986) s u g g e s t a Late P l e i s t o c e n e age, n o t older t h a n o.aa M a for t h e B o n a e r e n s e . T h e r e s u l t s o b t a i n e d for t h e Ens e n a d e n s e w e r e subject to t w o a l t e r n a t i v e i n t e r p r e t a t i o n s b o t h o f t h e m p a r t i a l l y coinci-

dent, s u g g e s t i n g a " s e n s u l a t o " u p p e r m o s t L a t e P l e i s t o c e n e to Late P l i o c e n e age (Fig.4b). A c c o r d i n g to this i n t e r p r e t a t i o n t h e E n s e n a d e n s e w o u l d e n c o m p a s s a m u c h l o n g e r time span t h a n w h a t w a s t r a d i t i o n a l l y considered.

36 Results obtained in Buenos Aires (Nabel and Valencio, 1981; Valencio and Orgeira, 1983) support this interpretation, The s e d i m e n t a t i o n p r o c e s s as o b s e r v e d in the Chapadmalal sequence

The exposures of the Pampeano Formation inland are generally limited to scattered and incomplete sections, only a few meters thick, outcropping in river banks, roadcuts and quarries, The Chapadmalal sea cliffs between Mar del Plata and Miramar (Figs.5 and 6) present the best exposed and most complete section of the Pampeano Formation (Teruggi, 1957). This unit is observed in 30km of almost continuous exposures in thickness ranging from 12 to 25 m. The locality constitutes a key geologic site not only as a record of the Plio-Pleistocene boundary in South America but also because of the abundance of mammal fossil remains. Kraglievich's stratigraphic nomenclature is still accepted and cited without modifications by both paleontologists and magnetostratigraphers. For this reason it will be used here as a reference framework to facilitate discussion. The geologic age assigned to the units corresponds to the unrevised land-mammal ages.

Lithofacies and paleoenvironments The continuous exposures of the sequence permits the observation of its lateral and vertical changes through space and time. The following main lithofacies and paleoenvironments (summarized in TableI) h a v e been recognized (from the bottom up):

Chapadmalal Formation This unit i s composed of reddish brown, sandy siltstones. Krotovinas up to 20 cm in diameter and 2 m long are very frequent; many of them contain remains of fossil rodents (Fig.7a,b). They are mainly associated with paleosols. The lower boundary of the unit is not observed while the upper boundary is a sharp erosional surface. Two zones can be

differentiated; the lower one which comprises loess-like deposits with secondary aqueous reworking. It is completely devoid of calcium carbonate accumulations except for localized nodules of diagenetic origin. The upper zone consists of mainly fluvial deposits among which channel and floodplain facies were observed. Floodplain deposits are usually highly bioturbated by invertebrate burrows (Fig.7c). Calcium carbonate accumulations are frequent, located along bedding planes, scour surfaces, bioturbations, etc. The occurrence of the so called escorias (slag) and tierras cocidas (fired earths) although also present in some other overlying Pliocene to Early Pleistocene units, is one of the most striking features of the Chapadmalal Formation (Fig.7d). The objects were considered to be of volcanic origin (Outes et al., 1908), while others believed them to be of anthropogenic nature. More recently Cortelezzi (1971) suggested an epigenetic origin resulting from metamorphic processes. Other investigators took in account natural' fires to explain the genesis of these features (San Cristobal, 1985).

Barranca de los Lobos Formation This entity unconformably overlies the Chapadmalal Formation. The lower section of the unit is composed of fluvial channel lithofacies (Fig.8a) with paleochannels up to 400 m wide, deeply carved in the Chapadmalal unit. It grades upwards into secondarily reworked loess deposits which are bounded by paleosols (Fig.5a). Loess layers are weathered and show grayish hues which contrast markedly with those of the underlying reddish Chapadmalal sediments. The upper contact of the unit is a smooth erosional surface, not always accessible in the vertical sea-cliffs. Where it could have been observed, it is overlain by Vorohue Formation.

Vorohud Formation The basal part of this unit consists of channel facies composed of calcrete clast

37

SantaIsabelFm

J_o S

Barranca

de facto

o •

•

, , ,.

•

. , -.,

, -

-

. . ,,7

ChapadmalalFm z( ~ 25:

.,

f

,

..

•

{

Fm Lo SecoFm

Loberia Arroyo

5

MiramarFm lO

Vorohu~ Fm

15

Barranca delos Lobos Fm ";

-----]floodplainl i t h o f a c i e s

~

~ I o e s s

nodular calcrete

~chonnel lithofacies ~sand

~rnain paJeosoJs

~

~

diamicton lithofacies~calcrete crusts

krotovinas

Fig.5. Schematic stratigraphic sections. (a) Barranca de Los Lobos profile. (b) Seco creek profile.

conglomerates. They form lenses of variable thickness between 0.2-1m. They grade laterally to sandstones or conglomeratic sandstones overlain by crossbedded, medium to

fine silty sandstones (Fig.8b and 9a). The channels are wide and fiat bottomed with extremely gently sloping sides. Floodplain facies are dominant in the upper part of this

38

Fig.6. Chapadmalal sequence. (a) General view towards the south: C= Chapadmalal Fro; B = Barranca de Los Lobos Fm; p = paleosol level. (b) Seco creek section: V~ Vorohu~ Fm; S = San AndrOs Fro; M = Miramar Fm; A ~ Arroyo Seco Fm. u n i t (Fig.10b). T h e y are c o m p o s e d m a i n l y of massive siltstones with p o o r l y defined horizontal bedding. Rootcasts, i n v e r t e b r a t e b u r r o w s and d e s s i c a t i o n c r a c k s are common. M i n o r c h a n n e l s are present n e a r the top.

San

Andres

Formation

This u n i t is made up of floodplain facies c o n s i s t i n g of massive to locally l a m i n a t e d or slightly stratified c l a y e y siltstones with subor-

39 TABLE I Chapadmalal sequence: characteristics and relative age of the lithostratigraphic units Stratigraphic unit

Land-mammal age

General characteristics

Loberia Santa Isabel

-Lujanian

Loess deposits covered by coastal dunes Eolian sediments at the top; fluvial lithofacies with deep paleochannelsat the bottom

Arroyo Seco

Lujanian

Miramar

Ensenadan

Reddish. siltstone; local ponds deposits with hydromorphic paleosols; floodplain environment. Upper section" reddish to tan siltstone deposited by both eolian and fluvial agents; floodplain environment Lower Section: diamicton lithofacies; many large and steep sided paleochannels

San AndrOs

Uquian

Vorohu~

Uquian

Barranca de los Lobos

Uquian

Chapadmalal

Late Montehermosan Chapadmalalan

dinated, crossbedded, sandy siltstone lenses, Subsequent pedogenetic processes and calcareous accumulations have masked many of the primary depositional features. Sedimentation probably took place in shallow water ponds, The presence of mottles, Fe-Mn nodules and the general morphology of the paleosols and calcretes indicates either more humid conditions t h a n those prevailing during the deposition of Vorohue Formation or perhaps the effect of a perched water table,

Miramar Formation The lower section of this Formation is composed of channel facies made up of sandy coarse siltstone, containing angular calcrete clasts, either floating in a sandy pelitic matrix

Clay siltstone lithofacies of mainly eolian origin with some shallow water pond deposits; intense pedogenesis Upper section: dominant floodplain deposition Lower section: channel and floodplain ' lithofacies; large paleochannels common Upper section: weathered loess deposits bounded by paleosols Lower section:fluvial lithofacies, deep and wide paleochannels Upper section: fluvial deposition with channel and floodplain environments Lower section: sandy siltstones (loess?) bounded by conspicuous paleosol levels

or arranged in stringers and lenses (Fig.9b). The upper part of the unit is composed of tan to reddish brown, sandy siltstone, locally showing a slight parallel stratification. It is interpreted as floodplain facies with some loess deposits and minor aqueous reworking of the latter.

Arroyo Seco Formation This unit shows lithological features similar to those mentioned above for the upper part of Miramar Formation; it is separated from the latter by a calcrete crust.

Santa Isabel Formation Channel and floodplain lithofacies characterize this formation. The channel facies are

40 ~

-

i~

--~i

¸

- -

O

Fig.7. Chapadmalal Formation. (a) Krotovinas(k) cutting a Bt horizon of a paleosol(p). (b) Rodent jaw in a krotovina. (c) Invertebrate burrows presumably of freshwater oligochaetes (?). (d) Escoria ("slag") with its typical "vesicular structure". Coin diameter is 2 cm.

composed of u p w a r d fining sequences, with brechoid, c o n g l o m e r a t e lenses o f c a l c r e t e clasts~ a t the base, o v e r l a i n by low angle, crossbedded sandstones. Loess deposits are d o m i n a n t n e a r the top. T h e c h a n n e l s show depths of up to 8 m with steep sides. T h e floodplain facies consist of n u m e r o u s pond deposits composed of silty c l a y s t o n e with fresh water mollusks.

Loberia Formation It consists of a loess l a y e r g e n e r a l l y truncated and buried by d u n e sand. T o g e t h e r with S a n t a Isabel F o r m a t i o n it encompasses the L a t e P l e i s t o c e n e - H o l o c e n e interval. A c c o r d i n g to i n t e r r e g i o n a l c o r r e l a t i o n s the two units m a y r e p r e s e n t a time span not l o n g e r t h a n some 20,000 years.

41

O

O

Fig.8. Fluvial lithofacies. (a) Channel crossbedded sandstone (ch) in the lower section of Barranca de Los Lobos Fm; c= carbonate accumulation following a krotovina. (b) Vorohu4 Fm, channel lithofacies(s) crossbedded sandstone; f= floodplain lithofacies. Scale is 1.4 m long.

Paleosols Almost all the sedimentary units have been modified by pedogenesis, which left behind morphologic features such as root-traces, pedotubules, mottles and nodules. However, since in buried soils it is sometimes difficult to differentiate pedogenic from diagenetic features, some of the above (e.g. nodules and mottles) could also be related to diagenesis,

The paleosols are represented by incomplete, t r u n c a t e d profiles; the upper limit is usually an erosional surface and the lower, a gradational boundary. No A-horizons were recognized; in some places these could be masked by postburial changes. The profiles consist mostly :of either Bw, Bt, Btk or Bk horizons. The Bt and Btk horizons are more commonly preserved. Paleosols of the Chapadmalal Formation are represented by thick Bt horizons with well

42

Fig.9. (a) Vorohu~ Fm; c= conglomerate lense composed of calcrete clasts; dm---diamicton lithofacies; b=carbonte accumulations in invertebrate burrows. (b) Miramar Fm; drn=diamicton lithofacies in steep sided paleochannels; p = paleosol level; c = cutting filled up with laminated sandy siltstone (s). Scale is 1.4 m long. developed pedogenic f e a t u r e s (Fig.10a). Those o t h e r s which o c c u r in the o v e r l y i n g Pleistocene units, are c h a r a c t e r i z e d by b o t h Btk and Bk horizons (Fig.10b and c). M a n y paleosols o c c u r t h r o u g h o u t the seq u e n c e but only a few of t h e m are useful for s t r a t i g r a p h i c correlation. T h e s t r a t i g r a p h i c section located n e a r A r r o y o Seco (Fig.5) de-

picts t h a t c h a r a c t e r i s t i c . F r o m a total n u m b e r of 13 paleosols, only four h a v e a sufficient lateral p e r s i s t e n c e and degree of d e v e l o p m e n t to m a k e t h e m s t r a t i g r a p h i c a l l y significant. L a t e r a l l y e a c h paleosol level shows a v a r i a b l e degree of t r u n c a t i o n r e l a t e d to its paleotopographic location. The m o r p h o l o g y and m i c r o m o p h o l o g y of the

43

O n

_

.

Fig.10. Paleosols and calcretes. (a) Paleosols consisting of Bt horizons(p) in Chapadmalal Fro; (k) krotovinas. Ladder is 2,5 m long. (b) Vorohu~ Fin; regional paleosurfaces(P) in floodplain environments; t= Bt paleosol horizon; k - Bk paleosol horizon. (c) Vorohu4 Fin; detail of a regional paleosurface, (t)--Bt horizon; (k)=Bk horizon; pc=discontinuous platey calcrete; dm = diamicton lithofacies. (d) Pseudostromatolithic structure of a calcrete.

four paleosols indicate that they are the result of various superposed pedogenetic episodes (Zarate and Fasano, 1984; Zarate, 1985). Hence they are polygenetic paleosols (Morrison, 1978)

or profils complexes (Douchaufour, 1977). Towards the southwest, there is a slight but progressive deepening of the sedimentary basin during the deposition time of San Andres

44

Fig.ll. Microstructures and features of calcrete crusts. (a) Rhizoliths(r) with sparitic calcite in a dense micritic matrix(m) with organic matter. (b) Volcanic lithoclast(v) (andesite?) in a micritic matrix; o m = decomposed organic matter. (c) ooids(o) and organic filaments(]) in a bioturbation structure(?); h = host material. (d) Close up of organic filaments(D, some of them with a micritic coating; o = ooids.

45 and Miramar Formations. Accordingly, some paleosols are divided into several paleosols by intervening sediments, giving rise to a subdivided geosol (Morrison, 1978).

Calcretes The calcium carbonate accumulations, locally known as toscas, are widely distributed throughout the stratigraphic succession. The morphology of these calcretes varies considerably according to structural control by the host material (fractures, bedding planes, erosion surfaces, pedogenic structures, bioturbation structures) (Figs.5b 8a and 9a) and the different mechanisms that could have been involved in their genesis. Hence, calcretes display nodular, platey, reticular

and string-like

structural

patterns which sometimes occur in very close vertical association, thus producing striking and complex crusts (Figs.5b, 10b and c). To account for the origin of calcretes, several theories have been proposed. Gile et al. (1966) and Allen (1974) consider calcretes as part of soils. Van Zuidam (1975) proposes seven different mechanisms of formation while Goudie (1983) points out the existence of two main models of calcrete formation: the nonpedogenic or per ascensum model, and the pedogenic or per descensum model. Considering theircomplexmorphologies, the genesis of the Chapadmalal calcretes is thought to be related to combined mechanisms, i.e. both pedogenic and non-pedogenic processes. At the section near Arroyo Seco (Seco creek), five laterally persistent crusts have been recognized. The calcretes are named with letters from the bottom to the top (Fig.5). The petrographical and morphological features displayed by these calcrete crusts are indicators of the diagenetic conditions which existed in the vadose zone (Zarate, 1985), suggesting subaerial exposure (Figs.10b and 11).

Regional extent of the Chapadmalal sedimentary record The general features shown by the Chapadmalal succession are also present in other

localities of the Pampas. In the piedmont areas of Tandilia and Ventania, late Cenozoic sequences consist of several truncated paleosol profiles, frequently calcretized, overlain by a blanket of Late Pleistocene to Recent eolian sediments. In the surrounding area of Bahia Blanca, the Pleistocene sequences are similar to those of the Chapadmalal locality. Several paleosol levels made up of Bt horizons are developed on reworked loess in a semiarid fluvial environment. Towards the north, near Buenos Aires and La Plata, the sequence comprises the Ensenadense and Bonaerense. It consists of numerous paleosols composed of thick argillic horizons in close vertical association, while calcretes constitute nodule layers but not thick crusts. The difference is supposed to be the result of a general climatic gradient in the Pleistocene similar to the present one. Warmer and more humid conditions prevail to the north. Late Cenozoic c l i m a t i c i n d i c a t o r s

As many other regions of the world, the Pampas evolved under fluctuating climatic conditions during Late Cenozoic times. One of the first references to past climates was made by Ameghino (1876) who thought that dry conditions prevailed at the end of the Pampean Epoch, transforming the Pampas into a "true Sahara", in his own words. Later, Frenguelli (1957) recognized five climatic cycles consisting of pluvial and interpluvial periods which he correlated with glacials and interglacials. Each one of the stratigraphic units represented a complete climatic cycle with initially humid conditions (pluvial) during which primary loess was reworked, resulting in the origin of limos pampeanos (Pampean silt). More arid conditions followed (interpluvial) characterized by loess deposition. The Late Cenozoic record was established by alternating primary loess and reworked loess. Kraglievich (1952, 1953) also proposed cyclic climatic conditions to account for the characteristics of the Chapadmalal sequence. He used the same

46 general concept of climatic cycles as Frenguelli. However, being this an extraglacial, midlatitude region, the climatic cyclicity might have been different from the traditional scheme cited above. Tricart (1973) pointed out that the Pampas had undergone a climatic pattern similar to that of the southern part of Sahara in West Africa. Dry conditions prevailed in the region when glaciations were taking place in the Andes and humid conditions were dominant during the interglacial intervals, Pascual (1984) considers that the environmental and geological changes which took place in the southern part of South America at the end of the Tertiary were related to a major tectonic event, the uplift of the Andes. This event known as the Quechua phase caused a shift in the climatic pattern of the east-central part of Argentina. Following Pascual, the beginning of this environmental change is recorded by sediments pertaining to Friasian land-mammal age which extends from 16 to 12 m.y. At this time, a general trend towards more arid and cooler conditions which prevailed during the Pleistocene, should have started.

Vertebrates as climate indicators Fossil vertebrates have yielded a great deal of information about the Late Cenozoic climates. Judging from the Montehermosan assemblage the Pliocene environment was characterized by "seasonal differences in rainfall, somewhat similar to the present Chacoan phytogeographic province, with open xerophytic woodlands, but wetter" (Pascual, 1984, p. 19). The assemblage of the Uquian landmammal age suggests a generally warm ternperate climate. Pascual (1984) believes that throughout the rest of the Pleistocene as well as during the Holocene, the region was affected by the more variable cyclical climates, Tonni and Fidalgo (1982) studied a section exposed along the sea cliffs near Miramar. It yielded fossil remains corresponding to the Ensenadan and Lujanian land-mammal ages. Among others, they recovered remains of the

cricetid Reithrodon auritus, the marsupial Lestodelphis cf halli, the rodents Microcavia australis, members of the Edentata order such as Glyptodon clavipes, Mesotherium cristatum, Eutatus seguini and birds which belong to the orders Psittaciformes, Tinamiformes and Piciformes. All these fossils represent arid to semiarid open areas. According to the authors, the aridity was accompanied by a lowering of mean temperatures which at their lowest values permitted the presence of a Patagonian stock and at their highest, the Central and Pampean stocks. They also point out that the climatic fluctuations may have been related to glacial and interglacial epochs.

Flora Until now, palynological analyses carried out in the Chapadmalal sequence (Vorohue and San Andres Formations) have failed to identify pollen grains, presumably because the dominance of alkaline conditions prevented pollen preservation (A. Prieto, pers. comm., 1988). Siliceous epidermic cells of grasses (phytoliths) are frequently found in the Pampean sediments, suggesting steppe or prairie environments (Teruggi, 1957). This interpretation is supported by frequent grass root casts.

Paleosols and calcretes as climate indicators Several well developed paleosols occurring throughout the Pampean profiles are indicative of alternating periods of landscape stability and sedimentation. Thus, greatly changing environmental conditions can be assumed. It is still questionable to what degree are they caused by climatic changes. The sedimentation in a complex alluvial plain may have also been controlled by internal and external factors other than climate. No detailed studies about the genesis of the paleosols have been carried out as yet. The well developed illuviation features displayed may be interpreted as indicators of more humid periods, but this assumption is questionable, because not all the soil-forming factors are clearly understood.

47 Furthermore, postburial changes may be also responsible for some of the morphological features presently found. The only difference observed in the field is the presence of Bk and Btk horizons in Pleistocene paleosols and their complete absence in those of Pliocene age. This may be related to more acid soil environments which caused the leaching of calcium carbonate during Pliocene times. Calcretes are generally considered to be indicators of semiarid conditions. However, the main calcrete crusts are polygenetic, sometimes related to pedogenesis as well as to nonpedogenetic processes. In addition, micromorphology of some calcretes shows signs of intense biological activity. Although such calcretes may relate to generally semiarid conditions they may also indicate effects of increased seasonality of precipitation, or even more humid conditions than generally believed,

Lithologic climate indicators In terms of the dominant depositional processes, the morphology of the paleochannels and lithofacies records different types of aqueous transport agents. The most remarkable change is observed when passing from the fluvial lithofacies of Vorohue Formation, characterized by meander-like streams with occasional episodes of turbulent flow, to the diamicton lithofacies infilling the gullies of the Miramar Formation. The latter suggests dense flows (mud flows, debris flows) occurring probably under generally dry conditions, Based on mineralogical results, Teruggi (1957) pointed out that arid climate dominated during the transport and deposition of the eolian sediments. This statement is based on the lack of alteration shown by a great part of the loess minerals which are relatively unstable in weathering,

Geomorphology Deflation basins are widely distributed in a broad area north of Mar del Plata and the Salado river basin. The sediments, eroded and

transported by wind, form dunes and lunettes downwind. Dangavs (1979), describes clay dunes consisting of sand-size aggregates (pellet sands) with up to 30-40% clay which were formed in a Mid-Holocene deflation cycle when the marine regression began. In the same area silt dunes closely related to deflation basins are thought to be of Pleistocene age. Towards the western border of Buenos Aires province a complex sand dune field, presently stabilized, has been recognized (M. Gardenal, pers. comm., 1987). Various wind directions are observed within the dunes which might date back from the Pleistocene.

Discussion Van Andel (1981) suggested that the rocks record on land commonly represent only 1-10% of the total available time. Thus there would be short periods of deposition separated by long intervals during which erosion or soil formation could take place. Dott (1983) shows that the sedimentation episodes can be either positive or negative deviations, both of them capable of producing either surfaces or deposits. Paleosols are negative deviations whereas the scour surfaces and erosive unconfortuities mark positive deviations. The latter two features, paleosols and unconformities, document sedimentation gaps. The frequent occurrence of paleosurfaces consisting of paleosols and calcrete crusts throughout the Pampean profiles along with the erosive surface which usually bound each of the sedimentary units indicate that Late Cenozoic record of the Pampas is the result of an episodic sedimentation process. The physical record is far from complete. It was interrupted by several periods of landscape stability or on the contrary, landscape stability intervals may have been dominant and their general quiescence interrupted by depositional episodes. The diamicton lithofacies, which accounts for a significant part of the entire thickness of the Pleistocene sequence, is attributed to denseflow transport that normally takes place during

48

short intervals. Considering the geologic timescale, these depositional episodes may have represented almost instantaneous events. A look at the present geologic setting of the Pampas can give some clues for a better understanding of the Late Cenozoic record. The present landscape is characterized by highly energetic and catastrophic floods, lasting only several days, which occur about once in every 70-80 years. After these events, '~rare and abnormal" for a Pampean dweller, "stability" comes back. The processes during the PlioPleistocene might have been not very different from this situation; the sedimentary record seems to be the result of the superposition of low frequency events, probably catastrophic floods caused by unusual heavy rains that reworked and redeposited sediments, most of which w e r e p r i m a r y l o e s s , Dott (1983) pointed out that equating the thickness of a sequence with time to obtain a sedimentation rate is a very simplistic approach since multiple breaks may have taken place during the deposition. The Chapadmalal sequence demonstrates this well; some thick deposits were laid down very quickly while some paleosurfaces indicate long intervals of non-deposition. Finally, erosion can remove unknown thicknesses of former sediments. To obtain an accretion rate by the thickness/time relationship basically fails to recognize the real situation. To minimize errors, only accretion rates of similar sedimentary environments, with each section bracketed by paleosurfaces, should be used. The time recorded by paleosurfaces, could be estimated from the degree of development of paleosols and calcretes, assuming this degree to be time dependent with other factors being constant. To what extent is this assumption possible for the Pampean paleosurfaces? As it was mentioned previously, climatic fluctuations which took place in the region may have also produced changes in the biota. Hence the degree of development acquired by a paleosol profile is a function of the intensity of pedogenic process which in this case depends on both bioclimate and time, since relief and

parent material can be assumed to be constant (Zarate and Fasano, 1984). Coming back to present landscape, some Pampean soils, moderately developed, seem to have initiatedtheirevolutionduringthel0,5008000 yr B.P. interval. Later they underwent episodes of erosion and sedimentation (Zarate and Flegenheimer, in press). Thus 8000 years could be considered a minimum reference age for the fossil paleosols, which are polygenetic. With regard to the calcrete crusts, their degree of development is also a polyvariate function. Rates of calcretization vary according to the relative importance of each forming factor, and the genetic mechanism involved. Goudie (1983) gives the rates of calcretization ranging from a minimum of 0.5 mm/1000 yr, to 350 mm/1000 yr for pedogenic calcretes to a maximum of 3000 mm/1000 yr in some groundwater calcretes. Taking into account carbonate morphologies, the main calcrete crusts seem to be stages III and IV of Gile et al. (1966). Machette (1985) summarizes the approximate times required to attain the different morphological stages of calcretes, which depend on the rate of pedogenetic carbonate accumulation. Although his values cannot be considered as representative of the calcretization rates of the Pampean crusts, they are mentioned here only to demonstrate that the Pampean pedogenic calcretes may record i n t e r v a l s of many thousands of years. Howev.er, since Pampean crusts formed in a near-surface vadose environment by both pedogenic and nonpedogenic processes with the source of calcium carbonate not known, it is difficult if not impossible to determine the true formation rates. Future work will be concerned with determining whether the nonpedogenic processes (biological activity, fluctuations of shallow water table, evaporation from surface w a t e r s ) m a y have produced morphological horizons similar to those described by Gile et al. (1966). According to that previously mentioned, the time physically represented by the deposits of the entire Pampean sedimentary sequence may be far shorter than the length of the interval

49 during which the deposits were formed. The numerous paleosurfaces and unconformities which occur in the profiles may account for a dominant portion of the total Plio-Pleistocene interval. The length of the hiatuses is still not possible to evaluate. Because of this, the results of paleomagnetic studies must be interpreted cautiously and carefully checked with other dating methods. The direct correlation of polarity zones recorded by the sediments with the paleomagnetic chronostratigraphy involves a great deal of uncertainty since an entire paleomagnetic zone may fail to be represented in the sedimentary sequence. Misinterpretations could result. Furthermore, the absolute lack of radiometric control and the yet incomplete understanding of the stratigraphy makes the paleomagnetic results subject to more than one interpretation. In addition, the depositional environments and the pedologic and diagenetic processes involved in the evolution of the sequence must also be considered in future magnetostratigraphic work. Different types of indicators document the existence of climatic changes during Late Cenozoic time. During Montehermosan and Uquian mammal ages warm and wet conditions prevailed; the landscape might have been similar to present Chaco. Semiarid and cooler conditions alternated with warmer and more humid intervals during Ensenadan and LujanJan times; steppe and prairie environments probably dominated. In Late Pleistocene and Holocene times desert-like conditions existed in the western part of Buenos Aires province. Although these climatic fluctuations are probably related to the glacial and interglacial periods in the Andes, the available meager evidence does not allow yet a reliable extraregional correlation. Recently, Rabassa et al. (1985) suggested that a loess unit was deposited in the Pampas when "the Little Ice Age" occurred in the Patagonian Andes. Future work must be concerned with absolute dating of key localities and with their correlation across the region; only then can a reliable regional stratigraphy be established for the Pampas.

Conclusions

Aqueous transport agents played an important role in reworking and redepositing the originally eolian sediments on a flat landscape with low relief. The sedimentation process took place under fluctuating paleociimates. The degree of development acquired by paledsols was related to both bioclima*.e and time. The genesis of the calcrete crusts is related to surface or near-surface environments being the product of both pedogenic and non-pedogenic processes. There are also calcrete accumulations to be related to groundwater fluctuations or perched water tables; these affect buried Bt horizons. The paleosurfaces, indicated by paleosols and calcrete crusts record potentially long intervals of landscape stability and hence sedimentation gaps. Therefore, Pleistocene sedimentation was a discontinuous, episodic process with several intervals of long environmental stability. Depositional episodes of very short duration, such as marked by the diamicton lithofacies, played locally a dominant role in the sedimentation process. Sedimentation rates have to be estimated separately for sections bracketed by paleosurfaces, realizing that part of the record may be missing. Magnetostratigraphic results in this kind of environment must be especially carefully interpreted. Acknowledgements The authors wish to express their thanks to Dr. Peter W. Birkeland for his critical reading of an early version of the manuscript. We also are indebted to Dr. George Kukla for his constructive comments. Dr. Nathaniel Rutter made very valuable suggestions towards improving this paper. The manuscript benefited also from discussions in the field with Dr. Rosendo Pascual, Lic. Jose Prado, Lic. Aldo Prieto and Marcelo Gardenal. Constructive comments by professors and graduate students from the Department of Geology, University of Colorado at Boulder contributed to this paper

50

during the stay of one of our a u t h o r s (M. Z.). Figures were drafted by Virginia Bernasconi and J u l i a n a Bo. The linguistic aid of N o r a F l e g e n h e i m e r and Ellen W e n t is kindly acknowledged.

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