Long-term landscape development: a perspective from the southern Buenos Aires ranges of east central Argentina

Journal of South American Earth Sciences 19 (2005) 193–204 www.elsevier.com/locate/jsames

Long-term landscape development: a perspective from the southern Buenos Aires ranges of east central Argentina A. Demoulina,*, M. Zarateb, J. Rabassac a

Department of Physical Geography and Quaternary, University of Lie`ge, Sart Tilman, B11, 4000 Lie`ge, Belgium b CONICET/UNLPAM, Avenida Uruguay 151, 6300 Santa Rosa, La Pampa, Argentina c CONICET/CADIC, Laboratorio de Geologia del Cuaternario, Ushuaia, Argentina Received 1 December 2002; accepted 1 December 2004

Abstract Traditionally, the long-term landscape evolution of the southern Buenos Aires ranges of east central Argentina has been related to the influence of the Andean orogeny. We describe the large-scale morphological units and associated weathering products in the Tandilia and Ventania ranges. Two main planation surfaces are encountered at varying altitudes in different sectors of these ranges. The lower surface is characterized by roots of kaolinized weathering profiles in the Tandil area and silicified conglomerates around Sierra de La Ventana. In an interpretative model linking the range morphogenesis to the tectonosedimentary evolution of the bordering Salado and Colorado Basins, we suggest that the main morphogenetic stages are related to the late Jurassic-early Cretaceous south Atlantic rifting and Miocene tectonic reactivation induced by the Andean orogeny. Thus, the uplifted surfaces appear much older than commonly believed: pre-Cretaceous and Paleogene. Although they contradict recent results of apatite fission-track studies along the South America and South Africa passive margins, the implied low denudation rates (w4 m/My) can be explained by the limited Meso-Cenozoic uplift suffered by the southern Buenos Aires ranges. The discussion also shows the limits of the comparison that can be made with the South African planation surfaces. q 2005 Elsevier Ltd. All rights reserved. Keywords: Argentina; Denudation; Long-term landscape evolution; Passive margin; South America

1. Introduction During the past decade, apatite fission-track (AFT) studies often have led researchers to interpret landforms as much younger than previously thought (e.g. Miller and Lakatos, 1983; Brown et al., 2000; Kerr et al., 2000). Traditional geomorphological studies of planation surfaces frequently assign Mesozoic or early Cenozoic ages to extended surfaces in different parts of the world, mainly in passive margin contexts (South Africa, King, 1962; King, 1983; Partridge and Maud, 1987; Australia, Twidale, 1994; Sri Lanka, Bremer, 1981; South America, Bigarella and Ab’Saber, 1964; Zonneveld, 1985; NW Europe, Klein, 1990), justified by either the preservation of dated covering * Corresponding author. Tel.: C32 4366 5660; fax: C32 4366 5722. E-mail addresses: [email protected] (A. Demoulin), [email protected] (M. Zarate), [email protected] (J. Rabassa).

0895-9811/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2004.12.001

sediments (Demoulin, 1995) or, more questionably, geometrical relationships and associated weathering features. Such old ages imply very low denudation rates that have been contradicted by AFT data that yield, for example, rates on the order of 10–100 m/My for Brazil and South Africa (Gallagher et al., 1994; Harman et al., 1998; Brown et al., 2000). However, because the AFT technique’s resolution is unable to detect erosional signals of limited amplitude (!500 m) typical of areas of low tectonic uplift (Gunnell and Fleitout, 2000), the morphological approach remains an important tool for unraveling long-term landscape evolution in these regions. Unfortunately, the absence of correlative sediments on the land surfaces and uncertainty about their linkage to buried counterparts often requires information from offshore sedimentary basins or tectonic models. The ambiguity in such studies often comes from inferring tectonic phases from a pattern of stepped surfaces rather than explaining the pattern on the basis of a preestablished tectonic history (Partridge and Maud, 1987; Klein, 1990).

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This article aims to decipher the long-term morphological evolution of cratonic east central Argentina from a strict integration of landforms in a regional evolutionary scheme derived from independent tectonic and sedimentary data. We reconstruct the story from recent events and forms to more ancient events, which are decreasingly well documented. We then evaluate the reconstructed landscape evolution by examining how well the associated weathering features comply with the regional paleoclimatological data. Finally, we compare the denudation information provided by this tentative model with that obtained by AFT analysis in other areas. Our study is based on a field survey and the analysis of topographical maps and profiles. In the absence of high resolution DEM, terrain models such as GTOPO30 USGS EROS Data Center, 1996), with 30!30 in. mesh (w750!925 m in the study area) cannot reliably reveal the surface remnants preserved in the ranges, which makes a statistical extraction of topographical features at the desired scale unsatisfying.

2. The southern Buenos Aires ranges Located in the southern part of the Buenos Aires province, the ranges of east central Argentina contrast sharply with much higher ranges located farther north along the south Atlantic passive margin. Whereas the southeastern Brazil ranges have a mean elevation of approximately 1200 m (with elevations up to 2000 m) and form a margin escarpment, the Tandilia and Ventania ranges rise only

a few 100 m above the flat, low Pampean plain. They are isolated in a province of Cenozoic sedimentation (Fig. 1) and did not undergo episodes of strong rifting-related uplift similar to those that induced the development of great escarpments and high denudation volumes in Brazil or South Africa. Although the planation surfaces of central Argentina have received little attention, diverging opinions have been expressed about their evolution. Most authors relate the sequence of uplifted remnants found in the southern Buenos Aires ranges to tectonic pulses induced by the Andean orogeny. Thus, they ascribe the landscape evolution of the sierras chiefly to the Neogene and explain the stepwise arrangement of the surfaces by faulting (Keidel, 1916; Schiller, 1930; Teruggi and Kilmurray, 1975). However, Du Toit (1927), noting the similarities between the surfaces and weathering products of the Buenos Aires ranges and the corresponding features of the Cape province in South Africa, proposed a common Gondwanic origin for both landscapes and, therefore, an older, partly Mesozoic age for the Argentinean paleosurfaces. Such an old age of the main geomorphic features of the sierras also is acknowledged by Rolleri (1975). Furthermore, Bigarella and Ab’Saber (1964) attributed Cretaceous (i.e. Gondwanic) and Paleogene ages to the highest surface remnants (Pd4 and Pd3) that they described in southeastern Brazil. Recently, Pereyra and Ferrer (1995) have pointed out that the higher planation surface of the northeastern ranges of Ventania probably was formed in the time between the Permian collision of Patagonia and central Argentina and the late Jurassic

Fig. 1. Location map, with hatched basin areas. The dashed 200 m contour line roughly encircles the study area. Par, Parana Basin; Sa, Salado Basin; LP, La Plata craton; Co, Colorado and Macachin Basins; Ne, Neuquen Basin; Pat, Patagonian massif; SJ, San Jorge Basin; and Ma, Malvinas Basin.

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opening of the Colorado Basin. Rabassa et al. (1995) suggest that the relict landscapes of the Sierras de La Ventana and Tandil, along with the morphology of other cratonic areas of Argentina, should be reinterpreted with a Gondwanic perspective. The sequence of uplifted surfaces could be linked to the late Jurassic-early Cretaceous rifting of South America and Africa, and their intensely weathered bedrock might point to Mesozoic and Paleocene tropical conditions rather than to the cooler, drier Neogene.

3. Physiographical and geological setting The Sierras de Tandil (Tandilia) is a NW-trending, 350 km long, subdued mountain system consisting of ranges and hills that rise 50–250 m above the surrounding Pampean plain. The ranges are separated by valleys or the undulating plain (Fig. 2A). Maximum altitudes (O500 m) are found in the area of Tandil, progressively decreasing to 50 m a.s.l. to the SE and 70 m a.s.l. to the NW (Gonza´lez-Bonorino et al., 1956). The Tandilia range is made up of a Proterozoic crystalline basement that pertains to the Rio de la Plata craton (Dalla Salda, 1999), mainly composed of granitoids, migmatites, amphibolites, and hypabyssal igneous rocks unconformably overlain by Precambrian and lower Paleozoic sedimentary rocks (Teruggi and Kilmurray, 1975). According to Teruggi and Kilmurray (1980), the Sierras de Tandil are divided into three main sectors from NW to SE: the Sierras de Olavarrı´a, the Sierras de Azul and Tandil,

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and the Sierras de Loberı´a, Balcarce, and Mar del Plata (Fig. 2A). The Precambrian basement and a cover of upper Precambrian siltoquartzitic and dolomitic rocks crop out in the Sierras de Olavarrı´a. In the Sierras de Azul and Tandil, the sedimentary cover also includes Cambro-Ordovician quartzites. The Sierras de Balcarce and Loberı´a consist of granitoid rocks unconformably overlain by the subhorizontal cover of Cambro-Ordovician quartzites, which thickens to the SE and constitutes the bulk of the Sierra de Mar del Plata. A prominent NW-striking scarp (Costa de Heusser) that bounds the Balcarce-Mar del Plata ranges to the NE and corresponds to a major fault is interpreted as the boundary between the ranges and the Salado Basin (Teruggi and Kilmurray, 1975). Gradually merging into the plain to the SW, Tandilia is composed of faulted blocks whose altitudes increase from the ends of the chain toward the center. This arching may result from either differential block movements or the prefaulting development of a large anticline (Teruggi and Kilmurray, 1975). The morphological survey reported herein was carried out in the sectors that encompass the Sierras de Tandil and the Sierras de Loberı´a, Balcarce, and Mar del Plata. Sierra de La Ventana (Ventania) is a 180 km long, 60 km wide, NW-trending system of parallel ranges located approximately 200 km west of the Sierras de Tandil. It rises 400–700 m above the surrounding plain, with maximum elevations of up to 1240 m a.s.l. Ventania consists of two main elevation groups (Fig. 3A). The western group comprises the Sierras de Pua´n and Bravard and the higher ranges of Curamalal and Sierra de La Ventana. The eastern

Fig. 2. (A) Geological sketch map of the Tandilia range (LP, Los Padres; LV, La Vigilancia; Ba, Barrosa; ES, El Sombrero; LB, La Blanca; LJ, La Juanita; and AV, Alta de Vela). (B) Schematic longitudinal section of the Tandil range, 1, pre-Cretaceous surface; 2, Paleogene surface; and 3, Pliocene pediments. The Ordovician quartzite cover appears in grey. Hatched areas are weathered profiles within the granite. Circles on the main surface denote corestones. The flexure of the surfaces in the middle part of the section may be related to transverse structures in the nearby Salado Basin.

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group, including the Sierras de Pillahuinco´ and Las Tunas, does not form well-defined ranges but rather an area of rounded, subdued summits with altitudes of 600–750 m a.s.l. (Harrington, 1980). Our landform analysis mainly focused on the central and southern parts of the Ventania. Ventania is composed of Ordovician-Permian quartzites, sandstones, and shales overlying a poorly exposed Precambrian basement to the west (Andreis et al., 1989). The Paleozoic sequence underwent a NE-vergent folding phase followed by secondary shearing, faulting, and thrusting during the middle and late Permian (Von Gosen

and Buggish, 1989). In the Gondwana supercontinent, this folding also involved the Cape fold belt of South Africa and the Ellsworth and Pensacola fold belts of Antarctica (Du Toit, 1937; Dalziel and Elliot, 1973). It has been related to either the presence of an Andean-type margin with a wide magmatic arc-backarc system located SW of Sierra de La Ventana (Forsythe, 1982; Uliana and Biddle, 1987; Lo´pez Gamundı´, 1992; Lo´pez Gamundı´ et al., 1995) or a continent–continent collision with a subduction zone dipping under a separate Patagonian plate (Ramos, 1984).

Fig. 3. (A) Map of southern and central Ventania, with location of the topographical profile in (B). Light grey ranges are higher than 400 m a.s.l.; bold lines denote the main ridges. Hatched areas are 450 m surface. Stippled zones are Brecha Cerro Colorado outcrops. The Arroyo Pantanoso-Rio Sauce Grande fault zone separates differentially uplifted sectors of Ventania. (B) Relationships of the successive paleosurfaces across the central sector of Ventania.

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A gently rolling plain, known as Llanura Interserrana, extends between Ventania and Tandilia and is dissected by fluvial systems that mostly drain the nearby ranges and flow to the Atlantic Ocean. The elevation in its central part is approximately 200 m a.s.l. and gradually decreases toward the Atlantic coast. The scantily exposed sandstones and shales of the late Permian bedrock are overlain by a few tens of meters (locally up to 150 m) of Miocene marine sediments and late Miocene–Quaternary loess. Both mountain ranges, together with the intervening Llanura Interserrana, are regarded as a positive structural element: the Positivo Bonaerense (Yrigoyen, 1975), which separates two major tectonic basins, the Colorado to the south and the Salado to the north (Fig. 1). Extending offshore up to the continental slope, these faulted basins initiated during the late Jurassic-early Cretaceous in a complex system of interconnecting NNW- and WNWstriking horsts and grabens whose easternmost elements finally gave rise to the south Atlantic rift (Urien and Zambrano, 1973; Dingle et al., 1983; Nu¨rnberg and Mu¨ller, 1991). The direction of the Colorado Basin was determined by the NW–SE orientation of the Gondwanide fold belt accreted onto the SW margin of the Pampean–Brazilian shield (Urien and Zambrano, 1996). The orientation of the Salado Basin is inherited from late Precambrian structures. As failed rift arms (Burke, 1976; Juan et al., 1996), both basins display Cretaceous and younger sediments up to 6–7 km thick. At the base of the Salado infill, continental deposits interlayer with volcanic and volcanoclastic rocks associated with the early Cretaceous rifting phase. Above an angular unconformity, the next sequence corresponds to an incipient marine environment, still interrupted by continental episodes. From the Maastrichtian onward, both basins were repeatedly submitted to marine transgressions and regressions, with major phases of marine deposition taking place in the upper Cretaceous–Paleocene (thermal subsidence) and the Mio–Pliocene; a well-defined Eocene– Oligocene regression is documented by fluviodeltaic deposits.

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4. Planation surfaces and associated features of the Tandilia and Ventania ranges 4.1. Sierras de Tandil 4.1.1. Large-scale morphological units Along the southeastern segment of Tandilia, between Mar del Plata and Balcarce, the summit surfaces of the lower ranges, made up of Cambro–Ordovician quartzites (Los Padres, La Vigilancia, Barrosa), are fairly level at 200–250 m a.s.l. and stand 100–150 m above the surrounding Pampean plain. The elevation of these surface remnants progressively increases from the coast to Balcarce. They also display a southwestern tilt of approximately 28 following the dip of the Ordovician beds. Northwest of Balcarce, the crests of the eroded Precambrian hills belong to the same surface, which we call the main surface. They truncate, at altitudes of 300–350 m, Proterozoic granites whose quartzite capping rapidly thins and disappears (Fig. 2B). In this area, widely scattered remnants of a higher surface first occur (Cerro El Sombrero, 420 m a.s.l.; Fig. 4). Farther NW, the main surface remains at the same elevation to the Tandil area, where it is preserved on the summit of granitic hills. This change in its along-range slope evidences a slight warping of the surface, which is characterized by a gently rolling topography with a relief of a few tens of meters between 300 and 350 m. In the Tandil area, several remnants of the higher surface are preserved on the leveled summits of the highest sierras (La Juanita, Alta de Vela, Cerro La Blanca; Fig. 2). This older surface cuts into the Proterozoic granites at 450–500 m a.s.l., well above the altitude of the Ordovician quartzites of Balcarce. The third morphological unit encountered in Tandilia consists of alternating pediments and alluvial fans that surround the ranges and merge into the Llanura Interserrana at approximately 150 (Balcarce)–250 (Tandil) m. The present-day surface of the Llanura results from the accumulation of late Cenozoic loess.

Fig. 4. In the foreground, the main surface of the Tandilia range NW of Balcarce. In the right background, the Cerro El Sombrero (420 m a.s.l.), probably of the higher surface. Both are controlled by the presence of subhorizontal Ordovician quartzites. In the central hilly background, the main surface is slightly lower and becomes a pure erosional feature.

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4.1.2. Small-scale morphology and weathering features The higher surface developed on the granites of the Tandil area at 450–500 m a.s.l. is characterized by irregular topography. It is covered by granitic corestones but does not preserve any remnants of deep kaolinic weathering. The surface remnants appear as small massifs emerging from the main surface; their 25–308 regolithic slopes are similar to those described by Bremer (1981) in Sri Lanka. The main surface at 250–350 m displays various features depending on the lithology. In the Mar del Plata–Balcarce segment, the quartzites determine the occurrence of mesas. Due to the nature of the rocks, no trace of kaolinic weathering is present, but quartzitic gruss, sparse corestones, minor silica dissolution features, and some siliceous patinas are found. In the Balcarce, Tandil, and Olavarrı´a areas, thick altered sections of granitic rocks have been described and interpreted as the result of hydrothermalism (Di Paola, 1988). However, the many exposed weathering profiles in the sierras suggest that they belong to a once-ubiquitous weathering mantle. Near Balcarce, a few occurrences of kaolinized profiles, several meters thick, have been described in the granite that outcrops downslope of unweathered Cambro–Ordovician quartzites. Assuming that the bulk of the granite was weathered under the quartzitic capping, some authors have claimed that weathering dates back to pre-Ordovician times (Andreis and Zalba, 1986; Zalba et al., 1992). However, no direct observation of weathered granites overlain by fresh quartzites is available. Similar cases have been described in South Africa, where granites outcropping on hillsides below a quartzitic cover are surficially weathered but quite fresh a few meters inside the hill (Partridge and Maud, pers. comm.). We interpret these kaolinic occurrences as the deepest roots of an irregular weathering front, all the more likely because other profiles are observed in comparable locations in the Tandil area, whereas the quartzites are absent. On the granitoid basement west of Balcarce, the main surface shows a more lively topography, similar to that of the higher surface. Beyond the remnants of the latter, some subdued inselbergs dominate its gently rolling landscape. Moreover, the surface is covered by many granitic corestones that occasionally rest on a thin layer of gruss. Surrounding Tandil, the main surface is highly dissected, so some of its remnants at 300 m a.s.l. (Cerro La Movediza, Cerro El Centinela) appear to be boulder inselbergs rising above the lower landscape. The only weathering products associated with the main surface are located 50 m below it, in the hill slopes that lead to the lower surface (Fig. 2B). They consist of granitic corestones with concentric weathering rinds, embedded in situ within either more or less fresh granite (La Movediza) or slightly kaolinized granitic gruss (El Centinela). However, no pronounced kaolinization is present, which suggests that only the roots of a stripped irregular weathering mantle are exposed at this level. Higher on the hillside, flared slopes (Twidale, 1982) carved into the fresh granite confirm this interpretation. Uphill, the slopes are covered by regolith.

4.2. Sierra de La Ventana 4.2.1. Large-scale morphological units The various sierras that form the Ventania range culminate at varying altitudes and correspond to differentially uplifted blocks. The highest sierras are located west of a NNW-striking axial depression drained by the Rio Sauce Grande and the Arroyo Pantanoso and possibly corresponding to a fault zone (Fig. 3). To the west, the Sierras de Curamalal, de Bravard, and de la Ventana have preserved remnants of three paleosurfaces. The highest makes up the top surface of the ranges. Undulating between 800 and 900 m a.s.l. in midrange, it rises by 150 m in the southern part of the Sierra de la Ventana, where it is dominated by a few summits of up to 1240 m a.s.l. and descends to approximately 700 m in the northern Sierra de Bravard. The intermediate surface, of lesser extent, is carved at altitudes of 600–700 m (700–850 m in the southern Sierra de la Ventana) and cuts the steeply dipping lower Paleozoic strata some 150–200 m below the ridges. The extended lower surface corresponds to the Los Vertientes intramontane basin (sensu Bremer, 1967, 1981). Also called the main surface, it developed at 450–500 m a.s.l. within the axial depression of the Ventania. The valley of the upper Sauce Grande is incised by approximately 100 m in this surface. To the north, the main surface is formed by the 1 km wide longitudinal depression of Valle de las Grutas at 450 m a.s.l. and preserved in the external piedmonts of the eastern sierras (Las Tunas, Pillahuinco´) near the Llanura Interserrana. To the east, the Sierra de Las Tunas presents a pattern of stepped surfaces similar to that of the western ranges, except that the higher surface is at 700–750 m and the intermediate one at 550–600 m. However, only the higher paleosurface can be mapped on the still lower Pillahuinco´ range, in correspondence with its inclined top surface, with altitudes decreasing from 650 m in the west to 470 m in the east. Likewise, a single surface is mapped in the minor ranges located within the axial depression (Sierra Negra, Cordo´n Mambaches). Finally, pediments spanning outward into aggradation surfaces are developed along the external foots of the Ventania range, at 350–450 m altitude west of the Curamalal and Sierra de la Ventana and 300–350 m to the north, east, and south of the eastern Sierras, where they connect to the Llanura Interserrana. 4.2.2. Small-scale morphology and weathering features Due to a varied lithology, the paleosurfaces of the Ventania are less well leveled than those of the Tandilia for both the small-scale morphology of the surfaces and the extension of some levels. For example, the development of the Los Vertientes intramontane basin has been strongly controlled by the outcrop area of the easily erodible shales of the Devonian Lole´n Formation.

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Although kaolinitic remains are absent, other weathering features have been described. Silicified conglomerates and breccias occur at different places in the western ranges of the Ventania (Harrington, 1936) in close association with the main surface (Keidel, 1916; Zarate et al., 1995). Known as Conglomerado Rojo (Harrington, 1936) and then as Brecha Cerro Colorado (Andreis et al., 1971), these silcretes are present in the Valle de Las Grutas and on both sides of the Sierra de La Ventana (Fig. 3). Generally, they are located in front of small valleys that drain the ranges and consist of gravely–pebbly slope or alluvial fan deposits embedded in a white or reddish sandy matrix cemented by opal and Fe-sesquioxides (Andreis et al., 1971; Zarate et al., 1998). Recently, the Miocene age assigned to the silicification has been challenged by Zarate et al. (1995), who date the conglomerates from the Cretaceous or early Tertiary. However, the deposition of the conglomerates (in arid conditions?) and their silicification in a humid environment (as demonstrated by the presence of opal) were not contemporaneous. At Valle de Las Grutas, the silcretes determine the preservation of flat-topped remnants of the main surface along the eastern foot of the Curamalal. These silicified hills are cut by pediments covered by accumulated boulders (the Plio–Pleistocene Las Malvinas Formation).

5. Morphogenetic evolution: an interpretative model The erosional morphologies of the southern Buenos Aires ranges are very difficult to place within a consistent evolutionary frame. The relationships among surfaces in separate massifs are especially confusing because of the absence of datable deposits. Some authors have attempted to relate the surface pattern of the sierras to more or less remote events, such as the Andean orogeny (Keidel, 1916; Schiller, 1930; Teruggi and Kilmurray, 1975), or to comparable patterns in other areas of purportedly similar tectonic history, such as South Africa (Rabassa et al., 1995). However, we believe that it is more appropriate to relate the morphogenetic evolution of the sierras to the regional tectonic evolution and the associated sedimentation in the neighboring basins within the context of the south Atlantic rifting and South American passive margin evolution (Rolleri, 1975). Although not demonstrable, our correlation of the paleosurfaces in Tandilia and Ventania is based on the observation that both massifs belong to the same structural unit and extend along two basins of fairly similar Meso– Cenozoic evolution. They also display nearly the same sequence of stepped surfaces and associated weathering and connect at the same morphological level with the intervening Llanura Interserrana. Another problem is related to the arbitrary determination of the time at which the morphological evolution imprinted on the present-day landscape started. Many studies of stepped land surfaces start the morphological history of

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a given area at a seemingly ‘evident’ time of prolonged tectonic quiescence after a major tectonic event (Bigarella and Ab’Saber, 1964; Partridge and Maud, 1987; Demoulin, 1995). To avoid such an arbitrary start and in the absence of dated cover deposits, we try to establish gross links between the undated surfaces and regional tectonosedimentary events retrogressively, from the best documented recent landforms to poorly preserved older features. Only afterward do we propose a detailed evolutionary model based on these relationships. The three main morphogenetic stages we distinguish in the study area are (1) the present-day loess-covered Llanura to which the foot slope pediments connect, (2) the main surface at either 300 m (Tandilia) or 450 m (Ventania), and (3) the remnants of one (locally two) higher surface(s). We thus assume three periods during which denudation and planation were faster than tectonic uplift and two intervening periods of predominating uplift. Identifying the periods of increased uplift is the key to understanding the morphogenetic evolution of Tandilia and Ventania. Going back in time with the filling of the Salado and Colorado Basins, the first episode of increased sedimentation is found during the Mio–Pliocene (Urien and Zambrano, 1973). This period of rapid basin subsidence probably is a remote, slightly delayed consequence of the phase of intense shortening and uplift starting at approximately 26 Ma and culminating during the middle Miocene in the eastern Andean cordillera (Kennan, 2000; Quechua phase of Ramos, 1988). If we interpret this effect in terms of lithospheric buckling (Nikishin et al., 1997), the concomitantly induced uplift of the southern Buenos Aires ranges would have caused scarp development and initiated a new morphological ‘cycle’ in which the marginal pediments formed at the expense of the main surface. The latter ceased to evolve and thus may be dated from the Miocene. The other major period of sedimentation in the Salado and Colorado Basins is the upper Cretaceous (100–65 Ma), during which more than 2 km of continental red beds accumulated in both areas (Urien and Zambrano, 1973). This period of thermal subsidence followed the cessation of faulting in the Salado and Colorado rift arms after continental breakup occurred farther east during the Hauterivian (Burke, 1976; Nu¨rnberg and Mu¨ller, 1991; Brown et al., 2000). We assume that a significant part of the upper Cretaceous sediments was supplied by the erosion of the Ventania and Tandilia massifs, submitted to shoulder uplift when the Salado and Colorado grabens acted as developing rifts during the lower Cretaceous. A 0.5 km thick slice of rock, which corresponds to the height difference between the higher and main surfaces, was removed from the Ventania; denudation in the Tandilia and extended surroundings provided large amounts of sediments too. These regional details of the tectonic history of the south Atlantic opening yield the second period of increased uplift of the massifs needed to explain the older stair in the surface reconstruction. We therefore propose that the main

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Fig. 5. Morphogenetic evolution of the Pampean ranges and neighboring structural units from late Jurassic to present day exemplified by Ventania. I, preCretaceous surface; Ib, Intermediate (intra-Cretaceous) surface; II, Paleogene surface; and P, Pliocene pediments. Curved arrows indicate erosion leading to the formation of new surfaces.

surface of Ventania and Tandilia started to form during the Cretaceous, after the prerift landscape (higher surface) of the massifs had been uplifted. As a starting point of the Pampean range morphogenesis, we consider that the late Permian–early Triassic folding of the Ventania range (Lo´pez Gamundı´ and Rossello, 1998) was followed by an approximately 80 My period of tectonic quiescence that eroded eventually to form the higher surface of the sierras and ended with the late Jurassic–early Cretaceous south Atlantic rifting event. Although we do not know whether this prerift surface was unique or distinct levels existed (e.g. in the folded Ventania range, on the unfolded Permian sandstones of the Llanura interserrana), the relief of this ancient landscape likely was small except for some 200–300 m high inselbergs in the Ventania (Fig. 5A). In this sector of Gondwana, final breakup actually occurred along the eastern border of a mosaic of interconnecting grabens and horsts and thus resulted in a marked structural asymmetry between the margins (Brown et al., 2000). On the South African side, isostatic response to the rise of asthenospheric

material caused the development of the Great Escarpment. On the Argentinean side, no marked rift flank uplift was recorded. This might have responded to low heat production at a distance from the rift center or, more probably, to asymmetric rifting (Wernicke, 1985; Lister et al., 1986). Limited shoulder uplift associated with the pre-breakup subsidence of the Salado and Colorado Basins probably played a key role in isolating parts of the pre-Cretaceous landscape atop the NNW-striking Ventania and Tandilia uplifts. This hypothesis is supported by the slight ENE tilting of the higher surface and the altitudes of the different sierras in the Ventania range, which decrease from west to east and correspond to the outward tilting of an uplifted graben shoulder. On the external side (with respect to the Positivo Bonaerense) of the uplifted ranges, the new base levels were determined by the subsident Colorado and Salado Basins, whereas on their internal border, new surfaces began to develop, grading down to the prerift surface that had not been uplifted and continued to evolve within the more stable Llanura Interserrana (Fig. 5B and C). The presence of the

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intermediate, less extended surface on the highest ranges of the Ventania suggests that the uplift of the blocks was not continuous. Moreover, the occurrence of similarly stepped surfaces at varying altitudes in various ranges emphasizes the differential character of uplift in the Ventania. These events occurred after the opening of the south Atlantic, when the Salado and Colorado grabens had turned into aulacogens. An approximately 100 My period of stability, from the early Cretaceous to the late Oligocene, allowed the main surface to be carved from the ranges’ outskirts inward at the expense of the uplifted pre-Cretaceous landscape. Then, during the early Miocene, the increased rate of Nazca-South American plate convergence and resulting compressive phase in the Andean domain (Ramos, 1988) caused remote epeirogenic movements, notably the reactivation of basin subsidence and range uplift in east central Argentina (Fig. 5D). The main surface ceased to evolve. In the Ventania, unlike the higher surface, it occurs at similar altitudes everywhere in the range; thus, the faults that separate various blocks in the massif were active mainly during the early Cretaceous rifting phase, whereas the Ventania was uniformly uplifted during the Miocene, in accordance with the different styles of deformation recorded in the nearby Colorado Basin (Tavela and Wright, 1996). Again, while the sierras were uplifted, the Llanura Interserrana remained at a lower elevation. Thus, the Paleogene surface was buried under Mio–Pliocene continental deposits, mainly loess of the La Norma Formation (Fidalgo et al., 1975) (Fig. 5E). The last morphogenetic stage is characterized by the development of late Tertiary aggradation surfaces and simultaneous pedimentation at the foot of the uplifted Paleogene surface. These pediments connect down with either the top surface of the Mio–Pliocene sequence in the Llanura Interserrana or the surface of the Colorado and Salado Basins in the outer rim of the massifs.

6. Morphoclimatic consistency Our tentative morphogenetic model can be evaluated by considering its consistency with the available morphoclimatic and weathering data. The prerift higher surface, which is presently devoid of any trace of deep weathering in either range, has been submitted to planation processes that left a smooth topography littered with corestones and dominated by inselbergs. This characterization indicates that a late episode of surface stripping cleared the surface of all previous weathering products, isolating the inselbergs above the exhumed weathering front. Although surface stripping may respond to tectonic uplift as well as arid conditions, the latter agree with the warm, dry paleoclimates assigned to southern South America during the Jurassic (Hallam, 1985; Frakes et al., 1992; Sellwood and Valdes, 1997), which

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indicates that the removal of the weathering mantle may have started before the surface was uplifted in the Cretaceous. However, renewed weathering and denudation may have affected the surface during the Tertiary. The postrift main surface has preserved no further remains of deep weathering in the Tandil area, but especially where it cuts granites, it is covered by innumerable core boulders that indicate such weathering once existed. Moreover, the deeply weathered granite occurrences located somewhat lower than the surface may be interpreted as the roots of the kaolinic profiles associated with it. These observations fit paleoclimatic reconstructions of hot conditions, seasonally wet during the Cretaceous (Hallam, 1985) and more humid during the Paleocene, that are favorable to deep weathering. Later, Eocene and especially Oligocene cooling and drying of the climate may have been the cause of the weathering mantle’s erosion. Therefore, perhaps at the end of the Oligocene, the main land surface would have almost overtaken its weathering front, leaving only corestones in large areas and some weathered granite in more local sheltered places. However well this reconstructed morphogenetic evolution of the area agrees with its paleoclimatic evolution, we must remember that later episodes of weathering and surface stripping may have affected all surfaces to some extent, but generally speaking, the higher (and older) a surface is, the more complete the removal of its weathering mantle has been. A similar evolution may be assumed for the Ventania range. However, in this area, the weathering products veiling the main surface have been evacuated more completely. The lithologically controlled Los Vertientes Basin also indicates that pedimentation prevailed in the last planation stages of the drier Oligocene, before the Miocene uplift of the range interrupted its development. Furthermore, the gravely–pebbly nature of the silicified material on the main surface of the Ventania area and the absence of weathered underlying bedrock indicate that deposition took place in dry conditions, whereas later silicification occurred in a warm climate with marked seasonality (Zarate et al., 1998). The ages inferred from the integration of the planation surfaces into a regional tectonosedimentary evolutionary frame therefore are supported by the associated morphoclimatic data, which also fit with independent paleoclimatic reconstructions. In contrast, a young age for the surfaces is contradicted by the inconsistency between the Neogene climates and traces of extended bedrock kaolinization in Tandilia.

7. Long-term denudation rates In the last decade, estimates of long-term denudation rates, especially in passive margin settings, have been inferred from AFT studies (e.g. Brown et al., 1994; Summerfield, 2000). In several cases, AFT-derived

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denudation amounts appear to contradict the previously assumed antiquity of paleosurfaces (Miller and Lakatos, 1983; Brown et al., 2000; Cockburn et al., 2000; Kerr et al., 2000). For the South American passive margin, Harman et al. (1998) conclude that 3–7 km of rocks were eroded from the cratons of NE Brazil mainly after continental breakup I.E. at rates that reached 50 m/My. In SE Brazil, Gallagher et al. (1994) and Brown et al. (2000) claim more than 3 km of denudation on the coastal plain but only a little more than 1 km in the hinterland. However, recent AFT results of Tello Saenz et al. (2003) in Precambrian areas of SE Brazil are more compatible with the old ages of the preserved erosion surfaces. Similarly, the denudation we infer from our paleosurface reconstruction for the Ventania and Tandilia ranges is much smaller. If assimilated to the height difference between the pre- and postrift surfaces (to which we should add the stripping of the prerift landscape), the postrift denudation amounts to approximately 400 m in the Ventania and 200 m in the Tandilia during a 100 My period (130–25 Ma). Miocene uplift reactivation caused the erosion of a 100–150 m rock slice in approximately 25 My. All cases yield denudation rates of 2–6 m/My, or one order of magnitude lower than most AFT-derived rates. However, such small rates of erosion are not exceptional. In SE Australia, geomorphic studies (Bishop, 1985; Bishop and Goldrick, 2000) and numerical modeling (Van der Beek et al., 1999) indicate denudation rates of 1–10 m/My. Calculations by Summerfield and Hulton (1994) also yield low present-day fluvial denudation rates for several drainage basins of Africa and South America. In SW Africa, Cockburn et al. (2000) combine AFT and cosmogenic isotope studies and conclude that, in previously strongly denudated coastal areas, the denudation rate has fallen to approximately 5 m/My since the end of the Eocene. The very low denudation amounts deduced from our morphogenetic model are easily explained by the tectonic evolution of east central Argentina. The Ventania and Tandilia ranges did not suffer any significant syn- or postrifting uplift, probably because they were far from the rift center on a lower plate passive margin (Lister et al., 1986). According to Ahnert (1970), denudation depends on the slope gradient (extrapolating the curve of Granger et al. (1996) for small gradients gives denudation rates of 3–4 m/ My for slopes typical of planation surfaces). Steep slopes in turn are strongly correlated with high local relief (Summerfield, 1991), which depends on the importance of differential tectonics and subsequent lowering of base too levels. Therefore, the driving factor of any episode of kilometer-scale denudation remains an initial base-level change of some importance, often related to strong differential uplift. The limited uplift suffered by the Ventania and Tandilia massifs could not develop a relief sufficiently high to promote active denudation. Subsequent denudational rebound (Gilchrist and Summerfield, 1990)

was thus insignificant too, and the total amount of denudation remained small.

8. Conclusion The landform analysis of the Ventania and Tandilia ranges leads us to propose a morphogenetic model that emphasizes the old age (Paleogene or even Mesozoic) of the greatest part of the landscape. This model includes two range uplift episodes that match phases of increased subsidence and sedimentation in the Salado and Colorado Basins. They responded, respectively, to the south Atlantic rifting and the Neogene reactivation of structures located in the remote foreland of the Andean orogen. The chiefly detritic origin of the basin fills supports the importance of continental erosion during these periods, marked by the stripping of thick weathering mantles and the development of new surfaces. This landscape antiquity consistently integrates available morphoclimatic data. It also confirms the interpretation by Rabassa et al. (1995) that the main tectonic event interfering with Pampean longterm morphogenesis has been late Jurassic–early Cretaceous rifting of the south Atlantic. Therefore, these authors are justified in unifying the pre-Cretaceous surfaces on both sides of the south Atlantic into a single topography. Theoretically, such a correlation can hold only for the prerift surface, but the conclusion of Ollier and Marker (1985) that the Great Escarpment of South Africa separates two surfaces of distinct age leads to a perfect match of the surface succession on both sides of the south Atlantic, though the respective postrift surfaces had a long separate history subsequently. The asymmetric character of the south Atlantic rifting induced different uplift amplitudes in South Africa and Argentina, opposing the Great Escarpment on one side to insignificant along-rift slopes on the other. Denudation was consequently much smaller in east central Argentina than along the South African coast. Finally, the poor preservation of kaolinized bedrock on the Argentinean surfaces with respect to the extended weathering mantles covering African or Brazilian surfaces may point to a superimposed climatic influence on denudation effectiveness.

Acknowledgements This article is dedicated to the memory of the late Drs. Marcelo Yrigoyen and Miguel Uliana, who recently passed away and played leading roles in understanding the evolution of southern South America. Field trips were supported by the Comision de Investigaciones Cientı´ficas de la provincia de Buenos Aires. The authors gratefully acknowledge funding by CONICET/Argentina and the Belgian National Fund for Scientific Research. This joint work has been carried out within the framework of

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the Belgo-Argentinian Agreement on Cooperation in Scientific Research.

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