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Upper Quaternary geology of the lower Carcarañá Basin, North Pampa, Argentina
Quaternary International 57/58 (1999) 135—148
Upper Quaternary geology of the lower Carcaran a´ Basin, North Pampa, Argentina Daniela M. Kro¨hling CONICET — Facultad de Ingenierı& a y Ciencias Hı& dricas, Universidad Nacional del Litoral, C.C. 495, (3000) Santa Fe, Argentina
Abstract The geological evolution of the lower Carcaran a´ Basin reflects a sequence of dry and humid climates. From a geomorphological point of view, the area belongs to the Pampean Eolian System. The geomorphological units comprise several fluvial geoforms, partially masked by a loess carpet, on which eolian erosive forms developed. The sedimentary record is not simple. The typical profile begins with a predominantly eolian unit, composed of silty sand and including two pedogenic levels (Carcaran a´ Fm). The formation is the result of the reworking of a dune field in the Pampean Sand Sea (IS4). It is upper Pleistocene in age (IS 3). Another, somewhat different, climatic deterioration followed (related to the begining of the Last Glacial Maximum -IS 2), which caused the deposition of a loessic mantle (Tezanos Pinto Fm). Subsequent evolution of the climate involved a shift to humid conditions during the Hypsithermal period, generating a well-developed soil at the top of the formation. An upper Holocene eolian layer (San Guillermo Fm) covers the whole sequence. At the river margins of the basin the late Pleistocene/lower Holocene eolian deposits are replaced by a complex silty—clayey sequence, mainly paludal, with a soil complex in the middle of the section (Lucio Lopez Fm). 1999 INQUA/Elsevier Science Ltd. All rights reserved.
1. Introduction The lower basin of the Carcaran a´ river constitutes a favorable area for research on geomorphology and Quaternary stratigraphy of North Pampa (Kro¨hling, 1995a and b). This study is part of a research project on the paleoclimatic sequence of the pampean region during the upper Quaternary, building upon reconstructions of scenarios for different climatic periods which affected in the plain (Iriondo and Garcia, 1993; Iriondo and Kro¨hling, 1995, 1996; Kro¨hling and Iriondo, 1995). The overall results of that study are presented in another contribution of this volume (see Iriondo). The main objective of this contribution is the synthesis of evidence for environmental changes in the area during the last 65,000 yr, based on interpretations of the sedimentary record and geomorphological analyses, carried out as part of our own research. The study area occupies 14,400 km in the south-east of the province of Co´rdoba and south-center of the province of Santa Fe (approx. lat. 32°15 S to 33°30 S and long. 60°45 W to 62°30 W). The Carcaran a´ river lies
within the Rı´ o de la Plata basin. It drains region extending from the eastern slope of the Sierras Grandes and Comechingones (Sierras Pampeanas) to the lower Parana´ river (Fig. 1). The area has a subtropical humid climate, with a mean temperature of 17°C and precipitation increasing from west to east; averages oscillate between 900 and 1000 mm. The natural vegetation is a pseudosteppe, in general developed on Argiudolic soils. According to Iriondo (1987) the geological evolution of the region can be explained in terms of sedimentary systems: the Pampean Eolian System, alluvial fans of the Sierras Pampeanas and the Parana´ river belt. The first of these had a considerable influence in the study area during the Upper Quaternary. The geomorphological units of the area comprise fluvial geoforms (mostly developed on the older unit of the Pampean Sand Sea), partially masked by a loess mantle on which mainly eolian erosive forms were developed. The sedimentary column of the area reveals fluctuating environmental conditions, characterized by dry intervals, associated with episodes of formation and remobilization of the Pampean Sand Sea and of the Peripheral Loessic
1040-6182/99/$20.00 1999 INQUA/Elsevier Science Ltd. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 9 8 ) 0 0 0 5 5 - X
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investigation of the processes responsible for their generation. During nine expeditions to the area, involving 46 days of intensive fieldwork, a rigorous analysis of each identified geomorphological unit was carried out. 2.2. Regional outline Fig. 1 shows the main units of the study area, taken from the Geomorphological Map of the Plains of South America (Iriondo, 1990a, 1992). The principal tectonic and sedimentary characteristics of the region are the following: Fractures several hundred of kilometers long, with modest topographical expression of a few meters. Two of these major fractures cross the basin (Fig. 1): the TostadoSelva fault (Middle Pleistocene in age and reactivated in the Holocene) and a fracture controlling the course of the Parana´ river in the sector analyzed (Iriondo, 1989). Minor fractures, parallel to the larger ones, form an orthogonal pattern, and define a series of tectonic blocks tilted towards the east. From a sedimentological point of view, the major part of the area is dominated by the Peripheral Loessic Belt of the Pampean Eolian System (PES), except the southwest border, which is part of the transition to the Pampean Sand Sea (Fig. 1). The northwest sector of the area is covered by sediments belonging to the alluvial fan of the Tercero river. Sediments deposited by the Parana´ river are present below surface in the eastern sector of the area. The delta of the Parana´ river is located immediately to the east of the area, and was developed during the Hypsithermal Period (Iriondo and Scotta, 1979). 2.3. Geomorphological units Fig. 1. Geological Map of the region — taken from Iriondo (1992).
Belt during the stages of Glacial Maximum, alternating with humid intervals characterized by pedogenesis and the development of fluvial belts. General outlines of the Quaternary of South America are given by Clapperton (1993) and Markgraf (1993).
2. Geomorphology of the area 2.1. Methodology The methodology used in the geomorphological study of the basin involved small-scale analyses using satellite images and detailed photointerpretation, supported by the study of topographical maps. These studies were controlled by fieldwork, involving the classification, description and interpretation of geoforms, including
The area in the province of Santa Fe was mapped in terms of the geomorphological classification developed by Iriondo (1990a) and by Iriondo and Ramonell (1993), which has a hierarchical structure. The area conforms to the following regional pattern: E E
E
First level. Geomorphological Province: Pampa Plain. Second level. Type of Geomorphological System: Pampean Eolian System (Iriondo, 1987). Third level. Geomorphological Subsystems: Pampean ¸oessic Subsystem and Pampean Sand Sea Subsystem.
Most of the area is part of the Pampean Loessic Subsystem (PLS); only the southwestern sector corresponds to the zone of transition to the Pampean Sand Sea (PSS). Eight geomorphological units (GU) occur in the mapped area (Fig. 2): 2.3.1. G.U. Fluvial terraces of the CarcaranJ a& river These form the fluvial belt of the Carcaran a´ river above a fracture of NW—SE strike, located at 32°45 S and 61°15 W, which impeded terrace development
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Fig. 2. Geomorphological scheme of the Carcaran a´ river basin in the province of Santa Fe.
downstream. Two levels of continuous terraces are associated with the present river channel; intermediate terraces appear locally. The present river channel has not developed a floodplain. The significant elements of the unit are: an upper terrace level (mean width: 3000 m) and the outer edge of this plane, a lower terrace (common width: 300 m) and its outer edge, and the discontinuous planes of intermediate terraces. Numerous palaeochannels of the Carcaran a´ river cut the upper terrace. The present channel of the river, bounded by cliffs (in parts cut by gullies), shows a meandering pattern inherited from a period in which its energy was smaller than at present. Associated elements are inactive alluvial fans (related to fluvial tributary
valleys) and deflation hollows, some of which hold temporary lakes. 2.3.2. G.U. Fluvial belt of the lower CarcaranJ a& river This extends from Unit I to the younger channel of the Carcaran a´ river. It is formed by an upper continuous terrace (mean width: 2500 m) and a narrow lower level (of variable width between 100 and 300 m) in which the river flows. This latter level was partially filled by the Lucio Lopez Fm, which possess thixotropic characteristic. Large soil flows of this formation toward the river occurred in recent times, leaving numerous depressions on the slope of this belt (depressions are 1—3.5 m deep and 50—300 m long). The bottom of these depressions is
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horizontal, with gullies and swamps. The lower narrow belt constitutes the distinctive element of the unit; it is the downstream prolongation of the lower terrace of Unit I. The river channel flows at the centre of this level, whose edge is well marked. Elements associated with the channel are cliffs, gullies and a waterfall. Some significant elements of the unit are: the terrace with its outer margin and palaeochannels of the Carcaran a´ river. A palaeochannel of the Parana´ river, buried at shallow depth, is visible in the superficial geoforms. Elements associated with Unit I also appear in this Unit. 2.3.3. G.U. Fluvial valleys of Villa Eloı& sa Typical elements of this unit are tributary valleys of the Carcaran a´ river. Elements of the original valley landscape are covered by a mantle of loess, with superimposed episodic channels. Valleys are in general asymmetric with typical widths between 800 and 1200 m, represented largely by lateral slopes, and characteristic depths between 5 and 7 m. The largest valley is 2700 m wide and 15 m deep. At the junction with Unit I the old valleys are linked to it by inactive alluvial fans. The high frequency of valleys makes a landscape of well-defined hills, formed by simple, long slopes. A small number of deflation hollows are excavated in the top of the loess formation. 2.3.4. G.U. Fluvial paleovalleys of CanJ ada de Gomez This unit is characterized by a well-defined palaeodrainage pattern, formed by a series of paleovalleys of the Carcaran a´ river, which are approximately parallel to the present general direction of the river. They are asymmetric, complex valleys, masked by a loess cover; they are interpreted as polygenetics belts, affected by neotectonics. The main valley is 4900 m wide (represented mostly by lateral slopes) with a typical depth of 18 m. Some episodic channels and gullies are developed, mainly in the bottom of the paleovalleys; numerous palaeochannels are present. Other associated elements are deflation hollows and depressions generated by neotectonics. 2.3.5. G.U. Old plain of Bustinza This is a relict landscape, not much affected by fluvial processes, and masked by a loess mantle. There are minor palaeovalleys and parallel palaeochannels with a general direction towards the Carcaran a´ river; deflation depressions are conspicuous forms. In the field, the elementary geoforms of the unit are gentle and long slopes with low relief. Episodic channels and gullies were developed in the valley bottoms; some depressions and scarps of structural origin affect the regularity of the plain.
A minor terrace level formed by neotectonics follows the younger segment of the Carcaran a´ river next to the river outlet. In the western sector of the area, near the Tortugas tributary, a series of well-defined hills appear in the landscape; these are interpreted as partially eroded dunes, covered by a loess mantle. 2.3.6. G.º. Fluvial palaeovalleys of Berabevu& This unit is formed by allochthonous palaeovalleys with palaeochannels in their floors. Small channels, older than the palaeovalleys, form a colinear pattern. These geoforms are covered by eolian sediments. The largest palaeovalley has a symmetrical profile, 1.5—3 km wide and 6—18 m deep. The bottom is flat, and occupied by a 300 m wide palaeochannel. Iriondo (1987) interpreted these palaeovalleys as geoforms generated by the Quinto river, with headwaters in the Sierras de San Luis (Sierras Pampeanas). The southwestern sector of the area comprises the transition between the Pampean Sand Sea (PSS) and the Pampean Loessic Subsystem (PLS) of the Last Glacial Maximum. The transition zone has a width of the order of 10 km. The landscape is dominated in some areas by partially dissipated dunes (typical of the PSS), alternating with practically level sectors corresponding to the PLS. Numerous deflation depressions occur in these sediments, being more frequent and better developed in the sectors where the eolian sand predominates, especially in palaeochannels, where they reach 500 m in diameter. Some depressions have become semi-permanent and permanent shallow lakes. Gullies and episodic channels occupy the palaeovalleys. 2.3.7. G.U. Palaeochannels of Luis Palacios This unit is defined by a series of gentle, aligned depressions, which form a large arc of 10—15 km radius. The depressions, with typical widths of 1.5 km and maximum widths of 3.5 km, are partially flooded. The similarity of the pattern of depressions with the channel of the Parana´ river near the study area is noteworthy. One can therefore conclude that a palaeochannel of the Parana´ river, covered by a thin layer of loess, is responsible for the unit. A system of minor palaeochannels, parallel to the general course of the Carcaran a´ river, is older than these large, aligned depressions. Numerous deflation hollows affect the palaeochannels; some of them are occupied by semipermanent shallow lakes. Tectonic depressions, episodic channels and gullies are common in this unit. 2.3.8. G.U. Fluvial valley of Tortugas This comprises the asymmetric valley of a small river, the Tortugas, which exhibits strong structural control. The valley is covered by eolian deposits.
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A fluvial belt in the bottom of the valley is partially occupied by the channel; the belt includes palaeochannels and deflation depressions. The lateral slopes of the valley are well defined; the left corresponds to an attenuated fault scarp, 50 m high, the other constitutes a simple slope. Four kilometer long tributary valleys have developed by headward erosion on both sides of the fluvial belt.
3. Upper Quaternary of the area 3.1. Methodology Extensive fieldwork in the course of nine expeditions resulted in the recognition and description of several geological formations of Quaternary age. The Palaeopedology Manual of INQUA (Catt, 1990) was used in the study of the palaeosols. The colour of the sediments was recorded in accordance with the Standard Soil Colour Chart of the Agriculture Ministry of Japan. The investigations were complemented by routine granulometric and mineralogical analyses in order to obtain a preliminary characterization of the geological units. A series of datings by thermoluminescence has established the absolute ages of the formations. Datings were made in the Thermoluminescence Laboratory of Peking University (China). 3.2. Geological formations A general characterization of the geological units is presented here. Some outcropping formations have already been defined by other authors; the remainder are defined in this paper (Fig. 3). 3.2.1. CarcaranJ a& formation This is composed of very fine to fine silty sand, and silt with very fine sand, with clays rarely present, little visible muscovite, bright reddish-brown wet colour (5YR 5/6), and dull orange dry colour (7.5 YR 6/4). It has very thick horizontal beds, not well defined and generally with concordant contacts. The sediment varies in consistency between friable and consolidated. It has a strong, fine to medium blocky structure. In general, it is non-calcareous. It is affected locally by bioturbations, such as crotovinas (up to 0.75 m diameter) and root casts (up to 0.60 m long and 0.10 m diameter). In some places sedimentary structures relating to internal translocation of material are visible, marked by differences in concentrations of colloidal materials. These structures are characterized by an irregular wavy pattern. This unit forms the lower sections of cliffs along river channels, forming gentle to subvertical slopes, which reflect its resistance in comparison with the upper units. The maximum outcrop thickness is some 5.50 m. It is
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discordantly covered by the Tezanos Pinto Fm in the interfluves and by the Lucio Lopez Fm in the main valleys of the area. The formation, mainly of eolian origin, is the result of the reworking by erosion of a dunefield in the Pampean Sand Sea (PSS; IS4). Primary structures of the dunes of the PSS appear in profiles located immediately to the south of the area. Locally different paludal and alluvial facies are present. A representative profile of the unit is present in the cliffs of the Can ada de Gomez river, at its junction with the Carcaran a´ river. From bottom to top the following sequence is present: 0.00—2.00 m: Stratum formed by very fine, slightly silty sand, bright reddish brown when moist, medium dense, with translocation structures visible in units 1.5 m long and 0.10 m thick. Non-calcareous. Locally, in units 0.15— 0.70 m thick and of 1—1.50 m in lateral extent, the sediment is covered with brown cutans, and contains medium to coarse angular hard nodules, coated with iron sesquioxides. There are rhizoconcretions of CaCO up to 5 cm long and 1.5 cm in diameter and abundant very fine macropores. The stratum is characterized by an orthogonal system of joints. Concordant upper contact. 2.00—4.50 m: Stratum sedimentologically similar to the lower unit, without internal structures, and with less resistance to erosion. Numerous rhizoconcretions of CaCO at the contact between units. 1 m below the contact crotovinas up to 0.50 m in diameter (some including carbonaceous remains) were observed. Close to the top there is a level formed by a succession of erosional geoforms comparable to yardangs, 0.50—0.80 m wide and 0.40—0.50 m deep. (Top of Carcaran a´ Fm). A representative section of the paludal and alluvial facies of the formation outcrops in the Carcaran a´ river cliffs, next to the city of Carcaran a´. These facies constitute a filling up sequence of a 100 m wide channel. From the water level to the top the following units are present: 0.00—2.90 m: Paludal facies, comprising very thick lenticular strata. It is composed of very fine sand with uniform silt and clay, dull orange (7.5 YR 6/4) in colour, partially covered by brown films (5 YR 4/6). Moderately consolidated, weakly structured in irregular peds, very firm. In some levels there are concentrations of fine to coarse hard nodules, compact, coated with films of iron sesquioxides, and associated with manganese dendrites. Numerous root casts covered by similar films. Abundant very fine and fine macropores. Crotovinas up to 0.30 m in diameter. Non-calcareous to very slightly calcareous. Locally, the sediment is laminated. The exposed surface contains spherical and ellipsoidal depressions up to 3 cm in diameter, developed in a less resistant epigenetic phase. The
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Fig. 3. General stratigraphic profile of the Carcaran a´ river basin.
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stratum is traversed by a system of vertical, orthogonal joints. The upper contact is represented by an intraformational unconformity. 2.90—5.30 m: Alluvial facies. At the base, lenses 5—6 m wide and 0.05—0.10 m thick, composed of very fine pebbles, derived from the lower stratum, are supported in an internally laminated sandy coarse matrix with abundant mica. The middle section is a succession of lenticular layers, 0.05—0.15 m thick and 0.10—0.50 m wide. Such lenses are individually composed of fine sand, with variable amounts of very fine, friable micaceous sand, silt and clay, dull brown (7.5 YR 6/3) in colour with olive mottles. Some lenses show discontinuous planar lamination. These lenses are intercalated with lenses of medium to coarse pebbles, which are reworked from the underlying facies. Abundant macropores, megapores and root casts are present. The upper section (1.5 m) has distinctive paludal characteristics, as are expected in the filling of an abandoned channel. The unit is covered by the Lucio Lopez Fm.
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Locally, this level is represented by palaeogullies filled with soil fragments. Fragments have medium to coarse pebble sizes, are rounded, consolidated, comminuted by epigenesis, with little matrix. They show bioturbation structures. At the top of the formation a palaeosoil has developed. It is represented by a poorly defined B horizon, 0.40 m thick, dark brown in colour, weakly structured and containing CaCO rhizoconcretions. The age of the formation is Upper Pleistocene (IS3). A TL date in the middle section indicates an age of 52,310$1200 yr BP. A deposit located in the same stratigraphic position outside of the area (Marull Fluvial Sands), has an age of 45,610$1990 yr BP. 3.2.2. Tezanos Pinto formation This was formally defined by Iriondo (1980). It constitutes the typical unit of the Quaternary in the Pampa plain, composed of eolian silts (silt: 71—81%; clay: 13—29%; sand: 1—6%). At the base and top erosional unconformities connect these deposits with the Carcaran a´ Fm and with the San Guillermo Fm, respectively. Iriondo and Manavella (1990) recognized a series of sedimentary facies: A primary loess facies or eolian facies. This has the greatest areal representation, with a typical thickness of 2—4 m, and is best developed on the interfluves (6—8 m thick) and in some river cliffs in the area. It is bright brown when moist (7.5 YR 5/6) and dull orange when dry (7.5 YR 7/3), consists of silt with subordinate clay and little sand (silt: 75%; clay: 23%; fine sand: 2%). It is porous, permeable and friable and has a coarse granular to medium-coarse blocky structure. It is calcareous throughout with powdery CaCO concentrations and hard concretions (of variable frequency, varied forms and centimetric sizes). Their interiors are crossed by fine rhizoid ramified canaliculi. It is stable in steep exposures, in parts subject to subsurface piping, and is characterized by columnar jointing. The mineralogy of the silt and clay fractions of this facies show a dominance of quartz compared with
The light minerals in the very fine sand fraction of the alluvial stratum indicate a dominance of materials of volcaniclastic origin — from the Cordillera de los Andes — (glass, polycrystalline quartz and lithic fragments), with a regular contribution from the Sierras Pampeanas (quartz, feldspars, lithic fragments and alterites; Table 1). Heavy minerals comprise less than 3% of the sediment. Sparse relicts of a palaeosoil developed within the formation appear discontinuously in the bottoms of gullies within fluvial palaeovalleys. This suggests a larger original relief than at present. Lateral variations are commont in the palaeosoil: the structure changes from coarse to very coarse blocky (strong) up to angular very coarse prismatic (strong, defined by coarse fissures, partly filled by CaCO precipitates). The sediment is weakly calcareous to calcareous. There are numerous epigenetic carbonate concretions and casts produced by partial dissolution. CaCO concretions of phreatic origin occupy coarse fissures between peds.
Table 1 Light mineral composition of very fine sand of Geological Units of the study area (relative percentage) p: pollycrystalline grains; m: monocrystalline grains Unit
Carcaran a´ (alluvial f.) Tezanos Pinto Eolian sands of Hansen Lucio Lopez (lower part) Lucio Lopez (upper part) Subactual fluvial sands
Quartz p
m
21.8 44.0 29.4 75.9 65.1 20.5
13.8 14.6 8.5 1.6 7.5 24.2
K-Feldspar
Plagioclase
Volcanic glass
Alterite
Litic fragments
Others
8.0 6.0 5.1 3.1 7.5 6.5
4.0 1.3 3.9 1.6 1.9 4.2
45.0 18.7 43.5 2.1 5.7 33.5
2.3 12.7 5.1 14.7 7.5 1.4
4.0 2.7 3.4 — 4.7 9.8
1.1 — 1.1 1.0 — —
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Fig. 4. X-ray diffraction of loess (Tezanos Pinto Fm) from Parana´ river cliff (Cu Ka radiation; oriented samples).
feldspars, with small amounts of mica (illite), montmorillonite, kaolinite, calcite, goethite, hematite and allophane. Halite, gibbsite, gypsum and anhydrite were presumably introduced later (Figs. 4 and 5). The thickest outcrop of this facies in the area is in a quarry next to Tortugas town. In the lower and middle parts of a profile of 8 m thickness there are numerous hollows produced by eolian erosion, interpreted as yardangs. They are 1—4 m wide and 0.30— 1.20 m deep, with intraformational filling of silt, generally laminated or weakly structured in coarse blocks, with differential concentrations of CaCO . The middle and upper part of the profile include numerous fossil nests, 2—3 m long and 0.50—2 m high, of varied forms, filled by silt, with black skins, strongly structured into fine peds of different shapes, with numerous CaCO rhizo concretions.
A light yellow to orange facies with nodules occurs frequently in the study area. It is composed of coarse silt (7.5 YR 8/3 moist), slightly clayey (on the average: 80% silt, 17% clay and 3% fine sand). When disturbed the sediment breaks into coarse, firm blocks. Calcareous in general, the CaCO is segregated locally into con cretions. Numerous silty nodules are present to varying extent in the matrix; they are medium to coarse, subrounded to subangular, hard, non-calcareous. Inside the nodules very fine and fine macropores and mottles of iron and manganese oxides occur. Such nodules are interpreted as a product of the activity of dung beetles. The depositional environment of this facies is subaerial. The light minerals in the very fine sand fraction of this facies are dominated by materials of volcaniclastic origin (polycrystalline quartz grains, glass and lithic fragments). Plutonic-metamorphic minerals derived from the Sierras
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Fig. 5. X-ray diffraction. a: loess (Tezanos Pinto Fm) from a quarry near Carcarn a´ river. b: upper section of the Lucio Lopez Fm from Carcaran a´ river cliff (oriented samples).
Pampeanas occur in significantly smaller proportions (quartz, feldspars and alterites; Table 1). Heavy minerals are rare (0.36%). A dull orange paludal facies is also present. It consists of clayey silts (5 YR 7/4), partly coated by reddish brown cutans. It forms tabular and lenticular bodies intercalated in the eolian facies. The facies was generated in non-permanent swamps, with intervals of subaerial oxidation. A greyish to yellow-brown paludal facies can also be recognized. It is composed of silty and clayey sediments (10 YR 5/2). It appears in gullies and small streams within fluvial palaeovalleys. It was formed in permanent swamps with a reducing environment. The above-mentioned facies are poorly represented within the area. A series of TL datings gave ages of 31,690$1620 yr BP and 32,000 yr BP in samples of the eolian facies, 4 m below the top of the unit. The facies with nodules gave an age of 35,890$1030 yr BP, near the base of the formation. 3.2.3. Eolian Sands of Hansen (informal name) In the southwestern sector of the area the loess of the Tezanos Pinto Fm is partially replaced by coeval eolian sand belonging to the Pampean Sand Sea (IS2 remobilization). This deposit extends outside the area to the southwest. The sediment appears in ditches and in the profiles at the Leones river, with a maximum visible thickness of 3.5 m near Hansen town. At the river, the unit uncomfor-
mably covers the alluvial facies of the Carcaran a´ Fm. The top is uncomformably covered by the San Guillermo Fm. It is composed of very fine silty sand, slightly plastic, orange (7.5 YR 6/6), friable, massive or with almost imperceptible sedimentary structures. Numerous large botrioidal CaCO concretions are present in the lower part of the outcrops. The sedimentary mass is also calcareous. The light mineralogy of the fine sand fraction shows a dominance of materials of volcaniclastic origin (glass, polycrystalline quartz, alterites, lithic fragments) over minerals from the Sierras Pampeanas (quartz and feldspars; Table 1). The heavy fraction comprises less than 0.33% of the sediment. 3.3.3. The Hypsithermal Palaeosoil The top of the Tezanos Pinto Fm and the top of the Eolian Sands of Hansen are marked by a palaeosoil, partially eroded, 0.30—1.10 m thick, typically represented by the Bt horizon. It can be exposed only by excavation and is truncated by an erosional unconformity, which separates it from the San Guillermo Fm. Micromorphological analyses confirm the existence of an argillic horizon. A representative profile of the soil on the Tezanos Pinto Fm follows (from top downward): 0.00—0.30 m: Bt horizon of clayey silt, dark brown (7.5 YR 3/4), with intense eluviation of cutans. Strongly developed structure in medium prisms, hard consistency, with very fine and fine cracks between peds. Nodules or mottles are absent. The sediment is
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non-calcareous. There are abundant root casts. The lower boundary is wavy and abrupt. 0.30—0.55 m: Bt horizon composed of coarse clayey silt, dull brown (7.5 YR 5/4), fairly weakly structured in fine prisms, peds bounded by very fine fissures. Abundant cutans, coated cracks and numerous root casts; very fine and medium macropores. Nodules or mottles absent. Two phases, differing in colour and texture are present: one is possibly a product of biological activity. The sediment is non-calcareous. Lower boundary wavy and abrupt. 0.55—0.80 m: BC transitional horizon, of coarse, slightly clayey silt, dull brown (7.5 YR 5/4). Weakly to moderately developed structure, represented by medium blocks, very firm, peds outlined by fine and very fine fissures. There are clay translocations indicating incipient pedogenesis. Contains no nodules or mottles, and displays fine macropores. Two phases, comparables to those of the upper horizon are present. The sedimentary mass is non-calcareous. Gradual transition into the parent material. The Hypsithermal Soil has been recognized by several investigators in different areas of the Pampa plain.
3.3.4. San Guillermo formation This was defined by Iriondo (1987). It is an eolian grey silt which occurs as a continuous mantle over the Tezanos Pinto Fm; it generally forms the top of the sedimentary sequence in the Pampa. This deposit is the product of deflation of the A horizon of the Hypsithermal Soil and the subsequent deposition of dust. It lies with an erosive contact on the B horizon of the palaeosoil developed at the top of the Tezanos Pinto Fm and the Eolian Sands of Hansen. Locally, the unit is covered by fluvial current or recent sediments, or by deposits of anthropogenic origin. The typical thickness is 0.30 m, with a maximum of 0.55 m. In the area where it covers the Tezanos Pinto Fm, it consists of coarse silt with small proportions of very fine sand and clay, and is of brownish grey colour (10 YR 5/1). It is unstratified and friable, porous, permeable, and moderately structured in very coarse prisms, firm, with peds bounded by very fine fissures. In some places the sediment is cemented (very strong prisms). It is without nodules or mottles, but contains numerous root casts. Abundant macropores, very fine and fine canaliculi and tubes formed by bioturbation, now filled with silt. The sedimentary mass is non-calcareous. In the sectors where the unit lies on the Eolian Sands of Hansen, the formation is composed by very fine silty sand, of brownish grey colour. A mineralogical analysis of thin sections shows a skeleton formed predominantly by quartz and plagioclase, this last differentiated into two populations: fresh and altered. Minor proportions of opaques (hematite
and pyrite), biotite, chlorite, zircon and volcanic glass are present. The formation was deposited during a Late Holocene dry period that occurred between 3500 and 1400 yr BP. (Iriondo, 1990b). 3.3.5. Lucio Lopez formation This unit has been formally described by Kro¨hling (1996). It constitutes a complex sequence, differentiated into three parts. Composed of clayey silt, with scarce very fine sand (silt: 58—77%; clay: 19—37%; sand: 3—6%), it is stratatified with variations in colour from olive to grey; pedogenic horizons are intercalated in the middle section. Locally, it includes high proportions of biogenic material (mainly Gastropoda and abundant vegetal remains). L. Lopez Fm outcrops in the cliffs of the main rivers of the basin, with most typical characteristics and the greatest thickness in the Carcaran a´ river cliffs. The thickness varies from 0.50 to 5.00 m, locally reaching several meters in lateral exposure. The lower and middle sections of the unit have a subvertical profile, locally forming a prominent slope partially affected by slides. The upper section forms a vertical profile. It discordantly overlies the Carcaran a´ Fm. The contact indicates a palaeorelief of channels and gullies. Locally the basal stratum of the formation shows flow structures. In general the formation is covered by contemporary or recent deposits of fluvial origin; locally the unit forms the top of the profile. The formation, with a clear paludal origin, constitutes the filling of erosive landforms located in a recent belt of the Carcaran a´ river (Geomorphological Unit II) and in the bottom of the major palaeovalleys. The type profile of the unit is in a cliff on the right bank of the Carcaran a´ river, 0.5 km downstream of the town of Lucio Lopez. From bottom to top it consists of: Lower section 0.00—1.00 m: Plastic clay with little silt and very fine sand, olive yellow when moist (5 Y 6/3), displaying diffuse fine to medium horizontal stratification. It is weakly structured into coarse to very coarse, hard blocks, partially coated by a dark brown film (7.5 YR 3/3). It contains abundant root casts, very fine and fine macropores and yellowish brown ferruginous mottles. It is non-calcareous to very slightly calcareous. Lenses several centimeters long, with a large number of Gastropoda in (¸ittordina) remains are intercalated. A smooth, gradual upper contact, is marked by limonitic mottles. 1.00—1.65 m: Plastic clay with minor silt, olive yellow (5 Y 6/3) in the lower part to light gray in the upper part (7.5 Y 8/1), with abundant dark brown cutans (10 YR 3/3). The aggregates are poorly defined, very firm to hard. Abundant root casts; very fine to medium
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macropores. Non-calcareous. Small lenses are intercalated with high concentrations of Gastropoda (¸ittoridina). Smooth and abrupt, conformable upper contact. 1.65—2.00 m: Stratum of peat, light grey (7.5 8/1), with mottles and root casts coated with brown films and vegetal stems. Parallel, planar internal lamination. Upper contact affected by recent bioturbation. Middle section 2.00—2.73 m: Palaeosol complex. 2.00—2.20 m: B horizon. Brown clayey silt (7.5 YR 4/3), strongly structured into fairly hard prisms, bounded by very fine fissures. Abundant root casts and very fine and fine macropores. Non-calcareous. Smooth and abrupt upper boundary. 2.20—2.45 m: Dull brown clayey silt (7.5 YR 5/3), with brown cutans, more consolidated than the lower horizon. Weakly to moderately structured into medium prisms separated by very fine fissures. Root casts and very fine macropores. Non-calcareous. Smooth, abrupt upper boundary. 2.45—2.63 m: B horizon. Clayey silt, dull brown (7.5 YR 5/3), with abundant cutans. Strongly structured into fine, hard prisms, separated by very fine fissures. Root casts and very fine macropores. Non-calcareous. Smooth, abrupt upper boundary. 2.63—2.73 m: B horizon. Greyish brown clayey silt (7.5 YR 5/2), weakly to moderately structured into fine to medium, hard prisms, bounded by very fine fissures. Abundant root casts and very fine macropores. Noncalcareous. Smooth, gradational upper boundary. (End of the soil complex) 2.73—2.85 m: Stratum of peat containing abundant silt, light grey (7.5 YR 8/2) with brown and yellow mottles, consolidated. It is structured into poorly defined, medium aggregates. Wavy, abrupt and conformable upper boundary. 2.85—3.22 m: Light brownish grey (7.5 YR 7/1) clayey silt with brown cutans and yellow mottles, weakly structured into medium, hard blocks, separated by very fine to fine fissures. Root casts and very fine and fine macropores. Non-calcareous. Smooth, abrupt and conformable upper contact. 3.22—4.15 m: Clayey silt with variations in colour from dull brown (7.5 YR 5/4) to brownish grey (10 YR 6/1) expressed in diffuse mottles with vertical elongation, formed by hydromorphic reduction related to gleying processes. Moderately to strongly structured into coarse to very coarse prisms, which break into fine to medium, hard prisms, separated by very fine to medium fissures. Root casts and very fine to medium macropores. Non-calcareous. It is affected by bioturbations up to 30 cm in diameter, some of which are crotovinas (5—9 cm diameter). Smooth form and abrupt upper contact.
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4.15—4.27 m: B horizon. Clayey silt, brownish grey (7.5 YR 6/1) in colour, with abundant cutans. Strongly structured into fine, hard prisms, bounded by very fine and fine fissures. Root casts and very fine to fine macropores. Non-calcareous. Smooth to wavy, unconformable upper boundary. Upper section 4.27—4.55 m: Stratum, probably of cinerite, brownish grey (10 YR 6/1) and light grey (2.5 Y 8/1) in colour, moderately consolidated. Some root casts and abundant very fine and fine macropores. Weakly structured into very coarse, hard blocks, separated by very fine and fine fissures. Non-calcareous. Smooth and clear upper contact. 4.55—4.67 m: Stratum of silty clay, light brownish grey (7.5 YR 7/1) in the exposed face, with some brown to grey films. Structured into medium blocks which break into minor peds, with fine fissures and some root casts. Smooth and clear upper contact. 4.67—4.77 m: Similar to the lower deposit of this section. 4.77—4.97 m: Similar to the second stratum of this section. 4.97—5.12 m: Similar to the lower stratum of this section. In the type profile the formation was deposited on a paludal deposit belonging to the Carcaran a´ Fm. The unconformity separating both units is marked by a calcareous level of phreatic origin in the lower unit. A recent stratum of 0.60 m thick, formed by a very fine silty sand, dull brown (7.5 YR 5/3), of fluvial origin, has been laid down on top of the Lucio Lopez Fm. Dominant minerals of the light fraction of the very fine sands are of volcaniclastic nature (pollycrystalline quartz, glass, alterites and lithic fragments); subordinate feldspars and quartz (Table 1). The heavy fraction forms a small percentage (0.45%). Fig. 5 shows the mineralogy of the sediments of the upper section (quartz, feldspars, calcite, hematite, mica -illite- and allophane). A TL date in the upper section of the formation (in the type profile) indicates an age of 1320$120 yr BP. The soil complex of the middle section belongs to the Hypsithermal period. Very probably the formation was deposited during much of the Holocene. 3.3.6. Little Ice Age Eolian Sands (informal name) A discontinuous deposit of eolian sand covers the sedimentary sequence in the western sector of the area (province of Co´rdoba). It represents a partially deflated dunefield, formed by the reworking of the fluvial sands of Tercero river. This deposit extends 50—80 km northwards of the northern limit of the PSS (established during the Last Glacial Maximum). The sediment is of variable thickness, varying from 0.50 up to 2.5 m within a few meters. It unconformably
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overlies the San Guillermo Fm or the upper palaeosol of the Carcaran a´ Fm in profiles along the Tercero river. It also unconformably covers the upper section of the Lucio Lopez Fm in profiles of the Saladillo and Carcaran a´ rivers (Co´rdoba). The sediment consists of very fine to fine sand, moderately sorted, with visible muscovite (5%), yellowish brown to greyish yellow in colour. It is friable, very calcareous to non-calcareous. Locally, the deposit preserves large duneforms. In other places it is bedded in fine horizontal parallel strata. Precepitates of iron oxides and root casts occur randomly at the base. The sediment contains fine sand lenses with abundant muscovite. It also includes numerous gypsum crystals. This unit was probably formed during the Little Ice Age. 3.3.7. Present and recent fluvial sands Several very young, predominantly sandy deposits are directly related to the channel of the Carcaran a´ river. Recent sands fill abandoned meander channels of the Carcaran a´ river, cut into the Carcaran a´ Fm. Such deposits outcrop in both cliffs of the river, cut into older formations; their lateral extent varies between a few tens and several hundreds of meters. Locally, such sands are covered by contemporary sands and other flood deposits. The recent sands are very fine to medium sand in texture, with abundant mica (5% muscovite), yellowish brown in colour, loose to friable. In general, the sands
Table 2 Quaternary sedimentary formations in Carcaran a´ river basin
were deposited in thick horizontal continuous strata, without unconformities. The light minerals of the very fine sand fraction represent a mixture of two populations, one of volcaniclastic origin, reworked over time by the wind (pollycrystalline quartz, volcanic glass and lithic fragments) and another, less significant, derived from the Sierras Pampeanas (quartz, feldspars, alterites and micas; Table 1). The heavy mineral fraction makes up 3.2% of the total. The present bedload of the Carcaran a´ river is composed of fine and very fine sand, with a little medium sand (mica: 10%). 3.4. Profile of the parana´ cliff at the carcaran a´ mouth The Parana´ cliff has a stratigraphic sequence which is not easy to correlate with the Quaternary of the basin. The column begins at the base with the Ituzaingo´ Fm (Herbst et al., 1976) deposited by the Parana´ during Pliocene-Holocene times. It is overlain unconformably by the Puerto San Martin Fm (Iriondo, 1987), of Upper Pleistocene age, comprising swamp deposits intercalated with typical loess strata and displaying pedogenic horizons near the top. Between the two formations, several paleochannels filled with alluvial and paludal deposits are intercalated. These were formed by succesive avulsions of the Carcaran a´ river. The landscape developed on the Puerto San Martı´ n Fm was later covered and smoothed by the Tezanos Pinto Fm. The sequence ends
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with the San Guillermo Fm, which lies on the Hypsithermal Soil.
4. Environmental evolution during the Upper Quaternary Investigations carried out in the lower basin of the Carcaran a´ river, combined with studies in neighboring zones, have led to the establishment of a regional stratigraphy for the Upper Quaternary in the northern Pampa. Numerous data obtained in the study area have contributed to the identification of different climatic periods which affected the Pampa plain, starting from IS4 (see Iriondo in this volume for a general report). The environmental evolution of the area for approximately the last 65,000 yr BP it is presented schematically in Table 2. Fig. 3 shows the regional stratigraphic column, beginning with the Carcaran a´ Fm, which is a part of the Pampean Sand Sea, generated during the IS4. The sediments of this unit were deposited in a subaerial environment under extremely cold and dry conditions. Later, geoforms were partially eroded during a phase of climatic amelioration in IS3. Differences between outcrops indicate that the climate of IS3 was complex. The stage begins with a rapid change to subtropical, humid conditions, favouring the development of a soil with a well-developed B horizon. This horizon appears discontinuously at different depths, reflecting the dune topography of the Pampean Sand Sea. Later, a semiarid climate was established, leading to the almost complete dissipation of the dunes, which left an extensive plain. The redistributed deposits constitute most of the Carcaran a´ Fm, and overlie the partially eroded remnants of soil filled gullies, suggesting a palaeorelief typical of badland conditions. A second phase of subtropical humid climate during IS3 is suggested by the soil at the top of the redistributed deposits, by the presence of CaCO precipitates of phreatic origin, and by the intercalation of alluvial and paludal strata in the upper part of the Carcaran a´ Fm. During IS3 fluvial belts of major size developed in the area. A new climatic deterioration linked to the ¸ast Glacial Maximum (IS2) is indicated by major remobilization of sands into a new dunefield immediately southwest of the study area. Silt transported in suspension generated the loessic belt to the northeast of the dunefield. Most of the area investigated is located in this belt. The deposition of Tezanos Pinto Fm obliterated fluvial belts developed during IS3, masking the pre-existing relief. In abandoned channels with low discharge, progressive filling took place, generating a paludal facies.
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Maximum climatic deterioration, marked by semiarid conditions, is reflected by the eolian (loess) facies. A climatic improvement, indicating the end of the ¸ast Glacial Maximum (around the 15,500 to 16,500 yr BP) is evidenced by pedogenic CaCO concretions in outcrops of the Tezanos Pinto loess. Semiarid/subhumid climatic conditions, homologous and coeval with the Allero¨d, are interpreted for this period. The upper part of the Tezanos Pinto Fm was deposited during the Late Pleistocene/Lower Holocene dry period, broadly equivalent to the European ‘Younger Dryas’. In the study area a semiarid and cold phase, characterized by widespread fluvial erosion and by modest eolian sedimentation accompanied by wind erosion, generated numerous deflation depressions. The Hypsithermal Humid Period (Climatic Optimum) of the Middle Holocene, characterized by a humid subtropical/tropical climate, is represented by a soil developed at the top of the eolian deposits of the LGM. During the Late Pleistocene/Lower Holocene dry period the main rivers of the basin would have been reduced to semipermanent swamps. The contribution of eolian sediments to the paludal environment is represented by the lower section of the Lucio Lopez Fm. During the Hypsithermal the soil complex of its middle section was developed. A semiarid pulse in the Upper Holocene is indicated by a series of erosional and depositional processes, e.g. generalized deflation of the A horizons of the palaeosoils, the excavation of deflation hollows, and the deposition over the whole area of the San Guillermo Fm. An arid period probably occurred during the Little Ice Age, leading to mobilization of fluvial sands of Tercero in Co´rdoba. Under the present climate, deflation basins became temporary-to permanent shallow lakes; some of the small river channels of the basin experienced reactivation of their fluvial dynamics. The San Guillermo Fm is being affected by incipient pedogenesis. Acknowledgements The author acknowledges the advice of Dr. M. Iriondo in this investigation. The research was partially supported by the Consejo Nacional de Investigaciones Cientificas y Te´cnicas (CONICET). The fieldwork was partially funded from Scientific Research Grant No 5194.94 of the National Geographic Society. References Catt, J.A., 1990. Paleopedology Manual. Quaternary International 6, 1—95. Clapperton, C., 1993. Quaternary Geology and Geomorphology of South America. Elsevier, Amsterdam, 779pp.
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Herbst, R., Santa Cruz, J., Zabert, M., 1976. Avances en el conocimiento de la estratigrafı´ a de la Mesopotamia Argentina, con especial referencia a la provincia de Corrientes. Rev. Asoc. Cienc. Nat. del Litoral, Santo Tome´, Vol. 7, pp. 101—121. Iriondo, M., 1980. El Cuaternario de Entre Rı´ os. Rev. Asoc. Cienc. Nat. del Litoral, Santo Tome´, Vol. 11, pp. 125—141. Iriondo, M., 1987. Geomorfologı´ a y Cuaternario de la provincia Santa Fe (Argentina). D’Orbignyana, Corrientes 4, 1—54. Iriondo, M., 1989. Major fractures of the Chaco-Pampa Plain. In: Mo¨rner, N. (Ed.), Bulletin of INQUA, Neotectonics Commission N.A., Vol. 12, pp. 42. Iriondo, M., 1990a. Map of the South American plains — Its present state. In: Rabassa, J. (Ed.), Quaternary of South America and Antarctic Peninsula, A.A. Balkema Publishers, Rotterdam, Vol. 6, pp. 297—308. Iriondo, M., 1990b. A late Holocene dry period in the Argentine plains. In: Rabassa, J. (Ed.), Quaternary of South America and Antarctic Peninsula, A.A. Balkema Publishers, Rotterdam, Vol. 7, pp. 197—218. Iriondo, M., 1992. Geomorphological Map of South American Plains, Wall Map, Escale 1 : 5 000 000. Grant No. 4127/88 National Geographic Society and PID 95100/88 CONICET, unpublished Iriondo, M., Garcia, N., 1993. Climatic variations in the Argentine plains during the last 18,000 yr. Palaeogeography, Paleoclimatology, Palaeoecology 101, 209—220. Iriondo, M., Kro¨hling, D.M., 1995. El Sistema Eo´lico Pampeano. Com. Mus. Prov. Cs. Naturales ‘Florentino Ameghino’ (N.S.), Santa Fe, Vol. 5(1), pp. 1—68. Iriondo, M., Kro¨hling, D.M., 1996. Los sedimentos eo´licos del noreste de la llanura pampeana (Cuaternario superior). XIII Congreso Geolo´gico Argentino y III Congreso de Exploracio´n de Hidrocarburos, Buenos Aires, Actas IV, pp. 27—48.
Iriondo, M., Mannavella, C., 1990. Facies sedimentarias de la Formacio´n Tezanos Pinto en el centro de la provincia de Santa Fe. In: Zarate, M. (Ed.), International Symposium on Loess, Properties, Chronology and Palaeoclimatic significance of loess, INQUA, Mar del Plata, Expanded Abstracts, pp. 74—77. Iriondo, M., Ramonell, C., 1993. San Luis. In: Iriondo, M. (Ed.), El Holoceno en la Argentina, CADINQUA, Parana, Argentina, Vol. 2, pp. 131—162. Iriondo, M., Scotta, E., 1979. The evolution of the Parana´ river delta. In: Suguio, K., Fairchild, T.R., Martin, L., Flexor, J.M. (Eds), Proceedings of the 1978 International Symposium on Coastal Evolution in the Quaternary, INQUA. San Pablo, pp. 405—418. Kro¨hling, D., 1995a. Geomorfologı´ a y Geologı´ a del Cuaternario de la cuenca del rı´ o Carcaran a´ en la provincia de Santa Fe. Informe Final de Beca de Iniciacio´n CONCIET, p. 228. (unpublished.) Kro¨hling, D., 1995b. Geomorphology and Upper Quaternary Stratigraphy of the lower Carcaran a´ basin, North Pampa, Argentina. XIV INQUA Congress, Berlin, Abstracts, p. 146. Kro¨hling D., 1996. La Formacio´n Lucio Lopez (holoceno), noreste de la llanura pampeana. XIII Congreso Geolo´gico Argentino y III Congreso de Exploracio´n de Hidrocarburos, Buenos Aires, Actas IV, p. 69. Kro¨hling, D., Iriondo, M., 1995. Upper Quaternary Paleoclimates of the Mar Chiquita area, North Pampa, Argentina. XIV INQUA Congress, Berlin, Abstracts, p. 146. Markgraf, V., 1993. Climatic History of Central and South America since 18,000 yr BP: Comparison of Pollen Records and Model Simulations. In: Wright, H.E., Kutzbach, J.E., Webb III, T., Ruddiman W.F., Street-Perrot, F.A., Bartlein, P.J. (Eds), Global Climates since the Last Glacial Maximum, Vol. 14, University of Minnesota Press, Minneapolis, MN, 14, 357—385.
Upper Quaternary geology of the lower Carcaran a´ Basin, North Pampa, Argentina Daniela M. Kro¨hling CONICET — Facultad de Ingenierı& a y Ciencias Hı& dricas, Universidad Nacional del Litoral, C.C. 495, (3000) Santa Fe, Argentina
Abstract The geological evolution of the lower Carcaran a´ Basin reflects a sequence of dry and humid climates. From a geomorphological point of view, the area belongs to the Pampean Eolian System. The geomorphological units comprise several fluvial geoforms, partially masked by a loess carpet, on which eolian erosive forms developed. The sedimentary record is not simple. The typical profile begins with a predominantly eolian unit, composed of silty sand and including two pedogenic levels (Carcaran a´ Fm). The formation is the result of the reworking of a dune field in the Pampean Sand Sea (IS4). It is upper Pleistocene in age (IS 3). Another, somewhat different, climatic deterioration followed (related to the begining of the Last Glacial Maximum -IS 2), which caused the deposition of a loessic mantle (Tezanos Pinto Fm). Subsequent evolution of the climate involved a shift to humid conditions during the Hypsithermal period, generating a well-developed soil at the top of the formation. An upper Holocene eolian layer (San Guillermo Fm) covers the whole sequence. At the river margins of the basin the late Pleistocene/lower Holocene eolian deposits are replaced by a complex silty—clayey sequence, mainly paludal, with a soil complex in the middle of the section (Lucio Lopez Fm). 1999 INQUA/Elsevier Science Ltd. All rights reserved.
1. Introduction The lower basin of the Carcaran a´ river constitutes a favorable area for research on geomorphology and Quaternary stratigraphy of North Pampa (Kro¨hling, 1995a and b). This study is part of a research project on the paleoclimatic sequence of the pampean region during the upper Quaternary, building upon reconstructions of scenarios for different climatic periods which affected in the plain (Iriondo and Garcia, 1993; Iriondo and Kro¨hling, 1995, 1996; Kro¨hling and Iriondo, 1995). The overall results of that study are presented in another contribution of this volume (see Iriondo). The main objective of this contribution is the synthesis of evidence for environmental changes in the area during the last 65,000 yr, based on interpretations of the sedimentary record and geomorphological analyses, carried out as part of our own research. The study area occupies 14,400 km in the south-east of the province of Co´rdoba and south-center of the province of Santa Fe (approx. lat. 32°15 S to 33°30 S and long. 60°45 W to 62°30 W). The Carcaran a´ river lies
within the Rı´ o de la Plata basin. It drains region extending from the eastern slope of the Sierras Grandes and Comechingones (Sierras Pampeanas) to the lower Parana´ river (Fig. 1). The area has a subtropical humid climate, with a mean temperature of 17°C and precipitation increasing from west to east; averages oscillate between 900 and 1000 mm. The natural vegetation is a pseudosteppe, in general developed on Argiudolic soils. According to Iriondo (1987) the geological evolution of the region can be explained in terms of sedimentary systems: the Pampean Eolian System, alluvial fans of the Sierras Pampeanas and the Parana´ river belt. The first of these had a considerable influence in the study area during the Upper Quaternary. The geomorphological units of the area comprise fluvial geoforms (mostly developed on the older unit of the Pampean Sand Sea), partially masked by a loess mantle on which mainly eolian erosive forms were developed. The sedimentary column of the area reveals fluctuating environmental conditions, characterized by dry intervals, associated with episodes of formation and remobilization of the Pampean Sand Sea and of the Peripheral Loessic
1040-6182/99/$20.00 1999 INQUA/Elsevier Science Ltd. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 9 8 ) 0 0 0 5 5 - X
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investigation of the processes responsible for their generation. During nine expeditions to the area, involving 46 days of intensive fieldwork, a rigorous analysis of each identified geomorphological unit was carried out. 2.2. Regional outline Fig. 1 shows the main units of the study area, taken from the Geomorphological Map of the Plains of South America (Iriondo, 1990a, 1992). The principal tectonic and sedimentary characteristics of the region are the following: Fractures several hundred of kilometers long, with modest topographical expression of a few meters. Two of these major fractures cross the basin (Fig. 1): the TostadoSelva fault (Middle Pleistocene in age and reactivated in the Holocene) and a fracture controlling the course of the Parana´ river in the sector analyzed (Iriondo, 1989). Minor fractures, parallel to the larger ones, form an orthogonal pattern, and define a series of tectonic blocks tilted towards the east. From a sedimentological point of view, the major part of the area is dominated by the Peripheral Loessic Belt of the Pampean Eolian System (PES), except the southwest border, which is part of the transition to the Pampean Sand Sea (Fig. 1). The northwest sector of the area is covered by sediments belonging to the alluvial fan of the Tercero river. Sediments deposited by the Parana´ river are present below surface in the eastern sector of the area. The delta of the Parana´ river is located immediately to the east of the area, and was developed during the Hypsithermal Period (Iriondo and Scotta, 1979). 2.3. Geomorphological units Fig. 1. Geological Map of the region — taken from Iriondo (1992).
Belt during the stages of Glacial Maximum, alternating with humid intervals characterized by pedogenesis and the development of fluvial belts. General outlines of the Quaternary of South America are given by Clapperton (1993) and Markgraf (1993).
2. Geomorphology of the area 2.1. Methodology The methodology used in the geomorphological study of the basin involved small-scale analyses using satellite images and detailed photointerpretation, supported by the study of topographical maps. These studies were controlled by fieldwork, involving the classification, description and interpretation of geoforms, including
The area in the province of Santa Fe was mapped in terms of the geomorphological classification developed by Iriondo (1990a) and by Iriondo and Ramonell (1993), which has a hierarchical structure. The area conforms to the following regional pattern: E E
E
First level. Geomorphological Province: Pampa Plain. Second level. Type of Geomorphological System: Pampean Eolian System (Iriondo, 1987). Third level. Geomorphological Subsystems: Pampean ¸oessic Subsystem and Pampean Sand Sea Subsystem.
Most of the area is part of the Pampean Loessic Subsystem (PLS); only the southwestern sector corresponds to the zone of transition to the Pampean Sand Sea (PSS). Eight geomorphological units (GU) occur in the mapped area (Fig. 2): 2.3.1. G.U. Fluvial terraces of the CarcaranJ a& river These form the fluvial belt of the Carcaran a´ river above a fracture of NW—SE strike, located at 32°45 S and 61°15 W, which impeded terrace development
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Fig. 2. Geomorphological scheme of the Carcaran a´ river basin in the province of Santa Fe.
downstream. Two levels of continuous terraces are associated with the present river channel; intermediate terraces appear locally. The present river channel has not developed a floodplain. The significant elements of the unit are: an upper terrace level (mean width: 3000 m) and the outer edge of this plane, a lower terrace (common width: 300 m) and its outer edge, and the discontinuous planes of intermediate terraces. Numerous palaeochannels of the Carcaran a´ river cut the upper terrace. The present channel of the river, bounded by cliffs (in parts cut by gullies), shows a meandering pattern inherited from a period in which its energy was smaller than at present. Associated elements are inactive alluvial fans (related to fluvial tributary
valleys) and deflation hollows, some of which hold temporary lakes. 2.3.2. G.U. Fluvial belt of the lower CarcaranJ a& river This extends from Unit I to the younger channel of the Carcaran a´ river. It is formed by an upper continuous terrace (mean width: 2500 m) and a narrow lower level (of variable width between 100 and 300 m) in which the river flows. This latter level was partially filled by the Lucio Lopez Fm, which possess thixotropic characteristic. Large soil flows of this formation toward the river occurred in recent times, leaving numerous depressions on the slope of this belt (depressions are 1—3.5 m deep and 50—300 m long). The bottom of these depressions is
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horizontal, with gullies and swamps. The lower narrow belt constitutes the distinctive element of the unit; it is the downstream prolongation of the lower terrace of Unit I. The river channel flows at the centre of this level, whose edge is well marked. Elements associated with the channel are cliffs, gullies and a waterfall. Some significant elements of the unit are: the terrace with its outer margin and palaeochannels of the Carcaran a´ river. A palaeochannel of the Parana´ river, buried at shallow depth, is visible in the superficial geoforms. Elements associated with Unit I also appear in this Unit. 2.3.3. G.U. Fluvial valleys of Villa Eloı& sa Typical elements of this unit are tributary valleys of the Carcaran a´ river. Elements of the original valley landscape are covered by a mantle of loess, with superimposed episodic channels. Valleys are in general asymmetric with typical widths between 800 and 1200 m, represented largely by lateral slopes, and characteristic depths between 5 and 7 m. The largest valley is 2700 m wide and 15 m deep. At the junction with Unit I the old valleys are linked to it by inactive alluvial fans. The high frequency of valleys makes a landscape of well-defined hills, formed by simple, long slopes. A small number of deflation hollows are excavated in the top of the loess formation. 2.3.4. G.U. Fluvial paleovalleys of CanJ ada de Gomez This unit is characterized by a well-defined palaeodrainage pattern, formed by a series of paleovalleys of the Carcaran a´ river, which are approximately parallel to the present general direction of the river. They are asymmetric, complex valleys, masked by a loess cover; they are interpreted as polygenetics belts, affected by neotectonics. The main valley is 4900 m wide (represented mostly by lateral slopes) with a typical depth of 18 m. Some episodic channels and gullies are developed, mainly in the bottom of the paleovalleys; numerous palaeochannels are present. Other associated elements are deflation hollows and depressions generated by neotectonics. 2.3.5. G.U. Old plain of Bustinza This is a relict landscape, not much affected by fluvial processes, and masked by a loess mantle. There are minor palaeovalleys and parallel palaeochannels with a general direction towards the Carcaran a´ river; deflation depressions are conspicuous forms. In the field, the elementary geoforms of the unit are gentle and long slopes with low relief. Episodic channels and gullies were developed in the valley bottoms; some depressions and scarps of structural origin affect the regularity of the plain.
A minor terrace level formed by neotectonics follows the younger segment of the Carcaran a´ river next to the river outlet. In the western sector of the area, near the Tortugas tributary, a series of well-defined hills appear in the landscape; these are interpreted as partially eroded dunes, covered by a loess mantle. 2.3.6. G.º. Fluvial palaeovalleys of Berabevu& This unit is formed by allochthonous palaeovalleys with palaeochannels in their floors. Small channels, older than the palaeovalleys, form a colinear pattern. These geoforms are covered by eolian sediments. The largest palaeovalley has a symmetrical profile, 1.5—3 km wide and 6—18 m deep. The bottom is flat, and occupied by a 300 m wide palaeochannel. Iriondo (1987) interpreted these palaeovalleys as geoforms generated by the Quinto river, with headwaters in the Sierras de San Luis (Sierras Pampeanas). The southwestern sector of the area comprises the transition between the Pampean Sand Sea (PSS) and the Pampean Loessic Subsystem (PLS) of the Last Glacial Maximum. The transition zone has a width of the order of 10 km. The landscape is dominated in some areas by partially dissipated dunes (typical of the PSS), alternating with practically level sectors corresponding to the PLS. Numerous deflation depressions occur in these sediments, being more frequent and better developed in the sectors where the eolian sand predominates, especially in palaeochannels, where they reach 500 m in diameter. Some depressions have become semi-permanent and permanent shallow lakes. Gullies and episodic channels occupy the palaeovalleys. 2.3.7. G.U. Palaeochannels of Luis Palacios This unit is defined by a series of gentle, aligned depressions, which form a large arc of 10—15 km radius. The depressions, with typical widths of 1.5 km and maximum widths of 3.5 km, are partially flooded. The similarity of the pattern of depressions with the channel of the Parana´ river near the study area is noteworthy. One can therefore conclude that a palaeochannel of the Parana´ river, covered by a thin layer of loess, is responsible for the unit. A system of minor palaeochannels, parallel to the general course of the Carcaran a´ river, is older than these large, aligned depressions. Numerous deflation hollows affect the palaeochannels; some of them are occupied by semipermanent shallow lakes. Tectonic depressions, episodic channels and gullies are common in this unit. 2.3.8. G.U. Fluvial valley of Tortugas This comprises the asymmetric valley of a small river, the Tortugas, which exhibits strong structural control. The valley is covered by eolian deposits.
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A fluvial belt in the bottom of the valley is partially occupied by the channel; the belt includes palaeochannels and deflation depressions. The lateral slopes of the valley are well defined; the left corresponds to an attenuated fault scarp, 50 m high, the other constitutes a simple slope. Four kilometer long tributary valleys have developed by headward erosion on both sides of the fluvial belt.
3. Upper Quaternary of the area 3.1. Methodology Extensive fieldwork in the course of nine expeditions resulted in the recognition and description of several geological formations of Quaternary age. The Palaeopedology Manual of INQUA (Catt, 1990) was used in the study of the palaeosols. The colour of the sediments was recorded in accordance with the Standard Soil Colour Chart of the Agriculture Ministry of Japan. The investigations were complemented by routine granulometric and mineralogical analyses in order to obtain a preliminary characterization of the geological units. A series of datings by thermoluminescence has established the absolute ages of the formations. Datings were made in the Thermoluminescence Laboratory of Peking University (China). 3.2. Geological formations A general characterization of the geological units is presented here. Some outcropping formations have already been defined by other authors; the remainder are defined in this paper (Fig. 3). 3.2.1. CarcaranJ a& formation This is composed of very fine to fine silty sand, and silt with very fine sand, with clays rarely present, little visible muscovite, bright reddish-brown wet colour (5YR 5/6), and dull orange dry colour (7.5 YR 6/4). It has very thick horizontal beds, not well defined and generally with concordant contacts. The sediment varies in consistency between friable and consolidated. It has a strong, fine to medium blocky structure. In general, it is non-calcareous. It is affected locally by bioturbations, such as crotovinas (up to 0.75 m diameter) and root casts (up to 0.60 m long and 0.10 m diameter). In some places sedimentary structures relating to internal translocation of material are visible, marked by differences in concentrations of colloidal materials. These structures are characterized by an irregular wavy pattern. This unit forms the lower sections of cliffs along river channels, forming gentle to subvertical slopes, which reflect its resistance in comparison with the upper units. The maximum outcrop thickness is some 5.50 m. It is
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discordantly covered by the Tezanos Pinto Fm in the interfluves and by the Lucio Lopez Fm in the main valleys of the area. The formation, mainly of eolian origin, is the result of the reworking by erosion of a dunefield in the Pampean Sand Sea (PSS; IS4). Primary structures of the dunes of the PSS appear in profiles located immediately to the south of the area. Locally different paludal and alluvial facies are present. A representative profile of the unit is present in the cliffs of the Can ada de Gomez river, at its junction with the Carcaran a´ river. From bottom to top the following sequence is present: 0.00—2.00 m: Stratum formed by very fine, slightly silty sand, bright reddish brown when moist, medium dense, with translocation structures visible in units 1.5 m long and 0.10 m thick. Non-calcareous. Locally, in units 0.15— 0.70 m thick and of 1—1.50 m in lateral extent, the sediment is covered with brown cutans, and contains medium to coarse angular hard nodules, coated with iron sesquioxides. There are rhizoconcretions of CaCO up to 5 cm long and 1.5 cm in diameter and abundant very fine macropores. The stratum is characterized by an orthogonal system of joints. Concordant upper contact. 2.00—4.50 m: Stratum sedimentologically similar to the lower unit, without internal structures, and with less resistance to erosion. Numerous rhizoconcretions of CaCO at the contact between units. 1 m below the contact crotovinas up to 0.50 m in diameter (some including carbonaceous remains) were observed. Close to the top there is a level formed by a succession of erosional geoforms comparable to yardangs, 0.50—0.80 m wide and 0.40—0.50 m deep. (Top of Carcaran a´ Fm). A representative section of the paludal and alluvial facies of the formation outcrops in the Carcaran a´ river cliffs, next to the city of Carcaran a´. These facies constitute a filling up sequence of a 100 m wide channel. From the water level to the top the following units are present: 0.00—2.90 m: Paludal facies, comprising very thick lenticular strata. It is composed of very fine sand with uniform silt and clay, dull orange (7.5 YR 6/4) in colour, partially covered by brown films (5 YR 4/6). Moderately consolidated, weakly structured in irregular peds, very firm. In some levels there are concentrations of fine to coarse hard nodules, compact, coated with films of iron sesquioxides, and associated with manganese dendrites. Numerous root casts covered by similar films. Abundant very fine and fine macropores. Crotovinas up to 0.30 m in diameter. Non-calcareous to very slightly calcareous. Locally, the sediment is laminated. The exposed surface contains spherical and ellipsoidal depressions up to 3 cm in diameter, developed in a less resistant epigenetic phase. The
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Fig. 3. General stratigraphic profile of the Carcaran a´ river basin.
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stratum is traversed by a system of vertical, orthogonal joints. The upper contact is represented by an intraformational unconformity. 2.90—5.30 m: Alluvial facies. At the base, lenses 5—6 m wide and 0.05—0.10 m thick, composed of very fine pebbles, derived from the lower stratum, are supported in an internally laminated sandy coarse matrix with abundant mica. The middle section is a succession of lenticular layers, 0.05—0.15 m thick and 0.10—0.50 m wide. Such lenses are individually composed of fine sand, with variable amounts of very fine, friable micaceous sand, silt and clay, dull brown (7.5 YR 6/3) in colour with olive mottles. Some lenses show discontinuous planar lamination. These lenses are intercalated with lenses of medium to coarse pebbles, which are reworked from the underlying facies. Abundant macropores, megapores and root casts are present. The upper section (1.5 m) has distinctive paludal characteristics, as are expected in the filling of an abandoned channel. The unit is covered by the Lucio Lopez Fm.
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Locally, this level is represented by palaeogullies filled with soil fragments. Fragments have medium to coarse pebble sizes, are rounded, consolidated, comminuted by epigenesis, with little matrix. They show bioturbation structures. At the top of the formation a palaeosoil has developed. It is represented by a poorly defined B horizon, 0.40 m thick, dark brown in colour, weakly structured and containing CaCO rhizoconcretions. The age of the formation is Upper Pleistocene (IS3). A TL date in the middle section indicates an age of 52,310$1200 yr BP. A deposit located in the same stratigraphic position outside of the area (Marull Fluvial Sands), has an age of 45,610$1990 yr BP. 3.2.2. Tezanos Pinto formation This was formally defined by Iriondo (1980). It constitutes the typical unit of the Quaternary in the Pampa plain, composed of eolian silts (silt: 71—81%; clay: 13—29%; sand: 1—6%). At the base and top erosional unconformities connect these deposits with the Carcaran a´ Fm and with the San Guillermo Fm, respectively. Iriondo and Manavella (1990) recognized a series of sedimentary facies: A primary loess facies or eolian facies. This has the greatest areal representation, with a typical thickness of 2—4 m, and is best developed on the interfluves (6—8 m thick) and in some river cliffs in the area. It is bright brown when moist (7.5 YR 5/6) and dull orange when dry (7.5 YR 7/3), consists of silt with subordinate clay and little sand (silt: 75%; clay: 23%; fine sand: 2%). It is porous, permeable and friable and has a coarse granular to medium-coarse blocky structure. It is calcareous throughout with powdery CaCO concentrations and hard concretions (of variable frequency, varied forms and centimetric sizes). Their interiors are crossed by fine rhizoid ramified canaliculi. It is stable in steep exposures, in parts subject to subsurface piping, and is characterized by columnar jointing. The mineralogy of the silt and clay fractions of this facies show a dominance of quartz compared with
The light minerals in the very fine sand fraction of the alluvial stratum indicate a dominance of materials of volcaniclastic origin — from the Cordillera de los Andes — (glass, polycrystalline quartz and lithic fragments), with a regular contribution from the Sierras Pampeanas (quartz, feldspars, lithic fragments and alterites; Table 1). Heavy minerals comprise less than 3% of the sediment. Sparse relicts of a palaeosoil developed within the formation appear discontinuously in the bottoms of gullies within fluvial palaeovalleys. This suggests a larger original relief than at present. Lateral variations are commont in the palaeosoil: the structure changes from coarse to very coarse blocky (strong) up to angular very coarse prismatic (strong, defined by coarse fissures, partly filled by CaCO precipitates). The sediment is weakly calcareous to calcareous. There are numerous epigenetic carbonate concretions and casts produced by partial dissolution. CaCO concretions of phreatic origin occupy coarse fissures between peds.
Table 1 Light mineral composition of very fine sand of Geological Units of the study area (relative percentage) p: pollycrystalline grains; m: monocrystalline grains Unit
Carcaran a´ (alluvial f.) Tezanos Pinto Eolian sands of Hansen Lucio Lopez (lower part) Lucio Lopez (upper part) Subactual fluvial sands
Quartz p
m
21.8 44.0 29.4 75.9 65.1 20.5
13.8 14.6 8.5 1.6 7.5 24.2
K-Feldspar
Plagioclase
Volcanic glass
Alterite
Litic fragments
Others
8.0 6.0 5.1 3.1 7.5 6.5
4.0 1.3 3.9 1.6 1.9 4.2
45.0 18.7 43.5 2.1 5.7 33.5
2.3 12.7 5.1 14.7 7.5 1.4
4.0 2.7 3.4 — 4.7 9.8
1.1 — 1.1 1.0 — —
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Fig. 4. X-ray diffraction of loess (Tezanos Pinto Fm) from Parana´ river cliff (Cu Ka radiation; oriented samples).
feldspars, with small amounts of mica (illite), montmorillonite, kaolinite, calcite, goethite, hematite and allophane. Halite, gibbsite, gypsum and anhydrite were presumably introduced later (Figs. 4 and 5). The thickest outcrop of this facies in the area is in a quarry next to Tortugas town. In the lower and middle parts of a profile of 8 m thickness there are numerous hollows produced by eolian erosion, interpreted as yardangs. They are 1—4 m wide and 0.30— 1.20 m deep, with intraformational filling of silt, generally laminated or weakly structured in coarse blocks, with differential concentrations of CaCO . The middle and upper part of the profile include numerous fossil nests, 2—3 m long and 0.50—2 m high, of varied forms, filled by silt, with black skins, strongly structured into fine peds of different shapes, with numerous CaCO rhizo concretions.
A light yellow to orange facies with nodules occurs frequently in the study area. It is composed of coarse silt (7.5 YR 8/3 moist), slightly clayey (on the average: 80% silt, 17% clay and 3% fine sand). When disturbed the sediment breaks into coarse, firm blocks. Calcareous in general, the CaCO is segregated locally into con cretions. Numerous silty nodules are present to varying extent in the matrix; they are medium to coarse, subrounded to subangular, hard, non-calcareous. Inside the nodules very fine and fine macropores and mottles of iron and manganese oxides occur. Such nodules are interpreted as a product of the activity of dung beetles. The depositional environment of this facies is subaerial. The light minerals in the very fine sand fraction of this facies are dominated by materials of volcaniclastic origin (polycrystalline quartz grains, glass and lithic fragments). Plutonic-metamorphic minerals derived from the Sierras
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Fig. 5. X-ray diffraction. a: loess (Tezanos Pinto Fm) from a quarry near Carcarn a´ river. b: upper section of the Lucio Lopez Fm from Carcaran a´ river cliff (oriented samples).
Pampeanas occur in significantly smaller proportions (quartz, feldspars and alterites; Table 1). Heavy minerals are rare (0.36%). A dull orange paludal facies is also present. It consists of clayey silts (5 YR 7/4), partly coated by reddish brown cutans. It forms tabular and lenticular bodies intercalated in the eolian facies. The facies was generated in non-permanent swamps, with intervals of subaerial oxidation. A greyish to yellow-brown paludal facies can also be recognized. It is composed of silty and clayey sediments (10 YR 5/2). It appears in gullies and small streams within fluvial palaeovalleys. It was formed in permanent swamps with a reducing environment. The above-mentioned facies are poorly represented within the area. A series of TL datings gave ages of 31,690$1620 yr BP and 32,000 yr BP in samples of the eolian facies, 4 m below the top of the unit. The facies with nodules gave an age of 35,890$1030 yr BP, near the base of the formation. 3.2.3. Eolian Sands of Hansen (informal name) In the southwestern sector of the area the loess of the Tezanos Pinto Fm is partially replaced by coeval eolian sand belonging to the Pampean Sand Sea (IS2 remobilization). This deposit extends outside the area to the southwest. The sediment appears in ditches and in the profiles at the Leones river, with a maximum visible thickness of 3.5 m near Hansen town. At the river, the unit uncomfor-
mably covers the alluvial facies of the Carcaran a´ Fm. The top is uncomformably covered by the San Guillermo Fm. It is composed of very fine silty sand, slightly plastic, orange (7.5 YR 6/6), friable, massive or with almost imperceptible sedimentary structures. Numerous large botrioidal CaCO concretions are present in the lower part of the outcrops. The sedimentary mass is also calcareous. The light mineralogy of the fine sand fraction shows a dominance of materials of volcaniclastic origin (glass, polycrystalline quartz, alterites, lithic fragments) over minerals from the Sierras Pampeanas (quartz and feldspars; Table 1). The heavy fraction comprises less than 0.33% of the sediment. 3.3.3. The Hypsithermal Palaeosoil The top of the Tezanos Pinto Fm and the top of the Eolian Sands of Hansen are marked by a palaeosoil, partially eroded, 0.30—1.10 m thick, typically represented by the Bt horizon. It can be exposed only by excavation and is truncated by an erosional unconformity, which separates it from the San Guillermo Fm. Micromorphological analyses confirm the existence of an argillic horizon. A representative profile of the soil on the Tezanos Pinto Fm follows (from top downward): 0.00—0.30 m: Bt horizon of clayey silt, dark brown (7.5 YR 3/4), with intense eluviation of cutans. Strongly developed structure in medium prisms, hard consistency, with very fine and fine cracks between peds. Nodules or mottles are absent. The sediment is
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non-calcareous. There are abundant root casts. The lower boundary is wavy and abrupt. 0.30—0.55 m: Bt horizon composed of coarse clayey silt, dull brown (7.5 YR 5/4), fairly weakly structured in fine prisms, peds bounded by very fine fissures. Abundant cutans, coated cracks and numerous root casts; very fine and medium macropores. Nodules or mottles absent. Two phases, differing in colour and texture are present: one is possibly a product of biological activity. The sediment is non-calcareous. Lower boundary wavy and abrupt. 0.55—0.80 m: BC transitional horizon, of coarse, slightly clayey silt, dull brown (7.5 YR 5/4). Weakly to moderately developed structure, represented by medium blocks, very firm, peds outlined by fine and very fine fissures. There are clay translocations indicating incipient pedogenesis. Contains no nodules or mottles, and displays fine macropores. Two phases, comparables to those of the upper horizon are present. The sedimentary mass is non-calcareous. Gradual transition into the parent material. The Hypsithermal Soil has been recognized by several investigators in different areas of the Pampa plain.
3.3.4. San Guillermo formation This was defined by Iriondo (1987). It is an eolian grey silt which occurs as a continuous mantle over the Tezanos Pinto Fm; it generally forms the top of the sedimentary sequence in the Pampa. This deposit is the product of deflation of the A horizon of the Hypsithermal Soil and the subsequent deposition of dust. It lies with an erosive contact on the B horizon of the palaeosoil developed at the top of the Tezanos Pinto Fm and the Eolian Sands of Hansen. Locally, the unit is covered by fluvial current or recent sediments, or by deposits of anthropogenic origin. The typical thickness is 0.30 m, with a maximum of 0.55 m. In the area where it covers the Tezanos Pinto Fm, it consists of coarse silt with small proportions of very fine sand and clay, and is of brownish grey colour (10 YR 5/1). It is unstratified and friable, porous, permeable, and moderately structured in very coarse prisms, firm, with peds bounded by very fine fissures. In some places the sediment is cemented (very strong prisms). It is without nodules or mottles, but contains numerous root casts. Abundant macropores, very fine and fine canaliculi and tubes formed by bioturbation, now filled with silt. The sedimentary mass is non-calcareous. In the sectors where the unit lies on the Eolian Sands of Hansen, the formation is composed by very fine silty sand, of brownish grey colour. A mineralogical analysis of thin sections shows a skeleton formed predominantly by quartz and plagioclase, this last differentiated into two populations: fresh and altered. Minor proportions of opaques (hematite
and pyrite), biotite, chlorite, zircon and volcanic glass are present. The formation was deposited during a Late Holocene dry period that occurred between 3500 and 1400 yr BP. (Iriondo, 1990b). 3.3.5. Lucio Lopez formation This unit has been formally described by Kro¨hling (1996). It constitutes a complex sequence, differentiated into three parts. Composed of clayey silt, with scarce very fine sand (silt: 58—77%; clay: 19—37%; sand: 3—6%), it is stratatified with variations in colour from olive to grey; pedogenic horizons are intercalated in the middle section. Locally, it includes high proportions of biogenic material (mainly Gastropoda and abundant vegetal remains). L. Lopez Fm outcrops in the cliffs of the main rivers of the basin, with most typical characteristics and the greatest thickness in the Carcaran a´ river cliffs. The thickness varies from 0.50 to 5.00 m, locally reaching several meters in lateral exposure. The lower and middle sections of the unit have a subvertical profile, locally forming a prominent slope partially affected by slides. The upper section forms a vertical profile. It discordantly overlies the Carcaran a´ Fm. The contact indicates a palaeorelief of channels and gullies. Locally the basal stratum of the formation shows flow structures. In general the formation is covered by contemporary or recent deposits of fluvial origin; locally the unit forms the top of the profile. The formation, with a clear paludal origin, constitutes the filling of erosive landforms located in a recent belt of the Carcaran a´ river (Geomorphological Unit II) and in the bottom of the major palaeovalleys. The type profile of the unit is in a cliff on the right bank of the Carcaran a´ river, 0.5 km downstream of the town of Lucio Lopez. From bottom to top it consists of: Lower section 0.00—1.00 m: Plastic clay with little silt and very fine sand, olive yellow when moist (5 Y 6/3), displaying diffuse fine to medium horizontal stratification. It is weakly structured into coarse to very coarse, hard blocks, partially coated by a dark brown film (7.5 YR 3/3). It contains abundant root casts, very fine and fine macropores and yellowish brown ferruginous mottles. It is non-calcareous to very slightly calcareous. Lenses several centimeters long, with a large number of Gastropoda in (¸ittordina) remains are intercalated. A smooth, gradual upper contact, is marked by limonitic mottles. 1.00—1.65 m: Plastic clay with minor silt, olive yellow (5 Y 6/3) in the lower part to light gray in the upper part (7.5 Y 8/1), with abundant dark brown cutans (10 YR 3/3). The aggregates are poorly defined, very firm to hard. Abundant root casts; very fine to medium
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macropores. Non-calcareous. Small lenses are intercalated with high concentrations of Gastropoda (¸ittoridina). Smooth and abrupt, conformable upper contact. 1.65—2.00 m: Stratum of peat, light grey (7.5 8/1), with mottles and root casts coated with brown films and vegetal stems. Parallel, planar internal lamination. Upper contact affected by recent bioturbation. Middle section 2.00—2.73 m: Palaeosol complex. 2.00—2.20 m: B horizon. Brown clayey silt (7.5 YR 4/3), strongly structured into fairly hard prisms, bounded by very fine fissures. Abundant root casts and very fine and fine macropores. Non-calcareous. Smooth and abrupt upper boundary. 2.20—2.45 m: Dull brown clayey silt (7.5 YR 5/3), with brown cutans, more consolidated than the lower horizon. Weakly to moderately structured into medium prisms separated by very fine fissures. Root casts and very fine macropores. Non-calcareous. Smooth, abrupt upper boundary. 2.45—2.63 m: B horizon. Clayey silt, dull brown (7.5 YR 5/3), with abundant cutans. Strongly structured into fine, hard prisms, separated by very fine fissures. Root casts and very fine macropores. Non-calcareous. Smooth, abrupt upper boundary. 2.63—2.73 m: B horizon. Greyish brown clayey silt (7.5 YR 5/2), weakly to moderately structured into fine to medium, hard prisms, bounded by very fine fissures. Abundant root casts and very fine macropores. Noncalcareous. Smooth, gradational upper boundary. (End of the soil complex) 2.73—2.85 m: Stratum of peat containing abundant silt, light grey (7.5 YR 8/2) with brown and yellow mottles, consolidated. It is structured into poorly defined, medium aggregates. Wavy, abrupt and conformable upper boundary. 2.85—3.22 m: Light brownish grey (7.5 YR 7/1) clayey silt with brown cutans and yellow mottles, weakly structured into medium, hard blocks, separated by very fine to fine fissures. Root casts and very fine and fine macropores. Non-calcareous. Smooth, abrupt and conformable upper contact. 3.22—4.15 m: Clayey silt with variations in colour from dull brown (7.5 YR 5/4) to brownish grey (10 YR 6/1) expressed in diffuse mottles with vertical elongation, formed by hydromorphic reduction related to gleying processes. Moderately to strongly structured into coarse to very coarse prisms, which break into fine to medium, hard prisms, separated by very fine to medium fissures. Root casts and very fine to medium macropores. Non-calcareous. It is affected by bioturbations up to 30 cm in diameter, some of which are crotovinas (5—9 cm diameter). Smooth form and abrupt upper contact.
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4.15—4.27 m: B horizon. Clayey silt, brownish grey (7.5 YR 6/1) in colour, with abundant cutans. Strongly structured into fine, hard prisms, bounded by very fine and fine fissures. Root casts and very fine to fine macropores. Non-calcareous. Smooth to wavy, unconformable upper boundary. Upper section 4.27—4.55 m: Stratum, probably of cinerite, brownish grey (10 YR 6/1) and light grey (2.5 Y 8/1) in colour, moderately consolidated. Some root casts and abundant very fine and fine macropores. Weakly structured into very coarse, hard blocks, separated by very fine and fine fissures. Non-calcareous. Smooth and clear upper contact. 4.55—4.67 m: Stratum of silty clay, light brownish grey (7.5 YR 7/1) in the exposed face, with some brown to grey films. Structured into medium blocks which break into minor peds, with fine fissures and some root casts. Smooth and clear upper contact. 4.67—4.77 m: Similar to the lower deposit of this section. 4.77—4.97 m: Similar to the second stratum of this section. 4.97—5.12 m: Similar to the lower stratum of this section. In the type profile the formation was deposited on a paludal deposit belonging to the Carcaran a´ Fm. The unconformity separating both units is marked by a calcareous level of phreatic origin in the lower unit. A recent stratum of 0.60 m thick, formed by a very fine silty sand, dull brown (7.5 YR 5/3), of fluvial origin, has been laid down on top of the Lucio Lopez Fm. Dominant minerals of the light fraction of the very fine sands are of volcaniclastic nature (pollycrystalline quartz, glass, alterites and lithic fragments); subordinate feldspars and quartz (Table 1). The heavy fraction forms a small percentage (0.45%). Fig. 5 shows the mineralogy of the sediments of the upper section (quartz, feldspars, calcite, hematite, mica -illite- and allophane). A TL date in the upper section of the formation (in the type profile) indicates an age of 1320$120 yr BP. The soil complex of the middle section belongs to the Hypsithermal period. Very probably the formation was deposited during much of the Holocene. 3.3.6. Little Ice Age Eolian Sands (informal name) A discontinuous deposit of eolian sand covers the sedimentary sequence in the western sector of the area (province of Co´rdoba). It represents a partially deflated dunefield, formed by the reworking of the fluvial sands of Tercero river. This deposit extends 50—80 km northwards of the northern limit of the PSS (established during the Last Glacial Maximum). The sediment is of variable thickness, varying from 0.50 up to 2.5 m within a few meters. It unconformably
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overlies the San Guillermo Fm or the upper palaeosol of the Carcaran a´ Fm in profiles along the Tercero river. It also unconformably covers the upper section of the Lucio Lopez Fm in profiles of the Saladillo and Carcaran a´ rivers (Co´rdoba). The sediment consists of very fine to fine sand, moderately sorted, with visible muscovite (5%), yellowish brown to greyish yellow in colour. It is friable, very calcareous to non-calcareous. Locally, the deposit preserves large duneforms. In other places it is bedded in fine horizontal parallel strata. Precepitates of iron oxides and root casts occur randomly at the base. The sediment contains fine sand lenses with abundant muscovite. It also includes numerous gypsum crystals. This unit was probably formed during the Little Ice Age. 3.3.7. Present and recent fluvial sands Several very young, predominantly sandy deposits are directly related to the channel of the Carcaran a´ river. Recent sands fill abandoned meander channels of the Carcaran a´ river, cut into the Carcaran a´ Fm. Such deposits outcrop in both cliffs of the river, cut into older formations; their lateral extent varies between a few tens and several hundreds of meters. Locally, such sands are covered by contemporary sands and other flood deposits. The recent sands are very fine to medium sand in texture, with abundant mica (5% muscovite), yellowish brown in colour, loose to friable. In general, the sands
Table 2 Quaternary sedimentary formations in Carcaran a´ river basin
were deposited in thick horizontal continuous strata, without unconformities. The light minerals of the very fine sand fraction represent a mixture of two populations, one of volcaniclastic origin, reworked over time by the wind (pollycrystalline quartz, volcanic glass and lithic fragments) and another, less significant, derived from the Sierras Pampeanas (quartz, feldspars, alterites and micas; Table 1). The heavy mineral fraction makes up 3.2% of the total. The present bedload of the Carcaran a´ river is composed of fine and very fine sand, with a little medium sand (mica: 10%). 3.4. Profile of the parana´ cliff at the carcaran a´ mouth The Parana´ cliff has a stratigraphic sequence which is not easy to correlate with the Quaternary of the basin. The column begins at the base with the Ituzaingo´ Fm (Herbst et al., 1976) deposited by the Parana´ during Pliocene-Holocene times. It is overlain unconformably by the Puerto San Martin Fm (Iriondo, 1987), of Upper Pleistocene age, comprising swamp deposits intercalated with typical loess strata and displaying pedogenic horizons near the top. Between the two formations, several paleochannels filled with alluvial and paludal deposits are intercalated. These were formed by succesive avulsions of the Carcaran a´ river. The landscape developed on the Puerto San Martı´ n Fm was later covered and smoothed by the Tezanos Pinto Fm. The sequence ends
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with the San Guillermo Fm, which lies on the Hypsithermal Soil.
4. Environmental evolution during the Upper Quaternary Investigations carried out in the lower basin of the Carcaran a´ river, combined with studies in neighboring zones, have led to the establishment of a regional stratigraphy for the Upper Quaternary in the northern Pampa. Numerous data obtained in the study area have contributed to the identification of different climatic periods which affected the Pampa plain, starting from IS4 (see Iriondo in this volume for a general report). The environmental evolution of the area for approximately the last 65,000 yr BP it is presented schematically in Table 2. Fig. 3 shows the regional stratigraphic column, beginning with the Carcaran a´ Fm, which is a part of the Pampean Sand Sea, generated during the IS4. The sediments of this unit were deposited in a subaerial environment under extremely cold and dry conditions. Later, geoforms were partially eroded during a phase of climatic amelioration in IS3. Differences between outcrops indicate that the climate of IS3 was complex. The stage begins with a rapid change to subtropical, humid conditions, favouring the development of a soil with a well-developed B horizon. This horizon appears discontinuously at different depths, reflecting the dune topography of the Pampean Sand Sea. Later, a semiarid climate was established, leading to the almost complete dissipation of the dunes, which left an extensive plain. The redistributed deposits constitute most of the Carcaran a´ Fm, and overlie the partially eroded remnants of soil filled gullies, suggesting a palaeorelief typical of badland conditions. A second phase of subtropical humid climate during IS3 is suggested by the soil at the top of the redistributed deposits, by the presence of CaCO precipitates of phreatic origin, and by the intercalation of alluvial and paludal strata in the upper part of the Carcaran a´ Fm. During IS3 fluvial belts of major size developed in the area. A new climatic deterioration linked to the ¸ast Glacial Maximum (IS2) is indicated by major remobilization of sands into a new dunefield immediately southwest of the study area. Silt transported in suspension generated the loessic belt to the northeast of the dunefield. Most of the area investigated is located in this belt. The deposition of Tezanos Pinto Fm obliterated fluvial belts developed during IS3, masking the pre-existing relief. In abandoned channels with low discharge, progressive filling took place, generating a paludal facies.
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Maximum climatic deterioration, marked by semiarid conditions, is reflected by the eolian (loess) facies. A climatic improvement, indicating the end of the ¸ast Glacial Maximum (around the 15,500 to 16,500 yr BP) is evidenced by pedogenic CaCO concretions in outcrops of the Tezanos Pinto loess. Semiarid/subhumid climatic conditions, homologous and coeval with the Allero¨d, are interpreted for this period. The upper part of the Tezanos Pinto Fm was deposited during the Late Pleistocene/Lower Holocene dry period, broadly equivalent to the European ‘Younger Dryas’. In the study area a semiarid and cold phase, characterized by widespread fluvial erosion and by modest eolian sedimentation accompanied by wind erosion, generated numerous deflation depressions. The Hypsithermal Humid Period (Climatic Optimum) of the Middle Holocene, characterized by a humid subtropical/tropical climate, is represented by a soil developed at the top of the eolian deposits of the LGM. During the Late Pleistocene/Lower Holocene dry period the main rivers of the basin would have been reduced to semipermanent swamps. The contribution of eolian sediments to the paludal environment is represented by the lower section of the Lucio Lopez Fm. During the Hypsithermal the soil complex of its middle section was developed. A semiarid pulse in the Upper Holocene is indicated by a series of erosional and depositional processes, e.g. generalized deflation of the A horizons of the palaeosoils, the excavation of deflation hollows, and the deposition over the whole area of the San Guillermo Fm. An arid period probably occurred during the Little Ice Age, leading to mobilization of fluvial sands of Tercero in Co´rdoba. Under the present climate, deflation basins became temporary-to permanent shallow lakes; some of the small river channels of the basin experienced reactivation of their fluvial dynamics. The San Guillermo Fm is being affected by incipient pedogenesis. Acknowledgements The author acknowledges the advice of Dr. M. Iriondo in this investigation. The research was partially supported by the Consejo Nacional de Investigaciones Cientificas y Te´cnicas (CONICET). The fieldwork was partially funded from Scientific Research Grant No 5194.94 of the National Geographic Society. References Catt, J.A., 1990. Paleopedology Manual. Quaternary International 6, 1—95. Clapperton, C., 1993. Quaternary Geology and Geomorphology of South America. Elsevier, Amsterdam, 779pp.
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