GIRA ANIZACATE Y CORRALITO

ANIZACATE AND CORRALITO FIELD TRIP

By Jorge Sanabria, Graciela L. Argello and Clarita DassoI. INTRODUCTION

Crdoba central region is characterized by the predominance of hydric erosive processes, which vary according to geomorphology and soil management. There are two very clearly distinguishable situations: one for the piedmont and the other for the plain.

Inside the environment of Crdoba Sierra Chica piedmont, catastrophic fluvial erosive processes are relatively frequent. They have recently left their imprint with fatal results and great damage on the economy. Several factors simultaneously contribute to the phenomenon magnification by producing the so called synergism (sinergia). Namely: abundant precipitations, falling in a short time; runoff favoured by the lack of protection on soils, caused by deforestation and cattle grazing; high slopes at the headwaters basins and urbanizations mostly related to tourism- which are often located without any territorial planning occupying zones affected by natural hazards.

Some events of the last decades of 20th century were dramatic examples on the matter. Flooding and erosion processes took place in Villa de Soto (1992); Villa General Belgrano (1993) and Mina Clavero (1993) (Barbeito and Ambrosino, 1995). But undoubtedly, the most important events were: Arroyo Seco swelling in La Falda, by January of 1995, which caused six deaths, and Arroyo Noguinet or Vilches overflowing, in January 1992, in San Carlos Minas, where about thirty five lives were lost. The last one is related to El Nio.

The event to be analyzed during the field trip, also happened in January 1992, on Anizacate River, affecting Costa Azul quarter.

In the plain, hydric erosive processes take place in the form of rills, sheet erosion and eventually as gullies and barrancos (gully more than five meters deep). The last ones, although certainly not too abundant, may be sometimes spectacular, and that was also a reason to select Corralito barranco as one of the sites to be visited.

Due to the anthropic factor combined with higher amount of precipitation, erosive processes significantly increased in the last 34 to 40 years. Fortunately, in this environment hydric dynamics do not produce human lives losses. Nevertheless, they cause great economic damages, through their incidence on soil productivity, and on road network destruction.

To mitigate these effects, some conservation practices are being applied by some producers. The field trip includes a visit to a rural establishment under exploitation owned by Engineer Fabin Mengo, who has implemented some of those practices.II. GENERAL GEOGRAPHIC LOCATION The area to be visited partly includes Santa Mara, Calamuchita and Tercero Arriba departments in Crdoba Province, Argentina. It is delimited by 64 10 and 64 30 W and 31 45 and 32 05S (See the area indicated by a rectangle in the detailed geological map).III. GENERAL FEATURES OF THE AREA.

IIIa) CLIMATEMain climatic characteristics in the area were analyzed, emphasizing temperature regime, precipitations variability as a function of time along an average hydrologic year, and intense storm occurrence. Climatic and hydrometeorological information was provided by Servicio Meteorolgico Nacional (National Weather Service) through measurements from Estacin Pilar Observatorio and pluvial records from Estacin Crdoba Observatorio (1985, 1992)Thermal Regime: It is considered as mesothermal with a warm summer, as indicated by temperature records for the interval 1971-1990. Annual average temperature was 17C; being the average thermal minimum around 10.5 C in June and July and maximal mean temperature, that of 23,5 C by mid January. Absolute maximal values correspond to December, with 42,6 C in average, whereas the absolute minima occurs in July with 7,3 C below zero.

There are three distinct thermal seasons along the year:

a) Temperate Season between May and August with average temperatures ranging from 0 C to 10 C.

b) Warm Season: occurring twice in a year, the first between March and May and then again from August to November, with average temperatures ranging from 10 C to 20 C.

c) Hot Season: with average temperatures that surpass 20 C in the lapse between November and March, comprising the summer.

Precipitation regime: Pluviometric series continuously recorded along 50 hydrological years (1941/42- 1989/90) at Estacin Pilar were available for the area. Its analysis yielded an estimated Normal Annual Precipitation value of 736mm. Interannual distribution shows alternating humid and dry years.

The most humid hydrologic year (1980/81) reached an annual total of 1169 mm, three times more than the driest one (1971/72) with only 398 mm, incidentally almost similar to the 362,9 mm precipitated in a single month (January) of 1981, according to data series recorded since January 1972 to March 1994 at Estacin La Suela.

Furthermore, two seasonal cycles are clearly distinguishable: one for the summer, from September to March, and the other from April to August, corresponding to winter.

During the summer cycle, rain average surpasses 65 mm/month, whereas during the autumn-winter one, it stays below 35mm.

As an average, every two years 70mm can be precipitated in a single day, and every 10 years that value surpasses 100 mm. For a return period of 50 years, extreme values rise up to 140 mm in a day. Nevertheless, sometimes the statistics fail to predict some events, as was registered in Estacin La Suela between 1974 and 1984 where the amount of 100 mm was surpassed three times, being the latest far beyond the 140 mm to be expected as absolute extreme. These anomalous rains occurred on November the 11th of 1981 with 101 mm, February the 19th of 1974 with 109 mm, and January, the 9th of 1984 with 192 mm. These features are typical within climatic transitional zones such as those exhibiting a semi-arid regime.

Intense storms. Based on studies of precipitations data series, internal temporary distribution patterns of intense rains (precipitated in Central Hydrometeorological Zone) were examined, and characterized by pluviographic series corresponding to 35 years (1943-1977) recorded at Estacin Crdoba Observatorio (Dasso et al., 2001; Dasso, C, 2003; Caamao et al.,1999).

Applying criteria such as: minimum precipitated lamina (20mm), minimum intensity (1mm/min) and 2 hours without rain to separate the events, 157 intense storms were found and then divided into sextils (1/6 of the total duration) for further analysis. Once five categories of storms were established according to their total duration, it was noticed that most of the events (56) lasted between 3 and 6 hours, whereas only 15 are longer. Also the greater intensity was inside the first sextil, both for short (2 hours or less) and long storms (more than 2 hours).III. b) GEOMORPHOLOGY

Three geomorphological associations were distinguished by applying the Lands Classification System proposed by Sayago (1982): 1. Vertiente Oriental Semirida de la Sierra Chica (Sierra Chica Semiarid Eastern Slope),

2. Depresin Perifrica (Peripheral Depression) and 3. Plataforma Basculada (Tilted Platform).

The first two constitute Sierra Chica piedmont, whereas the third is a relative flat plain, internally divided into two subassociations: Plataforma Basculada Ondulada (Undulated Tilted Platform) and Flat Tilted Platform (Plataforma Basculada Plana) (Sanabria et al., 2004).

1.The Sierra Chica Semiarid Eastern Slope Association (Sanabria, 2000), is a highly dissected structural slope eastwards tilted, crossed by antecedent rivers (Suqua, Los Molinos, Anizacate, Ctalamochita) where numerous smaller rivers and arroyos headwaters and high basins are located, namely Arroyo Almacuna, Falda del Carmen Alta Gracia and San Jos.

Its petrology is predominantly constituted by the Cambrian crystalline basement (75% metamorphic rocks, and 25% igneous ones). Some Cretaceous and Tertiary sedimentary rocks are also outcropping in small areas.

The inverse faults system trending southwards determines the main morphostructural style of the range, while the intercrossed systems trending NW-SE; NE-SW and E-W control those shapes generated by exogenous factors.

The general landscape is that of hills with acute summits and steep shoulders, generating V-shape valleys, along which numerous arroyos. In some places, small undulated tectonic pampas de altura (a sort of plateaus) where alluvial sediments can be found, are visible.

Main drainage pattern is angular- rectangular, whereas the subordinate one is dendritic or subdendritic.

2.Peripheral Depression Association, is a graben located east of Sierra Chica, in Crdoba Province, extending from Sierra Las Peas at the South, to Jess Mara in the North. Its west border is the bedrock and the east one is the Tilted Platform. This graben was first described by Capitanelli (1979), who explained it as being originated or enhanced by reactivation of block movements between two faults: that of Sierra Chica steep slope and another one located east of it.

Many geologic studies have been carried out in the area, but no one has yet established the age of the graben. Sanabria (2000) proposed that it would have been formed at some time between late Pleistocene and Holocene, and reactivated in middle Holocene.

This Association displays an irregular longitudinal profile, where flat surfaces from the central area, gentle slopes lying W-E, and rivers valleys and smaller streams, constitute the landscape.

Typical and retransported loess are the predominant materials, followed by fluvial and alluvial sediments. Slopes range from 1% to 3% in hills and alluvial fans, whereas they are less than 0.5% in flat sectors. The whole unit is strongly controlled by structures. As already mentioned for Sierra Chica Semiarid Eastern Slope Association, the southern faults system, responsible for the graben, is the main morphostructural component. At lower levels, landscape is controlled by pre-Quaternary and Quaternary aggradational landforms depending on climatic changes -dissected alluvial fans- and by criss cross faults systems with predominant NE-SW and NW-SE course. Presumably this structural control gave shape to the ancient surface on which loess was deposited.

There are noticeable evidences of neotectonic movements such as a knick point 4 to 6 meters high, facing west, and exhibiting N-S trend, easily seen in aerial photographs as a clear and undulated line, caused by strong hydric erosion that has exposed the Ck Horizon of soils. It seems to be a neotectonic reactivation of the Calera-Mal Paso fault (Gordillo and Lencinas, 1972). This lineament is also visible east of Despeaderos Highlands where its topographic expression is fuzzier. Hydric erosion processes occur in rills and sheets favoured by slopes, intense precipitations, soil management, and most of all by loessic sediments susceptibility to erosion.

Geomorphological Units of the Peripheral Depression Association in the area (Fig. 1).

2.1 Alluvial Paleocones. They are bordering the Tilted Platform Association (Plataforma Basculada) which lies easterly. They consist of conglomeradic outcrops, possibly Cretaceous and Plio-Pleistocene in age. They display convex hills with slopes exhibiting an average dip up to 3% and severely affected by rill erosion, intersected by tectonic valleys lying NW-SE and NE-SW, originating a fernlike drainage pattern

2.2 Dissected Alluvial Paleocones covered by loess. (Sanabria et al., 1996). They are constituted by Cretaceous, Tertiary and Quaternary conglomerates, covered by loess which was probably deposited during mid to late Pleistocene and Holocene. Loess thickness average varies from 0.50 to 50-55m.

2.3 Loessic central plain (Sanabria, 2000). It is a plain with very low slope, minor to 0.5 %, occupying both strips north and south of Anizacate and Los Molinos rivers. At the north, it gradually turns into Anizacate River high terrace through a fuzzy border. Constituent materials are typical and retransported loess

2.4 Dissected Alluvial Paleocones with a thin Aeolian Cover. These are small extent landforms located north of Despeaderos. They are convex rises, with a N-S strike which exhibit flat- concave and regular slopes with a mean value of 3%. They are covered by aeolian sediments less than 50 cm thick, loamy textured, with scattered fine gravels.

Locally, conglomerates from the paleocones, with sandy matrix and blocks up to one meter in diameter can be found. These landforms have been severely eroded in rills and sheets, so that CaCO3 belonging to Ck horizon became exposed on the surface.

2.5. Anizacate and Los Molinos Fluvial Valleys. Xanaes river results from the confluence of Anizacate and Los Molinos. Both of them are antecedent rivers and are clearly controlled by Sierra Chica main faults systems. Anizacate River strikes NW-SE, whereas Los Molinos flows from SW to NE, but both of them follow a rectangular drainage pattern.

There are three terrace levels, possibly as a response to climatic changes. The high terrace is covered by the most recent loess, with an average thickness of nearly three meters. Boundaries between the high terrace north of Anizacate River and the central plain are so diffuse that to establish the extent of each unit was only possible by taking into consideration the presence or absence of structural control in drainage. Wherever there is a lack of control the unit was named terrace. This criterion was selected to depict different behaviours of those materials underlying loess, when affected by tectonism. It was considered that alluvial cones, and eventually also that loess equivalent to the Pampean Fm, are more competent than fluvial sediments.3. The Tilted Platform Association, is a part of the ancient bedrock which was separated as a whole from the mountains, by the faults system that uplifted Sierra Chica while giving birth to the Peripheral Depression.

It was progressively buried by sediments increasingly thick towards the east, due to its gentle regional slope. Being both associations simultaneously generated, it can be assumed that it would have formed during late Pleistocene, with reactivations during the Holocene.

Same as in the Peripheral Depression, some clearly defined river basins are located within the association, namely, (mentioned from the northermost to the southern ones) Rafael Garca- Lozada; Xanaes (or Segundo) River- Laguna Larga; Cortada de Graso and Corralito. At the specific area to be visited, that is between Suqua and Xanaes rivers, and because of the different landforms, two distinct regions, considered as subassociations can be separated within the Association: Undulated Tilted Platform in the western sector, and easterly, the Flat Tilted Platform

3.1.The Undulated Tilted Platform Sub-association is located next to the Peripheral Depression and shares with it the already described following landforms: Alluvial Paleocones; Dissected Alluvial Paleocones with a thin Aeolian Cover and Dissected Alluvial Paleocones covered by loess.

3.2. Flat Tilted Platform. Sub-association As can be deduced from its name, it is a surface where the level differences between minor landforms are very small. The general eastwards slope does not surpass 0,5%. Surface materials are constituted by typical and retransported loess whih were deposited during late Pleistocene, (or perhaps Holocene?).

III. c QUATERNARY GEOLOGY

There is not any profile in the studied area comprising the whole Quaternary stratigraphic sequence and for this reason, the scheme that is analyzed in this paper was obtained by gathering data from profiles located at these and at other neighboring basins.

Sequence beginning at the older materials and including prequaternary rocks. Basement: Cambrian cristalline rocks, only visible as relative small and discontinuous outcrops at the westernmost sites of the area and becoming much more frequent towards the ranges.

Conglomerates: They may be seen on Xanaes River barranco and at small superficial outcrops north of Despeaderos. Cretaceous aged and probably equivalent to Saldn Formation sensu Santa Cruz (1978).

Red clays Calcrete bank: They are found at a gully on Arroyo Soconcho and are constituted by red clays, 3 to 4 meters thick, with no visible bottom, underlying a calcrete sheet, about 8 meters in thickness, panelled at the base, gradually changing to columns and then into a mantle at the top. They might represent the Tertiary -Quaternary boundary?

Conglomerates. They are located south and north of Despeaderos town. They are on top of those red conglomerates equivalent to the Saldn Fm, already described, lying on an erosive discordance. They include coarse gravels and blocks as big as 1 m in diameter, with sandy matrix and calcium carbonate scattered in the mass. These sediments could be related to Alpa Corral Formation. (Cant, 1992) and Estancia Belgrano Fm. sensu Santa Cruz. (1972, 1978), both Plio-Pleistocenic.

Clayey silts: They are loessic, loesslike and fluvio-.aeolian sediments. Loess and loesslike materials are at the base of those profiles located 6 km west of Lozada, on Route 45 and at Monte Ralo and Corralito barrancos. They do not have any visible base and are constituted by dense clayey silts which include fragipans. All the profiles exhibit pedocomplexes. On rivers benches, they are missing, apparently due to erosion, but they can be found below younger sediments, outside the fluvial terraces, within the units: Loessic Central Plain and Dissected Alluvial Paleocones covered by loess. Boreholes yielded average thicknesses around 55- 60 m (minimum thickness: 25 ms; maximum: 100 ms).

Silts would have been deposited under arid to semi-arid conditions, whereas paleosols would have developed during humid climates. They are related to Pampean Fm. (Pampiano) sensu Cant (1992) and possibly, to the lower member of Ro Primero River Fm. (Santa Cruz,1972, 1978). Their age is Mid to Late Pleistocene. TL (Thermoluminiscence), OSL (Optically stimulated luminiscence) and IRSL (Infra red stimulated luminiscence) datings produced ages of 99.7 17.5 ky at the base and 55.6 10 ky at the top of Corralito gully. In Lozada, the results range from 105 to 50.1 ky.

Sandy sediments with gravels and silts and clays intercalations: They are fluvial sediments that are located in the Anizacate and Xanaes rivers terraces, in flood plains and channels facies. They show laminated sands and silts intercalated sequences with U shaped cross-stratification, planar stratification and also gravel banks. In some sites there are up to three palaeosols. This sequence ends with two calcrete banks, being the lowest a mantle, while the uppermost consists of concretions. These deposits were developed under a humid climate and can be correlated to the Chocancharava Fm (Cant, 1992) and, possibly, with the upper member of Ro Primero Formation sensu Santa Cruz (1972, 1978), all of them from Late Pleistocene Silts:They are found in the northern terrace of Anizacate River, lying erosively discordant on sediments equivalent to Chocancharava Fm. They are aeolian sediments with 70% of silt, mostly coarse, regarded as typical and retransported loess (Argello et al., 1996). At the north barranco of Anizacate River they are approximately 3 meters thick, homogeneous, lacking in sedimentary structures, yellowish brown (7,5YR5/4), when wet, somehow biodisturbed, with abundant powdered calcium carbonate scattered in the mass. Paleosols are not found. A TL dating at the north barranco of Anizacate River yielded 10.01 0.66 ky. Loess would have been deposited under arid to semi-arid climate during an interval between Last Glacial Maximum and Pleistocene Holocene boundary.

According to other datings, available for the whole region, the sedimentary event would have lasted until mid Holocene. In Corralito, ages range from 41.4 6.1 to 7.4 1.0 ky when dated by TL, whereas using the IRSL method, the age at the top is 14 ky. In Lozada an age of 5.74 ky for the top of the sequence was established by OSL.

Sands and gravels. They lie ont the middle terraces of Anizacate and Los Molinos rivers, and they are heterogeneous fluvial deposits, composed by alternating layers of materials such as sands, fine silts, and sandy gravels which indicate climatic fluctuations within an interval regarded as humid to subhumid. These deposits would have been leid during the Hypsithermal, which according to the already quoted dating in Lozada, would have begun approximately 6 ky ago. They could be correlated with La Granja River Formation (Santa Cruz, 1972, 1978) and with Arroyo Las Lajas Fm. Sensu Cant. (1992), Mid Holocene in age.

Sands and gravels. They are fluvial sediments from the low terraces of Anizacate and Los Molinos rivers. Their chronostratigraphic position is not established yet, but they are regarded as late Holocene. III. d VEGETATION

III. d. 1 NATURAL VEGETATION

According to Luti. et al., (1979), the sector of the zone which lies within the plain environment, belongs to the "Espinal", which in turn would be almost entirely inside the "Distrito del Algarrobo (Carob Tree District") as described by Cabrera (1968).

The Espinal may be considered as an impoverished "chaco" with species intruding from the "pampas steppe ". Due to forest and agricultural practices, most of the formation has already disappeared. Only isolated remainders and bibliography allow a partial reconstruction of the original forest.

Dominant species in the Espinal are black and white carob trees (Prosopis chilensis, P. nigra), but there are also white quebracho (Aspidosperma quebracho blanco), maitn and itn. In environments holding thicker substrate, espinillo (Acacia caven) is dominant. There are also talas (Celtis tala), chaares (Geoffroea decorticans) and other species, generally xerophytic

Being agriculture the main resource in the region, deforestation has been very intense and only at some highlands of Altos de La Lagunilla, at the outskirts of Despeaderos and, in the benches of Xanaes River, a few representatives of arboreal species were preserved. Flora is mostly composed of grasses and scarce xerophytic monte which includes carob trees, talas, molles, garabatos, chilcas and jarillas.

Verena and Methol. (1979) in their Description of Hoja 20i, Alta Gracia, very much agree with that already exposed by Luti et al. (Op. cit.)

In the Argentine Republic Total Atlas (1981), three districts with different vegetations are described for Cordoba Province, considering the influence of geomorphology, climate and soils, on the characteristics and distribution of the vegetation. The Algarrobo District corresponds to the Peripheral Depression, and is a relictual one, very much exploited for economic purposes and to generate agricultural and grazing lands. Black and white carob trees, mixed with quebrachos at the contact with the chaqueo forest, espinillos, talas, piquillines, chaares and other xerophytic species featured this district which at overexploited areas gave birth to a renewed fachinal espinoso (thorny fachinal).

Bernardes (1981), divides Crdoba province into three units: 1) Mountain District, in which the vegetation is distributed in altitude levels influenced by winds, exposure and humidity, 2) Algarrobo District (Provincia del Espinal), next to the previous one, and, 3) Pampean District at southern and western Crdoba.

Prieto (1996), locates Southern Crdoba Province inside the vegetation units called Prairie pampa (Prairie Grassland) and Inland pampa. The last one is divided into two minor units: Pampa central o plana (central or flat Prairie) that occupies parts of Buenos Aires province, and Pampa Oeste (West Pampa), in Crdoba, northern La Pampa and southern San Luis, all of them with less available humidity.

The most relevant vegetation is related to the dunes (psammophytic steppe), where dominant species are: Panicum urvilleanum, Poa ligularis and Halis argentea.

Mid-western of Crdoba is inside Espinal Province (Cabrera, 1968), at Distrito del Caldn unit (Calden District), where Prosopis caldenia is dominant.

In Anizacate area the vegetation of the plains is in contact with the ranges vegetation, specifically with Floor 1 of the Mountain Forest- between 500 and 1,350 masl, which is characterized by the presence of white carob tree or algarrobo blanco (Prosopis chilensis); white quebracho (Aspidosperma quebracho blanco) and tala (Celtis tala). Additionally also espinillos (Acacia caven); chaar (Geoffroea decorticans), and other species can be found.

III. d. 2 CULTIVATIONS

The whole area is nowadays almost exclusively used for agriculture. A study carried out through a sequential analysis of air photographs of the years 1962, 1970 and 1987 and satellite images of the year 1994, by Apestegua and Luque. (1996), in a subbasin belonging to Rafael Garcia- Lozada mayor basin, that can be considered as representative, determined a fast advance of agriculture along the available land.

Land use in percentages changed as follows: monte covered 48% of the terrains by 1962; 32% in 1970; 27% in 1987 and only 13% in 1994. Agriculture increased up to 87% of the total area in 1994. South of this region, between the rivers Xanaes and Ctalamochita, the advance of the agricultural activities has very likely been even faster.

Balbis, et al. (1991) by comparing aerial photographs from years 1962 and 1987, determined the cultivated surface variation on each geomorphological unit for La Lagunilla -an area not far away from Anizacate- resulting the unit described as Gently undulated aeolian hillslopes, the most affected one: 17% of its total surface was cultivated by 1962, and in 1987, the exploited surface had grown up to 91 %. The 70`s marked a special phase in the development of the agriculture, by incorporating soya as one of the main crops.

Before this epoch, the most representative cultivations were wheat, corn, beans, sunflower, etc. and there was also cattle rearing, allowing alternation in soil management to a certain degree. The alternating system exploitation was replaced by permanent agriculture, especially with productive systems based on two annual crops.

Nowadays, the most extended systems are soya/ soya, with an Autumn break, or wheat/ soya. Wheat higly depends on the soil humidity during sowing, which determines that only 20% of the cultivated surface is used to grow it.

Also corn or sorghum can replace soya, but always covering low percentages of the land.

Conditioning of the sow beds is a very important factor in hydric erosion processes.

Until 1987, farming was conventional and involved multiple action double rake, chisel, field cultivator, and teeth rake. This farming procedure favoured by relief, climatic characteristics and, fundamentally, by loess susceptibility, facilitated water erosion processes which became significant and costly in many Geomorphological units.

Calculations carried out in a soya field at Corralito where a gully 500 meters long and 0.70 m deep had been formed, indicated that each centimeter of soil loss meant a decrease in soya production of about 35 kg per hectare/year (Apestegua et al., 1987).

Since 1987 direct sowing has been a current practice, and it may be estimated that 90% of the farmers use it. This practice produces a decrease in erosive processes, because the soil is protected at the beginning of summer rains. In addition, a few producers build terraces as a conservation strategy.

IV. ANIZACATE

Anizacate and Los Molinos rivers, both with headwaters within Sierras Grandes piedmont, together give birth to Xanaes River. Villa Anizacate is located at 31 45 South and 64 25 West by the homonymous river.IV. a. GEOMORPHOLOGY

The site to be visited lies within the already described Peripheral Depression Association (see Fig. 1) , and specifically within the Unit named as Anizacate River Fluvial Belt (Fig. 2 and 3).It exhibits a NW-SE orientation, remarkably controlled by tectonic structures. Its width ranges from 500 to 2,000 meters. Two facets were determined:

1. Anizacate River Ordinary Streambed

Anizacate River exhibits moderate slope and straight course due to a West-East structural control extending between the west border of the studied area and Costa Azul quarter. From this point to the confluence with Los Molinos River, its behaviour locally fluctuates between angular, rectangular and meandering.

2. Alluvial Terraces

This unit comprises three main levels and several minor levels, with ancient channels and evidences of erosion caused by overflowing in some places of the low terraces (Fig. 4).

Sediments include very fine sands, silts and gravels in a sandy matrix.

At the study site, a meander presently cut off by overflowing- is to be seen, followed by a wide bend and then a straight course striking NW-SE (Fig. 4).

Fig 2. Detail of Costa Azul quarter. Meander cut-off.

Fig 3. A: The channel nowadays, B: cut-off meander, C: Route 5 Bridge and D: Sierra Chica on the background.

Fig 4. High and intermediate terraces in Ro Anizacate. See destroyed houses

PROCESSESCosta Azul is a quarter belonging to Anizacate locality, with a very high percentage of week ends and summers houses. In 1992, a meander cut-off produced a very active erosion at the riverbank, which in turn caused great economic damage. At the high terrace an hotel swimming pool fell down, and the hotel itself was also partially affected. (At the intermediate terrace there were summers houses breakages and settlements as well. (Figs. 5-7).

Another effect was the migration of the riverside towards the route that connects Alta Gracia and Jos de la Quintana (R-5). This event took place during a very rainy year, related to El Nio phenomenon, with an annual total rainfall amount of 1001.3 mm, 120.5 of which, were precipitated in January. In order to protect the mentioned road, D.I.P.A.S. (a governmental agency of Crdoba province) built in 1994 an overflowing channel at the left bank of Anizacate River. Instead of solving the problem, this channel introduced a new disturbance into the system, and has by now practically disappeared.

At the present time the process is still in action and advances towards Costa Azul generating new damages as seen in Figs 8, 9 and 10.

In an attempt to remediate the losses, some land filling works were made on the intermediate terrace at the north margin, but due to their position within the active zone, their future is uncertain.

Fortunately there were no deaths to regret during the event, though many people wanted to leave the site, and many posters with the words for sale appeared on the front doors.

Fig 5. Anizacate River overflowing and swimming pool fallen down. Fig 6. Fallen house at the intermediate terrace. Fig.7. Same house seen from the front

Fig 8.Affected house on the intermediate terrace and land reclamation attempt by filling

Fig 9. Detail of fig. 8

Fig 10. Picture taken in 1994, when the fallen house seen in fig. 9 was still standingV. CORRALITO AND SURROUNDING AREACorralito is located in Tercero Arriba Department at 64 10 west longitude and 32 00 south latitude (Fig. 11).

V. a GEOMORPHOLOGYThe geomorphofological units visible at the undulated Tilted Platform- especially at the west sector - are almost the same as in the Peripheral Depression.

That is the case for the previously described Alluvial Paleocones, Dissected Alluvial Paleocones Covered by Loess, and Dissected Alluvial Paleocones with a thin Aeloian Cover

As a general feature, the paleocones exhibit N-S trend in the western areas, and are W-E oriented at the eastern zone. These hills become progressively fuzzier towards the east and they contour a series of wide river basins such as those named Caballero (the southernmost), Corralito, Monte Ralo and Cortada de Graso at the North. Inside every one of these river basins, large gullies and barrancos can be found.

The low hills are crossed by very noticeable draining courses, generally controlled by tectonism.

The Flat Tilted Platform consists of a monotonous plain, gently dipping eastowards, where draining lineaments are only hardly noticed and build a spider-like drainage pattern (braided centripetal)

The general slope conditions the rainwater runoff, causing overflowing between river basins, and leaving behind many non-functional elongated lowlands.

Fig. 11. On 1970 series the gully is not visible yet, but it is clearly developed on 95s series. Notice the difference between both flights. 1) Corralito Barranco (see Fig.15); 2) road on which the gully was developed; 3) Sedimentation area (plain alluvial cones?); 4) Corralito locality and 5) new reactivation level (see Fig. 16).V. b. EROSIVE PROCESSES AND CONSERVATION PRACTICES

Alluvial erosive processes in the Tilted Platform are basically rill and sheet erosion, gullies and barrancos

Conditioning factors which determine both the intensity of the erosion and the resulting landforms are: geomorphology (topography and sediments), climate, (precipitations) and vegetation (soya cultivations).

Soils were classified as Molisols (typic Haplustols) and have developed from a very friable loess with silt, predominantly coarse, percentage of around 70% (Argello, et al., 1996 and Argello and Sanabria, 2000) and highly susceptible to hydric erosion.

Precipitations take place mainly in summer, and are frequently caused by convective storms, summing up many milimeters in a few hours.

Agriculture is dominated by soya and has replaced natural vegetation. Nevertheless, at the present time, due to market fluctuations, also corn is expanding.

Soya was introduced in the region during the 70s and had an explosive expansion through the time.

Soils are prepared for soya cultivation at the beginning of the rainy season, when torrential storms eventually take place.

Therefore, landforms, and mostly the slopes, determine the erosive processes. The most intense ones take place in the Undulated Tilted Platform (rill and sheet erosion), whereas gullies and barrancos occur bordering the flat environment as the runoff waters gather to find the required slope to keep flowing.

By analyzing aerial photographs from the years 1970 and 1995, (approximate scales 1:20.000 and 1:50.000 respectively), the differences in erosive processes intensity can be appreciated (Fig. 11 and 17).

During the field trip some erosive effects will be seen affecting San Agustin-Corralito route and a rural road (Figs 12 and 13).

It is also planned to visit Corralito Barranco which was generated during a torrential rain in 1975 on a rural road which can be seen in Fig. 11. It cut through the route which connects Corralito and Monte Ralo and also destroyed the railroad. In several opportunities remediation works were made on the route, such as huge culverts which were always eroded, as can be seen at present (Fig. 14) Its approximated length is 25 km and displays two different levels, produced by later reactivations. One of them, near Corralito locality is about 10m wide and approximately 11 ms in depth. Along its profile, a sequence of two different loess including paleosols at the base is visible (Fig. 15).

About 5 km to the east, is the second and bigger floor. It is around 20 meters, both in width and depth (Fig. 16).

In order to mitigate these erosive processes, with the encouragement arising from Law N 22,428, related to Soils Conservation Promotion and its Regulating Decree N 681/81, some soil conservation practices are being applied (Fig. 17).

Fields systematization is made through the application of non plowing farming, cultivation along terraces, slope intersections or contour lines, altogether with the corresponding vegetated channels, cultivation rotation and, eventually, the construction of mud dams

Fig 12. Erosion affecting San Agustn Corralito route.

Fig 13. Gully on a rural road.

Fig 14 Engineering works breakage (culverts and railroads) at Corralito barranco on road to Monte Ralo.

Fig. 15 Corralito barranco, first reactivation level. 5 km from headwaters (see Fig. 11)

Fig. 16 Corralito barranco, second reactivation level. 5 km east of Fig. 15.

Fig 16. Vegetated channel on soya crop.

Fig 17. Aerial photographs from the years 1970 and 1995 where the differences in erosive processes intensity can be appreciatedVI. REFERENCES

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