LOESS IN ARGENTINA: TEMPERATE AND TROPICAL (Report on the International Joint Field Meeting of the INQUA Loess Commission and Climates of the Past (CLIP) in Argentina). May 15-21, 1998.
This field meeting, organised by Martin Iriondo and his co-workers, went from the heart of the Pampa (Carcara¤ a basin, Santa Fe province) to the very north-eastern end of Argentina (the Iguazu falls), crossing the provinces of Entre Dos Rios, Corrientes and Misiones. One and half day has been devoted for oral presentations in Parana. Organization of this meeting was perfect. Martin Iriondo and his co-workers must be thanked for all their efforts. Participants were supplied with detailed, didactic and well arranged field guide books.
The first three days, we toured the pampean loess. Participants unanimously agreed on their typic character, however many problems stand concerning the stratigraphy and the soil forming processes which have affected them. Participants could recognized that studying pampean loess is not an easy task. The Pampa is in fact a huge zone of subsidence in which aeolian materials have accumulated in consequence of which only very shallow sections are visible along river beds, such as along the Carcara¤ a river. One exception should be mentioned the east bank cliffs of the Parana river in which thick natural sections exist, however with thin loess cover. Consequently pampean loess have to be investigated in quarries which are rather rare. Only two quarries were visited during the excursion, both located west of Rosario city.
In spite of an almost flat landscape, pampean loess sections appear varied and complex. Precise correlation seems to be difficult. Presently only main sedimentary units have been defined for the whole pampean loess belt, moreover they are restricted to the Last Glacial Cycle. Sediments and paleosols dating from isotopic stage 5 have notyet been recognized. Truncations and reworked loess could be observed in the first quarry (Tortugas) while in the second (Carcaraņa) loess sedimentation was apparently more progressive, however a discordance was visible in the lower part. Calcium carbonate had aggraded in beds with a nodular morphology (toscas) in pampean loess during episodes of the last glacial period while others hardened layers (duripans) are supposed to be cemented by silica.
Geologists and pedologists have a somehow controversial point of view on the Holocene pedo-sedimentary history of the Pampa. Geologists distinguish : (1) during the lower and middle Holocene, the development, on earlier deposited loess, of a paleosol, the Hypsithermal paleosol, which is characterized by an argillic horizon, (2) an aeolian erosion of the Hypsithermal soil around 4000 yr. BP, (3) a present day pedogenesis characterized by a mollic epipedon which is developing on the 4000 yr. BP deflated gray silt. For pedologists, the argillic B and the mollic epipedon belong to a unique soil profile which eventually had experienced Holocene climatic fluctuations. Pedologists consider that the lighter texture of the mollic epipedon results of the process of clay eluviation.
Pampean loess differ largely from Eurasian and Mississippi basin loess, however they have some common characters, calcium carbonate and silica (duripans) accumulations with loess from the western United States (e.g. the Colombia plateau).
In conclusion, further investigations on pampean loess are strongly needed. These loess are the only loess with those of New Zealand from the southern hemisphere. Probably further studies will provide abundant and accurate data. The great thickness of loess supposes a good time span resolution. The wide variety of features present, possibilities of establishing hierarchies between these features will enable precise paleo-environmental reconstructions. However these investigations seem however difficult and expensive. Coring appears necessary to reach sediments and paleosols of the stage isotopic 5 and eventually older sediments. Probably many cores would be necessary because of the stratigraphy variability. Systematic, detailed micromorphological analysis should be performed in order to : (1) detect all the minerals susceptible to trace the origin of the parental materials of these loess, (2) to identify erosional and sedimentary features for reconstructing intensities of erosion and mode of sedimentation and ascertain abrupt events, (3) to recognize pedological features, (4) to establish hierarchy between all present features pedological as well as sedimentary in order to reconstruct sequences of pedo-sedimentary events. Tl dates are already available, however OSL should be preferred to TL. More generally more and more reliable dates are necessary, such as AMS on organic matter of loess and paleosols. Carbon stable isotopes should be tested on secondary calcium carbonates for deciphering environmental conditions of their accretion and on organic matter of loess and paleosols in order to reconstruct the ratio C4/C3 vegetation.
Two and a half days were devoted for the red materials lying on the Cretaceous basalts and sandstones of Corrientes and Misiones provinces which are considered by Martin Iriondo and co-workers to be tropical loess. The recently built, national highway n°14 which runs south to north on the main water divide between Parana and Uruguay rivers offers on hundred of kilometers of magnificent sections. Four of these sections (and a few additional ones) were visited during the excursion. These red materials consist, with only slight variations in thickness, of, from bottom to top : (1) a basalt progressively weathered in kaolinite and ferric oxides, (2) a stone line, (3) a red surficial deposits (10R in the Munsell colour chart).
The proposal of Martin Iriondo and his co-workers to consider this red surficial deposits as tropical loess was intensely discussed. All loess specialists agreed that these red deposits do not fulfil the petrologic criteria for loess. These red deposits for the pedologists are Oxisols merging locally .into Ultisols (following the US Soil Taxonomy) which cover huge surfaces in the humid to sub-humid tropics (Brazil, sub-Saharan Africa, Far-East, Australia) on old, stable surfaces. The confrontation between geologists which emphasize on erosional and sedimentary phenomenon and pedologists which consider essentially the soil forming processes, was positive for both. Both agreed that the weathered basalt results of an in-situ supergene weathering of monosiallitic type. Various hypothesis have been proposed for the origin of the stone-line which truncates the weathered basalt. Any of them appears fully satisfactory. The main discussions have concerned the red deposits lying above the stone-line which argentian geologists consider to be wind deposited. A sedimentation by water appears almost impossible because these red deposits occur on top of rounded hills which are frequently on dominant positions on water divides. A correlation between pampean loess and these red deposits is not possible because loess and these deposits are separated by a hiatus of some hundred of kilometers. The Martin Iriondo and co-workers investigations on these questionable tropical loess will undoubtedly force the pedologists to revise theirs concepts on the genesis of Oxisols-Ultisols sequences.