Plio-Pleistocene red clay deposits in the Pannonian basin: A review more

Quaternary International 240 (2011) 35e43 Contents lists available at ScienceDirect Quaternary International journal homepage: www.elsevier.com/locate/quaint Plio-Pleistocene red clay deposits in the Pannonian basin: A review } János Kovács a, *, Szabolcs Ákos Fábián b, Gábor Varga b, Gábor Újvári c, György Varga d, József Dezso e a Department of Geology, University of Pécs, Ifjúság u. 6, H-7624 Pécs, Hungary Department of Physical Geography, University of Pécs, Ifjúság u. 6, H-7624 Pécs, Hungary c Geodetic and Geophysical Research Institute, Hungarian Academy of Sciences, Csatkai E. u. 6-8, H-9400 Sopron, Hungary d Geographical Research Institute, Hungarian Academy of Sciences, Budaörsi út 45, H-1112 Budapest, Hungary e Institute of Environmental Sciences, University of Pécs, Ifjúság u. 6, H-7624 Pécs, Hungary b a r t i c l e i n f o Article history: Available online 23 December 2010 a b s t r a c t Terrestrial red clays underlying Quaternary loess deposits, or filling fissures and recently existing caves in limestone are named Tengelic Red Clay Formation and Kerecsend Red Clay Formation (Middle Pliocene to Lower Pleistocene). They occur in three types in Hungary. (1) The oldest red clays are mainly in situ weathering crusts rich in kaolinite, formed in warm, humid, subtropical or monsoon climate; (2) the younger type is rich in smectite and goethite; and (3) illite and chlorite dominant in the youngest part, which formed under warm and dry climates in savannah, steppe or forest steppe environments, and is of wind-blown origin. Representative samples were selected for study from a large number of profiles. Mineralogical, some micromorphological, and geochemical investigations of typical samples of red clays in Hungary were performed. This review focuses on the origin, development and distribution in the Pannonian basin. Ó 2010 Elsevier Ltd and INQUA. All rights reserved. 1. Introduction The red clay in Hungary (Tengelic Red Clay Formation: TRCF; Kerecsend Red Clay Formation: KRCF) is overlain by loess paleosol sequences (Sümeghy, 1944; Jámbor, 1980, 1997; Halmai et al., 1982; } Fekete et al., 1997; Schweitzer and Szöor, 1997; Viczián, 2002a,b; Kovács, 2003, 2008). The thickness of red clay ranges from 4 to 90 m (Jámbor, 1980, 1997; Halmai et al., 1982; Schweitzer and } Szöor, 1997; Földvári and Kovács-Pálffy, 2002; Viczián, 2002b; Kovács et al., 2008). Hungarian red clays had been recognized by Lóczy (1886), but research on them was subsequently intermittent. Different views on the formation, properties and distribution of red clays have been published. Early scientists described the red clay as a variety of loess, sediment formed by the deposition of windblown silt (Lóczy, 1886; Sümeghy, 1944). Some authors drew parallels between red clay formation and the process of bauxite formation (Vendl, 1957) or considered red clays to be the weathering product of bauxite (Vadász, 1956). Kubiena (1958) studied the formation of red clays thoroughly. In his opinion red clay soils are the products of different processes. He identified two main processes of formation: lateritization and rubefication. Whereas lateritization is associated with the mobilization and washing away of silicic acid, rubefication is the process whereby the iron hydrous oxides coagulate within a short time after the dissolution of iron from primary minerals. He explained the difference at the micromorphological level. Bárdossy and Aleva (1990) also distinguished bauxite, bauxitic clay and terra rossa. They considered bauxite to be a product of soil formation as well, which could develop in situ or be the result of redeposition. According to Fekete et al. (2005), red clays in Hungary are similar to the tropical and subtropical ferrolite soils regarding their formation and mineral characteristics. In the last decade, there have been geological, mineralogical, and } pedological studies (Jámbor, 1997; Schweitzer and Szöor, 1997; Fekete, 2002; Földvári and Kovács-Pálffy, 2002; Viczián, 2002a,b; Fekete et al., 2005; Kovács, 2007; Viczián, 2007). However, complex studies concerning the geology, geomorphology, mineralogy and geochemistry of red clays have been carried out by only } a few authors (Schweitzer and Szöor, 1997; Kovács, 2003, 2007; Vincze et al., 2005). Recent investigations also demonstrate the eolian origin of the red clay in Hungary (Kovács, 2006, 2008; Kovács et al., 2008). This paper reports the results of studies carried out on Plio-Pleistocene red clays and paleosols in Hungary. 2. Regional setting and stratigraphy The red clay sediments in the Pannonian basin are known from both exposures and boreholes. Sections selected for this study are * Corresponding author. Fax: þ36 72501 531. E-mail address: jones@gamma.ttk.pte.hu (J. Kovács). 1040-6182/$ e see front matter Ó 2010 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2010.12.013 36 J. Kovács et al. / Quaternary International 240 (2011) 35e43 Fig. 1. Schematic map showing the study areas and sites (see also in Figs. 3 and 4) of the red clay sediments in Hungary (modified after Horváth and Bada, 2005). located mainly in the foothills of the Hungarian mountains, except for those in the central part (Fig. 1). The age of the Tengelic Red Clay Fm. is w3.5e1.0 Ma (Gyalog and Budai, 2004; Koloszár, 2004), with w1.5e0.5 Ma for the Kerecsend Red Clay Fm. (Jámbor, 2001). Correlations with the neighboring area are shown in Fig. 2. Generally, red clay (5 YR 5/6) displays prismatic structure with slickensides, stress surfaces, and brown and yellowish spots. CaCO3 nodules, 3e5 cm in diameter, occur in the lower part of the red clay. Usually, the color of the lower part is lighter than that of the upper part. Black FeeMn stains are generally abundant throughout the entire red clay unit. The underlying (MioceneeLower Pliocene) strata are generally composed of thick grayish-yellow, mica-rich, cross-bedded sand, and sandy clay or sometimes limestone. In extensive areas of Central and SE Transdanubia and in certain occurrences east of the Danube River, the lower part of the Pleistocene is represented by the TRCF (Figs. 3 and 4B) made up of red and variegated clay, sand and silt of fluvial facies. It is underlain unconformably by Upper Pannonian (Zanclean) or older deposits and generally covered by loess. Loess is widespread in alluvial plains and to a lesser extent on slopes of the mountainous areas as well. However, it reaches its widest extent and greatest thickness in hilly regions of Central and Fig. 2. Correlation of the Pannonian basin lithostratigraphy with neighboring areas (after Ková et al., 2006; Kovács et al., 2008). c J. Kovács et al. / Quaternary International 240 (2011) 35e43 37 Fig. 3. Geochronological and stratigraphical framework of the Hungarian red clays with the stratigraphic position of the studied profiles. Global chronostratigraphy is from Gibbard and Cohen (2008). T ¼ Tarantian, Paks LF ¼ Paks Loess Formation, BM ¼ Beremend Member, TM ¼ Tengelic Member, BRC ¼ Basal Red Clays of the Paks Loess Formation (after Kretzoi, } 1987; Jámbor, 1997; Schweitzer and Szöor, 1997; Koloszár, 2004; Kovács et al., 2008). Ph ¼ Paks sandy soil complex, PD1,2 ¼ Paks Double, MN 16 zone ¼ European Land Mammal Mega Zone MN 16 (roughly coeval with the Piacenzian between 3.600 and 2.588 Ma). Legend: I ¼ loess, II ¼ sand, III ¼ sandy-loamy marl, IV ¼ paleosol, V ¼ (terra rossa)/red clays, VI ¼ basalt/bentonite, VII ¼ sandy clay. SE Transdanubia (Jámbor, 2001). Loess is a wind-blown sediment of silt size formed in the periglacial areas during cold and dry periods of the Pleistocene. It may have transitions to, or may alternate with, eolian sand deposits. Loess sequences can be interrupted by paleosol horizons (Figs. 3 and 4C) formed during milder interglacial periods. In hilly and mountainous regions the loess complex is called the Paks Loess Formation (PLF), which is underlain in SE Transdanubia by the TRCF. According to Jámbor (1997), loess was formed in hilly regions during the time span of 1.2 Ma to 12 ka. Stratigraphic relations of the terrestrial sediments are described in classical studies of the vertebrate fauna by Kretzoi (1956,1969) and Jánossy (1986) and in more recent summaries by Koloszár (2004), Koloszár and Marsi (2005), and Kovács et al. (2008). Schweitzer } and Szöor (1997) distinguished and characterized the subsequent periods set up by the former authors as Ruscinian (4.5e3.0 Ma), Villanyian (3.0e1.8 Ma), and Biharian (younger than 1.8 Ma). 2.1. Kerecsend Red Clay Formation In the mountain areas of the northern part of the country, red clay occurs on karstic limestone areas as depression and cave fills. Jámbor (2001) considered that this clay was formed during the Middle Pleistocene, or possibly even prior to the Quaternary. Red clay fillings including bone fossils have been described in several places in the caves of the northeastern mountains. Age determination is based on stratigraphical position and vertebrate fauna (Jánossy,1986; Kretzoi, 38 J. Kovács et al. / Quaternary International 240 (2011) 35e43 Fig. 4. Photoset of study sites. (A) Study site in northern Hungary (Atkár); (B) outcrop of red clay from southern Hungary (Beremend); (C) paleosol layer in loess, central Hungary (Szekszárd). 1987). The oldest vertebrate fossils are 700,000 years old, although they mark only the age of the accumulation, while the red clays could be significantly older. In the northern part of the country, but on the southern foothills of the mountains, cross-bedded sand or sandy clay is overlain by a 3e20 m thick red clay horizon (KRCF, Figs. 3 and 4A), having a thickness of about 3e6 m (Atkár site). 2.2. Tengelic Red Clay Formation Red clays are widespread in the hilly and mountainous areas of Hungary underlying the Pleistocene PLF. According to Schweitzer } and Szöor (1997), red clays can be subdivided into two compositional groups: one rich in kaolinite, and the other rich in illite and smectite. The kaolinite-rich variety seems to be the older one (“Beremend Member”, Koloszár, 2004, Fig. 4B), while the illiteesmectite rich variety is generally younger, more widespread, and occurs in hilly areas (“Tengelic Member”, Koloszár, 2004). Red clays (Beremend site, Figs. 3 and 4B) of Late PlioceneeEarly Pleistocene age (3.3e2.4 Ma, MN16 mammal biozone) were dated using vertebrate mammals by Jánossy (1986) and Kretzoi (1987). The red clay (in Tengelic-2 borehole, Fig. 3) is the uppermost bed of a 25e60 m thick sequence consisting from the bottom upward of alluvial sand, occasional bentonite derived from basalt tuff, eluvial-deluvial variegated clay and clayey silt, and finally the red clay which is of eluvial-deluvial and residual facies. The bentonite layer, derived from basalt, is important for age determination. These potassic volcanic rocks dated to 2.17 Æ 0.17 Ma (KeAr method) were recovered from boreholes at Bár (Balogh et al., 1986). At Bár, in Bá-4 borehole (Fig. 3), K-rich basalt and basalt pyroclastite intercalations can be found between red clay layers. The whole sequence was deposited after a considerable hiatus on the eroded surface of Upper Pannonian sediments. Its age is supposed to be Lower Pleistocene. Thickness of the red clay varies from a few meters up to nearly 20 m. The red clay beds are overlain by other red clay strata which are the lower members of the PLF (Fig. 4C). The color is actually less deep red and has been called } “reddish” by Schweitzer and Szöor (1997). 3. Materials and methods Red clay samples from the Pannonian basin were collected during fieldwork. A total of 80 samples were taken from the northern, southern and the central part of Hungary. The sequences were continuously sampled at 10e20 cm intervals for analyses. Grain-size distribution of all samples was measured by laser diffraction (Fritsch Analysette 22) methods according to the approach described by Konert and Vandenberghe (1997). Concentrations of major and trace elements were determined by XRF (Fisons Instruments ARL 8410 and Thermo ARL Advant’XPþ) as described by Kovács (2007) and Újvári et al. (2008). Quantitative data related to the mineralogy of the samples have been obtained by X-ray diffraction analysis (Philips PW 1710 and 1730 series). The XRD instrumental parameters and methods are described by Viczián (2007) and Kovács et al. (2008). The majority of the mineralogical data are used in this review are from Viczián (2002a,b, 2007). Scanning electronic microscopy (SEM) analyses were performed on quartz grains of the samples with JEOL JCM 5800. 4. Results and discussion 4.1. Micromorphology and grain-size of the red clay Grain-size distributions of detrital sediments are usually regarded as useful parameters in characterizing sedimentary environments and dynamics. Grain-size distributions of red clay were analyzed, compared with typical eolian loess and paleosols developed on loess (Guo et al., 2001; Lu et al., 2001; Kovács, 2006, 2008; J. Kovács et al. / Quaternary International 240 (2011) 35e43 7 6 5 Red clay Loess 39 4 3 2 1 0 14 12 10 8 6 4 2 0 Phi values Fig. 5. Grain-size distribution curves of the Pliocene red clay and Quaternary loess samples from Hungary. Varga, 2011). Grain-size distribution curves of the samples demonstrate a predominantly bimodal character with a primary maximum in the medium to coarse-silt (modal size: 5e7 phi) and a secondary maximum in the clay or very fine silt fraction (modal size: 7e9 phi) (Fig. 5). At the same time, the >63 mm (4 phi) fraction is almost insignificant in the sediments. The red clays are moderately sorted and show positively skewed to symmetric grain-size distribution. The positively skewed distribution indicates that the finer fraction is included in the grain-size distribution. The grain-size distribution curves of loess deposits show strong similarity with the red clays. The bimodal pattern could also be identified, indicating that two sediment populations have been involved in the loess formation. The fine-grained populations in the grain-size distribution curves of loess deposits have a lower percentage, compared to the red clay samples. This suggests that the proximal mineral material may have played much larger role in the sedimentation than did the background dust. However, this does not mean that the amount of the distal dust material was reduced, but the increased quantity of the local material caused a decrease in the relative proportion of the fine-grained particles. More detailed grain-size properties of red clays, paleosols can be found in Kovács (2008) and Varga (2011). SEM observations showed that the majority of the quartz grains are finer than 100 mm in diameter, mostly ranging from 10 to 40 mm. Grains >60 mm represent a very small fraction. Most of the quartz grains have irregular and angular shapes (Fig. 6A) and many are characterized by sharp edges, breakage and stepped surfaces (Fig. 6B and C), as well as conchoidal fractures (Fig. 6D). These grainmorphology features are highly similar to those of the Quaternary and Pliocene eolian deposits elsewhere (Liu, 1985; Lu et al., 2001; Vandenberghe et al., 2004) and they are considered characteristic of eolian dust deposits (Pye and Sperling, 1983; Pye, 1995; Wright, 2001). Angular grains are believed to be the product of mechanical collisions, salt disintegration and freeze-thaw weathering in desert regions (Pye and Sperling, 1983; Wright, 2001, 2007; Smith et al., 2002). As the dust was transported by wind in suspension, their angular sharps were not abraded. According to Pye and Sperling (1983), this kind of angular grain-morphology is only characteristic of eolian dust particles. Striking features of red clays in thin sections are: overall clayey texture (Fig. 7A), with few particles coarser than coarse-silt (Fig. 7C); no typical oriented coatings are developed, but there are some weakly arranged clay and soil conglomerations (Fig. 7B and E) and strips in channels and voids. Detrital calcium carbonate is rare, but there is a small quantity of weakly oriented micritic carbonate in channels and voids (Fig. 7D). As well, there are a few black-brown FeeMn stains which have no orientation (Fig. 7A). These characteristics show that red clay is a type of paleosol which has developed in a semiarid environment. The presented sedimentological data turned out to be highly consistent with the morphological features, supporting the eolian origin of red clay in the Pannonian basin. 4.2. Eolian origin of the red clay The time of a considerable drop of the level of Lake Pannon can be correlated with the lowering level of the Mediterranean Sea Frequency (%) Fig. 6. Scanning electron microscopic photographs of <60 mm quartz grains from Tengelic Red Clay (Hungary). 40 J. Kovács et al. / Quaternary International 240 (2011) 35e43 Fig. 7. Photomicrographs of some micromorphological features from red clays and red paleosols. (A) Black-brown FeeMn stains (plane polarized light PPL); (B) soil fragment in red clay, arrow shows organic matter (PPL); (C) angular morphology of the coarse grains in red clay (cross-polarized light XPL); (D) micritic carbonate (PPL); (E) clay coating around FeeMn nodule (see arrow) and clay aggregations in pedogenic carbonate matrix (PPL); (F) red clay with botryoid from karst fissure (PPL). } during the Messinian Stage (Schweitzer and Szöor, 1997). After desiccation, aeolian sedimentation became more intensive in the Pannonian basin. The accumulation of aeolian dust deposits started around the ZancleanePiacenzian boundary, w3.6 Ma (Kovács et al., 2008), a period which was characterized by a warm-humid climate, with Southeast Asian faunal elements, like Viverridae, Pteromys, Ailuridae (Kretzoi, 1969; Jánossy, 1986). Although the climate turned to more humid, the amount of mineral dust in the region was notable. Particle-size characteristics and micromorphological investigations suggest that the main part of red clay is wind-blown in origin (Kovács, 2008; Kovács et al., 2008; Varga, 2011). Detailed granulometric analyses of red clays show similarity in terms of their bimodal grain-size distribution patterns with loess horizons, as in the Chinese Loess Plateau (Yang and Ding, 2004). Two main components can be distinguished from the curves. The Late Miocene deposits (conglomerates, sandstones, sands, clays), eroded from the Eastern Alps (Kuhlemann et al., 2002; Willett, 2010) and the local Messinian sands could be the source material of the coarse sediment population of red clays (Kovács et al., 2008). According to Willett (2010), the warm climate of the early Pliocene was more erosive than the preceding Miocene climate, and the cold, more variable climate of the Pleistocene was even more erosive, given that each of these times is associated with an additional increase in sediment yield. At the same time, the background dust-load could be represented in the fine component. The source Fig. 8. Ternary diagrams showing the weathering trend of red clays (all in molar proportions); basic Al2O3eCaO þ Na2OeK2O (AeCNeK) ternary diagram with CIA values. Note positions of selected mineral compositions (Pl ¼ plagioclase, Kf: K-feldspars, Mu: muscovite, Il: illite, Ka: kaolinite, Gi: gibbsite, Ch: chlorite, Sm: smectite). GAL: global average loess (Újvári et al., 2008); SED: average sedimentary rocks (Ronov and Yaroshevsky, 1976); UCC: upper continental crust (Rudnick and Gao, 2003). J. Kovács et al. / Quaternary International 240 (2011) 35e43 41 Fig. 9. UCC-normalized spider diagrams for samples from the red clay profiles of Hungary. area of this fine (clay- and fine silt-sized) material is yet to be fully explored. The Pannonian basin located in the D1b zone at the “Saharan dustfall map” from Stuut et al. (2009), meaning that the Saharan dust material could be incorporated into the soil system and serve to increase the fine silt content. Since 1991, approximately 50 episodes of Saharan dust intrusion have been observed in the Pannonian basin (Borbély-Kiss et al., 2004; Szoboszlai et al., 2009), and this is the present situation, which is a less dusty interval in the region. The connection between large scale atmospheric patterns (El Niño Southern Oscillation, North Atlantic Oscillation) and the quantity of Saharan dust is controversial. According to Prospero and Lamb (2003) major dust outbreaks could be associated with major El Niño events. During the Pliocene, a permanent El Niño-like state (El Padre) has influenced the climatic conditions (Shukla et al., 2009), so the Saharan dust outbreaks could be dominant factors of the sedimentation in the Pannonian basin. Since 5 Ma, Sahara has been the main dust source area. Eolian dust material from Africa can be found in the Mediterranean and Atlantic marine sediments (e.g. deMenocal, 2004), and in the Terra Rossae around the Mediterranean Sea (Yaalon and Ganor, 1973). 4.3. Mineralogy Thin section observations (Fig. 7) show that the coarse fraction (>10 mm) mainly consists of quartz (50e60%) and feldspar and micas (>30%). Pyroxene, hornblende, goethite and hematite are also observed. The mineralogy of the red clay was determined for the <2 mm size fraction (XRD). The major components are illite, kaolinite, quartz, plagioclase, smectite and chlorite with a high } amount of amorphous matter (Schweitzer and Szöor, 1997; Földvári and Kovács-Pálffy, 2002; Viczián, 2002a,b, 2007; Kovács, 2007). 4.4. Geochemical properties of the red clay deposits weathering of the sediments resulted in removal of Ca and Na (primarily plagioclase) from the source rocks and less intense leaching of K, whereas the stronger chemical weathering in the sediments caused considerable dissolution of CaeNa host minerals and even K-bearing minerals (mainly K-feldspar) as well. The CIA values are around 70e80, highlighting the fact that chemical weathering was intensive in Central Europe during the formation of red clays. 4.4.2. Trace elements Trace element concentrations are recalculated on a volatile-free basis. An average upper continental crust (UCC)-normalized spider diagram for the samples from the Plio-Pleistocene red clay sequences is shown in Fig. 9. In the samples, the insoluble residues are characterized by remarkable depletion in Sc, Sr, Nb, Ba and precise enrichment in Cr and Co relative to UCC. Depletion of Sr is more related to feldspar weathering than to CaCO3 dissolving. In sedimentary processes, the distribution of Sr is affected by strong adsorption on clay minerals. Strontium is easily mobilized during weathering, especially in oxidizing acid environments, and is incorporated in clay minerals. Some of the analyzed red clay samples show strong variability in terms of chemical composition and elemental ratios. The reasons for this include: they are common on pediment surfaces; formed in situ, they may be easily eroded (change of relief) and washed down; they are superimposed upon each other along gentle slopes or intercalated with other deposits. Pedogenic processes have played an important role in the formation of red clay deposits. In general, pedogenic processes involve eluviations of carbonate unstable minerals and formation of new clay-sized minerals, which can be revealed by changes in the mineralogy, carbonate content, element } ratio (Schweitzer and Szöor, 1997; Kovács, 2007) and grain-size distribution (Lu et al., 2001; Kovács, 2008). 5. Conclusions 4.4.1. Major elements The chemical composition of the red clay deposits in Hungary is dominated by SiO2, Al2O3, Fe2O3, CaO, MgO and K2O (Kovács, 2007; Kovács et al., 2008). The chemical index of alteration (CIA) for the samples varies from 64 to 93 (Fig. 8). In the Al2O3eCaO þ Na2OeK2O (AeCNeK) triangular diagram (Fig. 8), insoluble residues are close to the area where illite and smectite joins, and all of the sediments lie parallel to the AeCN line. This pattern suggests that the chemical The particle-size characteristics of the Neogene red clay sediments are very similar to those of the Pleistocene loess deposits (Fig. 5), suggesting an aeolian origin for the red clay (Kovács, 2006, 2007; Varga, 2011). It appears from the sedimentological data that the main part of the red clay is of wind-blown origins, other is weathering crust of the underlying material. The Neogene red clay accumulated under persistent weak winds and a rather steady 42 J. Kovács et al. / Quaternary International 240 (2011) 35e43 warm-arid climate. Late Neogene changes in the palaeogeography and palaeotopography of Europe are mainly related to the AfricaneEurasian collision. These changes include the uplift of mountain ranges such as the Alps, Pyrenees, Carpathians and Caucasus, as well as the shrinking of the Paratethys Sea (Van Dam, 2006). Uplift within Europe results in further regional aridification and sharpening gradients, especially during the Pliocene. But the general consensus is that high topographies were not attained before Plio-Pleistocene times, implying that uplift might at least be held partly responsible for the ongoing aridification of parts of Central and Eastern Europe during the Pliocene. The shrinking of the Eastern Paratethys during the Early Pliocene (Dacian) to what are now the Caspian and Black Seas might additionally have explained aridification in that area (Popov et al., 2006; Van Dam, 2006). The red clay, as well as the loess, thickens from west to east in the Pannonian basin. Therefore, the clay transporting winds came from the west, maybe from central Europe, and these winds are interpreted as driven by the westerlies. Modern dust observations and simulation experiments have suggested the coarse grain population or silt fractions in aeolian sediments, with an average grain-size range of >20 mm, could only be suspended for short intervals at low altitude, even if entrained by strong winds. Thus, the >20 mm particles in red clay were transported by low-level winds (Guo et al., 2002; Wen et al., 2005). The upper westerlies have been active as a planetary circulation system in the middle latitudes of the northern hemisphere and generally transport the fine populations (grain-size less than 10 mm) or clay fraction in long-distance suspension, depositing dust in downwind areas far away from the source regions. More detailed analyses of spatial variation of grain-size distribution could help for the implication of depositional environment and paleowind directions. Red clay was modified by post-depositional weathering under warm-humid climate. These environmental characteristics accompanying the deposition and weathering of red clay are responsible particularly for the finer grain-size distributions and lower dustfall rate than the overlying loess. The red clay is an eolian deposit that has been subjected to strong pedogenesis during a warm and moist climate punctuated by small-amplitude oscillations of cold and dry climate inferred from field investigations in Hungary (color variations, abundance of clay coatings and carbonate nodules). Finally, these studies on red clays in the Pannonian basin lead to the following conclusions: (1) The older type (Beremend Mb.) of the TRCF is red kaolinitic clay containing typically disordered kaolinite, mixed-layer smectite/kaolinite, smectite and little gibbsite. It was formed mainly in local subaerial weathering crusts under warm, humid, subtropical or monsoon climate. (2) The younger member (Tengelic Mb.) of the TRCF contains red (or “reddish”) clay beds. It can be found not only on the karst surfaces but also in hilly areas of SE Transdanubia. It contains relatively fresh material (illite, chlorite). The weathering products are predominantly smectite and goethite formed under warm and dry climate in savannah and steppe or forest steppe. (3) The basal red clay layers of the Paks Loess Fm. contain similar material as the underlying red clays belonging to the younger member of the TRCF. This type of red clay is analogous to the KRCF of the northern hilly region of Hungary. The slightly but significantly lesser degree of weathering (more illite and chlorite, less smectite) indicates cooling of the climate. The Tengelic Mb. of TRCF and basal red clays and reddish paleosols of the PLF, and some part of the Beremend Mb. of TRCF and KRCF are of wind-blown origin, and that they were consequently affected by weathering processes in the PlioceneeEarly Pleistocene. Acknowledgements The authors are grateful to Prof. F. 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