Sedimentary characteristics and source of loess in Baranja (eastern Croatia)
Adriano Banaka*, Davor Pavelićb, Marijan Kovačićc, Oleg Mandicd
aCroatian Geological Survey, Department for Geology, Sachsova 2, HR-10000 Zagreb, Croatia
bFaculty of Mining, Geology and Petroleum Engineering, University of Zagreb, Pierottijeva 6, HR-10000 Zagreb, Croatia
cFaculty of Science, Department of Mineralogy and Petrology, University of Zagreb, Horvatovac 95, HR-10000 Zagreb, Croatia
dNatural History Museum Vienna, Departmentof Geology and Paleontology, Burgring 7, A-1010, Wien, Austria
*Corresponding author. Tel.: +3851 6160 708; fax: +385 1 6160 799.
E-mail address: (A.Banak).
Abstract
Loess is terrestrial clastic sediment, composed dominantly of silt-sized particles formed by the accumulation of wind-blown dust. It is usually inter-bedded with soil horizons forming loess-palaeosol successions (LPS). Thickest LPS in Croatia are found in Baranja, region bounded with two big rivers, the Danube and the Drava. The results of grain-size and modal analysis provide information about source material and wind direction in different time periods during Pleistocene. Grain-size distribution is in good accordance with other loess localities in the Pannonian Basin. Garnet, epidote and amphibole mineral group are most abundant heavy minerals in samples of Danube River sediment. Comparing heavy mineral assemblage (HMF) from southern and northern LPS with that data, it is obvious that main source area for loess in Baranja is from Danube flood plain sediments. Main transport direction was from North or North-West. Nevertheless the higher concentration of amphiboles in southern and northern LPS (mean 26.3% in HMF) then in the Danube plain suggests additional source area. Western Carpathians with Neogene calc-alkaline volcanic rocks is major source for amphiboles. Alternatively those minerals could be from locally exposed volcanic and metamorphic rocks of the southward Slavonian Mts. Mt. Krndija and Mt. Papuk, which are closest to Baranja of all Slavonian Mts., consist of amphibolites. In that case, small amount of silt material for Baranja loess would be transported by WSW winds. Results obtained from sedimentological and SEM analyses show fairly good congruence with results from other LPS in the Pannonian Basin, with some differences in mineral composition which imply diversity and shifting of source area for Baranja loess during Late Pleistocene.
Key words:loess, Baranja, aeolian, heavy mineral fraction,quartz, SEM images
1.Introduction
Quaternary sediments are widespread in Croatia. They cover about 35.7% of the Croatian teritory (Bognar, 1976). Baranja, region in Eastern part of Croatia (Fig. 1) is almost completely covered with Quaternary sediments (Pikija & Šikić, 1991). Special place within,regarding the origin, holds loess and it's derivatives. Loess is terrestrial clastic sediment, composed predominantly of silt-size particles, formed by the accumulation of wind-blown dust (Pye, 1995).
Fig. 1. Map showing the position of Croatia, Baranja and two investigated loess profiles at slopes Bansko brdo hill.
One of the most specific characteristcs of loess is grain size distribution of particles. Most authors (Bognar, 1976; Nemecz et al., 2000; Pecsi, 1990; Smalley, 1966b;Smalley et al.,2005; Wright, 1995, 2001) agree that typical loess has grain size distribution in range 20-60 μm, which corelates with silt-size grains. Origin of those silt size particles and mechanism of it's transport is the main question in over one century of loess exploration. Two main thesis which are opposite-pedogenetic and aeolian, describe formation of thick loess deposits. First one emphasizes diagentic processes in silty material as beeing crucial in loess formation, while second one favors aeolian transport of silt-size particles. The aim of this study isto determine source area for loess in Baranja by identifying mineral composition of heavy mineral fraction and morphology of quartz grains. Geochemical and mineralogical bulk loess sedimentanalyses along parts of the Danube in NE and E Romania have pointed out that the main sediment for loess deposits was transported by windsfrom a NNW/NNE direction, indicating forcing by northerlywinds from the Fennoscandian ice sheet (Buggle et al., 2008). Bokhorst et al. (2011) suggest a domination of western winds during the Early and Middle Pleniglacial in central and easternEurope, while the Late Pleniglacial was dominated by NW or northern winds. Additionally this paper will propose a complex, five-phase process necessary for thick loess formation.
2. Geological setting
Geology of Baranja (Fig.2) is simple at the surface, consisting of dominantly Pleistocene and Holocene sediments, with some outcrops of Miocene igneous rocks, limestones and marls (CGS, 2009). Holocene sediments are aluvial and marsh gravel, sand and silt. Pleistocene sediments are dominantly loess, loess-like sediment and flood plain sediments. Only high ground in Baranja region is Bansko brdo hill (244m asl), an tectonic complex horst, situated in northern part of region, stretching 20km in NE-SW direction and reaching banks of Danube river. Combination of active neotectonics (Hećimović, 1991) and Danube river erosion exposed big outrocps of Middle and Upper Pleistocene sediments (loess palaeosol sequences-LPS) at surface (Fig. 2).
The oldest exposed rocks at Bansko Brdo hill belong to the Miocene volcano-sedimentary complex, and include basalt-andesite and pyroclastic rocks comprising volcanic and tuffaceous breccias and conglomerates (Fig. 2). K-Ar radiometric, whole-rock measurements indicate an early Middle Miocene age (13.8±0,4 and 14.5±0,4 Ma, Pamić & Pécskay, 1996). The basalt-andesite intercalates Middle Miocene (Badenian) marine calcareous sand and marl, indicating the synsedimentary character of the volcanic activity (Pamić & Pikija, 1987; Lugović et al., 1990).The oldest loess in Bansko brdo hill is detected at the Zmajevac locality and infrared optically stimulated luminescence (IRSL) method produced age of 217±22 ka, while the youngest loess has an IRSL age of 16.7±1.8 ka (Galović et al., 2009).
Fig. 2. Geological map of Baranja region(CGS, 2009) with two loess profiles at Bansko brdo hill, encircled with red colour.
3. Material and methods
3.1. Field methods
Field investigation and sampling where carried out during winter months, because lush vegetation in spring and summer, disables finding and aproach to loess profiless. Aim was to include maximum thicknes of sediments. Bulk samples (8-10kg) were collected from Zmajevac (Zma), Kotlina (Kot), Podolje (Pod) and Branjina (Br) loess outcrops for sedimentological analysis.These four outcrops make two loess profiles: northern and southern, regarding their position on the slopes of Bansko brdo hill. Field investigation included taking samples, measuring thickness of sediment,defining litology, GPS positioning and photographing. Diferent horizons in loess where recognized and described. Depending on that in-situ horizon diversity, sampling was carried out. From typical loess, sampling frequency was 1.5m on southern slopes of Bansko brdo hill. On northern slopes sampling frequency in loess was arround 0.5m. Total of 30loess samples where taken.
3.2. Lab methods
Grain-size analyses combined wet sieving and the pipette method. Classification of the grain size distribution follows Wentworth (1922). Mineral abundances (modal analyses) used the 0.063-0.125 mm calcite-free fraction. Heavy and light mineral fraction (HMF and LMF) were separated in bromoform liquid (CHBr3, δ=2.86 gcm-3) by gravity. Qualitative and quantitative analyses of the fractions were based on 300-350 grains per sample and were conducted using a polarizing light microscope (Menge & Maurer, 1992). The carbonate (CaCO3) content was calculated from the weight difference before and after cold hydrochloric acid (5%) treatment.
Photographsof quartzgrainswere madeusing ascanning electronmicroscope (SEM)in the Laboratoryof Geochemistry, INA oilCo.Out of 30 loess samples two with previously extracted light mineral fraction were chosen for scanning electron microscopy.One samplefrom the southernloessprofiles(Zma1/2)and the otherfrom the northernloessprofile(Br1/12). More than40 differentgrains were photographed, and best photographsthat showcompletequartz grains were selected. Photographs were taken on quartz grains in order to see the shape of grains and detailed morphology of grain surface. Microscopy was madewithin magnificationrange ofX350-X3500.Selectedsamplesweregluedto a carrierdouble-sided tape, and thensteamed withgoldwhich thickness is 25nm.Thusprepared sampleswere placedin a containerand analyzedusinga scanning electronmicroscopeJEOL, model JSM-6510 LV.Microphotographswererecorded.
The microscopescans the surfaceof the sampleby very preciselyfocused beam ofelectrons.Source of electronsis atungstenfilamentthatgeneratesthehigh voltageelectrons.Thebeam of electronesexciteselectronswithin theatomsof the sample.Voltage is15kV during process of scanning.Energyof electrons fromthe beamis in directproportion tointeractivelyexcitedelectronsfrom the sample.Energyof electronsderivedfromsamplesis collectedand measuredwith specialdetectors andwith the help ofa microprocessorcreates apseudoimage with wavelengthof electronsunique to an elementthat is foundin the sample.Detectionof secondary electronsenabledimages with topographic contrast.In this study, the quartz grain
surface microtexture classification method and implemented terminology are based on studies of Mahaney (1995b) and Strand et al. (2003).
4. Results
Four loess outcrops were investigated. Two are located on southern slopes of Bansko brdo hill, a and other two are on northern, more steep slopes of Bansko brdo hill. The southern loess profile makes up total of 18m of loess (Fig. 3), while northern loess profile makes up total of 8.5m of loess (Fig. 3).Southern loess profile has four palaeosols intercalated within five loess horizons and is defined as typical loess. Northern loess profile has two palaeosols intercalated within three loess horizins and is defined as 'slope loess' or 'loess derivated coluvium', a terms previously decribed by several authors (Bognar, 1979; Pye 1995).
Fig. 3. Southern and northern loess profiles from Bansko brdo hill.
Detailed lithology of each loess horizon of southern profile aswell with chronological frame from MIS 6 – MIS 2, has been previously described (Banak et al., 2012; Galović et al., 2009). Grain-size analysisindicate silt as the dominant grain-size fraction in all 13 studied loess samples from southern profile (Fig 4).Average share of silt-size particles is 88.11%. Coarse-grained silt is dominant silt fraction with average percentage of 41.38%. The laminated unit is composed of 81% sand, 11% silt and 8% clay, with a median grain-size of 0.22 mm.Skewness is fairly constant in all 13 samples, averaging of 0.79. Sorting is dominantly poor, with an average value 1.64. The average CaCO3content in loess samples from the southern loess profile is 9.32%, with a maximum value of 23.3% that was recorded in the sample Kot 1/4, and the minimum value of 2.9% in a sample Zma 1/1.
Fig. 4. Grain size distribution and coefficients of southern loess profile from Bansko brdo hill.
Grain-size analysisindicates silt as the dominant grain-size fraction in all 17 studied loess samples from northern profile (Fig. 5). Average share of silt-size particles is 90.44%. Coarse-grained silt is dominant silt fraction with average percentage of 42%. Skewnes is constant with one significant peak towards 0.5 and average value is 0.81. Sorting is dominantly poor, with an average value of 1.49which areslightly positive compared to southern profile. The average CaCO3content in loess samples from the northern profile is 9.92%, with a maximum value of 15.8% recorded in the samples Pod 1/1 and Br 1/12 and the minimum value of 5% recorded in the sample Br1/11.
Fig. 5. Grain size distribution and coefficients of northern loess profile from Bansko brdo hill.
Modal analysis of the light (LMF) and heavy mineral fraction (HMF) was made on all samples of loess and shows the following results (Table 1 and 2). In the southern loess profile light mineral fraction is dominated by quartz with an average share of 58.38%, while in the northern loess profile average percentage of quartz is 60.29%. Heavy mineral fraction is dominated by amphibole, garnet and epidote while chlorite is present in small percentages. Each of the three major groups (highlited with colour in Table 1 and 2) of transparent heavy minerals in the southern and northern loess profile is represented approximately in the range of 25 to 30%, and the relations between them vary.
It is worth noticing that incerased percentage of garnet in some samples is related with decreasd percentage of amphibole and vice versa. This ratio is recorded in samples from both southern (Kot 1/2, Kot 1/9, Zma 1/4) and northern (Br 1/6, Br 1/8, Br 1/12) loess profiles (Table 1 and 2, numbers in red colour). In all other samples percentage ratio between this two groups has more narow range.
Table 1 Mineral assemblages from southern loess profile. Note: op=opaque minerals, do=dolomite, bi=biotite, ch=chlorite, thm=transparent heavy minerals, tu=tourmaline, zr=zirqonium, ru=rutile,am=amphibole, py=pyroksene, ep=epidote, ga=garnet, cy=cyanite, st=staurolite, tit=titanite, czt=clinocoisite, cto=chloritoide, csp=cromespinel, si=silimanite, x=undetermined, q=quartz, f=feldspar, rf=rock fragments, mu=muscovite
Table 2 Mineral assemblages from northern loess profile. Note: op=opaque minerals, do=dolomite, bi=biotite, ch=chlorite, thm=transparent heavy minerals, tu=tourmaline, zr=zirqonium, ru=rutile,am=amphibole, py=pyroksene, ep=epidote, ga=garnet, cy=cyanite, st=staurolite, tit=titanite, czt=clinocoisite, cto=chloritoide, csp=cromespinel, si=silimanite, x=undetermined, q=quartz, f=feldspar, rf=rock fragments, mu=muscovite
4.2. Scanning electron microscope (SEM) images
Electron scanning microscope analyzed two samples from Bansko brdo hill loess. All scannedand photographed grains in both samples are sand-sized particles (> 63 um). In both samples angular grains dominate. Their share is over 80% of the total number of grains in the samples. Smaller percentage of grains are very angular and partly rounded. Rounded and well-rounded grains are not detected. Over 70% of the grains in both samples are of low sphericity. On most grains schist-like fractures and ''V'' impresses are visible. Schist-like fractures were detected in over 40% of the grains in the samples. The smaller number of grains have schist-like fractures that are nearly the lenght of longer axis of grain, while the majority of grains haveschist-like fractureswith size 1/3 or 1/4 of a grain. ''V'' impresses on the surfaces are visible in 15% of total number of grains. The size of these textures is in range from 3 μm to 8 μm. They are usually clustered on smooth, flat surfaces of grains, and in smaller number of grains are present in form of individual'' V'' impress. Small percentage of grains displayed sets of parallel striations. Percentage of grains with striations marks is about 5% of total number of grains. Length of striation marks on average isbetween 15-20 μm and sets are consisting of ten straitions in average.
Sample Zma 1/1:
Fig. 6. SEM images of quartz grains from southern loess profile.Note: A: Detail of'' V'' impress (1) and lot of small dents and cracks in the surface of grain. B: Whole grain, schist-like fracture (2) and'' V'' impress (1) in the upper right part of the grain. C: The same grain (0104 pic.), with focus on the'' V'' impress (1). D: Whole grain, schist-like fractures (2). E: Almost completely rounded grains with a schist-like fractures (2). E: Almost completely rounded grains with a schist-like fractures (2). F: Grain surface displays the'' V'' impress (1). Parallel striations (3) are also visible.
Sample Br 1/12:
Fig. 7. SEM images of quartz grains from northern loess profile. Note: G: Subangular grain with partially visible schist-like fracture (2).H: Whole, semispherical grains with a schist-like fracture (2) and severe'' V'' impress (1). I: The same grain (0112 pic.), zoomed in on'' V'' impress (1).J: The lower part of the grain, partially rounded, with ''V'' impress (1). K: Subangular grain with a big schist-like fracture (2). L: Subangular grain with lots of striation marks and schist-like fracture (2).
5. Discusion
Almost 50% of loess profiles investigated by Nemecz et al. (2000) are constituated of coarse silt (20-45 μm) grain size. Loess profiles from Bansko brdo hill in most sedimentological characteristics can be corelated with other loess profiles in Pannonian Basin. Grain size distribution displays a dominant share of coarse silt, as in other loess profiles from the eastern Croatia, likeŠarengrad (Galović et al., 2011) and Vukovar (Wacha & Frechen, 2011) which are located in range of 50 km from Bansko brdo hill. The percentage of sand is in a similar range as in other loess profiles in Pannonian Basin and varies from 5% to 20%. Northern loess profile from Bansko brdo hill has slightly higher percentage of sand-size particles, especially in the base of LPS.
When reconstructing the loess deposition in a specific locality geologist must take into account various factors and concepts. Pye (1995) described the necessary conditions and processes that lead to formation of loess deposits. According to him there are two basic requirements for the formation of thick loess deposits: 1.) Source area sufficiently rich with silty material 2.) Presence ofa adequate 'entanglement' in areas in which silt is accumulating. If both conditions are met, accumulation of loess is possible and it takes place in four phases (Pye, 1995). The source area for the primary material are mountain chains, which in this case of loess from Bansko brdo hill could be the Alps, the Carpathians and nearby Slavonian Mts. surrounding Pannonian Basin (Banak et al., 2012). Kuenen (1960, 1969), Wright (2000, 2007) and Smalley et al. (2005) described physical and chemical processes of weathering of rocks, resulting in production of material for the later formation of loess. Weathering of parent rocks is a combination of several processes. Glacial erosion, fluvial erosion, freez-thaw of water/ice in rock crevasses, aeolian abrasion of rocks by sand particles and tectonic movements are the most important processes of weathering. This is the first phase in the creation of loess deposits. Subsequently, streams, floods and riverstransport the material in flood plains, which constitutes the second phase. In the third phase in the summer period, dried river sediment which is dominatedby silt-sized particles together with small amounts of fine-grained sand, isexposed at surface and can be subjected to deflation. In the fourth phase northern winds (Hobbs, 1942, 1943)blow away the sediment and transport it in large accomodation areas, like the steppes of Pannonian Basin. It should be noted that deflation of silt-size and sand-size particles was possible only if vegetation, which acts as a stabilizing factor of sediment, did not grow. At the same time, the presence of vegetation and microorganisms within, wich excrete polysaccharidesand create so-called Biological Loess Crust (BLC), is an essential factor in the stabilization and erosion prevention of wind deposited particles (Smalley et al., 2011). It is unlikely that the deflation was effective in the winter, because snow or/and ice covered the sediment. It is known that in Europe thickest loess deposits are regularly very close to major rivers such as the Danube, Rhine, Tisza, Dniepar and Dniestar (Smalley et al., 2009). It shows how much the alluvial, flood plain sediment sediments is important in the formation of loess deposits. Thickness of loess in the vicinity of large rivers (in average 50 km from river beds) suggests that most of the silt and sand particles, before final sedimentation,were transportedat relatively short distances thus, aeolian transport has proximal character. The predominant mode of aeolian transport is saltation and only the smallest particles, such as fine-grained silt, are transported in air suspension (Goossens, 1988; Smalley et al., 2009).There is also adivision ofthe processes thatresult in the formationof loessin threephases.So Smalleyetal.(2009)proposethe following threestages:1.)weathering2.)fluvialtransport/depositionofsilt and sand3.)aeoliandeflationof sediment fromfloodplains.They believe thatthe process can bedistinguishedin fourphases, as indicated in the previousconcept(Pye, 1995).In doing so,thesecondphaseshould be dividedin two separateprocesses: transport and sedimentation (Smalleyetal., 2009). Both processesare importantbecause theyenhance thesortingof particles, particularly because theyremovesthe smallestparticles offine-grained siltandclayand thusform a non-cohesive sedimentswithhigh ratio ofsilt/clay.That non-cohesive sediment can be easely subjected todeflation.
This researchproposes the introduction offive phasesin the process (Fig 8),describing theformation of loess-palaeosol sequences (LPS)in Baranja.The sameprinciple could beapplied to describe formation of thick LPS in theentirePannonian Basin.