ANNALS OF AGRARIAN SCIENCE, vol. 9, no. 3, 2011 ИЗВЕСТИЯ АГРАРНОЙ НАУКИ, Том 9, Ном. 3, 2011
AGRONOMY AND AGROECOLOGY АГРОНОМИЯ И АГРОЭКОЛОГИЯ
INVESTIGATIONS OF THE FLUVIAL DYNAMICS IN EASTERN GEORGIAGEORGIA AS A CONTRIBUTION TO PALAEOENVIRONMENTAL RESEARCH OF THE REGION
Hans von Suchodoletz***, Dominik Faust**, Daniel Wolf** ***Leipzig*Leipzig University, Institute of Geography Johannisallee 19a, Leipzig, 04103 Germany; hans.von.suchodoletz@uni leipzig.de **Technical University of Dresden, Institute of Geography Helmholtzstrasse 10, Dresden, D – 01069 Germany; [email protected] dresden.de; daniel_wolf@tu dresden.de Received: 02.12.10; accepted: 18.01.11
Fluvial systems are important recorders of environmental change. Thus, using field and laboratory methods the past fluvial dynamics in two regions in eastern Georgia (Marneuli and Alazani depression) is investigated. Based on the literature and first field studies in the Marneuli depression, a preliminary model of the Cenozoic landscape evolution could be built up for the region. Although physical age determinations are still largely missing, these first results point to a strong change of the fluvial dynamics in the Marneuli depression during the Late Quaternary, amongst others probably in connection with tectonic activity in the region.
INTRODUCTION Fluvial systems and their deposits are ubiquitously found and known to be very sensitive recorders of environmental change. Furthermore, due to their flat topography and fertile soils, fluvial plains are often areas with a concentrated population so that the fluvial dynamics has a strong impact on human activity and health. Thus, a deeper understanding of the fluvial dynamics, also derived from studies about their past, is an urgent need. However, reading fluvial archives is never simple, due to several factors influencing their dynamics as there are climate, tectonics, human influence and last but not least the internal behaviour of a river, causing a change of its sedimentation pattern without any external trigger [1]. In spite of these problems, a careful study of the change of incision and aggradation of a fluvial system during the past, as well as of its sedimentation pattern following a longer part of its course can give valuable information on changes of its influencing factors. In combination with data from neighbouring river systems or other palaeoenvironmental archives, exact triggers of fluvial change can thus be identified [2,3]. In Georgia as well as in the whole Caucasian area, only limited studies on palaeoenvironmental change exist until today, mainly focussing on lacustrine deposits and peat bogs [4,5]. Whereas some studies on fluvial deposits in Georgia were carried out during the Soviet period [6,7], only recently some fluvial systems were studied in detail in Abkhazia [8] and southern Armenia [9], whereas no studies were carried out in eastern Georgia during the last years. However, understanding the fluvial dynamics in eastern Georgia is essential, since most cities are located in bedrock river valleys and floodplains regularly hit by inundations (e.g. Tbilisi, Gori, Rustavi, Marneuli, Bolnisi). To do so, it is also necessary to know about past changes of fluvial systems: For example, archaeologists suggest that a change in the fluvial dynamics caused a significant change of the human settlement pattern already about 6 ka ago [10]. Furthermore, the eastern Caucasus area is tectonically highly active, showing a recent convergence rate of 14 mm/a and an uplift rate of 8 mm/a [11]. For example, from the study in the Lesser Caucasus in Southern Armenia it is known that tectonic activity dramatically changed fluvial regimes during the Pleistocene 9. A similar situation can be expected in eastern Georgia, where 30 40 % of regional crustal shortening has been taken up by the so called Kura thrust and fold belt during the last 5 Ma [12], a region containing and being adjacent to large river systems (fig. 1). This article will present background and first results of a project about past fluvial dynamics in eastern Georgia. The project aims to reconstruct palaeoenvironmental changes using fluvial sediments from different rivers of the region.
Fig. 1. Overview of the Republic of Georgia showing the study areas and the Kura thrust and fold belt. The area shown in figure 2 is indicated by a dotted rectangle
OBJECTIVES AND METHODS The study focuses on fluvial deposits of two regions in the eastern part of Georgia: 1. The Marneuli depression in the Kvemo Kartli region south of Tbilissi: This tectonic depression with an extension of about 10 * 25 km 2 contains up to 135 m of cenozoic sediments 7, whereof the upper layers are partly composed of gravely to clayey fluvial material derived from the rivers Mashavera, Khrami, Algeti, Debeda and Mtkvari (Kura) as well as their tributaries (fig. 1,2). These rivers, together with their deposits are embedded in redeposited gravelly molasse deposits and loess borne colluvial fluvial material, the latter covering the footslopes and partly underlying coarser fluvial sediments. The rivers outcropped both fluvial and loess borne sediments up to 40 m (fig. 3). 2. Fluvial sediments of the Alazani river in the Alazani depression in the Kakheti region in northeastern Georgia: The river flows in the about 15 km wide Alazani depression, a piggy back basin at the southern deformation front of the Greater Caucasus [12] (fig. 1). In its lower middle course close to the Azerbaijan frontier, this river shows a meandering behaviour and its sediments are outcropped up to 20 m. In its upper middle course close to the cities of Gurjaani and Telavi, the recent river bed shows braided characteristics although fine sediments are outcropped up to 10 m. These fines contain archaeological findings in their lower parts and indicate a less energetic fluvial environment during the past.
Fig. 2. Overview of the Marneuli depression (hatched area) and its margins with main topographic features. An assumed shift of the Mtkvari Kura river to the west due to tectonic activity is indicated by dashed arrows
Fig. 3. Fluvial sequence of 18 m containing gravely and silty layers outcropped by the Khrami river close to the village of Imiri (cf. fig. 2)
Field as well as laboratory methods are applied during the investigations 1. Field methods include geomorphologic mapping of river terraces and the surrounding landscape aided by hand held and differential GPS. Stratigraphic sequences are mapped and sampled using natural exposures along the river banks. 2. Laboratory methods include sedimentologic analyses as grain size and heavy mineral analyses, the determination of the content of carbonate, C org and heavy metals and environmental magnetic studies. In order to get the age of the sediments, 14 C and OSL dating are applied.
RESULTS AND ANALYSIS Based on data from the literature and first geomorphologic and stratigraphic field studies in the Marneuli depression, we are able to present a preliminary landscape model for that region. The landscape evolution model of the Marneuli depression includes six fundamental phases (fig. 4): 1. Phase 1: Beginning with the first uplift of the Caucasus mountains above the sea level from the upper Eocene (35 Ma), clayey marly gypsum bearing marine molasse and flysch sediments were deposited throughout the region [13]. 2. Phase 2: An acceleration of orogenetic uplift from the middle Miocene (from about 15 Ma) raised these deposits above sea level, and subsequently they were capped forming large pediplanation surfaces. Concomitantly, the strong middle/upper Miocene uplift caused the deposition of thick continental inner molasse gravels. These gravels partly contain gypsum accumulations that are locally mined e.g. in the Yagludzha mountains above the city of Rustavi (fig. 2). Molasse gravel deposition was especially active during the Sarmatian phase from 11 8 Ma [13]. 3. Phase 3: Due to a shift of main orogenetic activity from the Greater Caucasus mountains towards the Kura thrust and fold belt in their southern foreland during the Pliocene from 5 Ma 12 , gravely molasse deposits were eroded and redeposited on pediplanation surfaces. Here they form large scale glacis deposits today. 4. Phase 4: During the next phase we see strong aeolian activity with large scale loess deposition. However, this material was immediately redeposited so that it is found as silty slope sediments, also forming a large part of the marginal fluvio colluvial sedimentary infill especially in the north of the Marneuli depression e.g. along the Algeti river east of the city of Marneuli (fig. 2). To our knowledge, not reworked aeolian loess is rarely conserved in Georgia, however easily found in neighbouring regions of southern Russia, Armenia, southern Azerbaijan and northern Iran [6,14,15]. We assume a Pleistocene age for this phase, since a large quantity of silt was produced in the glaciated headwater areas of the rivers and neighbouring areas in the Lesser and Greater Caucasus [6,16], and probably an increased amount of aeolian dust was brought from other regions as e.g. the shelves of the Caspian and Black Sea [15,17]. However, a high atmospheric aerosol load is also known from the recent warm period in the Transcaucasus region during a part of the year [18]. The strong aeolian phase was obviously finished with an erosional event. 5. Phase 5: Subsequently, about 30 m of layered silty sandy sediments with several darker clayey layers were deposited in the south of the Marneuli depression, where the loess borne material was largely eroded. These layers are well outcropped along the Mtkvari (Kura) river between the village of Ilmazlo and the Azerbaijan border, as well as along the Khrami river from the Azerbaijan border to the village of Didi Muganlo (fig. 2). Sandy gravely and silty layers are fluvial in origin, whereas interbedded cross stratified sandy layers are probably aeolian in origin. A single OSL age of about 6.6 ka ca. 3 m below the recent land surface south of the village of Meore Kesalo (fig. 2) suggests a Middle/Late Pleistocene till may be early Holocene age of these deposits, although this is not backed by other data yet. We assume that these sediments originate from a calm sedimentation regime with the character of floodout sediments when mainly fine textured material was deposited, with singular aeolian phases when sand was blown out and formed dunes. However, the reason for this conspicuous kind of sedimentation is not clear now. Also this phase finished with an erosional event. 6. Phase 6: The last phase was characterized by several cycles of fluvial aggradation and incision, leaving behind different fluvial terraces consisting of distinct layers of gravely and silty material along all rivers of the Marneuli depression. At very few places they also contain clayey palaeosols or palaeosol sediments. Correlating these terraces yields a pattern of four distinct levels along the Khrami and Debeda rivers (21 18 m, 15 12 m, 7 5 m, 4 2 m). At the Algeti river, it seems that there are only the three lower levels found, possibly caused by a shift of the river course from north to south in the depression as already formerly assumed [7]. Consequently, coarse gravely sediments outcropped along the Mtkvari (Kura) river close to the village of Keshalo could possibly be explained by a former course of the Algeti river, aggrading sediments equivalent to the 21 18 m terrace at the Khrami and Debeda rivers before the Algeti river changed its course into its recent bed (figure 2) . A 14 C age obtained from charcoal found at the base of a 10 m terrace at the Mtkvari (Kura) river close to the confluence with the Algeti river near the village of Meore Kesalo yields about 2.5 ka, in agreement with several other 14 C datings obtained from this terrace along the Mtkvari river [19]. Consequently, this terrace can be surely placed into the Late Holocene and is tentatively correlated with the 7 5 m terrace level at the Khrami/Debeda rivers.
Fig. 4. Phases of landscape evolution in the Marneuli depression. Insets show palaeogeographic maps of phases 1 and 2[20]
CONCLUSION Up to now, these results are mainly based on field observations and hardly backed by numerical datings, but they give a first idea about the main stages of the development of the Marneuli depression. However, the reasons for the fluvial dynamcis during phases 5 and 6 are not at all understood yet: If the preliminary Holocene ages of those phases should be confirmed, it is hard to explain how the observed strong change between aggradation and incision was triggered. One factor was probably tectonics: Looking at figure 2 it is obvious that the Mtkvari (Kura) river has its position in the westernmost part of a large low lying plain stretching from lake Dzhandari about 12 14 km to the west, so that this position could possibly be explained by tectonic deflection due to strong anticlinal movements observed in the Beyuk Kyasin mountains close to lake Dzhandari (Forte et al. 2010). Such a migration of the river bed of the Mtkvari (Kura) river would cause a lowering of the erosional base for the rivers in the Marneuli depression during phase 6, a possible explanation for erosional phases. Another reason could be a shift of the ratio between sediment load and river transport capacity due to i) climatic (e.g. thawing of glaciers during the late glacial, little ice age, dry periods) ii) anthropogenic (land use modifications) and/or iii) tectonic (e.g. damming of river courses) changes. We will try to answer these questions during the project that will last from 2010 – 2013. A separation of different influencing factors on the fluvial dynamics will only be possible when comparing the results of the rivers from the Marneuli depression with those from the Alazani river and with those from other studies in the region. However, our first results from the Marneuli depression already demonstrate that dramatic changes of fluvial activity in that region occurred during the late Pleistocene and early Holocene. This can give hints on the future development of fluvial dynamics in the densely populated fluvial plains in eastern Georgia when environmental factors will be changed.
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ИЗУЧЕНИЕ ФЛЮВИАЛЬНОЙ ДИНАМИКИ ВВВ ВОСТОЧНОЙ ГРУЗИИ КАК ВКЛАД ВВВ ПАЛЕОЭКОЛОГИЧЕСКОЕ ИССЛЕДОВАНИЕ РЕГИОНА
Ханс фон Суходолец, Доминик Фауст, Даниель Волф
Флювиальные системы являются важным показателем изменения окружающей среды. В статье приведены результаты полевых т лабораторных исследований флювиальной динамики в Восточной Грузии (Марнеульская и Алазанская депрессии). На основе литературных источников и первых полевых исследований в Марнеульской депрессии была восстановлена первичная модель ценозоического ландшафта региона. Физический возраст объектов отсутствует. Первые результаты показали сильное изменение флювиальной динамики в Марнеульской депрессии в течение позднего четвертичного периода в связи с тектонической активностью в регионе.