History of Formation Palaeodeltas of Lower
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Georgy I. Rychagov1, Vladislav N. Korotaev1*, Aleksey V. Chernov2 1 Faculty of Geography, Moscow State University, Leninskie Gory, 119991, Moscow, Russia Tel. +7 495 9395469, fax +7 495 9395044, e-mail: [email protected] 2 Moscow State Pedagogical University, M. Pirogovskaya, 119991, Moscow, Russia Tel. +7 499 2450310, fax + 7 499 2480162, e-mail: [email protected] * Corresponding author GEOGRAPHY 4 HISTORY OF FORMATION PALAEODELTAS OF LOWER VOLGA DELTAS ABSTRACT heights, morphological types of floodplain The palaeo-Volga River valley existed topography, and present-day vegetation. within the present-day Lower Volga region during the last 600–700 ka. Its lower parts KEYWORDS: Volga-Akhtuba valley, periodically transformed into a long and palaeodelta, ancient floodplain, bay-head deep ingressional estuary with the apex delta location controlled by the amplitude of the Caspian Sea level rise. Between the Early INTRODUCTION Khvalinian highstand of +50 m and the Early Holocene Mangyshlak lowstand at –100 m, The Volga-Akhtuba part of the Volga River the apex of the Volga Delta has wandered valley (downstream from the Volgograd city) 700 km alongstream. The estuarine- is geomorphically different from its upstream marine and alluvial environments in the parts largely occupied by reservoirs. It is ingressional estuary in the area between characterized by a well-developed wide the present-day cities of Volgograd and floodplain, lack of prominent terraces and Astrakhan, were changing throughout a symmetric box-like cross-section shape. the entire Late Pleistocene and Holocene. The adjacent interfluvial areas of the Early The associated succession reflects Khvalinian marine accumulation plain have a complex history of the Caspian Sea a monotonous even surface with dendritic level oscillations. Only over the last 16 ka, network of flat-bottomed hollows. Within the there have been six marine (estuarine) Late Khvalinian plain areas, aeolian landforms phases within the Volga-Akhtuba valley and Baer’s mounds are very widespread correspondent to the Late Khvalinian characteristic landforms. and Novocaspian transgressions. The transgressions alternated with regressive Long existence of a complex multi-thread phases associated with the dominance channel system of the Lower Volga River of alluvial environments in the Lower determined the formation of two main Volga valley. There are pronounced traces surface types within the valley bottom that of three transgressive-regressive phase differ in height, location, morphology and alternations of the Late Khvalinian and age. These clearly distinguished surface Novocaspian ages in the modern Volga- types are: i) ancient (central) floodplain and Akhtuba floodplain topography, that ii) modern floodplains of the Volga River correlate with four generations of ancient main channel and the Akhtuba branch; they floodplain and delta surfaces distinguished can be further subdivided into different in this study. Surfaces of different age generations of local floodplain surface generations differ in absolute and relative types. gi209.indd 4 05.08.2010 16:19:13 The Lower Volga region is located within the valley nearby the Volgograd city – to almost Pricaspian tectonic depression – the largest 90° – is associated with the Akhtubinskiy and deepest within the Russian platform fault of the southeastern strike direction. with a folding structure of the Karpinskiy The Volga-Akhtuba part of the Lower Volga anticline in its far southern part. According to River valley is formed along the two large geophysical data, pre-Palaeozoic crystalline linear depression structures. In the northern basement of the depression is found at part, it follows the Arzgirskiy depression GEOGRAPHY depths exceeding 15 km. It is fractured into (down to the Cherniy Yar settlement), farther separated blocks at different elevations. The southward – the Nizhne-Volzhskiy depression. 5 rocks that compose crystalline basement Both of these structures coincide with the of the depression mainly belong to the deep fracture zone which remained active Archaean-Proterozoic metamorphic type until the Holocene. Tectonic depressions are and are overlain by sedimentary rocks of superimposed with halokinesis structures the Russian platform mantle that consists of such as salt domes Beketovskiy nearby the undersalt, oversalt, and superficial structural Volgograd city, Verkhne-Akhtubinskiy at the levels. The undersalt structural level is Akhtuba branch inlet, Kamennoyarskiy, etc. composed of the terrigenous-carbonate These and other similar active structures rocks of the Late Palaeozoic that form cause local deviations of the Lower Volga relatively large platform structures broken River valley from its general southern by fracture dislocations. direction and formation of narrowed valley sections. Such influence is often reflected Salt domes are the main structural in local characteristics of the Volga River elements of the Pricaspian tectonic channel morphology and morphometry depression sedimentary mantle. They were [Lower Volga River..., 2002; Nikolaev, formed as a result of plastic dislocations of 1962]. Thus, the Lower Volga River valley enormous masses of the Kungurian stage formation, geomorphic structure, and (Lower Permian) evaporites which initial channel morphology are largely controlled strata had a thickness exceeding 4 km. The by tectonic structures and dislocations of the Lower Volga region has large underground Pricaspian depression crystalline basement. salt bodies that form extensive salt ridges For example, the valley width upstream of or gigantic domes – Enotaevskiy, Soleno- Volgograd does not exceed 3–8 km, whereas Zaymichenskiy, etc. The height of salt farther downstream it increases to 30–35 km stocks in such domes reaches 6 to 8 km; with local relatively narrower sections up to they are overlain by Mesozoic or even 12–15 km wide (Fig. 1). Neogenic rocks. In the brocken salt domes, such as Inder or Elton, some stocks are The upper part of the Paleozoic-Cenozoic open on the surface. The oversalt structural sedimentary mantle is mainly composed level consists of the Upper Permian and of unconsolidated Upper Pliocene – Pleistocene sedimentary rock which is Quaternary deposits few hundred meters strongly deformed on limbs of salt domes thick. Most of these deposits were formed and eroded on top. as marine sediments during the Akchagylian and Apsheronian transgressions. These The modern valley of the Lower Volga deposits are dominated by relatively fine River inherited negative tectonic structures sediments (clays and silts) with layers of developed at least since the beginning of sand or, less often, marls with basal clays. The the Quaternary. The valley section between Akchagylian and Apsheronian deposits are the Kamyshin and Volgograd cities follows distinctively characterized by specific macro- the Volzhsko-Ergeninskiy fracture zone. Its and microfossil communities. southwestern strike direction coincides with that of the Volgograd flexure and the The sedimentary mantle is topped with a Bolshoy Volgograd fault. A sharp bend of the heterogeneous layer of Quaternary deposits ggi209.inddi209.indd 5 005.08.20105.08.2010 116:19:136:19:13 GEOGRAPHY 6 Figure. 1. Width Variability of the Volga-Akhtuba Valley. exceeding 100 m in thickness. Within this general history of the Lower Volga River layer, all major subdivisions of the Quaternary valley, and, in particular, the Volga-Akhtuba system (Lower, Middle, Upper Pleistocene, floodplain formation. This complex and and Holocene) can be distinguished1. long-term process was primarily controlled Quaternary deposits are dominated by by river flow variations, the Caspian Sea level marine sediments formed during the Baku, oscillation, and tectonic activity. Lower and Upper Khazarian, Lower and Upper Khvalinian, and Novocaspian transgressions. The aforementioned history of the Significant part of the geological section is development of the Volga River valley represented by heterogeneous non-marine suggests that a large river system comparable aquatic sediments, i.e., fluvial, lacustrine, to the present-day valley existed in the Lower lagoon deposits. Aeolian deposits are the Volga region during the entire Late Cenozoic. most widespread within sediments of non- This system had a continuously migrating aquatic environments, though alluvial fan mouth and specific landform complexes that deposits and some other types are also have partly remained prominent up to the present. For a long period of geological present time. Review of previously published time, the study area has been a territory data [Goretsky, 1966; Rychagov, 1977; Svitoch of continuous migration of the sea-land et. al., 2000, 2004] allowed us to conclude interaction zone and the Volga River mouth. with a high degree of confidence that the Such conditions determine the widespread Volga River has been draining into the presence of complex origin sediments: fluvial- Caspian Sea at least since the Late Neogene marine (deltaic), lacustrine-marine (lagoons, ap limans, kultuks), and fluvial-lacustrine (oxbow ( N ). The buried Volga palaeovalleys of the lakes, ilmens). Such deposits often consist of vd sk numerous different facies that alternate in Venedy ( Q ), Solikamsk ( Q ) and Early both lateral and vertical directions. kr Krivichi ( Q ) ages have been discovered to The history of the Volga River palaeodeltas the east from the modern Volga valley. The formation is closely connected with a Planforms of these palaeovalleys generally resemble