Southeast Caspian coast evolution during Holocene

In modern literature, there is an almost unanimous opinion that the Amu-Darya flowed into the CaspianSea in prehistoric times, and the channel that many researchers previously drawn for the Amu-Daryachannel is the Uzboy channel, along which water from the Sarykamysh Lake was repeatedly releasedduring the historical time during its filling. The detailed analysis of previously published works andcartographic materials, as well as the use of recent geological and geomorphological data have led to theconclusion that the Amu-Darya (or one of its branches) has flowed into the Caspian Sea in historical times.Later, the Uzboy, which occasionally flowed out of Sarykamysh, had inherited its already developedchannel only to a considerable extent. Where it was obstructed by encroaching sand dunes, the Uzboywater was making its way through new channels and its erosion activity was insignificant, as the Uzboywas functioning sporadically during overflow of the Sarykamysh depression. According to historical data,human activity was high in this area on the coastal plain in the IX-XIII centuries. Following eighteenth- andnineteenth century’s researchers, the author of this article believes that the Amu-Darya (or a separatebranch of it) flowed into the Caspian Sea in the middle Ages. In addition to historical evidence, thisconclusion can be confirmed by some geological and geomorphological data. Investigations in the ۲۰thcentury have shown that in the IX-X centuries, Derbent regression of the Caspian sea occured, when thelevel dropped to -۳۵ and even to -۴۸ m, according to new data. Obviously, in the XIX century, researchersdid not know that the sea regressed was so deep, and therefore there was a controversy. Many did notagree that the Amu Darya flowed into the Caspian Sea in the historical times, as they found no signs ofboth the river valley mouth and the delta itself on the coast. In their opinion, the entire space between thegulf and the Aktam consists of high barchans and there is nothing resembling a trace of any channel in theMikhailovsky gulf. However, the huge masses of sand in the coastal zone of the seas do justify the existenceof a river delta when the so-called transgressive dunes were formed as the sea level was rising. It must beremembered that at that time the shoreline was many tens of kilometers west of the modern one and thedelta was therefore connected to it. It is likely that settlements and towns were located on the coastal deltaplain at that time, according to the historical evidence. The subsequent rise of sea level led to disastrousconsequences - flooding and submergence of vast lowland areas, attenuation of erosion processes and, asa consequence, filling of the channel with sediments as well as aeolian sands. On the edge of the delta plain,as the sea level was rising, as it usually happens in the coastal zone, near-estuary coastal bars were formedthat gradually shifted towards the landmass, partly being flooded. In the low-lying delta plain, floodingresulted in the formation of an extensive gulf. This scenario is typical of most delta plains. OgurchinskiyIsland is one such coastal bars. Reliable records evidencing it near the Turkmen coast date back to the XIVcentury. Earlier there were several islands, some of which joined together, while others were submerged.Two large islands, later merged into one, were known to ancient geographers as Ogroicha and Ogus.Ogurchinskiy in the late XIX century was ۲.۵ kilometers wide (about ۳ kilometers) and about ۴۵ kilometerslong. Nowadays, its maximum width is about ۲ km and its minimum width is ۶۰ m. The island is graduallyshifting towards the land due to redistribution of sediments after erosion and changes in its length andconfiguration. Aeolian process also plays a major role in this: ۵ kg of sand is blown away from one meterof beach per hour with a wind speed of ۴.۹ m/s. The central high part of the island was not flooded by thesea during the Novocaspian transgression. Indirect evidence of the island's permanent elevated position isthe presence of fresh water (the best water on the entire eastern coast, as noted by travellers in the ۱۸th and۱۹th centuries) and relatively fertile soils. Turkmen came from Cheleken and sowed wheat, cotton andplanted watermelons on the island. Herds of horses, flocks of sheep, goats and camels grazed on the island,which the nomads occasionally visited. It is known that all major deltas are confined to dipping areas. Thus,Ogurchinskiy is not associated with any positive tectonic structure; it is confined to the sinking structureKyzylkum trough, which is experiencing intensive sinking. Previously, it was assumed that the island isconfined to the vault of the structure; however, this has not been confirmed by subsequent geophysicalworks. The monotonic sedimentary strata in the area of the island are widely developed. The thickness ofquaternary deposits increases from ۳۵۰ m (Cheleken), to ۱۱۷۰ m - at Ogurchinsky. Within West-Turkmendepression, intensive sinking, according to [Richter, ۱۹۶۵], is also expressed in the increased depths ofcoastal lines of different regressive stages as compared to other regions. Mikhaylovsky Bay, which islocated to the east of Ogurchinsky Island, gives resemblance to a large river delta: here the same branchingnarrow fairways flow out between the underwater spits. The bay, in all probability, together with BalkhanBay, constituted the mouth of the Amu-Daria, which flowed in here in three main arms. On maps of theXVIII-XIX centuries, the Gulf of Khiva, up to ۲۰-۳۰ versts wide, deeply penetrating into the continent andlocated opposite the Ogurcha Gulf, was shown to the south of the Balkhan Bay. To date, the Gulf of Khivahas been completely filled up with sediments and only the vast Kelkor salt marsh, where the irregularlyfunctioning Uzboy flowed into in the XIX century, and where two channels, Adzhaib to the south andAktam to the north, have been preserved. The sediment-filled channel can still be traced on aerialphotographs within the Kelkor lowland to the town of Balkanabad. In the western part of the Kelkor,alluvial mixed-grained sediments with freshwater fauna are revealed in boreholes under marinetransgressive sediments. The spore-pollen analysis confirms the existence of a shallow intensivelyovergrown freshwater basin: many seeds of plants typical of the modern Amu-Darya delta have beenfound, there is a flora which is only common in the mountainous areas of the upper Amu and the modernAmu-Darya delta. The Uzboy estuary is the former Amu-Darya valley. With three terraces and a floodplain, it is very wide, sometimes up to ۲۰ km wide, with a dry branching channel in some places up to ۲-۳km. It is evident that the Uzboy, which periodically flows out of the Sarykamish depression when itoverflows with river water, was not capable of creating such a developed valley. The existence of a muchlarger river than the Uzboy is also indicated by the large meanders. Judging by the geometry of the rivermeanders measured on the satellite images, the flow was quite significant and corresponded to thecontemporary Amu-Darya flow or was even ۱.۵ times greater. The former Amu-Darya valley wassubsequently inherited in many parts by Uzboy. When it could not overcome the extensive proluvial conesor barchans which blocked the valley, the Uzboy built another route. The wide near-entrance part of thevalley cuts through the sea walls of the maximum Novocaspian transgression, i.e. the river functioned later,until the middle Ages.

کلیدواژه ها:

Uzboy River ، Turkmen coast of the Caspian Sea ، Ogurchinskiy island ، Holocene ، sea-level fluctuations