Browsing by Author "Cagatay, M. Namik"

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Evidence of Extensive Carbonate Mounds and Sublacustrine Channels in Shallow Waters of Lake Van, Eastern Turkey, Based on High-Resolution Chirp Subbottom Profiler and Multibeam Echosounder Data
(Springer, 2015) Cukur, Deniz; Krastel, Sebastian; Cagatay, M. Namik; Damci, Emre; Meydan, Aysegul Feray; Kim, Seong-Pil
In Lake Van of eastern Turkey, the fourth largest soda lake in the world, high-resolution subbottom profiles and bathymetric data acquired in 2004 and 2012 revealed several hundreds of topographic mounds in shallow waters (< 130 m) off the historical town of Adilcevaz in the northern lake sector. These structures are characterized by strong top reflections of transparent internal character, and are 10-300 m wide and 0.5-20 m high. Consistent with previous work, they are interpreted as carbonate mounds formed by precipitation from CO2-rich groundwater discharge into the highly alkaline lake. Their age remains to be determined but their alignment along faults suggests tectonic control on their growth. Several sublacustrine channel networks were observed on the eastern shelf of the lake, which connects with onshore rivers. The channels are up to 500 m wide and 20 m deep, and plausibly were formed by fluvial processes during the major lake level drop reported to have occurred by 14 ka in earlier publications. Erosion is common on the channel walls flanked by levees. The channels are presently inactive or abandoned. At a water depth of 100 m, they all merge into a single larger channel; this channel has a sinuous course initially trending southwestward and then northwestward at a water depth of 130 m. Numerous closely spaced small channels (similar to 10-200 m wide, 1-10 m deep) are also seen on the eastern lacustrine shelf, interpreted as denditric and parallel channel systems formed during lake level fall terminating at similar to 14 ka. Bathymetric data provide evidence of numerous sublacustrine canyons on the western slope of the lake's northern basin, most likely remnants of relict rivers formed during this lowstand.
Seismic Evidence of Shallow Gas From Lake Van, Eastern Turkey
(Elsevier Sci Ltd, 2013) Cukur, Deniz; Krastel, Sebastian; Tomonaga, Yama; Cagatay, M. Namik; Meydan, Aysegul Feray
Analysis of multi-channel seismic reflection and chirp data from Lake Van (eastern Turkey) reveals various shallow gas indicators including seismic chimneys, enhanced reflections, bright spots, mud volcanoes, pockmarks, and acoustic blanking. The enhanced reflections, suggesting the presence of free gas, are most dominant and observed at more than 200 locations. They are characterized by very-high amplitude reflections and occur in both deep and shallow sedimentary sections. Some enhanced reflections are accompanied by very subtle seafloor expressions such as mounds, which may suggest active venting activity. Seismic chimneys or columnar zones of amplitude blanking have been observed in much of the surveyed area. Seismic chimneys in the study area cannot be associated with any known faults that would act as migration pathways for deep fluids. This suggests that the observed structures in Lake Van sediments allow the preferential emission of gases which might be for a large share of biogenic origin. The acoustic blanking, characterized by transparent or chaotic seismic facies, is seen in the eastern part of the lake. The lakeward edge of the acoustic blanking largely coincides with the 100 m water depth contour, indicating that (past) changes of the hydrostatic pressure may be responsible for the distribution of these anomalies. Mound-like features, interpreted as mud volcanoes, occur in a few locations. The presence of these features may suggest active gas emission. Very strong amplitude anomalies or bright spots with negative polarity, indicating gas-charged zones, are also seen in a number of locations. Pockmarks are observed only in the northeastern part of the study area. The scarce occurrence of pockmarks in the study area might be ascribed to a higher permeability of the lake sediments or to the absence of the substrate/reservoir providing the critical mass of gases necessary to produce such features. Turbidites, tephra layers, and deltaic deposits have the potential to provide ideal conditions to allow the sediments to act as a gas reservoir. (c) 2013 Elsevier Ltd. All rights reserved.
Seismic Stratigraphy of Lake Van, Eastern Turkey
(Pergamon-elsevier Science Ltd, 2014) Cukur, Deniz; Krastel, Sebastian; Schmincke, Hans-Ulrich; Sumita, Mari; Cagatay, M. Namik; Meydan, Aysegul Feray; Stockhecke, Mona
More than 1500 km of multi-channel seismic reflection profiles combined with ICDP (International Continental Scientific Drilling Program) drilling data, provide important insights into the stratigraphic evolution of Lake Van, eastern Turkey. Three major basins (Tatvan, Northern and the Deveboynu basins) comprise the main lake basin and are separated by morphological highs (Ahlat ridge and Northern ridge). Moreover, NE SW faults, parallel to the general tectonic lineament of the area, dominate the entire basin and are in charge of creating graben and half-graben structures. Well-developed prograding deltaic sequences on top of the basement were recognized by seismic stratigraphy analysis. Most likely, they formed during the initial flooding of Lake Van similar to 600 ka. The Tatvan basin sediments are dominated by mass-flow deposits of various origins alternating with undisturbed lacustrine sediments including distinct tephra layers. Faulting along the Tatvan basin margins may have triggered margin-wide slope failures. Ahlat ridge started to form between ca 340 ka-290 ka. Since then, Ahlat ridge was sheltered from major mass-flows due to its elevation. Hence, slow lacustrine sedimentation has prevailed throughout lake history on Ahlat ridge, which was the location of the main drill site during the ICDP. Several lake level fluctuations are evident on the eastern slope area but the deep basins were permanently covered by water. A significant lake-level low stand (ca 600 ka BP) was found at similar to 610 m below present lake level. The setting of the lake changed at about 30 ka. Tectonic activity appears to have waned significantly as the mass-transport deposition decreased across the Tatvan basin while normal undisturbed lacustrine sedimentation prevailed. A different setting is found in the Northern basin from ca 90 ka to Present, especially due to the strong influx of mostly volcaniclastic turbidites causing sedimentation rates to be about 3.5 times higher (drill Site 1), than at Site 2 (Ahlat ridge). (C) 2014 Elsevier Ltd. All rights reserved.