Morphological and Stratigraphic Investigation of a Holocene Subaqueous Shelf Fan, North of the İstanbul Strait in the Black Sea

International audienceIn 2002, the Bosphorus outlet was mapped using an EM 300 multibeam echo-sounder together with a Chirp sonar system. Th is survey, carried out on board the Ifremer RV 'Le Suroit' in the frame of the BlaSON project, completes the data previously acquired directly at the mouth of the Bosphorus by Di Iorio et al. (1999) in the frame of a NATO SACLANT Undersea Research project using jointly the NATO RV Alliance, and the Turkish Navy Survey ship 'Çubuklu'. Th is acoustic imaging has identifi ed a canyon system at the slope and a shallow marine fan, which contains shelf incisions extending the İstanbul Strait incision. Multibeam bathymetry, ultra-high resolution seismic profi ling and coring correlations on this subaqueous fan area allowed reconstruction of morphology and patterns of sediment distribution indicative of high energy sediment transport processes. Th e discovery of a shallow water/shelf type fan directly off shore from the Bosphorus and connected to its outlet is consistent with the theories of sudden discharges of large volumes of water. Age dating obtained at the bottom of this subaqueous shelf fan yielded an age of 6700 yr C14 BP (uncorrected age) for the fi rst marine mollusc encountered at the base. Th is is in accordance with a last and abrupt reconnection of the Marmara Sea to the Black Sea. A detailed morphological map of the shelf and slope along with seismic profi le interpretation and core correlation is presented here. A synthesis is proposed to explain the formation of this subaqueous fan and its relationship with the last connection between Black and Marmara seas aft er the Last Glacial Maximum. Th is interpretation can be summarized as follows: stage A corresponds to the fi rst erosion surface seen on the shelf related to the Last Glacial Maximum low stand; stage B is the ravine surface onlapping to ca. -30, -40 m; stage C is a second erosional surface related to a sea level fall and eroding most of underlying Unit 1B; and stage D corresponds to the onset of the fan deposit during a period of high water run-off from the Black Sea entering from the Bosphorus. Avulsion branches show that this fan has been active for a long time

Evidence for widespread creep on the flanks of the Sea of Marmara transform basin from marine geophysical data

"Wave" fields have long been recognized in marine sediments on the flanks of basins and oceans in both tectonically active and inactive environments. The origin of "waves" (hereafter called undulations) is controversial; competing models ascribe them to depositional processes, gravity-driven downslope creep or collapse, and/or tectonic shortening. Here we analyze pervasive undulation fields identified in swath bathymetry and new high-resolution multichannel seismic (MCS) reflection data from the Sea of Marmara, Turkey. Although they exhibit some of the classical features of sediment waves, the following distinctive characteristics exclude a purely depositional origin: (1) parallelism between the crests of the undulations and bathymetric contours over a wide range of orientations, (2) steep flanks of the undulations (up to ∼40°), and (3) increases in undulations amplitude with depth. We argue that the undulations are folds formed by gravity-driven downslope creep that have been augmented by depositional processes. These creep folds develop over long time periods (≥0.5 m.y.) and stand in contrast to geologically instantaneous collapse. Stratigraphic growth on the upslope limbs indicates that deposition contributes to the formation and upslope migration of the folds. The temporal and spatial evolution of the creep folds is clearly related to rapid tilting in this tectonically active transform basin

Investigation of the Black Sea outlet of Istanbul Strait

İstanbul Boğazı çıkışındaki GB Karadeniz şelfi ve kıtasal yamacıdan alınan yüksek ayrımlı jeofizik veriler ve şelfteki kritik noktalardan alınan karot örneklerinin analizleri, yaş tayini sonuçları ile yorumlanmıştır. Elde edilen sonuçların ışığında Karadeniz'in neotektoniğiyle ilgili olarak güncel sedimantasyonunu, son buzul döneminden itibaren. oluşan su seviyesi değişimleri, zamanı ve bunların tortul birimler üzerindeki etkileri araştırılmıştır. EM300 Çok Işınlı ekosounder sistemi kullanılarak çalışma alanının bu güne dek yapılmış en ayrıntılı batimetrik haritası oluşturulmuş, kıtasal yamaç, denizaltı kanyonları ve kaymalar gibi geniş jeomorfolojik yapılar belirlenmiştir. Yüksek ayrımlı sismik (Chirp) kesitlerden güncel sedimantasyonu etkileyen faylar, sığ gaz yapıları ve çamur volkanlarının varlığı saptanmıştır. Kıtasal yamaçta yer alan kanyonların duvarlarındaki, kaymaların, ikincil kanyon ve küçük kanal yapılarının oluşumunu ve şelfte karaya doğru ilerleyen kanyon başlarının gelişimini tetikleyen etkenler ortaya konmuştur. Bu ilerlemenin batıya keskin dönüşü Kuzey Sınır Fayı ile ilişkilendirilmiştir. Ayrıca bu bölgedeki şelf ve şelf kırığı bölgesinde ilk kez sığ gaz birikimleri gözlenmiştir. Assemblage Avrupa projesi kapsamında derin bölgeden alınan piston karot örneklemesinde karotun gaz nedeniyle deforme olduğu ve alanın KD bölgesinde yapılan gravite karot örneklemesinde ilk kez Gaz Hidratların varlığı saptanmıştır. Boğaz çıkışında yer alan kanal sisteminin sismik stratigrafik özellikleri ortaya konmaya çalışılmıştır. Bu kanal sisteminin varlığı İstanbul Boğazı ve Karadeniz arasındaki çift yönlü akıntı sisteminin kurulmasından önce Akdeniz suyunun Karadeniz'e girişinin yüksek enerji altında meydana geldiğini ifade etmektedir. Katastrofik ve antikatastrofik görüşler ışığında, deniz seviyesi değişimleri, karot yaşlandırmaları ile ilişkilendirilerek, son buzul döneminden bu yana Karadeniz ve Marmara denizleri arasındaki bağlantının tekrar sağlanmasıyla ilgili bir senaryo oluşturulmuştur. Shelf and continental slope region of southwestern Black Sea shelf in front of the Bosphorus outlet are investigated by using high resolution acoustic data. These data are correlated with analysis and dating of cores recovered from critical points on the shelf to determine recent sedimentation in relation with neotectonics, the horizons of sea level changes since Last Glacial Maximum (LGM) and to understand their effects on recent sediments. EM300 Multibeam Echosounder system provided most detailed bathymetric map of the area that ever obtained and extensive geomorphologic features such as continental slope, submarine canyons and slumps are determined. Very high resolution seismic (Chirp) system was used to determine recent sedimentation, active faulting, shallow gas accumulations and mud volcanoes. The triggering mechanisms for the formation of slumps, secondary canyons, gullies on the canyon walls at the continental slope and shelf incising canyon heads were discussed. The westerly abrupt turn of the canyon head, which is proceeding on the shelf, is associated with the North Border Fault. Also shallow gas accumulations observed at the shelf and shelf edge region. In piston coring which is recovered in the frame of European Assemblage project, the sediments are extremely disturbed due to gas existence. For the first time, Gas Hydrates were recovered in gravity core that was taken from NW part of the study area. The seismic startigraphy of the channel system at the exit of Istanbul Strait was betrayed. From the existence of the channel system at the exit of Bosphorus, it is concluded that, the entrance of Mediterranean water into Black Sea was originated in a high energy system before the two-way under water current system between Istanbul Strait and Black Sea was formed. On the basis of the catastrophic and anti-catastrophic hypotheses and sea level changes since last glacial maximum (LGM) in correlation with core dating, a scenario was developed to explain the last connection between Marmara Sea and Black Sea