Stacked debris flows offshore Sakarya Canyon, western Black Sea: morphology, seismic characterization and formation processes

Analysis of ca. 1400 km of multichannel seismic data indicate that the distal part of the Sakarya Canyon within the continental rise is an unstable region with sediment erosion. Fourteen buried debris flows (DB1-DB14), in the stacked form within Plio-Quaternary sediments between 1400 and 1950 m water depth, were observed in the surveyed area. Their run-out distances range from 3.8 to 24.4 km. The largest debris flow DB10 affects ca. 225 km(2) surficial area transporting ca. 15 km(3) of sediment in S to N direction. The debris flows in the area are considered as gravity flows of unconsolidated sediments mobilized due to the excess pore pressures occurred in the unconsolidated shallow sediments arising from the high sedimentation rate. We also suggest that extensive seismic activity of North Anatolian Fault (NAF) located ca. 140 km south of the of the study area along with the possible local fault activity is also a significant triggering factor for the flows. The stacked form of the debrites indicates that the excess pore pressure conditions are formed periodically over the time in the continental rise, which makes the region a potentially unstable area for the installation of offshore engineering structures

Morphology and potential geohazards using seismo-acoustic data in Sakarya Canyon, Western Black Sea Margin

Multi-channel seismic, 3.5-kHz Chirp seismic, and multibeam bathymetric data were collected along the western Black Sea margin, offshore Sakarya River, to investigate the morphology and to evaluate the potential geologic hazards. The multibeam bathymetric data show that the morphology of the margin is controlled by the Sakarya Canyon consisting of three distinct canyon heads, all incising the southern continental shelf. Deep-water sediment erosion along the canyon walls and scour marks along the distal canyon floors indicate that both Sakarya and Kefken Canyons may be active in terms of sediment erosion and turbidity currents. We identify the depositional and erosional features in the area by means of echo-character mapping. The distribution of different echo-types is mainly controlled by the morphology of the margin, as well as the shape, location, and structure of the major canyon systems. Erosional features, constituting 47% of the total surficial area, are classified as slides, erosional truncations, gravitational mass wasting, gullies, and outcropping seafloor while depositional features, constituting 53% of the total surficial area, comprise shelf sediments, turbidites, pelagic/hemi-pelagic sediments, and sediment waves. Different types of geohazards coexist along the Sakarya Canyon, which are classified as hazards linked to (1) local and/or regional tectonism, (2) morphology of the continental margin (turbidity currents, slope overstepeening), and (3) prevailing sedimentary processes (mass transports, submarine fluid flow, loss of support due to the truncation scarps and bedforms)

Stratigraphical and morphological aspects of the northern Marmara shelf based on seismic data

Recent sedimentation and Plio-Quaternary stratigraphy of northern Marmara shelf has been investigated by high-resolution seismic datasets. Seismic data indicate that a thin cover of Plio-Quaternary unit (termed Unit 1) overlies the Miocene/Oligocene age older sediments (termed Unit 2). The erosional surface between Unit 1 and Unit 2 is the regional unconformity for the shelf formed during the last sea level low stand. Unit 1 is subdivided into two sub-units as Unit 1a for Holocene deposits accumulated after the last glacial maximum (LGM) and Unit 1b for fluvial sediments deposited during the last low stand of the Marmara Sea. The thickness of the Holocene sediments is maximum at SW of Bosphorus outlet, in the Buyukcekmece Bay and along the coastal area between Silivri and Buyukcekmece reaching approximately 32 m. Mean sedimentation rate of the entire northern shelf is calculated as 0.4 m/1000 yr for the last 12,000 yr based on the Holocene sediment thickness. There are four depressions in the western part of the shelf, which correspond to palaeolakes during the LGM filled by Plio-Quaternary sediments. Transition from lacustrine to marine conditions in the palaeolakes occurred when the Marmara Sea level exceeded -62 m threshold depth during the sea level rise following the LGM at approximately 12,500-13,000 yr before present (BP)

Seismic identification of gas hydrates: a case study from Sakarya Canyon, western Black Sea

Multichannel seismic, 3.5-kHz Chirp subbottom profiler and multibeam bathymetric data were collected along the western Black Sea margin, offshore Sakarya River, to investigate the bottom-simulating reflections (BSRs), free gas accumulations, and mud volcanoes. Geometries from the seismic data indicate widespread BSRs along the continental rise between 750 and 1950 m water depths, 70 to 350 ms below the seafloor. Seismic attribute analyses have been applied to the seismic data to reveal the acoustic properties of the gas hydrates. According to the results from such analyses, we conclude that there are acoustically transparent zones beneath most of the BSRs in the area, which are interpreted as free gas accumulations, and the gas hydrate-bearing sediments are acting as seals for the free gas in the underlying sediments. Stability analysis of the gas hydrates from different BSR zones in the area suggests that the gas composition in the gas hydrates may change locally. As we do not have ground truth data from BSR zones, the exact composition of the gas forming the gas hydrates is unknown. However, hydrocarbon productivity of the area, chromatography results of the shallow sediment samples nearby, and stability analysis of the gas hydrates indicate the possible existence of a thermogenic gas component in the gas hydrate composition, resulting in a mixture of gas hydrates