Introduction

Continental margins are regions of large-scale sediment transportation from shallow continental shelves to deep basins, characterized by turbidity currents and debris flows as well as massive submarine landslides (e.g., Mulder et al. 2009; Loncke et al. 2009; Mouchot et al. 2010; Savini and Corselli 2010). High-resolution 3.5 kHz sub-bottom profiler data allow us to analyze shallow acoustic facies and recent sedimentation processes along the shallow and deep-water areas of continental margins (e.g., Lee et al. 2002a; Dondurur and Çifçi 2007; Carlino et al. 2013; Garcia et al. 2016). The classification of seafloor deposits using the specific echo-characters on the high-resolution Chirp seismic datasets has been made by several researchers to efficiently determine seafloor morphology and associated sediment characteristics as well as to identify seafloor processes such as turbidity current activity, possible geohazards, and sediment erosion or deposition (e.g., Damuth and Hayes 1977; Winguth et al. 2000; Lee et al. 2002a; Kottke et al. 2003; Garcia et al. 2006; Hernández-Molina et al. 2008; Domzig et al. 2009; Loncke et al. 2009; Mouchot et al. 2010; Savini and Corselli 2010; Carlino et al. 2013). The main purpose of these studies is to reveal the modern depositional architecture of the continental margins and to define potential slope instabilities at variable scales in order to reduce the risks to offshore infrastructure.

Engineering and environmental installations in offshore areas have moved into relatively deeper waters in the past few decades especially for the exploitation of offshore resources along continental slopes and deep basins. Evaluation of potential geologic hazards is necessary for safe installation and operation of the engineering structures such as submarine pipelines, cables, and drilling platforms. These geohazards can be defined as irregular seafloor topography, scarps, sediment removal caused by bottom currents, low bearing capacity of the sediments, faults, diapiric structures, submarine fluid flow, and landslides (Baraza et al. 1999; Nadim et al. 2005). Submarine slumps and slides triggered by earthquake loading can also produce destructive tsunamis, which constitute potential geohazards for offshore settlements and coastal installations.

The western Black Sea margin has become more important in the last two decades due to its potential for petroleum plays in deeper waters. A 320 billion m3 gas discovery has recently been announced by Turkish Petroleum in the Tuna-1 well penetrating into sandy channels of the Danube submarine fan, which indicates the hydrocarbon potential of the basin (see Fig. 1a for the well location). The Black Sea also hosts a number of deep-water natural gas pipelines such as Blue Stream, Turk Stream, and South Stream. One can also anticipate the installation of a submarine pipeline for the transportation of the gas produced at Tuna-1 well to land. Therefore, studying the shallow sedimentary structure as well as sediment erosion and mapping potential geohazards along the margin is of importance for potential offshore infrastructure along the margin.

Fig. 1
figure 1

(a) Major tectonic elements of the Black Sea and surroundings (modified from Finetti et al. 1988; Robinson et al. 1996; Spadini et al. 1996). WBSB and EBSB are western and eastern Black Sea sub-basins, respectively, and BS is the Bosphorus Strait. Tuna–1 is the location of the recent gas discovery of Turkish Petroleum Co. (b) Bathymetric map of the study area and tracklines of the data collected along the Sakarya Canyon. Locations of gravity cores 1, 9, and 16 (light blue dots) are from Algan et al. (1999, 2002, 2007), and gravity core K27 and box core BC15 (yellow dots) are from Duman (1992, 1994)

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Existing information on the morphological framework, recent sedimentation, and depositional environments along the western Black Sea Turkish margin including our study area, which comprises the Sakarya Canyon (SC), is quite poor. Algan et al. (2002) investigated the continental shelf offshore Sakarya River using single-channel high-resolution seismic and gravity core data. Their study, however, was limited to the continental shelf around the head zone of the SC, and did not include any information about the structure and morphology of the deeper parts of the canyon. Yiğitbaş et al. (2004) tried to map the local faults of the margin using four multi-channel seismic (MCS) lines from the deep-water zone of the SC. Yet their seismic lines were regional and could not be used to obtain depositional or erosional structures of the area in detail. Recently, Nasıf et al. (2020) demonstrated widespread fluid flow especially along the continental rise of our study area by means of the presence of bottom-simulating reflections (BSRs), shallow gas accumulations, and gas chimneys as well as mud volcanoes.

This study aims to identify the morphology and recent depositional and erosional processes along the SC using marine geophysical data. The objective of this paper is to assess possible natural geohazards for the offshore engineering installations through the classification and mapping of different echo-types on 3.5-kHz Chirp seismic records as well as MCS lines. We also show the effects of different components such as seafloor morphology or local tectonism on the depositional pattern along the margin.

Geological and oceanographic settings

The Black Sea is a marginal back-arc basin generated by the northward subducting Tethys Ocean (Finetti et al. 1988; Okay et al. 1994; Robinson et al. 1996; Spadini et al. 1996). Although it is bathymetrically a single depocenter today, the Black Sea comprises two sub-basins, the western and eastern basins, which are separated by a regional high, the Mid Black Sea Ridge subdivided into the Andrusov and Archangelsky Ridges to the north and south, respectively (Fig. 1a). The Black Sea has an extensional origin; however, the tectonic setting changed to a compressional system during the Eocene, and the margins of the Black Sea, aside from the NW margin, are currently characterized by compressive deformation (Robinson et al. 1996; Spadini et al. 1996) (Fig. 1a).

The present-day morphology of the Black Sea is characterized by an oval basin with max. 2200-m water depth surrounded by steep continental slopes with a seafloor inclination exceeding 25° except at the NW margin where the Danube submarine fan forms a relatively smooth continental slope (Ross 1977). The western Black Sea Turkish continental margin has the common morphological characteristics of a modern ocean margin with a narrow shelf platform, steep continental slope, continental rise with gentle slopes, and a smooth basin plain (Dondurur et al. 2013; Nasıf and Dondurur 2017; Sipahioğlu and Batı 2017; Nasıf et al. 2018). The continental shelf is narrow along the eastern and southern margins, e.g., less than 10 km along most of the margin, whereas it is much wider along the western and northwestern margins, reaching a maximum width of approx. 60 and 200 km, respectively.

The oceanographic conditions of the Black Sea are also interesting since it is the largest anoxic basin in the world with an oxic-anoxic interface lying at approx. 150-m depth (Murray et al. 2007). Its only connection with the Mediterranean for water exchange is the narrow Bosphorus Strait with a sill depth of 34 m at its southern end (Fig. 1a). Although sea-level variations in the Black Sea following the last glacial maximum (LGM) are still controversial, several researchers suggest that the sea level of the Black Sea was 120 m lower than the present-day (e.g., Lericolais et al. 2009; Panin and Popescu 2007; Yanchilina et al. 2017). At this stage, the Black Sea was an enclosed lake; the shelf area surrounding the basin was exposed and rivers eroded the continental shelf. Following the LGM, a rapid transgression occurred and the sea level of the Black Sea rapidly increased; the Mediterranean salty waters invaded the deep basin, and present-day oceanographic conditions initiated in the mid-Holocene (Ryan et al. 1997; Panin and Popescu 2007; Lericolais et al. 2009; Constantinescu et al. 2015; Yanchilina et al. 2017; Berndt et al. 2019).

The study area is located offshore Karasu on the western Black Sea margin (Fig. 1b) and comprises a total area of approx. 3300 km2. This area hosts a highly dissected meandering canyon system extending from the shallow shelf platform to the deep basin, known as the Sakarya Canyon (SC). The canyon is located at the mouth of the Sakarya River, which is the second largest river of Anatolia and has a discharge rate of 5.6 km3/year, that is, 14% of all Anatolian rivers (Algan et al. 1999, 2002). The annual average suspended load of the river is 3.8 million tonnes (Algan et al. 1999).

In spite of the significant terrigenous sediment input from the large rivers surrounding the basin, pelagic sedimentation still plays an important role in the deep basin (Ross 1977; Ergün et al. 2002; Dondurur and Çifçi 2007). The continental rise can be considered the main depocenter for the terrigenous sediments transported by the canyon systems (Ergün et al. 2002; Dondurur and Çifçi 2007; Dondurur et al. 2013). Ross (1977) and Çağatay (1999) suggested a sedimentation rate of >30 cm/1000 years for this part of the margin, while Duman (1994) proposed 100 to 200 cm/1000 years obtained from several sediment cores from the southern Black Sea margin. Duman (1994) also showed that turbidites are very common towards the basin around the distal parts of the SC, reaching 50-cm thickness for the last 200 years.

Datasets and methods

High-resolution multi-channel seismic (MCS), Chirp sub-bottom profiler, and multibeam bathymetric datasets were collected simultaneously during the surveys aboard the R/V K. Piri Reis of Dokuz Eylül University, Institute of Marine Sciences and Technology in 2012 and 2016 along the SC in the western Black Sea margin (Fig. 1b). A global DGPS system was utilized during the entire survey.

A total of approx. 1400 km of MCS data was recorded using a 168-channel, 1050-m-long digital streamer with 6-s and 1-ms record length and sampling interval, respectively. The seismic source was a 45+45 inch3 Generator–Injector (GI) gun fired at 25-m intervals. A conventional processing sequence was applied to the MCS data using SeisSpace/Promax data processing software. This included data loading, geometry definition, band-pass filtering (8 to 180 Hz), trace editing, f-k dip filtering, surface-related multiple elimination (SRME), sorting to CDP gathers, velocity analysis (approx. every 1000 m along the lines), normal move-out correction, stacking, post-stack time migration, and gain recovery.

Bathymetric data were collected using a hull-mounted SeaBeam 1050D system operating at 50 kHz, which utilizes 126 beams providing a total swath coverage of 153°. The multibeam data were processed using the Caraibes software with the following processing steps: beam editing and de-spiking, correction of navigation errors, data interpolation, digital terrain model (DTM) construction, and gridding at 100-m resolution.

A pole-mounted Chirp sub-bottom profiler utilizing a sweep signal between 2.75 and 6.75 kHz centered at 3.5 kHz was used to image shallow sub-surface sediments, and a total of approx. 1480 km Chirp seismic data were recorded. Delay-time correction, gain recovery, de-chirping, and amplitude envelope calculations were applied to raw Chirp data.

Results

Physiography

The margin consists of a shelf platform with a maximum inclination of approx. 1.5°, a steep continental slope with inclinations reaching up to 20°, a continental rise with relatively gentle slopes up to 3°, and a smooth basin plain with inclinations generally less than 2° (Fig. 2a). A relatively smooth continental shelf, lying between 0- and 120-m water depth, is connected to the continental slope immediately beyond the shelf break around the 120-m depth contour (Fig. 2a). The shelf is sub-divided into three areas on the basis of their morphological architecture as the western, eastern, and southern shelf zones. It is much wider at the eastern and western parts of the study area (approx. 16 and 30 km in N to S direction, respectively), while it is almost nonexistent around the head of the SC to the south because of the incision of SC heads carving into the shelf.

Fig. 2
figure 2

(a) Main physio-geographic zones based on the morpho-bathymetry of the area. (b) Multibeam bathymetric map of the study area with the axes of major canyon systems and other main morphological elements along the margin. Axes of the main canyons are indicated by black lines while the axis of the main Sakarya Canyon and its heads are shown by white lines. SC: Sakarya Canyon, KC: Kefken Canyon, KR: Kefken Ridge, NP: Northern Platform, EH: Eastern Head, MH: Middle Head, WH: Western Head, NS: Northern Slope, KBC: Karasu Blind Canyon. (c) Map of the seafloor inclination superimposed on the bathymetric map

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The most prominent morphological feature of the study area is the SC, which has a significant influence on the morphology of the continental slope, and it shows different depositional and erosional characteristics within its proximal (from shelf break to approx. 1600-m water depth) and distal sectors (from 1600- to 2000-m water depth). The continental slope from 120 to 1600 m (Fig. 2a) is highly dissected by the SC system, consisting of several interconnected secondary small-scale canyons, or tributaries (Fig. 2b). The SC can be traced in an approx. SSW-NNE direction from the continental shelf to 2000-m water depth to the north where it bifurcates into two branches: one to the north and the other to the northwest. Bathymetric data indicates that, towards the southern shelf, it has three heads, the eastern, middle, and western heads, all incising the shelf break (Fig. 2b). The middle head merges with the eastern one at 640-m water depth to form the proximal parts of the main SC, and then the western head joins this main channel at 860-m water depth. To the west, there exists another large canyon system, the Kefken Canyon (KC), which consists of two independent shelf-originating canyon heads eroding the western shelf. An elongated small ridge, known as the Kefken Ridge, extending in a SW to NE direction, with a mean height of approx. 200 m, separates the Kefken and Sakarya Canyons. Both canyons coalesce at 1460 m to form the distal part of the SC system (Fig. 2b).

Most of the tributaries of the SC originate on the shelf. Multibeam bathymetric data, however, reveal the existence of a large blind canyon system located on the NE slope, named the Karasu Blind Canyon (Fig. 2b). It originates from the upper slope at 340-m water depth and lies approx. in a N to S direction. The Karasu Blind Canyon becomes a very narrow confined channel along the lower slope before it merges with the SC in deeper waters to the north.

The continental slope to the NW is known as the Northern Slope (Fig. 2b), which hosts three canyons lying in a N-S orientation. These drain into the basin plain and are connected to the main SC in deep waters at a depth of approx. 1900 m. The deep basin of the Black Sea is located further north between 2000- and 2200-m water depth and only a small portion of our acoustic data covers the flat basin plain (Fig. 2a).

The inclination is typically less than 3° on the continental shelf, the distal parts of the SC, the basin plain to the northwest and on the Northern Platform, which is an elongated area with an almost flat surface located at the NE part of the study area. The inclination reaches up to 20° elsewhere, especially around the shelf break and along the upper continental slope (Fig. 2c).

Morpho-sedimentary features

The major morpho-sedimentary features of the margin are evaluated by analyzing the different reflection characteristics observed on the 3.5-kHz Chirp sub-bottom profiler data. The reflection types imaged in the Chirp data are classified into five main groups based on the form and geometry of the reflected echo from the seafloor, and the continuity and characteristics of the sub-bottom reflections (Table 1). This classification tentatively constitutes the link between echo-characters and the associated depositional or erosional processes and enables geohazard evaluation. Identified echo-types are classified as follows: I—limited/no penetration; II—hyperbolic echoes; III—parallel stratified echoes; IV—undulating reflections; and V—discontinuous reflections.

Table 1 Classification of different echo-types observed in the Chirp data. Interpretations are explained and justified in the “Echo chaeracters” section
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Most of the continental slope can be characterized by regular hyperbolic echoes (Fig. 3a). Parallel-stratified echoes are generally observed along the continental shelf, and in some limited areas on the canyon banks as well as over the sediment platforms (Fig. 3b), while undulating seafloor echo-types characterize the NW part of the continental rise. Rough and chaotic echoes are observed in the slide material while slide scars and areas of sediment truncations produce discontinuous echoes around the headwalls and canyon walls, respectively. The abundance of these various seafloor processes are grouped as depositional (53%) and erosional (47%), excluding the area covered by the canyon axes (Fig. 3c).

Fig. 3
figure 3

Echo-character map of the study area superimposed on the shaded relief bathymetry. Distribution of (a) gullies on the slope along with the major canyon floors, (b) main echo-character types deduced from the acoustic data, and (c) percentage distribution of different depositional and erosional processes relative to the total surficial area

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Continental shelf features

Along the continental shelf, Chirp sub-bottom profiler data reveal different seafloor and sub-surface characteristics such as a thin drape of recent sediments, outcrops, along with the three canyon heads to the south and a paleo-channel. In the eastern part, a thin (a maximum thickness of approx. 16 m) sediment drape as a topmost sedimentary unit is separated from the deeper erosional strata consisting of older, steeply inclined sediments by an angular unconformity (Fig. 4(a)). This part of the shelf shows almost no morphological features and consists chiefly of undeformed sediments with a moderate Chirp signal penetration of up to 30 m below the seafloor (mbsf). The drape consists of horizontal and undeformed layers with increasing thickness landwards, indicating a significant terrigenous sediment input, classified as Echo-type III-3 (Table 1). Also evident on the Chirp sub-bottom profiler data is the presence of acoustically transparent zones below the drape towards the south indicative of gassy sediments (Fig. 4(b)).

Fig. 4
figure 4

Example of Chirp seismic data from the shallow continental shelf showing (a) thin drape of Holocene sediments over an unconformity surface, (b) gassy sediments from SE shelf, (c) outcrop area from SW shelf, (d) a MCS line from southern shelf showing a paleo-channel and faulting on the shelf platform, and (e) E-W trending Chirp seismic line crosscutting the three heads of the SC. WH, MH, and EH are western, middle, and eastern heads of the SC, respectively. Gravity cores 1, 9, and 16 are from Algan et al. (1999, 2002, 2007)

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The most prominent morphological feature in the southern part of the western shelf is an outcrop area of irregular seafloor topography (Fig. 4(c)) classified as Echo-type I-2 (Table 1). Chirp and MCS data indicate that the outcrop area represents an erosional seafloor architecture with several small-scale peaks and troughs approx. 3 to 25 m high from the surrounding seafloor, and 20 to 350 m in width. Chirp data show almost no signal penetration through these peaks and a thin drape of sediments is found within the troughs (Fig. 4(c)).

A paleo-channel with a mean width of 2000 m cuts through parallel-bedded sediments on the SW shelf (Fig. 4(d)), widening in the NW direction towards the Kefken Canyon head. Available seismic data coverage does not permit mapping of the southernmost part of the paleo-channel towards the land, and therefore, a possible connection of the channel to land cannot be resolved. The base of the channel located approx. 90 mbsf can be traced in MCS data, which shows that the channel is filled with recent sediments which onlap onto the channel flanks (Fig. 4(d)).

The southern continental shelf is eroded by the three heads of the SC. The canyon heads in this zone are V-shaped in cross-section and have steep (with inclinations exceeding 20°) erosional walls dissected by gullies (Fig. 4(e)). The floors of these canyon heads are narrow and are mainly occupied by hyperbolic echoes of Echo-type II-2 (Table 1).

Continental slope features

Bathymetric data indicate that the morphology of the continental slope is controlled by the SC and its tributaries (Fig. 2), which are V-shaped in cross-section in the proximal zone, becoming U-shaped in the distal section. The SC shows meandering form in the proximal part down to approx. 500-m water depths, then its floor becomes wider downslope where it merges with its tributaries.

The reflectivity pattern of the canyon axes can be subdivided into two types. The first type is defined as strong reflections with an almost flat seafloor morphology in the Chirp data and is observed in the proximal section of all canyon systems along the upper and middle continental slope classified as Echo-type I-1 (Table 1). These areas appear as confined zones located between steeply inclined canyon walls and are distinguished by their distinct high reflectivity echoes (Fig. 5(a)) with very limited or no signal penetration in the Chirp data. In some places, hyperbolic echoes of Echo-type II-2 (Table 1) are observed from canyon walls towards the channel axes (Fig. 5(a)).

Fig. 5
figure 5

(a) Example Chirp seismic data through the SC axis, (b) MCS line showing turbidite deposits beneath the SC canyon axis, (c) description of the gravity core K27, (d) Chirp, and (e) its corresponding MCS data illustrating the truncated surfaces from distal parts of the SC. BSR indicates the bottom-simulating reflection corresponding to the base of gas hydrate accumulations. Description of core K27 is from Duman (1992, 1994). Blue dashed line in (b) locates the border of the turbidite deposits

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The second type of channel axis echo along the continental slope is defined as moderate-to-high amplitude parallel to sub-parallel reflections, classified as Echo-type III-2 (Table 1). Chirp data show moderate signal penetration reaching a maximum of approx. 20 m. These types of channel axis echoes are generally found close to the base of the continental slope where a gentle seafloor gradient exists or in the areas where the canyons are meandering (Fig. 5(b)). The base of these echoes is indicated by a yellow dashed line in Fig. 5(b), where the reflections of Echo-type III-2 show onlap terminations. Gravity core K27 from Duman (1992, 1994) recovered coarse-grained turbidites interbedded with coccolith ooze laminations (Fig. 5(c)).

The walls of the Kefken, Sakarya, and Karasu Blind Canyon systems in the middle and lower continental slope between 800- and 1600-m water depth show significant truncations of the adjacent sediments of Echo-type V-1 (Table 1). This type of truncation commonly appears in areas where these large canyons meander, and at both sides of the Kefken Ridge (Figs. 5(d) and 5(e)). The truncation typically creates steep scarp surfaces seen on the MCS data (Fig. 5(e)).

The canyon walls between the shelf break and canyon axes are incised by gullies at various scales, especially in the middle and upper continental slope, with the gullies draining into the main canyons (Fig. 3a). The widths of the gullies (the horizontal distance between the tops of the gully sides where the wall rolls over) vary between 260 and 1450 m, and their axial depth (the vertical distance between the thalweg depth and the depth to tops of the gully sides) varies from 18 to 90 m (Fig. 6). They are typically V-shaped in cross-section and the inclination of the gully walls is generally between 8 and 12°. They are oriented perpendicular to the local depth contours, and their local orientation appears to be controlled by the inclination of the seafloor. The gully axes are not parallel to each other and they are generally convergent in nature when traced downslope where they merge into the main canyon axis (Fig. 3b).

Fig. 6
figure 6

(a) Chirp and (b) MCS data examples showing gullies of various sizes from the eastern slope of the SC. (c) Chirp line corresponding the southern part of the MCS line in (b) showing extensive gullying as contiguous reflection hyperbolas. Gravitational mass wasting are also observed along the walls of the gullies

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The gullies are characterized on the Chirp data as contiguous low-amplitude diffraction hyperbolae (Echo-type II-1) (Table 1). The MCS data indicate erosional truncation along the flanks of the gullies (Fig. 6(b)). There is almost always limited/no signal penetration in Chirp lines along the gully zones (Figs. 6(a) and 6(c)) which is primarily due to the reflector rugosity. This factor also significantly degrades the seismic data quality in deeper parts of the MCS lines (Fig. 6(b)).

In the upper and middle continental slope where the seafloor gradient is relatively steep, exceeding 20°, reflections, either as elongated lens-shaped weak transparent or as low-amplitude masses of Echo-type II-2 (Table 1) with irregular upper surfaces, are observed in the Chirp data (Figs. 7(a) and 7(b)). We typically observe this type of reflection in oversteepened areas of highly inclined seafloor especially on the gully and canyon walls and along the highly inclined upper slope of both the Kefken and Sakarya Canyons. They constitute positive hummocky topography in en-echelon form (Figs. 7(b) and 7(c)). In these areas, Chirp data show transparent to semi-transparent internal facies and their upper surfaces are generally defined by diffraction hyperbolae (Fig. 7(c)). The data show contiguous reflections along the canyon walls, and in most cases, the base of the deposits generating this type of reflection pattern can be better distinguished only in the MCS data (Fig. 7(d)) due to the limited penetration of the Chirp signal. Around the shelf break, these deposits are located downslope of truncated sedimentary sequences (Echo-type V-1, Table 1) (Figs. 7(e) and 7(f)), which can be consistently traced along the uppermost continental slope just beyond the shelf break in the whole study area (Fig. 3b).

Fig. 7
figure 7

Different Chirp seismic data examples for gravitational mass wasting along the canyon walls from (a) proximal part of the SC, (b) head area of the Karasu Blind Canyon system, (c) northern head area of the KC, and (d) its corresponding MCS line. Example Chirp lines in (e) and (f) show truncation surfaces along the shelf break

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Along the continental slope, echoes from parallel stratified sediments are observed in two small zones located in the southeast over relatively flat areas of the canyon banks and over the Kefken Ridge (Fig. 3b), and are classified as Echo-type III-1 (Table 1). In addition, the Northern Platform and partly the slide structure bounding the western border of the platform are characterized by this type of reflection (Fig. 8).

Fig. 8
figure 8

(a) Chirp and (b) its corresponding MCS line across the slide structure, which extends along the western borderline of the Northern Platform. Blue arrows locate the glide plane, and BSR indicates the bottom-simulating reflection corresponding to the base of gas hydrate accumulations. Facies A indicates the pelagic or hemi-pelagic sediments of the Northern Platform while facies B corresponds to chaotic slide material (see text for details)

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Three separated slides are distinguished in the area (Fig. 3b) and are classified as Echo-type V-2 (Table 1). The largest of these is found at the distal part of the SC associated with the western boundary of the Northern Platform, with two smaller slides located close to the NE boundary of the study area along the eastern wall of the Karasu Blind Canyon system (Fig. 3b). These two cannot be mapped completely since the extent of our existing data does not cover the entire surface area of these two slides. The largest slide mapped, however, extends in a SW to NE direction along the eastern levee of the distal zone of the SC at a depth of 1600 m. A Chirp profile and its corresponding MCS line, which obliquely crosscut this slide in N-S direction, are shown in Figs. 8(a) and 8(b), respectively. The slide covers an elongated surficial area of approx. 70 km2 with a maximum runout distance of 3.7 km.

Although the glide plane or the base of the debris flow cannot be distinguished on the Chirp data, it can be clearly recognized on the MCS data as a continuous erosional surface, which separates the parallel to sub-parallel deeper sediments from the overlying transparent to chaotic slide material. The seismic data in Fig. 8(b) show that the slide is composed of two different facies. The SE part consists of parallel and relatively undeformed sediments with no undulation or rugosity, which form the topmost facies of the Northern Platform (termed Facies A in Fig. 8(b)), while the NW part contains highly chaotic slide material (termed Facies B in Fig. 8(b)). MCS data indicate that the upper part of Facies A overlying a semi-chaotic basal material remained intact during sliding. Facies B, however, contains semi-transparent chaotic sediments which slid towards the SC floor (Fig. 8(b)). These can be differentiated in the Chirp data by their blocky and irregular seafloor topography. The headwall erosion of the slide is clear both on Chirp and MCS data, with Facies A deformed around the headwall where the maximum scarp height reaches 45 m. Several extensional cracks indicate internal deformation within Facies A around the head, producing small-offset sub-parallel normal faults penetrating down to the glide plane, and a step-like geometry with discontinuities possibly generated during sliding.

Continental rise and basin plain features

The continental rise and deep basin of the study area generally exhibit the depositional characteristics of channel axis sediments, undulating deposits, and parallel stratified sediments. The corrugated seafloor morphology (Echo-type IV) is observed at the continental rise between 1700- and 2000-m water depths along the NW part (Fig. 3b). The wavelengths of the undulations typically vary from 120 to 510 m, and their height ranges from 6 to 55 m (Fig. 9), both increasing gradually towards the north and west. The orientation of their crestlines is generally parallel to the toe of the continental slope. The consecutive undulations are separated by well-defined, sub-parallel dipping surfaces generated by troughs, which resemble sub-parallel vertical fault surfaces due to the reflection convergence (Fig. 9).

Fig. 9
figure 9

(a) MCS and (b) its corresponding Chirp line illustrating the sediment waves from NW part of the area. MTDs show the mass transport deposits. Blue dashed line indicates the basal reflector for sediment waves. Description of the box core BC15 is from Duman (1992, 1994)

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On the Chirp data, the undulations appear as asymmetric regular wavy reflections of individual sediment packages with a steeper lee-side downslope (Fig. 9(b)). Overall Chirp signal penetration is good over this area, reaching 60 m and consisting of medium- to high-amplitude reflections with good lateral continuity, with the amplitudes of the reflections decaying gradually with increased depth. Box core BC15 from Duman (1992, 1994) recovered turbidites interbedded with coccolith ooze laminations (Fig. 9(b)). The MCS data reveal that the basal reflector of the undulating deposits is quite deep, lying at approx. 700 ms below the seafloor reflection (approx. 540 mbsf assuming a 1500 m/s sediment velocity) in the northernmost regions (Fig. 9(a)).

Along the continental rise, axes of the canyons show moderate-to-high amplitude parallel to sub-parallel reflections, with relatively limited Chirp signal penetration of 5 to 10 m, classified as Echo-type III-2 (Table 1). These type of reflections are clearer along the NW branch of the SC distal zone. MCS data in Fig. 10(a) shows an example of channel axis deposits from this zone, which reaches 65-m thickness in its central part (assuming a 1500 m/s sediment velocity). On the northern branch, however, the canyon axis reflections become irregular with diffraction hyperbolae resulting from the scour marks (Fig. 10(b)).

Fig. 10
figure 10

(a) MCS data example showing turbidites below the western branch of the SC to the north, (b) example Chirp line crosscutting the distal axis of the SC, (c) MCS, and (d) its corresponding Chirp line showing pelagic/hemi-pelagic sediments from the toe of the Northern Slope. Blue dashed line in (a) locates the border of the turbidite deposits, and MTDs in (c) indicate mass transport deposits

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The area between the two branches of the SC to the northernmost part corresponds to the basin plain (Fig. 2a). Only a small part of the deep basin sediments of the Black Sea is covered by our acoustic data, which is characterized by slightly undulating and semi-parallel sediment deposits classified as Echo-type III-1 (Figs. 10(c) and 10(d)). This type of sediment also exists within the continental rise at the toe of the northern slope (Fig. 3b). These areas typically show good Chirp signal penetration reaching 40 to 50 m and consisting of relatively high-amplitude reflections.

Discussion

Echo-characters

The classification of the seabed deposits by using specific echo-characters on the high-resolution Chirp seismic datasets has been made by several researchers and interpreted to indicate seafloor processes such as turbidity current activity, sediment erosion, or different types of sediment accumulation (e.g., Lee et al. 2002a; Kottke et al. 2003; Garcia et al. 2006; Domzig et al. 2009; Loncke et al. 2009; Mouchot et al. 2010; Savini and Corselli 2010; Carlino et al. 2013). Based on the analysis of available high-resolution seismic and bathymetric datasets, we classify the echo-types observed on the acoustic data, and suggest the following links between the echo-characters and the type of sediments with associated depositional and erosional features.

Echo-type I: limited/no penetration

The seafloor involved with this type of echo is capable of reflecting most of the seismic energy preventing the penetration of Chirp signal into the sub-bottom sediments. The echoes are attributed to either canyon floor reflections (Figs. 5(a), 8(a), and 10(b)) (Echo-type I-1) or outcrop area of the acoustic basement (Fig. 4(c)) (Echo-type I-2), and are interpreted to be originated from strongly reflective seafloor sediments.

We interpret Echo-type I-1 as attributable to the strongly reflective coarse-grained sediments transported by the turbidity currents along the canyon axes (e.g., Domzig et al. 2009; Loncke et al. 2009). Echo-type I-2, however, is produced by the basement outcrop at the SW shelf where strong erosional reflection surfaces exist in the Chirp data. Dondurur et al. (2018) observed the same rough seabed relief on the shelf extending further west of our study area, and based on the analysis of their MCS data, they suggested that the feature was formed by Cangaza volcanics of Cretaceous age outcropping on the seafloor.

Echo-type II: hyperbolic echoes

Hyperbolic echoes have two different interpretations depending on the location, shape, and size of the hyperbolae in the Chirp data. They are either interpreted as slope gullies (Fig. 6) when they appear as contiguous low-amplitude transparent hyperbolae with irregular hummocky and chaotic surfaces (Echo-type II-1), or classified as gravity-driven small-scale mass transport deposits (Fig. 7) if they are isolated semi-elongated hyperbolae with transparent to semi-transparent internal facies (Echo-type II-2). In some cases, it is not easy to differentiate both structures since their appearances are quite similar on the Chirp data. The MCS data, on the other hand, show the correct morphology of this type of echo since the data has been migrated during processing. It is easier to distinguish the gullies and gravity-driven mass transport since the collapsed hyperbolae become either reverse-V-shaped structures with steep erosive walls (gullies) or semi-elongated sheets (mass transport deposits) after the migration process (compare Figs. 6(a) and 6(b)).

Loncke et al. (2009) suggest that the occurrence and the shape of these hyperbolae are mainly controlled by the roughness of the seafloor topography. However, in contrast to their observations along the French Guiana Margin, small regular hyperbolae are commonly associated with gullies while relatively larger and elongated hyperbolae are generally linked with gravitational mass wasting in our study area (compare Figs. 6 and 7).

Several different models are proposed for gully origins, such as sea-level oscillations (Chiocci and Normark 1992), mass wasting and sediment erosion on the steep continental slopes (Field et al. 1999; Dowdeswell et al. 2004), and turbidity currents (Garcia et al. 2006). McHugh and Ryan (2000) suggested that gullies occur by overbank turbidity current activity in the Monterey Fan, whereas Greene et al. (2002) proposed an erosive nature for the same area. Depending on their geometry, sizes, distribution and erosive character, we interpret that the gullies are originated by mass wasting along the canyon walls and overbank turbidity currents as suggested by Dondurur et al. (2013) for offshore Amasra further east of the SC. In the proximal part immediately beyond the shelf break, the gullies are possibly formed by gravitational mass wasting on the oversteepened upper slope. In the distal part to the north, however, the cause of the gully formation is possibly overbank spillover of the turbidity currents as suggested by McHugh and Ryan (2000). The intensive erosional truncation at both sides of the gully walls indicates the erosive nature of overbank turbidity current activity in these areas (Fig. 6(b)).

In some cases, we also record acoustic scattering or side-swipe reflections, which appear quite similar to gullies on the seismic data (Fig. 6(b)) since the continental slope topography is highly irregular due to the excessive gullying. In these cases, the side-swipe reflections can be distinguished by analyzing the seafloor reflection just beneath the side-swipe: If it is continuous and compatible with the seafloor reflection lying at both sides, then it is classified as a side-swipe reflection (Figs. 6(b) and 6(c)).

Gravitational mass transport deposits are common along the Turkish margin of the Black Sea. Dondurur and Çifçi (2007) showed their periodic occurrence along the steep slopes of the eastern Black Sea, and Dondurur et al. (2013) indicated widespread gravitational mass wasting especially along the steep walls of the canyons for the western Black Sea slope, where they produce distinct erosional scarps behind the head zones. This is also the case along the shelf break and upper slope of our study area. In this zone, gravitational mass transport occurs with scarp surfaces around the flow heads (Figs. 7(e) and 7(f)), interpreted as fault surfaces by various authors (Algan et al. 2002; Nasıf and Dondurur 2017; Nasıf et al. 2018). We interpret that the most important agents responsible for the gravity-driven mass transport deposits observed at the canyon heads and the shelf break are the steep seafloor gradient exceeding 20° (Fig. 2c) and possible contribution of the local faults shown by Algan et al. (2002) around the shelf break.

Echo-type III: parallel stratified echoes

We suggest that parallel stratified reflections correspond with three different deposits depending on their internal structure and location. The first type (Echo-type III-1) is interpreted as reflections from pelagic or hemi-pelagic sediments, considering the conformity of their internal reflectors with the surface and underlying topography, their constant thickness (Figs. 8(a) and 10(d)) as well as their occurrence over topographic highs, flat platforms, and the toe of the continental slope along the northern slope (Fig. 3b). Pelagic/hemi-pelagic sediments in the form of parallel to sub-parallel undeformed sediment deposits are observed over the Northern Platform (Fig. 8), in the deep basin zone between the two branches of the SC at the northernmost area, at the toe of the Northern Slope (Figs. 10(c) and 10(d)), and in some limited areas between the canyons in the middle slope such as the Kefken Ridge.

Pelagic/hemi-pelagic sediments are the main deposition type of the deep basin in the Black Sea (Ross 1977; Çağatay 1999; Dondurur and Çifçi 2007; Dondurur et al. 2013), commonly interbedded with turbidites in areas close to the canyon floors. Several authors (e.g., Duman 1992, 1994; Çağatay 1999; Genov 2009; Akyüz et al. 2001) suggested an approx. 30-cm-thick topmost pelagic unit typically consisting of coccolith ooze deposited during the last 3000 years in the deep basin. We interpret that the pelagic/hemi-pelagic sediments in the study area are found in areas where the activity of the turbidity currents is limited except for the area of sediment waves to the NW. In these relatively quiet zones, pelagic/hemi-pelagic sediments mainly formed by coccolith ooze are deposited, where they are not truncated or reworked by turbidity current activity.

Reflections of Echo-type III-2 are interpreted as channel-fill deposits since we observe them along the channel axes, especially in areas where the SC is strongly meandering, and along the entire distal zones of the Sakarya and Karasu Blind Canyons (Fig. 3b), which exhibit parallel to sub-parallel and asymmetric stratigraphy (Figs. 5(b) and 10(a)). Because it is generally not possible to map their internal structure by Chirp data due to the limited/no signal penetration along the canyon floors, we use MCS data to show their stratigraphy (compare Figs. 5(a) and 5(b)). We interpret these deposits as coarse-grained turbidites interbedded with coccolith ooze as suggested by Duman (1992, 1994) from gravity core data. Similar reflection patterns along the distal parts of the canyon systems are also reported along the world’s ocean margins and are interpreted as turbidites, typically interbedded with mass transport deposits below the canyon floors (Lee et al. 2004; Hasiotis et al. 2005; Domzig et al. 2009; Nelson et al. 2011; Chaytor and Brink 2014; Li et al. 2016; Su et al. 2020).

Geological and geophysical data indicate the turbidity current activity along the SC. Primary evidence arises from the core data collected from the deep-water zone of the SC (Duman 1992, 1994) where the Sakarya and Kefken Canyons coalesce. Gravity core K27 recovered approx. 1.4 m of sediment which was completely composed of coarse-grained turbidite deposits (between 3.6 and 10.4 cm thick) interbedded with thin coccolith ooze layers, which characterize a stratigraphy of cyclic activity of the turbidity currents (Fig. 5(c)), where the ooze between the turbidites can be considered the background sedimentation. Secondary evidence for the turbidity current activity of the SC arises from the seismic data: the headwall erosion of the canyons (Figs. 7(e) and 7(f)), erosional form of the gully walls (Fig. 6), deep-water sediment truncations along the canyon walls (Figs. 5(d) and 5(e)), and scour marks at the distal parts of the SC axis (Fig. 10(b)) as well as turbidite deposits within the mid- and lower slope regions (Figs. 5(b) and 10(a)).

Turbidity currents can be erosional and/or depositional and ultimately shape the seafloor micro-relief. Hasiotis et al. (2005) interpreted the canyon floors with this type of sub-bottom reflection as erosional/depositional areas. The scour marks in the distal SC floor (Fig. 10(b)) indicate the erosive effect of the turbidity currents even in the distal locations. Seismic and bathymetric data indicate that the seafloor gradient changes abruptly in areas of turbidite deposition, eventually forcing the turbidity currents to slow down and turning them into depositional forms (e.g., Kneller and McCaffrey 1999; Gervais et al. 2006; Huang et al. 2012; Paull et al. 2013).

Meandering of the canyons also has an influence on turbidite deposition by decelerating the turbidity currents, which is why we commonly observe turbidites around the meandering zones of the canyons (Fig. 3b). These areas are also zones where strong canyon wall erosion occurs because of the erosive effect of the turbidity currents. When a channel bends, the current flows faster through the outer side of the bend, and conversely, it is slower along the inner edge of the bend. Therefore, slower currents precipitate the sediments they transport around the inner side of the bend while the faster flows at the outer edge cause truncation along the canyon walls. This process is especially clear in the mid- and lower slope section of the SC between the water depths of 900 and 1700 m (Fig. 3b).

Echo-type III-3 corresponds to seaward inclined reflections lying over a distinct erosional unconformity surface (Fig. 4(a)). Overall, the continental shelf, except the area covered by the outcrop of the Cangaza volcanics to the west, consists of this thin drape of parallel and undeformed recent sediments overlying inclined older reflectors. Three gravity cores (Algan et al. 1999, 2002, 2007) from the shelf platform over the areas classified as Echo-type III-3 recovered greenish-gray mud and 4–6-cm-thick discrete shelly layers as well as greenish-gray mud over a 8-cm-thick sandy shelly layer towards the bottom (core number 1, Fig. 4(d)), and dark gray muddy sediments with a sandy silt layer to the bottom (core number 9 and 16, Fig. 4(e)). This thin drape is interpreted as a Holocene unit, and its basal unconformity represents dry land during the LGM when the sea level of the Black Sea was approx. 120 m lower than present-day sea level. It is a well-defined interface on the seismic data along the continental shelves of the Black Sea, especially around the Bosphorus outlet as well as in the Romanian and Bulgarian offshore (e.g., Demirbağ et al. 1999; Aksu et al. 2002; Genov 2009; Lericolais et al. 2010; Yanchilina et al. 2017). Aksu et al. (2002) termed this unconformity as the α reflector and mapped it along the whole SW Black Sea shelf. Holocene sediments have been deposited onto this erosional surface following the last transgression period of the Black Sea after the LGM (Ryan et al. 1997; Çağatay 1999; Lericolais et al. 2009). The thickness of the Holocene unit increases towards land and we do not observe any coastal onlap on the Chirp lines within the coverage area of our dataset. This situation may indicate an abrupt transgression phase after the LGM as suggested by Algan et al. (2002).

Echo-type IV: undulating reflections

Along the NW part of the continental rise, we observe well-developed undulations or bedforms of Echo-type IV interpreted as sediment waves (Fig. 9). Although their physical size is beyond the resolution limit of our multibeam bathymetric data and prevents us mapping the morphology of the waves, the seismic data indicate that they are still developing and move upslope with a crest orientation parallel to the slope, typical for sediment waves formed by turbidity current activity (McCave 2017). Box core BC15 from Duman (1992, 1994), superimposed on the Chirp data in Fig. 9(b), recovered sediments with a similar composition to the sediments of gravity core K27 (Fig. 5(c)) collected from the canyon floor that is turbidites (5- to 11-cm thickness) interbedded with coccolith ooze laminations.

Sediment waves formed by either turbidity or contour currents are widespread all over the world’s ocean margins though they are not common in the Black Sea. Using their seismic and bathymetric data, Sipahioğlu et al. (2013) showed seafloor and buried sediment waves of much smaller size located at the distal canyon floors and walls formed by turbidity current activity in the eastern Black Sea. Dondurur et al. (2013) suggested a similar form of sediment wave field offshore Zonguldak, further east of the study area, suggesting that they are formed by unconfined turbidity currents in the deep basin of the Black Sea.

Different suggestions about the formation of the seabed bedforms have been proposed. For instance, Field et al. (1980) described the seafloor bedforms near the mouth of the Eel River as the Humboldt slide. However, it has been suggested later that these corrugations may actually be from a field of migrating sediment waves (Lee et al. 2002b). The bedforms might also represent surface expression of listric fault fans created by gravity-driven extension, which may also produce similar seafloor bedforms. In addition, growth faults formed simultaneously with the sediment deposition along the basin edges may create such en-echelon structures such as those observed in the Chirp line in Fig. 9(b) where the dipping surfaces generated by the troughs of the bedforms resemble extensional fault planes. On the other hand, the slip rate of the growth faults should increase downsection. In the seismic line in Fig. 9(b), however, apparent vertical displacement along these surfaces decreases with depth. In this case, a low-angle decollement or a listric fault would have to exist so that the strain would be accommodated at depth as suggested by Lee et al. (2002b). However, conventional deep seismic data show no decollement or listric faulting along the continental rise of the western Black Sea margin (Finetti et al. 1988; Nikishin et al. 2015a; Nikishin et al. 2015b). In addition, the basal reflector of the undulating reflections is clear in our high-resolution seismic data at approx. 700 ms below the seafloor (Fig. 9(a)), and there is no indication of a possible listric fault in the seismic lines. Therefore, the planes in Fig. 9(b) are interpreted as the sub-parallel surfaces formed between the consecutive undulations due to the reflection convergence.

Lee et al. (2002b) summarizes some of the diagnostic characteristics of turbidity-current sediment waves, which meet most of the characteristics of the sediment waves in the area. These include the following:

  1. (i)

    The sediment accumulates more rapidly in the upcurrent flanks, which causes the sediment waves to migrate upslope. Lee-sides of the sediment waves in the area are commonly so steep that it is not easy to compare the sediment thickness at both flanks in the Chirp lines (Fig. 9(b)). On the seismic lines, however, it is clear that the sediment thickness at the stoss-sides is higher than the lee-sides (Fig. 9(a)).

  2. (ii)

    From one wave to the next, sediment beds are more or less continuous unless there is erosion on the downstream flanks, and the structure of internal reflections is similar from one wave to the next.

    Our seismic data indicates that the internal reflections are continuous and consistent along the sediment waves with a more or less good trace-by-trace consistency although they appear to be discontinuous at the steep lee-sides in some places (Fig. 9(a)), which may also due to the limits of our seismic resolution.

  3. (iii)

    The reflections between the waves are either linear or convex upwards, and the overall sediment-wave field has a concave-upward surface. The boundaries between the sediment waves are convex upwards at depth between 200 and 700 ms from the seafloor, and they become more linear towards the seafloor (Fig. 9(a)).

  4. (iv)

    Wavelengths commonly decrease with distance from the source. The wavelengths and amplitudes of the sediment waves in our study area gradually decrease towards the north and west. This indicates that the source forming the bedforms is possibly the turbidity current along the canyons of the North Slope to the south or along the main SC to the southeast (Fig. 11(b)).

  5. (v)

    If the system is still receiving active turbidite deposition, continuous formation of the bedforms extends to the seafloor. Our seismic and Chirp data indicate that the formation of the sediment waves is still active in the continental rise, which indicates that the sediment wave field is still fed by the turbidity currents from the southern canyon systems.

Fig. 11
figure 11

(a) Epicenter distribution map of the margin for the eartquakes (M>2) occurred in last decade (from Kandilli Observatory and Earthquake Research Institute), and (b) potential geohazards and fault map of the study area. Gas and gas hydrate distribution is from Nasıf et al. (2020). NAF and AKFZ correspond to North Anatolian Fault and Adapazarı–Karasu Fault Zone, respectively. Location of AKFZ is from Yiğitbaş et al. (2004)

Full size image

According to the classification given by Tubau et al. (2015), depending on the effective process across them, the bedforms are either depositional or erosional, or a mixture of the two (Symons et al. 2016), which ultimately produces turbiditic sediment waves or scour holes, respectively. Based on the Chirp and MCS data, we observe no erosional structures both at and around the up- and downstream sides of the sediment waves (Fig. 9). In addition, sediment waves may occur on the levees of the channels representing migrating sediment waves on sedimentary channel overbank deposits formed by overbank turbidity currents (Domzig et al. 2009). However, we observe sediment waves at the toe of the Northern Slope and seismic data indicate no bedforms over the channel banks, indicating that unconfined turbidity currents, rather than overbank turbidity currents, seem to be effective in the formation of the sediment waves along the continental rise. We also suggest that the pelagic or hemi-pelagic sediments in this area are also reworked by these turbidity currents to form the sediment waves.

Echo-type V: discontinuous reflections

Discontinuous reflections are linked to either erosional truncation surfaces (Echo-type V-1) or sliding (Echo-type V-2). We attribute deep-water truncation surfaces, especially observed in areas where the canyons are bent (Figs. 5(d) and 5(e)), to the erosive effect of the turbidity currents carrying coarse-grained sediments. Similar deep-water sediment truncations along the canyon walls in the Black Sea are also evident in the seismic data shown by Sipahioğlu and Batı (2017) for offshore Bosphorus to the west and Dondurur and Çifçi (2007) for offshore Yeşilırmak to the east.

Discontinuous reflections also appear as scarp surfaces of the slides both along the shelf break formed upslope of the gravity-driven mass transports around the canyon heads (Figs. 7(e) and 7(f)), and in deep water larger-scale slides (Fig. 8). Dondurur et al. (2013) show similar headwall erosions and their associated slide scars due to the sliding in deep-waters of the western Black Sea.

Fluvio-marine systems

The bathymetric data indicate that the eastern head of the SC located at the mouth of the Küçükboğaz Lake (see Fig. 1b for location) may have a connection with a possible fluvial system today. On the other hand, the mouth of the Sakarya River is located close to the western head. The southernmost E-W trending Chirp line, which is the closest profile to the dry land, however, does not show evidence of incision of the shelf towards the Sakarya River mouth, or a buried paleo-channel, which presumably connects the western head to the fluvial system of the Sakarya River today. This is in accordance with the suggestion of Algan et al. (1999, 2007) that the origin and evolution of the SC cannot be associated with the fluvial incision of the Sakarya River during the LGM, but related to the interaction of tectonic and erosional processes. On the other hand, we observe a paleo-channel in an E-W trending MCS lines from the western shelf close to the western head of the SC (Fig. 4(d)), which runs in the NW direction towards the KC head. We tentatively interpret that this paleo-channel connected the KC to land during the LGM, suggesting a possible fluvial incision in the formation and evolution stage of the KC.

The fluvio-marine depositional system on the continental shelf consists of coastal and shallow-marine deposits which are mainly composed of mud-rich sediments according to the gravity core data collected by Algan et al. (1999, 2002, 2007) from the southern part of the shelf around the canyon heads. Gravity cores 9 and 16 (see Fig. 1b for location) show that the coastal to shallow-marine facies consist primarily of light and dark gray, fine-grained mud with laminated layers consisting of silt and/or dark colored fine sands and shell fragments (Fig. 4(e)). Algan et al. (2007) calculate a sedimentation rate of 80–90 cm/1000 years, typical for a deltaic environment, using C14 dating from the bottom sediments of the core 16 (Fig. 4(e)).

Acoustic data indicate that the main turbidity current of the SC initiates from either head of the canyon and extends into the deeper waters in a N-S direction. The main deep-water sedimentary and depositional systems in the margin are controlled by the flow regime that occurred along the SC and its tributaries as well as the KC and three major canyons located on the Northern Slope (Fig. 2b). These depositional systems seem to be fed from the southern shelf and slope through two separate pathways, both stemming from the shelf zone which is the most important source of the sediments: SC system to the south, and the Kefken Canyon to the western slope (Figs. 2b and 3a).

The canyon walls generally consist of either a few meter-thick mass wasting sediments, or very distinct scarps. In any case, no pelagic or hemi-pelagic drape exists over either the canyon walls or floors, most probably indicating present-day activity of turbidity currents. We interpret that this activity is initiated by gravity-driven mass wasting within the upper slope area just beyond the shelf break as suggested by several researchers (e.g., Alonso et al. 1995; Hampton et al. 1996; Ercilla et al. 2002; Dasgupta 2003; Mohring and Marr 2003). Once formed by (possibly successive) sliding events around the canyon heads along the steep slope zones of the upper slope, the turbidity currents can then give rise to the canyon incision towards basinal depths. This interpretation also explains the formation and development of the Karasu Blind Canyon system in the eastern continental slope. We also propose that sedimentary processes such as gravity-driven mass wasting along the upper slope and downslope turbidity current activity further modify the morphology of the canyon.

Potential geohazards on the margin

Different types of geologic factors that can be considered potential hazards coexist along the margin, which are controlled by (1) tectonic processes (local faulting and regional seismicity), (2) morphology of the margin (turbidity currents via canyon systems, slope overstepeening), and (3) sedimentary processes (mass transports, submarine fluid flow, loss of support due to truncation scarps and bedforms).

Tectonic processes

Geologic hazards related to the tectonic processes include folding and diapirism, faults, and seismicity-induced tsunamis. Available seismic data show no diapirism or large-scale folding in the area, but indicate local faulting along the continental rise. Seismic activity is considered the most important factor for all kind of submarine slumps and slides along the world’s ocean margins (e.g., Owen et al. 2007), and is one of the most important geohazards for offshore settlements and may ultimately cause tsunamis. Twenty-two tsunami events were recorded in the Black Sea from historical times to present, most of which were caused by earthquakes (Yalçıner et al. 2004; Papadopoulos et al. 2011). Nine of them occurred in the twentieth century which indicates an 11-year return period for the Black Sea. Since the magnitudes of the tsunamigenic earthquakes in the Black Sea are not too high, the impacts of the tsunamis have been generally local with typically less than 1.5 m of maximum water level (Yalçıner et al. 2004).

The current tectonism of the Black Sea Turkish margin is expressed by thrust faulting related to the Pontides thrust belt along the whole southern margin (e.g., Robinson et al. 1996; Spadini et al. 1996). However, there is no indication about the reverse faults in our MCS data on the continental shelf. This may be related to a number of factors: (1) the thrust faults or folds are located too deep in the sediments that the penetration depth of the MCS is not sufficient, (2) the Pontide belt is located more landward to the south than our MCS data coverage, and (3) the evidence of thrust faulting is eroded along the SC.

Today, the most significant seismic activity around the southern Black Sea margin is related to the right-lateral North Anatolian Fault (NAF) located approx. 140 km south of the area (see Fig. 1a for the location of the NAF), which has the potential to produce large-to-moderate size earthquakes. The last destructive earthquakes on NAF were Gölcük (MW=7.4) and Düzce (MW=7.2) earthquakes both occurred in 1999 at 100 and 75 km south of the study area, respectively. In addition, according to Yiğitbaş et al. (2004), NNE-SSW-trending strike-slip Adapazarı–Karasu Fault Zone on land to the south is still active and producing seismic activity as a transfer fault zone between compressional tectonics along the Black Sea margin in the north and transtensional tectonics in the south. The seismicity map of the region prepared from earthquakes that occurred in the last 10 years indicates that most of the seismic activity is directly related to the NAF and Adapazarı–Karasu Fault Zone on the land (Fig. 11(a)).

Local faulting is also observed offshore Sakarya River. The modern morphology of the SC is closely related to faulting both around the shelf (Algan et al. 2002; Nasıf et al. 2018) and in the deep-water zones (Yiğitbaş et al. 2004). The fault map obtained from MCS data (Fig. 11(a)) reveals the existence of subsurface normal faults. They display two different trends: To the north, faults show a NE-SW orientation, while they follow approx. an E–W trend in the shelf to the south, which is in accordance with the fault maps of Yiğitbaş et al. (2004) and Algan et al. (2002) although we do not observe offshore indications of the Adapazarı–Karasu Fault Zone possibly due to sediment erosion along the proximal part of the SC.

For consideration of geohazards, the existence of faults is important for two reasons: First, faults may produce scarps or vertical displacements of the seafloor; and second, they may trigger massive slope failures. The E–W trending normal faults around the shelf break to the south are considered to be active since they cause scarps at the seafloor around the shelf break (Figs. 7(e) and 7(f)) indicating a distinct vertical displacement. For the faults to the north, however, the activity status of the identified faults is not easily determined. They do not reach the seafloor and do not have any surface expression (Fig. 8(b)). We also interpret that some of them are differential compaction faults since they are located over the regional subsurface highs and they cannot be traced within the deeper sediments.

The seismic activity within the offshore area cannot be determined correctly due to the lack of data from a seismic network to the north (Fig. 11(a)). In the distal parts of the SC, however, our MCS data show local fault systems (Fig. 11(b)), which may also act as triggers for the slides.

Therefore, we tentatively interpret that one of the primary agent for all types of mass wasting along the SC may be (1) the seismic activity of the NAF, (2) the existence of local faulting within the margin around the shelf break and continental rise, and (3) the activity of Adapazarı–Karasu Fault Zone on the land. We conclude that the fault activity itself can be considered a potential geohazard for the shelf break zone since it generates fault scarps on the seafloor (Figs. 7(e) and 7(f)). The faults to the north (Fig. 10(c)), however, are regarded as a potential factor for submarine mass wasting, which is ultimately deemed as a geohazard.

Morphology of the margin

Oversteepening of the continental slope

The current tectonic setting of compressive deformation along the whole southern Black Sea margin gave rise to the formation of a steep continental slope as well as to the absence of a continental shelf platform in most regions along the Turkish margin. Most of the upper and middle slope is characterized by an oversteepened seafloor, which also exhibits gravitational mass wasting and widespread gullying (Fig. 11(b)). Although oversteepening itself may not be considered a serious geohazard, it is an important factor for different types of mass wasting at variable scales, and so can be considered a geohazard (e.g., Imbo et al. 2003; Mulder et al. 2009). It is especially evident around the upper slope and canyon heads and we interpret that it is likely the main preconditioning factor for the small-scale gravitational mass wasting observed on the canyon and gully walls.

Canyon systems and turbidity currents

The SC is unique along the whole Turkish continental margin, although several large canyons and canyon systems are reported throughout the Black Sea continental slope (e.g., Dondurur et al. 2013; Sipahioğlu et al. 2013; Sipahioğlu and Batı 2017; Jipa and Panin 2020), because it is a semi-confined meandering canyon system whereas most of the canyon systems are unconfined and open directly to the deep basin of the Black Sea. Based on the available sediment cores, seismic, and bathymetric datasets, we hereby suggest that the SC and its tributaries act as primary pathways for downslope sediment transport from the narrow shelf platform to the continental rise, along with the downslope turbidity currents flowing along the canyon axes (Fig. 11(b)).

Turbidity currents can be hazardous for submarine telecommunication cables and pipelines (Kneller and Buckee 2000). Following the Pingtung earthquake in 2006, turbidity currents reached 45 km/h speed along submarine canyons towards the Manila Trench and broke 22 cables (Carter et al. 2014). Although we do not know about the intensity, amplitude, and periods of the deep-sea currents within the middle and lower slope, the truncation surfaces on the seismic data indicate that the activity of the turbidity currents along the SC can be considered a potential geohazard for the submarine cables and pipelines.

Sedimentary processes

Submarine fluid flow

Gas in the shallow sediments is considered a potential geohazard because (a) it gives rise to reduction of the sediment strength and provokes potential sediment instability, and (b) sudden escape of a large gas volume into the water column may reduce the buoyancy of seafloor engineering structures (Baraza et al. 1999), producing collapses or large pockmarks on the seabed, ultimately resulting in loss of support for pipelines.

Nasıf et al. (2020) reported that the shallow gas accumulations, gas hydrates, and gas chimneys are widespread along the continental rise of the study area (Fig. 11(b)). Dissociation of gas hydrate accumulations may also cause excess pore pressures, and therefore, if the base of the gas hydrates indicated by bottom-simulating reflections (BSR) coincides with the glide plane, then overpressured sub-bottom sediments may be the cause of a sediment failure (Grozic 2010). Dondurur et al. (2013) proposed that the slides offshore Amasra, located approx. 70 km east of the SC, are triggered by excess pore pressures in shallow sediments due to submarine fluid flow possibly produced from gas hydrate dissociation. Therefore, we suggest that deep-water fluid flow may be an important pre-conditioning factor for sediment instability, and we propose that the occurrence of shallow gas and gas hydrates can be considered a geohazard especially for the drilling platforms since they may produce blowouts during the drilling.

Loss of support and mass transports

Large-scale slumps and slides can be an important hazard to offshore engineering structures and they can produce destructive tsunamis (Kawamura et al. 2014). Two types of sediment failure are distinguished in the area: gravity-driven mass transport deposits (Figs. 5(a), 6(b), and 7) and relatively larger submarine slides (Fig. 8). Seismic data indicate that the continental slope is highly unstable since we observe significant amount of gravitational mass transport deposits on the upper and middle slope.

Loss of support may be an important hazard for the offshore installations especially for the submarine cables and pipelines since they extend for long distances on the seafloor. Along the Sakarya margin, it can arise because of (a) sediment truncations due to the erosive seafloor currents (Fig. 5(e)), (b) bedforms of a mobile seafloor (Fig. 9), and (c) outcropping of older strata (Fig. 4(c)). Sediment erosion results in truncation scarps, which may fail under the weight of engineering structures, while bedforms cause the abrasion or burial of the structures lying on the bottom.

In the continental rise to the north, three relatively larger submarine slides indicate the potentially hazardous areas for offshore developments (Fig. 11(a)). We interpret that the largest one of them (Fig. 8) occurred due to the loss of support resulting from the sediment erosion along the distal SC. This slide consists of two conjoined internal facies, both sharing the same glide plane of Facies A and B (Fig. 8(b)). We suggest that the Facies B moved first towards the canyon floor possibly due to the loss of support because of the erosive effects of the turbidity currents along the SC floor, forming an irregular slide material deposited along a moderately inclined glide plane. This event then produced loss of support for Facies A, which then followed up the movement of Facies B. This also resulted in an extensional zone within the Facies A, which ultimately caused a small-scale graben structure with small-offset nearly vertical faults.

Conclusions

Both erosional and depositional processes of different types and scales coexist along the Sakarya margin. Erosional features, constituting 47% of the total surficial area, are classified as slides, erosional truncations, gravitational mass wasting, gullies, and outcropping of the Cangaza volcanics at the western shelf, while depositional processes comprising shelf sediments, turbidites, pelagic/hemi-pelagic sediments, and sediment waves form 53% of the total surficial area.

Different geologic hazards are identified offshore Sakarya River by means of high-resolution seismic profiles, which are categorized as the geohazards related to (1) local and/or regional tectonism, (2) continental margin morphology, and (3) sedimentary processes. Faulting related to the local and/or regional tectonism is an important source of geohazards since faults produce scarps at the seafloor (observed along the shelf break) or trigger massive submarine slides. Hazards linked with the margin morphology are slope oversteepening which results in gravitational mass wasting at different scales, and the existence of the Sakarya Canyon which seems to be actively involved in sediment erosion which creates loss of support for sediments at the distal parts. One of the most important sedimentary process which causes geologic hazards is mass transport, caused by oversteepening of the upper slope and local normal faults around the shelf break, whereas sliding occurs because of the loss of support in the distal parts of the canyon.

Based on the analysis of available geological and geophysical data, we suggest that both the Sakarya and Kefken Canyons may be actively involved in sediment erosion and subsequent turbidity currents. Considering the attention being placed on the western Black Sea margin by the hydrocarbon industry following the recent hydrocarbon discoveries in Tuna-1 well at the Danube submarine fan, our analysis can be regarded as a base study for the forthcoming geological, geophysical, and site survey studies for the assessment of geo-engineering installations such as oil and gas platforms or submarine natural gas pipelines along the SC.

The heads of the SC are located very close to the mouth of the Sakarya River. Therefore, floods of the river can generate hyperpycnal flows, which may provide a suitable zone for flow monitoring. Considering the low salinity of the Black Sea, the field conditions might be preferable to other sites around the world providing a better opportunity for sediment-laden river outflow which has a higher density than basinal waters of the Black Sea.