# Mass Transport Deposits Periodicity Related to Glacial Cycles and Marine-Lacustrine Transitions on a Ponded Basin of the Sea of Marmara (Turkey) Over the Last 500 ka

Céline Grall, Pierre Henry, Graham K. Westbrook, M Namik Çağatay, Yannick Thomas, Bruno Marsset, Daniel Borschneck, Hakan Saritaş, Günay Çifçi, Louis Géli

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Céline Grall, Pierre Henry, Graham K. Westbrook, M Namik Çağatay, Yannick Thomas, et al. Mass Transport Deposits Periodicity Related to Glacial Cycles and Marine-Lacustrine Transitions on a Ponded Basin of the Sea of Marmara (Turkey) Over the Last 500 ka. Sebastian Krastel; Jan-Hinrich Behrmann; David Völker; Michael Stipp; Christian Berndt; Roger Urgeles; Jason Chaytor; Katrin Huhn; Michael Strasser; Carl Bonnevie Harbitz. Submarine Mass Movements and Their Consequences. 6th International Symposium, 37, Springer International Publishing, pp.595-603, 2014, Advances in Natural and Technological Hazards Research, 978-3-319-00971-1. 10.1007/978-3-319-00972-8_53. hal-03455606

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Distributed under a Creative Commons Attribution 4.0 International License
Mass Transport Deposits periodicity related to
glacial cycles and marine-lacustrine transitions
on a ponded basin of the Sea of Marmara
(Turkey) over the last 500 ka

5 C. Grall¹,²*, P. Henry¹, G.K. Westbrook², M.N. Çağatay³, Y. Thomas², B.

6 Marsset², D. Borschneck¹, H. Saritas⁴, G. Cifçi⁴, L. Géli²

7 ¹CEREGE, CNRS, Aix Marseille University, Marseille, France

8 ²IFREMER - French Research Institute for Exploration of the Sea, Marine Geosciences,

9 Plouzané, France

10 ³Istanbul Technical University, EMCOL and Department of Geological Engineering,

11 Istanbul, Turkey

12 ⁴Institute of Marine Sciences and Technology, Dokuz Eylül University, Izmir, Turkey

13 *grall@cerege.fr

14 Abstract

15 The Sea of Marmara (SoM) is affected by large earthquakes occurring on the
16 North Anatolian Fault. Numerous submarine mass movements have occurred and
17 the most recent turbidites in the basins of the SoM have been related to historical
18 earthquakes. Within the SoM, the occurrence of submarine mass movements and
19 their size appears modulated by eustatic changes that can be accompanied by tran-
20 sitions between a salty marine environment and a brackish lake environment. De-
21 tailed analysis, using a 3D high-resolution seismic dataset, of stratigraphy over the
22 last 500 ka, within a ponded basin of the Western High, shows that intervals of
23 draped sedimentary reflectors alternate with onlap sequences that followed episodes
24 of rapid sea-level rise, with a periodicity of approximately 100,000 years (corre-
25 sponding to glacial cycles). Mass Transport Deposits (MTDs) occur within the
26 onlapping sequences. Detail analysis of the youngest large slide, which probably
27 followed the lacustrine transition during Marine Isotopic Stage 4 is presented; and
28 the possible triggering processes are discussed. The potential triggers of MTDs dur-
29 ing this transition, in the context of the SoM are: (i) gas hydrate dissociation by
30 pressure drop; (ii) changes in sediments supply and transport dynamics; (iii) varia-
31 tions in pressure and/or ionic strength in pores. The latter case appears the most
32 suitable hypothesis, as salt diffuses out of the pores of the marine clay-rich sediment
33 dominated by smectite at the beginning of low stand/lacustrine stages. The pore
34 water freshening induces clay swelling, which can potentially drive sediment slope
35 failure.
36

37 **Keywords:** High Resolution 3D seismic data, Mass Transport Deposits,

38 lacustrine/marine transitions, Sea of Marmara

39 **1. Introduction**

40 A great variety of factors can cause and/or promote submarine mass movements,

41 including earthquakes (Masson et al. 2006), environmental changes such as sea-

42 level changes and associated variations in sediment supply (Rothwell et al. 1998),

43 and gas hydrate dissociation (e.g. Maslin et al. 2004). The Sea of Marmara (SoM)

44 is an active seismotectonic setting, located along the western part of the North An-

45 atolian major continental transform fault (Wong et al. 1995; Le Pichon et al. 2001).

46 Submarine gas hydrates occur in the sediment at fluid emission sites (Bourry et al.

47 2009). It is likely that hydrates were more widely distributed in the past and were

48 dissociated massively during the deglaciation (Menot and Bard, 2010). The SoM

49 has also been subjected to a history of connections and disconnections with Black

50 and Aegean Seas via the Bosphorous and the Çannakale Straits respectively (Figure

51 1A, e.g. Cagatay et al. 2009). These oscillations are related to global eustatic

52 changes, and induce transitions between lacustrine and marine conditions. In such

53 an environment, submarine mass-movements are very common, and 75% of the

54 sedimentation in basins is composed of gravity-induced deposits (e.g. Beck et al.

55 2007) and many of which were triggered by earthquakes (McHugh et al. 2006).

56 Mass-wasting occurrence and size increased during the end of the glaciation period

57 which may be attributed to thermal destabilization of gas hydrate or variations in

58 sediment supply (Zitter et al. 2012).

59 During the last glacial period, the SoM was a mildly brackish lake (e.g. Londeix

60 et al. 2009), as the global sea-level dropped below the level of the Çannakale sill

61 (e.g. Çağatay et al. 2009) at the beginning of Marine Isotopic Stage 4 at ~70 ka

62 (thereafter MIS-4). Sedimentation rates were at least two times higher during these

63 glacial times than during modern marine environment (Cagatay et al. 2003; Vidal

64 et al. 2010). Pore fluid composition profiles indicate diffusion of salts, between the

65 lacustrine sediments and the seawater above, affecting a layer of 12-40 m thickness

66 below the seafloor, that does not strongly depending on the sedimentation rates in

67 the area considered (Zitter et al. 2008; Çağatay et al. 2004).

68 Little is known about older paleo-environmental changes induced by sea-level

69 variations. Considering global sea-level variations (e.g. Lisieck and Raymo, 2005),

70 marine to lacustrine transitions may not have occurred during all the glacial

71 100,000-year cycles, as the timing of connection and disconnection depends not

72 only on global sea-level but also on the depth of the Çannakale sill, which probably

73 varied with time (e.g. Çağatay et al. 2009; Badertscher et al. 2011).

74 In this paper, the relationship between mass wasting and sea-level changes over

75 the last 400-500 ka is evaluated using high resolution seismic records. The Mass-
76 Transport Deposits (MTDs) related to sea-level oscillations and marine/lacustrine
77 transitions are here characterized, and the possible MTDs trigger processes dis-
78 cussed.

79 **2. Main observations and interpretations**

80 **2.1. Overview of stratigraphic sequences and Mass-Transport**
81 **Deposits position**

82 The 3D seismic survey covers an area of 33 km² on the western part of the West-
83 ern High, along the main strike-slip fault branch of the Main Marmara Fault (MMF,
84 Le Pichon et al. 2001, Figure 1B). We focus here on the Eastern Fault-Zone Basin,
85 an asymmetric basin bounded on its southern side by the MMF (Figure 1B). The
86 seismic imaging (migration, bin size 6 m, Thomas et al. 2012) has a maximum sub-
87 seabed seismic penetration of about 600 m in this basin. H0, the shallowest seismic
88 horizon (Figure 2) encountered over most of the 3D HR survey, is at around 15 m
89 depth on core MD01-2430 location, at the same depth as an ash layer (Lericolais and
90 Henry, 2004) Mean sedimentation rates over this time-scale have been derived
91 from the H0 mapping. The sedimentation rates range between 0.3 and 1.1 mm/a, in
92 the Eastern Fault-Zone Basin. Considering the seismic penetration and the sedimen-
93 tation rates, the dataset gives access to the history of sedimentation over a time scale
94 of about 500 000 years.
95

95 Sedimentary sections consist of the repetition of four sequences, each sequence
96 has between 0 and around 80 meters of thickness and contains two seismic units
97 (Figure 2A and B) : (1) Draped units with moderate lateral variations of thickness,
98 either acoustically transparent or bearing laterally continuous internal sub-parallel
99 reflectors (2) Basin filling units displaying strong lateral variations of thicknesses
100 and laterally correlative with either condensed sections (Figure 2B) or hiatuses on
101 the slopes and topographic highs. Sedimentary bodies that contain irregular lenses
102 and chaotic reflections are observed at the base and/or within the filling sequences
103 (Figure 2A and B), and are interpreted MTDs (e.g. Alves et al. 2010).

104 Three remarkable MTDs thus occur within the basin filling
105 units (Figure 2A). Prominent reflective horizons (H1, H4 and H6; Figure 2B) which
106 present significant disruption, mark the base of the MTDs. The top horizons of the
107 MTDs (H1º, H3 and H5, Figure 2B), pinch out against a continuous basal horizons
108 and display small-scale topography. Horizon H5 displays erosive channel geometry
109 that may have developed concurrently with the deposition of the basin filling unit
110 (Figure 2A and B). Erosion affects the underlying MTDs and therefore must at least
111 in part post-date the MTDs events. Horizon H3 is relatively smooth but small ero-
112 sive channels affect H4 where H3 is absent, suggesting the MTD strata between H3
113 and H4 is correlative with an erosion surface (Figure 2A and B).
114 The high-resolution age model from ¹⁴C calibrated ages obtained from core
115 MD01-2430 (Figure 1B) indicates a sharp reduction of sedimentation rate from 0.6
116 to 0.2 mm/a following the reconnection at 14.7 ka (Vidal et al. 2010). In the Holo-
117 cene, sedimentation rates subsequently increased back to 0.4 mm/a, although still
118 not reaching the glacial sedimentation rate again (Vidal et al. 2010). We surmise
119 this has been the case during previous glacial cycles, at least in cases when the SoM
120 had been disconnected during a global sea-level low-stand, and possibly during all
121 episodes of rapid seal-level rise. In this case, the repetition of filling unit/draped
122 section corresponds to the sea level low and high stands oscillation at 100 ka time-
123 scale. In particular, the onlap sequence on H1' (Figure 2C and D), first low-
124 stand/filling units below the seafloor, probably results from a decrease of sedimen-
125 tation rate during the marine high stand at MIS-5 (~130 ka). The MTDs lying on
126 H1 thus occurred during the sea level fall at MIS-4 (~70 ka).

127 Variations of clay composition could explain differences of slope instabilities
128 frequency between marine and lacustrine rich-clay sediments. Powder of clay frac-
129 tion of 38 samples (of both marine and lacustrine muds) has been characterized by
130 X-ray diffraction. The relative amount of smectite, illite, kaolinite and chlorite were
determined by comparing the major XRD reflection peaks surfaces obtained on un-
131 treated, ethylene-glycolated, hydrazine monohydrate and heated smear slides. La-
132 custrine and marine clay compositions do not display significant differences. The
133 clays fraction is dominated by swelling clay as the clay fraction is composed of, on
134 average, 75% smectite, 13% illite, and around 5-7% kaolinite and chlorite.
135
136

136

## 2.2 Characterization of MTDs on H1

138

139 The MTDs above H1 form a thick unstratified lens with a rough upper surface.
140 Within this lens, we observed locally some highly reflective patches interpreted as
141 stratigraphic reflectors within coherent blocks floating in a matrix of flowed mud
142 that has lost coherence, at least at the scale of the HR data (Figure 3D). The top of
143 the MTDs is a highly reflective and rough reflector of reverse polarity to that of the
144 seafloor, suggesting that the deposit has probably a higher content of fluid than the
145 upper sediments. The top of the MTD is undulated, and we observed small-scale
146 ridges of higher reflectivity (in absolute value) with a NE-SW strike (Figure 3B and
147 C). The geometry of the structures suggests the mud flow occurred perpendicular to
148 the western ridges (Figure 3A) along a NW-SE sliding direction. The MTD covers
149 an area of about 0.47 km² and has a total volume of 5.5 hm³. Along the NW edge
150 of the basin, the MTD thickness is around 7-8 m. It increases toward the southeast,
151 reaching a maximum thickness of 23 m in the central part of the basin (Figure 3C).
152 H1 displays variable reflectivity below the MTDs. On the whole western edge of
153 the MTDs but also its northeastern edge, H1 has negative polarity and high reflec-
154 tivity (Figure 3A). Below the central part of the MTDs, H1 appears composite with
155 several sub-reflectors resulting in a concentric reflectivity pattern (Figure 3A). On
156 the slope west of the MTD, H1 is absent, or displays low reflectivity, over a 0.28
157 km² area (labeled eroded area in Figure 3A). This area corresponds to a place where
158 H1 but also others horizons above have been eroded and thus appears as the probable main source of the material forming the slide masses. The thickness of sediment 159 removed during the event is estimated to 11±5 m comparing the thickness of sediment on either side of the slide with the thickness of sediment in the scar (Figure 160 161 3D, section 1-1'). It also seems that most of the sediment deposited between H1 and 162 163 H0 has been removed. The volume of sediment removed is thus 3±1.4 x 10⁶ m³, 164 which represents only about 1/2 to 1/3 of the MTD volume. This paradox may be 165 resolved if sediments deposited above H1 in the basin have been displaced and in 166 part incorporated into the MTD. The eastern edge of the MTD is not sharp (Figure 167 168 3D, section 2-2') but appears as a gradual transition between disturbed sediments and an undisturbed layer of about 7-8 m thickness. Also, the base of the MTD 169 is not erosive in the deepest parts of the basin but appears to follow closely H1, 170 which may represent the basal sliding surface or layer immediately beneath it. In 171 fact the volume and geometry of the MTD may be accounted for, if a 7.4 m average 172 thickness layer of sediment covering both the area of sediment removal and the area 173 now covered by the MTD was redistributed by the sliding event to the deeper part 174 of the basin. This can suggest liquefaction occurred as the slide progressed, a common feature in slides and slumps, which can evolve into debris flow deposits 175 (Mulder and Cochonat, 1996). 176

## 3. Discussions about possible triggering processes and conclusions:
177
178

179 During the last glacial cycle, the draped sequence in the basin corresponds to 180 low-stand lacustrine deposits with a relatively high sedimentation rate. Parts of the 181 underlying deposits, presumably marine sediments from the last interglacial (MIS- 182 183 5), have been redistributed by gravitationally driven mass transport, resulting in lateral variations in sediment thickness. Sediment remobilization likely occurred after 184 185 sea-level dropped at MIS-4. This pattern of alternating draping sedimentation and 186 sediment redistribution is found to repeat cyclically in the stratigraphic record. This 187 suggests that comparable sequences of events occurred during earlier glacial cycles. 188 This is also supported by the thickness of these sequences, which would correspond 189 to about 100,000 years of sedimentation extrapolating the mean sedimentation rates 190 measured over the last ~29 ka.

191 Sediment fluxes into the deep basins of the Sea of Marmara from continental 192 shelves and slopes are higher during low stand than during high stand, and slope 193 instabilities clustered at the end of the glaciation (Zitter et al. 2012). However, the 194 Western High is relatively isolated from this influence, as canyons originating from 195 the shelf edges bypass the Western High and supply sediments to the deep Central 196 and Tekirdag basins (Figure 1A). Nevertheless, we still observe cyclic local redis-
196 tribution of sediments between slopes and ponded basins but probably with a different timing as the MTD in this study is inferred to correlate with the beginning of a low stand.

199 At the beginning of MIS-4 (~70 ka), sea-level fell from -25 m to -85 m below the modern sea level resulting in the disconnection of the Sea of Marmara (Çağatay et al. 2009). This also induced a pressure drop of 0.6 MPa and thus possible dissociation of gas hydrate (Liu & Flemings, 2009). Nonetheless, a temperature decrease of 1°C at the seafloor of the Western High associated to this pressure drop should be sufficient to keep hydrates in their stability field (e.g, Henry et al. 1999). It is thus unlikely that widespread gas hydrate dissociation occurred at this time. On the other hand, pore fluid freshening by salt diffusion must have occurred. Since the post-glacial reconnection, water column salinization (that has been progressive over 2000 years, Vidal et al. 2010) lead to the diffusion of salts downward into the pore fluids of the lacustrine sediments (Zitter et al. 2008) and the reverse process had occurred during the last glaciation after the disconnection. The diffusion length being proportional to the square root of time, the salt diffusion gradient below the seafloor affects a sediment layer of thickness 10 to 100 m for times ranging from 1000 to 100.000 years. A layer of 11±5 m of marine sediments has been remobilized, which appears consistent with the thickness of leached sediments after only a few thousand years. As the clay-fraction is dominated by smectite, clay swelling induced by osmotic stress during the desalinization of the pore water is expected (Charpentier and Bourrié, 1997), reducing the cohesion of sediment and promoting submarine slope failures (Masson et al. 2006). Consequently, systematic destabilization of marine sediments on slopes at the beginning of lacustrine periods should occur.

221 We remarked that the cyclic occurrence of MTDs observed in a ponded basin of the Western High in the SoM could correlate with glacial-interglacial sequences. Despite that slope instabilities are presumably triggered by earthquakes, their size and/or frequency appears to be modulated by glacio-eustatic oscillations and the lacustrine/marine transitions associated to them. The swelling of clays in marine sediments under low salinity brackish water can promote slope instability after marine to lacustrine transitions, and may be one factor contributing to long-term cyclicity of slope instability occurrence in the Sea of Marmara.

229
230 **Figure captions**

231 Figure 1: A. Tectonic map of the Sea of Marmara region with the Main Marmara Fault (MMF, Le Pichon et al. 2001) and secondary fault system (Grall et al. 2012). The red box locates the 3D seismic survey data. B. Seafloor mapping from the 3D-seismic data, and the location of the Eastern Fault-Zone basin (annotated EFZ-basin). Locations of the seismic lines present on Figure 2, MD01-2430 core, and sites where gas hydrates have been sampled are shown. The black box indicates the area shown in Figure 3A.

237 Figure 2: A. Seismic section across the northern edge of the Eastern Fault-Zone Basin. MTDs are highlighted in yellow, and numbers 1, 2, 3 refer to the three most remarkable MTDs discussed in the text. B. Line drawing of the seismic section above. Horizon numbers mentioned increase from top to bottom. Stratigraphic sequences contain one basin filling unit (in grey) in which MTDs occurred (indicate by black stars), and one draped section (in white). The four main stratigraphic sequences in this characteristic seismic section have a thickness of around 65-75 ms twt, which represent around 50-63 m (P-wave velocity ranges between 1515 (above H1) and 1660 m/s (between H6 and H4), within the sediments, Thomas et al, 2012). C: Seismic section across the Eastern Fault-Zone Basin, showing details of the first stratigraphic sequence below the seafloor. D. Line drawing of the seismic section above. The filling unit contains a condensed section (H1'-H1) on which MTDs occurred (between H1 and H1'). The condensed section extension corresponds to the basin depocentre (represented in Figure 3A) when the slides occurred. See Figure 1C for the location of the seismic lines.

250 Figure 3: Mapping and characteristics of the first remarkable MTDs below the seafloor, which probably occurred after the sea-level fall at MIS-4 (~70 ka). A. Amplitude map of the basal surface of the MTDs (horizon H1). This map is located in Figure 1B. Eroded area and area of deposits (which is detailed in Figure 2B and C) correspond to the main scars of the slides and the zone of slide depositions. The depocentre is derived from the mapping of the condensed section below the MTDs (Figure 2C and D). B. Amplitude map of the top horizon of the MTDs. Amplitude anomalies alignment is indicated by white dash lines. C. Topography of the top horizon of the MTDs. The white dash lines and the black arrows indicate the orientation of small undulations and the flow lines respectively. D. Seismic sections along the scars of the slides (1-1') and across the MTDs (2-2').

260

261 **Acknowledgements**

262 The Marmara-DM cruise was supported by ESONET NoE, Network of Excellence, coordinated by IFREMER and co-funded by EU as part of FP6 (2009). This work is also co-funded by CNRS. We would like to thank Cedric Amiel, for his participation to the XRD laboratory manipulation. We thank Cecilia McHugh and Lies Loncke for their constructive comments that improved this manuscript.

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328

329 **5. Figures:**

330

331

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