Slope failures and stability analysis of shallow water prodeltas in the active margins of Western Greece, northeastern Mediterranean Sea

Abstract

Sediment instabilities are common on the prodeltas of the seismically active continental margins of Western Greece. Sediment failures on the low-angle (0.5°-2°) prodelta slopes manifest themselves as successions of peripheral rotational block slumps restricted to the foresets of the late highstand systems tract (HST). The individual slump blocks are about 80-150 m long and are bounded by growth faults acting as curved slip planes that extend to a mean depth of 10-15 m below seafloor. Shear planes develop in the lower part of muddy and/or gas charged HST foresets. Deeper basal transparent muddy layers of the early HST bottomset, together with the late Pleistocene transgressive systems tract sequences (TST), are mostly unaffected. On the steeper (2°-6°) fan delta slopes of the western Gulf of Corinth debris flows and avalanches with a significant retrogressive component dominate slope destabilisation. Sediment cores taken from landslide scarps and slide planes penetrated gas bubble releasing sediments thereby indicating that failure planes are in the late HST foresets/upper part of the early HST bottomsets gas charged zone. The foresets of the HST prodelta deposits display high water content (30-80%), low bulk density (1.4-1.9 g cm -3 ) and relatively low values of undrained shear strength (3-20 kPa). The water content of the HST distal muddy bottomsets is relatively higher (50-110%) and bulk density relatively lower (1.3-1.7 g cm -3 ) with low values of shear strength (2-10 kPa). The shear strength of the gas releasing sediment layer displays lower values (2-9 kPa) relative to the overlying, post failure, muddy sediments of the late 100-300 years. Slope stability was calculated using the normalised soil parameter (NSP) method under undrained conditions for normally consolidated prodelta sediments. This analysis indicates that instabilities could be induced by critical earthquake ground accelerations of 26.6-29.6% g for the HST foresets and 12.4-14.1% g for the basal transparent layer belonging to the early HST bottomsets. Consequently the early HST bottomsets has to be considered a potentially unstable layer since the regional peak ground accelerations (PGAs) for the next 50 years are expected to range from 19 to 30% g. Moreover, our results show that new glide planes in the prodeltaic sediment bodies of the seismically active continental margins of Western Greece will likely develop from the gas charged sediments of the lower part of the HST foresets to the upper part of early HST bottomsets.

Influence of bottom currents on the sedimentary processes at the western tip of the Gulf of Corinth, Greece

“…1D), reflectors corresponding to these two transgressions have been identified by correlation with other studies in the western and central gulf (Bell et al, 2008(Bell et al, , 2009. Different sedimentary units tracing the last post-glacial sea level rise have also been highlighted from higher-resolution seismic data at the western tip of the Gulf (Lykousis et al, 2009). These authors describe that in the low-gradient prodelta areas, one unit of transgressive systems tract is overlain by a prodelta wedge of the late Holocene highstand systems tract.…”

“…Other sedimentary environments are related to the existence of Gilbert-type fan-deltas, already described in details by Piper et al (1990) and Lykousis et al (2009). These environments are described here as shelves, delta foresets and fan aprons (Fig.…”

“…Those rivers develop giant Gilbert-type deltas along the southern coast, while deltas on the northern coast are smaller and thinner (Piper et al, 1990;Ford et al, 2013). Slopes are highly unstable (Ferentinos et al, 1988;Lykousis et al, 2009). During the last centuries, submarine landslides have been triggered by earthquakes but others also occurred aseismically, most often during the rainy season because of sediment overloading near river mouths (Galanopoulos et al, 1964;Heezen et al, 1966).…”

“…A morphosedimentary map of the Holocene deposits was also produced through the interpretation of the bathymetry, the seismic facies and the integration of previous works ( Fig. 3B; Heezen et al, 1966;Lykousis, 1990;Piper et al, 1988Piper et al, , 1990Lykousis et al, 2007bLykousis et al, , 2009 and new coring data.…”

“…Sediment characteristics and sedimentary processes in the Gulf of Corinth have been studied for half a century by seismic reflection profiling and gravity coring (Heezen et al, 1966;Ferentinos et al, 1988;Piper et al, 1988Piper et al, , 1990Papatheodorou and Ferentinos, 1997;Perissoratis et al, 2000;Lykousis et al, 2007aLykousis et al, , 2009). South of the Gulf, in the Peloponnese, an extended drainage network cuts through a thick cover of uplifted synrift deposits, delivering large amounts of sediments to the Gulf.…”