G. Ferentinos

Marine Debris on the Seafloor of the Mediterranean Sea: Examples from Two Enclosed Gulfs in Western Greece

Abstract During two daylong surveys aboard fishing boats, one in Patras Gulf and the other in Echinadhes Gulf, marine debris retained in the trawl nets was examined. The marine debris concentration on the seafloor of Patras and Echinadhes Gulfs is 240 and 89 items/km2, respectively. The most abundant debris is plastic followed by metal. The high percentage of beverage packaging in Echinadhes Gulf is attributed to shipping traffic, whilst the high percentage of general packaging in Patras Gulf suggests that the source of this material may be on land and it is transported into the gulf by rivers and seasonal streams.

A pockmark field in the Patras Gulf (Greece) and its activation during the 14/7/93 seismic event

Abstract During a recent oceanographical-geophysical survey carried out in the southeastern part of the Gulf of Patras in Western Greece for the construction of an outfall, an active pockmark field was found. The pockmark field was formed in soft layered Holocene silts. The pockmarks are associated with acoustic anomalies attributed to gas-charged sediments. The pockmarks vary in size and shape from 25 to 250 m in diameter and from 0.5 to 15 m in depth and are among the largest and deepest observed in the world. On July 14th, 1993, during the survey, a major earthquake of magnitude 5.4 on the Richter scale occurred in the area. During the 24 hour period prior to the earthquake the bottom water temperature anomalously increased on three occasions, whilst for a few days after the earthquake it was noted that the majority of the pockmarks were venting gas bublles. It is considered that the three abrupt sea-water temperature increases were probably the result of upward migrating high-temperature gas bubbles in the water column. It is further suggested that the earthquake was the triggering mechanism and that the gas expulsion was caused by the reduction in the pore volume in the sediments resulting from changes in the stress regime prior to the earthquake. Therefore, it can be suggested that in seismic areas adjacent to pockmark fields, earthquake prediction may be achieved by monitoring the water temperature and/or the rate of gas venting in the pockmark field. Our analysis indicates that the pockmark field in the Patras Gulf has formed slowly during the Holocene by continuous gas venting, which is periodically being interrupted by short-duration events of enhanced gas seepage triggered by earthquakes.

Sediment Transport processes on an active submarine fault escarpment: Gulf of Corinth, Greece

Abstract Sediment transport processes on the escarpment of a listric fault in the tectonically active and terrigenous sediment-dominated Corinth graben in Greece are studied using 3.5 kHz and airgun profiles and sidescan sonar images. The whole of the slope area is affected by gravitational mass movements: slumps, debris/mud flows and liquified and turbidity flows. In canyon fan systems, large volumes of sediment are transported through slumping, debris/mud and turbidity flows into the basin. Here, they build coalescing fans which consist of intercalations of debris flow deposits and turbidites. In delta front-slope-fan apron systems, sediments are transported radially downslope through slumping, debris/mud flows, liquified flows and high-density turbidity flows into the basin where they build coalescing fan aprons. These consist of massive homogenized deposits (fluxoturbidites). In shelf slope systems retrogressive rotational slumping is the dominant instability feature. However, due to the sharp increase in the slope gradient caused by the slumping, debris and turbidity flows are triggered. The major factors responsible for triggering gravitational mass movements in the region are earthquakes, high sedimentation rates off the river mouths and heavy rainfalls and high seas. These mass movements appear to occur at least once every 2 yrs as indicated by the failure of submarine cables.

High sediment yields and cool, wet winters: Test of last glacial paleoclimates in the northern Mediterranean

Opposing models have been proposed for last glacial paleoclimates in the Mediterranean area. To discriminate between the alternative models, we calculate variations in sediment yield within a high-resolution stratigraphic framework developed for the Alkyonides basin, central Greece. Latest Quaternary highstand deposits are distinguised from lowstand Lake Corinth deposits on the basis of seismic reflection data and micropaleontological and palynological analyses of drop cores. After depth conversion and calculation of solid sediment masses in successive time slices, sediment discharge rates during the last glacial lowstand interval (ca. 70-12 ka) are shown to have exceeded discharge rates during the preceeding interglacial highstand interval (ca. 128-70 ka) by =60%, implying enhanced seasonality during the glacial period, with cool, dry summer and wet winter conditions in the region. Sediment yield calculations can thus provide an empirical test of paleoclimate models and offer input to sequence stratigraphic models.

Active faulting within the offshore western Gulf of Corinth, Greece: Implications for models of continental rift deformation

Discrimination between different lithospheric extension models focusing on the roles of low-angle vs. high-angle faulting, and how strain is distributed, requires high-fidelity imaging of brittle deformation. High-resolution seismic reflection and multibeam bathymetric data in the western Gulf of Corinth continental rift were collected to establish the contribution of offshore faults to extension. Onshore fault slip here is significantly less than expected from geodetic strain rates. The rift at this location is a half-graben tilted to the north by S-dipping faults within the uppermost crust. A basement horst on the northern margin is uplifted by the North and South Eratini faults, and the axial channel is fault controlled. Subsided lowstand shorelines in the hanging wall of the North Eratini and the well-studied Aigion fault suggest that the faults have similar displacements. Summed extension from the four major faults across this part of the rift (Eliki, Subchannel, South Eratini, North Eratini) is 8–16 mm/yr, thereby reconciling geologic and geodetic data sets. Distributed deformation across isolated multiple faults can model this part of the rift without recourse to, and potentially incompatible with, an underlying low-angle detachment.

Submarine and coastal sediment failure triggered by the 1995, Ms = 6.1 R Aegion earthquake, Gulf of Corinth, Greece

On June 15th, 1995 a locally destructive earthquake of magnitude Ms 6.1 on the Richter scale occurred offshore, 7.5 km NNE of the town of Aegion, in the western Gulf of Corinth (Greece). An offshore survey using 3.5 kHz subbottom profiling system and a remote operated vehicle (ROV) has shown that the earthquake caused small sized subaerial to submarine sediment failure in at least four sites, in three fan delta deposits: the Rododafni; the Eliki and the Tolofonas. The fan deltas were located within a radius of about 9 km from the epicentre. The areal size of the four sediment failure sites ranged from 2 × 104 m2 to 6 × 105 m2, whilst the volume of the failed masses ranged from 4 × 103 to 3 × 106 m3. The sediment deformation types identified at the failure sites consist of ground cracking, rotational slides, elongated slides, sediment gravity flows and extrusion of mixtures of water and sand (sand boils). The sediment failure in the four sites affected the upper 5–6 m of well layered Holocene (?) topset and foreset fan delta deposits. The failure occurred on slopes ranging from 0.2 ° to 23 ° and the slip planes were all bedding planes which have a gradient from 0.2 ° to 21 °. The dominant instability mechanism that caused the sediment failure in the Rododafni and Eliki fan deltas is considered to be liquefaction of a shallow sub-surface horizon. The liquefaction was caused by elevated pore pressure enhanced perhaps by the presence of gas, resulting from the cyclic loading induced by the earthquake. The liquefied layer is assumed to have temporarily provided a failure surface for sliding to take place on and caused movement of sand that was ejected onto the surface. In the Tolofonas fan delta the causative mechanism in the case of the multi-block rotational slide is considered to be deformation of the underlying sediments caused by remoulding and/or liquefaction while in the case of the elongated slide it is considered to be the result of a combination of shear stress increase and/or strength degradation of the unconsolidated sediment. Both of these conditions could have been generated during the cyclic loading resulting from the Aegion earthquake. The study of historical documents reveals that sediment failure like those described above have also occurred at least four times during the past 2500 years in the same locations. Therefore, it is suggested that they could be repeated in the future by any earthquake event with a magnitude greater than 6 R, depending upon the proximity of the site to the earthquake epicentre.

Methane and hydrogen sulfide seepage in the northwest Peloponnesus petroliferous basin (Greece): Origin and geohazard

Gas seepages along the Ionian coast of the northwestern Peloponnesus (Greece), at Killini, Katakolo, and Kaiafas reflect deep hydrocarbon-generation processes and represent a real hazard for humans and buildings. Methane microseepage, gas concentration in offshore and onshore vents, and gas dissolved in water springs, including the isotopic analysis of methane, have shown that the seeps are caused by thermogenic methane that had accumulated in Mesozoic limestone and had migrated upward through faults, or zones of weakness, induced by salt diapirism. A link between local seismicity and salt tectonics is suggested by the analyses of hypocenter distribution. Methane acts as a carrier gas for hydrogen sulfide produced by thermal sulfate reduction and/or thermal decomposition of sulfur compounds in kerogen or oil. Methane seeps in potentially explosive amounts, and hydrogen sulfide is over the levels necessary to induce toxicological diseases and lethal effects.

Gas-charged sediments in the Aegean and Ionian Seas, Greece

Abstract During the last fifteen years, marine seismic surveys in the Aegean and Ionian Seas have revealed numerous acoustic anomalies; i.e. acoustic turbid zones, gas pockets, gas plumes, enhanced reflectors, columnar disturbances, wipe outs and meso- to micro-morphological features such as pockmarks, domes, mud volcanoes and elongated depressions. These are attributed to the presence of gas in sediment interstices. The gas-charged sediments are found in Pleistocene and present-day fjord-like environments, Pleistocene and present-day deltaic environments, lakes and open sea environments. The gas found in the Quaternary fjord-like and deltaic depositional environments is assumed to be of biogenic origin. The gas found in the pre-Quaternary open sea environments is associated with faulting and salt doming and may, therefore, be of thermogenic origin. Although the sediment grain size is favourable for the formation of pockmarks, the lack of them is probably due to the high seismicity which characterises these regions.

Modern sedimentation in the N.W. Aegean Sea

Abstract The area under investigation is located within 22°45′ to 23°45′E, and from 39°15′ to 40°15′N. Sedimentological, bathymetric and shallow geophysical data were collected from here during the 7 78 cruise of R.R.S. “Shackleton”. Twenty-seven sea-bed (surface) samples were collected, using a Day grab, from different environments; these were analysed for grain-size distribution, carbonate content, and clay mineralogy. On the basis of the bathymetric and shallow geophysical data, the area has been classified into five major physiographic environments, according to their bathymetry and topography: (1) shelf (Thermaicon Plateau); (2) slope; (3) marginal plateau; (4) the “canyon system” and valleys; and (5) (Sporades) basin. Based on the sedimentological analyses, the surface sediments have been classified into four major Provinces, as follows: (1) Province 1 — shelf muds; (2) Province 2 — muddy sands; (3) Province 3 — outer margin muds; and (4) Province 4 — outer shelf/shelf break. It is concluded that modern sedimentation in the N.W. Aegean Sea is dominated by the (terrigenous) river inputs and by the circulation of both high and low salinity masses.

Earthquake-induced coastal sediment instabilities in the western Gulf of Corinth, Greece

Abstract On June 15 1995, a locally 6.2-R destructive earthquake occurred in the western Gulf of Corinth and caused extensive onshore and offshore sediment instabilities. A detailed offshore geophysical survey along the Tolofonas/Eratini coastline, soon after the earthquake, revealed the existence of two major sediment failures affecting the surficial 8 and 2 m of the recent sedimentary cover. The sizes of the two sediment failures are approximately 200 000 and 50 000 m 2 . The geotechnical study has shown that (1) the coastal sediments are stable under gravitational stresses and cyclic loading stresses induced by the 6.2-R earthquake and (2) the sediment failures were initiated due to liquefaction of subsurface layers. The failed sediments very quickly disintegrated and transformed to mass flows. The deformation of the failed sediment was caused by loss of sediment strength due to development of high pore water pressure induced by the earthquake. The study suggests that the alluvial fan deposits along the coastal zone of the western Corinth Gulf are susceptible to liquefaction by any earthquake event with a magnitude greater than 6 R, depending upon the proximity of the site to the earthquake epicenter. Earthquakes of magnitude 6 R are expected to occur every 22.7 years and therefore the recurrence interval for sediment failure events is about the same.