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Evidence for abrupt environmental change from coring in the Eliki hanging wall and coastal plain

Shallow (<10m) coring campaigns in two locations attempted to establish a depositional record of fault activity along the western Eliki Fault based on detecting abrupt environmental changes in the adjacent coastal plain. One location was a former lake located in the upper part of the plain in immediate hanging wall of the fault. Here a series of boreholes revealed surficial silts and clays typical of a low-energy depositional setting.

However, subsequent laboratory analysis of samples found that oxidation of the sediments had ensured there was no preservation of sensitive palaeo-ecological indicators (pollen, ostracods etc) of the depositional environment. Moreover, no material suitable for radiometric dating was obtained. This environment, probably a seasonal playa, was considered unlikely to preserve a useful sedimentological record of tectonic movements.

A second and more favourable depositional environment was found in the lower part of the Eliki deltaic plain. According to historical accounts of the 373BC and 1861 earthquakes, surface faulting on the Eliki Fault is accompanied by the metre-scale tectonic subsidence and marine inundation of the coastal plain. An integrated programme of GPR surveys and shallow boreholes, accurately levelled in with respect to sea level, were carried out. Stratigraphic descriptions were made in the field and basic geo-technical tests were made on the intact core material (e.g. penetrometer measurements of bearing strength; shear vane measurements of shear strength).

Subject to the limitations for core sample collection imposed by the archaeological permit under which the group was working, core material was obtained for laboratory analyses (particle size, trace element) and, where appropriate, radiometric dating (137Cs, 210Pb, 14C). GPR surveys in the coring sites proved to be unsuccessful in imaging the near-surface stratigraphy; in the heavily farmed fields of the plain the GPR profiles were found to be excessively degraded by offline reflectors (roots of olive trees, metal poles, irrigation channels, telegraph poles).

The results from the sediment cores show a complex stratigraphy typical of late Holocene deltaic plains but do exhibit evidence of abrupt environmental changes. In places, there is stratigraphic evidence of a transition from basal lagoonal muds into a wetland peaty soil followed by an abrupt and erosional return to a lagoonal depositional environment. Although consistent with a sudden deepening of the coastal plain, the date of the transition is awaiting radiometric dating. Along a separate core transect, two abrupt environmental changes are recognised, one at ~0m and one at ~-2m.

Sedimentological, geochemical and microfossil analysis indicates that at ~2 m below sea level a basal high-energy shoreline deposit abruptly changes into a low-energy lagoonal deposit. This change postdates a terrestrial shell age of 5th-6th centuries AD and may correspond to sudden subsidence event in Roman or Byzantine times, consistent with published coastal studies, archaeological evidence and palaeoseismic investigations.

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