EUSTATIC AND SUBSIDENCE SIGNALS MODIFIED BY SEDIMENT FAILURE: SUBMERGED ANCIENT GREEK SITES AS MARKERS OFF EGYPT'S NILE DELTA

Interpreting the stratigraphy of geologically young sediment sequences on the inner continental shelf in a relatively stable region is not necessarily a straight-forward exercise. A case in point is the late Holocene section on the Nile delta coast of Egypt's Mediterranean margin. Study of sediment substrate beneath 8 ancient archaeological sites (Greek and other 1st millennium B.C., to Byzantine in the 5th century A.D.) distributed along 225 km of coast shows that some young sequences do not present a laterally continuous stratal configuration.

Progressive submergence of these ancient coastal sites and their substrates during the past ~2500 years would be expected as a result of concurrent sea-level rise and land subsidence by isostatic lowering (>3000 m of deltaic strata underlie this sector) and sediment compaction. Coastal submergence is a normal response in this type of large delta with rapid depositional accumulation rates (to >3mm/yr). However, the range of submergence at the archaeological sites is considerably more than expected: from 3 to 8 m in <2500 years. Moreover, cores and seismic profiles show that sequences in settings where sites are located have been disrupted and offset by syn- and post-depositional failure of water-saturated deposits. Triggers include the Nile floods, earthquakes, tsunamis and human-induced impacts such as loading by large structures.

These findings have implications for present world coastal settings, especially deltaic environments where weak unconsolidated sediments support population settlements. Submerged sites and substrates beneath the waves off Egypt may well have potential near-future analogues at Venice, New Orleans and in other vulnerable regions.