Stratigraphy and sedimentation in the western Strabo Trench, Eastern Mediterranean

Abstract

Three piston cores ranging in length from 501 to 921 cm, raised from the adjacent Mediterranean Tyro and Kretheus basins and their intervening sill, confirmed previous findings of persistent anoxia and extended our knowledge of the stratigraphic record. The two cores from the basinal settings contain a Bronze-age homogenite. This tsunami-induced turbidite is only 28 cm thick in the shallower Kretheus Basin whereas it has a thickness of 235 cm in the deeper Tyro Basin. The recovery of the homogenite in these subbasins of the western Strabo Trench demonstrates that the tsunami wave turned west of Crete and arrived here with a velocity sufficient to remobilize unconsolidated sediments draping the basin slopes. The Holocene and uppermost Pleistocene sedimentary record is characterized by numerous millimetric to centimetric sand layers containing reworked Pliocene and Miocene foraminifers. The repetitive, siliciclastic turbidites interfered with the normal Eastern Mediterranean succession of sapropels and tephras and document a strong neotectonic activity, related to the presence of an active trench. On the floor of ponded, rimmed basins like the Tyro and Kretheus basins the sand layers are associated with anoxic sediments, whereas they are associated with normal, well-oxygenated marls on the sill which lies above the anoxic/oxic interface. Basin anoxia is interpreted as the result of submarine dissolution of Messinian evaporites, giving rise to deep-seated brines. Extraformational reworking documents the occurrence of pre-Messinian fossiliferous sediments exposed on the trench wall and subject to submarine erosion. The stratigraphic time framework is given by nannofossil biostratigraphy, corroborated by lithostratigraphic correlation of isochronous lithologies (sapropel and tephra-chronology). A drastic change in sedimentary regime is recorded at approximately 70 ka B.P. ( Emiliania huxleyi Acme- E. huxleyi zonal boundary). Older sediments which were only recovered in Core 37 from the sill separating the Tyro and Kretheus basins are pelagic, with an almost typical Eastern Mediterranean succession of sapropels and tephras. No siliciclastic turbidites and no extraformational reworking are noticed in sediments older than 70,000 years. We conclude that the trench system became active just at that time. A thick layer of plurimillimetric pumice bubbles was discovered within sapropel S-6, indicating a strong explosion of a nearby volcano at 180 ka B.P., but we have no analytical data yet to substantiate the provenance of the volcanogenic deposits.