Eratosthenes Seamount: an oceanographic yardstick recording the Late Mesozoic-Tertiary geological history of the Eastern Mediterranean.
Robertson, Alastair H F
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Three boreholes were drilled on the northern slope of the Eratosthenes Seamount during Leg 160. Lithological data were obtained back to the Early Cretaceous. Lithological correlation of shallow-marine sedimentary rocks reveals similarities in the Lower Cretaceous lithological successions of the Eratosthenes Seamount with the southern Levant passive margin. Bathyal chalks of Coniacian-Maastrichtian to middle Eocene age recovered from the Eratosthenes Seamount at Site 967 are also comparable with similar sedimentary units in the Levant. Both the southern margin of the Levant basin and the Eratosthenes Seamount subsided to bathyal depths during Late Cretaceous-Paleogene time. These correlated successions differ from contemporaneous units in southern Cyprus that are dominated by the Upper Cretaceous Troodos ophiolite and its deep-sea sedimentary cover. The presence of shallow-marine limestones of Miocene age on the Eratosthenes Seamount indicates uplift before, or during, the Miocene, whereas the overlying Pliocene-Pleistocene successions comprise unlithified hemipelagic sediments that accumulated in deeper water following tectonic subsidence. Previously obtained geophysical data suggest that the Eratosthenes Seamount has variable crustal characteristics. Seismic refraction data reveal an intermediate crustal layer of seismic velocity of 6.1-6.3 km/s beneath the seamount, which also extends under Cyprus, but then wedges out southward under the Levant basin. A large magnetic anomaly that underlies the seamount and its perimeter was previously correlated with the Troodos ophiolite. However, seismic reflection profiles reveal the existence of an important northward-dipping thrust fault separating Cyprus from the Eratosthenes Seamount. Taken together, the geophysical and geological evidence indicate that the tectonic evolution of the Eratosthenes Seamount is linked to that of the North African continental margin from early Mesozoic time onward. The seamount was in a shallow-marine depositional setting in the Early Cretaceous, then subsided to bathyal depths in the Late Cretaceous. It was later uplifted, as indicated by the Miocene shallow-marine limestones recovered from Sites 965 and 966. Then, during Pliocene-Pleistocene time, the Eratosthenes Seamount was thrust beneath Cyprus because of collision of the African and the Eurasian plates.