Quaternary volcanic eruptions of the Santorini Island and associated gravity flow deposits in the Cretan Basin, South Aegean SeaEfthymios K. Tripsanas, Aristomenis P. Karageorgis, Christos Anagnostou, Vasilis Lykousis, Dimitris Sakelariou A large number of sediment cores acquired from the central Cretan Basin are characterized by the presence of three sand beds that consist of volcanic glass. The geochemistry of these sand beds indicates that they represent the Z-2, Y-2, and Y4 tephra layers, which are associated with three volcanic eruptions of Santorini Island at 3.3, 20, and 30 cal ka, respectively. On topographic highs, these sand beds are less than 0.1 m thick and bioturbated throughout their thickness, indicating fall out from volcanic ash clouds, and placid settling through the water column. On basin floor, the sand beds are much thicker (up to 0.3 m), display laminations and ripples, and their bases are sharp and erosionnal. This indicates that their deposition has been caused by turbidity currents initiated on the Cyclades Plateau or upper slope. The Y4 volcanic-ash turbidite is differentiated from the other ash layers by the existence of a succession of thick (> 0.5 m tick) homogenous mud zones (homogenites), which are based by laminated to rippled laminated sand layers. The chemical composition of the sand material from the homogenites is almost identical to that of the Y-4 ash layer, suggesting a strong connection with the 30 cal ka volcanic eruption. However, the Z-2 and Y-2 volcanic-ash turbidites are not accompanied by thick homogenous mud zones, suggesting that the flows that resulted in their formation were of lower energy, and thus of different origin. It is proposed that the homogenites originate from pyroclastic flows during the Y-4 eruption that were strong enough to travel through the Cyclades Plateau into the Cretan Basin. Such a catastrophic event would have been able to incorporate into its flowing body significant amounts of seafloor muddy sediments through basal erosion that would be able to justify the thick mud homogenite deposits. Sediment failures on the slopes triggered by the tremors of the volcanic eruption involve fine-grained sediment, and are unlikely to have the time or the travel distance to fully transform into turbidity currents. On the other hand, the younger volcanic-ash turbidites probably originate from postgenerated sediment instabilities in the thick, sandy ash layers on the Cyclades Plateau and upper northern Cretan slope. The triggering mechanism for these failures was probably ground shaking by the common earthquakes that occur in this area. |
