Sedimentology of the Minoan deep-sea tephra layer in the Aegean and Eastern Mediterranean

Abstract

The Minoan eruption of Santorini resulted in deposition of pyroclastic material over a large area of the Aegean Sea and Eastern Mediterranean. The eruptive activity commenced with a plinian phase, which was followed by a phreatomagmatic phase, and ended with numerous pyroclastic flows. We present a sedimentological study of the Minoan ash deposits found in cores taken throughout the region. The cores are divided into three groups: (1) those from the sub-marine slopes immediately surrounding Santorini; (2) those from the Aegean Sea where the topography consists of many steep-sided basins; and (3) those from the Eastern Mediterranean where the topography is relatively flat and smooth. This last group is designated the Distal Ash Zone. The deposits from the Aegean Sea have been remobilised as turbidity currents. Grain-size distribution analysis reveals that these deposits are bimodal, the Md ø of the coarse population increasing away from source, whilst that of the fine population remains uniform irrespective of distance from source. The bimodality is caused by a combination of aggregation of fine particles in ash clouds produced during the last two phases of the eruptive activity and mixing of the coarse plinian and fine phreatomagmatic and co-ignimbrite deposits within the turbidity current. An unusual feature found in two of the cores is a coarse, density sorted layer beneath the Bouma “a” division of several turbidite units. This is interpreted as an example of a flow head deposit as described by Sparks and Wilson (1983). The deposits from the Distal Ash Fall Zone are fine grained and unimodal, containing phreatomagmatic and co-ignimbrite material. They show only very minor remobilisation of the original air-fall ash, due to the flat topography of the area. The Md ø of the deposit remains uniform irrespective of distance from source and is very similar to that of the fine population found in the bimodal Aegean deposits. This is interpreted as the result of aggregation of fines in the phreatomagmatic and co-ignimbrite ash clouds. The deposits in both areas have been extensively bioturbated. A study of dispersed ash in the cores to reconstruct the original thickness of the ash layers suggests that ash layers less than 0.62 cm thick will be completely dispersed in the surrounding sediment, whilst those thicker than 2.5 cm will be preserved as discrete layers, the base being unaffected by bioturbation. The deposits on the submarine slopes of Santorini were not originally deposited as air-fall ash, but are derived from the erosion of coastal exposures of non-welded ignimbrite. They form storm-sand layers and are typical of the type of deposit that may be formed by rapid erosion of fresh pyroclastic material after the eruption of an island-arc volcano.