Syn-eruptive chaotic breccia on Kos, Greece, associated with an energetic pyroclastic flow

The climax of the Kos Plateau Tuff (KPT) eruption (eastern Aegean, Greece) generated a highly energetic, coarse-grained, lithic-rich, pyroclastic flow . In most places on Kos, the deposit from this event is an ignimbrite (igni mbrite El) that comprises a basal, coarse-grained, lithic breccia and overl ying pumiceous part, above a planar, strongly erosional lower contact. Howe ver, along the northern coast of central Kos, "normal" ignimbrite El overli es a hummocky, 6-m-thick layer of chaotic breccia comprising mingled-to-per vasively mixed ignimbrite El and unconsolidated sediment. The surface morph ology of the chaotic breccia and its internal texture resemble those of a d ebris-avalanche deposit, but the breccia is neither proximal nor downcurren t of steep topography. The lower part of the chaotic breccia comprises dist inct domains of unconsolidated sediment or lower KPT units that are deforme d and/or mingled with pumiceous ignimbrite. The upper part is dominated by a matrix of mingled-to-pervasively mixed ignimbrite and sediment that conta ins sediment domains as large as 2-10 m in diameter. Such large intact allo chthonous domains are best preserved at the top of the chaotic breccia and form the hummocks. The chaotic breccia formed synchronously with the passag e of a highly energetic pyroclastic flow where it traversed wet, unconsolid ated sediment. Shear-induced liquification, together with possible ground s haking associated with the eruption, probably caused failure. Part of the u nconsolidated substrate and basal part of ignimbrite El were dislodged and re-sedimented a short distance (tens to hundreds of metres) downcurrent. Th e lower part records deformation and disintegration of the substrate induce d by the overriding, shearing flow. Mingling and deformation of the poorly consolidated material occurred as a result of within-flow lateral shear. At tenuated worm burrows within the sediment domains, and pinch-and-swell and flame structures within the mingled domains, preserve evidence of shear in the lower part. The upper part was transported downcurrent above a zone of shear failure. Internal heterogeneities in physical properties resulted in variable strain rates causing some domains to be pervasively mixed while ot hers remained intact. Intact large unconsolidated domains at the top were t ransported mostly above the zone of shearing.