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

Abstract

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 (ignimbrite El) that comprises a basal, coarsegrained, lithic breccia and overlying pumiceous part, above a planar, strongly erosional lower contact. However, along the northern coast of central Kos, “normal” ignimbrite El overlies a hummocky, 6-m-thick layer of chaotic breccia comprising mingled-to-pervasively mixed ignimbrite El and unconsolidated sediment. The surface morphology of the chaotic breccia and its internal texture resemble those of a debris-avalanche deposit, but the breccia is neither proximal nor downcurrent of steep topography. The lower part of the chaotic breccia comprises distinct domains of unconsolidated sediment or lower KPT units that are deformed and/or mingled with pumiceous ignimbrite. The upper part is dominated by a matrix of mingled-to-pervasively mixed ignimbrite and sediment that contains sediment domains as large as 2–10 m in diameter. Such large intact allochthonous domains are best preserved at the top of the chaotic breccia and form the hummocks. The chaotic breccia formed synchronously with the passage of a highly energetic pyroclastic flow where it traversed wet, unconsolidated sediment. Shear-induced liquification, together with possible ground shaking associated with the eruption, probably caused failure. Part of the unconsolidated substrate and basal part of ignimbrite El were dislodged and resedimented a short distance (tens to hundreds of metres)downcurrent. The lower part records deformation and disintegration of the substrate induced by the overriding, shearing flow. Mingling and deformation of the poorly consolidated material occurred as a result of within-flow lateral shear. Attenuated 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 others remained intact. Intact large unconsolidated domains at the top were transported mostly above the zone of shearing.