Facies analysis and depositional mechanisms of hydroclastite breccias, Acicastello, eastern Sicily

Hydroclastite beds are interbedded with pillow lavas in an overturned secti on exposed near Acicastello, on the Ionian coast of Sicily. Two distinct hy droclastite facies are recognized in a continuous 15-m section in a cliff f ace overlying pillow lavas. The first, an hyaloclastite facies, comprises b lack hyaloclasts and pillow fragments, many of which retain a partial pillo w shape and glassy rim, in a matrix of smaller, partially devitrified and p alagonitized hyaloclasts. Beds of this facies an predominantly inversely gr aded, exhibit both matrix and clast support, and commonly contain outsize c lasts at or near the bed top. Beds of the hyaloclastite facies beds are int erpreted as the deposits of noncohesive submarine debris flows transporting fragments formed primarily by cooling-contraction granulation. The second, a pillow-fragment breccia facies, forms wedge-shaped beds that are interbe dded with the hyaloclastite facies. Pillow-fragment breccias comprise non-g raded breccias of basalt blocks that formed by spalling of joint blocks fro m pillow lavas. Beds of this facies are clast-supported and contain a matri x of marry sediment. This facies formed by accumulation of pillow fragments along the sides of pillow-lava ridges, creating wedges of talus which inte rfinger with the debris-flow deposits of the hyaloclastite facies. (C) 1999 Elsevier Science B.V. All rights reserved.