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

Abstract

Hydroclastite beds are interbedded with pillow lavas in an overturned section exposed near Acicastello, on the Ionian coast of Sicily. Two distinct hydroclastite facies are recognized in a continuous 15-m section in a cliff face overlying pillow lavas. The first, an hyaloclastite facies, comprises black hyaloclasts and pillow fragments, many of which retain a partial pillow shape and glassy rim, in a matrix of smaller, partially devitrified and palagonitized hyaloclasts. Beds of this facies are predominantly inversely graded, exhibit both matrix and clast support, and commonly contain outsize clasts at or near the bed top. Beds of the hyaloclastite facies beds are interpreted 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 interbedded with the hyaloclastite facies. Pillow-fragment breccias comprise non-graded breccias of basalt blocks that formed by spalling of joint blocks from pillow lavas. Beds of this facies are clast-supported and contain a matrix of marly sediment. This facies formed by accumulation of pillow fragments along the sides of pillow-lava ridges, creating wedges of talus which interfinger with the debris-flow deposits of the hyaloclastite facies.