Carbonate production by coralline algae and bryozoans in the Early Pleistocene of Castelluccio (Eastern Sicily)

Coralline algae and bryozoans can form dense populations in shelf environments. Possessing mineralised skeletons with relatively stable mineralogies, their remains usually contribute significantly to sediment production. The resulting rhodalgal and bryomol facies, nearly unanimously considered typical of cool water settings, are widespread in Neogene sediments from the Mediterranean area.Rhodolith- and bryozoan-rich layers commonly crop out along the margins of the Hyblean Plateau and a spectacular exposure occurs on the NE margin, along present-day Ionian coastline, near Castelluccio, North of Augusta (Syracuse, Sicily). These carbonate sediments have been dated to the Early Pleistocene.At Castelluccio, rhodolith- and bryozoan-rich layers lay on alcalibasalts and volcanoclastics. The base is marked by a discontinuous gravelly layer deposited on un uneven, partly erosive surface.Rhodolith-rich layers, up to 1 m thick, are rudstones to grainstones, have basal sharp flat to undulating surfaces. Rhodoliths, spheroidal to ellipsoidal in shape and smoothto- lumpy surfaced, range from few to 12 cm in diameter. They are mono- to oligo-specific, and dominated by Lithothamnion spp., associated with rare Mesophyllum, Lithophyllum and Peyssonnelia. Bryozoans and serpulids are minor contributor to rhodolith formation, and rare fragments occur in the sediment.Bryozoan-rich layers, 30-50 decimetres thick, are packstone, with sharp-to-transitional basal surfaces. Their gravelly fraction almost entirely consists of erect delicate branching bryozoans, mostly oligo-to-monospecific (Smittina cervicornis and Hornera frondiculata). Individual layers show size selection of bryozoan branches thickness that are lightly fragmented and unworn, although mostly sub-horizontally placed.Some decimetre-up to more than 1 metre-thick, intensely bioturbated wackestone layers are interposed in between, with few skeletal remains, except for sparse nearly entire several centimetre sized bryozoan colonies. Few pectinid lags, up to some cm thick, and alignments of volcanic pebbles, are present at different heights along the section.The rhodalgal and bryomol facies presumably formed in a carbonate ramp, at or near to the middle-outer ramp transition. A certain rhythmicity in the succession, points to repeated slight changes in palaeoenvironmental conditions (water temperature/salinity, light penetration, hydrodynamic energy, nutrient/food availability, sediment input and sedimentation rate) resulting from the interplay of local uplift and relative sea-level variations linked to slight climatic fluctuations. Bryomol facies resulted from the in situ dismantling of erect branching bryozoan carpets possibly flourishing during deepening phases, while rhodagal largely consists of the rhodolith beds formed during shallowing phases, whose sharp basal contacts could indicate a partial sediment erosion at their inception, possibly within the storm weather wave base.