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The record of tidal cycles in mixed silici–bioclastic deposits: examples from small Plio–Pleistocene peripheral basins of the microtidal Central Mediterranean Sea



The Pliocene–Pleistocene peripheral marine basins of the Mediterranean Sea in southern Italy, from Basilicata and western Calabria to northern and eastern Sicily, represent tectonically formed coastal embayments and narrow straits. Here, units of cross-stratified, mixed silici–bioclastic sand, 25 to 80 m thick, record strong tidal currents. The Central Mediterranean Sea has had a microtidal range of ca 35 cm, and the local amplification of the tidal wave is attributed to tides enhanced in some of the bays and to the out-of-phase reversal of the tidal prism in narrow straits linking the Tyrrhenian and Ionian basins. The siliciclastic sediment was generated by local bedrock erosion, whereas the bioclastic sediment was derived from the contemporaneous, foramol-type cool-water carbonate factories. The cross-strata sets represent small to medium-sized (10 to 60 cm thick) two-dimensional dunes with mainly unidirectional foreset dip directions. These tidalites differ from the classical tidal rhythmites deposited in mud-bearing siliciclastic environments. Firstly, the foreset strata lack mud drapes and, instead, show segregation of siliciclastic and bioclastic sand into alternating strata. Secondly, the thickness variation of the successive silici–bioclastic strata couplets, measured over accretion intervals of 2 to 3 m and analysed statistically, reveal only the shortest-term, diurnal and semi-diurnal tidal cycles. Thirdly, the record of diurnal and semi-diurnal tidal cycles is included within the pattern of neap-spring cycles. Differences between these sediments and classical tidal rhythmites are attributed to the specific palaeogeographic setting of a microtidal sea, with the tidal currents locally enhanced in peripheral basins. It is suggested that this particular facies of mud-free, silici–bioclastic arenite rhythmites in the stratigraphic record might indicate a specific type of depositional sub-tidal environment of straits and embayments and the shortest-term tidal cycles.