THE MOLLUSCAN ASSEMBLAGES IN THE FLUVIO-LACUSTRINE SUCCESSION OF THE PLIO-PLEISTOCENE MUGELLO BASIN (TUSCANY, ITALY)

Abstract

New geologic studies on the fluvio-lacustrine Mugello Basin (Florence, Italy) stimulated a revision of the continental molluscan assemblages known since the last century. The fluvio-lacustrine succession has been subdivided in four synthems composed of fluvio-lacustrine (Torrente Ensa synthem, STE) and alluvial deposits (Scarperia, Luco di Mugello and Sagginale synthems). Two progressive angular unconformities in the STE allowed to distinguish three depositional sequences (S1, S2 and S3) composed of fan-delta gravels and sands, lacustrine silty clays (S1 and S2) and alluvial-fan gravels and sands (S3). Molluscs have been collected in various localities where S1 and S2 fan-delta and lacustrine deposits are exposed. The paleoecologic analysis of the molluscan assemblages is in accordance with the fluvio-lacustrine environment inferred from facies analysis. Different types of humid habitats, ranging from swamps, ponds, to channel-related environments (banks, leeves etc.), and open woody habitats have been recognized. The presence of Villafranchian extinct taxa such as Prososthenia oblonga, Emmericia cf. umbra and Tournouerina belnensis is in general agreement with the vertebrate fauna collected in the fluvio-lacustrine deposits since the last century and referred to the Tasso and Farneta faunal units (Late Villafranchian). A detailed integrated analysis of a 15 m thick gravelly-silty facies section of the S2 sequence reveals alternating depositional conditions in the subaerial portion of the fan deltas. Following a relative rise of base-level (i.e the lake level) flood-channels were disactivated with the formation of a floodplain-like environment dominated by fine-grained deposition, where localized poorly-drained areas created favourable habitats for molluscan taxa loving humid conditions. The sourrounding zones were characterized by open forests inhabited by terrestrial taxa. Low-magnitude, overland flows mixed the molluscan faunas of the different biotopes. The cyclic arrangement of gravels and silty clays reflects high-frequency uplift/denudational cycles during which biotopes for the molluscan fauna were alternatively activated.

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