THE MANCIANO SANDSTONE - A SHOREFACE DEPOSIT OF MIOCENE BASINS OF THENORTHERN APENNINES, ITALY

Well exposed, diamond-line cut, quarry-exposures of the Manciano Sands tone allow a detailed analysis of sandy, fossiliferous, nearshore depo sits of the shelf of the Northern Apennines. The Manciano Sandstone is characterized by medium to very coarse, washed, fairly well sorted, l ithic sandstone, with thin interlayers of sandy conglomerates. It disp lays two principal, rhythmically alternating sandy facies: (a) slightl y burrowed (mostly Macaronichnus, Ophiomorpha, Skolithos) units, troug h cross-bedded, locally showing possible tidal bundles with few whole Scutella (echinoid) shells reworked on foresets, or occasional large-s cale (approximately 2 m) planar cross-bedded, bar-accretion units; and (b) slightly finer, darker-coloured reddish-brown sandstone units, he avily bioturbated (Cruziana-Skolithos ichnofauna) representing slightl y more sheltered settings. Large oysters are present in near-living po sition in a few thin layers and, more commonly, as reworked, comminute d fragments in sandy layers. Many calcareous pebbles and oyster fragme nts are bored. Other fossils consist of echinoids (Scutella), some bal anids and reworked foraminifera and bryozoa. The Manciano sands were d eposited primarily in a wave-dominated shoreface, containing migrating bars/ridges and affected by wave-induced, possibly tidal-enhanced cur rents. This tidal influence confirms the opening of the Miocene Apenni nic Sea to oceans, both the developing Atlantic Ocean to the west and, through a long, narrow seaway, the Asian portion of the Tethys Sea to the east.