Paleoenvironmental control on the morphology of Nummulites fabianii (Prever) in the late Priabonian parasequences of the Mortisa sandstone (Venetian Alps, Northern Italy)

The distribution of Nummulites fabianii fabianii and Nummulites fabianii retiatus in the Mortisa sandstone is discussed. The Mortisa sandstone is a terrigenous unit of late Priabonian age occurring south of the Asiago Plateau (Venetian Alps, northern Italy). The facies analysis allowed to recognize thickening-coarsening upward cycles interpreted as shallowing upward parasequences corresponding to depositional regressions. In each cycle five different facies were recognized. The first and the last facies of every cycle contain larger foraminiferal assemblages with numerous specimens of Nummulites fabianii. Two different morphotypes of this species were attributed to the "subspecies" N. fabianii fabianii (Prever) and N. fabianii retiatus Roveda. Their distribution was influenced by the paleoenvironmental conditions. The flat "subspecies" N. fabianii retiatus occurs in the base-cycle marly facies, deposited under low-energy, low-light conditions. The more inflated N. fabianii fabianii characterises the shallower top-cycle limestone, deposited under high-energy, high-light conditions.Our preliminary observations suggest there is no obvious link between the morphology and the stratigraphic position, so the biostratigraphic significance of the two "subspecies" (currently utilized to divide the Priabonian in a lower and an upper part) remains uncertain.