PALEOENVIRONMENTAL CONTROL ON THE MORPHOLOGY OF NUMMULITES FABIANII (PREVER) IN THE LATE PRIABONIAN PARASEQUENCES OF THE MORTISA SANDSTONE (VENETIAN ALPS, NORTHERN ITALY)

Abstract

The distribution of Nurnmulites fabianii fabianii and Nummulites fabi.anii reti.dtas in the Mortisa sandstone is discussed. The Monisa 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 corre- sponding to depositional regressions. In each cycle five different fa- cies were recognized. The first and the last facies of every cycle con- tain 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. /a- bianii retiatus Roveda. Their distribution was influenced by the pa- leoenvironmental conditions. The flat subspecies N. fabianii retia- lrs occurs in the base-cycle marly facies, deposited under low-energy, lowlight conditions. The more inflated N. fabianii fabianii charac- terises the shallower top-cycle limestone, deposited under high-energn highlight conditions. Our observations suggest there is no obvious link between the morphology and the stratigraphic position, so the biostratigraphic si- gnificance of the two subspecies (currently used to divide the Pria- bonian in a lower and an upper part) remains uncertain.