DASYCLADALEANS AND DEPOSITIONAL-ENVIRONMENTS OF THE UPPER TRIASSIC-LIAASIC CARBONATE PLATFORM OF THE GRAN-SASSO (CENTRAL APENNINES, ITALY)

Sedimentological and paleontological studies carried out on the Upper Triassic Dolomia Principale and on the Lower Liassic Calcare Massiccio carbonate platform successions of the Gran Sasso area (Central Apenni ne, Italy) allowed to recognize several microfacies types arranged in three different types of high frequency cycles, subtidal, peritidal an d diagenetic. The cycles can be attributed to high frequency sea-level oscillations on a fifth and fourth order scale.Five algal assemblages have been identified: 1. Gyroporella vesiculifera, 2. Griphoporella c urvata, 3. Palaeodasycladus spp., 4. Palaeodasycladus and Thaumatopore lla, and 5. Tersella genoti and Fanesella sokaci assemblage. In this l atter assemblage two new species (Tersella genoti n.sp. and Fanesella sokaci n.sp.) are described, both from Liassic beds. Tersella genoti d iffers from the other Tersella species for a less 'differentiated' clu b shaped thallus, the shape of secondary branches and size. Fanesella sokaci differs from other species of the same genus in shape and size of the branches, size of the thallus and degree of calcification. A re -evaluation of the genus Tersella MORELLET considers Morellet's origin al diagnosis to be apter than the subsequent emendations. A discussion on several ''Terselliform'' taxa (Dissocladella crefica, Linoporella lucasi, Dissocladella iberica and Dissocladella ebroensis) is supplied . Some comments on the genus Fanesella CROS & LEMOINE and Palaeodasycl adus gracilis CROS & LEMOINE are given. Several facies units have been identified on the basis of frequency and distribution of algal assemb lages and type of cycle, each unit recording a different depositional paleoenvironment. In the Dolomia Principale Formation two main deposit ional environments have been recognized: -a shelf lagoon facies charac terized by subtidal and peritidal cycles dominated by the Gyroporella vesiculifera algal assemblage; - a backmargin facies marked by peritid al and diagenetic cycles in wihich the Griphoporella curvata algal ass emblage is widespread. In the Calcare Massiccio Formation the followin g depositional environments have been recognized: -a back-margin facie s characterized by diagenetic cycles with high energy subtidal units a nd the Palaeodasycladus spp. algal assemblage. The presence of the Pal aeodasycladus and Thaumatoporella algal assemblage indicates a local t ransition to a more protected open lagoon environment. -a margin facie s characterized by oolites and bioclastic sandy shoals, with minor con tributions of reef-building organisms, and by a very poor cyclothemic organization. This facies is marked by the widespread occurrence of th e Tersella genoti and Fanesella sokaci algal assemblage. The environme ntal distribution of subtidal, peritidal and diagenetic cycles within the facies units were probably controlled more by the relative positio n to the margin than by the amplitude of the high frequency sea level fluctuations. The distribution of facies units allowed the reconstruct ion of the paleoenvironmental evolution of the shallow water facies in this area during the Late Triassic-Early Liassic interval. In the Lat e Triassic a shallow water carbonate facies (from lagoonal to backmarg in) extended in the western and southern parts of the Gran Sasso area. In the Early Liassic the platform facies retrograded, and the margin moved back to the west and south of the Gran Sasso area. In the Middle Liassic, extensional tectonics fragmented the Calcare Massiccio platf orm. Large platform areas were drowned, and shallow water sedimentatio n retreated to the South of the Gran Sasso range.