SEQUENCE DEVELOPMENT AND BIOTIC ASSEMBLAGES ON AN ACTIVE CONTINENTAL-MARGIN - THE TURONIAN-CAMPANIAN OF THE NORTHERN CALCAREOUS ALPS, AUSTRIA

The ?Middle Turonian to Lower Campanian Lower Gosau Subgroup at Gosau and Brandenberg (northern Calcareous Alps, Austria) provides an exampl e for mixed siliciclastic-carbonate sequence development in a converge nt plate tectonic setting along the leading margin of the Austroalpine microplate. Two groups of parasequences were distinguished, (a) silic iclastic-dominated parasequences, and (b) carbonate parasequences. The siliciclastic-dominated parasequences were deposited in shelf, beach to lagoon, and marsh environments, and contain coal seams. The carbona te parasequences were deposited in shelf subtidal to lagoonal supratid al environments, and contain different types of bioconstructions. Each group of parasequences contains several fossil assemblages that each characterizes a specific segment of the depositional surface from oute r shelf to marsh. In the Upper Turonian-Lower Campanian at Gosau, four sequences were recognized. In Brandenberg, three sequences were disti nguished in the ?Middle Turonian to Coniacian succession. The sequence s are between 50 metres to more than 400 metres in thickness, and embr ace intervals of time estimated between 1 m.y. to 3 m.y. (third-order cycles). The LST/TST of the sequences consists of local alluvial fan d eposits. These are overlain, in the TST, by siliciclastic-dominated pa rasequences that record a deepening from fluvial/paralic to shelf envi ronments up-section. The HST typically consists of siliciclastic-domin ated parasequences. Up-section, the HST shoals from outer to mid-shelf depths to inner shelf and, locally, to lagoonal-marsh environments. A lternatively, the HST consists of stacked carbonate parasequences that become both progressively thinner up-section and record a shallowing of facies. The stratigraphic development of the Turonian p.p.-Lower Ca mpanian succession does;not fit the second-order sea-level curve of Ha q et al. [1988]. With the possible exception of two sequences, the thi rd-order sequences recognized in Brandenberg and at Gosau seem not to correlate in time. These discrepancies suggest that the dynamics of th e accretionary wedge strongly influenced sequence development. Silicic lastic input was generally high of carbonates deposited. Both in the T ST and HST of the sequences, at several levels fossils from the temper ate palaeobiogeographic realm are present together with fossils from t he Tethyan realm. In addition, the rudist fauna is markedly impoverish ed. This suggests an influence of environmental changes that may be re lated to the position of the Gosau depocenters along the northern limi t of the Tethyan palaeobiogeographic realm.