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 example for mixed siliciclastic-carbonate sequence development in a convergent plate tectonic setting along the leading margin of the Austroalpine microplate. Two groups of parasequences were distinguished, (a) siliciclastic-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 carbonate parasequences were deposited in shelf subtidal to lagoonal supratidal 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 outer shelf to marsh. In the Upper Turonian-Lower Campanian at Gosau, four sequences were recognized. In Brandenberg, three sequences were distinguished in the ?Middle Turonian to Coniacian succession. The sequences are between 50 metres to more than 400 metres in thickness, and embrace 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 deposits. These are overlain, in the TST, by siliciclastic-dominated parasequences that record a deepening from fluvial/paralic to shelf environments up-section. The HST typically consists of siliciclastic-dominated parasequences. Up-section, the HST shoals from outer to mid-shelf depths to inner shelf and, locally, to lagoonal-marsh environments. Alternatively, 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 Campanian succession does not fit the second-order sea-level curve of Haq et al. [1988]. With the possible exception of two sequences, the third-order sequences recognized in Brandenberg and at Gosau seem not to correlate in time. These discrepancies suggest that the dynamics of the accretionary wedge strongly influenced sequence development. Siliciclastic input was generally high and determined the location and amount of carbonates deposited. Both in the TST and HST of the sequences, at several levels fossils from the temperate palaeobiogeographic realm are present together with fossils from the Tethyan realm. In addition, the rudist fauna is markedly impoverished. This suggests an influence of environmental changes that may be related to the position of the Gosau depocenters along the northern limit of the Tethyan palaeobiogeographic realm.