LOWER CAMPANIAN MARGINAL MARINE DEPOSITS (GOSAU GROUP, AUSTRIA): SEDIMENTATION AND COAL DEPOSITION AT AN ACTIVE CONTINENTAL MARGIN

The Gosau Group of the Northern Calcareous Alps records terrestrial to deep marine sedimentation on top of the Eoalpine orogenic wedge of the Alps. Due to later phases of the Alpine orogeny the present day distribution of Gosau Group sediments are frequently confined to narrow synclines at the boundaries of tectonic nappes. In this study we investigate the Gosau sediments of Grünbach at the eastern boundary of the Northern Calcareous Alps at the transition to the southern part of the Vienna Basin. Lithologically, the syncline comprises terrestrial conglomerates, sandstone and shallow marine limestone at the base, followed by coastal siliciclastic sediments with up to 8 major coal horizons, succeeded by deeper water sandstone, shale, and marl. The total thickness reaches around 1200 m with a stratigraphic range from Upper Cretaceous (Late Santonian) to Paleogene (Paleocene). The general structure of the Grünbach Gosau is a non-cylindrical, tight, inclined, parallel fold. In its western part the syncline strikes more or less W-E with axial surfaces dipping around 60° towards North, while in the Eastern part the syncline trends SW-NE with axial surfaces dipping towards southeast around 60°. Temperature conditions during deformation can be estimated from vitrinite reflectance values of 0.56-0.61 %. Due to the generally poor outcrop conditions of Gosau sediments, a trench has been excavated to study the lithostratigraphy and depositional environment of coal bearing intervals of the Grünbach Formation (Lower Campanian) and to obtain fresh rock samples for correlation with other Gosau sediments from the Eastern Alps, hydrocarbon wells in the Vienna Basin, and Carpathians, based on lithological, geochemical, heavy mineral and faunal analysis. The trench was 140 m long and exposed almost 45 m of Lower Campanian stratigraphy dominated by sandstone/clay/marl alternations with eight coal horizons ranging in thickness from 1 to 25 cm. The documented deposits allow insights into the relationship between Early Campanian marginal marine coal deposition and clastic sedimentation.