Paleoenvironmental variations in the coal-bearing deposits of the Lower Gosau Subgroup (Turonian to Campanian) in the Northern Calcareous Alps, Austria.

The Gosau Group is a terrestrial to deep-water succession formed on the northern margin of the Austroalpine microplate during the early stages of Alpine orogenesis. The sedimentary characteristics of the Gosau Group reflect the paleoenvironment of the Northern Calcareous Alps from the Late Cretaceous to the Eocene. It has been divided into the Upper Gosau Subgroup and the Lower Gosau Subgroup based on the sedimentary facies. The Lower Gosau Subgroup indicates a transitional environment from terrestrial to shallow water. It has been subdivided into the Lower Gosau Subgroup, composed of fluvial, lacustrine to shallow marine sediments during the Turonian to Santonian-Campanian, and the Upper Gosau Subgroup, characterized by deep-water succession up to Eocene in age. The shallow to deep environmental transition was derived from the subsidence of the continental margin, associated with the subduction of the South Penninic Ocean. However, details of the Late Cretaceous environmental variations have not been understood enough for reconstructing the formation of the Gosau Group. This study investigates environmental signals by analyzing coal-bearing deposits in the Lower Gosau Subgroup. We selected seven outcrop sites including coal seams ranging from the Turonian to Campanian in Austria, which showed fluvial and/or lacustrine depositional settings, and collected fourteen coal samples. Based on the industrial analysis and vitrinite reflectance, the samples were evaluated as low to medium ranks. Our results indicate a wide range of variations in silicon oxides, organic matters, and clay contents that are related to changes in depositional environment and water energy. These correspond to occasional occurrences of pyrite and carbonates, which could have been caused by interaction with seawater. Changes in the marcel contents also could be influenced by varying water level. Our study suggests that the coal-bearing sediments of the Lower Gosau Subgroup in Austria were deposited in unsteady environments and occasionally affected by seawater, which might correspond to the short-term sea-level fluctuations.