BASIN FORMATION AND COAL DEPOSITION

Within the proposed project, we plan to study basin formation, coal deposition and their relation in Miocene basins along the Noric Depression (Eastern Alps). We selected the Fohnsdorf, Leoben and Trofaich basins for our investigations. The basins formed during the final stages of the Alpine orogeny along the northern margin of an eastward extruding crustal wedge. The Fohnsdorf and Leoben basins host thick coal seams. No economic coal formed in the Trofaiach Basin. Detailed coal geological investigations will help to evaluate geometry, ash yield, sulphur content, and petrography of coal seams, and to determine their depositional environments. Geophysical (high-resolution reflection seismics, gravity), tectonical and sedimentological investigations will clarify the structure, architecture and basin formation mechansims. Main aims of the proposed project are: > Determination of the factors which control basin formation, depositional environments and coal properties in the Fohnsdorf and Leoben basins. Determination of the factors which prevented economic coal deposition in the Trofaiach Basin. > To contribute to a better understanding of the structural evolution of the extruding crustal block in the Eastern Alps. We will compare the results of the proposed study with results from former investigations on Neogene coal deposits, which formed in different geotectonic settings in the Eastern Alpine realm. This comparison will contribute to > A better understanding of the causal links between basin formation, depositional environments and coal properties, especially in basins forming during the late stage of continental collision.

"Pull-apart" basins are an important type of sedimentary basin. Nevertheless, relatively few data on coal and hydrocarbon source rocks in this basin type were were avaialble so far. Within the frame of the present project, the relation between processes resulting in the formation of pull-apart basins, and the deposition of coal seams and source rocks was studied using the example of Neogene (17 - 14 million years) pull-apart basins within the Eastern Alps. In a first step, the geological history of the Fohnsdorf, Leoben, Aflenz and Trofaiach basins has been studied using borehole data, geophysical, geochemical, and geological investigations. Sedimentation in the first three basins commenced with the deposition of fluvio-deltaic rocks. Thereafter, increased subsidence rates caused the formation of mires and the accumulation of coal seams, up to 16 m thick. Finally, the mires drowned and lakes several hundred meters deep were formed. At least the Fohnsdorf Basin was influenced by marine waters entering the central Alps through the Lavant Valley. This shows that parts of the Eastern Alps were located near sea level only 15 million years ago. Subsidence rates within the Trofaiach Basin were probably lower. Therefore, neither a thick coal seam, nor a deep lake was formed. Despite similar rank (sub-bituminous coal), the properties of the coals in the Fohnsdorf and Leoben basins differ significantly. Coal petrological and geochemical investigations show that the clean Leoben coal was formed in a raised mire. Small lakes were present within the raised mire during late stages of peat accumulation. Volcanic ashes caused high silica contents in the lake water and stimulated the growth of siliceous sponges. The depositional environment of the Leoben seam was similar to that of modern raised mires in Indonesia. In contrast, the high-ash, high-sulphur Fohnsdorf coal accumulated in a low-lying mire close to a brackish lake. The basins along the Noric Depression provide general models for the distribution of source rocks in pull-apart basins, which were formed in humid climates. The coal seams represent a potential source for gas. Sapropelic rocks overlying the seam, and sediments which accumulated during deep lake stages of basin evolution are very good sources for oil. The source potential of late stage, shallow lacustrine sediments is reduced because of dilution of the organic matter by detrital minerals.