Kalkowskyʹs stromatolites revisited (Lower Triassic Buntsandstein, Harz Mountains, Germany)

The term ‘stromatolite’ was proposed by Kalkowsky (1908. Z. dt. geol. Ges. 60, 68–125) for structures he discovered in the carbonates of the Lower Triassic Buntsandstein of the Harz Mountains in central Germany. We have re-examined these localities to clarify Kalkowskyʹs descriptions and to deduce the environmental conditions of stromatolitic growth. The Lower Buntsandstein was deposited in a closed lacustrine basin, the Central European Permo-Triassic Basin. The depositional environment was a playa lake with variable salinity. In the centre of the basin oolite beds are intercalated in the mainly siliciclastic succession which is composed of >20 fining upward cycles thought to have been produced by climatic variations. Stromatolites occur almost exclusively on the surface of oolite beds. Stromatolitic structures consist of millimetre-thick laminated crusts and centimetre-high branching columnar forms. Smaller and bigger (>2 m high) domes are composed of secondary order columns and intercalated crusts. Occasionally, upper surfaces of the domes are pitted due to syndepositional dissolution. Despite of the overall intense sparitization, two groups of stromatolitic microstructures can be distinguished, spongy-fenestrate and fan-like fabrics. The spongy-fenestrate fabric is similar to that one caused by the recent cyanobacteria Phormidium, Calothrix and Dichothrix, and the fan-like fabric to Rivularia. Laminated crusts called stromatoid by Kalkowsky are interpreted as being of cement origin and formed by quick precipitation. The change from columnar to domical forms may reflect changes of hydrodynamic conditions. Clear photoautotrophism suggests that cyanobacteria or eukaryotes were the dominant organisms involved in the formation of Kalkowskyʹs stromatolites. Influx of ooids or siliciclastic mud interrupted or terminated the stromatolitic growth. The morphology, growth pattern and fabrics of stromatolites were most likely controlled by ecological factors which all seem to be related to the general hydro-climatic evolution of the Lower Buntsandstein.