Isovolumetric replacement and aeolian deposition contributed to Terrae calcis genesis in Franconia (central Germany)

Abstract. We investigated Terrae calcis on limestone and dolomite in Franconia, as well as the red fill of deep cracks in the rock (Karstschlotten). SEM images of the rock-soil transition zones supported by EDS found amorphous clays along fissures that could be products of metasomatic, authigenic clay neoformation within microfossils, calcite, and dolomite grains, or of replacing deposition of amorphous clays inside the calcite, probably due to percolating waters (illuviation). In the SEM-images, the replacement appears as exchange process characterized by substitution of Ca and Mg against Si, Al, and Fe. There is no crystalline clay deposited within rock fissures, and the transition between calcareous minerals and amorphous clay is gradual. This and the presence of Fe let it seem possible that plant roots play a major role for the transport of elements and neoformation of clays, similar to clay pavements along eucalyptus roots in Western Australia. In this context, more or less uniform Fe(d/t) ratios contradicting other weathering indicators could be the result of neoformed phyllosilicates containing Fe³⁺. Bulk soil and bedrock analyses indicate that the solum of the investigated Terrae calcis does mainly not represent insoluble bedrock residue. Dust deposition and bioturbation are evident due to sand grains coming from a loess surface cover, which buried pre-existing Terrae calcis and contributed to their substrate, apparently supplying quartz and clay-rich pseudosand aggregates.