Another one bites the quartz? How silica dissolution contributes to the development of ruiniform relief in the Central European sandstone tablelands?

The research conducted in recent years has revealed that processes operating at depth play an important role in the morphogenesis of stepped tablelands in Central Europe (SW Poland, N Czechia), underlain by alternating layers of sandstone and fine-grained rocks of the Upper Cretaceous age. It has been demonstrated that underground erosion significantly contributes to the in situ disintegration of the marginal parts of sandstone plateaus and mesas, thereby producing distinctive ruiniform assemblages. They comprise chaotic boulder clusters replacing the cliff-lines, joint-aligned corridors, clefts opened due to the loss of mass from beneath, as well as plazas and courtyards with negligible surface runoff. While the widespread occurrence of sandy cones at the outlets of vertical fissures and allochthonous sandy aprons on the subjacent slopes testifies well to the efficiency of underground erosion, the mechanisms of detachment of sand grains remain unknown. This research aims to elucidate which processes are making the sandstone mechanically incoherent and susceptible for mechanical erosion at depth.

In older views weathering processes acting upon sandstone cliff-lines have either remained unspecified or a priori assumed to be mechanical breakdown. Yet, the presence of joint-aligned and highly disintegrated zones, often mimicking the grikes of the ‘classic’ karst terrains, as well as the widespread evidence of sandy detritus removal via subsurface drainage, allowed us to hypothesise that dissolution may be an important but neglected factor responsible for the loss of coherence of the sandstone rock mass.

Sandstone samples were collected from both disintegrated and non-disintegrated sandstone cliff-lines, as well as fresh debris originating from a recent rockfall event in one of the rock cities in Czechia. Loose grains of sand, already removed from the caprock and deposited at the foot of rock walls, were investigated too. Scanning electron microscopy equipped with energy dispersive spectrometer (SEM-EDX) was used to study micromorphological changes, diagnostic of intensive chemical weathering. This was supplemented by studies using the polarizing light microscopy, i.e. study of thin sections as well as x-ray diffraction (XRD) accompanied by thermal analysis (DSC-TG) to evaluate mineralogical composition of sandstone and to assess the secondary rock porosity.

The preliminary results revealed high degree of etching, mainly of syntaxial quartz overgrowths, with a number of v-shaped or irregularly shaped pits as well as large embayments. Interestingly, SEM-EDX analysis showed the presence of kaolinite, which might conform to the advanced chemical weathering of sandstone as well. It is supposed that solutional processes not only have prepared the rock for further erosional processes and the resultant development of ruiniform relief, but they also contribute to a variety of catastrophic mass movements due to the significant decrease of intact rock strength.

The results presented are the first outcomes of the new Q-MESA project (no. 2020/39/D/ST10/00861) funded by the National Science Centre, Poland.