Rock fragments with dark coatings in slope deposits of the Famenne region, southern Belgium

Abstract

Quaternary slope deposits near the Lesse river in the Famenne region, south Belgium, contain scattered rounded rock fragments (5-15 cm diameter) that are typically characterized by a dark coating or patina. This dark surface resembles rock varnish occurrences. To establish the nature of the coatings, chemical, mineralogical and petrographical analyses were performed. The coatings are essentially Fe/Mn oxide features, developed as surface deposits on the rock fragments with associated impregnation of the rock substrate. The Fe/Mn compounds are amorphous or at least poorly crystalline. The coating also shows enrichment in clay minerals compared to the matrix of the covered rock fragments. Some of these characteristics are typical for rock varnish, but the coating does not show a layered structure. We hypothesize that the coating was formed by precipitation of Mn and Fe supplied by water, after burial of the rock fragments in the surrounding slope deposits, and was not affected by atmospheric deposition. Despite its strong macroscopic resemblance to rock varnish the patina on the Famenne rocks must most likely be classified as a heavy-metal skin.