The role of loamy sediment ( terra rossa) in the context of steady state karst surface lowering

Abstract

Reddish, loamy material ( terra rossa), found on many karstified surfaces, has long been accepted as a characteristic karst feature. Two basic views of terra rossa formation were distinguished, related to either a residual or a detrital origin. More recently it has been suggested that it could derive from isovolumetric reactions between the parent carbonate rock and airborne material. This paper reviews possible sources of terra rossa, explores its behaviour on the karst surface from the karst geomorphology viewpoint, and considers whether its existence is better explained in terms of a closed or open karst geomorphic system. Two approximately west–east traverses were laid out across Slovenia and the Czech Republic, comprising nine sample locations in each country previously known to be characterized by terra rossa. General geomorphic/speleomorphic conditions were estimated, and loamy material and parent rock samples were collected. Insoluble residues extracted from the rock were processed in the same ways as the loamy material. Basic geochemical and mineralogical investigations were run. The data obtained were processed statistically. Results show that statistical relationships between the adjacent rock insoluble residues and adjacent terra rossa bodies exist only at sampling sites recognised as “vertical”. Most such sites are cutters, which are “karren-like grooves formed beneath the soil, more commonly referred to as subsoil karren” (US Environmental Protection Agency, 2002. A Lexicon of Cave and Karst Terminology. Field, M.S. (Ed.). <http://www.karstwaters.org/files/glossary.pdf>, p.53). Possible sources of terra rossa material as well as the possibilities of material accumulating on the karst surface are discussed in detail, with special emphasis upon cutters. Theoretical considerations indicate that cutters are the only features that can collect sufficient insoluble residue to be detectable after a period of evolution within a surface that is lowering under steady state conditions. All other accumulations of similar appearance must be admixed materials of diverse origins or be completely allogenic in origin. The most recent approach, which considers terra rossa as an active karst surface agent, is temporarily put aside, though its explanatory potential in the future is acknowledged.