Sandstone Modellation Across Climatic Zones and Lithofacies: The Concept of Porokarst

Radek Mikuláš

Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 135, 165 02 Praha 6, Czech Republic

Landscapes showing characteristic geomorphology and hydrography bound to porous massive sandstones are usually denoted by the term of “sandstone pseudokarst”. This term, however, is not apposite for processes and forms subjected to a non-live component of the sandstone phenomenon. The first, though the least important reason is a degrading prefix pseudo- which indicates a “pinchbeck” of another phenomenon (in this case, karst). However, sandstone meso- and microrelief show peculiar forms, which can be aesthetically fully compared with karst. Major reason lies in the factual inaccuracy of account of relevant processes. The term of karst is an aggregate for territories build of carbonates and other dissolvable rocks and adjacent typical underground and surface forms and characteristic hydrography. A subordinate term parakarst is used for features in non-carbonate rocks formed by solution; it is subdivided, among others, to bradykarst (quartzites and other silicious rocks) and tachykarst (evaporites). The term pseudokarst is reserved for features that originated at the same time as the host rock (e.g., cavities left by gases in lavas), and again for forms made by tectonic effects, mechanic disintegration and/or water, ice, and wind erosion. Versatility of forms of sandstone phenomenon is, however, caused mostly by numerous combinations and successions of the two following phenomena: 1, salt erosion made by salt solutions within the rock massive; 2, precipitation in the vicinity of the rock surface, which tends to the case hardening (i.e., origin of a rock crust of various thickness, shape and durability). The mass of the secondary cement comes from within the rock massive, but in contrast to karst, it precipitates only inside the substrate. Neither the first process, nor the second one fall to the definition of karst, parakarst or pseudokarst; they thereby require an additional term. Because the porosity of the substrate (specific in size of capillary space) is one of the key factors of hydrography of the sandstone phenomenon, the term of porokarst is designed herein. The porokarst can be defined by the presence of the two above-mentioned processes. Likewise karst and other related phenomena, also demarcation of porokarst is fuzzy. Across the climatic zones, poor conditions for the origin of porokarst are in arctic zone (e.g., East Greenland) because of weak vaporization and strong mechanic (i.e. pseudokarst) effects. Moderately humid temperate zone (e.g., Czech Republic and Germany) is the optimum. Towards subtropical and tropic arid areas, aeolian erosion dominates (e.g., Namibia Desert) but porokarst effects will probably be also locally important. Conditions of humid tropical zone bring to sandstone areas significant solution and therefore a dominance of parakarst processes (Tepuys, Venezuela). Across the lithofacies, medium-grained quartzose sandstones represent an optimum for porokarst. Effects of both main porokarst principles decline with decreasing or increasing of grain size (e.g., salt erosion and case hardening are not of use in conglomerates) as well as with increasing of amount of carbonate or clay cement. 

The study is a part of the Research Programme of the Institute of Geology, AS CR, Praha (No. CEZ: 23-013-912).