Naturalna degradacja wysadu solnego w Inowrocławiu

Katarzyna Poborska-Młynarska

Abstract


 Nowa mapa strukturalno-geologiczna zwierciadła solnego na wysadzie w Inowrocławiu umożliwia bliższe poznanie procesów podziemnego rozpuszczania skat czapy gipsowej i górnych części złoża solnego. Podziemne zjawiska krasowe są zasadniczą przyczyną tworzenia się na powierzchni terenu nad wysadem niecek osiadania i zapadlisk. Na tej podstawie dokonano próby ilościowego scharakteryzowania procesów subrozji, a co za tym idzie określenia szybkości degradacji wysadu.

 

NATURAL DEGGRADATION OF SALT DOME AT INOWROCŁAW

 

Natural deggradation of salt dome at Inowrocław is due to advancement of subrosion process in dome cap and at salt table. The process was traced and quantitatively characterized with reference to a new structural-geological map of the salt table (Fig. 1). The studies showed that it is most advanced at subcrops of pure chloride salts and the salt table. This phenomenon, connected with underground leaching of soluble rocks of salt deposits, resembles some known halurgic processes (halurgy is the field of mining technology, mainly dealing with exploitation of salt deposits by leaching chloride sals).

The process of natural deggradation appears to develop continuously but with varying intensity, presumably depending on climate. It became artificially intensified in the XIX and the beginning of XX c. in connection with exploitation of brines as well as in the last 25 years in connection with some activities related to extension of water conduicts and soil drainage. Intense underground leaching and development of karst phenomena in the uppermost part of the dome result in lowering of the salt table. Moreover, the processes are the major reason of ground surface subsidence and origin of collapse basins above the dome. The influence of continuing exploitation of salt in the dome on origin of deformations of terrain surface is difficult to be estimated with the use of geological-mining methods.

However, such influence seems rather negligible, especially in the light of data on terrain surface subsidence in this area before the beginning of mining works.

Close connections between the surface phenomena and the process of salt dome degradation made possible estimations of the rate of subrosion and mean rate of lowering of the salt table, i.e. that of annual degradation of the dome. The latter value, equal about 6 mm, is relative and it gives only some estimate of the rate of lowering of the table.

In geological time scale, the natural halurgic degradation is compensated by halokinetic squeezing of salt masses upwards. Total thickness of gypsum-clay cap may be the measure of degradation interpreted in this way. When we known the share of non-chloride rocks in the cap in relation to thickness of the whole evaporitic complex in stratigraphic column, that value may show us the length of dome section which became degraded. Estimations made for the salt dome at Inowrocław indicate degradation of a section of some hundred meters during the Quaternary.

 


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