Dolomityzacja osadów wapienia cechsztyńskiego w rejonie Wrześni

Abstract

Osady wapienia cechsztyńskiego z otworu Grondy Górne IG 1 uległy całkowitej dolomityzacji. Z wyjątkiem łupku miedzionośnego i dolnej części wapienia cechsztyńskiego, w których w składzie dolomitu zaznacza się przewaga jon6w wapnia nad magnezem, stwierdzono obecność bardzo dobrze uporządkowanych dolomitów o stosunku stechiometrycznym. Zawartość strontu wskazuje na powstanie dolomitów w obecności roztworów rozcieńczonych; do tego samego Wniosku prowadzą rozważania paleogeograficzne, źródłem wody meteorycznej były opady atmosferyczne. Woda ta spływała w dół stoku strefy Kalejów - Czeszewa, wypiętrzonej w wapieniu cechsztyńskim, w wyniku czego następowało mieszanie się wody słodkiej z wodą morską, co spowodowało dolomityzację osadów.

DOLOMITIZATION OF THE ZECHSTEIN UMESTONE DEPOSITS OF THE WRZEŚNIA REGION (FORESUDETIC AREA, WESTERN POLAND)

In the condensed successions of the Zechstein Limestone in the northern Fore-Sudetic area limestones prevail. as opposed to the normal, successions, characteristic of somewhat deeper areas compared to those in which the condensed successions ,originated. In the normal successions the dolomite content is usually higher. In the Grundy Górne IG 1 well, located south of Września, the lithological development of the, Zechstein Limestone horizon (developed as dolomites) (Fig. 1; Tabl. I, II) is similar to that of other successions in the Fore-Sudeticarea (Fig. 2).

A distinct evolution in dolomite composition was recorded (Fig. 1), but usually the dolomites are stoichiometric and very well ordered. The increase in strontium content (Fig. 1, 3) reflects the trend to reach the stoichiometric compoSition of dolomite. The characteristics of dolomites and the preservation of fabrics testify to the existence of only one main period of dolomitization. As in result of it the entire Zechstein Limestone succession has been dolomitized, the doiomitization evidently took place after deposition of the Zechstein Limestone succession and before deposition of the chicken-wire anhydrites, constituting the lowermost part of the Lower Anhydrite. The non-stoichiometric dolomites occurring in the Lower Anhydrite horizon originated in the sabkha environment. The dolomites of the Zechstein Limestone originated in the presence of dilute solutions, 'as suggested by the strontium content and the stoichiometric composition of dolomites.

The Grundy Górne IG 1 well is located on the slope of the Kaleje - Czeszewo zone that constituted the shoal during the Zechstein Limestone deposition (T.M. Peryt, H. Ważny, 1978). The shoal has been subaerially expoed during the periods of lowering of sea level (T.M. Peryt, 1981b), as presented in Fig. 4, and the meteoric ,"water could flow down the slope, what led to the mixing of fresh and sea water, and hence to the dolomitization of the Zechstein Limestone deposits. The meteoric water derived most probably from rains - although during the Zechstein Limestone deposition the climate in the Fore-Sudetic area was similar to that prevailing now in the Persian Gulf area (T.M. Peryt, 1978), there are some indications that there existed more humid periods.

Concluding, the data indicate that the dolomites of the Zechstein Limestone succession in the Grundy Gqme IG 1 well originated from mixing of sea and meteoric waters (mixing water mechanism, Fig. 4). This mechanism explains many cases of dolomitization of the Zechstein Limestone deposits in the central part of the. Zechstein Limestone basin although numerous other cases should be connected with other mechanisms of dolomitization (T.M. Peryt, 1978, 1981c).

The support of Alexander von Humboldt-Stiftung is gratefully acknowledge.