Wczesnodiagenetyczny cement w dewońskich skałach węglanowych północno-wschodniego obrzeżenia Górnośląskiego Zagłębia Węglowego

Abstract

Mimo silnej rekrystalizacji dewońskich skał węglanowych północno-wschodniego obrzeżenia Górnośląskiego Zagłębia Węglowego, zdołano stwierdzić relikty wczesnodiagenetycznego cementu w wapieniach organogenicznych i organodetrytycznych. Relikty tego cementu najlepiej zachowane są we wnętrzach elementów szkieletowych, rzadziej w wolnych pierwotnie porach pomiędzy nimi. Jednym z najważniejszych kryteriów identyfikacji reliktów wczesnodiagenetycznego cementu w tak zrekrystalizowanych skałach węglanowych jest zachowane jeszcze zróżnicowanie strukturalne poszczególnych generacji cementu. Cement wczesnodiagenetyczny tworzy w omawianych skalach dwie dobrze indywidualizujące się strukturalnie generacje. Są to: cement palisadowy (cement A, wcześniejszy, obrastający brzegi por) i cement mozaikowy (cement B, późniejszy, wypełniający wewnętrzne części por). Rozwój rekrystalizacji prowadzi do zatarcia różnic strukturalnych pomiędzy poszczególnymi generacjami cementu wczesnodiagenetycznego. EARLY-DIAGENETIC CEMENT IN DEVONIAN CARBONATE  ROCKS FROM NORTH-EASTERN MARGIN OF THE UPPER SILESIAN COAL BASINPrimary features of Devonian carbonate rocks from north-eastern margin of the Upper Silesian Coal Basin are strongly obliterated due to advanced diagenetic transformations such as dolomitization, recrystallization, and silification. Therefore, the original character of groundmass (including early- diagenetic cement) usually appears impossible to define in any unequivocal way. This is even the case when a suggestive and selective (i.e. not affecting allochems) development of isometric pseudosparite is recorded. Relics of early diagenetic cement were found in some rocks (organogenic and organodytrital limestones) only. They are best preserved inside skeletal components or, sometimes, in originally free spaces between them (Figs. 1-4). Usually two clearly different generations of the cement relics may be identified:- an older cement A, growing directly or indirectly, through a thin micritic coating at margins of original pores (sparite of the bladet type - R.L. Folk, 1965; the ratio of length to width of crystals, d:s, ranging from 2:1 to 3.5:1, or sparite of the fibrous type - R.L. Folk, 1965, the ratio d:s = 6.1); and- younger cement B, completely or partly infilling the remaining, more internal parts of pores (cement built of sparite of the equant type - R.L. Folk, 1965; the ratio d:s = 1:1).The boundary of the two generations of cement is a sharp, strongly toothed line, easy to trace because of differences in structure (cement A - radial structure, cement B - mosaic structure - figs. 1-4). However, the differences become more and more obliterated along with advancement of recrystallization (Figs. 5 - 7).Relics of recrystallized early-diagenetic cement are characterized by some features which may be treated as criteria for its identification. The features include: - gradual increase in size of crystals, from the outermost parts of pore space towards the inner ones;- lack of relics of micrite or any greater amount of ingrowths and contaminations in areas originally occupied by early-diagenetic cement; - preservation of relics of palisade structure in some places in outer part of pore space;- sharp and straight boundaries of crystals infilling the pore space;- a clear difference in transparency of areas previously occupied by cement of the first (A) and second (B) generations.It should be stated that the above mentioned features are completely obliterated in very strongly recrystallized rocks. In such a case we may speak about pseudosparite only.