Przedmioceńska strefa wietrzenia (pstre utwory) w stropie warstw załęskich Rybnickiego Okręgu Węglowego

Abstract

 Opisano przejawy wietrzenia i późniejszych zmian epigenetycznych na wychodniach warstw załęskich (westfal dolny) w południowo-wschodniej części Rybnickiego Okręgu Węglowego. Zmianom wietrzeniowym towarzyszyły lokalne pożary pokładów węgla, gdzie indziej węgiel wietrzał w warunkach tropikalnego klimatu. Degradacja wietrzeniowa prowadziła do powstawania gibbsytu ze skał pierwotnych, kaolinit owo-illit owych. Równolegle syderyt i markasyt przechodziły w hematyt, maghemit i getyt.Procesy agradacyjne zachodziły początkowo w warunkach niskiego stężenia jonów sodu i potasu przy wzrastającym stężeniu krzemionki (powstanie hydrohaloizytu), później nastąpiło stopniowe podwyższenie stężenia tych jonów. Stąd kolejnymi produktami agradacji są hydrohaloizyt, illit, montmorillonit, zeolity i kwarc. PRE-MIOCENE WEATHERING ZONE (RED BEDS) IN TOP PART OF THE ZAŁĘŻE BEDS IN THE RYBNIK COAL MINING REGION The paper presents new results of the author's studies (W.M. Kowalski 1971, 1973, 1977, 1979,1981) on mineral composition and origin of mottled deposits found at Miocene subcrops of sedimentary Namurian and Westphalian rocks in the Rybnik Coal Mining Region (Upper Silesia). The traces of weathering and subsequent epigenetic changes found at subcrops of the Załęże Beds (Lower Westphalian) are described. The traces of weathering seem mainly related to Pa1eogene weathering and older than the epigenetic changes. There is no evidence for the presence of intraformational (Namurian or Westphalian) red beds. The weathering was sometimes accompanied by fires of coals whereas in other places the coals were weathering under tropical climate conditions.The studies covered samples from the Manifest Lipcowy (samples 1-10) and XXX-Iecia PRL mines (samples 11­–13, Fig. 1). In the sampled sections, sedimentary rocks are characterized by smaller thickness of wedging-out coal seams than in the Upper Silesian Sandstone Series (W.M. Kowalski, 1977) and markedly higher share of claystones and siderite nodules. Hematite is the most common product of weathering of these nodules.The red beds from the top of the Załęże Beds are represented by clay-ferruginous rocks ) which are the relics of non-coal material in places of secondary lack of coal seams – samples 1–5), breccias consisting of detritus of top parts of disappearing coal seams (sample 6), weathered siderite nodules (samples, 9, 10 and 12), and weathered pelite (samples 7, 11, 13) and silty (samples 3 and 8) claystones. The succession of secondary changes is here similar as in previously studied sections of the Upper Silesian Sandstone Series. The final products of the weathering and fires include hematite, hydrohematite, maghemite, geothite and gibbsite. The origin of gibbsite was facilitated by oxidation of coal accompanying marcasite and other sulfides. Gibbsite was here accompanied by allophane. Hydrohalloysite, halloysite and kaoline originated due to resilification processes in the red beds. Resilification is known to develop at present in bauxites in Guinea (W.M. Kowalski, 1970), France (J. Nicolas, P. Bildgen 1973) and other countries. This process was here accompanied by deferrifucation of the mottled deposits, proceeding under conditions of low value (about 4) of log Na/H isopleth in the P.C. Hess (1966, fig. 2A) phase diagramme. Along with increase in concentration of SiO2 in fissure solutions, gibbsite and hematite began to disappear and minerals of the kaolinite and halloysite group (mainly hydrohalloysite) and, subsequently, smectite and quartz started to be formed.The development of smectitization processes may be partly related to the second stage of aggradational changes, proceeding under conditions of continuing, steady increase in content of silica in fissure solutions. At that time, concentration of sodium was higher than in sea water (log Na/H about 8.5) and potassium ions of the order of log K/H equal 6 (Fig. 2B). This stage primarily resulted in illitization of sandstones as well as crystallization of zeolites in fissures.X-ray studies (Tables 1–4) made possible analysis of mineral composition of the red beds and they gave further support to the above discussed directions of processes.