PROBLEM GENEZY KAOLINITU W DOLNOJURAJSKICH IŁACH CERAMICZNYCH I OGNIOTRWAŁYCH Z REGIONU ŚWIĘTOKRZYSKIEGO

Abstract

A PROBLEM OF KAOLINITE ORIGIN IN THE LOWER JURASSIC CERAMIC AND REFRACTORY CLAYS FROM THE HOLY CROSS MTS. REGION

Abstract. This paper summarizes the results of research performed so far on the origin of kaolinite in the Lower Jurassic (Hettangian) ceramic and refractory clays from the Holy Cross Mts. margin. Clay mineral assemblages from the analysed sections contain predominantly detrital kaolinite and illite with subordinate chlorite and only trace amounts of smectite. In the refractory and ceramic clay deposits (and in many other Hettangian strata), a very high kaolinite content was observed. The clay mineral composition was controlled mostly by climatic conditions and weathering regime. The claystones and mudstones were deposited in continental and marginal marine palaeoenvironments in warm climatic conditions, mostly with year-round humidity. Pronounced greenhouse conditions strongly intensified chemical weathering in the source areas. Extensive alluvial-lacustrine and delta-lagoonal systems acted as traps for kaolinite formed on the hinterland. Reworking and redeposition of ancient sediments (caused by tectonic processes and/or by sea-level changes) could modify the clay mineral composition. Expressive changes of the kaolinite/illite ratio in the Zagaje Fm. (lower Hettangian) might have been influenced by a sea-level lowstand and tectonic reactivation at the Triassic/Jurassic boundary, coupled with possible arid episodes and subsequent degradation of Norian or Rhaetian deposits. The next Skłoby Fm. (lower-middle Hettangian) is dominated by sandstones and heteroliths. Kaolinite is the most abundant clay mineral particularly in the Przysucha Ore-bearing Fm. (middle-late Hettangian). The tectonic quiescence and low surface relief, combined with high temperatures and intense rainfall, greatly favoured chemical weathering over mechanical erosion. In many cases, the original clay mineral assemblage was modified by early diagenetic processes mainly in swampy environments. The burial diagenesis and telodiagenesis changed the mineral composition only on a local scale. However, the telodiagenetic transformations might have been of great importance in the formation of some high-quality refractory clay deposits.