Clay minerals of a German-type middle triassic sequence, bore hole Nagykozár 2, Mecsek Mts., S. Hungary

The mineralogical composition of Middle Triassic sedimentary rocks in the bore hole sequence Nagykozar-2 and in few surface outcrops in the western Mecsek Mts. was investigated by X-ray diffraction methods. The composition of the bulk rock, insoluble residue and the <2 |im fraction was considered...

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Bibliographic Details
Main Author: Viczián István
Format: Article
Published: Szegedi Egyetem Ásványtani, Geokémiai és Kőzettani Tanszéke Szeged 2000
Series:Acta mineralogica-petrographica 41
Kulcsszavak:Kőzettan, Ásványtan, Földtan
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Online Access:http://acta.bibl.u-szeged.hu/24889
Description
Summary:The mineralogical composition of Middle Triassic sedimentary rocks in the bore hole sequence Nagykozar-2 and in few surface outcrops in the western Mecsek Mts. was investigated by X-ray diffraction methods. The composition of the bulk rock, insoluble residue and the <2 |im fraction was considered. Triassic sediments of the Mecsek Mts. Show remarkably analogous features with those of the German-type Triassic. Middle Triassic was deposited in a transgressive period of evolution of a flat coastal plain and later of a carbonate ramp environment. Buntsandstein-type sandstones of Schythian age are overlain by Rot-type sediments. Red to green laminated siltstones of tidal flat facies contain the detrital clay assemblage of illite (mica), Fe-Mg-chlorite, and the detrital feldspars K-feldspar and plagioclase. Anhydritic evaporites alternating with dolomite and siltstone beds of coastal sabkha origin are characterised by illite and Mg-rich chlorite or mixed-layer chlorite/smectite of presumably neoformational origin. The carbonate mineral of the evaporites is dolomite sometimes associated by magnesite. Corrensite, Mg-chlorite and illite occur in the overlying marly limestones and dolomites of inner and mid ramp, periodically dysaerobic carbonate environment. The genesis of corrensite is connected with the restricted nature of this sedimentary basin. Carbonate sediments analogous to Germanic Muschelkalk are represented by Middle Anisian to Middle Ladinian formations. They are mostly characterised by the dominance of the single clay mineral illite-lMd and by presumably authigenic K-feldspar. In lower part of the sequence a dolomitised variegated peritidal facies exists in which sometimes kaolinite may be enriched indicating subaerial weathering on the top of periodically emerging small carbonate sand bars. Another kaolinitic weathering seems to have taken place during the Middle Muschelkalk emersion. In contrast to the Germanic Triassic, the Mecsek Mts. rocks underwent relatively strong burial diagenetic transformation which is reflected by the widths of the illite basal reflections (KOBLER indexes). Another reason of sharpening of the illite reflections is the reorganisation in the evaporitic environment. Analogies with the clay mineralogy of a formerly studied bore hole sequence in the Mecsek Mts., Pecs-DC. and with that of Triassic formations of the Germanic basin are discussed.
Physical Description:9-29
ISSN:0365-8066