Die jurassische hydrothermale Überprägung der sedimentären Deckgebirgsserien im Schwarzwald (Süddeutschland) The Jurassic hydrothermal alteration of sedimentary cover rocks of the Black Forest in Southern Germany

The extent of pervasive hydrothermal alteration of the sedimentary cover rock in the Black Forest is revealed by geochemical analyses together with illite K-Ar dating, thin section microscopy, microthermometry, X-ray diffractometry and infrared spectroscopy of bulk samples and clay separates. The alteration extends from faults in the five intramontane troughs into the Permo-Carboniferous sediments and further into the overlying Bunter deposits. Within the troughs, extensive alteration occurred by complete illitisation of detrital feldspar grains and recrystallisation of the illitic detritus. Authigenic illite particles are commonly grown in pore spaces and intersperse palisadian like as small authigenic illite flakes detrital mica particles encrusting feldspar and quartz grain surfaces of Bunter sandstones that are partly coated later by quartz overgrowth. The degree of intensive alteration within the intramontane troughs decreases from the northern to the southern Black Forest, and equally the smectite component in illite/smectite mixed-layered minerals of the <2 μm fractions increases from <5 % up to 25 %. Hydrothermal dioctahedral Al-rich chlorite from feldspar is widely observed as a minor component in the southern areas. Illite and chlorite authigenesis was triggered by hot and low saline solutions (150–250 °C, <10 wt% NaCleq), while the partly silicification was caused by low temperature and high saline fluids (<150 °C, 20–25 wt% NaCleq). Temperature of both fluid types decreases by about 50 °C from the northern to the southern Black Forest and likewise the ascending fluid volume appears to have been reduced. According to K-Ar ages of authigenic illite of the <2 and <0.2 μm fractions pervasive alteration occurred in the Upper Jurassic at about 145 Ma, and locally in addition Cretaceous hydrothermal events and – next to the Rhine Graben boundary fault – even those of Tertiary age are evident, indicating multiple hydrothermal activities. © 2015 E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart, Germany DOI: 10.1127/zdgg/2015/0033 www.schweizerbart.de 1860-1804/0033 $ 9.45 Z. Dt. Ges. Geowiss. (German J. Geol.), 166 (3), p. 275–295, 9 figs., 3 tables Article Published online June 2015