Response of clastic sediments to episodic hydrothermal fluid flows in intramontane troughs: a case study from Black Forest, Germany

Abstract

Permo-Triassic sandstones of the Breisgau and Schramberg troughs from the central Black Forest and the adjoining Rhine Graben were hydrothermally illitized during post-Variscan times. Illite K-Ar dating outlines a multi-episodic activity during Late Jurassic (about 145 Ma ago), Early Cretaceous (about 130 Ma ago) and Tertiary (about 60 Ma ago). The last episode appears to be restricted to the foothill zone along the Rhine Graben boundary fault. Detrital feldspars, micas, illite and authigenic kaolinite are the percursors of the hydrothermal illite that is Al-rich when derived from feldspars and authigenic kaolinite, or Al-depleted when formed from micas and detrital illite. The chemical composition of authigenic illite is therefore not an appropriate geothermometer to be used on immature sandstones. Furthermore, the occurrence of mixed-layered illite/smectite and the limited extent of intensive alteration suggest low fluid-rock ratios. Illitization was induced by hot, low saline, metamorphic and/or formation fluids (150– 250 °C, eq ). A pulse of high saline and low temperature fluids (about 100 °C, 20–25 wt% NaCl eq ) derived from mixtures of deep seated saline basement brines and meteoric waters caused quartz overgrowths only in the Triassic sandstones. In addition, kaolinite deriving from detrital feldspars in the Rhine Graben area, could have resulted from interaction with thermal, acidic and low-saline fluids. The results indicate tectonically controlled fluid transport, mainly along reactivated NE and NNE trending faults, due to large-scale tectonic processes, namely: (1) rifting in the central Atlantic and western Tethys oceans, (2) Alpine orogenesis, and (3) rifting of the Rhine Graben.