Reactivated basement structures allecting the sedimentary facies in a tectonically quiescent epicontinental basin: an example from NW Switzerland

The Jurassic deposits in the southern part of central Europe accumulated in a shallow epicontinental sea, and their deposition has usually been believed to have corresponded to a phase of tectonic quiescence; neither on conventional seismic records nor in outcrops obvious indications of synsedimentary tectonics were found. Nonetheless, subtle variations in lithofacies and thickness occur above faults in the crystalline basement. In fact, preexisting structures became reactivated in the Jurassic during major extensional phases when the Tethys and the North Atlantic opened. This reactivation led to differential subsidence and/or to rotation of fault-bounded blocks, but the sediments were deformed mainly flexurally as vertical movements in the basement were dissipated by Triassic salt. Thus, the depositional area was morphologically differentiated into swells and depressions. In siliciclastic muddy environments, swells were characterized by hardbottoms and hiatus beds; depressions were filled by distal tempestites and gravity deposits. In carbonate settings, reefs nucleated on swells; marls and muds were deposited in depressions (many of them gravity deposits). Reactivation of faults occurred only during short episodes and was not synchronous throughout the study area, and reactivation of individual faults was episodic and probably controlled by the palaeostress field and the faults' orientation. Reactivation of deep-rooted faults is also documented by hydrothermal activity which led to vein mineralization and alteration of minerals-today exposed in nearby basement units. The chronostratigraphic ages of the hydrothermal processes coincide with phases of enhanced subsidence during the Sinemurian, Aalenian, Bajocian/Bathonian, and Oxfordian. In turn, facies changes of the sedimentary cover should be useful to predict basement structures when no seismic records are available. (C) 2002 Elsevier Science B.V. All fights reserved.