Interaction between bimodal volcanism, fluvial sedimentation and basin development in the Permo-Carboniferous Saar-Nahe Basin (south-west Germany)

Widespread bimodal-calcalkaline magmatism and contemporaneously active faulting reflect the combined effects of late-orogenic collapse and strike-slip faulting during syn-rift development of the late-Variscan Saar-Nahe Basin. The intrabasinal drainage systems experienced major but differing influences from three prominent styles of volcanic extrusives: extensive basic lava flows; localized acidic domes; and widespread pyroclastic units. Sedimentary sections including basic to intermediate lavas show the transformation of pre-eruptive meandering fluvial systems into lacustrine systems following lava eruptions. These relationships are interpreted in terms of damming of the drainage systems by the topographic barriers represented by the basic lavas themselves. Siliciclastic sediments re-entered and ultimately filled the lakes through prograding, small-scale crevasse channel/mouth bar complexes. Uplift associated with the intrusive/extrusive emplacement of acidic domes affected both the accommodation space and the internal topography of the basin. Following an extrusive pulse, each dome collapse caused the progradation of voluminous volcaniclastic aprons onto the surrounding alluvial plain. Locally, the overload by large volumes of unconsolidated detritus converted the regional meandering drainage system into a braided one. The overall duration of acidic dome activity could have been longer than 10⁶ years, but was interrupted by repose periods varying probably between a few hundred years to several thousand years. Pyroclastic activity accompanied dome emplacement and registers their episodic activity in the stratigraphic record. Fine-grained ashes of phreatoplinian eruptions were spread widely within the basin and overloaded streams in the area of fallout. Rapid reworking of huge amounts of unconsolidated material supplied an abundance of volcanic detritus, which tended to fill topographic lows that were usually fault-controlled. The relationships between basin evolution, intrabasinal volcanism and contemporaneously active faulting, identified in the Saar-Nahe Basin, are important. They emphasize and further the understanding of how similar volcanic products in sedimentary basins worldwide impacted on the depositional system.