Sedimentation and reworking of volcanic detritus in Frasnian and lower Famennian carbonate rocks of the Velbert Anticline (Rhenish Massif, Germany)

This investigation describes different sedimentation phases of a very special clastic sediment, deposited before and during the drowning of the Velbert Anticline reef complex around the time of Frasnian/Famennian transition. The occurrence of siliciclastic and especially volcanic detritus embedded in Frasnian sandstones and reef limestones at the Velbert Anticline is rather unique for the Devonian of the Rhenish Massif. Rounded quartz grains with mineral-filled resorption hollows together with broken quartz pieces are accompanied by small amounts of pre-Devonian metaquartzite, chert grains, well-rounded pebbles of vein quartz and quarzitic sandstone. Although the mineral fillings of the resorption hollows gave no geochemical evidence for the type of their volcanic origin, it seems likely that the quartz grains derived from phenocrysts of a rhyolitic volcanic rock. After their erosion and marine sedimentation during the hassi and jamieae conodont zones, semi-consolidated sandstones were affected by tectonic movements leading to their uplift and erosion in a timespan beginning shortly before the triangularis time gap. The reworked material built up a new type of sediment, mixed siliciclastic and carbonate clastic, ranging from the Upper triangularis to Lower crepida conodont zones (Hülsbeck East). The uplift locally culminated in tilting and non-sedimentation finally resulting in an angular unconformity between Massenkalk (Wülfrath Formation) and overlying Famennian crinoidal limestones. At Hülsbeck West, where strata of the Lower crepida Zone are lacking, a gap in sedimentation occurs from the Upper rhenana to the Uppermost crepida/Lower rhomboidea zones. A detailed provenance discussion of the volcanic quartz grains is given herein. Though it may be obvious to refer the grains to the same-age volcanism of the Lahn-Dill Synclines, this sediment source seems unlikely. Instead, the siliciclastic grain component suggests an input from a Frasnian successor area of the Givetian Krefeld High.