New palynological dating of the coral bioherm «Gisliflue Reef» and the implication for the stratigraphy of northern Switzerland

Abstract The present study investigates the coral bioherm and the associated «Echinodermenbrekzie» of the «Gisliflue Reef» at Gisliflue and Homberg, located northeast of Aarau, Switzerland. Sedimentological and palaeontological analyses were carried out on four outcrops along the prominent ridge, two outcrops each at Gisliflue and Homberg. Four stratigraphic sections were analysed palynologically using detailed dinoflagellate cyst stratigraphy to determine the biostratigraphic age of the coral reef development and the deposition of the coarse, echinoderm-rich, bioclastic limestone to the west. The investigation reveals that the lower part of the «Gisliflue Reef» consists of marly, strongly arenitic limestone with isolated platy corals, transitioning upwards into rock-forming platy corals such as Isastrea and Thamnasteria that form the distinctive cliff up to 20 to 40 m high. The top few metres of the summit of Gisliflue feature mainly branching corals, capped by a bored hardground that has formed at the earliest following the end of coral growth. The bioherm can be traced westward to the midsection of Homberg before it abruptly wedges out and transitions into an arenitic, bioclastic limestone that can be traced to the western end of the Homberg, where it has a fairly constant thickness of around 8 to 12 m. This stratigraphic succession, referred to as «Echinodermenbrekzie», is recognised both in the literature and on geological maps. Regional comparisons with neighbouring stratigraphic sections indicate a gradual lateral thinning of the «Echinodermenbrekzie» to the west. The palynological analyses revealed well-preserved dinoflagellate cysts in many of the samples, which date the «Gisliflue Reef», including the «Echinodermenbrekzie», to the Early Bajocian Humphriesianum Zone. The above following oolitic limestones of the Hauptrogenstein, which directly overlie the hardground at the summit of the Gisliflue, are dated to the Late Bajocian, Niortense Zone. Within the palynological samples a new stratigraphic marker species, Hypolytodinium argoviense , gen. nov. & sp. nov. is formally described. The species is considered a useful stratigraphic marker for the late Early Bajocian Humphriesianum Zone, with its main occurrence in the Romani to Humphriesianum subzones.