Ancient Swiss Coral Reef Redated by Tiny Fossils, Reshaping Jurassic Timeline

High on a forested ridge northeast of the Swiss town of Aarau, a wall of limestone cliffs hides one of the most complete records of a tropical sea that covered northern Switzerland roughly 170 million years ago. The Gisliflue Reef, a massive coral build-up exposed at Gisliflue and the neighbouring Homberg ridge, has long been a landmark for geologists mapping the Jurassic strata of the Swiss Plateau. A new study published in the Swiss Journal of Geosciences now pins down its age with unprecedented precision, using microscopic fossils that were invisible to earlier generations of field geologists, and in doing so forces a rethink of how the rock sequence of northern Switzerland is subdivided.

The research, carried out by Andri Degiacomi, Susanne Feist-Burkhardt and Stephan Wohlwend, combined classical sedimentological fieldwork with a technique rarely applied to such reef limestones: palynology, the study of organic microfossils, in this case the resistant cysts produced by ancient dinoflagellates, single-celled plankton that drifted in the Jurassic seas. Four stratigraphic sections, two on the Gisliflue ridge and two on the Homberg, were sampled and analysed in detail. The well-preserved dinoflagellate cyst assemblages recovered from many of the samples allowed the team to date the entire reef complex, together with a peculiar overlying deposit known in the Swiss literature as the Echinodermenbrekzie, to the Early Bajocian stage of the Middle Jurassic, specifically the Humphriesianum ammonite Zone.

The reef itself tells a vivid story of growth and death on a shallow sea floor. In its lower part, the researchers documented marly, strongly arenitic limestone containing isolated platy corals, evidence of a phase in which coral colonies were still sparse and sediment input relatively high. Upward, the facies changes dramatically: rock-forming platy corals of the genera Isastrea and Thamnasteria come to dominate, building the massive, resistant frame that now forms the striking cliffs, up to 20 to 40 metres high, that give the Gisliflue its silhouette above the Aare valley. At the very top of the summit, the final few metres are dominated mainly by branching corals, a shift that may reflect changing environmental conditions as the reef approached sea level.

The end of reef growth is recorded in a feature that palaeontologists find particularly evocative: a bored hardground capping the summit. A hardground is a seafloor surface that became cemented while exposed at or near the sediment-water interface, and the borings that riddle it were made by organisms excavating into the cemented surface in the earliest interval following the end of coral growth. In other words, the summit of the Gisliflue preserves a fossilised snapshot of the moment the reef finally stopped growing, a cemented sea floor colonised immediately by hard-substrate dwellers before the next phase of deposition began.

Tracing the bioherm laterally proved just as informative as dating it. The reef body can be followed westward across the ridge to the midsection of the Homberg, where it abruptly wedges out and passes into an arenitic, bioclastic limestone. This coarse, echinoderm-rich rock, packed with fragments of crinoids, sea urchins and other echinoderms, continues to the western end of the Homberg, where it maintains a fairly constant thickness of around 8 to 12 metres. This stratigraphic succession, long recognised both in the published literature and on official geological maps as the Echinodermenbrekzie, or echinoderm breccia, is thus shown to be the lateral equivalent of the coral reef rather than an unrelated unit stacked above it.

Regional comparisons with neighbouring stratigraphic sections add a further dimension to the picture. The analyses indicate a gradual lateral thinning of the Echinodermenbrekzie to the west, suggesting that the coarse bioclastic facies was deposited in a belt that tapered away from the reef margin. Such geometry is typical of reef-flank and fore-reef settings, where wave action and currents break up reef-dwelling organisms, especially the delicate echinoderms, and redistribute the debris as aprons of bioclastic sand and rubble flanking the living reef. The new data therefore tie the breccia genetically to the Gisliflue Reef, resolving a relationship that had remained ambiguous in the regional stratigraphy.

The palynological results carry implications well beyond the two ridges themselves. Because the reef and the Echinodermenbrekzie are dated to the Early Bajocian Humphriesianum Zone, while the oolitic limestones of the Hauptrogenstein that directly overlie the bored hardground at the summit are dated to the Late Bajocian Niortense Zone, the succession at Gisliflue now provides a tightly bracketed reference section for the Early to Late Bajocian transition in northern Switzerland. The Hauptrogenstein, a formation of oolitic limestones deposited in agitated, shallow water and famous as a building stone across the region, rests directly on the hardground, meaning the hiatus between reef growth and oolite deposition can be constrained to a narrow interval of ammonite zones.

A highlight of the study is the formal description of a new dinoflagellate cyst species, Hypolytodinium argoviense, designated as a new genus and new species. 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. Marker species of this kind are the workhorses of biostratigraphy: because dinoflagellate cysts evolved rapidly and dispersed widely, their first and last appearances allow sedimentary successions to be correlated over large distances, even in rocks, such as these reef limestones, that lack the ammonite fossils traditionally used to define Jurassic zones.

For the stratigraphy of northern Switzerland, the consequences are substantial. The Gisliflue Reef and its associated deposits had previously been assigned to various levels within the regional rock classification, and the new dating demonstrates that the reef complex, the Echinodermenbrekzie and the overlying Hauptrogenstein form a coherent, precisely dated succession spanning the Early to Late Bajocian. This allows geologists to refine the boundaries of lithostratigraphic units on maps and in the subsurface, where the same Jurassic rocks are encountered in boreholes and are relevant to groundwater flow, geothermal exploration and the assessment of potential nuclear waste repository host rocks in the region.

The study also illustrates how modern microfossil techniques can rejuvenate classic field localities. The cliffs of the Gisliflue have been examined for well over a century, yet the application of detailed dinoflagellate cyst stratigraphy has extracted a chronological framework that field observation alone could never deliver. As the authors show, a ridge of coral limestone above the Aare valley, once a puzzle of conflicting age assignments, now stands as one of the most securely dated Middle Jurassic reef successions on the Swiss Plateau, a reference point against which the reefal and bioclastic limestones of the entire region can be calibrated.

Subject of Research: Palynological dating of the Middle Jurassic Gisliflue coral reef and its stratigraphic implications in northern Switzerland

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

Article References: New palynological dating of the coral bioherm «Gisliflue Reef» and the implication for the stratigraphy of northern Switzerland. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Gisliflue Reef, palynology, dinoflagellate cysts, Bajocian, Jurassic, Switzerland, coral bioherm, Echinodermenbrekzie, Hauptrogenstein, stratigraphy, Humphriesianum Zone, biostratigraphy