Hirnantian icebergs in the subtropical shelf of Baltica: Evidence from sedimentology and detrital zircon provenance

The widespread glaciomarine record of the Hirnantian (end-Ordovician) glaciation is confined to peri-Gondwanan terranes in Variscan Europe and, hence, is of little use for assessing the equatorward range of icebergs that calved from the North Gondwana ice sheet into the Rheic Ocean. New sedimentological investigations aided by laser ablation-inductively coupled plasma-mass spectrometry detrital zircon dating of Hirnantian diamictites from the East European craton in Poland demonstrate that the iceberg pathways extended to subtropical Baltica. The diamictites occur in a series of north-thinning (0.4-9.6 m) pods that are traceable for a 700 km subcrop length within the marly portion of a carbonate-shelf ramp. Dropstone-bearing textures, soft-sediment deformation style, and thickness trend point to deposition from floating and grounded icebergs driven northward by along-shelf circulation. Density plots of detrital zircon U-Pb ages from the diamictites reveal a North Gondwana provenance suggestive of iceberg-calving sites located on the North African margin. This implies a minimum 3500 km drifting distance for icebergs that reached the western shelf of Baltica. Our discovery provides a new constraint for modeling Late Ordovician climate and ocean-circulation patterns.