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Anomalous massive water-column carbonate precipitation (whitings) as another factor linking Silurian oceanic events

Abstract The geological record of the mid-Homerian Mulde event in the deep-water settings of the East European Platform is characterized by a profound graptolite extinction event and a low amplitude positive δ13C excursion. It is also associated with a negative natural gamma excursion occurring in a monotonous shaly succession, visible on well logs and traceable across the entire basin. Nevertheless, its origin has not been clarified yet. We present new data on the geophysical and microfacies record across the stratigraphic interval of this event in northern Poland. The present paper tests a hypothesis that the anomalies are related to the appearance of syngenetic and/or detrital carbonates at the onset of this Silurian event. New data reveal that both: (1) natural gamma radiation and magnetic susceptibility negative anomalies and (2) facies changes manifested in the increased carbonate content and ingression of ‘varve-like’ laminated calcisiltite containing large amounts of ‘sparoids’ (interpreted as carbonate suspensoids) occur simultaneously in the middle Homerian. Despite different palaeogeographic situations of the basin in the Wenlock and Ludlow due to the progressive collision of Baltica with Avalonia and the evolution of Caledonian foredeep, the geophysical and facies record of the Mulde Event is strikingly similar to that of the Ludfordian Lau Event record from the same basin. This leads to the conclusion that carbonate hypersaturation, resulting in the mass appearance of water-column carbonate precipitates (whitings) played a significant role during both Mulde and Lau events and represents palaeoceanographic phenomena rather than single case local anomalies.