UPPER SILURIAN LOESS-LIKE SEDIMENTS – A CASE OF EXCEPTIONAL PRESERVATION OR INDEED A CLIMATIC RECORD?

Upper Silurian (upper Ludfordian to lowermost Přídolí) Rykhta and Prigorodok Formations in Podolia, Ukraine, record a major regression, which is globally recognized as the Lau Event and associated with a pronounced positive carbon isotope excursion. Based on palaeotemperature indicators and theoretical models, a short-term glaciation was recalled as possible mechanism driving the regression; to date, however, no direct sedimentological evidence exists for a cooling event in this interval. Upper Silurian marginal-marine carbonates of Podolia, developed on the cratonic end of rapidly subsiding foreland basin, provide a sensitive record of sea-level changes during this interval. Lowstand and early TST deposits of the Prigorodok Formation are interpreted here as representing hypersaline, restricted coastal carbonate lakes dominated by microbial mats. These sediments contain an important admixture of detrital silt-sized quartz and dolomite, trapped by mats or forming distinct sheets and event beds. These deposits share many petrographic characteristics with Quaternary and putative upper Palaeozoic loess deposits. They were not observed in underlying HST rocks capped by emersion surface, interpreted as the major Lau Event disconformity, nor in late TST rocks represented by tempestites. The unique sequence-stratigraphical context and depositional setting of these loess-like deposits might be the sole factor responsible for their preservation. On the other hand, appearance of eolianites in this interval is in agreement with contemporaneous increase in abundance of terrestrial spores noted in shallow-water sections by several authors, and supports some of the palaeoclimatic models proposed for the Lau Event and similar, presumably glacioeustatically driven, global regressions in the Silurian.