A sharp positive δ34S anomaly occurs in a 0–5 cm siltstone that contains syndepositional pyrite and was deposited below wave base as part of a basinal off-reef succession at Medicine Lake, Alberta. The bed rests on bioturbated, oxygenated dolomitic siltstones that are positively correlated with biostromal banks containing the last Frasnian shelly benthos over a wide region. Above, laminated anoxic argillaceous lime-mudstones grade upward into beds containing Famennian conodonts and brachiopods. Geochemical and sedimentological data suggest flooding of the craton by basin-derived anoxic water at the extinction horizon as a result of a sudden deep basin or ocean turnover. Comparable positive δ34S values are found in sedimentary pyrite in Frasnian black shales from a passive margin basin in the Yukon to the north; values decrease in the Famennian.