Sediments of the Lea Park Formation in south-central Saskatchewan were deposited in the Claggett sea during the Late Cretaceous (Campanian) Claggett marine cycle. Rocks of the Lea Park Formation have remained virtually unaffected by diagenetic alteration; hence, variations in mineralogical and chemical compositions result primarily from changes in the source of detritus. Quartz, feldspar, discrete illite, kaolinite, and chlorite were derived from weathering of the western highlands, whereas illite–smectite (I–S), the dominant clay phase, originated from volcanic ash. Changes in the proportions of these minerals suggest a large-scale volcanic episode during deposition of the Lea Park sediments, an episode that affected not only the detritus delivered to the seaway but also the chemical and isotopic compositions of the sea water itself. δ13C values of organic matter increase with little change in the quantity of organic carbon preserved through both the transgressive and regressive phases of the Claggett cyclothem, unlike other cyclothems in which δ13C values generally decrease through the regressive phase. The increase in δ13C values in the Claggett cyclothem suggests a partial contribution from volcanic emanations to the 13C/12C ratio of the sea. Dissolution of Mg-rich volcanic debris from a major volcanic episode resulted in anomalously high Mg/Ca ratios in the sea and in the shells of molluscs living in the sea at that time. Rocks containing the shells with anomalously high Mg/Ca ratios are not themselves unusually Mg rich, but a general increase in Mg/SiO2 ratios in the rocks points to an increase in volcanic influx. The similarity between the mineralogical composition of the Lea Park Formation in Canada and that of the coeval lower part of the Pierre Shale in the United States indicates that volcanic ash was an important detrital source for both formations.