The Upper Cretaceous (Campanian) Lea Park Formation of south-central Saskatchewan, a product of continuous deposition in the Claggett sea of the Western Interior basin, contains unusually well-preserved fossil shells ideally suited for analysis of their chemical and isotopic compositions.Although both ammonites and bivalves secreted shells in isotopic equilibrium with the waters in which they lived, ammonites usually have higher δ18O and lower δ13C values than coexisting bivalves. These values point to the ammonites and bivalves having inhabited isotopically distinct waters within the Claggett sea. The benthonic bivalve Inoceramus is inferred to have lived in a thin bottom layer, the isotopic composition of which was modified by that of pore waters expelled at the sediment–water interface following compaction of sediments. The nekto-benthonic ammonites Scaphites and Baculites are inferred to have lived mostly at successively higher levels in the water column, the isotopic composition of which was modified by an influx of meteoric waters.Variations in trace-element/Ca ratios in molluscan shells are attributed to changes in the chemical composition of the Claggett sea, with essentially constant 87Sr/86Sr ratios suggesting that most chemical variations resulted from processes operating within the basin rather than at mid-ocean ridges. Sr concentrations in the sea decreased during Lea Park deposition, presumably as a result of weathering of increased proportions of igneous rocks relative to carbonate rocks in the western sourcelands. Anomalous Mg/Ca ratios are present in molluscan shells at a biostratigraphically correlative level. These ratios are primary and attributed to weathering of Mg-rich igneous rocks on the western flank of the basin. Probably produced in less than 40 000 years, the anomalous Mg/Ca ratios may form a chemostratigraphic and chronostratigraphic marker for the correlation of upper Campanian rocks.