The 87Sr/86Sr of Sr in macrofossil carbonate from Upper Cretaceous strata deposited in the Western Interior Seaway, USA, provides a Sr-isotope curve for Late Cretaceous time (Cenomanian-Early Maastrichtian). The curve is calibrated biostratigraphically against the most refined ammonite zonation known for the interval, and calibrated numerically with 39Ar/40Ar dates for 20 bentonites within the sequence. Marine 87Sr/86Sr decreased from 0.70743 in the late Middle Cenomanian to a Late Cretaceous minimum of 0.70730 in the Late Turonian (89 Ma). From the minimum, 87Sr/86Sr increased through a Middle Campanian inflexion (minimum at 77 Ma, maximum at 78-80 Ma) to reach 0.70772 at the Campanian/Maastrichtian boundary. Thereafter 87Sr/86Sr increased through a very short inflexion in the latest Early Maastrichtian to the limit of our sampling in the earliest Late Maastrichtian. The inflexions provide global event markers for correlation. For most of the period covered by our curve resolution in dating and correlation of ≤0.8 Ma should be achievable with our 2 s.d. precision in measurement of ±18 × 10-6. Our 87Sr/86Sr data show no evidence of having been affected by influxes of freshwater into the US Western Interior Seaway from rivers, including those that drained the Sevier Orogenic Belt. © 1994.
