STRONTIUM ISOTOPE STRATIGRAPHY FOR LATE CRETACEOUS TIME - DIRECT NUMERICAL CALIBRATION OF THE SR ISOTOPE CURVE BASED ON THE UNITED-STATES WESTERN INTERIOR

The Sr-87/Sr-86 of Sr in macrofossil carbonate from Upper Cretaceous s trata deposited in the Western Interior Seaway, USA, provides a Sr-iso tope 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 w ith Ar-39/Ar-40 dates for 20 bentonites within the sequence. Marine Sr -87/Sr-86 decreased from 0.70743 in the late Middle Cenomanian to a La te Cretaceous minimum of 0.70730 in the Late Turonian (89 Ma). From th e minimum, Sr-87/Sr-86 increased through a Middle Campanian inflexion (minimum at 77 Ma, maximum at 78-80 Ma) to reach 0.70772 at the Campan ian/Maastrichtian boundary. Thereafter Sr-87/Sr-86 increased through a very short inflexion in the latest Early Maastrichtian to the limit o f our sampling in the earliest Late Maastrichtian. The inflexions prov ide global event markers for correlation. For most of the period cover ed by our curve a resolution in dating and correlation of less-than-or -equal-to 0.8 Ma should be achievable with our 2 s.d. precision in mea surement of +/- 18 x 10(-6). Our Sr-87/Sr-86 data show no evidence of having been affected by influxes of freshwater into the US Western Int erior Seaway from rivers, including those that drained the Sevier Orog enic Belt.