INTEGRATED STRATIGRAPHY OF THE WESTERN INTERIOR SEAWAY (SANTONIAN-CAMPANIAN)

The biostratigraphy of late Cretaceous Western Interior (WI) ammonite index fossils is well documented but lacks complete age control. While magnetic polarity reversals and K/Ar isotopic ages constrain upper Campanian to lower Maastrichtian biozones, no such integrated dataset exists for the upper Santonian through lower Campanian, representing biozones Scaphites leei III-Baculites perplexus. Within a stratigraphic context, we combine paleomagnetic and biostratigraphic data from 9 outcrops in South Dakota, Wyoming, and Montana, preliminarily suggesting a paleolatitude of ca. 40 – 50 degrees for this part of the North American craton and further constraining the biostratigraphy of lower Campanian ammonites of the WI as described by Gill & Cobban (1966).

Index ammonites and oriented samples for magnetostratigraphy were sampled from Rosebud County, MT (Scaphites leei III); Manderson, WY (Scaphites hippocrepis I); Greybull, WY (Scaphites hippocrepis I or III); Casper, WY (Baculites sp.(smooth)-Baculites sp.(weak flank ribs)); Belle Fourche, SD (B. sp.(smooth)-B. sp.(weak flank ribs)); Oral, SD (B. obtusus-B. mclearni); Ardmore, SD (B. obtusus); Poison Spider, WY (B. asperiformis); and Oral (north), SD (B. perplexus-B. scotti). The presence of molluscan fossils with iridescent aragonite at most sites demonstrates that regional thermochemical alterations were well below the aragonite-calcite replacement boundary of ~ 100 C. Rock-magnetic and demagnetization experiments indicate that fine-grained (SD to PSD) magnetite is the most likely carrier of the stable magnetization. Magnetochron C33r, whose base is at or closely below the GTS 2004 working definition of the Santonian-Campanian stage boundary, may span the interval 83.9-79.3 Ma. Preliminary interpretation of our magnetic reversal stratigraphy locates normal polarity, either the base of C32n or a small normal chron in C33r, at Ardmore and Oral, SD within the zones of B. obtusus or B. maclearni. Correlation between outcrops constrains the base of C33r to between the zones of Scaphites leei III and Scaphites hippocrepis III.