Macrofossil magnetobiostratigraphy for the upper Santonian–lower Campanian interval in the Western Interior of North America: comparisons with European stage boundaries and planktonic foraminiferal zonal boundaries

Abstract

Magnetostratigraphic samples were collected from the Milk River and Pakowki formations of southeastern Alberta to determine the polarity chron 34–33 r boundary. This boundary was located in the upper half of the Deadhorse Coulee Member, and is below the first local occurrence of the Western Interior ammonite Baculites obtusus. Comparison with the laterally equivalent Eagle Sandstone of Montana and Niobrara Formation of Colorado indicates that the 34–33 r contact is within either the Baculites sp. (smooth) or Baculites sp. (weak flank ribs) ammonite zone in the Western Interior of North America. Direct correlation of the Desmoscaphites erdmanni through Baculites asperiformis Western Interior ammonite zones with magnetobiostratigraphic sections of the upper Santonian–lower Campanian stages in Europe suggests that (i) the Santonian–Campanian boundary as defined in Europe is within the lower Baculites obtusus Zone and (ii) the Western Interior appearance of some ammonites and crinoids (Scaphites hippocrepis, Trachyscaphites spiniger, Marsupites, Uintacrinus) is profoundly time transgressive relative to the appearance of these same species in Europe. The date of 84 Ma for the Santonian–Campanian boundary is too old, and a date of around 80 Ma may be more accurate. The Santonian–Campanian boundary defined by ammonites, crinoids, and belemnites in Europe is within the lower part of chron 33 r, and is younger than the Santonian–Campanian boundary as determined by planktonic foraminifera at Gubbio, Italy.