Biostratigraphy of the Campanian-Maastrichtian, Mid-Maastrichtian, and Cretaceous-tertiary boundaries in the Raton Basin: Implications for models of early Laramide tectonism and for the origin of the enigmatic Disturbed Zone

Abstract

The Campanian-Maastrichtian stage boundary is placed within the upper part of the Pierre Shale about 100 m below the base of the Trinidad Sandstone at Berwind Canyon, Colorado. Correlation of ammonite biostratigraphy with palynostratigraphy places this boundary near the top of the Vermejo Formation in the southwestern part of the Raton Basin, northeastern New Mexico. The informal mid-Maastrichtian boundary should be placed at the level of the Trinidad Sandstone at Berwind Canyon, although it should be placed at the level of the Vermejo-Raton unconformity in northeastern New Mexico based upon palynostratigraphy. High-resolution megafloral biostratigraphy of the K-T boundary is also discussed. Chronostratigraphic and paleoenvironmental indicator species reveal differential sedimentation rates across the basin. The magnitude of the Vermejo-Raton unconformity varies primarily in a north-south direction across the Raton Basin and was greatest in northeastern New Mexico and least in south-central Colorado, which may affect interpretations of the early Laramide history of the Raton Basin. The results of this study preclude correlation of the basal part of the Vermejo Formation with the Disturbed Zone of southwestern South Dakota.