Baculites (Ammonoidea) and the age of the Pierre Shale in the eastern Raton Basin, south-central Colorado

The Pierre Shale is a marine deposit that accumulated in the Cretaceous Western Interior Seaway during the onset of Laramide tectonism in the southern Rocky Mountains region. In the eastern part of the Raton Basin near Trinidad, Colorado, ammonite biostratigraphy suggests that the base of the Pierre Shale lies within or slightly above the Lower Campanian Scaphites hippocrepis III ammonite range zone (81.8–80.5 Ma), and the top of the Pierre Shale corresponds with the Lower Maastrichtian Baculites clinolobatus Zone (69.59 + 0.36 Ma). These data are consistent with previous estimates for the age of the base of the Pierre Shale in the Raton Basin, and indicate that the top of the Pierre Shale (base of the overlying Trinidad Sandstone) in the eastern part of the basin lies near the Lower-Upper Maastrichtian substage boundary. A Late Maastrichtian age for the Trinidad Sandstone near Trinidad has a bearing on the timing of geological events associated with the eastward retreat of the Western Interior Seaway from the region during the Late Cretaceous. Introduction The Pierre Shale was deposited within the Western Interior Seaway during an early phase of Laramide tectonism, which resulted in areas of regional subsidence and uplift associated with subduction of the Farallon tectonic plate beneath western North America (Baltz, 1965; Cather, 2004; Slattery et al., 2015; Heller and Liu, 2016). In the Raton Basin of northeastern New Mexico and south-central Colorado (Fig.1), the Pierre Shale is underlain by the Niobrara Formation and overlain by the Trinidad Sandstone (Lee, 1917). The age of the base of the Pierre Shale is constrained by ammonite biostratigraphy of the uppermost part of the Niobrara Formation in the Raton Basin and surrounding regions (Scott et al., 1986; Molenaar et al., 2002; Merewether et al., 2011). The age of the top of the Pierre Shale is similarly constrained by ammonite biostratigraphy of the upper part of the formation.