Inoceramid fauna and biostratigraphy of the upper Middle Coniacian–lower Middle Santonian of the Pueblo Section (SE Colorado, US Western Interior)

The Middle Coniacian to Middle Santonian inoceramid succession of the Pueblo section, southeastern Colorado, US Western Interior, is reviewed in the context of the zonal scheme applied in Europe. The Middle Coniacian record starts with the upper Middle Coniacian Volviceramus involutus Zone, because the base of the succession is characterised by a hitherto unsuspected stratigraphical gap, spanning the topmost Lower and lower Middle Coniacian in European inoceramid terms. The base of the Upper Coniacian is recognised by the entry of Magadiceramus; this substage is divided into a Magadiceramus subquadratus Interval Zone and a Magadiceramus crenelatus Taxon Range Zone. The base of the Santonian is taken at the base of the Cladoceramus undulatoplicatus Taxon Range Zone. The Middle Santonian is assigned to the Cordiceramus bueltenensis Zone, and the base of the Upper Santonian is placed at the base of the zone of Cordiceramus muelleri. The inoceramid faunas of the Pueblo succession are similar to those known from southern Europe, south of the regular occurrence of sphenoceramids. The apparent differences reported in the previous literature between the inoceramid faunas of Europe and of the Western Interior at this level resulted mainly from differences in the taxonomic concepts applied. The new data enable correlation between the proposed inoceramid zonal scheme and the scaphitid ammonite zones.