Cretaceous plants from southwestern Colorado

Abstract

In southwestern Colorado a sequence of strata between the Jurassic carnotite-bearing Morrison formation (fMcElmo) and the Upper Cretaceous marine Mancos shale contains fossil plants that are described here and used for determining the most likely position of the Lower Cretaceous-Upper Cretaceous boundary. The plants fall into two groups, the older, with the conifer Frenelopsis varians Fontaine, indicating Lower Cretaceous age, and the younger, with numerous ferns and well-developed dicotyledons, characterizing the early Upper Cretaceous. The boundary is probably at the top of the unit that Coffin called fPost-McElmo and at the base of his overlying Dakota.