THE STRATIGRAPHY AND SEDIMENTOLOGY OF NEW VERTEBRATE LOCALITIES IN THE MENEFEE FORMATION OF NEW MEXICO

The San Juan Basin in northwestern New Mexico contains a relatively complete Late Cretaceous sedimentary sequence. One of the component units, the Allison Member of the Menefee Formation, is an early Campanian floodplain deposit that has produced fragmentary vertebrate material. We recently discovered a very productive area of exposure that has yielded hadrosaur, theropod, ankylosaur, crocodylomorph, and turtle material as well as a substantial amount of petrified wood. The new vertebrate material will increase our knowledge of vertebrate evolution in the Western Interior during the early Campanian, and so it is critical to place it into a stratigraphic and sedimentologic framework. The geologic composition of our field area consists of alternating layers of medium-grained sandstone and organic-rich, laminated mudstone. Vertebrate fossils are abundant in the area and are generally preserved in the mudstone, and less commonly in the sandstone. Sideritic and calcareous concretions are common and are found eroded on the surface or as nodules in the mudstone layers. The geology is consistent with deposition in a fluvial floodplain swamp. In addition to performing a detailed stratigraphic analysis of the localities totaling more than ninety meters of measured section, we have analyzed the fossiliferous layers at the various vertebrate sites using petrographic thin sections and X-ray diffraction. When the stratigraphic columns are compiled, it reveals that the majority of the vertebrate material is found in less than fifteen meters of section in organic-rich mudstone composed primarily of quartz, clay, and feldspars of varying proportions. The main difference among sites is the organization of the minerals relative to each other rather than their composition. However, one site has bone preserved within sideritic concretions; this site produced a very different mineralogic makeup despite the fact that such nodules are found in similar mudstone at other sites. This demonstrates that there were multiple preservational environments in the Allison Member in our area. By understanding the stratigraphy and sedimentology, we will better understand the interactions between the organisms and their surroundings and the factors leading to their preservation.