PALEOENVIRONMENTAL CONTROLS ON ANGIOSPERM ABUNDANCE AND DIVERSITY IN THE FRUITLAND-KIRTLAND FORMATION, SAN JUAN BASIN, NEW MEXICO

A macrofloral assemblage within the Late Cretaceous Fruitland and Kirtland Formations in northwestern New Mexico was examined for abundance and diversity within a detailed sedimentological and paleoenvironmental framework. This provided the ability to compare plant habitat preferences along a Cretaceous floodplain landscape, and characterize those areas within the floodplain that preserve plant material. Twenty-five sites were sampled for morphotypes as a measure of species richness and vegetative cover as a measurement of abundance. At each site sedimentological data from plant-bearing beds and adjacent strata were collected and microsections were completed. Sedimentary samples were analyzed for total organic carbon and grain size distribution. Based on this qualitative and quantitative information, the sites were classified into six different subenvironments. When combining the total results of the sites, angiosperms were the most diverse and abundant group. However, when comparing the sites to one another based on their designated floodplain subenvironments, variations in taxonomic diversity and ecological composition discern significant differences among the sites. Multivariate analyses support the correlation of different morphotypes to particular subenvironments on the floodplain. Dicot morphotypes, such as those in the Lauraceae, were both the most abundant and diverse group at sites interpreted as swamps with an influx of coarser-grained sediment. Overbank splay-levee sites proximal to the channel were dominated by dicots, but also supported a variety of other monocot, conifer and fern morphotypes. Angiosperms were least abundant at ashfall sites in distal backswamps. In these sites pteridophytes, such as those in the Osmundaceae and Schizaeaceae, were the most dominant and diverse. By using a combination of both detailed sedimentological and paleobotanical data, environmental characteristics that were favorable for the preservation of different plant taxa can be determined. In particular, angiosperms were most diverse and abundant in subenvironments that were proximal to the channel and/or at more distal sites undergoing disturbance. These environmental factors have important implications for understanding the radiation of angiosperms during this time period.