DIFFERENT MODES OF WOOD PRESERVATION IN AN ISOTAPHONOMIC SETTING OF THE MORENO HILL FORMATION (TURONIAN, “MIDDLE” CRETACEOUS) OF NEW MEXICO

Wood is generally preserved in one of two ways, through permineralization or as carbonized wood (coalified or charcoalified). These modes of preservation can be found together in the same deposits, however the relative timing of these modes of preservation and the conditions which favor one mode over another are poorly understood (with the exception of charcoal). Knowing how such factors influence wood preservation is important for paleoecological and paleoenvironmental studies which use preserved wood to reconstruct forest densities and paleoclimate. In order to compare the rates and mechanisms controlling the different modes of preservation, it is ideal to study fossil wood in an isotaphonomic setting. This constrains the number of extrinsic factors which can affect the mode of preservation, such as sediment types and regional water chemistry. The terrestrial Moreno Hill Formation presents an isotaphonomic setting in which carbonized and permineralized wood are commonly found within the same sediments.

Methods used to compare and characterize the preservational modes included thin section, reflectivity and maceral analyses, as well as a couple of non-destructive chemical techniques; cathodal luminescence and electron microprobe analysis. Preliminary reflectance analyses of representative samples indicate that the most common forms of carbonaceous wood in the Moreno Hill Formation have a reflectance of about 0.64% Rrandom, indicating that they are high volatile B bituminous rank coals. The samples analyzed were mostly telinite with infilling resinite, and retained cell structure. These coals occur frequently as coal inclusions. Some carbonized wood samples measured have a reflectance of around 2.49% and retain excellent cell structure, indicating that they are samples of charcoal. These samples occur within fine sandstone beds and were transported some distance before deposition. Additionally, field work revealed several examples of the different modes of wood preservation occurring within the same sedimentary layer, and even within the same fossil. Some specimens of coalified stumps were found which contain a permineralized ring of wood within the coal. Preservation in a couple of these exceptional specimens suggests a bias towards permineralization in the sapwood of preserved wood.