Fossil wood from the middle Cretaceous Moreno Hill Formation: Unique expressions of wood mineralization and implications for the processes of wood preservation

Abstract

Abstract The Cretaceous (Turonian) Moreno Hill Formation of west–central New Mexico provides an opportune isotaphonomic setting to study wood preservation because both carbonized and mineralized wood are commonly found within the same sediments and often within individual specimens. Petrographic, reflectance, maceral, and chemical analyses were used to compare and characterize mineralized and carbonized wood from the study site to investigate biases which influence mode of preservation. Coal (in the form of coal inclusions) is the most common form of fossil wood, and mineralized wood is also widespread in the formation. Two striking specimens of in situ coalified stumps are particularly informative because each includes a mineralized ring that appears to represent the former location of sapwood. Most of the mineralized samples are primarily composed of quartz, but a few specimens are predominantly mineralized with apatite. Calcite is present in charcoal samples where it permineralized, but never replaced organic carbon. This study points to two key factors that influence mineralization; wood permeability and the chemical alteration of the wood prior to incorporation into the sediment. The patterns of mineralization are consistent with hypothesized microbial involvement in the mineralization process. The coexistence of coal and mineralized tissues in some samples suggests that the processes of silicification and phosphatization co-occurred with coalification and that these processes are competitive within individual pieces of wood. As such, various stages of wood degradation have been preserved in mineralized samples from the Moreno Hill Formation. The study suggests that coal inclusions and mineralized wood/coal associations may also be common in other ancient sediments even though weathering may obscure their presence. Such information is important for paleoecological and paleoenvironmental studies which utilize the occurrence of fossil wood to help reconstruct floral composition or climatic conditions.