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

摘要

Wood is generally preserved in one of two ways, through mineralization or as carbonized wood (coalified or charcoalified). The objective of this study is to investigate the biases which influence whether wood is mineralized. Such information is important for paleoecological and paleoenvironmental studies which aim to reconstruct floral diversity or climatic conditions from fossil wood. The Cretaceous (Turonian) Moreno Hill Formation provides an opportune isotaphonomic setting to study wood preservation because carbonized and mineralized wood are commonly found within the same sediments and even within the same specimens.; The study area is in the Zuni Basin of west central New Mexico. The distribution and types of fossil wood were noted, and petrographic, reflectance, maceral, and chemical analyses were used to compare and characterize mineralized and carbonized wood collected from the Moreno Hill Formation. Coal is the most common form of fossil wood (in the form of coal inclusions) and mineralized wood is also widespread. Two striking specimens of in situ coalified stumps were found which contain a mineralized ring of wood within coalified wood. Analyses of 13 specimens demonstrated that most of the mineralized samples are primarily composed of quartz, and that a few specimens were primarily mineralized with fluorapatite. Calcite only occurred as cement in sediment or in charcoal samples where it permineralized, but never replaced, organic carbon. This study revealed that factors which affect wood mineralization include wood permeability and the chemical alteration of wood prior to mineralization. Permeability of the wood appeared to affect the likelihood of wood mineralization, and chemical alteration of wood prior to mineralization affected the pattern of mineralization and the mineral phase present. Furthermore, the pattern of mineral and carbon in some samples suggests that mineralization by quartz and fluorapatite likely co-occurred with the process of coalification and that these processes were competitive. The rate of mineralization was apparently variable, resulting in the variety in the state of degradation observed in mineralized wood samples.