Paleoenvironments include wood transported by lahars (Ginkgo Petrified Forest State Park), fluvial and palludal environments (Saddle Mountain and Yakima Canyon fossil localities), and standing forests engulfed by advancing lava (Yakima Ridge fossil forest). These fossil localities are well-dated based on radiometric ages determined for the host lava. Silicified wood occurs abundantly in Middle Miocene flows and sedimentary interbeds of the Columbia River Basalt Group (CRBG) in central Washington State, USA. Our report describes the geologic setting and our research mineralogy of specimens at both localities, and discusses conservation strategies. The environmental complexities at the Tak and Khorat fossil wood localities challenge conservators, but their creative attempts provide useful lessons for future preservation efforts. Many specimens are preserved indoors in the Khorat Fossil Museum in Mueang Nakhon Ratchasima District, both as indoor exhibits and as outdoor displays in a garden plaza. At Khorat, compositions of individual specimens range from pure quartz and pure opal to mixtures of the two polymorphs. A variety of methods have been employed in attempts to reduce the damage, including the construction of various styles of shelters to protect the fossil logs from direct precipitation. These factors make the fossils vulnerable to weathering.
Excavation of the huge logs means that they occupy topographically low positions prone to flooding during the monsoon season. Hydration swelling of these components results in stress. The resulting permeability allows penetration of moisture, and allows introduction of microbes and the accumulation of clays, iron oxides, soluble salts and other materials that may cause discoloration and deterioration. Incomplete mineralization is characteristic of the Tak wood, with intercellular spaces commonly remaining open. At Ban Tak fossil logs are primarily mineralized with crystalline quartz. Deposits in the Khorat Plateau in northeast Thailand contain a multitude of wood fossils that span a probable age range of Miocene to Pleistocene. Pleistocene fluvial sediments in the Tak region in the northern highlands contain silicified trunks of large trees. Thailand contains two notable fossil forest regions. Cryptocrystalline quartz (chalcedony) in cell walls appears to have formed from a process of diagenetic transformation from an opaline parent material, but chalcedony in lumina, vessels, and rot cavities sometimes originated as a primary precipitate. In Nevada specimens, crystalline quartz was only observed as a secondary mineral precipitated in fractures and void spaces, though in these locations quartz crystals may coat individual fibers that face the open areas (e.g., Figure 8A,B). This mineralogy has been interpreted as evidence that quartz precipitated on oriented cellulose fibrils within the cell wall, and that larger quartz crystals grew in open spaces.
In Permian wood from Chemnitz, Germany, cell walls contain quartz crystals and cell lumina are filled with cryptocrystalline silica.
Another discrepancy involves the possibility of quartz as a primary agent of silicification. In contrast, the empty lumina conflict with hypotheses that assert silicification begins with precipitation of porous silica within cell lumina. This observation is consistent with the experimental studies that show silica is actively precipitated in cell walls because of the chemical affinity of lignin and cellulose for silicic acid. For example, Nevada fossil wood commonly has permineralized cell walls, with lumina remaining empty ( Figure 10).
Nevada Late Tertiary specimens yield information that is markedly different from some previous hypotheses based on studies of Paleozoic and Mesozoic wood. petrified wood localities provide an important opportunity for studying the fossilization process in Late Tertiary wood, where the sequential steps in the mineralization process can be directly observed, rather than attempting to reconstruct early mineralization sequences by inference from wood from older deposits that may have had a complex diagentic history.
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