Abstract
Late Tertiary fossil woods from the state of Nevada provide an opportunity for observing the mineralization sequences that cause buried wood to become permineralized. Oligocene and Miocene caldera basins contain abundant petrified wood that ranges in composition from incipient silicification to complete permineralization. Examination of specimens from 21 localities reveals that the petrifaction sequence can follow multiple pathways. Fossil wood specimens from a single stratum may have different mineralization; silicification may vary even within a single specimen. Despite these variations, several trends are evident. Features in Nevada specimens suggest that two fundamental processes are involved: early mineralization of cell walls, and later silica deposition in lumina, vessels, and rot pockets from groundwater that permeated these open spaces. The process of open-space filling may be analogous to the genesis of geodes and veins, where multiple episodes of hydrothermal precipitation may produce opal, chalcedony, and quartz as deposits within a single cavity. Silica polymorphs may coexist as primary precipitates, or they may originate from solid-state transformation of a single parent material. Relic lepisphere textures observed in some chalcedony wood specimens are evidence of opal→chalcedony transition. In Nevada, specimens that contain crystalline quartz, this mineral appears to have been formed by direct precipitation in open spaces, not from recrystallization of chalcedony. Opal-A has seldom been reported in fossil wood, but this amorphous material is fairly common in Nevada specimens.
Highlights
Previous interpretations of wood petrifaction have involved several approaches
Opal-A has seldom been reported in fossil wood, but this amorphous material is fairly common in Nevada specimens
The results in this study indicate that opal-A, opal-CT, and chalcedony may occur in vitreous form
Summary
One strategy utilizes observations of silicification of wood in modern hot-springs [1,2,3,4]. A second research strategy involves experimental studies where modern wood is exposed to silica-bearing solutions [5]. A third strategy is based on petrographic observations of petrified wood, often from well-known fossil forests. Paleozoic and Mesozoic silicified woods almost invariably contain cryptocrystalline or microcrystalline quartz as the principal components, in contrast to opal that comprises many Cenozoic woods. This chronologic trend suggests that the composition of many Mesozoic woods have been transformed from their original mineralogy. Cathodolumninescence microscopy has been used in attempts to better understand the mineralogy of Mesozoic specimens [6,7,10,11], but determination of the original mineralization remains subjective
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