- freely available
Geosciences 2015, 5(4), 286-309; doi:10.3390/geosciences5040286
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.
Previous interpretations of wood petrifaction have involved several approaches. One strategy utilizes observations of silicification of wood in modern hot-springs [1,2,3,4]. Mineralization occurs rapidly in these geothermal environments, but extremely high dissolved silica concentrations, elevated temperatures, and strongly oxidizing conditions provide a dissimilar match for depositional environments that produce most fossil wood. A second research strategy involves experimental studies where modern wood is exposed to silica-bearing solutions . The practical need to produce silicification in a brief time period commonly involves use of synthetic silica compounds not found in nature (e.g., silanes or organosilicates), and elevated reaction temperatures.
A third strategy is based on petrographic observations of petrified wood, often from well-known fossil forests. Examples include Czech Republic (Late Pennsylvanian) , Chemnitz, Germany (Late Permian) [5,7,8], and Petrified Forest, Arizona, USA (Triassic) . The study of very old petrified wood to infer initial phases of petrifaction poses significant analytical challenges. For example, 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.
This report describes silification processes based on analyses of Late Tertiary wood from 21 locations in Nevada, USA.These specimens record mineralization of plant tissue beginning at incipient stages and continuing thorough subsequent phases that resulted in complete permineralization.
2. Geologic Setting
The widespread abundance of silicified wood in the state of Nevada is related to the availability of silica as a result of episodes of rhyolitic volcanism. A fundamental tectonic process responsible for these volcanic events was the subduction of the Farallon Plate beneath the North America continental margin. Episodic eruptions produced large volumes of rhyolitic tephra that engulfed forests; caldera collapse produced topographic basins that allowed extensive accumulation of volcaniclastic sediments that preserved wood transported into ancient lakes by streams and mudflows. Twenty-three calderas have been identified in the Western Nevada Volcanic Field, and 14 additional rhyolitic ash-flow tuffs that presumably represent as-yet undiscovered calderas . Radiometric ages indicate eruption and collapse of Western Nevada calderas occurred between 34.4 and 23.3 Ma. Calderas in the Southwestern Nevada Volcanic Field have ages ranging from ˃15 Ma to 7.5 Ma. . Ash-flow tuffs in Western Nevada that were deposited during a 2 million year interval between 25.5 and 23.5 Ma . These Oligocene-Miocene caldera deposits contain many fossil wood localities. In some locations, preservation of fossil wood continued into the early Pliocene, as lake basins created by caldera collapse continued to accumulate volcaniclastic sediment eroded from the adjacent uplands.
In northern Nevada, caldera development was triggered by the movement of the North America Plate over the Yellowstone Hot Spot, a body of magma situated in the upper mantle. Volcanic features include the High Rock Caldera Complex [15,16,17] and the McDermitt Caldera [18,19]. These calderas include two of Nevada’s best-known fossil wood deposits: the Lund Petrified Forest (Figure 1) in the Black Rock Desert [20,21] and the precious opal beds at Virgin Valley (Figure 2) in Humboldt County. Other wood occurrences are well-known to collectors [22,23]. Botanical diversity in these deposits resulted from episodes of climatic change that affected local plant communities [24,25]. Nevada leaf floras have received much attention [25,26,27,28,29,30], but only a few studies have been published on permineralized cones  and wood .
Scanning Electron Microscopy (SEM) was performed using a Vega Tescan SEM. Samples were attached to aluminum stubs and sputter coated with palladium to provide electrical conductivity. Elemental analyses were performed using an EDAX energy-dispersive X-ray detector using Genesis software. These spectra were used to determine silica and carbon content in fossil wood, and to identify zeolite, barite, and gypsum as accessory minerals in several specimens. X-ray diffraction patterns were obtained from packed powders with a Rigaku Geigerflex diffractometer using Ni-filtered Kα radiation. Specimens used in this study (Figure 3, Table 1) are archived at the WWU Geology Department. Specific sites are not described in this report to protect research localities from recreational collecting. Schorn et al. provide information on Nevada paleontology locations .
|1||McDermitt||Humboldt Co., NV/Malheur Co./OR||Middle Miocene||Chalcedony|
|2||Virgin Valley||Humboldt||Middle Miocene||opal-A, opal-CT|
|4||Blowout Mountain||Washoe||Middle Miocene||opal-CT, chalcedony, quartz|
|6||Black Rock Desert||Washoe||Middle Miocene||opal-CT|
|7||Goose Creek||Elko||Middle Miocene||opal-CT, quartz. Chalcedony|
|8||Texas Spring||Elko||Middle Miocene||Chalcedony|
|11||Hubbard Basin||Elko||Middle Miocene||Chalcedony|
|12||Cherry Creek||White Pine||Oligocene||Chalcedony|
|15||Rawhide||Mineral||Middle Miocene||opal-CT, quartz|
|17||Gabbs||Nye||Lower Miocene||opal-CT, quartz|
|20||Miller Mountain||Mineral||Lower Miocene||opal-CT|
|21||Stonewall Pass||Nye||Lower Miocene||Chalcedony|
4. Forms of Silica
Silicified wood may contain any of four silica polymorphs, forms of SiO2 that have different crystallinity. These forms include opal-A (amorphous), opal-CT (incipient crystallization of cristobalite and tridymite), chalcedony (micro-fibrous quartz), and microcrystalline quartz. Opal-A commonly encrusts and permeates wood in modern hot springs, but it rarely been reported in petrified wood. However, opal-A is present at several Nevada Late Tertiary locations. “Opalized” wood usually consists of opal-CT. “Agatized” wood, the variety most prized for lapidary purposes, is mineralized with chalcedony. Less commonly, silicified wood contains microcrystalline quartz.
These silica polymorphs can be recognized by a variety of analytical methods, although no single method provides reliable characterization. X-ray diffraction (Figure 4) provides structural information, with two major limitations. Pure opal-A can be recognized by the absence of diffraction peaks, but the presence of this amorphous silica is not evident when other crystalline phases are present. Opal-CT yields a distinctive weak diffraction pattern, but quartz and chalcedony usually cannot be distinguished, because chalcedony is structurally a fibrous variety of cryptocrystalline quartz intergrown with varying amounts of moganite, a monoclinic SiO2 polymorph.
Polarized light optical microscopy is useful for identifying quartz and chalcedony, but is seldom reliable for distinguishing opal-A from opal-CT. Higher magnifications available the scanning electron microscope are useful for discerning these silica polymorphs. Secondary electron images primarily show topography. Opal-A can be recognized as smooth-surfaced lepispheres, with individual diameters in the range of 0.5–2 microns (Figure 5A–C), and as smooth-surfaced botryoidal encrustations (Figure 5D). Opal-CT commonly exists as tabular microcrystals (Figure 6). Both varieties of opal may also occur as porous or vitreous masses. Opal-A has seldom been reported in fossil wood, but this amorphous material is fairly common in Nevada specimens. Opal-A is a major constituent of siliceous hot spring deposits, and the presence of this mineral in wood from Nevada caldera deposits is perhaps related to warm groundwater temperatures caused by local volcanism. Opal of both types is usually considered to represent an early stage in the silicification process, but an Eocene Texas locality has been reported where opal-CT is inferred to have been produced during weathering of quartz-mineralized wood .
Chalcedony often appears as a vitreous solid mass that may preserve anatomical details as color variations visible in transmitted light optical microsocopy (Figure 7E,F). At high magnification, the fibrous structure of chalcedony may be visible (Figure 7A,B). Anatomical structure may be visible in SEM images in specimens where intercellular spaces have not been mineralized (Figure 7C). Chalcedony may also form layers lining cavities or fractures (Figure 7D).
Quartz can be recognized as hexagonal crystals. These usually occur as coatings on fracture surfaces or fillings in cavities (Figure 8). Development of euhedral crystals requires open space. When quartz is present in Nevada specimens, microtextures suggest that the mineral was precipitated as a late-stage hydrothermal precipitate, not as a primary component of petrifaction. In other regions, quartz crystallization has sometimes been interpreted as a primary stage in wood petrifaction [6,10,35,36].
5. Anatomical Preservation
Preservation of cellular detail is highly variable among specimens from different localities, and even among individual specimens from a single locality. Anatomical preservation is related to the rate of mineralization vs. the rate of tissue degradation. Limb casts result when opal or chalcedony is deposited within a void left by a twig that has been completely destroyed, leaving only a mold in volcanic or sedimentary rocks. More commonly, gradual mineralization occurs in tissue that is simultaneously experiencing microbial degradation. Some Nevada specimens show evidence of this degradation (Figure 7C,D and Figure 8C). However, anatomical details are commonly well-preserved. During the life of a plant, a primary function of wood is the conduction of nutrient-bearing water, and after death and burial this conductive architecture favors petrifaction. Groundwater is readily absorbed by tracheids, and transmitted from cell to cell by circular pits. The resulting permineralization may replicate cellular anatomy with high fidelity (Figure 9).
The preceding figures illustrate the range of mineral compositions and textures present in Late Tertiary woods from Nevada localities. These photographs were selected as representative examples from more than 500 SEM photos made from specimens from 21 different localities. Figure 5, Figure 6, Figure 7 and Figure 8 show features observed in multiple specimens, commonly including specimens from at least two localities. The data have several implications for understanding how petrifaction occurs.
Once wood has been complexly mineralized, the original mineralization sequence is seldom evident. The Late Tertiary localities in this study include many specimens that are in early or intermediate stages of petrifaction. Nevada specimens provide clear evidence that wood silicification does not proceed along a single pathway. Even at a single locality, mineralogy may vary among individual specimens, or even within a single specimen. Indeed, mineralization may vary from one cell to the next. Despite these variations, some general compositional trends can be observed at wood from various localities. As a result, evidence from Nevada petrified wood provides important new insights into the silicification process.
Observations and interpretations are summarized as follows:
6.1. Silicification Commonly Begins with Precipitation of Silica within Cell Walls, with Lumina Remaining Unmineralized (Figure 10)
Mineralization processes are related to anatomical structure. Because fluid transport is a primary function of this stem tissue, tracheids readily allow groundwater to permeate buried wood. The typical first step in petrifaction is precipitation of silica within cell walls, facilitated by the affinity of lignin and hemicellulose for silica . Silicified cell walls and empty lumina are very common features of Nevada Late Tertiary wood. Presence of silica in cell walls can be by recognized by Si peaks in X-ray fluorescence spectra (SEM/EDS analysis), and by visual evidence of silica polymorphs at high magnification.
Subsequent mineralization occurs when silica is deposited in cell interiors (lumina), vessels, spaces between adjacent cells, and voids caused by decay or insect damage. Some Nevada localities contain wood with lumina filled with porous opal (Figure 5D and Figure 11). The sequence of mineralization is uncertain for these specimens, but they may provide evidence that precipitation of opal can sometimes occur in cell lumina very early in the silicification process. This porosity allows the possibility of later penetration of mineral-bearing groundwater.
SEM secondary electron images predominately show surface topography, and silica minerals are most likely to show characteristic features when these materials border open spaces, allowing three-dimensional expression of structural characteristics. Commonly, silica is present in smooth-textured porcelain-like form, as seen in SEM images. However, many massive specimens contain local regions that have topography useful for recognizing silica phases. X-ray diffraction also provides useful information for distinguishing between silica polymorphs. The results in this study indicate that opal-A, opal-CT, and chalcedony may occur in vitreous form. Although vitreous regions may appear relatively featureless in SEM images, color variations may reveal anatomical details when the specimens are examined by optical microscopy (Figure 7C,D).
6.3. Intercellular Spaces Commonly Remain Unmineralized (Figure 13)
The exchange of silica-bearing groundwater occurs relatively easily among tracheids because of the presence of inter-cellular pits (Figure 9). Spaces between tracheids are less likely to be exposed to groundwater, which may explain why these spaces commonly remain unmineralized in Late Tertiary wood, even though cell walls and lumina are well-silicified.
6.4. Mineralization May Be Different among Individual Cells within a Single Small Specimen (Figure 14)
One of the most notable characteristics of Nevada Late Tertiary woods is the wide variability of mineralization. Mineralogy may vary among individual specimens, or even among adjacent cells.
The Virgin Valley Formation (locality 2) provides an excellent example. Wood is preserved in middle Miocene lacustrine sediments having a total stratigraphic thickness of approximately 300 m [38,39,40,41]. The middle zone of the formation varies in thickness from 58 to 85 m, and silicified twigs and occasional logs are found in random throughout this interval. Nearly a century of opal mining has resulted in numerous excavations within these strata; more than 200 specimens were obtained for this study, collected from the major opal mines and many smaller claims. Virgin Valley specimens are illustrated in many of the figures. Some of the best stratigraphic exposures occur at Royal Peacock Mine, where a 2 m thick wood-bearing section is exposed over a lateral distance of more than 100 m. Within this single horizon, fossil wood is abundant, including carbonized wood, ordinary opal-CT wood, and the rarest and most-prized material: “fire opal”, wood mineralized with well-ordered opal-CT lepispheres (Figure 2). Mineralogy may be variable within a single limb, and even from one cell to the next.
6.5. Carbonized Wood Is Unlikely to Become Silicified (Figure 15)
Wood buried in an anaerobic environment (e.g., impervious clay) may be protected from microbial degradation, preserved in an unmineralized state [42,43]. Carbonized wood specimens were observed at only one locality, the Royal Peacock opal mine at Virgin Valley (locality 2). These specimens commonly consist of organic tissue that had been reduced to carbon, with later precipitation of opal-CT lepispheres or clay minerals in some of the lumina. These siliceous minerals are not bonded to the adjacent carbonaceous cell walls.
6.6. Non-Silica Minerals May Be Present (Figure 16)
If groundwater contains other components besides silica, other minerals may be precipitated during petrifaction. At Virgin Valley, zeolites are a common accessory mineral, typically occurring in cells in the outer regions of wood where chemical breakdown of tephra provide a source of ions to recombine as zeolites in cell lumina. Intergrowths of zeolite and opal-Ct demonstrate that the two minerals were forming contemporaneously (Figure 16A,B). At other sites, gypsum (location 16) and barite (location 11) were observed as accessory minerals (Figure 16C,D). The intergrowth of opal-CT and zeolite are evidence that these minerals were concurrently precipitated. Barite and gypsum appear to have formed on silicified tissue as a result of secondary mineralization.
6.7. Primary Precipitation versus Diagenetic Transformations
Remaining observations relate to the common belief that wood silicification follows a transformational sequence, where opal is the initial silica phase, with chalcedony and quartz originating as successive transformations during long burial [44,45,46,47]. Evidence for this hypothesis is largely circumstantial. Diagenetic transformation of opal-A→opal-CT→quartz is well-documented for two geologic environments: biogenic sediments [48,49,50,51,52,53,54,55,56] and siliceous hot spring sinter [57,58,59,60,61,62,63,64,65,66,67].
If gradual mineral transformation is a common process during petrifaction, it would seem likely that many deposits would reveal petrified wood that contains both opal and chalcedony. However, this association is surprisingly rare. Opal-CT and chalcedony coexist in late Eocene specimens from the Florissant, Colorado fossil forest, but these silica phases appear to have been formed during separate stages of mineral deposition, not as a transformative sequence . Nevada specimens provide further evidence that silicification may proceed along a variety of pathways.
6.8. Multiple Silica Phases May Coexist as a Result of Successive Episodes of Precipitation (Figure 17)
Some individual Nevada wood specimens contain several forms of silica that appear to have formed in separate precipitation events. One example is the presence of opal-A or opal-CT in different physical forms (Figure 17A,C). Quartz and opal-CT typically coexist because quartz was deposited in fractures or open other spaces in wood that had previously been permineralized with opal. Evidence for this hypothesis comes from the presence of quartz along fracture planes that crosscut botanical structures, a geometry that can only occur if the wood has been rendered brittle because of mineralization.
Based on comparison to geodes and vein deposits, in these circumstances, mineral deposition may be related to dissolved silica concentrations. High silica concentrations favor rapid precipitation of opal or chalcedony; low silica concentrations favor precipitation of quartz, when slow crystallization rates allow formation of well-ordered lattices and open spaces allow development of terminated crystals. Quartz has been observed as a primary agent of petrifaction at a few locations in other regions, e.g., the Eocene Yegua Formation of Texas  and Cretaceous and Paleocene wood in Alabama . Like crystalline quartz, chalcedony may occur as a filling material in vugs and fractures, (Figure 17D); chalcedony is the main constituent of “agatized” wood at locations 1, 3, 9–12, and 21.
6.9. Relict Textures Suggest Opal May Transform to Chalcedony (Figure 18)
Opal and chalcedony rarely occur together in Nevada specimens, though chalcedony mineralized wood is very common. Several locations yielded specimens where chalcedony appears to preserve relict lepispheres that are evidence of the original opal mineralization. This mode of occurrence contrasts the chalcedony that appears to have formed as a primary mineral (e.g., Figure 8C and Figure 17D).
Nevada 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.
Nevada Late Tertiary specimens yield information that is markedly different from some previous hypotheses based on studies of Paleozoic and Mesozoic wood. For example, Nevada fossil wood commonly has permineralized cell walls, with lumina remaining empty (Figure 10). 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. In contrast, the empty lumina conflict with hypotheses that assert silicification begins with precipitation of porous silica within cell lumina [11,70,71,72,73].
Another discrepancy involves the possibility of quartz as a primary agent of silicification. In Permian wood from Chemnitz, Germany, cell walls contain quartz crystals and cell lumina are filled with cryptocrystalline silica. 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 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). 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. Though chalcedony has not yet been synthesized as a primary silica phase in laboratory, this silica polymorph commonly occurs as a primary phase in geode deposits from many locations in North America (example: Figure 19A).
Incipient silicification involves organic templating within cell walls, but subsequent episodes of silicification mostly involve open space filling. The most useful analogy may be the precipitation of silica in geodes and veins, where silica is deposited in successive layers within empty spaces in a host rock . For vein deposits, these spaces are created by brittle fracturing. For geodes, the spaces may be vesicles in a volcanic rock, or erosional spaces in a sedimentary rock. Mineralization occurs when groundwater enters via fractures or porosities in the host rock. This physical setting is similar to the absorption of groundwater by buried wood, the main difference being the relative sizes and shapes of the open spaces. Tracheids and vessels typically have diameters of 50 microns or less, with tube-like architecture. Variations in mineralogy within these cells may result from microhydrology. The flow of silica-bearing groundwater into tracheids and vessels is influenced by anatomical characteristics. Damage to pits or cell walls may inhibit permeability. Other preservational factors may also play a role, e.g., crushing of tissue, or local decay. These structural variations may explain specimens of the type illustrated in Figure 14, where neighboring cells have differing mineralogy.
The geode/vein analogy accounts for many instances where a single fossil wood specimen contains several different silica polymorphs. This phenomenon is common in geodes, where successive episodes of mineralization may precipitate opal, chalcedony, or quartz, sometimes in interlayered form (Figure 19 and Figure 20). Veins may likewise be filled with several different minerals precipitated in separate events. Physical and chemical factors that control mineral formation may include the concentration of dissolved silica, pH, and temperature. Crystalline quartz is commonly present in geodes and veins as a primary product, not derived from an earlier silica precursor. Well-terminated quartz crystals are likely to form when there is empty interior space to allow unrestricted growth.
The most important inference that can be made from Nevada specimens is that wood silicification does not follow a single pathway. Instead, mineralization may result from a diverse variety of processes, including successive stages of silica precipitation under varying geochemical conditions, and diagentic transformation of opal to chalcedony.
This study greatly benefitted from permission from property owners to visit opal mines at Virgin Valley, Nevada in 2004. I particularly thank Joy and Harry Wilson (Royal Peacock Mine), John Church (Swordfish Mining), Glenn Hobson (Rainbow Ridge Mine), and Pat and Scott Ryals (Opal Negra Mine). Informal conversations with mine owners were a valuable source of geologic and historical information regarding the opal deposits. Nevada petrified wood specimens were contributed by many individuals. Richard Rantz, Jim Mills, and Patrica Caplette were particularly generous. Western Washington University Scientific Technical Services staff members Erin Macri and Charles Wandler deserve special credit for their expertise in maintaining the scanning electron microscope at Western Washington University, and for providing many hours of access to this instrument. I thank Thomas Yancey and Chris Ballhaus for providing detailed manuscript commentary, supplementing helpful contributions from three anonymous reviewers. This paper is dedicated to the late Howard Schorn (1933–2013). A Museum Associate at University of California Museum of Paleontology at Berkeley, Howard Schorn was a leading expert on Nevada plant fossils who always generously shared his extensive knowledge.
Conflicts of Interest
The author declares no conflict of interest.
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