Multi-Stage Silicification of Pliocene Wood: Re-Examination of an 1895 Discovery from Idaho, USA

Abstract

The 1895 discovery of a petrified tree near Clover Creek in south-central Idaho, USA, attracted worldwide attention and resulted in the naming of a new species of ancient oak, Quercinium pliocaenicum Schuster. For more than a century, the discovery has largely been forgotten, even though specimens reside in reputable museums. Reinvestigation of the locality in 2014/2015 resulted in newly-collected specimens and a wealth of new data. Optical microscopy confirms the cellular anatomy used for the original taxonomic study. X-ray diffraction, scanning electron microscopy, energy-dispersive electron spectroscopy, Raman spectroscopy and cathodoluminescence microscopy reveal details of the mineralization, showing the presence of opal-CT as the primary component, with chalcedony as a lesser constituent. This mineralogy suggests petrifaction occurred in at least two stages, beginning with opalization of cellular tissue, leaving open vessels that became filled with chalcedony during a later mineralization episode. Clover Creek oak represents relict flora growing in a wetter climate before the uplift of the Cascade Range created a rain shadow that caused profound desertification of the inland Pacific Northwest.

Wood Petrifaction: A New View of Permineralization and Replacement

“…SEM images and HF etched specimens show silicification commonly begins at cell walls, but subsequent mineralization may occur via multiple pathways [7][8][9]34,35]. The preservation of relict organic matter is highly variable, but commonly very low.…”

“…During wood petrifaction, where silica precipitation may occur in several stages, with different polymorphs deposited in separate episodes [7][8][9], these silica minerals may undergo diagenetic transformation, e.g., amorphous opal opal-A transforms to microcrystalline opal-Ct, which may convert to chalcedony. When wood has been fully petrified, mineralization involves all the anatomical features, producing the dense, homogeneous material favored by collectors because the material can be polished to high luster.…”

The Bruneau Woodpile: A Miocene Phosphatized Fossil Wood Locality in Southwestern Idaho, USA

“…This possible variation is reminiscent of wood silicification, which can result from a variety of sequential processes [61,62]. SEM images of examples of apatite-mineralized wood from other North American Cenozoic formations are shown in Figure 19.…”

Mineralogy of Non-Silicified Fossil Wood

“…In the latter case, if compositions of pore fluids fluctuate, if Eh or pH conditions shift, or if burial temperatures change, complex mineral assemblages may be produced. This variation is well-documented for SiO 2 polymorphs silicified wood [3,89,90]. The potential for [16] compositional variation may be even greater for non-silica petrified wood, where multivalent ions have the capacity for several oxidation states, and where wood replacement is not dominated by organic templating of silica in cell walls, where mineral precipitation is related to the stereochemistry of functional groups organic constituents of wood tissue.…”