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

Abstract

Abstract:The Bruneau Woodpile site has long been popular among fossil collectors; however, the deposit has received scant attention from scientists. Our research reveals that the fossilized wood was deposited ca. 6.85 Ma, within the Chalk Hills Formation, and was mineralized with carbonate-fluorapatite. The diverse assemblage of conifers and hardwoods is representative of the warm temperate forests that flourished in southwest Idaho, USA during the late Miocene. Limb and trunk fragments preserved in a single thin sandstone bed appear to represent woody debris that was transported by streams. One possible explanation is that wood, pumice, and sandy volcaniclastic sediment arrived separately as a result of ordinary stream action, and later were combined into a single assemblage during a subsequent high-energy sedimentation event. We favor an alternate hypothesis: a catastrophic event (e.g., a windstorm) damaged trees on slopes bordering the ancient lake. Branches and small trunk fragments were carried by wind and rain into local streams and ponds where they became waterlogged. After a delay that allowed pumice and wood to become saturated, storm water transported these materials, along with finer volcaniclastic sediment, into a lake. The resulting density current produced a fining-upward sedimentary cycle where wood was preserved in the lowest, coarsest stratum.

Neogene Tree Trunk Fossils from the Meshgin Shahr Area, Northwest Iran

“…Research from other regions provides abundant examples of petrified wood that was resulted from multiple episodes of mineral deposition [49,[63][64][65][66][67][68][69]. At Meshgin Shahr Fossil Forest, mineral features suggest that the following processes were involved in wood petrifaction:…”

“…In both specimens, the presence of these minerals is evidence that cell interiors (lumen) remained open after the cell walls had been mineralized with opal. Calcium phosphate has previously been reported as a major constituent in fossil woods that range in age from Carboniferous to Holocene (summarized by [69]), and as a minor constituent of Eocene carbonized wood in Eocene wood from California, USA [70]. The precipitation of calcium phosphate is favored by the presence of dissolved Ca and P, and low pH that favors the precipitation of Ca 5 (PO 4 ) 3.…”

Mineralogy of Non-Silicified Fossil Wood

“…Abundant phosphatized wood in Miocene lakebeds at Bruneau, Idaho, USA are homogeneous in composition, consisting of carbonate-fluorapatite [28]. At this locality, fossilized driftwood ( Figures 4B and 5A) occurs within a single thin sandstone bed that was deposited as a cataclysmic debris torrent [28].…”

“…At this locality, fossilized driftwood ( Figures 4B and 5A) occurs within a single thin sandstone bed that was deposited as a cataclysmic debris torrent [28]. The fossil wood displays moderately well-preserved anatomical detail ( Figures 5B and 6).…”

“…These fossil limbs were recognized as being non-silicified because of their low hardness, and in the absence of analytical data they were described in as being mineralized with calcite and gypsum [91]. In 2017, the Bruneau wood was finally correctly identified as being composed of carbonate-fluorapatite based on XRD and SEM/EDS data [28]. In summary, future research can follow two possible paths: a search for new fossil sites, and careful analyses of specimens in museums and research collections.…”