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

Mike Viney,George Mustoe,Paul Link,Thomas Dillhoff

doi:10.3390/geosciences7030082

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.

Highlights

The Bruneau Woodpile is an informal name used by petrified wood aficionados to describe a fossil collecting locality in southwest Idaho, approximately 20 km south of the town of Bruneau (Figure 1 [1])
This long-accepted terminology is confusing because the Bruneau Formation, is a younger (Pleistocene) stratigraphic unit; Bruneau Woodpile occurs in strata of the late Miocene Chalk Hills Formation (Figure 2 [2])
The Bruneau Woodpile is an unusual deposit, providing a rare opportunity to collect abundant specimens of fossil wood mineralized with carbonate-fluorapatite

Summary

Introduction

The Bruneau Woodpile is an informal name used by petrified wood aficionados to describe a fossil collecting locality in southwest Idaho, approximately 20 km south of the town of Bruneau (Figure 1 [1]).This long-accepted terminology is confusing because the Bruneau Formation, is a younger (Pleistocene) stratigraphic unit; Bruneau Woodpile occurs in strata of the late Miocene Chalk Hills Formation (Figure 2 [2]). The Bruneau Woodpile is an informal name used by petrified wood aficionados to describe a fossil collecting locality in southwest Idaho, approximately 20 km south of the town of Bruneau (Figure 1 [1]). Taxonomic affinities of the Bruneau Woodpile specimens supplement pollen and leaf studies of similar aged deposits that help establish the vegetative types making up late Miocene ecosystems of southwest Idaho [3,4,5,6]. This is the first scientific investigation that provides a comprehensive overview of this unusual North American fossil wood locality. Our research focuses on two puzzling aspects: the occurrence of limb and small wood fragments in a single stratum within a stratigraphic section that otherwise contains no petrified wood, and the phosphatic mineralization of wood

Methods

Results

Discussion

Conclusion