geosciences

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

Mike Viney 1,*, George E. Mustoe 2 ID , Thomas A. Dillhoff 3 and Paul K. Link 4 1 College of Natural Sciences Education and Outreach Center, Colorado State University, Ft. Collins, CO 80523, USA 2 Geology Department, Western Washington University, Bellingham, WA 98225, USA; [email protected] 3 Evolving Earth Foundation, P.O. Box 2090, Issaquah, WA 98027, USA; [email protected] 4 Department of Geosciences, Idaho State University, Pocatello, ID 83209, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-970-481-0225

Received: 3 June 2017; Accepted: 5 September 2017; Published: 9 September 2017

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.

Keywords: apatite; carbonate-fluorapatite; Chalk Hills Formation; Chalk Hills Lake; ancient Lake Idaho; Idaho; Mount Saint Helens; Snake River Plain; phosphatized wood; paleobotany; Bruneau Woodpile; petrified wood

1. 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 (Figure1[ 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 (Figure2[ 2]). Located near the town of Bruneau, Idaho, the site preserves abundant petrified wood in the form of limbs and driftwood mineralized with carbonate-fluorapatite. The fossil wood assemblage contains a diverse mix of at least 15 angiosperm and gymnosperm types. 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 [3–6]. 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.

Geosciences 2017, 7, 82; doi:10.3390/geosciences7030082 www.mdpi.com/journal/geosciences Geosciences 2017, 7, 82 2 of 25 Geosciences 2017, 7, 82 2 of 25 singleGeosciences stratum2017, 7 ,within 82 a stratigraphic section that otherwise contains no petrified wood, and2 ofthe 25 single stratum within a stratigraphic section that otherwise contains no petrified wood, and the phosphatic mineralization of wood. phosphatic mineralization of wood.

Figure 1. Location map adapted from Lewis et al. [1]. Figure 1. Location map adapted from Lewis et al.al. [[1].1].

Figure 2. Generalized Stratigraphic Section [2]. Figure 2. Generalized Stratigraphic Section [[2].2].

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1.1. Previous Work In the late 1960s, Lt. Col. Quincy D. Howell and J. V. Root enlisted the help of members of the GeosciencesGem Club2017 ,of7, Boise 82 to identify fossil wood from Bruneau Woodpile and other sites in southern Idaho3 of 25 and eastern Oregon. This group published their findings in the October 1971 issue of Gems and Minerals [7]. They concluded that the wood had washed into an ancient lake during the late Pliocene. 1.1.In Previous the absence Work of analytical data, the petrified wood was presumed to consist of calcium sulfate (CaSOIn the4) and late calcium 1960s, Lt.carbonate Col. Quincy (CaCO3 D.). While Howell polished and J. V.transverse Root enlisted wood thesections help were of members used to of investigate taxonomic affinities of the Bruneau Woodpile assemblage, corroboration based on the Gem Club of Boise to identify fossil wood from Bruneau Woodpile and other sites in southern microscopic thin sections was not attained until now. Individual taxa have been previously Idaho and eastern Oregon. This group published their findings in the October 1971 issue of Gems and described from the Bruneau Woodpile, including a bracket fungus [8] and pine cones [9]. Our Minerals [7]. They concluded that the wood had washed into an ancient lake during the late Pliocene. investigation is a long-delayed successor to the work begun by the original Bruneau Woodpile study In the absence of analytical data, the petrified wood was presumed to consist of calcium sulfate (CaSO ) group. 4 and calcium carbonate (CaCO3). While polished transverse wood sections were used to investigate taxonomic1.2. Geologic affinities Setting of the Bruneau Woodpile assemblage, corroboration based on microscopic thin sections was not attained until now. Individual taxa have been previously described from the Bruneau The Bruneau Woodpile is located in the western Snake River Plain, a broad arcuate depression Woodpile, including a bracket fungus [8] and pine cones [9]. Our investigation is a long-delayed that extends across southern Idaho, USA. The Neogene volcanic history of southern Idaho was successor to the work begun by the original Bruneau Woodpile study group. dictated by the passage of the North American Plate over a stationary mantle plume that is currently 1.2.situated Geologic beneath Setting Yellowstone National Park, Wyoming [10]. The western part of the Snake River Plain is a NW trending fault-bounded graben, while the eastern part is a structural downwarp createdThe Bruneau by the weight Woodpile of overlying is located volcanic in the and western sedimentary Snake Riverrocks interacting Plain, a broad with arcuate the formation depression of thata mid-crustal extends across sill southernand thermal Idaho, contraction USA. The after Neogene passage volcanic over the history plume-generated of southern hotspot Idaho was [11]. dictated The bywestern the passage section of thecontains North extensive American rhyolitic Plate over tuffs a stationaryand ash flows mantle of the plume Miocene that isIdavada currently Volcanic situated beneathGroup, Yellowstone ranging in age National from 15 Park,to 11 Ma Wyoming [12,13]. [10]. The western part of the Snake River Plain is a NW trendingThese volcanic fault-bounded materials graben, are overlain while the by eastern younger part mafic is a structural volcanic downwarprocks and createdfluvial byand the weightlacustrine of overlying sediments volcanic (Chalk and Hills sedimentary and Glenns rocks Ferry interacting formations) with thethat formation contain ofvolcanic a mid-crustal and sillhyaloclastic and thermal interbeds contraction (Figures after 2 passage and 3). overSedimentation the plume-generated began at approximately hotspot [11 ].9 TheMa, westernwhen several section containslakes developed. extensive rhyolitic One of these tuffs andwas ash Chalk flows Hills of theLake, Miocene in which Idavada the Bruneau Volcanic Woodpile Group, ranging sediments in age fromwere 15 deposited. to 11 Ma [ 12These,13]. lakes expanded and retreated in size, eventually coalescing to form a single enormousThese volcanic lake, ancient materials Lake are Idaho overlain (represented by younger by the mafic Glenns volcanic Ferry rocks Formation, and fluvial Figure and 2), lacustrine which 2 sedimentsultimately (Chalk extended Hills over and an Glenns area of Ferry several formations) thousand that km contain [2,14]. During volcanic the and next hyaloclastic 6.5 million interbeds years, thick basin-fill sediment accumulated in the lake basin. This sediment contains material eroded from (Figures2 and3). Sedimentation began at approximately 9 Ma, when several lakes developed. One of nearby mountain ranges, as well as rhyolitic tephra. These sediments locally preserve plant and these was Chalk Hills Lake, in which the Bruneau Woodpile sediments were deposited. These lakes animal fossils. expanded and retreated in size, eventually coalescing to form a single enormous lake, ancient Lake The petrified wood assemblage investigated in our study is preserved in lacustrine sediments Idaho (represented by the Glenns Ferry Formation, Figure2), which ultimately extended over an area of the Chalk Hills Formation. This Miocene-age lake is here referred to as the Chalk Hills Lake to 2 of severaldistinguish thousand it from km the[ 2Pliocene,14]. During ancient the Lake next 6.5Idaho. million Other years, important thick basin-fill Snake River sediment Plain accumulatedfossil sites ininclude the lake the basin. late ThisMiocene sediment Pickett contains Creek leaf material and po erodedllen deposit from nearby[6] and mountainthe Hagerman ranges, Fossil as wellBeds as rhyoliticNational tephra. Monument, These sedimentswhere Plioce locallyne strata preserve preserve plant a multitud and animale of vertebrate fossils. fossils [15].

Figure 3. Modern views of the Snake River Plain, near Salmon Falls (A) and Shoshone Falls (B); Figure 3. Modern views of the Snake River Plain, near Salmon Falls (A) and Shoshone Falls (B); Idaho USA. Idaho USA.

The petrified wood assemblage investigated in our study is preserved in lacustrine sediments of the Chalk Hills Formation. This Miocene-age lake is here referred to as the Chalk Hills Lake to distinguish it from the Pliocene ancient Lake Idaho. Other important Snake River Plain fossil sites include the late Miocene Pickett Creek leaf and pollen deposit [6] and the Hagerman Fossil Beds National Monument, where Pliocene strata preserve a multitude of vertebrate fossils [15]. Geosciences 2017, 7, 82 4 of 25

1.3. Paleoenvironment Southern Idaho is now an arid region where extensive lower elevations are sagebrush steppe flora, bordered by barren mountains (Figure3). The region typically receives less than 250 mm of rain each year, with temperatures that range from −34 ◦C in winter to 43 ◦C in summer [5]. Neogene environmental conditions were very different. By the late Miocene lakes occupied a large area of southern Idaho, eventually expanding in the Pliocene to produce ancient Lake Idaho; the origin of which was related to formation of the western Snake River Plain graben [16]. Topography of the surrounding watershed was relatively gentle; much of the lakebed sediment consists of small pebble to clay-sized fractions transported by local stream tributaries as well as larger rivers. Lake levels fluctuated over time, due to both tectonic and climatic changes. Ultimately, ancient Lake Idaho drained when the outlet river incised Hells Canyon, breaching the lake basin. In place of the ancient lake, the Snake River now traverses southern Idaho [17,18]. The late Miocene Chalk Hills Formation, host to the Bruneau Woodpile, represents a time when Pacific Northwest forests were more uniform and more widespread than today, in large part because the Cascade Range had not yet reached an elevation sufficient to cause a pronounced rain shadow. The Chalk Hills Formation paleoflora originated during the late Miocene, when the northwest climate was transitioning from the warm, humid regime of the middle Miocene to the more arid, summer-dry climate of today. Low elevation forests bore a resemblance to the modern broadleaved hardwood forests of north central USA. Annual precipitation was in the range of 1200–1300 mm, with a mean annual temperature of approximately 13.5 ◦C[5]. Analyses of fossil pollen show Pinus as the dominant conifer, but Abies, Picea, and Tsuga were present. Deciduous trees were abundant and taxonomically diverse [3,5]. The Pickett Creek leaf and pollen deposit [6] of the Chalk Hills Formation documents a flora with a high incidence of summer-dry tolerant species, including white oaks, evergreen oaks, pine and legumes, similar to the wood types found in the Bruneau Woodpile assemblage. The lake sediments also preserve fish remains that include cold water salmon and trout and warm water sunfish and catfish, indicative of warm, moist climate with cool summers and mild winters [19,20]. Two fish taxa, Mylocheilus inflexus (a minnow) and Paleolox larsonia (a salmon) found in the Chalk Hills Formation do not occur in the overlying lacustrine sediments of the Pliocene Glenns Ferry Formation [2]. Chalk Hills Formation strata also preserve remains of beaver, rhinoceros, camel, horse, sloth, and various molluscs [19]. By the mid Pliocene, the development of the Cascade Range rain shadow [5] and global climatic change [21] profoundly altered the flora and fauna of the Snake River Plain. Temperate forests were replaced by pine/oak woodlands with areas of grassland. The mid-Pliocene climate was drier, with perhaps half the annual rainfall of the Chalk Hills Formation. Lake shore, ponds, open woodlands and grasslands were inhabited by a diverse vertebrate fauna that included swans, grebes, and other water birds, rodents, mustelids, saber-tooth tigers, camels, and horses [14]. The Pleistocene brought increased aridity to the Snake River Plain. By 1.3 Ma, annual rainfall had decreased to approximately 250 mm, and the landscape became dominated by sagebrush (Artemisia), saltbush (Sarcobatus) and grasses, i.e., semi-desert sagebrush/bunchgrass steppe flora that flourishes in modern times [5] (Figure3).

2. Methods and Materials SEM/EDS images were obtained for 10 specimens using a Vega Tescan SEM (Tescan, Brno, Czech Republic) equipped with an EDAX Genesis XRF spectrometer (EDAX, Mahwah, NJ, USA). X-ray diffraction (XRD) patterns were made for two specimens with a Rigaku Geigerflex diffractometer using Ni-filtered Cu K-α radiation. Tephra geochronology is based on electron microprobe analysis of Bruneau Woodpile volcanic ash collected by Paul Link, using the elemental data to correlate the tephra with previously-analyzed specimens from other Snake River Plain strata of known radiometric age. Glass shards recovered from a vitric ash bed at the Bruneau Woodpile site were analyzed using electron Geosciences 2017, 7, 82 5 of 25 microprobe by B.P. Nash (University of Utah, UT, USA) who interpolated the age by chemostratigraphic comparisonGeosciences with 2017, 7, western 82 North American vitric tuffs spanning 16 to 6 Ma [22]. 5 of 25 Fossil wood thin sections were prepared using standard techniques and examined and photographedinterpolated using the age a Nikon by chemostratigraphic Eclipse 50i microscope comparison (Nikon, with western Tokyo, North Japan) American coupled vitric with tuffs a Nikon spanning 16 to 6 Ma [22]. DS-Fi1 camera. Thin sections described in this study were deposited at the University of Washington Fossil wood thin sections were prepared using standard techniques and examined and Burke Museum in Seattle, WA, USA. Identifications were aided by standard references, including the photographed using a Nikon Eclipse 50i microscope (Nikon, Tokyo, Japan) coupled with a Nikon TextbookDS-Fi1 of Wood camera. Technology Thin sections[23] anddescribed the InsideWood in this study imagewere deposited database at [the24]. University of Washington Burke Museum in Seattle, WA, USA. Identifications were aided by standard references, including 3. Results the Textbook of Wood Technology [23] and the InsideWood image database [24]. 3.1. Site Description 3. Results The wood-bearing beds occur at near-surface depths at several localities, where collecting has 3.1. Site Description been popular since the 1950s [7,25]. Our research was conducted at a site accessible via several km of rough dirtThe road wood-bearing at coordinates beds of occur 42◦45 at0 47.19near-surface00 N, 115 depths◦53029.46 at several00 W. localities, where collecting has Phosphatizedbeen popular since wood the 1950s is preserved [7,25]. Our in research a sandy was bed conducted that marks at a site the accessible bottom via of several a 15 km cm ofthick fining-upwardrough dirt road sequence at coordinates (Figure4 of). Specimens42°45′47.19″ areN, 115°53 abundant′29.46 for″ W. those who are willing to first remove Phosphatized wood is preserved in a sandy bed that marks the bottom of a 15 cm thick more than a meter of partially-cemented sediment to reach the wood-bearing stratum (Figures5–7). fining-upward sequence (Figure 4). Specimens are abundant for those who are willing to first The stratum is rich in volcanic material that includes numerous pumice clasts and obsidian granules in remove more than a meter of partially-cemented sediment to reach the wood-bearing stratum a fine(Figures tephra-rich 5–7). matrixThe stratum (Figure is7 rich) containing in volcanic well-rounded, material that moderatelyincludes numerous well-sorted pumice quartz clasts grains and of similarobsidian size and granules shape toin equallya fine tephra-rich abundant matrix obsidian (Figure granules. 7) containing Disarticulated well-rounded, fish bones moderately are sparsely distributedwell-sorted within quartz the wood-bearinggrains of similar stratum. size and shape to equally abundant obsidian granules. TheDisarticulated majority fish of fossilbones woodare sparsely specimens distributed are within elongated the wood-bearing limbs or irregular stratum. fragments of trunk wood thatThe have majority characteristics of fossil wood indicative specimens of fluvial are el transport.ongated limbs Exterior or irregu surfaceslar fragments have been of smoothedtrunk by erosion,wood that bark have and characteristics side branches indicative have beenof fluvia removed,l transport. and Exterior terminations surfaces arehave rounded been smoothed (Figure 8). The wood-bearingby erosion, bark bed and is side overlain branches by ahave light been gray removed, layer of and white terminations fine sand are with rounded parallel (Figure laminations, 8). The wood-bearing bed is overlain by a light gray layer of white fine sand with parallel laminations, capped by a thin layer of whitish silt. This interval completes the first fining-upward sequence. capped by a thin layer of whitish silt. This interval completes the first fining-upward sequence. A A second, 30-cm upward fining succession begins with a bed of cross bedded vitric ash that passes second, 30-cm upward fining succession begins with a bed of cross bedded vitric ash that passes upwardupward to rippled to rippled ash and ash to and massive to massive white white argillaceous argillaceous siltstone. siltstone. The secondThe second upward upward fining fining sequence is overlainsequence by is a overlain brown cementedby a brown layercemented and layer a massive and a massive white white clay. Aboveclay. Above that that is a is fallout a fallout ash ash layer consistinglayer ofconsisting gray fine of pumicegray fine sand pumice and sand vitric and gray vitric rhyolitic gray rhyolitic ash. Geochemical ash. Geochemical analysis analysis and correlation and suggestscorrelation that this suggests younger that ashthis younger layer can ash be layer assigned can be aassigned chemostratigraphic a chemostratigraphic age of age 6.84 of± 6.840.05 ± Ma (late Miocene)0.05 Ma (late [26 Miocene)]. [26].

Figure 4. Generalized stratigraphic section of the Chalk Hills Formation beds at Bruneau Woodpile Figure 4. Generalized stratigraphic section of the Chalk Hills Formation beds at Bruneau locality. Woodpile locality.

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Figure 5. Bruneau Woodpile site. (A) Revealing the wood-bearing stratum requires excavation of one Figure 5. Bruneau Woodpile site. (A) Revealing the wood-bearing stratum requires excavation of Figuremeter of5. Bruneaucemented Woodpile sediment; site. (B )( ALimb) Revealing sections the and wood-bearing trunk fragme strantstum occur requires in a weaklyexcavation cemented of one one meter of cemented sediment; (B) Limb sections and trunk fragments occur in a weakly cemented metersandstone of cemented bed that sediment;contains large (B) Limbpumice sections clasts. and Some trunk wood fragme specimensnts occur have in aa thinweakly black cemented exterior sandstone bed that contains large pumice clasts. Some wood specimens have a thin black exterior sandstonecoating composed bed that of contains manganese large oxide, pumice as clasts.evidenced Some by wood SEM/EDS specimens analysis; have (C a) thinThe wood-bearingblack exterior coating composed of manganese oxide, as evidenced by SEM/EDS analysis; (C) The wood-bearing coatinglayer is composedoverlain by of a manganese~7 cm white oxide, fine sandy as evidenced ash with by parallel SEM/EDS laminations. analysis; Note(C) The the wood-bearingabundance of layer is overlain by a ~7 cm white fine sandy ash with parallel laminations. Note the abundance of layerpumice is overlainclasts in theby asandy ~7 cm sediment white fine below sandy the ash elongate with parallelfossil twig. laminations. Note the abundance of pumice clasts in the sandy sediment below the elongate fossil twig. pumice clasts in the sandy sediment below the elongate fossil twig.

Figure 6. Phosphatized wood at the Bruneau Woodpile site. Elongate limbs and larger wood Figure 6. Phosphatized wood at the Bruneau Woodpile site. Elongate limbs and larger wood fragmentsFigure 6. Phosphatized are positioned wood in different at the Bruneau orientations. Woodpile site. Elongate limbs and larger wood fragments fragments are positioned in different orientations. are positioned in different orientations.

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Figure 7. Bruneau Woodpile sedimentology. (A,B) are sediments from wood-bearing stratum. (A) Figure 7. Bruneau Woodpile sedimentology. (A,B) are sediments from wood-bearing stratum. Black obsidian granules and translucent gray quartz grain; (B) Pumice clast; (C) fine-grained ash-rich Figure 7. Bruneau Woodpile sedimentology. (A,B) are sediments from wood-bearing stratum. (A) (A) Blackbed that obsidian overliesgranules wood-bearing and translucentstratum; (D) grayfish vertebra quartz in grain; sandy( Bsediment) Pumice containing clast; (C quartz) fine-grained and Black obsidian granules and translucent gray quartz grain; (B) Pumice clast; (C) fine-grained ash-rich ash-richobsidian bed thatgranules. overlies wood-bearing stratum; (D) fish vertebra in sandy sediment containing quartz and obsidianbed that overlies granules. wood-bearing stratum; (D) fish vertebra in sandy sediment containing quartz and obsidian granules.

Figure 8. Bruneau Woodpile specimens have characteristics indicative of fluvial transport. Two morphotypes are common: elongate limbs (A–C) and irregular fragments of trunk wood (D–F). For Figure 8. Bruneau Woodpile specimens have characteristics indicative of fluvial transport. Two Figureboth 8. types,Bruneau exterior Woodpilesurfaces have specimens been smoothed have by characteristics erosion, bark indicativeand side branches of fluvial have transport.been morphotypes are common: elongate limbs (A–C) and irregular fragments of trunk wood (D–F). For Two morphotypesremoved, and terminations are common: are elongate rounded. limbs Elongate (A–C limbs) and (A irregular) break into fragments shorter ofsegments trunk wood during (D –F). both types, exterior surfaces have been smoothed by erosion, bark and side branches have been excavation from the matrix because of the brittleness of the phosphatized wood. These morphologies For bothremoved, types, and exterior terminations surfaces are have rounded. been smoothedElongate limbs byerosion, (A) break bark into and shorter side segments branches during have been are identical to modern fluvial driftwood (G). removed,excavation and terminationsfrom the matrix are because rounded. of the brittlen Elongateess of limbs the phosphatized (A) break into wood. shorter These morphologies segments during excavationare identical from theto modern matrix fluvial because driftwood of the brittleness(G). of the phosphatized wood. These morphologies

are identical to modern fluvial driftwood (G).

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3.2. Age Geosciences 2017, 7, 82 8 of 25 The Bruneau Woodpile was originally assigned a late Pliocene age [7]. Geochemical analysis [26] of rhyolitic3.2. Age glass shards in the tuff bed ~80 cm above the wood bed (stratigraphic location of sample shown inThe (Figure Bruneau4), matches Woodpile an unnamedwas originally peralkaline assigned ash a late bed Pliocene below the age Blacktail [7]. Geochemical Creek Tuff analysis [ 27] from the Heise[26] of volcanic rhyolitic field glass of shards the eastern in the tuff Snake bed River~80 cm Plain. above The the Blacktailwood bed Creek (stratigraphic Tuff is 6.61location± 0.01 of Ma (Ar-Arsample age from shown welded in (Figure ash 4), of matches sample an PTH4 unnamed [28]). peralkaline Our sample ash of bed the below unnamed the Blacktail ash 80 Creek cm above Tuff the Bruneau[27] from Woodpile the Heise bed volcanic was chemostratigraphically field of the eastern Snake correlated River Plain. by The B.P. Blacktail Nash (University Creek Tuff is of 6.61 Utah) ± to 0.01 Ma (Ar-Ar age from welded ash of sample PTH4 [28]). Our sample of the unnamed ash 80 cm an age of 6.84 ± 0.05 Ma [26]. Based on these dates, the Bruneau Woodpile occurs in the Chalk Hills above the Bruneau Woodpile bed was chemostratigraphically correlated by B.P. Nash (University of Formation (late Miocene) [29]. Utah) to an age of 6.84 ± 0.05 Ma [26]. Based on these dates, the Bruneau Woodpile occurs in the Chalk Hills Formation (late Miocene) [29]. 3.3. Mineralogy and Geochemistry While3.3. Mineralogy the Bruneau and Geochemistry Woodpile fossil wood specimens have long been identified as being mineralizedWhile with the calciumBruneau carbonateWoodpile fossil or calcium wood spec sulfateimens [7 ],have new long analytical been identified data reveal as being that the mineralizationmineralized is with phosphatic. calcium carbonate or calcium sulfate [7], new analytical data reveal that the X-raymineralization diffraction is phosphatic. patterns (Figure9) show that the only discernable crystalline phase is a memberX-ray of thediffraction apatite patterns family, (Figure comprising 9) show members that the thatonly falldiscernable within crystalline the general phase formula is a of member of the apatite family, comprising members that fall within the general formula of Ca5(PO4)3(F, OH, Cl) [30,31]. The three members, fluorapatite, hydroxylapatite, and chlorapatite Ca5(PO4)3(F, OH, Cl) [30,31]. The three members, fluorapatite, hydroxylapatite, and chlorapatite are are not easily distinguishable from XRD data, but SEM/EDS spectra (Figure 10) show that fluorine not easily distinguishable from XRD data, but SEM/EDS spectra (Figure 10) show that fluorine is is present at levels of ~1% by weight. Cl is below detection limits, and the inability of EDS to detect present at levels of ~ 1% by weight. Cl is below detection limits,− and the inability of EDS to detect H H andand poor poor quantification quantification ofof OO preventprevent recognition of of OH OH−. However,. However, the thepresence presence of a ofsignificant a significant fluorinefluorine spectral spectral peak peak suggests suggests that that calcium calcium phosphate phosphate is is present present as as fluorapatite, fluorapatite, which which is sometimes is sometimes givengiven the mineral the mineral name namefrancolite francolite. .

FigureFigure 9. X-ray 9. X-Ray diffraction diffraction (XRD) (XRD) pattern. pattern. AllAll peaks are are from from an an apatite apatite group group mineral. mineral.

The large carbon peak in the SEM/EDS spectrum (Figure 10) suggests that the mineralization is Thecarbonate-fluorapatite. large carbon peak The in the450 SEM/EDS °C LOI (loss spectrum on ignition) (Figure (Table 10) suggests1) indicates that an the organic mineralization carbon is ◦ carbonate-fluorapatite.content of several percent, The 450 but Cthe LOI EDS (loss spectrum on ignition) was obtained (Table from1) indicates microcrystalline an organic material carbon within content of severala vessel percent, lumen, but where the the EDS analyzed spectrum material was obtained was separated from from microcrystalline any possible materialrelict organic within matter a vessel lumen,in the where cell walls. the analyzed This occurrence material is evidence was separated that carbon from is present any possible as an inorganic relict organicconstituent matter within in the cell walls.the phosphatic This occurrence structure. is Carbonate-fluorapatite evidence that carbon results is present from as the an partial inorganic substitution constituent of CO within3−2 in the place of PO4−3, equivalent to a generalized formula of Ca5(PO4, CO3)3(F, OH, Cl) [30]. −2 phosphatic structure. Carbonate-fluorapatite results from the partial substitution of CO3 in place of The amount of carbonate in carbonate-fluorapatite can be estimated based on major element PO −3, equivalent to a generalized formula of Ca (PO , CO ) (F, OH, Cl) [30]. 4 geochemistry (Table 2). Carbon concentrations5 cannot4 be3 3 accurately estimated by SEM/EDS Thespectrometry amount because of carbonate of the inelement’s carbonate-fluorapatite low atomic mass can that be makes estimated it undetectable based on by major element geochemistry (Table2). Carbon concentrations cannot be accurately estimated by SEM/EDS spectrometry because of the element’s low atomic mass that makes it undetectable by Geosciences 2017, 7, 82 9 of 25

Geosciences 2017, 7, 82 9 of 25 wavelength-dispersive XRF spectrometry. However, when wt. % values are recalculated as atomic % values,wavelength-dispersive the ratio of Ca:P provides XRF spectrometry. an important However, compositional when wt. %indication values are recalculated (Table2). For as atomic pure apatite, % the Ca:Pvalues, proportion the ratio of is Ca:P 1.67. provides A Ca:P ratioan important that exceeds compositional this value indication provides (Table evidence 2). For that pure the apatite, mineral is the Ca:P proportion is 1.67. A Ca:P ratio that exceeds this value provides evidence that the mineral is deficient in P, suggesting that CO3 is present in an amount sufficient to provide a neutral molecular structure.deficient Both in BruneauP, suggesting Woodpile that CO samples3 is present have in an ratios amount that sufficient exceed that to provide of ideal a apatite,neutral molecular substantiating structure. Both Bruneau Woodpile samples have ratios that exceed that of ideal apatite, the existence of a carbonate component. For this reason, we identify the mineralizing material of the substantiating the existence of a carbonate component. For this reason, we identify the mineralizing carbonate-fluorapatite. Bruneaumaterial Woodpile of the Bruneau fossil wood Woodp specimensile fossil wood as specimens as carbonate-fluorapatite.The small The amount small ofamount SiO2 may be causedof SiO by2 may substitution be caused by of substitution silica for phosphorus of silica for phosphorus in the mineral in the structure; mineral structure; the absence the absence of quartz or otherof silicate quartz mineralor other peakssilicate in mineral the XRD peaks pattern in the (FigureXRD pattern9) suggests (Figure that 9) suggests Si is not that present Si is not as apresent secondary mineralas a phase.secondary mineral phase.

FigureFigure 10. SEM/EDS 10. SEM/EDS spectrum, spectrum, 15 KeV.15 KeV. Pd peaksPd peaks are are spectral spectral artifacts artifacts from from sputter-coating sputter-coating the the specimen withspecimen Pd to provide with Pd electrical to provide conductivity. electrical conductivity. Calculated Calculated fluorine content fluorine =content 1.03 wt. = 1.03 %(= wt. 1.11 % (= atomic 1.11 %); chlorineatomic content %); chlorine = 0.0 wt. content % (= = 0.00.0 atomicwt. % (= %). 0.0 atomic %).

Table 1. Major element analysis by X-Ray Fluorescence Spectrometry. Elemental results are Tablecalculated 1. Major as element wt. % oxide. analysis by X-ray Fluorescence Spectrometry. Elemental results are calculated as wt. % oxide. Sample SiO2 Al2O3 TiO2 Fe2O3 * MnO MgO CaO K2ONa2OP2O5 LOI ** 1 14.53 0.12 0.00 1.88 0.45 0.98 44.36 0.01 0.94 37.79 3.83 Sample2 SiO13.692 Al20.04O3 TiO0.002 Fe2O1.733 * MnO0.50 MgO0.55 CaO45.67 K0.002O Na0.002 OP33.332O5 LOI5.83 ** 1 14.53* total iron0.12 calculated 0.00 as 1.88Fe2O3. ** 0.45 wt % loss 0.98 on ignition44.36 0.01after 450 0.94 °C heating.37.79 3.83 2 13.69 0.04 0.00 1.73 0.50 0.55 45.67 0.00 0.00 33.33 5.83 Table* Total 2. iron Conversion calculated of as% Fe2O3;oxide values ** wt. %to lossatomic on ignition%, and calculated after 450 ◦ CCa:P heating. ratios. Weight % Atomic % Sample Ca:P Ratio Table 2. Conversion of %CaO oxide P values2O5 toCa atomic P %, and calculated Ca:P ratios. 1 44.36 37.79 31.72 16.48 1.92 2 45.67Weight 33.33 % 32.65 Atomic14.53 % 2.25 Sample Ca:P Ideal formula: Ca5(PO4)3 Ca:P = 5:3 = 1.67. Ratio CaO P2O5 Ca P 1 44.36 37.79 31.72 16.48 1.92 High magnification images (Figure 11) show that the carbonate-fluorapatite is present in 2 45.67 33.33 32.65 14.53 2.25 crystalline form, consistent with the strong peaks observed in the XRD pattern (Figure 9). No relict Ideal formula: Ca (PO ) Ca:P = 5:3 = 1.67. organic matter is visible; anatomical features 5are preserved4 3 as inorganic replacements. Quality of anatomical preservation is variable. Many specimens show preserved cellular features (Figure 12). HighFeatures magnification of a few specimens images include (Figure spiral 11 )fibril show patterns that thein the carbonate-fluorapatite middle cell wall (Figure is 12B,C), present in crystallineevidence form, of trees consistent that have with anatomical the strong asymmetry peaks because observed of oblique in the XRD gravitational pattern forces, (Figure e.g.,9). a No tree relict organicgrowing matter on isa visible;slope, or anatomical on branched features limbs [23] are. preservedOne specimen as inorganic(Figure 12D) replacements. shows evidence Quality of of anatomicalmicrobes preservation that bored isbranched variable. galleries Many specimensin the exterior show surface. preserved Possible cellular organisms features include (Figure 12). Features of a few specimens include spiral fibril patterns in the middle cell wall (Figure 12B,C), evidence of trees that have anatomical asymmetry because of oblique gravitational forces, e.g., a tree Geosciences 2017, 7, 82 10 of 25 growing on a slope, or on branched limbs [23]. One specimen (Figure 12D) shows evidence of microbes that boredGeosciences branched 2017, 7, 82 galleries in the exterior surface. Possible organisms include cyanobacteria,10 of 25 algae, and fungi. These microbes are known to penetrate the phosphate-enriched skeletons of corals as Geosciences 2017, 7, 82 10 of 25 a sourcecyanobacteria, of phosphorus, algae, and a macronutrient fungi. These microbes that is often are known scarce to in penetrate pristine naturalthe phosphate-enriched environments [32]. skeletons of corals as a source of phosphorus, a macronutrient that is often scarce in pristine natural Chemolithotrophiccyanobacteria, algae, bacteria and are fungi. also aThese possibility, microbes particularly are known phosphobacteria, to penetrate the phosphate-enriched which use phosphorus environments [32]. Chemolithotrophic bacteria are also a possibility, particularly phosphobacteria, reductionskeletons as a of source corals ofas metabolica source of energyphosphorus, [33]. a macronutrient that is often scarce in pristine natural which use phosphorus reduction as a source of metabolic energy [33]. environments [32]. Chemolithotrophic bacteria are also a possibility, particularly phosphobacteria, which use phosphorus reduction as a source of metabolic energy [33].

Figure 11. SEM photomicrographs of Bruneau Woodpile wood show microcrystalline structure of Figure 11. SEM photomicrographs of Bruneau Woodpile wood show microcrystalline structure of carbonate-fluorapatite. (A) Longitudinal view of two tracheids showing circular pits; (B) Fractured Figure 11. SEM photomicrographs of Bruneau Woodpile wood show microcrystalline structure of carbonate-fluorapatite.cell walls of a tracheid; (A) Longitudinal(C) Interior surface view of twoa tracheid tracheids showing showing numerous circular pit pits;apertures; (B) Fractured (D) carbonate-fluorapatite. (A) Longitudinal view of two tracheids showing circular pits; (B) Fractured cell wallsCarbonate-fluorapatite of a tracheid; crystals (C) Interior bordering surface a pit aperture. of a tracheid showing numerous pit apertures; cell walls of a tracheid; (C) Interior surface of a tracheid showing numerous pit apertures; (D) (D) Carbonate-fluorapatite crystals bordering a pit aperture. Carbonate-fluorapatite crystals bordering a pit aperture.

Figure 12. SEM images of Bruneau Woodpile specimens. (A) Conifer wood, radial orientation, showing tracheids with well-preserved bordered pits; (B,C) Radial view of specimens of reaction Figure 12. SEM images of Bruneau Woodpile specimens. (A) Conifer wood, radial orientation, Figurewood, 12. whereSEM tracheids images ofhave Bruneau spiral archit Woodpileecture in specimens. the secondary (A cell) Coniferwall; (D) wood,Exterior radial surface orientation, of this showing tracheids with well-preserved bordered pits; (B,C) Radial view of specimens of reaction limb shows galleries produced in the mineralized wood by chemolithotrophic microbes (see text for showingwood, tracheids where tracheids with well-preserved have spiral archit borderedecture in the pits; secondary (B,C) Radial cell wall; view (D) Exterior of specimens surface ofof this reaction discussion). wood,limb where shows tracheids galleries produced have spiral in the architecture mineralized in wo theod secondaryby chemolithotrophic cell wall; microbes (D) Exterior (see text surface for of this limbdiscussion). shows galleries produced in the mineralized wood by chemolithotrophic microbes (see text

for discussion).

Geosciences 2017, 7, 82 11 of 25

3.4. Paleobotany Taxonomic information from fossil wood collected at the Bruneau Woodpile is limited due to poor anatomical preservation, especially in the radial and tangential sections. Even so, a number of types can be assigned to family or genus based on available features. The best representatives of each type were selected for sectioning from more than 200 specimens. Larger images and anatomical descriptions are provided in Supplement 1. Taxa recognized from the Bruneau Woodpile are as follows (Figure 13):

Conifers • Cupressaceae • Pinaceae

Piceoxylon (Spruce/Douglas fir type) Pinus sp. (Pine)

Angiosperms • Berberidaceae

cf. Berberis (Barberry/Oregon grape type) • Fabaceae

cf. Robinia (Black locust type) • Fagaceae

Quercus sp. (White oak group) Quercus/Lithocarpus (Live oak group) • Juglandaceae

Carya sp. (True hickory type) • cf. Salicaceae

Populus/Salix (Poplar/willow type) • Sapindaceae

Acer sp. (Soft maple group) • Ulmaceae

Ulmus sp. (Elm) • Undetermined angiosperms

Undetermined hardwood 1 Undetermined hardwood 2 Undetermined hardwood 3 Undetermined hardwood 4 Geosciences 2017, 7, 82 12 of 25 Geosciences 2017, 7, 82 12 of 25

FigureFigure 13. 13.Fossil Fossil wood types types from from the Bruneau the Bruneau Woodpile Woodpile (University (University of Washington of WashingtonBurke Museum Burke specimen numbers in parentheses). (A) Cupressaceae (UWBM PB98590); (B) Piceoxylon (UWBM Museum specimen numbers in parentheses). (A) Cupressaceae (UWBM PB98590); (B) Piceoxylon PB98565); (C) Pinus sp. (UWBM PB98568); (D) cf. Berberis (UWBM PB98560) (E) cf. Robinia (UWBM (UWBM PB98565); (C) Pinus sp. (UWBM PB98568); (D) cf. Berberis (UWBM PB98560) (E) cf. Robinia PB98587); (F) Quercus sp. (UWBM PB98591); (G) Quercus/Lithocarpus (UWBM PB98589); (H) Carya sp. (UWBM(UWBM PB98587); PB98577); (F )(IQuercus) cf. Salicaceaesp. (UWBM(UWBM PB98557); PB98591); (J) ( GAcer) Quercus/Lithocarpus sp. (UWBM PB98558);(UWBM (K) Ulmus PB98589); sp. (H) Carya(UWBMsp. PB98559); (UWBM (L PB98577);) Undetermined (I) cf. hardwood Salicaceae #1 (UWBM (UWBM PB98567); PB98557); (M (J)) UndeterminedAcer sp. (UWBM hardwood PB98558); (K) Ulmus#2 (UWBMsp. (UWBM PB98570); PB98559); (N) Undetermined (L) Undetermined hardwood hardwood #3 (UWBM #1 (UWBM PB98576); PB98567); (O) (UndeterminedM) Undetermined hardwoodhardwood #2 (UWBM #4 (UWBM PB98570); PB98556). (N ) Undetermined hardwood #3 (UWBM PB98576); (O) Undetermined hardwood #4 (UWBM PB98556). 4. Discussion 4. Discussion 4.1. Taphonomy and Sedimentology 4.1. TaphonomyThe origin and Sedimentologyof the Bruneau Woodpile can be considered using a multiple working hypotheses approach, considering two alternatives. Hypothesis 1: Fossilized wood originated from normal The origin of the Bruneau Woodpile can be considered using a multiple working hypotheses processes of fluvial transport and lacustrine deposition. This scenario involves the wood and approach,sediment considering being delivered two alternatives.separately to the Hypothesis lake. They 1:were, Fossilized subsequently wood reworked originated into froma single normal processesbed. Hypothesis of fluvial transport2: Fossilized and wood lacustrine resulted deposition. from an unusual This scenarioevent, when involves limbs theand woodwood and sedimentfragments being from delivered catastrophically separately damaged to the lake. treesThey were were,transported subsequently and deposited reworked with intopumice a single and bed. Hypothesisobsidian 2: Fossilizedin a single wood bed. resultedThese hypotheses from an unusual are considered event, when with limbs respect and woodto stratigraphic, fragments from catastrophicallysedimentologic, damaged and paleontologic trees were features transported of the and site. deposited with pumice and obsidian in a single bed. TheseA hypothesesbrief review areof the considered depositional with characteristics respect to stratigraphic, are in order. At sedimentologic, Bruneau Woodpile, and fossilized paleontologic featureslimbs of and the small site. trunk fragments occur in the basal unit of a fining-upward sequence. This mode of occurrence is very different from a typical lacustrine lahar deposit, where wood occurs in A brief review of the depositional characteristics are in order. At Bruneau Woodpile, fossilized fine-grained strata at the top of a graded sequence (Figure 14). At Bruneau Woodpile, the fossil limbs and small trunk fragments occur in the basal unit of a fining-upward sequence. This mode of wood comprises limbs and small trunk fragments stripped of their bark—both types having abraded occurrencedriftwood-like is very differentshapes with from tapered a typical ends. lacustrine This morphology lahar deposit, is whereevidence wood that occurs the wood in fine-grained had strataundergone at the top fluvial of a graded transport. sequence Fossil wood (Figure is present 14). At in Bruneauonly one bed Woodpile, in the stratigraphic the fossil wood sequence comprises at limbsthe and study small site. trunk Pollen fragments and leaf fo strippedssils provide of their ample bark—both regional evidence types having of the existence abraded of driftwood-like Miocene shapesforests with in tapered the hills ends. bordering This morphology Chalk Hills is evidenceLake [4,5]. that Hypotheses the wood had1 and undergone 2 involve fluvial different transport. Fossilassumptions wood is present regarding in the only processes one bed that in transported the stratigraphic wood into sequence the ancient at the lake. study They site.also require Pollen and leaf fossilsdifferent provide diagenetic ample conditions regional that evidence resulted ofin thefossilization. existence of Miocene forests in the hills bordering Chalk Hills Lake [4,5]. Hypotheses 1 and 2 involve different assumptions regarding the processes that transported wood into the ancient lake. They also require different diagenetic conditions that resulted in fossilization. Geosciences 2017, 7, 82 13 of 25 Geosciences 2017, 7, 82 13 of 25

Figure 14. Distal lacustrine lahar deposits in the Miocene Wilkes formation in southeast Washington, Figure 14. Distal lacustrine lahar deposits in the Miocene Wilkes formation in southeast Washington, USA [34]. (A) Streambank exposure reveals three successive wood mats (arrows). (B) These lahar USAdeposits [34]. (A contain) Streambank rough-surfaced exposure fragments reveals of three mummified successive wood woodin a wide mats range (arrows); of sizes. (EachB) These wood lahar depositsmat containis preserved rough-surfaced at the top of a fragmentsfining-upward of mummified cycle. wood in a wide range of sizes. Each wood mat is preserved at the top of a fining-upward cycle. Hypothesis 1. “ordinary deposition” HypothesisThis 1. model“ordinary assumes deposition” that wood was gradually released from watershed forests, transported into the lake by normal streamflow, and deposited in the lake over a long period of time. Wood is Thisconsidered model to assumes have been that continually wood was introduced gradually into released Chalk from Hills watershed Lake, rather forests, than by transported a unique into the lakehigh-energy by normal sedimentary streamflow, event. and deposited Fossilization in the of lake wood over in a longthe Bruneau period of Woodpile time. Wood stratum is considered is to haveconsidered been continually to be the result introduced of unique into geochemica Chalk Hillsl conditions; Lake, rather at other than stratigraphic by a unique levels high-energy wood sedimentaryoriginally event. incorporated Fossilization in the sediments of wood was in thedestroyed Bruneau during Woodpile diagenesis. stratum is considered to be the The natural buoyancy of wood causes it to float until cellular spaces become water saturated, result of unique geochemical conditions; at other stratigraphic levels wood originally incorporated in resulting in a slow settling rate. Similarly, pumice settles only after vesicles become waterlogged, the sedimentswhich is a was slow destroyed process for during large diagenesis.clasts. Experimental evidence reveals that 4–20 mm diameter Thepumice natural clasts buoyancyrequire 50-100 of wood hours causesof water it exposu to floatre untilto become cellular saturated spaces [35]. become The wood-bearing water saturated, resultinglayer incontains a slow pumice settling clasts rate. up Similarly, to 7 cm pumicein diameter, settles a size only that after indicates vesicles these become porous waterlogged, clasts whichexperienced is a slow process prolonged for largemoisture clasts. contact Experimental prior to their evidence transport reveals into that the 4–20lake. mm At diameterthe Bruneau pumice clastsWoodpile require 50–100site, fossil h of wood water occurs exposure only to in become a single saturated stratum, [the35 ].lowest The wood-bearing bed of a fining-upward layer contains pumicesedimentary clasts up cycle, to 7 cm where in diameter, rhyolitic volcaniclastic a size that indicates material is these the dominant porous clasts constituent. experienced prolonged moistureAlthough contact prior hypothesis to their 1 is transport described into herein the as lake. “ordinary At the deposition”, Bruneau Woodpile a complex site,sequence fossil of wood occursprocesses only in is a required single stratum, to explain the the lowest origin bed of the of Bruneau a fining-upward Woodpile sedimentary bed. The presence cycle, of where wood and rhyolitic large pumice clasts in the basal bed requires both materials to have been water-saturated prior to volcaniclastic material is the dominant constituent. final deposition. Hypothesis 1 presumes that wood and pumice were independently deposited on Althoughthe lake bottom, hypothesis subsequently 1 is described intermixed hereinwith volcaniclastic as “ordinary sand deposition”, during an episode a complex of high-energy sequence of processessediment is required transport. to To explain complete the the origin genetic of thesequ Bruneauence, wood Woodpile in this stra bed.tum The became presence phosphatized; of wood and largeconditions pumice clasts necessary in the for basal phosphatization bed requires bothdid materialsnot exist in to haveall of beenthe other water-saturated stratigraphic prior levels, to final deposition.consequently Hypothesis the wood 1 presumes decomposed that during wood diagenesis. and pumice were independently deposited on the lake bottom,Hypothesis subsequently 2. “catastrophic intermixed deposition” with volcaniclastic sand during an episode of high-energy sediment transport. To complete the genetic sequence, wood in this stratum became phosphatized; conditions necessaryA for very phosphatization different model is did that not the exist wood-bearing in all of thestratum other originated stratigraphic by an levels,unusual consequently event, when the watershed forests were damaged by a major storm, producing a subaerial reservoir of branches, wood decomposed during diagenesis. single limbs and wood fragments that became waterlogged prior to or during transport. This organic debris arrived at the lake concurrently with volcaniclastic sediment, producing a density current. “catastrophic deposition” HypothesisThe first 2. materials to settle were coarse clastic grains, waterlogged pumice fragments, and waterlogged woods. Smaller clastic grains settled at a slower rate, producing a fining-upward Asequence. very different model is that the wood-bearing stratum originated by an unusual event, when watershedHypothesis forests were 2 involves damaged a scenario by a major where storm, wood producing is introduced a subaerial into Chalk reservoir Hills Lake of as branches, the result single limbsof and a catastrophic wood fragments incident that that became released waterlogged large volumes prior of woody to or duringdebris. transport.This scenario This presumes organic an debris arrivedevent at thethat lakeresulted concurrently in severe structural with volcaniclastic damage to forests sediment, in the producing local watershed. a density One current.possibility The is first materialsthe onset to settle of botanical were coarse disease, clastic akin to grains, the pine waterlogged bark beetle epidemic pumice fragments, currently decimating and waterlogged large forest woods. Smallerarea clastic in western grains North settled America at a slower(Figure rate,15). producing a fining-upward sequence.

Hypothesis 2 involves a scenario where wood is introduced into Chalk Hills Lake as the result of a catastrophic incident that released large volumes of woody debris. This scenario presumes an event Geosciences 2017, 7, 82 14 of 25 that resulted in severe structural damage to forests in the local watershed. One possibility is the onset of botanical disease, akin to the pine bark beetle epidemic currently decimating large forest area in western North America (Figure 15). GeosciencesGeosciences 2017, 2017,7, 82 7, 82 14 of 2514 of 25

Figure 15. Lodgepole Pine (Pinus contorta) killed by Mountain Pine Beetle (Dendroctonus), Montana, Figure 15.USA.Lodgepole This beetle Pine only (attacksPinus certain contorta Pinaceae) killed species by Mountain [36]. Pine Beetle (Dendroctonus ), Montana, USA. This beetle only attacks certain Pinaceae species [36]. FigureForest 15. Lodgepole destruction Pine caused (Pinus by contorta disease) killedor insect by Mountaininfestation Pine is inconsistent Beetle (Dendroctonus with the taxonomic), Montana, USA.diversity This of beetle fossil only woods attacks at Bruneau certain PinaceaeWoodpile; species plant [36].pathogens and invading insects are typically Forestspecific destruction to a particular caused host. by In addition, disease tree or death insect does infestation not directly is result inconsistent in stripping withof branches, the taxonomic diversity ofForestwhich fossil requiresdestruction woods a subsequent at caused Bruneau storm by diseaseevent. Woodpile; However, or insect plant some infestation pathogensBruneau Woodpile is andinconsistent specimens invading with show insects the evidence taxonomic are typically specificdiversity toof a insect particular of fossiland fungal woods host. attack at In Bruneau(Figure addition, 16); Woodpile; Fossilized tree death br plantacket does fungipathogens not have directly previously and invading result beenin reported insects stripping [8,25,37]; are typically of branches, these fungi are typically found on the trunks of dead or dying trees in modern forests. whichspecific requires to aa subsequentparticular host. storm In addition, event. However, tree death some does Bruneaunot directly Woodpile result in specimensstripping of show branches, evidence which requires a subsequent storm event. However, some Bruneau Woodpile specimens show evidence of insect and fungal attack (Figure 16); Fossilized bracket fungi have previously been reported [8,25,37]; of insect and fungal attack (Figure 16); Fossilized bracket fungi have previously been reported [8,25,37]; these fungi are typically found on the trunks of dead or dying trees in modern forests. these fungi are typically found on the trunks of dead or dying trees in modern forests.

Figure 16. Insect galleries are commonly preserved in specimens from Bruneau Woodpile. (A) Galleries in limb wood just below the bark layer probably made by insect larvae; (B) Galleries penetrating deeply into a hardwood limb; (C) close up view of insect gallery aligned subparallel to annual rings.

An alternate possibility to a pest epidemic is that trees were damaged by an intense wind storm, Figuresimilar 16.to the Insect forest galleries destruction are causedcommonly by wind preserved blast from in specimensthe 1980 eruption from Bruneau of Mount Woodpile. Saint Helens (A ) Figure 16. A GalleriesInsect in gallerieslimb wood are just commonly below the preserved bark layer in specimensprobably made from by Bruneau insect larvae; Woodpile. (B) Galleries ( ) Galleries

in limbpenetrating wood just deeply below into the a bark hardwood layer probably limb; (C) madeclose up by view insect of larvae; insect gallery (B) Galleries aligned penetrating subparallel deeplyto into aannual hardwood rings.limb; (C) close up view of insect gallery aligned subparallel to annual rings.

An alternate possibility to a pest epidemic is that trees were damaged by an intense wind storm, similar to the forest destruction caused by wind blast from the 1980 eruption of Mount Saint Helens

Geosciences 2017, 7, 82 15 of 25

Geosciences 2017, 7, 82 15 of 25 An alternate possibility to a pest epidemic is that trees were damaged by an intense wind storm, (WA, USA). During this explosive eruption, air blasts toppled thousands of trees, stripping away similar to the forest destruction caused by wind blast from the 1980 eruption of Mount Saint Helens limbs to leave a carpet of fallen trunks oriented by the wind direction (Figure 17A,B). In other more (WA, USA). During this explosive eruption, air blasts toppled thousands of trees, stripping away limbs protected areas, dead trees were left standing, with many intact branches. Subsequently, the to leave a carpet of fallen trunks oriented by the wind direction (Figure 17A,B). In other more protected branches and small trunk fragments were removed, swept downslope by the combined effects of areas, dead trees were left standing, with many intact branches. Subsequently, the branches and small wind and rain, leaving behind the parent logs (Figure 17C). trunk fragments were removed, swept downslope by the combined effects of wind and rain, leaving behind the parent logs (Figure 17C). Interpretation of past geologic events can seldom be done with certainty, but the principle of Interpretation of past geologic events can seldom be done with certainty, but the principle of parsimony (“Occam’s razor”) potentially provides a useful tool. This principle states that among parsimony (“Occam’s razor”) potentially provides a useful tool. This principle states that among competing hypotheses, the one with the fewest assumptions is preferred [38]. competing hypotheses, the one with the fewest assumptions is preferred [38]. In our study, hypothesis 1 presumes the delivery of wood, pumice, and sandy sediment In our study, hypothesis 1 presumes the delivery of wood, pumice, and sandy sediment occurred occurred as independent events, with subsequent reworking of these materials producing a as independent events, with subsequent reworking of these materials producing a well-defined stratum well-defined stratum at the base of a fining-upward sedimentary cycle. If wood was routinely at the base of a fining-upward sedimentary cycle. If wood was routinely delivered to the lake by delivered to the lake by streams, this wood underwent decomposition in all beds except for the Bruneaustreams, thisWoodpile wood underwentstratum, where decomposition the tissues in instead all beds became except forpetrified. the Bruneau Compare Woodpile this complex stratum, sequencewhere the to tissues the relatively instead becamesimple “catastrophic” petrified. Compare scenario this of complex hypothesis sequence 2: A major to the destructive relatively simple event “catastrophic”damages trees scenarioon watershed of hypothesis hill slopes; 2: A Wood major and destructive pumice eventclasts damagesbecome saturated trees on watershed from contact hill withslopes; moist Wood soil and or standing pumice clastswater. become A subsequent saturated precipitation from contact event with causes moist streams soil or to standing carry a water.pulse ofA subsequentwater-saturated precipitation wood and event pumice causes to streams the lake to to carry produce a pulse a density of water-saturated current. Suspended wood and material pumice settlesto the lakeon the to producelake floor a densityas a fining current. upward Suspended sediment material cycle where settles wood on the and lake pumice floor as are a fining deposited upward in thesediment basal bed. cycle Fossil where wood wood does and not pumice occur are in deposited other strata in thebecause basal the bed. transport Fossil wood of woody does notdebris occur was in notother an strata ongoing because process. the transportInstead, this of woody transport debris was was preceded not an by ongoing an unusual process. forest Instead, destruction this transport event. waspreceded by an unusual forest destruction event.

Figure 17. Destruction of trees from May 18, 1980 eruption of Mount Saint Helens (WA, USA). (A,B) Figure 17. Destruction of trees from May 18, 1980 eruption of Mount Saint Helens (WA, USA). “Blast zone” where fallen logs are oriented in the direction of wind created during explosive (A,B) “Blast zone” where fallen logs are oriented in the direction of wind created during explosive eruption. In more protected areas some trees were left standing, and even on fallen trees fractured eruption. In more protected areas some trees were left standing, and even on fallen trees fractured limbs limbs remained on the ash-covered ground surface; (C) Streams flowing down blast zone slopes remained on the ash-covered ground surface; (C) Streams flowing down blast zone slopes transported transported ash, pumice, and broken limbs, but lacked sufficient energy to move fallen logs. Photos ash, pumice, and broken limbs, but lacked sufficient energy to move fallen logs. Photos courtesy of courtesy of United States Forest Service. United States Forest Service.

“Catastrophic hypothesis 2” is consistent with the Neogene geologic history of the Snake River Plain.“Catastrophic In late Miocene hypothesis time, a 2”seri ises consistent of rhyolite with volcanic the Neogene eruptions geologic produced history periodic of the Snakeinfluxes River of rhyoliticPlain. In volcaniclastic late Miocene time,debris a seriesand tephra. of rhyolite In volcanicthis geologic eruptions setting, produced the distinction periodic influxes between of “ordinary’” and “catastrophic” processes is blurred; from the perspective of geologic time, “rare” events may become relatively common. The sequence of late Miocene events that produced the

Geosciences 2017, 7, 82 16 of 25 rhyolitic volcaniclastic debris and tephra. In this geologic setting, the distinction between “ordinary’” Geosciences 2017, 7, 82 16 of 25 and “catastrophic” processes is blurred; from the perspective of geologic time, “rare” events may unusualbecome relativelyBruneau common.Woodpile Thepetrifie sequenced wood of lateoccurrence Miocene cannot events be that reconstructed produced the with unusual certainty, Bruneau but Woodpilethe principle petrified of parsimony wood occurrence favors hypothesis cannot be2. A reconstructed simplified depiction with certainty, of this scenario but the principleis shown ofin parsimonyFigure 18. favors hypothesis 2. A simplified depiction of this scenario is shown in Figure 18.

Figure 18. This reconstruction illustrates streams carrying woody debris and sediment to Chalk Hills Figure 18. This reconstruction illustrates streams carrying woody debris and sediment to Chalk Hills Lake in the late Miocene. Original artwork by Jessie Thorenson. Lake in the late Miocene. Original artwork by Jessie Thorenson. 4.2. Mineralogy and Geochemistry 4.2. Mineralogy and Geochemistry The first detailed description of francolite (fluorapatite) was provided by Sandell et al. [39], The first detailed description of francolite (fluorapatite) was provided by Sandell et al. [39], though the mineral name was used as early as 1850. Apatite has previously been reported as a though the mineral name was used as early as 1850. Apatite has previously been reported as a common common mineral in phosphatic sediments and fossils [40]. mineral in phosphatic sediments and fossils [40]. Studies of lacustrine phosphatic oolite in the Pliocene Glenns Ferry Formation, which overlies Studies of lacustrine phosphatic oolite in the Pliocene Glenns Ferry Formation, which overlies the the Chalk Hills Formation have demonstrated the presence of fluorapatite (“francolite”) as an Chalk Hills Formation have demonstrated the presence of fluorapatite (“francolite”) as an important important constituent [41,42]. SEM/EDS analyses show that isolated fish vertebra in the constituent [41,42]. SEM/EDS analyses show that isolated fish vertebra in the wood-bearing stratum wood-bearing stratum at Bruneau Woodpile are composed of carbonate-fluorapatite. at Bruneau Woodpile are composed of carbonate-fluorapatite. 4.3. Source of Phosphorus 4.3. Source of Phosphorus Phosphorus has been described as the 10th [43] or 11th [44] most abundant element in the Phosphorus has been described as the 10th [43] or 11th [44] most abundant element in the Earth’s crust, but under pristine environmental conditions this element is typically present only at Earth’s crust, but under pristine environmental conditions this element is typically present only at low low concentration. In the USA., background orthophosphate concentrations from 400 shallow wells concentration. In the USA., background orthophosphate concentrations from 400 shallow wells were were found to average 0.03 mg/L [45]. Although, in regions affected by anthropogenic contributions: found to average 0.03 mg/L [45]. Although, in regions affected by anthropogenic contributions: agricultural fertilizer, leaking septic systems, and animal manure, the levels can be higher [46]. In agricultural fertilizer, leaking septic systems, and animal manure, the levels can be higher [46]. typical terrestrial settings phosphorus is a limiting factor for primary producers. Yet, phosphorus is In typical terrestrial settings phosphorus is a limiting factor for primary producers. Yet, phosphorus is a macronutrient needed by all living organisms for making membranes, nucleic acids, and ATP-life’s a macronutrient needed by all living organisms for making membranes, nucleic acids, and ATP-life’s energy currency. energy currency. Phosphorus enters the food chain through primary producers who obtain their phosphorus Phosphorus enters the food chain through primary producers who obtain their phosphorus from from both the residues of organisms and the weathering and erosion of phosphate-containing both the residues of organisms and the weathering and erosion of phosphate-containing minerals and minerals and rock, most of which owes its origin to ancient shallow marine environments. All rock, most of which owes its origin to ancient shallow marine environments. All consumers obtain their consumers obtain their phosphorus by eating other organisms. Thus, food webs allow for the phosphorus by eating other organisms. Thus, food webs allow for the bioaccumulation of phosphorus. bioaccumulation of phosphorus. Fish obtain their phosphorus from ingesting other organisms; Fish obtain their phosphorus from ingesting other organisms; seabirds that consume these fish produce seabirds that consume these fish produce guano that releases the accumulated phosphorus back into guano that releases the accumulated phosphorus back into the environment. Similarly, bats excrete the environment. Similarly, bats excrete guano that is phosphorus-rich because of their consumption guano that is phosphorus-rich because of their consumption of insects. The cycling of phosphorus of insects. The cycling of phosphorus in the natural environment is complex, involving processes in the natural environment is complex, involving processes where living organisms interact with where living organisms interact with and depend upon the rock cycle; the phosphorus cycle is a and depend upon the rock cycle; the phosphorus cycle is a topic that has paleontologic relevance. topic that has paleontologic relevance. For example, Arena [47] suggested that some Australian For example, Arena [47] suggested that some Australian phosphatized wood owed its origin to the phosphatized wood owed its origin to the close proximity of caves that contained bat guano. close proximity of caves that contained bat guano. Both organic and inorganic sources of phosphorus are relatively rare in terrestrial environments making the origin of phosphorus needed for the formation of nonmarime-phosphatized rocks or the mineralization of phosphatized fossils in many localities enigmatic. Likewise, the origin of carbonate-fluorapatite mineralized wood at the Bruneau Woodpile poses a mystery. It is tempting to consider that the Bruneau area may have received phosphorus from regionally extensive bedrock

Geosciences 2017, 7, 82 17 of 25

Both organic and inorganic sources of phosphorus are relatively rare in terrestrial environments making the origin of phosphorus needed for the formation of nonmarime-phosphatized rocks or the mineralization of phosphatized fossils in many localities enigmatic. Likewise, the origin of carbonate-fluorapatite mineralized wood at the Bruneau Woodpile poses a mystery. It is tempting to consider that the Bruneau area may have received phosphorus from regionally extensive bedrock source, the Phosphoria Formation, which contains abundant Ca and P. These Permian marine rocks are part of a regional stratigraphic sequence known as the Western Phosphate Field. The Phosphoria Formation, which represents shallow subtidal to intertidal deposition, originating at least in part from intertidal oolitic shoals, has been studied in detail [48–51]. Because present-day surface outcrops of the Phosphoria Formation in Idaho are restricted to the eastern part of the state, it is doubtful this could be the source of phosphorus in the deposits at Bruneau Woodpile. A massive fish kill could also act as a source of phosphorus and while disarticulated fish remains are mixed with fossil wood at the Bruneau Woodpile they are sparse.

4.4. Mineralization Process Phosphatization of wood has received little study, but evidence from other research is useful for understanding the mineralization processes. These occurrences reveal that phosphate mineralization can occur in diverse geologic settings (Table3). Mineralogic aspects of phosphatized wood have received little previous attention, but more than one pathway may be involved. 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. Briefly previewed here, these phosphatized specimens are from localities that will be described in detail in a future report. The specimen from the Virgin Valley Formation (NV, USA) (Figure 19A,B) comes from a late Miocene lakebed deposit where opalized fossil wood is abundant; this phosphatized wood presumably represents a highly localized geochemical anomaly. High magnification images show that the apatite crudely preserves anatomical features of cell walls, but the network of relatively large crystals has poor fidelity for fine details. This crystallinity suggests that the organic matter provided a favorable chemical Eh/pH environment for inorganic precipitation of Ca5(PO4)3, but that molecule-by-molecule organic templating was not involved. An alternate possibility is that original fine-grained apatite was later subject to diagenetic recrystallization. Phosphatized wood from Santa Fe River (FL, USA) (Figure 19C,D) has been fluvially transported from the original locality, so the geologic origin is enigmatic. Like the Nevada wood, tracheid boundaries and cell surface features are visible, but the coarse crystallinity obscures fine details. In this Florida wood, apatite is commonly present as botryoidal encrustations made up of radiating crystals.

Table 3. Known phosphatized wood occurrences.

Location Age Setting Reference Illinois, USA Jurassic marine [52] Svalbard, Boreal Realm Jurassic marine [53,54] Swindon, U.K. Jurassic marine [55] Antarctica Cretaceous continental [56] New Mexico, USA Cretaceous continental [57] California, USA Eocene continental This report Australia Miocene/Oligocene continental [47] Nevada, USA Miocene continental This report Idaho, USA Miocene continental This report Uganda Miocene/Pliocene continental [58] Florida, USA Neogene continental This report France Pliocene/Pleistocene continental [59] Pacific sea floor Holocene marine [60] Geosciences 2017, 7, 82 18 of 25 Geosciences 2017, 7, 82 18 of 25

Figure 19. Phosphatized wood from other CenozoicCenozoic localities. (A,B) Radial views of MioceneMiocene woodwood from Virgin ValleyValley Formation,Formation, northernnorthern Nevada,Nevada, USA,USA, mineralizedmineralized withwith fluorapatite;fluorapatite; (C,D) Neogene fluorapatitefluorapatite wood from Sante Fe River (FL, USA); (E,F) Apatite crystals on carbonized cell walls, Eocene, YubaYuba River, River, Nevada Nevada County County (CA, (CA, USA). USA). Specimens Specimens from from G. Mustoe G. Mustoe research research collection, collection, apatite compositionsapatite compositions determined determined by SEM/EDS. by SEM/EDS.

A very different form of apatite occurs in EoceneEocene wood from the Yuba River region in central California (USA)(USA) (Figure (Figure 19 E,F).19E,F). Samples Samples came came from from a large a carbonizedlarge carbonized log found log in found alluvial in sediment. alluvial Cellsediment. walls Cell consist walls of consist relict organic of relict matter organic that matter has fusedthat has into fused a single into homogenousa single homogenous layer. Interior layer. surfacesInterior surfaces of these of cells these contain cells contain abundant abundant hexagonal hexagonal microcrystals. microcrystals. SEM/EDS SEM/EDS spectra spectra reveal reveal their compositiontheir composition as pure as apatite.pure apatite. Thelocation The location and morphologiesand morphologies of these of these crystals crystals are evidence are evidence that theythat they originated by inorganic precipitation of Ca5(PO4)3 from groundwater that permeated the porous originated by inorganic precipitation of Ca5(PO4)3 from groundwater that permeated the porous wood long after the cellcell wallswalls werewere carbonized.carbonized. Their origin may have been a result of changes in groundwater chemistry, such as inin increaseincrease inin phosphorus.phosphorus. Alternately, apatite crystallization may have resulted in a change in Eh/pH conditions conditions that that favored favored precipitation. precipitation. Microbial processes may play a role inin phosphatizationphosphatization of faunal remains and phosphaticphosphatic mineral deposits deposits [63,64], [63,64], but but possible possible involvement involvement of ofmicrobes microbes during during phosphatization phosphatization of wood of wood has hasnot notbeen been studied. studied. Previous Previous reports reports provide provide abundant abundant evidence evidence for for the the recognition recognition of of microbial associations in aquatic mineral deposits where mineralmineral deposition is facilitated by microbial mats or biofilms.biofilms. These include spring sintersinter [[65–69]65–69] and clastic lake bed sediment [[70–72].70–72]. An SEM image of one Bruneau Woodpile fossilfossil wood specimen exhibits etching on its surface; these traces are inferred to have been mademade byby aa phosphorus-seekingphosphorus-seeking endolithendolith after the wood had already been petrifiedpetrified (Figure 1212D).D). Other fossilfossil woodwood specimensspecimens inin thisthis studystudy showshow nono evidenceevidence ofof microbes.microbes. Degradation of relict organic matter may have been important for controlling the Eh and pH conditions that favored phosphatephosphate precipitation.precipitation. Phosphatization of organic matter under geologicgeologic conditions has primarily been studied in depositsdeposits thatthat containcontain fossilizedfossilized invertebrates,invertebrates, particularly in occurrencesoccurrences were softsoft tissuestissues areare preservedpreserved [[4040,73–76].,73–76]. These fossils typically were preserved in marine sedimentary environments, likely explained by the greater abundance of phosphorus in ocean water and sea floorfloor sediment, compared to the low concentrationsconcentrations typical of terrestrial environments. Furthermore, animalanimal decay decay also also adds adds a source a source of phosphate of phosphate ions. Rapid ions. burial, Rapid a reducing burial, environmenta reducing andenvironment anaerobic and decay anaerobic are identified decay as are necessary identified parameters as necessary for phosphatization parameters for ofphosphatization animal soft tissue. of Onceanimal these soft conditions tissue. Once have these been conditions met evidence have suggests been thatmet aevidence steep chemical suggest gradients that a resulting steep chemical in a pH dropgradient triggers resulting the calciumin a pH carbonate—calciumdrop triggers the calcium phosphate carbonate—calcium switch, favoring ph apatiteosphate precipitation switch, favoring [74]. apatite precipitation [74]. Plant tissues are less likely to be phosphatized because they are relatively phosphorus deficient [40]; however, numerous occurrences have been reported and the chemical

Geosciences 2017, 7, 82 19 of 25

Plant tissues are less likely to be phosphatized because they are relatively phosphorus deficient [40]; however, numerous occurrences have been reported and the chemical conditions necessary for their formation may have important parallels to phosphatization of marine organisms. One detailed experimental study [77] involved shrimp carcasses that were incubated in a marine medium under aerobic conditions, introducing sulfate-reducing, sulfide-oxidizing and fermentative bacteria. Levels of introduced sulfate, glucose, microbes and sediment were varied among individual samples, as was buffering capacity of the culture medium. In most experiments, oxygen was quickly depleted, pH decreased, and sulfate accumulated. The result was anaerobic decay, accompanied by steep chemical gradients near the shrimp carcasses. In areas where decay was intense, pH decreased markedly, and some muscle tissue was replaced by Ca5(PO4)3. In less decayed areas, CaCO3 crystals precipitated. Local pH appeared to be the most important factor for determining whether CaCO3 or Ca5(PO4)3 was deposited. These experimental results resemble observations of phosphatization of organic matter under natural conditions. Phosphate-mineralized wood from the Pacific sea floor [60] appears to have resulted from decomposition of organic matter that resulted in decreases in Eh and pH. Because Eh and pH conditions that control solubility of CaCO3 and Ca5(PO4)3 are similar, high phosphorus concentration in the water apparently favored apatite deposition. Ca5(PO4)3 was deposited when pH values were around 6.2. At higher pH, mineralization was dominated by CaCO3 [60]. At the Bruneau Woodpile, all fossil wood is mineralized with carbonate-fluorapatite, but phosphate minerals are not detectable in the surrounding sandstone matrix. This mode of occurrence is evidence that unique geochemical conditions were present within the buried plant tissue. The hyaloclastic lake sediment would have potentially provided an abundant source of dissolved silica: pumice and volcanic ash are commonly associated with silicified wood, and experimental studies have shown obsidian to be a good source of dissolved silica for wood mineralization [78]. The presence of diatom fossils in the lakebed sediment is evidence that dissolved silica was available, and mineralization of the wood with calcium phosphate rather than silica is probable evidence that that Eh/pH conditions within the buried tissue were not favorable for silica precipitation. This is consistent with experimental evidence [78] demonstrating that when obsidian powder is used as a silica source, dissolved Si levels were elevated at pH values > 9. For wood that contained absorbed silica, a decrease in pH caused breakdown of hydrogen bonds between silica complexes and organic molecules, releasing silica back into aqueous solution. These experiments suggest that the low pH values typical of calcium phosphate precipitation would not have been favorable for silicification. The absence of phosphate minerals in the matrix that encloses the fossil wood is an indication that phospatization involved local geochemical gradients rather than more general diagenetic conditions. The mode of phosphate mineralization is unclear. One possibility is that at the Bruneau Woodpile carbonate-fluorapatite was deposited via process of organic templating, where deposition of calcium phosphate was facilitated by an affinity with cell wall constituents, analogous to the hydrogen bonds that form between silica and hydroxyl functional groups on organic wood molecules [56,79]. This organic templating possibility is consistent with SEM views of Bruneau wood, which show that cell walls contain microcrystalline carbonate-fluorapatite that preserves anatomical details with many vessel lumen remaining unmineralized (Figure 11).

4.5. Paleoclimate The Cenozoic climate followed a general cooling trend largely attributable to changes in ocean circulation patterns, but factors such as volcanic emissions and seabed methane releases also played roles. The Paleocene-Eocene Thermal Maximum (PETM) and Pleistocene glacial advances and retreats are notable examples of Cenozoic climatic fluctuations (Figure 20). Geosciences 2017, 7, 82 20 of 25 Geosciences 2017, 7, 82 20 of 25

FigureFigure 20.20. CenozoicCenozoic average average global global temperatures temperatures based ba onsed oxygen on oxygen isotope isotope values in values marine in sediments. marine sediments.Adapted from Adapted [80]. from [80].

Neogene paleoclimatic conditions in southern Idaho have been studied using paleontologic Neogene paleoclimatic conditions in southern Idaho have been studied using paleontologic evidence. Plant fossils provide information about regional climatic trends, and fish fossils can be evidence. Plant fossils provide information about regional climatic trends, and fish fossils can be used used to study seasonality. to study seasonality. The taxonomic composition of extant taxa found within a fossil assemblage can be compared to The taxonomic composition of extant taxa found within a fossil assemblage can be compared modern taxon-climate calibrated floras to infer past climate. In the Pacific Northwest, fossil floras to modern taxon-climate calibrated floras to infer past climate. In the Pacific Northwest, fossil floras record climatic changes that resulted from the uplift of the Cascade Range in the late Miocene and record climatic changes that resulted from the uplift of the Cascade Range in the late Miocene and early Pliocene [5]. early Pliocene [5]. Middle to late Miocene-aged fossil floras between latitudes 41° N and 46° N in the states of Middle to late Miocene-aged fossil floras between latitudes 41◦ N and 46◦ N in the states of Washington and Idaho are composed of broadleaved hardwoods and conifers with some Washington and Idaho are composed of broadleaved hardwoods and conifers with some assemblages assemblages including swamp elements such as Taxodium-type vegetation. These palaeofloras have including swamp elements such as Taxodium-type vegetation. These palaeofloras have strong affinities strong affinities to modern deciduous forests of eastern and southeastern North America as well as to modern deciduous forests of eastern and southeastern North America as well as eastern Asia. eastern Asia. These modern analogs suggest that a summer-wet climate existed in the Snake River These modern analogs suggest that a summer-wet climate existed in the Snake River Plain during the Plain during the middle to late Miocene, with annual precipitation of >1000 mm, and warm winters, middle to late Miocene, with annual precipitation of >1000 mm, and warm winters, and an estimated and an estimated mean annual temperature (MAT) of 12–13 °C. mean annual temperature (MAT) of 12–13 ◦C. Deposition of the late Miocene Chalk Hills Formation, the host sediment for the Bruneau Deposition of the late Miocene Chalk Hills Formation, the host sediment for the Bruneau Woodpile, Woodpile, occurred near the end of this climatic period and the fossil floras of this period indicate a occurred near the end of this climatic period and the fossil floras of this period indicate a transition transition towards a more summer-dry climate regime. Primary evidence for this comes from the towards a more summer-dry climate regime. Primary evidence for this comes from the Pickett Creek Pickett Creek flora, which is in the lower Chalk Hills Formation and has been dated at flora, which is in the lower Chalk Hills Formation and has been dated at approximately 8.5–10.5 Ma approximately 8.5–10.5 Ma based on geochemical correlation of volcanic ash layers present in the based on geochemical correlation of volcanic ash layers present in the deposit [6]. The Pickett Creek deposit [6]. The Pickett Creek flora lacks many of the exotic taxa that require high humidity and flora lacks many of the exotic taxa that require high humidity and summer rainfall found in older summer rainfall found in older middle Miocene floras of the Latah, Sucker Creek, and Mascall middle Miocene floras of the Latah, Sucker Creek, and Mascall Formations, with increasing dominance Formations, with increasing dominance of more drought tolerant types such as pines, white oaks, of more drought tolerant types such as pines, white oaks, live oaks, and Robinia. The Bruneau Woodpile live oaks, and Robinia. The Bruneau Woodpile is thought to be slightly younger than Pickett Creek is thought to be slightly younger than Pickett Creek and the wood types found at Bruneau Woodpile and the wood types found at Bruneau Woodpile closely mirror the Pickett Creek assemblage. Even though the growth of the Cascade Range had not yet created a significant rain shadow, there is

Geosciences 2017, 7, 82 21 of 25 closely mirror the Pickett Creek assemblage. Even though the growth of the Cascade Range had not yet created a significant rain shadow, there is evidence to suggest that the climate of the northwest interior was becoming more arid towards the late Miocene [81]. Pliocene-aged fossil floras in the Snake River Plain indicate a continued transition to a more arid climate, resulting from the rain shadow created by the newly-uplifted Cascade Range. These younger paleofloras show the decline of hardwoods, and the appearance of abundant grasses and dryland shrubs, particularly Artemisia (sagebrush). By the early Pleistocene, the transition to modern sagebrush/bunchgrass steppe flora was well under way [5]. Miocene and Pliocene fish faunas that populated Lake Idaho and its Miocene predecessors can be used as proxy sources for climatic parameters based upon their distributions and the limiting temperature for nearest living relatives. During the late Miocene, the cold month mean temperature is estimated to be 10 ◦C, compared to 0 ◦C for the Pliocene, and a reduction in the number of frost-free days (<0 ◦C) from 270 to 175. The seasonal range or difference between the mean for coldest and warmest month increased from 11 ◦C during the late Miocene to 21 ◦C during the Pliocene [20]. In summary, evidence from previous studies of fossil leaf, pollen and fish fossils suggests that the trees that produced the Bruneau Woodpile came from forests that existed during the climatic transition that occurred near the close of the Miocene Epoch. This plant community represents an assemblage that contains more summer-dry adapted taxa than the earlier mid-Miocene floras of the region; very different from the arid conditions that exist today.

5. Conclusions The Bruneau Woodpile is an unusual deposit, providing a rare opportunity to collect abundant specimens of fossil wood mineralized with carbonate-fluorapatite. The taxonomic diversity of the specimens provides a glimpse of an ancient forest growing in a region that was on the brink of climatic change. The region would soon transition from warm temperate to semi-arid. One long-standing enigma of this locality is the abundance of wood in the form of small limbs and trunk fragments, with no larger specimens. The driftwood-like morphology of the specimens is evidence of fluvial transport. As described above, two possible hypotheses are considered. Hypothesis 1 assumes that the wood-bearing stratum resulted from a sequence of ordinary processes, where wood was continuously introduced to the lake, but only preserved in a single stratum, where it was intermixed with waterlogged pumice and volcaniclastic sediment. Hypothesis 2 invokes a catastrophic event that severely damaged local forest, producing a reservoir of branches and wood fragments that gradually became water saturated from soil moisture or in shallow ponds. This material was later transported to the lake during a storm event when a high-energy stream flow simultaneous carried pumice, sand, and silt. This suspended sediment entered Chalk Hills Lake as a turbidity current, the source of the fining-upward sediment cycle that contains wood in the coarse basal bed. The principle of parsimony favors hypothesis 2. Chemical conditions in the deposit caused mineralization of the wood by replacement with carbonate fluorapatite. While the exact mechanism could not be determined, the process may have been similar to the silicification of wood in which the wood acts as a template for the mineralization process. Microbes do not appear to have been directly responsible for phosphatization, but inorganic phosphate precipitation may have been the result of Eh/pH gradients that were strongly influenced by the decomposition of relict organic matter.

Supplementary Materials: The following are available online at www.mdpi.com/2076-3263/7/3/82/s1. Atlas of wood types from the Bruneau Woodpile, with transverse section images and anatomical descriptions. Acknowledgments: Rick Dillhoff and Mary Klass assisted with excavation and specimen collecting. Elisabeth Wheeler provided helpful advice for interpreting photomicrographs. Nancy and Larry Dillon provided guidance in how to work the fossil site and provided some samples Richard Rantz contributed specimens collected in 1997 for initial chemical analyses. Yuba River, California USA specimens (Figure 19) provided by David Lawler, Farwest Geoscience Foundation Director. Jessie Thoreson provided the (Figure 18) sketch. We thank Barbara Nash for determining the age of the ash. Geosciences 2017, 7, 82 22 of 25

Author Contributions: The investigation was conceived and directed by Viney. Link provided stratigraphic data and age determinations. Link, Viney, Dillhoff participated in field work; taxonomic identifications were provided by Dillhoff. Mustoe provided graphic art., photomicrographs, SEM/EDS data, XRD and XRF analyses. The manuscript was written by Mustoe and Viney, with contributions from the other coauthors. Conflicts of Interest: The authors declare no conflicts of interest.

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