Composition of the Rex Chert and Associated Rocks of the

Permian Phosphoria Formation: Soda Springs Area, SE Idaho

B y

James R. Hein1

, Brandie McIntyre1, Robert B. Perkins1, David Z.

Piper1

, James Evans2

Open-File Report 02-345

2002

Prepared in Collaboration With

U.S. Bureau of Land Management

U.S. Forest Service

Agrium U.S. Inc.

Astaris LLC

J.R. Simplot Company

Rhodia Inc.

Monsanto Co.

This report is preliminary and has not been reviewed for conformity with U.S. Geological

Survey editorial standards or with the North American Stratigraphic Code. Any use of

trade, firm, or product names is for descriptive purposes only and does not imply

endorsement by the U.S. Government.

U. S. DEPARTMENT OF THE INTERIOR

U. S. GEOLOGICAL SURVEY 1

U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA, 94025

2

U.S. Geological Survey, 904 W. Riverside Ave., Spokane, WA, 99201
2

CONTENTS

ABSTRACT 4

INTRODUCTION 5

Background 5

Location and General Geology 5

Lithostratigraphy 7

METHODS 9

Field Sampling 9

Rock-Sample Preparation 1 0

Analyses 10

RESULTS 16

Petrography 1 6

Mineralogy 18

Chemical Composition 1 8

Stratigraphic Changes in Chemical Composition 2 3

Phase Associations of Elements 2 3

DISCUSSION AND CONCLUSIONS 2 8

ACKNOWLEDGMENTS 2 9

REFERENCES CITED 2 9
3

FIGURES

Figure 1. Index map of SE Idaho and locations of sections of Rex Chert.

Figure 2. Three measured stratigraphic sections of the Rex Chert Member of the

Phosphoria Formation.

Figure 3. Q-mode rotated factor scores and phase association of elements for chemical data

from sections 1, 5, and 7.

Figure 4. Q-mode rotated factor scores and phase association of elements for chemical data

from section 1.

Figure 5. Q-mode rotated factor scores and phase association of elements for chemical data

from section 7.

TABLES

Table 1. GPS locations of Rex Chert sections in the vicinity of Dry Valley, SE Idaho

Table 2. X-ray diffraction mineralogy of samples of the Rex Chert, cherty shale, and

Meade Peak Members of the Phosphoria Formation

Table 3. Chemical Composition of samples from the Rex Chert, cherty shale, and Meade

Peak Members of the Phosphoria Formation

Table 4. Statistics of chemical data for 39 Rex Chert and cherty shale samples from

sections 1, 5, and 7

Table 5. Statistics of chemical data for 14 Rex Chert and cherty shale samples, section 1

Table 6. Statistics of chemical data for 4 Rex Chert samples, section 5

Table 7. Statistics of chemical data for 21 Rex Chert samples, section 7

Table 8. Correlation coefficient matrix of Rex Chert and cherty shale member chemical data

combined from measured sections 1, 5, and 7

Table 9. Correlation coefficient matrix of Rex Chert and cherty shale members chemical

data from measured section 1

Table 10. Correlation coefficient matrix of Rex Chert Member chemical data from measured

section 7
4

ABSTRACT

This study, one in a series, reports bulk chemical and mineralogical compositions, as

well as petrographic and outcrop descriptions of rocks collected from three measured

outcrop sections of the Rex Chert member of the Phosphoria Formation in SE Idaho. The

three measured sections were chosen from ten outcrops of Rex Chert that were described in

the field. The Rex Chert overlies the Meade Peak Phosphatic Shale Member of the

Phosphoria Formation, the source of phosphate ore in the region. Rex Chert removed as

overburden comprises part of the material disposed in waste-rock piles during phosphate

mining. It has been proposed that the chert be used to cap and isolate waste piles, thereby

inhibiting the leaching of potentially toxic elements into the environment. It is also used to

surface roads in the mining district. The rock samples studied here constitute a set of

individual chert beds that are representative of each stratigraphic section sampled. The

informally named cherty shale member that overlies the Rex Chert in measured section 1

was also described and sampled. The upper Meade Peak and the transition zone to the Rex

Chert were described and sampled in section 7. The cherts are predominantly spicularite

composed of granular and mosaic quartz, and sponge spicules, with various but minor

amounts of other fossils and detrital grains. The cherty shale member and transition rocks

between the Meade Peak and Rex Chert are siliceous siltstones and argillaceous cherts with

ghosts of sponge spicules and somewhat more detrital grains than the chert. The

overwhelmingly dominant mineral is quartz, although carbonate beds are rare in each

section and are composed predominantly of calcite and dolomite in addition to quartz.

Feldspar, mica, clay minerals, calcite, dolomite, and carbonate fluorapatite are minor to

trace minerals in the chert.

The mean concentrations of oxides and elements in the Rex Chert and the cherty shale

member are dominated by SiO2, which averages 94.6%. Organic-carbon contents are

generally very low in the chert, but are up to 1.8 wt. % in cherty shale member samples

and up to 3.3% in samples from the transition between the Meade Peak and Rex Chert.

Likewise, phosphate (P2O5) is generally low in the chert, but can be up to 3.1% in

individual beds. Selenium concentrations in Rex Chert and cherty shale member samples

vary from <0.2 to 138 ppm, with a mean concentration of 7.0 ppm. This mean Se content

is heavily dependent on two values of 101 and 138 ppm for rocks from the transition zone

between the Meade Peak and Rex Chert. Without those two samples, the mean Se

concentration would be <1.0 ppm. Other elements of environmental interest, As, Cr, V,

Zn, Hg, and Cd, are generally near or below their content in average continental shale.

Stratigraphic changes (equivalent to temporal changes in the depositional basin) in chemical

composition of rocks are notable either as uniform changes through the sections or as

distinct differences in the mean composition of rocks that compose the upper and lower

halves of the sections.

Q-mode factors are interpreted to represent the following rock and mineral components:

chert-silica component consisting of Si (± Ba); phosphorite-phosphate component

composed of P, Ca, As, Y, V, Cr, Sr, and La (± Fe, Zn, Cu, Ni, Li, Se, Nd, Hg); shale

component composed of Al, Na, Zr, K, Ba, Li, and organic C (± Ti, Mg, Se, Ni, Fe, Sr,

V, Mn, Zn); carbonate component (dolomite, calcite, silicified carbonates) composed of

carbonate C, Mg, Ca, and Si (± Mn); tentatively organic matter-hosted elements (and/or

sulfide-sulfate phases) composed of Cu (± organic C, Zn, Mn Si, Ni, Hg, and Li).

Selenium shows a dominant association with the shale component, but correlations and Qmode factors also indicate that organic matter (within the shale component) and carbonate

fluorapatite may host a portion of the Se. Consideration of larger numbers of factors in Qmode analysis indicates that native Se (a factor containing Se (± Ba)) may also comprise a

minor component of the Se compliment.
5

INTRODUCTION

We have initiated a study of the Rex Chert, which conformably overlies the Meade

Peak Phosphatic Shale Member of the Permian Phosphoria Formation. It comprises part of

the overburden that is removed to reach the phosphate ore at mines in SE Idaho and

therefore comprises a part of the waste dumps. In addition, the Rex Chert is used to

surface roads in the mining district. It has been proposed that the chert be used to cap and

isolate waste dumps to prevent the release of selenium (Se) and other potentially toxic

elements to the environment. Previous spot analyses indicated that Se might occur in high

concentrations in some chert beds, so we sampled several outcrop sections to determine the

composition of the Rex Chert. A parallel report has been produced by Herring et al. (2002)

on channel samples through the Rex Chert from the active Rasmussen Ridge and Enoch

Valley phosphate mines. Those channel samples comprise a continuous composite record

of Rex Chert composition through the entire section. In contrast, our outcrop samples

provide the compositions of a series of individual beds through each outcrop section, with

intervening beds being unsampled. One outcrop section sampled also includes the

overlying, informally named, cherty shale member of the Phosphoria Formation and

another section includes the upper part of the Meade Peak Phosphatic Shale Member and

the transition zone between the Meade Peak and Rex Chert.

Background

U.S. Geological Survey (USGS) geologists have studied the Phosphoria Formation in

SE Idaho and the Western U.S. Phosphate Field throughout much of the twentieth century.

In response to a request by the U.S. Bureau of Land Management (BLM), a new series of

resource and geoenvironmental studies was initiated by the USGS in 1998. Present studies

involve many scientific disciplines within the USGS and consist of: (1) integrated,

multidisciplinary research directed toward resource and reserve estimations of phosphate in

selected 7.5-minute quadrangles; (2) element residences, mineralogical and petrochemical

characteristics; (3) mobilization and reaction pathways, transport, and disposition of

potentially toxic trace elements associated with the occurrence, development, and use of

phosphate rock; (4) geophysical signatures; and (5) improving our understanding of the

depositional origin and evolution of the deposit.

To carry out these studies, the USGS formed cooperative research relationships with

the BLM and the U.S. Forest Service (USFS), which are responsible for land management

and resource conservation on public lands, and with five private companies currently

leasing or developing phosphate resources in southeastern Idaho. These companies include

Nu-West (Agrium U.S. Inc.) (Rasmussen Ridge mine), Astaris Production LLC (Dry

Valley mine), Rhodia Inc. (Wooley Valley mine-inactive), J.R. Simplot Co. (Smoky

Canyon mine), and P4 Production LLC (Monsanto and Solutia) (Enoch Valley mine).

Because raw data acquired during the project will require time to interpret, data are released

in open-file reports for prompt availability to other workers. The open-file reports

associated with this series of resource and geoenvironmental studies are submitted to each

of the Federal and industry collaborators for technical comment; however, the USGS is

solely responsible for the data contained in the reports.

Location and General Geology

Ten sections of Rex Chert located in the vicinity of Dry Valley, northeast of Soda

Springs in SE Idaho (Fig. 1; Table 1), were described in the field. Three of those sections

(1, 5, 7) were measured (Figs. 2-4) and sampled in detail for chemical, mineralogical, and

petrographic analyses. This region of SE Idaho has supported extensive phosphate mining

over the past several decades and currently has four active mines. Service (1966) evaluated
6

Figure 1. Index map of S. E. Idaho and location of sections of Rex

Chert (1-5, 7-11); section 1, 5, and 7 were measured and sampled

Table 1. GPS coordinates and elevations; both accurate

within <15 meters (49 ft)

Section

Number

Latitude

(N)

Longitude

(W)

Elevation

(m)

Section 1 42° 42.16’ 111° 29.07’ 2122

Section 2 42° 41.78’ 111° 24.61’ 1967

Section 3 42° 42.82’ 111° 21.93’ 1974

Section 4 42° 42.17’ 111° 22.04’ 1961

Section 5 1

42° 39.38’ 111° 19.76’ 2042

Section 5 2

42° 39.39’ 111° 19.42’ 2042

Section 7 42° 43.91’ 111° 17.37’ 2248

Section 8 42° 42.15’3 111° 17.07’3 25603

Section 9 42° 44.85’ 111° 17.79’ 2100

Section 10 42° 37.55’ 111° 21.13’ 2314

Section 11 42° 36.89’ 111° 20.25’ 2134

 1 SW end, 2 NW end, 3 taken from topographic map
7

the western phosphate industry in Idaho and provided a brief description of the mining

history, ore occurrence, and geology, which has recently been updated by Lee (2001).

McKelvey and others (1959) discussed in detail the Phosphoria Formation in the Western

Phosphate Field. Cressman and Swanson (1964) provided detailed stratigraphy and

petrology of these same rock units located in nearby SW Montana. Gulbrandsen and Krier

(1980) discussed general aspects of the large and rich phosphate resources of the

Phosphoria Formation in the vicinity of Soda Springs. Gulbrandsen (1966, 1975, 1979)

summarized bulk chemical compositions for various rock types of the phosphatic intervals

of the Phosphoria Formation.

Lithostratigraphy

Three stratigraphic sections of the Rex Chert Member were measured and those three

sections along with seven others were described in the field in SE Idaho (Fig. 1). Samples

were collected from the three measured sections and their stratigraphic positions are

indicated in Fig. 2.

The Phosphoria Formation in the Soda Springs area of SE Idaho consists of three

members, which in ascending order are the Meade Peak Phosphatic Shale Member, the Rex

Chert, and the informally named cherty shale (McKelvey and others, 1959; Montgomery

and Cheney, 1967; Brittenham, 1976; Oberlindacher, 1990). The measured sections of

this report focus on the Rex Chert in addition to the cherty shale member in one measured

section. The Meade Peak unconformably overlies the Grandeur Tongue of the Permian

Park City Formation, and the cherty shale member of the Phosphoria is overlain by the

Triassic Dinwoody Formation.

The contacts between the Meade Peak and the Rex Chert and between the Rex Chert

and the cherty shale member are gradational. The transitional rocks generally contain

carbonates or carbonate-rich beds.

Section 1 was measured through a road cut that exposed the transition zone between the

upper part of the Meade Peak and the Rex Chert and an adjacent quarry wall that exposed

the cherty shale member. Section 5 was measured in a small quarry where the Rex Chert

was being mined for surfacing roads. The underlying Meade Peak was not exposed and

the overlying cherty shale was not present. Section 7 was measured along the exposed face

of the now-abandoned South Maybe Canyon Mine and includes the upper Meade Peak

rocks and transition rocks to the Rex Chert. Measurements record true thickness of strata.

In section 1 , the Meade Peak Member is covered and the lowermost beds exposed

include limestone alternating with black chert (Fig. 2, unit A). Most of the lower part of

the Rex Chert is thin-bedded black and gray chert, which gives way up section to thickbedded massive gray chert (units B-F). Load casts are seen at the base of the thick chert.

The uppermost massive chert beds are composed of composite beds displaying pinch-andswell structures (unit F). Above the uppermost massive thick chert bed are several thin,

iron-rich, weathered chert beds (unit G). Sponge spicules are clearly seen in some of the

chert beds, but thin sections reveal that most of the Rex Chert in this section is spicularite.

The transition to the overlying cherty shale member is covered in the measured section, but

in the outcrop on the opposite (south) side of the road, that position may be occupied by a

chert-pebble conglomerate that is not seen on the north side of the road. The lower third of

the cherty shale member consists of thin-bedded (up to 6 cm) siliceous shale, with some

beds displaying load casts (unit H1). Beds generally thicken up section. The middle third

of the section consists of thin-bedded (6-15 cm) argillaceous chert with thinner (<1-3 mm)

interbedded siliceous shale (unit H2). The upper third of the section is variable and

consists of moderately thick-bedded (10-30 cm) argillaceous chert with sheared siliceous

shale partings (unit H3). Near the top of the section occurs a body of pale-brown siliceous

dolostone that appears to be the hinge of a fold and the remainder of the bed(?) is not

exposed laterally from that body. This was probably originally a dolostone bed or laterally
8

Figure 2. Measured and sampled sections of Rex Chert; units H1-H3 in

section 1 is cherty shale member, units A-D in section 7 is Meade Peak

Member

T

ξ

ξ

ξ

X X

M

T

T

ξ

M

M

ξ

X X

M

ξ

ξ

SECTION 7

SECTION 5

H3

H2

H1

G

F

E

D

C

B

A

SECTION 1 

LEGEND

Symbols

26-1A

26-1B

26-1D

26-1C

27-1A

27-1B

27-1C

27-1D

27-1E

27-1F

27-1G

27-1H

27-1I

27-1J

27-1K

27-1L

27-1M

27-1N

28-1N

28-1M

28-1L

28-1K 28-1I 28-1J

28-1H

28-1G

28-1F

28-1E

28-1D

28-1C

28-1B

28-1A

28-1P

28-1Q

28-1R

Black, gray chert

Limestone/dolostone

Black shale/phosphorite

Siliceous siltstone/shale

Breccia

Rock Types

A

B

C

I

H

G

F

E

D

C

B

A

Fractures

Fault

Thick bedding

Laminated

Covered section

Carbonate lenses

Intraformational breccia

Concretions with 

wrapped laminae

Burrows

Spicules

Load cast

Fossils

Samples collected

Pinch and Swell

structure

Informal unit 

designator H1

scale (m)

Siliceous dolostone

White chert

B

B

Brown chert

 0

 10

 5

Marker bed

14-2A

14-2B

14-2C

14-2D

14-2E

14-2F

14-2G
9

extensive lens. A bed (40 cm thick) of laminated and sheared black siliceous shale occurs

near the top of the section, which is capped by a nodular siliceous shale bed. The upper

part of the section is highly sheared and is likely bounded by a fault. It is likely that the

thickness of the Rex Chert and cherty shale members have been altered by folding and

faulting. Bedding (S0) in much of the Rex Chert has been obscured by development of

generally low-relief stylolites (S1) oriented subparallel to bedding. Locally, bedding has

been transposed (S2) so that it parallels axial planes of isoclinal folds that may have formed

during compaction, or possibly during emplacement of the Paris thrust plate. S2 remained

an active structural element as shown by quartz veins that formed perpendicular to S2 and

are offset along S2.

Section 5 was measured at its NW end in a quarry where the thickness of the Rex Chert

is a minimum because the lower part of the section is covered and the upper part has been

removed by quarrying. The Rex Chert consists of dark-gray, medium- to thick-bedded

chert that is divided into two sections by an intervening friable brown layer of ferruginous

chert (unit B) that has an earthy texture. This 30-40 cm-thick brown zone may be alteration

along a fault or leaching of a chert bed. The cherts in this section are spicularites, as

determined from thin sections. The chert shows bedding-parallel pressure-solution

cleavage as indicated by truncated microfossils and accumulations of relatively insoluble

components such as sericite and hematite.

In section 7 , the upper part of the Meade Peak Member (units A-D) and the transition to

the Rex Chert (units E-F) are well exposed. Alternating beds of black shale, phosphorite,

and carbonate characterize the Meade Peak exposure. Carbonates (calcite and dolomite)

occur as beds, horizons of disconnected concretions, and as isolated concretions in some

shale beds. Generally the carbonates thicken up section from nodules to thin beds to

thicker beds, except near the base of the exposed section where a 0.6 m thick carbonate

"marker bed" occurs. These marker beds (see below also) crop out over a distance of

about 3 miles (4.8 km) in the North, Middle, and South Maybe Canyon mines in the Dry

Ridge Mountains. Shale and phosphorite laminae wrap around the carbonate concretions

indicating an early diagenetic origin (pre-compaction). The Meade Peak is overlain by a

0.8 m thick marker bed of siliceous siltstone (Fig. 2, unit E). That bed is overlain by a

section that is partly covered, but consists predominantly of thin-bedded siliceous siltstone

(unit F). That transition zone of siliceous siltstones is overlain by thin- to medium-bedded

(5-30 cm) black chert with brown upper and lower margins on each bed (unit G). Load

casts are seen at the base of some beds. A few thin siliceous siltstone beds also occur in

unit G. Those cherts are overlain by 1 m of thin-bedded white chert that is overlain by a 1

m thick white chert marker bed (unit H). The white cherts are overlain by 10 m of thin- to

medium-bedded (to 30 cm) black and gray chert (unit I) that is very similar to cherts in unit

G. The section is capped by a 8 m thick, discontinuous chert megabreccia with a calcitebearing quartz cement. This breccia body may be a slump deposit or fault-zone deposit.

METHODS

Field Sampling

The samples within the measured sections obtained for analyses were collected as

individual beds, where possible. In several places, where beds were too thick to collect in

their entirety, portions of beds were collected. This approach provides an opportunity to

determine the changes in composition of like samples through the history of deposition of

stratigraphic sections. The choice of beds sampled is intended to provide uniform

representation of each section. In addition, unusual rock types were sampled, for example
10

a large dolomitic body in the cherty shale member. About 0.1-1 kg of rock was collected

for each bed.

Rock Sample Preparation

A representative slab was cut through the entire thickness of each chert bed sample and

was crushed in a mechanical jaw crusher, and then powdered in a roller mill. An aliquot of

the powdered material was analyzed by X-ray diffraction for mineral content and a second

aliquot was sent to a contract laboratory for chemical analyses. All splits were obtained

with a riffle splitter to ensure similarity with the whole sample. Splits of about 15-50 g

were sent to the contract laboratory and the remaining powders for all samples are archived

at the USGS. A second slab of each chert bed sample was cut into one or more thin

sections for petrographic analysis.

Analyses

Samples were analyzed for 40 major, minor, and trace elements using acid digestion in

conjunction with inductively coupled plasma-atomic emission spectrometry (ICP-AES).

For the 40-element analysis (referred to as ICP-40), a split was dissolved using a lowtemperature (<150o

 C) digestion with concentrated hydrochloric, hydrofluoric, nitric, and

perchloric acids (Jackson and others, 1987). The analytical contractor modified this

procedure to shorten the digestion time (P. Lamothe, USGS, oral communication). The

acidic sample solutions were taken to dryness and the residue was dissolved with 1 ml of

aqua regia and then diluted to 10.0 g with 1% (volume/volume) nitric acid. Strontium and

Ba concentrations were determined by both ICP-40 and ICP-16 (see below) techniques and

the two data sets are comparable (R2 = 0.999 for Sr and 0.997 for Ba) and are reported in

Table 3. Manganese concentrations were also provided by both ICP techniques and they

have comparable accuracy and precision, however, the ICP-40 data set is the only one

reported because it has a much lower detection limit, 4 parts per million (ppm) compared to

100 ppm for ICP-16. The ICP-40 technique measures the following elements above the

indicated ppm limits: Au 8 ppm, Bi 50 ppm, Sn 50 ppm, Ta 40 ppm, and U 100 ppm;

however, no samples had concentrations above those quantification limits and those

elements are not included in Table 3.

Another split of the each sample was fused with lithium metaborate then analyzed by

ICP-AES after acid dissolution of the fusion mixture. This technique, referred to as ICP16, provides analysis of all major elements, including Si, and a few minor and trace

elements. Cherts are very high silica rocks yet the accuracy of Si determinations is quite

good, probably about 2-4% based on the total-oxide sums. Si measurement is not possible

using the 4-acid digestion ICP-40 technique because it is lost as a volatile fluoride

compound during digestion. Analysis of major elements using the fusion technique also

provides a compositional check on the concentrations of these same elements as measured

by acid digestion. Titanium and Cr were analyzed using both ICP techniques, but only

data from the ICP 16 technique are given in Table 3 because the fusion technique more

completely digests resistant minerals that might contain those elements.

Selenium concentrations were determined using hydride generation followed by atomic

absorption (AA) spectrometry. Selenium is not reported using either of the ICP

techniques, as it is generally volatilized during sample preparation. The hydride generation

combined with AA technique was also used to determine concentrations of As, Sb, and Tl.

The hydride analytical technique is considered to be more sensitive than the acid digestion

ICP-AES analytical technique for As and are the data reported here. Mercury was

determined by cold vapor atomic absorption spectrometry.

Total S and total C were measured using combustion in oxygen followed by infrared

measurement of the evolved gas. For the other forms of carbon, carbonate carbon was
mposition of rocks from three sections (1, 5, 7; Figs. 1, 2) of Rex Chert and adjacent rocks; samples are listed in

ajor oxides, C, and S in wt.%, others in ppm

ple Description Lithology SiO2 Al2O3 Fe2O3 TiO2 CaO K2O MgO Na2O P2O5

ICP-16 ICP-16 ICP-16 ICP-16 ICP-16 ICP-16 ICP-16 ICP-16 ICP-16

-1I; CHERTY SHALE MEMBER 1J-1N

n, calcareous Cherty-calcareous shale bed 87.5 3.27 0.77 0.22 2.28 0.61 1.09 0.63 0.48

k Laminated siliceous shale 78.7 8.41 2.66 0.55 1 1.95 0.8 0.59 0.7

brown to gray=fresh Siliceous dolostone 48.8 1.1 0.7 0.07 16.2 0.16 9.91 0.22 0.16

n, earthy, porous Argillaceous chert bed 92.6 2.74 1.92 0.17 0.43 0.3 0.18 0.61 0.32

n, calcareous Argillaceous chert bed 92 2.4 0.64 0.17 1.41 0.43 0.73 0.31 0.2

n Shaly interbed 90.9 3.48 0.96 0.23 0.8 0.64 0.32 0.42 0.44

n, calcareous Cherty shale bed 86.2 3.82 1.16 0.23 2.01 0.78 0.95 0.49 0.44

n, Fe stained Phosphatic-ferruginous chert bed 83.2 2.55 4.68 0.12 3.69 0.64 0.45 0.08 3.05

Chert bed 98.2 0.79 0.76 0.02 0.5 0.13 0.07 0.04 0.37

 spicular Base of massive thick chert bed 95.6 1.19 1.4 0.03 0.97 0.22 0.12 0.03 0.85

Chert bed 99.9 0.26 0.23 <0.02 0.39 0.02 <0.02 0.01 0.28Chert bed 96 0.62 0.44 <0.02 1.27 0.08 0.05 0.03 0.96

k Chert bed 98.2 0.6 0.33 0.02 0.21 0.1 0.05 0.04 0.09

k, calcareous Chert bed 97.5 0.43 0.19 0.02 1.1 0.06 0.18 0.04 0.16k, calcareous Chert bed 93.5 0.59 0.17 0.03 2.31 0.07 0.93 0.05 0.28

Siliceous limestone bed 44.3 0.2 0.06 <0.02 30.9 0.04 0.48 0.04 0.05

 gray, fractured Chert bed 98.6 0.55 0.31 0.02 0.22 0.07 0.05 0.04 0.14

y, brown Leached zone in chert 93.9 1.49 2.65 0.08 0.43 0.17 0.12 0.04 0.3

 gray, fractured Chert bed 97.5 1.04 0.47 0.03 0.29 0.16 0.1 0.05 0.18

 gray, fractured Chert bed 97.5 1.1 0.49 0.05 0.25 0.18 0.08 0.07 0.14

01-28-1A-1C: REX CHERT 601-28-1D THROUGH 601-28-1R1

brown-gray chert clast Breccia 99.5 0.4 0.07 <0.02 1.05 0.05 0.55 0.05 0.14

 gray, fractured Chert bed 101 0.4 0.06 <0.02 0.46 0.04 0.02 0.05 0.28

Chert bed 95.1 0.55 0.11 0.03 0.41 0.05 0.02 0.08 0.25

k, fractured Chert bed 95.81 0.6 0.11 0.03 0.43 0.06 0.03 0.08 0.25

n Chert bed 94.29 0.72 0.21 0.03 0.49 0.08 0.03 0.07 0.3

 mottled Chert bed 96.7 0.25 0.04 <0.02 2.92 0.01 0.7 0.04 0.09

e, upper 15 cm margin 1N-1K=parts of a 80 cm thick 102 0.28 0.17 <0.02 0.3 0.04 0.02 0.04 0.18

e, middle 7 cm white chert bed 101 0.32 0.04 <0.02 0.18 0.01 0.02 0.03 0.09

e, 7-15 cm above base chert 101 0.23 0.13 <0.02 0.27 <0.01 0.02 0.03 0.16

e, lower margin (8 cm) chert 101 0.36 0.1 <0.02 0.35 0.02 0.02 0.03 0.2

brown, upper bed margin Chert 100 0.28 0.1 <0.02 0.3 0.02 0.02 0.04 0.18

e-gray, main bed, dense Chert 101 0.32 0.04 <0.02 0.2 0.02 <0.02 0.03 0.11

e-gray, lower bed margin Chert 101 0.42 0.09 <0.02 0.34 0.02 0.02 0.03 0.21

k-brown Chert bed 96.7 0.77 0.21 0.03 0.5 0.1 0.03 0.08 0.32

brown, silty Chert bed 99.5 0.43 0.17 0.02 0.34 0.05 0.03 0.07 0.2

brown, upper bed margin Chert 96 1.38 0.37 0.05 1.01 0.22 0.08 0.12 0.7

n, main bed Chert 90.7 0.93 0.26 0.05 0.67 0.14 0.05 0.11 0.39

brown, lower bed margin Chert 95.4 1.64 0.6 0.08 1.48 0.27 0.12 0.12 1.01
12

ple Description Lithology SiO2 Al2O3 Fe2O3 TiO2 CaO K2O MgO Na2O P2O5

ICP-16 ICP-16 ICP-16 ICP-16 ICP-16 ICP-16 ICP-16 ICP-16 ICP-16

-brown, fossiliferous Phosphatic cherty shale bed 89 3.48 1.24 0.18 2.46 0.69 0.3 0.15 1.72

k, thin-bedded Siliceous siltstone 75 10.7 2.09 0.78 0.17 2.58 0.65 1.2 0.09

-brown, mid. Thick bed Siliceous siltstone, marker bed 78.5 11.1 0.96 0.82 0.34 2.57 0.61 1.32 0.05

k Phosphatic-calcareous shale 48.3 8.35 2.75 0.48 17.1 1.88 0.7 0.66 5.5

k, carbonaceous Phosphorite 3.96 0.59 0.13 0.03 46.8 0.12 0.07 0.16 33.1

k nodule Carbonate nodule in black shale 3.53 0.47 0.16 0.03 47.8 0.08 2.74 0.22 2.82

k, carbonaceous Phosphatic shale 16.1 2.48 0.81 0.13 35.1 0.65 0.91 0.63 15.4

k nodule Carbonate nodule in black shale 1.1 0.15 0.06 <0.020 48.7 0.01 3.3 0.11 2.8

k, carbonaceous Phosphatic shale 23.8 3.87 1.3 0.23 31.5 0.96 1.09 0.74 9.56k, carbonaceous Phosphatic shale 46 7.29 2.37 0.48 11.1 1.94 1.44 1.2 4.42

otal CT C C Corg. ST Ag As Ba Ba Be Cd Ce Co

-- Comb. Acid. Difference Comb. ICP-40 Hydride ICP-40 ICP-16 ICP-40 ICP-40 ICP-40 ICP-40

8.6 1.32 0.46 0.86 0.2 <2 3.1 593 611 <1 <2 19 <2

5.4 1.81 0.01 1.8 0.05 <2 12.6 243 243 2 <2 36 3

01 6.36 6.39 -0.03 <0.05 <2 1.1 28 27 <1 <2 6 <2

9.3 0.11 0.01 0.1 <0.05 <2 3.8 57 60 <1 <2 12 <29.5 0.77 0.31 0.46 0.12 <2 2.5 480 439 <1 <2 13 <28.5 0.76 0.06 0.7 <0.05 <2 3.8 138 125 <1 <2 19 2

7.7 1.57 0.43 1.14 0.07 <2 3.1 123 124 <1 <2 20 3

8.5 0.31 0.02 0.29 <0.05 <2 13.2 78 81 2 <2 87 <2

00 0.13 0.01 0.12 <0.05 <2 6.1 86 80 <1 <2 9 <200 0.13 0.01 0.12 <0.05 <2 5.1 39 48 <1 <2 10 <2

01 0.01 <0.003 0.007 <0.05 <2 0.9 18 16 <1 <2 <5 <2

9.7 0.27 0.01 0.26 <0.05 <2 2.5 30 30 <1 <2 <5 <29.7 0.39 0.01 0.38 <0.05 <2 1.3 50 49 <1 <2 <5 <2

00 0.38 0.22 0.16 <0.05 <2 0.7 26 27 <1 <2 <5 <2

00 0.91 0.59 0.32 <0.05 <2 1 25 24 <1 <2 <5 <2

01 7.06 6.85 0.21 <0.05 <2 <0.6 8 <10 <1 <2 <5 <200 0.09 0.01 0.08 <0.05 <2 1.6 273 261 <1 <2 <5 <2

9.2 0.11 <0.003 0.107 <0.05 <2 14.5 85 84 <1 <2 8 <2

9.9 0.13 0.01 0.12 <0.05 <2 2.7 127 115 <1 <2 5 <2

9.9 0.19 <0.003 0.187 0.05 <2 2.4 423 415 <1 <2 6 <2

02 0.38 0.3 0.08 <0.05 <2 4.3 27 32 <1 <2 <5 <2

02 0.11 0.01 0.1 <0.05 <2 5.4 25 26 <1 <2 <5 <2

6.6 0.09 0.01 0.08 <0.05 <2 3.7 31 35 <1 <2 <5 <2

7.4 0.17 0.01 0.16 <0.05 <2 1.1 39 37 <1 <2 <5 <2

6.3 0.05 0.01 0.04 <0.05 <2 2.2 63 64 <1 <2 <5 <2

03 0.81 0.76 0.05 <0.05 <2 <0.6 17 23 <1 <2 <5 <2

03 0.02 <0.003 0.017 <0.05 <2 0.9 21 28 <1 <2 <5 <2

02 0.01 <0.003 0.007 <0.05 <2 <0.6 24 26 <1 <2 <5 <2
13

otal CT C C Corg. ST Ag As Ba Ba Be Cd Ce Co

-- Comb. Acid. Difference Comb. ICP-40 Hydride ICP-40 ICP-16 ICP-40 ICP-40 ICP-40 ICP-40

02 0.03 <0.003 0.027 <0.05 <2 2.8 21 22 <1 <2 <5 <202 0.02 0.01 0.01 <0.05 <2 0.9 25 33 <1 2 <5 <2

01 0.03 <0.003 0.027 <0.05 <2 2.3 35 37 <1 2 <5 <202 0.02 <0.003 0.017 <0.05 <2 0.8 39 44 <1 <2 <5 <2

02 0.03 <0.003 0.027 <0.05 7 1.2 37 40 <1 <2 <5 <2

8.8 0.15 0.01 0.14 <0.05 <2 2.1 43 43 <1 <2 <5 <2

01 0.12 0.01 0.11 <0.05 <2 0.6 29 33 <1 <2 8 <200 0.18 0.01 0.17 <0.05 3 6.6 51 50 <1 4 7 <2

3.3 0.35 0.01 0.34 0.05 <2 4.1 62 56 <1 4 5 <2

01 0.21 0.02 0.19 <0.05 3 11 62 59 <1 11 10 <2

9.3 0.18 0.03 0.15 <0.05 3 16.8 108 108 1 22 19 <2

3.1 3.35 <0.003 3.347 0.42 <2 14.2 342 336 1 <2 47 <26.3 0.76 <0.003 0.757 0.18 6 1.9 365 351 1 7 46 <2

3.9 4.62 2.23 2.39 0.29 <2 23.2 367 346 1 5 62 8

6.4 8.84 0.4 8.44 1.08 <2 3.8 56 49 1 79 21 <2

6.6 12.1 10.6 1.54 0.28 <2 2.5 22 17 <1 22 6 <2

4.5 13.8 3.36 10.4 1.86 8 26.7 88 91 1 35 29 3

5.4 13.8 10.7 3.07 0.27 <2 3.3 14 11 <1 15 <5 <2

5.8 15.7 3.49 12.2 2.52 14 42.5 108 106 2 194 27 21.7 13.3 1.5 11.8 2.97 15 32.2 223 212 2 37 52 4

Eu Ga Hg Ho La Li Mn Mo Nb Nd Ni Pb Sb

40 ICP-40 ICP-40 CVAA ICP-40 ICP-40 ICP-40 ICP-40 ICP-40 ICP-40 ICP-40 ICP-40 ICP-40 Hydride

<2 <4 0.05 <4 20 14 81 <2 <4 25 37 9 0.7

<2 14 0.15 <4 31 27 76 <2 <4 37 97 7 0.6

<2 <4 0.02 <4 8 16 298 <2 6 10 14 4 <0.6<2 <4 0.04 <4 10 29 333 <2 <4 13 45 <4 <0.6<2 <4 0.04 <4 10 17 68 3 <4 14 29 5 <0.6

<2 4 0.03 <4 16 13 60 <2 <4 22 40 7 0.6

<2 5 0.03 <4 17 12 91 <2 <4 22 44 6 0.9

8 8 0.14 7 176 43 31 <2 4 197 21 13 2.2

<2 <4 0.02 <4 18 8 13 <2 <4 21 10 <4 <0.6<2 <4 0.04 <4 14 8 16 <2 <4 30 13 <4 <0.6

<2 <4 <0.02 <4 12 5 16 <2 <4 12 4 <4 <0.6

<2 <4 0.05 <4 19 6 38 <2 <4 23 21 <4 <0.6

2 <2 <4 0.04 <4 3 9 42 2 <4 <9 20 9 0.6

<2 <4 <0.02 <4 3 4 37 <2 <4 <9 8 <4 <0.6<2 <4 <0.02 <4 7 4 50 <2 <4 <9 11 <4 <0.6

<2 <4 <0.02 <4 3 4 148 <2 <4 <9 4 4 <0.6

<2 <4 0.02 <4 5 6 29 <2 <4 <9 8 <4 0.9

<2 <4 0.1 <4 10 10 1710 <2 <4 15 98 5 1

<2 <4 0.03 <4 4 13 36 <2 <4 11 11 <4 0.7
14

Eu Ga Hg Ho La Li Mn Mo Nb Nd Ni Pb Sb

40 ICP-40 ICP-40 CVAA ICP-40 ICP-40 ICP-40 ICP-40 ICP-40 ICP-40 ICP-40 ICP-40 ICP-40 Hydride

<2 <4 0.03 <4 6 10 22 <2 <4 9 13 <4 0.7

<2 <4 <0.02 <4 3 5 106 <2 <4 <9 6 <4 0.8

<2 <4 <0.02 <4 3 4 21 <2 <4 <9 5 <4 <0.6<2 <4 <0.02 <4 7 4 48 <2 <4 <9 6 <4 <0.6

<2 <4 0.03 <4 9 4 23 <2 <4 <9 7 <4 <0.6

<2 <4 0.02 <4 9 6 143 <2 <4 <9 16 <4 <0.6

<2 <4 <0.02 <4 3 4 160 <2 <4 <9 23 <4 <0.6

8 <2 <4 <0.02 <4 5 5 92 <2 <4 <9 8 <4 <0.6

<2 <4 <0.02 <4 3 <2 57 <2 <4 <9 4 <4 <0.6

<2 <4 <0.02 <4 4 3 191 <2 <4 <9 10 <4 <0.6

<2 <4 <0.02 <4 6 13 221 <2 <4 <9 11 <4 <0.6

2 <2 <4 <0.02 <4 4 4 299 <2 <4 <9 20 <4 1

<2 <4 <0.02 <4 3 4 250 <2 <4 <9 15 <4 <0.6<2 <4 <0.02 <4 6 7 227 <2 <4 <9 21 <4 <0.6

<2 <4 0.03 <4 9 6 48 <2 <4 <9 13 <4 0.7

<2 <4 0.03 <4 6 6 107 <2 <4 <9 13 58 11.4

4 <2 <4 0.04 <4 18 12 55 <2 <4 13 31 6 1

<2 <4 0.06 <4 12 8 51 2 <4 <9 29 <4 <0.6

6 <2 <4 0.05 <4 29 16 58 <2 <4 21 45 <4 0.8

<2 6 0.05 <4 48 18 34 <2 <4 37 86 5 1.5

<2 16 0.16 <4 29 21 32 12 <4 18 33 14 1.8

<2 13 0.1 <4 26 18 24 <2 5 20 25 13 0.9

4 12 0.17 <4 140 21 496 14 5 93 251 13 1.8

<2 <4 0.08 <4 92 3 10 6 <4 57 40 13 <0.6

<2 <4 0.03 <4 8 <2 102 7 <4 10 16 6 <0.6

3 <4 0.3 <4 141 7 59 24 4 81 223 12 4.1

<2 <4 0.03 <4 11 <2 69 18 4 16 27 6 0.9

4 <2 8 0.64 <4 86 14 82 117 6 53 387 15 9.23 2 15 0.59 <4 84 31 146 65 11 59 361 14 6.2

Sr Sr Th Tl V Y Yb Zn Zr

de ICP-40 ICP-16 ICP-40 Hydride ICP-40 ICP-40 ICP-40 ICP-40 ICP-16

85 92 <6 0.1 32 36 2 110 95

66 70 7 0.5 86 49 3 154 164

124 126 <6 <0.1 14 13 <1 28 31

23 26 <6 0.3 27 20 2 112 7064 65 <6 0.1 20 16 <1 49 72

52 56 <6 0.1 31 31 2 98 101

63 65 <6 0.1 37 32 2 99 91

215 231 <6 0.1 188 321 13 52 42

27 30 <6 <0.1 24 30 1 43 <10

34 40 <6 0.3 38 54 3 135 18
15

Sr Sr Th Tl V Y Yb Zn Zr

de ICP-40 ICP-16 ICP-40 Hydride ICP-40 ICP-40 ICP-40 ICP-40 ICP-16

16 17 <6 <0.1 4 16 <1 30 <10

44 47 <6 <0.1 21 46 2 78 15

14 14 <6 <0.1 19 8 <1 133 <10

2 23 23 <6 <0.1 5 6 <1 57 13

34 36 <6 <0.1 14 13 <1 81 16

2 188 183 <6 <0.1 5 3 <1 32 <10

22 23 <6 <0.1 11 10 <1 29 <10

24 28 <6 0.3 35 20 1 244 4524 27 <6 <0.1 15 12 <1 45 27

51 54 <6 <0.1 18 13 <1 50 23

2 19 21 <6 <0.1 5 5 <1 38 12

17 17 <6 <0.1 3 5 <1 24 <1022 22 <6 <0.1 5 10 <1 35 13

2 21 22 <6 <0.1 9 13 <1 36 22

23 24 <6 <0.1 14 13 <1 63 21

2 15 15 <6 <0.1 <2 4 <1 73 <10

2 11 15 <6 <0.1 3 7 <1 121 13

2 8 <10 <6 <0.1 <2 3 <1 28 <10

2 10 11 <6 <0.1 3 6 <1 109 12

2 12 13 <6 <0.1 5 8 <1 94 21

2 14 15 <6 <0.1 3 6 <1 153 <102 12 15 <6 <0.1 <2 4 <1 62 <102 15 17 <6 <0.1 4 8 <1 79 <10

29 32 <6 <0.1 13 15 <1 91 14

16 17 <6 <0.1 5 9 <1 90 <10

43 46 <6 0.7 47 29 1 253 1936 35 <6 0.4 27 19 <1 123 1358 60 <6 0.8 63 44 2 437 24

95 97 <6 0.9 130 76 4 569 52

80 83 12 1.3 122 10 2 96 394

78 82 9 1 63 9 2 115 397

2 275 282 <6 1.1 110 206 10 658 233

920 914 <6 2.6 579 191 10 483 62

8 425 420 <6 0.8 176 13 <1 284 30

1010 1030 <6 1 486 256 11 1070 114

696 699 <6 <0.1 280 21 <1 201 23

9 749 788 <6 2.4 586 147 8 3060 1345 437 436 <6 1.1 196 144 8 1460 243
16

measured as evolved CO2 after acidification of the sample, and organic carbon was

calculated as the difference between total and carbonate carbon. The compilations by

Arbogast (1996) and Baedecker (1987) include additional discussions about the various

types of analytical methodologies used here.

The concentration of each element is reported in the chemistry table as received from the

analysts. However, qualified data (detection limit values) were modified for use in

statistical analyses. An element was not used in statistical analyses if more than 30% of the

data points for that particular element were qualified. If there were fewer than 30%

qualified values for an element, then the qualified values were multiplied by 0.5 and data

for that element were used in the statistical analyses.

Mineral compositions were determined by X-ray diffraction using a Philips

diffractometer with a graphite monochromator and CuKα radiation. Samples were run

from 4-70° 2Θ at 40 kV, 45 mA, and 10 counts per second. Semiquantitative mineral

contents were determined and are grouped in Table 2 under the classifications of major

(>25%), moderate (5-25%), and minor (<5%).

Statistical analyses were performed on 3 data sets including all data (except data for the

Meade Peak in section 7 and the two carbonate beds in section 1); data solely for section 1

and data solely for section 7. The usual Pearson product moment correlation coefficient

was used to calculate correlation coefficient matrices. A 99% confidence level was used to

calculate the zero-point of correlation. For Q-mode factor analysis, each variable

percentage was scaled to the percent of the maximum value before the values were rownormalized and cosine theta coefficients calculated. Factors were derived from orthogonal

rotations of principal component eigenvectors using the Varimax method (Klovan and

Imbrie, 1971). All communalities are ≥0.90.

RESULTS

Petrography

The dominant characteristic of the chert beds is the presence of sponge spicules, which

vary from relatively well preserved to faint ghosts. Most of the chert beds can be classified

as spicularites (spongolites). These spicularites are laminated and commonly show a

preferred orientation of elongate grains parallel to bedding. One sample (601-28-1H)

shows preferred orientation in some laminae but not in others, indicating that the alignment

of grains was caused by bottom currents rather than by compaction or tectonics. The

sparse to common occurrence of rhombs characterize most spicularite beds. These rhombs

are likely quartz-replaced dolomite rhombs. Glauconite, mica, and feldspar are present in

some beds. Various combinations of bivalves, fish debris, radiolarians(?), and calcareous

algae(?), are seen in some beds. Spicularite beds in section 7 show sedimentary structures

that include cross bedding and cut-and-fill scouring.

The upper part of section 7 does not consist of spicularite beds, but rather consist of

replaced carbonates. These chert beds are generally white to grayish, centimeters to a meter

thick, and in thin section consist of abundant rhombs partly to completely replaced by

quartz. Some replaced rhombs show relict carbonate twinning. Laminae are compacted

around some large rhombs, which indicates that they formed during early diagenesis prior

to compaction. The textures indicate that carbonate and silica fossils were deposited on the

seafloor, dolomite rhombs formed in unconsolidated sediment during early diagenesis,

compaction took place with increasing burial, and finally carbonate grains were replaced

and cement was precipitated during silica diagenesis, thereby producing chert.
17

Table 2. X-Ray diffraction mineralogy of Rex Chert, cherty shale, and Meade Peak

samples

SAMPLE LITHOLOGY MAJOR MODERATE MINOR/ TRACE

Section 1

601-27-1N Cherty-calcareous shale bed quartz feldspar dolomite, gypsum, smectite(?)

601-27-1M Laminated siliceous shale quartz feldspar illite, smectite, CFA

601-27-1L2 Siliceous dolostone quartz dolomite calcite, feldspar, smectite(?)

601-27-1L1 Argillaceous chert bed quartz feldspar goethite

601-27-1K2 Argillaceous chert bed quartz -- dolomite, feldspar, smectite, gypsum(?)

601-27-1K Shaly interbed quartz -- feldspar, smectite, illite, CFA

601-27-1J Cherty shale bed quartz -- feldspar, illite, CFA, calcite, dolomite

601-27-1I Phosphatic-ferruginous chert bed quartz CFA clay mineral

601-27-1H Chert bed quartz -- illite, CFA, smectite(?)

601-27-1G Base of massive thick chert bed quartz -- CFA, heulandite(?)

601-27-1F Chert bed quartz -- smectite(?)

601-27-1E Chert bed quartz -- CFA, smectite(?), chlorite or kaolinite(?)

601-27-1D Chert bed quartz -- clay mineral

601-27-1C Chert bed quartz -- dolomite, calcite, smectite(?)

601-27-1B Chert bed quartz -- dolomite, calcite, smectite(?)

601-27-1A Siliceous limestone bed calcite quartz dolomite, smectite(?)

Section 5

601-26-1A Chert bed quartz -- --

601-26-1B Leached zone in chert quartz -- kaolinite or chlorite, goethite, bixbyite

601-26-1C Chert bed quartz -- illite, chlorite or kaolinite

601-26-1D Chert bed quartz -- illite, chlorite or kaolinite

Section 7

601-28-1R1 Breccia quartz -- dolomite

601-28-1Q Chert bed quartz -- CFA(?)

502-14-2G Chert bed quartz -- CFA

502-14-2F Chert bed quartz -- CFA, clay minerals

502-14-2E Chert bed uartz -- CFA, clay minerals

601-28-1P Chert bed quartz -- calcite, dolomite, clay minerals

601-28-1N 1N-1K=parts of a 80 cm thick quartz -- --

601-28-1M white chert bed quartz -- clay minerals

601-28-1L chert quartz -- smectite(?)

601-28-1K chert quartz -- smectite(?)

601-28-1J3 Chert quartz -- CFA(?)

601-28-1J2 Chert quartz -- --

601-28-1J1 Chert quartz -- smectite(?)

601-28-1I Chert bed quartz -- CFA(?)

601-28-1H Chert bed quartz -- CFA(?), smectite(?)

601-28-1G1 Chert quartz -- CFA(?), gypsum

601-28-1G2 Chert quartz -- CFA, clay minerals

601-28-1G3 Chert quartz -- CFA, smectite

601-28-1F Phosphatic cherty shale bed quartz CFA illite, smectite(?)

601-28-1E Siliceous siltstone quartz plagioclase Illite

601-28-1D Siliceous siltstone, marker bed quartz feldspar illite, smectite(?), jarosite(?)

601-28-1C Phosphatic-calcareous shale quartz CFA, calcite, feldspar illite, natrojarosite

502-14-2D Phosphorite CFA -- quartz, clay minerals

502-14-2C Carbonate nodule in black shale calcite CFA, dolomite --

502-14-2B Phosphatic shale CFA calcite, quartz dolomite, plagioclase, illite

601-28-1B Carbonate nodule in black shale calcite dolomite, CFA quartz

502-14-2A Phosphatic shale CFA, calcite quartz, plagioclase pyrite (?), dolomite, mica, clay minerals

601-28-1A Black phosphatic shale quartz feldspar, CFA calcite, dolomite, illite, pyrite
18

 The cherty shale member from section 1 consists of siliceous siltstone beds and one

siliceous dolostone bed. Beds are laminated, contain ghosts of spicules, and some beds

contain various combinations of sparse rhombs, bivalves, fish debris, feldspar, fibrous

clay minerals, and calcite. Carbonate minerals are more common than they are in the

underlying chert. Grains are well sorted. Uncommon sedimentary structures include

burrows and reverse grading.

The siliceous siltstone from the Meade Peak-Rex Chert transition zone in section 7 is

somewhat different from siliceous siltstone in section 1. The section 7 siltstone is more

compacted and shows a preferred fabric created by aggregate extinction of clay minerals, or

by parallel orientation of thin wavy iron-rich or organic-matter-rich lenses. Grains are

moderately well sorted and range from angular to subrounded, but are predominantly

subangular. Similar to section 1, siltstone beds are laminated, contain ghosts of spicules,

and some beds contain various combinations of bivalves, fish debris, feldspar, and fibrous

clay minerals, as well as mica and chlorite; however no rhombs or calcite were seen.

Mineralogy

The mineral content of the Rex Chert and cherty shale member is dominated by quartz

(Table 2). Several beds also have major amounts of carbonate minerals: the lowermost bed

in section 1 (601-27-1A), which contains major calcite as well as quartz; and a siliceous

dolostone (601-27-1L2) in section 1 contains major dolomite as well as quartz. Feldspar

(combined K-feldspar and plagioclase), dolomite, and carbonate fluorapatite (CFA) occur

in moderate amounts in a few beds (Table 2). Clay minerals occur in minor amounts. In

the upper part of the Meade Peak Member in section 7, calcite and CFA are major phases

along with quartz in some samples.

Chemical Composition

The mean concentrations of elements in the Rex Chert and the cherty shale member are

overwhelmingly dominated by silica, which averages 94.6% for the three sections studied

and ranges from 92.2% for section 1 to 96.9% for section 5 (Table 3). The sums of the

major oxides are reasonable close to 100%, but are a little high for most of the

stratigraphically highest 13 samples from section 7, up to 103% (Table 3). These high

values provide a measure of the analytical accuracy of silica determinations for these very

high silica rocks. Samples with low sums of the major oxides results from not including

organic carbon and sulfur compounds in those totals, which occur predominantly for the

Meade Peak rocks in section 7. 

Organic carbon contents are generally very low in the chert, but are up to 1.8% in

samples from the cherty shale member in section 1 and up to 3.4% in rocks from the

transition between the Meade Peak and Rex Chert in section 7 (Table 3). Likewise,

phosphate (P2O5) is generally very low in the chert, but can be high (up to 3.1%) in

individual beds; these beds do not consistently occur at any particular stratigraphic level in

the sections.

Selenium concentrations for the Rex Chert and cherty shale member vary from the

detection limit (<0.2) to 138 ppm, with a mean concentration of 7 ppm (Table 4), or <1.0

ppm if two outliers are removed.

Mean Se concentrations vary from 0.65 ppm in section 5 (Tables 6) to 12 ppm for section 7

(Table 7; this does not include values from Meade Peak rocks). The mean Se concentration

for section 5 rocks is equivalent to that of mean shale, 0.6 ppm (Krauskopf, 1979),

whereas that of section 1 (Table 5) is 2.2 times the concentration in mean shale and section

7 is 20 times the mean shale concentration. The reason for these differences is that section

7 includes the lowermost Rex Chert, which contains rocks of transitional character, and

section 1 includes the cherty shale member. All the chert beds in the upper part of the Rex

Chert in section 7 have Se concentrations of <0.2 to 0.8 ppm and the mean concentration of
19

Table 4. Statistics for 39 Rex Chert and cherty shale samples collected from

measured sections 1, 5, and 7

N Mean Median SD1 Minimum Maximum

SiO2 wt% 39 94.6 96.3 6.79 74.9 102

Al2O3 39 1.82 0.72 2.64 0.23 11.1

Fe2O3 39 0.70 0.31 0.96 0.04 4.68

TiO2 39 0.11 0.03 0.19 0.01 0.82

CaO 39 0.88 0.46 0.86 0.17 3.69

K2O 39 0.35 0.10 0.63 0.005 2.58

MgO 39 0.25 0.08 0.32 0.01 1.09

Na2O 39 0.19 0.05 0.31 0.01 1.32

P2O5 39 0.42 0.25 0.54 0.05 3.05

∑CO2 39 0.31 0.02 0.67 0.005 2.78

Total 39 99.7 99.9 2.51 93.1 103

Ct 39 0.42 0.17 0.65 0.01 3.35

Cc 39 0.09 0.01 0.18 0.0015 0.76

Corg 39 0.33 0.12 0.61 0.007 3.347

St 39 0.05 0.025 0.07 0.025 0.42

As ppm 39 4.22 2.50 4.45 0.30 16.8

Ba 39 112 50 143 17 593

Ba 39 111 49 140 16 611

Ce 39 11 3 17 2.5 87

Cr 39 143 67 227 5 1100

Cu 39 46 31 58 5 296

Hg 39 0.04 0.03 0.04 0.01 0.16

La 39 16 9 28 3 176

Li 39 10 8 8 1 43

Mn 39 128 55 272 13 1710

Nd 39 17 5 31 4.5 197

Ni 39 25 16 23 4 98

Sb 39 0.9 0.3 1.8 0.3 11.4

Se 39 7.1 0.8 26.8 0.1 138

Sr 39 39 24 37 8 215

V 39 30 15 40 1 188

Y 39 26 13 51 2 321

Zn 39 110 90 107 24 569

Zr 39 48 16 89 5 397

 1Standard Deviation
20

Table 5. Statistics for 14 Rex Chert and cherty shale samples collected from

measured sections 1

N Mean Median SD1 Minimum Maximum

SiO2 wt % 14 92.2 93.1 6.28 78.7 99.9

Al2O3 14 2.23 1.80 2.18 0.26 8.41

Fe2O3 14 1.17 0.77 1.24 0.17 4.68

TiO2 14 0.13 0.08 0.15 0.01 0.55

CaO 14 1.32 1.05 0.96 0.21 3.69

K2O 14 0.43 0.26 0.51 0.02 1.95

MgO 14 0.42 0.25 0.39 0.01 1.09

Na2O 14 0.24 0.07 0.25 0.01 0.63

P2O5 14 0.62 0.41 0.74 0.09 3.05

∑CO2 14 0.56 0.06 0.77 0.01 2.15

Total 14 99.3 99.6 1.5 95.4 101

Ct 14 0.63 0.39 0.58 0.01 1.81

Cc 14 0.15 0.02 0.21 0.0015 0.59

Corg 14 0.48 0.31 0.50 0.007 1.8

St 14 0.05 0.03 0.05 0.025 0.2

As ppm 14 4.3 3.1 4.0 0.7 13.2

Ba 14 142 68 179 18 593

Ba 14 140 70 177 16 611

Ce 14 17 11 22 2.5 87

Cr 14 239 138 310 19 1100

Cu 14 38 31 35 11 152

Hg 14 0.05 0.04 0.04 0.01 0.15

La 14 25 15 44 3 176

Li 14 14 11 11 4 43

Mn 14 68 46 80 13 333

Nd 14 31 22 49 4.5 197

Ni 14 29 21 24 4 97

Sb 14 0.6 0.3 0.5 0.3 2.2

Se 14 1.3 1.1 0.8 0.1 2.8

Sr 14 54 39 51 14 215

V 14 39 26 47 4 188

Y 14 48 31 80 6 321

Zn 14 88 90 38 30 154

Zr 14 51 30 49 5 164

 1Standard Deviation
21

Table 6. Statistics for 4 Rex Chert and cherty shale samples collected from

measured sections 5

N Mean Median SD1 Minimum Maximum

SiO2 wt % 4 96.9 97.5 2.05 93.9 98.6

Al2O3 4 1.05 1.07 0.39 0.55 1.49

Fe2O3 4 0.98 0.48 1.12 0.31 2.65

TiO2 4 0.05 0.04 0.03 0.017 0.08

CaO 4 0.30 0.27 0.09 0.22 0.43

K2O 4 0.15 0.17 0.05 0.07 0.18

MgO 4 0.09 0.09 0.03 0.05 0.12

Na2O 4 0.05 0.05 0.01 0.04 0.07

P2O5 4 0.19 0.16 0.08 0.14 0.3

∑CO2 4 0.02 0.02 0.01 0.01 0.03

Total 4 100 100 0.37 99.2 100

Ct 4 0.13 0.12 0.04 0.09 0.19

Cc 4 0.01 0.01 0.00 0.0015 0.01

Corg 4 0.12 0.11 0.05 0.08 0.187

St 4 0.03 0.03 0.01 0.025 0.05

As ppm 4 5.3 2.6 6.2 1.6 14.5

Ba 4 227 200 154 85 423

Ba 4 219 188 152 84 415

Ce 4 5 6 2 2.5 8

Cr 4 70 73 13 51 81

Cu 4 32 28 20 13 60

Hg 4 0.05 0.03 0.04 0.02 0.1

La 4 6 6 3 4 10

Li 4 10 10 3 6 13

Mn 4 449 33 841 22 1710

Nd 4 10 10 4 4.5 15

Ni 4 33 12 44 8 98

Sb 4 0.8 0.8 0.2 0.7 1.0

Se 4 0.7 0.6 0.2 0.5 0.9

Sr 4 30 24 14 22 51

V 4 20 17 11 11 35

Y 4 14 13 4 10 20

Zn 4 92 48 102 29 244

Zr 4 25 25 16 5 45

 1Standard Deviation
22

Table 7. Statistics for 21 Rex Chert and cherty shale samples collected from

measured sections 7

N Mean Median SD1 Minimum Maximum

SiO2 wt % 21 95.9 96.7 7.38 74.9 102

Al2O3 21 1.69 0.43 3.15 0.23 11.1

Fe2O3 21 0.34 0.14 0.51 0.04 2.09

TiO2 21 0.11 0.02 0.23 0.01 0.82

CaO 21 0.70 0.41 0.74 0.17 2.92

K2O 21 0.34 0.05 0.76 0.005 2.58

MgO 21 0.16 0.03 0.24 0.01 0.7

Na2O 21 0.18 0.07 0.36 0.03 1.32

P2O5 21 0.33 0.23 0.39 0.05 1.72

∑CO2 21 0.21 0.02 0.64 0.005 2.78

Total 21 99.9 101 3.2 93.1 103

Ct 21 0.34 0.12 0.73 0.01 3.35

Cc 21 0.06 0.01 0.17 0.0015 0.76

Corg 21 0.28 0.08 0.72 0.007 3.347

St 21 0.05 0.03 0.09 0.025 0.42

As ppm 21 4.0 2.2 4.6 0.3 16.8

Ba 21 70 37 97 17 365

Ba 21 70 39 93 22 351

Ce 21 8 3 13 2.5 47

Cr 21 93 16 158 5 601

Cu 21 54 33 73 5 296

Hg 21 0.03 0.01 0.04 0.01 0.16

La 21 12 6 12 3 48

Li 21 8 6 6 1 21

Mn 21 107 58 85 21 299

Nd 21 9 5 9 4.5 37

Ni 21 20 15 19 4 86

Sb 21 1.1 0.3 2.4 0.3 11.4

Se 21 12.1 0.2 36.2 0.1 138

Sr 21 30 19 26 8 95

V 21 25 5 39 1 130

Y 21 14 8 17 3 76

Zn 21 128 91 136 24 569

Zr 21 51 13 115 5 397

 1Standard Deviation
23

12 ppm is heavily dependent on two sample values of 101 and 138 ppm. Without those

two samples, the mean Se concentration in section 7 would be 0.8 ppm Se, close to its

crustal mean content (Table 3).

Other elements of environmental interest include As, Cr, V, Zn, Hg, and Cd. Arsenic

concentrations vary by a factor of 56, from 0.3 to 16.8 ppm, mean 4.2 ppm, which is

slightly less than the concentration in average shale of 6.6 ppm (Govett, 1983). Chromium

concentrations vary by a factor of 220, from 5 to 1100 ppm, with a mean of 143 ppm,

which is higher than the 100 ppm concentration in average shale. Vanadium concentrations

vary by a factor of 188, from 1 to 188 ppm, mean 30 ppm, which is much lower than the

130 ppm concentration in average shale. Zinc concentrations vary by a factor of 24, from

24 to 569 ppm, mean 110 ppm, which is somewhat greater than the concentration in

average shale of 80 ppm. Mercury concentrations vary by a factor of >8, from 0.01 to

0.16 ppm, mean 0.04 ppm, which is an order of magnitude less than the concentration in

average shale of 0.4 ppm. Cadmium concentrations are uniformly below the limit of

quantification (<2 ppm) except for rocks from the lowermost part of section 7; we are

unable to ascertain whether the average may be higher than that of average shale, 0.3 ppm.

Stratigraphic Changes in Chemical Composition

Stratigraphic changes (equivalent to temporal changes in the depositional basin) in

chemical composition of rocks are notable either as uniform changes through the sections

or as distinct differences in the mean composition of rocks that compose the upper and

lower halves of the sections. In this regard, most elements increase up section in section 1,

whereas they decrease up section in sections 5 and 7. These increases up section in section

1 still occur for about half of the elements if the cherty shale member is not included in the

analysis; the other half of the elements do not vary uniformly up section. Silica has the

opposite trend of the other elements. For section 1 (including the cherty shale member),

the following elements increase up section: Al, Fe, Ti, K, Na, As, Ba, Ce, Cr, Hg, La,

Li, Ni, Sc, Sr, V, and Zr; in contrast, silica decreases up section and Ca, Mg, C, and Mn

decrease to near mid-section then increase farther up section. For section 5, the following

elements decrease up section: Al, Fe, Ti, K, Na, C, Ce, Ni, Sr, V, Y, and Zn; whereas Si

increases up section. For section 7, the following elements decrease up section: Al, Fe,

Ti, K, Na, organic C, Ba, Ce, Cr, La, Li, Ni, Se, Sr, Tl, V, Y, Zn, and Zr; whereas Si

and Mn increase up section.

Phase Associations of Elements

The phase associations of elements were determined by comparing results from element

correlations determined from correlation coefficient matrices (Tables 8-10), rotated factor

loadings from Q-mode factor analyses (Figures 3-5), and mineralogy as determined by

XRD (Table 2). Three data sets were analyzed including data from sections 1, 5, and 7

combined, section 1 data, and section 7 data (excluding Meade Peak and Rex Chert

carbonate beds).

We consider four to five Q-mode factors that are interpreted to represent the following

rock and mineral components: Factor 1, Chert-silica component consisting solely of Si,

except for the combined data set where Ba shows a minor but statistically significant factor

loading. Factor 2, phosphorite-phosphate component comprised of P, Ca, As, Y, V, Cr,

Sr, and La (± Fe, Zn, Cu, Ni, Li, Se, Nd, and Hg depending on the data set). Factor 3,

shale component comprised of Al, Na, Zr, K, Ba, Li, and organic C (± Ti, Mg, Se, Ni,

Fe, Sr, V, Mn, and Zn depending on the data set). Factor 4, carbonate component

(dolomite, calcite, silicified carbonates) comprised of carbonate C, Mg, Ca, and Si (± Mn).

Factor 5 we tentatively interpret as representing organic matter (and/or sulfide-sulfate

phases) hosted elements comprised of Cu (± organic C, Zn, Mn Si, Ni, Hg, and Li

depending on the data set). Copper correlates only with Zn, but Zn also correlates with Ni,

As and some elements associated with the shale component. Silica is dominantly in the
oefficient matrix for 39 samples from sections 1, 5, and 7 combined listed in Table 3; the point of zero

 the 99% confidence level is |0.4047|

3 TiO2 CaO K2O MgO Na2O P2O5 ∑CO2 C T C C C org As Ba Cr Cu La Li Mn

6

1 0.022

3 0.992 0.062

2 0.550 0.626 0.544

9 0.953 -0.003 0.916 0.570

7 0.021 0.682 0.099 0.111 -0.080

3 -0.031 0.647 -0.054 0.742 0.045 -0.108

5 0.768 0.223 0.772 0.739 0.737 -0.042 0.342

5 -0.034 0.645 -0.058 0.739 0.042 -0.110 1.000 0.340

2 0.825 0.044 0.836 0.564 0.769 -0.011 0.065 0.960 0.063

7 0.457 0.382 0.522 0.240 0.315 0.670 -0.196 0.415 -0.199 0.501

7 0.570 0.094 0.547 0.553 0.612 -0.062 0.170 0.517 0.166 0.499 0.164

6 0.472 0.565 0.547 0.404 0.331 0.813 -0.076 0.425 -0.080 0.475 0.826 0.230

7 0.016 0.016 0.030 -0.115 -0.013 0.147 -0.164 -0.044 -0.168 0.003 0.255 -0.097 0.095

9 0.225 0.618 0.298 0.221 0.130 0.909 -0.092 0.130 -0.095 0.166 0.620 0.081 0.864 0.061

3 0.535 0.426 0.565 0.386 0.500 0.660 -0.106 0.368 -0.110 0.423 0.668 0.356 0.854 0.118 0.770

6 -0.163 -0.157 -0.214 -0.113 -0.051 -0.222 0.030 -0.183 0.030 -0.204 -0.253 -0.233 -0.225 0.028 -0.211 -0.008

7 0.518 0.361 0.534 0.448 0.442 0.377 0.046 0.448 0.043 0.463 0.675 0.295 0.645 0.242 0.267 0.585 0.052

2 0.838 -0.160 0.827 0.290 0.815 -0.131 -0.108 0.664 -0.110 0.737 0.304 0.402 0.197 -0.007 0.111 0.271 -0.176

6 0.453 0.669 0.508 0.476 0.382 0.823 0.051 0.371 0.046 0.380 0.671 0.409 0.883 0.050 0.916 0.816 -0.315

2 0.557 0.526 0.623 0.356 0.433 0.796 -0.126 0.442 -0.130 0.509 0.874 0.259 0.940 0.165 0.855 0.832 -0.284

2 0.068 0.647 0.144 0.169 -0.018 0.938 -0.070 0.019 -0.073 0.043 0.550 0.011 0.823 0.041 0.977 0.719 -0.1932 0.141 0.292 0.162 0.029 0.072 0.415 -0.101 0.004 -0.104 0.035 0.609 -0.031 0.347 0.620 0.176 0.253 -0.014

0 0.980 -0.039 0.963 0.500 0.958 -0.050 -0.035 0.746 -0.038 0.803 0.369 0.557 0.365 -0.007 0.180 0.463 -0.157

V Y Zn

2

8 0.778

1 0.457 0.1433 0.495 0.015 0.069
25

icient matrix for 14 samples from section 1 listed in Table 3; the point of zero correlation for n=14 at the 99%

TiO2 CaO K2O MgO Na2O P2O5 ∑CO2 C T C c C org As Ba Ce Cr Cu Hg La Li Mn

0.105

0.968 0.190

0.580 0.608 0.535

0.814 0.055 0.672 0.592

0.050 0.704 0.212 0.012 -0.148

0.053 0.502 -0.027 0.806 0.233 -0.257

0.800 0.322 0.776 0.863 0.669 -0.114 0.551

0.048 0.498 -0.032 0.803 0.229 -0.261 1.000 0.547

0.912 0.167 0.918 0.670 0.683 -0.023 0.224 0.937 0.220

0.566 0.383 0.707 0.151 0.220 0.732 -0.377 0.236 -0.379 0.434

0.482 0.247 0.395 0.654 0.605 -0.103 0.429 0.547 0.422 0.461 0.076

0.393 0.695 0.516 0.260 0.180 0.894 -0.151 0.186 -0.157 0.282 0.850 0.128

0.541 0.589 0.681 0.273 0.221 0.827 -0.216 0.307 -0.222 0.451 0.927 0.089 0.961

0.052 -0.215 0.090 -0.107 -0.059 -0.117 -0.164 0.122 -0.166 0.212 -0.036 -0.070 -0.026 0.058

0.638 0.415 0.763 0.243 0.295 0.699 -0.309 0.357 -0.313 0.547 0.912 0.198 0.817 0.919 0.163

0.094 0.717 0.244 0.074 -0.097 0.967 -0.204 -0.069 -0.209 0.008 0.741 -0.043 0.946 0.866 -0.061 0.700

0.511 0.446 0.550 0.204 0.429 0.702 -0.252 0.140 -0.258 0.271 0.795 0.188 0.870 0.854 -0.001 0.809 0.766

0.291 -0.163 0.121 0.086 0.648 -0.167 -0.010 0.031 -0.008 0.039 -0.023 0.067 -0.026 -0.038 -0.100 0.022 -0.137 0.415

0.101 0.708 0.254 0.063 -0.091 0.976 -0.226 -0.074 -0.231 0.011 0.756 -0.037 0.949 0.871 -0.066 0.709 0.997 0.768 -0.142

0.967 -0.008 0.930 0.471 0.807 0.009 -0.050 0.739 -0.052 0.883 0.536 0.382 0.312 0.486 0.149 0.641 0.027 0.502 0.402

0.310 0.527 0.302 0.449 0.462 0.407 0.209 0.377 0.201 0.356 0.249 0.580 0.451 0.355 -0.087 0.311 0.369 0.355 0.025

0.264 0.843 0.378 0.355 0.120 0.904 0.035 0.188 0.028 0.207 0.706 0.251 0.950 0.862 -0.091 0.715 0.939 0.766 -0.118

0.343 0.663 0.493 0.181 0.082 0.927 -0.229 0.129 -0.234 0.249 0.883 0.025 0.982 0.969 0.030 0.858 0.952 0.850 -0.064

0.054 0.714 0.209 0.039 -0.131 0.984 -0.225 -0.108 -0.229 -0.029 0.731 -0.073 0.930 0.848 -0.065 0.686 0.994 0.744 -0.150

0.526 -0.176 0.510 0.214 0.484 -0.136 -0.070 0.455 -0.067 0.559 0.207 0.108 -0.020 0.143 0.502 0.337 -0.213 0.135 0.298

0.976 0.145 0.908 0.627 0.890 0.023 0.140 0.801 0.134 0.878 0.469 0.569 0.363 0.470 -0.004 0.547 0.061 0.491 0.355

Sr V Y Zn

0.920

0.929 0.947

0.137 0.047 -0.177

0.274 0.287 0.025 0.501
26

efficient matrix for 21 samples from section 7 listed in Table 3; the point of zero

 99% confidence level is |0.542|

CaO K2O MgO Na2O P2O5 ∑CO2 C T C org As Ba Cr Cu La Li Mn

9

4 -0.079

4 0.485 0.661

3 -0.151 0.990 0.645

1 0.580 -0.030 -0.056 -0.146

4 0.718 -0.127 0.623 -0.121 -0.140

3 0.017 0.776 0.689 0.757 -0.158 0.141

7 -0.154 0.811 0.544 0.790 -0.124 -0.098 0.971

7 0.390 0.447 0.325 0.336 0.735 -0.139 0.486 0.523

4 -0.107 0.995 0.629 0.989 -0.037 -0.160 0.746 0.788 0.419

0 0.402 0.584 0.404 0.469 0.721 -0.137 0.529 0.565 0.930 0.562

8 0.148 0.016 -0.109 -0.007 0.410 -0.170 -0.072 -0.030 0.338 0.000 0.230

9 0.409 0.611 0.373 0.514 0.765 -0.187 0.386 0.434 0.855 0.605 0.945 0.3476 0.218 0.762 0.477 0.698 0.480 -0.178 0.570 0.616 0.726 0.753 0.813 0.347 0.869

6 -0.133 -0.379 -0.225 -0.365 -0.302 0.120 -0.277 -0.307 -0.426 -0.362 -0.416 -0.009 -0.473 -0.332

1 0.620 0.337 0.294 0.221 0.862 -0.003 0.209 0.211 0.800 0.332 0.883 0.344 0.899 0.709 -0.200

4 0.337 0.768 0.520 0.692 0.601 -0.144 0.532 0.570 0.815 0.766 0.908 0.245 0.964 0.895 -0.5306 0.331 0.722 0.483 0.625 0.627 -0.156 0.609 0.650 0.897 0.705 0.977 0.261 0.958 0.890 -0.4617 0.580 0.107 0.048 -0.008 0.984 -0.136 -0.047 -0.014 0.766 0.103 0.796 0.414 0.848 0.583 -0.3371 0.569 0.127 0.071 0.018 0.933 -0.109 -0.043 -0.016 0.739 0.115 0.752 0.622 0.820 0.607 -0.179

4 -0.155 0.992 0.651 0.996 -0.145 -0.121 0.781 0.814 0.354 0.988 0.490 -0.021 0.518 0.707 -0.339

Y Zn

26 0.951

9 -0.008 0.018
27

Figure 3. Q-mode factors for Rex Chert and

cherty shale samples from sections 1, 5, and 7

Figure 4. Q-mode factors for Rex Chert and

cherty shale samples from section 1

Factor 3:Shale component

30.6% of sample variance

0

0.1

0.2

0.3

0.4

0.5

0.6

Na

Zr

Mn Ni Al

Se Ba Li K Corg Zn

Ti

Varimax Rotated Factor Scores

Factor 2: Phosphorite-phosphate

component

11.4% of sample variance

-0.2

-0.1

0

0.1

0.2

0.3

0.4

As P Y V Nd Cr La Hg Sr Ca Fe

Se

Na Mn

Varimax Rotated Factor Scores

Factor 5: Organic Carbon component 

6.0% of sample variance

-0.2

0

0.2

0.4

0.6

0.8

Ba Na

Hg Corg

Zn

Cu

Varimax Rotated Factor Scores

Factor 4: Carbonate 

component

17.3% of sample variance

0.1

1

Si

Ca

Mg

CC

0.5

Varimax Rotated Factor Scores

Factor 1: Chert-silica 

component

29.7% of sample variance

-0.2

0

0.2

0.4

0.6

0.8

1

Mg Zn

Si

Varimax Rotated Factor Scores

Factor 3:Shale component

30.6% of sample variance

0

0.1

0.2

0.3

0.4

0.5

0.6

Na

Zr

Mn Ni Al

Se Ba Li K Corg Zn

Ti

Varimax Rotated Factor Scores

Factor 2: Phosphorite-phosphate

component

11.4% of sample variance

-0.2

-0.1

0

0.1

0.2

0.3

0.4

As P Y V Nd Cr La Hg Sr Ca Fe

Se

Na Mn

Varimax Rotated Factor Scores

Factor 5: Organic Carbon component 

6.0% of sample variance

-0.2

0

0.2

0.4

0.6

0.8

Ba Na

Hg Corg

Zn

Cu

Varimax Rotated Factor Scores

Factor 4: Carbonate 

component

17.3% of sample variance

0.1

1

Si

Ca

Mg

CC

0.5

Varimax Rotated Factor Scores

Factor 1: Chert-silica 

component

29.7% of sample variance

-0.2

0

0.2

0.4

0.6

0.8

1

Mg Zn

Si

Varimax Rotated Factor Scores
28

Figure 5. Q-mode factors for Rex Chert and cherty shale

samples from section 7

chert fraction (factor 1) and does not show up in the aluminosilicate fraction (factor 3 shale

fraction) even though it is clearly part of the feldspars and clay minerals that comprise that

fraction. Likewise, organic C is dominantly in Factor 3 and does not show up in factor 5

(except for section 1) even though it is likely that the elements in factor 5 are hosted by

organic matter. These characteristics are an artifact of analyzing a data set that is

overwhelmingly dominated by one variable, silica, and the distribution of some elements in

more than one phase. Selenium shows a dominant association with the shale component,

but correlations and Q-mode factors also indicate that organic matter (within the shale

component) and carbonate fluorapatite may host a portion of the Se. Consideration of

larger numbers of factors in Q-mode analysis indicates that native Se (a factor containing Se

± Ba) may also comprise a minor component of the Se compliment.

DISCUSSION AND CONCLUSIONS

The chert beds generally have Se concentrations of <1 ppm. The cherty shale member

rocks have a somewhat higher Se contents, mean 1.8 ppm. Siliceous siltstone and

argillaceous chert that comprise the zone of transition between the Meade Peak and Rex

Chert have Se concentrations up to 138 ppm.

The low Se contents determined for the chert-bed samples here are not characteristic of

Se concentrations found for composite channel samples of chert at the Rasmussen Ridge

and Enoch Valley Mines (Herring et al., 2002). Those weighted (for stratigraphic

thickness) mean Se concentrations are 63 ppm for the Rasmussen Ridge samples and 18

ppm for the Enoch Valley samples. These differences may result from several factors,

such as weathering of the outcrop samples we analyzed and the inclusion of shale and

siliceous shale beds that are interbedded with cherts in the composite samples. This latter

influence is supported by the lower silica and higher Al2O3, K2O, etc. contents of the

composite samples. In the outcrop sections studied here, shale interbedded with the chert

consists only of thin partings, except in the cherty shale member, and those partings would

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4 Factor 3:Shale component

11.9% of sample 

variance

Varimax Rotated Factor Scores

V

Sr Corg

Fe

Li Mg

Ba Al Na K Zr

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8 Factor 4: Carbonate component

9.5% of sample variance

Varimax Rotated Factor Scores

Mn Si

Ca

Mg

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4 Factor 2: Phosphorite-phosphate

component

18.4% of sample variance

Varimax Rotated Factor Scores

Sr Cr Li Ca V La Ni As

P Y Cu

Zn

-0.5

0

0.5

1

Factor 1: Chert-silica 

component

22.2% of sample 

variance

Varimax Rotated Factor Scores

Si

Mn Cu

-0.2

0

0.2

0.4

0.6

0.8 Factor 5: Organic carbon component 

35.4% of sample variance

Varimax Rotated Factor Scores

Mg

Li

Cu

Si

Mn
29

not likely contribute significantly to the mean Se concentration for each outcrop section. It

is not known why there are more (or thicker) shale interbeds, or more argillaceous cherts in

the composite sections than those studied in outcrops. Chert is very resistant to weathering

and little Se should be leached by weathering of the outcrops.

Other elements of environmental interest include As, Cr, V, Zn, Hg, and Cd. Of these

elements, only mean Cr and Zn values are higher than their respective values in average

shale, 30% and 27% higher, respectively. Cadmium could not be evaluated because most

concentrations are below the limit of quantification of 2 ppm.

ACKNOWLEDGMENTS

We thank Dick Grauch for reviewing this report.

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