Petrographic Descriptions of Selected Rock Specimens 

From the Meade Peak Phosphatic Shale Member, 

Phosphoria Formation (Permian), Southeastern Idaho 

Scientific Investigations Report 2007–5223 

U.S. Department of the Interior 

U.S. Geological Survey 
Cover: Rasmussen Ridge mine, Caribou County, Idaho
Petrographic Descriptions of Selected 

Rock Specimens From the Meade Peak 

Phosphatic Shale Member, Phosphoria 

Formation (Permian), Southeastern Idaho

By Edward A. Johnson, Richard I. Grauch, and James R. Herring

Scientific Investigations Report 2007-5223

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

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Suggested citation:

Johnson, E.A., Grauch, R.I., and Herring, J.R., 2007, Petrographic descriptions of selected rock specimens from the 

Meade Peak Phosphatic Shale Member, Phosphoria Formation (Permian), southeastern Idaho: U.S. Geological Survey 

Scientific Investigations Report 2007–5223, 17 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Phosphoria Formation...................................................................................................................................1

Previous Petrographic Studies....................................................................................................................3

Sampling..........................................................................................................................................................3

Terminology and Classification....................................................................................................................8

Phosphorite............................................................................................................................................8

Apatite. ....................................................................................................................................................8

Peloids. ....................................................................................................................................................9

Phosphate Lithofacies .........................................................................................................................9

Petrographic Observations ..........................................................................................................................9

Peloidal Phosphorite ............................................................................................................................9

Peloid Composition......................................................................................................................9

Peloid Structure .........................................................................................................................10

Peloid Types................................................................................................................................10

Phosphatic Mud.........................................................................................................................11

Peloidal Lithofacies ...................................................................................................................11

Sedimentary Structures............................................................................................................12

Diagenesis ..................................................................................................................................12

Organic Claystone...............................................................................................................................12

Muddy Siltstone ..................................................................................................................................13

Dolomitized Calclithite .......................................................................................................................14

Miscellaneous Findings.....................................................................................................................15

Petrographic Observations of Selected Marker Beds ..........................................................................15

Summary........................................................................................................................................................16

Acknowledgments.......................................................................................................................................16

References Cited..........................................................................................................................................16

Figures

1. Map showing locations of phosphate mines and associated measured 

stratigraphic sections from which specimens were selected 

for petrographic study..................................................................................................................2

2. Diagram showing average thicknesses of formal and informal members of

the Phosphoria Formation and average thicknesses of informal mining

zones in the Meade Peak Phosphatic Shale Member ...........................................................4

3. Photomicrograph showing opaque peloids in a specimen of silty, 

organic claystone collected from middle waste zone at Enoch 

Valley mine...................................................................................................................................10
iv

4. Photomicrograph showing complex peloids in a specimen of grainstone 

phosphorite collected from upper ore zone (D bed) at Enoch 

Valley mine...................................................................................................................................11

5. Photomicrograph showing translucent peloids in a specimen of phosphatic, 

dolomitic calclithite collected from lower ore zone (false cap) at 

Dry Valley mine ................................................................................................................................. 11

Tables

1. Lithofacies in the Meade Peak Phosphatic Shale Member as represented 

by the thin sections used in this study ......................................................................................5

2. Number of thin sections examined from the five mined zones in the 

Meade Peak Phosphatic Shale Member in the two stratigraphic sections 

sampled at each of the four mines studied ..............................................................................8
Abstract

Based on petrographic observations of 135 thin sections, rocks in the Meade Peak Phosphatic Shale Member 

of the Permian Phosphoria Formation in southeastern Idaho 

can be placed into one of four major lithofacies: organic 

claystone, muddy siltstone, peloidal phosphorite, and dolomitized calclithite—in order of decreasing abundance. 

Organic claystones are the most common lithofacies in the 

Meade Peak. Many of these rocks contain sufficient amounts 

of silt to make silty, organic claystones a common subtype. 

Organic claystones commonly contain crystals of muscovite 

and bioclasts as accessory components, and they are typically 

parallel laminated. Muddy siltstones are composed primarily of quartz silt, but some feldspar and rare carbonate silt are 

also present; some rocks are parallel laminated. Phosphate 

peloids are composed of varying amounts of opaque, complex, 

and translucent material, and observed internal structures are 

classified as simple, banded, cored, zoned, oolitic, nucleated, 

and polynucleated. Opaque, complex, and translucent peloids 

form the framework grains of three peloidal phosphorite rocks: 

wackestone phosphorite, packstone phosphorite, and grainstone phosphorite. Wackestone phosphorite is phosphatic-mud 

supported and contains more than 10 percent peloids; it is the 

most common type of phosphorite. Packstone phosphorite 

is peloid supported and contains interstitial phosphatic mud; 

it is also a common type. Grainstone phosphorite is peloid 

supported but lacks phosphatic mud; it is the least common 

type. Dolomitized calclithites contain three types of carbonate 

grains: macrocrystalline, microcrystalline, and crystalline with 

a microcrystalline nuclei—in order of decreasing abundance. 

Based on chemical staining and X-ray diffraction analyses, 

most of the carbonate is dolomite. Sufficient amounts of 

quartz silt or muddy material allow some rocks to be called 

silty dolomitized calclithite or muddy dolomitized calclithite, 

respectively. Sedimentary structures are absent except in some 

muddy dolomitized calclithites. Organic claystones, muddy 

siltstones, and dolomitized calclithites are detrital deposits. 

Many rocks in the Meade Peak contain calcite and apatite as 

fracture fillings and vug linings. As expected, peloidal phosphorites are most common in ore zones, and detrital rocks are 

most common in waste zones. Mine-specific marker beds are 

mostly composed of dolomitized calclithite.

Introduction

As part of the U.S. Geological Survey (USGS) Western 

United States Phosphate (WUSP) Project—established primarily to study the presence of trace elements, particularly selenium, in the Permian Phosphoria Formation in southeastern 

Idaho—a subtask was established to describe the petrologic 

features of rocks collected from the Meade Peak Phosphatic 

Shale Member (hereafter shortened to Meade Peak). Specimens were selected as splits from several extensive sets of 

samples collected from a broad study area (fig. 1) for use in 

various petrologic and geochemical analyses conducted within 

other tasks and subtasks in the project. In making selections, 

the objective was to obtain representatives of the major rock 

types, or lithofacies, present in the Meade Peak. Thin sections cut from 135 rock specimens resulted in 199 individual 

petrographic descriptions. From these data, it is apparent that 

four major lithofacies dominate the Meade Peak: organic claystone, muddy siltstone, peloidal phosphorite, and dolomitized 

calclithite, in decreasing order of apparent abundance.

Phosphoria Formation

The Lower to Upper Permian (Leonardian through Guadalupian age) Phosphoria Formation (Wardlaw and Collinson, 

1986; Wardlaw, 1995) was deposited in the central part of 

an epicontinental marine embayment on the west margin of 

the North American craton. This embayment, known as the 

Sublet Basin (as used by Maughan, 1979), extended over an 

area of about 100,000 mi2, and the Phosphoria sea (as used by 

Sheldon, 1963) occupied its central part. The Phosphoria Formation is now present in parts of six states: Idaho, Wyoming, 

Petrographic Descriptions of Selected Rock Specimens 

From the Meade Peak Phosphatic Shale Member, 

Phosphoria Formation (Permian), Southeastern Idaho

By E.A. Johnson1, R.I. Grauch1, and J.R. Herring1

1

 U.S. Geological Survey, Denver Federal Center, Box 25046, Denver, 

CO 80225
2 Petrographic Descriptions of Selected Rock Specimens from the Meade Peak Phosphatic Shale Member

Figure 1. Locations of phosphate mines and associated measured stratigraphic sections in 

southeastern Idaho, from which specimens were selected for petrographic study.

26

34

89

89

30

30

34

Georgetown

Soda Springs

43°

111°30’ 111°

42°

30’

Blackfoot

Reservoir

Grays

Lake

Palisades

Reservoir

Snake River

Alpine

Afton

Montpelier

Paris Border

[wps, western phosphate section]

IDAHO

AREA OF

FIGURE

0 5

0 5 10 15 KILOMETERS

10 MILES

Enoch Valley mine

Rasmussen Ridge mine

Dry Valley mine

Smoky Canyon mine

Sections wpsA, wpsB, 

and wpsJ

Sections wpsE and wpsF

Sections wpsC

and wpsD

Sections wpsG

and wpsH

IDAHO

WYOMING

34
Sampling 3

Montana, Utah, Nevada, and Colorado—in order of decreasing 

area.

Richards and Mansfield (1912) first used the term Phosphoria Formation to separate out the upper two members of 

the previously named Permian Park City Formation (Boutwell, 

1907) and designated Phosphoria Gulch, Bear Lake County, 

Idaho, as the type locality. After many years of confusing 

modifications to the Permian stratigraphic nomenclature in the 

region, McKelvey and others (1959) published what became 

the definitive work on the Park City and Phosphoria Formations, and in the process established (or reconfirmed) six 

formal or informal members for the Phosphoria. In ascending 

order, they are: lower chert member, Meade Peak Phosphatic 

Shale Member, Rex Chert Member, cherty shale member, 

Retort Phosphatic Shale Member, and Tosi Chert Member 

(fig. 2A). Within the boundaries of this study, only the Meade 

Peak, Rex Chert, and cherty shale members are present. In 

southeastern Idaho, the Meade Peak has been subdivided into 

five informal zones by the phosphate mining industry. These 

zones are, in ascending order: lower waste (including the fishscale marker bed at the base), lower ore, middle waste, upper 

ore, and upper waste (fig. 2B). In addition, many individual 

mines have local names that apply to specific marker beds in 

the zones exposed on their properties. Examples include the B 

mudstone in the lower ore zone at the Dry Valley mine and the 

twin limes in the middle waste zone at the Rasmussen Ridge 

mine.

Previous Petrographic Studies

Although many reports have been written on the Phosphoria Formation, petrographic studies are few in number, and 

most of these focus mainly on diagenetic processes. The earliest significant petrographic study is that conducted by Donald 

Emigh of the Idaho Bureau of Mines and Geology; in the late 

1950s, he studied the Phosphoria phosphorites from selected 

sites in Idaho, Montana, and Utah. As part of this work, Emigh 

(1958) proposed a broad classification scheme for sedimentary phosphate deposits. He also proposed a nomenclature for 

phosphate pellets in the Phosphoria, but he did not present a 

lithologic classification of phosphorites. This work was later 

summarized, with slight modifications, in a paper published by 

the Intermountain Association of Geologists (Emigh, 1967). 

The next significant study was conducted by Curtis Mabie 

and Hal Hess of the U.S. Bureau of Mines in the early 1960s. 

Mabie and Hess (1964) studied 21 ore samples collected from 

the Phosphoria in Idaho, Montana, and Utah. These workers 

presented their own nomenclature for phosphate pellets and 

proposed two lithologic classification schemes for phosphorites, one based on mineral composition and the other on 

metallurgical characteristics, neither of which has been widely 

accepted. Also in the early 1960s, Dick Sheldon of the USGS 

conducted an extensive study of the Permian rocks in western 

Wyoming. As part of this effort, Sheldon (1963) made 

petrographic observations of phosphorites and carbonate rocks 

in the Phosphoria. Although Sheldon described his findings, 

he did not propose a classification scheme for pellets or phosphorites. A following study by Peter Cook, as a Ph.D. dissertation at the University of Colorado, was based on the examination of 100 thin sections cut from specimens of the Meade 

Peak, specifically the upper ore zone. Cook (1969) applied his 

own pellet nomenclature but offered no lithologic classification of phosphorites. The most recent work was done by Eric 

Hiatt, as part of a Ph.D. dissertation at the University of Colorado. Hiatt (1997) does not list a nomenclature for phosphate 

pellets but does adopt and slightly modify a lithologic classification proposed by Cook and Shergold (1986). 

Sampling

A regional sampling program of the Meade Peak was 

designed by the WUSP Project to provide rocks for various petrologic and geochemical studies. In order to obtain 

relatively unweathered samples, collections were made from 

the pit floors and highwalls of four active phosphate mines. 

The mines sampled—Enoch Valley, Rasmussen Ridge, Dry 

Valley, and Smoky Canyon—are all located within several 

tens of miles north to east of Soda Springs, Idaho (fig. 1). 

At each mine, two stratigraphic sections were measured and 

described, a less-weathered section and a more-weathered section, determined by the pre-mining depth of the rocks below 

the land surface. In addition, one core of the Meade Peak was 

measured and described. Each of the stratigraphic sections, 

ranging in thickness from 118 to 215 ft, includes most of the 

Meade Peak; in several cases some of the overlying Rex Chert 

Member is included and in one case some of the underlying 

Grandeur Member of the Park City Formation is included. 

Grab samples were collected from each of the stratigraphic 

sections, and the number of samples per section ranged from 

33 to 87. Sample selections were made to assure representation of all of the major lithofacies (table 1). It is important to 

note that no systematic approach was used in this sampling 

activity. Therefore, only the broadest generalizations can be 

drawn in regard to the vertical and horizontal distribution of 

lithofacies. Specimens singled out for thin sectioning were 

labeled in the field, or in the case of the core, selected out 

of core boxes at the USGS facility in Lakewood, Colo. The 

specimens were then shipped to a commercial petrographic 

company for standard thin sectioning. In addition, several thin 

sections were stained using sodium cobaltinitrate for potassium feldspar and plagioclase feldspar, and several others were 

stained using alizarine red for calcite and dolomite. Table 2 

tabulates the number of thin sections examined from the five 

mined zones encountered in the two stratigraphic sections 

sampled at each of the four mines studied.
4 Petrographic Descriptions of Selected Rock Specimens from the Meade Peak Phosphatic Shale Member

Tosi Chert Member 

Retort Phosphatic 

Shale Member 

cherty shale member

Rex Chert Member

Meade Peak Phosphatic 

Shale Member 

lower chert member

Phosphoria Formation

upper waste

upper ore

middle waste

lower ore

lower waste

0'

 100'

200'

300'

400'

500'

600'

700'

800'

900'

1000'

50'

100'

150'

0'

A.

B.

Figure 2. A, Average thicknesses of formal and informal members of the Phosphoria Formation in 

Western Phosphate Field, southeastern Idaho (McKelvey and others, 1959). B, Average thicknesses of 

informal zones of the Meade Peak Phosphatic Shale Member as used by phosphate mining industry. 

Based on stratigraphic sections measured for this study.
Sampling 5

LITHOFACIES SECTION A SECTION B SECTION C SECTION D SECTION E SECTION G SECTION J

Enoch 

Valley

Enoch 

Valley

Dry 

Valley

Dry 

Valley

Rasmussen 

Ridge

Smoky 

Canyon

Enoch 

Valley

Wackestone phosphorite wpsA023T wpsB073T wpsC016 wpsE075 wpsG110-C wpsJ17.4

wpsA122T wpsB099T wpsC031 wpsJ128.6

wpsA153T wpsB115T

Silty, wackestone phosphorite wpsA123T wpsC036 wpsD199 wpsE065 wpsG016 wpsJ17.4

wpsA147T wpsC090 wpsE066

wpsC096 wpsE075.5

wpsC122

wpsC130

Sandy, wackestone phosphorite wpsD207

Packstone phosphorite wpsA001T wpsB001T wpsC090 wpsD109 wpsG105 wpsJ106.5

wpsA154T wpsB009T wpsC112 wpsG110-A

wpsA158T wpsB024T wpsG110-D

wpsB033T

wpsB139T

Silty, packstone phosphorite wpsA163T wpsB026T

Grainstone phosphorite wpsB136T wpsC029 wpsG010

wpsC031 wpsG110-E

wpsC032

Organic claystone wpsA023T wpsB068T wpsC155 wpsD046 wpsE114 wpsG038 wpsJ49.4

wpsA029T wpsB073T wpsD049 wpsJ52.2

wpsA118T wpsB091T wpsD068 wpsJ57.8

wpsA147T wpsB115T wpsD069 wpsJ61.3

wpsB131T wpsJ64.8

wpsJ67.1

wpsJ67.3

wpsJ66.8

wpsJ71.0

wpsJ72.2

wpsJ72.9

wpsJ73.5

wpsJ106.5

wpsJ122.5

wpsJ129.8

wpsJ130.2

wpsJ132.5

Table 1. Lithofacies in the Meade Peak Phosphatic Shale Member of the Phosphoria Formation, as represented by the thin sections 

used in this study (for location of sections, see figure 1). Some thin sections are listed in more than one place on the table, a result of 

several lithofacies being present as individual stratum on one thin section. Details of the stratigraphic sections are in Tysdal and others 

(1999), Tysdal and others (2000a), Tysdal and others (2000b), Tysdal and others (2000c), and Grauch and others (2001).

[Each thin section has three notations: wps, an abbreviation for western phosphate section; a single capital letter identifying the measured stratigraphic section; 

and a number indicating the footage above the base of the section. Some sample notations end with a hyphenated letter, which represents a further subdivision, 

and some notations end with the letter T, which stands for thin section. As an example, wpsA023T is a western phosphate study specimen collected 23 ft above 

the base of the Meade Peak stratigraphic section A and was intended for thin sectioning. A few footage values listed in the table vary from the others in the 

placement of zeros, but the intended footage is apparent]
6 Petrographic Descriptions of Selected Rock Specimens from the Meade Peak Phosphatic Shale Member

LITHOFACIES SECTION A SECTION B SECTION C SECTION D SECTION E SECTION G SECTION J

Enoch 

Valley

Enoch 

Valley

Dry 

Valley

Dry 

Valley

Rasmussen 

Ridge

Smoky 

Canyon

Enoch 

Valley

Silty, organic claystone wpsA023T wpsB046T wpsC153 wpsD036 wpsE065 wpsJ49.4

wpsA029T wpsB062T wpsD046 wpsE067 wpsJ51.1

wpsA078T wpsB073T wpsD049 wpsE073 wpsJ52.2

Silty, organic claystone (cont.) wpsA118T wpsB078T wpsE108 wpsJ58.3

wpsA122T wpsB099T wpsJ61.3

wpsA127T wpsB115T wpsJ64.8

wpsA147T wpsB125T wpsJ71.0

wpsB129T wpsJ72.9

wpsB148T wpsJ73.6

wpsB157T wpsJ122.5

wpsJ130.2

wpsJ131.9

wpsJ132.5

wpsJ163.7

Phosphatic, organic claystone wpsC075 wpsE073 wpsJ131.9

wpsC090 wpsE082

wpsC112

wpsC130

Muddy siltstone wpsA153T wpsC153 wpsD068 wpsE065 wpsJ52.2

wpsD069 wpsE073 wpsJ57.8

wpsD071 wpsE075 wpsJ67.3

wpsD207 wpsE76.5 wpsJ72.2

wpsE086 wpsJ73.5

wpsE090 wpsJ106.5

wpsE093 wpsJ128.6

wpsJ129.8

wpsJ136.8

wpsJ165.9

wpsJ176.9

Sandy, muddy siltstone wpsA163T wpsJ73.6

wpsJ165.9

wpsJ176.9

wpsJ181.7

Phosphatic, muddy siltstone wpsC075 wpsD199 wpsE093

wpsC150 wpsD06

Table 1. Lithofacies in the Meade Peak Phosphatic Shale Member of the Phosphoria Formation, as represented by the thin sections 

used in this study (for location of sections, see figure 1). Some thin sections are listed in more than one place on the table, a result of 

several lithofacies being present as individual stratum on one thin section.—Continued
Sampling 7

LITHOFACIES SECTION A SECTION B SECTION C SECTION D SECTION E SECTION G SECTION J

Enoch 

Valley

Enoch 

Valley

Dry 

Valley

Dry 

Valley

Rasmussen 

Ridge

Smoky 

Canyon

Enoch 

Valley

Dolomitic calclithite wpsB003T wpsD017 wpsJ58.3

wpsB038T wpsD025 wpsJ66.8

Dolomitic calclithite (cont.) wpsB065T wpsD033 wpsJ67.1

wpsB121T wpsD121 wpsJ79.2

wpsD201 wpsJ85.9

wpsJ92.3

wpsJ97.6

Muddy, dolomitic calclithite wpsB068T wpsD058 wpsE087 wpsG046-B wpsJ2.5

wpsD062 wpsE122 wpsJ49.4

wpsD069 wpsJ52.4

wpsJ54.8

wpsJ58.3

wpsJ63.6

wpsJ72.9

Silty, dolomitic calclithite wpsC096 wpsG093

Phosphatic, dolomitic calclithite wpsD017 wpsG014

wpsD031 wpsG106

Sandstone wpsC146

Table 1. Lithofacies in the Meade Peak Phosphatic Shale Member of the Phosphoria Formation, as represented by the thin sections 

used in this study (for location of sections, see figure 1). Some thin sections are listed in more than one place on the table, a result of 

several lithofacies being present as individual stratum on one thin section.—Continued
8 Petrographic Descriptions of Selected Rock Specimens from the Meade Peak Phosphatic Shale Member

Table 2. Tabulations of the number of thin sections examined from the five mined zones 

in the Meade Peak Phosphatic Shale Member of the Phosphoria Formation, encountered 

in the two stratigraphic sections sampled at each of the four mines studied in this report 

(for location of sections, see figure 1).

Measured Sections

Unit 

sampled Enoch Valley Dry Valley Rasmussen

Ridge4

Smoky

Canyon4

wps 

A1

wps 

B2

wps 

J3

wps 

C1

wps 

D2

wps 

E2

wps 

G2

Upper waste 1 2 3 4 3 4 0

Upper ore 5 4 1 1 1 1 6

Middle waste 5 11 30 5 9 10 3

Lower ore 2 6 1 5 5 0 3

Lower waste 1 2 1 0 0 0 0

1

 More-weathered sample

2

 Less-weathered sample

3

 Core

4

 More-weathered samples not collected

Terminology and Classification

Phosphorite

Defining the word phosphorite is problematic. According to the Glossary of Geology (Jackson, 1997), phosphorite 

is a sedimentary rock with a high enough content of phosphate minerals to be of economic interest. Many workers 

consider this definition too general and would prefer a more 

quantitative version. Suggestions as to the minimum amount 

of phosphate required for a rock to be termed a phosphorite 

has been set as low as 5 percent P2O5 (Slansky, 1986), but 

minimum values of 19 percent (Cressman and Swanson, 

1964) or 20 percent (Boggs, 1987) are most commonly cited. 

However, adding such a requirement to the definition can 

cause problems in two ways. First, using a P2O5 percentage 

in the definition requires a chemical analysis to legitimately 

classify a rock as a phosphorite, and this procedure takes time 

and is costly. Second, a set P2O5 minimum would most likely 

exclude some Florida deposits that are economical at values as 

low 10 percent P2O5 in the bulk rock. But those deposits are 

an exception—most mined phosphorites in the U.S. contain at 

least 20 percent P2O5.

As proposed by Slansky (1979), and as adopted by Cook 

and Shergold (1986) and Trappe (2001), a value of 18 percent 

P2O5 equates to about 50 percent phosphate components by 

visual inspection. Using Slansky’s definitions, rocks with 

greater than 50 percent phosphate components are termed 

phosphorite and rocks with less than 50 percent phosphate 

components are termed phosphate rock. However, Slansky 

stated that the actual P2O5 percentage can range from 10 to 22, 

depending on the types of phosphate components. Although 

less than perfect, this scheme provides a quick hand-specimen 

method that more or less matches an anticipated analytical 

result. Slansky’s (1979) scheme does not solve the problem 

of phosphatic mud in peloidal rocks, which can contain a 

substantial amount of P2O5. The scheme also does not solve 

the problem of phosphatic argillaceous rocks that show few 

megascopic grains in hand specimens but can contain a substantial amount of P2

O5.

Apatite

Apatite group minerals are complex in their composition—general formula Ca5

(PO4,CO3)

3

(F,OH,CL)—and can be 

present in either igneous, metamorphic, or sedimentary rocks. 

The word collophane was used for many years as the finegrained apatite group present in marine phosphates. However, 

collophane is not a distinct mineral but a mixture of varying 

components, and in this way use of this term is analogous to 

use of the term limonite to describe various iron oxide/hydroxide compounds. Gulbrandsen (1960) reported that the apatite 

group mineral in the Meade Peak is intermediate among 

francolite, dehrnite, wilkeite, and ellestadite in composition, 

and suggested calling it carbonate fluorapatite instead of collophane. The substitution of carbonate (CO3

) for phosphate 

(PO4) is a principal characteristic of marine apatite, thus the 

descriptive compositional name. The term carbonate fluorapatite has since been widely accepted as the marine apatite in the 

Phosphoria. However, carbonate fluorapatite is shortened to 

simply apatite in this report. The material that was interpreted 

as apatite during this petrographic study is an integral component of peloidal phosphorites, and it is ubiquitous within 

peloids and as part of the phosphatic mud. It is light amber 
Petrographic Observations 9

to black in plane light and typically black (pseudoisotropic) 

under crossed nicols because of the aggregate polarizing effect 

resulting from its cryptocrystalline character. A more microcrystalline character in slightly coarser specimens can cause 

the apatite to appear weakly birefringent as the microscope 

stage is rotated.

Peloids

Traditionally, the round particles observed in phosphorites have been referred to as pellets, but a more definitive 

word is needed. The standard dictionary definition of pellet is 

too general to be of use in describing phosphorite components, 

and the Glossary of Geology (Jackson, 1997) restricts its 

usage to a structureless, homogeneous component of carbonate rock, such as a fecal pellet. Some workers have adapted 

the term phosphate allochems to describe the coarser components in phosphorites, and this general term is acceptable. 

The word phosphate links the term directly to phosphorites (as 

opposed to carbonates) and the word allochem is a collective 

term that includes rounded and non-rounded particles such as 

pellets, ooliths, fossils (and their fragments), and intraclasts. 

However, to be more specific, the term phosphate peloids (also 

spelled pelloids) is used in this report to describe the rounded, 

phosphatic aggregates that constitute the majority of particles 

in phosphorites. In doing so, the definition of peloid in the 

Glossary of Geology has been modified here by restricting the 

type of particles that can be included in the term. Again, the 

word phosphate links the term directly to phosphorites. In this 

report, the term phosphate peloid has been shortened to simply 

peloid (peloidal in adjective form).

Phosphate Lithofacies

The only concerted attempts at an original classification of phosphorites in the Phosphoria are the two schemes 

proposed by Mabie and Hess (1964)—one based on mineral 

composition and the other based on metallurgical characteristics. The first of the two schemes was designed to categorize 

ore by grade, and the second was designed for use in the ore 

beneficiation process. Neither of these two schemes, however, 

has applicability to this study. Instead, this study uses a classification scheme based on the original textural classification of 

carbonate rocks by Dunham (1962) as later modified by Cook 

and Shergold (1986) for suitability with phosphorites. This 

scheme is also the basic classification scheme used by Hiatt 

(1997) in his petrographic study of Phosphoria phosphorites. 

An important feature of this classification scheme is that it can 

be used with either hand specimens or thin sections. To Cook 

and Shergold’s scheme, two minor modifications are added 

for this study: (1) the word peloid is substituted for grain 

(as originally used by Dunham, 1962) because, in the case 

of phosphorites, peloids usually far outnumber any bioclasts, 

lithoclasts, or fragments of minerals that might be present; and 

(2) the word phosphatic is used as an adjective for mud (as 

originally used by Dunham, 1962) to more closely define the 

type of mud. Accordingly, the resulting classification adopted 

for this study, although not intended to replace the classification used by Cook and Shergold (1986), is as follows:

Phosphatic-mud supported 

Mudstone phosphorite 

 (less than 10 percent peloids)

 Wackestone phosphorite 

 (more than 10 percent peloids)

Peloid supported

Packstone phosphorite 

 (peloid supported with phosphatic mud)

 Grainstone phosphorite 

 (peloid supported without phosphatic mud)

Petrographic Observations

Only four major lithofacies were observed during this 

petrographic study of the Meade Peak: organic claystone, 

muddy siltstone, peloidal phosphorite, and dolomitized 

calclithite. Because the specimens from which the thin sections were cut were not collected systematically, the order 

of relative abundance of these four lithofacies is unknown; 

however, field observations indicate that this listing might be 

in the decreasing order of abundance. This ranking is for the 

entire Meade Peak, but relative abundance of lithofacies varies 

by mining zone; for example, ore zones contain more peloidal phosphorite than waste zones. In the following sections 

of this report, peloidal phosphorite is discussed first because 

of its obvious economic importance; following that, the other 

lithofacies are discussed in their perceived order of decreasing 

relative abundance.

Peloidal Phosphorite

Peloid Composition

Peloids are the major framework constituents of peloidal 

phosphorites. Peloids appear as featureless particles in hand 

specimens, but when viewed in thin sections with a polarizing 

microscope they are complex and varied. In hand specimens, 

peloids are seen to be spherical or ovoid in shape and seem 

to range in size from medium grained to very coarse grained 

(Wentworth, 1922). However, under the microscope the shape 

and size of the peloids are a function of the orientation of a 

particular peloid to the plane of the thin section. For example, 

an ovoid peloid can appear spherical if cut normal to its long 

axis, and a coarse-grained peloid can appear finer grained if 

not cut at its maximum diameter. Because of this, the reporting of shapes or of ranges in diameters would be misleading, 

so shapes are not described and only maximum diameters are 

cited. From petrographic observations, it is apparent that peloids are made up of varying amounts of three basic materials:
10 Petrographic Descriptions of Selected Rock Specimens from the Meade Peak Phosphatic Shale Member

Opaque material.––This peloidal component is the most 

common. It appears more or less homogeneous when viewed 

under a microscope, is very dark brown or black in plane light, 

and is totally opaque in polarized light. It is probably composed of some mixture of organic matter and cryptocrystalline 

phosphate mineral, possibly apatite.

Complex material.––This peloidal component is also 

common. It appears heterogeneous when viewed under a 

microscope, is typically some shade of light to dark brown 

or orange brown in plane light, and is dark brown to black 

in polarized light. It is probably composed of various forms 

of phosphate mineral, possibly apatite with or without some 

amount of organic matter.

Translucent material.––This peloidal component is rare. 

It appears more or less homogeneous when viewed under a 

microscope, and is bright brown or bright yellow in plane light 

and black in polarized light.

Peloid Structure

Also from petrographic observations, the internal structures of peloids can be placed into one of eight categories:

Simple.––Commonly, peloids lack any noticeable internal 

structure.

Banded.––This term is applied when the outer perimeter 

of a peloid displays one (single) or several (multiple) concentric bands of peloidal material differing from that of the main 

body of the peloid.

Cored.––Many peloids display a central core composed 

of a peloidal material differing from that of the main body. 

As defined here, these internal spherical masses must have a 

diameter less than half the diameter of the whole peloid.

Zoned.––This structure, which is uncommon, describes a 

peloid when its main body displays several concentric zones of 

differing material.

Oolitic.––Rarely, a peloid will be so tightly zoned that 

it appears oolitic in form. Such peloids are probably not true 

ooliths; that is, the structure is most likely diagenetic in origin 

rather than a recording of the primary outward growth of a 

peloid from a central nucleus.

Nucleated.––In rare cases, a peloid will contain a single 

nucleus that differs significantly in composition from normal 

peloidal material (for example, a particle of quartz or a fossil 

fragment). By convention, the diameter of the nucleus must be 

less than half the diameter of the peloid.

Polynucleated.––Even rarer, a peloid will contain two or 

more nuclei, and in these cases the combined nuclei can be of 

any diameter.

Composited.––Some rare peloids show an encasement of 

two or more individual peloids within a single body.

Peloid Types

A review of the pellet nomenclature of Emigh (1958) 

and Mabie and Hess (1964), as used in their respective 

petrographic studies of the Phosphoria, led to the conclusion 

that neither set of terms was wholly applicable to this study. 

Consequently, a simple peloid nomenclature was developed 

using the three basic materials that compose peloids. In almost 

all cases, the peloid name is accompanied by a structural 

modifier (for example, banded).

Opaque peloids.––Simple opaque peloids (fig. 3) are 

the most common type observed in thin sections. Many 

opaque peloids show some inclusions of quartz or carbonate 

silt, as well as small patches of translucent amber-colored 

material that is probably cryptocrystalline phosphate mineral. 

Opaque peloids with a single band of translucent material are 

common. A few opaque peloids were observed to show single 

bands of complex material, and opaque peloids in one thin 

0.5 mm

0 0.1 0.2 0.3 0.4 0.5

Figure 3. Photomicrograph showing a patch of opaque 

peloids in a specimen of silty, organic claystone (wpsJ131.9) 

collected from middle waste zone at Enoch Valley mine (for 

explanation of specimen notation, see table 1). This view in 

plane light shows black opaque peloids separated by white 

carbonate material. White patches within some peloids are 

quartz silt. 

section display a single band of randomly orientated, siltsized carbonate minerals. Rare opaque peloids are cored with 

complex material.

Because opaque peloids are dark in color and commonly 

ovoid in shape, it is tempting to assume that they are fecal in 

origin. McKelvey and others (1953) argued against such an 

origin for any pellets in the Phosphoria by pointing out that 

the pellets (1) have a wide range in size, (2) are well sorted 

within individual beds, (3) conform to regional trends in 

average size, (4) commonly have a compound nature, and (5) 

are oolitic and even pisolitic in some cases. If one considers 

the first point—the wide range in size—it seems possible that 

different size invertebrates would have produced different size 

fecal pellets. In regard to points two and three—good sorting 

within individual beds and regional trends in sorting—both of 
Petrographic Observations 11

these phenomena could possibly be explained by low-energy 

currents moving along the sea floor. Concerning points four 

and five—the compound nature of some pellets and the fact 

that some are oolitic or pisolitic—it is possible that most of 

these features are a function of diagenesis. Thus, a fecal origin 

for some opaque peloids cannot be excluded, an opinion 

generally shared by Emigh (1958), Cook (1969), and Tucker 

(1981). 

Complex peloids.––Complex peloids (fig. 4) are the 

most interesting and probably provide the best evidence for 

diagenesis in peloidal phosphorites. Most complex peloids 

are simple—that is, they lack any organized internal structure. However, many complex peloids display a single band 

of less complex material or translucent material. Rare complex peloids show a single band of opaque material, and even 

rarer complex peloids are cored with opaque material. Some 

complex peloids contain a few silt-sized, angular fragments of 

quartz and carbonate. Zoned and oolitic structures were rarely 

observed, but where noted they are associated with complex 

peloids. 

Translucent peloids.––Translucent peloids (fig. 5) are relatively uncommon, but are easily spotted by their bright color 

and clear appearance at first glance; however, most are cloudy 

to some degree when examined closely. Simple translucent 

Phosphatic Mud

The term phosphatic mud is defined here as the interstitial material separating individual peloids. Emigh (1967) 

referred to this material as cementing matrix [sic] because it 

forms the binding agent that holds the framework grains in 

position to form a rock. Under the microscope, phosphatic 

mud is typically some shade of dark brown in plane light and 

black in polarizing light. Because of its dark color, phosphatic 

mud is probably composed predominately of organic-rich clay. 

However, on the edges of the thin sections where the rock slice 

was less than 30 μm thick, phosphatic mud was observed to 

be complex, and it undoubtedly contains varying amounts of 

phosphatic mineral. In many cases, phosphatic mud contains 

a variety of silt-sized particles such as quartz and feldspar 

fragments, carbonate lithoclasts, mica crystals, and bioclasts. 

Silt-sized peloids are also common in the phosphatic mud. 

What is usually thought of as mineral cement was not readily apparent in the thin sections but is undoubtedly present in 

small amounts in the form of recrystalized phosphate mineral, 

calcite, and in special cases, silica.

Peloidal Lithofacies

Three types of peloidal phosphorite were observed: (1) 

wackestone phosphorite, (2) packstone phosphorite, and (3) 

grainstone phosphorite—in order of decreasing amounts of 

phosphatic mud. Most of the thin sections of peloidal phosphorite show only one of the three lithofacies, but many also 

show several lithofacies as individual stratum. Each thin 

0.5 mm

0 0.1 0.2 0.3 0.4 0.5

Figure 4. Photomicrograph showing complex peloids 

in a specimen of grainstone phosphorite (wpsB136T) 

collected from upper ore zone (D bed) at Enoch Valley 

mine. This view in plane light shows brown, simple 

(structureless), banded, and zoned complex peloids 

separated by light-brown, discontinuous areas of 

siliceous material. White, angular patches are holes in 

thin section. 

peloids are the most common type, but many are cored with 

opaque peloidal material, indicating a transitional relation; 

some rare peloids are cored with complex peloidal material. 

Rare translucent peloids display a single band of opaque peloidal material or a single band of complex peloidal material.

0.5 mm

0 0.1 0.2 0.3 0.4 0.5

Figure 5. Photomicrograph showing translucent 

peloids in a specimen of phosphatic, dolomitic calclithite 

(wpsD031) collected from lower ore zone (false cap) at 

Dry Valley mine. This view in plane light shows lightorange, translucent peloids separated by light-yellow, 

carbonate material. Dark brown patches within some 

peloids are opaque peloid material possibly being altered 

to translucent material. 
12 Petrographic Descriptions of Selected Rock Specimens from the Meade Peak Phosphatic Shale Member

section showing these lithofacies contains a unique array of 

peloid types, peloid structures, and phosphatic mud and its 

components, making the thin sections almost kaleidoscopic in 

their variations. Many of these rocks contain silt, but this study 

required the presence of at least 20 percent of these grains 

before attaching the word silty as a modifier. For the rare 

sandy rocks, a minimum value of 20 percent was also required 

before attaching the word sandy.

Wackestone phosphorite.––Wackestone phosphorite 

was a commonly observed peloidal rock. Several mediumand coarse-grained peloids were observed in thin sections 

of these rocks, but all other peloids were either very fine or 

fine grained. Thus, the average maximum peloid size might 

be classed as fine grained. Almost half of the thin sections 

contain sufficient amounts of silt to warrant the term silty 

wackestone phosphorite. Quartz is by far the most common 

silt fragment, but some feldspar and carbonate fragments 

are also present. In addition, one specimen was observed to 

contain enough sand to warrant the term sandy wackestone 

phosphorite, and in this case the sand grains are all quartz. 

The maximum sand size observed was very fine grained. The 

common presence of silt (and sand) is probably related to the 

higher phosphatic mud content of these rocks as compared to 

packstone phosphorites and grainstone phosphorites. Wackestone phosphorites and silty wackestone phosphorites were 

collected from both the ore zones and the waste zones.

Packstone phosphorite.––Packstone phosphorite was 

another commonly observed peloidal rock. Several coarsegrained and very coarse grained peloids are represented in 

thin sections of these rocks, but all others are either very fine, 

fine, or medium grained. Thus, the average maximum peloid 

size might be medium grained. Only a few of the thin sections 

contain sufficient amounts of silt to warrant the term silty 

packstone phosphorite. Packstone phosphorites were collected 

from both the ore zones and the waste zones. The fish-scale 

marker bed (Yochelson, 1968) at the base of the lower waste 

zone is unlike any other in the Meade Peak. Two specimens 

of this unit from the Enoch Valley mine are silica-cemented, 

packstone phosphorites composed of complex and translucent 

peloids. The complex peloids are almost always banded with 

complex material darker than that of the peloid, and translucent peloids and lithoclasts are also banded with complex 

material. Nucleated and polynucleated peloids are common. 

In addition to the peloids, lithoclasts of quartz and quartzite 

are common grains. Both medium- and coarse-grained peloids 

and lithoclasts are present, making coarse the possible average 

maximum grain size.

Grainstone phosphorite.––Grainstone phosphorite was 

an uncommonly observed peloidal phosphorite. Although 

some medium-grained peloids are present, most are either 

very fine or fine grained. Thus, the average maximum peloid 

size might be fine grained. None of the thin sections contain 

sufficient amounts of silt to include silty in the rock name. 

The scarcity of detrital silt is probably a function of the 

lower phosphatic mud content of these rocks as compared to 

packstone phosphorites and wackestone phosphorites. All of 

the grainstone phosphorites were collected from ore zones.

Sedimentary Structures

Observing sedimentary structures under the microscope, 

even with the lowest power objective, is difficult because most 

features are larger than the view seen through the eyepiece. 

However, by placing the thin sections on a light table and 

scanning them with a hand lens, sedimentary structures can 

be identified. Many of the peloidal phosphorites observed 

using this method are planar, parallel laminated, albeit weakly 

in some cases, and a few specimens display wavy, parallel 

laminations (as defined by Reineck and Singh, 1975). In hand 

specimens of peloidal phosphorites, the thickness of individual 

lamina is seen to range from very thin to very thick (as defined 

by Reineck and Singh, 1975). However, under the microscope, 

the thickness of lamina is a function of the orientation of the 

stratification to the plane of the thin section. Because of this, 

reporting lamina thickness would be misleading.

Diagenesis 

The diagenetic history of the various lithofacies in the 

Meade Peak is beyond the scope of this study. However, a 

brief mention of diagenesis as it affects the appearance of 

peloids is important. Based on the work of Emigh (1958) and 

Cook (1969), the vast array of peloid views seen through the 

microscope is mostly a function of the amount and arrangement of organic matter. In general, as diagenesis proceeds, the 

original content of organic matter gradually mobilizes and is 

either rearranged within the peloid or is selectively removed 

from the peloid. As the organic matter migrates outward, the 

peloid becomes a lighter overall color and one or more darker 

shells form on the circumference. If a significant amount of 

the organic matter leaves the peloid, one or more lighter-colored shells will remain around a relatively darker-colored core. 

In special cases, peloids become zoned or even develop oolitic 

structure as organic matter segregates as concentric shells 

within the main body of the peloid. This change in color tone 

is probably enhanced by the introduction of secondary phosphate mineral that fills the spaces left by the migrating organic 

matter. The fact that organic matter can freely move toward the 

edge of the peloid and beyond, and that phosphate-mineralrich solutions can enter the peloid, indicate that what appears 

to be a dense, solid material is in reality a porous body. It is 

possible that most peloids were originally structureless bodies 

with only subtle differences in composition. With time, diagenetic processes acted on these differences giving each peloid, 

or group of peloids, a unique appearance.

Organic Claystone

Organic-rich claystones (hereafter called organic claystones) are black in hand specimens and show only a few 
Petrographic Observations 13

discernable grains when viewed with a hand lens. Under the 

microscope, they appear almost totally black in both plane and 

polarized light. The term claystone, as defined by the Glossary of Geology (Jackson, 1997), is a sedimentary rock with 

greater than 67 percent clay-sized minerals, and the modifying 

term organic is added here because of the rock’s ubiquitous 

black color. Microscopic observations made in reflected light 

reveal that the organic matter is primarily solid bitumen (M.J. 

Pawlewicz, oral commun., 2002). Thin section observations 

disclosed that much of the organic matter is present within 

densely packed, opaque peloids. Because more than one-half 

of the organic claystones are peloidal, this characteristic is 

probably the rule rather than the exception. Several mediumand coarse-grained organic peloids were observed, but all 

others were either very fine or fine grained. Thus, the average 

maximum peloid size might be fine grained. On the edges 

of some thin sections, where the rock slice is relatively thin, 

complex patterns in various shades of amber are visible in 

plane light. This feature indicates that some organic claystones 

are in fact a mixture of organic matter and phosphate mineral 

that is present as phosphatic mud or as a component of the 

opaque peloids. In some of these cases the name phosphatic 

organic claystone might be more appropriate. This combination of organic matter and phosphate mineral might place 

some of these rocks in the category of mudstone phosphorites 

(phosphatic-mud supported with less than 10 percent peloids) 

as defined by Cook and Shergold (1986).

Almost all of the organic claystones contain accessory 

components. The most common of these is silt-sized quartz, 

with lesser amounts of feldspar. In more than one-half of the 

rocks, silt-sized material exceeds 20 percent and the modifying term silty was applied. Some organic claystones contain 

phosphate peloids, but rarely in excess of 20 percent. Also 

present in some organic claystones are crystals of muscovite 

aligned parallel to lamination. Perhaps the most interesting 

accessory components of some organic claystones are siltsized bioclasts. Based on observations by R.R. Charpentier 

(oral commun., 2002), the most common bioclasts are forams 

(in both polar and equatorial views), brachiopod shell fragments (including some with pseudopuncate shell structure), 

and sponge spicules. In addition, C.A. Sandberg (oral commun., 2002) recognized fragments of fish teeth.

As viewed on a light table, almost all organic claystones 

are parallel laminated. Planar parallel lamination is the most 

common type, but discontinuous planar parallel laminations 

and wavy parallel laminations were also noted (as defined by 

Reineck and Singh, 1980). Graded bedding was seen in some 

thin sections, and, where observed in orientated specimens, 

inverse grading was found to be more common than normal 

grading. Rare examples of soft-sediment deformation and cutand-fill structures were also present.

Organic claystone is present throughout the Meade Peak. 

However, as might be expected, these rocks are most common 

in the waste zones, particularly in the middle waste zone.

Four samples of organic claystone were arbitrarily 

selected for organic geochemistry and vitrinite reflectance 

(Ro) analysis in an effort to acquire a cursory impression of 

the Meade Peak’s past potential as a source rock for petroleum. Two of the samples were from a measured stratigraphic 

section at the Smoky Canyon mine (one from the lower ore 

zone and one from the upper ore zone), and two of the samples 

were from a core drilled at the Enoch Valley mine (one from 

the lower ore zone and one from the upper ore zone). The 

samples were analyzed by M.J. Pawlewicz (written commun., 

2001) at the USGS organic geochemistry laboratory in Denver, 

Colo. Vitrinite reflectance, an indicator of thermal maturity, 

was measured on solid bitumens from two of the samples—the 

other two samples did not contain enough organic matter to 

be measured. The values obtained were 1.41 percent and 1.88 

percent, both numbers indicating that the rocks have passed 

through the oil window and are overmature in regard to oil. 

Thermal maximum (Tmax), another indicator of thermal maturity, was measured on three of the samples—the fourth sample 

yielded no results. The values obtained were 449°C, 451°C, 

and 563°C. The higher of the three numbers indicates that the 

rock has passed through the oil window and is overmature, but 

the lower numbers indicate that the rock is still within the oil 

window. Total organic carbon (TOC) was measured on all four 

of the samples, with values of 2.33 percent, 2.72 percent, 8.85 

percent, and 11.08 percent. Although this diminutive sampling 

of the geochemical properties of the organic claystones has 

no statistical merit, it does imply that these rocks were good 

source rocks for petroleum generation.

Muddy Siltstone

Organic-rich siltstones (hereafter informally called 

muddy siltstones) are black in hand specimens and show 

many discernable grains when viewed with a hand lens. 

Under the microscope, in both plane and polarized light, siltsized fragments appear bright against an almost totally black 

background. The term siltstone, as defined by the Glossary of 

Geology (Jackson, 1997), is a sedimentary rock with greater 

than 67 percent silt-sized material; here, the modifying term 

muddy is added because organic-rich mud always exceeded 20 

percent of the rock. The framework grains are subrounded to 

subangular, with quartz and feldspar being the most abundant. 

Some rocks contain small portions of organic-claystone peloids and rarely phosphate peloids; the average maximum size of 

these constituents might be fine grained.

The quartz grains are clearly recognizable by standard 

optical properties such as a colorless appearence in plane 

light and a white or first-order gray appearance in polarized 

light, the display of undulatory extinction, and the absence 

of cleavage. Based on these characteristics, quartz dominates 

the framework grains in the siltstones. The feldspar grains are 

also colorless in plane light and white or first-order gray in 

polarized light, but they lack undulatory extinction and their 

outer margins are commonly transected cleavage planes. Two 

thin sections of muddy siltstone were stained for potassium 

feldspar and plagioclase feldspar. In both thin sections the yellow coloration indicative of potassium feldspar was noted on 
14 Petrographic Descriptions of Selected Rock Specimens from the Meade Peak Phosphatic Shale Member

some grains. However, despite the rare observation of grains 

displaying albite twinning in other thin sections of muddy siltstone, the pink coloration indicative of plagioclase feldspar is 

absent in both of these thin sections. Powder X-ray diffraction 

(XRD) analyses by Knudsen and others (2002), using Rietveld 

quantification software for siltstones collected from the wpsJ 

core, indicated the presence of three types of feldspar: albite, 

buddingtonite, and orthoclase (for explanation of specimen 

notation, see table 1). In these samples, any one of the three 

feldspars could dominate, but commonly buddingtonite is the 

most abundant and orthoclase the least abundant. Knudsen and 

others (2002) also identified the presence of relatively small 

amounts of dolomite and calcite, and probably small amounts 

of these minerals are present among the framework grains.

Some muddy siltstones contain very fine grained sand as 

streaks or patches in the rock. In rare cases, this size fraction 

exceeds 20 percent and the name sandy muddy siltstone is 

applicable. (One specimen of very fine grained sandstone was 

also noted during this study.) 

Although the intergranular material in the muddy siltstones is composed primarily of black organic-rich mud, this 

material in most rocks contains common grains of various 

siliceous minerals at the lower end of the range of silt sizes, 

with quartz and feldspar predominating. In addition, fragments 

of muscovite crystals are present in some of this material, as 

are rare bioclasts similar to those present in some organic claystones. The intergranular material in some muddy siltstones 

contains amber-colored material that is probably phosphate 

mineral. In rare cases where this component exceeds 20 percent (at the expense of the organic-rich mud), the name phosphatic siltstone is appropriate, especially if phosphate peloids 

are also present in the same rock.

All of the framework grains observed in the muddy 

siltstones are most likely detrital fragments that were deposited in the depositional site as the distal part of a subaqueous, 

siliciclastic sedimentary wedge. If so, then the prevalent silt 

size resulted from the natural sorting that took place as the 

wedge entered deeper water and wave and current energy was 

reduced. Alternatively, Carroll and others (1998) suggest that 

the silt fragments settled into the depositional site after being 

blown into the vicinity by prevailing winds. 

Many muddy siltstones are planar parallel laminated, 

and one example of inverse graded bedding was observed. 

As might be expected, muddy siltstones are most commonly 

associated with waste zones, and the phosphatic siltstones are 

most commonly associated with ore zones.

Dolomitized Calclithite

Carbonate rocks in the Meade Peak are dark or medium 

gray to medium brown in hand specimens. They are commonly aphanitic when viewed with a hand lens, but very fine 

grains are visible in a few specimens. The rocks are well 

indurated and generally emit a fetid odor when struck with a 

rock hammer. Outcrop debris are blocky to chip-like. Many 

specimens display intersecting, light-colored, calcite veinlets. 

Rare specimens are parallel laminated and others are fossiliferous. Some of these rocks are readily susceptible to weathering, 

and where it has taken place the rocks are an ochre color and 

are altered to a punky, clay-like material.

As viewed under the microscope, the dolomitized calclithites consist of light-colored, silt-sized grains separated 

from each other by varying amounts of dark-colored intergranular material. The grains are of two types: carbonate and 

quartz (though probably there are some feldspars as well). In 

plane light the carbonate grains appear complex and varied. 

They are typically colorless and cloudy, and rhombic cleavage 

is commonly observed. In polarized light the carbonate grains 

show high birefringence and appear pearl gray to pale pastel. 

The carbonate grains can generally be placed into one of three 

broad categories, listed in order of decreasing abundance: (1) 

partially rectangular or trapezoidal, single- or multiple-crystalline units; (2) poorly rounded, microcrystalline masses; or 

(3) crystalline units with faint, microcrystalline nuclei. Some 

of these rocks contain all three grain types, but most have only 

two types, and in rare cases, just one. The poorly rounded, 

microcrystalline masses (category 2) are interpreted as detrital 

fragments, and it is postulated that the single- or multiple-crystalline units (category 1) are the result of selective recrystallization of the detrital fragments, probably following deposition. 

Those crystalline units with microcrystalline nuclei (category 

3) would then represent a transitional phase in this process. A 

detrital interpretation, if correct, would eliminate the necessity 

of conceiving a scheme to explain the unique and transient 

chemical environment needed for primary carbonates to be 

precipitated in the middle of the Phosphoria sea. The established environment for the Meade Peak would remain one of 

phosphate-mineral precipitation (peloidal phosphorites), with 

intermittent influxes of detrital clay-sized material (organic 

claystones) and silt (muddy siltstones); the carbonate rocks 

would then represent just another type of detrital influx. Two 

conceivable process-source scenarios for the detrital carbonate are speculated: (1) debris flows extending seaward from 

carbonate reefs marginal to the Phosphoria sea that are now 

lithified as the Permian Park City Formation, or (2) stream 

loads consisting of eroded carbonate originally precipitated in 

sabkhas in what is now central Wyoming and now lithified as 

the Permian-Triassic Goose Egg Formation. Either possibility 

could have been facilitated by eustatic falls in sea level.

The term calclithite, as generally defined by the Glossary 

of Geology (Jackson, 1997), is a fragmental carbonate rock 

consisting of more than 50 percent extraclasts derived from the 

erosion of older lithified carbonates. All of the rocks referred 

to as calclithite in this report contain at least 50 percent carbonate grains, but some rare specimens also contain quartz silt 

in excess of 20 percent; for these rocks, the modifying term 

silty is applied.

Differentiating between calcite and dolomite under 

the microscope is generally difficult, so two thin sections 

(wpsB003T and wpsJ97.6) were stained with Alizarian 

Red-S. Almost all of the carbonate grains in the stained thin 

sections are a subtle beige color that is typically associated 
Petrographic Observations of Selected Marker Beds 15

with dolomite. Red coloration associated with stained calcite 

is present as veinlet material and in the interiors of some bioclasts. All of this calcite is undoubtedly the result of secondary mineralization. However, red coloration is also present as 

minute, irregular patches within grains and amidst intergranular material, but whether this calcite is primary or secondary 

is questionable. Knudsen and others (2002) conducted powder 

X-ray diffraction analyses on samples labeled as dolostones 

(their terminology) collected from the wpsJ core. For those 

samples that qualified as carbonate rock, about 67 percent of 

the total rock is dolomite and about 16 percent is calcite.

How dolomite became the dominate carbonate in the 

calclithites is indeterminable. The detrital grains could have 

arrived at the depositional site as a mixture of primary calcite 

(from eroded reef complexes) and primary dolomite (from 

sabkha supersaline carbonate flats) and remained so. More 

likely, they could have arrived solely as calcite grains and later 

selectively dolomitized. If dolomitization did take place, the 

timing of this process is also indeterminable—did it take place 

at the same time as the carbonate recrystallization or later?

The intergranular material in the dolomitized calclithites 

is similar to that contained in the muddy siltstones, being 

composed primarily of black organic-rich mud with dispersed 

grains of carbonate, quartz, and feldspar, all at the lower end 

of the range of silt sizes. In addition, bioclasts are present in 

some of this material. More than one-half of the dolomitized 

calclithites contain 20 percent or more organic-rich mud, so 

the name muddy, dolomitized calclithite is applied to these 

rocks. For those containing phosphate peloids in excess of 20 

percent, which are uncommon, the termed phosphatic, dolomitized calclithites is applied.

Sedimentary structures are generally absent from the 

dolomitized calclithites. However, these structures seem to 

increase with an increase in intergranular material. Planar 

parallel laminations are common in muddy dolomitized calclithites, and graded bedding (both normal and inverse) was 

observed in some of these rocks. 

Dolomitized calclithites and muddy dolomitized calclithites are present in all of the mining subdivisions of the 

Meade Peak, but as expected, they are particularly common in 

the waste zones. However, some of the thicker units of these 

rocks are present in ore zones, and because they are somewhat unique in these zones, they are named and used as local 

marker beds at some mines. All of the phosphatic, dolomitized 

calclithites came from ore zones, as would be expected.

Miscellaneous Findings

Two types of authigenic minerals were observed in thin 

sections, and both are present as fracture fillings and vug 

linings. The more common of the two is calcite, and the less 

common is a translucent-orange phosphate mineral. Because 

all of the specimens observed were included in subsets of 

either less-weathered or more-weathered pairs collected at 

specific mines, it was expected that a petrographic distinction 

would be apparent between these two degrees of weathering. 

Although there are undoubtedly subtle variations between the 

two categories, no striking differences were noted. However, if 

certain paired subsets of less-weathered and more-weathered 

thin sections were viewed on a light table, the less-weathered 

thin sections tended to have an overall gray tone, whereas the 

more-weathered thin sections had an overall brown tone. This 

contrast in color is at least partly the result of differing oxidation history, but the specific components involved were not 

determined.

Petrographic Observations of Selected 

Marker Beds

When observing the Meade Peak in the highwalls of 

phosphate mines, it can be difficult to pick out the various 

mining zones because everything appears black. However, 

personnel at specific mines faced with this problem typically 

utilize mine-specific marker beds, which in themselves can be 

difficult to pick out. In some cases, marker beds used at one 

mine are laterally discontinuous and do not extend beyond the 

mine property, so they cannot be correlated with marker beds 

at adjacent mines. In other cases, such as the twin limes in the 

middle waste at the Rasmussen Ridge and Dry Valley mines, 

correlation is tempting.

 Specimens from several of these marker beds were 

collected during the course of field work, and a brief discussion of the result of thin section observations follow. A pair 

of marker beds is present in the middle waste zone at the 

Rasmussen Ridge and Dry Valley mines, referred to by mine 

personnel as the twin limes. Personnel at the Rasmussen Ridge 

mine identified these beds in the highwall while stratigraphic 

section wpsE was being measured (Tysdal and others, 2000b). 

Based on thin sections, the lower bed is a silty organic claystone and the upper bed consists of a wackestone phosphorite 

and a muddy siltstone.

A marker bed at the Enoch Valley mine termed the false 

cap by mine personnel is present in the lower ore zone (Tysdal 

and others, 1999). A thin section of this bed collected from 

wpsA shows the rock to be a dolomitized calclithite.

Three marker beds are recognized by mine personnel in 

the lower ore zone at the Dry Valley mine. In ascending order, 

they are: A cap, B mudstone, and false cap (Tysdal and others, 

2000a). A thin section of a specimen of the A cap in wpsD 

contains two lithofacies: dolomitized calclithite and phosphatic, dolomitized calclithite. A thin section of the B mudstone in wpsD shows the rock to be a dolomitized calclithite. 

The relation, if any, between the false cap at the Dry Valley 

mine and the false cap at the Enoch Valley mine was not determined. A thin section of the false cap in wpsC shows the rock 

to be a wackestone phosphorite. Two thin sections were cut 

from specimens of the false cap in wpsD. One shows the rocks 

to be a phosphatic, dolomitized calclithite, and the other shows 

the rock to be a dolomitized calclithite.
16 Petrographic Descriptions of Selected Rock Specimens from the Meade Peak Phosphatic Shale Member

The cap rock is a term used by mine personnel at the 

Smoky Canyon mine to designate a marker bed in the lower 

ore zone (Tysdal and others, 2000c). A thin section cut from a 

specimen collected from the unit in wpsG shows the rock to be 

a phosphatic, dolomitized calclithite. Another thin section cut 

from a specimen of the unit in wpsG contains two lithofacies 

identified as muddy siltstone and silty, wackestone 

phosphorite.

Summary

1. The Meade Peak Phosphatic Shale Member of the Permian Phosphoria Formation in southeastern Idaho contains 

four major lithofacies: organic claystone, muddy siltstone, 

peloidal phosphorite, and dolomitized calclithite—in 

order of decreasing abundance. 

2. Phosphate peloids are composed of varying amounts of 

opaque, complex, and translucent material.

3. The internal structures of peloids are classified as simple, 

banded, cored, zoned, oolitic, nucleated, polynucleated, or 

composited.

4. Peloidal phosphorites are placed in one of three categories: wackestone phosphorite, packstone phosphorite, or 

grainstone phosphorite.

5. Wackestone phosphorites are phosphatic-mud supported 

and contain more than 10 percent peloids, packstone 

phosphorites contain phosphatic mud but are peloid supported, and grainstone phosphorites are peloid supported 

and lack phosphatic mud.

6. Organic claystone is the most common lithofacies in the 

Meade Peak and commonly contains quartz silt, muscovite, and bioclasts as accessory components; it generally 

is parallel laminated.

7. Muddy siltstone is composed primarily of quartz silt; 

parallel laminations are common.

8. Dolomitized calclithite contains three types of carbonate 

grains: macrocrystalline, microcrystalline, and crystalline with microcrystalline nuclei—in order of decreasing 

abundance. Sedimentary structures are generally absent.

9. Secondary calcite and apatite, observed as fracture fillings 

and vug linings, are common in all rock types.

10. Peloidal phosphorites are most common in the ore zones, 

and detrital rocks—organic claystones, muddy siltstones, 

and dolomitized calclithites—are most common in the 

waste zones.

11. Mine-specific marker beds are most commonly dolomitized calclithites.

Acknowledgments

Several USGS geologists provided guidance during this 

study. Among them are Ron Charpentier and Charlie Sandberg 

with micropaleontology, Joe Hatch with carbonate deposition, 

and Mark Pawlewicz with organic geochemistry.

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