Conodont Color Alteration (CAI) as an Aid to Structural 

Interpretation in the Black Pine Mountains, Idaho

By J. Fred Smith, Jr. and Bruce R. Wardlaw

Open-File Report 2012–1100

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

For product and ordering information:

World Wide Web: http://www.usgs.gov/pubprod

Telephone: 1–888–ASK–USGS

For more information on the USGS—the Federal source for science about the Earth,

its natural and living resources, natural hazards, and the environment:

World Wide Web: http://www.usgs.gov

Telephone: 1–888–ASK–USGS

Suggested citation:

Smith, J.F., Jr., and Wardlaw, B.R., 2012, Conodont color alteration (CAI) as an aid to structural interpretation in the 

Black Pine Mountains, Idaho: U.S. Geological Survey Open-File Report 2012–1100, 5 p., available only at 

http://pubs.usgs.gov/of/2012/1100.

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

endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.
iii

Contents

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

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

Geology ......................................................................................................................................................................... 2

Conodont Color Alteration Indices in the Black Pine Mountains .................................................................................... 4

Interpretation ................................................................................................................................................................. 4

Summary ....................................................................................................................................................................... 4

Acknowledgments ......................................................................................................................................................... 5

References Cited........................................................................................................................................................... 5

Figures

1. Index map showing location of Black Pine Mountains (Strevell 15-minute quadrangle), 

Cassia County, Idaho......................................................................................................................................... 8 

2. A, Generalized geologic map of Paleozoic rocks in the Black Pine Mountains (Strevell 15-minute 

quadrangle) modified from Smith (1982); B, Map of outcrop area of Paleozoic rocks in the Black Pine 

Mountains showing conodont localities .............................................................................................................. 9 

Table

1. Conodonts reported from samples submitted, Strevell 15-minute quadrangle,

Cassia County, Idaho................................................................................................................. In a separate file
iv

Conversion Factors

Inch to SI

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

mile, nautical (nmi) 1.852 kilometer (km)

yard (yd) 0.9144 meter (m)

SI to Inch/Pound 

 Multiply By To obtain

 Length

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

kilometer (km) 0.5400 mile, nautical (nmi) 

meter (m) 1.094 yard (yd) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32
1

Conodont Color Alteration (CAI) as an Aid to Structural 

Interpretation in the Black Pine Mountains, Idaho

By J. Fred Smith, Jr.1 and Bruce R. Wardlaw

Abstract 

The Black Pine Mountains, southeastern Cassia County, Idaho, consist of southern and northern 

blocks separated by a northeast-trending, high-angle fault. Differences in conodont color alteration 

values distinguish the two blocks. The southern block has significantly higher organic maturation levels 

than the northern block and is interpreted to have been thrust northeastward adjacent to the northern 

block. 

Introduction

The Black Pine Mountains (Strevell 15-minute quadrangle), Cassia County, Idaho (fig. 1), are 

transected by a northeast-trending fault that separates the range into two blocks. Values of CAI (color 

alteration index) of conodonts from Mississippian to Permian rocks are very different on opposite sides 

of this fault. This difference in values is useful to understanding the relative movement on the fault. 

 Conodont CAI values have been shown to be useful indicators of organic metamorphism in 

Ordovician through Triassic rocks (Epstein and others, 1977). Harris and others (1978) used CAI values

for indicating thermal maturity in assessing hydrocarbon potential in Paleozoic rocks of the Appalachian 

basin. Harris and others (1980) applied the same technique to the Great Basin for assessing thermal 

maturity. CAI analysis of collections from relatively small areas may also aid in structural 

interpretations by indicating contiguous structural blocks with similar thermal histories and by 

indicating other, perhaps adjacent, blocks with markedly different thermal histories. The use of CAI 

values in structural interpretation in eastern Tennessee has been demonstrated by Epstein and others 

(1977, fig. 15). This report on the Black Pine Mountains demonstrates the usefulness of this technique 

in a small area in the Great Basin-Rocky Mountains region. 

Color or CAI values are expressed as numbers from 1 to 8 (Epstein and others, 1977; Rejebian 

and others, 1987). Values recorded from the Black Pine Mountains range from 3.0 to 6.5, which equate 

to temperatures ranging from 110o

C to 500oC. A low- to high-temperature range is given for each value, 

as Epstein and others (1977) determined that color alteration in conodonts is both time and temperature 

dependent. 

 

1

Deceased. 
2

Figure 1. Index map showing location of Black Pine Mountains (Strevell 15-minute quadrangle), Cassia County, 

Idaho. 

The trace amount of organic matter encased in the apatite skeleton of individual conodont 

elements gradually changes color from amber to black as carbon is fixed within the temperature range of 

50oC to 300oC. At higher temperatures (400oC–500oC), the carbon is gradually burned off and the 

apatite skeleton gradually decomposes, resulting in color changes from black to gray to white to crystalclear before the conodont element is completely destroyed. Conodonts, therefore, can indicate minimum 

and maximum temperatures attained by their host rocks through a wide thermal range (50oC–500oC). 

These temperatures relate chiefly to depth and duration of burial and to geothermal gradients. 

Geology

Paleozoic rocks in the Black Pine Mountains range in age from Middle Devonian to Middle 

Permian (fig. 2). They consist of siltstone, limestone, dolostone, quartzite, all gradations between 

limestone and sandstone and siltstone, and chert. The mountain range is separated into two structural 

blocks by a northeast-trending fault near West Dry Canyon (fig. 2). Rock units from which conodont 

collections were made south of this fault include the Upper Mississippian and Lower Pennsylvanian 

Manning Canyon Shale and the Lower Pennsylvanian (Bashkirian) to Lower Permian (Artinskian) 

Oquirrh Formation of which four informal members are recognized. Four units from which conodonts 

were collected north of the fault and in the northeastern part of the Strevell quadrangle range from 

Upper Pennsylvanian (Gzhelian) to Middle Permian (Roadian). South of the range-transecting fault, all 

contacts between mapped units are low-angle faults. North of the range-transecting fault, one contact 

appears to be a fault and the others appear to be conformable (fig. 2A).
3

Figure 2. A, Generalized geologic map of Paleozoic rocks in the Black Pine Mountains (Strevell 15-minute 

quadrangle) modified from Smith (1982); north block map units: a, limestone, silty limestone, dolostone, siltstone 

and sandstone with chert; b, chert, siltstone, and limestone; c, calcareous siltstone, calcareous sandstone, and silty 

limestone; d, dark-gray limestone; e, silicified limestone, limestone, and limestone breccias. South block map units: 

J, Jefferson Formation; M, Manning Canyon Shale; and informal members of the Oquirrh Formation: 1, limestone, 

sandstone, and quartzite; 2, limestone; 3, limestone and dolostone; 4, sandstone and siltstone. B, Map of outcrop 

area of Paleozoic rocks in Black Pine Mountains showing conodont localities; open circles indicate CAI values of 

3.0 to 4.0, solid circles indicate CAI values of 4.5 to 5.5, and solid diamonds indicate CAI values of 5.5 to 6.5.
4

Conodont Color Alteration Indices in the Black Pine Mountains

CAI values of two conodont collections south of the fault transecting the Black Pine Mountains 

(fig. 2B) have a range from 4.5 (240oC–400oC) to 6.5 (450oC–500oC); most of the collections have a 

value of 5.0 (300oC to 400oC). CAI values of 10 conodont collections north of the range-transecting 

fault have a range from 3.0 (110oC–200oC) to 4.0 (190oC–300oC) with all but one value being 4.0 (table 

1). There is no overlap in CAI values north and south of the fault. 

Interpretation

The difference in CAI values and inferred temperatures on the two sides of the range-transecting 

fault is interpreted to indicate distinct thermal histories for the two structural blocks. It is inferred that 

the sediments of the blocks were deposited at some distance from each other, underwent different 

general geologic and thermal histories, and were subsequently juxtaposed structurally. Rocks of the 

northern block are younger than and partly equivalent to those of the southern block (fig. 2). Differences 

in lithology and CAI values in correlative (Gzhelian to Artinskian) units of the two blocks indicate 

somewhat different depositional environments and thermal histories. The correlative units are unit 4 in 

the southern block and unit c in the northern block (fig. 2A). 

The youngest beds of the northern block are younger than those of the southern block (Roadian 

versus Artinskian; early Middle Permian versus late Early Permian); these beds, however, do not exceed 

a thickness of 2,300 meters (m), and it seems doubtful that the differences in CAI values resulted simply 

from differences in overburden. Furthermore, CAI values south of the fault are similar through a 

stratigraphic thickness of about 5,500 m, and those north of the fault are similar through a stratigraphic 

thickness of about 2,800 m. This indicates that the elevated temperatures probably were pervasive 

through at least these rock thicknesses.

Overturned folds in some units of the southern block indicate that at least part of the deformation 

was related to generally eastward-directed movements (Smith, 1983). The southern block is interpreted 

to have moved northeastward relative to the northern block and to have formed an upper plate that cut 

across the northern block along the northeast-trending, essentially vertical (strike-slip) part of the fault. 

It also rode up and over the northern block along the northwest-trending part of the fault inferred to be 

covered by Quaternary deposits east of the Black Pine Mountains (fig. 2). The differences in CAI values 

of conodonts in the two blocks suggest that the southern block originated in an area of higher heat flow 

than that of the northern block. This higher heat flow is of uncertain origin, although it may possibly 

have been related to development of the metamorphic complexes of the Raft River Mountains and 

Albion Range (fig. 1). Temperatures attained by rocks in the northern block may have been related to 

some part of the same complexes but farther removed from the heat source than the southern block. 

CAI values can be used to assess organic maturation in the Black Pine Mountains. These values 

suggest that post-mature or high-temperature methane stage of petroleum generation was reached (for 

example, see Hood and others, 1975, p. 993–994; Epstein and others, 1977, p. 22–25; Milner and others, 

1977, p. 107–108) and that occurrence of liquid hydrocarbons is unlikely. Although the possibility that 

there is a lower structural plate consisting of rocks of lower organic maturation cannot be disproved, it

also seems unlikely.

Summary 

Conodont CAI values provide data that can be used in structural interpretations, although 

relations of indices may not always be as clearly drawn as they seem to be in the Black Pine Mountains. 
5

Differences in CAI values in the two principal structural blocks of these mountains suggest that these 

blocks originated in different thermal regimes before being juxtaposed by faulting. The southern block 

is interpreted to have been thrust adjacent to the northern block. Geologic data from the Black Pine 

Mountains are not adequate to demonstrate whether or not the lower part of the northern block is 

autochthonous. 

Acknowledgments

Anita G. Harris and John E. Repetski wrote initial conodont reports on a few samples. This 

paper was prepared in 1981 but was set aside upon the death of the senior author. Recent inventory of 

the USGS collections at the National Museum of Natural History identified matrix collections that were 

collected for fusulinids and etched for conodonts in 1979 by Wardlaw. These collections made up about 

half of those reported here. The discovery of the sampled matrix collections led to the discovery of the 

last version of the manuscript, which is updated here. As the senior author originally pointed out, this 

paper makes for a good case study in the use of conodont derived thermal maturity in structural 

interpretations.

References Cited

Epstein, A.G., Epstein, J.B., and Harris, L.D., 1977, Conodont color alteration; An index to organic 

metamorphism: U.S. Geological Survey Professional Paper 995, 27 p. 

Harris, A.G., Harris, L.D., and Epstein, J.B., 1978, Oil and gas data from Paleozoic rocks in the 

Appalachian basin; Maps for assessing hydrocarbon potential and thermal maturity (conodont color 

alteration isograds and overburden isopachs): U.S. Geological Survey Miscellaneous Investigations 

Series Map I–917–E, scale 1:2,500,000. 

Harris, A.G., Wardlaw, B.R., Rust, C.C., and Merrill, G.K., 1980, Maps for assessing thermal maturity 

(conodont color alteration index maps) in Ordovician through Triassic rocks in Nevada and Utah and 

adjacent parts of Idaho and California: U.S. Geological Survey Miscellaneous Investigations Series 

Map I–1249, scale 1:2,500,000. 

Hood, A., Gutjahr, C.C.M., and Heacock, R.L., 1975, Organic metamorphism and the generation of 

petroleum: American Association of Petroleum Geologists Bulletin, v. 59, no. 6, p. 986–996. 

Milner, C.W.D., Rogers, M.A., and Evans, C.R., 1977, Petroleum transformations in reservoirs: Journal 

of Geochemical Exploration, v. 7, no. 2, p. 101–153. 

Rejebian, V.A., Harris, A.G., and Huebner, J.S., 1987, Conodont color and textural alteration; An index 

to regional metamorphism, contact metamorphism, and hydrothermal alteration: Geological Society 

of America Bulletin, v. 99, no. 4, p. 471–479. 

Smith, J.F., Jr., 1982, Geologic map of the Strevell 15-minute quadrangle, Cassia County, Idaho: U.S. 

Geological Survey Miscellaneous Investigations Series Map I–1403, scale 1:62,500. 

Smith, J.F., Jr., 1983, Paleozoic rocks in the Black Pine Mountains, Cassia County, Idaho: U.S. 

Geological Survey Bulletin 1536, 36 p.