DOE/ID-22223

Prepared in cooperation with the U.S. Department of Energy

Paleomagnetic Correlation and Ages of Basalt Flow Groups 

in Coreholes At and Near the Naval Reactors Facility, 

Idaho National Laboratory, Idaho

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013-5012
Cover: Kyle Champion, USGS volunteer, using drill press to subcore basalt samples for paleomagnetic analysis; 

subcores marked with orientation; box of core from corehole NRF 15. (Photographs courtesy of Kyle Champion, and 

the USGS Idaho National Laboratory Project Office.)
Paleomagnetic Correlation and Ages of 

Basalt Flow Groups in Coreholes At and 

Near the Naval Reactors Facility, 

Idaho National Laboratory, Idaho

By Duane E. Champion, Linda C. Davis, Mary K.V. Hodges, and Marvin A. Lanphere

DOE/ID-22223

Prepared in cooperation with the U.S. Department of Energy

Scientific Investigations Report 2013–5012

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Champion, D.E., Davis, L.C., Hodges, M.K.V., and Lanphere, M. A., 2013, Paleomagnetic correlation and ages of basalt 

flow groups in coreholes at and near the Naval Reactors Facility, Idaho National Laboratory, Idaho: U.S. Geological 

Survey Scientific Investigations Report 2013-5012 (DOE/ID-22223), 48 p.
iii

Contents

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

Introduction.....................................................................................................................................................2

Purpose and Scope ..............................................................................................................................4

Previous Investigations .......................................................................................................................4

Basalt Flow and Flow Group Definition and Labeling Conventions..............................................8

Geologic Setting....................................................................................................................................8

Analytical Methods......................................................................................................................................10

Paleomagnetic Results ...............................................................................................................................10

Corehole USGS 133.............................................................................................................................11

Corehole NRF B18-1............................................................................................................................11

Corehole NRF 89-05 ............................................................................................................................11

Corehole NRF 89-04 ............................................................................................................................11

Corehole NRF 6P .................................................................................................................................11

Corehole NRF 7P .................................................................................................................................11

Corehole NRF 15..................................................................................................................................11

Corehole NRF 16..................................................................................................................................11

Age Data........................................................................................................................................................18

Limitations of Radiometric Dating of Olivine Tholeiite Basalts ...................................................19

Methods of Analysis...........................................................................................................................19

K-Ar Age Experiments...............................................................................................................19

40Ar/39Ar Age Experiments........................................................................................................19

Results of Age Experiments by Basalt Flow Group.......................................................................20

West of Advanced Test Reactor Complex (ATRC) Flow Group ..........................................20

Advanced Test Reactor Complex (ATRC) Unknown Vent Flow Group .............................20

Central Facilities Area (CFA) Buried Vent Flow Group ........................................................20

State Butte Flow Group ............................................................................................................20

Atomic Energy Commission (AEC) Butte Flow Group .........................................................20

North Late Matuyama Flow Group .........................................................................................21

Stratigraphy .................................................................................................................................................21

West of Advanced Test Reactor Complex (ATRC) Flow Group....................................................21

Advanced Test Reactor Complex (ATRC) Unknown Vent Flow Group .......................................21

North Idaho Nuclear Technology and Engineering Center (INTEC) 

Buried Vent Flow Group .......................................................................................................23

Central Facilities Area (CFA) Buried Vent Flow Group..................................................................23

State Butte Flow Group......................................................................................................................23

Atomic Energy Commission (AEC) Butte Flow Group ...................................................................23

Late Basal Brunhes Flow Group.......................................................................................................23

East Matuyama Middle Flow Group ................................................................................................24

North Late Matuyama Flow Group...................................................................................................24

Additional Basalt Flow Groups and Sedimentary Interbeds Found at Depth in 

Corehole NRF 15 ....................................................................................................................24

Summary and Conclusions.........................................................................................................................24

Acknowledgments.......................................................................................................................................25
iv

References Cited..........................................................................................................................................26

Appendix A. Paleomagnetic Inclination Data for Selected Coreholes At and Near the Naval 

Reactors Facility, Idaho National Laboratory, Idaho...............................................................29

Appendix B. 40Ar/39Ar Analytical Data for Selected Coreholes, Naval Reactors Facility, Idaho 

National Laboratory, Idaho ...........................................................................................................41

Figures

1. Map showing location of the eastern Snake River Plain, the boundary of the

Idaho National Laboratory (INL), the location of the Naval Reactors Facility

(NRF), and other selected facilities ...................................................... 3

2. Map showing location of the Naval Reactors Facility (NRF) at the Idaho

National Laboratory, Idaho, the location of eight coreholes described in this

report, and the line of cross-section through the eight coreholes .................. 5

3. Pliocene-Holocene geomagnetic time scale............................................. 9

4. Log of the core stratigraphy and mean paleomagnetic inclination data for 

corehole USGS 133 at the Idaho National Laboratory, Idaho ........................ 12

5. Log of the core stratigraphy and mean paleomagnetic inclination data for

corehole NRF B18-1 at the Idaho National Laboratory, Idaho ........................ 13

6. Log of the core stratigraphy and mean paleomagnetic inclination data for

corehole NRF 89-05 at the Idaho National Laboratory, Idaho ........................ 13

7. Log of the core stratigraphy and mean paleomagnetic inclination data for

corehole NRF 89-04 at the Idaho National Laboratory, Idaho ........................ 14

8. Log of the core stratigraphy and mean paleomagnetic inclination data for

corehole NRF 6P at the Idaho National Laboratory, Idaho ........................... 15

9. Log of the core stratigraphy and mean paleomagnetic inclination data for

corehole NRF 7P at the Idaho National Laboratory, Idaho. ........................... 16

10. Log of the core stratigraphy and mean paleomagnetic inclination data for

corehole NRF 15 at the Idaho National Laboratory, Idaho. ........................... 17

11. Log of the core stratigraphy and mean paleomagnetic inclination data for

corehole NRF 16 at the Idaho National Laboratory, Idaho.............................. 18

12. Map showing location of coreholes in the southern part of the Idaho National

Laboratory, Idaho (modified from Champion and others, 2011) ........................ 22

Plate

Plate 1. Cross-section of the subsurface stratigraphy interpreted from paleomagnetic 

inclination data from coreholes near the Naval Reactors Facility, Idaho National 

Laboratory, Idaho.

Contents—Continued
v

Tables

1. Weighted mean potassium-argon ages and analytical data for basalt samples

from coreholes NRF 89-05, NRF 89-04, NRF 6P, and NRF 7P at the Naval Reactors

Facility, Idaho National Laboratory, Idaho................................................ 6

2. Summary of 40Ar/39Ar ages and analytical data for basalt samples from 

coreholes NRF 6P and NRF 7P, Naval Reactors Facility, Idaho National 

Laboratory, Idaho ........................................................................... 6

3. Summary of selected previous investigations on geology, paleomagnetism, 

and stratigraphy of the eastern Snake River Plain and Idaho National 

Laboratory, Idaho ........................................................................... 7

Conversion Factors, Datums, and Abbreviations and 

Acronyms

Conversion Factors

SI to Inch-Pound

Multiply By To obtain

centimeter (cm) 0.3937 inch (in.)

cubic kilometer (km3) 0.2399 cubic mile (mi3)

cubic meter (m3) 35.3145 cubic foot (ft3) 

kilometer (km) 0.6214 mile (mi)

meter (m) 3.2808 foot (ft)

square kilometer (km2) 0.3861 square mile (mi2)

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

°F= (1.8×°C) +32.

Inch-Pound to SI

Multiply By To obtain

cubic mile (mi3) 4.168 cubic kilometer (km3)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

square mile (mi2) 2.590 square kilometer (km2)

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

°C=(°F-32)/1.8.

Datums

Vertical coordinate information is referenced to National Geodetic Vertical Datum of 1929 

(NGVD 29). 

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

Altitude, as used in this report, refers to distance above the vertical datum. 
vi

Abbreviation or 

Acronym

Definition

AF Alternating field

ATRC Advanced Test Reactor Complex (formerly known as RTC, Reactor 

Technology Complex, and TRA, Test Reactor Area)

CFA Central Facilities Area

DOE U.S. Department of Energy

ESRP Eastern Snake River Plain

ICPP Idaho Chemical Processing Plant

INEL Idaho National Engineering Laboratory (1974–97)

INEEL Idaho National Engineering and Environmental Laboratory (1997–2005)

INL Idaho National Laboratory (2005-present)

INTEC Idaho Nuclear Technology and Engineering Center

IRM Isothermal remanent magnetization

ka Thousand years old

k.y. Thousand years

Ma Million years old

NRF Naval Reactors Facility

NRTS National Reactor Testing Station (1949–74)

RWMC Radioactive Waste Management Complex

TAN Test Area North

USGS U.S. Geological Survey

Abbreviations and Acronyms
Abstract

Paleomagnetic inclination and polarity studies were 

conducted on subcore samples from eight coreholes located at 

and near the Naval Reactors Facility (NRF), Idaho National 

Laboratory (INL). These studies were used to characterize and 

to correlate successive stratigraphic basalt flow groups in each 

corehole to basalt flow groups with similar paleomagnetic 

inclinations in adjacent coreholes. Results were used to extend 

the subsurface geologic framework at the INL previously 

derived from paleomagnetic data for south INL coreholes. 

Geologic framework studies are used in conceptual and 

numerical models of groundwater flow and contaminant 

transport. Sample handling and demagnetization protocols 

are described, as well as the paleomagnetic data averaging 

process. 

Paleomagnetic inclination comparisons among NRF 

coreholes show comparable stratigraphic successions of 

mean inclination values over tens to hundreds of meters of 

depth. Corehole USGS 133 is more than 5 kilometers from 

the nearest NRF area corehole, and the mean inclination 

values of basalt flow groups in that corehole are somewhat 

less consistent than with NRF area basalt flow groups. Some 

basalt flow groups in USGS 133 are missing, additional basalt 

flow groups are present, or the basalt flow groups are at depths 

different from those of NRF area coreholes. 

Age experiments on young, low potassium olivine 

tholeiite basalts may yield inconclusive results; paleomagnetic 

and stratigraphic data were used to choose the most reasonable 

ages. Results of age experiments using conventional potassium 

argon and argon-40/argon-39 protocols indicate that the 

youngest and uppermost basalt flow group in the NRF area is 

303 ± 30 ka and that the oldest and deepest basalt flow group 

analyzed is 884 ± 53 ka.

A south to north line of cross-section drawn through 

the NRF coreholes shows corehole-to-corehole basalt flow 

group correlations derived from the paleomagnetic inclination 

data. From stratigraphic top to bottom, key results include the 

following:

• The West of Advanced Test Reactor Complex (ATRC) 

flow group is the uppermost basalt flow group in the 

NRF area and correlates among seven continuously 

cored holes in this study under surficial sediments. The 

West of ATRC flow group is also found in coreholes 

near the ATRC, the Idaho Nuclear Technology 

and Engineering Center (INTEC), and in corehole 

USGS 129.

• The ATRC Unknown Vent flow group correlates among 

seven continuously cored holes in this study underlying 

the West of ATRC flow group and a sedimentary 

interbed. Additional paleomagnetic inclination and 

stratigraphic data derived from the NRF coreholes 

changed the previously reported interpretation of the 

subsurface distribution of this basalt flow group. The 

ATRC Unknown Vent flow group also is found in 

coreholes near the ATRC and INTEC.

• The Central Facilities Area (CFA) Buried Vent flow 

group correlates among all eight coreholes in the NRF 

area. It also is found in coreholes near the CFA and the 

Radioactive Waste Management Complex (RWMC) 

to the south. This basalt flow group is thickest near the 

CFA, which may indicate proximity to the vent. The 

State Butte flow group is found below the CFA Buried 

Vent flow group in the four northern NRF coreholes. 

It correlates to the State Butte surface vent located just 

northeast of the NRF. It is not found in coreholes south 

of the NRF.

Paleomagnetic Correlation and Ages of Basalt Flow 

Groups in Coreholes at and near the Naval Reactors 

Facility, Idaho National Laboratory, Idaho

By Duane E. Champion, Linda C. Davis, Mary K.V. Hodges, and Marvin A. Lanphere
2 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

• The Atomic Energy Commission (AEC) Butte flow 

group is found in coreholes USGS 133, NRF 6P, and 

NRF 7P. It probably underlies coreholes NRF B18-1, 

NRF 89-05, and NRF 89-04, but those coreholes were 

not drilled deeply enough to penetrate the flow group. 

The AEC Butte flow group vent is exposed at the 

surface near the ATRC, and its flows are found in many 

coreholes near the ATRC and INTEC. The AEC Butte 

flow group abruptly pinches out against the Matuyama 

Chron reversed polarity flows of the East Matuyama 

Middle flow group between coreholes NRF 7P and 

NRF 15.

• The East Matuyama Middle flow group correlates 

between coreholes NRF 15 and NRF 16 and may 

correlate to coreholes NPR Test/W-02 and ANLOBS-A-001.

• The North Late Matuyama flow group correlates 

among coreholes USGS 133, NRF 6P, NRF 7P, 

NRF 15, and NRF 16. It probably underlies coreholes 

NRF B18-1, NRF 89-05, and NRF 89-04, but those 

coreholes were not drilled deeply enough to penetrate 

the flow group. The vent that produced the North Late 

Matuyama flow group may be located in the general 

NRF area because it is thickest near corehole NRF 6P.

• The Matuyama flow group is found in coreholes in the 

southern INL from south of the RWMC to corehole 

USGS 133 and may extend north to corehole NRF 15. 

The Matuyama flow group is thickest near the RWMC 

and thins to the north.

• The Jaramillo (Matuyama) flow group is found in 

corehole NRF 15, which is the deepest NRF corehole, 

and shows that the basalt flow group is thick in the 

subsurface at NRF. This flow group is thickest between 

the RWMC and INTEC and thins towards the ATRC 

and NRF.

Introduction

The U.S. Atomic Energy Commission [(AEC), now the 

U.S. Department of Energy (DOE)] established the National 

Reactor Testing Station (NRTS) on about 2,300 km2 of the 

eastern Snake River Plain (ESRP) in southeastern Idaho 

in 1949. The NRTS was established to develop peacetime 

atomic energy, nuclear safety research, defense programs, 

and advanced energy concepts. The name of the laboratory 

has been changed to reflect changes in the research focus of 

the laboratory. Names formerly used for the laboratory, from 

earliest to most recent, were the National Reactor Testing 

Station [(NRTS) 1949–74], the Idaho National Engineering 

Laboratory [(INEL) 1974–97], and the Idaho National 

Engineering and Environmental Laboratory [(INEEL) 

1997– 2005]. Since 2005, the laboratory has been known as 

the Idaho National Laboratory (INL) (fig. 1). 

U.S. Geological Survey (USGS) scientists have been 

studying the geology, petrography, paleomagnetism, age 

of basalt lava flows, and hydrology of the ESRP for more 

than 100 years beginning with Russell (1902). Studies of 

the geologic framework of the ESRP at and near the INL 

intensified in 1949 when feasibility studies for siting of 

the NRTS began. Studies included evaluation of hydraulic 

properties of the aquifer, seismic and volcanic hazards, 

facility design and construction, and the evolution of basaltic 

volcanism on the ESRP. 

Wastewater that contained chemical and radiochemical 

wastes was discharged to ponds and wells, and solid and liquid 

wastes were buried in trenches and pits excavated in surficial 

sediments at the INL. Some wastewater continues to be 

disposed to infiltration and evaporation ponds. Concern about 

subsurface movement of contaminants from these wastes 

increased the number and variety of studies of subsurface 

geology and hydrology to provide information for conceptual 

and numerical models of groundwater flow and contaminant 

transport (Anderson and Lewis, 1989; Anderson, 1991; 

Anderson and Bowers, 1995; Anderson and Liszewski, 1997; 

Anderson and others, 1996a, 1996b; Ackerman and others, 

2006, 2010). 

In 1974, studies began to evaluate potential volcanic 

hazards at the INL. Subsequently, these studies were 

expanded in an evaluation of subsurface stratigraphy as it may 

have influenced hydrologic properties of the saturated and 

unsaturated zones of the ESRP aquifer. These studies have 

included geologic mapping and various studies of cored and 

surface basalt flows such as petrologic and paleomagnetic 

investigations and radiometric age measurements. Many 

investigations were carried out on selected drill cores from 

different facilities at the INL, such as the Radioactive 

Waste Management Complex (RWMC), the Idaho Nuclear 

Technology and Engineering Center (INTEC), and Test Area 

North (TAN).
Introduction 3

Figure 1. Location of the eastern Snake River Plain, the boundary of the Idaho National Laboratory (INL), the location of the 

Naval Reactors Facility (NRF), and other selected facilities.

tac12-0786_fig 01

Mud

Lake

Birch

Big

Lost

River

Little

Lost

River

Creek

Little Lost

River Sinks

Big Lost

River

Spreading

Areas

Big Lost River

Sinks and 

Playas

BITTERROOT

RANGE

LEMHI RANGE

LOST RIVER RANGE

PIONEER

MOUNTAINS

Howe

Mud

Lake

Terreton

Atomic

City

Arco

Base from U.S. Geological Survey digital data, 1:24,000 and 

1:100,000. Projection: UTM, Zone 12N, datum is North American 

Datum of 1983.

Selected facilities at the Idaho National Laboratory

Advanced Test Reactor Complex

Central Facilities Area

Idaho Nuclear Technology and Engineering Center

Naval Reactors Facility

Radioactive Waste Management Complex

Test Area North

TAN

ATRC

INTEC 

NRF

RWMC

CFA

113°

44°

43°

30’

113°30’ 112°30’

0 10 Miles

0 10 Kilometers

IDAHO

BOISE

EASTERN 

SNAKE RIVER

PLAIN IDAHO 

NATIONAL

LABORATORY

Twin

Falls

Pocatello

Idaho

Falls

CFA

INTEC

NRF

RWMC

ATRC

TAN

EXPLANATION

Idaho

National

Laboratory

Map area

Figure 2

Map area

State Figure 12

Butte

Vent

AEC

Butte

West of

ATRC

Vent

Idaho National Laboratory boundary

 Vent exposed at land surface with label AEC Butte
4 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Paleomagnetic data records the Earth’s magnetic field 

at the time of eruption and is not unique. The remanent 

magnetization recorded by ferromagnetic minerals in basalt 

lava flows aligns with the geomagnetic field vector as the 

basalt crystallizes and cools. The local geomagnetic field 

vector varies with an average angular motion of 4 to 5 degrees 

per 100 years in latest Pleistocene and Holocene basalt flows, 

with extreme variance from 0 to 10 degrees per 100 years 

(Champion and Shoemaker, 1977). The ESRP monogenetic 

volcanic fields with volumes of 4 km3 or more record a single 

direction of remanent magnetization, which indicates that 

individual basalt flows that belong to that basalt flow group 

erupted in a sufficiently brief period of time (less than 100 

years) such that any change in the local geomagnetic field 

vector was too small to detect. Each volcano most likely 

erupted for only a few days to a few decades. 

Paleomagnetic inclination was used to correlate 

subsurface basalt flow groups based on similar inclination 

values and polarity. The subcore plugs taken from drill cores 

and used in this study only yield paleomagnetic inclination 

data because the original declination was not preserved in 

the drill cores during drilling. Other data, such as lithology, 

petrology, geophysical logs, and geochemistry, can be used 

in conjunction with paleomagnetic inclination and age data to 

confirm or reject correlations.

Age dates for this study were obtained through 

conventional potassium-argon (K-Ar) or argon-40/argon-39 

(40Ar/39Ar) age experiments. Ten samples from four coreholes 

were chosen for analysis. Four were dated by using both the 

K-Ar and the 40Ar/39Ar methods, five were analyzed solely 

by the K-Ar method, and one was analyzed solely by the 

40Ar/39Ar method.

Purpose and Scope

This report presents paleomagnetic and radiometric age 

information for basalt flow groups from coreholes drilled 

at and near the Naval Reactors Facility (NRF) at the INL. 

Coreholes used in this study were drilled between 1989 and 

2009. The objective of this study was to describe subsurface 

stratigraphy derived from paleomagnetic properties and 

selected ages of basalt flow groups from eight drill cores 

(USGS 133, NRF B18-1, NRF 89-05, NRF 89-04, NRF 

6P, NRF 7P, NRF 15, and NRF 16) taken along a southto-north transect from corehole USGS 133 through the 

NRF to corehole NRF 16 at the INL (fig. 2). Subsurface 

basalt flow groups were then correlated based on their 

mean paleomagnetic inclinations (appendix A), ages, and 

stratigraphic position. K-Ar age experiment results from 

coreholes NRF 89-04 and NRF 89-05 were previously 

released by Lanphere and others (1993). K-Ar ages for 

coreholes NRF 6P and NRF 7P are presented in table 1 and 

40Ar/39Ar ages for samples from coreholes NRF 6P and 

NRF 7P were published by Champion and others (2002). 

Results and analytical data from all these age experiments are 

presented here for completeness (tables 1, 2, and appendix B). 

Results of this study will be used to extend a threedimensional subsurface stratigraphic framework of the INL 

(Champion and others, 2011) at and near the NRF for geologic 

and hydrologic studies, including the evaluation of potential 

volcanic hazards to the INL. This study will also help to refine 

the geologic framework of the conceptual and numerical 

models of groundwater flow and contaminant transport 

through the ESRP aquifer at and near the INL.

Previous Investigations 

Numerous geologic, paleomagnetic, and stratigraphic 

investigations on surface and subsurface basalts at and 

near the INL and the ESRP have been conducted. Selected 

publications that resulted from these investigations are 

summarized in table 3. 

Kuntz and others (1980) described the petrology, 

subsurface stratigraphic framework, and potential volcanic 

hazards of the Radioactive Waste Management Complex 

(RWMC) by using drill core data, which include K-Ar ages, 

paleomagnetic inclination and polarity data, and petrography. 

Anderson and Lewis (1989) expanded the stratigraphic 

framework of the unsaturated zone at the RWMC to numerous 

other boreholes by using natural gamma geophysical logs as 

a primary correlation tool in conjunction with core described 

by Kuntz and others (1980). Anderson (1991) and Anderson 

and Bowers (1995) used natural gamma logs and sparse core 

data to describe the stratigraphy of the unsaturated zone and 

upper part of the ESRP aquifer near the Idaho Chemical 

Processing Plant [ICPP—now the Idaho Nuclear Technology 

and Engineering Center (INTEC)], Test Reactor Area [TRA, 

now the Advanced Test Reactor Complex, (ATRC)] and Test 

Area North (TAN) areas of the INL. Anderson and others 

(1996a), Anderson and Liszewski (1997), and Champion 

and others (2011) summarized the regional stratigraphic 

relationships of the unsaturated zone and the ESRP aquifer 

at and near the INL. The Champion and others (2011) report 

represents the most quantitative interpretation of the regional 

stratigraphy of the south INL area to date because it is based 

on paleomagnetic inclination and polarity measurements on 

cores not previously published, rather than interpretations 

made from natural gamma geophysical logs.
Introduction 5

Figure 2. Location of the Naval Reactors Facility (NRF) at the Idaho National Laboratory, Idaho, the 

location of eight coreholes described in this report, and the line of cross-section through the eight 

coreholes.

tac12-0786_fig02

NRF 6P

NRF 7P

NRF 16

NRF 15

NRF 89-05

NRF 89-04

Big Lost River

USGS 133

LINCOLN BOULEVARD

A’

A

NRF B18-1

USGS 133

A A’

Corehole identifier

Line of cross-section

State

Butte

Vent

Naval Reactors 

Facility (NRF)

0 1 Miles

0 1 Kilometers

EXPLANATION

 Vent exposed at land surface with label 

State Butte

Vent
6 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Table 1. Weighted mean potassium-argon ages and analytical data for basalt samples from coreholes NRF 89-05, NRF 89-04, NRF 6P, 

and NRF 7P at the Naval Reactors Facility, Idaho National Laboratory, Idaho.

[Shaded bands indicate results for samples from different cores or depths within the same basalt flow group. Basalt flow group: The flow group from which the 

core sample was collected. ATRC, Advanced Test Reactor Complex; CFA, Central Facilities Area; AEC, Atomic Energy Commission. Depth (m/ft): Sample 

depth is in meters and feet (m/ft). K2O wt%: Mean and standard deviation of four measurements in weight percent of potassium oxide measured in each sample. 40Arrad (10-13 mol/g): absolute number of moles/gram (mol/g) of 40Ar measured in each extraction. 40Arrad (percent): percentage of radiogenic 40Ar measured 

in each extraction. Weighted mean flow group age (103 years): weighted mean ages in red italics represent the age deemed reasonable for a particular flow 

group, see text and table 2 for explanations; 103 years, 1000s of years]

Core name Basalt flow group Experiment 

number

Depth

(m/ft)

K2

O

wt%

40Arrad

(10-13 mol/g)

40Arrad

(percent)

Calculated

extraction age 

(103

 years)

Weighted mean1,2

flow group age 

(103

 years)

NRF 89-053 West of ATRC 89-05_79a 24/79 0.522±0.002 2.1 2.7 279 ± 45 303 ± 30

89-05_79b 2.435 3.6 323 ± 41

NRF 89-053 ATRC Unknown Vent 89-05_179a 55/179 0.332±0.001 3.204 4.3 669 ± 75 712 ± 53

89-05_179b 3.61 4.7 754 ± 74

NRF 6P ATRC Unknown Vent 6P_155a 47/155 0.398±0.004 3.483 7.8 609 ± 52 491± 30

6P_155b 2.395 6.4 418 ± 50

6P_155c 2.585 2.6 451 ± 53

NRF 89-043 CFA Buried Vent 89-04_212a 65/212 0.448±0.002 3.82 1.6 591 ± 149 492 ± 56

89-04_212b 3.067 3.1 475 ± 61

NRF 89-053 CFA Buried Vent 89-05_235a 72/235 0.646±0.003 5.138 4 552 ± 55

89-05_235b 7.047 5.1 758 ± 58 598 ± 31

89-05_235c 4.815 4 518 ± 50

NRF 89-043 State Butte 89-04_231a 70/231 0.576±0.001 7.161 6 864 ± 58 819 ± 39

89-04_231b 6.478 6.2 782 ± 53

NRF 6P AEC Butte 6P_372a 113/372 0.526±0.001 3.946 1.9 521 ± 110

6P_372b 2.597 1.1 343 ± 119 384± 65

6P_372c 2.178 1.1 287 ± 108

NRF 7P North Late Matuyama 7P_440 134/440 0.456±0.003 4.475 2.2 682 ± 121 4604 ± 89

7P_486 148/486 0.463±0.006 3.415 1.6 512 ± 131

1

Weighted mean of the calculated extraction ages, where weighting is by the inverse of the variance, and weighted standard deviation.

2

λε = 0.581 × 10-10yr-1, λB = 4.962 × 10-10yr-1, 40K/K = 1.167 × 10-4 mol/mol. Errors are estimates of the standard deviation of analytical precision.

3

Data from Lanphere and others (1993).

4

Weighted mean age of two samples from different depths in same flow.

Table 2. Summary of 40Ar/39Ar ages and analytical data for basalt samples from coreholes NRF 6P and NRF 7P, Naval Reactors Facility, 

Idaho National Laboratory, Idaho.

[Basalt flow group: The flow group from which the core sample was collected. ATRC, Advanced Test Reactor Complex; AEC, Atomic Energy Commission. 

Depth (m/ft): sample depth, in meters and feet (m/ft). Plateau 39Ar* (percent [steps]): corrected percentage of 39Ar released (100 percent total) during [n of m] 

diffusion steps. Plateau age: weighted mean age using [n of m] diffusion steps. Plateau ages in red italics represent the age deemed reasonable for a particular 

flow group, see text and table 1 for explanations. Spectrum description: qualitative description of the flatness and continuity of the diffusion steps. Isochron 

age: derived from the slope of the best fit line on an isochron diagram plotting 40Ar/36Ar versus 39Ar/36Ar isotopic ratios. Isochron intercept: 40Ar/36Ar axis 

ratio of the best fit isochron line on the isochron diagram. Abbreviation: 103 years, 1000s of years]

Core name Basalt flow group Depth

(m/ft)

Plateau 39Ar*

(percent [steps])

Plateau age

(103

 years)

Spectrum

description

Isochron age

(103 years)

Isochron

intercept

NRF 6P ATRC Unknown Vent 47/155 98[7 of 8] 395±25 Good plateau 372±38 296.6±1.7

NRF 6P State Butte 74/241 90[5 of 8] 546±47 Good plateau 484±89 297.0±1.8

NRF 6P AEC Butte 113/372 89[4 of 10] 727±31 Good plateau 752±63 294.9±1.6

NRF 7P North Late Matuyama 134/440 50[6 of 10] 884±53 Fair plateau 878±242 295.7±3.4

NRF 7P North Late Matuyama 148/486 94[12 of 18] 1,176 ± 27 Fair plateau 1,178±168 295.5±2.8
Introduction 7

Reference Area of investigation Reference summary 

Anderson and Lewis, 1989 INL, RWMC Stratigraphy of the unsaturated zone and upper aquifer at the RWMC, based on 

natural gamma logs and selected core data.

Anderson, 1991 INL, ICPP and TRA Stratigraphy of the unsaturated zone and upper aquifer at ICPP and TRA, based 

on natural gamma logs and selected core data.

Anderson and Bowers, 1995 INL, TAN Stratigraphy of the unsaturated zone and upper aquifer at ICPP and TRA based 

on natural gamma logs and selected core data.

Anderson and Liszewski, 1997 INL INL unsaturated and ESRP aquifer stratigraphy, based on natural gamma logs 

and selected core data.

Anderson and Bartholomay, 1995 INL, RWMC RWMC stratigraphy based on natural gamma logs and selected core data. 

Anderson and others, 1996a INL Stratigraphic data for wells at and near the INL. 

Anderson and others, 1999 ESRP, INL, and vicinity Geologic controls on hydraulic conductivity. 

Champion and others, 1981 INL, RWMC Potassium-argon dating and paleomagnetic data from RWMC basalt cores 

indicate a 0.46 Ma magnetic reversal. 

Champion and others, 1988 INL Radiometric ages and paleomagnetism at corehole Site E (NPR Test), 

description of Big Lost cryptochron.

Champion and Herman, 2003 INL, INTEC Paleomagnetism of basalt from drill cores. 

Champion and others, 2002 ESRP, INL Accumulation and subsidence based on paleomagnetism and selected core data. 

Champion and others, 2011 INL Paleomagnetic correlation of surface and subsurface basalt from drill cores. 

Grimm-Chadwick, 2004 INL, CFA Stratigraphy, geochemistry, and descriptions of high K2O flow in cores. 

Hackett and Smith, 1992 ESRP Description of ESRP volcanism including development of Axial Volcanic Zone. 

Kuntz, 1978 INL, RWMC Geology of RWMC area. 

Kuntz and others, 1980 INL, RWMC Radiometric dating, paleomagnetism on cores from RWMC. 

Kuntz and others, 1986 ESRP Radiocarbon dates on Pleistocene and Holocene basalt flows. 

Kuntz and others, 1992 ESRP ESRP basaltic volcanism, including eruption styles, landforms, petrology, and 

geochemistry. 

Kuntz and others, 1994 INL Geologic map of INL, including radiometric ages and paleomagnetism. 

Lanphere and others, 1994 INL, TAN Petrography, age and paleomagnetism of basalt flows at and near TAN. 

Lanphere and others, 1993 INL, NRF Petrography, age and paleomagnetism of basalt flows at and near NRF. 

Mazurek, 2004 INL Genetic alteration of basalt in SRP aquifer. 

Miller, 2007 INL, RWMC Geochemistry, and descriptions of the B flow, and stratigraphy of corehole 

USGS 132.

Morse and McCurry, 2002 INL Base of the aquifer, alteration in basalts. 

Reed and others, 1997 INL, ICPP Geochemistry of lava flows in cores at ICPP. 

Rightmire and Lewis, 1987 INL, RWMC Unsaturated zone geology, geochemistry of sediment and alteration. 

Scarberry, 2003 INL, RWMC Geochemistry of the F flow (now referred to as the Big Lost Reversed Polarity 

Cryptochron flows) and distribution in several coreholes at the INL.

Shervais and others, 2006 INL, TAN Cyclic geochemical variations in basalt in TAN drill cores. 

Tauxe and others, 2004 SRP Paleomagnetism of the Snake River Plain. 

Twining and others, 2008 INL Construction diagrams, lithological, and geophysical logs for coreholes. 

Welhan and others, 2002 INL, ESRP Morphology of inflated pahoehoe flows. 

Wetmore and Hughes, 1997 INL Model morphologies of subsurface lava flows. 

Wetmore and others, 1999 INL Axial Volcanic Zone construction. 

Table 3. Summary of selected previous investigations on geology, paleomagnetism, and stratigraphy of the eastern Snake River Plain 

and Idaho National Laboratory, Idaho. 

[Abbreviations: INL, Idaho National Laboratory; RWMC, Radioactive Waste Management Complex; INTEC, Idaho Nuclear Technology and Engineering 

Center (also known as ICPP [Idaho Chemical Processing Plant]); TRA, Test Reactor Area; TAN, Test Area North; ESRP, eastern Snake River Plain; Ma, million 

years; NPR, New Production Reactor; CFA, Central Facilities Area; NRF, Naval Reactors Facility; USGS; U.S. Geological Survey] 
8 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Basalt Flow and Flow Group Definition and 

Labeling Conventions

In this report, a basalt flow is defined as a single, 

continuous outpouring of basalt on the Earth’s surface. A 

basalt flow group is defined as any and all basalt flows from a 

single monogenetic volcanic eruption. The ESRP monogenetic 

volcanic fields record a single direction of paleomagnetic 

inclination, which indicates that individual basalt flows that 

belong to that flow group were emplaced during a few days 

to a few decades of time (Kuntz and others, 1992). In this 

report, basalt flow groups were identified by boundaries where 

polarity or mean inclination values changed. Some boundaries 

were identified by sedimentary interbeds, oxidized flow 

tops, or areas where a significant change in petrography was 

evident.

Whenever possible, basalt flow groups were labeled 

for their vents, such as “AEC Butte flow group,” and are so 

labeled on cross-sections. Flow groups without known surface 

vents were labeled for chemical, spatial, or paleomagnetic 

identifiers. For example, the Jaramillo (Matuyama) flow group 

is named for the Jaramillo Normal Polarity Subchron of the 

Matuyama Reversed Polarity Chron (fig. 3). Other names were 

derived from proximity to INL facilities, such as the ATRC, 

and may have designations such as West of ATRC flow group. 

For convenience, the labeled basalt flow groups are referred to 

as a “flow group,” rather than a “basalt flow group.”

Geologic Setting

The INL covers an area of about 2,300 km2 on the 

ESRP, which is a northeast-trending structural trough that 

contains Neogene volcanic rocks and interbedded sediments 

(fig. 1). Tertiary rhyolites are generally exposed only along 

the margins of the ESRP, but several Quaternary rhyolite 

domes surrounded by basalt flows are located on the ESRP. 

Late Pleistocene and Holocene basalt fields cover most 

of the ESRP. The axis of the ESRP is a locus of the basalt 

shield vents from which young basalt flows erupted. Kuntz 

and others (1994) showed that the surface area of the INL is 

dominated by basalt flows of the Qbb (basaltic lava flows and 

pyroclastic deposits, upper to middle Pleistocene, estimated 

to be 15-200 ka) and Qbc (basaltic lava flows and pyroclastic 

deposits, middle Pleistocene, estimated to be 200-400 ka) 

relative age classification units, which suggests that they are 

15 to 400 thousand years (ka) in age.

Basalt flows at the INL are very similar petrographically. 

All are tholeiitic olivine basalt that contains olivine, 

plagioclase, clinopyroxene, ilmenite, magnetite, glass, and 

accessory apatite. Basalt flows may have significantly different 

textures despite having similar mineralogy and chemistry. 

In the subsurface, sedimentary interbeds separate some 

basalt flows; the distribution and thickness of these interbeds 

varies. Sedimentary interbeds are important in the subsurface 

correlation at the NRF.

Basalt flow groups are composed of one or more 

individual basalt flows that range from 1.5 to about 28 m 

thick. Sequences of several thin flows may indicate proximity 

to the vent for that basalt flow group. A basalt flow of greater 

than about 9 m may indicate a ponded or inflated flow that 

accumulated in a depression where the outlet was restricted 

or blocked. These depressions may have formed in strictly 

geomorphic ways, such as a closed basin between several 

shield volcanoes, or a depression along the course of an 

ephemeral stream. They may have formed due to topography 

created by earlier basalt flows of the same eruption of which 

they are part. The depressions may also have formed in 

structural ways, such as slumping of surface blocks in volcanic 

rift zones, slumping related to compaction of underlying 

sedimentary or volcanic deposits, or perhaps faulting.

Most flows investigated in this study do not have 

significant amounts of cinder or rubble at or near their 

uppermost surfaces. This suggests that most of the flows are 

pahoehoe and not slab pahoehoe or aa flows in morphology. 

Surface basalt flows and vents in the south and central 

parts of the INL have normal magnetic polarity (Kuntz and 

others, 2007) and were erupted during the Brunhes Normal 

Polarity Chron (less than 0.78 Ma) (fig. 3; Ogg and Smith, 

2004). Some surface basalt flows and vents located just to the 

north and west of the NRF have reversed polarity (Lanphere 

and others, 1994) and were erupted during the Matuyama 

Reversed Polarity Chron (2.58–0.78 Ma) (fig. 3; Ogg and 

Smith, 2004). A reversed polarity basalt flow group is 

sometimes found at depths of from 107 to 152 m in coreholes 

from the southern INL area and was identified as the Big Lost 

Polarity Cryptochron basalt flow group as shown in figure 3

and by Champion and others (1988). This basalt flow group 

was not found in any of the coreholes described from the NRF 

area. 
Introduction 9

Figure 3. Pliocene-Holocene geomagnetic time scale.

tac12-0786_fig 03

0

Big Lost (R)

(~0.565 Ma)

Jaramillo (N) 

(1.07 to 0.99 Ma)

Olduvai (N) 

(1.95 to 1.77 Ma)

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

3.8

Cryptochron Polarity subchron Polarity chron Age, millions of

 years (Ma)

Kaena (R) 

Mammoth (R)

(3.330-3.207 Ma)

EXPLANATION

Brunhes (N)

(onset at 0.78 Ma)

Matuyama (R)

(2.581 to 0.78 Ma)

Gauss (N)

(3.596 to 2.581 Ma)

Gilbert (R)

(termination at 3.596 Ma)

(3.116-3.032 Ma)

Black bars in age column indicate 

normal polarity (N - normal polarity)

White bars in age column indicate 

reversed polarity (R - reversed 

polarity)
10 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Analytical Methods

The drill cores from coreholes USGS 133, NRF B18-1, 

NRF 89-04, NRF 89-05, NRF 6P, NRF 7P, NRF 15, and 

NRF 16, were carefully logged and sampled by using INL 

Lithologic Core Storage Library protocols as described in 

Davis and others (1997). Prior to sampling, the core material 

was described, and the tops and bottoms of basalt flows were 

identified. Depths were measured by tape measure from 

known marks recorded on foam markers or depth markings 

on the cores in the core boxes. The foam markers or depth 

markings were placed by the drillers at the time of coring and 

recorded the measured depths logged at the end of each coring 

run. Detailed petrographic descriptions were not made for the 

cores in this study.

For paleomagnetic studies, an attempt was made to take 

seven samples of core from each basalt flow identified. For 

each sample, a 2.5-cm diameter subcore was drilled at right 

angles to the axis of the original core to provide material for 

paleomagnetic analysis. The samples were trimmed to 2.2-cm 

lengths, and the inclination and intensity of magnetization 

were measured with a cryogenic magnetometer. Progressive 

alternating-field demagnetization was performed on nearly 

all samples to remove any components of secondary 

magnetization. A few samples from coreholes NRF 6P, 

NRF 15, and NRF 16, particularly near polarity transitions, 

were thermally demagnetized to better remove opposite 

polarity that overprinted magnetizations. The primary or 

characteristic remanent inclinations measured on individual 

basalt samples are shown in figures 4 through 11. Mean 

inclination values for each basalt flow group and 95-percent 

confidence limits about the mean value were calculated by 

using the method of McFadden and Reid (1982) and are listed 

in appendix A. The subcore plugs taken from drill cores and 

used in this study only yield inclination data because the 

original declination was not preserved in the drill cores during 

drilling.

To facilitate paleomagnetic interpretations, the coreholes 

were assumed to be vertical in their original drilling 

orientation. Gyroscopic deviation logs of coreholes NRF 15 

and NRF 16 were made at 0.3-m intervals, and they record 

moderate deviations to the east-northeast, no greater than 

2.0 and 2.4 degrees from vertical, respectively, for those 

coreholes. A gyroscopic deviation log also was made for 

corehole USGS 133 with data recorded at 1.5-m intervals and 

recorded deviation no greater than 1.5 degrees from vertical to 

the northwest. Because these small deviations are not within 

the north-to-south vertical plane, no correction needed to be 

applied to the measured paleomagnetic remanent inclination 

values for samples from these coreholes. No gyroscopic logs 

were available for coreholes NRF B18-1, NRF 89-05, NRF 

89-04, NRF 6P, and NRF 7P. Deviation from vertical for any 

particular depth interval in INL coreholes typically is less than 

one degree and does not significantly affect paleomagnetic 

remanent inclination interpretations. 

Paleomagnetic Results

Paleomagnetic measurements were made on 749 basalt 

samples from the eight coreholes. The inclinations for the 

individual basalt flows were used to determine the mean 

for the basalt flow group. The mean inclination values for 

individual basalt flow groups ranged from -40.3 to 77.0 

degrees within the typical range of inclinations owing to 

geomagnetic secular variation (Champion and Shoemaker, 

1977). The precision of these mean inclination values, which 

were reported as two standard errors of the mean (α95), ranges 

between 0.6 and 17.0 degrees, while the mean, median, and 

mode of this two standard error population are identical at 

1.4 degrees. Data of this precision allow the differentiation 

and correlation of basalt flows or basalt flow groups combined 

with the stratigraphic constraints from the physical core 

logging and qualitative petrographic assessments. 

Basalt flow group assignments were made according 

to mean paleomagnetic inclination and polarity data and 

stratigraphic position. Similar mean inclination values, 

however, can be randomly acquired by two successive 

basalt flows or flow groups, even though they may be tens 

of thousands of years different in age. Flows with different 

petrography that have the same mean inclination values and 

that are not separated by a sedimentary interbed may have 

erupted from two different vents and represent two different 

magma batches. This phenomenon has been demonstrated in 

the surface paleomagnetic and petrographic/chemical work 

presented in Kuntz and others (2002). For the basalt flow 

groups analyzed in this report, it was sometimes difficult to 

distinguish between these interpretive alternatives. These 

petrographic assessments were qualitative and do not replace 

any future quantitative chemical and petrographic studies of 

these coreholes. Changes in phenocryst abundances, however, 

were noted for the different basalt flows in the NRF coreholes 

and were used to help identify significant boundaries between 

these flows.

As shown in plate 1, 13 basalt flow groups were identified 

in the 8 coreholes and range in thickness from 2.4 to 56 m. 

Of the 13 basalt flow groups identified in the 8 coreholes, 

7 were erupted during the Brunhes Normal Polarity Chron 

(0.78–0 Ma), and 6 were erupted during the latter part of 

the Matuyama Reversed Polarity Chron (2.58– 0.78 Ma), 

which included 1 that was erupted and magnetized during 

the Jaramillo Normal Polarity Subchron (1.07–0.99 Ma) of 

the Matuyama Reversed Polarity Chron (fig. 3; pl. 1; Ogg 

and Smith, 2004). The results for individual coreholes are 

discussed below.
Paleomagnetic Results 11

Corehole USGS 133

Corehole USGS 133 is a 248-m-deep corehole located 

about 5 km south-southwest of the NRF (fig. 2; pl. 1). 

Corehole USGS 133 contains eight basalt flow groups 

separated by conformable contacts and sedimentary interbeds. 

A log of the core stratigraphy, paleomagnetic inclinations, and 

the depths of samples used for paleomagnetic measurements is 

shown in figure 4. The individual and mean inclination values 

and uncertainties and depth ranges for the inclination values 

for corehole USGS 133 are given in appendix A. 

Corehole NRF B18-1

Corehole NRF B18-1 is a 77-m-deep corehole located 

122 m southwest of the NRF (fig. 2; pl. 1). Corehole NRF 

B18-1 contains three basalt flow groups separated by 

conformable contacts and sedimentary interbeds. A log of the 

core stratigraphy, paleomagnetic inclinations, and the depths 

of samples used for paleomagnetic measurements is shown 

in figure 5. The individual and mean inclination values and 

uncertainties and depth ranges for the values for corehole NRF 

B18-1 are given in appendix A. 

Corehole NRF 89-05

Corehole NRF 89-05, which was completed in September 

1989, is a 74-m-deep corehole located near the center of the 

NRF (fig. 2; pl. 1). Corehole NRF 89-05 contains three basalt 

flow groups separated by sedimentary interbeds. A log of the 

core stratigraphy, paleomagnetic inclinations, and the depths 

of samples used for paleomagnetic measurements is shown 

in figure 6. The individual and mean inclination values and 

uncertainties, and depth ranges for the values for corehole 

NRF 89-05 are shown in appendix A. 

Corehole NRF 89-04

Corehole NRF 89-04, which was completed in September 

1989, is a 76-m-deep corehole also located near the center 

of the NRF (fig. 2; pl. 1). Only the deepest part of the core 

was sampled because of the proximity of this corehole to 

corehole NRF 89-05, which was sampled in its entirety. The 

sampled interval of corehole NRF 89-04 ranges from 59.7 

to 75.6 m below land surface (BLS) and contains two basalt 

flow groups separated by a sedimentary interbed. A log of the 

core stratigraphy, paleomagnetic inclinations, and the depths 

of samples used for paleomagnetic measurements is shown 

in figure 7. The individual and mean inclination values and 

uncertainties and depth ranges for the values for corehole NRF 

89-04 are given in appendix A. 

Corehole NRF 6P

Corehole NRF 6P is a 152.4-m-deep corehole located 

about 425 m north-northwest of the center of the NRF (fig. 2; 

pl. 1). Corehole NRF 6P contains six basalt flow groups 

separated by conformable contacts and sedimentary interbeds. 

A log of the core stratigraphy, paleomagnetic inclinations, and 

the depths of samples used for paleomagnetic measurements is 

shown in figure 8. The individual and mean inclination values 

and uncertainties and depth ranges for the values for corehole 

NRF 6P are given in appendix A.

Corehole NRF 7P

Corehole NRF 7P is a 153-m-deep corehole located about 

1,040 m northeast of the center of the NRF (fig. 2; pl. 1). 

Corehole NRF 7P contains seven basalt flow groups separated 

by conformable contacts and sedimentary interbeds. A log 

of the core stratigraphy, paleomagnetic inclinations, and the 

depth of samples used for paleomagnetic measurements is 

shown in figure 9. The individual and mean inclination values 

and uncertainties and depth ranges for the values for corehole 

NRF 7P are given in appendix A.

Corehole NRF 15

Corehole NRF 15 is a 231-m-deep corehole located 

1.5 km north of the center of the NRF (fig. 2; pl. 1). Coring 

began at a depth of 4.9 m, recovering basalt samples; 

sediments are present above that depth. Corehole NRF 15 

contains 10 basalt flow groups separated by conformable 

contacts and sedimentary interbeds. A log of the core 

stratigraphy, paleomagnetic inclinations, and the depths of 

samples used for paleomagnetic measurements is shown in 

figure 10. The individual and mean inclination values and 

uncertainties, and depth ranges for the values for corehole 

NRF 15 are given in appendix A. 

Corehole NRF 16

Corehole NRF 16 is a 129-m-deep corehole located 

2.6 km north of the center of the NRF (fig. 2; pl. 1). Corehole 

NRF 16 contains six basalt flow groups separated by 

conformable contacts and sedimentary interbeds. A log of the 

core stratigraphy, paleomagnetic inclinations, and the depths 

of samples used for paleomagnetic measurements is shown 

in figure 11. The individual and mean inclination values and 

uncertainties and depth ranges of the values for corehole NRF 

16 are given in appendix A. 
12 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Figure 4. Log of the core stratigraphy and mean paleomagnetic inclination data for corehole USGS 133 at the 

Idaho National Laboratory, Idaho.

tac12-0786_fig 04

-68.9o ± 0.9o

Petrographic boundary

Flow contact

Remagnetized samples

Samples not included in the paleomagnetic inclination mean calculation

EXPLANATION

Black dots represent the paleomagnetic sample location. Numbers represent the 

mean inclination value for the group of samples, black vertical lines indicate 

the 95-percent confidence limits around the mean inclination values

Grey represents a sedimentary interbed, solid horizontal black lines are flow contacts

 Atomic Energy Commission

Advanced Test Reactor Complex

Central Facilities Area

Idaho Nuclear Technology and Engineering Center

CFA

INTEC

ATRC

AEC

-20o-40o-60o-80o 0o 20o 40o 60o 80o 100o

100

50

150

110

120

130

140

150

160

170

180

190

0

200

250

250

240

230

220

210

260

300

350

400

450

500

550

600

650

700

750

800

850

Depth, in feet below land surface

Depth, in meters below land surface

Paleomagnetic inclination, in degrees

Uncored surficial sediments

74.6o ± 1.1o

51.3o ± 1.3o

54.4o ± 1.9o

55.2o ± 1.0o

67.3o ± 2.2o

65.1o ± 1.7o

49.9o ± 2.3o

51.6o ± 1.6o

54.4o ± 1.1o

63.8o ± 1.7o

-47.2o ± 1.2o

-71.6o ± 2.6o

-68.9o ± 0.9o

Total depth 812 ft

USGS 133

No core

No core

No core

No core

Unrecovered core and sediment

Unrecovered core and sediment

Unrecovered core and sediment

West of ATRC

Late Basal Brunhes

Matuyama

North Late Matuyama

CFA Buried Vent

AEC Butte

North INTEC Buried Vent

ATRC Unknown Vent

Basalt flow group 

100

200

0

10

20

30

40

50

60

70

80

90
Paleomagnetic Results 13

Figure 5. Log of the core stratigraphy and mean paleomagnetic inclination data for corehole NRF B18-1 at 

the Idaho National Laboratory, Idaho.

Figure 6. Log of the core stratigraphy and mean paleomagnetic inclination data for corehole NRF 89-05 

at the Idaho National Laboratory, Idaho. tac12-0786_fig05

NRF B18-1

-66.9o ± 2.4o

Petrographic boundary

Flow contact

EXPLANATION

Black dots represent the paleomagnetic sample location. Numbers represent the 

mean inclination value for the group of samples, black vertical lines indicate 

the 95-percent confidence limits around the mean inclination values

Grey represents a sedimentary interbed, solid horizontal black lines are flow contacts

 Advanced Test Reactor Complex

Central Facilities Area

 Naval Reactors Facility

CFA

NRF

ATRC

-20o-40o-60o-80o 0o 20o 40o 60o 80o 100o

100

0

200

300

Depth, in feet below land surface

Paleomagnetic inclination, in degrees Basalt flow group 

76.9o ± 1.3o

54.8o ± 1.2o

66.9o ± 2.4o

64.8o ± 1.8o

Uncored surficial sediment

Total depth 251 ft

ATRC Unknown Vent 

West of ATRC

CFA Buried Vent

100

75

50

25

0

Depth, in meters below land surface

75

50

25

0

Depth, in meters below land surface

-66.5o ± 3.0o

Petrographic boundary

Flow contact

Samples not included in the paleomagnetic inclination mean calculation

EXPLANATION

Black dots represent the paleomagnetic sample location. Numbers represent the 

mean inclination value for the group of samples, black vertical lines indicate 

the 95-percent confidence limits around the mean inclination values

Grey represents a sedimentary interbed, solid horizontal black lines are flow contacts

 Advanced Test Reactor Complex

Central Facilities Area

 Naval Reactors Facility

CFA

NRF

ATRC

-20o-40o-60o-80o 0o 20o 40o 60o 80o 100o

100

0

200

300

Depth, in feet below land surface

Paleomagnetic inclination, in degrees Basalt flow group 

Total depth 241 ft

73.3o ± 1.5o

55.6o ± 0.7o

61.5o ± 17.0o

66.5o ± 3.0o

ATRC Unknown Vent

West of ATRC

CFA Buried Vent

Uncored surficial sediments

NRF 89-05
14 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Figure 7. Log of the core stratigraphy and mean paleomagnetic inclination data for corehole NRF 89-04 

at the Idaho National Laboratory, Idaho.

tac12-0786_fig07

-66.1o ± 3.0o

Petrographic boundary

Flow contact

EXPLANATION

Black dots represent the paleomagnetic sample location. Numbers represent the 

mean inclination value for the group of samples, black vertical lines indicate 

the 95-percent confidence limits around the mean inclination values

Grey represents a sedimentary interbed, solid horizontal black lines are flow contacts

 Central Facilities Area

 Naval Reactors Facility

CFA

NRF

-20o-40o-60o-80o 0o 20o 40o 60o 80o 100o

100

0

200

300

Depth, in feet below land surface

Paleomagnetic inclination, in degrees Basalt flow group 

Total depth 248 ft

Sediment and basalt 0–196 ft

not sampled for paleomagnetic analysis

State Butte

66.1o ± 3.0o CFA Buried Vent

71.2o ± 2.1o

NRF 89-04

100

75

50

25

0

Depth, in meters below land surface
Paleomagnetic Results 15

Figure 8. Log of the core stratigraphy and mean paleomagnetic inclination data for corehole NRF 6P at the 

Idaho National Laboratory, Idaho.

tac12-0786_fig08

-46.9o ± 1.7o

Petrographic boundary

Flow contact

Remagnetized samples

Samples not included in the paleomagnetic inclination mean calculation

EXPLANATION

Black dots represent the paleomagnetic sample location. Numbers represent the 

mean inclination value for the group of samples, black vertical lines indicate 

the 95-percent confidence limits around the mean inclination values

Grey represents a sedimentary interbed, solid horizontal black lines are flow contacts

 Atomic Energy Commission

Advanced Test Reactor Complex

Central Facilities Area

 Naval Reactors Facility

CFA

ATRC

AEC

NRF

100

125

150

200

75

50

25

0

Depth, in meters below land surface

-20o-40o-60o-80o 0o 20o 40o 60o 80o 100o

100

0

200

300

400

500

Depth, in feet below land surface

Paleomagnetic inclination, in degrees NRF 6P

Basalt flow group 

West of ATRC

North Late Matuyama

AEC Butte

State Butte

CFA Buried Vent

ATRC Unknown Vent

Total depth 500 ft

75.3o ± 2.0o

54.6o ± 1.1o

64.8o ± 1.5o

71.3o ± 1.0o

56.6o ± 1.6o

-46.9o ± 1.7o

Uncored surficial sediments
16 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

tac12-0786_fig09

-63.0o ± 2.9o

Petrographic boundary

Flow contact

Samples not included in the paleomagnetic inclination mean calculation

EXPLANATION

Black dots represent the paleomagnetic sample location. Numbers represent the 

mean inclination value for the group of samples, black vertical lines indicate 

the 95-percent confidence limits around the mean inclination values

Grey represents a sedimentary interbed, solid horizontal black lines are flow contacts

 Atomic Energy Commission

Advanced Test Reactor Complex

Central Facilities Area

 Naval Reactors Facility

CFA

ATRC

AEC

NRF

100

125

150

175

75

50

25

0

Depth, in meters below land surface

-20o-40o-60o-80o 0o 20o 40o 60o 80o 100o

100

0

200

300

400

500

Depth, in feet below land surface

Paleomagnetic inclination, in degrees NRF 7P

Basalt flow group 

West of ATRC

Late Basal Brunhes

North Late Matuyama

AEC Butte

State ButteState Butte

CFA Buried Vent

ATRC Unknown Vent

67.1o ± 13.4o

72.6o ± 1.0o

Total depth 501 ft

75.8o ± 1.4o

57.3o ± 1.8o

56.8o ± 2.8o

63.0o ± 2.9o

-46.2o ± 1.4o

Uncored surficial sediments

Figure 9. Log of the core stratigraphy and mean paleomagnetic inclination data for corehole NRF 7P at the Idaho 

National Laboratory, Idaho. 
Paleomagnetic Results 17

tac12-0786_fig10

-57.9o ± 1.2o

Petrographic boundary

Flow contact

Samples not included in the paleomagnetic inclination mean calculation

EXPLANATION

Black dots represent the paleomagnetic sample location. Numbers represent the 

mean inclination value for the group of samples, black vertical lines indicate 

the 95-percent confidence limits around the mean inclination values

Grey represents a sedimentary interbed, solid horizontal black lines are flow contacts

 Advanced Test Reactor Complex

Central Facilities Area

 Naval Reactors Facility

CFA

ATRC

NRF

-20o-40o-60o-80o 0o 20o 40o 60o 80o 100o

100

0

200

300

400

500

600

700

800

Depth, in feet below land surface

Paleomagnetic inclination, in degrees NRF 15

Basalt flow group 

77.0o ± 1.4o

57.6o ± 0.6o

64.1o ± 1.2o

73.4o ± 1.2o

72.0o ± 1.3o

-41.5o ± 2.1o

-46.1o ± 1.8o

-47.5o ± 1.5o

-57.9o ± 0.9o

-68.5o ± 5.0o

53.7o ± 1.1o

-57.9o ± 1.2o

West of ATRC

North Late Matuyama

<Unnamed>

Matuyama

Matuyama 1.21 Ma

Jaramillo (Matuyama)

East Matuyama Middle

State Butte

CFA Buried Vent

ATRC Unknown Vent

Total depth 759 ft

Uncored surficial sediments

100

125

150

175

75

50

25

0

200

225

250

Depth, in meters below land surface

Figure 10. Log of the core stratigraphy and mean paleomagnetic inclination data for corehole NRF 15 at the 

Idaho National Laboratory, Idaho. 
18 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Age Data

Age experiments that use conventional K-Ar and 

40Ar/39Ar techniques were obtained from samples from four 

of the coreholes (NRF 89-05, NRF 89-04, NRF 6P, and NRF 

7P) and are presented in this report to help better constrain 

stratigraphic interpretation. A total of 10 samples from the 

four coreholes were chosen according to the criteria of 

acceptability for K-Ar dating (Dalrymple and Lanphere, 1969; 

Mankinen and Dalrymple, 1972) or 40Ar/39Ar age dating. 

Thin sections of basalt from these coreholes were examined 

Figure 11. Log of the core stratigraphy and mean paleomagnetic inclination data for corehole NRF 16 at the 

Idaho National Laboratory, Idaho.

tac12-0786_fig11

100

125

150

75

50

25

0

Depth, in meters below land surface

-47.3o ± 1.4o

Petrographic boundary

Flow contact

Samples not included in the paleomagnetic inclination mean calculation

EXPLANATION

Black dots represent the paleomagnetic sample location. Numbers represent the 

mean inclination value for the group of samples, black vertical lines indicate 

the 95-percent confidence limits around the mean inclination values

Grey represents a sedimentary interbed, solid horizontal black lines are flow contacts

 Advanced Test Reactor Complex

Central Facilities Area

 Naval Reactors Facility

CFA

ATRC

NRF

-20o-40o-60o-80o 0o 20o 40o 60o 80o 100o

100

0

200

300

400

500

Depth, in feet below land surface

Paleomagnetic inclination, in degrees NRF 16

Basalt flow group 

72.2o ± 1.5o

-47.3o ± 1.4o

West of ATRC

North Late Matuyama

East Matuyama Middle

State Butte

CFA Buried Vent

ATRC Unknown Vent

Total depth 424 ft

Uncored surficial sediments

No core

No core

75.8o ± 1.4o

54.8o ± 1.4o

64.0o ± 3.9o

72.9o ± 0.8o

73.8o ± 1.5o

-40.3o ± 1.7o

petrographically to determine those most suitable for age 

dating. The samples were selected to minimize the amount 

of glass in the groundmass. In these samples, the volume 

of glass in the groundmass was considered to be too small 

to affect the measurements. The samples were crushed and 

ground to a size of 0.5 to 1 mm. Of these 10 samples, 4 were 

dated by using K-Ar and 40Ar/39Ar methods, 5 were analyzed 

solely by the K-Ar method, and 1 was analyzed solely by the 

40Ar/39Ar method (tables 1, 2; appendix B). Results of five of 

the conventional K-Ar experiments on samples from coreholes 

NRF 89-04 and NRF 89-05 are shown in table 1 and were 

previously reported by Lanphere and others (1993). 
Age Data 19

Limitations of Radiometric Dating of Olivine 

Tholeiite Basalts

ESRP olivine tholeiite basalts have generally less 

than 2 weight percent potassium (Stout and Nicholls, 1977, 

table 9), and the basalts used in this study are young, less 

than 1 Ma. Age experiments rely on accurate analyses of 

small quantities of potassium and the daughter products of its 

radiogenic isotope, which may be difficult in basalts with little 

potassium. Conventional K-Ar age experiments can be skewed 

to old ages owing to excess 40Ar embedded in phenocryst 

phases that grew in the magma chamber because the method 

uses a single argon gas extraction. 

Discrepancies between conventional K-Ar and 40Ar/39Ar 

age experiments arise from inherent procedural constraints of 

the two dating methods. In conventional K-Ar experiments, 

independent chemical and mass spectroscopic analyses 

are conducted on separate aliquots in precise and complete 

extractions of the elements being studied. The argon part of 

the analysis is a single thermal diffusion presumed to be a total 

extraction. Concentrations of potassium and argon by analysis 

are combined mathematically, and the age of the sample is 

calculated. Although conventional K-Ar age experiments can 

be accurate (Lanphere, 2000), the expectation that a single 

thermal extraction will yield argon gas of homogeneous 

isotopic ratios is sometimes violated. The 40Ar/39Ar method 

avoids using this analytical assumption by performing 

progressive thermal diffusion extractions to prove that the 

argon isotopic ratios are homogeneous. Also, the analysis is 

conducted on a single, small-mass sample after it has been 

irradiated by fast neutrons to convert some of the potassium to 

argon.

Therein lies the potential flaw in 40Ar/39Ar dating 

experiments on young, fine-grained basalt samples. Fast 

neutron irradiation can cause the resulting 39Ar, 37Ar, and 

36Ar ions to “recoil” out of their crystallographic domain and 

into adjacent domains that may thermally diffuse at higher 

or lower temperatures. This causes progressive change and 

abrupt discontinuities in the argon isotopic ratios being 

produced in successive temperature steps of extraction. The 

irradiation timeframe must be held to a minimum, gas samples 

being extracted and analyzed on the mass spectrometer 

must be of reasonably uniform size, and active knowledge 

of the mass discrimination value of the instrument must be 

maintained. Despite the refinements in technique listed above, 

some aphanitic basalt samples cannot be dated by using the 

40Ar/39Ar method. 

The actual age of a basalt flow at the INL may be known 

only to the nearest 100 thousand years (k.y.) or so despite the 

analytical uncertainty from any single age experiment. Given 

the assumptions and uncertainties of both methods, the results 

for each method must be evaluated and the age date that seems 

most reasonable based on known stratigraphy and other data 

must be determined. Numerous age dates derived by both 

methods are in proper depth order and replication of ages 

on correlative flows in different coreholes has been shown 

(Champion and others, 2002, 2011). Repeated age experiments 

using the conventional K-Ar and 40Ar/39Ar age methods on 

different samples of the same basalt flow may identify the 

potential flaws in both approaches and allow a reasonable age 

to be chosen. 

Methods of Analysis

K-Ar Age Experiments

An aliquot (10 g) of the 0.5- to 1-mm master sample was 

pulverized to less than 74 microns. This powdered material 

was used for the potassium oxide (K2O) measurements. The 

K2O measurements were made in duplicate on each of two 

splits of powder by flame photometry after lithium metaborate 

fusion and dissolution (Ingamells, 1970). Argon analyses were 

made by isotope dilution mass spectrometry using techniques 

described by Dalrymple and Lanphere (1969). Aliquots of 

the 0.5- to 1-mm master sample were baked overnight in a 

vacuum in an argon extraction system at 280°C. Argon mass 

analyses were done on a computerized multiple-collector mass 

spectrometer with a 22.86-cm radius and a nominal 90-degree 

sector magnet (Stacey and others, 1981). The analytical error 

for an individual age measurement was calculated by using the 

method of Cox and Dalrymple (1967). Weighted mean ages 

for basalt flows were calculated by using the method of Taylor 

(1982). Results from K-Ar experiments for the four coreholes 

are shown in table 1.

40Ar/39Ar Age Experiments

Splits of whole-rock samples were packaged in copper 

foil and placed in cylindrical vials with mineral grains of 

known age. Samples were irradiated with fast neutrons 

along with a monitor mineral of known age [sanidine 

from the Taylor Creek Rhyolite, 27.92 Ma (Dalrymple and 

Duffield, 1988)], to induce the nuclear reaction 39K (neutron, 

proton)39Ar. Ages of samples were calculated from the 

measured 40Ar/39Ar ratio after determining the fraction of 

39K converted to 39Ar by analyzing the monitor mineral. 

Corrections for interfering argon isotopes produced from 

potassium and calcium and for atmospheric argon were also 

made. 
20 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Results of Age Experiments by Basalt Flow 

Group

West of Advanced Test Reactor Complex (ATRC) 

Flow Group 

The West of ATRC flow group (pl. 1) was sampled for 

K-Ar age experiments from corehole NRF 89-05 from 24.1-m 

BLS. The experiment produced a weighted mean age of 303 ± 

30 ka from two separate extractions [Experiment numbers 

(Exp. nos.) 89-05_79a and 89-05_79b; table 1]. Paleomagnetic 

inclination correlation of this sample to coreholes south of the 

NRF indicates that this age seems reasonable (Champion and 

others, 2011).

Advanced Test Reactor Complex (ATRC) 

Unknown Vent Flow Group 

The ATRC Unknown Vent flow group (pl. 1) was 

sampled for K-Ar analyses from corehole NRF 89-05 at 55 m 

BLS (Exp. nos.89-05_179a and 89-05_179b; table 1), and 

from corehole NRF 6P at 47 m BLS (Exp. nos. 6P_155a, 

b, and c; table 1). The sample from corehole NRF 89-05 

yielded a weighted mean age of 712 ± 53 ka. The sample from 

corehole NRF 6P yielded a weighted mean age of 491 ± 30 ka. 

The sample from NRF 6P at 47 m was also analyzed by 

using the 40Ar/39Ar method. The plateau age was 395 ± 25 ka 

(table 2; appendix B). 

The 395 ± 25 ka age from the 40Ar/39Ar experiment 

seems reasonable (table 2). It is more than 200 k.y. younger 

than the oldest K-Ar age (Exp. no. 6P_155a; table 1) on the 

same sample. The K-Ar weighted mean age from the sample 

from corehole NRF 89-05 from 55 m BLS derived from two 

extractions (Exp. nos.89-05_179a and 89-05_179b, table 1) 

seems too old at 712 ± 53 ka. The NRF area is underlain by 

four more normal polarity flows and, thus, is much younger 

in age than the 0.78-Ma boundary of the Brunhes Normal 

Polarity Chron (fig. 3). This may be an example of an age 

that is too old owing to excess 40Ar imbedded in phenocryst 

phases. 

Central Facilities Area (CFA) Buried Vent Flow 

Group 

Two sets of K-Ar age experiments were conducted on 

two samples from the Central Facilities Area (CFA) Buried 

Vent flow group at the NRF (pl. 1). One K-Ar experiment was 

performed on a sample from corehole NRF 89-04 from 65 m 

BLS and yielded a potentially reasonable weighted mean age 

of 492 ± 56 ka (table 1), from two extractions, which yielded 

different results (Exp. nos. 89-04_212a and 89-04_212b; 

table 1). The other K-Ar experiment was conducted on a 

sample from corehole NRF 89-05 from 72 m BLS and yielded 

a weighted mean age of 598 ± 31 ka (table 1), from three 

extractions one of which (Exp. no. 89-05_235b) was much 

older than the other two extractions (Exp. nos. 89-05_235a 

and 89-05_235c; table 1).

 The 492 ± 56 ka weighted mean age for the sample 

from corehole NRF 89-04 at 65 m is about 100 k.y. younger 

than the 617 ± 22 ka age for the CFA Buried Vent flow group 

in south INL coreholes that also were correlated by using 

paleomagnetic inclination (Champion and others, 2011). The 

598 ± 31 ka weighted mean age is in much closer agreement. 

The more-than 280-k.y. span (Exp. no. 89-05_235b versus 

Exp. no. 89-04_212 b; table 1) of the five extraction ages for 

the samples from both coreholes (Exp. nos. 89-04_212a and b, 

89-05_235 a, b, c), however, suggests caution in the use of this 

mean age.

State Butte Flow Group 

The State Butte flow group (pl. 1) was sampled for a 

K-Ar experiment from corehole NRF 89-04 at 70 m BLS. The 

experiment yielded a weighted mean age of 819 ± 39 ka from 

two extractions (Exp. nos. 89-04_231a, b; table 1). 

 An 40Ar/39Ar age experiment was done on a sample from 

the State Butte flow group in the subsurface at the NRF. The 

experiment on a sample from corehole NRF 6P at 74 m BLS 

yielded a plateau age of 546 ± 47 ka (table 2; appendix B).

The K-Ar age (819 ± 39 ka; table 1) may indicate that the 

flow was emplaced during the Matuyama Reversed Polarity 

Chron (older than 0.78 Ma; fig. 3); the State Butte flow group, 

however, has normal magnetic polarity and is underlain by 

another normal polarity basalt flow group. Thus, the age is 

probably unreliable. The 40Ar/39Ar age of 546 ± 47 ka (table 2) 

seems to be reasonable. 

Atomic Energy Commission (AEC) Butte Flow 

Group 

The AEC Butte flow group (pl. 1) was sampled for a 

K-Ar age experiment from corehole NRF 6P from 113.4 m 

BLS. The K-Ar experiment yielded a weighted mean age of 

384 ± 65 ka from three separate extractions which yielded 

different ages (Exp. nos. 6P_372 a, b, c; table 1).

An 40Ar/39Ar age experiment was also conducted on this 

sample. The 40Ar/39Ar age experiment yielded a plateau age of 

727 ± 31 ka (table 2; appendix B). 

The K-Ar weighted mean age of 384 ± 65 ka seems too 

young in stratigraphic context, and the 40Ar/39Ar plateau age 

of 727 ± 31 ka may be somewhat too old but is close to the 

637 ± 35 ka age (Champion and others, 2011, p. 21) for this 

basalt flow group when uncertainties are taken into account. 
Stratigraphy 21

North Late Matuyama Flow Group 

The North Late Matuyama flow group (pl. 1) was 

sampled for two K-Ar age experiments from corehole NRF 

7P at 134 m and 148 m BLS. The two K-Ar experiments, each 

from one extraction (Exp. nos. 7P_440 and 7P_486; table 1), 

yielded a weighted mean age of 604 ± 89 ka.

40Ar/39Ar experiments were also performed on the same 

samples from corehole NRF 7P. These age experiments 

yielded plateau ages of 884 ± 53 ka and 1,176 ± 27 ka, 

respectively (table 2; appendix B). 

The K-Ar weighted mean age of 604 ± 89 ka is too 

young because this reversed polarity basalt flow group must 

be older than 0.78 Ma (onset of the Brunhes Normal Polarity 

Chron) (fig. 3). The 40Ar/39Ar experiment that yielded the 

1,176 ± 27 ka age seems too old because this flow is the 

shallowest Matuyama Reversed Polarity Chron flow in 

coreholes NRF 6P and NRF 7P and underlies flows of early 

Brunhes Normal Polarity Chron age (fig. 3; pl. 1). If the 

1,176 ± 27 ka age were correct, then that would indicate an 

approximate 500- k.y. gap in basalt flow accumulation in the 

NRF area, which is unlikely (Champion and others, 2002). 

Additionally, this flow overlies a normal polarity flow that is 

61 m deeper in corehole NRF 15 that was correlated to similar 

cored flows of Jaramillo Normal Polarity Subchron age farther 

to the south (Champion and others, 2011). The Jaramillo 

Normal Polarity Subchron (onset at 1.072 Ma, termination 

at 0.988 Ma) (fig. 3) is younger than 1,176 ± 27 ka, which 

suggests that this age experiment is not reasonable. The 

40Ar/39Ar age of 884 ± 53 ka from corehole NRF 7P at 134 m 

BLS appears to be reasonable. 

Stratigraphy 

Plate 1 shows an interpretation of the subsurface 

stratigraphy in a south-to-north line of cross-section 

constructed through eight coreholes that span approximately 

9 km (fig. 2). Paleomagnetic inclination and polarity 

determinations document that 13 basalt flow groups (pl. 1) 

were identified in coreholes USGS 133, NRF B18-1, NRF 

89-05, NRF 89-04, NRF 6P, NRF 7P, NRF 15, and NRF 16. 

The basalt flow groups have normal and reversed magnetic 

polarities and were emplaced during the late Matuyama 

Reversed Polarity Chron, the Jaramillo Normal Polarity 

Subchron, and the Brunhes Normal Polarity Chron (fig. 3). Tie 

lines of correlation of sedimentary interbeds or the upper and 

lower surfaces of basalt flow groups were drawn between the 

coreholes on the basis of measured paleomagnetic inclination 

values (appendix A), polarity, and (or) age dates (tables 1

and 2) determined from samples of core. 

 Comparisons of paleomagnetic inclination and polarity 

measurements between samples from coreholes located only 

kilometers apart show comparable stratigraphic successions 

of mean inclination values with depth (pl. 1). At distances of 

greater than a few kilometers, correlation of mean inclination 

values is less consistent because basalt flow groups may be 

missing or additional basalt flow groups may be present and 

(or) found at different depth intervals. Some sedimentary 

interbeds found between some basalt flows and flow groups 

are correlative through parts of the cross-section; others pinch 

out between coreholes. The stratigraphy shown in the crosssection (pl. 1) is described from top to bottom and south to 

north. All coreholes began in surficial sediments of variable 

thickness. Surficial sediments in the NRF area are thinner than 

those found farther south at and near the INTEC, the CFA, and 

the RWMC (Anderson and others, 1996b). 

West of Advanced Test Reactor Complex (ATRC) 

Flow Group

The West of ATRC flow group is found at the top 

of seven of the eight coreholes (USGS 133, NRF B18-1, 

NRF 89-05, NRF 6P, NRF 7P, NRF 15, and NRF 16) under 

sediments that range from about 3 to 10 m thick (pl. 1). 

Although it is also likely in corehole NRF 89-04, the upper 

60 m of that hole was not sampled because of its close 

proximity to corehole NRF 89-05. The basalt flow group has 

mean inclination values that range from 73 to 77 degrees, 

is between about 15 m and 34 m thick, and is found as a 

single flow or multiple flows in the different coreholes. This 

basalt flow group is believed to have erupted from a vent to 

the west of the ATRC and is found in many southern INL 

coreholes (fig. 1; Champion and others, 2011). To date, no 

surface paleomagnetic sites in the flows of this vent have been 

sampled to compare with subsurface corehole inclination data. 

Advanced Test Reactor Complex (ATRC) 

Unknown Vent Flow Group

The West of ATRC flow group is underlain by the ATRC 

Unknown Vent flow group (Champion and others, 2011). 

It is so labeled because it is thickest in coreholes near the 

ATRC (pl. 1) near corehole USGS 133. In corehole USGS 

133, it has inclinations that range between 51 and 54 degrees 

separated by a petrographic boundary and unrecovered core 

(fig. 4; pl. 1; appendix A). In coreholes NRF B18-1, NRF 

89-05, NRF 6P, NRF 7P, NRF 15, and NRF 16, it has mean 

inclinations that range from 55 to 58 degrees. A petrographic 

boundary similar to that in USGS 133 is found in coreholes 

NRF 89-05, NRF 6P, NRF 7P, and NRF 15 at similar depths 
22 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

when land surface elevation and the thickness of the overlying 

West of ATRC flow group are considered. This basalt flow 

group is also likely also in corehole NRF 89-04; the upper 60 

m of that hole, however, was not sampled for paleomagnetic 

analysis. The ATRC Unknown Vent flow group is also found 

in coreholes near the INTEC area (fig. 12; Champion and 

others, 2011).

Champion and others (2011) correlated this basalt 

flow group with the North INTEC Buried Vent flow group 

in corehole NRF 7P. Paleomagnetic inclination values, 

stratigraphic position, and thickness in the north NRF area 

coreholes in this report, however, suggest that correlation of 

this basalt flow group to the ATRC Unknown Vent flow group 

is more probable (figs. 5–11; pl. 1). 

Spreading

Areas

Big Lost River

Big

Lost River

ATRC

INTEC 

NRF

MFC

RWMC

CFA

A

A'

Atomic

City

C-1A

NRF 7P

NRF 16

NRF 89-04

NRF 89-05

NRF 15

 135

 134

133

132

129

127

127

131

130

121

NPR TEST/W-02 Middle 1823

Middle

2050A

ANL-OBS-A-001

ARA-COR-005

STF-AQ-01

ICPP 023

ICPP

214

123

Middle 2051

128

A A'

NRF 6P

NRF B18-1

Base from U.S. Geological Survey digital data, 1:24,000 and 

1:100,000. Projection: UTM, Zone 12N, datum is North American 

Datum of 1983.

Selected facilities at the Idaho National Laboratory

Advanced Test Reactor Complex

Central Facilities Area

Idaho Nuclear Technology and Engineering Center

Materials and Fuels Complex

Naval Reactors Facility

Radioactive Waste Management Complex

0 5 Miles

0 5 Kilometers

CFA

INTEC

MFC

NRF

ATRC

RWMC

EXPLANATION

Idaho National Laboratory boundary

Corehole (number only indicates USGS corehole, other names

are local corehole names) 

Cross-section line (this report) 

Direction of regional

 groundwater flow

East

Butte

Middle

Butte

LOST RIVER RANGE

113°10' 113° 112°50' 112°40'

43°

40'

43°

35'

43°

30'

43°

25'

Figure 12. Location of coreholes in the southern part of the Idaho National Laboratory, Idaho (modified from Champion and others, 

2011).
Stratigraphy 23

North Idaho Nuclear Technology and 

Engineering Center (INTEC) Buried Vent Flow 

Group

The North INTEC Buried Vent flow group underlies 

the ATRC Unknown Vent flow group and is found only in 

corehole USGS 133 in the NRF area (fig. 4; pl. 1). It has a 

mean inclination of 55 degrees and also is found in coreholes 

USGS 121, ICPP 023, USGS 123, and USGS 128 to the 

south near the INTEC (fig. 12; Champion and others, 2011). 

Champion and others (2011) correlated the North INTEC 

Buried Vent flow group to a 57-degree mean inclination 

basalt flow group in corehole NRF 7P. Data from the NRF 

area coreholes in this report suggest that correlation of the 

57-degree basalt flow group to the ATRC Unknown Vent flow 

group is more probable.

Central Facilities Area (CFA) Buried Vent Flow 

Group

The CFA Buried Vent flow group underlies the North 

INTEC Buried Vent flow group in corehole USGS 133 (pl. 1) 

in the NRF area. It also is found in coreholes to the south near 

the INTEC (fig. 12; Champion and others, 2011). This basalt 

flow group is thickest at corehole USGS 128 about 2 km 

south of the INTEC and extends west to the RWMC and north 

to corehole NRF 7P (fig. 12; Champion and others, 2011). 

The CFA Buried Vent flow group has inclinations that range 

from 65 to 67 degrees in corehole USGS 133 (fig. 4; pl. 1). 

The CFA Buried Vent flow group in corehole USGS 133 

likely correlates to basalt flow groups with mean inclinations 

that range from 62 to 67 degrees in coreholes to the north 

from NRF B18-1 to NRF 16 (pl. 1). This basalt flow group 

is thinner in the northern coreholes, which implies that the 

source is relatively far from the NRF area. 

State Butte Flow Group

The State Butte flow group is found in five coreholes–

NRF 89-04, NRF 6P, NRF 7P, NRF 15, and NRF 16 (pl. 1). 

The mean inclination values for this basalt flow group range 

from 71 to 74 degrees. These flows range from 44 to 59 m 

thick and thicken to the north, thus indicating a nearby source 

in that direction. Surface paleomagnetic sites at State Butte 

and adjacent surface basalt flows correlate well with this 

basalt flow group, and these flows probably erupted from the 

State Butte vent (fig. 1; Champion and others, 2011). The 

State Butte flow group probably pinches out to the south 

of corehole NRF 89-04 and is not found as far south as 

corehole USGS 133. State Butte flows may underlie coreholes 

NRF B18-1 and NRF 89-05 on the basis of their proximity to 

corehole NRF 89-04; these coreholes were not cored deeply 

enough to sample the State Butte flow group. 

The State Butte flow group is the deepest cored basalt 

flow group in corehole NRF 89-04. In coreholes NRF 15 

and NRF 16, the State Butte flow group is the oldest normal 

polarity basalt flow group emplaced after the BrunhesMatuyama transition (fig. 3). In coreholes NRF 15 and NRF 

16, reversed polarity basalt flow groups underlie the State 

Butte flow group.

Atomic Energy Commission (AEC) Butte Flow 

Group

The AEC Butte flow group is found in coreholes USGS 

133, NRF 6P, and NRF 7P (pl. 1). This basalt flow group is 

likely in the subsurface near coreholes NRF B18-1, NRF 

89-05, and NRF 89-04; these holes, however, were not cored 

deeply enough to sample it. The mean inclination values 

for this basalt flow group are in agreement at 57 degrees in 

coreholes NRF 6P and NRF 7P. In corehole USGS 133, the 

basalt flow group is thick and has mean inclinations that range 

from 50 to 54 degrees. Champion and others (2011) correlated 

this flow group to AEC Butte, which is found at the surface 

just north of the ATRC (fig. 1). The AEC Butte flow group 

also is found in the subsurface at the ATRC, the CFA, and the 

INTEC in coreholes USGS 134, USGS 133, USGS 121, ICPP 

023, USGS 123, USGS 128, and Middle 1823 (Champion and 

others, 2011, pl. 1), and NPR Test/W-02 (fig. 12; Champion 

and others, 1988). Because the AEC Butte flow group is not 

found in coreholes NRF 15 and NRF 16, the northern edge of 

the basalt flow group is probably in the subsurface at or near 

corehole NRF 7P. 

Late Basal Brunhes Flow Group

The Late Basal Brunhes flow group is found in corehole 

USGS 133, may be found in corehole NRF 7P, and does not 

correlate to any other coreholes in the NRF area (pl. 1). In 

corehole USGS 133, it has a mean inclination of 64 degrees. 

In corehole NRF 7P, the possible correlative basalt flow group 

has a mean inclination value of 63 degrees and is found below 

the AEC Butte flow group. The basalt flow group labeled the 

“Late Basal Brunhes flow group” in corehole NRF 7P (pl.1) 

could be a single flow from a different vent with similar mean 

inclination, or it could correlate to the Late Basal Brunhes 

flow group of Champion and others (2011) in coreholes ICPP 

023, USGS 121, USGS 133, USGS 134, and Middle 1823 

(fig. 12). That basalt flow group is found directly beneath 

flows from AEC Butte in coreholes proximal to the AEC Butte 

vent (fig. 1). The Late Basal Brunhes flow group is thickest in 

corehole USGS 134 west of the ATRC (fig. 12), and the vent is 

probably located in the subsurface near the ATRC (Champion 

and others, 2011). This flow is the deepest cored Brunhes 

Normal Polarity Chron (fig. 3) basalt flow group in coreholes 

NRF 7P and USGS 133.
24 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

East Matuyama Middle Flow Group

The East Matuyama Middle flow group underlies the 

State Butte flow group in coreholes NRF 15 and NRF 16 

(pl. 1). It has reversed polarity and a mean inclination value of 

-42 degrees in corehole NRF 15, and -40 degrees in corehole 

NRF 16. This basalt flow group is the uppermost Matuyama 

Reversed Polarity Chron (fig. 3) basalt flow group in the NRF 

area and may correlate to the East Matuyama Middle flow 

group in coreholes NPR Test/W-02 and ANL-OBS-A-001 

(fig. 12) to the south and east of the NRF area, which have 

similar inclinations (Champion and others, 2011). 

North Late Matuyama Flow Group

The North Late Matuyama flow group is found in 

coreholes USGS 133, NRF 6P, NRF 7P, NRF 15, and NRF 16 

(pl. 1). It has reversed magnetic polarity and mean inclination 

values that range from -46 to -48 degrees. Other NRF area 

coreholes were not cored deeply enough to sample this basalt 

flow group. 

Additional Basalt Flow Groups and Sedimentary 

Interbeds Found at Depth in Corehole NRF 15

Corehole NRF 15 was cored more than 76 m deeper 

than other coreholes near the NRF and, therefore, penetrates 

additional basalt flow groups below the North Late Matuyama 

flow group. The stratigraphic order below the North Late 

Matuyama flow group is shown on plate 1. A 6-m-thick 

sedimentary interbed separates the North Late Matuyama flow 

group from the next underlying 4-m thick basalt, which has a 

mean inclination value of -58 degrees. This flow is underlain 

by a 5-m-thick sedimentary interbed that overlies a 2-m-thick 

basalt flow with a mean inclination of -69 degrees. The 

-69-degree basalt flow group possibly correlates to the south to 

the Matuyama flow group near the base of corehole USGS 133 

that has -69-and -72-degree mean inclinations. The Matuyama 

flow group is extensive in the south INL and is thickest near 

the RWMC (Champion and others, 2011).

 A 12-m- thick sedimentary interbed underlies the 

Matuyama flow group, which is in turn underlain by a 22-mthick basalt flow group that comprises multiple basalt flows, 

has normal magnetic polarity, and has a mean inclination 

value of 54 degrees. This normal polarity basalt flow group 

probably correlates to the Jaramillo (Matuyama) flow group 

of Champion and others (2011, pl. 1), which has similar mean 

inclinations and is found in six coreholes of the south INL area 

(USGS 134, Middle 2050A, Middle 1823, Middle 2051, C1A, 

and USGS 132) (fig. 12). This Jaramillo (Matuyama) flow 

group thickens to the south, which may indicate proximity 

to the eruptive vent (Champion and others, 2011, pl. 1). The 

Jaramillo (Matuyama) flow group in corehole NRF 15 now 

lies nearly 183 m (600 ft) higher in elevation in the NRF area 

than in the southern INL because of subsidence of the ESRP 

during the past 1 Ma (Champion and others, 2002). 

The Jaramillo (Matuyama) normal polarity basalt flow 

group in corehole NRF 15 is underlain by a sedimentary 

interbed that overlies a reversed polarity basalt flow group 

that comprises two basalt flows with a mean inclination value 

of -58 degrees (pl. 1). The reversed polarity basalt flow group 

may correlate to the Matuyama 1.21-Ma flow group that has 

mean inclination values that range from -57 to -59 degrees 

and is found beneath the Jaramillo (Matuyama) flow group 

in coreholes Middle 1823, Middle 2050A, NPR Test/W-02, 

and ANL-OBS-A-001 (fig. 12) (Champion and others, 2011, 

pl. 1). This is the deepest cored basalt flow group in corehole 

NRF 15 and is underlain by more than 8.8 m of sedimentary 

interbeds.

Summary and Conclusions

Paleomagnetic inclination and polarity studies on samples 

from subsurface drill cores provide data that can help constrain 

the age and the extent of basalt flows and basalt flow groups. 

Data from paleomagnetic inclination and polarity studies were 

used to refine Pleistocene and Holocene stratigraphy at and 

near the Naval Reactors Facility (NRF) of the Idaho National 

Laboratory (INL).

Eight coreholes at and near the NRF were studied, and 

749 subcores were collected and analyzed for paleomagnetic 

inclination and polarity. Samples of individual basalt flows 

from drill cores were collected in accordance with INL 

Lithologic Core Storage Library protocols. Paleomagnetic 

samples were collected from coreholes at depths from a few 

meters to 222 m. 

Mean paleomagnetic inclination and polarity data 

from the samples were compared among the coreholes 

and basalt flow groups were determined. The basalt flow 

group determinations were then used to correlate subsurface 

basalt stratigraphy. Results demonstrate that stratigraphic 

successions of basalt flow groups correlate over distances of a 

few kilometers. Correlation of basalt flow groups in corehole 

USGS 133 to basalt flow groups to the north is less certain 

because it is more than 5 km from the nearest NRF area 

corehole. 
Acknowledgments 25

 Conventional K-Ar and 40Ar/39Ar age experiments were 

conducted on core samples and results were evaluated to 

determine the most reasonable ages and uncertainty levels for 

the different basalt flow groups of the NRF area. The selection 

of the most reasonable age for basalt flow groups that had 

multiple age experiments was based on stratigraphic position 

and paleomagnetic polarity. 

Basalt shield volcanoes and their basalt flow groups 

have a wide range of eruptive volumes and areal extents. This 

study presented a detailed stratigraphic analysis of the NRF 

area and ties paleomagnetic data on these coreholes to earlier 

paleomagnetic data of other southern INL coreholes. A southto-north cross-section shows correlation of basalt flow groups 

in the subsurface of the NRF area. Important correlations 

include the following:

• The West of Advanced Test Reactor Complex (ATRC) 

flow group is the uppermost basalt flow group in the 

NRF area and correlates among seven continuously 

cored holes in this study under surficial sediments. The 

West of ATRC flow group is also found in ATRC and 

INTEC coreholes and corehole USGS 129 (Champion 

and others, 2011).

• The ATRC Unknown Vent flow group correlates 

among seven continuously cored holes in this study 

that underly the West of ATRC flow group and a 

sedimentary interbed. Additional paleomagnetic 

inclination and stratigraphic data derived from the 

NRF coreholes changed the previously reported 

interpretation of the subsurface distribution of this 

basalt flow group (Champion and others, 2011). The 

ATRC Unknown Vent flow group also is found in 

coreholes near the ATRC and the INTEC.

• The Central Facilities Area (CFA) Buried Vent flow 

group correlates among all eight coreholes in the NRF 

area. It is also found in coreholes near the CFA and the 

Radioactive Waste Management Complex (RWMC) 

to the south. This basalt flow group is thickest near the 

CFA, which may indicate proximity to the vent. The 

State Butte flow group is found below the CFA Buried 

Vent flow group in the four northern NRF coreholes. 

It correlates to the State Butte surface vent located just 

northeast of the NRF. It is not found in coreholes south 

of the NRF.

• The Atomic Energy Commission (AEC) Butte flow 

group is found in coreholes USGS 133, NRF 6P, and 

NRF 7P. It probably underlies coreholes NRF B18-1, 

NRF 89-05, and NRF 89-04, but those coreholes were 

not drilled deeply enough to penetrate the flow group. 

The AEC Butte flow group vent is exposed at the 

surface near the ATRC, and its flows are found in many 

coreholes near the ATRC and the INTEC (Champion 

and others, 2011). The AEC Butte flow group abruptly 

pinches out against the Matuyama Chron reversed 

polarity flows of the East Matuyama Middle flow 

group between coreholes NRF 7P and NRF 15.

• The East Matuyama Middle flow group correlates 

between coreholes NRF 15 and NRF 16 and may 

correlate to coreholes NPR Test/W-02 and 

ANL-OBS-A-001 (Champion and others, 2011).

• The North Late Matuyama flow group correlates 

among coreholes USGS 133, NRF 6P, NRF 7P, 

NRF 15, and NRF 16. It probably underlies coreholes 

NRF B18-1, NRF 89-05, and NRF 89-04, but those 

coreholes were not drilled deeply enough to penetrate 

the flow group. The vent that produced the North Late 

Matuyama flow group may be located in the general 

NRF area because it is thickest near corehole NRF 6P.

• The Matuyama flow group is found in coreholes in the 

southern INL from south of the RWMC to corehole 

USGS 133 and may extend north to corehole NRF 15. 

The Matuyama flow group is thickest near the RWMC 

and thins to the north.

• The Jaramillo (Matuyama) flow group is found in 

corehole NRF 15, which is the deepest NRF corehole, 

and shows that the basalt flow group is thick in the 

subsurface at NRF. This flow group is thickest between 

the RWMC and the INTEC and thins towards the 

ATRC and the NRF.

Acknowledgments

Potassium measurements were made by L. Espos and 

S.T. Pribble; argon measurements and age calculations were 

done by J.C. Von Essen. Assistance in the paleomagnetic 

sampling was provided by Dean Miller, Kyle Champion, and 

Joel Robinson. We gratefully acknowledge their help. We also 

wish to thank our manuscript peer reviewers. Their comments 

helped to significantly clarify the report. Funding for this 

research was provided by the U.S. Department of Energy, 

through the USGS INL Project Office, and by the USGS 

Volcano Science Center. 
26 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

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Appendix A 29

Table A1 contains depth and paleomagnetic inclination 

data from coreholes in cross-section A–A’ presented in this 

report. The table presents inclination data that are organized 

alphabetically by corehole name. 

Information in table A1 includes the corehole name, 

sample depth, and identification of sample groups that 

were used to determine the mean inclination for flows and 

95-percent uncertainty. Each sample has a characteristic 

remanent inclination in degrees and an alternating field 

demagnetization level in milliTeslas or an alternative 

demagnetization approach. Positive inclination values indicate 

normal paleomagnetic polarity, and negative inclination values 

indicate reversed paleomagnetic polarity.

Petrographic boundaries denote a significant change in 

mineralogy in a sequence of flows. Unrecovered core and 

sediment intervals also were recorded in the depth column.

Some samples are labeled “NIIM”, which stands for “Not 

Included In Mean.” NIIM samples may have been thermally 

overprinted by overlying flows, tilted by endogenous inflation, 

struck by lightning when on the surface, or otherwise had 

Appendix A. Paleomagnetic Inclination Data for Selected Coreholes At and Near 

the Naval Reactors Facility, Idaho National Laboratory, Idaho

their orientations disturbed so that they do not yield usable 

paleomagnetic inclination data. 

Some samples carry the notation “therm,” which is 

direction found by the difference vectors between sample 

magnetizations during the course of stepwise thermal 

demagnetizations. Stepwise thermal demagnetizations were 

done only on samples of particular interest. 

The designation “TO” stands for “thermal overprinting.” 

Samples that are thermally overprinted may require extra 

processing to discover their original orientation. If thermal 

overprinting was extensive, then the sample may not yield 

useful paleomagnetic data, and those samples were not 

included in means. 

The “line fit” label means that the entire progressive 

alternating field demagnetization sequence was fit with a 

line on a vector component diagram to find the characteristic 

remanent inclination. Depth measurements are in feet and 

alternate field demagnetization level is in milliTeslas because 

that is how the raw data were collected. 
30 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole USGS 133

28 72.3 20

38 72.7 30

47 74.5 30

57 77.0 20

67 76.6 20

77 77.5 30

86 75.6 30

96 74.9 30

106 73.6 20 74.6±1.1

113 76.5 30

120 71.6 30

121–128 Sediments

130 77.6 30

132 74.7 30

135 70.8 30

137 75.0 40

139 73.2 30

141 52.5 30

145 55.3 20

146 49.8 30

148 50.3 40

153 50.9 30

154 51.8 20

156 55.4 30

158 54.2 30

162 50.9 30

165 52.8 30

168 50.0 30 51.3±1.3

170 47.1 30

171 48.5 30

174 53.3 30

176 46.7 30

178 45.4 20

179 51.5 30

182 50.9 30

185 53.0 30

187 54.6 20

187–190 Unrecovered core and petrographic boundary

192 52.8 30

193 52.4 30

195 56.1 30

201 51.1 30

207 57.3 30

211 59.2 30 54.4±1.9

Table A1. Paleomagnetic inclination values for basalt samples from coreholes at and near the Naval Reactors Facility, Idaho National 

Laboratory, Idaho.

[Green shading marks start of the list of values for a different corehole. Grey shading delimits groups of samples used to determine the mean flow inclination 

and 95 percent uncertainty level. Sample depth: measured to nearest 1 foot. Characteristic remanent inclination: in degrees, down (negative value) or up 

(positive value) from the horizontal obtained by demagnetization. Abbreviations: ft, foot; AF, Alternating-Field; mT, milliTesla; Petrographic boundary, 

indicates noted change in mineralogy; NIIM, the sample was not included in the group mean inclination--see appendix explanation; Therm, stepwise 

thermal demagnetization was applied to the sample; TO, sample was thermally overprinted by overlying flow; line fit, the entire progressive alternating field 

demagnetization sequence was fit with a line on a vector component diagram to find the characteristic remanent inclination]

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole USGS 133—Continued

213 57.9 30

216 54.3 30

223 51.8 30

225 51.7 30

228 54.1 30

229–245 Unrecovered core and sediment

247 54.3 30

250 56.5 30

252 53.5 20

253 55.4 20

255 56.4 30

258 59.9 20

262 58.7 30

266 56.6 30

270 57.5 30

272 55.6 30

275 53.6 40

278 52.6 30

282 54.2 30 55.2±1.0

287 53.5 30

291 52.9 30

294 53.3 30

300 50.5 30

302 53.4 30

303 53.5 30

306 55.2 30

309 58.9 30

312 56.7 30

314 55.9 30

315–317 Unrecovered core and petrographic boundary

319 59.9 30 NIIM

324 63.9 30

330 68.0 30

334 69.7 30 67.3±2.2

338 67.1 30

342 67.4 30

346 67.6 30

349 Petrographic boundary

351 62.5 30

354 64.0 30

359 66.9 20

364 65.4 30 65.1±1.7

369 64.1 30

373 65.2 30
Appendix A 31

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole USGS 133—Continued

378 67.5 30

379–382 Unrecovered core and sediment

383 57.2 30 NIIM

386 48.2 30

388 51.1 20

391 48.1 30 49.9±2.3

393 48.1 20

397 51.8 30

398 52.3 30

400–402 Unrecovered core and petrographic boundary

404 50.0 30

409 49.5 30

414 52.7 30

420 53.9 30 51.6±1.6

427 53.1 40

432 49.4 30

437 53.1 30

445 50.7 30

447 Petrographic boundary

448 55.2 20 NIIM

452 45.5 20 NIIM

453 41.7 30 NIIM

455 39.9 20 NIIM

458 38.9 50 NIIM

462 45.5 20 NIIM

463 47.7 30 NIIM

467 50.6 50 NIIM

471 50.6 20 NIIM

475 55.3 30

479 54.1 30

482 56.9 30

486 53.5 30

488 46.9 20

492 59.4 30

494 53.6 30

497 56.3 30 54.4±1.1

501 55.3 20

503 55.1 10

505 53.8 20

510 53.6 20

513 57.0 30

520 52.3 30

525 56.2 30

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole USGS 133—Continued

531 55.4 30

537 55.5 30

544 54.9 30

550 57.0 30

556 53.4 30

561 54.8 30

564 50.6 20

566 50.6 30

567 Petrographic boundary

569 53.9 30

575 59.5 20 NIIM

582 55.4 30 NIIM

588 61.9 30

594 65.3 20 63.8±1.7

604 65.2 30

610 64.1 30

617 62.2 30

628 64.1 30

630–662 Unrecovered core and sediment

663 -42.0 50 NIIM

665 -50.5 50

666 -48.5 30

669 -48.4 20

672 -48.4 30

676 -47.7 30

679 -45.8 30 -47.2±1.2

683 -45.5 30

687 -45.2 30

689 -46.8 30

692 -44.3 30

695 -48.9 20

697 -45.8 30

698–700 Sediments

701 -66.2 20

704 -70.6 20

708 -71.6 30

712 -68.6 20

717 -69.4 30

720 -74.6 20 -71.6±2.6

723 -75.7 20

724 Petrographic boundary

726 -75.2 20

Table A1. Paleomagnetic inclination values for basalt samples from coreholes at and near the Naval Reactors Facility, Idaho National 

Laboratory, Idaho.—Continued

[Green shading marks start of the list of values for a different corehole. Grey shading delimits groups of samples used to determine the mean flow inclination 

and 95 percent uncertainty level. Sample depth: measured to nearest 1 foot. Characteristic remanent inclination: in degrees, down (negative value) or up 

(positive value) from the horizontal obtained by demagnetization. Abbreviations: ft, foot; AF, Alternating-Field; mT, milliTesla; Petrographic boundary, 

indicates noted change in mineralogy; NIIM, the sample was not included in the group mean inclination--see appendix explanation; Therm, stepwise 

thermal demagnetization was applied to the sample; TO, sample was thermally overprinted by overlying flow; line fit, the entire progressive alternating field 

demagnetization sequence was fit with a line on a vector component diagram to find the characteristic remanent inclination]
32 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Table A1. Paleomagnetic inclination values for basalt samples from coreholes at and near the Naval Reactors Facility, Idaho National 

Laboratory, Idaho.—Continued

[Green shading marks start of the list of values for a different corehole. Grey shading delimits groups of samples used to determine the mean flow inclination 

and 95 percent uncertainty level. Sample depth: measured to nearest 1 foot. Characteristic remanent inclination: in degrees, down (negative value) or up 

(positive value) from the horizontal obtained by demagnetization. Abbreviations: ft, foot; AF, Alternating-Field; mT, milliTesla; Petrographic boundary, 

indicates noted change in mineralogy; NIIM, the sample was not included in the group mean inclination--see appendix explanation; Therm, stepwise 

thermal demagnetization was applied to the sample; TO, sample was thermally overprinted by overlying flow; line fit, the entire progressive alternating field 

demagnetization sequence was fit with a line on a vector component diagram to find the characteristic remanent inclination]

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole USGS 133—Continued

730 -72.4 20

733 -67.5 20

737 -70.6 20

740 -66.8 30

743 -71.3 20

746 -67.7 30

750 -68.9 20 -68.9±0.9

754 -67.7 20

759 -67.9 20

764 -69.7 20

771 -68.8 20

776 -68.9 20

782 -70.5 20

Corehole NRF B18-1

39 75.2 20

47 75.1 20

55 76.2 20

61 77.8 20 76.9±1.3

67 76.2 20

74 78.9 20

81 77.6 40

88 Petrographic boundary 

93 78.4 20

97 59.0 40

100 54.5 40

104 53.2 30

107 59.0 30

111 54.3 40

114 55.6 20

117 50.1 30

119 50.5 30

121 55.3 30

123 54.2 30

127 52.4 30

130 52.3 30

136 55.7 30 54.8±1.2

137 56.2 40

141 55.5 40

145 56.2 30

150 55.4 30

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole NRF B18-1—Continued

154 51.9 50

159 56.7 50

163 55.6 40

167 54.6 40

170 56.4 50

175 55.7 40

177 45.4 30

179 58.9 30

182 57.1 30

187 56.9 20

187–190 Probably sediment 

195 67.4 20

198 66.5 20

203 65.3 20

209 63.5 20 64.8±1.8

213 62.4 30

219 63.0 30

221 65.7 30

225 68.6 30

227 68.2 20

231 71.0 30

233 67.1 30

236 61.1 40 66.9±2.4

240 68.7 40

242 67.7 30

244 64.1 20

246 62.5 30

249 70.0 40

Corehole NRF 89-05

22 45.6 20 NIIM

27 45.5 20 NIIM

32 68.6 30

35 62.5 10 NIIM

39 74.9 10

44 74.3 20

54 73.9 20 73.3±1.5

64 72.9 20

75 74.8 20

85 71.2 10

91 75.0 10

99 74.3 20
Appendix A 33

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole NRF 89-05—Continued

106 73.5 20

107–119 Sediment 

122 56.8 20

125 54.3 20

129 56.2 20

131 55.3 20

137 56.8 30

144 56.0 30

150 54.7 30

156 54.8 20

157 Petrographic boundary 55.6±0.7

159 44.3 30 NIIM

162 47.3 20 NIIM

165 52.3 20

170 56.6 20

173 57.8 20

179 55.7 20

182 55.4 20

188 56.2 20

191 55.7 20

192–196 Sediment

198 60.9 20

202 59.5 20 61.5±17.0

208 64.0 10

212 65.3 20

216 64.2 10

221 63.9 20

225 71.0 20 66.5±3.0

230 66.5 20

235 64.1 20

238 70.6 20

Corehole NRF 89-04

192–196 Sediment 

198 63.3 20

201 66.0 10

204 62.8 20 66.1±3.0

208 67.0 20

212 68.1 20

217 69.3 10

218–221 0 Sediment 

223 67.6 10

227 70.1 10

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole NRF 89-04—Continued

231 69.4 20

234 72.1 20 71.2±2.1

237 74.9 20

241 70.9 20

244 70.3 20

247 74.3 20

Corehole NRF 6P

14 71.0 20

25 72.0 20

34 73.9 20

45 78.3 30

54 76.7 30

64 79.1 20 75.3±2.0 

73 78.2 30

84 74.3 30

93 75.2 30

94–95 Sediment

98 74.0 30

103 67.6 30 TO

108 66.8 30 TO

114 60.1 40 TO

120 54.5 20

125 55.8 30

132 57.0 20

138 54.1 10

146 53.8 30 54.6±1.1 

155 54.7 20

161 52.8 30

165 Petrographic boundary

169 54.2 20

173 63.8 30

179 64.5 30

186 63.9 30

191 64.4 30 64.8±1.5 

199 64.6 30

206 67.4 30

210 Petrographic boundary

215 61.0 20 NIIM

219 70.1 20

223 66.4 30

Table A1. Paleomagnetic inclination values for basalt samples from coreholes at and near the Naval Reactors Facility, Idaho National 

Laboratory, Idaho.—Continued

[Green shading marks start of the list of values for a different corehole. Grey shading delimits groups of samples used to determine the mean flow inclination 

and 95 percent uncertainty level. Sample depth: measured to nearest 1 foot. Characteristic remanent inclination: in degrees, down (negative value) or up 

(positive value) from the horizontal obtained by demagnetization. Abbreviations: ft, foot; AF, Alternating-Field; mT, milliTesla; Petrographic boundary, 

indicates noted change in mineralogy; NIIM, the sample was not included in the group mean inclination--see appendix explanation; Therm, stepwise 

thermal demagnetization was applied to the sample; TO, sample was thermally overprinted by overlying flow; line fit, the entire progressive alternating field 

demagnetization sequence was fit with a line on a vector component diagram to find the characteristic remanent inclination]
34 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole NRF 6P—Continued

229 67.5 20

235 70.2 30

241 66.2 30

247 73.0 30

253 73.6 30

259 70.2 30

265 72.1 20

272 72.0 20

280 72.4 30 71.3±1.0 

284 71.7 30

289 73.0 30

296 72.3 30

305 68.9 30

310 Petrographic boundary

312 73.3 30

317 70.8 30

323 72.8 30

328 72.8 30

333 70.5 30

337 72.6 30

342 72.1 30

347 74.5 30

352 73.1 20

353–354 Sediment

356 66.3 30 NIIM

360 62.9 50 NIIM

364 61.0 30

370 56.5 30

372 57.0 30

377 59.1 30

384 50.6 30

387 54.6 20 56.6±1.6 

391 54.6 30

394 53.9 20

397 56.8 30

401 56.9 30

407 57.1 30

409 55.9 30

413 56.3 30

416 61.6 20

418 Petrographic boundary

427 -48.4 Therm

433 -48.4 Therm

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole NRF 6P—Continued

437 -48.0 Therm

449 -50.8 Therm

454 -49.4 Therm

459 -48.0 Therm -46.9±1.7 

467 -45.9 Therm

474 -44.2 Therm

481 -43.5 Therm

492 -43.7 Therm

499 -45.5 Therm

Corehole NRF 7P

27 74.8 20

30 72.6 30

32 76.5 20

39 77.1 30

44 75.2 20

49 74.4 20

54 74.1 20 75.8±1.4

56 76.7 30

62 79.6 30

67 71.6 20

76 77.3 20

83 77.7 30

89 77.4 20

92–108 Sediment 

106 65.5 30 NIIM

112 60.3 20

115 54.7 20

119 55.4 30

127 56.8 30

131 54.3 20 57.3±1.8

133 Petrographic boundary

133 59.4 20

137 59.6 30

143 60.1 30

147 57.7 20

151-152 Sediment 

155 54.3 30

159 65.1 30

165 68.6 30 67.1±13.4

170 67.7 50

173 71.3 30

Table A1. Paleomagnetic inclination values for basalt samples from coreholes at and near the Naval Reactors Facility, Idaho National 

Laboratory, Idaho.—Continued

[Green shading marks start of the list of values for a different corehole. Grey shading delimits groups of samples used to determine the mean flow inclination 

and 95 percent uncertainty level. Sample depth: measured to nearest 1 foot. Characteristic remanent inclination: in degrees, down (negative value) or up 

(positive value) from the horizontal obtained by demagnetization. Abbreviations: ft, foot; AF, Alternating-Field; mT, milliTesla; Petrographic boundary, 

indicates noted change in mineralogy; NIIM, the sample was not included in the group mean inclination--see appendix explanation; Therm, stepwise 

thermal demagnetization was applied to the sample; TO, sample was thermally overprinted by overlying flow; line fit, the entire progressive alternating field 

demagnetization sequence was fit with a line on a vector component diagram to find the characteristic remanent inclination]
Appendix A 35

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole NRF 7P—Continued

175 73.2 30

181 72.0 20

190 69.7 30

200 70.4 30

210 71.6 20

220 71.7 20

229 70.0 30

240 69.7 30

251 66.5 30

254 Petrographic boundary

258 57.3 50

261 65.4 30

265 76.6 20

270 73.6 30

275 71.9 30

281 74.1 20

287 74.5 20

291 74.1 20 72.6±1.0

294 76.8 20

300 74.1 30

301 76.3 30

306 76.4 30

308 Petrographic boundary

309 72.8 30

313 71.7 30

318 69.8 30

321 68.4 30

326 71.7 20

330 70.0 30

336 77.8 30

341 69.5 20

345 77.0 30

350 76.8 30

353 74.8 30

356-358 Sediment

360 72.4 20

367 69.9 30

370 60.2 20

374 55.2 20

379 53.9 20

382 50.9 30 56.8±2.8

Table A1. Paleomagnetic inclination values for basalt samples from coreholes at and near the Naval Reactors Facility, Idaho National 

Laboratory, Idaho.—Continued

[Green shading marks start of the list of values for a different corehole. Grey shading delimits groups of samples used to determine the mean flow inclination 

and 95 percent uncertainty level. Sample depth: measured to nearest 1 foot. Characteristic remanent inclination: in degrees, down (negative value) or up 

(positive value) from the horizontal obtained by demagnetization. Abbreviations: ft, foot; AF, Alternating-Field; mT, milliTesla; Petrographic boundary, 

indicates noted change in mineralogy; NIIM, the sample was not included in the group mean inclination--see appendix explanation; Therm, stepwise 

thermal demagnetization was applied to the sample; TO, sample was thermally overprinted by overlying flow; line fit, the entire progressive alternating field 

demagnetization sequence was fit with a line on a vector component diagram to find the characteristic remanent inclination]

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole NRF 7P—Continued

386 59.7 20

390 57.4 20

394 58.5 30

400 58.5 30

403-404 Sediment

405 63.9 20

409 61.2 20

412 61.8 30 63.0±2.9

416 67.2 20

420 60.1 30

424 63.6

425-428 Sediment

430 -43.8 40

435 -44.8 20

440 -45.6 10

447 -44.7 30

456 -48.6 30 -46.2±1.4

463 -49.6 20

470 -44.4 20

476 -46.2 20

486 -46.2 10

496 -47.6 50

Corehole NRF 15

17 75.5 20

20 75.2 40

23 75.6 40

26 72.6 30

27 80.6 40

32 80.4 30

35 78.1 30

40 80.5 40 77.0±1.4

44 78.8 40

48 77.9 30

52 80.1 40

54 74.5 40

58 67.7 40 NIIM

59 74.9 30

65 73.7 40

68 76.8 40

71 76.0 30

71–76 Probably sediment 

78 56.0 40
36 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Table A1. Paleomagnetic inclination values for basalt samples from coreholes at and near the Naval Reactors Facility, Idaho National 

Laboratory, Idaho.—Continued

[Green shading marks start of the list of values for a different corehole. Gray shading delimits groups of samples used to determine the mean flow inclination 

and 95 percent uncertainty level. Sample depth: measured to nearest 1 foot. Characteristic remanent inclination: in degrees, down (negative value) or up 

(positive value) from the horizontal obtained by demagnetization. Abbreviations: ft, foot; AF, Alternating-Field; mT, milliTesla; Petrographic boundary, 

indicates noted change in mineralogy; NIIM, the sample was not included in the group mean inclination--see appendix explanation; Therm, stepwise 

thermal demagnetization was applied to the sample; TO, sample was thermally overprinted by overlying flow; line fit, the entire progressive alternating field 

demagnetization sequence was fit with a line on a vector component diagram to find the characteristic remanent inclination]

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole NRF 15—Continued

81 56.6 30

86 57.8 40

89 60.0 40

92 61.6 40

95 56.9 30

97 58.4 30

99 56.3 40

103 56.3 30

106 55.9 40

108 57.9 40

110 56.4 40 57.6±0.6

113 59.6 30

115 58.1 40

118 58.0 30

126 56.3 40

131 56.1 30

133 56.4 40

136 58.1 30

138 58.0 30

142 56.4 40

143 59.3 40

147 57.7 40

150 58.0 40

155 Petrographic boundary 

156 56.7 Therm

160 64.4 Therm

164 62.6 40

169 65.3 30 64.1±1.2

172 63.0 30

174 64.8 40

175 64.7 40

176–181 Probably sediment 

182 73.8 30

184 68.0 40

186 72.2 40

190 70.7 40

192 61.9 40 NIIM

195 72.0 40

202 73.8 40

203 74.9 30

206 75.0 40

212 73.7 40

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole NRF 15—Continued

214 77.3 30 73.4±1.2

216 75.4 40

220 74.8 30

225 73.5 40

230 78.8 40

235 69.4 40

239 75.5 40

243 74.8 40

247 75.0 40

251 70.7 30

256 71.4 40

260 71.5 30

264 71.7 40

266 Petrographic boundary 

269 55.8 40 NIIM

274 73.1 40

276 72.6 40

279 75.3 40

283 76.6 40

286 69.3 40

289 71.9 40

292 74.7 40

296 74.1 40

300 74.5 40 72.0±1.3

304 72.3 40

308 73.5 40

310 69.3 40

314 69.9 30

317 72.5 40

322 65.8 30

326 68.9 40

331 71.2 30

335 73.9 40

337–341 Probably sediment 

344 -36.8 Therm

348 -44.9 Therm

353 -45.5 40

358 -47.8 40

363 -46.5 30

368 -36.5 40

373 -44.7 40

379 -45.7 40

384 -44.7 30 -41.5±2.1

389 -37.8 40
Appendix A 37

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole NRF 15—Continued

392 -38.0 40

394 -38.3 40

397 -33.6 40

400 -35.7 40

403 -43.1 40

406 -39.6 40

409 -41.9 40

413 -44.4 40

416 -42.8 40

416–419 Probably sediment 

421 -50.9 40

425 -47.3 30

429 -46.0 40

433 -44.4 40

438 -47.9 30

440 -45.5 40

442 -46.2 40 -46.1±1.8

445 -39.2 30

447 -43.4 40

451 -47.5 40

452 -50.5 30

456 -45.6 40

458 -45.4 40

460 Petrographic boundary 

462 -44.5 40

465 -46.9 40

470 -49.9 30

473 -53.4 30

476 -46.4 30

480 -47.9 30

483 -46.4 40 -47.5±1.5

484 -44.6 40

485 -48.3 30

487 -46.1 40

488 -50.1 40

490 -46.5 30

491 -46.8 30

491–512 Probably sediment 

514 -57.8 30

516 -56.8 line fit

519 -57.1 line fit

521 -57.8 line fit -57.9±0.9

522 -59.6 line fit

524 -58.3 line fit

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole NRF 15—Continued

525 -57.9 line fit

526–542 Probably sediment 

542 -65.2 line fit

545 -70.6 line fit -68.5±5.0

546 -68.9 line fit

548 -69.1 line fit

550–589 Probably sediment 

590 47.3 line fit NIIM

594 51.1 line fit

598 53.0 line fit

603 53.7 line fit

608 48.1 line fit NIIM

609 52.7 line fit

610 49.3 line fit NIIM

614 50.7 line fit

619 56.6 line fit

623 54.1 line fit

629 54.0 line fit 53.7±1.1

633 54.9 line fit

637 53.2 line fit

641 48.8 line fit NIIM

642 48.5 line fit NIIM

643 47.4 line fit NIIM

646 49.0 line fit NIIM

648 53.8 line fit

650 58.0 line fit

654 53.8 line fit

656 51.6 line fit

657 Petrographic boundary 

659 54.5 line fit

663 -54.2 line fit

669 -58.2 line fit

675 -55.9 line fit

681 -56.7 line fit

684 -56.4 line fit

690 -58.7 line fit -57.9±1.2

696 -61.0 line fit

701 -59.5 line fit

709 -59.3 line fit

715 -58.3 line fit

722 -58.6 line fit

727 -57.5 line fit

730–759 Probably sediment 

Table A1. Paleomagnetic inclination values for basalt samples from coreholes at and near the Naval Reactors Facility, Idaho National 

Laboratory, Idaho.—Continued

[Green shading marks start of the list of values for a different corehole. Gray shading delimits groups of samples used to determine the mean flow inclination 

and 95 percent uncertainty level. Sample depth: measured to nearest 1 foot. Characteristic remanent inclination: in degrees, down (negative value) or up 

(positive value) from the horizontal obtained by demagnetization. Abbreviations: ft, foot; AF, Alternating-Field; mT, milliTesla; Petrographic boundary, 

indicates noted change in mineralogy; NIIM, the sample was not included in the group mean inclination--see appendix explanation; Therm, stepwise 

thermal demagnetization was applied to the sample; TO, sample was thermally overprinted by overlying flow; line fit, the entire progressive alternating field 

demagnetization sequence was fit with a line on a vector component diagram to find the characteristic remanent inclination]
38 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Table A1. Paleomagnetic inclination values for basalt samples from coreholes at and near the Naval Reactors Facility, Idaho National 

Laboratory, Idaho.—Continued

[Green shading marks start of the list of values for a different corehole. Gray shading delimits groups of samples used to determine the mean flow inclination 

and 95 percent uncertainty level. Sample depth: measured to nearest 1 foot. Characteristic remanent inclination: in degrees, down (negative value) or up 

(positive value) from the horizontal obtained by demagnetization. Abbreviations: ft, foot; AF, Alternating-Field; mT, milliTesla; Petrographic boundary, 

indicates noted change in mineralogy; NIIM, the sample was not included in the group mean inclination--see appendix explanation; Therm, stepwise 

thermal demagnetization was applied to the sample; TO, sample was thermally overprinted by overlying flow; line fit, the entire progressive alternating field 

demagnetization sequence was fit with a line on a vector component diagram to find the characteristic remanent inclination]

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole NRF 16

16 32.9 line fit NIIM

18 68.6 line fit NIIM

20 78.5 line fit

26 75.2 line fit

29 81.4 line fit

32 73.9 line fit

34 76.0 line fit

35 78.8 line fit

37 73.9 line fit 75.8±1.4

40 74.4 line fit

43 74.5 line fit

46 81.0 line fit

49 77.7 line fit

51 73.9 line fit

53 75.7 line fit

55 77.9 line fit

56 77.0 line fit

64 72.7 line fit

65–69 Probably sediment 

69 -15.2 line fit NIIM

72 20.7 line fit NIIM

75 45.1 line fit NIIM

79 66.5 line fit NIIM

82 58.0 line fit

89 51.9 line fit

90 58.1 line fit

92 58.1 line fit

94 54.7 line fit

99 55.1 line fit 54.8±1.4

102 55.7 line fit

105 53.5 line fit

110 56.1 line fit

114 52.5 line fit

117 53.9 line fit

120 51.6 line fit

65–69 Probably sediment 

125 52.5 Therm

128 61.0 line fit

130 62.3 line fit

131 59.2 line fit

134 62.0 line fit 64.0±3.9

136 69.6 line fit

138 64.9 line fit

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole NRF 16—Continued

141 69.1 line fit

141–144 Petrographic boundary 

147 69.2 line fit

152 70.0 line fit

159 74.0 line fit

167 72.3 line fit

174 74.5 line fit 72.2±1.5

180 73.1 line fit

187 70.6 line fit

192 71.1 line fit

199 75.6 line fit

206 71.7 line fit

208–210 Petrographic boundary 

212 73.3 line fit

216 73.5 line fit

220 70.4 line fit

224 73.0 line fit

227 73.6 line fit

228 75.1 line fit

230 72.2 line fit

232 72.3 line fit

235 75.3 line fit

237 71.7 line fit

240 70.4 line fit

245 70.7 line fit

249 71.2 line fit

254 72.3 line fit

258 73.0 line fit 72.9±0.8

262 67.4 line fit

264 75.5 line fit

266 71.8 line fit

269 76.6 line fit

272 71.4 line fit

275 72.4 line fit

278 73.5 line fit

280 76.6 line fit

284 75.3 line fit

289 72.0 line fit

291 70.2 line fit

293 74.1 line fit

294 74.4 line fit

296 73.8 line fit

299 75.2 line fit

300 Petrographic boundary 
Appendix A 39

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole NRF 16—Continued

302 70.1 line fit

305 74.0 line fit

309 72.8 line fit

312 75.8 line fit

319 74.9 line fit

321 77.4 line fit 73.8±1.5

322 76.3 line fit

324 76.2 line fit

326 75.5 line fit

329 72.7 line fit

332 70.7 line fit

336 70.8 line fit

337 Sediment 

339 -35.3 Therm

342 -37.7 Therm

345 -45.8 Therm

348 -41.2 line fit

350 -38.4 line fit

353 -34.4 Therm

356 -44.3 Therm

359 -38.8 line fit

362 -42.8 line fit -40.3±1.7

364 -40.1 line fit

Table A1. Paleomagnetic inclination values for basalt samples from coreholes at and near the Naval Reactors Facility, Idaho National 

Laboratory, Idaho.—Continued

[Green shading marks start of the list of values for a different corehole. Gray shading delimits groups of samples used to determine the mean flow inclination 

and 95 percent uncertainty level. Sample depth: measured to nearest 1 foot. Characteristic remanent inclination: in degrees, down (negative value) or up 

(positive value) from the horizontal obtained by demagnetization. Abbreviations: ft, foot; AF, Alternating-Field; mT, milliTesla; Petrographic boundary, 

indicates noted change in mineralogy; NIIM, the sample was not included in the group mean inclination--see appendix explanation; Therm, stepwise 

thermal demagnetization was applied to the sample; TO, sample was thermally overprinted by overlying flow; line fit, the entire progressive alternating field 

demagnetization sequence was fit with a line on a vector component diagram to find the characteristic remanent inclination]

Sample 

depth 

(ft) 

Characteristic 

remanent 

inclination 

(degrees) 

AF demagnetization 

level (mT) 

or alternative 

demagnetization 

approach

Mean flow 

inclination and 

95% uncertainty 

level for sample 

groupings (degrees) 

Corehole NRF 16—Continued

366 -37.7 line fit

369 -40.0 line fit

371 -45.8 line fit

374 -43.8 line fit

376 -41.4 line fit

378 -38.5 line fit

378–381 Probably sediment 

382 -38.8 Therm

386 -45.9 line fit

390 -47.3 line fit

392 -67.0 line fit NIIM

397 -50.4 line fit

399 -46.1 line fit

401 -48.3 line fit

404 -49.3 line fit -47.3±1.4

407 -50.8 line fit

409 -47.2 line fit

413 -44.1 line fit

417 -44.5 line fit

420 -47.6 line fit

424 -45.5 line fit
40 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

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Appendix B 41

Appendix B. 40Ar/39Ar Analytical Data for Selected Coreholes, Naval Reactors 

Facility, Idaho National Laboratory, Idaho

Table B1 shows analytical data associated with 

determination of the plateau ages of samples from coreholes 

NRF 6P and 7P, Naval Reactors Facility, Idaho National 

Laboratory, Idaho.

J is the neutron fluence constant calculated from the 

irradiation of a known age monitor mineral. Plateau age is 

the weighted mean age using [n of m] diffusion steps. The 

isochron age is derived from the slope of the best fit line 

on an isochron diagram plotting 40Ar/36Ar versus 39Ar/36Ar 

isotopic ratios; Inverse isochron age is derived from the slope 

of the best fit line on an isochron diagram plotting 36Ar/40Ar 

versus 39Ar/40Ar isotopic ratios; total gas age is calculated by 

summing all diffusion steps.

Temp (°C) is the temperature of each progressive argon 

diffusion step. 40Ar/39Ar, 37Ar/39Ar, and 36Ar/39Ar are the 

ratios of the abundances of argon isotopes 40Ar, 39Ar, 37Ar, and 

36Ar. Moles 40Arrad is the absolute number of moles of 40Ar 

measured in each diffusion step. 40Arrad (%) is the percentage 

of radiogenic 40Ar measured in each step; 39ArCa (%) and 

36ArCa (%) are the percentages of 39Ar and 36Ar correcting 

those isotope abundances for calcium interference for each 

step. K/Ca is the potassium/calcium ratio calculated for each 

step from the 39Ar and 37Ar isotope measurements. 39Ar (%) 

is the percentage of 39Ar released (100% total) during each 

step. Age±s.d. (Ma) is the age and standard deviation in age, in 

millions of years, calculated for each step. 

Figures in appendix B show the forward and inverse 

isochron diagrams and argon age plateau diagrams for each 

basalt sample analyzed by the 40Ar/39Ar method.

Isochron Diagrams

Forward isochron diagrams plot the 40Ar/36Ar ratio 

against the 39Ar/36Ar ratio for each temperature step of the 

argon age experiment. The inverse isochron diagram plots the 

36Ar/40Ar ratio against the 39Ar/40Ar ratio for each temperature 

step of the argon age experiment. The age of the isochron 

with uncertainty, 40Ar/36Ar intercept value with uncertainty, 

and Mean Square of Weighted Deviates (Sums/(N-2)) are also 

shown. Open circle data points were used in the isochron fit, 

cross data points were not used.

Argon Age Plateau Diagrams

Argon age plateau diagrams plot the age of the sample in 

millions of years (Ma) against the cumulative percent of 39Ar 

released in the argon age experiment. Rectangular boxes show 

the age and the 1 sigma uncertainty in age plotted against the 

span of 39Ar released during each successive temperature step 

of the argon age experiment. The small italic numbers are 

temperatures (°C) of the progressive steps. The horizontal 

double-headed arrowed line with enclosed number shows the 

age and 1 sigma uncertainty in age, and the percent span of 

39Ar used in the weighted mean age calculation for this age 

experiment.
42 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Table B1. 40Ar/39Ar analytical data, with plateau, forward and inverse isochron diagrams for basalt samples from coreholes 

NRF 6P and NRF 7P, Naval Reactors Facility, Idaho National Laboratory, Idaho.

[J is the neutron fluence constant calculated from the irradiation of a known age monitor mineral. Plateau age is the weighted mean age using [n of 

m] diffusion steps; Isochron age is derived from the slope of the best fit line on an isochron diagram plotting 40Ar/36Ar versus 39Ar/36Ar isotopic 

ratios; Inverse isochron age is derived from the slope of the best fit line on an isochron diagram plotting 36Ar/40Ar versus 39Ar/40Ar isotopic ratios; 

total gas age is calculated by summing all diffusion steps. Temp (°C): temperature of each progressive argon diffusion step; 40Ar/39Ar, 37Ar/39Ar, 

and 36Ar/39Ar: ratios of the abundances of argon isotopes 40Ar, 39Ar, 37Ar, and 36Ar. Moles 40Arrad: absolute number of moles of 40Ar measured in 

each diffusion step. 40Arrad (%): percentage of radiogenic 40Ar measured in each step. 39ArCa (%) and 36ArCa (%): percentages of 39Ar and 36Ar 

correcting those isotope abundances for calcium interference for each step. K/Ca: potassium/calcium ratio calculated for each step from the 39Ar 

and 37Ar isotope measurements. 39Ar (%): percentage of 39Ar released (100% total) during each step. Age±s.d.: age and standard deviation in age, 

in millions of years (Ma), calculated for each step]

NRF 6P (155 ft depth) [J=0.00038155]

Plateau age=0.395±0.025 Ma; isochron age=0.372±0.038 Ma; 

inverse isochron age=0.371±0.037 Ma; total gas age=0.382±0.028 Ma

Temp 

(C°)

40Ar/39Ar 37Ar/39Ar 36Ar/39Ar Moles40Arrad

40Arrad

(%)

39ArCa

(%)

36ArCa 

(%) K/Ca

39Ar 

(%)

Age±s.d.

(Ma)

450 9.818 8.480 0.03308 1.701-14 5.6 0.55 5.3 0.057 53.6 0.382 ± 0.041

500 6.176 6.000 0.02015 7.022-15 9.4 0.39 6.1 0.081 21.0 0.402 ± 0.038

600 6.853 6.144 0.02240 2.728-15 8.8 0.40 5.6 0.079 7.9 0.417 ± 0.078

650 12.318 5.371 0.04072 1.948-15 4.9 0.35 2.7 0.091 5.6 0.420 ± 0.109

700 14.252 4.703 0.04792 9.497-16 2.6 0.30 2.0 0.104 4.4 0.257 ± 0.136

750 33.28 5.951 0.11170 1.327-15 1.9 0.38 1.1 0.082 3.7 0.435 ± 0.179

800 42.57 12.376 0.14233 1.314-15 3.0 0.80 1.8 0.039 1.8 0.874 ± 0.340

900 66.98 64.36 0.2414 -5.334-16 -0.7 4.2 5.5 0.007 2.0 -0.326 ± 0.413

NRF 6P (241 ft depth) [J=0.00037585]

Plateau age=0.546±0.047 Ma; isochron age=0.484±0.089 Ma; 

inverse isochron age=0.485±0.084 Ma; total gas age=0.561±0.065 Ma

Temp 

(C°)

40Ar/39Ar 37Ar/39Ar 36Ar/39Ar Moles40Arrad

40Arrad

(%)

39ArCa

(%)

36ArCa

(%) K/Ca

39Ar 

(%)

Age±s.d.

(Ma)

450 28.18 1.8562 0.09276 1.254-14 3.1 0.014 0.12 0.264 50.2 0.596 ± 0.078

500 9.723 2.072 0.03107 2.249-15 6.8 0.039 0.13 0.236 11.9 0.451 ± 0.100

575 12.900 8.698 0.04262 3.606-15 6.4 0.030 0.56 0.056 15.2 0.565 ± 0.087

625 23.75 9.709 0.07940 1.911-15 3.7 0.016 0.63 0.050 7.6 0.597 ± 0.164

675 35.92 12.040 0.12237 6.825-16 1.4 0.011 0.78 0.040 4.9 0.336 ± 0.256

725 52.88 10.674 0.17381 1.742-15 4.1 0.007 0.69 0.046 2.8 1.478 ± 0.431

900 198.49 123.02 0.6973 -1.971-16 -0.1 0.002 7.9 0.0037 5.6 -0.840 ± 0.763

NRF 6P (372 ft depth) [J=0.00036836]

plateau age = 0.727 ± 0.031 Ma; isochron age = 0.752 ± 0.063 Ma;

inverse isochron age = 0.754 ± 0.063 Ma; total gas age = 0.635 ± 0.077 Ma

Temp

(C°)

40Ar/39Ar 37Ar/39Ar 36Ar/39Ar Moles40Arrad

40Arrad

(%)

39ArCa

(%)

36ArCa

(%) K/Ca

39Ar

(%)

Age±s.d.

(Ma)

450 47.68 4.756 0.15898 3.923-14 2.1 0.31 0.61 0.103 58.2 0.658 ± 0.130

500 16.082 0.95151 0.05072 1.503-14 7.1 0.061 0.38 0.515 19.2 0.764 ± 0.042

550 12.936 7.491 0.04187 5.351-15 7.8 0.48 3.7 0.065 7.7 0.677 ± 0.054

600 14.550 7.894 0.04717 2.846-15 7.5 0.51 3.4 0.062 3.8 0.726 ± 0.087

700 20.58 11.058 0.07012 1.116-15 2.5 0.71 3.2 0.044 3.1 0.351 ± 0.111

750 29.46 10.879 0.09960 8.001-16 2.3 0.70 2.2 0.045 1.7 0.460 ± 0.187

800 37.65 11.369 0.12872 2.998-16 0.8 0.73 1.8 0.043 1.5 0.202 ± 0.222

850 72.22 17.075 0.2444 1.220-15 1.4 1.1 1.4 0.028 1.7 0.691 ± 0.247

900 120.57 84.03 0.4274 -1.125-15 -0.5 5.4 4.0 0.0055 2.4 -0.451 ± 0.462

950 114.13 102.02 0.4048 3.099-16 0.6 6.6 5.2 0.0045 0.6 0.485 ± 0.668
Appendix B 43

NRF 7P (440 ft depth) [J=0.00036836]

plateau age = 0.884 ± 0.053 Ma; isochron age = 0.878 ± 0.242 Ma;

inverse isochron age = 0.875 ± 0.211 Ma; total gas age = 0.724 ± 0.079 Ma

Temp

(C°)

40Ar/39Ar 37Ar/39Ar 36Ar/39Ar Moles40Arrad

40Arrad

(%)

39ArCa

(%)

36ArCa

(%) K/Ca

39Ar

(%)

Age±s.d.

(Ma)

450 48.00 3.740 0.16095 1.677-14 1.4 0.24 0.48 0.131 48.4 0.500 ± 0.147

500 33.52 7.246 0.10990 1.985-14 4.4 0.47 1.4 0.067 25.6 1.117 ± 0.105

550 22.63 13.692 0.07551 6.071-15 5.0 0.88 3.7 0.035 10.1 0.864 ± 0.090

600 20.06 17.674 0.06824 3.391-15 4.8 1.1 5.3 0.027 6.7 0.732 ± 0.108

650 30.09 17.631 0.10146 2.090-15 3.9 1.1 3.6 0.027 3.4 0.892 ± 0.164

700 56.23 26.15 0.19286 8.484-16 1.5 1.7 2.8 0.0184 2.0 0.624 ± 0.284

750 77.76 96.11 0.2818 3.859-16 0.4 6.2 7.0 0.0048 2.1 0.264 ± 0.493

800 54.02 198.98 0.2348 -1.445e-15 -6.1 12.9 17.4 0.0021 0.7 2.85 ± 1.06

900 43.63 150.79 0.17210 5.432-16 4.4 9.7 18.0 0.0029 0.5 1.590 ± 1.091

1000 42.60 126.01 0.14899 1.659-15 14.6 8.1 17.3 0.0036 0.5 5.07 ± 1.06

NRF 7P (486 ft depth) [J=0.00035049]

plateau age = 1.176 ± 0.027 Ma; isochron age = 1.178 ± 0.168 Ma;

inverse isochron age = 1.181 ± 0.154 Ma; total gas age = 1.204 ± 0.031 Ma

Temp

(C°)

40Ar/39Ar 37Ar/39Ar 36Ar/39Ar Moles40Arrad

40Arrad

(%)

39ArCa

(%)

36ArCa

(%) K/Ca

39Ar

(%)

Age±s.d.

(Ma)

410 98.15 41.46 0.3833 -1.589-16 11.9 2.9 3.0 0.0115 0.1 7.62 ± 7.63

450 52.74 13.935 0.16906 7.994-16 7.5 0.98 2.3 0.035 1.0 2.51 ± 0.47

500 41.53 9.121 0.13580 4.002-15 5.2 0.64 1.9 0.053 8.9 1.375 ± 0.108

525 37.27 8.054 0.12288 2.958-15 4.4 0.57 1.8 0.060 8.8 1.032 ± 0.098

550 33.41 8.646 0.10948 4.899-15 5.3 0.61 2.2 0.056 13.4 1.128 ± 0.087

575 29.24 9.816 0.09430 5.290-15 7.5 0.69 2.9 0.050 11.7 1.391 ± 0.077

600 25.55 11.339 0.08210 4.568-15 8.7 0.80 3.9 0.043 9.9 1.418 ± 0.075

625 21.97 13.821 0.07265 2.878-15 7.5 0.98 5.3 0.035 8.4 1.049 ± 0.076

650 20.31 16.534 0.06769 2.367-15 8.3 1.2 6.9 0.029 6.8 1.073 ± 0.085

700 20.79 16.767 0.06916 2.696-15 8.4 1.2 6.8 0.029 7.4 1.120 ± 0.081

750 23.10 19.316 0.07787 2.286-15 7.3 1.4 7.0 0.025 6.5 1.083 ± 0.092

800 27.98 18.409 0.09377 1.910-15 6.4 1.3 5.5 0.026 5.1 1.150 ± 0.112

850 34.19 18.507 0.11588 1.279-15 4.3 1.3 4.5 0.026 4.1 0.952 ± 0.137

900 68.27 17.976 0.2312 9.933-16 2.1 1.3 2.2 0.027 3.3 0.932 ± 0.208

950 124.34 42.95 0.4217 1.967-15 2.7 3.0 2.9 0.0111 2.8 2.15 ± 0.34

1000 161.07 119.85 0.5745 3.471-16 0.8 8.5 5.9 0.0037 1.2 0.869 ± 0.585

1100 126.00 244.1 0.5015 -1.598-16 -1.5 17.2 13.7 0.0017 0.3 1.472 ± 1.665

Table B1. 40Ar/39Ar analytical data, with plateau, forward and inverse isochron diagrams for basalt samples from coreholes 

NRF 6P and NRF 7P, Naval Reactors Facility, Idaho National Laboratory, Idaho.—Continued

[J is the neutron fluence constant calculated from the irradiation of a known age monitor mineral. Plateau age is the weighted mean age using [n of 

m] diffusion steps; Isochron age is derived from the slope of the best fit line on an isochron diagram plotting 40Ar/36Ar versus 39Ar/36Ar isotopic 

ratios; Inverse isochron age is derived from the slope of the best fit line on an isochron diagram plotting 36Ar/40Ar versus 39Ar/40Ar isotopic ratios; 

total gas age is calculated by summing all diffusion steps. Temp (°C): temperature of each progressive argon diffusion step; 40Ar/39Ar, 37Ar/39Ar, 

and 36Ar/39Ar: ratios of the abundances of argon isotopes 40Ar, 39Ar, 37Ar, and 36Ar. Moles 40Arrad: absolute number of moles of 40Ar measured in 

each diffusion step. 40Arrad (%): percentage of radiogenic 40Ar measured in each step. 39ArCa (%) and 36ArCa (%): percentages of 39Ar and 36Ar 

correcting those isotope abundances for calcium interference for each step. K/Ca: potassium/calcium ratio calculated for each step from the 39Ar 

and 37Ar isotope measurements. 39Ar (%): percentage of 39Ar released (100% total) during each step. Age±s.d.: age and standard deviation in age, 

in millions of years (Ma), calculated for each step]
44 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Figure B1-A. Forward and inverse isochron diagrams for sample NRF-6P-155.

Figure B1-B. Argon age plateau diagram for sample NRF-6P-155.
Appendix B 45

Figure B2-A. Forward and inverse isochron diagrams for sample NRF-6P-241.

Figure B2-B. Argon age plateau diagram for sample NRF-6P-241. 
46 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Figure B3-A. Forward and inverse isochron diagrams for sample NRF-6P-372.

Figure B3-B. Argon age plateau diagram for sample NRF-6P-372.
Appendix B 47

Figure B4-A. Forward and inverse isochron diagrams for sample NRF-7P-440.

Figure B4-B. Argon age plateau diagram for sample NRF-7P-440.
48 Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho

Figure B5-B. Argon age plateau diagram for sample NRF-7P-486.

Figure B5-A. Forward and inverse isochron diagrams for sample NRF-7P-486.
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Champion and others—Paleomagnetic Correlation and Ages of Basalt Flow Groups in Coreholes, Idaho National Laboratory, Idaho—Scientific Investigations Report 2013-5012