Geology and Geophysics Applied to Groundwater Hydrology at Fort Irwin, 

California

David C. Buesch, Editor

Laboratory Electrical Resistivity Analysis of Geologic 

Samples from Fort Irwin, California

By Benjamin R. Bloss and Paul A. Bedrosian

Open-File Report 2013–1024–E

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Bloss, B.R., and Bedrosian, P.A, 2015, Laboratory electrical resistivity analysis of geologic samples from Fort Irwin, 

California, chap. E of Buesch, D.C., ed., Geology and geophysics applied to groundwater hydrology at Fort Irwin, 

California: U.S. Geological Survey Open-file Report 2013-1024, 104 p., http://dx.doi.org/10.3133/ofr20131024E.

ISSN 2331-1258 (online)
iii

Acknowledgments

Sample collection was undertaken as part of an integrated program funded by the Fort Irwin National Training 

Center. All laboratory resistivity measurements were carried out in the U.S. Geological Survey (USGS) 

Petrophysics Laboratory in Denver, Colorado. Joseph Nawikas is thanked for collecting many of the surface 

samples. Thanks to Robert Horton for guidance and assistance in measurement techniques and data reduction. Bill 

Benzel is thanked for running XRD analysis on select drill-core and surface sediment samples, the results of which 

informed our interpretation. Eric Anderson provided the petrologic description for all samples. Discussions with 

Chris Menges, David Miller, David Buesch, and Jill Densmore provided the geologic and hydrologic context for this 

work. Thanks to Robert Horton and Lyndsay Ball for constructive reviews of this manuscript. A hearty thanks goes 

to Carole Kaouane of École Polytechnique de Montréal for many discussions on laboratory setup and data 

interpretation as well as a technical review of this document.
iv

Contents

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

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

Approach .................................................................................................................................................................... 3

Experimental Setup .................................................................................................................................................... 4

Measurements............................................................................................................................................................ 6

Sources of Error.......................................................................................................................................................... 7

Representing Complex Resistivity .............................................................................................................................. 7

Results...................................................................................................................................................................... 11

Discussion ................................................................................................................................................................ 15

Origin of High Conductivity.................................................................................................................................... 15

Correlation with Ground TEM Models ................................................................................................................... 16

Conclusions .............................................................................................................................................................. 18

References Cited...................................................................................................................................................... 19

Appendix A. Core Rock Resistivity Versus Frequency ............................................................................................. 20

Appendix B. Core Sediment Resistivity Versus Frequency....................................................................................... 46

Appendix C. Surface Rock Resistivity Versus Frequency......................................................................................... 64

Appendix D. Surface Sediment Resistivity Versus Frequency.................................................................................. 86

Appendix E. Laboratory Borehole Resistivity Comparison to TEM and Downhole Resistivity Logs.......................... 95

Supplemental Data ................................................................................................................................................. 103

Data .................................................................................................................................................................... 103

Supplement A ..................................................................................................................................................... 103

Supplement B ..................................................................................................................................................... 103

Supplement C ..................................................................................................................................................... 104

Supplement D ..................................................................................................................................................... 104

Figures

 1. Map of Fort Irwin National Training Center, California, showing boreholes used in this report, sample sites, 

time-domain electromagnetic sites, and known faults.................................................................................... 2

 2. Photographs showing the sample holder used for sediments. A, Disassembled sediment sample holder. B, 

Assembled sediment sample holder attached to the LCR (inductance, capacitance, resistance) test leads . 5

 3. Photographs showing the sample holder used for hard rocks. A, Disassembled rock sample clamp. B, 

Assembled rock sample clamp attached to the LCR (inductance, capacitance, resistance) test leads. ........ 6

 4. Simple frequency-dependant circuit diagram representative of many geologic samples............................... 8

 5. Complex impedance plots of the Mio4b surface sediment sample, where Z" is the imaginary component 

and Z' is the real component ......................................................................................................................... 9

 6. Bode phase plots of the three states of saturation for surface rock sample Crash-4a-weld_R. Plots 

presented are dry state, humid state, and submerged state........................................................................ 11

 7. Graph showing resistivity versus frequency for core sediment sample CRTH1-2c-2................................... 12

 8. Graph showing resistivity versus frequency for core rock sample GOLD2-2c-1T_R. .................................. 13

 9. Histograms showing resistivity of all core sediment samples. Top panel represents the as-received state; 

second panel represents the saturated state............................................................................................... 14

10. Graph showing resistivity comparison for samples from borehole LL04, time-domain electromagnetic (TEM) 

sounding LN2, and long-normal borehole resistivity log. ............................................................................. 17
v

Tables

 1. Borehole locations, nearest time-domain electromagnetic (TEM) sounding locations, and distance in 

meters between boreholes and nearest TEM soundings............................................................................. 16
vi

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

inch (in.) 2.54 centimeter (cm)

square inch (in2) 6.452 square centimeter (cm2)

pound per square inch (lb/in2) 6.895 kilopascal (kPa) 

SI to Inch/Pound

Multiply By To obtain

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as °F = (1.8 × °C) + 32.

Datum

Vertical coordinate information is referenced North American Vertical Datum of 1988 (NAVD88).

Horizontal coordinate information is referenced to the World Geodetic System of 1984 (WGS 84).

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

Abbreviations

[Listed in order of appearance.]

USGS U.S. Geological Survey

Ω·m ohm-meter

TEM time-domain electromagnetics

NTC National Training Center

m meter

cm centimeter

in. inch

ρ electrical resistivity

TDS total dissolved solids

f frequency

Hz hertz

kHz kilohertz

|Z| electrical impedance amplitude

φ phase

Z* complex electrical impedance
vii

Z’ real component of the complex electrical impedance

Z” imaginary component of the complex electrical impedance

e Euler’s number

i imaginary unit

LCR inductance, capacitance, resistance

ZOA zero offset adjustment

mrad milliradians

AR as-received

WA water-added

psi pounds per square inch

°C degrees Celsius

K geometrical factor

A area

L length

Zb bulk impedance element

Cdl double layer capacitor

Zct charge transfer impedance element

W Warburg impedance element

DC Direct Current

XRD X-ray diffraction

UTM Universal Transverse Mercator, zone 11

BH borehole
Laboratory Electrical Resistivity Analysis of Geologic

Samples from Fort Irwin, California

By Benjamin R. Bloss and Paul A. Bedrosian

Abstract

Correlating laboratory resistivity measurements with geophysical resistivity models helps 

constrain these models to the geology and lithology of an area. Throughout the Fort Irwin National 

Training Center area, 111 samples from both cored boreholes and surface outcrops were collected and

processed for laboratory measurements. These samples represent various lithologic types that include

plutonic and metamorphic (basement) rocks, lava flows, consolidated sedimentary rocks, and 

unconsolidated sedimentary deposits that formed in a series of intermountain basins. Basement rocks, 

lava flows, and some lithified tuffs are generally resistive (≥100 ohm-meters [Ω·m]) when saturated.

Saturated unconsolidated samples are moderately conductive to conductive, with resistivities generally 

less than 100 Ω·m, and many of these samples are less than 50 Ω·m. The unconsolidated samples can 

further be separated into two broad groups: (1) younger sediments that are moderately conductive,

owing to their limited clay content, and (2) older, more conductive sediments with a higher clay content 

that reflects substantial amounts of originally glassy volcanic ash subsequently altered to clay. The older 

sediments are believed to be Tertiary. Time-domain electromagnetic (TEM) data were acquired near 

most of the boreholes, and, on the whole, close agreements between laboratory measurements and 

resistivity models were found.

Introduction

The Fort Irwin National Training Center (hereafter called NTC) is located in southern California

within the Eastern California Shear Zone and Mojave strike-slip province, and most of the NTC is in the 

east-striking fault domain (fig. 1; Miller and Yount, 2002). The NTC consists of mountains separated by 

a series of topographic basins floored by volcanic, plutonic, and metamorphic rocks. The complex 

geologic history of the NTC is described in Miller and others (this volume, chap. B). Miocene and 

younger sediments within the basins reflect a range of depositional environments, resulting in alluvial, 

lacustrine, eolian, paludal, and groundwater-discharge deposits. Sediment provenance varies both 

throughout the NTC and within individual basins; primary sediment sources are grus-rich material

derived from Jurassic to Cretaceous granite and volcaniclastic sediments derived from both local and 

regional sources. The region has further been affected by multiple generations of Cenozoic faulting that 

includes transverse, thrust, normal, and oblique faults (fig. 1).
2

Figure 1. Map of Fort Irwin National Training Center, California, showing boreholes used in this report, sample 

sites, time-domain electromagnetic sites (Burgess and Bedrosian, this volume, chap. F), and known faults (Miller 

and others, this volume, chap. B). The index map shows the Eastern California Sheer Zone (ECSZ), the San 

Andreas Fault (SAF), the Garlock Fault (GF), and the Mojave strike-slip province (MSSP), as well as the map area.

Given the complex geologic and tectonic history, establishing a lithostratigraphic framework 

from limited borehole samples and geophysical data is challenging (D. Buesch, written commun., 2014). 

Physical property measurements on surface and borehole sediment samples provide a means to 

discriminate between different lithologies and aid in the interpretation of geophysical models. 

Additional measurements on consolidated rock samples offer context for airborne geophysical surveys 
3

and, in turn, serve as a complement to traditional geologic mapping (Bedrosian and others, this volume, 

chap. G).

From 2009 to 2012, 21 sites within the NTC have been drilled; 15 of these boreholes have 

segments of recovered drill core (Kjos and others, 2014), and 14 of these boreholes with recovered drill 

core have had laboratory resistivity measurements performed on them. (See table 1, appendix E, and 

supplemental data for a complete list of laboratory-processed borehole drill cores.) Laboratory

resistivity measurements on these samples give constraints on the physical properties of basement rocks, 

lava flows, and lithified sedimentary volcanic rocks, in addition to consolidated sedimentary rocks and 

unconsolidated sediment deposits. Basement here is defined as plutonic and metamorphic rocks, 

Mesozoic and older, that are exposed in the ranges and floor the intervening basins. Consolidated is 

defined herein as sedimentary rocks (volcanic or epiclastic) that are at least partially lithified and 

capable of retaining shape as a hand sample or core sample.

A sample’s bulk electrical resistivity has four contributing factors: (1) mineralogy, (2) porosity, 

(3) saturation, and (4) conductivity of pore fluid. Laboratory resistivity measurements on samples of 

representative geology are important in relating geophysical resistivity measurements, both groundbased and airborne, to structure and geology. When borehole samples are available, laboratory 

resistivity measurements deliver a close approximation to the in situ bulk resistivity, albeit at very 

different scales than is approximated by borehole geophysical logs and surface-based resistivity 

soundings. The ability to control saturation in the laboratory environment permits investigation of the 

effect of water content on sample resistivity. Furthermore, if estimates of water quality are available, we 

can further constrain the resistivity signature of various lithologies above and below the local water 

table. These signatures can be used to constrain ground-based and airborne resistivity models 

(Bedrosian and others, this volume, chap. G; Burgess and Bedrosian, this volume, chap. F) and identify 

changes in lithology and saturation.

Approach

To evaluate the resistivity of various sedimentary units and basement rocks found throughout the 

NTC, laboratory resistivity measurements were performed on 54 drill-core and 57 surface samples (see

supplemental data for details). The 54 drill-core samples were subdivided into two groups: 34

unconsolidated sediment deposit samples (hereafter known as sediment) and 20 consolidated, lithified, 

or inherently competent rock samples (hereafter known as rock), which are primarily large clasts within 

a sedimentary unit (Kjos and others, 2014). The 57 surface samples were likewise separated into 16 

sediment samples and 41 rock samples. Note that these classifications are not necessarily based on 

lithology or geology but on the mechanical properties of the sample. Because of the inherent differences 

between sediment and rock, these samples were prepared differently for subsequent measurements.

Also, the condition of the sample in the laboratory may be considerably different than the condition of 

the sample in situ. Collection, storage, and preparation of the samples may change the level of 

compaction, disturb sample-scale sedimentary structure, and alter the moisture content. Each borehole 

core is composed of several ~1.0- to ~1.5-meter (m) (~3- to ~5-foot) segments. These cored segments

were subdivided into sections that varied in length between 8 and 65 centimeters (cm) (3 and 26 inches 

[in.]). A sample was taken from the top and bottom of each section, except for the extremely short 

sections that only produced one sample. Core samples may been contaminated by infiltration of drilling 

fluids; however, samples were chosen that were not in contact with the core housing (thus decreasing 

the radius) or core cap (through removal of 2 to 3 cm of material from the core section ends).

Electrical resistivity (ρ) is an intrinsic electrical property, independent of a material's size and 

shape, that characterizes its opposition to the flow of electric current. In sedimentary environments, the 
4

dominant factors that determine a sample's bulk ρ are clay content, pore water ρ, and the degree of 

saturation. In the absence of clay, dry materials are typically more resistive than wet materials, and 

material saturated with water containing low total dissolved solids (TDS) will be more resistive than 

material saturated with high-TDS water. 

Bulk ρ, as measured and treated by many geophysical techniques, is considered to be frequency

(f) independent. In reality, ρ is frequency dependent. We aim to constrain ground-based time-domain 

electromagnetic, airborne electromagnetic, and borehole ρ measurements, which are all considered to be 

low frequency, or low induction number, systems. We characterized ρ through impedance spectroscopy 

over a frequency bandwidth from 100 hertz (Hz) to 100 kilohertz (kHz). Resistivity anisotropy was not 

measured and is not considered here.

Impedance spectroscopy consists of measuring the impedance amplitude (|Z|) and the phase (φ)

of the complex electrical impedance (Z*). Phase measurements refer to the angle that an observed sine 

wave lags, or leads, a reference sine wave. For these data, a time-varying current was passed through the 

sample, and a time-varying voltage was measured. The phase of the received voltage waveform was

calculated using the current waveform as a reference. Impedance phase is a measure of energy storage, 

also called polarization.

Z∗ = |Z|eiφ = Z′ + iZ" (1)

where Z’ is the real part of the complex impedance, or resistance, in ohms; and

Z” is the imaginary part of the complex impedance, or reactance, in ohms.

Experimental Setup

All measurements were made using a Hewlett Packard LCR (inductance, capacitance, 

resistance) meter controlled via a computer using an in-house LabVIEW interface. The LCR impedance 

measurements were carried out using two wire-leads and two electrodes in what is commonly referred 

to as a two-electrode measurement. Each electrode was attached to one current lead. The LCR meter 

sweeps through a series of measurement frequencies and at each frequency stores the sample’s 

impedance, resistance, and phase measurements. It is worth noting that the LCR meter reports 

impedance values with the effects of capacitance and inductance included, whereas resistance values are 

reported with the effects of capacitance and inductance removed.

When making resistivity measurements, the electrical response of the measurement system must 

be accounted for. The LCR meter, test cables, and samples holders all have an associated impedance 

caused by stray capacitance, residual inductance, and system resistance. An instrument calibration 

procedure, referred to as the zero offset adjustment (ZOA), was performed to compensate for these 

effects. It consists of two parts: An open-circuit ZOA measures stray capacitance and conductance of 

the test cables with an empty sample holder, and a short-circuit ZOA measures the residual inductance 

and resistance of the system with the current electrodes shorted. Subsequent measurements 

automatically compensated for these system-response measurements. Calibration of the LCR meter also 

included a self-diagnostic test to confirm normal system operation. Finally, a verification procedure was

carried out in which parallel, resistor-capacitor circuits with a known resistance, impedance, and phase 

response were measured. These equivalent circuits provide a means with which to compare measured 

and theoretical circuit responses.

The accuracy of the LCR meter varies as a function of signal level and measurement frequency. 

In general, the LCR meter is less accurate at low measured voltages and at high measurement

frequencies. The smallest instrument impedance and phase errors occur at low frequencies with low 

measured impedances (impedance: ±0.1 to ±0.3 percent; phase: ±0.87 milliradians [mrad]), while larger 
5

instrument errors occur at high frequencies and high measured impedances (impedance: ±1 to ±3

percent; phase: ±17.45 mrad). Very high measured impedance values (>1,000 kilohms) can result in 

instrument errors as high as ±3 to ±5 percent. Given the low ρ of many of our samples, the instrument 

impedance errors generally fell within the range of ±0.1 to ±3 percent, while instrument phase angle 

measurement errors were typically within the range of ±0.87 to ±17.45 mrad.

Sample preparation depended on the degree of lithification. The sample holders used for 

sediments, both assembled and disassembled, are shown in figure 2. These parallel-plate sample holders 

consist of two platinum foil electrodes with a clear Plexiglas cylinder sandwiched in between. The 

cylinders are 5.08 cm (2.00 in.) in length with an inner diameter of 4.45 cm (1.75 in.). The current 

electrodes are 0.0127-cm (0.005-in.) thick, bright platinum foil. Each electrode is backed with a 0.32-cm

(1⁄8-in.) thick, hard rubber pad, which, in turn, is backed with a rigid, 1.27-cm (1⁄2-in.) thick, plastic tile.

The entire sample holder assembly is held together with a bar clamp. To ease the filling of the sample 

holder, an electrode is glued to one end of the Plexiglas cylinder with a silicon rubber adhesive. Fixing 

the electrode and the Plexiglas cylinder together also allows the sample holder to be watertight until the 

clamp can apply pressure.

Figure 2. Photographs showing the sample holder used for sediments. A, Disassembled sediment sample 

holder. B, Assembled sediment sample holder (empty) attached to the LCR (inductance, capacitance, resistance) 

test leads.

Hard-rock samples were slabbed prior to measurement to obtain two parallel surfaces. Each slab 

was cut to an approximate thickness of 1.91 cm (3⁄4 in.). The electrodes were a 0.32-cm (1⁄8-in.) thick,

hard rubber pad wrapped in aluminum foil, which was, in turn, backed with a rigid, 1.27-cm (1⁄2-in.)

thick, plastic tile. The rock slab and electrodes were held together with a bar clamp. The sample clamp

disassembled and assembled is shown in figure 3.
6

Figure 3. Photographs showing the sample holder used for hard rocks. A, Disassembled rock sample clamp 

(with sample slabbed rock). B, Assembled rock sample clamp (with sample slabbed rock) attached to the LCR 

(inductance, capacitance, resistance) test leads.

Geometrical factors were calculated by measuring sample dimensions (thickness and crosssectional areas for slabs, diameter and height of sample holder for sediments) using a dial caliper with

0.0254-millimeter (0.001-in.) resolution and by visually verifying complete contact between each of the 

sample’s areas and their respective electrodes. The cross-sectional areas were calculated by digitally 

scanning the rocks at 300 pixels-per-inch, counting the pixels, and multiplying by pixel area (1/90000 

square inches). The transparent Plexiglas sample holder permits visual inspection of the sampleelectrode contact.

Measurements

Sediment samples were typically measured twice: once in as-received (AR) condition, and once 

after deionized water was added (WA) to saturate the sample. This AR to WA measurement series often

produces a broad range of ρ and φ values, indicating the degree to which electrical properties vary as a 

function of water content. Surface sediment samples were generally received very dry, whereas the core 

sediment samples were generally received fully saturated. Many samples from the core sediment sample 

group could only be measured in their AR state due to an inability to add more water. For the AR

measurement, the sample holder was filled and packed with a representative sample from the sample 

container. For the samples that were able to have water added to them, a WA measurement was 

conducted by adding deionized water to a sample already mounted in the sample holder. To increase the 

amount of water imbibed by the sample, the wetted sample was placed in a vacuum jar and the vacuum 

pressure cycled from 0 to 15 pounds-per-square-inch (psi) several times. The process of adding water

and applying a vacuum was repeated until the sample was visibly saturated and transient effects

(bubbling, swelling) had stopped. Starting from the sample’s AR state, the volume of water added to the 

sample was recorded by weight, providing a minimum estimate of sample porosity. Many of the 

samples expanded when water was added, requiring removal of material from the sample holder, thus

further skewing the porosity estimates. The minimum porosity estimates are expected to significantly 

underestimate the true porosity of the core samples, but are thought to be comparable to the true 

porosity for all other samples.
7

The slabbed rock samples were generally measured four times: (1) once in their AR condition,

(2) once fully dried in an oven at 100 degrees celcius (°C) while under a vacuum at 15 psi (dry 

measurement), (3) once after extensive exposure to a 100-percent relative humidity environment (humid 

measurement), and (4) after being submersed in deionized water with all air removed under vacuum

(submerged measurement). For the surface samples, the AR measurements were omitted, owing to 

environmental changes during the time between collection and processing. In general, the AR

measurements typically have resistivity values that fall in between the dry and humid measurements.

Impedance measurements were converted to resistance by the LCR meter, which took into 

account the effects of capacitance and inductance; both resistance and impedance datasets were saved. 

Resistance data were converted to resistivity using a geometrical factor (K) based on the effective area 

(A) of the electrodes and the length or thickness (L) of the sample holder or slabbed rock. Note that 

errors in K contribute to errors in ρ values; a 1-percent error in the geometrical factor produces a 1-

percent error in the resistivity value.

ρ = KK, where (2)

K = �A1A2

L (3)

and K is the sample’s geometrical factor in meters,

A1,2 are the sample's effective cross-sectional areas in contact with each electrode in 

square meters,

L is thickness or height of the sample in meters,

ρ is resistivity in Ω·m, and

is the resistance, as reported, in ohms.

Sources of Error

The LCR meter has an upper and lower limit to its measurement range. Occasionally, a sample 

was found to be too resistive to measure given the finite input impedance of the LCR meter. Typically, 

this is observed when the resistance of the sample exceeds 19.999 megaohms. Where observed, these 

measurements are flagged as -99999 in the supplemental data. Electrode impedance effects at the 

electrode-sample interface may further complicate two-electrode impedance and resistance 

measurements. These effects are most prominent at low frequencies, where they tend to increase the 

observed impedance amplitude measurement and alter the impedance phase, ultimately altering the 

resistance measurement. The effect on impedance is most pronounced in dry, resistive samples, whereas 

distortion in the phase measurement, associated with electrode polarization, is strongest in wet,

conductive samples. These effects can often be observed at low frequencies on a complex impedance

plot, which is discussed further in the next section. 

Representing Complex Resistivity

Complex impedance plots allow us to dissect the complex, frequency-dependent impedance into 

its component pieces. Simplified electronic circuits (for example, fig. 4) can model the complex 

impedance of many earth materials.
8

 Zb is the bulk impedance element of the sample,

Cdd is the double layer capacitor,

Zcc is the charge transfer impedance element of the sample, and

W is the Warburg impedance element of the sample (inversely proportional to �f).

Figure 4. Simple frequency-dependant circuit diagram representative of many geologic samples. Equivalent 

circuit models exist that would produce the same complex impedance versus frequency behavior. At low 

frequencies (f), the capacitor is open, and the primary current path is through Zct and W. At high f, the capacitor 

is shorted, and the primary current path is through Cdl.

The impedance element, Zb, represents the frequency-independent sample impedance and is an 

intrinsic property dependent upon the type and number density of charge carriers and their ionic 

mobilities. The two parallel branches of the circuit represent two different paths by which current can be 

carried across the sample-electrode contact. A faradaic conduction path involves electrochemical charge 

transfer via a combination of oxidation and reduction and ionic diffusion across the interface. Current 

can also be passed via a non-faradaic, electrokinetic path associated with the capacitance of the fixed 

and diffuse layers at the sample-electrode interface. The electrokinetic conduction pathway is much 

faster and tends to carry more of the current at high frequencies compared to the faradaic conduction 

pathway. This electrokinetic pathway is represented by the double-layer capacitor, Cdl, in figure 4. The 

faradaic conduction path contains both a charge transfer impedance, Zct, associated with a kinetically

controlled charge transfer mechanism, and the Warburg impedance, W, associated with ionic diffusion 

across the interface (Flekkøy, 2013). The Warburg impedance is unique in that it cannot be represented 

by a traditional circuit element. Its magnitude varies inversely with �f, and it has a constant phase of π

4

radians; that is, it approximates a straight line at low frequencies on a complex impedance plot (fig. 5).
9

Figure 5. Complex impedance plots of the Mio4b surface sediment sample, where Z" is the imaginary 

component and Z' is the real component. The Warburg impedance (the polarization effect – red dashed line) 

dominates at low frequencies (100 Hz), while the complex impedance arcs (blue dashed line) represent different 

conduction mechanisms within the sample.

The correspondence between the complex impedance plots in figure 5 and the equivalent circuit 

in figure 4 can be considered as follows. At high frequencies (100 kHz), the capacitor (Cdl in fig. 4) is 
10

shorted, and the complex impedance becomes purely a function of the capacitor and the bulk impedance 

element (Cdl and Zb in fig. 4). As the input frequency is lowered, the capacitor’s ability to transmit 

current is reduced, allowing current to flow through the other impedance elements and traces out the arc 

in the complex impedance plots (dashed blue lines in fig. 5). At very low frequencies, the capacitor (Cdl

in fig. 4) is open, forcing current to pass solely through the three impedance elements (Zb, Zct, and W in

fig. 4), including the constant-phase Warburg impedance, the influence of which dominates at the 

lowest frequencies (100 Hz). Samples with additional mechanisms for ionic charge transfer (conduction 

mechanisms) can be described by additional Zct and Cdl circuit elements and can exhibit multiple arcs in 

a complex impedance plot. In closing, we note that the intercepts of the complex impedance arc with the 

real axis are of special significance—the low frequency intercept defines the direct-current (DC), or 0 

Hz, sample resistance, while the width of the semicircle is related to the frequency dependence of the

dielectric constant of the sample, and the high frequency intercept relates to the Zb of the sample. When 

relating laboratory measurements to field measurements, the lower frequency ρ measurements often 

reflect both sample and electrode effects. At higher frequencies, the real impedance component (Z’) 

measurements are most comparable to field ρ measurements. 

One other aspect of evaluating complex impedance data comes from evaluating the phase 

relationship with the measured ρ. A peak in the phase often corresponds with the greatest slope on a 

resistivity versus frequency plot. Many of our dry rock samples exhibited relatively high, yet consistent,

phase values. When phase is plotted against frequency (a Bode phase plot), a flat curve suggests the 

conduction mode is largely frequency independent, while a phase peak suggests the center frequency of 

one conduction mechanism. Owing to the narrow bandwith of the measured frequencies, we were 

unable to identify many conduction mechanism bandwidths. Phase data varies with the degree of 

saturation of a sample, indicating that the modes of conduction are also related to water content (for 

example, fig. 6). We present only one example in figure 6; however, all phase data are released in the 

supplemental data.
11

Figure 6. Bode phase plots of the three states of saturation for surface rock sample Crash-4a-weld_R. Plots 

presented, from top down, are dry state, humid state, and submerged state. Resistivity data (blue curve) align with 

the left axes, and phase data (green curves) align with the right axes.

Results

We grouped the samples into the following categories, also reflected in the supplemental data:

core sediments, core rocks, surface sediments, and surface rocks. Core rocks consist of competent 

material recovered in the drill core and are typically either volcanic flows or large clasts, often 

crystalline, within sedimentary units. (See supplemental data [SupplementA_CoreRock_table.xlsx,

SupplementC_SurfaceRock_table.xlsx] for delineation between clast or in-place rock as well as a brief 

petrologic description.) Surface rocks span the gamut of intrusive and extrusive igneous rocks as well as 

consolidated sediments found throughout the region. Some samples were collected directly from 

outcrops, while others have likely been transported, as float, some distance from their source. The latter 

are believed to be representative of regionally extensive basement rock units that do not outcrop within 

the survey area. Although the sediment provenance may lie outside the immediate study area, surface 
12

and core sediment samples are assumed to be deposited in place. The sediment samples reflect a range 

of depositional environments but are primarily alluvial or lacustrine in origin; a single sample (GWdis)

was collected from a spatially limited groundwater-discharge deposit.

Representative resistivity-frequency plots for core sediment and rock samples are shown in 

figures 7 and 8, respectively. Resistivity-frequency plots for all samples are provided in appendixes A, 

B, C, and D. In general, and as expected, lower sample resistivities were observed with water added

compared to AR or dry conditions. For core sediment samples, however, this variation in resistivity is 

less pronounced, reflecting the wet condition of the AR samples and, as discussed later, the presence of 

clay. Sample resistivity tends to either decrease or remain constant as frequency increases. The strongest 

frequency dependence was observed in the rock samples, which tend to be quite resistive.

 

Figure 7. Graph showing resistivity versus frequency for core sediment sample CRTH1-2c-2. Triangles pointing 

up indicate the sample was taken from the top of the core segment, whereas triangles pointing down indicate the 

sample was taken from the bottom of the core segment.
13

 

Figure 8. Graph showing resistivity versus frequency for core rock sample GOLD2-2c-1T_R.

Considering the sample set as a whole, a broad separation exists between electrically resistive 

rock samples (including volcanic flows encountered in the core samples) and conductive sediments

(from both surface or core samples). Based on the suite of measured rock samples (disregarding data 

points that exhibit Warburg effects), all of the rock types that floor the basins of the NTC were observed 

to be electrically resistive (here considered to be resistivities greater than about 100 Ω·m). In actuality, 

all of the plutonic and metamorphic rocks and lava flows (from both surface and core samples), which 

includes most of the rock or clast samples, have resistivities around or above 1,000 Ω·m, the exception 

being altered volcanic rock encountered in boreholes in the Goldstone Basin (fig. 1). The most resistive 

basement rock was a diorite with measured resistivity on the order of 100,000 Ω·m. These results are 

consistent with an airborne electromagnetic survey in Leach Lake Basin (Bedrosian and others, this 

volume, chap. G), where the resistivity of all basement rock types was found to be quite high (greater 

than 200 Ω·m), with the exception of altered mafic volcanic rocks (20–100 Ω·m).

By and large, the sediment samples are fairly conductive, typically with resistivities less than 

100 Ω·m and often closer to 10 Ω·m. Examining the core sediment resistivities in aggregate reveals no 

clear trends; however, the majority of the boreholes show core resistivities of around 10 Ω·m at all 

sample depths. Several boreholes (CCT1, GOLD2, LL04, NELT2, NELT3, and NELT7) have a

relatively shallow core sample and show resistivity decreasing with depth. In these boreholes, the 

resistivity of the near-surface samples is typically several tens of Ω·m, transitioning at depth to 
14

resistivities of about 10 Ω·m. An example of core resistivity values in comparison to additional data is 

shown in figure 10; all core resistivity comparisons are included in appendix E.

A resistivity separation, though subtle, is evident in histograms of log resistivity for the asreceived and saturated core samples (assuming that if a WA measurement was not made, then the 

sample was received saturated), where two peaks are suggested (fig. 9). Groups centered around 13 Ω·m

and 23 Ω·m are consistent between the two histograms, suggesting that ionic conduction through pore 

fluids may not be the dominant conductivity mechanism. Recall, however, that most of the core samples 

were received saturated; hence, the difference in water content between as-received and saturated 

samples may be quite small. A small number of samples were also observed to have resistivities 

between 40 and 150 Ω·m.

 

Figure 9. Histograms showing resistivity of all core sediment samples. Top panel represents the as-received 

state; second panel represents the saturated state (assuming that if a water-added measurement was not 

conducted that the sample was received fully saturated). Note: the resistivity values are binned and plotted 

logarithmically; values presented are in ohm-meters.
15

The suite of surface sediment samples are believed to be representative of older 

lithostratigraphic units exposed in the eastern half of the NTC. The samples collected, however, are 

from isolated exposures and have not been definitively correlated with a particular lithostratigraphic 

unit. When saturated, the majority of these samples are characterized by resistivities below 10 Ω·m and 

in several cases as low as 1 Ω·m. The low resistivity of these surface sediment samples suggest they 

may be correlative with the more conductive, and generally deeper, group of core sediment samples. 

The exposures from which these samples were collected may be part of a regional and highly 

conductive stratigraphic unit identified in resistivity models from both ground and airborne time-domain 

electromagnetic (TEM) measurements (Bedrosian and others, this volume, chap. G; Burgess and 

Bedrosian, this volume, chap. F).

Discussion

Origin of High Conductivity

Measurements on the core sediments show the majority of the samples to be rather conductive, 

regardless of the degree of saturation. This suggests that sample conductivity may be governed 

primarily by clay content. To evaluate this possibility, four core sediment samples from three boreholes

were selected for quantitative analysis using X-ray diffraction (XRD) to identify the mineralogy and 

clay content of the samples. Samples analyzed include NELT1-3c-1_top, NELT2-1c-2_top, NELT2-2c1b_bottom, and NELT3-3c-1_top. Three of the samples (the exception being NELT2-1c-2, the 

shallowest of the samples) have measured resistivities below 20 Ω·m. These three conductive samples 

were found to have both a high clay fraction (14–26 weight percent) and a high, yet variable, percentage 

of amorphous glass (6–29 weight percent). In contrast, the less conductive sample has 9 weight percent

clay content and 8 weight percent amorphous glass. Further analysis found the clay fraction in all 

samples to be almost entirely composed of expandable smectite, with trace amounts of kaolinite and 

mica. Drilling mud contains bentonite, a smectite clay, which raises the question of whether the 

expandable smectite is directly associated with the sediments or is simply the result of infiltration of 

drilling mud into the core samples. This question led to the collection and analysis of the surface 

sediment samples, where the concern of drilling mud contamination is absent. As discussed previously, 

laboratory resistivity measurements reveal that surface sediments have comparable resistivities to the 

drill core sediments when saturated. The comparable resistivity values between these two sample sets 

suggests, but does not prove, that the older surface sediments contain a significant clay fraction and that

the clay in the drill core samples is not solely the product of drilling contamination.

An additional XRD analysis on a suite of surface samples was also conducted on four additional 

surface sediment samples and one rock sample (Crash2a [sediment], Mio3a [sediment], Mio5b

[sediment], Whale-Sed [sediment], and Crash4a-weld-R [rock]) as well as a sample of the drilling mud 

from borehole NELT6 to evaluate clay and amorphous glass content. These five surface samples came 

from three distinct areas (fig. 1): Nelson Lake (Crash samples), Central Corridor (Mio samples), and just 

north of Cronise Basin (Whale sample). The results from these four sediment samples show high clay 

content, varying from 10 to 45 weight percent, and amorphous glass content ranging from 8 to 27 

weight percent. The amorphous glass and clay contents are in the same range as the three core sediment 

samples from the Nelson Lake area. Although this indicates that the high clay content in the core 

samples is inherent to the basin sediments, three of the four core sediment samples (NELT1-3c-1, 

NELT2-1c-2, and NELT3-3c-1) have an XRD signature that is similar to the drilling mud’s XRD 

signature. Because of this, we cannot rule out the possibility of drilling mud contamination. However,
16

the content of clay and amorphous glass is consistent between the core and surface samples, and we 

believe that the core resistivity values are representative of the host lithologies.

We speculate that the three conductive, clay-rich core samples (NELT1-3c-1, NELT2-2c-1b, and 

NELT3-3c-1) are Tertiary. Local and regional volcanism during the Miocene is the most likely source 

of amorphous glass in the sediments, and we hypothesize that some of this glass, in the presence of 

water, has been chemically weathered to smectite. The reaction of glass with water of moderate pH and 

low ionic strength is known to produce clay, predominantly smectite, as a secondary alteration product 

(Vaniman, 2006). Studies by Giorgetti and others (2006, 2009) find volcanic glass within felsic and 

mafic volcanic rocks to be more susceptible to low-temperature hydrothermal alteration than the 

crystalline phases. The less conductive core sediment sample, which is also the shallowest of the four

samples, is interpreted to be Quaternary. The lower deeper samples’ clay and amorphous glass fractions 

reflect a regional decrease in volcanism, hence a decrease in volcanic source material, following the 

Miocene.

Correlation with Ground TEM Models

Twelve of the fourteen boreholes where core samples were collected are close to TEM 

resistivity sounding locations. Names and locations for boreholes, nearest TEM sounding, and the 

distance from each borehole to the nearest TEM sounding are shown in table 1 (NELT3 and NELT6 

both had two TEM soundings nearby). This provides an opportunity to compare laboratory 

measurements to geophysical resistivity models and borehole resistivity logs (Kjos and others, 2014; 

Burgess and Bedrosian, this volume, chap. F). A sample figure showing the resistivity correlation 

between these three datasets for borehole LL04 is shown in figure 10; comparison plots for the 

remaining boreholes are presented in appendix E. For all comparisons, both AR and WA values are 

shown; if multiple samples were measured within a cored interval (typically one sample from the top

and one sample from the bottom of core, a distance of ~0.3 m), top and bottom resistivity values are 

shown separately. Overall, the laboratory resistivity data match the TEM models quite well.

Table 1. Borehole locations (Kjos and others, 2014), nearest time-domain electromagnetic (TEM) sounding 

locations (Burgess and Bedrosian, this volume, chap. F), and distance in meters (m) between boreholes (BH) and 

nearest TEM soundings.

[UTM, Universal Transverse Mercator projection, zone 11; BH, borehole; TEM, time-domain electromagnetics; m, meter]

Borehole

(BH) BH UTM Easting BH UTM Northing TEM sounding TEM UTM Easting TEM UTM Northing Distance, in meters

BLA5 533775 3903859 BL6 534151 3904739 957

CCT1 534643 3913459 CC1 534773 3913921 480

CRTH1 549819 3893511 CR6 548720 3893207 1140

CRTH2 548135 3899568 CR2 547852 3899664 299

GOLD1 511457 3913224 AU3 510965 3913383 518

GOLD2* 511489 3913255 -- -- -- --

LL04 534331 3890648 LN2 534206 3890727 147

NELT1 522429 3920367 NL03 521700 3920438 733

NELT2 526952 3919008 NL14 526969 3919264 257

NELT3 518265 3921007 NL11 518237 3921137 133

NELT3 518265 3921007 NL20 518236 3920854 156

NELT4 528440 3917291 NL13 528649 3916821 514
17

Borehole

(BH) BH UTM Easting BH UTM Northing TEM sounding TEM UTM Easting TEM UTM Northing Distance, in meters

NELT5* 514184 3920199 -- -- -- --

NELT6 518661 3918538 NL04 518576 3918340 215

NELT6 518661 3918538 NL30 518499 3918431 194

NELT7 520618 3925021 NL18 521343 3925173 741

*

Boreholes GOLD2 and NELT5 did not have nearby TEM soundings.

 

Figure 10. Graph showing resistivity comparison for samples from borehole LL04, time-domain electromagnetic 

(TEM) sounding LN2, and long-normal borehole resistivity log.
18

Several boreholes encountered lava flows within the sedimentary section. Typically, the lava 

flows are a few meters to tens of meters thick, and core samples from these flows were of moderate to 

high electrical resistivity, the exception being altered volcanic rocks within the Goldstone Basin cores. 

The generally resistive volcanic flows are not reflected in the low to moderate resistivities of the TEM 

models (for example, CRTH2). This discrepancy reflects the sensitivity and resolution of the TEM 

method, which is insensitive to thin, horizontal resistors. As a rule of thumb, a resistive layer must have 

a thickness comparable to its depth in order to be resolved by a surface TEM sounding (Orange, 1989). 

For example, the electrically resistive volcanic flow encountered at 293-m depth in CRTH2 would need 

to have a thickness of 293 m or more in order to be resolved by a TEM sounding. Forward modeling has 

verified that surface TEM data are consistent with, yet insensitive to, the volcanic flows or dacite 

encountered in boreholes CRTH2 (Burgess and Bedrosian, chapter F, this volume), GOLD1, and 

NELT2 (see appendix E; Kjos and others, 2014).

In addition to volcanic flows encountered in higher cored intervals, the deepest cored interval in 

five boreholes (NELT4, GOLD2, GOLD1, CRTH2, NELT7) contained igneous rock or, at the very 

least, clasts of igneous parent material. According to Kjos and others (2014), borehole NELT4 

encountered crystalline gneissic bedrock, and the borehole interval containing GOLD2-3c and GOLD2-

4c is interpreted as weathered and fractured bedrock. Boreholes GOLD1 and CRTH2 both terminate in 

extrusive igneous material (possible interbasin flows as opposed to bedrock), while borehole NELT7 

encountered only igneous clasts within the deepest cored interval.

Based on the core, NELT4 encountered basement at a minimum of 265 m. Gravity-based 

estimates of depth to basement (Jachens and Langenheim, this volume, chap. H) are in close agreement 

and suggest a depth to basement of 250 m. The deep cores from GOLD2 are more variable: Rock 

samples in the top and bottom of GOLD2-3c-1 (GOLD2-3c-1T, GOLD2-3c-1B; 122-m depth) are

coarse-grained clasts sourced from an intrusive body, and the segment at the bottom of the borehole

(GOLD2-4c-1; 137-m depth) is a clast of very fine grained metamorphic rock. (Kjos and others, 2014,

list this clast as aphanitic basalt; however, our evaluation suggests metamorphic. See 

SupplementA_CoreRock_table.xlsx for a complete description.) The corresponding gravity-based 

basement estimate of 375 m is significantly greater at this location.

Conclusions

Laboratory resistivity measurements provide a means to link geophysical resistivity models to 

geology and lithology. Measurements were carried out on 54 core samples from 14 boreholes in 

addition to 57 surface samples within the Fort Irwin NTC. The samples span a range of sediment 

lithologies and rock samples which represent the major basement rock types flooring the basins of the 

NTC. All of the basement rock types were found to be resistive, typically greater than 1,000 Ω·m, 

whereas sediment samples were typically less than 50 Ω·m. Surface and borehole samples are 

subdivided into two broad groups: moderately conductive Quaternary sediments with low clay content 

and minimal volcanic input and highly conductive, older (Tertiary?) sediments with higher clay content 

and significant volcanic input. Excellent correlation is found between TEM resistivity models and core 

resistivity samples, further supporting a prominent geophysical distinction in resistivity between 

Quaternary and Tertiary sediments.
19

References Cited

Bedrosian, P.A., Ball, L.B., and Bloss, B.R., 2014, Airborne electromagnetic data and processing within 

Leach Lake Basin, Fort Irwin, California, chap. G of Buesch, D.C., ed., Geology and geophysics 

applied to groundwater hydrology at Fort Irwin, California: U.S. Geological Survey Open-File Report 

2013–1024 (this volume).

Burgess, M.K., and Bedrosian, P.A., 2014, Time-domain electromagnetic surveys at Fort Irwin, San 

Bernardino County, California, 2010–2012, chap. F of Buesch, D.C., ed., Geology and geophysics 

applied to groundwater hydrology at Fort Irwin, California: U.S. Geological Survey Open-File Report 

2013–1024 (this volume).

Flekkøy, E.G., 2013, A physical basis for the Cole-Cole description of electrical conductivity of 

mineralized porous media: Geophysics, v. 78, no. 5, p. D355–D368.

Giorgetti, G., Monecke, T., Kleeberg, R., and Hannington, M.D., 2006, Low-temperature hydrothermal 

alteration of silicic glass at the PACMANUS hydrothermal vent field, Manus Basin—an XRD, SEM, 

and AEM-TEM study: Clays and Clay Minerals, v. 54, no. 2, p. 240–251.

Giorgetti, G., Monecke, T., Kleeberg, R., and Hannington, M.D., 2009, Low-temperature hydrothermal 

alteration of trachybasalt at Conical Seamount, Papua New Guinea—formation of smectite and 

metastable precursor phases: Clays and Clay Minerals, v. 57, no. 6, p. 725–741.

Jachens, R.C., and Langenheim, V.E., 2014, Gravity survey and interpretation of Fort Irwin and 

vicinity, Mojave Desert, California, chap. H of Buesch, D.C., ed., Geology and geophysics applied to 

groundwater hydrology at Fort Irwin, California: U.S. Geological Survey Open-File Report 2013–

1024 (this volume).

Kjos, A.R., Densmore, J.N., Nawikas, J.M., and Brown, A.A., 2014, Construction, water-level, and 

water-quality data for multiple-well monitoring sites and test wells, Fort Irwin National Training 

Center, San Bernardino County, California, 2009–12: U.S. Geological Survey Data Series 788, 139 p.,

https://dx.doi.org/10.3133/ds788.

Miller, D.M., Menges, C.M., and Lidke, D.J., 2014, Generalized surficial geologic map of the Fort 

Irwin area, San Bernardino County, California, chap. B of Buesch, D.C., ed., Geology and geophysics 

applied to groundwater hydrology at Fort Irwin, California: U.S. Geological Survey Open-File Report 

2013–1024 (this volume).

Miller, D.M., and Yount, J.L., 2002, Late Cenozoic tectonic evolution of the north-central Mojave 

Desert inferred from fault history and physiographic evolution of the Fort Irwin area, California: 

Geological Society of America Memoir, v. 195, p. 173–197.

Orange, A.S., 1989, Magnetotelluric exploration for hydrocarbons: Proceedings of the Institute of 

Electrical and Electronics Engineers, v. 77, no. 2, p. 287–317, doi: 10.1109/5.18627.

Vaniman, David, 2006, Tuff mineralogy, chap. 2.2 of Heiken, G., ed., Tuffs—their properties, uses, 

hydrology, and resources: Geological Society of America Special Papers, v. 408, p. 11–15, doi: 

10.1130/2006.2408(2.2).
20

Appendix A. Core Rock Resistivity Versus Frequency

Figures, one per sample, showing sample resistivity, in ohm-meters, versus frequency, in hertz. Each 

curve represents a different degree of saturation.
36
37
38
39
40
41
42
43
44
45
46

Appendix B. Core Sediment Resistivity Versus Frequency

Figures, one per sample, showing sample resistivity, in ohm-meters, versus frequency, in hertz.

Each curve represents a different degree of saturation.
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
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Appendix C. Surface Rock Resistivity Versus Frequency

Figures, one per sample, showing sample resistivity, in ohm-meters, versus frequency, in hertz.

Each curve represents a different degree of saturation.
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66
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68
69
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71
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73
74
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Appendix D. Surface Sediment Resistivity Versus Frequency

Figures, one per sample, showing sample resistivity, in ohm-meters, versus frequency, in hertz.

Each curve represents a different degree of saturation.
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88
89
90
91
92
93
94
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Appendix E. Laboratory Borehole Resistivity Comparison to TEM and Downhole 

Resistivity Logs

Figures showing consolidated and unconsolidated core sample resistivity data, in ohm-meters, in 

comparison to the nearest inverted TEM resistivity sounding and long-normal resistivity borehole data. 

Depth is relative to the land surface.
96
97
98
99
100
101
102
103

Supplemental Data

Data

The supplemental data include frequency-dependent resistivity, complex impedance magnitude, 

phase, and real and imaginary components of impedance. Frequency is specified in hertz, resistivity is 

specified in ohm-meters, phase is specified in radians, and all impedance values (magnitude, real, and 

imaginary components) are specified in ohms.

All of the processed data are presented in one of four spreadsheets in the supplemental data. The 

four spreadsheets have been broken up based on the origin and type of sample: core rocks

(SupplementA_CoreRock_spreadsheet.xlsx), core sediments (SupplementB_CoreSed_spreadsheet.xlsx),

surface rocks (SupplementC_SurfaceRock_spreadsheet.xlsx), and surface sediments

(SupplementD_SurfaceSed_spreadsheet.xlsx). In addition to the spreadsheets, there is a table for each 

group that contains sample locations and, for the rock samples, a brief petrologic description 

(SupplementA_CoreRock_table.xlsx, SupplementB_CoreSed_ table.xlsx, SupplementC_SurfaceRock_

table.xlsx, and SupplementD_SurfaceSed_ table.xlsx). Appendixes A, B, C, and D present each of these 

four groups and show the data on a resistivity versus frequency graph, one for each core segment or each 

sample.

The core-rock spreadsheet (SupplementA_CoreRock_spreadsheet.xlsx) presents the data 

separated out by borehole and then by sample. The data presented include as-received, dry, humid, and 

submerged resistivities; impedance; phase; and real and imaginary components of the complex 

impedance at each frequency.

The core-sediment spreadsheet (SupplementB_CoreSed_spreadsheet.xlsx) presents the data 

separated out by borehole and then by sample. The data presented include as-received and water-added 

resistivities; impedance; phase; and real and imaginary components of the complex impedance at each 

frequency. For each core segment, measurements are separated into top and bottom samples.

The surface-rock spreadsheet (SupplementC_SurfaceRock_spreadsheet.xlsx) presents the data 

separated by sample and, in a few cases, by subsample (for example, sample LAIR3 was slabbed twice, 

and two subsamples, LAIR3-a and LAIR3-b, were created). The data presented includes dry, humid, and 

submerged resistivities; impedance; phase; and real and imaginary components of the complex 

impedance at each frequency.

The surface-sediment spreadsheet (SupplementD_SurfaceSed_spreadsheet.xlsx) presents the 

data separated by sample. The data presented include as-received and water-added resistivities;

impedance; phase; and real and imaginary components of the complex impedance at each frequency.

Additionally, appendix E compares the borehole geophysical logs to the laboratory 

measurements and time-domain soundings, one plot for each borehole.

Supplement A

CoreRock table (coordinates and description of samples) and CoreRock spreadsheet (containing all 

data). See FtIrwin_README.doc (in supplemental data folder) for additional details.

Supplement B

CoreSed table (coordinates and depth of samples) and CoreSed spreadsheet (containing all data). See 

FtIrwin_README.doc (in supplemental data folder) for additional details.
104

Supplement C

SurfaceRock table (coordinates and description of samples) and SurfaceRock spreadsheet (containing all 

data). See FtIrwin_README.doc (in supplemental data folder) for additional details.

Supplement D

SurfaceSed table (coordinates of samples) and SurfaceSed spreadsheet (containing all data). See 

FtIrwin_README.doc (in supplemental data folder) for additional details.