OSL dating of a lacustrine to fluvial transitional sediment
sequence in Valle Toledo, Valles caldera, New Mexico
Kenneth Lepper, Department of Geosciences, North Dakota State University, Fargo, North Dakota 58105;
Steven L. Reneau, Environmental Geology and Spatial Analysis Group, Los Alamos National Laboratory, Los Alamos, New Mexico 87545;
Jennifer Thorstad, Boone Pickens School of Geology, Oklahoma State University, Stillwater, Oklahoma 74078;
Anne Denton, Computer Sciences Department, North Dakota State University, Fargo, North Dakota 58105
Abstract
Quaternary lacustrine deposits exist within
several valleys in the Valles caldera in northcentral New Mexico. These deposits contain
potentially valuable paleoclimatic records.
We report OSL ages for a section of unconsolidated Quaternary sediments exposed in
the southwest part of Valle Toledo within
Valles caldera. The sequence represents the
transition over time from lacustrine to fluvial
deposition at the site. We present a stratigraphically coherent depositional chronology
for the Valle Toledo section consistent with
the available radiocarbon constraint based on
analysis of bootstrapped dose distributions
derived from data collected by IRSL MAAD
procedures. The ages suggest the existence of
a late Pleistocene lake in Valle Toledo from
at least 48.5 ka to ~44 ka, which is considerably younger than age interpretations based
on correlation to a >500 ka rhyolite dam
across San Antonio Creek. This study helps to
emphasize the need for additional research to
decipher the geologic history of the intra-caldera lakes as well as to correlate the records
of climate and environmental change among
the lacustrine deposits within Valles caldera.
Introduction
The Valles caldera, in the Jemez Mountains
of northern New Mexico (Fig. 1), was formed
at ca. 1.25 Ma following eruption of voluminous ignimbrites of the Tshirege Member of
the Bandelier Tuff (Smith and Bailey 1968;
age from Phillips 2004). After breach of the
caldera rim associated with resurgence at
≥ 1.2 Ma, multiple lakes formed at different times when post-resurgence volcanic
eruptions dammed drainages within the
caldera (Reneau et al. 2007). The lacustrine
FIGURE 1—Digital elevation model (DEM) map of the Valles caldera
showing estimated maximum lake extent (blue) in Valle Grande and along
San Antonio Creek, and other locations mentioned in text. SAC = San
112
deposits in the caldera contain potentially
valuable paleoclimatic records (e.g., Sears
and Clisby 1952; Fawcett et al. 2006), and
accurate geochronologic data are required
to best interpret these records and understand the history of the lakes.
The most extensive outcrops of lacustrine
sediment in the Valles caldera are found
in the northern moat along San Antonio
Creek and its tributaries, as first mapped
by Smith et al. (1970; see also Gardner et
al. 2006 and Goff et al. 2006). Topographic
relations and the distribution of lacustrine sediment are consistent with most
of these deposits being associated with a
lake impounded behind ca. 557 ka rhyolite
flows from San Antonio Mountain (Reneau
et al. 2007; age from Spell and Harrison
1993). The easternmost outcrops along San
Antonio Creek are found in Valle Toledo,
Antonio Creek; VG = Valle Grande; VSA = Valle San Antonio; VT = Valle
Toledo; star indicates OSL sample location. Modified from Reneau et al.
2007; reprinted by permission of the New Mexico Geological Society.
New Mexico Geology
November 2007, Volume 29, Number 4
where lacustrine deposits are conformably
overlain by fluvial deposits (Fig. 2) recording filling of a lake with sediment and the
transition to a terrestrial environment. The
lacustrine and overlying fluvial deposits
are capped by a buried soil and overlain by
thick stream terrace deposits. Fluvial and
lacustrine sediment from Valle Toledo were
sampled for optically stimulated luminescence (OSL) analyses to test the utility of
the OSL method for dating lacustrine sediment in the Valles caldera and to better constrain the lacustrine and fluvial history of
this valley.
Methods
FIGURE 2—Stratigraphic section of lacustrine and overlying fluvial deposits in Valle Toledo, showing 14C and OSL sample locations; upper elevation ~2,645 m (~8,678 ft). Unit 1 is dominated by clay
and is well laminated in the upper half; Unit 2 is dominated by diatomaceous silt; Unit 3 represents
the transition from shallow-water lacustrine deposition to fluvial deposition; Unit 4 represents an
overlying fluvial terrace deposit. From Reneau et al. 2007; reprinted by permission of the New Mexico
Geological Society.
November 2007, Volume 29, Number 4
New Mexico Geology
Four samples for OSL dating were collected from an 11-m (36-ft) section exposed
in a gully in the southwestern part of Valle
Toledo (Fig. 2). The sequence represents
the transition over time from lacustrine to
fluvial depositional processes. Samples VT03 and VT-04, from the lower part of the
sequence, were collected from lacustrine
silty clays. Samples VT-01 and VT-02, from
the upper part of the section, were collected from interstratified very fine to coarse
fluvial sands. These two upper samples
bracket a buried soil, which was also sampled for radiocarbon dating (Fig. 2). VT-01,
above the buried soil, is from the base of
an extensive stream terrace deposit that
records fluvial aggradation in Valle Toledo
after filling of the lake.
Quartz sand in the grain size range from
150 to 250 µm was obtained from the fluvial samples VT-01 and VT-02 using common procedures for luminescence dating
studies, which include wet sieving, digestion of organic matter by H2O2, aggressive treatment with HF acid to etch quartz
grains surfaces and dissolve feldspars,
followed by HCl and Na-pyrophosphate
rinses to remove precipitates and particulates. After drying, the clean sand grains
were attached to stainless steel planchets
for OSL measurements using a non-luminescent medical adhesive. These prepared
sub-samples are referred to as aliquots.
Because sand was not recovered from the
fine-grained lacustrine samples VT-03 and
VT-04, polymineral fine-grained silts were
extracted from all four field samples. Again,
common luminescence dating pretreatment
procedures were employed, which in this
case included HCl and H2O2 treatments to
remove carbonates and digest organic matter followed by Stoke’s Law settling and
centrifugation to isolate the 4–11 µm size
fraction. Measurement aliquots were prepared by evaporation plating.
All measurements and irradiations were
conducted in the Optical Dating and Dosimetry (ODD) Lab at North Dakota State University using a Risø DA-15 automated TL/
OSL reader system. The system is equipped
with a 40 mCi 90Sr/90Y beta-source, a blue
diode array (OSL; 470 ± 30 nm), an infrared
laser diode assembly (IRSL; 830 ± 10 nm),
113
and an EMI model 9235QA PMT. Stimulated luminescence was measured in the UV
emission range (7.5 mm Hoya U-340) for all
data sets in this report.
Data were obtained from the fluvial sands
(150–250 µm) using OSL single aliquot
regenerative dose (SAR) procedures as presented in Murray and Wintle (2000, 2006)
with the minor modification of maintaining
a uniform cut heat and preheat of 160˚C for
10s (Lepper et al. 2000). Dose response calibration was conducted for every aliquot,
and equivalent doses (De) were interpolated by linear local slope approximation. The
resulting De data sets were analyzed keying
in on the shape properties of the distribution as a guide for selection of a sedimentologically appropriate age-representative
dose (Lepper and McKeever 2002; Lepper
2004; Lepper et al. 2007).
Data were collected from the fine-silt
fraction (4–11 µm) of all four field samples
using infrared stimulation and multialiquot additive dose procedures (IRSL
MAAD total-bleach method: Wintle 1997;
Aitken 1998; Forman et al. 2000). Equivalent dose calibration for each sample was
extrapolated using a saturating exponential model applied to a 24 aliquot data set,
which consisted of four aliquots in each
of six treatment groups. The data were
dose normalized following primary IRSL
measurements. Traditional analyses of
MAAD data result in a single De determination extrapolated from the full data set.
So, in contrast to the SAR-based analysis
described above in which each aliquot
yields an independent dose assessment
(De), MAAD analysis requires multiple aliquots for one dose assessment (De). In this
study we have also applied a resampling
or “bootstrapping” technique to analyze
the IRSL MAAD data (Lepper and Denton
2006). Bootstrap resampling, also called
bootstrapping, is a popular technique for
estimating the sampling distribution of a
population parameter, such as in this case
De. We derive resampling distributions by
drawing from the distribution of experimental data. “Sampling with replacement”
is used, meaning that the same experimental value can contribute to the De calculation multiple times. One De value is derived
for each set of resampled data points. The
resampling approach allows tens of thousands of equivalent doses to be determined
from the MAAD data sets. These MAADderived De distributions can then be statistically interrogated in the same manner as
SAR dose distributions.
Dose rates for samples in this investigation were determined from elemental
concentrations of potassium (K), rubidium
(Rb), thorium (Th), and uranium (U) by
the method presented by Aitken (1998).
Elemental analysis was obtained via
instrumental neutron activation (INAA)
at the Ohio State University research reactor (Table 1). The cosmic ray dose at depth
was calculated using the equations of
114
TABLE 1—Results of INAA elemental analysis for the dosimetrically significant elements K, Rb, Th,
and U as well as estimated water content1.
Sample
ID
VT-01
K (ppm)
Rb (ppm)
Th (ppm)
U (ppm)
42,751 ± 5,481
156.42 ± 11.16
23.98 ± 2.53
7.13 ± 0.82
H2O1 (%)
8±3
VT-02
33,531 ± 4,318
162.49 ± 11.89
22.20 ± 1.95
8.17 ± 0.94
8±3
VT-03
15,334 ± 2,215
123.61 ± 8.60
24.38 ± 2.28
7.52 ± 0.86
10 ± 3
VT-04
20,859 ± 3,011
140.03 ± 10.25
25.23 ± 2.30
9.91 ± 1.04
15 ± 3
1Pore
water content estimated in the field based on sediment texture, consistency, and landscape
position.
Prescott and Hutton (1988, 1994), taking
into account a hard component dose rate
of 0.30 Gy/ka in the Jemez Mountains. An
alpha-efficiency value of 0.055 was used
for all fine-grained silt-age calculations in
this investigation based on the average of
measured values from past dating studies
using fine silts in the area (Berger 1999; S.
Forman and J. Pierson unpubl. data). The
average pore water content of the samples
over their burial lifetime was estimated in
the field based on sediment texture, consistence, and landscape position (Table 1).
The relevance of sediment water content to
dosimetric calculations is discussed in Aitken (1998).
Radiocarbon dating
A single sample consisting of multiple small
charcoal fragments was collected from the
buried soil within the Valle Toledo section
(Fig. 2) and submitted for accelerator mass
spectrometry (AMS) radiocarbon dating to
Beta Analytic, Inc. (Reneau et al. 2007). The
AMS date, 38,940 ± 440 14C yr b.p. (Beta208305), is close to the limit of radiocarbon
dating and is potentially a minimum-limiting age because of the possibility of contamination with small amounts of younger
carbon. Additional uncertainty exists in
the true age of the sample because this is
beyond the limits of reliable radiocarbon
calibration, and the calibrated calendar
age could be different by several thousand
years. However, the apparent consistency
of this date with OSL analyses, discussed
below, and with soils in the overlying fluvial deposit (Reneau et al. 2007), provisionally indicates that the date is reasonable,
pending further analyses.
Results and discussion
Our initial approach to assigning a depositional chronology to the Valle Toledo
sequence was based on a composite OSL
chronology using OSL SAR ages for the fluvial sediments and traditional IRSL MAAD
ages for the lacustrine sediments (top part
of Table 2; Thorstad et al. 2004). The appeal
of this chronology is that it supports an
increasing depositional rate over time as
might be anticipated as lacustrine deposition is replaced by fluvial deposition during final filling of a lake.
However, this composite chronology has
deficiencies. The SAR ages from the fluvial
sands (13.9 ± 1.3 and 14.9 ± 1.5 ka) are not
in agreement with the radiocarbon date
(38,940 ± 440 14C yr b.p.) from charcoal in
the buried soil found between them. SAR
age underestimates have been reported in
the OSL literature, particularly when using
TABLE 2 —OSL dating results.
Sample
ID
Depth
(m)
Aliquots
measured
Des
calculated
Equivalent
dose (Gy)
Dose rate
(Gy/ka)
Age (ka)
Composite chronology
OSL SAR —dose distribution analysis
VT-011
4.6
90
90
100.8 ± 3.2
7.24 ± 0.55
13.9 ± 1.3
VT-022
5.4
96
96
97.8 ± 4.8
6.55 ± 0.47
14.9 ± 1.5
IRSL MAAD—traditional analysis
VT-033
8.9
24
1
175.5 ± 8.0
6.48 ± 0.43
27.1 ± 2.8
VT-043
10.1
24
1
390.1 ± 4.7
7.46 ± 0.48
52.3 ± 4.8
Single-method chronology
IRSL MAAD—bootstrapped distribution analysis
VT-01
4.6
24
10,000
344.8 ± 12.8
9.08 ± 0.65
38.0 ± 4.0
VT-02
5.4
24
10,000
362.3 ± 5.9
8.40 ± 0.56
43.2 ± 4.1
VT-03
8.9
24
10,000
300.9 ± 17.4
6.48 ± 0.43
46.4 ± 5.1
VT-04
10.1
24
10,000
361.7 ± 11.3
7.46 ± 0.48
48.5 ± 4.7
1leading
edge method; 2mean De; 3cumulative plateau method (Duller 2003)
New Mexico Geology
November 2007, Volume 29, Number 4
Fluvial samples
VT-01
VT-02
100
40
N
60
20
20
200
400
600
800
200
400
600
800
Lacustrine samples
VT-03
60
VT-04
100
radiocarbon age suggest the existence of a
late Pleistocene lake in Valle Toledo. However, these ages also pose a geologic puzzle. Field relations are consistent with the
lacustrine deposits in Valle Toledo being
associated with damming of San Antonio
Creek by ca. 557 ka rhyolite flows from San
Antonio Mountain (Reneau et al. 2007),
therefore the dating results presented here
suggest either a very long lived lake or a
younger damming event. The relatively
thin lacustrine section in Valle Toledo (~34
m) would seem to argue against a lake persisting for >500 k.y., yet no evidence for a
younger dam has been found. Additional
work is warranted to resolve the geologic
history of this basin.
Conclusions
N 40
60
20
20
200
600
1000
Dose (Gy)
200
400
600
Dose (Gy)
FIGURE 3—Dose distributions obtained from bootstrapping analysis of the IRSL MAAD data sets
collected from the polymineral fine-silt sediment fractions of each field sample.
low temperature preheats, such as the
160˚C used in this study (Murray and Wintle 2006). Our own past work in the area
(Lepper et al. 2003; Gardner et al. 2003) also
indicates that quartz sands derived from
the volcanic deposits in this region commonly exhibit signal characteristics, such
as premature saturation, that can preclude
accurate OSL SAR age determinations.
Luminescence dating of polymineral fine
silts has been employed in several studies
in the Jemez volcanic field (e.g., Kelson
et al. 1996; Reneau et al. 1996; McCalpin
2005). Like these earlier studies our alternate depositional chronology is based on
IRSL MAAD measurements of the fine-silt
sediment fraction for all four samples, but
it capitalizes on the power of the bootstrapping technique to generate large De data
sets for more robust analysis. In this study
we have analyzed dose distributions composed of 10,000 unique Des (Fig. 3).
The positively asymmetric dose distributions obtained from the fluvial samples,
VT-01 and VT-02 (Fig. 3), are consistent
with SAR dose distributions from fluvial
sands reported in many studies (e.g., Murray et al. 1995; Olley et al. 1998; Lepper et
al. 2000; Rowland et al. 2005). The dose
distributions obtained from the lacustrine
silts, VT-03 and VT-04, were polymodal
with a distinct subordinate mode at lower
De values (Fig. 3). We hypothesize that this
type of distribution could result from a
small population of grains that were well
reset before deposition and a larger population of grains that were incompletely reset.
One could further speculate that this type
November 2007, Volume 29, Number 4
of distribution would be consistent with
lacustrine depositional processes that yield
laminated sediments containing alternating
layers of sediment grains deposited rapidly
from turbid water (dominant higher dose
population in the De distribution) as well
as sediment grains that settle more slowly out of suspension or that were derived
from eolian input (subordinate lower dose
population in the De distribution). However, the nature of the laminations in the Valle
Toledo section, defined in the field by color
variations, has not been evaluated.
All of the bootstrapped fine-silt IRSL
dose distributions could be adequately
modeled as a combination of two Gaussian
populations. The ages presented in the bottom part of Table 2 are based on the mean
and standard error of the lower dose population. This chronology, based on analysis
of the bootstrapped distributions, is stratigraphically coherent and consistent with
the available 14C date. The strong agreement between the radiocarbon age (38,940
± 440 14C yr b.p.) and the IRSL ages from
the fluvial sediments that bracket it (VT-01,
38.0 ± 4.0 ka and VT-02, 43.2 ± 4.1 ka), were
obtained without correction for anomalous
fading (Huntley and Lamothe 2001; Wintle
1973). Therefore, fading corrections were
not applied to the lacustrine sediment ages
in the lower part of the profile (VT-03, 46.4
± 5.1 ka and VT-04, 48.5 ± 4.7 ka). This chronology for the Valle Toledo sequence offers
increased confidence in all ages because it
does not rely on compositing of multiple
experimental and analytical techniques.
Our OSL analyses and the available
New Mexico Geology
We have developed a coherent depositional chronology for lacustrine and fluvial
deposits in Valle Toledo consistent with
the available radiocarbon constraint based
on analysis of bootstrapped dose distributions derived from data collected by IRSL
MAAD procedures. The bootstrapping
analytical technique affords the benefits of
equivalent dose distributions analysis to
dating of polymineral fine silts and has the
potential to extend the utility of OSL dating
in studies of lacustrine deposits.
Our age results suggest the existence
of a late Pleistocene lake in Valle Toledo,
although such a young lake may be at odds
with interpretations based on correlation to
a rhyolite dam across San Antonio Creek.
These results emphasize the need for additional research to decipher the geologic history of the intra-caldera lakes as well as to
correlate the records of climate and environmental change among the lacustrine
deposits within Valles caldera. OSL dating
can be a valuable member of a suite of dating methods used to provide geochronologic control for these deposits.
Acknowledgments
A portion of this work was supported by
the North Dakota NSF EPSCoR Program
through an Advanced Undergraduate
Research Award (AURA) for J. T. (who
was, at the time, a student at North Dakota
State University) and by the Los Alamos
National Laboratory Community Relations
Office. The authors would like to recognize
Mr. Joe Talnagi of the Ohio State University
Research Reactor for INAA, which was supported by the DOE Reactor Sharing Grant
Program. The authors would also like to
thank Dr. Steve Forman and Mr. James Pierson of the University of Illinois at Chicago
as well as Dr. Glenn Berger of the Desert
Research Institute for providing unpublished and/or limited availability dosimetry data from their previous luminescence
dating research efforts in the region. We also
thank personnel from the Valles Caldera
National Preserve for their support, Paul
115
Drakos and Danny Katzman for their work
on the Valle Toledo lacustrine deposits, Jeff
Heikoop for funding a 14C date, Rick Kelley, Mark Lesh, and Jim Riesterer for figure
preparation, and Emily Schultz-Fellenz for
helpful review comments. Bruce Allen and
an anonymous reviewer also read an earlier version of this manuscript and made
helpful suggestions for its improvement.
This document is approved for unlimited
public release: LA-UR-07-2834.
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November 2007, Volume 29, Number 4