Early Pleistocene incision of the San Juan River, Utah, dated with 26Al and 10Be: Comment and Reply
COMMENT
Thomas C. Hanks
U.S. Geological Survey, MS 977, Menlo Park, California 94025, USA
Robert C. Finkel
Lawrence Livermore National Laboratory, MS L-206, Livermore,
California 94550, USA
Wolkowinsky and Granger (2004) analyze gravels deposited by the
San Juan River near Mexican Hat and Bluff, Utah, for their abundances of
26
Al and 10Be and the ratio 26Al/10Be as a function of depth. Using primarily the Bluff data because of the greater depth of sampled section (11.7 m),
and assuming that this section is a single depositional unit, Wolkowinsky
and Granger find that these gravels were deposited at 1.36 ± 0.20 Ma and
have eroded at a rate of 14 ± 4 m/m.y. From a sample apart from the depth
profile, Wolkowinsky and Granger also determine an “effective surface
exposure age” for the Bluff deposit to be 660 ± 84 ka, assuming no surface
lowering. Finally, Wolkowinsky and Granger determine an average incision rate for the San Juan River since 1.36 Ma to be 110 ± 14 m/m.y.
The Wolkowinsky and Granger “burial age” calculations employ a
χ2-minimization procedure to determine the age of the deposit, its surface
erosion rate, its bulk density, additional surface cover on the deposit, and
the nuclide inheritance of each sample; a manifold of nonunique solutions
should exist in this complex parameter space. Our principal disagreement
with Wolkowinsky and Granger, however, lies in their assumption that the
river gravels at Bluff are a single depositional unit.
Evidence against the one-stage deposition model lies in Figure 3 of
Wolkowinsky and Granger, redrafted here as Figure 1A to exclude, for
clarity, the nondefinitive Mexican Hat data and the Bluff 1.5-m outlier.
In contrast to the Wolkowinsky and Granger model curve calculated with
uniform inheritance, the 26Al/10Be data for both Bluff and Mexican Hat
are indistinguishable from a constant value down to a depth of 4.3 m. The
Wolkowinsky and Granger one-stage model crosses this “vertical line” with
a very different slope, suggesting to us that the material above ~4.3 m is
significantly younger than that below it. Moreover, a significant unconformity exists in the Bluff section at ~4 m depth: coarse, bedded river gravels
lie above it with a massive sand unit below (D.E. Granger, 2004, personal
commun.). Finally, we find the results of Wolkowinsky and Granger to be
inconsistent with their assumption of one-stage deposition. The great variation in the model inheritances found by Wolkowinsky and Granger for the
six Bluff data (a factor of ~20) suggests multiple episodes of deposition of
materials experiencing very different exposure histories. Neither does it
seem likely that at least 31 m of river gravels and sands, about the height of
a ten-story building, were deposited by the San Juan River as a single unit.
Our two-stage depositional model consists of a basal unit deposited
at 1.5 Ma subject to an erosion rate of 16 m/m.y. for the next 0.84 m.y. At
0.66 Ma, the exposure age of the lag deposit dismissed by Wolkowinsky
and Granger, the upper unit is deposited and subjected to an erosion rate
of 18 m/m.y., such that now it is 4.5-m thick. With respect to other parameters used in Wolkowinsky and Granger, we use a density of 1.8 g/cm3
(1.5 g/cm3 in Wolkowinsky and Granger), no added surface material (6.0
cm in Wolkowinsky and Granger) and inherited sample abundances of
10
Be ranging from 80 to 1200 kiloatom/gm SiO2 (55–1000 kiloatom/gm
SiO2 in Wolkowinsky and Granger). These inheritance values are given
for the time of deposition, not for today as presented in Wolkowinsky and
Granger’s Table 2; the Table 2 data have also been corrected for transcription errors (D.E. Granger, 2004, personal commun.). We follow Wolkowinsky and Granger in using the equations of Granger and Muzikar (2001)
to calculate the 10Be and 26Al production from neutron spallation and from
slow and fast muon reactions.
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Figure 1. A: 26Al/10Be ratios. B: 10Be abundances for Bluff river gravels.
Data in A are shown as open circles with error bars. Continuous
curves (dashed—Wolkowinsky and Granger; solid—this study)
use uniform inheritance of 180 kiloatom/gm SiO2. Point-by-point
inheritance calculations: x—Wolkowinsky and Granger; plus
sign—this study. Measured abundance data in B are open circles
with error bars: x—calculated values of Wolkowinsky and Granger;
plus sign—calculated values of this study.
We compare predicted 26Al/10Be ratios and 10Be abundances for both
of these models, together with the Wolkowinsky and Granger data in Figure 1. Figure 1A shows model predictions for both uniform inheritance
(continuous curves) and for point-by-point determinations of sample
nuclide inheritance. For uniform inheritance, the two-stage depositional
model fits the data somewhat better than the one-stage model of Wolkowinsky and Granger. For point-by point inheritance, both models fit the data
reasonably well. The 10Be abundances that correspond to the point-bypoint inheritances are shown for both deposition models in Figure 1B.
We do not claim that the two-stage deposition model presented here
correctly portrays the history of deposition and incision by the San Juan
River at Bluff over the past ~1.5 Ma; indeed, the model inheritance data,
ours as well as Wolkowinsky and Granger’s, suggest multiple stages of
deposition, not just two. Neither do we claim that the two-stage model presented here is the best two-stage model that can be constructed; our exploration of its entire parameter space is preliminary. We do claim, however,
that this two-stage deposition model is a viable alternative to the one-stage
model assumed by Wolkowinsky and Granger. That different models with
different implications can fit the same data set is hardly news in the earth
sciences; even so, we are surprised how poorly constrained burial-age calculations can be if the number of depositional units is itself a variable.
Specifically, the two-stage deposition model allows for the San Juan
River to be depositing river gravels at the Bluff site at least as recently
as 660 ka, ~140 m above its present elevation. The minimum average
incision rate of the San Juan since that time exceeds 200 m/m.y. Finally,
while Wolkowinsky and Granger correctly state that their results are incompatible with the results of Hanks et al. (2001) and Garvin (2004), we
are convinced that it is not the data of Wolkowinsky and Granger that are
incompatible with Hanks et al. (2001) and Garvin (2004), only the assumption of a single-stage deposition model.
ACKNOWLEDGMENTS
We appreciate several communications with D.E. Granger as we
developed the ideas and calculations of this study and the constructive
reviews of it that we received from A. Matmon and R.H. Webb. This work
was supported in part by U.S. Department of Energy/Lawrence Livermore
Laboratory Contract No. W-7405-Eng-48.
REFERENCES CITED
Garvin, C.D., 2004, Long-term differential bedrock incision rates of the Colorado
River in Grand Canyon, Utah, and headward erosion rates of tributaries
(M.S. thesis): Hanover, New Hampshire, Dartmouth College, 59 p.
Granger, D.E., and Muzikar, P.F., 2001, Dating sediment burial with in situ–
produced cosmogenic nuclides: Theory, techniques and limitations: Earth
and Planetary Science Letters, v. 188, p. 269–281, doi: 10.1016/S0012821X(01)00309-0.
Hanks, T.C., Lucchitta, I., Davis, S.W., Davis, M.E., Lefton, S.A., and Garvin,
C.D., 2001, The Colorado River and the age of Glen Canyon, in Young,
R.A., and Spamer, E.E., eds., The Colorado River: Origin and Evolution:
Grand Canyon Monograph 12, Grand Canyon, Arizona.
Wolkowinsky, A.J., and Granger, D.E., 2004, Early Pleistocene incision of the
San Juan River, Utah, dated with 26Al and 10Be: Geology, v. 32, p. 749–752,
doi: 10.1130/G20541.1.
REPLY
Amy J. Wolkowinsky
Darryl E. Granger
Department of Earth and Atmospheric Sciences, Purdue University,
West Lafayette, Indiana 47907, USA
Hanks and Finkel attempt to discredit our cosmogenic nuclide profile
dating of a gravel-capped terrace at Bluff, Utah, by suggesting that the
gravels could have been deposited in two stages separated by a hiatus of
nearly a million years. The cosmogenic nuclide data alone cannot eliminate
this possibility. Nor can the data exclude an arbitrarily complex history, if
just the right amount of erosion occurred and new material was added with
just the right amount of inheritance. However, our cosmogenic nuclide
profile unambiguously indicates the minimum age of the base of the deposit. In addition, all of our data and field observations are consistent with
the sediments being deposited in a single episode that occurred quickly
with respect to radioactive decay of 26Al (i.e., <105 yr). In the absence of
evidence to the contrary, we therefore conclude that a single depositional
episode is the most likely explanation (Wolkowinsky and Granger, 2004).
We would like to elaborate first by reviewing the capabilities and
weaknesses of dating sediments with cosmogenic nuclide profiles, and
second by discussing the absence of evidence for multistage deposition at
the Bluff terrace.
When analyzing cosmogenic nuclides in sediments from a profile, it
is important to realize what can and cannot be determined. As shown by
Granger and Smith (2000), sediments at depths >5–10 m have 26Al/10Be
ratios that depend strongly on time since deposition, and weakly on the
erosion rate of the deposit. The 26Al and 10Be concentrations at shallower
depths depend only weakly on depositional age, but are sensitive to the
erosion rate of the past few meters of sediment removed. The erosion rate
determined from the upper part of the profile can then be used to refine
the burial age determined for the lower part of the profile. A cosmogenic
nuclide profile indicates with certainty only the minimum age of the lowermost sediments. It is therefore essential that the cosmogenic data be
interpreted in the context of field observations.
Given the possibility, however remote, that sediment erosion and
deposition can occur and leave no cosmogenic trace, we must ask if there
is any evidence for multiple erosion and deposition episodes at the Bluff
terrace. Hanks and Finkel offer four potential lines of evidence, none of
which we find convincing.
1. Hanks and Finkel suggest that a two-stage deposition model
improves the data fit. Examination of the model fits to the data (rather
than their idealized smooth curves) indicates that both our single-stage
model and their two-stage model agree with the data to well within analytical uncertainty. There is no part of the data that is not accounted for
by our simpler model, and thus no compelling reason to invoke a more
complex model.
2. Hanks and Finkel identify a “significant unconformity” in the
profile, based upon a photograph exchanged through e-mail (Fig. 1). The
gravel/sand transition in Figure 1 is simply the top of a local sand lens,
hardly evidence for a million-year erosional unconformity as Hanks and
Finkel suggest.
3. Inherited cosmogenic nuclide concentrations vary significantly
among samples in the profile. We believe that this is due primarily to
variation in the source area, where quartz pebbles are derived from a highrelief, partially glaciated mountain range. Part of the variation could also
reflect production as the sediments accumulated on the terrace over thousands of years. Such variability does not seriously affect our burial ages.
4. Hanks and Finkel extrapolate our erosion rate of 14 m/m.y. to
infer an original terrace thickness of 31 m, a value they consider unlikely.
However, the cosmogenic erosion rate reflects only the recent past, and
may not necessarily be extrapolated for the life of the terrace. Moreover,
it is certainly possible that thick gravel deposits can accumulate during a
single glacial cycle; terrace sediments elsewhere in the region have thicknesses ranging up to 40 m (Patton et al., 1991).
Given that both cosmogenic nuclide data and field evidence are consistent with a single stage of deposition, it is difficult to justify invoking
a million-year hiatus in sediment deposition. We therefore maintain our
conclusion (as stated in our paper) that although such a scenario is conceivable, we consider it unlikely.
ACKNOWLEDGMENTS
We thank M. Caffee for discussion and for joining us in the field.
REFERENCES CITED
Figure 1. Photograph of upper portion of Bluff terrace gravels. Hanks
and Finkel suggest that top of 2 m-thick sand body may represent
an erosional unconformity of nearly a million years. Our field
observations indicate that sand is local lens within larger deposit.
Granger, D.E., and Smith, A.L., 2000, Dating buried sediments using radioactive
decay and muogenic production of 26Al and 10Be: Nuclear Instruments and
Methods in Physics Research, v. B172, p. 822–826.
Patton, P.C., Biggar, N.E., Condit, C.D., Gillam, M.L., Love, D.W., Machette,
M.N., Mayer, L.A., Morrison, R.B., and Rosholt, J.N., 1991, Quaternary
Geology of the Colorado Plateau, in Quaternary nonglacial geology;
conterminous U.S.: Boulder, Colorado, Geological Society of America,
Geology of North America, v. K-2: p. 373–406.
Wolkowinsky, A.J., and Granger, D.E., 2004, Early Pleistocene incision of the
San Juan River, Utah, dated with 26Al and 10Be: Geology, v. 32, p. 749–752,
doi: 10.1130/G20541.1.
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