TECHNICAL COMMENT
Comment on “Apatite 4He/3He
and (U-Th)/He Evidence for an
Ancient Grand Canyon”
Ivo Lucchitta1,2
Flowers and Farley (Reports, 21 December 2012, p. 1616; published online 29 November 2012)
use thermochronometry to propose that the western paleo–Grand Canyon was nearly as deep
70 million years ago (Ma) as today. However, lithologies, facies relations, geomorphology, and
paleotopography of Miocene interior-basin deposits near the mouth of the Grand Canyon show that
no paleocanyon existed in that area during filling of the basin, ~17 to ~5 Ma.
F
U.S. Geological Survey, Flagstaff, AZ 86001, USA. 2Museum
of Northern Arizona, Flagstaff, AZ 86001, USA. E-mail: ilucchitta@
gmail.com
Fan deposits are the chief constituent of the
Muddy Creek. One fan, derived from the west,
contains clasts of the distinctive Gold Butte Granite.
This fan locally is in contact with Paleozoic rocks
of the Grand Wash Cliffs (Fig. 1). According to
Longwell (2), who saw the area before deposition of the Lake Mead deltaic sediments, the fan
nearly reaches the canyon mouth, where it is in
contact with the Pierce Canyon fan. The latter not
only spills westward from a relatively small canyon cut into the Grand Wash Cliffs (Fig. 1) but
also extends across the mouth of the Grand Can-
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lowers and Farley (1) use thermochronometry data to propose that the western
paleo–Grand Canyon resembled the present
canyon in location, configuration and depth
70 million years ago (Ma) and further suggest that
its carving was separate from integration of the
Colorado River. However, the area near the mouth
of the western Grand Canyon in the Grand Wash
Trough contains critical geologic evidence bearing on the problem because a 70 Ma paleocanyon that coincides with the Grand Canyon would
have transited the area and would have affected
Miocene deposits there. The area was studied
in reconnaissance by Longwell (2) and later
by Lucchitta (3–5), Faulds et al. (6, 7), and
Wallace et al. (8).
The mouth of the canyon is at the Grand
Wash Cliffs, which were formed by down-tothe-west movement on the Grand Wash normal
fault starting ~17 Ma. Faulting produced a halfgraben, the Grand Wash Trough, whose lowest
point was near the cliffs; the graben received the
well-dated ~17 to 5 Ma Muddy Creek interiorbasin deposits. Post-5 Ma erosion by the Colorado
River has produced 530 m of fine exposures
of these deposits but mostly not of the bedrock beneath them. However, the bedrock exposures on the Grand Wash Cliffs are excellent
and show no major canyons other than the
Grand Canyon, which therefore must coincide
with the proposed prefaulting paleocanyon.
The modern canyon is narrow and steep, which
is not in keeping with an ancient feature.
Because the proposed paleocanyon would
have existed throughout deposition of the Muddy
Creek, it should have contributed sediments because its drainage direction at that time, whatever it had been previously, would have been
toward the newly formed deep graben immediately to the west, yet no evidence for such a
drainage is present in the Muddy Creek deposits
(Fig. 1).
yon itself. Consequently, either the paleocanyon did not exist in Muddy Creek time, or the
paleoriver within it was completely inactive.
The distribution of Muddy Creek deposits illustrates how the fans, which formed high ground,
encircle the mouth of the proposed paleocanyon,
whose mouth was thus blocked (Fig. 1). Therefore, no paleodrainage could flow from a paleocanyon westward into the Muddy Creek basin in
Miocene time.
The low areas between fans were occupied by
playas whose deposits were derived principally
from the west; these deposits show no contributions from a paleoriver to the east. Shallow and
probably ephemeral lakes occupied parts of the
playas. The lakes were fed by groundwater or
springs (9). With time, the lakes expanded and
transgressed over the other lithologies. There is
no connection between the lake deposits and the
mouth of the proposed paleocanyon. These lake
beds are the youngest deposits that predate integration 5 to 6 Ma.
Flowers and Farley (1) propose a notable
separation in time between the carving of a
paleocanyon coincident with the Grand Canyon
and the establishment of an integrated stream
flowing though it. Other advocates of an old
paleocanyon have similarly resorted to various
explanations to account for the lack of evidence for such a canyon in Miocene time. These
1
Fig. 1. Muddy Creek facies near mouth of Grand Canyon. (A) Map view. (B) Ground-level view.
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TECHNICAL COMMENT
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so it should have contributed sediment while
it was being carved. Furthermore, the Pierce
Canyon fan was deposited across the supposed
eastward-eroding paleodrainage, which thus
could not exist.
Geologic data from the western Grand Canyon
region do not support establishment of a Grand
Canyon in nearly its present form 70 Ma but
instead indicate that it was the result of various geologic processes that took place after 5
to 6 Ma.
References
1. R. M. Flowers, K. A. Farley, Science 338, 1616 (2012).
2. C. R. Longwell, Geol. Soc. Am. Bull. 47, 1393 (1936).
3. I. Lucchitta, thesis, Pennsylvania State University, State
College, PA (1966).
4. I. Lucchitta, The mouth of the Grand Canyon and edge of
the Colorado Plateau in the upper Lake Mead area,
Geological Society of America Centennial Field
Guide—Rocky Mountain Section (1987), pp. 365–370.
5. I. Lucchitta, in Geologic Evolution of Arizona: Arizona
Geological Society Digest 17, J. P. Jenney, S. J. Reynolds,
Eds. (1989), pp. 701–715.
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6. J. E. Faulds, M. A. Wallace, L. A. Gonzalez, M. T. Heizler,
in Colorado River Origin and Evolution,
R. A. Young, E. E. Spamer, Eds. (Grand Canyon
Association, Grand Canyon, AZ, 2001), pp. 81–87.
7. J. E. Faulds, L. M. Price, M. A. Wallace, in Colorado River
Origin and Evolution, R. A. Young, E. E. Spamer, Eds.
(Grand Canyon Association, Grand Canyon, AZ, 2001),
pp. 93–99.
8. M. A. Wallace, J. E. Faulds, R. E. Brady, 2001, Geologic
Map of the Meadview North quadrangle, Arizona and
Nevada: Nevada Bureau of Mines and Geology Map 154,
Scale 1:24,000.
9. J. C. Lopez Pearce et al., in CRevolution 2—Origin and
Evolution of the Colorado River System, Workshop
Abstracts, Beard, Karlstrom, Young, Billingsley, Eds.
(U.S. Geological Survey Open-File Report 2011-1210,
2011), p. 180.
10. D. P. Elston, R. A. Young, J. Geophys. Res. 96, 12,389
(1991).
11. R. A. Young, in Late Cenozoic Drainage History of the
Southwestern Great Basin and Lower Colorado River
Region: Geologic and Biologic Perspectives, M. C. Reheis,
R. Herschler, D. M. Miller, Eds. (Geological Society of
America Special Paper 439, 2008), pp. 319–333.
27 December 2012; accepted 25 February 2013
10.1126/science.1234567
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explanations are mostly of two kinds: (i) the
paleocanyon existed but was choked with debris repeatedly owing to an arid climate, one
of these choking events taking place in Muddy
Creek time (10), and (ii) a younger canyon was
formed by headward erosion eastward from
the Grand Wash cliffs in Muddy Creek time but
became dammed after being carved so could not
contribute sediment to the Muddy Creek deposits (11).
The first explanation is negated by the immature character of the Grand Canyon, which is
not consonant with an old age, by the improbability of the paleocanyon being choked repeatedly and completely, and by the Pierce
Canyon fan, which shows that there was enough
precipitation in Muddy Creek time for even a
small canyon to efficiently transport large quantities of debris.
The second explanation is negated by the
fact that the canyon had to be carved before it
could be dammed late in Muddy Creek time,
Comment on ''Apatite 4He/3He and (U-Th)/He Evidence for an Ancient Grand Canyon''
Ivo Lucchitta
Science 340 (6129), 143.
DOI: 10.1126/science.1234567
http://science.sciencemag.org/content/340/6129/143.1
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