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Journal List > Proc Natl Acad Sci U S A > v.95(17); Aug 18, 1998
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Proc Natl Acad Sci U S A. 1998 August 18; 95(17): 9744-9749.
PMCID: PMC21407
Copyright [copyright] 1998, The National Academy of Sciences
Geology
K-Ar ages of Pleistocene lava dams in the Grand Canyon in Arizona
G. Brent Dalrymple^*^[dagger] and W. K. Hamblin^[double dagger]
^*College of Oceanic and Atmospheric Sciences, 104 Ocean
Administration Building, Oregon State University, Corvallis, OR 97331;
and ^[double dagger]Department of Geology, Brigham Young University,
Provo, UT 84602
^[dagger]To whom reprint requests should be addressed. e-mail:
gbd/at/oce.orst.edu.
Contributed by G. Brent Dalrymple
Accepted June 17, 1998.
* Other Sectionsv
+ Abstract
+ References
Abstract
At least 13 times during the Pleistocene Epoch lava flowed into the
inner gorge of the Grand Canyon and formed lava dams, as high as 600
m, that temporarily blocked the flow of the Colorado River. K-Ar ages
on these lava dams indicate that the seven youngest formed within a
short period of time between about 0.6 and 0.4 mega-annum (Ma). The
physiography of the lava dam remnants within the canyon shows that
each dam was destroyed by erosion, the Colorado River rapidly reaching
its pre-existing grade level, before the next dam was emplaced by new
eruptions. The total time for emplacement and destruction for an
individual lava dam was probably as little as 0.01 --0.02 million
years. The K-Ar ages of the two oldest dams, the Lava Butte dam (0.577
[plus minus] 0.054 Ma) and the Prospect dam (0.684 [plus minus] 0.051
Ma) are somewhat younger than the physiography of their remnants
suggest.
Keywords: radiometric dating/volcanism/basalt/potassium-argon
* Other Sectionsv
+ Abstract
+ References
One of the most significant recent events in the history of the Grand
Canyon was the formation of a series of basaltic lava dams in the area
of Toroweap Valley, Prospect Canyon, and Whitmore Canyon approximately
200 km west of the national park headquarters (Fig. (Fig.1).1 Figure 1
). More than 150 lava flows poured into the canyon during the
Pleistocene and formed a series of 13 major dams ranging from 60 to
600 m high. The lava dams impounded the water from the Colorado River
to form a series of temporary lakes upstream. The shorelines of the
deeper lakes were located near the base of the Redwall Limestone in
the area of the park headquarters and extended far upstream into Utah
beyond the present shores of Lake Powell. These lakes were larger in
areal extent and impounded more water than Lake Mead and Lake Powell
combined. Sediment deposited in these lakes still remain as isolated
exposures, the largest being the silt deposits that fill Havasu
Canyon. The lava dams have been described by previous workers,
including Powell (1), Dutton (2), Moore (3), and McKee and Schenk (4),
and discussed in detail by Hamblin (5 --8); we include only a summary
description in this paper.
Figure 1
Figure 1
Figure 1
Map showing general locations of Pleistocene lava dams in the Grand
Canyon in Arizona, and the locations of places mentioned in the text.
Mile markers, measured downstream from Lee's Ferry, are the
traditional way to locate features along the (more ...)
Numerous remnants of the lava dams are preserved from Vulcan's Throne
(mile 179) downstream to mile 263, a distance of 135 km. (Locations
along the Colorado River in the Grand Canyon traditionally are
measured in miles downstream from Lee's Ferry and nearly all of the
relevant literature uses this location system. We also use the mile
system for location.) Remnants range from a few kilometers long to
small patches a few meters thick clinging precariously to the near
vertical cliff of the inner gorge. Some dams were formed from a single
lava flow, others were composed of multiple flows, some of which are
interbedded with river gravel.
Hydrologic data from Lake Mead surveys suggest that both the formation
and destruction of the lava dams occurred in a remarkably short period
of time (9). The small single-flow dams were formed in a matter of
days. Larger complex dams involving multiple flows may have required
several thousand years to form and undoubtedly included several cycles
of partial erosion between periods of lava extrusion and dam
reconstruction. The lakes behind the dams were formed in a matter of
years and probably filled with sediment in several hundred years.
Destruction of the dams was also rapid. The overflow of the Colorado
River at the downstream end of the dam undoubtedly formed rapids and
waterfalls that quickly migrated upstream. After the lakes filled with
sediment the normal traction load transported over the dam must have
resulted in vigorous downcutting.
The relative ages of most of the lava dams are clearly expressed by
juxtaposition (Fig. (Fig.2).2 Figure 2 ). Remnants of older basalt
flows are adjacent to the canyon walls with succeeding younger units
stacked in sequence side by side, with the youngest flows closest to
the river channel. In addition, some expression of relative age is
seen in the degree of erosion of remnants of dams that fill tributary
valleys. Older remnants that fill major tributary valleys have been
dissected by renewed side-canyon cutting whereas younger remnants that
fill similar side valleys are only slightly modified.
Figure 2
Figure 2
Figure 2
Relative age of major dams as indicated by juxtaposition insofar as
known. Paleozoic sedimentary units that form the walls of the Grand
Canyon in the vicinity of the lava dams are shown on the left. This is
a composite diagram constructed from numerous (more ...)
Despite the importance of these lava dams to the recent history of the
Grand Canyon, there have been few attempts to determine their age.
McKee and others (10) obtained a K-Ar age of 1.16 [plus minus] 0.18
mega-annum (Ma) (1 [sigma]) for the basalt at the base of the Toroweap
Dam. Their age was based on three Ar analyses whose radiogenic ^40Ar
content varied by 79% and whose atmospheric ^40Ar content ranged from
0.7 to 3.1%. Paul Damon (personal communication, quoted in ref. 8)
measured K-Ar ages of 0.993 [plus minus] 0.097 Ma on one of the middle
flows in the Whitmore Dam, and one of 0.549 [plus minus] 0.032 Ma on
the flow that formed the Black Ledge Dam. Some of our preliminary
results were referred to briefly by Hamblin (7, 8), but this is the
first paper to document our radiometric ages for the lava dams.
During field work in 1971 and 1992 we collected samples for dating
from more than 80 flows representing 13 of the major lava dams. The
purpose of this paper is to present K-Ar ages for some of these
samples and to relate these data to the Pleistocene history of the
Grand Canyon.
Analytical Methods
Samples consisted of both hand samples and one-inch cores. All samples
were examined in thin section and many were rejected as unsuitable for
K-Ar dating, a few because of alteration but most because of large
amounts of glass in the ground mass. Those samples that were selected
for dating were free of alteration and were either holocrystalline or
contained relatively small amounts of isotropic ground-mass glass.
Two methods of sample preparation were used: small blocks cut with a
diamond saw and 0.5 --1.0 mm crushed material. For the former, small
chips were sawed from each end for the K[2]O analyses whereas for the
latter an aliquant was taken with a sample splitter. The material
taken for the K[2]O analyses was further crushed to [minus sign]75
[mu]m before analysis. Many of the basalts contain xenoliths
(primarily dunite) and carbonate and as much care as possible was
taken during sample preparation to avoid incorporating either of these
into the sample analyzed. Both the carbonate, which interferes with
the clean-up process during Ar extraction and whose results are
unpredictable, and the older inclusions, which contribute unknown and
varying amounts of inherited ^40Ar, would be expected to result in
calculated ages that are inconsistent and, where inherited ^40Ar is
present, too old. Despite our precautions, the inconsistency of the
results for several of the samples suggests that we were not entirely
successful for several of the flows.
Potassium analyses were by flame photometry using a lithium metaborate
flux (11). Argon analyses were by isotope dilution using ^38Ar tracers
calibrated with our intralaboratory standard biotite SB-3 (162.9 Ma)
and methods described previously (12). Errors assigned to the
individual ages are SDs of precision unless specified otherwise.
Weighted means and weighted SDs ([sigma][best]), were calculated
following Taylor (13). For some samples the dispersion of the measured
ages is greater than expected from random analytical errors, i.e., the
mean square of the weighted deviates (MSWD) is greater than one. This
finding indicates that "geological" error, perhaps because of
carbonate, xenoliths, or other factors, is present. For these samples
we have adjusted the uncertainties by multiplying each individual
error by the square root of MSWD before calculating [sigma][best] (14,
15). In general, our results are not sufficiently precise to
distinguish differences in age between flows within a single dam, and
so we treat all of the ages from such flows as a single population and
use the weighted mean of the ages to represent the age of the dam.
Description of the Lava Dams and Their K-Ar Ages
The Prospect Dam.
A single remnant of the Prospect Dam is preserved in a large alcove
just east of the mouth of Prospect Canyon on the south side of the
river at mile 179. It consists of three exceptionally thick units of
basalt preserved in the ancestral Prospect Canyon and is capped by a
thick horizontal basalt layer that extends westward beyond the inner
gorge of Prospect Canyon. The Prospect Dam had a height above the
present Colorado River of 699 m and an estimated lava volume of 4.0
km^3 (7, 8). Younger flows believed to be equivalent to the Ponderosa
and Toroweap dams are juxtaposed against the Prospect flows, but this
correlation is not certain. The relative antiquity of the Prospect Dam
is suggested by the degree of erosion of the flows in Prospect Canyon.
A V-shaped gorge over 2 km long has been reexcavated into the basalt
fill in Prospect Canyon whereas remnants of all of the other dams that
fill tributary valleys, such as those preserved in Toroweap and
Whitmore canyons, have been subjected to much less erosion. Thus, the
physiographic evidence suggests that the Prospect Canyon dam is
perhaps the oldest of the lava dams.
We measured K-Ar ages on all three of the flows that form the Prospect
Dam (Table 1). The 1.86-Ma result from one sample of the oldest flow
is clearly anomalous when compared with the results from the other two
samples of this flow, and we suspect that the anomalous sample may
have contained an undetected xenolith. If we exclude the one anomalous
result the weighted mean age of the results from the three Prospect
flows is 0.679 [plus minus] 0.047 Ma (MSWD = 6.057). If the anomalous
result is included, the weighted mean age is 0.684 [plus minus] 0.019
Ma (MSWD = 7.099).
Table 1
Table 1
Analytical data for K-Ar ages on Pleistocene lava flows of the Grand
Canyon, Arizona
We also measured an age of 0.500 [plus minus] 0.047 Ma on a flow at
the head of Prospect Canyon (Upper Prospect Canyon flow). There is no
stratigraphic or geomorphic evidence to indicate whether this flow is
part of the Prospect Dam or is a younger flow that flowed onto the
surface of the dam at some later time. In view of its K-Ar age,
however, we think that the latter explanation is most likely. In
addition, this flow is cut by the Toroweap Fault, indicating that the
fault was active less than about 0.5 Ma.
Lava Butte Dam.
Several remnants of a sequence of thin flows interbedded with river
sediment are preserved high on the north wall of the inner gorge at
mile 180.8 about 4 km downstream from Vulcan's Throne. One remnant
caps an isolated butte that stands approximately 130 m above the
surrounding area. If this remnant was formed by topographic inversion,
i.e., by erosion along the flow margins, it suggests that the Lava
Butte Dam is one of the oldest dams preserved in the canyon. The
remnants are not juxtaposed with other flows, however, so its relative
age cannot be determined. The Lava Butte Dam reached a height of 560 m
above the present river but there are insufficient remnants to permit
an accurate estimate of the original lava volume.
We were unable to sample the basalt on the lava-capped butte, but
samples from four flows from the Lava Butte dam were collected from
remnants on the north canyon wall and dated. The results are
reasonably consistent with a weighted mean age of 0.577 [plus minus]
0.054 Ma (MSWD = 12.20).
Toroweap Dam.
A remnant of the Toroweap Dam is juxtaposed against the lower basalt
flows that fill Toroweap Valley and against the adjacent Paleozoic
rocks between miles 178 and 180. This remnant, which consists of at
least five major flows, is over 2 km long and indicates that the
Toroweap Dam had a maximum thickness of 424 m. The Toroweap Dam is
older than the Lava Falls dam and, if correlation with similar basalts
across the river is correct, it is also younger than the Prospect and
Ponderosa dams. The Toroweap Dam had an estimated volume of 3.7 km^3.
Several attempts were made to measure the age of the Toroweap dam by
using multiple samples from two flows (E and F), but the results are
highly inconsistent (Table 1), indicating that the analyses were
affected by older inclusions, carbonate, or both. The weighted mean of
all of our measurements is 0.557 [plus minus] 0.074 Ma (MSWD = 3.517),
but in view of the poor reproducibility we do not consider this to be
a reliable age. The age of 1.16 [plus minus] 0.18 Ma obtained by Damon
(quoted in ref. 8) falls within the range of our ages but we consider
it no more reliable than ours.
Whitmore Dam.
A number of large remnants of the Whitmore Dam are preserved at the
mouth of Whitmore Canyon and on both sides of the inner gorge of the
Grand Canyon between miles 187 and 190. The Whitmore Dam consisted of
more than 40 thin flows, commonly interbedded with river gravel. What
appears to be a remnant of the Ponderosa flow (the identification is
uncertain) is juxtaposed against, and therefore younger than, the
basalt flows of the Whitmore dam as are the flows that formed the
Black Ledge, Massive Diabase, and Gray Ledge dams.
We attempted to measure K-Ar ages on two of the older flows from the
Whitmore Dam but the argon extracted in three attempts was all
atmospheric and so it was not possible to calculate K-Ar ages. The age
measurements on the stratigraphically older Lava Butte (0.577 [plus
minus] 0.054 Ma) and the stratigraphically younger Ponderosa (0.607
[plus minus] 0.024 Ma) dams seem to indicate that the age of the
Whitmore Dam is probably about 0.6 Ma. Damon (quoted in ref. 8),
however, obtained a single K-Ar age of 0.993 [plus minus] 0.097 Ma
from a Whitmore flow. We cannot explain this discrepancy, but it has
been our experience in studying the Grand Canyon flows that
consistency from multiple measurements, preferably on multiple flows,
is necessary before a K-Ar age can be considered truly reliable. We
hesitate, therefore, to place much significance on a single age
measurement from a single flow and suggest that the age of the
Whitmore Dam may be about 0.6 Ma.
Ponderosa Dam.
A large remnant of the Ponderosa Dam, which was constructed by a
single flow at least 300 m thick with a volume of 2.5 km^3, is
preserved on the north side of the river at mile 182. The base of the
flow is near river level, indicating that the Grand Canyon was cut to
its present depth at the time the Ponderosa dam was formed. Remnants
of the Esplanade Dam are juxtaposed against the Ponderosa flows,
indicating that the Ponderosa is one of the older dams in the Canyon.
Its age relative to the Prospect Canyon and Lava Butte dams, however,
is unknown.
Only one sample from the Ponderosa dam was suitable for dating. That
flow gave reproducible results with a weighted mean age of 0.607 [plus
minus] 0.024 Ma (MSWD <1).
Esplanade Dam.
Between miles 181 and 182 a large remnant of a sequence of basalt
flows is preserved beneath younger lava flows (the Esplanade Cascades)
that flow from the north canyon rim. The Esplanade dam was formed by 6
--8 major flows, each over 50 m thick, and had an estimated volume of
1.8 km^3. Remnants of the younger Lava Falls dam (discussed below) are
juxtaposed against the Esplanade, indicating that the Esplanade Dam is
at least older than the five youngest dams in the Grand Canyon.
We made a single age measurement (0.110 [plus minus] 0.053 Ma) on one
flow of the Esplanade dam but because the result is not verified
either by replication or by an age on another flow we do not consider
it to be reliable.
Buried Canyon Dam.
At mile 183 a large exposure of a sequence of basalt flows fills an
ancient canyon through which the Colorado River once flowed. This
remnant is almost a perfect cross section through a complex dam and
shows how the canyon was filled with basalt and river gravel during
the construction of the dam. This dam consisted of eight major flows
with a total volume of 1.7 km^3. The basalt flows of the Lava Falls
Dam (discussed below) are juxtaposed against the flows of the Buried
Canyon Dam but there is no evidence of the relative age between the
Buried Canyon and Esplanade dams.
We attempted to measure K-Ar ages on six samples from five of the
Buried Canyon flows, and the results are highly inconsistent, both
within individual flows and between flows whose stratigraphy is known
(Table 1, Fig. Fig.3).3 Figure 3 ). The weighted mean of the results
is 0.907 [plus minus] 0.066 Ma (MSWD = 1.474) but because of the lack
of reproducibility and consistency we do not consider this age to be
reliable.
Figure 3
Figure 3
Figure 3
Weighted mean K-Ar ages for lava dams in the Grand Canyon. Errors are
shown at the 68% confidence level ([sigma][best]) by vertical bars. *,
Ages considered reliable. [open circle], Ages considered unreliable.
[square, square], Ages from McKee and (more ...)
D Dam.
Several small remnants of a sequence of thin basalt flows are
preserved on both sides of the canyon below Vulcan's Throne. McKee and
Schenk (4) called these the D flows of their Lower Canyon Series.
These basalt flows formed a dam 191 m high consisting of as many as 40
thin flows with a total volume of approximately 1.1 km^3. On the basis
of juxtaposition the D Dam is clearly younger than the Toroweap Dam
and older than the Lava Falls and Black Ledge dams.
We obtained reproducible K-Ar ages on a single sample from one of the
flows that formed the D dam. The results have a weighed mean age of
0.582 [plus minus] 0.031 Ma (MSWD <1).
Lava Falls Dam.
Several large remnants of a single basalt flow 191 m thick and with an
estimated volume of 1.2 km^3 are preserved between Lava Falls Rapids
and Whitmore Canyon. It is the oldest of the five most recent dams,
whose relative ages are indicated by juxtaposition. Of the samples
collected from this flow none was suitable for dating because of
excessive amounts of glass. The Lava Falls Dam is thus not dated
directly but it is tightly bracketed by indistinguishable ages for the
D Flows (0.582 [plus minus] 0.031 Ma) and the Layered Diabase (0.620
[plus minus] 0.052 Ma, discussed below) dams.
Black Ledge Dam.
One of the most distinctive remnants of basalt in the Grand Canyon is
a dense black flow that typically forms a distinctive ledge along the
canyon walls. This dam was 111 m high, was the longest flow in the
canyon extending 135 km downstream from the Toroweap area, and had an
estimated volume of 2.1 km^3. None of the samples we collected from
the Black Ledge flow was suitable for dating because of excessive
amounts of glass but, like the Lava Falls Dam, the age of the Black
Ledge Dam is constrained by the ages of the flows of the D Dam (0.582
[plus minus] 0.031 Ma) and the Layered Diabase Dam (0.620 [plus minus]
0.052 Ma). The age of 0.549 [plus minus] 0.032 Ma obtained by Damon
for the Black Ledge Dam (quoted in ref. 8) thus is consistent with,
and not significantly different from, our ages for the two older and
younger dams.
Layered Diabase Dam.
A distinctive sequence of 15 --20 flows of coarse-grained basalt
formed the Layered Diabase Dam, which had a thickness of 89 m and a
volume of only 0.3 km^3. Two flows, from near the bottom and top of
the sequence exposed at mile 183.2 on the south side of the canyon,
were analyzed. The results from the two flows are reasonably
consistent and have a weighted mean of 0.620 [plus minus] 0.052 Ma
(MSWD <1).
Massive Diabase Dam.
This dam was formed by a single flow of very coarse-grained basalt. It
was only 68 m high with a volume of 0.2 km^3 and had only a slight
effect on the river compared with most of the other major lava dams.
Two samples from two different localities were dated with reproducible
results. The weighted mean age is 0.443 [plus minus] 0.041 Ma (MSWD
<1).
Gray Ledge Dam.
Remnants of a single basalt flow characterized by an abnormally thick
basal colonnade structure commonly is juxtaposed against the remnants
of the Black Ledge Dam. This flow is covered by a thick layer of
coarse gravel and typically weathers into a distinctive gray ledge.
The dam's height above the present river was 61 m, and its estimated
volume was only 0.3 km^3.
A single age of 0.788 [plus minus] 0.128 Ma was obtained on one sample
from this flow. Because this age is based on a single measurement we
do not consider it to be reliable. The maximum age of the Gray Ledge
Dam, however, is constrained by the age of 0.443 [plus minus] 0.041 Ma
for the Massive Diabase Dam, which is stratigraphically older.
A Possible Dam at Toroweap Valley.
The huge volume of basalt that fills Toroweap Valley suggests the
possibility of an additional high-level dam in the inner gorge of the
Grand Canyon. These flows are all relatively thin (4 --5 m) and are
similar in almost every way to the flows that form the Whitmore Dam.
The cumulative thickness of these flows is 504 m. They apparently
originated from lava that issued from vents on the Uinkaret Plateau
and flowed into Toroweap Valley; many undoubtedly cascaded into the
inner gorge. The eroded face of these flows in the inner gorge is
slightly more than 600 m back from the river channel. Inasmuch as most
flow units are nearly horizontal, it seems reasonable to suppose that
they formed a barrier across the Grand Canyon. Without equivalent
flows preserved on the south wall of the canyon, however, the
existence of the dam cannot be demonstrated conclusively, but dikes
cutting these flows suggest that the flows once filled the inner gorge
and formed the country rock into which the dikes were injected. If a
dam was constructed from these flows, it was older than the Toroweap
Dam and was probably similar in geometry and internal structure to the
Whitmore Dam.
We made three age measurements on a sample from the uppermost flow
near the base of Vulcan's Throne. One of the ages is anomalously high,
perhaps because of the inadvertent inclusion of a xenolith, but the
other two are consistent with a weighted mean age of 0.776 [plus
minus] 0.138 Ma (MSWD <1). Because of the relatively large analytical
uncertainty, this age is not significantly different from the ages
found for any of the other dated dams (Fig. (Fig.3),3 Figure 3 ), but
the result is not inconsistent with the hypothesis that the Toroweap
Valley fill is older than the Toroweap Dam.
Dike, Prospect Canyon.
We also sampled and dated a dike that occurs at an elevation of 805 m
at mile 179.5 on the south side of the Colorado River and in the west
wall of Prospect Canyon. This dike is about 15 m thick and is one of a
large number of similar dikes, some of which intrude remnants of lava
dams. Lava remnants clearly show that the canyon was cut to its
present depth before the oldest dams were formed (8). Because the dike
had to have been intruded into some body of rock that since has been
removed by erosion, it seems likely that the dike intruded lava flows
that formed a dam in the canyon rather than the older sedimentary
rocks. If this is so, then the top of the dam must have exceeded an
elevation of 805 m.
The duplicate analyses of this dike are relatively reproducible with a
mean age of 0.827 [plus minus] 0.058 Ma (MSWD <1). This age is
significantly older, at the 95% level of confidence, than the age we
obtained for the Prospect Dam and suggests the existence of volcanic
activity, and perhaps even older dams, within the area of the present
Grand Canyon before the formation of the 13 Pleistocene lava dams
discussed above.
Discussion
We have obtained reasonably consistent K-Ar age data on lava flows
from more than half of the 13 major lava dams that partly filled the
Grand Canyon and temporarily blocked the flow of the Colorado during
the Pleistocene Epoch (Fig. (Fig.3).3 Figure 3 ). Four of the six
youngest dams (Ponderosa, D Flows, Layered Diabase, and Massive
Diabase) have K-Ar ages that fall within a very narrow range, 0.61
--0.43 Ma, indicating that these dams, as well as the Black Ledge,
Lava Falls, Buried Canyon, Esplanade, and Toroweap dams, were formed
within a very short period of time, probably 0.2 Ma or so. We did not
obtain a reliable K-Ar age for the youngest (Gray Ledge) dam, but its
flows are stratigraphically younger than those of the Massive Diabase
dam and so it must be less than about 0.43 Ma in age, but not
necessarily much less.
The K-Ar ages for the Lava Butte (0.574 [plus minus] 0.054 Ma) and
Prospect Canyon (0.684 [plus minus] 0.051 Ma) dams, although
stratigraphically and radiometrically consistent with the ages of the
younger dams, pose somewhat of a problem. The physiography of these
two dams suggests that they may be much older, rather than slightly
older, than the younger dams. The remnant of the Prospect Dam that
partly fills Prospect Valley has been eroded into a V-shaped gorge
nearly 2 km in length. Neither the Toroweap nor the Whitmore dams,
both of which also fill major tributary valleys, have been eroded
similarly. This finding suggests that the Prospect Dam is much older
than the K-Ar ages indicate. An alternate explanation is that
pre-existing and temporary hydrologic conditions in Prospect Valley
may have eroded the Prospect Dam preferentially.
The antiquity of the Lava Butte Dam is suggested by the degree to
which it has been eroded by minor intermittent streams on the sides of
the inner gorge. A remnant of a series of basalt flows at mile 180.5
caps a high butte rising 120 m above the surrounding area and 529 m
above river level. The butte has been separated from the wall of the
inner gorge by headward erosion of two short tributary streams. No
other remnant of a lava dam has been eroded by tributary streams to
such a degree that it forms an isolated butte separated from the
canyon wall.
We cannot resolve the apparent conflict between the K-Ar data and the
physiography of the Lava Butte and Prospect dams. Either the K-Ar ages
are incorrect or the relative degree of erosion is an imperfect
indicator of elapsed time for these lava dams. As discussed below,
erosion in the Grand Canyon can be extremely rapid under certain
conditions and it is possible that the physiography suggesting the
passage of substantial time was produced, instead, in a very short
interval of time. Because the K-Ar ages are consistent with the known
stratigraphy and are the only quantitative measure of time available
we adopt, with the reservations expressed above, the K-Ar ages for the
Lava Butte and Prospect flows in the discussion that follows.
Based on the K-Ar data (Fig. (Fig.3)3 Figure 3 ) all 13 of the major
lava dams, representing a minimum of 22 km^3 of lava and a cumulative
vertical thickness of more than 3.5 km, were formed by copious
eruptions and were rapidly destroyed by the Colorado River within a
very short period of time. All available evidence indicates that
before the extrusions of lava into the Grand Canyon the Colorado River
had cut down to its present gradient and stratigraphic position and
that the size and shape of the canyon walls at the time of basalt
extrusions were essentially the same as those we see today (8). This
is clear from the position of numerous thin remnants of various dams
that remain as slivers next to the original canyon wall, some of which
extend down to present-day river level. Thus, after each lava dam was
formed, the Colorado River eroded through the dam down to its original
profile, but no farther. This process of re-excavating the canyon took
place at least 13 times during the Pleistocene Epoch.
The K-Ar ages are not sufficiently precise to permit an estimate of
the times between the various episodes of dam formation, but the
clustering of ages for the younger dams between about 0.6 and 0.43 Ma
shows that dam erosion was very rapid. The data indicate that the nine
younger dams that are either dated directly or bracketed by dated
flows (Ponderosa through Massive Diabase) were erupted within a period
of only about 0.2 million years. This finding suggests that on the
average an individual dam was emplaced and destroyed by erosion in
only about 0.01 --0.02 million years. Apparently major rivers, like
the Colorado River, are extremely efficient at removing blockage and
re-establishing a former gradient, even when the blockage is by
relatively resistant rocks like basaltic lava flows.
Acknowledgments
We thank Skip Kover and Jerry Von Essen for their assistance in the
field and in the laboratory and Robert Fleck for a most helpful review
of the manuscript. We also thank Myron Best and Sherman Gromme, who
were part of our field party and with whom we spent much time
discussing the lava dams and their bearing on the Pleistocene history
of the Grand Canyon. Tom and Doreen Frost assisted us with sampling
some of the forbidding cliff faces of the lava dam remnants and we
thank them for their help.
ABBREVIATIONS
Ma mega-annum
MSWD mean square of the weighted deviates
* Other Sectionsv
+ Abstract
+ References
References
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Tributaries Explored in 1869 --1872. Washington, DC: Smithsonian
Institution; 1873. pp. 94-196.
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3. Moore R C. US Geol Surv Water-Supply Paper. 1925;556:125-171.
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