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EVIDENCES FOR RAPID FORMATION AND FAILURE
OF PLEISTOCENE "LAVA DAMS" OF THE
WESTERN GRAND CANYON, ARIZONA
SCOTT H. RUGG
RUGG & ASSOCIATES
1221 OLIVER AVE.
SAN DIEGO CA 92109 STEVEN A. AUSTIN
INSTITUTE FOR CREATION RESEARCH
10946 N. WOODSIDE AVE.
SANTEE CA 92071
Presented at the Fourth International Conference on Creationism
Pittsburgh, PA, August 3-8, 1998
KEYWORDS
Arizona, Grand Canyon, catastrophic erosion, dam breachment, lava dam,
geomorphology, excess argon, K-Ar dating, Pleistocene Epoch.
ABSTRACT
Over 200 isolated outcrops of horizontally stratified, basaltic lava flows
within the inner gorge of western Grand Canyon indicate that several natural
"lava dams" blocked the flow of the Colorado River during the Pleistocene,
resulting in the formation of several lakes within the canyon. The largest
lake was 90 m above the high water level of present-day Lake Powell and
backed up a distance of over 480 km to Moab, Utah . Although early studies
indicated that three or less dams once blocked the inner gorge, work
completed in 1994 indicated that at least 13 distinct lava dams may have
blocked the Colorado River. Comparison with modern erosion rates of cliff
retreat (Niagara Falls) indicate that the 13 dams would have required a
minimum of 250,000 years to erode during the Pleistocene. However, geologic
features and relationships not previously considered indicate that the dams
formed rapidly (hours, days, or months) and failed catastrophically soon
after formation. Excess radiogenic argon is contain within many basalts of
Grand Canyon. This initial argon invalidates K-Ar model ages which are
assumed by many geologists to require an age of more than one million years
for the oldest lava dams. We envision that the entire episode of the lava
dams can easily be reconciled within a time-frame of less than two thousand
years. Our observations and interpretations reveal serious flaws in the
current long-age time-scale of the Pleistocene Epoch.
INTRODUCTION
The western Grand Canyon contains a unique and spectacular sequence of
Pleistocene volcanic flows. The basaltic flows are particularly
captivating because of their stark contrasting jet-black color against
the light brown and red hues of the underlying Paleozoic sedimentary
rocks. The Pleistocene flows appear as "frozen" lava falls cascading
down the walls of the inner gorge to the Colorado River below. They
also have a much more unique aspect which was first observed by John
Wesley Powell in 1887. Powell noted that many of the inner gorge flows
are horizontally bedded, indicating that they once extended across the
entire width of the inner gorge, damming the Colorado River and
forming an immense lake within the Grand Canyon. Later geologic
studies showed that there were possibly several separate lava dams
within the western Grand Canyon during the Pleistocene. Recently, W.
Kenneth Hamblin [6] evaluated over two hundred lava-dam remnants
within the inner gorge between miles 177 and 254 (river miles measured
downstream from Lee's Ferry, Arizona - See Figure 1) and concluded
that at least 13 separate and distinct lava dams once blocked the
Colorado River spanning a length of time between approximately 1.8 Ma
(million years ago) to as recently as 0.45 Ma.
The remnants of lava dams outcrop at elevations from river level (500
m) up to near the top of the inner gorge rim (1200 m),and vary in size
from a few meters to over 2.5 km long. The tallest and oldest lava dam
had a crest of 700 m above the Colorado River and backed up a lake to
near Moab, Utah (a distance of over 480 km) which would have been 90 m
above the high water level of present-day Lake Powell. The dams were
all at least several kilometers long, with the longest extending a
total distance of over 138 kilometers. Based on present rates of
retreat of Niagara Falls, Hamblin [6] suggested that the individual
dams required from 10,000 to 40,000 years to erode. Using an
intermediate value of 20,000 years, Hamblin [6] concluded that the
Colorado River would have been dammed a total of up to 250,000 years
during the period between 1.8 Ma to 0.45 Ma of the Pleistocene.
Lava dams figure prominently in the rendition of Grand Canyon in the
popular press. Hamblin and Hamblin [7] have recounted the naturalist's
common perception of Grand Canyon's lava dams being "more than one
million years old." Davis Young [17], a Christian geologist writing
about Noah's Flood, has reiterated the notion that Noah's Flood could
not have been involved in forming the Grand Canyon, because the canyon
was already present "1.16 million years ago" when lava flowed in and
blocked the river. Young's very precise "age" for the lava dam comes
from potassium-argon (K-Ar) dating of the basalt [12].
This long time-frame potentially presents a problem to those who hold
to a Biblical view of a young earth and a short time-frame for Earth
history. If the Pleistocene is a post-Flood epoch, then the episode of
the volcanic dams needs to be reconciled within only a
several-thousand-year time-frame, and not the "more than one million
years" of the uniformitarian time scale. Does the geologic field
evidence support a short or long time-frame scenario for the
development and subsequent erosion of the Pleistocene lava dams? We
believe that the evidence overwhelmingly supports a short time-frame,
and we will examine several important details not previously
considered.
Figure 1. Location and Geologic Map of Grand Canyon, Arizona.
GEOLOGIC SETTING
The volcanic rocks of the western Grand Canyon are part of the
Uinkaret Volcanic Field. This volcanic field extends northward from
the Colorado River approximately 80 kilometers to near the Vermilion
Cliffs, and contains up to 160 volcanic cones [8]. The cones range
from 15 to 250 meters in height. The volcanic flows are generally less
than 8 meters thick and cover an area of several hundred km^2.
Maxson [10] noted that the volcanic rocks consist of olivine basalt
flows and basaltic cinders . The flows erupted in association with two
north/south trending fissures on the Uinkaret Plateau which extend
north from near the rim of the inner gorge. Only a few relatively
small eruptive sources occur on the platform south of the inner gorge.
The flows average between approximately 1 to 2 meters thick. Some
individual flows cover areas of up to several square kilometers. The
thin and extensive lateral coverage of the flows indicates that they
were highly fluid upon eruption. Many of the flows poured southward
into the inner gorge as lava cascades. The most spectacular cascades
occur between miles 179 and 182 on the north wall of the inner gorge.
One cascade (near mile 181) almost reaches the bank of the Colorado
River [2].
The classic Grand Canyon sequence of Paleozoic rocks (Tapeats
Sandstone through Kaibab Formation) all outcrop in the western Grand
Canyon. The rim of the inner gorge is composed of the Esplanade
Sandstone (Supai Group). The wall of this inner gorge exposes strata
as deep as Tapeats Sandstone. The broad Esplanade Platform occurs
above the inner gorge and is overlain by the Hermit through Kaibab
Formations.
The Toroweap and Hurricane faults are the most prominent structural
features of this region of the western Grand Canyon. The Toroweap
Fault, which crosses the Colorado River near mile 179, displays about
250 meters of displacement and has controlled the development of
Toroweap Valley on the north side of the inner gorge and Prospect
Valley on the south. The Hurricane Fault exhibits up to 400 meters of
offset. The fault runs parallel with the Colorado River starting at
mile 188 (Whitmore Canyon) where the river makes a southward bend, and
eventually crosses the river near mile 191, where the river makes
another turn toward the west. Like Toroweap Valley, Whitmore Canyon
has allowed the lava flows on the Esplanade to be channeled southward
toward the inner gorge.
LAVA DAMS
McKee and Schenk [11] first studied the lava-dam remnants and
concluded that they were part of a large solitary dam structure. After
a more detailed study, Maxson [10] concluded that up to three separate
dams, two of which coexisted, once filled the inner gorge. Hamblin [6]
has concluded, in the most detailed study to date, that at least 13
separate lava dams, none of which coexisted, filled the inner gorge
during a period between 1.8 Ma to 0.45 Ma of the Pleistocene. Hamblin
noted that the remnants displayed several distinctive types of
depositional features (texture and flow thickness) which he relied
upon to correlate the individual dam remnants.
One of the most interesting aspects of the remnants is that many,
including some of the oldest, occur near the present elevation of the
Colorado River. For example, a large outcrop of Toroweap Dam occurs
within only 15 m of the present river level. This shows that there has
not been significant additional downcutting of the canyon in this area
since the time of formation of even the oldest dams. The pattern of
preservation of dam remnants also shows that the inner gorge has not
undergone noticeable widening during the Pleistocene.
Concepts of uniformitarian geologists regarding the very long ages of
the lava dams within Grand Canyon come from three areas: (1) the
stratigraphic relationships of the different flow remnants of ancient
dams, (2) the durability of slopes within the canyon against which
these dams have accumulated, and (3) K-Ar dating of the basalt. The
first two methods are strongly tied to the geomorphic presuppositions
of the geologist making the interpretation. For example, were multiple
dams each eroded slowly at the rate at which the Niagara River of New
York is now eroding back the falls?. The third (K-Ar dating) appears
to be less dependent on geomorphic presuppositions.
K-Ar Dating of Lava Dams
The first basalt dam to be dated using the K-Ar method was Toroweap
Dam by McKee, Hamblin and Damon [12]. The lowest part of that dam gave
a K-Ar model age of 1.16 +/- 0.18 Ma (million years). These earliest
workers admitted that their age could be in error because of "excess
argon", a process whereby the magmatic argon is occluded within basalt
as it cools making the sample appear exceedingly old. Other
investigators since have also dated basalts within Grand Canyon.
Hamblin [5, p. 199] described numerous basalt samples collected during
1972 and dated by G. B. Dalrymple. Concerning these rocks, Hamblin
noted that four basalt flows gave "reliable dates" (0.14, 0.57, 0.64,
and 0.89 Ma). However, Hamblin noted "many had excess argon" [5, p.
199]. The "ages" for those with "excess argon" have not been reported
in any publication. Also, there has been no publication of which
criteria were used to select the "reliable" from the more-frequently
occurring "unreliable" ages. Recently, Wenrich, Billingsley and
Blackerby [18, p. 10,421] reported other "ages" for basalt dams within
Grand Canyon, but none exceeds the "age" of Toroweap Dam (supposedly
1.16 +/- 0.18 Ma).
In order to test the K-Ar dating of the lava dams, we collected
another sample of the Toroweap Dam about 300 meters downstream from
the site sampled by McKee, Hamblin and Damon [12]. Our new sample of
Toroweap Dam (called QU-16) comes from the north side of the river
just above Lava Falls Rapid (mile 179.4) at somewhat higher elevation
than the sample of McKee, Hamblin and Damon [12]. This new sample is
very fine-grained and uniform black, without phenocrysts and without
xenoliths. It may be classified as a "basanite" (44.3wt % SiO2, 5wt%
total alkalis and significant olivine). In every way it appears
suitable for K-Ar dating. The one-kilogram sample was milled to
-230/+270 mesh particles (63 to 53 microns) and separated into heavy
and light fractions by centrifugation in methylene iodide, a heavy
liquid "cut" to a specific gravity of 3.20 with ethyl alcohol. The
float fraction (called QU-16FG) is dominated by plagioclase and glass.
The sink fraction was separated magnetically into weakly magnetic
olivine (called QU-16HN) and strongly magnetic orthopyroxene with some
Fe-Ti oxides (called QU-16HM). The three new samples were submitted to
Geochron Laboratories (Cambridge, Massachusetts) for conventional K-Ar
analysis. The results are listed in Table 1 and plotted graphically in
Figure 2.
Table 1. Potassium and Argon Data for Toroweap Dam.
%K ^40K ^40Ar* ^40Ar* 40 Ar*/^40K "Age"
ppm % ppm Ma
A-Flow 0.9475 1.130 3.1 0.780e-4 0.690e-4 1.19+/- 0.18
QU-16FG 1.468 1.751 5.9 3.49 e-4 2.00 e-4 3.4 +/- 0.2
QU-16HM 0.693 0.826 5.0 1.49 e-4 1.80 e-4 3.1 +/- 0.3
QU-16HN 0.253 0.302 5.0 3.65 e-4 12.07 e-4 20.7 +/- 1.3
New K-Ar analyses on the Toroweap Dam lava are listed with the sample
"A-Flow" (our name for the published data of McKee, Hamblin and Damon
[12]). We recalculated the abundance of ^40K and the resulting "model
age" in "A-Flow" using the new constants [14]. The recalculated age is
1.19 +/- 0.18 Ma. However, the three mineral concentrates from sample
QU-16 contain significantly more ^40Ar* than the whole rock analysis
of "A-Flow". Mineral concentrates from QU-16 have 1.49 to 3.65 x 10^-4
ppm ^40Ar*, whereas "A-Flow" has only 0.78 x 10^-4 ppm ^40Ar*. "Model
ages" for QU-16 are 3.4 +/- 0.2 Ma (feldspar-glass), 3.1 +/- 0.3 Ma
(orthopyroxene + FeTi oxides), and 20.7 +/- 1.3 Ma (olivine). These
ages are strongly discordant with that from the whole rock of "A-Flow"
(1.19 +/- 0.18 Ma). Most interesting is the olivine in QU-16, which of
all the analyses has the lowest ^40K (0.302 ppm), but has the highest
^40Ar* (3.65 x 10^-4ppm).
Figure 2. K-Ar plot for basalts of Toroweap Dam. If the lava dam has
an "age" of 1.2 Ma, the three QU-16 mineral concentrates should plot
as a line on the 1.2 Ma reference isochron with whole rock sample
"A-FLOW" (arrows indicate where each mineral concentrate should plot).
Instead the mineral concentrates plot significantly above the 1.2 Ma
reference isochron, arguing that the lava dam contains significant
"excess radiogenic argon." Can any basalt sample from the Toroweap Dam
be assumed to be free of "excess radiogenic argon?"
If "A-Flow" is actually 1.19 +/- 0.18 Ma, then the mineral
concentrates from QU-16 should each lie on the line in Figure 2
describing an isochron through "A-Flow". The new data do not lie on
that line, but significantly above that line. Why does the basalt of
Toroweap Dam give discordant K-Ar "ages"? There must be "excess argon"
in the olivine of QU-16. Are we sure there is not "excess argon" in
the olivine in "A-Flow" sampled and analyzed as a whole rock by McKee,
Hamblin and Damon [12]? Because many basalts of Grand Canyon have been
shown to contain "excess argon" (e.g., admission by Hamlin [5, p.
199]), we can ask a more important general question. Has any Grand
Canyon lava dam been demonstrated not to contain "excess argon"?
The ages of the remnants and dams were deciphered by Hamblin [6] using
both the relative dating method of juxtaposition and the "absolute
ages" determined by K-Ar dating. However, the K-Ar dates in many cases
do not match the relative sequence worked out by juxtaposition. This
may be why most have been discarded after K-Ar analysis as containing
"excess argon". The results of Hamblin's work concerning the relative
sequence of development for his 13 dams, along with other important
details, are listed in Table 2.
Table 2. Characteristics of Lava Dams of the western Grand Canyon
(after Hamblin [6]).
K-Ar Number Dam Lake Water Sediment
Elevation Height Age of length length fill fill
Dam (m) (m) (Ma) Flows (km) (km) time time
-------- --------- ------ ---- ------ ----- ---- ---- -------
Prospect 1200 699 1.8 3 ? 518 23 yr 3018 yr
Lava Butte 1050 560 ? Several ? ? ? ?
Toroweap 927 424 1.2 5 16 283 2.6 yr 345 yr
Whitmore 750 270 0.99 40+ 29 173 240 d 88 yr
Ponderosa 840 339 0.61 1 19 202 1.5 yr 163 yr
Buried Cany 744 255 0.89 8 ? 173 231 d 87 yr
Esplanade 780 288 ? 6-8 13 174 287 d 92 yr
"D" Dam 689 191 0.58 40 ? 123 87 d 31 yr
Lava Falls 678 180 ? 1 35 123 86 d 30 yr
Black Ledge 610 111 0.55 1 138+ 85 17 d 7 yr
Layered Diabase 581 89 0.62 20 22 67 8 d 3 yr
Massive Diabase 548 68 0.44 1 16 64 5 d 1.4 yr
Gray Ledge 544 61 0.78 1 21 59 2 d 0.9 yr
The tens of thousands of years Hamblin [6] has interpreted for each of
the 13 dams to form and then erode are seemingly impossible to
reconcile within the several thousands of years of the post-Flood
period. However, several geologic relationships indicate that the dams
actually formed rapidly and failed catastrophically within a period of
less than several hundred years. Furthermore, it is also evident that
several of the 13 dams coexisted, as previously interpreted by Maxson
[10]. We shall highlight these important conclusions in the following
sections by addressing: (1) duration of dam formation (the amount of
time required for each of the individual dams to form); (2) duration
of the dams (the amount of time each dam was in existence after
formation); and (3) the temporal relationship of dams (the amount of
time that transpired between erosion of one dam and the formation of
the next dam).
DURATION OF DAM FORMATION
Hamblin [6] estimated that the total volume of all 13 lava dams was
near 25 km^3. The flows are composed of olivine basalt, nearly
identical to those expelled during the highly fluid, late Cenozoic
eruptions of the western United States and other regions of the world.
Fissure eruptions of the Columbia River Basalt resulted on occasion in
the expulsion of hundreds and even thousands of cubic kilometers of
lava in individual flow events [16]. During the historic Lakagigar
eruption of June 8, 1783 in Iceland, a total volume of approximately
12.2 km^3 of olivine basalt lava was expelled over a period as short
as eight months [15]. This eruption resulted in a complex sequence of
thin vertically stacked lava flows very similar to flows seen in the
Uinkaret Volcanic Field.
The single flow lava dams of the western Grand Canyon (refer to Table
2) could, therefore, have formed within periods as short as several
hours or days. The most extraordinary example is Black Ledge Dam,
which consists of a solitary flow up to 111 m thick and over 138
kilometers long. The Black Ledge lava must have been fast flowing in
order to spread over such a long distance. The appreciable thickness
of the flow probably resulted from damming along the front edge of the
flow as it cooled and hardened. Three other single flow dams (Lava
Falls, Massive Diabase and Gray Ledge) have been identified. Thickness
are from 61 m to 180 m and lengths are between 16 km to 35 km.
Obviously these dams also could have formed over a very short period
of time.
Five dams (Prospect, Toroweap, Ponderosa, Buried Canyon and Esplanade)
where formed by as few as 3 to 8 flows. Most of these flows are near
100 m in thickness. Prospect Dam consists of three major flows ranging
from 180 to 250 meters thick. The main remnant of Ponderosa Dam
contains one major flow over 300 m thick. Esplanade Dam actually
contains laminated tephra that passes laterally into at least three
lava flow units. This shows that the tephra was deposited
contemporaneously and at near the same rate of the adjacent flows. The
multiple flow dams would have taken longer to form than the single
flow dams, but could have still formed within a very short period of
several months, as demonstrated by the development of stacked multiple
flows in the Lakagigar eruption.
The remaining four dams (Lava Butte, Whitmore, `D', and Layered
Diabase) are composed of numerous thin flows from 10 to 40 in number.
These dams probably took the longest time to form. However, the total
time required could have been still very short, probably as short as
several years. Only a short amount of time (the time required for the
upper surface of a flow to cool) is necessary before a subsequent flow
covers the previous flow and creates a bedding plane between them.
Many of the dam remnants show evidence of erosion between flows, and
also contain interstratified and capping gravel beds. Remnants of
Whitmore Dam along the south wall of the inner gorge contain several
interstratified gravel beds. Although many of the gravel beds lie on
top of flows that exhibit little if any undulatory relief, areas of
moderate scouring indicate erosion did occur. Similar patterns of
interbedded gravels and moderate scouring are found in many of the
other dam remnants, including Prospect Dam and Esplanade Dam. The main
remnant of Buried Canyon Dam is capped with a massive stratified unit
of coarse gravel 60 m thick and contains blocks up to 1 m in size.
Remnants of Gray Ledge Dam are overlain with very coarse cross-bedded
gravel deposits up to 45 m thick and contain clasts as large as 15 m.
Erosion and deposition are typically used as a uniformitarian
indicator of the passage of a significant amount of time. Therefore,
based on this interpretation, the dams would have taken at least
several hundreds of years, if not thousands to build-up to account for
such erosion and deposition within the dam structures. However, it is
peculiar that thick gravel deposits are found at all within the dam
structures, and we contend that this actually is an indicator for a
rapid process of dam erosion and gravel deposition.
The addition of the volcanic flows into the course of the Colorado
River would have raised the stream bed above the previously
established base level. This would mean that the regions occupied by
the dam would have been subjected to an interval of sustained erosion
until the structure of the dam was worn down to the original base
level. The dam structure could have grown only by the addition of
lava, and not by gravel from stream bedload accumulation. The stream
bed across the dam would have been relatively clean of gravel, except
for relatively small quantities of gravel material in transport. Thick
accumulations of gravel could not have occurred under normal stream
flow conditions.
Clearly, the only process that could account for both the evidence of
erosion, and, the accumulation of thick gravels, would be periodic
catastrophic flooding. During the initial stages of the flooding
episode, erosion of the dam would have been taking place by flood
bedload scouring and cavitation. During the waning stages of the
flood, the sediment load would have dropped out and accumulated on top
of the dam structure, where it then could have been covered by
subsequent lava flows. Rogers and Pyles [13] have suggested that many
of the gravels are the result of high energy/flow breachment or
catastrophic breakout of a dam crest. The coarse cross-bedded gravel
deposit with blocks of up 15 m seen on Gray Ledge flows was clearly
formed by high energy water flow, probably resulting from a dam
breachment event.
DURATION OF DAMS
The best test to determine how long an ancient dam was in existence is
to ascertain the degree to which the lake behind the dam was filled
with sediment. The sediment that a river normally carries along its
coarse will be caught and deposited within the lake created behind the
dammed river. The length of time required for siltation can be
determined if both the volume of the lake and the sediment transport
load of the river are known. The larger the lake, the longer it will
take for the sediment to fill completely that lake. This test can only
be used to place a minimum number of years for dam longevity, because
once the dam is completely silted-in, the sediment that the river is
carrying will then be transported over the dam. The siltation time
required for each of the lakes formed behind the 13 dams has been
calculated by Hamblin [6] and is based on the sediment load carried by
the modern Colorado River into Lake Mead (refer to Table 1).
Recent surficial deposits related to fluvial-type processes are
relatively sparse within the Grand Canyon. The Geologic Map of the
Eastern Part of the Grand Canyon (1996) identifies two main types of
surficial deposits; river gravels and alluvium. The river gravels are
limited to very recent deposition along the banks of the Colorado
River. The alluvium occurs in isolated outcrops primarily within the
broad valley floors of Nankoweap Creek, Kwagunt Valley, Sixtymile
Creek and Chuar Valley, and is found on terraces at elevations up to
1500 m (645 m above river level). Remnants of a thin gravel and
boulder deltas are found on terraces at 930 m elevation on both sides
of the Colorado River downstream of Comanche Creek (miles 67 to 73)
[9]. This was probably a temporary delta into the lake behind the
Toroweap Dam. Other relatively large bodies of surficial-type alluvial
deposits are found within Havasu Canyon, at Lee's Ferry, and at
several locations in the Lake Powell region. Hamblin [6] believed that
this alluvium was derived from deposition within the larger
Pleistocene lava-dam lakes. An extraordinary deficiency of lake
sediments exists in the canyon of the Little Colorado River. Apart
from these few areas, other significant deposits of supposed lake
deposits are peculiarly absent.
The alluvium (lake sediments) in the eastern Grand Canyon consists of
several small to large gravel deposits located mostly on the west
(left) side of the Colorado River. The larger deposits consist of four
outcrops within the upper basins of Nankoweap Creek, Kwagunt Valley,
Sixtymile Creek and Chuar Valley (refer to Figure 1). These outcrops
range in size from 2 km^2 to 5 km^2 and are up to several tens of
meters thick, extending up-basin to elevations ranging from 1285 m to
1500 m. These elevations indicate that the gravel deposits are most
likely related to Prospect Lake. Local uplift across one or several of
the normal faults of the Grand Canyon, sometime after failure of
Prospect Dam, has probably raised these deposits above the 1200 m
level of Prospect Lake.
Typical gravels contain clasts derived locally from each particular
depositional basin. Therefore, most deposits are the result of gravel
deltas that built outward into the main lake body, and are not derived
from material transported down the Colorado River. Elston [4] believed
that they may record aggradation by flash flooding. These gravels
probably once extended all the way down to the Colorado River, where
similar small isolated gravel deposits occur. One small outcrop,
located where Nankoweap Creek enters the Colorado River, is overlain
by silty alluvium material and underlain by gravel which contains
exotic clasts derived well upstream of the Colorado River. Numerous
other similar small isolated deposits occur downstream all the way to
Big Bend (mile 75). West of Big Bend these types of alluvial gravels
are not found. The lower gravel units containing exotic clasts may
represent the initial lake deposits transported down the Colorado
River into Prospect Lake.
The up-basin gravels are overlain along their edges by several small
to very large units of talus and landslide debris. These debris
deposits are not known to underlie the gravels in any significant
quantities. The onlapping relationship of the talus and landslide
debris indicates that mass wasting was a post-lake event and may have
resulted from slope instability caused by rapid lake drawdown.
The next large lake sediment deposit occurs within Havasu Canyon. The
main unit consists of a long thin deposit extending 8 km up Havasu
Canyon from Beaver Falls to the Havasupai Indian village. Smaller
isolated outcrops occur both downstream and upstream of the main
deposit. The sediments are composed primarily of silt and fine sand
with interbeds of travertine. Travertine has also armored the surfaces
of the deposits in many areas, particularly along the course of Havasu
Creek. The main deposit reaches a high elevation at 960 m, with small
isolated outcrops preserved on the upper canyon walls at as high as
1032 m. Hamblin [6] believed that these deposits may include material
from several lava-dam lakes, the highest from Prospect or Lava Butte
Lake. The main deposit at 960 m may be from Toroweap Lake. Hamblin
stated that the sediment contains thin horizontal laminae similar to
lake deposits in Lake Mead and Lake Bonneville. However, the sediments
also contain medium- to micro-scale cross-bedding, showing that they
were also influenced by current flow. This suggests that deposition
may have occurred down Havasu Canyon as an aggrading delta, and not up
canyon from material derived from down-river transport of the Colorado
River. Therefore, the deposits at Havasu Canyon are an isolated,
localized unit and not the result of the complete infilling of a large
lava-dam lake.
A sequence of gravel, sand and silt occurs just west of Lee's Ferry,
near the confluence of the Paria and Colorado Rivers. This deposit
occurs at and elevation of 1080 m and consists of an upper gravel unit
with clasts over 6 inches in size overlying laminated sand and silt.
Hamblin [6] argues that the sand and silt are indicative of lake
deposits and could not be the result of deposition from the high
energy flow of the Colorado River. However, his explanation does not
address the coarse gravel cap which would have required swiftly moving
currents. We contend that the proximity of this deposit near the
confluence of the Paria is no coincidence and that they are
genetically linked. Swiftly moving currents from flash floods could
have readily transported the entire unit (gravel, silt and sand) in
one or several phases of deposition. Here again, this material is the
result of an aggrading delta fed by material down a tributary drainage
(Paria River) into the lava-dam lake.
The water level of the lava-dam lakes ranged in elevation from a low
of 544 m (61 m above the river) for Gray Ledge Lake to a high of 1200
m (699 m above the river) for Prospect Lake. The majority of these
lakes would have been confined to the thin long channel of the steep
sided inner canyon gorge. The exceptions would have been Prospect
Lake, Lava Butte Lake, and Toroweap Lake. These three lakes would have
been high enough to extend a considerable distance up many of the side
canyons, including Havasu and Kanab Canyons. Prospect Lake was by far
the largest and within the Grand Canyon it would have covered more
than three times the surface area of Toroweap Lake and extended well
up into all of the side canyons including those of the eastern Grand
Canyon, all the way through the Little Colorado River gorge, and over
z of the distance up Havasu and Kanab Canyons. Lake Prospect would
have also extended past present day Lake Powell, approximately 90 m
higher then the present high water elevation of the lake. Below Grand
Canyon Village, Prospect Lake would have completely inundated the
Tonto Platform by over 90 m up to the base of the Redwall Limestone.
The sediment fill time for Prospect Lake (3,000 years) is well below
the 10,000-year duration of Prospect Dam as determined by Hamblin [6],
and would therefore have had more than enough time to fill completely
with sediment under uniformitarian conditions.
Prospect Lake sediments should have been preserved in literally
thousand of locations, ranging from very small to large remnants, if
in fact Prospect Lake was completely sediment filled. Likely areas of
preservation would be within the myriad of protected pockets of small
and large side canyons, and on top of elevated flat-lying surfaces
such as the Tonto Platform where erosion is at a minimum within the
canyon. Appreciable sediment preservation within protected areas would
have also occurred from accumulation in Lava Butte and Toroweap Lakes.
Because the remainder of the lakes were confined to the steep sided
inner gorge, sediment preservation would have been less likely.
The most interesting characteristic of the lake deposits is that,
nearly without exception, they all occur in low-lying drainages which
are areas of the most active erosion (aside from the main Colorado
River channel). Lake deposits are not found in areas protected from
erosion such as within the thousands of tributary canyons or, most
puzzling, on top of the Tonto Platform such as below Grand Canyon
Village where sediment depths of over 90 m should have occurred. In
fact, the pattern of occurrence of the lake sediments is exactly
opposite of what would be expected. This pattern indicates that the
lake deposits are not remnants left over from erosion of a
sediment-filled lake, but are relatively intact uneroded depositional
units formed by aggrading deltas building outward from the side canyon
tributaries into the main lake body. This shows that the lava-dam
lakes were never completely filled with sediments, and, therefore,
were very short-lived features. We estimate that this relatively small
quantity of lake sediment could have been deposited in a period of
less than 100 years. Multiplying this value by 13 for the number of
total possible lava dams, we obtain a total of 1300 years for the
duration of lava dams blocking the flow of the Colorado River.
TEMPORAL RELATIONSHIP OF DAMS
Early investigators [10] [11] concluded that only a small number (one
to three) of dams blocked the Colorado River. Maxson [10] determined
that only three lava dams existed, based on the presence of finely
laminated lake-deposited tephra interbedded with volcanic flows which
strongly indicated dam coexistence (found between miles 180.5 and
194.5). Hamblin [6] discounted Maxson's [10] contemporaneous lava dam
theory and proposed that the tephra developed within temporary lakes
formed by landslide dams. Hamblin offered no substantive evidence for
the landslide dams.
McKee and Schenk's [11] and Maxson's [10] premise that only a small
number of lava dams once blocked the inner gorge was founded on the
fact that nearly all lava remnants are composed of a similar olivine
basalt. Although they undoubtedly noted the depositional and textural
differences between individual outcrops, the compositional similarity
obviously was paramount in their interpretation.
Hamblin's work does suggest strongly that there have been numerous
lava dams within the inner gorge. However, his belief that all 13 dams
were separate and non-contemporaneous features is not necessarily
supported by the data. First of all, the finely laminated tephra
observed by Maxson [10] is clear evidence of dam coexistence.
Secondly, many of the remnants are compositionally and texturally
similar, which underscores the potential for error in Hamblin's
correlations. Finally, many of the K-Ar dates obtained from dam
remnants are completely ambiguous and yield dates entirely out of
sequence from that determined by the reliable relative dating method
of juxtaposition. Referring to Table 1, the 9 youngest dams (Ponderosa
through Gray Ledge Dams) yield K-Ar dates that are out of sequence.
Our own date determined from a remnant of Toroweap Dam is older than
the oldest date determined for Prospect Dam. We believe that it is not
only possible, but highly probable (based on tephra deposits and K-Ar
dates), that several lava dams coexisted as either separate dam
structures, or even overlapping dam structures.
Figure 3 is based on Hamblin's [6] geologic map, and shows the
overlapping relationships between different dam remnants that were
observed in contact in at least one locality. For example, the first
mark in the upper left of the table indicates that one or more
remnant(s) of Gray Ledge Dam overlies a younger remnant(s) of Massive
Diabase Dam. The table shows that there are a total possible 78
different combinations of dam remnant overlap. However, as the table
also shows, only 19 overlap combinations were actually found by
Hamblin. Hamblin questioned two of these overlap relationships. These
are indicated on the table by the queries. Therefore, only 17 dam
remnant overlap combinations are known with certainty. Hamblin worked
out his interpretation of the relative sequence of lava dams from
these 17 dam remnant overlap combinations.
Figure 3. Relative Age /Juxtaposition Relationships of Lava Dams.
The upper section of the table shows the contact relationship between
the 6 youngest dams (indicated on the table as: Juxtaposition
Relationship Between 6 Younger Dams - Gray Ledge through D-Dam). Out
of a total of 15 possible overlap combinations, there are 8 (53%)
found for the younger dam remnants. This is a high percentage and
indicates that the relative sequence for the younger dams has a high
degree of reliability. The lower section of the table shows the
overlap relationship for the older dams (Juxtaposition Relationships
Between 7 Older Dams - Buried Canyon through Prospect). Only 2 overlap
contacts out of a total possible 21 combinations (10%) are found in
the inner gorge. This is a very low percentage, and the only relative
sequence that can be determined from these two contacts is that
Esplanade Dam is older than Ponderosa Dam, which is older than
Prospect Dam. Therefore, the relative ages of Buried Canyon, Whitmore,
Toroweap and Lava Butte Dams cannot be worked out amongst these older
dams based on juxtaposition. Therefore, it is possible that these four
dams could have been part of a one large single dam complex, or any
other combination of one or more of these four dam units.
OTHER EVIDENCES FOR CATASTROPHIC DAM FAILURE
Slope Failures Upstream of Lava Dams
The majority of the slopes of the Grand Canyon are devoid of a notable
build-up of talus from mass wastage or landsliding (slope failures).
This general absence of talus deposits is an indicator of the long
term stability of the Grand Canyon slopes. Several large deposits of
talus do occur and are primarily isolated at two localities. The first
location is at Surprise Valley between miles 134-139, and the second
is the eastern Grand Canyon in the same areas of the previously
described lake deposits. Only a few mappable talus deposits are found
in the region between these two localities.
The Surprise Valley landslide, which is the largest slope failure in
the Grand Canyon, stretches along the Colorado River for over 8 km and
is over 2 km wide. It is estimated that the slide has a volume of over
5.5 billion cubic yards.
The slope failures in the eastern Grand Canyon, although much smaller
then the Surprise Valley landslide, are also enormous in dimension.
The largest occurs along the south wall of Chuar Valley, and is 5 km
long and up to 1 km wide. Other large slope failures occur in Unkar
Creek, Kwagunt Valley, and Nankoweap Creek.
Rogers and Pyles [13] assert that water saturation from the
Pleistocene lava-dam lakes was the instrumental factor in the failure
of these large slope areas. Although rapid drawdown of the lake waters
was not necessary for slope failure to occur, it would have
facilitated failure if drawdown occurred prior to complete slope
saturation. The failure would result from what Rogers and Pyles [13]
describe as the development of large hydrostatic forces that act on
the free face of a slope as the water tries to reestablish equilibrium
conditions after a sudden lowering of the water level by dam failure.
The slope failure talus is almost without exception always overlying
the lacustrine deposits where they occur together. In relation to
this, Elston [4] makes the following statement: " The relations thus
seem to indicate that gravel had accumulated along the course of the
river prior to the episode of catastrophic landsliding, and that it
was a time that the Colorado River was not actively removing detritus
from the area. The pre-landsliding episode of aggradation thus appears
to parallel the episode of aggradation seen in the eastern Grand
Canyon, and landsliding can be inferred to have occurred during the
episode of aggradation." This relationship, therefore, shows that
landsliding occurred only after deposition of the isolated lake
deposits and that it is related to rapid lake drawdown.
Inner Gorge Widening from Flooding
The inner gorge of canyon widens noticeably at mile 181 and again at
mile187.5. The widening at mile 181 is directly downstream of the
larger and older dams (Prospect, Toroweap, Ponderosa, Lava Butte). The
widening at mile 187.5 is also downstream of these four dams, as well
as the Buried Canyon, Whitmore, and Esplanade Dams. The widening can
be easily explained by catastrophic dam failure and subsequent
flooding.
Near mile 183, a large side canyon on the south side of the inner
gorge opens along an upstream alignment at the first bend in the river
downstream of the older dams. This upstream alignment is anomalous to
the normal alignment of side canyons, which is typically perpendicular
to the inner gorge. The upstream side canyon alignment is probably the
result of flood waters impinging upon the outer inner gorge wall of
the river bend, causing erosion and formation of the side canyon.
MECHANISM OF CATASTROPHIC DAM FAILURE
Natural dams are typically prone to catastrophic failure by
overtopping. Costa [3] gives two examples of historical breakouts
(1982 in Mexico and 1912 in Alaska) from failure of volcanic dams. In
both of these cases, the dams failed within a year of formation. The
flood from failure of the Alaskan volcanic dam caused scouring of 1 to
2 m and transported coarse gravel with clasts up to 50 cm diameter
over a 20 km distance.
Catastrophic overtopping or breachment of the lava dams was probably
caused by movement on the Toroweap and Hurricane Faults. Movement
along these faults could have caused both mechanical fracturing of the
dam structures leading to failure, and lake seiches resulting in dam
overtopping. The main remnant of Toroweap Dam shows a decreasing
amount of up-section fault offset across the Toroweap Fault, showing
that the fault was active during the formation of Toroweap Dam. In
fact, both the Toroweap and Hurricane Faults are probably still active
today [6, p. 4].
CONCLUSIONS
Several important geologic features, which have been previously
overlooked, give strong indication that the Pleistocene lava dams of
the western Grand Canyon formed rapidly and were destroyed
catastrophically within several tens to hundreds of years after
formation. We believe that the entire span of time from the formation
of the first dam to the destruction of the last dam could have
transpired over a time-frame of less than 2000 years. We consider our
time estimate to be generous, leaving open the probability that the
total time-frame could have been considerably less.
It is undisputed, by even uniformitarian geologists, that the several
single flow lava dams formed in a length of time as little as several
hours to days. The larger multiple flow dams (consisting of 3 to over
40 flows) are commonly stacked one atop the other with no signs of
significant erosion. Although it is clear that in many instances
interflow erosion has occurred, we have shown that the presence of
interflow gravels actually indicates catastrophic flooding, rapid
erosion, and deposition, and, therefore, does not require us to
accommodate hundreds to thousands of years for these erosional
features. Catastrophic flooding is clearly represented by the coarse
cross-bedded gravel on top of Gray Ledge remnants.
The most convincing evidence that the dams where short-lived
structures is the presence of relatively small isolated
depositionally-intact aggraded delta deposits within tributary
drainages of the eastern and central Grand Canyon. The fact that these
relatively uneroded deposits occur within the most active erosive
areas, and the absence of lake deposits on the least erosive areas
(Tonto Platform and protected side canyons), reveals that the larger
lava-dam lakes were not in existence long enough to allow for complete
sediment infilling. The small quantity of delta deposits that are
present could have accumulated easily in less than one hundred years.
Hamblin [6] believes that 13 separate lava dams once blocked the inner
gorge. The relative age of the 7 older dams were determined by only
two overlap relationships. This allows for the possibility that
several of these dams may have coexisted as a complex mega-dam
structure . The presence of tephra deposits within several dam
remnants is hard evidence that several of the dams coexisted.
K-Ar dates for many of the lava dams are out of sequence from that
determined by juxtaposition. These essentially "impossible" dates show
the difficulty in assessing the sequence of the dam remnants, and
reveals the possibility that many of the correlations proposed by
Hamblin may be in error. Furthermore, a sample of Toroweap Dam
retrieved and dated in this study yielded dates of 3.1, 3.4 and 20.7
Ma, which are significantly older then the date (1.8 Ma) of the oldest
dam (Prospect) determined in Hamblin's study. Either Hamblin's dates
should be much older or the samples of Toroweap dam contain excess
argon. In any case, the K-Ar dates obtained in this and Hamblin
studies reveal the inherent problems of this dating method, casting
doubt on the standard interpretation of 1.8 Ma for the Pleistocene
Epoch.
The presence of lava-dam remnants near the present level of the
Colorado River reveals that the canyon has undergone only negligible
deepening since the time the dams originally formed. Furthermore, the
normal flow of the Colorado River has not appreciably widened the
inner gorge. Under a uniformitarian interpretation, this means that
the Grand Canyon has not undergone appreciable erosion at least for
the 1.8 million year period of the Pleistocene. A better
interpretation [1] would be that the Grand Canyon is a relic
flood-formed feature, and, likewise, that the lava dams were
short-lived, catastrophically formed and eroded features.
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