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The Grand Age of Rocks: The Numeric Ages for Rocks Exposed within Grand Canyon
Allyson Mathis and Carl Bowman, 2006
Grand Canyon National Park
[see Photo 1] is one of the best places in the world to gain a sense
of geologic, or “deep,” time
because the canyon exposes a great swath of geologic history. Rocks
exposed in Grand Canyon are truly ancient, ranging from 1840 million
years old (m.y.), or 1.84 billion years old (b.y.), to 270 m.y. The
Grand Canyon landscape is geologically young, being carved within just
the last 6 m.y. There are younger geologic deposits in Grand Canyon too,
such as the Ice Age fossils found in caves, a 1000-year-old lava flow
in the western canyon, and even the debris flow deposits that continue
form each year.
Yet, it is the canyon’s rock walls that allow people to develop their greatest perspective on geologic time, because of these rocks’ immense age, their fossil record, and because these rocks formed in environments far different than those found in northern Arizona today. With a rock record that spans more than 1500 m.y., Grand Canyon is truly a panoramic view into the geologic past.
Nonetheless, how geologists determine the age of rocks
is a mystery to many members of the public, and even to many park rangers
[see Photo 2], guides, and others who share the
canyon’s geologic story with others. It is natural to wonder “How
do you know that?” when geologists say a Grand Canyon rock formed
270 million years ago. Further confusion arises when one publication
or geologist says, for example, that the Kaibab Formation is 270 m.y.
and another says 255 m.y. The same questions arise for the other rock
units at Grand Canyon. Which are the more correct ages and why?
This article will answer these questions by providing a short primer on geologic dating methods and how they were applied to Grand Canyon rocks. We then describe Grand Canyon rocks as belonging to three “sets,” or packages of rocks, each with unique geologic histories, and present a compilation of “best” numeric ages for Grand Canyon rocks to use when explaining Grand Canyon geology to the public.
Figure 1: Relative dating diagram. more »
Photo 1: Many people consider Grand Canyon National Park the world’s premiere geologic landscape and a “geologic wonder
”. Grand Canyon contains many important geologic resources, including a vast fossil record ranging from Precambrian stromatolites to Ice Age mammal bones and dung found in caves; a potentially active volcanic field in the western Grand Canyon; a geologic history ranging more than 1.7 billion years; and the canyon landscape itself as the greatest example of arid land erosion.
Photo 2: Park Rangers like Stacy Wagner present geology walks and talks daily at Grand Canyon National Park.
There are two major categories of geologic dating techniques: relative dating and absolute age determinations . Relative dating determines the order in which a sequence of past events occurred, but does not determine exactly when the geologic events happened. Absolute age determinations are numeric and identify when, in years, specific events happened. Both techniques are important in different geologic situations. And both techniques are used together to develop the geologic time scale and to discern the ages of rocks exposed in Grand Canyon.
Relative dating can be easy. In flat-lying
sedimentary rocks it is simply the Principle of Superposition [see Photo
3] (or the “rule
of pancakes”): the rocks at the bottom are oldest, and the rocks
on top are the youngest. It’s not who’s on first, but what’s
on top. When rocks are folded, faulted, overturned, or intruded by igneous
rocks, it is still possible to determine the relative age of the rocks.
The Principle of Superposition and the Principle of Original Horizontality
[see Photo 4] (sedimentary rocks are
always deposited flat, i.e., horizontally) allow geologists to reconstruct
the sequence of events. Cross cutting relationships [see Photo 5] also
demonstrate relative ages because what does the cutting (whether it is
a fault or a dike) must be younger than what is cut. You can’t
cut into a stack of pancakes until they’re stacked up! See Figure
1 for illustrations of other relative age relationships at Grand Canyon.
Correlation determines if rocks are the same age, and
is a key tool that geologists use to identify the relative ages of rocks
where a rock layer is not continuously exposed. It is also how geologists
relate rocks from one region to another. Rock type (or lithology) is
usually not a good basis for correlation. Some rock types look much the
same regardless of their age (the 275 m.y. Coconino Sandstone in Grand
Canyon looks similar to the 160 m.y Navajo Sandstone in southern Utah)
[see Photos 6a&b].
Lithology is also a poor basis for correlation because different rock
types can be deposited at the same time in adjacent areas because of
different environments. For example, in Grand Canyon, there are many
examples of sandstone deposition on a beach while limestone was forming
Geologists usually correlate sedimentary rocks based on their fossil assemblages, particularly on the presence of index
fossils. William “Strata” Smith first recognized in the early 1800s that the fossil record has changed systematically
through time. Smith used this observation to correlate rock units throughout
England and make the world’s first geologic map. Again, the principle
is simple: for example, all rocks with trilobite fossils [see Photo 7] in them
are Paleozoic in age, whereas rocks containing dinosaur fossils are Mesozoic.
No rock layers contain both trilobites and dinosaurs (or dinosaurs and humans),
so these distinctions can be very clear. In fact, the names of the eras in
the Geologic Time Scale reflect the relative ages of fossil assemblages: Paleozoic means “ancient
life,” Mesozoic means “middle life,” and Cenozoic means “new
life.” Further subdivisions of the time scale, such as Periods and Epochs
are also based on changes within fossil assemblages with time (for example,
dinosaurs are in rocks of the Mesozoic Era, Tyrannosaurus rex is only found
in the Cretaceous Period of the Mesozoic).
Index fossils are usually from microscopic organisms that lived in widespread environments for a short period of time. A good collection of index fossils in a rock layer allows a very precise assignment of relative age. An “index fossil” in the realm of clothing fashion could be polyester leisure suits – they were widespread, and thankfully, only around for a brief time. Someone looking at a photograph with men dressed in leisure suits would be able to correlate that photo to others of the same time period in which men are wearing leisure suits. But to know that these photos were taken in the 1970’s requires another type of dating, an “absolute age determination.”
Photo 3: Given the "layercake" geology (as it is commonly called) of Grand Canyon, the Principle of Superposition is easy to apply.
Photo 4: Sedimentary rocks are typically deposited as vast horizontal layers. Since the Grand Canyon region has undergone very little structural deformation since the deposition of these rocks, the original horizontal layering is still evident.
Photo 5: This exposure of the Grand Canyon Supergroup clearly shows a cross-cutting relationship. The basalt dike cuts the red Hakatai Shale indicating that the dike must be younger than the shale.
Photo 6a&b: Rock type is usually not a good basis
for age correlation. Some rock types, such as sandstones, look nearly identical
regardless of their age. Both the 275 million-year-old Coconino Sandstone
and the 160 million-year-old Navajo Sandstone were deposited in large sand
dune fields, but at different times and in different locations (northwestern
Arizona versus southern Utah). Although these two formations have nearly
identical lithologies, the Coconino Sandstone is about 115 million years
older than the Navajo Sandstone.
Photo 7: Trilobite fossils are found in several Paleozoic rock units in Grand Canyon, such as the Cambrian Bright Angel Shale and the Permian Kaibab Formation. But, there are no trilobites found in the Precambrian Grand Canyon Supergroup Rocks, or in the Mesozoic rocks found in Zion and Arches National Parks in Utah. Trilobites only lived during the Paleozoic - they evolved near the start of the Paleozoic and became extinct at the end of the era.
|Absolute Age Determinations
Although geologists were certain that
the Earth was old, at least on the order of 100s of millions of years,
by the time John Wesley Powell [see Photo 8] first explored
the Grand Canyon, it wasn’t until the 1950s that absolute dating
techniques were reliable and widely in use. Most absolute ages in geology
are obtained through radiometric dating techniques [LINK TO KellerLynn’s
Geologic Time Knowledge Center]. The constant rate of radioactive decay
of radioisotopes (such as uranium isotopes, potassium-40 and others)
provides the clock required to calculate the time when a rock formed.
This time of formation, whether it is the crystallization of an igneous
rock, the alteration of a metamorphic rock, or in rare circumstances,
the growth of a mineral in sedimentary environment, starts the clock.
The steady rate of radioactive decay constituting the clock “ticks.” Although
the radioactive decay is not a linear process like sand in an hourglass,
an hourglass provides a good analogy. Turning the hourglass over starts
the clock and the rate of sand falling through the hourglass provides
the “ticks.” The
ratio of sand remaining in the top portion to the sand at the bottom
allows us to calculate the amount of time since the clock started The
same type of measurement--comparing the amount of the parent radioisotope
and the amount of the daughter isotope, and using with the known rate
of radioactive decay for each radioisotope--allows geologists to calculate
when a rock formed.
In detail, the application of radiometric dating techniques
is complicated, see USGS
Most importantly, as in any scientific analysis, the right technique
must be applied to the right rock. And not every radiometric dating technique
will work on every rock.
Potassium-argon dating provides a relatively simple
example of a radiometric dating technique. This technique is useful for
dating many volcanic rocks. One isotope of potassium, potassium-40, decays
to argon-40, with a half life of 1.25 b.y. (This means that in 1.25 b.y.,
half the potassium-40 will have decayed to argon-40; in another 1.25
b.y., half again will have decayed, and so on, see Figure 2). Prior to
crystallization of the volcanic rock, argon-40 just bubbles out of the
magma. Solidification starts the clock, because the crystal structure
traps any argon-40 produced by radioactive decay. Dating the time of
crystallization of a volcanic rock requires analysis for the amounts
of potassium and argon-40 in the rock, and calculation of the age using
the known half life of potassium-40. Potassium-argon dating will not
work on sedimentary or metamorphic rocks, or on intrusive igneous rocks,
and is even problematic to use on young volcanic rocks (because of the
long half life of potassium-40), or on rocks with little potassium. But
it remains a very commonly used technique on a large number of volcanic
rocks that has provided many accurate absolute age determinations at
Grand Canyon and elsewhere [see Photo 9].
Some rocks in Grand Canyon that have been radiometrically
dated directly, using a variety of techniques (potassium-argon, uranium-lead,
and others) include the igneous and metamorphic rocks of the Vishnu basement
rocks and Grand Canyon Supergroup. These techniques reveal that the schists
of the Inner Gorge are about 1.7 billion years old [see Photo 10]. But
most rocks exposed in Grand Canyon can not be directly dated because
they are sedimentary rocks. Sedimentary rocks can only be radiometrically
dated in rare circumstances, such as when datable clay minerals grew
during deposition of the sediment. So how can we say, for example, that
the Kaibab Formation is 270 m.y? To do this, we must use the Geologic
which is the product of both relative and absolute dating, and worldwide
correlation of rock units. We can then correlate rock units exposed in
Grand Canyon to this time scale using their index fossils and other tools.
Radioactive decay, exponential decay graph. more »
Photo 8: Geologist, and one-armed Civil War veteran, John Wesley Powell, lead the first exploration through Grand Canyon in 1869. Major Powell planned the expedition to explore what he called the greatest exposure of the rock record on the continent. But, by the time he arrived in Grand Canyon, their supplies were so low and the crew so ragged, the journey was more an epic of survival than a scientific expedition. However, it led to the second Powell expedition through Grand Canyon. Powell later helped establish the US Geological Survey. Powell, and the other 19th and early 20th century geologists who followed him, did not know in absolute terms how old Grand Canyon rocks are, but recognized the great antiquity of the earth. More information on Powell's career and contributions to the nation can be found here
Photo 9: The young basalt lava flows in the western Grand Canyon are difficult to date using the potassium-argon (K-Ar) technique because these rocks contain little potassium, because of the long half-life (1.25 billion years) of 40K, and because these rocks contain a small amount of "inherited argon" (argon present in the magma prior to crystallization). A variation of the K-Ar technique, called argon-argon (Ar-Ar) allows for more precise age determinations (even on very young rocks) and also provides a self-checking mechanism for inherited argon. Ar-Ar and other dating techniques on these basalts, such as the ones that made the lava dams in western Grand Canyon, are revising the timing of volcanism in the western Grand Canyo
n and show how refinements in dating techniques can yield more precise radiometric dates and can increase understanding of the timing and relationship of geologic events.
Photo 10: Metamorphism of the Vishnu Basement Rocks occurred between 1706 - 1697 million years ago based on uranium-lead dating on a type of mineral (monazite) that grew in the schist during the metamorphism. The error range of individual dates was only 1 million years. The approximately 1700 million year old age for these schists is further supported by 1680 million year old granite dikes that cross cut the schists (using the principle of cross-cutting relationship).
Geologic Time Scale
The development of the Geologic Time Scale (Table 1) has been a cumulative undertaking by geologists, beginning in Europe, over the last 200+ years. Early geologists determined the relative age of rocks based on fossil correlation, and began to develop a worldwide generalized rock column. Geologists named subdivisions of this generalized rock column for areas with prominent rock outcrops from that time interval. For example, Permian rocks were first described in the Perm area of Russia, Pennsylvanian for the excellent coal-bearing outcrops in Pennsylvania, or Devonian for Devon in England’s west country.
Geologists began trying to calibrate this relative time
scale with absolute age determinations as early as the 1930s, when radiometric
dating techniques were first developed. Although most fossil-bearing
sedimentary rocks cannot be radiometrically dated themselves, their age
can be constrained by dating igneous rocks and using relative dating
techniques. Dating volcanic ash beds and lava flows interlayered with
the sedimentary rocks, and intrusive igneous rocks such as dikes and
sills that cross cut the sedimentary rocks allow this numeric calibration
[see Photo 5]. For example, a sedimentary rock lying between two lava
flows that were dated at 10 m.y. and 12 m.y. must have been deposited
between 10 and 12 m.y. The Geologic Time Scale continues to be refined
with more precise absolute age determinations, and with better worldwide
correlation of rock units and fossil assemblages.
Geologic time scales can be confusing. Although geologists
all use the same basic divisions of geologic time (like eras and periods),
there are many local scales, such as the North American chronostratigraphic
scale, used around the world for finer subdivisions. These different
scales work well in describing regional geologic histories, but they
are difficult to correlate on a world-wide basis. The International
Stratigraphic Chart is a chronostratigraphic (time-rock) chart
that emerged from the worldwide generalized rock column. It refers
to actual bodies of rocks that serve as the reference sections for
all rocks formed during that time unit. The International Stratigraphic
Chart is the basis for the
Geologic Time Scale 2004, which is the most accurate and up-to-date
time scale available. We used these charts as our basis for determining
the numeric ages for rocks in Grand Canyon that have not been assigned
absolute ages by researchers.
The Geologic Time Scale
Photo 5: This exposure of the Grand Canyon Supergroup clearly shows a cross-cutting relationship. The basalt dike cuts the red Hakatai Shale indicating that the dike must be younger than the shale.
Numeric Ages of Rocks Exposed in Grand Canyon
We compiled the overall age range for three sets of rocks exposed in Grand Canyon (Table 2), and a set of numeric ages for individual rock units at the formation and/or group level (Table 3), using absolute dates where available. For sedimentary rocks without absolute age determinations available in the literature, we based numeric ages on their reported chronostratigraphic ages correlated to the International Stratigraphic Chart (2004), and Geologic Time Scale 2004. We also consulted with geologists who have worked on Grand Canyon rocks (notably with Ronald
Blakey at Northern Arizona University who is a stratigrapher who works
with Mesozoic and Paleozoic rocks of the Colorado Plateau, and Michael
Timmons, at the New Mexico Bureau of Geology and Mineral Resources, who
did his PhD research on the Precambrian Grand Canyon Supergroup).
Numeric ages for rocks exposed in Grand Canyon National Park
Grand Canyon's Three Sets of Rocks
Canyon’s Three Sets of Rocks
Geologists, beginning with Powell [see Photo 8], have long recognized
that there are three main packages or “sets” of rocks exposed
in Grand Canyon (Figure 3) and yardstick strat column: the crystalline
rocks of the Inner Gorge, the tilted rocks of the Grand Canyon Supergroup,
and the layered sedimentary rocks in the upper two-thirds of the canyon.
As knowledge of Grand Canyon geology progressed, individual rock units,
or formations, were identified, and ultimately more than 100 formal
stratigraphic names were applied to mappable rock units found within
Descriptions of the canyon’s geologic history often focus more on individual rock layers, particularly the easily recognized, flat-lying sedimentary rock layers, and less on the overall stories of the three sets of rocks exposed in the canyon. The individual rock layers are like snapshots of the geologic past. Using the three sets places these snapshots into an album that gives an overall context to the story of each rock unit. For example, the Redwall Limestone was deposited in a shallow ocean approximately 340 m.y., yet it is only one of many sedimentary rock layers deposited in, along, or near a coastline that moved across what is now northern Arizona for about 250 million years, when this area was free of major geologic upheavals. Focusing on only the flat-lying sedimentary rocks of the upper part of the canyon overlooks the rich and dramatic geologic stories of the other rocks exposed deeper within Grand Canyon. Using the three sets of rocks can make this whole story easier to tell.
We use the informal term “sets” to indicate the three main packages of rocks exposed within the canyon. (“Set” is used because it is not a part of the formal stratigraphic hierarchy like “group,” “series,” or “complex.”) The three sets of rocks are differentiated on the basis of not only stratigraphic position, but also age, rock type, and overall geologic setting in which they formed (Table 2). The Vishnu Basement Rocks consist of all the crystalline rocks exposed near the bottom of Grand Canyon. The Grand Canyon Supergroup Rocks include sedimentary and volcanic rocks deposited in coastal basins and were tilted as joined continents separated. The Layered Paleozoic Rocks include the 3,000 – 4,000 feet (900 – 1200 meters) of flat-lying sedimentary rocks that make up the “stairstep” canyon (Figure 4).
Reproduction of Powell Figure 79. more »
Photograph of Grand Canyon with 3 sets of rocks labeled. more »
The Redwall Limestone was deposited 340 million years ago in a shallow ocean basin and is one of several sedimentary rock units (formations) deposited in what is now northern Arizona during the Paleozoic Era.
Photo 12:The Vishnu Basement Rocks contain all the igneous and metamorphic rocks found in the Inner Gorge of Grand Canyon.
The foreground is made up of the tilted rocks of the Grand Canyon Supergroup Rocks exposed near Nankoweap Butte.
The Layered Paleozoic Rocks below Yavapai Point.
Basement Rocks 1840 – 1680 m.y.
The making of the continent
The Vishnu Basement Rocks include the
Precambrian metamorphic (recrystallized due to heat and pressure) and
igneous (“fire-born”) rocks exposed within what Powell named
the “Granite Gorge.” [see Photo 15] This
rock assemblage records the building of North America by collisions,
in this case, of volcanic island chains with the continental landmass (to see more information about this click here).
These collisions generated intense heat and pressure deep within the
crust, forming metamorphic and igneous rocks while mountain ranges formed
at the surface. Like the basement of a house, basement rocks are the
foundation of continents. No older rocks exist beneath the Vishnu Basement
No single formal stratigraphic term refers to all igneous
and metamorphic rocks exposed in Grand Canyon. Therefore, we use the
informal term “Vishnu Basement Rocks” to refer to these rocks: “Vishnu” because
the public is familiar with the Vishnu Schist, and “basement” to
indicate the type of rock assemblage, and to imply that these rocks are
exposed at the bottom of Grand Canyon. The names “Vishnu Schist” and “Zoroaster
Granite” are well known to the general public, but they are only
two of at least 25 named rock units found in the Inner Gorge (GSA Bulletin article).
Therefore, these names can not refer to the full suite of diverse crystalline
rocks exposed in Grand Canyon. Even the formal term “Granite Gorge
Metamorphic Suite” (Ilg, et al, 1996) only includes the Vishnu,
Brahma, and Rama Schists, and excludes all igneous rocks found in the
basement complex. And unlike the metamorphic rocks, no single term includes
all the igneous rocks (“plutons”). Each pluton is a discrete
igneous intrusion with its own specific crystallization history, and
its own name [see Photo 16]. Referring to all of the
plutonic rocks as the “Zoroaster Granite” incorrectly oversimplifies
this complex assemblage of igneous rocks.
Absolute age determinations constrain the age of the Vishnu Basement Rocks. For igneous rocks, radiometric age determinations reveal the time of crystallization. For metamorphic rocks, radiometric dates can determine the time of metamorphism, or in specific cases, can even reveal the time of original crystallization of an igneous rock that was later metamorphosed.
Detailed geologic reconstruction and radiometric age
determinations have unraveled the geologic history of the Vishnu Basement
Rocks (Karlstrom et al, 2003). The oldest rock in Grand Canyon is the
Elves Chasm Gneiss, only found near Elves Chasm [see Photo 17]. It
formed 1840 m.y. and is 90 m.y. older than any other rock found in
the canyon. Although the specific origin of the Elves Chasm Gneiss
is unclear, it may be a fragment of older continental crust, or may
be part of the “basement for the basement.” Most
of the Vishnu Basement Rocks formed between 1750 and 1680 m.y. They
record a complex geologic history of volcanic island chains near a
continent between 1750 and 1730 m.y, in a region like the East Indian
island chains adjacent to Asia today. In the American southwest, the
ancient islands began colliding with each other about 1740 m.y., with
peak metamorphism and igneous intrusion occurring 1700 to 1680 m.y.
when the island chains themselves collided with the continent [see
Photo 18]. Each of the three schists formed
from different rock types. The Vishnu Schist formed from metamorphosed
sedimentary rocks, and the Brahma and Rama Schists are metamorphosed
volcanic rocks. The different igneous plutons likewise have different
geologic origins. Some are crystallized magma chambers from volcanic
islands, and others formed when the islands crashed into the continent.
John Wesley Powell’s flare for naming features at Grand Canyon did not always yield names that were geologically correct. The “Granite Gorge” contains more metamorphic schist than granite, and “ Marble Canyon” has walls of limestone, not marble.
Photo 16: The Ruby Pluton, exposed between River Miles 102 and 108, consists mostly of diorite, an igneous rock with a different composition than granite. Igneous rocks exposed in Grand Canyon rocks actually have a very wide range of composition and geologic history. Ruby Pluton formed as a magma chamber for the volcanoes of an island arc, prior to their collision with the main continental landmass. Other igneous rocks, like these pink granite (pegmatite) dikes [link to second photo (scenic2.jpg)], formed during the actual collision between the island chains and the continent.
The Elves Chasm Gneiss is only found between River Miles 112 and 118, and is substantially older than all other dated rocks in the Vishnu Basement.
Photo 18: These highly metamorphosed rocks are found in Phantom Canyon, a few miles from Phantom Ranch. These migmatites form under extreme metamorphic conditions that actually cause the rocks to start melting. Migmatites are properly thought of as being intermediate between igneous and metamorphic rocks.
Photo 19: The Cardenas Basalt forms dark colored slopes and ledges above the red shales in the Grand Canyon Supergroup. Lavas are commonly erupted in tectonically active basins, such as the ones in which the Supergroup rocks were deposited.
Photo 20: Desert View and Lipan Points are the best places to view the Supergroup from the South Rim. Since the Supergroup consists predominately of soft shales, the canyon is much wider at river level than where the extremely resistant Vishnu Basement Rocks form the Inner Gorge.
Canyon Supergroup Rocks 1200 – 740 m.y.
The rifting of continents
The Grand Canyon Supergroup Rocks [LINK TO strat column] consist of two main groups of rocks separated by nearly 300 million years of time, the older Unkar Group and younger Chuar Group. Separating these two groups of rock is a thin section of sandstone and mudstone called the Nankoweap Formation. Capping the Chuar Group is another thin sequence of mostly sandstone called the Sixtymile Formation. Collectively, these units are made up of mostly sedimentary rocks with only a few interlayered igneous rocks, such as the Cardenas Basalt [LINK TO photo 19 that includes Cardenas Basalt]. These rocks were deposited in tectonically active basins formed by faulting, or rifting, somewhat like the modern basins of the Basin and Range in Nevada. But the Supergroup basins were at about sea level during deposition.
At least some of these basins, such as the ones in
which the rocks of the Chuar Group were deposited, formed as an ancient
supercontinent called Rodinia separated and a new ocean basin opened. The 10 degree dip of these rocks resulted from tilting
and faulting that occurred during and after deposition of these sedimentary
rocks [LINK TO photo 20 showing tilted rocks]. Active faulting during
deposition allowed great thicknesses of Supergroup sediments to accumulate.
The cumulative thickness of the entire Grand Canyon Supergroup is about
12,000 feet (3,700 meters). The Supergroup rocks are visible today
in places along the Colorado River, especially from Carbon Canyon to
Hance Rapid, and again from Bedrock to Tapeats, where down-faulted
blocks protected them from subsequent erosion prior to the deposition
of the Layered Paleozoic Rocks. From the South Rim, Supergroup Rocks,
particularly the bright red Hakatai Shale of the Unkar Group, are visible
below Desert View or Lipan Point [LINK TO photo 20 Supergroup from
DV or Lipan Point] and from the Village area [see Photo 21].
Normally, sedimentary rock units are assigned numeric ages based
on the presence of index fossils, worldwide correlation, and the
geologic time scale. Unfortunately, the Precambrian Grand Canyon
Supergroup Rocks are so ancient that they formed before life diversified
and developed hard parts. There are few, if any, index fossils identified
in Precambrian rocks. The few fossils [see Photo 22] that have been
found in Supergroup rocks suggest their great antiquity, but they
cannot be used to pinpoint their age. However, the igneous Cardenas
Basalt and a few volcanic ash beds [see Photo 23] or zircon grains
in sedimentary rocks have been successfully dated. They bracket the
ages of the Unkar and Chuar Groups at approximately 1200 – 1100 m.y. (Timmons, 2005 – 2003,
personal communication) and 770 – 740 m.y. respectively (Dehler,
et al, 2005). The age of base of the Unkar Group was recently estimated
to be closer to 1200 m.y. in age rather than the previously reported
1250 m.y. based on a new age determination (Timmons, et al, 2005)
of zircon grains found near the base of the Unkar Group. The age
of the base of the Chuar Group is equally elusive, but emerging data
suggest that it is formed at about 770 Ma. This is an area of active
research and uncertainties remain for the age of these hard-to-date
The Nankoweap Formation was tentatively dated at 900
m.y. using paleomagnetism, which measures the natural magnetism preserved
in rocks from their time of formation and utilizes worldwide correlation
to infer geologic age (Timmons, 2005 – 2003, personal communication).
These absolute age determinations provide important age constraints
on the Grand Canyon Supergroup, yet our knowledge of these rocks’ history
is incomplete. The Sixtymile Formation [see Photo 24] has not been
dated, and, in fact, may not contain any datable material. Its age
may remain only as inferred as younger (but how much younger is unclear)
than 740 m.y.
Grand Canyon's three sets of rocks, showing lithology, age and thickness of individual units. For more information on the geology of each of the formations exposed in Grand Canyon see these USGS sites: Grand Canyon
and Colorado Plateau
The Grand Canyon Supergroup is visible on the north side of the river where down faulted blocks have preserved a wedge of these rocks. The bright red shale is the Hakatai Shale of the Unkar Group. This photo was taken from Plateau Point, which is directly below the Grand Canyon Village.
A larger boulder from the Chuar Group that is full of stromatolite fossils. Stromatolites form from cyanobacteria that grow in layered columns. Stromatolites were common in Precambrian seas.
Photo 23: Ash beds, such as this one in the Chuar Group, allow geologists to determine the age of this mostly sedimentary rock unit. Minerals within volcanic ash deposits can be dated using radiometric techniques, and used to constrain the age of the encompassing sedimentary rock assuming that the ash was deposited at the time of the volcanic eruption. Without ash beds and other datable units, the precise age of the Grand Canyon Supergroup Rocks would be unknown because of the lack of index fossils in these Precambrian rocks.
Photo 24: The Sixtymile Formation exposed on Nankoweap Butte. Since it contains no ash beds, or other datable material, and contains no known fossils, its precise age may never be known. As it is the uppermost unit of the Grand Canyon Supergroup Rocks, the only age constraint for this unit is that it is younger than the uppermost Chuar Group, which is approximately 740 million years old.
The stairstep pattern of Grand Canyon results from alternating harder and softer formations within the Layered Paleozoic Rocks. Vertical cliffs form where harder sandstones and limestones are exposed, and softer shales and siltstones make the slopes.
|Layered Paleozoic Rocks 525 – 270
The quiet edge of a continent
As with the Vishnu Basement Rocks, no single term delineates all the flat-lying
sedimentary rocks of Paleozoic age that are exposed in the cliff-and-slope
pattern of the upper canyon walls [see Photo 25].
Although each geologic formation in the Layered Paleozoic Rocks [LINK TO
strat column] records its individual depositional environment, together
these rock layers also form a comprehensive set based on rock type, age,
and overall geologic setting. For example, the Coconino Sandstone [See
Photo 26a, b]
reveals only an ancient coastal sand dune field, yet all the flat-lying
sedimentary rocks deposited during the Paleozoic Era reveal a quiet edge
of North America. And although each
individual rock layer of what we call the “Layered Paleozoic Rocks” is
more easily identifiable to most canyon visitors than the individual formations
within the other two sets, it is still important to place these geologic
snapshots into the context of their set or album.
Determining numeric ages for the Layered Paleozoic Rocks was more difficult
than that of the other two sets because no reliable absolute age determinations
exist for these sedimentary rocks. However, many of these rock units,
particularly marine units like the Kaibab Formation [see Photo 27], contain
abundant fossils from the Paleozoic Era. Through fossil correlation,
particularly using index fossils and relative age relationships, geologists
assigned chronostratigraphic (time-rock) ages for each rock unit (Beus
and Morales, eds., 2003). We then used these chronostratigraphic ages
and the Geologic Time Scale 2004 to assign numeric ages for each rock
unit. For example, the Redwall Limestone [see Photo 11] is late Early
to Middle Mississippian (Beus and Morales, eds., 2003). Given the boundary
between the Early and Middle Mississippian is at 345 m.y., and that more
of the Redwall was deposited in the Middle Mississippian than the early
part of the period, we estimated that the Redwall Limestone was deposited
approximately 340 m.y. We assigned numeric ages for the other formations
and/or groups within the Layered Paleozoic Rocks in a similar manner.
Of course, a single, inferred numeric age for sedimentary rocks is imperfect and cannot be entirely precise. It obscures the reality that most sedimentary rock units were deposited over long periods of geologic time. Still, the numeric ages are valuable as a way to communicate the age of rocks to people who are not familiar with the Geologic Time Scale and the relative age of geologic periods like Mississippian and Pennsylvanian. Nonetheless, our compilation of numeric ages for each rock unit of the Layered Paleozoic Rocks (Table 3) provides a single, reasonable, and standardized age to use for each layer.
It is important to note that deposition of sediments was not continuous
in the Grand Canyon region during the Paleozoic (between 525 and 270
m.y.), leaving significant gaps in age between some adjacent rock layers.
These gaps in the geologic record, produced by erosion or nondeposition
of sediment, are known as “unconformities.” Unconformities
[see Photo 28a, b, c, d, e] not only exist between the 3 sets
of rocks, but also within individual sets like the Layered Paleozoic
Rocks. For example, a large unconformity exists between the 505 m.y.
Muav Limestone and the overlying 385 m.y. Temple Butte Formation [see
Photo 29]. An unconformity is also present between the Redwall Limestone
and the Surprise Canyon Formation, which also was the last rock unit
to be identified within the Layered Paleozoic Rocks [see Photo 30].
Photo 26a, b: Rock type (in this case, a sandstone
made up of frosted round sand grains), fossils, and other geologic information
allow reconstruction of the ancient environment of deposition—a coastal
sand dune field. While the Coconino Sandstone was deposited above sea level,
other formations in the Layered Paleozoic Rocks such as the Kaibab Formation
were deposited below sea level.
Photo 27: The presence of brachiopods and crinoids such as the ones in this photograph, plus the occurrence of other fossils particularly microscopic index fossils reveal that the Kaibab Formation formed in the Roadian age of the Permian Period, which was approximately 270 million years ago.
Photo 28a: The Great Unconformity at Grand Canyon is among the world’s best known unconformities. The Great Unconformity is at the base of the Tapeats Sandstone, which is the bottom of the Layered Paleozoic Rocks. The Tapeats Sandstone sits either directly on top of the Vishnu Basement Rocks, where the time gap between the two sets is 1.2 billion years Tapeats Sandstone overlies the Grand Canyon Supergroup Rocks, it is an angular unconformity, and the time gap can be as short as about 220 million years.
Unconformities represent missing time periods. It is impossible to completely reconstruct what happened during that time period. For example, where the Tapeats Sandstone overlies the Vishnu Basement Rocks, clearly, at a minimum, a great deal of erosion had to happen after the formation of the basement rocks deep in the earth’s crust to expose them on the surface prior to the deposition of the Tapeats Sandstone. Gravel made up of cobbles of eroded Vishnu Basement Rocks may be present at the surface of the unconformity.
Photo 28c: Sometimes Grand Canyon Supergroup Rocks are preserved between the Vishnu Basement Rocks and Layered Paleozoic Rocks. This indicates that these rocks were deposited and tilted by faulting in the time interval between the formation and erosion of the basement rocks, and the deposition of the Layered Paleozoic Rocks.
Photo 28d: The unconformities between Grand Canyon’s 3 sets of rocks include the unconformity between the Vishnu Basement Rocks and Grand Canyon Supergroup Rocks between the Supergroup and the Layered Paleozoic Rocks , and between the Vishnu Basement Rocks and the Layered Paleozoic Rocks in areas where the Supergroup is missing.
Photo 29: The 385 million-year-old Temple Butte Formation was deposited in a channel cut into the 505 million-year-old Muav Limestone. This type of unconformity is called a disconformity, and indicates that there was a period of erosion between the deposition of the Muav Limestone and the Temple Butte Formation. The Temple Butte Formation is only present in these channel cuts in the eastern Grand Canyon, but thickens into a continuous layer up to 450 feet thick in the western Grand Canyon. The unconformity between the Temple Butte Formation and the Muav Limestone is one of several disconformities within the Layered Paleozoic Rocks.
The Surprise Canyon Formation also is bounded by unconformities on both its upper and lower contacts. It is never exposed as a continuous layer and is present in channel cuts into the underlying Redwall Limestone and as fill into a karst topography developed on top of the Redwall Limestone. This photo also shows a breccia pipe, which is another dissolution feature related to the Redwall Limestone. Caves that had formed in the Redwall Limestone later collapsed, forming pipe-like columns of brecciated rock. Because of their high porosity, these breccia pipes are locations of uranium mineralization.
Since each of Grand Canyon’s three sets of rocks is unique, different dating techniques were used to determine the age of rocks in each set. Radiometric dating techniques revealed the absolute age of the igneous and metamorphic rocks of the Vishnu Basement Rocks, and also provided dates on volcanic ash beds and other datable units in the mostly sedimentary Grand Canyon Supergroup. Relative dating, index fossils, and geologic correlation were used to determine the geologic age of the Layered Paleozoic Rocks, and numeric ages were then inferred.
A wide variety of numeric ages for Grand Canyon rocks, particularly for the sedimentary rocks which usually cannot be absolutely dated, exist in both the technical and popular literature. When someone’s objective is really just to learn how long ago these rocks formed, it is very confusing to sort through subdivisions of geologic periods, the scientific names of microscopic index fossils, and the nuances of radiometric dating techniques. Terms such as “Roadian” or “Leonardian” are very accurate and meaningful to a geologist, but they do not say how old the Kaibab Formation is in numerical terms (such as 270 m.y.). Most audiences will not find a description of the Kaibab Formation as “Leonardian” or “Roadian” meaningful, but will be able to comprehend the numeric value of 270 m.y. (at least to the degree that geologic time is understandable to humans).
Tables 2 and 3 are compilations of what we believe to be the best numeric ages of Grand Canyon rocks given the current knowledge of Grand Canyon geology and the Geologic Time Scale. Our goal was to be as accurate as possible in assigning numeric ages, even though it was impossible to be entirely precise. Additionally, where the science allowed, we wanted to have rounded numbers for easier interpretive use and for better retention by the public. We think that these numeric ages are an important translation for the public for the age of rocks at Grand Canyon and are useful for interpretive purposes. It is our hope that people—rangers, guides, authors, students, and others—who interpret or study the age of Grand Canyon rocks will use these numeric ages.
These numeric ages may need to be revised as knowledge of Grand Canyon improves, new or improved absolute dating techniques are developed, and/or the geologic time scale is modified. Although the ages in the chart were “set by stones,” it is important to remember that, like all scientific findings, they are not “set in stone.” Regardless of whatever revisions to the ages of Grand Canyon rocks occur with further scientific inquiry, most changes will only be on the order of a few million years, a very short period geologically. Grand Canyon will remain a great window into the deep history of our planet.
Mike Timmons, Ron Blakey, and Karl Karlstrom provided valuable insight
into the ages of rocks exposed in Grand Canyon and assisted us with our
compilation of best numeric ages.
Previous versions of this article were published in Nature Notes (http://www.grandcanyon.org/canyonviews/NNWinter05.pdf and http://www.grandcanyon.org/canyonviews/NNSpring05.pdf), published by Grand Canyon National Park in cooperation with the Grand Canyon Association, and in Boatman’s Quarterly Review, published by Grand Canyon River Guides.
For more information on the geology of each of the formations exposed in Grand Canyon see https://3dparks.wr.usgs.gov/coloradoplateau/grandcanyon_strat.htm.
Grand Canyon: Yardstick of Geologic Time, published by Grand Canyon Association is an interpretive publication that helps people put the age of Grand Canyon rocks into the context of geologic time. It is available from Grand Canyon Association at http://www.grandcanyonassociation.org/grand_canyon_bookstore_featured.html
Beus, Stanley. S., and Morales, Michael, eds., 2003, Grand Canyon Geology, Second Edition, Oxford University Press, 432 p.
Blakey, Ronald, 2004, personal communication.
Dehler, C.M., Elrick, M., Block, J.D., Crossey, L.J., Karlstrom, K.E., Des Marais, D.J., 2005, High-resolution δ13 stratigraphy of the Chuar Group (ca. 770 – 742 Ma), Grand Canyon: Implications for mid-Neoproterozoic climate change: Geological Society of America Bulletin, v. 117, p. 32 – 45.
Dehler, Carol, Porter, Susannah, and Karlstrom, Karl, 1999, Grand Canyon Supergroup: Geologic Follow-up to Part 1 – Winter 1998-99 BQR: Boatman’s Quarterly Review, v. 12, n. 3, p. 31 – 36.
Geological Society of America, 1999, Geological time Scale: http://www.geosociety.org/science/timescale/timescl.htm
Ilg, Bradley R., Karlstrom, Karl E., Hawkins, David P., and Williams, Michael L., 1996, Tectonic evolution of Paleoproterozoic rocks in the Grand Canyon: Insights into middle-crustal process: Geological Society of America Bulletin, v. 108, p. 1149 – 1166.
International Commission on Stratigraphy, 2004, Geologic Time Scale 2004: Cambridge University Press and www.stratigraphy.org/scale04.pdf.
International Commission on Stratigraphy, 2004, International Stratigraphic Chart: www.stratigraphy.org/.
Karlstrom, K.E., Ilg, B.R., Williams, M.L, Hawkins, D.P., Bowring, S.A., and Seaman, S.J., 2003, Paleoproterozoic rocks of the Granite Gorges, in Grand Canyon Geology, second edition, Beus, Stanley. S., and Morales, Michael, eds., Oxford University Press, p. 9 – 38.
Karlstrom, Karl, 2004, personal communication.
Ogg, James G., 2004, Status of division of the International Geological Time Scale: Lethaia, v. 37, p. 183 – 199.
Powell, J.W., 1875, Exploration of the Colorado River of the West and its tributaries. Explored in 1869, 1870, 1871, and 1872, under the direction of the Secretary of the Smithsonian Institution: Washington, DC: US Government Printing Office, 218 p.
Timmons, Michael, 2005 - 2003, personal communication.
Timmons, J.M., Fletcher, K., Karlstrom, K., Heizler, M., Gehrels, G., Bowring, S., and Crossey, L., 2005, The Mesoproterozoic Grand Canyon Unkar Group: A detailed record of intracratonic basin formation and deformation during Grenville-age orogenesis: Geological Society of America Annual Abstracts with Programs, v. 37, n. 7, p. 494. Paper No. 224-8, Abstract ID 95694.
Timmons, Mike, Karlstrom, Karl, and Dehler, Carol, 1998 – 1999, Grand Canyon Supergroup: Six Unconformities make one “great unconformity:” A record of Supercontinent Assembly and Disassembly: Boatman’s Quarterly Review, v. 12, n. 1, p. 28 – 32.
About the authors
Allyson Mathis is a geologist by training and an interpretive park ranger at Grand Canyon National Park. Carl Bowman is the Air Quality Specialist for Grand Canyon National Park’s Science Center. Allyson Mathis is also the lead author of “Grand Canyon: Yardstick of Geologic Time” published by Grand Canyon Association, $5.95 (available through the Grand Canyon Association Bookstore).