Determining the Stage of the Navajo Sandstone
Dennis Titterton
Amy Lombari
Zoe Gentes
Jen Sullivan
AJ Infante
November 12, 2009
ABSTRACT
The Navajo Sandstone is part of the Glen Canyon Group that is located in Utah, Colorado,
Arizona, and New Mexico. To determine the stage of the sedimentary rocks comprising the Navajo
Sandstone, we used biostratigraphy and lithostratigraphy. Body fossils are rare in the Navajo
Sandstone, so we drew conclusions largely from the trace fossils consisting of trackways. Grallator,
Eubrontes, Anchisauripus, Navahopus, and Brasilichnium were the ichnotaxa included in this report
along with the coinciding body fossils, Ammosaurus, Segisaurus, Protosuchus, and an unnamed
tritylodontid protomammal . Lithostratigraphy is conducted using the Carmel formation of the San
Rafael group from above and the Kayenta formation also of the Glen Canyon group from below.
INTRODUCTION
The Navajo Sandstone, part of the Glen Canyon group, is an eolian formation in the southwest
United States. It is home to an abundance of trace fossils, or ichnotaxa, and corresponding body fossils.
To properly date this formation, these fossils will be used to determine the period and stage(s) of the
Navajo Sandstone. Along with the use of biostratigraphy by studying the fossils, we will also use
lithostratigraphy by studying the surrounding strata above and below the Navajo formation. Since the
Navajo Sandstone formation is the earliest layer of the Glen Canyon group, the layer used above the
Navajo is one that of the San Rafael group, the Carmel formation. The layer below the Navajo formation
is known as the Kayenta formation and is also included in the Glen Canyon group. Through the studies of
the biostratigraphy and lithostratigraphy of the Navajo Sandstone formation, we are determined to
come to a conclusion as to which period and stage(s) this formation is from.
BIOSTRATIGRAHY
Trace Fossils
Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative
ages of rock strata by using the fossil assemblages contained within them. There are many fossilized
remains in the Navajo Sandstone, mostly trace fossils, because of the erosive nature of the sandstone
which destroyed most of the body fossils. The major trace fossils found in the Navajo Sandstone
formation consist of the ichnotaxa Grallator, Brasilichnium, Eubrontes, Navahopus, and Anchisauripus.
Ichnotaxon is a taxon based on the fossilized work of organisms. Ichnofossils are known by most simply
as trace fossils. Genus and species are given to the ichnotaxa since most trace fossils can’t be linked
solely to one body fossil genus, in some cases no body fossils are particularly linked to an ichnogenus
(Rainforth, 1997).
The ichnogenus Grallator are small tridactyl tracks measuring up to 18 cm in length and have
well-defined pad and claw impressions (Figure 6B) (Rainforth, 1997). The ichnogenus Eubrontes are
large, longer than 25 cm, tridactyl tracks which have well-defined pad and claw impressions (Figure 7A)
(Rainforth, 1997). The ichnogenus Anchisauripus are medium-sized tridactyl tracks ranging from 15-30
cm in length with well-defined pad and claw impressions (Rainforth, 1997). These three ichnogenus are
considered uncommon relatives since they have many similarities, primarily just differing in length
(Rainforth, 1997). The ichnogenus Navahopus, being track fossils, have subcircular pes which are larger
than the manus and are believed to be either tridactyl or tetradactyl. It has been suggested this
ichnogenus is of a larger Brasilichnium, but the morphological differences are too great to attribute
them together (Rainforth, 1997). The ichnogenus Brasilichnium consists of a quadrupedal trackway in
the Navajo Sandstone formation with subcircular pes and are pentadactyl, digits preserved by claw
impressions only. Dating of ichnotaxa is easier if corresponding body fossil genus and species can be
identified as the creator of the tracks, such as the ones found in the Navajo Sandstone formation. This
correspondence is noted below in the body fossils section. Radiometric dating is also used such as
uranium to lead isotopic dating, but is flawed due to the fact it is dating the grains of the rock which
would have formed way before they came to settle there.
Body Fossils
Body fossils are fossils that are the preserved remains or traces of remains of living organisms
from the past. “Few body fossils are known from the Navajo, but they include Protosuchus (a
crocodylomorph), Ammosaurus (a plateosaurid prosauropod), Segisaurus (a ceratosaurian theropod
known from a single subadult specimen) and an unnamed tritylodontid protomammal.” (Rainforth,
1997). The crocodylomorph, Protosuchus, was about 1 meter in length and around 90 lbs in weight
on average (Rainforth, 1997). Protosuchus is the only body fossil identified in the Navajo Sandstone
formation that may or may not be also represented by a corresponding trace fossil. The prosauropod,
Ammosaurus, was about 4 meters in length and is currently believed to be synonymous with the
prosauropod, Anchisaurus. Ammosaurus fossils have also been recorded in the Portland formation of
the Newark Supergroup in Connecticut. The Portland formation is the same age as the Navajo
Sandstone formation, dated primarily due to their prosauropod relationships (Galton, 1971). The
ichnogenus, Navahopus, is the corresponding trace fossil with Ammosaurus within the Navajo
Sandstone since it is associated with a quadrupedal tetrapod vertebrate (Galton, 1971). The theropod,
Segisaurus, was about 1 meter in length and the only dinosaur excavated in the Navajo Sandstone
formation. Being a bipedal theropod, Segisaurus’ corresponding ichnogenus in the Navajo formation
is Grallator (Rainforth, 1997). The last body fossil discovered in the Navajo Sandstone formation
was that of an unnamed tritylodontid protomammal. Tritylodontid were quadrupedal mammal-like
reptiles that varied in size. Being quadrupedal, this body fossil has been found to correspond with the
Brasilichnium ichnogenus (Rainforth, 1997).
Ammosaurus has been dated to be existent during the Early to Middle Jurassic period. The
unnamed tritylodontid has been dated to have existed from the Late Triassic to Mid-Cretaceous
period. Both the Protosuchus and Segisaurus have been dated to only be from the Early Jurassic
period. Protosuchus is considered an index fossil, a fossil used to identify and define geologic
periods, due to its short lived existence. The fossil has also been dated using uranium to lead
radiometric dating to further identify Protosuchus to be only of the Early Jurassic period (Lucas,
2008). The body fossils of the Navajo Sandstone formation have all been dated by the use of index
fossils such as Protosuchus and radiometric dating of uranium to lead primarily on zircon grains
found in the strata (Lucus, 2008). Being that both the Protosuchus and Segisaurus body fossils found
in the Navajo Sandstone are only found in the Early Jurassic Period, we are able to firmly place the
forming of the Navajo Sandstone formation within the Early Jurassic Period as well.
LITHOSTRATIGRAPHY
Carmel Formation
Above the Navajo Sandstone lies the formation known as the Carmel formation. The Carmel
formation is a member of the San Rafael group that is spread across the U.S. states of Wyoming, Utah,
Colorado, north east Arizona and New Mexico. The Carmel formation formed during the Middle Jurassic,
more specifically during the Callovian, Bathonian, and late Bajocian stages. Being that the Bajocian stage
is the second stage of the Middle Jurrasic, after the Aalenian stage, we can safely keep the Navajo
Sandstone’s age within the Early Jurrasic period, backing this theory up with the information provided
by the biostratigraphy of the Navajo Sandstone formation.
Kayenta Formation
Below the Navajo Sandstone lies the formation known as the Kayenta formation. The Kayenta
formation is a thin layer of fluvial deposited sandstone between the Navajo Sandstone and the Wingate
formation and is part of the Glen Canyon Group. The Kayenta is dated to be of the Early Jurassic period
and has been dated using the index fossil, Syntarsus (Rowe, 1989). Syntarsus was a dinosaur of the
theropod family Coelophysidae. Syntarsus measured up to 10 ft in length and lived during the the
Hettangian, Sinemurian, and Pliensbachian stages of the Early Jurassic period (Rowe, 1989). The fact
that the Kayenta formation interfingers with the Navajo Sandstone formation in particular areas
throughout the Glen Canyon group, one can conclude that the Navajo had to have begun to form during
the Pleinsbachian stage of the Early Jurassic period since this stage is the younger of the 3 stages
identified in the Kayenta formation. This fact also allows us to include the Navajo Sandstone formation
in the Toarchian stage of the Early Jurassic period since it is the youngest stage of the Early Jurassic and
begins immediately after the Pleinsbachian stage.
CONCLUSION
The Navajo Sandstone formation can be concluded to be that of the Pleinsbachian and
Toarchian stages of the Early Jurassic period. Using the biostratigraphy of trace and body fossils found
within the Navajo formation along with the lithostratigraphy provided by the adjacent layers of the
Navajo formation, the Carmel and Kayenta formations, we can safely come to this conclusion. The use of
the index fossils in the Navajo Sandstone formation (Protosuchus) and the Kayenta formation
(Syntarsus) provided a basis for this conclusion, only to be better solidified with the accompanying trace
and body fossils also found in the Navajo formation. By using biostratigraphy to place the Navajo
Sandstone formation within a given period, we were able to narrow the formation down to its
respective stages within the Early Jurassic using lithostratigraphy. The Carmel formation, positioned
above the Navajo, is that of the Middle Jurassic period, keeping the Navajo restrained within the Early
Jurassic period. The Kayenta formation is also of the Early Jurassic, forming at the beginning of the
period, but also interfingers with the Navajo formation. This would lead us to conclude that the last
stage of the Kayenta formation is shared with the Navajo, the Pleinsbachian stage. By coming to this
conclusion, the only other stage left before the start of the Middle Jurassic period is the Toarchian stage.
This conclusive date allows us to state that the age of the Navajo Sandstone formation, found in the
Glen Canyon group, is of the Pleinsbachian and Toarchian stages in the Early Jurassic period.
Figures
References Cited
Galton, Peter M., 1971, The Prosauropod Dinosaur Ammosaurus, the Crocodile Protosuchus, and Their
Bearing on the Age of the Navajo Sandstone of Northeastern Arizona, Journal of Paleontology, Vol. 45,
No. 5, pp. 781-795.
Spencer G. LUCAS, 2008, Volumina Jurassica, Vol. VI, pp 99-108.
Rainforth, E.C., 1997, Vertebrate Ichnological Diversity and Census Studies, Lower Jurassic Navajo
Sandstone PDF ver... Unpublished masters thesis, Department of Geological Sciences, University of
Colorado, Boulder.
T. Rowe. 1989. A new species of the theropod dinosaur Syntarsus from the Early Jurassic Kayenta
Formation of Arizona. Journal of Vertebrate Paleontology 9(2):125-136 [M. Carrano/M. Carrano/M.
Carrano]