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]