Fossil Walk at Grand Canyon National Park
Crinoids, and Brachiopods, and Worms, Oh My!
Location: Grand Canyon Village, South Rim Grand Canyon National Park
Geological Formations: Outcrops of Permian marine fossils in the Kaibab Formation.
Other Attractions: Spectacular South Rim views and overlooks nearby.
Hiking Difficulty: Easy walking along the paved South Rim Trail.
Access: Via the South Rim Trail. Guided fossil walks are conducted at Grand Canyon Village as interpretive programs by park rangers.
USGS 7.5 Minute Arizona Topo Map: Grand Canyon
There are numerous opportunities to enjoy finding and viewing fossils at Grand Canyon National Park. Unfortunately many park visitors are so overwhelmed by the Grand vistas unfolding before them they fail to notice and appreciate some of the smaller and more subtle wonders immediately beneath their feet. While at Grand Canyon during an October 2002 visit I attended several guided fossil walks, which are one of many interpretive programs freely provided by the Park Service. These fossil walks are popular and frequently conducted by park rangers at Grand Canyon Village along with several other geology related talks and programs.
The rim rock at Grand Canyon is composed of the Kaibab Formation, also popularly referred to as the "Kaibab Limestone". A relatively shallow, ancient sea that cyclically encroached and retreated over the region covering portions of present day Arizona, Utah and Nevada deposited the Kaibab during the late Permian Period at the end of the Paleozoic Era, approximately 250 million years ago. The transgressions and regressions of the Kaibab Sea were probably driven by climatic (glacial) induced oscillations in the global sea level. During Kaibab time the North American crustal plate was still located near the equator and incorporated in the monolithic supercontinent of Pangea - as yet unseparated into the modern continents through ensuing continental drift. Ancient life was well developed and represented on land with plants, insects, amphibians and both herbivorous and carnivorous reptiles, but the dinosaurs had yet to appear.
Modern geologists have divided the Kaibab Formation into two members, the overlying Harrisburg, and the thicker, underlying Fossil Mountain Member.
|This exposure on the west side of Mohave Point was photographed from a vantage point on the West Rim Trail between Pima and Mohave Points.
|KFh = Kaibab Formation - Harrisburg Member
KFf = Kaibab Formation - Fossil Mountain Member
TF = Toroweap Formation
CS = Coconino Sandstone
Shown at left is the Fossil Mountain Member of the Kaibab Formation. Note the numerous darker brown features peppering the Fossil Mountain Member, which are chert nodules surrounding fossilized sponges.
The Fossil Mountain Member is normally concealed at rim level by 80 to 300 feet of the overlying Harrisburg Member. Approximately 200 feet of relative uplift on the west side of the Bight Angel Fault has caused outcrops of the Fossil Mountain Member to be exposed at the rim surface along the South Rim Trail just west of Grand Canyon Village. These exposures are located an easy 10 minute or so walk from the Bright Angel Lodge in Grand Canyon Village where visitors initially assemble to attend the ranger led fossil walks.
The strata of the Fossil Mountain Member were deposited by the ancient Kaibab sea as it cyclically transgressed and regressed across the Grand Canyon region and consist principally of limestone, dolostone and cherty sandstone. It is heavily loaded with countless fossil sponges, brachiopods, corals, crinoids, and trace fossils such as worm burrows. It typically ranges between 250 to 300 feet thick and weathers to form distinctive pinnacles and hoodoos. The Fossil Mountain Member exhibits a thickening trend towards the western regions of the Grand Canyon. Its strata in the eastern regions of the Grand Canyon are composed of interleaved layers of dolostone and cherty sandstone with sandstone becoming increasingly predominate to the east. The carbonate facies (dolostone) were deposited during west to east transgressive cycles of the ancient Kaibab Sea and record a rise in sea level. The siliciclastic facies (sandstone) were deposited during east to west regressive cycles and record a fall in sea level.
The lateral facies of the Fossil Mountain Member exhibit pronounced changes in the rock types, grading from the interleaved dolomite/sandstone strata characteristic of shallow marine and shore depositional environments in the eastern regions, to more homogenous limestone characteristic of deeper, open marine depositional environments in the western regions.
Fossils within the Fossil Mountain Member in western Grand Canyon are often complete and without abrasions, indicating calm water conditions and little to no transport. Large spined brachiopods, which are often silicified and found along bedding planes in life-position and silicified sponges occurring at the center of spherical chert nodules are characteristic fossils in the western regions. The frequency of brachiopods and sponges tends to decrease towards the top of the Fossil Mountain Member, where the frequency of bryozoans and crinoid debris increases.
Bright Angel Fault
Ranger James Mimosa
October 14th, 2002