The Kaibab Formation
The Kaibab Formation, popularly referred to as the 'Kaibab Limestone', comprises a continuous layer of cap rock and the final chapter in geologic history recorded in the rocks of the Grand Canyon region. Its depositional environment was that of a relatively shallow, ancient sea that cyclically encroached and retreated over the Grand Canyon region approximately 250 million years ago during the Middle Permian period near the end of the Paleozoic Era. The transgressions and regressions of the Kaibab Sea were probably driven by climatic (glacial) induced oscillations in the global sea level.
The Kaibab Formation was subsequently overlain with great layers of Mesozoic strata which are preserved in adjacent areas, but which have been eroded away in the Grand Canyon vicinity due to an uplift of the region during the Laramide Orogeny, beginning at the the end of the Mesozoic era approximately 65 million years ago.
The rubble surface of the Kaibab Formation in the vicinity of Cedar Mountain, which stands in the background several miles distant to the south. Cedar Mountain, together with nearby Gold Hill, and Red Butte between Tusayan and Valle, are scarce, isolated ramparts of Mesozoic Era Moenkopi Formation surviving atop the Kaibab Formation in the immediate vicinity of the Grand Canyon. In the foreground is archeological site AZC:13:512, the remnants of an Indian "mescal pit". When in use the stones were piled up and arranged to create an oven with a fire built within. Agave hearts and stalks were roasted in the hot stones for several days prior to consumption. This site is located right off the old jeep trail (now closed except to hikers) from the Desert View Ranger Station to Cape Solitude, located by my GPS receiver at UTM coordinates 12S 0430132mE, 3992813mN, elevation 6544 feet.
The Kaibab and underlying Toroweap Formation were considered by early geologists as a single formation, the 'Aubrey Limestone'. More recent work has differentiated the Kaibab from the Toroweap on the basis of lithologic types. The Kaibab forms a gray stepped cliff at the very rim of the Grand Canyon which ranges between 300 and 500 feet in thickness. The general zone of contact with the underlying, slope forming Toroweap Formation is visually discernable as the first major cliff-to-slope gradient change as the canyon walls are descended.
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
The Harrisburg Member
The strata of the Harrisburg Member were deposited by the ancient Kaibab Sea as it retreated from east to west across the Grand Canyon region. The Harrisburg Member has been informally divided into six stratigraphic units deposited in marine/shoreline environments during the cyclic western regression of the Kaibab Sea, producing alternating beds of carbonates (limestones/dolostones) and siliciclastics (redbeds/sandstones). Western facies of this member also contain evaporite deposits of gypsum.
The uppermost beds of the Harrisburg form the erosion surface across much of the Grand Canyon region and make up unit six, which exhibits significant lateral variation east to west in the general composition of its facies and characteristic fossils. In the eastern regions unit six is composed primarily of dolostone with fossil populations shifting towards bivalves, gastropods and peloids forming packstone. In western regions unit six grades to cherty, dolomitic sandstone with the fossil populations shifting to predominately molluscan fauna, including whole, japserized Bellerophon gastropods reported from the vicinity west of Kanab Creek.
Unit five also exhibits lateral facies variations east to west. In eastern regions it is approximately 20 feet thick and composed of ripple laminated sandstone that pinches out east of the Grand Canyon. West of Kanab Creek unit five grades to gypsum and interstratified siliclastic redbeds composed of hematite bearing sandstone and siltstone approaching 80 feet in thickness.
Unit four is composed of sandy, dolomitic mudstone containing thin lenses of fossils debris and hash, eroded limestones and cryptalgal laminated horizons. It averages about 20 feet in thickness and is generally less sandy towards the west and towards its top layers. Fossil fragments found in this unit include bivalves, gastropods, ostracods, and less commonly, crinoids, bryozoans and brachiopods.
Unit three is similar to unit five in its lateral east to west facies variations. In eastern regions it is an approximately 20 foot thick sequence composed of sandstone, sandy dolomitic mudstone, and cryptalgal-laminated dolomitic mudstone. West of Kanab Creek unit three grades to a sequence of contorted red sandstone with considerable bedded gypsum which can exceed 100 feet in thickness.
Unit two consists of thin, carbonate deposits five to twelve feet thick with facies grading from dolomitic mudstone textures exhibiting cryptalgal laminations and calcite filled vugs in the east, to limestone characterized by packstone and whackestone textures in the west. The western facies of unit two contains fossil fragments of bivalves, gastropods, crinoids, bryozoans, foraminifers and ostracods.
Unit one, at the base of the Harrisburg Member, varies from twenty to forty feet thick and conformably overlies fossiliferous facies of the Fossil Mountain member. Unit one consists of sandstone and sandy dolostone overlying a bed of marker chert at its horizon with the Fossil Mountain member. The chert contains silicified sponge spicules, scattered peloids, varying amounts of transported quartz grains, and sparse skeletal remains.
|Above left is a colorful cherty dolomite outcrop found on the erosion surface of the Harrisburg Formation near the Hermit Trail trailhead, located by my GPS receiver at UTM coordinates 12S 0390839mE, 3991069mN, 6604 feet elevation. Cherty nodules of different varieties and sizes are distributed throughout the Kaibab Formation and are typically concentrated along bedding planes. There are prominent outcrops of several varieties of chert nodules along horizons in the Kaibab that are readily appreciated from the Hermit Trail as it traverses the formation. Above right is a common variety of chert nodule which had weathered completely out of the bedrock and fallen with numerous others to the base of a cliff nearby the trail.
The Fossil Mountain Member
The strata of the Fossil Mountain Member were deposited by the ancient Kaibab sea as it cyclically transgressed from west to east across the Grand Canyon area and consists principally of limestone, dolostone and cherty sandstone. This cliff forming member of the Kaibab Formation underlies the Harrisburg and weathers to form distinctive pinnacles and hoodoos. It typically ranges between 250 to 300 feet thick and exhibits a thickening trend towards the western regions of the Grand Canyon. The Fossil Mountain Member 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.
|Above left is a portion of several pronounced chert nodule horizons in the Fossil Mountain Member of the Kaibab Formation which are beautifully exposed in walls along Hermit Trail. These nodules form around silicified sponges. Above right is a close-up of one of the chert nodules with a 55mm lens cap held up to it for scale.