Cambrian System of the Grand Canyon

Cambrian System of the Grand Canyon

The purpose of this work is to research and examine the relevant information available in relation to the Cambrian System of the Grand Canyon.

Just above the break in the historical record of rocks which is referred to as "the unconformity" in the Grand Canyon noticed in 1869 by John Wesley Powell while on his pioneer voyage, are the Cambrian Tapeats Sandstone. The Great Uncomformity is stated to represent about 1/2 billion years of missing time! According to some the answer is stated as follows:

"During Grand Canyon Supergroup time, a hypothesized supercontinent called Rodinia assembled at about 1.1 Ga, and then broke apart around 0.75 Ga. The first life forms with a nucleus, single-celled creatures called Eukaryotes, were becoming more diverse, testing the waters for their future evolutionary extravaganza known as the 3 Cambrian Explosion.2 Changes in global seawater composition reflects complex changes in climate, tectonics, atmosphere, and biosphere."

Whatever the answer may be, the Cambrian System itself is an amazing system to study in relation to its' formation.

I. The Cambrian System

The stratigraphy of the Grand Canyon is illustrated in the chart below: Precambrian sedimentological layer is a "complex group of highly metamorphic and intensively folded rock of the "Vishnu Group," inclusive of chlorite-mica schist, with minor amount of amphibolites, gneiss and calc-silicate rocks. Occurrences of Zoroaster pink feldspars granite is noted with intrusion of Vishnu rocks vertically and pegmatic veins that are as thick as multiples of ten meters. The Zoroaster and Vishnu rocks the compositional materials of the Lower Proterozoic. These rocks are laid over by tilted Upper Proterozoic volcanic0sedimentary Grand Canyon Supergroup in some tectonic depressions with the Tonto Group of the Cambrian System overlaying the Vishnu and the Zoroaster rocks. (Grand Canyon, 1989)

II. Stratigraphy of the Grand Canyon

Shown below in the chart labelled Figure 1.0 are the layers of sediment that compose the Stratigraphic column of the Grand Canyon strata. (Berkhault, 2004)

(1) Schist;

(2) Granite;

(3) Pebble and boulder conglomerate;

(4) Sandstone;

(5) Siltstone;

(6) Shale;

(7) Limestone

Figure 1.0

Source: Berkhault (2004)

III. The Tonto Group Formations and Layers

There have been three formations in the Tonto Group recognized as belonging which are the Tapeats Sandstone, the Bright Angel Shale, and the Muav Limestone. The Characteristics of the Tapeats Sandstone is that it is the lowest horizontal formation of enormous lateral extent in the entire canyon with a medium to course grained sandstone that is rich in quartz and is generally between 40 to 100 meters thick with a base that is usually pebbles and boulders mixed.

The mid-portion of the formation is mostly sandstone that is coarse with cross beds in the westward and south-westward with dips providing the indication that water currents were flowing westward. The top layer is mostly plane beds of sand that is rippled and other thinner and finely grained sand as well as beds of silt that form a gradational contact with the Bright Angel Shale that overlays it. The Bright Angel Shale is greenish-gray and is 100 to 120 meters in thickness with sandy dolomite and silty limestone beds throughout the canyon. The Muav Limestone is yellowish-brown and is impure with silty and sandy limestone that is around 100 up to 300 meters in thickness with small inclusions of clay that is irregular. Berkhault (2004) states that, "Because the Tapeats, Bright Angel, and Muav are not separated by unconformities but grade into each other, they have been collectively called the Tonto Group. The deposits are overlain by the sequence of sandstones, siltstones, shales, and carbonate rocks of Devonian, Carboniferous and Permian Systems."

IV. Zones in the Tonto Group Strata

The Tapeats, Bright Shale Angel and Muav Limestone "superposed each other and juxtaposed when erosion produced clasts of all sizes, including particles of quartz, clay, pebbles and boulders. As indicated by Lebediev (1959) the velocity of incipient motion relative to the depth of water was in relation to the clast size. As the advancement of the transgression occurred the water increased in deepness which resulted in a reduction of the current but still a capacity to erode remained causing clasts that were smaller than boulders and were transported as far away as Zone 2.

Zone One & Two

The shallow zone or Zone 1 experienced current that was greatly 'diminished' (Berkhault, 2004) and the erosion level was diminished as well but transported 'clasts of terrigeneous which is grave, sand, silt and clay as well as carbonate material such as lime. Berkhault (2004) states that, "A regressive current started, which carried westwards the largest particles in a bed load, and the smallest in a suspended load. The first deposited from zone 2 to 6, and the second in zones 5 and 6 only."

Zone Three

Characterized in composition by sand in waves that were formed by thin cross sand beds, this zone is composed of sand waves forming thinly cross-bedded sands, which compose the middle of the Tapeats. Water velocity was approximately 1.0 meter per second.