nazSTUFF - Grand Canyon Ancient Environments 02

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Grand Canyon Geology

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The Grand Canyon in Arizona is known as a living laboratory displaying the oldest rock formations on earth. Science, art and adventure come together in this magnificant place. This page presents some elementary information about Grand Canyon geology. JVroom

Ancient Environments Part Two

The Supai Group - 300 million years old

The nearly 1000 feet of redbeds which make up the Supai Group form a series of alternating cliffs and slopes which reflect the widely changing environments during their deposition. To the naked eye the Supai Group is easily distinguished from the soft, dark red slopes of the Hermit Shale above and the sheer cliff of the Redwall Limestone below. These amalgamated layers represent a variety of changing environments, including marginal marine (delta, estuary, lagoon and tidal flat) and shallow marine, similar to the Texas Gulf today. Rocks of the Supai Group straddle the Pennsylvanian / Permian boundary. Four separate formations are present in this group (Esplanade Sandstone, Wescogame Formation, Manakacha Formation and Watahomigi Formation). Fossils in the Supai Group include ferns and animal tracks.

This scene attempts to interpret these marginal marine environments with scant vegetation (ferns and calamities- giant horsetails) and animal tracks of an unknown species.

The Redwall Limestone - 335 million years old

The Redwall Limestone forms one of the most conspicuous cliffs in the Grand Canyon, just below the redbeds of the Hermit Shale and the Supai Group, and above the soft green slopes of the Tonto Platform. It averages 500 feet thick. The predominantly reddish color of the Redwall Limestone is in fact a feature of weathering and is the result of iron oxides which have formed from the (above) overlying iron-rich strata of the Hermit Shale and the Supai Group. The Mississippian Redwall Limestone is separated from the underlying Muav Limestone (of Cambrian Age) by a major unconformity (missing layers). From the rim this contact is not always easy to distinguish. This layer is almost entirely pure limestone (calcium carbonate) which accumulated in a broad shallow quiet sea far from nearshore silts and clays. The unit is fossiliferous and has many solution features such as caves and collapsed places.

This scene shows that Redwall Limestone was the result of an environment that was completely underwater. This is similar to the Kaibab Limestone except that it is much earlier and the lifeforms are different. The straight-coiled animal above is a form of cephalopod. Crinoids, bryozoa, brachiopods and coral fill out the community of life.

The Muav Limestone - 530 million years old

The Muav Limestone forms the base of the large Redwall Limestone cliff. Sometimes it is difficult to distinguish from the overlying Redwall, particularly in areas where the red iron oxide stain from above masks its true gray-green color. Look for the resistant cliff just above the green, sparsely vegetated slopes of the Bright Angel Shale. Across the canyon its exposure ranges from 150 to 1400 feet thick.

The Muav Limestone is the uppermost (youngest) unit of the Tonto Group), which includes Muav Limestone, Bright Angel Shale and Tapeats Sandstone.

Collectively, these three units, which grade into one another, represent a classic example of what geologists call a transgressive sequence: rock units which gradually change from shallow water, nearshore environments, upwards into shales, deeper water, and limestones, deeper water yet, indicating a gradual advance of ocean waters over the land. Muav Limestone is the top unit of this transgressive sequence and is marine limestone.

This scene features trilobytes, glass sponges and crinoids in deeper water far from shore.

The Bright Angel Shale - 540 million years old

The thin-bedded, weak and easily eroded fine-grained shales of the Bright Angel Shale are well exposed on the broad shelf which stretches from the base of the Redwall/Muav cliff to the top of Tapeats Sandstone at the top edge of the Inner Gorge. This broad shelf is known as the Tonto Platform. Sparsely vegetated with blackbush, sagebrush and desert scrub, the green color of these gentle slopes is due partly to the vegetation present there and partly to the pale green color of the Bright Angel Shale itself. Faint trails (occasionally visible from the rim) criss-cross the Bright Angel Shale on the Tonto Platform; most of these are game trails or dry water courses. Most prominent amongst them today is the Tonto Trail, which roughly parallels the base of the cliff for a distance of 70 miles.

The Bright Angel Shale is the intermediate unit of the Tonto Group.

The Bright Angel Shale is made up of deposits which accumulated somewhat offshore in this shallow muddy sea, intermediate between the nearshore deposits of the Tapeats Sandstone and the offshore marine limestones of the Muav Limestone.

This scene features a trilobyte, jellyfish, brachiopods and worm burrows found in a shallow marine environment, probably a mud flat.

The Tapeats Sandstone - 550 million years old

In Grand Canyon the Tapeats Sandstone is recognizable as the thin, evenly-bedded outcrop at the edge of the Tonto Platform above the Inner Gorge, just beneath the gentle green slopes of the Bright Angel Shale. Beneath the Tapeats Sandstone lies the Great Unconformity, the large gap (missing layers) in the geologic record which separates Precambrian rocks of the Inner Gorge from the overlying Paleozoic rocks above. Tapeats Sandstone is the lowest and the oldest unit of the Tonto Group. It represents a nearshore environment at the base of this transgressive sequence.

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