Toroweap Formation
| Toroweap Formation | |
|---|---|
| Stratigraphic range: late early Permian,[1] | |
 Toroweap Formation in Grand Canyon National Park | |
| Type | Geological formation |
| Sub-units | Seligman, Brady Canyon, and Woods Ranch members (oldest to youngest) |
| Underlies | Kaibab Limestone |
| Overlies | Coconino Sandstone |
| Thickness | 200 feet (61 m) approximate maximum. |
| Lithology | |
| Primary | red beds, gypsum, and fossiliferous limestone[2] |
| Other | dolomite[2] |
| Location | |
| Coordinates | 36°12′19.34″N 113°05′24.40″W / 36.2053722°N 113.0901111°W |
| Region | (southwest)-Colorado Plateau Northern Arizona, southeast Utah |
| Extent | Grand Canyon, North Rim, (Kaibab Plateau), also South Rim, and elsewhere in Arizona, Utah, Nevada |
| Type section | |
| Named for | Toroweap Valley, Arizona[3] |
| Named by | McKee (1938)[3] |
The Toroweap Formation outcrops as a thin layer of generally darker, interlayered slope- and cliff-forming strata lying between the brighter colored cliffs of the Kaibab Limestone above, and Coconino Sandstone below. It outcrops in Grand Canyon, Arizona, Southwestern United States, found throughout walls of the South Rim, Grand Canyon, and the North Rim, of the Kaibab Plateau. Also, it outcrops in the Kaibab's southeast extension to Cape Royal, the Walhalla Platea. The formation is also found in southeast Utah and west-central Nevada.[2][3][4]
Description
The Toroweap Formation exhibits well-defined lateral and vertical changes in lithofacies over its outcrop. In the western extent of its outcrop in the Grand Canyon region and adjacent parts of Utah and Nevada, the Toroweap Formation is readily subdivided, in ascending order, into the Seligman, Brady Canyon, and Woods Ranch members. Two of these members (Seligman and Woods Ranch members ) consist of red beds and evaporites (gypsum]) each is separated by a fossiliferous limestone member (Bardy Canyon Member). The red beds of the Seligman and Woods Ranch members are largely of soft, friable sediments, which rapidly weather into slopes, The Bardy Canyon Member is a resistant limestone which characteristically stands up as a prominent cliff between the slopes of the Seligman and Woods Ranch members. Further eastward, the Brady Canyon Member disappears, and the two red bed members merged together into an undivided Toroweap Formation. Further east, the red beds grade laterally into cross-bedded sandstones similar to the Coconino Sandstone.[3][4][5]
The Seligman Member of the Toroweap formation largely consists of fine-grained, red and yellow sandstone. It typically exhibits flat or irregular bedding. Its maximum observed thickness is about 50 ft (15 m) and in most places is no thicker than 45 ft (14 m). At its upper contact, the Seligman Member grades upwards through a transitional zone of alternating beds of sandstone and limestone into the fossiliferous limestone of the Brady Canyon Member. In Grand Wash Canyon on Mead Lake, the sandstones of the Seligman Member contain a very conspicuous layer of breccia, interpreted to be an intraformational conglomerate, only a few feet above the top of the Coconino Sandstone. The basal layer of the Seligman Member is a red sandstone or siltstone composed of Coconino-like quartz grains scattered through finer-grained sediment. The Seligman Member appears to interfinger with and lie conformable with the underlying Coconino Sandstone.[3][4][5]
Overlying the Seligman Member is the Brady Canyon Member. It consists of cliff-forming limestone and dolomite. Laterally, the Brady Canyon Member is divisible into two facies grading from one through a third transitional facies into the other using differences in lithology and fossil content. The first facies is exposed in an area from the extreme western edge of its outcrop belt east to Toroweap Valley and southeast almost to Seligman, Arizona. This facies consists of a marine limestone that is mostly coarsely crystalline and cherty in some beds. This facies contains a fauna dominated by brachiopods and echinoids. The second facies of the Brady Canyon Member is exposed in outcrops eastward past Seligman, Arizona to where it merges into the enclosing red beds. In consist of fine-grained, mostly sand-, silt-, and clay-free limestone. It contains a fauna composed almost exclusively of abundant, but poorly preserved, pelecypods and gastropods. It apparently accumulated nearer the coastline and likely under brackish-water conditions. The transition zone between the two facies consists of an unfossiliferous, thin-bedded (1 to 2 in (2.5 to 5.1 cm) thick), uniform-textured dolomite. This limestone weathers into smooth, small, angular cobbles. In western Grand Canyon region, it is thickest, as much as to 280 ft (85 m) thick. The Brady Canyon Member thins uniformly to the east where it is of approximately 220 ft (67 m) thick in the type section in Toroweap Valley and disappears near Marble Canyon as it merges with the overlying Woods Ranch Member. Beds of the third (dolomite) facies recur between overlying red beds of the Woods Ranch Member and the other facies of the Brady Canyon Member as part of a gradational contact between these members. Below Desert View Point in Grand Canyon, the Brady Canyon Member is about 20 ft (6.1 m) thick and is entirely missing in outcrops along the Little Colorado Canyon and in Sycamore Canyon.[3][4][5]
The red beds of the Woods Ranch Member consist of interbedded layers of gypsum, thin-bedded dolomite, and sandstone. Eastward of Havasu Canyon this member lack gypsum and dolomite and contains beds of white, cross-bedded sandstone. Breccias or intraformational conglomerates occur in many places throughout the entire outcrop of the Woods Ranch Member. Associated with these breccias in some places are lacustrine travertines. A prominent feature found throughout the entire outcrop of the Woods Ranch Member is a fossil-bearing limestone, It occurs over a remarkably wide area without appreciable variation with a thickness of only 3 to 4 ft (0.91 to 1.22 m). The fossils found everywhere in with this marker bed consist only of a pelecypod of the genus Schizodus. This member forms distinctive slopes and attains a maximum thickness of about 180 ft (55 m).[3][4][5]
- )
- Lucchitta, 2001. Hiking Arizona's Geology, Ivo Lucchitta, c 2001, )
Depositional environments
Marine transgression, and continental wind-blown sand, laid down the three units of Kaibab, Toroweap, and Coconino. The oceanic sea came from the west, and receded and re-transgressed. The Coconino Sandstone being made of sand dunes, continental, means that near-shore Toroweap deposits, and off-shore Kaibab Limestone, can all have the same actual ages in different regions where they were deposited, (continental, near-shore, open sea). In Arizona, the Toroweap formation is found in two transgressions eastwards,[6] Then shallow Kaibab and Toroweap seas advanced from the west, each depositing a wedge of marine limestone bordered with nearshore sandstone, siltstone, and gypsum. Desert conditions still prevailed on the shores, so dune sandstone there is of the same age as the marine limestones and nearshore deposits.[7]
-
North Rim, Grand Canyon:
4-Permian formations:
-4-Kaibab Limestone
-3-Toroweap Formation
-2-Coconino Sandstone
-1-Hermit Formation
-X-(upon Supai Group "redbeds") -
Toroweap Formation in Walnut Canyon, east of Flagstaff, Arizona
Darker middle section is Toroweap: above: (3): Kaibab Limestone; below: (1): Coconino Sandstone (showing fossil dunes)-(note erosion of dunes compared to horizontal bedding of Kaibab Limestone)
See also
References
Citations
- ^ Lassiter, S.L., Tweet, J.S., Sundberg, F.A., Foster, J.R., and Bergman, P.J., 2020. Chapter 5. Paleozoic Invertebrate Paleontology of Grand Canyon National Park. In: Santucci, V.L., Tweet, J.S., ed., pp. 109-236, Grand Canyon National Park: Centennial Paleontological Resource Inventory (Non-sensitive Version) . Natural Resource Report NPS/GRCA/NRR—2020/2103. National Park Service, Fort Collins, Colorado, 603 pp.
- ^ a b c Connors, T.B., Tweet, J.S., and Santucci, V.L., 2020. Chapter 3. Stratigraphy of Grand Canyon National Park. In: Santucci, V.L., Tweet, J.S., ed., pp. 54–74, Grand Canyon National Park: Centennial Paleontological Resource Inventory (Non-sensitive Version) . Natural Resource Report NPS/GRCA/NRR—2020/2103. National Park Service, Fort Collins, Colorado, 603 pp.
- ^ a b c d e f g McKee, E. D., 1938, The environment and history of the Toroweap Formation and Kaibab formations of northern Arizona and southern Utah. Publication, no. 492. Carnegie Institution of Washington, Washington, DC. 268 pp.
- ^ a b c d e Sorauf, J. E. and G. H. Billingsley, 1991, Members of the Toroweap and Kaibab Formations, Lower Permian, northern Arizona and southwestern Utah. The Mountain Geologist, 28(1):9–24.
- ^ a b c d Hopkins, R. L., and K. L. Thompson, 2003, Kiabab Formation. in: Beus, S.S., Morales, M., eds., pp. 196–211, Grand Canyon Geology, 2nd. Oxford University Press, New York. ISBN 978-0-19-512299-2, 448 pp.
- ^ Chronic, Halka. Roadside Geology of Arizona, c. 1983, 23rd printing, Mountain Press Publishing Co. 322 pages. pp. 229–232. (US 89A Marble Canyon – Fredonia), pp. 179–180. softcover, ISBN 978-0878421473
- ^ Lucchitta, 2001. Hiking Arizona's Geology, Mountaineers's Books. softcover, pp. 207. ISBN 0898867304
External links
